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OCR for page 184
EFects of Beta'Adrenergic Agonists on
Growth and Carcass Characteristics of Animals
LARRY A. MUIR
Until recently, few mechanisms were
known through which a drug could promote
the growth performance or improve the
carcass characteristics of livestock and poul-
try (Muir, 1985~. Antimicrobial agents, such
as antibiotics and antibacterials, improve
growth performance of livestock and poultry
by killing or inhibiting the growth of micro-
organisms (Muir et al., 1977~. Estrogenic
agents improve growth performance and
carcass characteristics of cattle and sheep,
but the specific mechanism is not well
understood (Burroughs et al., 1954; Dinus-
son et al., 1950; Muir et al., 1983~. Proges-
tational agents improve the growth perform-
ance of cyclic heifers by inhibiting estrus
and therefore its adverse affects, such as
hyperactivity and reduced feed consump-
tion (Davis, 1969~. Androgenic agents im-
prove growth performance and carcass char-
acteristics of cattle and swine, especially
females, supposedly through a direct, re-
ceptor-mediated action on skeletal muscle
cells (Heitzman, 19804. In addition, exoge-
nous growth hormone administration re-
portedly improves growth performance and
carcass characteristics of livestock (Machlin,
1972; Wagner and Veenhuizen, 19784.
184
Now a new mechanism has been found
through which the growth performance and
carcass characteristics of all poultry and
livestock species are dramatically improved
(Baker et al., 1984; Beermann et al., 1986;
Dalrymple et al., 1984; Moser et al., 1986;
Muir et al., 1985; Ricks et al., 1984~. This
mechanism involves the activation by beta-
adrenergic agonists (beta-agonists) of spe-
cific beta-adrenoceptors on the surface of
adipocytes and skeletal muscle cells. This
paper describes what is known about beta-
agonists and the mechanisms through which
they work.
WHAT ARE BETA-AGONISTS?
Beta-agonists are structural analogs of the
catecholamines epinephrine and norepi-
nephrine. Epinephrine and norepinephrine
are very similar in structure, and both bind
to four different cell surface receptors called
adrenoceptors (specifically, the alpha l, alphas,
betel, and betas receptors). Of special in-
terest are the effects of beta-agonists on
adipose and muscle tissues. The adipose
tissue of most species contains beta-recep-
tors that, when activated, stimulate lipoly
OCR for page 185
BETA-ADRENERGIC AGONISTS
sis. Most muscle tissue contains primarily
betas or betas receptors, which, when ac-
tivated, cause a specific muscular function.
Skeletal muscle is known to have betas
receptors, but their response function is not
well understood.
The structures of the beta-agonists that
will be discussed in this paper isoproter-
enol, clenbuterol, cimaterol, L-640,033, add
BRL3513~are shown in Figure 1. Isopro-
tereno} is a very potent beta/beta agonist
that is not orally active but is very effective
in vitro. Clenbutero} and cimaterol (Amer-
ican Cyanamid) ant! L-640,033 (Merck) are
orally active beta-agonists that have been
shown to stimulate animal growth and change
carcass characteristics (Dalrymple et al.,
1984; Muir et al., 1985; Ricks et al., 19844.
BRL35135 (Beecham) is an orally active
HO
HO~CH-CH2-NH-CH (CH3)2
C1
a>=\ OH
/ \ 1
H2N - \ /^ CH-CH2-NH-C (CH3)3
C1
OH H
H9N~ C CH2-NH- IC-CH2-CH2-C6H5
C - N
i4: OH
H2N~CH-CH2-NH-CH (CH3)2
i=: OH C\H3 ~ ~
CH-CH2-NH-C-CH2~ OCH2-CO2-CH3
185
beta-agonist that has been shown to stim-
ulate lipolysis (Arch et al., 1983, 1984~.
EFFECTS OF BETA-AGONISTS ON
GROWTH PERFORMANCE AND
CARCASS CHARACTERISTICS
Numerous growth trials have been con-
ducted with different beta-agonists at vary-
ing dose levels in poultry, swine, sheep,
and, to a lesser extent, cattle (Baker et al.,
1984; Beermann et al., 1986; Dalrymple et
al., 1984; Moser et al., 1986; Muir et al.,
1985; Ricks et al., 1984~. The results of these
trials are summarized in Tables 1 and 2. In
general, beta-agonists work best when used
during the finisher period, regardless of
species. Optimum responses are obtained
when these drugs are administered right to
Isoproterenol
CIonbuterol
L - 40,033
Cimaterol
BRL35135
FIGURE 1 Structures of the beta-adrenergic agonists isoproterenol,
clenbuterol, L-640,033, cimaterol, and BBL35135.
OCR for page 186
186
TABLE 1 Profile of a Beta-Adrenergic Agonist Product for Livestock Growth
Promotion Growth Performance
APPENDIX
Characteristic Poultry RuminantSwine
Dietary use level (ppm) 0.~2 l_l0a0.2 -
Growth rate (% increase) 4 ~20b0~
Feed conversion (% improvement) 5 0-2050 6
a Sheep and cattle data.
bSheep data only; cattle data not available.
SOURCE: Based on studies by Muir et al. I_,, . _ . ~,, _
Moser et al. (1986), and Dalrymple et al. (1984) using different beta-agonists.
the time of marketing. How close to mar-
keting time that beta-agonists will actually
be used will Repent] on the withdrawal time
for each cirug; actual withdrawal times have
not yet been established.
In poultry, the dietary use levels for beta-
agonists range from 0.2 to 2 ppm in the
feed. When given during the final 2 to 4
weeks of the 7-week period before slaugh-
ter, improvements in growth rate and feed
conversion of 4 ant] 5 percent, respectively,
are usually obtainer] with broilers. Also,
total carcass protein is increased approxi-
mately 6 percent, while total carcass fat is
reduced. Abdominal fat is reduced, but the
reduction is less than expected. In addition,
the eject of beta-agonists on abclominal fat
appears to differ between sexes, with males
showing little or no reduction and females
a reduction of 5 to 20 percent. As a result
of these changes, carcass yield of broilers is
usually increased by approximately 1 per
cent.
(1985). Ricks et al. (1984). Baker et al. (1984), Beermann et al. (1986),
In ruminants, the dietary use levels for
beta-agonists range from 1 to 10 ppm in the
feed. In sheep, feeding of 1 to 2 ppm for
the last 3 to 6 weeks of the finishing period
appears to be most elective. In most sheep
growth trials, responses in growth rate and
feed conversion of 20 percent are obtained,
although occasionally no response is ob-
servecI. In terms of carcass composition,
sheep respond] with a 10 percent increase
in total carcass protein and a 15 to 30 percent
increase in the loineye area. Total carcass
fat is reduced 20 to 30 percent, with even
larger decreases in back fat and abdominal
fat.
Data on the effects of beta-agonists in
cattle are extremely limiter! but do show
changes in carcass composition that are
similar to those observer] for sheep.
Swine appear to be more sensitive to
beta-agonists than other species, with 0.2
to 4 ppm in the feed appearing to yield
optimum results. Unlike other species, swine
TABLE 2 Profile of a Beta-Adrenergic Agonist Product for Livestock Growth
Promotion Carcass Characteristics
Characteristic Poultry Ruminanta Swine
Carcass protein (% increase) 6 10 4-8
Loineye area (% increase) 15-20 9-15
Carcass fat (% decrease) 4-8 20~0 10-16
Back fat (% decrease) 2~50 10-17
Abdominal fat (% decrease) 2-8 20~5
aSheep and cattle data.
SOURCE: Based on studies by Muir et al. (1985), Ricks et al. (1984), Baker et al. (1984), Beermann et al. (1986),
Moser et al. (1986), and Dalrymple et al. (1984) using different beta-agonists.
OCR for page 187
BETA-ADRENERG1C AGONISTS
have failed in most reported trials to respond
with improved growth rate or feed conver-
sion. The studies in which improved growth
performance was observed have used short-
duration treatment (4 weeks or less). Swine
do show very consistent improvement in
carcass characteristics when medicated with
beta-agonists. Total carcass protein is in-
creased 4 to 8 percent, and loineye area
muscle protein is increaser] 9 to 15 percent.
Total carcass fat and back fat are reduced
10 to 17 percent.
In addition to food animal species, beta-
agonists are also very effective in laboratory
animals. For example, clenbuterol has been
shown to improve the growth performance
and shift the carcass composition of young,
rapidly growing male rats (Table 3; Rickes
et al., 1985). Apparently, a dose of 10 ppm
in the feed produces the maximum re-
sponse: a 9 percent improvement in weight
gain, a 10 percent improvement in feed
conversion, a 9 percent increase in total
carcass protein, and a 20 percent reduction
in total carcass fat. These responses to beta-
agonists in the rat are very similar to those
observer! in food-producing animals. Thus,
the rat appears to be an excellent model for
studying beta-agonists as growth promoters.
Beta-agonists have been examined for
TABLE 3 Effects of a Beta-Adrenergic
Agonist, Clenbuterol, on Growth
Performance and Carcass Composition of
the Rat (percent change over control)
Clenbuterol, ppm in diet
Characteristic
21050
Weight gain (g/day)
Feed intake (g/day)
Feed conversion
(g feed/g gain)
Carcass protein
(g/carcass) 5.1*8.5**9.2**
Carcass fat (g/carcass) - 8.5*- 19.9**- 23.3**
4.8* 9.6*
0.4 -3.4
8.5*
1.7
-3.6 - 10.7*
- 7.2*
*P < 0.05 compared with control.
**P < 0.01 compared with control.
SOURCE: Ricks et al. (1985).
187
their effects on milk production by dairy
cows. Cows producing 17 to 18 kg of milk
per day were medicated with the beta-
agonists formoterol, zinterol, or Z1170. The
beta-agonists were fed at 20 mg per head
per day for 10 clays. Milk production on
days 5 to 10 of treatment was not different
from that of controls or from milk production
before or 5 days after treatment. In addition,
the composition of the milk was not altered.
These data suggest that beta-agonists, unlike
growth hormone, apparently are not able
to stimulate milk production, even though
both beta-agonists and growth hormone ap-
pear to function through a repartitioning of
nutrients.
EFFECTS OF BETA-AGONISTS ON
LIPID METABOLISM
Free fatty acid (FFA) synthesis is the
conversion of glucose, acetate, or both to
free fatty acids. Lipogenesis is the sum of
FFA synthesis and the esterification of FFAs
to triglycerides (TGs). Lipolysis is the break-
clown of TGs to FFAs and glycerol. The
rate of glycerol production can be used to
estimate lipolysis because the glycerol pro-
ducecl during lipolysis cannot be reused for
FFA esterification since adipocytes lack the
necessary enzyme for phosphorylation of
glycerol (phosphokinase). A scheme for the
regulation of lipolysis by beta-agonists through
specific adrenoceptors is shown in Figure
2. The activation of the beta-receptor on
the outer surface of the aclipocyte plasma
membrane activates the chain of events that
eventually leads to the breakdown of stored
triglycerides to FFAs and glycerol.
Many beta-agonists effectively reduce lipic!
accumulation in adipose tissue. The mech-
anisms through which they act were studied
at Merck in an in vitro system (Duquette
and Muir, 1984~. Adipose tissue was taken
from an animal source, for example, rat
epicliclymal or perirenal fat, using the pro-
ceclure of Rodbell (1964~. Aclipocytes were
incubated with treatment for 2 hours at
OCR for page 188
188
APPENDIX
Adipose
Plasma
Membrane
Adrenergic Recept pAC~t~de;
a Adrenergic Receptor | Inhibiting
Peps -
Guanino
Nucleo~dde
,4N
Phosphodl~terase
c IMP ' 5'AMP
l ATP
Guanine
Nucleotide
Protein Klnase
Active Hormone
Inactive Hormone
Sensitive Upase ~ Sensitive Upa"
TrIglycerld" ~ · - Free Fatty Acids
Glycerol
FIGURE 2 Scheme for the regulation of lipolysis by beta-agonists through
specific adrenoceptors. Source: Adapted from I. A. Garcia-Sarnz and
J. N. Fain. 1982. Regulation of adipose tissue metabolism by catechol-
amines: Roles of alphas, alphas, and beta-adrenoceptors. Trends Phar-
macol. Sci. 3:201.
37°C. Lipolysis was estimated by measuring
glycerol production. Glycerol was measured
by a fluorometric mollification of the enzy-
matic method of Wieland (1974). Lipoge-
nesis was estimated by measuring the in-
corporation of i4C-acetate into fatty acids
from TGs.
Isoprotereno} is very effective for stimu-
lating lipolysis (glycerol release) in rat a(li-
pose tissue in vitro. At a concentration of
0.001 ,uM, isoprotereno} had no effect on
the basal rate of glycerol release; maximum
stimulation occurred at 1 ,uM. The half-
maximal effect dose was 0.017 ,uM. The
idea that beta-agonists stimulate lipolysis
through a specific beta-receptor is sup-
portec! by the observation that the stimu-
lation of lipolysis by isoproterenol can be
blocked by beta-antagonists, such as pro-
pranolol or betaxolol.
Isoprotereno} was also studied for its
effects on lipogenesis (~4C-acetate incorpo-
ration into FFAs in TGs) in rat adipose
tissue in vitro (Duquette and Muir, 1985~.
Insulin was used in this study to increase
lipogenesis and to simulate in viva condi-
tions. Isoprotereno! at concentrations of
0. 01, 0. OS, and 0.25 EM reduced i4C-acetate
incorporation into TGs in a close-related
manner. In addition, when the TGs were
hydrolyzes! and the resulting FFAs and
glycerol were tested for i4C activity, the
results showed that the effect of isoproter-
enol was primarily to reduce the incorpo-
ration of i4C-acetate into FFAs with only
minor effects on glycerol. These observa-
tions support the validity of this test system
for estimating lipogenesis.
These in vitro test systems for lipolysis
and lipogenesis were used to compare the
activities of four different beta-agonists. Iso-
proterenol, clenbuterol, L-640,033, and
BRL35135 were dose-titratec! in rat adipose
tissue to study their effects on lipolysis and
lipogenesis (Duquette ant] Muir, 1985). All
four inhibited lipogenesis and stimulated
OCR for page 189
BETA-ADRENERGIC AGONISTS
lipolysis. The intrinsic activity of all beta-
agonists was similar for both actions. This
means that the maximum effects of the beta-
agonists, inclepenclent of dose, were of sim-
ilar magnitude for each eject. Three of the
four beta-agonists were 5 to 10 times more
potent as inhibitors of lipogenesis than as
stimulators of lipolysis; BRL35135 was equally
potent for both. A comparison of the poten-
cies of these drugs for inhibition of lipoge-
nesis, as measured by 50 percent effective
concentration, were isoproterenol >
BRL35135 > L-640,033 > clenbuterol. Sim-
ilar comparisons for stimulation of linolvsis
animal
were BRL35135 > isoproterenol > L-640,033
> clenbuterol. These observations suggest
that in the animal a drug's efficacy for
reducing belly lipids may be even more
depenclent on that drug's activity for block-
ing lipogenesis than for stimulating lipolysis.
They also indicate that there is considerable
variation between beta-agonists in their po-
tencies for blocking lipogenesis and stimu-
lating lipolysis.
The ability of aclipocytes from different
species to initiate lipolysis in re-
sponse to similar concentrations of isopro-
terenol was investigated by Muir et al.
(1985~. Adipocytes from sheep, pigs, and
rats all responder! with increases of 380 to
2,300 percent over their controls. Aclipo-
cytes from chickens failer! to respond.
Because chicken adipocytes failed to re-
spond to isoproterenol, a more cletailec]
study was conductecI. The beta-agonists iso-
proterenol, clenbuterol, and L-640,033 and
the positive control glucagon were all tested
at concentrations of 0. 01, 0. 05, 0. 25, 1. 25,
and 6.25 EM for effects on lipolysis in
isolated chicken adipocytes (Muir et al.,
1985~. As expected, the chicken adipocytes
responded to glucagon with increased li-
polysis. However, none of the three beta-
agonists stimulated lipolysis at any of the
concentrations tested.
Since the chicken, unlike the other spe-
cies studied, synthesizes FFAs in the liver,
the ejects of beta-agonists on lipogenesis
189
in isolated chicken hepatocytes were inves-
tigated. Isoproterenol, clenbuterol, and L-
640,033 and the positive control glucagon
were tested on chicken hepatocytes in a
design identical to the one describer! for
chicken adipocytes (Muir et al., 1985~. As
expected, glucagon caused a close-related
inhibition in i4C-acetate incorporation into
FFAs in TGs (that is, inhibition of lipoge-
nesis). All three beta-agonists also caused a
dose-relatec] inhibition of lipogenesis. Thus,
beta-agonists block body fat accumulation
in chickens by inhibiting lipogenesis in the
liver, but they are not able to stimulate
lipolysis in adipose tissue.
In summary, beta-agonists have been
shown to stimulate lipolysis and inhibit
lipogenesis in the adipose tissue of the rat
(Muir et al., 1985). In adipose tissue from
sheep and swine, beta-agonists can stimu-
late lipolysis, but no data are available
regarding their effects on lipogenesis in
these species. Beta-agonists are ineffective
in chicken adipocytes. However, they in-
hibit lipogenesis in chicken hepatocytes,
the primary site for fatty acid synthesis in
poultry.
EFFECTS OF BETA-AGONISTS ON
SKELETAL MUSCLE PROTEIN
METABOLISM
Skeletal muscle celDs have beta-receptors,
ant! beta-agonists increase skeletal muscle pro-
tein in animals. Ibus, beta-agonists might exert
a direct effect on skeletal muscle cells, or their
effect could be indirect through changes in
plasma hormone concentrations or nutrient
partitioning. Herbert et al. (1986) measurer]
the effects of clenbutero! on urinary nitrogen
excretion by sheep by infusing clenbutero!
together with fee(lstuffs directly into the abom-
asum. Within 6 hours after the initiation of
infusion, nitrogen excretion dropped about 25
percent anc] remained clepresse ;1 over the entire
7-(lay test. These results suggest that clenbu-
terol causer! an immediate improvement in
nitrogen retention.
OCR for page 190
190
Muscle protein accumulation is the net
balance of protein synthesis minus protein
degradation. A drug like clenbutero} that
dramatically increases muscle protein ac-
cumulation might be expected to act by
altering the rate of muscle protein synthesis,
degradation, or both. This was examined by
Reeds et al. (1986), who fed clenbuterol at
O or 2 ppm in the diet to young male rats.
The rates of protein synthesis in two skeletal
muscles the gastrocnemius and soleus-
were estimates] by the method of Garlick
et al. (1980~. Rats were injected! with a large
dose of labeled phenylalanine ant] killed 10
minutes later. The rate of protein synthesis
was estimated from the amount of labeled
phenylalanine in the muscle protein and
from the specific activity of the free phen-
ylalanine. Protein deposition was calculated
from the slope of the line of the log of the
protein content versus time. Protein content
was estimates! from bocly weight. The rate
of protein degradation was estimated from
the differences between rates of synthesis
and deposition. The effects of clenbuterol
on muscle protein mass and rates of protein
synthesis and degradation were determiner!
on days 4, 11, ant! 21 of treatment.
After only 4 days of clenbuterol treat-
ment, the protein masses of the gastrocne-
mius and soleus muscles were increased
17.7 and 50.6 percent, respectively, over
the controls. These larger muscle protein
masses were maintained throughout the 21-
ciay test period. In both the gastrocnemius
and soleus muscles, the rate of protein
degradation was decreased on day 4 by 55
percent, with no change in the rate of
protein synthesis. By clay 11, this decrease
in the rate of protein degraciation relative
to the controls was still evident, but the
magnitude of the decrease was slightly less
(39 en c] 25 percent for the gastrocnemius
and soleus muscles, respectively). The rates
of protein synthesis were still the same as
for the controls. At clay 21, the rates of
protein degradation were still reduced (20 related manner (Rickes et al., 1985~. Feed
and 30 percent, respectively), but the rates consumption was increased only at the two
APPENDIX
of protein synthesis had decreased 20 per-
cent relative to the controls.
These observations suggest that clenbu-
tero} increases skeletal muscle protein in
the rat by reducing the rate of muscle
protein degradation. Apparently, after mus-
cle protein mass is increased by a certain
amount, the rate of protein synthesis is
reduced. At this point, the rate of muscle
protein accumulation is reduced to normal,
but the extra muscle protein mass is main-
tained.
SEPARATION OF LIPID AND PROTEIN
EFFECTS OF BETA-AGONISTS
The information presented thus far clem-
onstrates that beta-agonists reduce the con-
tent of lipids in the carcass, increase the
accumulation of skeletal muscle protein, and
improve the growth rate and feed conver-
sion of the animal. To understand the mode
of action of beta-agonists, studies were un-
dertaken to determine whether the growth
and feet] conversion responses were asso-
ciated with the ejects of the beta-agonists
on lipid metabolism, protein metabolism,
or both.
L-640,033 is an excellent growth-pro-
moting beta-agonist. BRL35135 is a potent
inhibitor of lipogenesis and a potent stim-
ulator of lipolysis, but it does not appear to
affect muscle protein metabolism. Both beta-
agonists were evaluated in similar rat growth
trials. In each trial, 110 young mate rats (10
per treatment and 20 controls) were feel the
beta-agonist at 0, 0.25, 1.0, 2.5, 5.0, 10,
15, 25, or 50 ppm for 2 weeks. Clenbuterol
at 10 ppm was used as a positive control.
Rate of gain, feed intake, and feed conver-
sion were determined. In addition, the
weights of the gastrocnemius muscle and
epididymal fat pads were measured to assess
the ejects of the drugs on skeletal muscle
protein and carcass fat.
L-640,033 increased rate of gain in a dose
OCR for page 191
BETA-ADRENERGIC AGONISTS
highest doses (25 and 50 ppm). Feed con-
version was improved 4 to 6 percent, in-
dependent of dose. Also, L-640,033 in-
creased gastrocnemius muscle weight and
decreased epididymal fat pad weight, both
in dose-related manners (Rickes et al., 1985~.
Both responses were similar to those ob-
served with Clenbuterol at 10 ppm.
BRL35135 had no effect on either rate of
gain or feed conversion at any of the doses
tested. Clenbuterol, the positive control,
increased rate of gain and improved feed
conversion. As expected, BRL35135 re-
duced epididymal fat pad weight in a dose-
related manner, with the maximum effect
at 5 ppm. The reduction in epididymal fat
pad weight with BRL35135 was significantly
greater than that with clenbuterol. BRL35135
did not increase gastrocnemius muscle
weight, while Clenbuterol increased it 17
percent. Thus, BRL35135 reduced carcass
lipids in the rat without increasing skeletal
muscle protein or improving rate of gain or
feed conversion.
These observations suggest that improve-
ments in growth rate and feed conversion
obtained with beta-agonists are associated
with the effects of the drugs on skeletal
muscle protein metabolism and not with
their effects on lipid metabolism. Thus,
while the ability of a beta-agonist to reduce
carcass fat is an important benefit, this
activity does not appear to be related to any
growth-promoting activity.
EFFECTS OF BETA-AGONISTS ON
PLASMA HORMONES
The effect of beta-agonists on plasma
hormone levels is an important considera-
tion when assessing the possible modes of
action of these drugs in promoting growth.
Thus, a study was carried out in which 50
young male rats (10 per treatment and 20
controls) were fed Clenbuterol at 10 ppm,
BRL35135 at 15 ppm, or L-640,033 at 15
ppm for 2 weeks (L. A. Muir, unpublished
data). At necropsy, gastrocnemius muscle
191
and epididymal fat pad weights were meas-
ured to assess the effects of the beta-agonists
on skeletal muscle protein and carcass lipids.
In addition, blood samples were obtained
and assayed for plasma growth hormone,
insulin, somatomedin-C (SM-C), and glu-
cose.
All three beta-agonists produced the ex-
pected changes in gastrocnemius muscle
and epididymal fat pad weights. Clenbuterol
and L-640,033 increased gastrocnemius
muscle weight and decreased epididymal
fat pad weights. BRL35135 decreased epi-
didymal fat pad weight, but did not alter
gastrocnemius muscle weight. Plasma in-
sulin levels were decreased approximately
30 percent by Clenbuterol and L-640,033,
but were not decreased by BRL35135. Sim-
ilar effects were observed for SM-C. Plasma
growth hormone was decreased by all three
drugs, especially BRL35135, but the re-
sponses were so variable that none of these
growth hormone reductions was statistically
significant. Clenbuterol decreased plasma
glucose, while the other beta-agonists had
no effect on glucose.
Beermann et al. (1985) reported that
cimaterol fed to lambs for 12 weeks reduced
plasma insulin and elevated plasma T4 (thy-
roid hormone thyroxine) but did not alter
plasma levels of T3 (thyroid hormone tri-
iodothyronine), cortisol, or prolactin.
SUMMARY
Beta-adrenergic agonists are analogs of
the catecholamines epinephrine and nor-
epinephrine. They appear to work through
specific beta-adrenoceptors on the surface
of adipocytes and skeletal muscle cells. Beta-
adrenergic agonists that are known to pro-
mote growth, such as clenbuterol, cima-
terol, and L-640,033, improve the growth
rate and feed conversion of sheep and poul-
try. Effects on swine are more variable,
while definitive data on cattle are not yet
available. These drugs have also been shown
to decrease total carcass fat and to increase
OCR for page 192
192
total carcass protein in all four animal spe-
cies. Many beta-adrenergic agonists reduce
carcass lipids by stimulating lipolysis and
blocking lipogenesis in adipose tissue. The
exception occurs in poultry, where these
drugs inhibit lipogenesis in the liver but do
not stimulate lipolysis in adipose tissue.
Less is known about the effects of beta-
adrenergic agonists on protein metabolism
in skeletal muscle. However, recent studies
suggest that some of these drugs increase
skeletal muscle protein accretion by reduc-
ing the rate of protein degradation without
altering the rate of protein synthesis. Stud-
ies in rats comparing the growth-promoting
and carcass-altering effects of two beta-
acirenergic agonists, L-640,033 and
BRL35135, indicate that improvements in
growth rate and feed conversion with beta-
adrenergic agonists are associates! with im-
proved protein accretion rather than altered
lipid metabolism. Finally, growth-promot-
ing beta-adrenergic agonists were found to
reduce plasma levels of insulin and soma-
tomedin-C in the rat but did not elevate
plasma growth hormone levels. These ob-
servations support the concept that growth-
promoting beta-adrenergic agonists work
directly through skeletal muscle cell recep-
tors and not indirectly through the elevation
of plasma growth hormone or insulin con-
centrations. In addition, beta-adrenergic ag-
onists that reduce carcass lipids appear to
work directly through beta-adrenoceptors
on the surface of aclipocytes in livestock and
hepatocytes in poultry.
ACKNOWLEDGMENT
The author wishes to acknowledge the
work of Paul Duquette, Eric Rickes, and
Sandra Wien, who contributed to numerous
aspects of Me beta-agonist research at Merck,
and Dr. Y. T. Yang, whose ideas and re-
search finclings supported our beta-agonists
research program.
APPENDIX
REFERENCES
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Arch, J. R. S., A. T. Ainsworth, M. A. Cawthorne, V.
Piercy, M. V. Sennitt, V. E. Thody, C. Wilson, and
S. Wilson. 1984. Atypical beta-adrenoceptor on
brown adipocytes as target for anti-obesity drugs.
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Baker, P. K., R. H. Dalrymple, D. L. Ingle, and C.
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Beermann, D. H., W. R. Butler, D. E. Hogue, R. H.
Dalrymple, and C. A. Ricks. 1985. Plasma metabolic
hormone, glucose, and free fatty acid concentrations
in lambs fed the repartitioning agent, cimaterol (CL
263,780~. J. Anim. Sci. 61(Suppl. 1~:254 (Abstr.).
Beermann, D. H., D. E. Hogue, V. K. Fishell, R. H.
Dalrymple, and C. A. Ricks. 1986. Effects of cima-
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Anim. Sci. 62:370.
Burroughs, W., C. C. Culbertson, J. Kastelic, E.
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Dalrymple, R. H., P. K. Baker, P. E. Gingher, D. L.
Ingle, J. M. Pensack, and C. A. Ricks. 1984. A
repartitioning agent to improve performance and
carcass composition of broilers. Poultry Sci. 63:2376.
Davis, L. W. 1969. MGA- A new concept in heifer
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Dinusson, W. E., F. N. Andrews, and W. M. Beeson.
1950. The effects of stilbesterol, testosterone, thyroid
alteration and spaying on the growth and fattening
of beef heifers. J. Anim. Sci. 9:321.
Duquette, P. F., and L. A. Muir. 1984. Effects of
ovine growth hormone and other anterior pituitary
hormones on lipolyis of rat and ovine adipose tissue
in vitro. J. Anim. Sci. 58:1191.
Duquette, P. F., and L. A. Muir. 1985. Effect of the
beta-adrenergic agonists isoproterenol, clenbuterol,
L-640,033 and BRL35135 on lipolysis and lipogenesis
in rat adipose tissue in vitro. J. Anim. Sci. 61(Suppl.
1~:265 (Abstr.).
Garcia-Sarnz, J. A., and J. N. Fain. 1982. Regulation
of adipose tissue metabolism by catecholamines:
Roles of alphas, alphas and beta-adrenoceptors. Trends
Pharmacol. Sci. 3:201.
Garlick, P. S., M. A. McNurlan, and V. R. Preedy.
1980. A rapid and convenient technique for meas
OCR for page 193
BETA-ADRENERGIC AGONISTS
uring the rate of protein synthesis in tissues by
injection of"3H" phenylalanine. Biochem. J. 192:719.
Heitzman, R. J. 1980. Manipulation of protein metab-
olism, with special reference to anabolic agent. Pp.
193-203 in Protein Deposition in Animals, P. J.
Buttery and D. B. Lindsay, eds. Boston: Butter-
worth.
Herbert, F., F. D. DeB. Hovell, and P. J. Reeds.
1986. Some preliminary observations on the im-
mediate effects of clenbuterol on heart rate, body
temperature and nitrogen retention in lambs wholly
nourished by intragastric infusion. Br. J. Nutr.
56:483 (Abstain.
Machlin, L. J. 1972. Effect of porcine growth hormone
on growth and carcass composition of the pig. J.
Anim. Sci. 35:794.
Moser, R. L., R. H. Dalrymple, S. G. Cornelius, J.
E. Pettigrew, and C. E. Allen. 1986. Effect of
cimaterol (CL 263,780) as a repartitioning agent in
the diet for finishing pigs. J. Anim. Sci. 62:21.
Muir, L. A. 1985. Mode of action of exogenous sub-
stances on animal growth an overview. J. Anim.
Sci. 61(Suppl. 2):154.
Muir, L. A., M. W. Stutz, and G. E. Smith. 1977.
Feed additives. Pp. 27-37 in Livestock Feeds and
Feeding, D. C. Church, ed. Corvallis, Oreg.: ORB
Books.
Muir, L. A., S. Wien, P. F. Duquette, E. L. Rickes,
and E. H. Cordes. 1983. Effects of exogenous growth
hormone and diethylstilbesterol on growth and car
193
cass composition of growing lambs. J. Anim. Sci.
56:1315.
Muir, L. A., S. Wien, P. F. Duquette, and G. Olson.
1985. Effect of the beta-adrenergic agonist L-640,033
on lipid metabolism, growth and carcass character-
istics of female broiler chickens. J. Anim. Sci.
61(Suppl. 1~:263 (Abstr. ).
Reeds, P. J., S. M. Hay, P. M. Dorwood, and R. M.
Palmer. 1986. Stimulation of muscle growth by
clenbuterol: Lack of effect on muscle protein bio-
synthesis. Br. J. Nutr. 56:249.
Rickes, E. L., L. A. Muir, and P. F. Duquette. 1985.
Effect of the beta-adrenergic agonist L-640,033 on
growth and carcass composition of growing male
rats. J. Anim. Sci. 61(Suppl. 1):264 (Abstr. ).
Ricks, C. A., R. H. Dalrymple, P. K. Baker, and D.
L. Ingle. 1984. Use of beta-agonist to alter fat
and muscle deposition in steers. J. Anim. Sci.
59:1247.
Rodbell, M. 1964. Metabolism of isolated fat cells. I.
Effects of hormone on glucose metabolism and
lipolysis. J. Biol. Chem. 239:375.
Wagner, J. F., and E. L. Veenhuizen. 1978. Growth
performance, carcass deposition and plasma hor-
mone levels in wether lambs when treated with
growth hormone and thyroprotein. J. Anim. Sci.
47(Suppl. 1):397.
Wieland, O. 1974. Glycerol UV-method. Pp. 140(
1409 in Methods of Enzymatic Analysis, H. U.
Bergmeyer, ed. New York: Academic Press.
OCR for page 194
Anabolic EFects of Porcine
Somatotropin on Pig Growth
TERRY D. ETHERTON
Animal agriculture must develop ways to
enhance the growth performance of animals
raised for meat production in order to pro-
vide consumers with a product that is leaner
and, therefore, more nutritious. Because
leaner meat products will be sought by
consumers concerned about the relation
between the consumption of saturated fatty
acids and the incidence of coronary heart
disease, strategies to increase growth rate
and improve feed efficiency (ratio of feed
consumed to bocly weight gained) will eco-
nomically benefit producers. The central
question is, what research options are avail-
able now and in the foreseeable future that
may provide effective ways to manipulate
meat animal growth performance?
This paper focuses on the concept that an
elevation of blood concentrations of growth
hormone (GH, or somatotropin) in meat
animals markedly increases growth rate,
improves feed efficiency, and dramatically
increases muscle mass while decreasing adi-
pose tissue (fat) mass (Chung et al., 1985;
Etherton et al., 1986a, 1986b, 1987; Mach-
lin, 1972~. Table 1 shows the extent to which
growth hormone can affect the growth per-
formance of pigs. The stimulatory effects of
GH on growth performance have created
great interest in developing a GH-based
product for practical use in animal agricul-
ture. In fact, it is likely that such a product
will be available for use within the next 2
to 3 years. The mechanisms by which GH
works are discussed in the following section,
since a better understancling of them may
leac! to ways to improve the effectiveness of
GH or of alternative strategies for enhancing
growth performance.
Growth hormone is a protein that is
synthesized in the anterior pituitary gland
of mammals. It plays a central role in
stimulating normal growth and is both an-
abolic ant! catabolic in that it stimulates
growth rate ant! muscle accretion ant! con-
currently decreases adipose tissue growth
(Etherton et al., 1986b, 1987~. The positive
effects of GH on growth rate are indirect,
being mecliated largely by the GH-clepend-
ent insulin-like growth factor I (IGF-I, or
somatomedin-C) (Etherton ant! Kensinger,
1984~. The effects of GH on adipose tissue
growth and metabolism are direct, not being
mediated by IGF-I (Walton and Etherton,
1986; Walton et al., 1986, 1987a).
Observations by Etherton and coworkers
194
OCR for page 231
COMPOSITION OF MILK FROM DAIRY COWS
Biosynthesis
The synthesis of milk proteins has been
extensively reviewer! (Larson, 1979, 1985;
Mercier ancl Gaye, 1983~. In general, pro-
tein synthesis in mammary alveolar cells is
similar to other protein synthesis systems
in which DNA controls protein synthesis.
Messenger RNA carries the encoded DNA
message from the nucleus to the ribosomes
located in the rough encloplasmic reticulum
(RER) and cytoplasm. Ribosomes are com-
posec3 of ribosomal RNA and several pro-
teins combiner! into a ribonucleoprotein
complex, which, in conjunction with trans-
fer RNA, combines amino acids into peptide
chains. As the polypeptide chains are elon-
gatecl to form proteins, they pass out of the
HER, through the lumen, and into the
region of the Golgi apparatus where they
accumulate and polymerize into different
milk protein molecules. Casein must be
phosphorylated, bound with calcium, and
stabilized by calcium phosphate linkages
and other ionic bonds before being released
from the vesicles. The presence of alpha-
lactalbumin in the region of the Golgi ap-
paratus promotes synthesis of lactose. The
secretory vesicles containing essentially
nonfat milk constituents leave the cell by
moving to the apical surface and fusing with
the plasma membrane and discharging the
vesicular contents into the cell lumen.
Most of the proteins present in milk are
synthesized in the mammary gland, al-
though some immunoglobulins and albu-
mins are transferred from the blood (Larson,
19791. Blood leukocytes can also cross mam-
mary barriers either by passing between
secretory cells or by pushing secretory cells
directly into the lumen. Urea diffuses freely
across mammary cells, so there is a high
correlation between blood plasma and milk
urea concentrations (Thomas, 1980~.
The synthesis of milk protein requires
that both essential and nonessential amino
acids be supplied to the mammary gland
(Clark et al., 1978; Mepham, 1982~. Uptake
231
of free amino acids from the blood by the
mammary gland can occur via several trans-
port systems (Baumrucker, 1985~. Mepham
(1982) has classifier] essential and nonessen-
tial amino acids into three groups according
to uptake by the mammary gland. Group I
essential amino acids (methionine, histi-
dine, phenylalanine, tyrosine, and trypto-
phan) are taken up in amounts just sufficient
to meet milk protein synthesis needs. Group
II essential amino acids (valine, leucine,
isoleucine, arginine, lysine, and threonine)
are taken up in excess. However, some data
(Thomas, 1983) suggest that lysine and pos-
sibly leucine, isoleucine, and threonine
should also be included in group I. Group
III is the nonessential amino acicls. The
amounts taken up vary with animal, time,
and availability. In addition to free amino
acid uptake from blood, there is evidence
that rec] blood cells and the recycling of
amino acids also contribute to the cellular
amino acid pool (Baumrucker, 1985~.13reak-
down of red blood cell glutathionine can
make a significant contribution to the amount
of cysteine, glycine, and glutamic acic! avail-
able in the cell. Recycling of casein proteins
is reporter! to account for at least 7 percent
of the protein synthetic capacity in the
mammary gland.
Factors Affecting Milk Protein Content
Bree~lGenetics
Breeds differ in total milk protein per-
centage and type of milk protein produced.
Jersey and Guernsey cattle have the highest
percentages of total protein, casein, and
whey. Variability of the major protein frac-
tions within breeds has also been reporter]
(Roller) et al., 1956), with Holstein milk
containing less of the major caseins and
more gamma-casein than milk from other
breecis. Genetic variants have been dem-
onstrated for the milk protein groups, and
breed differences have been found for the
frequency of occurrence of these variants
(Gaunt, 19801.
OCR for page 232
232
Genetic selection would increase the per-
centage of protein in milk 0.075 percentage
units but decrease milk yield 231 pounds.
loins selection for milk yield, protein, and
fat is recommencled if the desired result is
increased yield of protein and fat (Gaunt,
1980; Van VIeck, 1978; Wilcox, 1978~. Gaunt
(1980) estimates] that it would take about 11
generations for milk protein percentages to
equal milk fat percentages if protein yield
with no change in fat percentage were used
as the selection criterion.
EnvironmentlManagement
ness, 1985; Ng-Kwai-Hang et al., 1982;
Rogers ant] Stewart, 19821. Milk protein
percentage declines in cows older than 3
years, with a 0.4 percentage unit drop being
reported over five lactations (Rogers and
Stewart, 1982~. This decline appears to be
primarily in the casein fraction; however,
changes in whey protein fractions have also
been reported (Kroeker et al., 1985~. Sug-
gestec! reasons for the change are deterio-
ration of unpiler tissue, selective culling for
high production, and increased incidences
of mastitis. The increase in immunoglobu-
lins with advancing age reported by Kroeker
et al. (1985) supports the latter suggestion.
Stage of lactation has a considerable in-
fluence on milk protein concentration (Dav-
ies et al., 1983; Ng-Kwai-Hang et al., 1982,
1985; Rogers and Stewart, 1982~. At the
beginning of lactation, colostrum is excep-
tionally rich in protein containing large
quantities of immunoglobulins and about
twice the levels of casein, beta-lactoglobu-
lin, ancl alpha-lactalbumin fount] in micI-
lactation milk. Total protein amounts fall
rapidly during the first few days of transition
from colostrum to normal milk ant! reach a
minimum about 5 to 10 weeks into lactation,
corresponding inversely to maximum milk
yield. Thereafter, the amount of protein
tends either to increase gradually as lacta
APPENDlX
lion progresses or to rise sharply when the
cow becomes pregnant.
Milk protein percentage (Ng-Kwai-Hang
et al., 1982) and yield (Keown et al., 1986)
are higher during fall and winter than spring
and summer. However, stage of lactation
and feeding practices confound these ob
servations as cows on spring pasture have
elevated milk protein concentrations (Rog
ers and Stewart, 1982~. Whey proteins have
been found to have no definite seasonal
variations (Kroeker et al., 1985~. High en
vironmental temperatures, above 29°C, have
been suggester! to depress milk protein
percentage, but cows offered cold water
Age has a significant e~ecton milk protein(10°C) cluring heat stress do not show in
neroenta~f, an] nomno.sition in onw.s (~en-creased milk protein concentrations over
cows offered 28°C water (Milam et al., 1986~.
Variations in milking procedure or fre
quency have a minor effect, if any, on milk
protein percentage. Milk protein or SNF
percentages clo not change during the milk
ing process (lenness, 1985~. Extended milk
ing intervals do not change milk protein or
SNF percentages until intervals exceed 16
hours (Rogers and Stewart, 1982~. Increas
ing milking frequency from twice to three
times daily for more than 15,000 Holstein
cows dill not change the percentage of SNF
(Gist et al., 1986~. Similar results were
reported by Amos et al. (1985) and DePeters
et al. (1985~.
HealthlPhysiology
Mastitis has very little effect on total milk
protein percentage; however, it drastically
alters the composition of milk protein
(Kitchen, 1981; Schultz, 1977~. The general
effect of mastitis is to impair milk synthesis
and loosen the connections between cells,
thereby increasing permeability of blood!
constituents Jenness, 1985; Wheelock, 1980~.
Milk proteins synthesized in the mammary
gland (caseins, beta-lactogIobulin, and al
pha-lactalbumin) decrease (Kitchen, 1981;
Schultz, 1977), whereas blood serum pro
teins (whey proteins) increase (Kitchen, 1981;
OCR for page 233
COMPOSITION OF MILK FROM DAIRY COWS
Kroeker et al., 1985; Poutrel et al., 1983;
Schultz, 1977~. Grappin et al. (1981) re-
ported a whey protein to total protein ratio
increase of 2.08 percent and a casein to total
protein ratio decrease of 1.85 percent for
every 1 log unit increase in somatic cell
count. The same change in somatic cell
count was reporter! by Ng-Kwai-Hang et al.
(1982) to decrease the ratio of casein to total
protein by 2.79 percent.
The hormone requirement for milk syn-
thesis and secretion is prolactin, acirenocor-
ticotrophic hormone, and estrogens and the
relative absence of progesterone. Of partic-
ular importance to milk protein synthesis is
prolactin (Tucker, 1985~. Current studies
(Bauman et al.. 1985; Peel et al., 1985) on
administration of exogenous growth hor-
mone have generally shown increases in
milk yield without significant changes in
composition. However, Eppard et al. (1985)
observed a slight decrease in milk protein
percentage and an increase in alpha-lactal-
bumin as a percentage of total milk protein
with increasing dosage levels (0 to 100 IU/
day) of bovine growth hormone.
Nutrition
Dietary crude protein affects milk yield
and consequently milk protein yield more
than milk protein percentage (Emery, 1978;
Kaufman, 1980; Thomas, 1980, 1983~. A
small effect of dietary crude protein con-
centration on milk protein percentage was
reported by Emery (1978~: a 0.02 percentage
unit increase in milk protein with every 1
percentage unit increase in clietary crude
protein between 9 and 17 percent. More
recently, Cragle et al. (1986) reported an
increase of 0.1273 Meal in milk protein
energy content per 1 Mcal gross energy
increase in feed protein. Neither of these
studies, however, considered source of di-
etary crude protein or change in milk pro-
tein composition. Thus, the increases in
milk protein observed may have been in
milk NPN and not true milk protein. Ele
233
vate(1 milk protein concentrations from cows
fed cliets high in rumen-clegradable protein
or NPN most likely will be from increased
milk urea or NP~ levels (Oltner et al., 1985;
Thomas, 1980~. On the other hand, diets
low in rurnen-degraciable protein or bal-
anced for optimal microbial protein synthe-
sis should increase supplies of amino acids
available to the mammary gland for protein
synthesis, and thus, more true milk protein
should be proclucec! (Kaufman, 1980; Old-
ham, 1984; Thomas, 1980~. However, the
proportions between true milk proteins (cas-
eins, beta-lactoglobulin, and alpha-lactal-
bumin) (lo not appear to change with in-
creases or decreases in milk protein synthesis
(Thomas, 1983~.
In experiments where protein (usually
casein) has been abomasally infused to in-
crease amino acid supplies to the tissue,
increases in milk protein percentage along
with milk yield have been reporter! (Clark,
1975; Clark et al., 1977~. Abomasal infusions
of amino acid mixtures also increased milk
protein percentage, with methionine and
lysine accounting for more than 68 percent
of the observed increase (Schwab et al.,
1976~. Baser! on these responses, it could
be concluded that increasing the intestinal
supply of amino acids through increaser!
rumen protein synthesis or low rumen-
degradable protein sources would increase
milk protein percentage and probably milk
yield. However, on a practical feeding basis,
milk protein responses to dietary proteins
with clifferent rum en ciegradabilities have
been quite variable but generally of no
effect. A number of studies (Crawford and
Hoover, 1984; Crooker et al., 1983; Forester
et al., 1983; Henderson et al., 1985; Holter
et al., 1985; Kung ant! Huber, 1983; Lund-
quist et al., 1986) reported no increases in
milk protein when protected proteins were
fed. Madsen's (1982) study, however, re-
ported significant increases. Again, none of
the studies cited determined milk protein
composition except that of LuncIquist et al.
(1986), which showed no change in milk
OCR for page 234
234
NPN content within equal dietary crude
protein percentages due to feeding formal-
dehyde-treated soybean meal compared with
feeding an untreated soybean meal.
Kaufman (1980) summarized the effects
of dietary protein supply on milk protein
concentration. Insufficient amounts of die-
tary protein will reduce milk protein con-
centrations, but the reduction is minimized
when low rumen-degradable protein sup-
plements are fed. Increasing dietary crude
protein supply has little effect on milk
protein percentage.
The amount of energy consumed, ctens~ty
of energy in the flier, and the source of
energy in the diet all influence milk protein
percentage and yield. Cragle et al. (1986)
compared 59 percent versus 49 percent
concentrate feeding and found that cows fed
rations containing 59 percent concentrate
procluced an average of 11 percent more
milk, 13 percent more protein, 3 percent
more fat, and 11 percent more lactose than
cows fed 49 percent concentrate rations. Of
the increase in milk protein, 85 percent was
attributed to increased yield and only 15
percent to increased percentage in the milk.
Emery (1978) reported that milk protein
percentage increases 0.015 percent for each
Meal of additional net energy fed from 9 to
40 Mcal/ciay and that the increaser! protein
percentage was usually accompanied by an
increased milk yield. Mild energy malnu-
trition has been reported to slightly reduce
milk protein percentage; however, uncler
severe energy malnutrition, milk protein
percentage is unaltered but yields (decrease
drastically (Thomas, 1980, 1983~.
Rogers and Stewart (1982) reviewed the
effects of various forage sources in the diet
on milk composition. Cows grazing early
spring pastures were reported to have in-
creasec3 milk protein percentages. How-
ever, the confounding of energy, protein,
and condition of the cow in most forage
studies where milk composition is reported
prohibits the (lrawing of definite conclu-
s~ons.
APPENDIX
Thomas (1980, 1983) discussed the notion
that increasing propionic acid in the rumen
through increased concentrate feeding or
reduced forage particle size affects milk
protein percentage. To summarize, there is
a strong positive correlation between rumen
production of propionic acid and milk pro-
tein; however, the exact mechanism is un-
known. One suggestion is that propionate
increases glutamic acid availability to the
mammary gland and, through its role in
amino acid transamination, enhances syn-
thesis of nonessential amino acids. A second
hypothesis is that propionate through in-
sulin couIcl enhance plasma concentrations
of glutamine and alanine. Propionate could
also enhance glutamate output from the
liver by increasing its synthesis or reducing
usage in gluconeogenesis.
Intake of energy can also be increased
through inclusion of fats or oils in the diet.
Beetling protected lipids, vegetable fats, or
vegetable oils to lactating cows depressed
milk protein percentage, whereas animal
fats had no effect or minimal effect on milk
protein percentages (Linn, 1983; Palmquist
and Jenkins, 1980~. Dunkley et al. (1977)
indicated that the depressing effect was on
the casein fraction. Although the exact de-
pressing mechanism is unknown, it may be
through altered glucose metabolism
(Palmquist and Jenkins, 1980), changes in
rumen metabolism Jenkins and Palmquist,
1984), or both. Thus, the source of increased
dietary energy (carbohydrate versus lipid)
fed to lactating cows has a significant eject
on milk protein percentage changes.
CARBOHYDRATES
The predominant carbohydrate in milk is
the disaccharide lactose. It is composed of
one molecule of glucose and one molecule
of galactose joined in a 1-4 carbon linkage
as beta-galactoside. The principal biological
function of lactose in milk is the regulation
of water content and, thus, the regulation
OCR for page 235
COMPOSITION OF MILK FROM DAIRY COWS
of osmotic content (Davies et al., 1983;
lenness, 1985~. Because of this function,
lactose is the most constant constituent in
milk, averaging 4.6 percent.
Carbohydrates other than lactose that are
found in milk are monosaccharides, sugar
phosphates, nucleoticle sugars, free neutral
and acid oligosaccharides, ant] glycosyl groups
of peptizes and proteins (lenness, 19854.
Free glucose and galactose and the sugar
alcohol myo-inositol are also present in milk.
However, the amounts of these carbohy-
drate fractions are minor compared with
that of lactose.
Biosynthesis
Glucose is the primary substrate for lac-
tose synthesis, with 85 percent of the carbon
secreted in lactose derived from blood glu-
cose (Thomas and Chamberlain, 1984~. Lac-
tose synthesis is initiated in the Golgi ap-
paratus and continues in the vesicles with
an influx of water and ionic constituents that
causes the vesicles to swell as they pass
toward the cell surface. Glucose and uridine
cliphosphate (UDP)-galactose, derived from
glucose, combine to form lactose under the
action of the enzyme lactose synthetase.
The milk protein beta-lactalbumin must be
present for glucose and UDP-galactose to
combine. Thus, beta-lactalbumin appears to
be a prime regulator of lactose synthesis
(Kuhn, 1983; Larson, 1985~. Entry of water
into the vesicle is linked with lactose syn-
thesis to maintain osmotic equilibrium with
surrounding fluids. Thus, the rate of lactose
synthesis regulates water secretion and con-
sequently milk yield.
MINERALS
Factors Affecting Milk Mineral Content
The mineral content of milk is derived
from minerals fount] in circulating body
fluids. The factors influencing mineral con-
tent of milk are discusser] below.
235
BreedlGenetics
Cerbulis and Farrel (1976) reported the
ash, calcium, phosphorus, and magnesium
contents of milk from different breeds of
dairy cattle. The average ash content varied
from 0.74 percent for Holsteins to 0.83
percent for Jerseys. The highest calcium
and phosphorus contents in milk were re-
ported for lerseys.
EnvironmentlManagement
It is well documented that the mineral
composition of colostrum is higher than that
of milk. Calcium, phosphorus, potassium,
and chloride concentrations follow the same
lactation curves as fat and protein-that is,
high in colostrum, lowest at peak milk yield,
and then gradually increasing as lactation
progresses (Iyengar, 1982; Jenness, 1985~.
Milk inorganic phosphorus levels were shown
to be higher in first lactation cows than in
multiparous cows, and milk phosphate lev-
els were lowest during the summer (Forar
et al., 1982).
HealthlPhysiology
Mastitis increases the percentages of so-
dium and chloride in milk and decreases
the percentage of potassium (Kitchen, 1981;
Peaker and Faulkner, 1983; Schultz, 1977~.
Bacterial infection of the udder results in
damage to the ductal and secretory epithe-
lium and increases the permeability of blood
capillaries. Thus, sodium-and chloride, which
are higher in blood, pour into the lumen of
the alveolus, and in order to maintain os-
molarity, potassium is decrease propor-
tionally. Fernando et al. (1985) reported the
decline in lactose and potassium and in-
crease in sodium and chloride in mastitic
milk was most prominent in strippings of
milk after milking.
The percentages of calcium and phospho-
rus in milk decline with mastitis infections
(Kitchen, 1981; Schultz, 1977~. Most likely
OCR for page 236
236
this reflects lower casein levels, since both
ions are complexed with casein micelles.
Contradictory evidence exists regarding the
effect of mastitic infections on levels of
magnesium. Trace elements may increase
slightly in mastitic milk (TalIamy and Ran-
dolph, 19704.
Administration of exogenous growth hor-
mone has relatively little effect on the per-
centages of minerals in milk, but yields of
minerals increased with increasing milk pro-
cluction (Epparc] et al., 1985~.
Nutrition
Normal dietary regimes have little influ-
ence on the mineral composition of milk,
especially the macromineral constituents.
Forar et al. (1982) fed two levels of phos-
phorus (0.31 and 0.54 percent) and two
levels of calcium (1.0 and 1.8 percent) in
four diets to lactating cows and found no
differences in milk inorganic phosphorus
percentages. Diets depressing milk fat per-
centage have been shown to lower the
percentage of citrate and soluble calcium in
milk (Davies et al., 1983~. Changes in milk
phosphorus en cl calcium percentages woulc!
not be expected, since very few of these
ions are in the free form in milk. Dietary
factors affecting citrate ant! casein contents
of milk would be expected to correspond
with small changes in calcium, since calcium
is complexed and secreted with these sub
stances.
Fettman et al. (1984) observed decreases
in milk chloride percentage when cows were
fed chloricle-deficient rations during early
lactation. Milk potassium percentage de-
clinec] along with chloride levels, reflecting
altered mineral metabolism in chloricle-de-
ficient cows. A recent report (Schneider et
al., 1986) evaluating dietary sodium and
potassium effects on heat-stressed cows found
no change in milk potassium percentages
based on quantity of potassium fed or source
of sodium fed. However, cows offered shacle
had higher milk potassium percentages than
APPENDIX
cows given no shade. Percentages of sodium
in milk were lowered significantly by feed-
ing cows sodium bicarbonate and only slightly
by feeding salt or high levels of potassium,
as compared with results for control cows.
The lower milk sodium percentages corre-
spondec3 with lower plasma sodium levels
in cows fee} sodium bicarbonate.
Milk iodine levels have been shown to
increase with increased beetling of iodine.
Franke et al. (1983) observed progressive
increases in milk iodine concentrations dur-
ing lactation when as little as 4 ppm of
organic iodine were a(lded to the (lies.
Larson et al. (1983) found cow age, season
of calving, milk production, and health sta-
tus to have no effect on the concentration
of iodine in milk.
Iyengar (1982) reported that iocline, man-
ganese, molybdenum, selenium, zinc, and
cobalt concentrations in milk could be al-
tered by dietary means. However, very
limited research has been directed toward!
this end. Most of the changes that have
been observed are the result of marked
dietary changes. The effects of slightly un-
clerfeeding or overfeeding a required dietary
mineral or the effects of mineral interactions
on the mineral composition of milk are not
well known.
OTHER MILK SOLUBLES
There are many other components in milk
in adclition to those already discussed. They
can be categorized as either natural or
contaminant. They appear in milk both from
leakage during the normal secretory process
and by actual secretion. Whatever the mode
of entry, their concentrations can vary con-
siderably, but their significance and purpose
remain largely unknown.
The natural compounds that have been
detected in milk are gases, alcohol, alde-
hydes, ketones, carboxylic acicI, sulfur-con-
taining compounds, nucleotide material,
hormones, phosphate esters, glucose, ace-
tate, and citrate. Many of these are products
OCR for page 237
COMPOSITION OF MILK FROM DAIRY COWS
of intermediary metabolism ofthe mammary
gland (Davies et al., 1983; lenness, 1985;
Peaker and Faulkner, 1983~. The reasons
for changes in the concentrations of these
compounds in milk are unknown.
An exception to the above is the com-
pounc] citrate. The concentration of citrate
in milk is moclifiable and is important from
a milk-processing standpoint. Alteration of
citrate concentrations changes the amount
of free calcium in the soluble phase of milk,
which, in turn, affects the precipitation of
milk proteins. Milk citrate concentrations
are highly correlates] with fat percentage,
ant] therefore diets that lower milk fat per-
centage also decrease the citrate content of
milk (Faulkner ant] Peaker, 1982~. Stage of
lactation and season of the year also affect
milk citrate levels. It appears that the source
of milk citrate is from synthesis within
secretory cells and that secretion into milk
is similar to that of lactose and casein (Faulk-
ner and Peaker, 1982~.
The other category of components-con-
taminants inclucles compounds that are
not normally found in milk but that enter
acciclentally or by design. Included here are
chemicals, pesticides, herbicides, fungi-
cides, heavy metals, and drugs. These items
are mentioned as a reminder that milk can
contain compounds other than those of nu-
tritional importance to humans and that
maintaining a nutritious, wholesome milk
supply is of utmost importance.
MANIPULATING MILK
CONSTITUENT~SUMMARY AND
CONCLUSIONS
Variations in milk composition arise from
differences in relative rates of synthesis ant]
secretion of milk components by the mam-
mary gland. The processes involved for
lactose, protein, and fat synthesis and se-
cretion are independent but regulated
through nutrient or substrate availability
ant! hormonal control of nutrient utilization.
Thus, genetics, which mediates hormonal
237
effects, and diet, which regulates nutrient
availability, are the major factors affecting
milk composition.
The most variable milk constituent is fat.
Consiclerable variation exists between and
within dairy cattle breeds. Genetic selection
for fat percentage can change fat content of
-milk but will also affect other constituents
since there is a high correlation between
the percentage of components in milk. Ge-
netic selection for fat content would! alter
the quantity of fat produced but not the
composition of the fat. The best hope for
altering composition is through diet. Changes
in fat percentage and composition can be
accomplished by altering the flier to produce
changes in fermentation patterns or the
composition of fat absorbe(1 from the diges-
tive tract. Diets that increase the proportion
of propionate in the rumen depress milk fat
percentage, but changes in fat composition
are minimal, including slight increases in
Cog polyunsaturated fatty acids and slight
decreases in C~60 and C~80 fatty acids. In-
clusion of fats in the diet, particularly ru-
men-protected fats, is the most effective
way to alter milk fat composition. Significant
increases in long-chain fatty acids can be
achieved by inclucling-fat containing these
acids in the diet. However, the amount and
composition of fats in the diet Heel] to be
controlled to avoid impairment of digestion
of other (lietary constituents in the rumen.
Unsaturated fatty acids are hydrogenated
extensively in the rumen.
Milk protein percentage and composition
can be manipulates] through genetic selec-
tion. Variations in the casein, beta-lacto-
globulin, and alpha-lactalbumin fractions
are known to exist. Heritability estimates
of protein percentage range from 0.3 to 0.7.
Increasing milk protein percentage through
genetic selection is feasible; however, in-
creasing milk protein yield! through selec-
tion is more desirable. The percentage of
true proteins in milk cannot be manipulated
through feecling. Total protein percentage
in milk can be lowered by including fats in
OCR for page 238
238
the diet or raised in relation to milk fat
percentage by feeding high-concentrate diets.
Dietary protein percentage has a minimal
eject on milk protein percentage when it
is within practical feeding ranges.
Levels of other nutritional components of
milk lactose, vitamins, and minerals are
rather constant and not subject to large
changes through genetic or nutritional ma-
nipulation.
The manipulation of milk components
through changes in dairy management prac-
tices, breeding, feecling, health, environ-
ment, and general management appears to
be rather limited. Milk fat percentage and
composition can be changed through feed-
ing, whereas milk protein percentage is best
changed through genetics. Any changes will
be slow in coming and minor compared to
those achieved through processing and man-
ufacturing. The goal of milk producers should
be to modify composition as much as pos-
sible to meet market demand but to em-
phasize maximum yield of components in
high-quality, wholesome milk.
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APPENDIX
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Representative terms from entire chapter:
growth hormone
