Click for next page ( 14


The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 13
Appendix A Dose-Effect, Dose-Response Concepts of Toxicology The dose-effect, dose-response concepts of toxicology provide a framework for examining the biologic effects of toxic metals. These concepts and their application to human exposures to heavy metals are discussed fully in "Effects and Dose-Response Relationships of Toxic Metals." Here we will initially summarize these concepts through a series of definitions and then discuss their specific application to lead. Critical Effect - The critical effect is not the most serious, but rather the most sensitive and specific biologic change, beyond acceptable physiologic variation, which is caused by the presence of a toxic substance. Although many different effects may occur, the critical effect is defined as the first measurable adverse effect. "Sub-critical effects" are measurable biologic changes which do not impair cellular function, but which are directly related to the concentration of a toxic substance. Critical Site - The critical site is the location in the body where the critical effect occurs. It may be a system, organ, cell type or cell component. Dose - In experimental animals, an administered dose is readily quantified but this Is not true of humans. For humans we can estimate the amount taken in and for this we use the term "external dose." Since this is an indefinite amount for humans we must relate response to a tissue level such as blood concentration and we use the term "internal dose" for this tissue level. The "external dose" is the quantity of a toxic agent which enters the organism through the lungs, gastrointestinal tract, skin, etc., a portion of which may be excreted before reaching the "critical site." The "internal dose" is the quantity of a toxic agent which is absorbed and reaches the critical site. Since the concentration of a toxic agent at the critical site can rarely, if ever, be measured in human studies, the concentration is measured in a body fluid such as blood or urine. The concentration of a toxic agent in blood or urine is then used as an indicator of the internal dose. Dose-Effect Relationship - The dose-effect relationship is a relationship in which a quantitative change in a metabo- lite Affect) is directly related to the concentration (dose) of a toxic substance. A typical dose-effect relationship is graphically illustrated by an "s" shaped curve when dose is plotted on the abscissa and degree of effect on the ordinate. 13

OCR for page 13
Dose-Response Relationship - The dose-response relationship is a relationship in which the percentage (response) of a population exhibiting an effect is related to the con- centration (dose) of a toxic substance. Those exhibiting an effect are termed "reactors," and those not exhibiting an effect, "non-reactors." A typical dose-response relationship is illustrated by an "s" shaped curve when dose is plotted on the abscissa and percent positive reactors is plotted on the ordinate. DOSE-EFFECT RELATIONSHIPS FOR LEAD The effects of lead are seen in the neurologic, hematopoietic and renal system. Acute adverse functional effects in the kidney are generally seen in association with symptoms and high levels of lead exposure in adults. Similarly, late lead nephropathy is also associated with prolonged high levels of lead exposure. Therefore, the kidney is not currently considered the first organ affected by lead. ''' Derangement of hemoglobin synthesis in the erythroid cells of the bone marrow is currently considered the critical effect for lead.20,36,70,71,105 increases in urinary 6-aminolevulinic acid (ALA-U), urinary coproporphyrin (CP-U) and erythrocyte protoporphyrin (EP) are indicators of lead's effect in the hematopoietic system. In lead poisoning (and iron deficiency), it is the metalloporphyrin, zinc protoporphyrin, rather than free protoporphyrin IX which is present in excess in the circulating erythrocytes. Increased ALA-U and EP reflect in vivo inhibition of the enzymes ALA-D and ferro chelatase by lead.70,71 The mechanisms responsible for the coproporphyrinuria of plumbism are not well understood. Inhibition of ALA-D by lead has been extensively studied. ''^ In a number of epidemiological studies in which ALA-D activity is measured in vitro in hemolysates of peripheral blood, a significant negative log-normal relationship has been found between ALA-D activity and whole blood lead concentration. This relationship is found over a wide range in blood lead concentration, including the normal range (5-,to approximately 40 yg Pb/dl whole blood). More recently, Granic et al have developed an assay for ALA-D in which the ratio of activated to non-activated ALA-D activity can be measured. With this particular assay, a positive linear relationship between the ratio of activated to non-activated ALA-D activity and blood lead concentration is found over a range of 20-90 yg Pb/dl whole blood. On the basis of this and kinetic studies, they propose that inhibition of ALA-D by lead is non-competitive and that there is probably no interaction with lead at concentrations of <15 yg Pb/dl whole blood. Under physiologic conditions in man, however, accumulation and increased excretion of ALA-U, the substrate of ALA-D, does not begin to occur until Pb-B exceeds approximately 40 yg Pb/dl whole blood, a level at which most in vitro assays for ALA-D indicate substantial inhibition. This has been interpreted as evidence that there is a substantial reserve of ALA-D adequate to meet physiologic needs at lower concentrations of lead in whole blood. For this reason, reduction in AlA-D activity, as measured 14

OCR for page 13
in vitro in peripheral blood, is considered a sub-critical effect within the context of the above definitions; while increases in ALA-U, CP-U and EP are considered indicators of lead's critical effect on hema- topoiesis. 70,7l,105 Significant dose-effect relationships are found between these metabolic precursors of heme and the concentration of lead in body fluids. As an example, a dose-effect curve, using blood lead concen- tration (Pb-B) as an indicator of internal dose and quantitative 24- hour output of ALA-U as an indicator of effect, is given in Figure 1. One can begin to see the typical s-shaped curve usually associated with this relationship. Differences in individual susceptibility can also be seen at each dose (Pb-B) level. Zielhuis105 and others78'81'86 have found similar relationships for lead's effect on the hematopoietic system. These effects begin to occur when Pb-B levels reach the 40-60 yg Pb-B range, although preliminary data suggest that EP begins to in- crease as Pb-B rises above 30 yg Pb/dl whole blood.105 Adverse effects in the hematopoietic system are reversible. At the present time, there is no well-defined set of sensitive biochemical indicators of lead's effect on the nervous system. However, preliminary studies in rats use neurochemical tests which may become useful as measures of neurologic changes in humans. Published reports to date have used functional tests to measure lead's effect on the human nervous system. A relationship has been tentativelv suggested between decreased intelligence,3,23,24,73 hyperactivity,^ behavioral and psychological changes, 24 ancj loss of fine motor function,'" in young children with blood lead concentrations in the 50-70 yg Pb-B range. In some instances, the effects of lead on the nervous system are clearly irreversible. Sequelae are related to the severity and duration of signs and symptoms. The risk of permanent neurological complications increases with repeated acute clinical episodes of lead poisoning. Whether the effects reported in subclinical lead poisoning are reversible is unknown; however, de la Burdens most recent study suggests that they 24 are not. Currently, no data exist to show whether neurochemical or neurophysiological changes precede changes in the hematopoietic systeiu. The hematopoietic system is currently considered the "critical" or first system to be affected. If this is true, then medical intervention based on evidence of reversible effects in the hematopoietic system should prevent possible irreversible effects in the nervous system. DOSE-RESPONSE RELATIONSHIPS FOR LEAD The results of population surveys which measure both dose and effect in each individual may be expressed as dose-response curves. Figure 21 illustrates this relationship for lead when Pb-B is used as an indicator of internal dose and erythrocyte protoporphyrin as an indicator of effect. Positive reactors are those individuals who exhibit an effect greater than the expected mean plus two standard deviations. 15

OCR for page 13
The percentage of positive reactors for each blood lead group is plotted. Highly susceptible individuals show a positive response at relatively low blood lead levels, while highly resistant individuals show a normal response at relatively high blood lead levels. Similar dose-response curves can be plotted for each measure of lead's effect, so that a series of dose-response relationships may be shown.105,106 For the groups charged with recommending "safe levels" of toxic substances, this approach to analysis of the data is helpful. However, appropriate interpretation of the data requires that the population under study be well-defined for factors such as age, sex, concurrent illnesses, etc., which may influence test results. Background response must also be considered. Background response refers to the percentage of the population exhibiting an effect caused by factors other than the specific agent under consideration. When using erythrocyte proto- porphyrin as a measure of lead's effect, some "background response" may be expected due to the presence of iron deficiency in the population. Separate dose-response curves may be drawn to correct for background response. Based on inspection of Figure 2, an epidemiologist seeking to prevent early hematologic effects in 90 percent of the preschool- age population, would recommend that environmental lead sources not exceed a limit known to raise Pb-B levels to the neighborhood of 30 yg. Dose-response relationships for lead are generally not available because of inadequately designed population surveys which consist solely of the collection and analysis of biologic samples. The population characteristics, including those which influence background response, must be known. No published data are available for children less than one year of age. 16

OCR for page 13
lOQOr ICO 1.0 o o o o SYMPTOMATIC PLUMBISM • ASYMPTOMATIC 20 100 fjg Pb/lOOml WHOLE BLOOD 1000 FIGURE 1. Dose-effect relationship between an indicator of internal dose (Pb-B) and an indicator of effect (urinary excretion of ALA, ALA-D). (From Chisolm, J. Julian Jr., Barrett, Maureen B., and Mellits, E. David:^0 Dose-effect and dose- response relationships for lead in children. J. Pediatr. 87.: ll52-ll60, 1975. Reprinted with permission.) Note: 2 indicates 2 superimposed data points. 17

OCR for page 13
100 I I 80 I 60 S 40 20 FIGURE 2. TOTAL N = 155 N=34 N=54 M=38 N=20 <30 30-39 40-49 50-59 560 BLOOD LEAD frgPb/IOOml WHOLE BLOOD Dose-response relationship for the effects of internal doses of lead as Pb-B on erythrocyte protoporphrin. (From Chisolm, J.J., Jr.18 Arh. Hig. Rada Toksikol. (Archives of Industrial Hygiene & Toxicology), Suppl. to vol. 26, 1976 [in press]. Reprinted by permission.) 18