balance due to improved Na+ conservation at the level of the kidney and sweat gland (Allan and Wilson, 1971).
Similar arguments and rationale can be made for the pattern of PRA levels observed in the current experiments. For example, in the control group, mean PRA peaked on day I of heat acclimation (9.04 ng per ml per hour) followed by a declining trend (6.8, 6.6, and 5.3 ng per ml per hour) on days 4, 8, and 10, respectively, of heat acclimation. Moreover, in the low-salt group, by using the rationale developed above for ALD, maximal effects of the low-salt diet and exercise in the heat would be expected on day 4 of heat acclimation (15.2 ng per ml per hour) followed by moderation on day 8 (9.6 ng per ml per hour) with further reductions on day 10 (7.3 ng per ml per hour). The endocrinological adaptations that occur early in the acclimation process are necessary to maintain electrolyte balance and ultimately to expand extracellular fluid volume and may be closely related to the acquisition of full acclimation. Moreover, when the physiological strain of exercise in the heat has been reduced after full acclimation and expansion of plasma volume have been achieved, these hormonal responses are obviated.
In the current experiments, subjects on the low-salt diet gradually achieved the same state of acclimation as the control group over the 10-day acclimation period as suggested by significant reductions in heat-and exercise-induced heart rate, rectal temperature, urinary Na+, and perceived exertion. Further, neither these variables nor the distance walked during the 10-day acclimation period differed significantly between the control and low-salt groups. The moderation of the endocrinological responses during the latter phase of the acclimation interval probably reflects, most importantly, the attendant plasma volume expansion and achievement of electrolyte balance. However, successful acclimation to recurrent and prolonged exercise in the heat on 4 g per day of NaCl was accomplished under the specific conditions of this study. It is unknown whether successful adaptation to this low-salt diet could have been accomplished if the exercise or environmental conditions were more intense.
It is generally agreed that AVP is most responsive to dehydration and increments in plasma osmolality (Von Ameln et al., 1985). Accordingly, Greenleaf et al. (1983) reported that when water intake was increased during an acclimation period from 450 ml per hour on day 1 to approximately 1000 ml per hour on days 5 to 8, there also occurred a significant decrement in plasma AVP. In the current experiments, subjects were weighed every 30 minutes during each of the 8-hour workdays, were encouraged to drink, and were provided fresh, cool water at 30-minute intervals. Thus, dehydration was not a significant observation in either group of subjects, and AVP levels remained generally consistent throughout both the stabilization and the heat acclimation intervals. In fact, the range of AVP for both groups over all