lization period prior to salt restriction and recurrent exercise in the heat. Thus, during the first 7 days of the experiment all subjects remained under the 24-hour control of test investigators with obligations only to the study (as was the case throughout the experiment). During this interval, subjects consumed the same test diets—which delivered 8 g of NaCl daily for each volunteer—and adhered to a structured and consistent schedule of activity that included completion of questionnaires (for example, environmental symptoms, thermal comfort), simulated target acquisition and firing, a maximal oxygen uptake test on day 4, and at least 4 hours per day of light recreational activity that included reading, videos, and games. All of this activity took place after blood sampling on all days.
During the stabilization period, circulating levels of all three hormones were generally consistent between groups, with some minor variation over time. All values were well within the normal range for young adult males, except for ALD levels on day 4, which were approximately 4 ng per dl above the upper limit of the normal range. This increment was probably due to the reduced NaCl content of the stabilization diet (8 g per day) as compared to the normal salt content of military dining hall rations, which is usually 11 to 15 g per day (Szeto et al., 1987). By day 7, mean ALD levels had returned to within the normal range.
Francesconi et al. (1983, 1985) and others (Finberg and Berlyne, 1977; Finberg et al., 1974) have reported—and current results confirm—that the acquisition of acclimation attenuates the response to exercise in the heat of both PRA and ALD. In the current experiments, the effects of acclimation were manifested in mean daily ALD levels of 79 and 84.9 ng per dl on days 8 and 11 (days 1 and 4 of heat acclimation), respectively, which were reduced by days 15 and 17 (days 8 and 10 of heat acclimation) to 44.3 and 51.3 ng per dl, respectively, in the control group. Even in the low-salt group, the mean plasma ALD concentration on day 11 was a markedly elevated 167.3 ng per dl, which was reduced to 133.5 and 78.7 ng per dl on days 15 and 17, respectively. By day 17 there were no significant differences in plasma ALD either between the control and low-salt groups at any of the individual sampling times or between the T1, T2, and T3 values for either group. These results suggest that the chronicity of both the heat acclimation regimen and the low-salt diet engender temporally related endocrinological accommodations to these experimental manipulations. Thus, the attenuated hormonal responses over time may be related to the following: increased plasma volume of heat acclimation (Bonner et al., 1976), generally reduced adrenocorticotrophic activity with decreased physiological strain (that is, a reduced stress response to the experimental conditions) (Francesconi et al., 1984), decreased sensitivity to adrenocorticotrophic hormone (McDougall, 1987), increased renal sensitivity to ALD-induced sodium reabsorption mechanisms (Smiles and Robinson, 1971), and a general improvement of the Na+