To ensure that both the duration of heat exposure and the amount of work done in the heat were adequate to elicit effects of the low-salt diet, subjects remained in the hot environment for approximately 8.5 hours per day. They entered a large environmental chamber (41°C dry bulb, 21 percent relative humidity, 1.1 to 1.2 meters per second wind speed) at approximately 7:30 a.m. on each of the 10 heat acclimation days and remained standing for at least 20 minutes to stabilize body fluid compartments prior to blood sampling (Hagan et al., 1978). At approximately 8:00 a.m. on each day of acclimation (days 8 to 17), subjects began exercise (treadmill, 5 percent grade, 5.6 km per hour) in this hot environment. They then walked for 30 minutes and rested for 30 minutes each hour for the next 8 hours with water available (temperature of water, 10° to 15°C). Fluid consumption and body weight were monitored at 30-minute intervals, and subjects were instructed to maintain euhydration levels by consuming the full complement (based on body weight measures) that was allowed during each 30-minute interval. Lunch was presented at approximately 12:30 p.m. (fifth rest period), and subjects were allowed to exit the chamber shortly after the eighth walk upon completing other physiological and behavioral measures (approximately 4:00 p.m.). If a person was unable to complete all of the work cycles on a particular day, he remained in the chamber and participated in all other aspects of the protocol (for example, diet, drinking, other tests). Thus, the continuity of the heat exposure component of the acclimation process was uninterrupted. After completing the daily chamber exposure, subjects returned to their dormitory setting until the regimen was repeated the next morning.
During the dietary stabilization period, on days 1, 4, and 7 a 6-ml sample of blood was removed by venipuncture from a superficial arm vein by a trained phlebotomist using aseptic techniques. This sample was removed at approximately 7:45 a.m. after subjects had been standing for 20 minutes; room temperature was 21° ± 1°C during the entire stabilization period. During the heat-work period (days 8 to 17) three samples were taken on each of experimental days 8, 11, 15, and 17, which corresponded to days 1, 4, 8, and 10 of heat acclimation. Because of the requirement for repeated blood sampling on these days, a catheter was aseptically inserted into each subject's superficial arm vein. The first blood sample of the day (T1) was taken after subjects had stood in the heat for 20 minutes, prior to exercise, at approximately 7:45 a.m. to correspond with the time of blood sampling during the dietary stabilization period. The second blood sample