et al., 1977). However, it is unknown whether these adaptive responses are sufficient to acquire and sustain acclimation during unaccustomed restriction of dietary salt intake. For example, there have been no comprehensive studies on the minimal daily consumption of salt necessary to acquire and sustain heat acclimation in individuals who ordinarily ingest 8 to 15 g per day of NaCl. This question is particularly important to military planners. When troops are rapidly deployed from garrison to field conditions, their normal salt intake in many cases drops precipitously due to the altered salt content of the issued field rations and the generally reduced total ration consumption, especially during the first several days of the deployment. If the mobilization destination is a desert or jungle environment, the problem is compounded by enhanced salt losses in sweat, especially during the early stages of heat acclimation.
It has been extensively documented, however, that reduced consumption of salt in the human diet results in hormonal adaptations designed to reduce urinary and sweat losses of sodium (Na+). Initially, reductions in Na+ content of the glomerular filtrate are perceived by cells of the distal tubules, thus stimulating the biosynthesis of renin (Rowell, 1986). Elevations in plasma renin activity (PRA) are followed rapidly by an increased biosynthesis and release of aldosterone (ALD), which promotes reabsorption of Na+ by the distal tubules, with obligatory retention and return of water to the extracellular space. Thus, PRA and ALD, in conjunction with argininevasopressin (antidiuretic hormone, AVP), are the humoral factors most instrumental in the regulation of fluids and electrolytes, especially under conditions of reduced availability of either.
The responses of these hormones have been extensively studied during passive heat exposure (Adlerkreutz et al., 1977; Kosunen et al., 1976), during exercise in the heat (Francesconi et al., 1983, 1985), and during periods when these stressors have been combined with restricted or supplemented sodium intake (Brandenberger et al., 1986; Davies et al., 1981). Armstrong et al. (1987) reported that during heat acclimation subjects who consumed a diet providing 5.7 g per day of NaCl had higher heart rates and rectal temperatures, as well as attenuated increments in plasma volume, than when the same group repeated the acclimation while ingesting 23 g per day of NaCl. Endocrinological responses were not described.
The current study offered a unique opportunity to assess and quantitate the endocrinological responses during, and perhaps integral to, the acquisition of heat acclimation and prolonged periods of work in a desert environment (Costill et al., 1976). Further, consumption of a low-salt diet during the acclimation period was expected to amplify these hormonal adaptations (Follenius et al., 1979) and provide important information on their role in the individual's response to recurrent and prolonged exercise in the heat. A