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FLUID REPLACEMENT AND HEAT STRESS
are not known. Specific high-affinity binding sites for ANF have been found in many areas of the body, including the kidneys, the adrenal gland, smooth muscles of blood vessels, and the hypothalamus. Increases in plasma ANF have been shown to inhibit aldosterone production in the adrenal zona glomerulosa (Laragh and Atlas, 1988) and thus might contribute to the altered sodium regulation that occurs with aging. ANF also has an antagonist role to many of the actions of angiotensin II. It inhibits water intake induced by the administration of angiotensin II to the central nervous system. It has the potential to block the formation and secretion of both ADH and angiotensin II (Kramer, 1988; Laragh and Atlas, 1988), and it modulates sympathetic activity by inhibiting epinephrine release and reducing baroreceptor responsiveness. We are just beginning to understand the role of ANF in the regulation of body fluids and electrolytes.
On the basis of two facts, that (1) acute heat exposure provokes rapid changes in body fluids and (2) older individuals have an impaired ability to regulate body fluids, we hypothesized that older subjects would have difficulty maintaining plasma volume and osmolality during prolonged heat exposure. By comparing the time courses of body fluid responses to heat exposure of young and older individuals, the mechanisms for altered body fluid regulation in older healthy men may become apparent.
The experiment outlined below is described in more detail in Miescher and Fortney (1989).
The plasma volume, protein, and osmolality responses of six young men (age, 24-29 years) were compared with those of five older men (age, 61-67 years). The subjects were normotensive, non-smokers who were not taking any medications. The subjects had an average level of aerobic fitness for their age (Astrand, 1960). Each subject had an active life-style but did not participate in routine exercise training or sports. The two groups were matched for height, body surface area, and surface area/weight ratio (Table 16-1).
The tests were performed in the winter months in Baltimore. The subjects reported to our laboratory at 8 a.m. after a light breakfast and after