Effect of propranolol on aldosterone response to heat exposure in sodium-restricted men

To assess the role of the renin-angiotensin system in the control of aldosterone reease in response to heat exposure, 6 sodium restricted subjects were studied on three random experimental days: a control day and two heat exposure days (46C, 35 mbar, 90 min) with and without propranolol. Plasma aldosterone, renin activity, ACTH and K+ were determined from plasma samples taken every 20 min from 08:00 to 14:00. After propranolol administration, plasma aldosterone responsiveness to heat exposure increased, though plasma renin activity was depressed. Concurrently, propranolol reduced heat tolerance, leading to an increased ACTH and cortisol release in 3 of the subjects. Plasma levels of K+ were not significantly different during both heat exposure days. The enhanced response of plasma aldosterone may in part be related to the concurrent rises in ACTH, but a similar sustained aldosterone response was observed in subjects without ACTH release. Except for a significantly lower heart rate, propranolol induced no changes in hemodynamic and thermal response to heat. Plasma volume, blood pressure, sodium excretion, mean skin and rectal temperature rises and body weight losses were not modified by prior administration of the drug. The dissociation between aldosterone and renin activity after propranolol administration suggests that the aldosterone response to heat exposure is not primarily mediated by changes in activity of the renin-angiotensin system. Propranolol may lead to a reduced metabolic clearance rate of aldosterone or increase the sensitivity of the adrenal cortex to concomitant changes in the known stimuli, but the involvement of an additional factor in aldosterone regulation during heat exposure cannot be excluded.