THE EFFECT OF SODIUM-BALANCE ON SWEAT SODIUM-SECRETION AND PLASMA-ALDOSTERONE CONCENTRATION

The effect of manipulating sodium intake upon sweat sodium secretion w as investigated during heat acclimation. Twenty-five male subjects wer e confined to an environmental chamber at a temperature of 25 degrees C for 3 days, and then acclimated to heat by a further 5 days at 40 de grees C. The subjects' daily sodium intake was controlled throughout a s follows: high (HNa), 348.4 (0.8) mmol.day(-1) n = 7; moderate (MNa), 174.1 (0.6) mmol.day(-1) n = 9; or low (LNa), 66.3 mmol.day(-1) n = 9 . Sodium losses were estimated from urinary, faecal and sweat collecti ons using a whole-body washdown method. Plasma aldosterone concentrati on was also measured from venous blood sampled each morning. Measureme nts of body temperature and heart rate during the heat exposure phase indicated a degree of heat acclimation. During this heat phase there w as a reduction (P < 0.01) in sweat sodium secretion for all three cond itions which was greatest for the LNa condition, although this finding was not significant (P < 0.1). In the LNa condition, plasma aldostero ne concentration increased (P < 0.05) prior to heat exposure, and the secretion of aldosterone was potentiated (P < 0.01) during the heat ex posure in comparison with the MNa condition. In contrast, the HNa diet produced a fall(P < 0.05) in plasma aldosterone concentration prior t o heat exposure and an attenuation of aldosterone secretion thereafter . These findings are inconsistent with the hypothesis that retention o f sweat sodium is dependent upon a net body sodium deficit, but demons trate that aldosterone secretion is potentiated under such conditions.