Plasma renin activity and serum aldosterone during prolonged physical strain

Summary Plasma renin activity (PRA), serum aldosterone and the serum and urinary levels of sodium and potassium have been investigated in 24 young men participating in a 5-day military training course with heavy continuous physical exercise, energy and sleep deprivation. The subjects were divided into three groups. Group 1 did not get any extra sleep or food, group 2 were compensated for the energy deficiency, and group 3 slept 3 h each night. The basic diet given to all the subjects was about 5,000 kJ and 2 g NaCl·24 h^–1·cadet^–1. The high calorie diet contained approximately 25,000–35,000 kJ and 20 g of NaCl·24 h^–1·cadet^–1.The study showed that serum aldosterone and PRA were extremely activated during such prolonged physical strain combined with lack of food and salt, whereas sleep deprivation did not seem to have any large influence. Only small variations were found in the serum levels of sodium and potassium and the urinary level of potassium during the course, whereas a decrease was seen in urinary sodium concentration. The fairly good correlations between the decrease in urinary sodium levels and the increase in PRA (r=0.7) and further between PRA and serum aldosterone (r=0.8) during the course indicate that there is a causal connection between the decrease in urinary sodium excretion and the increase in PRA and serum aldosterone. An increased response was seen during a short term exercise test in groups 1 and 3, whereas no such increase was seen in the subjects of group 2, probably because the higher salt intake abolishes the renin-aldosterone response to exercise. In spite of high catecholamine levels during the course, the serum potassium response to physical exercise was unchanged. The circulating catecholamines did not seem to have any significance for renin secretion and sodium reabsorption during the course. Further, there was no sign of exhaustion in Na/K homeostasis during the experiment.