Prolonged isometric exercise. Part 2: Effect on circulation and on renal excretion of sodium and potassium in young males genetically predisposed to hypertension.

The effect of stress, in the form of prolonged isometric exercise, on the circulation and on the renal excretion of sodium and potassium was studied in 16 male medical students whose parental blood pressure (BP) was less than 140/85 mm Hg, and in 17 male students with one or two parents who had BPs greater than 150/95 mm Hg. After the subjects rested initially for 90 minutes, basal measurements were made of heart rate, BP, and the rates of sodium and potassium excretion. The subjects then underwent a 1-hour period of intermittent isometric exercise involving all four limbs in rotation, during which BP and heart rate were measured. A 5-hour period of rest followed, during which BP, heart rate, and the rate of electrolyte excretion were measured at half-hourly intervals for the first 2 hours and at hourly intervals for the last 3 hours. The precise protocol was repeated on another day in the absence of the period of isometric exercise. The electrolyte excretion responses of each subject were then expressed as the ratio of the changes from basal values observed on exercise and rest days. At no time was there any difference in systolic and diastolic BP, heart rate, and rate of sodium and potassium excretion following exercise when sons of normotensive parents were compared to the sons of hypertensive parent(s). These results indicate that the retention of sodium and potassium following isometric exercise seen in patients with hypertension does not occur in subjects genetically predisposed to hypertension and suggest that the effect is a consequence of, rather than a predisposing factor to, hypertension.