Sequential responses of the renin-angiotensin-aldosterone axis to acute postural change: effect of dietary sodium.

The simultaneous levels of plasma renin activity (PRA), angiotensin II (A II), and aldosterone (PA) were frequently assessed in 13 normal subjects following acute postural change (assumption of upright posture or returning to the supine position) on low (10 mEq.) and high (200 mEq.) sodium (Na+) intakes. The rate of response of aldosterone secretion was also correlated with changes in the metabolic clearance rate (MCR) of aldosterone. Significant increments of PRA and A II on either sodium intake occurred within 5 to 20 minutes; the peak values occurred within 90 minutes and tended to plateau until the end of the study (240 minutes). The mean absolute peak levels were approximately 2 to 3.5-fold greater than control. Increments of PA were initially delayed 20 to 30 minutes, but peak levels also were achieved by 90 minutes. The secretion rate of aldosterone increased 4-fold on the 10 mEq. Na+ and 2-fold on the 200 mEq. Na+ intake even though MCR declined 30 to 40 per cent in the upright posture. Sodium restriction enhanced the rate and magnitude of response of all parameters. Specifically, the slope of the regression relationship between PRA and PA was more than 4-fold steeper in the sodium-restricted than sodium-loaded subjects. From the rate of decline in PRA following resumption of supine posture, the half-life of PRA was estimated to be 14 to 15 minutes. The present study demonstrates that acute changes in posture are associated with closely correlated changes in PRA or A II. To varying degrees, it appeared that sympathetic activity, intravascular volume, diurnal secretion, and the sodium ion play a role in the sequential responses of these parameters to acute postural alterations.