A diurnal rhythm in plasma renin activity in man.

Although it has been known for many years that the renin-angiotensin system is responsive to decreases in renal perfusion pressure (1-3), it was only recently discovered that angiotensin is an important regulator of aldosterone secretion (4-6). Before it was known that angiotensin had a role in the regulation of aldosterone secretion, a large number of studies indicated that experimental or pathological conditions that resulted in a reduction of effective blood volume would stimulate al-dosterone secretion (7). Studies have already been published indicating that plasma renin activity and aldosterone excretion both increase in response to sodium deprivation (8-10) and assumption of upright posture (11-13). In the course of studies attempting to correlate plasma renin activity with aldosterone excretion we found that the rises in plasma renin activity and in urinary aldosterone that occur after the assumption of upright posture early in the day are both of limited duration and that they diminish late in the day despite continued maintenance of upright posture (14). The observed afternoon fall in plasma renin activity is not readily explained by existing knowledge of the physiology of renin secretion. The present study, therefore, focuses attention for the first time on the question of whether there might be a consistent diurnal rhythm in plasma renin activity. In order to characterize diurnal variations in plasma renin activity that might occur independently of dietary and postural influences , we studied normal subjects during continuous recumbency while they received constant diets in equal portions at regular intervals throughout the day and night. To determine whether the rise in plasma renin activity that occurs in response to upright posture is due to "loss" of blood into the lower extremities, we observed the effect of bandaging the lower abdomen, hips, and legs before the subjects assumed upright posture. In order to determine whether the afternoon fall in plasma renin activity of upright subjects was a function of the time of day rather than the duration of upright posture, we compared the effect of rising at noon with the effect of rising at 8 a.m. We also performed studies to assess the importance of sodium and water intake and of al-dosterone secretion in bringing about the afternoon decline in plasma renin activity.