A diurnal rhythm in plasma renin activity in man.

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Abstract
Plasma renin activity of recumbent normal subjects exhibits a diurnal rhythm that is not dependent upon diurnal variations in posture or diet. Highest values are observed between 2 a.m. and 8 a.m. and lowest values between noon and 6 p.m. A change from recumbency to upright posture leads to a greater increase in plasma renin activity in the forenoon than it does in the afternoon. The posturally induced increase in plasma renin activity can be prevented by bandaging the lower abdomen, hips, and lower extremities. When a normal subject rises at 8 a.m., his plasma renin activity increases to peak values at 10 a.m. or noon and then falls despite continuation of upright posture. The afternoon fall in plasma renin activity is not dependent upon changes in adreno-cortical function, nor is it entirely dependent upon retention of salt and water during the forenoon. In subjects who are upright during the day, the diurnal rhythm mechanism appears to work in combination with postural factors to elevate plasma renin activity in the forenoon and to work in opposition to postural factors to depress plasma renin activity in the afternoon.