A Kinetic Study of Plasma Renin and Aldosterone During Changes of Posture in Man

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Abstract
Cardiovascular parameters, hematocrit (Ht), plasma electrolytes, renin activity (PRA) and aldosterone concentration (PAC) were measured in 12 normal human subjects (6 males and 6 females) eating an ad lib diet. At 8 am, volunteers assumed the following postural changes: 1 hour supine, then 2 hours upright and finally 1 hour supine. Orthostatism induced the following changes: heart rate, systolic and diastolic blood pressure increased immediately; Ht rose significantly at the 5th min in males but not in females; Plasma sodium showed no variations but potassium increased after 30 min; PRA rose significantly at the 5th min and, after 120 min of orthostatism, was found to be 3 times greater than its value after recumbency; and PAC increased significantly at the 15th min and exhibited a plateau 4.5 times its basal values after 90 min of upright posture. When subjects returned to the supine position all the parameters, except PAC decreased. During active orthostatism, a significant correlation was found between PAC and plasma potassium, but correlation was closer between PAC and PRA and between PAC and PRA + potassium. It can be concluded that the renin-angiotensin system is a more potent stimulus for aldosterone secretion than plasma potassium in normal man assuming postural changes. The results presented here can be applied to the development of a short posture test in non-hospitalized patients.

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