Plasma Renin Activity in Relation to Serum Sodium Concentration and Body Fluid Balance

The relationship of peripheral plasma renin activity to changes in body fluid volume and to serum sodium concentration was studied in normal subjects and in hypertensive patients during changes in hydration or changes in sodium intake. There was an inverse relationship between renin activity and fluid volume; e.g., renin activity increased (168 to 833 ng/100 ml in normal subjects) following loss of body fluid (−2.1%) by diuresis. In contrast, the relationship of renin activity to serum sodium concentration could be reversed; e.g., renin activity decreased (109 to 62 ng/100 ml) during overhydration in spite of a decrease (140 to 129 mEq/1) in serum sodium concentration. Expansion of intravascular volume in a patient with a defect in renal sodium conservation resulted in suppression of plasma renin activity. It thus appears that some factor related to body fluid balance, perhaps intravascular volume, may be more important than serum sodium concentration in determining plasma renin activity.