Renin Response to Upright Posture in Varying Sodium Balance in Normal Young Japanese Men

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Abstract
For the quantitative analysis of the effect of Na balance on renin secretion, plasma renin activity (PRA) was measured in 9 healthy young males in the respective 3- or 4-day periods of Na repletion, mild Na restriction, moderate and severe Na depletion. In the latter 2 periods, Na balance was made negative by low salt diet and furosemide administration (120 mg/day, p.o. [per os]) for 1 or 3 days. Blood samples for PRA determination were obtained at supine position (0800 h) and after 1, 2 and 4 h upright posture. PRA was also measured after furosemide stimulation (80 mg, p.o.) plus 2 h upright posture in the period of Na repletion. Change in the supine PRA was directly correlated with that in hematocrit (r [correlation coefficient] = 0.81) and inversely with that in body weight (r = - 0.64). The PRA rose significantly from 1 to 2 and 4 h upright posture in the period of mild Na restriction, whereas it reached the maximum level at 1 h upright posture and remained unchanged thereafter in either period of negative Na balance. The upright PRA after furosemide stimulation in the period of Na repletion was almost identical to 2 and 4 h upright RPA in the period of mild Na restriction and less than 1 h upright RPA in either period of negative Na balance. One hour''s upright posture is apparently enough to evaluate PRA when Na balance is negative.