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

For the quantitative analysis of the effect of sodium balance on renin secretion, plasma renin activity (PRA) was measured in 9 healthy young males in the respective 3- or 4-day periods of sodium repletion, mild sodium restriction, moderate and severe sodium depletion. In the latter 2 periods, sodium balance was made negative by low salt diet and furosemide administration (120mg/day, p.o.) for 1 or 3 days. Blood samples for PRA determination were obtained at supine position (8:00 a.m.) and after 1-, 2-, and 4-hour's upright posture. PRA was also measured after furosemide stimulation (80mg, p.o.) plus 2-hour's upright posture in the period of sodium repletion.Change in the supine PRA was directly correlated with that in hematocrit (r=0.81) and inversely with that in body weight (r=-0.64). The PRA rose significantly from 1- to 2- and 4-hour's upright posture in the period of mild sodium restriction, whereas it reached the maximum level at 1-hour's upright posture and remained unchanged thereafter in either period of negative sodium balance. The upright PRA after furosemide stimulation in the period of sodium repletion was almost identical to 2- and 4-hour's upright PRA in the period of mild sodiom restriction, and less than 1-hour's upright PRA in either period of negative sodium balance.These data suggest that 1-hour's upright posture is enough to evaluate PRA when sodium balance is negative.