The influence of posture on renal tubular function in man

The acute effects of changes in posture on renal function were investigated in healthy volunteers. Proximal tubular function was assessed using two methods simultaneously: lithium clearance and urine flow rate during maximal water diuresis. Changing from the standing to the supine posture was associated with significant increases in urine flow rate and in the clearances of sodium, potassium, creatinine and lithium (p less than 0.01 in each case). Increases in fractional water excretion and fractional lithium excretion (using creatinine clearance as an estimate of glomerular filtration rate) were also observed (p less than 0.01). The increases in lithium clearance and urine flow rate, both absolute and fractional, were significantly correlated. These findings suggest that the acute increase in sodium excretion following assumption of the supine posture results largely from an increase in end-proximal fluid delivery, due partly to an increase in glomerular filtration and partly to a decrease in fractional reabsorption in the proximal tubule. The results also provide circumstantial support for the use of lithium clearance as a marker of end-proximal delivery in man.