Effect of direct increases in renal interstitial hydrostatic pressure on sodium excretion

This study examined the effect of increases in renal interstitial hydrostatic pressure (PI) on sodium excretion (UNaV) utilizing a direct technique for increasing renal interstitial volume. PI was increased by renal interstitial volume expansion (RIVE) via injection of 50 microliters of a 2% albumin in saline solution into the renal interstitium through a chronically implanted interstitial catheter. RIVE resulted in a stable increase in PI (4.6 +/- 0.4 to 9.4 +/- 0.8 mmHg) that was sustained over a 30- to 40-min period without significant changes in renal blood flow or glomerular filtration rate. Increases in PI were associated with significant increases in urine flow (13.8 +/- 3.4 to 31.7 +/- 5.0 microliters/min) and UNaV (2.3 +/- 0.6 to 6.2 +/- 1.1 micro eq/min) and fractional excretion of Na (2.6 +/- 0.8 to 6.9 +/- 1.5%). To determine the importance of albumin in maintaining an elevated PI, the effects of renal interstitial injections of saline were compared with albumin in saline solution. Injection of 50 microliters of saline into the renal interstitium had no sustained effect on PI. Injection of 2% albumin in saline solution in the same group of rats resulted in significant elevations in PI and UNaV. These data indicate that direct increases in PI via renal interstitial volume expansion result in significant increases in UNaV, thus supporting a role for PI in controlling UNaV.