Postglomerular vascular hydrostatic and oncotic pressures during acute saline volume expansion in normotensive man

The importance of the peritubular physical factors as mediators of the natriuretic response to saline volume expansion was examined in twenty normotensive, hydropenic indivduals. Intrarenal venous pressure (IRVP), used as a measure of peritubular capillary hydrostatic pressure, and efferent arteriolar colloid osmotic pressure, calculated from arterial colloid osmotic pressure and the filtration fraction, were measured before and during sustained volume expansion with 0.9% NaCl, increasing the body weight by 3% and plasma volume about 20%. During expansion there was a significant increase in urine flow from 1.1 +/0 0.1 to 3.1 +/- 0.4 ml/min and sodium excretion from 161 +/- 12 to 551 +/- 61 microEq/min. Efferent colloid osmotic pressure fell from 31.9 +/- 0.6 to 23.6 +/- 0.5 mmHg (P less than 0.001) while IRVP changed from 24.8 +/- 0.8 to 25.1 +/- 0.9 mmHg (P greater than 0.10). In eight individuals IRVP increased during saline loading but later fell during sustained expansion. Glomerular filtration rate and renal blood flow did not change significantly. It is concluded that increase in peritubular capillary hydrostatic pressure is not necessary either to induce or to maintain the natriuresis of a moderate saline volume expansion. Although the fall in postglomerular vascular colloid osmotic pressure is a possible mediator of the natriuretic response, the change in peritubular transcapillary net driving force produced by a modest saline volume expansion is probably small.