Influence of serum proteins on net fluid reabsorption of isolated proximal tubules

Abstract

Influence of serum proteins on net fluid absorption of isolated proximal tubules. To determine the direct effect on net fluid absorption of serum proteins in the peritubular fluid, isolated segments of proximal convoluted (PCT) and straight (PST) tubules were studied in vitro . The tubular lumen was perfused at one end with a micropipet; the other end was occluded. Net transtubular isosmotic fluid absorption at 37° C was determined from the clearance of 3 H 2 O out of the perfusion pipet. In a bath of rabbit serum or 6 g/100 ml bovine albumin in Ringer's media, net absorption was approximately three-fold greater in PCT's than in PST's. Net absorption was directly proportional to tubular length and was markedly inhibited by ouabain. Increased intralumenal hydrostatic pressure did not affect net absorption significantly. Rapid lowering of the bath protein to less than 6 g/100 ml caused net absorption to decrease; in both proximal convoluted and straight tubules net absorption was reduced about 40% in a bath containing 0.3% serum protein. When the bath protein was increased from 6% to 10%, net fluid absorption decreased slightly. On the basis of these studies we conclude that net fluid absorption of proximal tubules is relatively independent of the difference in either hydrostatic or oncotic pressure between the tubular lumen and peritubular fluid. It is suggested that peritubular serum proteins may influence net absorption at the antilumenal side of the epithelial cell, perhaps by facilitating the movement of absorbate across the tubular basement membrane.