Rate of formation and composition of absorbate from proximal nephron segments

Proximal convoluted (PCT) and proximal straight (PST) tubules from rabbit kidney were isolated and subsequently perfused under light mineral oil so that absorbate could be collected. Volume absorption rates in both segments were comparable to those observed during conventional perfusion in aqueous bathing solution. However, volume absorption in the PCT was significantly decreased by reduction or elimination of preferentially absorbed solutes from the perfusate. As previously observed in the PST, glucose was actively absorbed by the PCT against concentration gradients of up to 26 mM. In contrast, the Cl- concentration of the PCT absorbate was less than that in the lumen, presumably due to preferential HCO3- absorption. In the PST, the Na+ concentration of the absorbate exceeded that in the lumen and, given the negligible measured transepithelial voltage, active transepithelial Na+ transport was demonstrated. In contrast, in the PCT, when preferentially absorbed solutes were present in the perfusate at normal concentrations, the absorbate Na+ concentration was significantly less than that in the lumen. This low absorbate Na+ concentration may be due to the dilutional effect of more rapid fluid absorption in the presence of the preferentially absorbed solutes. When the concentrations of these solutes in the perfusate were reduced or eliminated, the absorbate Na+ concentration was not significantly different from that in the lumen.