Peritubular uptake and transepithelial transport of glycine in isolated proximal tubules

We examined transport of glycine from the bathing solution into cells and across the epithelium of perfused and nonperfused segments of isolated proximal straight (PST) and proximal convoluted (PCT) tubules. The cell-to-bath distribution ratio in about 0.15 mM glycine was 10.0 +/- 0.7 (SE) in nonperfused PST and 4.3 +/- 1.2 in PCT. Rapid luminal perfusion reduced these values to 4.6 +/- 0.3 and 2.0 +/- 0.5, respectively, but cellular accumulation in the PST still confirmed the presence of active basolateral uptake which was Na+ dependent. Bath-to-lumen glycine fluxes in both segments were linear over a wide range of bathing solution glycine concentrations, and the apparent permeability of this nonsaturable pathway was not significantly different from the nonsaturable component of the lumen-to-bath flux, evaluated by nonlinear fitting. Removal of Na+ from perfusing and bathing solutions reduced the cellular glycine concentration by more than 60% in the PST, yet this had no effect on bath-to-lumen glycine fluxes. We conclude that backleak of glycine occurs by a paracellular path.