Kinetics of Na+ transport in Necturus proximal tubule.

Abstract

The dependence of proximal tubular Na and fluid reabsorption on the Na+ concentration of the luminal and peritubular fluid was studied in the perfused Necturus maculosus kidney. Fluid droplets, separated by oil from the tubular contents and identical in composition to the vascular perfusate, were introduced into proximal tubules, reaspirated and analyzed for Na+ and 14C-mannitol. Fluid transport was measured in short-circuited fluid samples by observing the rate of change in length of the split droplets in the tubular lumen. Both reabsorptive fluid and calculated Na fluxes were simple, saturable functions of the perfusate Na+ concentration (Km = 35-39 mM/l, Vmax = 1.37 control value). Intracellular Na+, determined by tissue analysis, and open-ciruit transepithelial electrical potential differences were also saturable functions of extracellular Na+. Net reabsorptive fluid and Na+ fluxes were linearly dependent on intracellular Na+ and showed no saturation, even at sharply elevated cellular Na concentrations. These concentrations were achieved by addition of amphotericin B to the luminal perfusate, a maneuver which increased the rate of Na+ entry into the tubule cells and caused a proportionate rise in net Na+ flux. Active peritubular Na transport in proximal tubule cells of Necturus was normally unsaturated and remained so even after amphotericin-induced enhancement of luminal Na+ entry. Transepithelial movement of NaCl may be described by a model with a saturable luminal entry step of Na+ or NaCl into the cell and a 2nd, unsaturated active transport step of Na+ across the peritubular cell boundary.