Effects of ouabain on the electrophysiological properties of proximal tubular cells.

The properties of the Na-K pump were studied by electrophysiological techniques in the proximal tubule of Necturus kidney. Peritubular capillaries were perfused for a short time with different artificial solutions, then ouabain (50 microM-1 mM) was added to each perfusate and its effects on cell membrane potential were assessed. Addition of ouabain, 1 mM, in a physiologic Ringer's solution (K concentration = 3 mM) failed to produce immediate depolarization. The introduction of the cardiac glycoside (50 microM) in solutions in which external K+ concentration was lowered to 0.75 or 0.3 mM resulted in depolarization by 4.1 +/- 0.8 and 9.5 +/- 1.4 mV, respectively. Furthermore, ouabain brought about an increase of membrane input resistance when added in a solution of lowered K+ concentration, but not in the presence of physiologic K+ concentrations. Readmission of K+ into a K-free perfusate flowing through the renal capillaries for more than 15 min resulted in hyperpolarization, 7.2 + 2.0 mV. These and other observations suggest that the Na-K pump operates in electrogenic fashion when external K+ concentration is lowered. Yet, part of the depolarizing response to ouabain under these conditions is mediated via ouabain-elicited changes of cell membrane permeabilities. The stoichiometry of the Na-K pump at physiologic external K concentrations cannot be assessed from the present experiments.