Uncoupled H transport by the Na/K pumps expressed in Xenopus oocytes

Abstract

The Na/K pump (NKA) is a P‐type ATPase that maintains ion gradients across the plasma membrane of animal cells by exporting 3 Na in exchange for 2 K, per each ATP hydrolyzed. The molecular mechanism of ion selectivity at the binding sites is not fully understood. Two out of three ion binding sites bind both Na and K (shared sites) and another exclusively binds Na. Without external Na (Nao) or K (Ko) the pump transports H passively through a pathway and mechanism that is, as yet, uncharacterized (H‐leak). In order to better understand this atypical transport mode and the interaction of ions at the binding sites we investigated the characteristics of this H‐leak. We characterized external [H] dependence, inhibition by Nao and Ko and voltage dependence of NKA heterologously expressed in Xenopus oocytes via two electrode voltage clamp and giant patch clamp. Increasing [H]o increased inward current with a strongly V‐dependent K0.5 (0.4 μM at −180 mV and 1.3 μM at −80 mV). At pH ≤ 5.6, the rapid increase in current was followed by a slower current inhibition (80% inhibition at pH 5 in steady state). At pH 7.6 and 6, the leak was inhibited by Nao or Ko with a mild V‐dependence (K05 ≃ 10 mM and 0.1 mM, respectively at −180 mV). At pH 5, Na had a biphasic effect, increasing the current at < 5 mM and inhibiting at higher [Na], indicating relief of the H dependent inhibition. Supported by AHA BGIA2140172 to PA & NIH GM061583 to CG.