Kinetics of hyperosmotically stimulated Na-K-2Cl cotransporter in Xenopus laevis oocytes

Abstract

A detailed study of hypertonically stimulated Na-K-2Cl cotransport (NKCC1) in Xenopus laevis oocytes was carried out to better understand the 1 K+:1 Cl−stoichiometry of transport that was previously observed. In this study, we derived the velocity equations for K+influx under both rapid equilibrium assumptions and combined equilibrium and steady-state assumptions and demonstrate that the behavior of the equations and curves in Lineweaver-Burke plots are consistent with a model where Cl−binds first, followed by Na+, a second Cl−, and then K+. We further demonstrate that stimulation of K+movement by K+on the trans side is an intrinsic property of a carrier that transports multiple substrates. We also demonstrate that K+movement through NKCC1 is strictly dependent upon the presence of external Na+, even though only a fraction of Na+is in fact transported. Finally, we propose that the larger transport of K+, as compared with Na+, is a result of the return of partially unloaded carriers, which masks the net 1Na+:1K+:2Cl−stoichiometry of NKCC1. These data have profound implications for the physiology of Na-K-2Cl cotransport, since transport of K-Cl in some conditions seems to be uncoupled from the transport of Na-Cl.