Transport of K+ by Na(+)-Ca2+, K+ exchanger in isolated rods of lizard retina (original) (raw)

1995, Biophysical Journal

Transport of K+ by the photoreceptor Na'-Ca2",K' exchanger was investigated in isolated rod outer segments (OS) by recording membrane current under whole-cell voltage-clamp conditions. Known amounts of K+ were imported in the OS through the Ca2`-activated K+ channels while perfusing with high extracellular concentration of K+, [K+]O. These channels were detected in the recordings from the OS, which probably retained a small portion of the rest of the cell. The activation of forward exchange (Na+ imported per Ca2+ and K+ extruded) by intracellular K+, Kt+, was described by first-order kinetics with a Michaelis constant, K8PP(Kj+), of about 2 mM and a maximal current, /max, of about-60 pA. [Na+], larger than 100 mM had little effect on KaPP(Kj+) and /max, indicating that Na+ did not compete with K:+ for exchange sites under physiological conditions, and that Na+ release at the exchanger intracellular side was not a rate-limiting step for the exchange process. Exchanger stoichiometry resulted in one K+ ion extruded per one positive charge imported. Exchange current was detected only if Ca2+ and K+ were present on the same membrane side, and Na+ was simultaneously present on the opposite side. Nonelectrogenic modes of ion exchange were tested taking advantage of the hindered diffusion found for Ca?+ and K:+. Experiments were carried out so that the occurrence of a putative nonelectrogenic ion exchange, supposedly induced by the preapplication of certain extracellular ion(s), would have resulted in the transient presence of both Ca?+ and K:+. The lack of electrogenic forward exchange in a subsequent switch to high Na+, excluded the presence of previous nonelectrogenic transport.