Voltage dependence of the Ca²⁺-activated K⁺ conductance of human red cell membranes is strongly dependent on the extracellular K⁺ concentration

Summary

The conductance of the Ca²⁺-activated K⁺ channel (g _K(Ca)) of the human red cell membrane was studied as a function of membrane potential (V _m ) and extracellular K⁺ concentration ([K⁺]_ex). ATP-depleted cells, with fixed values of cellular K⁺ (145mm) and pH (≈7.1), and preloaded with ≈27 μm ionized Ca were transferred, with open K⁺ channels, to buffer-free salt solutions with given K⁺ concentrations. Outward-current conductances were calculated from initial net effluxes of K⁺, correspondingV _m , monitored by CCCP-mediated electrochemical equilibration of protons between a buffer-free extracellular and the heavily buffered cellular phases, and Nernst equilibrium potentials of K ions (E _K) determined at the peak of hyperpolarization. Zero-current conductances were calculated from unidirectional effluxes of⁴²K at (V _m −E _K)≃0, using a single-file flux ratio exponent of 2.7. Within a [K⁺]_ex range of 5.5 to 60mm and at (V _m −E _K) ≥ 20 mV a basic conductance, which was independent of [K⁺]_ex, was found. It had a small voltage dependence, varying linearly from 45 to 70 μS/cm² between 0 and −100 mV. As (V _m −E _K) decreased from 20 towards zero mVg _K(Ca) increased hyperbolically from the basic value towards a zero-current value of 165 μS/cm². The zero-current conductance was not significantly dependent on [K⁺]_ex (30 to 156mm) corresponding toV _m (−50 mV to 0). A further increase ing _K(Ca) symmetrically aroundE _K is suggested as (V _m −E _K) becomes positive. Increasing the extracellular K⁺ concentration from zero and up to ≈3mm resulted in an increase ing _K(Ca) from ≈ 50 to ≈ 70 μS/cm². Since the driving force (V _m −E _K) was larger than 20 mV within this range of [K⁺]_ex this was probably a specific K⁺ activation ofg _K(Ca). In conclusion: The Ca²⁺-activated K⁺ channel of the human red cell membrane is an inward rectifier showing the characteristic voltage dependence of this type of channel.