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

Abstract

The conductance of the Ca2+-activated K+ channel (gK(Ca)) of the human red cell membrane was studied as a function of membrane potential (Vm) and extracellular K+ concentration ([K+]ex). ATP-depleted cells, with fixed values of cellular K+ (145 mM) and pH (approximately 7.1), and preloaded with approximately 27 microM 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+, corresponding Vm, 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 (EK) determined at the peak of hyperpolarization. Zero-current conductances were calculated from unidirectional effluxes of 42K at (Vm-EK) approximately equal to 0, using a single-file flux ratio exponent of 2.7. Within a [K+]ex range of 5.5 to 60 mM and at (Vm-EK) greater than or equal to 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 microS/cm2 between 0 and -100 mV. As (Vm-EK) decreased from 20 towards zero mV gK(Ca) increased hyperbolically from the basic value towards a zero-current value of 165 microS/cm2. The zero-current conductance was not significantly dependent on [K+]ex (30 to 156 mM) corresponding to Vm (-50 mV to 0). A further increase in gK(Ca) symmetrically around EK is suggested as (Vm-EK) becomes positive. Increasing the extracellular K+ concentration from zero and up to approximately 3 mM resulted in an increase in gK(Ca) from approximately 50 to approximately 70 microS/cm2. Since the driving force (Vm-EK) was larger than 20 mV within this range of [K+]ex this was probably a specific K+ activation of gK(Ca). 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.