Ca2+-activated K+ conductance of human red cell membranes exhibits two different types of voltage dependence

The voltage dependence for outward-going current of the Ca-activated K+ conductance (gK(Ca] of the human red cell membrane has been examined over a wide range of membrane potentials (Vm at constant values of K+]ex, K+]c and pHc, the intact cells being preloaded to different concentrations of ionized calcium. Outward-current conductances were calculated from initial net effluxes of K+ and the corresponding (Vm - EK) values. The basic conductance, defined as the outward-current conductance at (Vm - EK) greater than or equal to 20 mV and K+]ex greater than or equal to 3 mM (B. Vestergaard-Bogind, P. Stampe and P. Christophersen, J. Membrane Biol. 95:121-130, 1987) was found to be a function of cellular ionized Ca. At all degrees of Ca activation gK(Ca) was an apparently linear function of voltage (Vm range -40 to +70 mV), the absolute level as well as the slope decreasing with decreasing activation. In a simple two-state model the constant voltage dependence can, at the different degrees of Ca activation, be accounted for by a Boltzmann-type equilibrium function with an equivalent valence of approximately 0.4, assuming chemical equilibrium at Vm = 0 mV. Alternatively, the phenomenon might be explained by a voltage-dependent block of the outward current by an intracellular ion. Superimposed upon the basic conductance is the apparently independent inward-rectifying steep voltage function with an equivalent valence of approximately 5 and chemical equilibrium at the given EK value.