Ca-activated K conductance of the human red cell membrane: Voltage-dependent Na block of outward-going currents.

Human red cells were prepared with various cellular Na and K concentrations at a constant sum of 156 mm. At maximal activation of the K conductance, g(Ca), the net efflux of K was determined as a function of the cellular Na and K concentrations and the membrane potential, V, at a fixed [K] of ∼3.5 mm. V was only varied from ( V− E)≈25 mV and upwards, that is, outside the range of potentials with a steep inward rectifying voltage dependence (Stampe & Vestergaard-Bogind, 1988). g(Ca) as a function of cellular Na and K concentrations at V=−40, 0 and 40 mV indicated a competitive, voltage-dependent block of the outward current conductance by cellular Na. Since the present Ca-activated K channels have been shown to be of the multi-ion type, the experimental data from each set of Na and K concentrations were fitted separately to a Boltzmann-type equation, assuming that the outward current conductance in the absence of cellular Na is independent of voltage. The equivalent valence determined in this way was a function of the cellular Na concentration increasing from 0.5 to 1.5 as this concentration increased from 11 to 101 mm. Data from a previous study of voltage dependence as a function of the degree of Ca activation of the channel could be accounted for in this way as well. It is therefore suggested that the voltage dependence of g(Ca) for outward currents at ( V− E)>25 25 mV reflects a voltage-dependent Na block of the Ca-activated K channels. [ABSTRACT FROM AUTHOR]