The Ca 2+-sensitive K +-conductance of the human red cell membrane is strongly dependent on cellular pH

Abstract

The conductance of the Ca 2+-sensitive K +-channels in human red cell membranes has been determined as a function of the intracellular pH. A sudden increase in the intracellular concentration of ionized calcium was established by addition of ionophore A23187 to a suspension of cells in buffer-free, Ca 2+-containing salt solution. At the various cellular pH-values cellular concentrations of ionized Ca, saturating with respect to activation of the Ca 2+-sensitive K +-conductance, were obtained by the use of varied concentrations of extracellular Ca 2+ and added ionophore A23187. Changes in membrane potential was monitored as CCCP-mediated changes in extracellular pH. Initial net effluxes of K +, cellular K + contents and the K + Nernst equilibrium potentials were calculated from flame photometric measuruments. Cellular Ca-contents were determined by aid of 45Ca. With cellular Ca 2+ at the saturating level with respect to activation of the K +-channel the K +-conductance calculated from these data was independent of extracellular pH and a steep function of cellular pH with a half maximal conductance of 31 μS/cm 2 at a cellular pH of 6.1. The K +-conductance is not a simple function of cellular pH (pH c). From pH c = 6.5 and down to pH c = 6.0 a Hill-coefficient of 2.5 was found, indicating cooperativity between at least two sites regulating the conductance. Below pH c=6.0 an extremely high Hill-coefficient of 11 was found, probably indicating that the additional titration of the channel protein leads to an increased cooperativity. The importance, as a physiological regulatory mechanism, of a K +-conductance increasing from zero to maximal conductance within less than one unit of pH, is discussed.