Spontaneous inactivation of the Ca2+-sensitive K+ channels of human red cells at high intracellular Ca2+ activity.

Ionophore A23187-mediated Ca2+-induced oscillations in the conductance of the Ca2+-sensitive K+ channels of human red cells were monitored with ion specific electrodes. The membrane potential was continuously reflected in CCCP-mediated pH changes in the buffer-free medium, changes in extracellular K+ activity were followed with a K+-selective electrode, and changes in the intracellular concentration of ionized calcium were calculated on the basis of cellular 45Ca content. An increased cellular 45Ca content at the successive minima of the oscillations where the K+ channels are closed indicates that the activation of the channels might be a (dCa2+/dt)-sensitive process and that accommodation to enhanced levels of intracellular free calcium may occur. An incipient inactivation of the K+ channels at intracellular ionized calcium levels of about 10 microM and a concurrent membrane potential of about -65 mV was observed. At a membrane potential of about -70 mV and an intracellular concentration of about 2 X 10(-4) M no inactivation of K+ channels took place. Inactivation of the K+ channels is suggested to be a compound function of the intracellular level of free calcium and the membrane potential. The observed sharp peak values in cellular 45Ca content support the notion that a necessary component of the oscillatory system is a Ca2+ pump operating with a significant delay in the activation/inactivation process in response to changes in cellular concentration of ionized calcium.