Properties of the CA2+-activated K+ conductance of human red cells as revealed by the patch-clamp technique.

Application of Ca2+ to the inner surface of red-cell membranes activates unitary currents that can be measured in cell-attached and cell-free membrane patches. Ca2+ can be replaced by Pb2+ to activate the single channels. In addition to internal Ca2+ external K+ has to be present. The channels are preferentially permeable to K+ with a selectivity ratio PK:PNa of about 15:1 as estimated from measurement of reversal potentials. The dependence of channel activity on Ca2+ is compatible with the conception that the binding of two Ca2+ is necessary to open a single channel. Both the channel activity and the single-channel conductance exhibit inward rectification. External and internal Na+ inhibit the K+ currents. The reported results suggest that the unitary current events are responsible for the Ca2+-dependent K+ permeability known from measurement on cell suspensions. Therefore, comparison of the two techniques allows calculation of the number of K+ channels per red cell, which on average is about 10.