Modulation of Ca2+-mediated K+-gating of erythrocyte ghosts by external Ca-EGTA.

Using 86Rb+ as a marker for K+ permeability, we find that extracellular Ca-EGTA influences the rate of 86Rb+ efflux from erythrocyte ghosts preloaded with 86Rb+ and "buffered" Ca2+. At an internal free Ca2+, where the rate of 86Rb+ efflux is minimal and uninfluenced by either external EGTA or external Ca2+, external Ca-EGTA at 0.2-0.5 mM can raise the flux rate to as high as can be attained by raising internal Ca2+, in the presence of an excess externally either of Ca2+ or of EGTA. Higher concentrations of Ca-EGTA (up to 1-2 mM) diminish the flux rate. External Ca-EDTA or Mg-EDTA can substitute for Ca-EGTA in enhancing and suppressing flux rate. The peak rate is insensitive to external free Ca2+ but depends on internal Ca2+; internal Mg-EDTA does not substitute for internal Ca-EGTA. Thus, the erythrocyte membrane is asymmetric with respect to its interaction with Ca2+ and Ca-EGTA. Also, 22Na+ does not substitute for 86Rb+. The peak rate of 86Rb+ flux produced by external Ca-EGTA is diminished by chlorpromazine (0.1 mM) and augmented by 1-propranolol (25 microM), in the same way as the rate produced by increasing internal Ca2+. The results suggest that external Ca-EGTA enhances the affinity of internal Ca2+ for its receptor(s) which operate the K+-gate at the inner surface of the membrane. At external concentrations of Ca-EGTA above 1-2 mM, 86Rb+ flux rate again rises with increase of Ca-EGTA. This phenomenon does not depend upon internal Ca2+, is not affected by chlorpromazine or by 1-propranolol, and is associated with an enhanced permeability to 22Na+, inulin, and haemoglobin.