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

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Abstract
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 μM), 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.