Calcium-potassium-stimulated net potassium efflux from human erythrocyte ghosts

In the presence of 8 mM external Ca++, the K+ permeability of human red cell ghosts increases provided K+ is also present in the medium. This increase does not represent K+/K+ exchange but a stimulation of net K+ efflux. The stimulation is half-maximal at 0.7 +/- 0.15 mM (n=5). At concentrations above 4.0 mM, external K+ inhibits net K+ efflux. Similar stimulatory and inhibitory effects of external K were also observed in intact cells after exposure to Pb++ or to Ca++ in the presence of fluoride, iodoacetate plus adenosine, or propranolol, suggesting that a common K+ -activated K+ -specific transfer system may be involved under all of these various circumstances. Internal K+ also stimulates net K+ efflux from ghosts, but it is uncertain whether internal K+ is an absolute requirement for the K+ permeability increase. In contrast to external Na+ which slightly stimulates K+ efflux, internal Na+ inhibits. The inhibition by internal Na+ is abolished by sufficiently high concentrations of external K+, showing that K+ binding to the outer membrane surface and Na+ binding to the internal surface are mutually interdependent. In red cell ghosts the Ca++ -K+ -stimulated net K+ efflux increases with increasing pH until a plateau is reached between pH 7.2 and 8.0. In fluoride-poisoned intact cells, the Ca++-K+ stimulated flux passes through a maximum around pH 6.8. Neither internal nor external Mg++ interferes with the combined effects of Ca++ and K+. Similarly, external EDTA has no influence at concentrations which are far lower than the Ca++ concentration required to produce a maximal response. In contrast, low concentrations of internal EDTA prevent the permeability change.