, Volume 59, Issue 1, pp 57-63

Effects of quinine on Ca++ -induced K+ efflux from human red blood cells

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The Ca++-mediated increase in K+-permeability of intact red blood cells (Gardos effect) was initiated by exposing cells to known concentrations of Ca++ (using EGTA buffers) in the presence of the ionophore A23187. The potency of quinine, an inhibitor of the response, was found to depend on the external K+ concentration. In K+-free solutions the concentration of quinine to achieve 50% inhibition (K 50) was 5 μm, but at 5mm K+ the required concentration was increased 20-fold to 100 μm. An increase in internal Na+ had the opposite effect, allowing a high potency of quinine despite the presence of external K+. Alterations in the internal K+ level, on the other hand, were without effect on theK 50, suggesting that the membrane potential is not a factor. This conclusion is supported by the lack of effect on quinine inhibition of substitution of Cl by NO 3 , a considerably more permeant anion. The data are consistent with the hypothesis that quinine inhibits by competitively displacing K+ from an external binding site, the reported K+-activation site for the Ca++-mediated K+-permeability.