Differences in the actions of some blockers of the calcium-activated potassium permeability in mammalian red cells

1 The actions of some inhibitors of the Ca2+-activated K+ permeability in m ammalian red cells have been compared. 2 Block of the permeability was assessed from the reduction in the net loss of K+ that followed the application of the Ca2+ ionophore A23187 (2 mu M) to rabbit red cells suspended at a haematocrit of 1% in a low potassium sol ution ([K](0) 0.12-0.17 mM) at 37 degrees C. Net movement of K+ was measure d using a K+-sensitive electrode placed in the suspension. 3 The concentrations (mu M +/- s.d.) of the compounds tested causing 50% in hibition of K+ loss were: quinine, 37+/-3; cetiedil, 26+/-1; the cetiedil c ongeners UCL 1269, UCL 1274 and UCL 1495, similar to 150, 8.2+/-0.1, 0.92+/ -0.03 respectively; clotrimazole, 1.2+/-0.1; nitrendipine, 3.6+/-0.5 and ch arybdotoxin, 0.015+/-0.002. 4 The characteristics of the block suggested that compounds could be placed in two groups. For one set (quinine, cetiedil, and the UCL congeners), the concentration-inhibition curves were steeper (Hill coefficient, n(H), grea ter than or equal to 2.7) than for the other (clotrimazole, nitrendipine, c harybdotoxin) for which n(H)approximate to 1. 5 Compounds in the first set alone became less active on raising the concen tration of K+ in the external solution to 5.4 mM. 6 The rate of K+ loss induced by A23187 slowed in the presence of high conc entrations of cetiedil and its analogues, suggesting a use-dependent compon ent to the inhibitory action. This was not seen with clotrimazole. 7 The blocking action of the cetiedil analogue UCL 1274 could not be overco me by an increase in external Ca2+ and its potency was unaltered when K+ lo ss was induced by the application of Pb2+ (10 mu M) rather than by A23187. 8 These results, taken with the findings of others, suggest that agents tha t block the red cell Ca2+-activated K+ permeability can be placed in two gr oups with different mechanisms of action. The differences can be explained by supposing that clotrimazole and charybdotoxin act at the outer face of t he channel whereas cetiedil and its congeners may block within it, either a t or near the K+ binding site that determines the flow of K+.