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The actions of some inhibitors of the Ca2+‐activated K+ permeability in mammalian red cells have been compared. 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 μM) to rabbit red cells suspended at a haematocrit of 1% in a low potassium solution ([K]0 0.12–0.17 mM) at 37°C. Net movement of K+ was measured using a K+‐sensitive electrode placed in the suspension. The concentrations (μM±s.d.) of the compounds tested causing 50% inhibition of K+ loss were: quinine, 37±3; cetiedil, 26±1; the cetiedil congeners UCL 1269, UCL 1274 and UCL 1495, ∼150, 8.2±0.1, 0.92±0.03 respectively; clotrimazole, 1.2±0.1; nitrendipine, 3.6±0.5 and charybdotoxin, 0.015±0.002. 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, nH, 2.7) than for the other (clotrimazole, nitrendipine, charybdotoxin) for which nH∼amp;1. Compounds in the first set alone became less active on raising the concentration of K+ in the external solution to 5.4 mM. The rate of K+ loss induced by A23187 slowed in the presence of high concentrations of cetiedil and its analogues, suggesting a use‐dependent component to the inhibitory action. This was not seen with clotrimazole. The blocking action of the cetiedil analogue UCL 1274 could not be overcome by an increase in external Ca2+ and its potency was unaltered when K+ loss was induced by the application of Pb2+ (10 μM) rather than by A23187. These results, taken with the findings of others, suggest that agents that block the red cell Ca2+‐activated K+ permeability can be placed in two groups with different mechanisms of action. The differences can be explained by supposing that clotrimazole and charybdotoxin act at the outer face of the channel whereas cetiedil and its congeners may block within it, either at or near the K+ binding site that determines the flow of K+. British Journal of Pharmacology (1999) 126, 169–178; doi:

Cetiedil ((+/-)-2-cyclohexyl-2-(3-thienyl)ethanoic acid 2-(hexahydro-1 H-azepin-1-yl) ethyl ester) possesses anti-sickling and analgesic, antispasmodic, local anaesthetic and vasodilator activities. A total synthesis and circular dichroism spectra of the enantiomers of cetiedil is described, together with a comparison of their effectiveness as blockers of the Ca(2+)-activated K+ permeability of rabbit erythrocytes; the contractile response of intestinal smooth muscle to acetylcholine; the Ca(2+)-dependent contraction of depolarized intestinal muscle; and the cell volume-sensitive K+ permeability (Kvol) of liver cells. The enantiomers did not differ substantially in their ability to block the Ca(2+)-activated K+ permeability of rabbit red cells or in their effectiveness as blockers of the contractile response of depolarized smooth muscle to externally applied Ca2+. There was a clear difference in the muscarinic blocking activity of the enantiomers, as assessed by inhibition of the contractile response of intestinal smooth muscle to acetylcholine; (+)-cetiedil was 7.7 +/- 0.2 (s.d.) times more active than the (-) from. The enantiomers also differed in their potency as blockers of the increase in membrane conductance which occurs when liver cells swell. The concentration of (+)-cetiedil needed to reduce the conductance increase by 50% was 2.04 +/- 0.54 (s.d.) microM; (-)-cetiedil was 2.6 +/- 0.8 (s.d.) times less active (IC50 of 5.2 +/- 1.2 microM). Differences in the biological actions of the enantiomers of cetiedil indicate that a more extensive study could be rewarding in relation to the use of the enantiomers both in therapeutics and in the study of K+ channels.