Calcium antagonist properties of diclofurime isomers. II. Molecular aspects: allosteric interactions with dihydropyridine recognition sites.

Trans-diclofurime has been shown to be a very potent class II calcium antagonist (see preceding report), and we have examined its molecular interactions with the different receptor sites at the Ca2+ channel. Trans-diclofurime did not affect [3H]nitrendipine binding to rat cortical membranes at 37 degrees C and showed weak inhibitory effects at 25 degrees C, whereas at 0 degrees C 80% of the binding was inhibited noncompetitively (IC50, 13 nM); cis-diclofurime was 22-fold less potent. Trans-diclofurime, like diltiazem, blocked the inhibitory effects of verapamil on [3H]nitrendipine binding. Trans-diclofurime is a potent displacer of [3H]diltiazem binding (IC50, 15 nM; IC50 for diltiazem, 55 nM); the diclofurime isomers showed high stereoselectivity, with high Hill coefficients (0.85-1.0). In contrast, the stereoselectivity of the isomers was lower as inhibitors of [3H]verapamil binding, as were the Hill coefficients (0.55-0.65). It is proposed that the functional potency of the diclofurime isomers as calcium antagonists can be explained on the basis of their relative affinities for the diltiazem site and that this site is coupled to the dihydropyridine site in a positive heterotropic allosteric manner. A model for the interaction of group II calcium antagonists with the Ca2+ channel is proposed.