Pharmacologic and radioligand binding studies of 1,4-dihydropyridines in rat cardiac and vascular preparations: Stereoselectivity and voltage dependence of antagonist and activator interactions

The pharmacologic and radioligand-binding properties of 1,4-dihydropyridines in an activator (Bay K 8644) and an antagonist (nifedipine) series were studied in rat tail artery, heart membrane, and neonatal rat ventricular myocytes. The S-enantiomers of the activator series contracted rat tail artery in the presence of 15 mM K⁺ (EC₅₀ values of 10^-8 to 10^-5 M). (S)-Bay K 8644 (I) and its o-difluoromethoxy analog (III) were the most potent members of the activator series examined. The abilities of the activators to stimulate maximum tension response of the artery differed with structure; thus, the efficacy of (S)-Bay K 8644 was 70% that of the analog lacking the 3-carbomethoxy group. The R-enantiomers of the activator series and a series of achiral nifedipine analogs were inhibitory in the same tissue. The intact-cell binding assay revealed the binding affinities of 1,4-dihydropyridine antagonists in depolarized cells (50 mM K⁺) to be higher than those in polarized cells (5 mM K⁺). The ratio K_D (polarized)/K_D (depolarized) was 77 for nifedipine (IC₅₀ = 5.4 × 10^-9 M) but was only 2.9 for the weak 3-methoxy nifedipine analog (IC₅₀ = 4.8 × 10^-6 M); an approximately linear relationship exists between this ratio and the antagonist potency. In marked contrast, and in confirmation of previous work [Mol. Pharmacol. 35:541-552 (1989)], the binding affinities of activators were not significantly affected by membrane potential, regardless of potency. We conclude that the S-enantiomers of Bay K 8644 analogs are activators with different potency and efficacy and that the R-enantiomers are antagonists, that the binding of 1,4-dihydropyridine antagonists is voltage dependent, whereas binding of the activators is not, and that the voltage-dependence of binding of the antagonists is correlated with the potency of the antagonist.