Correlation between the negative inotropic potency and binding parameters of 1,4-dihydropyridine and phenylalkylamine calcium channel blockers in cat heart

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Abstract
Summary Partially purified plasma membranes were prepared from cat ventricle. The purification factors for the calcium channel ligands (+)-3H-PN 200-110, 3H-nimodipine (1,4-dihydropyridines) and (−)-3-H-desmethoxyverapamil (a phenylalkylamine) were 3.1-, 3.4- and 2.9-fold, respectively, whilst β-adrenoceptors labelled with (−)-3H-dihydroalprenolol were purified 3.0-fold. (+)-3H-PN 200-110 bound to 930±140 fmol/mg of membrane protein with a dissociation constant of 70 pmol/l at 25°C. Under the same conditions 3H-nimodipine bound to 490±24 fmol/mg of sites with a K D of 120 pmol/l. (−)-3-H-desmethoxyverapamil bound to 530±55 fmol/mg of sites with a K D of 2.47 nmol/l. Twelve 1,4-dihydropyridines were evaluated for binding inhibition constants (K i) with (+)-3H-PN 200-110 and 13 phenylalkylamines with (−)-3-H-desmethoxyverapamil in radioligand binding assays. Of the twelve 1,4-dihydropyridines evaluated (±)-nitrendipine was the most potent with a K i-value of 280 pmol/l, nifedipine had a K i-value of 500 pmol/l and the weakest drug tested, (±)-Bay b4328, had a K i-value of 14.3 nmol/l. The EC50-values of the same 1,4-dihydropyridines to inhibit the electrically driven cat papillary muscle were 77- to 3,450-fold higher and little correlation existed between K i and EC50-values. Thirteen phenylakylamines were tested for their potency to inhibit (−)-3-H-desmethoxyverapamil binding. The most potent phenylalkylamine with respect to negative inotropy was (±)-D 595 with an EC50-value of 794 nmol/l, the least potent substance was (±)-Sz 45 with an EC50-value of 39.8 μmol/l. The binding inhibition constants for the phenylalkylamines were 13-to 322-fold lower than the values for negative inotropy, but a significant positive correlation between the K i and EC50-values (n=12, r=0.84) was observed. The absolute differences may reflect the state-dependent binding of phenylalkylamines to the channel. QSAR analysis revealed nearly identical correlations between physicochemical substituent properties on the one hand and binding affinities or functional potency on the other hand. In both cases the electronic properties (F-constant) of ring substituents mainly determine the variance in potency.