Comparative aspects and temperature dependence of [3H]1,4-dihydropyridine Ca2+ channel antagonist and activator binding to neuronal and muscle membranes

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Abstract
Binding of [3H]nitrendipine, [3H]nimodipine, and (+)[3H]PN 200-110 to microsomal preparations of guinea pig smooth and cardiac muscle and brain synaptosomes revealed high affinity interaction with KD values in the sequence, (+)PN 200-110 > nitrendipine > nimodipine. Bmax values for a particular tissue were independent of the 1,4-dihydropyridine employed in radioligand binding at 25 °C. The temperature dependence of [3H]nitrendipine binding in cardiac and smooth muscle microsomal preparations and brain synaptosomes was measured from 0° to 37 °C and for skeletal muscle preparations from 0° to 30 °C. Bmax values increased with temperature for cardiac membranes, but did not vary in other tissues, van't Hoff plots were nonlinear in all tissues, enthalpy and entropy changes becoming increasingly negative with increasing temperature. Competition binding of the activator–antagonist enantiomeric 1,4-dihydropyridine pairs of Bay k 8644 and PN 202-791 for [3H]nitrendipine in smooth muscle did not reveal significant thermodynamic differences between activator and antagonist molecules.