(-)-[³H]Desmethoxyverapamil labels multiple calcium channel modulator receptors in brain and skeletal muscle membranes: Differentiation by temperature and dihydropyridines

The verapamil-like calcium channel modulator, (-)-[³H]desmethoxyverapamil binds to multiple sites in microsomal membrane preparations from brain and skeletal muscle. In brain the K(d) values of the sites are 0.55 ± 0.1 and 61.8 ± 18 nM for the high and low affinity sites and the maximum binding values are 0.22 ± 0.04 and 4.6 ± 1.0 pmol/mg of protein, respectively. Equilibrium analysis of saturation data in skeletal muscle membranes shows only one site with an affinity of 7.2 ± 0.8 nM and a maximum binding of 2.96 ± 0.32 pmol/mg of protein. However, a low affinity site with an estimated K(d) of 152 nM is indicated in dissociation kinetic studies. Dihydropyridine calcium channel modulators regulate the binding of desmethoxyverapamil in a temperature-dependent fashion with (+)-PN 200110 decreasing (-)-[³H]desmethoxyverapamil binding more at 0°C than at higher temperatures and, at 37° C, enhancing binding in skeletal muscle. The influence of (+)-desmethoxyverapamil on (+)-[³H]PN 200110 binding is unchanged by temperature variations, whereas interactions of the (-)-enantiomer are altered markedly with more inhibition at 0°C than at higher temperatures and, in skeletal muscle, stimulation of binding at 37° C. Dissociation studies indicate that the two sites labeled by (-)-[³H]-desmethoxyverapamil in skeletal muscle interact in a negative heterotropic cooperative fashion. Dihydropyridines appear to slow the dissociation of ligand from the low affinity site, whereas diltiazem accelerates the dissociation of (-)-[³H]desmethoxyverapamil from the high affinity site. These results suggest that the high and low affinity sites labeled by (-)-[³H]desmethoxyverapamil, respectively, represent the verapamil and diltiazem receptors in brain and skeletal muscle.