Abstract
The presence of a diltiazem recognition site within the macromolecular complex of the calcium channel in the brain had been hypothesized on the basis of binding studies with [ 3H]dihydropyridine calcium channel antagonists. In the present study, we therefore characterized [ 3H]diltiazem binding sites in the rat cerebral cortex. Saturable high affinity (K d = 50−170 nM) [ 3H]diltiazem binding to the rat cerebral cortex was stereospecifically inhibited by the enantiomers of diltiazem according to their activity as calcium channel antagonists and modulators of [ 3H]dihydropyridine binding. An association between the [ 3H]diltiazem binding site and the calcium channel was further corroborated by the effects of chemically heterogenous calcium channel antagonists on [ 3H]diltiazem binding. Dihydropyridines appeared to allosterically affect [ 3H]diltiazem binding according to their pharmacological effects; e.g. at 37°C nitrendipine enhanced whereas the calcium agonist Bay K 8644 failed to affect [ 3H]diltiazem binding at concentrations fully inhibiting [ 3H]nitrendipine binding. The effect of nitrendipine may, at least in part, be explained by an increase in the affinity of [ 3H]diltiazem.
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