Binding Sites

Although all three types of selective calcium channel blockers interact with the alpha1 subunit of the L-type calcium channel, each binds to a different receptor site. A complex allosteric relationship exists among these receptor sites. For example, drugs binding at the dihydropyridine site appear to increase the affinity of diltiazem for the benzothiazepine site, and vice versa. In contrast, the binding of verapamil at the phenylalkylamine site appears to reduce the affinities of diltiazem and the dihydropyridine calcium channel blockers for binding at their respective sites.

The binding sites for all three chemical types of calcium channel blocker are present in many tissues, including myocardium, smooth muscle, skeletal muscle, and glandular tissue. However, the activity of each calcium channel blocker in a particular tissue varies. Nifedipine and other dihydropyridines act preferentially on vascular smooth muscle, exerting potent peripheral vasodilating effects. Verapamil and diltiazem are less specific for peripheral vascular smooth muscle and more active in the myocardium and cardiac conductive tissues.

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