Calcium Channel Blockers

The L-type calcium channel, acted on by calcium channel blockers, consists of five different subunits, designated a\, a2, fi, y, and 8. The a1 subunit provides the central pore of the channel (Fig. 19.6). Calcium channel blockers can be divided conveniently into the three different chemical classes of the prototype drugs that have been used: phenylalkyl-amines (verapamil), 1,4-dihydropyridines (nifedipine), and benzothiazepines (diltiazem). These prototype compounds sometimes are termed the "first generation" of calcium channel blockers because two of the groups of drug classes have been expanded by the introduction of a "second generation" of more potent analogs (Table 19.3).

The specific Ca2+ channel antagonists verapamil, nifedipine, and diltiazem interact at specific sites on the calcium channel protein. These blockers do not occlude the channel physically but bind to sites in the channel, because they can promote both channel activation and antagonism. Affinity for binding sites on the channel varies, depending on the status of the channel. The channel can exist in either an open (O), resting (R), or inactivated (I) state, and the equilibrium between them is determined by stimulus frequency and membrane potential (Fig. 19.7). Verapamil

TABLE 19.3 First- and Second-Generation Calcium Channel Blockers

Chemical

First

Second

Classification

Generation

Generation

Phenylalkylamines

Verapamil

Anipamil

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