Interaction Between Calcium Channel Ligands and Calcium Channels

Distinct drug receptors for 1,4-dihydropyridines, phenylalkylamines, and the benzothiazepine d-cis-diltiazem exist on voltage-dependent calcium channels. The drug receptors show reciprocal allosteric communication and are linked to calcium binding sites. The 1,4-dihydropyridine-selective receptor (probed with [H]nimodipine) has a size (measured by radiation inactivation) identical in heart, skeletal muscle, and brain (180 kDa). To compare the sizes of 1,4-dihydropyridine receptors in different tissues, pure tritiated enantiomers of the arylazido photoaffinity probe [H]azidopine were used to irreversibly label the purified 1,4-dihydropyridine receptor (155,65, and 32 kDa) from guinea pig skeletal muscle transverse tubules and the membrane-bound cardiac receptor. The 155 kDa polypeptide region, but not the 65 or 32 kDa bands, was specifically labelled by ( - )-[H]azidopine. (+ )-[H]Azidopine did not label any of the polypeptides in the purified receptor preparation. In contrast with the results from other investigators, a 155 kDa polypeptide was also specifically labelled in cardiac membranes by ( - )-[H]azidopine. A 34 kDa photolabelled band carries a low-affinity 1,4-dihydropyridine binding site that has no obvious relation to the channel but is abundant in heart membranes and has apparently led to erroneous results in previous affinity or photoaffinity labelling experiments. Antibodies raised against the purified skeletal muscle channel precipitate the I-labelled 155 kDa channel polypeptide from skeletal muscle and precipitate the three drug receptor sites from both crude and purified channel preparations. We conclude that all three drug receptor sites are localized on this polypeptide.