Calcium channels: basic properties as revealed by radioligand binding studies.

The structural features and molecular properties of calcium channels have been explored with 1,4-dihydropyridines, d-cis diltiazem and verapamil in radiolabeled form. The concept of multiple (at least three) drug receptor sites of the channel (sites 1, 2, and 3) has been proposed. Site 1 is labeled by 1,4 dihydropyridines, site 2 by (+/-) verapamil (in skeletal muscle) and by (-) desmethoxyverapamil (in brain, skeletal muscle, and heart), site 3 by d-cis diltiazem (in skeletal muscle). The three distinct drug receptor sites communicate with each other via reciprocal allosterism and are linked to divalent cation binding sites. The concept of the reciprocal allosteric communication between the receptor sites has been proven by individual labeling of each of the drug receptor sites in the skeletal muscle t-tubule membrane where the density of calcium channels is extremely high. In brain and heart, labeling of the receptor sites 1 and 2 has also been successful. Three novel ligands for the calcium channel are presented: 125I-iodipine, a 1,4 dihydropyridine ligand, which has an exceptionally high specific activity (2,200 Ci/mmol), 3H-azidopine, a high-affinity, photoaffinity ligand and finally (-) 3H-desmethoxyverapamil, a high-affinity radioligand for receptor site 2. These ligands will aid in the isolation and final characterisation of the calcium channel. 125I-iodipine, 3H-azidopine and (-) 3H-desmethoxyverapamil bind to detergent-solubilized channels from skeletal muscle t-tubules. Whereas the 1,4-dihydropyridines label only a 12.9 S macromolecule, (-) 3H-desmethoxyverapamil additionally labels a approximately 5.0 S peak, where almost no high-affinity 1,4 dihydropyridine binding is found. Taken together with the results from target-size analysis experiments and photoaffinity labeling, an oligomeric structure of the channel is most likely. One polypeptide chain (Mr = 145,000) in reduced form has been unambiguously identified and is currently further characterized. It remains to be seen if the all of the earlier postulated four elements of the channel (alpha, beta, delta, gamma) are on different polypeptide chains or if the 145,000 peptide corresponds to the (beta, delta) or the (delta, gamma) element.