Molecular pharmacology of the calcium channel: evidence for subtypes, multiple drug-receptor sites, channel subunits, and the development of a radioiodinated 1,4-dihydropyridine calcium channel label, [125I]iodipine.
Radiolabeled Ca2+ antagonists (1,4-dihydropyridines, verapamil, and D-cis-diltiazem) were used to study voltage-operated Ca2+ channels in different excitable tissues. The concept of three subtypes of Ca2+ channels, represented by brain, heart, and skeletal-muscle isoreceptors for 1,4-dihydropyridines, is developed. The three subtypes are characterized by a variety of criteria. Despite the biochemical differences between the subtypes, they have the same Mr in situ by target-size analysis (Mr approximately equal to 180,000, when evaluated by [3H]nimodipine). The concept of the metalloprotein nature of the channel and the interaction of channel drugs with the Me2+ binding sites of the ionic pore is demonstrated. Distinct but interacting drug-receptor sites of the Ca2+ channel are found by direct labeling as well as indirectly by drug competition studies. We distinguish between the 1,4-dihydropyridine site, the verapamil site, and the D-cis-diltiazem site. Each receptor site can exist in high and low-affinity state; the distribution of receptor sites in these states is regulated by temperature, ions, and drugs. The concept of intrinsic activity of drugs to stabilize the high-affinity state is exemplified for the 1,4-dihydropyridines. A change in the channel architecture is induced by binding of D-cis-diltiazem to its drug receptor site. This is proven by target-size analysis of the channel in situ. Partially purified t-tubule membranes from skeletal muscle are an extremely rich source of Ca2+ channel drug-receptor sites. The stoichiometry was determined in this preparation and found to be four verapamil:two 1,4-dihydropyridine:one D-cis-diltiazem site. A novel Ca2+ channel probe, [125I]iodipine (2,200 Ci/mmol), was synthetized, and the properties of this ligand are presented.
D000818:Animals; D001001:Anura; D001345:Autoradiography; D002121:Calcium Channel Blockers / Q000494:pharmacology*; D015220:Calcium Channels; D002417:Cattle; D004110:Diltiazem / Q000494:pharmacology; D006168:Guinea Pigs; D006801:Humans; D006863:Hydrogen-Ion Concentration; D066298:In Vitro Techniques; D007457:Iodine Radioisotopes; D007462:Iodobenzenes / Q000378:metabolism*; D007473:Ion Channels / Q000187:drug effects*; D008024:Ligands; D008566:Membranes / Q000378:metabolism; D008667:Metalloproteins / Q000378:metabolism; D008861:Microsomes / Q000378:metabolism; D051381:Rats; D011978:Receptors, Nicotinic / Q000378:metabolism*; D013045:Species Specificity; D014700:Verapamil / Q000494:pharmacology
D002121:Calcium Channel Blockers; D015220:Calcium Channels; D007457:Iodine Radioisotopes; D007462:Iodobenzenes; D007473:Ion Channels; D008024:Ligands; D008667:Metalloproteins; D011978:Receptors, Nicotinic; C041028:iodipine; D014700:Verapamil; D004110:Diltiazem