title: Target size analysis and molecular properties of Ca2+ channels labelled with [3H]verapamil.
abstract: [3H]Verapamil was employed to label the drug receptor sites within the Ca2+-channel in skeletal muscle microsomes which are coupled in a negative heterotropic allosteric manner to the previously characterized 1,4-dihydropyridine receptors. At 2 degrees C the Kd of a high-affinity receptor site is 45 nM and the maximum density of binding sites is 37 pmol/mg of protein. Established subcellular fractionation procedures were used to isolate transverse tubule membranes from rabbit and guinea-pig skeletal muscle. [3H]Verapamil, d-cis-[3H]diltiazem as well as 1,4-[3H]dihydropyridine receptors copurify with t-tubule membranes. The ratio of high-affinity verapamil: 1,4-dihydropyridine d-cis-diltiazem Ca2+ channel receptor sites is 4:2:1. The verapamil drug receptors are heat-labile and have essential sulfhydryl groups since they are inactivated by p-chloromercuriphenylsulfonic acid and N-ethylmaleimide. The receptors recognize the main classes of Ca2+ antagonists and agonists in a stereoselective manner. Divalent cations (Mn2+ greater than Ca2+ greater than Mg2+) are inhibitory. Target size analysis with high-energy electrons was performed and the Mr of the verapamil drug receptor site is 110000.
mesh_terms: D000408:Alamethicin / Q000494:pharmacology; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D015220:Calcium Channels; D006168:Guinea Pigs; D066298:In Vitro Techniques; D007473:Ion Channels / Q000378:metabolism*; D007700:Kinetics; D008861:Microsomes / Q000378:metabolism; D009132:Muscles / Q000378:metabolism; D011487:Protein Conformation; D011817:Rabbits; D011978:Receptors, Nicotinic / Q000187:drug effects / Q000378:metabolism*; D013696:Temperature; D014700:Verapamil / Q000378:metabolism*
chemical_list: D015220:Calcium Channels; D007473:Ion Channels; D011978:Receptors, Nicotinic; D000408:Alamethicin; D014700:Verapamil; D002118:Calcium