Identification, characterization, and photoaffinity labeling of the dihydropyridine receptor associated with the L-type calcium channel from bovine adrenal medulla

Abstract

The dihydropyridine receptor associated with the L-type Ca2+ channel in adrenal medulla membranes has been identified and characterized. [3H]PN200-110 binds in a stereoselective, saturable manner to a single class of high affinity sites in adrenal medulla membranes, with a Kd of 0.1 nM and a Bmax of 141 fmol/mg of protein. Dihydropyridines inhibited [3H]PN200-110 binding with the rank order (+)-PN200-110 greater than nifedipine greater than nimodipine greater than usoldipine greater than or equal to nitrendipine greater than BayK8644 greater than (-)-PN200-110. [3H] PN200-110 binding was sensitive to divalent cations, as examined by the effects of Ca2+, Mg2+, and the chelators ethylene glycol bis-(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid and EDTA. [3H]PN200-110 binding was modulated by various classes of L-type Ca2+ channel effectors. Benzothiazepines modulated binding of [3H]PN200-110 in a negative or positive manner that was temperature dependent, whereas phenylalkylamines weakly inhibited [3H]PN200-110 binding. Bepridil stimulated [3H] PN200-110 binding, whereas phencyclidine was without effect. The photoaffinity probe [3H]azidopine labeled a single polypeptide that migrated with an apparent molecular weight of 185,000-190,000 in sodium dodecyl sulfate gel electrophoresis. The dihydropyridine receptor was found to bind specifically to wheat germ agglutinin columns. These results demonstrate the presence of a Ca2+ channel blocker complex in adrenal medulla. The drug receptor sites reside on a glycoprotein complex in which a polypeptide analogous to the alpha 1-subunit of the L-type Ca2+ channel from skeletal muscle has been identified.