title: Characterization of binding of the Ca++ channel antagonist, [3H]nitrendipine, to guinea-pig ileal smooth muscle.
abstract: A chemically heterogeneous group of compounds, the Ca++ channel antagonists, which includes verapamil, diltiazem and nifedipine inhibits excitation-contraction coupling in smooth and cardiac muscle by blocking Ca+ entry at a specific class of Ca++ channels. The binding of the nifedipine analog, [3H]nitrendipine, to a microsomal fraction from guinea-pig longitudinal smooth muscle has been characterized. Specific binding was saturable, linear with protein concentration and reversible. The apparent equilibrium dissociation constant was 1.63 +/- 0.06 X 10(-10)M and the maximum site density was 1.13 +/- 0.03 pmol/mg of protein determined from Scatchard analysis of equilibrium binding at 25 degrees C. Inhibition of binding was specific and stereoselective for Ca++ channel antagonist drugs and was unaffected by a variety of receptor active ligands. Correlations between binding and inhibition of mechanical response to methylfurmethide- and K+-stimulation in a series of nifedipine analogs were determined. A 1:1 correlation was found for the K+ tonic response, but for the phasic component of the K+ response and for both components of the methylfurmethide response the antagonists were more active as inhibitors of [3H]nitrendipine binding than as inhibitors of mechanical response. [3H]Nitrendipine binding was sensitive to other Ca++ channel antagonists including verapamil, D-600, diltiazem, flunarizine, lidoflazine and bepridil. Interaction with these agents suggests, consistent with previous reports, that more than one binding site for Ca++ antagonists exists. A variety of inorganic divalent and trivalent cations (Mn++, Co++, Ni++, Pb++, UO2++, Zn++, Cd++, Cu++, Tm+++ and La+++) inhibit specific [3H]nitrendipine binding. The data suggest that [3H]nitrendipine binding in smooth muscle is to a site which mediates the pharmacologic response.
mesh_terms: D000818:Animals; D001665:Binding Sites; D001667:Binding, Competitive / Q000187:drug effects; D002121:Calcium Channel Blockers / Q000494:pharmacology*; D002936:Cinnarizine / Q000031:analogs & derivatives / Q000494:pharmacology; D003035:Cobalt / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D004357:Drug Synergism; D005444:Flunarizine; D006168:Guinea Pigs; D007082:Ileum; D007700:Kinetics; D008297:Male; D009119:Muscle Contraction / Q000187:drug effects*; D009130:Muscle, Smooth / Q000378:metabolism*; D009539:Nicotinic Acids / Q000378:metabolism / Q000494:pharmacology; D009543:Nifedipine / Q000031:analogs & derivatives / Q000378:metabolism*; D009553:Nimodipine; D009568:Nitrendipine; D011725:Pyridines / Q000378:metabolism*; D013329:Structure-Activity Relationship; D014700:Verapamil / Q000494:pharmacology
chemical_list: D002121:Calcium Channel Blockers; D009539:Nicotinic Acids; D011725:Pyridines; D002936:Cinnarizine; D003035:Cobalt; D009553:Nimodipine; D009568:Nitrendipine; D014700:Verapamil; C018021:cobaltous chloride; D009543:Nifedipine; D005444:Flunarizine