CHARACTERIZATION OF BINDING OF THE CA++ CHANNEL ANTAGONIST, [H-3]-LABELED NITRENDIPINE, TO GUINEA-PIG ILEAL SMOOTH-MUSCLE

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  • 1 January 1983
    • journal article
    • review article
    • Vol. 225  (2) , 291-309
Abstract
A chemically heterogeneous group of compounds, the Ca2+ channel antagonists, which includes verapamil, diltiazem and nifedipine inhibits excitation-contraction coupling in smooth and cardiac muscle by blocking Ca2+ entry at a specific class of Ca2+ channels. The binding of the nifedipine analog, [3H]nitrendipine, to a microsomal fraction from a guinea pig longitudinal smooth muscle was characterized. Specific binding was saturable, linear with protein concentration and reversible. The apparent equilibrium dissociation constant was 1.63 .+-. 0.06 .times. 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.degree. C. Inhibition of binding was specific and stereoselective for Ca2+ 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 and for both components of the methylfurmethide reponse the antagonists were more active as inhibitors of [3H]nitrendipine binding than as inhibitors of mechanical response. [3H]Nitrendipine binding was sensitive to other Ca2+ channel antagonists including verapamil, D-600 [methoxyverapamil], diltiazem, flunarizine, lidoflazine and bepridil. Interaction with these agents suggests, consistent with previous reports, that more than one binding sites for Ca2+ antagonists exists. A variety of inorganic divalent and trivalent cations (Mn2+, Co2+, Ni2+, Pb2+, UO22+, Zn2+, Cd2+, Cu2+, Tm3+ and La3+) inhibit specific [3H]nitrendipine binding. [3H]nitrendipine binding in smooth muscle may be to a site which mediates the pharmacologic response.