Differential effect of verapamil on noradrenaline, carbachol, tyramine and 5-hydroxytryptamine induced contractions of the rat anococcygeus muscle.

The effect of verapamil and dantrolene sodium on the contractile responses evoked by noradrenaline, carbachol, tyramine and 5-hydroxytryptamine on the rat anococcygeus muscle was studied. Dantrolene sodium did not significantly antagonize noradrenaline, carbachol, tyramine and 5-hydroxytryptamine-induced contractions. Verapamil on the other hand, competitively antagonized 5-hydroxytryptamine-induced contractions while non-competitive antagonism was observed with tyramine and carbachol. Noradrenaline evoked contractions were unaffected. All agonists tested failed to produce a contraction in Ca++-free medium indicating that a transmembrane influx of Ca++ is involved in the contractile response to all these agonists. It is suggested that there are verapamil-sensitive and verapamil-insensitive calcium channels in the rat anococcygeus muscle.