In vitro effects of calcium entry blockers, chlorpromazine and fenoterol upon human pregnant myometrium contractility.

The inhibitory effects of nifedipine, verapamil, cinnarizine (calcium entry blockers), chlorpromazine (a putative calmodulin antagonist) and fenoterol (a beta 2-adrenoceptor agonist) on contractility in human isolated pregnant myometrium were studied. Spontaneous contractions (present in 93% of the preparations) were inhibited in a concentration-related manner by these compounds in the following order of potency: nifedipine greater than verapamil much greater than cinnarizine greater than chlorpromazine. Cinnarizine was effective only at a concentration greater than 100 microM. Fenoterol, at 10 microM, did not produce an inhibitory effect but decreased the frequency of spontaneous contractions. All drugs, except fenoterol, produced a concentration-dependent relaxation of K+-induced contractions in the following order of sensitivity: nifedipine greater than verapamil much greater than chlorpromazine. Cinnarizine produced only about 40% of relaxation. Under these conditions nifedipine and verapamil were about 80 and 5 fold more potent respectively than when tested against spontaneous contractions. The potencies of chlorpromazine and cinnarizine did not differ in the two experimental conditions. Both the spontaneous and K+-induced contractions were inhibited in a time-dependent manner in Ca2+-free media and the responses were almost completely abolished in 70-100 min. Calcium addition to the medium rapidly restored both spontaneous or K+-induced contractions. To investigate further the role of intracellular calcium, K+-depolarized preparations contracted by calcium 3 mM (40-60% of maximal contractions) were relaxed by these compounds. Nifedipine and verapamil showed a relaxation time course similar to that induced by calcium removal. Cinnarizine and fenoterol had no relaxant effect while chlorpromazine induced a slight and slow relaxation. 6 These findings suggest that calcium influx and calmodulin are involved in spontaneous contractions of pregnant human myometrium in vitro. Since nifedipine and verapamil were more potent against K+-induced than spontaneous contractions, calcium channels activated by these conditions could be different. Finally, fenoterol, a beta 2-adrenoceptor agonist, widely used as a tocolytic agent, blocked neither spontaneous nor K+-induced contractions.