Inhibition of in vitro contractions of human myometrium by mibefradil, a T-type calcium channel blocker: support for a model using excitation-contraction coupling, and autocrine and paracrine signaling mechanisms.

Objective: To investigate the importance of T-type calcium channels in human myometrial contractility and to refine a model of contractility.
Methods: We performed in vitro muscle bath experiments on human myometrial tissue strips while simultaneously monitoring bioelectrical activity with loose-contact electrodes. Tissue was obtained by myometrial biopsy from term pregnant women at the time of cesarean delivery and studied after 1 to 7 days in organ culture. Tissue strips were exposed to 5 nM oxytocin to obtain regular contractions. Tissue was then exposed to the T-type calcium channel blocker mibefradil (1 microM), the L-type calcium channel blocker nifedipine (5 nM), or the cyclooxygenase (COX) inhibitor indomethacin (30 microM). All study drugs reduced the strength of contractions. Data were analyzed using a model composed of two phases of force production--an electrical phase (E), which encompassed the first 3 seconds of each contraction, and a paracrine phase (P), which continued until the peak of the contraction. For each phase, the relative force reduction was calculated by the ratio of forces measured after and before drug exposure (RE and RP, respectively). For each drug, experiments were performed in at least triplicate.
Results: Myometrial contractions and bioelectrical signals were routinely observed following oxytocin exposure. Spike-like bioelectrical signals occurred only at the beginning of each contraction. Nifedipine and indomethacin slightly modulated the bioelectrical signals, but mibefradil appeared to block the spike component. Mibefradil caused a similar reduction of E and P (RE = 58 +/- 10%, RP = 62 +/- 22%). Nifedipine also reduced E and P, although P was reduced more than E (RE = 72 +/- 12%, RP = 38 +/- 8%, P = .006). Indomethacin yielded variable results, with some experiments showing similar RE and RP values, while other experiments showed RP > RE.
Conclusion: Mibefradil inhibition of the bioelectrical signal and uterine contractile forces suggest that T-type calcium channels are important in the initiation of each contraction. Some results using indomethacin suggest that for a specific tissue or under some conditions, autocrine stimulation by prostaglandins may be important for recruitment of myocytes in the electrical phase. Stimulation by prostaglandins is likely important in the paracrine phase. Enhanced inhibition by nifedipine in the paracrine phase suggests the mechanism of action of nifedipine tocolytic effect is block of prostaglandin F2alpha (PGF2alpha) paracrine stimulation. Our initial model proposed at least two mechanisms (electrical and nonelectrical) for the recruitment of myocytes. Our modified model suggests that the electrical phase contains a prostaglandin autocrine mechanism in addition to excitation-contraction coupling, and the mechanism in the nonelectrical phase is prostaglandin paracrine signaling.