Mechanism of action of sparteine sulfate on myometrial activity in vitro

The in vitro response of uterine muscle to sparteine sulfate was studied in 104 strips of human and 65 strips of rat uterine muscle. Sparteine sulfate influences the duration and frequency but not the maximum tension development of spontaneous activity in the rat myometrium. In all studies of electrically stimulated human and rat myometrium an increase of maximum tension was observed after sparteine sulfate stimulation. In the human no increase in sensitivity to sparteine sulfate was observed with advancing gestation. However, the gradual increase in sensitivity to Pitocin as pregnancy progresses has been observed by us as well as by several other investigators. The rat uterus, unlike the human uterus, exhibits an increased sensitivity to sparteine sulfate during advancing stages of pregnancy. A greater response is demonstrable during late pregnancy and is relatively refractile during early and midgestation. The response of muscle strips obtained from placental implantation sites to sparteine sulfate were less marked than uterine muscle strips from the opposite wall of the same uterus (nonplacental sites). Furthermore, the dose response range of human uterine muscle to increasing concentrations of sparteine sulfate was narrower than with Pitocin. Larger doses of sparteine sulfate produced myometrial contracture in normal Krebs' solution. Lower concentrations had a tendency to cause myometrial contracture on partially depolarized muscle. These studies suggest the major action of sparteine sulfate is directed at the cell membrane rather than directly on the contractile system of the uterine muscle cell.