Contractile activity, membrane potential, and cytoplasmic calcium in human uterine smooth muscle in the third trimester of pregnancy and during labor

OBJECTIVE: This study was undertaken to investigate in human tissue samples the mechanisms underlying spontaneous and prostaglandin F-2 alpha-induced contractions during the final trimester of pregnancy and labor. STUDY DESIGN: Membrane potential and cytoplasmic calcium were recorded simultaneously with contraction in uterine strips obtained from the lower segment during cesarean delivery. RESULTS: Between week 28 of gestation and term there was a progressive increase in the frequency of spontaneous contractions and a decrease in the negative potential of the membrane. The response to prostaglandin F-2 alpha was biphasic. The initial excitatory component remained stable toward term. A later inhibitory component, which was underpinned by increased activity of the sodium-potassium adenosine triphosphatase pump, decreased at the time of labor. CONCLUSIONS: There is a gradual increase in excitability in uterine muscle throughout the third trimester of human:pregnancy. The initial component of the prostaglandin response is a large contraction that is kept brief by a subsequent inhibitory component of the response, which ensures that full relaxation occurs between contractions.