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 F2α–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 F2α 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. (Am J Obstet Gynecol 1999;181:1145-51.)