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(2alpha) 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.

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