Contractile activity, membrane potential, and cytoplasmic calcium in humanuterine 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 simu ltaneously with contraction in uterine strips obtained from the lower segme nt during cesarean delivery. RESULTS: Between week 28 of gestation and term there was a progressive incr ease in the frequency of spontaneous contractions and a decrease in the neg ative potential of the membrane. The response to prostaglandin F-2 alpha wa s 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 re laxation occurs between contractions.