Effects of magnesium, oxytocin, and prostaglandin F2 alpha on the generation and propagation of excitation in the longitudinal muscle of rat myometrium during late pregnancy.

In order to find out the changes in myometrial properties towards parturition, effects of Mg, oxytocin, and prostaglandin F2 alpha (PGF2 alpha) were investigated on longitudinal muscle strips taken from rat uteri on Days 20 and 22 of pregnancy. With intracellular recording by microelectrode, membrane potential was 50.5 +/- 1.4 mV on Day 20, and 49.9 +/- 1.0 mV on Day 22 in the Mg-free Krebs solution. The slow potential was larger, and the spike potentials during a burst discharge exhibited variable amplitude on Day 20, whereas these were regular on Day 22. With extracellular recording by the rubber gap method, spike potentials discharged in a synchronized manner on Day 22, suggesting a better coordination between cell groups. When 1.2 mM Mg was applied, the spontaneous activity was depressed more strongly on Day 22. The excitatory effect of PGF2 alpha faded sooner, whereas that of oxytocin persisted. When the fading of PGF2 alpha had occurred, the excitatory effect of oxytocin was weaker than when it was given alone. Because propagation of spontaneous activity, occurring either in the Mg-free solution or in the presence of oxytocin and PGF2 alpha in normal Krebs solution, did not show a definite direction from ovarian to vaginal side of the longitudinal muscle, it was concluded that generation of spontaneous activity was not localized in a given site of the muscle, but originated anywhere in Days 20 and 22 preparations.