Changes in the adrenergic effects and membrane activity of the circular muscle of rat uterus during late pregnancy and postpartum.

Circular muscle strips of rat uterus were obtained in the morning on Days 21 and 22 of pregnancy, and on the first postpartum day. Changes in the effects of noradrenaline and isoprenaline due to pregnant stage and incubation period in vitro were investigated, taking the contractile response and membrane activity as indicators. Plateau potential was dominant in Day 21 and postpartum preparations, and spike potential on Day 22. Noradrenaline (3 X 10(-7), 10(-6) M) caused excitation in Day 21 and postpartum preparations, whereas it caused inhibition in Day 22 preparations during early exposure to Krebs solution. When contractile potentiation was caused, plateau potential was prolonged, and when contractile depression was caused the plateau potential was suppressed. By incubation in vitro with Krebs solution, a transformation of the effect of noradrenaline from excitation to inhibition was caused in Day 21 preparations, and from strong to weak inhibition in Day 22 preparations. Isoprenaline of 3 X 10(-10) M concentration markedly depressed the contractile response in Day 22 preparations, whereas 10(-8) M isoprenaline was not enough to suppress the spontaneous contractions in postpartum preparations.