Blockade by cycloheximide of the prepartum changes in membrane activity and adrenergic response of the circular muscle of rat uterus.

A study was made of the effects of cycloheximide, an inhibitor of protein synthesis, on the transformations of electrical activity and adrenergic response of the circular muscle of rat uterus during pregnancy and postpartum. Electrical activity proceeded from plateau type on Day 18 of gestation to spike type on Day 22. Excitatory reaction of the contraction to noradrenaline (10(-7), 3 X 10(-7) M) on Day 18 altered to an inhibitory one on Day 22. When pregnant rats were injected intraperitoneally with 50 micrograms cycloheximide daily from Day 18, the changes mentioned above were suppressed and parturition delayed. In the preparations obtained on Day 21, the excitatory effect of 10(-7) M noradrenaline on the contraction changed to an inhibitory one by in vitro incubation for 7 h with Krebs solution. This transformation was prevented by the incubation with 2 micrograms/ml cycloheximide for 7 h. Membrane activity and adrenergic response turned to plateau-dominant action potential with long duration and potentiation of the contraction by noradrenaline (10(-7), 3 X 10(-7) M) 48 h after parturition, respectively. Cycloheximide (50 micrograms/day, i.p., immediately after delivery and on the next day) failed to inhibit these postpartum changes. The results suggest that the process of protein synthesis is involved in the alterations in electrical activity and adrenergic response of the circular muscle of rat uterus during late pregnancy stage.