Changes in rat myometrial plasma membrane protein kinase A are confined to parturition.

We have previously shown that pregnant rat myometrial plasma membrane-associated cyclic adenosine monophosphate (cAMP)-dependent protein kinase A (PKA) decreases prior to delivery, coincident with a decline in the inhibitory effect of cAMP on contractant-stimulated parameters. We now find that rat myometrial membrane-associated PKA concentrations in early to mid-pregnancy are equivalent to those in cycling rats. Following the decline associated with parturition, membrane PKA recovers within 1 to 2 days postpartum. Treatment with the antiprogestin onapristone caused a decrease in myometrial membrane PKA catalytic and regulatory subunits compared to untreated controls by 12 hours. This coincided temporally with recently reported increases in electrical and contractile activity. In unilaterally pregnant rats, the decline in plasma membrane PKA was observed in both nonpregnant and pregnant horns but was more rapid in the pregnant horns. These data indicate that the myometrial plasma membrane PKA pattern before and during most of pregnancy is not consistent with progesterone exerting a primary influence on PKA membrane localization. Rather, the fall in membrane PKA associated with parturition may contribute to or be influenced by the increased contractile and electrical activity of labor that is a consequence of the loss of progesterone influence and is not absolutely dependent on the presence of fetuses.