Increased myometrial responsiveness to oxytocin during term and preterm labor.

The contractile effects of oxytocin on the longitudinal and circular muscle of rat uteri from animals undergoing term and preterm labor were studied in vitro to define the action of the hormone. The maximal tension and slopes of oxytocin dose contractile-response curves, but not the pD2 values (-log ED50), were higher in both muscle layers at term delivery and also during preterm delivery induced with antiprogesterones compared with responses from tissues of nondelivering animals. These data indicate that the myometrium is hyperresponsive and hyperreactive to oxytocin during term and preterm birth but not hypersensitive. The increase in oxytocin responsiveness of the myometrium may be fundamental to the initiation of labor and regulated by the increases in oxytocin receptors and postreceptor events, including gap junctions.