Changes in content of mRNA encoding oxytocin in the pig uterus during the oestrous cycle, pregnancy, at parturition and in lactational anoestrus.

The aim of this study was to show that the pig uterus synthesizes oxytocin. Uteri were obtained from 2-7 pigs at regular intervals during the oestrous cycle, throughout pregnancy, at parturition and in lactational anoestrus. Localization of mRNA encoding oxytocin was by in situ hybridization and oxytocin concentrations were measured by radioimmunoassay. As reproductive status changed, mRNA encoding oxytocin varied significantly (P < 0.05). Uterine tissue type was a significant factor in determining synthesis of mRNA encoding oxytocin (P < 0.001). In luminal epithelia, concentrations of mRNA encoding oxytocin were greater at oestrus than during day 14 of the luteal phase (P < 0.01) or at any stage of pregnancy (P < 0.05), with concentrations minimal at parturition. This trend was also exhibited in uterine circular muscle. In longitudinal muscle, concentrations of mRNA encoding oxytocin were lower during late pregnancy than at oestrus (P < 0.05) or during the luteal phase (P < 0.05). Concentrations were minimal at parturition. The oxytocin content in endometrial and myometrial tissue was positively correlated across reproductive status (P < 0.02, r = 0.402, n = 35). These data are the first indication that the uterine endometrium and musculature of the pig express mRNA encoding oxytocin. The luminal epithelium of animals at oestrus was particularly rich in mRNA encoding oxytocin, whilst late pregnant and parturient animals did not show a rise in mRNA encoding oxytocin. Local uterine synthesis of oxytocin may therefore be more important in control of the oestrous cycle than in pregnancy or at parturition in pigs.