CHANGES IN CONTENT OF MESSENGER-RNA ENCODING OXYTOCIN IN THE PIG UTERUS DURING THE ESTROUS-CYCLE, PREGNANCY, AT PARTURITION AND IN LACTATIONAL ANESTRUS

The aim of this study was to show that the pig uterus synthesizes oxyt ocin. Uteri were obtained from 2-7 pigs at regular intervals during th e oestrous cycle, throughout pregnancy, at parturition and in lactatio nal anoestrus. Localization of mRNA encoding oxytocin was by in situ h ybridization and oxytocin concentrations were measured by radioimmunoa ssay. As reproductive status changed, mRNA encoding oxytocin varied si gnificantly (P < 0.05). Uterine tissue type was a significant factor i n determining synthesis of mRNA encoding oxytocin (P < 0.001). In lumi nal epithelia, concentrations of mRNA encoding oxytocin were greater a t 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 parturit ion. This trend was also exhibited in uterine circular muscle. In long itudinal muscle, concentrations of mRNA encoding oxytocin were lower d uring late pregnancy than at oestrus (P < 0.05) or during the luteal p hase (P < 0.05). Concentrations were minimal at parturition. The oxyto cin content in endometrial and myometrial tissue was positively correl ated across reproductive status (P < 0.02, r = 0.402, n = 35). These d ata are the first indication that the uterine endometrium and musculat ure 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 mR NA encoding oxytocin. Local uterine synthesis of oxytocin may therefor e be more important in control of the oestrous cycle than in pregnancy or at parturition in pigs.