Intrauterine migration of the porcine embryo--interaction of embryo, uterine flushings and indomethacin on myometrial function in vitro.

Twenty-four pregnant Landrace and Yorkshire gilts were utilized to examine the relationship between myometrial activity and intrauterine embryo migration. On d 2 (1st day of estrus = d 0), embryos were flushed from one oviduct and transferred to the opposite oviduct. Uterine horns were ligated at the uterotubal junction on the flushed side and 40 and 50 cm posterior to the uterotubal junction on each side. On d 6, 9 or 12 (n = 8), embryo migration was determined by flushing segments of the excised uterus. Strips of myometrium from the pregnant and nonpregnant horn of each gilt were subsequently removed and assigned to one of the three in vitro experiments to examine the effects of the embryo and uterine flushings on myometrial contractility. Myometrial contractility increased concomitantly with embryo migration through the uterine ligations (day x side interaction, P less than .10). Uterine flushings from pregnant horns contained a short-acting substance that mimicked, in part, the stimulatory influence(s) of the in situ embryo on in vitro myometrial contractility. However, only flushings from the uterine segment containing the d 12 embryos could overcome the in vitro inhibitory effects of indomethacin (P less than .01) on myometrial contractility. The porcine embryo coincubated with myometrial strips could not directly stimulate contractions of the myometrium. Results of these experiments indicate that in gilts the stimulatory influence(s) of the migrating embryo on myometrial function may involve a "hormonal" factor of short half-life that does not directly affect the smooth muscle cell.