Relationship between uterine secretion of prostaglandin F2 alpha induced by oxytocin and endogenous concentrations of estradiol and progesterone at three stages of the bovine estrous cycle.
The purpose of this experiment was to determine whether differences among cows in the ability of oxytocin to stimulate uterine secretion of prostaglandin F2 alpha (PGF2 alpha) were related to the endogenous ovarian steroid environment. Sexually mature heifers were treated with oxytocin (.33 IU/kg BW) at three stages of the estrous cycle: early (d 3 to 5; n = 5), middle (d 10 to 11; n = 5) or late (d 16 to 17; n = 5). To assess uterine responsiveness to oxytocin, concentrations of 13,14-dihydro-15-keto-PGF2 alpha (PGFM) were quantified in jugular venous plasma samples collected at 1/2-h intervals for 8 h postinjection. The ovarian steroid environment at the time of injection was estimated by measuring the concentrations of progesterone and estradiol in jugular venous plasma samples collected at 4-h intervals for 12 h immediately prior to injection. Concentrations of PGFM increased immediately following injection of oxytocin either early or late in the estrous cycle. The response was much less during the middle of the estrous cycle. The magnitudes of response, early and late in the estrous cycle, were similar and greater than that observed during the middle of the estrous cycle (P less than .05). There was a positive relationship (R2 greater than .8; P less than .05) between magnitude of the response to oxytocin and ratio of estradiol to progesterone both early and late in the estrous cycle. Thus, individual differences in uterine secretion of PGF2 alpha in response to oxytocin were related to stage of the cycle and to differences in the endogenous ovarian steroid environment within each stage of the estrous cycle.