Changes in Uterine Oxytocin Receptor Concentrations Throughout the Estrous Cycle of the Cow1

The amount of oxytocin (OT) bound by plasma membrane fractions from the bovine myometrium and endometrium on different days of the estrous cycle was determined. In the myometrium, OT was bound to a single class of independent sites with an apparent KD (+/- SE) of 1.2 +/- 0.16 nM (n = 34). There were no differences in the apparent KD for different days of the cycle. The amount of OT bound per mg of membrane protein, however, was greatest near the time of estrus and the least in the luteal phase. Nearly ten times more OT was bound to myometrial membranes on Day 21 than on Day 7 (p less than 0.05). OT was bound to endometrial plasma membranes with KD values ranging from 1.3 to 25.7 nM. The mean apparent KD (+/- SE) with 11 samples was 14.9 +/- 3.2 nM. No consistent differences in KD values were detected among days of the cycle. Because of the large variation in the apparent KD values, we were unable to determine if there were differences in the amount of OT bound to endometrial membranes at different stages of the cycle. The correspondence between myometrial OT receptor concentrations and sensitivity to OT during the estrous cycle suggests that the myometrial response to OT in vivo is regulated at the receptor level as well as or rather than by circulating OT concentrations.