OXYTOCIN AND BOVINE PARTURITION - A STEEP RISE IN ENDOMETRIAL OXYTOCIN RECEPTORS PRECEDES ONSET OF LABOR

Oxytocin receptors were measured in myometrium and intercaruncular endometrium of cows during pregnancy and parturition. Concentrations of estradiol- 17beta, estrone, and progesterone in peripheral blood were also measured. Receptor concentrations in the endometrium rose almost 200-fold from Day 20 to term (p < 0.0001, ANOVA), from 40 +/- 11 to 7300 +/- 1430 fmol/mg protein. Myometrial receptor concentrations increased 10-fold from 180 +/- 36 fmol/mg on Day 20 to 1850 +/- 360 fmol/mg protein at term (p < 0.0001, ANOVA). During labor, endometrial receptors (6600 +/- 1300 fmol/mg) remained at pre-labor values, whereas myometrial receptor concentrations had decreased to 1190 +/- 316 fmol/mg (not significant) and declined further postpartum. Plasma concentrations of progesterone declined from 4-5 ng/ml to about 2 ng/ml between Days 250 and 282 and dropped to < 0.2 ng/ml shortly before delivery. Plasma concentrations of estrone and estradiol-17beta were below 10-20 pg/ml until Day 230. Estrone concentrations were significantly (p < 0.05) increased by Day 250 and estradiol-17beta by Day 270, and then both rose rapidly. During labor, plasma estrone was 1135 +/- 245 pg/ml and plasma estradiol-17beta was 226 +/- 131 pg/ml. The molar ratio of estrone and estradiol-17beta to progesterone rose from less than 0.01 to 4.4 during labor, and was correlated with oxytocin receptor concentrations in endometrium (r = 0.5160, p < 0.001), but not those in myometrium (r = 0.0122). The regulation of oxytocin receptors by ovarian hormones in the two tissues may therefore differ. The results suggest that oxytocin participates in the regulation of myometrial contractility and endometrial prostaglandin F2alpha release in parturient cows and possibly in luteolysis.