Relationships among Endometrial Oxytocin Receptors, Oxytocin-Stimulated Phosphoinositide Hydrolysis and Prostaglandin F2α Secretion in Vitro, and Plasma Concentrations of Ovarian Steroids before and during Corpus Luteum Regression in Cyclic Heifers1

Oxytocin stimulates secretion of endometrial prostaglandin (PG) F2α and induces endometrial phosphoinositide hydrolysis around the time of regression of the corpus luteum (CL) in cows. This study investigated the relationship between endometrial oxytocin receptors, oxytocin-stimulated phosphoinositide hydrolysis and PGF2α secretion in vitro, and plasma concentrations of ovarian steroids before and during CL regression in cyclic heifers (i.e., Days 13–19 post-estrus). Reproductive tracts were collected from 16 cyclic heifers on Days 13 (n = 5), 16 (n = 5), and 19 (n = 6). Decreases in mean CL weight (4.7, 4.6, and 3.0 ± 0.6 g for Days 13, 16, and 19, respectively; p < 0.05) and plasma progesterone (12.2, 10.5, and 4.3 ± 1.5 ng/ml for Days 13, 16, and 19, respectively; p < 0.01) were detected between Days 16 and 19, indicating that CL regression had been initiated in the group of Day 19 heifers. Mean concentration of plasma estradiol (3.4, 3.8, and 5.6 ± 1.7 pg/ml for Days 13, 16, and 19, respectively) did not differ significantly (p > 0.5) among days of the estrous cycle. Concentration of endometrial oxytocin receptors increased (p < 0.06) during the estrous cycle (36, 49, and 789 ± 234 fmol/mg protein on Days 13, 16, and 19, respectively), but the greatest increase (p < 0.05) occurred between Days 16 and 19. Similarly, incorporation of [3H]inositol into endometrial inositol phosphates induced in vitro with 100 nM oxytocin also increased (p < 0.01) during the estrous cycle (5121 vs. 5819, 4529 vs. 7239, and 5888 vs 68,096 ± 18,564 dpm/g tissue for control vs. oxytocin-treated endometrium on Days 13, 16, and 19, respectively) with the greatest increase (p < 0.01) in responsiveness to oxytocin occurring between Days 16 and 19. Endometrial PGF2α secretion in response to 100 nM oxytocin increased linearly (p < 0.05) during the estrous cycle (905 vs. 1151, 783 vs. 1947, and 639 vs. 2485 ± 510 pg/g tissue/min for control vs. oxytocin-treated endometrium on Days 13, 16, and 19, respectively). On Day 19, CL regression was not yet apparent in 3 of 6 heifers, whereas luteolysis was clearly occurring in the remaining 3 heifers (2.1 vs. 4.0 ± 0.4 g, p < 0.05, for CL weight and 1.1 vs. 7.6 ± 0.6 ng/ml, p < 0.01, for plasma progesterone in heifers with regressing and nonregressing CL, respectively). Mean concentrations of plasma estradiol did not differ significantly (p > 0.25) between heifers with a regressing CL (8.3 ± 3.1 pg/ml) and heifers with a functional CL (2.8 ± 3.1 pg/ml), although 2 of 3 heifers with a regressing CL and 0 of 3 heifers with a functional CL had elevated concentrations of estradiol. Concentration of endometrial oxytocin receptors was greater (p < 0.05) in heifers undergoing luteolysis than in heifers with CL maintained on Day 19 (1490 vs. 88 ± 265 fmol/mg protein, respectively). Incorporation of [3H]inositol into endometrial inositol phosphates in response to oxytocin also increased (p < 0.06) in those heifers undergoing luteolysis compared with those still having a functional CL (6170 vs. 120 353 and 5606 vs. 15 837 ± 18 564 dpm/g tissue for control vs. oxytocin-treated endometrium from regressing and maintained heifers, respectively). In contrast, endometrial PGF2α secretion in response to oxytocin did not differ significantly between the two groups. These results indicate that 1) endometrial concentration of oxytocin receptors and oxytocin-induced phosphoinositide hydrolysis increased similarly during luteolysis with the greatest increases occurring around the time of onset of luteolysis; 2) endometrial PGF2α secretary response to oxytocin coincides with the early gradual increases in endometrial oxytocin receptor concentration and oxytocin-induced phosphoinositide hydrolysis between Days 13 and 16, but precedes the large increase in activity of the oxytocin receptor-phospholipase C system occurring 16–19 days post-estrus; and 3) oxytocin receptors and oxytocin- induced phosphoinositide hydrolysis are markedly increased during CL regression in cows. We suggest that a modest increase in the oxytocin receptor-phospholipase C system between Days 13 and 16 may be responsible for initiating endometrial sensitivity to oxytocin required for onset of luteolytic PGF2α secretion, whereas the large increase in the oxytocin receptorphospholipase C system occurs late in the estrous cycle as a consequence of luteolysis.