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

Abstract

Oxytocin stimulates secretion of endometrial prostaglandin (PG) F2 alpha 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 PGF 2 alpha 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.(ABSTRACT TRUNCATED AT 250 WORDS)

Secretion of Prostaglandins E₂ and F_2&alpha; by the Bovine Endometrium in Response to Oxytocin during the Estrous Cycle

“…This OT concentration was comparable to the affinity of the bovine endometrial OT receptor reported previously [7,11]. Consequently, our present results indicate that PGF 2α secretion induced by OT in the endometrium occurred through ligand binding to the OT receptor.…”

Effect of Administration of Oxytocin During Diestrus on Corpus Luteum Function and Endometrial Oxytocin Receptor Concentration in Cycling Mares

“…The oxytocin-binding assay was performed as described previously [20,21], with slight procedural modifications. The [ 3 H]-oxytocin (OT) was diluted in oxytocin diluting buffer #3 (25 mM Tris-HCl, 0.01% NaN 3 , 0.2% bovine serum albumin, 20 mM manganese chloride, pH 7.4) to a concentration of 2 pmol/50 mL (approximately 163,000 disintegrations/min/50 mL), from which serial dilutions between 0.125 and 2.0 pmol [ 3 H]-OT/50 mL were made.…”

Nongenomic Inhibition of Oxytocin Binding by Progesterone in the Ovine Uterus1

“…Endometrial tissue was evaluated for OTR binding by use of a modified procedure of Hazzard and Stormshak [14] adapted from original methods of Mirando et al [16]. Specific binding of OT to its receptor was determined with [ 3 H] OT (range of concentrations 0.625-10 nM; 44.5 Ci/ mmol, New England Nuclear, Boston, MA) and labeled ligand plus a 200-fold excess of unlabeled OT to correct for nonspecific binding.…”