Effect of oxytocin on concentration of PGF2 alpha in the uterine lumen and subsequent endometrial responsiveness to oxytocin in pigs.

The aim of this study was to determine the effect of oxytocin on PGF2 alpha secretion into the uterine lumen of pigs and subsequent endometrial responsiveness to oxytocin in vitro. Cyclic, pregnant and oestradiol-induced pseudopregnant gilts were injected i.v. with vehicle or 20 iu oxytocin 10 min before hysterectomy on day 16 after oestrus. Concentrations of PGF2 alpha and 13,14-dihydro-15-keto PGF2 alpha (PGFM) were significantly increased in uterine flushings collected at hysterectomy (P < 0.05) in pregnant oxytocin-injected gilts. Concentrations of PGF2 alpha and PGFM were greater (P < 0.001) in pregnant than in pseudopregnant and cyclic gilts, and greater (P < 0.01) in pseudopregnant than in cyclic gilts. The ratio of PGFM:PGF2 alpha tended to be greater in cyclic (P < 0.06) and pseudopregnant gilts (P < 0.1) than in pregnant gilts. At 85 +/- 5 min after oxytocin injection, endometrium from each gilt was incubated for 3 h for determination of phosphoinositide hydrolysis and PGF2 alpha secretion in response to treatment with 0 or 100 nmol oxytocin l-1. Endometrial phosphoinositide hydrolysis in response to 100 nmol oxytocin l-1 in vitro was greater (P < 0.05) in cyclic oxytocin-injected gilts than in cyclic vehicle-injected gilts. Treatment with oxytocin in vitro did not stimulate phosphoinositide hydrolysis significantly in vehicle- or oxytocin-injected pregnant gilts or pseudopregnant gilts. Endometrial PGF2 alpha secretion increased after treatment with 100 nmol oxytocin l-1 in vitro in cyclic vehicle-injected (P < 0.01), cyclic oxytocin-injected (P < 0.01), pregnant vehicle-injected (P = 0.06), pseudopregnant vehicle-injected (P < 0.05) and pseudopregnant oxytocin-injected (P < 0.05) gilts, but not in pregnant oxytocin-injected gilts. The increase in PGF2 alpha in pseudopregnant oxytocin-injected gilts was less (P < 0.05) than that in cyclic oxytocin-injected gilts. These results indicate that oxytocin increases the concentration of PGF2 alpha and PGFM in the uterine lumen during pregnancy and may upregulate endometrial responsiveness to oxytocin during late dioestrus in pigs, but does not have the latter effect during early pregnancy or oestradiol-induced pseudopregnancy.

Exogenous oxytocin stimulates uterine secretion of prostaglandin F2 alpha in cyclic and early pregnant swine.

Oxytocin (OT) stimulates endometrial secretion of prostaglandin (PG) F2 alpha around the time of corpus luteum regression in ruminants, but the stimulus for luteolytic PGF2 alpha release in cyclic pigs is not clear. We previously reported that OT stimulates endometrial phosphoinositide hydrolysis and PGF2 alpha release in vitro, and that exogenous OT administered on Days 10-16 caused a uterine-dependent reduction in interestrous interval. In this study, the development of endometrial responsiveness to OT in cyclic, early pregnant, and ovarian-intact hysterectomized gilts was investigated. On Day 7 (onset of estrus = Day 0), 26 gilts were fitted with indwelling jugular catheters, and 5 of these gilts were hysterectomized. Cyclic (n = 5), pregnant (n = 6), and ovarian-intact hysterectomized (n = 5) gilts received i.v. injections of 20 USP units OT (equivalent to 20 IU or 40 micrograms/ml) on Days 10, 12, 14, and 16; and cyclic controls (n = 5) and pregnant controls (n = 5) received i.v. injections of vehicle. Concentration of 13,14-dihydro-15-keto-PGF2 alpha (PGFM; the major stable metabolite of PGF2 alpha) was measured in jugular venous plasma collected at 10-min intervals, from 20 min before to 120 min after i.v. injections of OT or vehicle. Plasma progesterone was measured in blood samples collected daily from Day 9 through return to estrus (cyclic gilts) or through Day 30 (pregnant and hysterectomized gilts). Vehicle-treated and OT-treated cyclic gilts were not responsive to OT on Days 10 and 12, and had similar plasma PGFM profiles on these days. However, OT-treated cyclic gilts were responsive (p < 0.01) to OT on Days 14 and 16, and peak concentrations of PGFM were detected in jugular plasma 10 min after OT injection. Concentrations of PGFM did not increase after vehicle injection on any day in controls. Similarly, PGFM in ovarian-intact hysterectomized gilts did not increase on any day after OT injection, indicating that the uterus was probably the source of OT-induced PGFM in cyclic gilts. Pregnant vehicle-treated gilts also did not have increased PGFM on any day after injection of vehicle. Pregnant OT-treated gilts had increased (p < 0.01) PGFM concentrations after OT injection on all days that were higher than concentrations in cyclic gilts on Days 10 and 12, but lower than those in cyclic gilts on Days 14 and 16 (p < 0.01). The concentration of plasma progesterone in cyclic gilts did not decrease until Days 15-16 (p < 0.01). Plasma progesterone was maintained in pregnant and hysterectomized gilts and was not influenced by OT treatment. These results indicated that 1) endometrial responsiveness to OT in cyclic gilts developed between Days 12 and 14 postestrus, 2) endometrial responsiveness to OT developed before luteolysis was initiated, and 3) endometrial responsiveness to OT in pregnant pigs developed before Day 10 of pregnancy and was attenuated on Days 14 through 16. These results are consistent with the hypothesis that OT may promote pulsatile luteolytic secretion of PGF2 alpha during corpus luteum regression in swine and that responsiveness to OT was suppressed to maintain corpus luteum function during early pregnancy.

Exogenous oxytocin decreases interestrous interval of cyclic gilts.

Oxytocin (OT) stimulates pulsatile secretion of uterine PGF2 alpha in ruminants, but the role of OT in regulation of the estrous cycle of pigs is not clear. four experiments were performed to examine the effect of exogenous OT on interestrous interval of intact cyclic and hysterectomized gilts. In Exp. 1, i.v. injections of 20 USP units (equivalent to 20 IU) of OT, once/day via an ear vein on d 10, 12, 14, and 16 after estrus, decreased (P < .01) interestrous interval (19.9 +/- .2 d) compared with vehicle-injected control gilts (20.8 +/- .2 d), without affecting ovulation rate (12.1 vs. 12.0 +/- .7 corpora lutea; OT vs control gilts) at subsequent estrus. In Exp. 2, i.v. infusions of 20 USP units of OT, twice/day via an indwelling jugular catheter on d 10 to 16 after estrus, did not alter interestrous interval (20.6 +/- .3 d) compared with control gilts (20.4 +/- .3 d). Concentrations of progesterone in jugular vein plasma did not differ between treatment groups on d 9 to 21 after estrus. In Exp. 3, i.m. injections of 20 USP units of OT, twice/day on d 10 to 16 after estrus, decreased (P < .05) interestrous interval (20.6 +/- .4 d) compared with control gilts (22.3 +/- .4 d). In Exp. 4, i.m. injections of 20 USP units of OT, twice/day on d 10 to 16 after estrus, decreased (P < .05) interestrous interval (20.7 +/- .3 d) compared with control injections in uterine-intact gilts (21.8 +/- .3 d). None of the gilts hysterectomized on d 7 and treated on d 10 to 16 after estrus with either OT or control injections returned to estrus by d 28, and all had increased plasma progesterone on d 21 to 27. Mean weight of individual corpora lutea (502 vs 449 +/- 28 mg; OT vs control gilts) and total weight of corpora lutea (5,758 vs. 5,126 +/- 298 mg; OT vs control gilts) of hysterectomized gilts did not differ between treatment groups at ovariectomy on d 28. These results indicate that 1) exogenous OT administered on d 10 to 16 shortened the interestrous interval of intact cyclic gilts and 2) the effect of OT was uterine-dependent.

A proposed role for oxytocin in regulation of endometrial prostaglandin F2 alpha secretion during luteolysis in swine.

Pulsatile secretion of endometrial prostaglandin (PG)F2 alpha is stimulated by oxytocin (OT) during late diestrus in domestic ruminants (i.e., cattle, sheep and goats) and results in corpus luteum (CL) regression leading to the onset of a new estrous cycle. Pulsatile PGF2 alpha release is also responsible for CL regression in swine, but the stimulus for its secretion from the uterine endometrium is not known. We propose that OT binds to specific OT receptors (OTR) on the endometrium to stimulate phosphoinositide (PI) hydrolysis, thereby activating the inositol trisphosphate (IP3)-diacylglycerol (DAG) second-messenger system to promote pulsatile PGF2 alpha secretion. Exogenous OT administered to cyclic gilts during late diestrus (days 10-16) decreased interestrous interval in three of four experiments. However, OT did not promote CL regression in hysterectomized gilts indicating that the effect of OT was uterine-dependent. Circulating concentrations of 13,14-dihydro-15-keto PGF2 alpha (the major stable metabolite of PGF2 alpha) were increased (p < 0.01) 10 min after i.v. injection of OT on days 14 and 16 in cyclic gilts and on days 10-16 in pregnant gilts, but the magnitude of the response to OT on all days in pregnant gilts was markedly reduced compared to the response in cyclic gilts on days 14 and 16. Mean density and Kd of OTR detected on endometrium of cyclic pigs 15 days post-estrus were 29.2 +/- 5.5 fmol/mg protein and 1.59 +/- 0.23 nM, respectively. Density of OTR was correlated with OT-stimulated PI hydrolysis (r = 0.83, p < 0.05) and PGF2 alpha secretion (r = 0.87, p < 0.10). Endometrial IP3 was increased within 30 seconds after OT treatment and preceded the increase in PGF2 alpha release stimulated by OT. Endometrial PI hydrolysis and PGF2 alpha secretion were similarly increased by AIF4-(phospholipase C activator), but not by cholera toxin (adenylyl cyclase activator). Although OT binding to OTR could be displaced by lysine-vasopressin and lysine-vasopressin stimulated PI hydrolysis, lysine-vasopressin did not stimulate PGF2 alpha release. Distinct receptors for OT and lysine-vasopressin on pig endometrium were confirmed by treatment with 100 nM OT + 100 nM lysine-vasopressin which stimulated PI hydrolysis more than 100-200 nM OT or lysine-vasopressin alone. These results support the hypothesis that OT stimulates phospholipase C to hydrolyze PI, yielding IP3 and DAG second-messengers which promote endometrial PGF2 alpha release during CL regression in pigs.