Oxytocin receptor concentrations, inositol phosphate turnover and prostaglandin release by endometrium from ewes induced to ovulate during the early post-partum period

Resumption of oestrous and ovarian cyclicity during the post-partum period in autumn-lambing ewes is not influenced by age or dietary protein content

The objective was to determine the effect of age and dietary protein content on post-partum oestrous and ovarian cyclicity in autumn-lambing ewes. Twenty-four multiparous (age 4·5 (s.e. 0·61) years) and 24 primiparous (age 1·5 years) ewes which lambed on 28 October (s.e. 0·2) and were rearing 2·0 and. 1·7 (s.e. 0·13) lambs, respectively, were equally allocated to two dietary treatments in a 2 × 2 factorial design. Diets comprised a d libitum hay supplemented with concentrate (12·5 MJ metabolizable energy per kg dry matter (DM) and either 210 or 140 g crude protein per kg DM), offered at a daily allowance of 1·2 kg per ewe from lambing to weaning (18 December) and 0·4 kg per ewe thereafter. Throughout the study (3 November to 24 May) ewes were group-housed in straw-bedded pens under natural photoperiod at 57°N and their live weights and body condition scores were assessed at 2-week intervals. A raddled vasectomized ram was continually present with each group and ewes marked by the ram were recorded thrice weekly. Blood samples were obtained thrice weekly for measurement of peripheral progesterone concentrations and ovulation rate was determined by laparoscopy during January and March. There was no significant effect of dietary protein content on any of the variables measured. For multiparous and primiparous ewes, mean live weights and body condition scores at lambing were 79·5 (s.e. 1·41) v. 58·4 (s.e. 1·54) kg (P < 0·001) and 2·5 (s.e. 0·05) v. 2·2 (s.e. 0·05) (P < 0·001) and daily live-weight losses over the suckling period were 153 (s.e. 19·3) v. 85 (s.e. 21·1) glday (P < 0·05). Intervals from parturition to first overt oestrus were 39 (s.e. 3·2) v. 33 (s.e. 2·3) days and to the onset of ovarian cyclicity were 29 (s.e. 4·9) v. 25 (s.e. 1·5) days. Durations of ovarian cyclicity were 133 (s.e. 27·1) and 110 (s.e. 5·7) days (P < 0·01) during which 7·6 (s.e. 0·32) and 6·5 (s.e. 0·34) (P < 0·05) ovarian cycles were recorded respectively. Fifteen ewes had initial short cycles (≤10 days), six had prolonged cycles (≥21 days) and 11 had extended inter-luteal phase intervals (≥8 days). All ewes had recent ovulations at the January laparoscopy but only 19 multiparous and nine primiparous ewes had ovulations in March. Mean ovulation rates were: January 2·2 (s.e. 0·14) and 1·4 (s.e. 0·13) (P < 0·001), and March 1·6 (s.e. 0·23) and 1·2 (s.e. 0·15) for multiparous and primiparous ewes, respectively. It is concluded that, for ewes lambing at their normal time of mating, (i) the strong seasonal drive for reproduction results in the resumption of oestrous and ovarian cyclicity during the early post-partum period; (ii) initial luteal phases may be of short or extended duration; (Hi) there is no effect of age or dietary protein content, within the range studied, on the onset of ovarian cyclicity; and (iv) multiparous ewes experience a greater number of ovarian cycles and achieve higher ovulation rates than primiparous ewes.

Cobalt complex with GnRH stimulates the LH release and PKA signaling pathway in pig anterior pituitary cells in vitro

Metal complexes with GnRH were shown to interact with GnRH receptors in pituitary cells. In the present study we examined the effects of GnRH and its cobalt complex form (Co-GnRH) on LH secretion and generation of second messengers, namely inositol phosphates (IPs) and cAMP, in porcine pituitary cells in vitro. The cells were obtained from gilt pituitary at the pre-ovulatory phase of estrous cycle and cultured for 72 h before challenge with GnRH or Co-GnRH. Both substances induced a significant increase in LH release that was detectable after 60 min (P<0.05) of treatment, with the Co-GnRH complex being more efficient than GnRH at 180 min (P<0.01). GnRH and Co-GnRH were equally effective at 10(-8)M (P<0.01), however, at the lowest (10(-9)M) as well as the highest (10(-7)M) concentrations tested, Co-GnRH was more potent than its native counterpart (P<0.01). Interestingly, Co-GnRH revealed twice more efficient than GnRH at stimulating cAMP production, an effect which was detectable in cells after 1h-incubation (P<0.001). In contrast, while native GnRH induced a rapid increase (P<0.05) in IPs no such effect of Co-GnRH was observed. These data demonstrate that Co-GnRH and GnRH differentially effect on the signaling pathway in porcine gonadotropes and suggest that in these cells, the releasing action of Co-GnRH results from the mediation via the cAMP/protein kinase A second messenger system.

Effect of diet and GnRH administration on post-partum ovarian cyclicity in autumn-lambing ewes

Using autumn-lambing ewes, this study investigated (i) the effects of diet on gonadotrophin secretion and responsiveness of the hypothalamic-pituitary-ovarian axis to exogenous GnRH during the early post-partum period; and (ii) whether ovulation prior to completion of uterine involution results in an increased incidence of aberrant ovarian cycles. Thirty-two ewes rearing 1.9+/-0.12 lambs were equally allocated to two dietary treatments at lambing (22 October +/-0.2 day). Diets comprised ad libitum hay and 1.5 kg per ewe per day of one of two concentrates (11.5 MJ ME, 195 g CP per kg) containing 300 g kg(-1) cracked maize grain (M) or 300 g kg(-1) sugar beet pellets (S). Half of the ewes on each diet (G) received 25 i.v. injections of 250 ng GnRH in 2 ml 0.9% saline at 2 h intervals from days 12-14 post-partum while remaining ewes (N) were monitored for the resumption of spontaneous ovarian cyclicity. Blood samples were obtained from all ewes throughout the study (lambing to 18 December) for measurement of circulating hormone concentrations and the uteri and ovaries of all ewes were examined via laparoscopy on day 21 post-partum. There were no effects of dietary treatment on ewe daily live weight loss, lamb daily live weight gain or the immediate post-partum increase in circulating FSH concentrations. Diet did not affect insulin concentrations or LH pulse frequency on day 12 post-partum but LH pulse amplitude was lower in ewes fed concentrate M compared to concentrate S (1.4+/-0.10 versus 1.7+/-0.12 ng ml(-1), respectively, P<0.05) and this was associated with an increased interval to the resumption of spontaneous ovarian cycles (35+/-3.1 versus 26+/-2.1 day, respectively, P<0.05). Administration of exogenous GnRH increased (P<0.05) the proportion of ewes on both diets that ovulated within 20 days of parturition and advanced the onset of ovarian cyclicity in ewes fed concentrate M by 9.5 days (significance of interaction, P<0.05). Four ewes, all of which ovulated before day 22 post-partum, had extended luteal activity while in remaining ewes, duration of the first luteal phase was inversely related to the time of first ovulation (r(2)=0.16, P<0.05). Results demonstrate that (i) the onset of ovarian cyclicity is influenced by diet and can be advanced by administration of exogenous GnRH; and (ii) ovulation during the early post-partum period is associated with an increased incidence of extended luteal activity.

Second messenger systems in the action of LH and oxytocin on porcine endometrial cells in vitro

LH/hCG as well as oxytocin receptors are present in the porcine endometrium. Oxytocin increases phosphatidylinositol hydrolysis in this tissue, but its action on adenylate cyclase activity is disputed. The second messenger system responding to LH/hCG in endometrial cells has not been established. In this study, we investigated the involvement of protein kinase A and C signaling mechanisms in the action of LH on porcine endometrial cells in vitro. The possibility of cAMP accumulation after treatment of endometrial cells with oxytocin was also investigated. Endometrial tissue was obtained from gilts during Days 12-15 of the estrous cycle. To study the adenylate cyclase system, endometrial cells were cultured for 48 h and then incubated with different doses of LH or oxytocin for 15, 30, 60, and 180 min. To study the phospholipase C system, dispersed cells were first labeled with myo-[3H]inositol and then treated with increasing doses of LH or 100 nM of oxytocin for 30 min. Time- and dose-dependent effect of LH and oxytocin on cAMP concentration was observed. After 30 min of incubation only the highest dose of LH (100 ng/ml) was able to increase cAMP concentration in medium (P < 0.05). Longer periods (1 and 3 h) caused increased cAMP accumulation after treatment with 10 and 100 ng/ml of LH (P < 0.001). Oxytocin-stimulated cAMP concentration was observed after 1 h when only the highest dose (1000 nM) of hormone was used (P < 0.01) and after 3 h of incubation with doses of 10-1000 nM (P < 0.01). LH (10 and 100 ng/ml) increased inositol phosphates (IPs) accumulation in endometrial cells after 30 min of incubation (P < 0.01). Oxytocin involvement in IPs synthesis was more apparent than was LH (P < 0.001 versus P < 0.01). This is the first demonstration that LH receptor signaling leads to increased cAMP generation as well as IPs turnover in porcine endometrium. Oxytocin-dependent cAMP production in endometrial cells of swine was found after longer periods (3 h) of incubation. Our observations lead to the conclusion that both protein kinase A and C second messenger systems are involved in LH action and that oxytocin is able to stimulate adenylate cyclase activity in porcine endometrial cells.

The control of prostaglandin production by the endometrium in relation to luteolysis and menstruation

Oestradiol acting on a progesterone-primed uterus stimulates prostaglandin (PG) F2 alpha synthesis by the endometrium. In some species (notably the sheep, cow and goat) oxytocin released from the ovary also forms part of the physiological stimulus for increased endometrial PGF2 alpha production. The corpus luteum contains high concentrations (> 1 microgram/g tissue) of this peptide in these species. The intracellular mechanisms by which these three hormones control endometrial PGF2 alpha synthesis and release are far from clear. Oxytocin stimulates the synthesis of inositol phosphates and diacylglycerol in the endometrium of some species, but whether this pathway is involved in endometrial PGF2 alpha synthesis is still open to question. There is evidence that increased endometrial PGF2 alpha synthesis is dependent upon increased endometrial protein synthesis but, apart from the recorded effects of steroid hormones on the concentrations of phospholipase A2, prostaglandin H synthase and oxytocin receptors, it is not known what other endometrial proteins are involved. Some disorders of menstruation are associated with abnormal PG production by the endometrium, but the reasons for this abnormality are not clear. During early pregnancy an increase in PGF2 alpha synthesis by the endometrium is prevented, except in the pig where the PGF2 alpha produced is directed from the venous drainage to the uterine lumen. In those species in which endometrial PGF2 alpha synthesis is dependent upon oxytocin secreted by the ovary, the conceptus secretes an interferon-tau (previously named trophoblast protein-1) which prevents oestradiol and oxytocin acting on a progesterone-primed uterus from stimulating endometrial PGF2 alpha synthesis. The identities of the factors produced by the conceptus which prevent endometrial PGF2 alpha synthesis during early pregnancy in other species are not known, although it is clear that they are not interferons.