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Summary. The vasoconstrictor activity of the ovarian vascular bed in vitro was investigated during the oestrous cycle and early pregnancy. Gilts were killed during the follicular phase (Days 20 to + 1; N = 5) or luteal phase (Days 11 to 13; N = 4) of the oestrous cycle, or on Day 13 of pregnancy (N = 5). Immediately before death, a sample of vena cava blood was obtained for determination of progesterone and oestrogen (oestrone and oestradiol-17β) concentrations. One ovary was removed, cannulated, perfused in vitro, and subjected to 10-min infusions of saline (vehicle control) and noradrenaline. Vasoconstriction was provoked by electrical stimulation at the end of each infusion. Ovaries from luteal-phase gilts exhibited greater (P < 0·01) vasoconstriction than did ovaries from follicular-phase and pregnant gilts at the end of saline and noradrenaline infusions. The oestrogen to progesterone ratio was less (P < 0·01) for luteal-phase and pregnant than for follicular-phase gilts. Vasoconstriction was negatively correlated (r = −0·99, P < 0·01) with the oestrogen to progesterone ratio in systemic blood of gilts during the oestrous cycle but not during early pregnancy (r = +0·39, P > 0·10), possibly due to an effect of the conceptuses.

Summary. Five gilts each received intraluteal implants releasing about 4·4 ± 1·1 (Group 1), 15·0 ± 1·1 (Group 2) or 22·4 ± 1·0 (Group 3) μgoestradiol/day as determined by in-vitro incubation of implants of similar weight and oestradiol content. On Day 11 of the oestrous cycle (Day 0 = first day of oestrus), 3 CL in one ovary received oestradiol implants, 3 CL in the other ovary received vehicle implants, and all other CL in both ovaries served as uninjected control CL. An additional group of 6 animals served as controls and included 4 animals receiving bilateral vehicle implants (3 CL per ovary) on Day 11 and 2 unoperated gilts. All animals were slaughtered on Day 19 of their oestrous cycle, and the weight, progesterone content and concentration of each CL were determined. In Group 3 gilts, luteal weight, progesterone content and concentration were greater by 68·7 ± 24·0 mg, 6·54 ± 1·33 μg and 7·54 ± 2·00 ng/ml respectively (P < 0·01) in oestradiol-implanted CL than in vehicle-implanted CL, which appeared to be similar to uninjected control CL. No differences were seen between oestradiol-17β and vehicle-treated CL in CL from gilts in Groups 1 or 2. All CL of Group 3 gilts were heavier and contained a greater content and concentration of progesterone (P < 0·01) than gilts in Groups 1 and 2, and in controls for which the values were similar. These results demonstrate that exogenous oestradiol can directly stimulate the growth of an individual CL; however, the oestradiol release rate must be high enough to maintain all CL bilaterally before this effect is seen.

Keywords: corpus luteum; oestradiol-17β; luteotrophin; progesterone; pigs

Summary. Implants containing vehicle or oestradiol-17β (10 mg) were placed into pairs of corpora lutea (CL) with and without prostaglandin F-2α (PGF-2α) (100 μg) on Day 11 and CL were collected on Day 19, in cyclic gilts (Exp. 1). The results demonstrated that CL implanted with PGF-2α with or without oestradiol-17β had a markedly lower (P < 0·01) weight (mg) and progesterone concentration (ng/mg) than CL with vehicle-or oestradiol-17β-implanted or unimplanted CL, which were similar (149 and 7·2 vs. 304 and 49·6, respectively). In Exp. 2, CL implanted with vehicle, oestradiol-17β or PGE-2 remained fully functional until Day 19, whereas CL implanted with oestradiol-17β ± PGF-2α and PGE-2 + PGF-2α exhibited lower (P < 0·05) weight and progesterone concentrations; CL implanted with PGE-2 + PGF-2α were heavier (P < 0·05) and tended (P < 0·10) to have greater progesterone concentrations than CL implanted with oestradiol-17β + PGF-2α. In Exp. 3, a dose-dependent (P < 0·05) effect of PGE-2 on preventing regression induced by PGF-2α was observed on Day 19. These data demonstrate a direct effect of PGE-2, but not of oestradiol-17β in protecting the CL against luteolysis induced by PGF-2α.

Keywords: oestradiol-17β; prostaglandin E-2; prostaglandin F-2α; intraluteal implants; corpus luteum; pig

Summary. Total ovarian blood flow of cows (sum of flows through the ovarian artery and uterine branch of the ovarian artery), measured by electromagnetic flow probes, followed a consistent cyclic pattern, being highest from Day 10 of the oestrous cycle (oestrus = Day 0) until 4 days before the subsequent oestrus. During this period of high ovarian blood flow, blood of uterine origin flowed to the ovary through the uterine branch of the ovarian artery and was found to comprise 20–40% of the ovarian blood flow. Between Days —4 and —3, ovarian blood flow dropped (P < 0·01) from 3·2 ± 0·5 to 1·0 ± 0·4 ml/min, coincident with a reversal of flow through this anastomosis. Uterine arterial blood flow was highest (P < 0·05) from 1 day before oestrus to the day of oestrus when compared to the remaining days of the oestrous cycle. Throughout the oestrous cycle, ovarian blood flow was positively correlated with systemic concentrations of progesterone (r = 0·55, P < 0·01) and negatively correlated with systemic concentrations of oestradiol-17β (r = —0·42, P < 0·01). In contrast, uterine arterial blood flow was negatively correlated with systemic progesterone (r = —0·25, P < 0·05) and positively correlated with systemic oestradiol (r = 0·45, P < 0·05). These data provide direct evidence in the cow that a portion of the blood flowing to the ovary containing a fully functional corpus luteum is contributed by the ipsilateral uterine artery.

Summary. Concentrations of oestradiol-17β and oestrone were measured in the uterine arterial and venous blood of anaesthetized sows on Days 11, 13 or 15 of the oestrous cycle or of pregnancy. Uterine arterial blood flow and the amounts of oestradiol-17β and oestrone in uterine flushings were determined in the same animals. In pregnant animals arterial and venous concentrations were significantly different (P < 0·05) for oestradiol-17β on Days 11 and 13 and for oestrone on Day 15. Uterine content of both steroids was consistently greater in pregnant than in non-pregnant animals with a peak on Day 13. Uterine arterial blood flow increased from Day 11 to 13 of pregnancy then declined (P < 0·08) by Day 15; no change in uterine blood flow occurred on the corresponding days of the oestrous cycle.

Summary. The administration of oestrogen results in increased arterial blood flow in all mammalian species studied to date, but its mechanism of action has not been elucidated. Because an interval of 30–60 min is observed between oestrogen injection and uterine hyperaemia, it has been suggested that a vasoactive intermediate is involved and recent evidence suggests that catechol oestrogens are the vasoactive oestrogen intermediates. Uterine peroxidase catalyses the conversion of oestrogens to their catechol forms and thus may play an important role in oestrogen-induced uterine hyperaemia. The present studies evaluated the time course and dose–response effects of oestrogen on uterine peroxidase activity and related these to changes in uterine blood volume, an index of uterine hyperaemia in immature rats. These data demonstrated that the minimal effective hyperaemic dose of oestradiol also increased (P < 0·05) uterine peroxidase activity. The oestradiol-induced increase in uterine peroxidase activity preceded significant increases in uterine blood volume (1 h versus 2 h, respectively). These data are consistent with a role for peroxidase-mediated conversion of oestradiol to catechol oestradiol in facilitating uterine hyperaemia in rats.

Keywords: peroxidase; uterus; oestrogen; hyperaemia; rat

Summary. Non-pregnant heifers (4/group) received intrauterine infusions of vehicle, oestradiol-17β (150 ng), PGE-2 (250 μg), or oestradiol + PGE-2 every 6 h from 12:00 h on Day 13 to 06:00 h on the day of subsequent oestrus or 06:00 h on Day 21 (day of oestrus = Day 0). Ten of 12 heifers receiving vehicle, oestradiol or PGE-2 returned to oestrus by Day 21, whereas none of the heifers receiving oestradiol + PGE-2 returned to oestrus by Day 21. Jugular venous progesterone concentrations of vehicle- and PGE-2-treated heifers declined rapidly after Day 15 and were basal (< 1 ng/ml) by Day 20. For heifers receiving oestradiol infusions, systemic progesterone levels did not decline until after Day 18, but were again basal by Day 20. Heifers treated with oestradiol + PGE-2 maintained elevated systemic progesterone levels until Day 21 after oestrus. In addition, the corpora lutea of the heifers treated with oestradiol + PGE-2 were heavier (P < 0·01) and contained more (P < 0·05) progesterone than did corpora lutea of the heifers in the other 3 groups on Day 21 (3·4 g and 19·52 μg/g and 1·2 g and 1·65 μg/g, respectively). It is concluded that oestradiol-17β and PGE-2, both of which are produced by the bovine conceptus and secreted from the gravid uterus, may act synergistically to maintain luteal function during early pregnancy in the cow.

Summary. Blood flow to each uterine horn of cows during the oestrous cycle and early pregnancy was determined daily by use of electromagnetic blood flow probes placed around both middle uterine arteries. The pattern of blood flow to uteri of pregnant and non-pregnant cows was similar until Day 14 after mating or oestrus. Between Days 14 and 18 of pregnancy blood flow to the uterine horn containing the conceptus increased (P < 0·01) 2- to 3-fold, whereas blood flow to the non-gravid uterine horn in these cows remained constant. No corresponding increase in blood flow to the uterine horn ipsilateral to the ovary bearing the CL was observed in non-pregnant cows during this 4-day period. By Day 19 of pregnancy, blood flow to the gravid uterine horn had returned to a level similar to that observed on Day 13. Blood flow to both uterine horns of pregnant cows remained constant from Days 19 to 25 and then increased to the gravid horn (P < 0·01) markedly until Day 30 whereas blood flow to the non-gravid horn remained low. Uterine blood flow during the oestrous cycle of non-pregnant cows was positively correlated (P < 0·01) with systemic concentrations of oestradiol and the ratio of oestradiol (pg/ml) to progesterone (ng/ml). There was no association between oestradiol concentrations and blood flow to the gravid uterine horn. These data indicate local control of uterine blood flow by the bovine conceptus which may function to create optimal conditions for the continuation of pregnancy.

Day 12 blastocysts from Meishan gilts contain fewer cells than do day 12 blastocysts from Yorkshire gilts. The purpose of this study was to evaluate the effect of breed on the relative numbers of inner cell mass and trophectoderm cells in Meishan and Yorkshire embryos at similar stages. Embryos were collected on days 5.5–6.5 of gestation and were subjected to image analysis and differential cell staining. No breed differences were detected in the thickness of zona pellucida or in the areas of the perivitelline space, embryo proper, blastocoel and inner cell mass at any of the developmental stages examined (compact morula, early blastocyst or blastocyst). However, differences were observed in the pattern of growth of embryos from Meishan versus Yorkshire gilts. The total number of cells of Meishan embryos from Meishan gilts increased progressively from the compact morula through the blastocyst stage, whereas the total number of cells of embryos from Yorkshire gilts remained constant from compact morula through to early blastocyst, and then increased markedly from the early blastocyst to the blastocyst stage. At the blastocyst stage, Meishan embryos contained fewer (P < 0.05) cells than did Yorkshire embryos, and this lower number of cells was due entirely to fewer (P < 0.05) trophectoderm cells. As the number of inner cell mass cells increased during embryonic growth, Meishan embryos exhibited a slower (P < 0.02) increase in the number of trophectoderm cells than did Yorkshire embryos. These results demonstrate that the reduced number of cells present in Meishan embryos results from a selective reduction in the number of trophectoderm, but not inner cell mass, cells.

The expression of cytochromes P450 17α-hydroxylase (P450c17) and aromatase (P450arom) was compared between preimplantation Chinese Meishan and domestic Yorkshire conceptuses during the period encompassing maternal recognition of pregnancy. Individual conceptuses were recovered on days 10.5, 11.0, 11.5, 12.0, and 14.0 of gestation. Diameter (spherical blastocysts), length (elongated blastocysts), DNA, protein and oestradiol content, as well as the amounts of P450c17 and P450arom (western analysis) were determined in individual conceptuses. Comparisons were made only between conceptuses of similar diameters on each day which restricted analyses to blastocysts 6 mm or less in diameter on days 10.5–12.0. Nonetheless, both DNA and protein content were greater in Yorkshire than in Meishan conceptuses. Oestradiol content also tended to be greater in Yorkshire than in Meishan conceptuses across days. A significant effect of breed and breed by day interaction was detected for P450c17. Expression of P450c17 in Yorkshire conceptuses increased markedly above that in Meishan conceptuses by day 11, remained high until day 11.5 and returned to values similar to those of Meishans by day 12. The expression of P450arom was also greater in Yorkshire than in Meishan conceptuses, but no breed by day interaction was detected. These data suggest that differences in development between Meishan and Yorkshire conceptuses include trophoblastic differentiation during preattachment stages. The significance and impact of this divergence in development on subsequent growth and survival remains to be determined.