Changes in uterine secretion of prostaglandin F2 alpha and luteal secretion of progesterone in response to oxytocin during the porcine estrous cycle.

The purpose of this experiment was to determine whether the ability of oxytocin to stimulate uterine secretion of prostaglandin F2 alpha (PGF2 alpha) and luteal secretion of progesterone changes during the porcine estrous cycle. Nineteen multiparous sows were observed for estrus. After one estrous cycle of normal length, sows were assigned randomly to receive an injection of oxytocin (30 IU, i.v.) in the EARLY (Days 4-6; n = 6), MID (Days 9-11; n = 7), or LATE (Day 15; n = 6) stage of the estrous cycle. Concentrations of 13, 14-dihydro-15-keto-PGF2 alpha (PGFM) and progesterone were determined in jugular venous serum samples collected at -60, -45, -30, -15, 0, 2, 5, 10, 15, 30, 45, 60, 90, and 120 min after injection of oxytocin. The magnitudes of the PGFM and progesterone responses and the area under the respective response curves (AUC) were calculated for each sow. Concentrations of PGFM did not change in response to oxytocin administered during the EARLY or MID portions of the estrous cycle. Concentrations increased rapidly in 4 of 6 sows that received oxytocin LATE in the estrous cycle. Both magnitude and AUC were greater LATE in the estrous cycle than at either EARLY or MID cycle (p less than 0.05). Thus, uterine secretory responsiveness to oxytocin develops between Days 11 and 15 postestrus in the sow. For progesterone, a transient increase was observed immediately following injection of oxytocin at MID cycle (p less than 0.05), but not at the other times examined. Therefore, oxytocin appears to be capable of stimulating secretion of progesterone from the functionally mature corpus luteum.     

Effect of induced suprabasal progesterone levels around estrus on plasma concentrations of progesterone, estradiol-17beta and LH in heifers

A controlled study was carried out to investigate the effects of suprabasal plasma progesterone concentrations on blood plasma patterns of progesterone, LH and estradiol-17beta around estrus. Heifers were assigned to receive subcutaneous silicone implants containing 2.5 g (n=4), 5 g (n=4), 6 g (n=3), 7.5 g (n=3) or 10 g (n=4) of progesterone, or implants without hormone (controls, n=5). The implants were inserted on Day 8 of the cycle (Day 0=ovulation) and left in place for 17 d. The time of ovulation was determined by ultrasound scanning. Blood was collected daily from Days 0 to 14 and at 2 to 4-h intervals from Days 15 to 27. Control heifers had the lowest progesterone concentrations on Days 20.5 to 21 (0.5 +/- 0.1 nmol L(-1)); a similar pattern was observed in heifers treated with 2.5 and 5 g of progesterone. In the same period, mean progesterone concentrations in the heifers treated with 6, 7.5 and 10 g were larger (P < 0.05) than in the controls, remaining between 1 and 2.4 nmol L(-1) until implant removal. A preovulatory estradiol increase started on Days 16.4 to 18.4 in all the animals. In the controls and in heifers treated with 2.5 and 5 g of progesterone, estradiol peaked and was followed by the onset of an LH surge. In the remaining treatments, estradiol release was prolonged and increased (P < 0.05), while the LH peak was delayed (P < 0.05) until the end of the increase in estradiol concentration. The estrous cycle was consequently extended (P < 0.05). In all heifers, onset of the LH surge occurred when progesterone reached 0.4 to 1.2 nmol L(-1). The induction of suprabasal levels of progesterone after spontaneous luteolysis caused endocrine asynchronies similar to those observed in cases of repeat breeding. It is suggested that suprabasal concentrations of progesterone around estrus may be a cause of disturbances oestrus/ovulation.     

Increased ovulation rate in gilts after oral administration of epostane

In Phase I of this study to enhance ovulation rate and hence litter size, gilts received 0 (sham control), 0.625, 1.25, 2.5 or 5.0 mg epostane/kg body weight on Days 10, 11 and 12 of the oestrous cycle (5 gilts/group). After epostane treatment, plasma progesterone concentrations were reduced (P less than 0.01) in a dose-related manner, % progesterone decline = 21.30 x square root of (dose) + 10.45, R2 = 0.70, but recovered to pretreatment levels by 24 h. In Phase II the effects of epostane on ovulation rate and litter size were tested at two study centres. At each centre 108 gilts were treated with the same doses of epostane as used in Phase I and the doses were given for 7 days (Days 15-21) or 12 days (Days 10-21) during the first oestrous cycle. Gilts were inseminated twice during the oestrus after treatment and were slaughtered 30 days later. Mean (+/- s.d.) ovulation rate was 16 +/- 2.7 (N = 8) and 21 +/- 4.0 (N = 61) for control and epostane-treated gilts in Centre A and 12 +/- 2.4 (N = 5) and 17 +/- 3.8 (N = 55) respectively in Centre B (P less than 0.01 for both) and was dose related (ovulation rate = 3.38 x square root of (dose) + 16.17, R2 = 0.31). The effects of 7- or 12-day epostane treatment on ovulation rate were not different (P greater than 0.05), indicating that effects of treatment after Day 14 of the oestrous cycle are most important to subsequent ovulation frequency.(ABSTRACT TRUNCATED AT 250 WORDS)     

Luteinizing hormone secretion and corpus luteum function in cows receiving two levels of progesterone

The objectives of this experiment were to determine if subnormal levels of progesterone (P4) indicative of luteal insufficiency influence (1) pulsatile release of luteinizing hormone (LH), (2) the interval to the preovulatory surge of LH after removal of P4, and (3) the secretion of P4 during the estrous cycle subsequent to administration of subnormal levels of P4. On Day 5 (Day = 0 day of estrus) of the estrous cycle, cows received P4-releasing intravaginal devices (PRID) to produce normal (2 PRIDs; n = 7) or subnormal (0.5 PRID; n = 6) concentrations of P4. Five cows served as controls. On Day 10, serial blood samples were collected from all cows. Collection of blood samples was again initiated on Day 17 in cows receiving PRIDs. The PRIDs were removed and blood collection continued for 78 h. Daily blood samples were collected from all animals for 42 days subsequent to estrus (estrous cycles 1 and 2, respectively). During estrous cycle 1, mean concentration of P4 was lower (p less than 0.05) and frequency of pulses of LH was higher (p less than 0.05) in cows receiving subnormal P4 than in cows receiving normal P4 and control cows. Plasma concentrations of estradiol (E2) were higher (p less than 0.05) on Days 9-16 of estrous cycle 1 in cows receiving subnormal P4 than in cows receiving normal P4 or in control cows. Concentrations of E2 were greater (p less than 0.05) at 6, 18, and 30 h following removal of PRIDs in cows receiving subnormal P4 than in cows receiving normal P4.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of oxytocin on plasma concentrations of 13,14-dihydro-15-keto prostaglandin F and the oxytocin-associated neurophysin during the estrous cycle and early pregnancy in the ewe

This study was undertaken to determine the effect of exogenous oxytocin on plasma concentrations of the prostaglandin (PG) F metabolite 13,14-dihydro-15-keto-PGF (PGFM) and the oxytocin-associated neurophysin (OT-N) during the estrous cycle and early pregnancy in the ewe. Ewes were given oxytocin (250 mU, i.v.) on Days 3 (n = 4), 8 (n = 5), 13 (n = 4) or 14 (n = 5) of the estrous cycle, and a further 6 ewes were injected on Days 13 (n = 2) and 14 (n = 4) of pregnancy. No significant rises in plasma concentrations of PGFM were observed on Days 3 and 8 of the estrous cycle and on Days 13 and 14 of pregnancy. A marked increase in plasma PGFM concentrations occurred on Day 14 of the estrous cycle with the PGFM levels rising from a mean basal value of 120 pg/ml to a mean maximum value of 415 pg/ml within 2-10 min of administering oxytocin (P less than 0.001). No increases in plasma OT-N concentrations were found in early pregnancy and only 1 of 4 ewes at Day 14 of the cycle showed any significant increase in OT-N concentrations. It is concluded that there is an increase in the responsiveness of the uterine-PGF secretory system to oxytocin during the latter stages of the estrous cycle. During early pregnancy this response was blocked by the presence of the embryo.(ABSTRACT TRUNCATED AT 250 WORDS)     

Characterization of a distinctive pattern of periovulatory leptin secretion and its relationship with ovulation rate and luteal function in swine with obesity/leptin resistance

Patterns of leptin secretion during the estrous cycle and the possible relationship of changes in circulating leptin during the periovulatory period with ovarian function in sows of obese (Iberian breed) and lean genotype (Large White×Landrace) were evaluated in two consecutive experiments. Plasma leptin concentrations throughout the estrous cycle in lean sows remain unchanged, but Iberian females showed a periovulatory increase in circulating leptin levels without associated changes in body condition and fatness. In these sows, plasma leptin concentrations at Days -1 and 0 of the cycle were found to be positively correlated with the ovulation rate (r=0.943 and r=0.987, respectively; P<0.05 for both), but the levels of leptin at Day 0 were negatively correlated with the progesterone release from Day 3 (r=-0.557; P<0.05) and, became more evident at Day 5 of the estrous cycle (r=-0.924; P<0.005). Such relationships were not observed in the females of the lean genotype. In conclusion, the present study indicates the existence of a distinctive pattern in the periovulatory leptin secretion in swine with obesity and leptin resistance, which is associated with the number and functionality of the corpora lutea present in the subsequent cycle.     

Quantification of receptors for estradiol-17 beta and progesterone in the pituitary and hypothalamus of gilts during the estrous cycle and early pregnancy

Nuclear and cytoplasmic exchange assays were utilized to quantify receptors for estradiol-17 beta (E2) and progesterone (P4) in hypothalamic and pituitary tissues from 4-6 gilts each on Days 1, 5, 10, 15 and 18 of the estrous cycle and from 4-5 gilts each on Days 5, 10, 15, 21 and 30 of pregnancy. No differences in the number of cytoplasmic E2 or P4 receptors in the pituitary were found from Days 1 to 15 of the estrous cycle (P greater than 0.05). However, on Day 18, the quantities of E2 and P4 receptors were 64-fold and 25-fold lower (P less than 0.01) than those found during Days 1 to 15 of the estrous cycle. No differences in the number of nuclear receptors for E2 in the pituitary were observed from Days 1 to 18 of the estrous cycle, but nuclear receptors for P4 were 2-fold higher (P less than 0.01) on Day 1 than Days 5 to 18. In hypothalamic tissue, the numbers of cytoplasmic and nuclear receptors for E2 and P4 were lower (P less than 0.05) on Day 18 than Day 10 of the cycle. The quantity of most steroid receptors decreased between Days 15 and 18 in nonpregnant gilts as luteolysis occurred and a new follicular phase was initiated. Pregnant pigs on Days 5, 10 and 15 had decreased pituitary receptors for E2 and P4 when compared with cycling animals on these days. In general, numbers of receptors in hypothalamic tissue did not differ between pregnant and nonpregnant pigs except for decreased (P less than 0.01) nuclear P4 receptors on Day 15.(ABSTRACT TRUNCATED AT 250 WORDS)     

Oxytocin-stimulated inositol phosphate turnover in endometrium of ewes is influenced by stage of the estrous cycle, pregnancy, and intrauterine infusion of ovine conceptus secretory proteins

Three experiments (Exp) assessed the influence of stage of the estrous cycle, pregnancy, and intrauterine infusion of ovine conceptus secretory proteins (oCSP) on turnover of inositol trisphosphate (the putative second-messenger for oxytocin-stimulated secretion of prostaglandin F2 alpha) in ovine endometrium during luteolysis and maternal recognition of pregnancy. In Exp 1, endometrium was collected from 5 cyclic (Cy) and 6 pregnant (P) ewes on Day 16 after onset of estrus. In Exp 2, endometrium was collected from Day 12 Cy (n = 5), Day 12 P (n = 3), Day 16 Cy (n = 4), and Day 16 P (n = 3) ewes. In Exp 3, 12 Cy ewes were allotted randomly, in a 2 x 2 factorial arrangement, to receive serum protein (SP), or oCSP and estradiol-17 beta (E2), or vehicle treatments. Ewes were injected i.v. with 0.5 mg E2 or vehicle on Day 12 and received twice-daily infusions of 1.5 mg SP or oCSP (containing 25 micrograms ovine trophoblast protein-1 by radioimmunoassay [RIA]) + SP (1.5 mg total protein) into each uterine horn on Days 12, 13, and 14. Blood samples for RIA of plasma progesterone were collected on Days 10-15 (before treatment on each day) and endometrium was collected on Day 15. For each Exp, 100 mg endometrium was incubated, in duplicate, for 2 h with 10 microCi [3H] inositol and treated with 0 or 100 nM oxytocin (OT) for 20 min, then [3H]inositol mono-, bis-, and trisphosphates (IP1, IP2, and IP3, respectively) were quantified.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of day of estrous cycle, time of day, luteolysis, and embryo on uterine contractility in mares

One-minute continuous ultrasonic scans of longitudinal sections of the uterine body were videotaped, and contractility scores (1 to 5, minimal to maximal contractility) were assigned without knowledge of mare identity, day of the estrous cycle or pregnancy status. Contractility was assessed, and plasma progesterone concentrations were determined for each of 3 daily examinations (at 0800, 1600 and 2400 hours) from Day 9 to Day 19 (Day 0 = day of ovulation). For both the nonbred (n=11) and pregnant (n=11) mares, there was no effect of hour of scan on the extent of uterine contractility. When data for the nonbred mares were normalized to the onset of luteolysis (defined for each mare as the first >/=25% decrease in plasma progesterone concentrations between successive samples), there was an abrupt increase (P<0.05) in contractility 24 hours prior to the onset of luteolysis. Contractility was also assessed daily in 20 nonbred and 27 pregnant mares from Day 0 to Day 17. For the nonbred mares, a biphasic profile in contractility occurred during the estrous cycle as indicated by the following significant changes: a decrease between Days 0 and 2, an increase between Days 2 and 4, a plateau between Days 4 and 7, a decrease between Days 7 and 11, an increase between Days 11 and 13, and a decrease between Days 14 and 16. For pregnant mares, contractility increased (P<0.05) prior to the late-diestrous increase for nonbred mares. In addition, a significant reduction in contractility was detected on Day 5 in these mares compared with that in the nonbred mares. Contractility in the uterine body in 7 mares was assessed every 5 minutes after departure of the embryonic vesicle from the uterine body. Levels of contractility in the uterine body were lower (P<0.05) 55 minutes after the vesicle had exited the body than 相似文献   

Prolactin, progesterone and luteinizing hormone secretion after bromocriptine (CB-154) treatment in cyclic sows

The effect of bromocriptine (CB-154) on prolactin, progesterone and luteinizing hormone (LH) secretion was studied in cyclic sows. Four sows were given subcutaneous injections of bromocriptine on Day 14 of the estrous cycle (70 mg CB-154) and again on Day 16 of the cycle (50 mg CB-154). Two control sows were injected with vehicle at similar time intervals. Blood samples were taken four times daily (0700, 1100, 1500 and 1900 h) from Day 11 of the estrous cycle to Day 2 of the following estrous cycle. Prolactin peaks during the estrous cycle were not observed after CB-154 treatment. CB-154 treatment had no effect on plasma LH concentration, but plasma progesterone concentrations appeared to fluctuate more and slowly decreased.     

Extended function of the corpus luteum and earlier development of the second follicular wave in heifers treated with bovine somatotropin

Effects of recombinant bovine somatotropin (bST) on growth of the corpus luteum (CL) and development of ovarian follicles were tested. Starting at estrus (Day=0), the following treatments were administered: control (saline injected Days 0 to 19, n=5); bST[0-9] (25 mg bST injected Days 0 to 9, saline injected Days 10 to 19, n=5); bST[10-19] (saline injected Days 0 to 9, 25 mg bST injected Days 10 to 19, n=5); and bST[0-19] (25 mg bST injected Days 0 to 19, n=6). Blood was collected daily for progesterone analysis, and ultrasound examinations were performed daily for measurement of follicles and CL. Compared with the heifers treated with saline, those treated with bST had larger CL and more progesterone during the early (/=10 mm) follicles was greater (P<0.01) and largest follicles were smaller (P<0.001) in bST than in saline-treated heifers. Estrous cycle length and ovulation rate were similar for each group. In conclusion, bST increased initial development of the CL and extended its function. Furthermore, the second follicular wave was earlier with bST.     

Changes in ovarian oxytocin secretion as an indicator of corpus luteum response to prostaglandin F(2alpha) treatment in cattle

Exogenous prostaglandin F(2alpha) (PGF(2alpha)) rapidly increases ovarian oxytocin (OT) release and decreases progesterone (P4) secretion in cattle. Hence, the measurement of OT secretion (the area under the curve and the height of the peak) after different doses of Oestrophan - PGF(2alpha) analogue (aPGF(2alpha)) on Days 12 and 18 of the estrous cycle (estrus = day 0), could be a suitable indicator of corpus luteum (CL) sensitivity to PGF(2alpha) treatment. Mature heifers (n = 36) were used in this study. Blood samples were collected from the jugular vein for the estimation of OT, P4 and 13, 14-dihydro-15-keto-prostaglandin F(2alpha) (PGFM). In Experiment 1, different doses of aPGF(2alpha) (400, 300, 200 and 100 microg) given on Day 12 of the estrous cycle (n = 8) shortened (P < 0.05) the cycle duration (15.2 +/- 0.6 d) compared with that of the control (21.7 +/- 0.4 d). Successive heifers were also treated on Day 12 with 200 (n = 2), 100 (n = 2), 75 (n = 2) or 50 microg aPGF(2alpha) (n = 2). Only the 50 microg aPGF(2alpha) dose did not cause CL regression, although it increased OT concentrations to levels comparable to those observed during spontaneous luteolysis (50 to 70 pg/ml). In Experiment 2, on Day 18 of the cycle heifers (n = 8) were treated with 50, 40, 30 and 20 microg aPGF(2alpha). There was a dose-dependent effect of aPGF(2alpha) on OT secretion on Day 18 of the estrous cycle (r = 0.77; P < 0.05). In Experiment 3, an injection of 500 microg aPGF(2alpha) on Day 12 (n = 4) and 50 microg aPGF(2alpha) on Day 18 (n = 4) caused a similar (P > 0.05) increase in the OT concentration (288.5 +/- 23.0 and 261.5 +/- 34.7 pg/ml, respectively). Thus the effect of the same dose of aPGF(2alpha) (50 microg) on OT secretion was different on Days 12 and 18 of the cycle. To evoke similar OT secretion on Days 12 and 18 the dose of aPGF(2alpha) on Day 18 could be reduced 10-fold, confirming that CL sensitivity to PGF(2alpha) appears to increase in the late luteal phase.     

Passive immunization of the pig against gonadotropin releasing hormone during the follicular phase of the estrous cycle

Three experiments were conducted to determine the effects of passively immunizing pigs against gonadotropin releasing hormone (GnRH) during the follicular phase of the estrous cycle. In Experiment 1, sows were given GnRH antibodies at weaning and they lacked estrogen secretion during the five days immediately after weaning and had delayed returns to estrus. In Experiment 2, gilts passively immunized against GnRH on Day 16 or 17 of the estrous cycle (Day 0 = first day of estrus) had lower (P<0.03) concentrations of estradiol-17beta than control gilts, and they did not exhibited estrus at the expected time (Days 18 to 22). When observed three weeks after passive immunization, control gilts had corpora lutea present on their ovaries, whereas GnRH-immunized gilts had follicles and no corpora lutea. The amount of GnRH antiserum given did not alter (P<0.05) serum concentrations of LH or pulsatile release of LH in sows and gilts. In Experiment 3, prepuberal gilts were given 1,000 IU PMSG at 0 h and GnRH antiserum at 72 and 120 h. This treatment lowered the preovulatory surge of LH and FSH, but it did not alter serum estradiol-17beta concentrations, the proportion of pigs exhibiting estrus, or the ovulation rate. These results indicate that passive immunization of pigs against GnRH before initiation of or during the early part of the follicular phase of the estrous cycle retards follicular development, whereas administration of GnRH antibodies during the latter stages of follicular development does not have an affect. Since the concentration of antibodies was not high enough to alter basal or pulsatile LH secretion, the mechanism of action of the GnRH antiserum may involve a direct ovarian action.     

Effects of progesterone and estradiol on uterine secretion of prostaglandin f(2alpha)in response to oxytocin in ovariectomized sows

Thirty ovariectomized sows were used in an experiment designed to determine whether the ability of the porcine uterus to release prostaglandin (PG) F(2alpha) in response to oxytocin is regulated by progesterone (P(4)) and estradiol (E(2)). Sows were assigned to one of four treatment groups: 1) no steroids (ovariectomized controls; n = 8), 2) E(2) (n = 8), 3) P(4) (n = 7), or 4) E(2) + P(4) (n = 7). P(4) and E(2) were administered so as to mimic the normal temporal changes that occur in these hormones during the estrous cycle. A group of intact sows (n = 9) was included for comparison. All sows received an injection of oxytocin (30 IU, i.v.) on Days 12, 15, and 18 postestrus. Jugular venous blood samples were collected from 60 min before through 120 min after injection of oxytocin for quantification of 13,14-dihydro-15-keto-PGF(2alpha) (PGFM). Preinjection baseline concentrations of PGFM, the magnitude of the PGFM response above baseline, and area under the PGFM response curve (AUC) were calculated for each sow on each day and compared among treatment groups by ANOVA. Among the ovariectomized sows receiving steroid replacement, baseline concentrations of PGFM were low on Day 12 postestrus in all four groups. On Days 15 and 18, baseline concentrations remained low in the two groups that did not receive P(4) but increased in those that did. Both the magnitude of the response to oxytocin and AUC were small on Day 12 postestrus in all 4 groups. By Day 15, the magnitude of the response and AUC increased in the group that received both P(4) and E(2) but remained low in the other three groups. By Day 18, responses to oxytocin were greater in both groups that received P(4) than in those that did not. Baseline concentrations were similar in intact sows and in those that received both P(4) and E(2) on all three days examined. The magnitude of the response and the AUC were greater in the ovariectomized sows receiving P(4) and E(2) replacement than in the intact control sows on Days 15 and 18 postestrus. From these results, we conclude that P(4) and E(2) interact to control the time when the uterus begins to secrete PGF(2alpha) in response to oxytocin and the amount of PGF(2alpha) secreted.     

Failure of the LH-releasing hormone agonist, deslorelin, to prevent development of a persistent follicle in heifers synchronized with norgestomet

The use of exogenous progestagens for estrus synchronization in cattle can result in a persistent dominant follicle which is associated with reduced fertility. We examined whether the LHRH agonist, deslorelin, would prevent the formation of a persistent follicle in heifers synchronized with norgestomet. The estrous cycles of heifers were synchronized with cloprostenol, and on Day 7 of the ensuing cycle the heifers received one of the following treatments for 10 d: Group C (n = 5), untreated control; Group N (n = 6), injection of a luteolytic dose of cloprostenol on Days 7 and 8 and implant of norgestomet from Day 7 to Day 17 (i.e. typical 10-day norgestomet implant period); Group D (n = 6), injection of cloprostenol on Days 7 and 8 and implants of deslorelin from Day 7 to Day 17; Group ND (n = 6), injections of cloprostenol and both norgestomet and deslorelin implants as above. Follicle growth was monitored using ultrasonography. Group-N heifers showed continued follicle growth and had larger follicles on Day 17 of the cycle than Group-C heifers (16.8 +/- 1.6 and 10.4 +/- 1.6 mm). Follicle growth for Group-D and ND heifers was similar and variable, and seemed to depend on follicle status at the initiation of treatment. Heifers with follicles of 5 to 10 mm (n = 9) in diameter either showed no follicle growth (2 9 ) or developed large follicles (7 9 ), while heifers with follicles approximately 12 mm (n = 3) in diameter showed follicle atresia with no further significant growth. On Day 17, size of the largest follicle was similar for Group-ND (14.3 +/- 2.9) and Group-D (16.8 +/- 1.6) heifers. Heifers in Group N showed estrous behavior 1.8 +/- 0.2 d after treatment, whereas heifers in Groups D and ND did not show estrus for 2 to 4 wk. The results show that combined treatment with progestagen and an LHRH agonist does not consistently prevent the development of a persistent dominant follicle and that return to estrus can be delayed after treatment with an LHRH agonist.     

Gonadotropin-releasing hormone-induced alteration of bovine corpus luteum function

Two experiments were conducted to evaluate effects of gonadotropin-releasing hormone (GnRH) on the function of the bovine corpus luteum during the estrous cycle. In Experiment 1, 10 beef heifers were assigned randomly into two groups; each heifer served as her own control. Heifers in Group I (n = 5) were injected i.v. with vehicle (saline) on Day 2 of the cycle (Day 0 = day of estrus) followed by an i.v. injection of 100 micrograms GnRH on Day 2 of the subsequent estrous cycle. Group II (n = 5) heifers were treated similarly except injections were given on Day 10 of the estrous cycle. All heifers were bled via the jugular vein at 15 min intervals beginning 30 min prior to injection and for 3 h after injection. Blood samples were also taken on alternate days after injection through Day 16 of the cycle. Gonadotropin-releasing hormone caused a significant release of luteinizing hormone (LH) on both treatment days with the peak occurring at 15 to 30 min postinjection. Treatment with GnRH on either Day 2 or 10 caused a reduction in serum progesterone levels on Days 12, 14 and 16 of the cycle (Group I, control 3.99, 3.97; 4.07 vs. treated 2.63, 3.45, 2.87; Group II, control 3.18, 3.82, 4.13 vs. treated 2.50, 2.82, 3.17 ng/ml, respectively; common SE = 0.24 p less than 0.03). Length of the estrous cycle did not differ between groups (Group I, control 20.7 vs. treated 20.9; Group II, control 20.7 vs. treated 21.1 days, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)     

Morphometric quantification of mitochondria in the two steroidogenic ovine luteal cell types

Progesterone secretion is regulated by different mechanisms in large and small steroidogenic ovine luteal cells. Large cells secrete approximately 7-fold more progesterone in an unstimulated state than small cells. Since cholesterol side-chain cleavage, which is catalyzed by an inner mitochondrial membrane enzyme complex, is a major rate-limiting step in progesterone synthesis, mitochondrial components were quantified in the two steroidogenic cell types throughout the estrous cycle. Corpora lutea collected on Days 4 (n = 4), 8 (n = 4), 12 (n = 5), and 16 (n = 6) of the estrous cycle were prepared for electron microscopy. Volume densities of cell types within corpora lutea and mitochondrial densities within cell types were estimated by point-counting; nuclear and cytoplasmic volume densities were estimated by planimetric analysis. A total of 570 micrographs (magnification 5300 X) were analyzed. Large cell volume density was unchanged during the cycle (35 +/- 1%) while small cell volume density increased (p less than 0.05) from 13 +/- 1% on Day 4 to 20 +/- 3% on Day 12. Large cell mitochondrial volume density increased (p less than 0.05) from 13 +/- 1% on Day 4 to 23 +/- 1% on Day 16 accompanied by an increase in cytoplasmic volume density such that nuclear to cytoplasmic ratio increased (p less than 0.05) from 1:14 to 1:34 between Days 4 and 16. Small cell mitochondrial volume density increased from 11 +/- 1% on Day 4 to 14 +/- 1% (p less than 0.05) for the rest of the cycle while the nuclear to cytoplasmic ratio remained at 1:14.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of plane of nutrition on endometrial sex steroid receptor expression in ewes

The effect of plane of nutrition on progesterone receptor (PR) and estrogen receptor alpha (ERalpha) expression in ovine endometrium was investigated. Rasa Aragonesa ewes (n=26) were fed diets to provide either 1.5 (Group C) or 0.5 (Group L) times the daily maintenance requirement and were slaughtered at Days 5 or 10 of the estrous cycle (Day 0=estrus). PR and ERalpha immunoreactivity were analyzed in eight endometrial cell compartments, defined by cell type and location. Group L had less PR immunostaining on Day 5 (P<0.05), which is consistent with lesser endometrial content of progesterone found in such animals. Most cell types of Group C had down regulation of PR at Day 10, but in Group L, this pattern was observed only in three cell compartments. The lesser PR contents found at Day 5 in Group L ewes may explain the lack of inhibition of PR. No effect of treatment or day of the estrous cycle was observed in ERalpha. Results indicate that endometrial PR is affected in a cell type, in specific manner, by plane of nutrition.     

Effects of oxytocin on cloprostenol-induced luteolysis, follicular growth, ovulation and corpus luteum function in heifers

Twenty-five normally cyclic Holstein heifers were used to examine the effects of oxytocin on cloprostenol-induced luteolysis, subsequent ovulation, and early luteal and follicular development. The heifers were randomly assigned to 1 of 4 treatments: Group SC-SC (n=6), Group SC-OT (n=6), Group OT-SC (n=6) and Group OT-OT (n=7). The SC-SC and SC-OT groups received continuous saline infusion, while Groups OT-SC and OT-OT received continuous oxytocin infusion (1:9 mg/d) on Days 14 to 26 after estrus. All animals received 500 microg, i.m. cloprostenol 2 d after initiation of infusion (Day 16) to induce luteolysis. Groups SC-OT and OT-OT received oxytocin twice daily (12 h apart) (0.33 USP units/kg body weight, s.c.) on Days 3 to 6 of the estrous cycle following cloprostenol-induced luteolysis, while Groups SC-SC and OT-SC received an equivalent volume of saline. Daily plasma progesterone (P4) concentrations prior to cloprostenol-induced luteolysis and rates of decline in P4 following the induced luteolysis did not differ between oxytocin-infused (OT-OT and OT-SC) and saline-infused (SC-SC and SC-OT) groups (P >0.1). Duration of the estrous cycle was shortened in saline-infused heifers receiving oxytocin daily during the first week of the estrous cycle. In contrast, oxytocin injections did not result in premature inhibition of luteal function and return to estrus in heifers that received oxytocin infusion (OT-OT). Day of ovulation, size of ovulating follicle and time of peak LH after cloprostenol administration for oxytocin and saline-treated control heifers did not differ (P >0.1). During the first 3 d of the estrous cycle following luteal regression, fewer (P <0.01) follicles of all classes were observed in the oxytocin-infused animals. Day of emergence of the first follicular wave in heifers treated with oxytocin was delayed (P <0.05). The results show that continuous infusion of oxytocin during the mid-luteal stage of the estrous cycle has no effect on cloprostenol-induced luteal regression, timing of preovulatory LH peak or ovulation. Further, the finding support that an episodic rather than continuous administration of oxytocin during the first week of the estrous cycle results in premature loss of luteal function. The data suggest minor inhibitory effects of oxytocin on follicular growth during the first 3 d of the estrous cycle following cloprostenol-induced luteolysis.     

Luteotrophic influence of early bovine embryos and the relationship between plasma progesterone concentrations and embryo survival

This study was carried out to evaluate the luteotrophic influence of early (before Day 7 as well as after Day 7; Day 0=estrus) bovine embryos and the relationship between plasma progesterone (P4) concentrations and embryo survival. Virgin Holstein dairy heifers (n=325) from a single herd were randomly allocated to be nonbred, bred by artificial insemination (AI) or by embryo transfer (ET). Bred heifers were either treated with 1500 IU human chorionic gonadotrophin (hCG) on Day 7 of the estrous cycle or received no hCG treatment. Plasma P4 concentrations on Days 0, 5, 7, 10, 13, 15, 17, 19 and 21 were similar in pregnant AI- and ET-bred heifers and, this was observed in both hCG-treated and untreated females. Nonbred, AI- and ET-bred nonpregnant heifers (both hCG-treated and untreated) presented similar plasma P4 concentrations. Plasma P4 concentrations of pregnant heifers significantly deviated from those of nonpregnant and nonbred heifers on Day 17. In hCG-treated heifers, plasma P4 concentrations and Day 28 pregnancy rate were significantly higher in females with an induced accessory corpus luteum (CL) than in those females without an induced accessory CL. Treatment with hCG, although inducing the formation of accessory CL and significantly increasing plasma P4 concentrations had no significant effect on Day 28 pregnancy rate. In conclusion, this study does not support the existence of any peripherally detectable luteotrophic influence from early embryos (Days 5-7). Plasma P4 was only significantly related to embryo survival on Day 17, the time of expected onset of luteolysis.