Involvement of gonadotropins in induction of luteolysis in pigs

In our previous study we have demonstrated that treatment of endometrial explants with LH increased 13,14-dihydro-15-ketoprostaglandin F(2alpha) (PGFM) accumulation in pigs. This was particularly visible on Days 14-16 of the estrous cycle. Action of gonadotropin in porcine endometrium appears to be mediated by LH/hCG receptors whose number is dependent on the day of the estrous cycle. In the current study i.v. infusion (1 hour) of hCG (200 IU) performed on Days 10 (n=4) and 12-14 (n=4) of the porcine estrous cycle did not affect plasma PGFM (ng/ml+/-SEM) concentrations. In contrast, administration of hCG on Days 15-17 produced, depending on plasma PGFM level before the infusion period, three different types of response: I. plasma PGFM surge of amplitude 0.62+/-0.15 was observed when the mean basal pre-infusion PGFM plasma level was 0.23+/-0.05 (n=6 gilts); II. the delayed PGFM surge of amplitude 0.62+/-0.15 was determined when basal pre-infusion PGFM level was 0.80+/-0.20 (n=6); and III. lack of PGFM response to hCG was found when basal pre-infusion PGFM level was 1.09+/-0.61 (n=6). Concentrations of plasma PGFM before and after saline infusion did not differ on Days 12-14 and 16 of the estrous cycle. In the next experiment blood samples were collected every 1 hour on Days 12-19 of the estrous cycle to determine concentrations of LH, PGFM and progesterone in four gilts. In particular gilts, plasma peaks of LH closely preceded surges of PGFM in 72.7, 84.6, 75.0 and 66.6 percent, respectively. The highest PGFM surges followed a decline in plasma progesterone concentration. We conclude that the increased PGF(2alpha) metabolite production after hCG infusion during the late luteal phase of the estrous cycle as well as the relationship between plasma LH and PGFM peaks suggest the LH involvement in the elevation of endometrial PGF(2alpha) secretion in pigs, and, in consequence, induction of luteolysis.     

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.     

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.     

Effects of tumor necrosis factor-alpha on secretion of prostaglandins E2 and F2alpha in bovine endometrium throughout the estrous cycle

To determine the physiological significance of tumor necrosis factor-alpha (TNFalpha) in the regulation of endometrial prostaglandin (PG) release in cattle, we investigated the effects of TNFalpha on the secretion of PGE2 and PGF2alpha by bovine endometrium during the estrous cycle. Bovine uteri were classified into six stages (estrus: Day 0, early luteal 1: Days 2 to 3, early luteal 11: Days 5 to 6, mid-luteal: Days 8 to 12, late luteal: Days 15 to 17 and follicular: Days 19 to 21). After 1 h of pre-incubation, endometrial tissues (20 to 30 mg) were exposed to 0 or 0.6 nM TNFalpha for 4 h. The PGE2 concentrations in the medium were higher in the luteal stages than in the follicular stage and in estrus. In contrast, PGF2alpha concentrations were higher in the follicular stage and in estrus than in the luteal stages. The ratio of the basal concentrations of PGE2 and PGF2alpha (PGE2/PGF2alpha ratio) was higher in the luteal stages than in the follicular stage and in estrus. Although TNFalpha stimulated both PGE2 and PGF2alpha secretion during the entire period of the estrous cycle, the level of stimulation of TNFalpha on PGE2 output by the bovine endometrium does not show the same cyclical changes as that shown on PGF2alpha output. The stimulation of TNFalpha resulted in a decrease in the PGE2/PGF2alpha ratio only in the late luteal stage. Furthermore, TNFalpha stimulated PGE2 secretion in stromal, but not epithelial cells. The overall results suggest that TNFalpha is a potent regulator of endometrial PGE2 secretion as well as PGF2alpha secretion during the entire period of estrous cycle, and that TNFalpha plays different roles in the regulation of secretory function of bovine endometrium at different phases of the estrous cycle.     

Reduction by human chorionic gonadotropin of the luteolytic effect of prostaglandin F2 alpha in ewes

The ability of human chorionic gonadotropin (HCG) to reduce the luteolytic effect of prostaglandin (PGF2 alpha) was demonstrated in cycling ewes. As expected, treatment with 10 mg of PGF2 alpha alone on Day 10 of the estrous cycle exerted a potent negative effect on the function and structure of corpus luteum (CL) as indicated by reduced plasma progesterone, CL progesterone, and CL weight. However, the identical PGF2 alpha treatment failed to significantly reduce either luteal function or luteal weight when administered to ewes that were also treated with HCG on Days 9 and 10 of the estrous cycle. Treatment with HCG alone had a positive effect on CL as indicated by increased plasma progesterone, CL progesterone, and CL weight. Treatment with HCG did not render the CL totally insensitive to the negative effects of PGF2 alpha because plasma progesterone was reduced when the dose of PGF2 alpha was doubled. Whether CL regressed or continued to function after treatment with both HCG and PGF2 alpha appeared to depend upon a balance between the positive and negative effects of the two hormones.     

Trypanosome-induced increase in prostaglandin F(2alpha) and its relationship with corpus luteum function in the goat

Plasma progesterone and 13,14-dihydro-15-keto Prostaglandin F(2alpha) (PGFM) were measured in normal (uninfected) and Trypanosoma congolense -infected adult goats for a period of 121 d, from May to August, during the breeding season in Kenya. Chronic trypanosomiasis rapidly increased the baseline plasma PGFM levels and the occurrence of irregular PGFM peaks in several infected goats. Progesterone luteal levels declined rapidly from the second and subsequent cycles post patency. Estrous cycles also became irregular but predominately shorter (8 to 19 d) before cessation from the second to fourth cycle following infection. The PGFM levels were still high during the acyclic period in all goats when progesterone levels were very low (1.4 to 2.4 nmol/l). The reciprocal increase in peripheral PGFM and decline in progesterone in these goats would suggest, in part, a trypanosome-induced PGF(2alpha) mediated luteolysis, and the possible involvement of prostaglandins in trypanosome-induced infertility in female goats.     

Prostaglandin F2 alpha, oxytocin and progesterone secretion by bovine luteal cells at several stages of luteal development: effects of oxytocin, luteinizing hormone, prostaglandin F2 alpha and estradiol-17 beta

Bovine luteal cells from Days 4, 8, 14 and 18 of the estrous cycle were incubated for 2 h (1 x 10(5) cells/ml) in serum-free media with one or a combination of treatments [control (no hormone), prostaglandin F2 alpha (PGF), oxytocin (OT), estradiol-17 beta (E) or luteinizing hormone (LH)]. Luteal cell conditioned media were then assayed by RIA for progesterone (P), PGF, and OT. Basal secretion of PGF on Days 4, 8, 14 and 18 was 173.8 +/- 66.2, 111.1 +/- 37.8, 57.7 +/- 15.4 and 124.3 +/- 29.9 pg/ml, respectively. Basal release of OT and P was greater on Day 4 (P less than 0.01) than on Day 8, 14 and 18 (OT: 17.5 +/- 2.6 versus 5.6 +/- 0.7, 6.0 +/- 1.4 and 3.1 +/- 0.4 pg/ml; P: 138.9 +/- 19.5 versus 23.2 +/- 7.5, 35.4 +/- 6.5 and 43.6 +/- 8.1 ng/ml, respectively). Oxytocin increased (P less than 0.01) PGF release by luteal cells compared with control cultures irrespective of day of estrous cycle. Estradiol-17 beta stimulated (P less than 0.05) PGF secretion on Days 8, 14 and 18, and LH increased (P less than 0.01) PGF production only on Day 14. Prostaglandin F2 alpha, E and LH had no effect on OT release by luteal cells from any day. Luteinizing hormone alone or in combination with PGF, OT or E increased (P less than 0.01) P secretion by cells from Days 8, 14 and 18. However on Day 8, a combination of PGF + OT and PGF + E decreased (P less than 0.05) LH-stimulated P secretion. These data demonstrate that OT stimulates PGF secretion by bovine luteal cells in vitro. In addition, LH and E also stimulate PGF release but effects may vary with stage of estrous cycle.     

Sensitivity of bovine corpora lutea to prostaglandin F2alpha is dependent on progesterone, oxytocin, and prostaglandins.

Prostaglandin (PG) F2alpha that is released from the uterus is essential for spontaneous luteolysis in cattle. Although PGF2alpha and its analogues are extensively used to synchronize the estrous cycle by inducing luteolysis, corpora lutea (CL) at the early stage of the estrous cycle are resistant to the luteolytic effect of PGF2alpha. We examined the sensitivity of bovine CL to PGF2alpha treatment in vitro and determined whether the changes in the response of CL to PGF2alpha are dependent on progesterone (P4), oxytocin (OT), and PGs produced locally. Bovine luteal cells from early (Days 4-5 of the estrous cycle) and mid-cycle CL (Days 8-12 of the estrous cycle) were preexposed for 12 h to a P4 antagonist (onapristone: OP; 10(-4) M), an OT antagonist (atosiban: AT; 10(-6) M), or indomethacin (INDO; 10(-4) M) before stimulation with PGF2alpha. Although OP reduced P4 secretion (p < 0.001) only in early CL, it reduced OT secretion in the cells of both phases examined (p < 0.001). OP also reduced PGF2alpha and PGE2 secretion (p < 0.01) from early CL. However, it stimulated PGF2alpha secretion in mid-cycle luteal cells (p < 0.001). AT reduced P4 secretion in early and mid-cycle CL (p < 0.05). Moreover, PGF2alpha secretion was inhibited (p < 0.05) by AT in early CL. The OT secretion and the intracellular level of free Ca2+ ([Ca2+]i) were measured as indicators of CL sensitivity to PGF2alpha. PGF2alpha had no influence on OT secretion, although [Ca2+]i increased (p < 0.05) in the early CL. However, the effect of PGF2alpha was augmented (p < 0.01) in cells after pretreatment with OP, AT, and INDO in comparison with the controls. In mid-cycle luteal cells, PGF2alpha induced 2-fold increases in OT secretion and [Ca2+]i. However, in contrast to results in early CL, these increases were magnified only by preexposure of the cells to AT (p < 0.05). These results indicate that luteal P4, OT, and PGs are components of an autocrine/paracrine positive feedback cascade in bovine early to mid-cycle CL and may be responsible for the resistance of the early bovine CL to the exogenous PGF2alpha action.     

Effect of intrauterine iodine infusion on luteal function and blood PGF2a concentration in cycling goats

Studies were conducted to determine the effect of iodine infusion on the luteal function of goats, as evident by blood progesterone concentration, and on plasma PGF2a levels. Ten cycling mixed breed goats were synchronized for estrus by PGF2a (5 mg) and given a single intrauterine iodine infusion on day 5 and on day 15 of the estrous cycle. Iodine infusion on day 5 (group II) resulted in shorter estrous length (8.2 days) and a 7-fold increase in plasma PGF2a concentration as compared to control animals (group I) given distilled water infusion. Similar infusion on day 15 (group III), on the other hand, failed to alter the estrous cycle length but induced a moderate increase in PGF2a concentration which lasted only for a brief period. The progesterone levels declined concomitantly as PGF2a levels rose after iodine infusion in group II animals but failed to decline until after 24 hours in group III animals. The studies indicate that the endometrium reacts to the chemical stimuli and releases PGF2a which, in turn, alters the luteal function.     

Influence of dietary-induced weight changes on serum luteinizing hormone, estrogen and progesterone in the bovine female

The working hypothesis in the present study was that changes in concentrations and secretory patterns of luteinizing hormone (LH), 17 beta estradiol (E2), and progesterone in sexually mature beef heifers fed diets deficient in energy are related to changes in body weight of the animals. Another important component of the study was to determine if concentrations and secretion patterns of the reproductive hormones changed over time as feeding of the experimental diets continued. Twelve Red Angus X Hereford heifers (20 mo of age; 355 +/- 7 kg) were assigned randomly to receive a low- (L, n = 7) or high- (H, n = 5) energy diet for 100 days (Day 0 = day of initiation of dietary treatment). All heifers were exhibiting estrous cycles at regular intervals when the experiment was initiated and continued to exhibit estrous cycles at regular intervals throughout the study. Stage of the estrous cycle was synchronized in all 12 heifers by administration of prostaglandin F2 alpha (PGF2 alpha) on two occasions (Days 45 and 75) during the experiment. Serial blood samples (taken at 12-min intervals for 4 h) were collected at 0, 12, 24, 36, 48, and 60 h after the PGF2 alpha injections (Days 45-47 and 75-77) to determine patterns of LH secretion during the follicular phase of the estrous cycle. In addition, serial blood samples (taken at 20-min intervals for 18 h) to monitor LH secretion during the luteal phase of the estrous cycle, in which the stage of the cycle was standardized between heifers, were obtained (Days 59 and 89).(ABSTRACT TRUNCATED AT 250 WORDS)     

Associations among progesterone, estradiol-17 beta, oxytocin and prostaglandin in cattle treated with hCG during diestrus to extend corpus luteum function

Treatment of cattle during the middle of the luteal phase with appropriate doses of human chorionic gonadotropin (hCG) causes a 5 d extension of the estrous cycle. Three experiments were conducted to determine how treatment with hCG affected the pattern of secretion of prostaglandin F2 alpha, as indicated by blood levels of 13,14-dihydro-15-keto-prostaglandin F2 alpha (PGFM). In experiment 1, Holstein cows were given saline (Sal) or hCG (10,000 IU, im) on d 10 of the estrous cycle and blood samples were collected over a 6 h period on d 14 and 18 during which oxytocin (10 and 100 IU, iv) was given at 2 and 4 h. Concentrations of PGFM before and after oxytocin were similar between Sal and hCG-cycles, but PGFM was higher on d 18 than d 14 (P less than 0.05). In experiment 2, episodic PGFM was measured from d 16 to 20 in cows given Sal or hCG on d 10. There was tendency for hCG to reduce PGFM baseline and pulse amplitude (P = 0.22). In experiments 1 and 2, estradiol increased during d 16 to 20 of Sal-cycles, but did not change during this period of hCG-cycles. Therefore, in experiment 3, Holstein heifers were given Sal or hCG (5000 IU, im) on d 10, followed by corn oil (Oil) or estradiol benzoate (EB; 200 micrograms, im, 2X/day) on d 15 to 18. No difference in progesterone secretion was observed between Sal-Oil and Sal-EB heifers; however, EB hastened luteolysis in hCG-treated heifers (P less than 0.05), without causing an increase in PGFM. Although subtle differences were seen in pulsatile PGFM, we conclude that hCG altered the pattern of estrogen secretion, and this led to delayed luteolysis.     

Oxytocin (OT) action in uterine tissues of cyclic and early pregnant gilts: OT receptors concentration, prostaglandin F(2)alpha secretion, and phosphoinositide hydrolysis

Oxytocin (OT) is involved in the regulation of luteolysis in pigs. However, it is still not clear if OT is responsible for initiation of luteal regression in this species. The objectives of the study were: (1) to compare OT receptors (OTr) concentrations in endometrium and myometrium of cyclic and early pregnant pigs, (2) to examine the effect of OT on plasma PGF(2)alpha secretion during the progressive luteal regression, (3) to ascertain the effect of OT on inositol phosphates (IPs) accumulation in endometrial and myometrial cells of cyclic and early pregnant pigs. Concentrations of OTr on the endometrium and myometrium of cyclic (n = 33) (days 2-4; 11-13; 14-16; 18-20; day 21) and early pregnant (n = 4) (days 14-16) gilts were determined and they ranged from 7 +/- 3 (days 11-13) to 377 +/- 113 fmol/mg protein (day 21) in the endometrium and from 33 +/- 11 (days 2-4) to 167 +/- 28 fmol/mg protein (days 18-20) in the myometrium. In both tissues, concentrations of OTr were low during the luteal phase and increased (P < 0.01) during the follicular phase. In contrast to myometrial OTr, endometrial OTr during pregnancy were undetectable. In next experiment, mature gilts (n = 12) were injected with OT (20IU; i.v.) for three consecutive days starting on days 14 and 15 of the oestrous cycle and plasma PGF(2)alpha metabolite-13,14-dihydro-16-keto PGF(2)alpha (PGFM) concentration was determined. On days 15-16 and 16-17, OT increased plasma PGFM level. This effect was not observed on days 14-15 of the estrous cycle. A negative correlation was noticed between plasma concentrations of PGFM and progesterone (r = -0.3; P < 0.05). In last experiment, OT (100 nM) augmented (P < 0.01) an accumulation of inositol phosphates (IPs) in isolated myometrial cells on days 14-16 (n = 4) and 18-20 (n = 3) of the estrous cycle and on days 14-16 (n = 4) of pregnancy. Oxytocin-stimulated accumulation of IPs was not observed in endometrial cells. In summary: (1) concentrations of OTr on both the endometrium and myometrium were the highest during perioestrus-period in pigs, (2) myometrium of early pregnant sows possessed functional OTr, (3) oxytocin increased plasma PGFM concentration after initiation of luteolysis; and (4) OT-stimulated accumulation of IPs in myometrial, but not in endometrial cells. In conclusion, OT appears to not be involved in the initiation of luteal regression in sows and functional OTr are still present in the myometrium during early pregnancy (days 14-16).     

Effect of exogenous progesterone on prostaglandin F2 alpha release and the interestrous interval in the bovine

The present study was developed to determine if administration of progesterone, early in the estrous cycle of the cow, stimulated an advanced pulsatile release of PGF2 alpha from the uterine endometrium resulting in a decreased interestrous interval. Twenty-three cyclic beef cows were randomly assigned to receive either sesame oil or progesterone (100 mg) on Day 1, 2, 3 and 4 of the estrous cycle. Peripheral plasma concentrations of progesterone and the metabolite of prostaglandin F2 alpha, 15-keto-13,14-dihydro-prostaglandin F2 alpha (PGFM) were measured by radioimmunoassay. Administration of exogenous progesterone increased peripheral plasma concentration of progesterone in treated (3.67 ng/ml) compared to control (1.28 ng/ml) cows from Day 2 through 5 of the estrous cycle. Progesterone administration shortened the interestrous interval (16.7 d) compared to controls (21.6 d). The shortened interestrous intervals in treated cows resulted from an earlier decline in peripheral plasma progesterone. Decline of peripheral plasma progesterone concentrations is coincident with an increased pulsatile release of PGFM in both progesterone treated and control cows. Results indicate that administration of exogenous progesterone stimulates an earlier maturation of endometrial development, causing an advanced release of PGF2 alpha which shortens the interestrous interval of the cow.     

Is tumor necrosis factor alpha a trigger for the initiation of endometrial prostaglandin F(2alpha) release at luteolysis in cattle?

To determine the physiological significance of tumor necrosis factor alpha (TNFalpha) in the regulation of luteolytic prostaglandin (PG) F(2alpha) release by the bovine endometrium, the effect of TNF-alpha on PGF(2alpha) output by the endometrial tissues in vitro was investigated and compared with the effect of oxytocin (OT). Furthermore, the presence of specific receptors for TNFalpha in the bovine endometrium during the estrous cycle was determined. Endometrial slices (20-30 mg) taken from six stages of the estrous cycle (estrus: Day 0; early I: Days 2-3; early II: Days 5-6; mid-: Days 8-12; late: Days 15-17; and follicular: Days 19-21), as determined by macroscopic examination of the ovaries and uterus, were exposed to TNFalpha (0.06-6 nM) and/or OT (100 nM). OT stimulated PGF(2alpha) output at the follicular stage and at estrus (P < 0.001), but not at the late luteal stage. On the other hand, the stimulatory effects of TNFalpha on PGF(2alpha) output were observed not only at the follicular stage but also at the late luteal stage (P < 0.001). When the endometrial tissues at late luteal stage were simultaneously exposed to TNFalpha (0.6 nM) and OT (100 nM), the stimulatory effect on PGF(2alpha) output was higher than the effect of TNFalpha or OT alone (P < 0.05). Specific binding of TNFalpha to the bovine endometrial membranes was observed throughout the estrous cycle. The concentration of TNF-alpha receptor at the early I luteal stage was less than the concentrations at other luteal stages (P < 0.01). The dissociation constant (K(d)) values of the endometrial membranes were constant during the estrous cycle. The overall results lead us to hypothesize that TNFalpha may be a trigger for the output of PGF(2alpha) by the endometrium at the initiation of luteolysis in cattle.     

Temporal relationship between plasma concentrations of 13,14-dihydro-15-keto-prostaglandin F and neurophysin I/II around luteolysis in sheep

Plasma concentrations of neurophysin I/II (N-I/II), 13,14-dihydro-15-keto-prostaglandin F (PGFM) and progesterone were measured by radioimmunoassay in plasma samples collected from four sheep at hourly intervals between 0700 and 1900 h from Days 12–17 of the estrous cycle. Plasma samples were also collected from a fifth sheep at 2-hourly intervals during Days 12–16 of the cycle. In all sheep, intermittent surges in the plasma concentrations of PGFM and N-I/II occurred during the period of luteal regression. On at least one occasion in each sheep a surge in the plasma concentration of N-I/II was observed coincident with a rise in PGFM concentrations. In general, the highest levels of N-I/II were observed early in luteolysis (Days 13–14 of the cycle) while the corresponding levels of PGFM in plasma were maximal around Day 15 when luteolysis was well advanced.It is suggested from this temporal data that oxytocin, which is considered to be released in association with N-I/II, may play an important role in ovine luteolysis by stimulating the secretion of prostaglandin F from the uterus during Days 13–15 of the estrous cycle.     

Prostaglandin F in reproductive tissues collected during the luteal phase of the estrous cycle and at parturition in Virginia opossums (Didelphis virginiana )

Prostaglandin F(2alpha) (PGF(2alpha)) plays a role in the regression of the corpus luteum (CL) in a number of placental mammals. However, the mechanism of luteal regression has not been extensively studied in marsupials. The objectives of this study were to characterize changes in concentrations of PGF(2alpha) within utero-ovarian (UO) tissue/venous plasma during the luteal phase of the estrous cycle in Virginia opossums, to correlate these changes with those of plasma progesterone (P(4)), and to characterize the peripheral pattern of 13,14-dihydro-15-keto-PGF(2alpha) (PGFM) in parturient opossums. Ovaries, uteri, UO venous plasma and peripheral plasma were collected on Days 5, 9 and 12 after induced ovulation (n = 3 to 4 opossums/group). In addition, concentrations of PGFM were measured in peripheral plasma collected from two opossums during late gestation (Days 7,9,11 and 12) and at parturition (Day 13). Concentrations of P(4), PGFM and PGF(2alpha) in tissue homogenates and plasma samples were estimated by radioimmunoassay. In nonpregnant opossums, peripheral P(4) levels were highest on Day 5 (38.8 +/- 11.1 ng/ml, x +/- SEM) declined on Day 9 (22.6 +/- 7.4 ng/ml), and were at basal levels by Day 12 (2.4 +/- 0.7 ng/ml). Endometrial concentrations of PGF(2alpha) increased (P = 0.056) from Day 5 (15.7 +/- 4.1 ng/g) to Day 9 (92.1 +/- 61.0 ng/g) and were maintained to Day 12 (97.2 +/- 25.7 ng/g). Prostaglandin F(2alpha) concentrations in UO plasma increased (P < 0.01) from Day 5 (143.1 +/- 32.7 pg/ml) to Day 12 (333.0 +/- 32.4 pg/ml). Prostaglandin F(2alpha) concentrations in ovarian tissue followed a similar pattern and were correlated with UO concentrations (r = 0.708, P < 0.05). In pregnant opossums, the highest levels of peripheral PGFM were recorded in the peripartum period, when luteal regression would also be expected to occur. The negative temporal relationship between peripheral concentrations of P(4) and concentrations of PGF(2alpha) in UO tissue/venous plasma observed in this preliminary study is consistent with the notion that PGF(2alpha) from the ovary and/or uterus may play a role in CL regression in the opossum.     

Effect of luteinizing hormone (LH), pregnancy-specific protein B (PSPB), or arachidonic acid (AA) on secretion of progesterone and prostaglandins (PG) E (PGE; PGE(1) and PGE(2)) and F(2alpha) (PGF(2alpha)) by ovine corpora lutea of the estrous cycle or pregnancy in vitro

LH regulates luteal progesterone secretion during the estrous cycle in ewes and cows. However, PGE, not LH, stimulated ovine luteal progesterone secretion in vitro at day 90 of pregnancy and at day 200 in cows. The hypophysis is not obligatory after day 50 nor the ovaries after day 55 to maintain pregnancy in ewes. LH has been reported to regulate ovine placental PGE secretion up to day 50 of pregnancy and by pregnancy-specific protein B (PSPB) after day 50 of pregnancy. The objective of this experiment was to determine if and when a switch from LH to PGE occurred as the luteotropin regulating luteal progesterone secretion during pregnancy in ewes. Ovine luteal tissue slices of the estrous cycle (days 8, 11, 13, and 15) or pregnancy (days 8, 11, 13, 15, 20, 30, 40, 50, 60, and 90) were incubated in vitro with vehicle, LH, AA (precursor to PGE(2) and PGF(2alpha) synthesis), or PSPB in M199 for 4 h and 8 h. Concentrations of progesterone in jugular venous plasma of bred ewes increased (P< or =0.05) after day 50 and continued to increase through day 90. Secretion of progesterone by luteal tissue of non-bred ewes on days 8, 11, 13 and 15 and by bred ewes on days 8, 11, 13, 15, 20, 30, 40, and 50 was increased (P< or =0.05) by LH, but not by luteal tissue from pregnant ewes after day 50 (P> or =0.05). LH-stimulated progesterone secretion by luteal tissue from day 15 bred ewes was greater (P< or =0.05) than day 15 luteal tissue from non-bred ewes. Concentrations of progesterone in media were increased (P< or =0.05) when luteal tissue from pregnant ewes on day 50, 60, or 90 were incubated with AA or PSPB. Concentrations of PGE in media of non-bred ewes on days 8, 11, 13, or 15 and bred ewes on days 8 and 11 did not differ (P> or =0.05). Concentrations of PGE were increased (P< or =0.05) in media by luteal slices from bred ewes on days 13, 15, 20, 30, 40, 50, 60, and 90 of vehicle, LH, AA or PSPB-treated ewes. In addition, PSPB increased (P< or =0.05) PGE in media by luteal slices from pregnant ewes only on days 40, 50, 60, and 90. Concentrations of PGF(2alpha) were increased in media (P<0.05) of vehicle, AA, LH, or PSPB-treated luteal tissue from non-bred ewes and bred ewes on day 15 and by luteal tissue from bred ewes on days 20 and 30 after which concentrations of PGF(2alpha) in media declined (P< or =0.05) and did not differ (P> or =0.05) from non-bred or bred ewes on days 8, 11, or 13. It is concluded that LH regulates luteal progesterone secretion during the estrous cycle of non-bred ewes and up to day 50 of pregnancy, while only PGE regulates luteal progresterone secretion by ovine corpora lutea from days 50 to 90 of pregnancy. In addition, PSPB appears to regulate luteal secretion of progesterone from days 50 to 90 of pregnancy through stimulation of PGE secretion by ovine luteal tissue.     

Effects of progesterone and estradiol-17 beta on uterine secretion of prostaglandin F2 alpha in response to oxytocin in ovariectomized ewes

Twenty ovariectomized ewes were used in an experiment designed to examine the interaction of progesterone, estradiol, and oxytocin in the regulation of uterine secretion of prostaglandin F2 alpha (PGF2 alpha). All ewes underwent a steroid pretreatment that mimicked the changes in progesterone and estradiol which occur during the six days immediately prior to estrus. After pretreatment, ewes were randomly assigned to 1 of 4 treatment groups: 1) control (n = 4); 2) estradiol-17 beta (n = 6); 3) progesterone (n = 4); and 4) progesterone and estradiol-17 beta (n = 6). Progesterone was injected twice daily for 15 days. The dose of progesterone varied with day postestrus in a manner designed to simulate endogenous luteal secretion of progesterone. Estradiol-17 beta was administered in s.c. Silastic implants. The implants maintained circulating concentrations of estradiol at 3 pg/ml. On Days 5, 10, and 15 of treatment, ewes were injected with oxytocin (10 IU in 1.0 ml saline, i.v.). Jugular venous blood samples were collected beginning one-half hour prior to and continuing for 2 hours post-oxytocin injection for quantification of 13,14-dihydro-15-keto-prostaglandin F2 alpha (PGFM). No changes in concentration of PGFM following injection of oxytocin were observed on Day 5 or 10 in any treatment group. Concentrations of PGFM increased following injection of oxytocin on Day 15 only in groups receiving progesterone. Both the area under the PGFM response curve (p = 0.08) and peak response (p = 0.06) were greater in ewes treated with progesterone and estradiol-17 beta than in those receiving progesterone alone.(ABSTRACT TRUNCATED AT 250 WORDS)     

Influence of luteinizing hormone and prostaglandin F(2alpha) on progesterone secretion in superfused minced bovine luteal tissue in the early stage of the estrous cycle

Minced luteal tissue of bovine corpora lutea from Day 4, 5, and 6 of the estrous cycle (n = 4 corpora lutea each) was superfused for 9 h, and the progesterone secretion under the influence of 100 ng luteinizing hormone (LH)/ml and/or 1,000 ng prostaglandin F(2alpha) (PGF(2alpha))/ml was determined. In vivo, this period of the estrous cycle is characterized by a transition from PGF(2alpha) refractoriness to PGF(2alpha) sensitivity. The investigations were carried out in order to examine whether this transition is reflected by a change in the hormone secretion pattern in vitro. The basal secretion was higher on Day 6 than on Day 4 and 5 (P < 0.01). PGF(2alpha) slightly increased the progesterone secretion, but there was no statistically significant difference (P > 0.05). LH, however, stimulated the progesterone secretion by about 30% in luteal tissue collected from Day 4 and 5 (P < 0.01). In luteal tissue collected from Day 6, the LH-induced increase in hormone secretion was not statistically significant due to two corpora lutea that showed no response at all to LH. The progesterone secretion of the two other corpora lutea, however, was increased by 30% (P < 0.01). When PGF(2alpha) and LH were simultaneously added, the LH-induced progesterone secretion was not inhibited; PGF(2alpha) even seemed to intensify the action of LH. The difference between the hormone secretion under the influence of LH alone and that under the influence of a combination of LH and PGF(2alpha), however, was not statistically significant. It is concluded that in cattle the end of the refractoriness to PGF(2alpha) in vivo is not reflected by a corresponding change of the hormone secretion pattern in vitro.     

Differential effects of progesterone treatment on the oxytocin-induced prostaglandin F2 alpha response and the levels of endometrial oxytocin receptors in ovariectomized ewes.

The oxytocin-induced uterine prostaglandin (PG) F2 alpha response and the levels of endometrial oxytocin receptors were measured in ovariectomized ewes after they had been given steroid pretreatment (SP) with progesterone and estrogen to induce estrus (day of expected estrus = Day 0) and had subsequently been treated with progesterone over Days 1-12 and/or PGF2 alpha over Days 10-12 postestrus. The uterine PGF2 alpha response was measured after an i.v. injection of 10 IU oxytocin on Days 13 and 14, using the PGF2 alpha metabolite, 13,14-dihydro-15-keto-PGF2 alpha (PGFM), as an indicator for PGF2 alpha release. The levels of oxytocin receptors in the endometrium were measured on Day 14. During the treatment with progesterone, the peripheral progesterone concentrations were elevated and remained above 1.8 ng/ml until the morning of Day 14. The PGFM responses to oxytocin in untreated controls and SP controls were low on both Days 13 and 14 whereas the levels of endometrial oxytocin receptors in the same ewes were high. Treatment with progesterone either alone or in combination with PGF2 alpha significantly (p less than 0.04) increased the PGFM response on Day 14 and reduced the levels of endometrial oxytocin receptors; treatment with PGF2 alpha alone had no effect. It is concluded that progesterone promotes the PGFM response to oxytocin while simultaneously suppressing the levels of endometrial oxytocin receptors. PGF2 alpha treatment had no effect on either the uterine secretory response to oxytocin or the levels of oxytocin receptors in the endometrium.(ABSTRACT TRUNCATED AT 250 WORDS)