Concentrations of 13, 14-dihydro-15-keto prostaglandin F2 alpha after pulsatile progesterone administration at the time of luteolysis of heifers

Progesterone was administered in pulses to 12 dairy heifers from days 17.5 to 22.5 post-estrus in order to determine its ability to modify secretion of PGF2 alpha around the time of luteolysis. Control heifers exhibited pulses of PGFM concomitant with a sharp decline in progesterone concentrations and thus these pulses were temporally associated with luteolysis. Additional pulses of PGFM were observed in heifers receiving exogenous progesterone, but these were not statistically predictable by either dose of progesterone (50 or 100 micrograms) or time of administration (3 or 6 hour intervals). However, all heifers (4/4) treated with progesterone at 3 hour intervals had additional pulses of PGFM as compared to only one heifer (1/4) treated at 6 hour intervals. When pulses of PGFM were induced by exogenous progesterone there was a substantial lag time between the initiation of progesterone treatment and their occurrence. The limited response to progesterone administration and the lack of synchrony is not consistent with an ability of exogenous progesterone to directly stimulate secretion of PGF2 alpha at the time of luteolysis.     

Effects of progesterone administered to dairy heifers on sensitivity of corpora lutea to PGF(2)alpha and on plasma LH concentration

To determine whether progesterone facilitates PGF(2)alpha-induced luteolysis prior to day 5 of the estrous cycle, 48 Holstein-Friestian heifers were assigned at random to four treatments: 1) 4 ml corn oil/day + 5 ml Tris-HCl buffer (control); 2) 25 mg prostaglandin F(2)alpha (PGF(2)alpha); 3) 100 mg progesterone/day (progesterone); 4) 100 mg progesterone/day + 25 mg PGF(2)alpha (combined treatment). Progesterone was injected subcutaneously daily from estrus (day 0) through day 3. The PGF(2)alpha was injected intramuscularly on day 3. Estrous cycle lengths were decreased by progesterone: 20.2 +/- 0.56, 19.2 +/- 0.31 (control and PGF(2)alpha); 13.2 +/- 1.40, and 11.7 +/- 1.27 (progesterone and combined). The combination of progesterone and PGF(2)alpha did not shorten the cycle any more than did progesterone alone (interaction, P>0.05). PGF(2)alpha treatment reduced progesterone concentrations on day 6 (P<0.05) and both progesterone and PGF(2)alpha reduced plasma progesterone on day 8 (P<0.01 and P<0.05, respectively). LH was measured in blood samples collected at 10- min intervals for 4 hr on day 4 from three heifers selected at random from each of the four treatment groups. Mean LH concentration for control heifers ranged from 0.35 to 0.63 ng/ml (overall mean, 0.49 ng/ml) and for progesterone-treated heifers ranged from 0.12 to 0.30 ng/ml (overall mean, 0.23 ng/ml). LH concentrations were greater in control heifers (P<0.01). The mean LH pulse rate for control heifers was 2.7 pulses/heifers/4 hr, while that for the progesterone-treated heifers was 1.7 pulses/heifer/4 hr. The mean pulse amplitude for control and progesterone treatments was 0.47 ng/ml and 0.36 ng/ml, respectively. Neither pulse amplitude nor frequency were different between treatment groups.     

Temporal associations among pulses of 13,14-dihydro-15-keto-PGF2alpha, luteal blood flow, and luteolysis in cattle

Luteal blood flow was studied in heifers by transrectal color-Doppler ultrasound. Data were normalized to the decrease in plasma progesterone to <1 ng/ml (Day 0 or Hour 0). Blood flow in the corpus luteum (CL) was estimated by the percentage of CL area with color flow signals. Systemic prostaglandin F2alpha (PGF) treatment (25 mg; n=4) resulted in a transient increase in CL blood flow during the initial portion of the induced decrease in progesterone. Intrauterine treatment (1 or 2 mg) was done to preclude hypothetical secondary effects of systemic treatment. Heifers were grouped into responders (luteolysis; n=3) and nonresponders (n=5). Blood flow increased transiently in both groups; induction of increased blood flow did not assure the occurrence of luteolysis. A transient increase in CL blood flow was not detected in association with spontaneous luteolysis when examinations were done every 12 h (n=6) or 24 h (n=10). The role of PGF pulses was studied by examinations every hour during a 12-h window each day during expected spontaneous luteolysis. At least one pulse of 13,14-dihydro-15-keto-PGF2alpha (PGFM) was identified in each of six heifers during the luteolytic period (Hours -48 to -1). Blood flow increased (P<0.02) during the 3-h ascending portion of the PGFM pulse, remained elevated for 2 h after the PGFM peak, and then decreased (P<0.03) to baseline. Results supported the hypothesis that CL blood flow increased and decreased with individual PGFM pulses during spontaneous luteolysis.     

Release of prostaglandin F-2 alpha and the timing of events associated with luteolysis in ewes with oestrous cycles of different lengths

Ewes (N = 32) were bled every 2 h from 5 days before expected oestrus until the end of oestrus. Plasma concentrations were determined for progesterone to monitor luteal activity and for the prostaglandin F-2 alpha (PGF-2 alpha) metabolites, 15-keto-13,14-dihydro-PGF-2 alpha and 11-ketotetranor-PGF to determine uterine synthesis and release of PGF-2 alpha. Most of the variation in cycle length was associated with the time of onset of luteolysis, the timing of events after luteolysis being constant and not related to cycle length. The time of occurrence of the first PGF-2 alpha pulse and the interval between this pulse and the start of luteolysis were the two main determinants responsible for oestrous cycle length. Several PGF-2 alpha pulses with interpulse intervals of 15.9 h occurred before the onset of functional luteolysis compared with 7.7 h for pulses associated with luteolysis. The numbers of PGF-2 alpha pulses and interpulse intervals were similar for oestrous cycles of different lengths. While a gradual decline in progesterone concentrations was observed before functional luteolysis in the ewes with longer cycles, this did not appear to be an integral part of the stimulus which initiates the pulse frequency of PGF-2 alpha required for luteolysis. We therefore suggest that differences in oestrous cycle length in the ewe are determined by the time of the onset of PGF-2 alpha pulsatile release, and especially by the time of increased pulse frequency.     

Effect of prostaglandin F2alpha dosage and stage of estrous cycle on the estrous response and corpus luteum function in beef heifers

Ninety-five normal cyclic crossbred beef heifers were used to determine if the proportions of heifers showing estrus, intervals to estrus and corpus luteum (CL) function were influenced by PGF(2alpha) dosage and (or) the stage of luteal phase when PGF(2alpha) was administered. Heifers were assigned randomly to treatments in a 4 x 3 factorial arrangement. Treatments were 5, 10, 25 or 30 mg PGF(2alpha) injected either in early (5 to 9 d), mid (10 to 14 d) or late (15 to 19 d) stages of the luteal phase. Jugular samples were taken at 0 h and at 8 h-intervals for 48 h and again at 60 h after PGF(2alpha) treatment for progesterone assay. Heifers were observed for estrus continuously for 120 h PGF(2alpha) treatment. The proportion of heifers showing estrus was dependent upon (P<0.05) both dosage of PGF(2alpha) and stage of luteal phase. Heifers given 5 mg of PGF(2alpha) showed estrus only if treated during the late stage, while those given 10 mg of PGF(2alpha) showed a progressive increase of heifers in estrus as stage of luteal phase advanced. The proportion of heifers showing estrus after 25 and 30 mg of PGF(2alpha) increased from 56% for the early stage to 100% for the mid and late stages. Interval to estrus in heifers showing estrus within 120 h after PGF(2alpha) treatment did not differ (P>0.05) among dosages but tended (P=0.10) to be longer in heifers treated during the mid luteal stage (67 h) than in heifers treated in the two other stages (56 h). A greater proportion of heifers (P<0.05) showed estrus by 60 h after PGF(2alpha) when treated during the early and late luteal stages (75.5%) than for heifers treated during the mid luteal stage (30.4%). Patterns of progesterone concentrations were influenced (P=0.08) by the three way interaction of dosage, stage and time. In heifers that showed estrus, rate of decline in progesterone tended (P=0.07) to be slower during the mid luteal stage than during the early and late stages. Progesterone did not drop below 1 ng/ml until 32 h in heifers treated during the mid luteal stage; whereas progesterone dropped below 1 ng/ml by 24 h in heifers treated during the early and late stages. These data may be useful in designing more efficient systems for using PGF(2alpha) or its analogues in estrus synchronization of beef cattle.     

Modification of prostaglandin F-2 alpha synthesis and release in the ewe during the initial establishment of pregnancy

Pregnant (N = 10) and non-pregnant (N = 10) ewes were bled every 2 h from Days 12 to 17 after oestrus (oestrus = Day 0). Plasma concentrations of progesterone, 15-keto-13,14-dihydro-PGF-2 alpha and 11-ketotetranor-PGF metabolites were determined in all samples. The number of PGF-2 alpha pulses in non-pregnant ewes was 8.2 +/- 0.4 (mean +/- s.e.m.) with an interpulse interval of 10.7 +/- 0.7 h. Two or 3 pulses of low frequency (interpulse interval = 13.4 +/- 1.6 h) occurred in most non-pregnant ewes before the onset of luteolysis; the interpulse interval then decreased to 7.9 +/- 0.4 h for the 6.0 +/- 0.3 pulses temporally associated with luteolysis. In contrast, the number of PGF-2 alpha pulses in pregnant ewes was lower (2.5 +/- 0.7, 0-8) and the interpulse intervals longer (18.9 +/- 6.1 h). Most pulses occurred on Days 14 and 15 in the pregnant and non-pregnant ewes. The mean concentrations of both PGF-2 alpha metabolites in non-pregnant ewes were highest on Day 15 while basal levels of both metabolites remained constant at all times. In pregnant ewes, the mean concentrations of both metabolites were highest on Day 14; basal concentrations of both metabolites were also highest on Day 14. The mean concentrations of 15-keto-13,14-dihydro-PGF-2 alpha were higher in pregnant than in non-pregnant ewes on Days 13 and 14 (P less than 0.05) and higher in non-pregnant than pregnant ewes on Day 15 (P less than 0.05). The basal concentrations of the 15-keto metabolite were higher in pregnant than non-pregnant ewes at Days 13, 14, 15, 16 and 17 (P less than 0.05). Both the mean and the basal concentrations of 11-ketotetranor-PGF metabolites were higher in pregnant than in non-pregnant ewes on Day 14 (P less than 0.05). It is concluded that uterine production of PGF-2 alpha peaks at Days 14-15 after oestrus in pregnant and non-pregnant ewes. Patterns of release differ, however, in that non-pregnant ewes have a pulsatile PGF-2 alpha pattern superimposed on a constant baseline, while pregnant ewes have an increasing basal secretory pattern which is more nearly continuous, i.e. not pulsatile in form. Modification of pulsatile PGF-2 alpha synthesis and release is therefore a key aspect of prolongation of luteal function at the beginning of pregnancy in the ewe.     

Response of dairy heifers to Prostaglandin F2alpha after treatment with human chorionic gonadotropin

After the observation of estrus following administration of Prostaglandin F(2alpha) (PGF(2alpha)), 79 dairy heifers were randomly either injected with 2500 IU of human chorionic gonadotropin (hCG) 48 h postestrus or maintained as controls with no injection at that time. Five to 9 d later, after a blood sample for progesterone determination was taken, all heifers were injected with 25 mg of PGF(2alpha). Heifers observed in estrus within the next 5 d were inseminated about 12 h after initial observation and were palpated for pregnancy 45 to 60 d postinsemination. Heifers treated with hCG had higher progesterone concentrations, reduced and delayed estrual responses, and lower insemination fertility rates when compared with control heifers.     

Effect of PGF2alpha and 13,14-dihydro-15-keto-PGF2alpha (PGFM) on corpora luteal function in nonpregnant mares

The objective of this study was to determine if the primary circulating metabolite of PGF2alpha, 13,14-dihydro-15-keto-PGF2alpha (PGFM), is biologically active and would induce luteolysis in nonpregnant mares. On Day 9 after ovulation, mares (n = 7/group) were randomly assigned to receive: 1) saline control, 2) 10 mg PGF2alpha or 3) 10 mg PGFM in 5 mL 0.9% sterile saline i.m. On Days 0 through 16, blood was collected for progesterone analysis. In addition, blood was collected immediately prior to treatment, hourly for 6 h, and then at 12 and 24 h after treatment for progesterone and PGFM analysis; PGFM was measured to verify that equivalent amounts of hormone were administered to PGF2alpha- and PGFM-treated mares. Mares were considered to have undergone luteolysis if progesterone decreased to < or = 1.0 ng/mL within 24 h following treatment. Luteolysis was induced in 0/7 control, 7/7 PGF2alpha-treated, and 0/7 PGFM-treated mares. There was no difference (P>0.1) in the occurrence of luteolysis in control and PGFM-treated mares. More (P<0.001) PGF2alpha-treated mares underwent luteolysis than control or PGFM-treated mares. There was no difference (P>0.1) in progesterone concentrations between control and PGFM-treated mares on Days 10 through 16. Progesterone concentrations were lower (P<0.01) on Days 10 through 14 in PGF2alpha-treated compared with control and PGFM-treated mares. There was no difference (P>0.05) in PGFM concentrations between PGF2alpha- and PGFM-treated mares; PGFM concentrations in both groups were higher (P<0.001) than in control mares. These results do not support the hypothesis that PGFM is biologically active in the mare, since there was no difference in corpora luteal function between PGFM-treated and control mares.     

Norgestomet- and oestradiol valerate-induced luteolysis is dependent upon the uterus

Beef heifers were assigned to three groups: (1) untreated controls (n=4), (2) Syncro-Mate B(R) (SMB)-treated (n=5), and (3) hysterectomized and SMB-treated (n=4). SMB was administered 8 or 9 days after oestrus, approximately 30 days after hysterectomy. This study was conducted to determine if the uterus was necessary for SMB to induce luteolysis. SMB induced premature luteolysis as only 20% of the intact SMB-treated heifers had >/=0.75 ng/ml of progesterone 7 days after the time of SMB treatment, compared to all (100%) of the untreated heifers (p<0.05). By 9 days after the time of SMB treatment, 25% of the untreated heifers and none (0%) of the intact SMB-treated heifers had >/=0.75 ng/ml of progesterone; however, all (100%) of the hysterectomized SMB-treated heifers had >/=0.75 ng/ml of progesterone (p<0.05). Therefore, SMB-induced luteolysis required the involvement of the uterus. The luteolysin, prostaglandin F(2alpha), is probably the secretion from the uterus that mediates the SMB-induced luteolysis. SMB treatment, however, required 7-8 days to induce luteolysis.     

Decreased pulsatile LH secretion in heifers superovulated with eCG or FSH

This study was designed to test the hypothesis that treatment with super-ovulatory drugs suppresses endogenous pulsatile LH secretion. Heifers (n=5/group) were superovulated with eCG (2500 IU) or FSH (equivalent to 400 mg NIH-FSH-P1), starting on Day 10 of the estrous cycle, and were injected with prostaglandin F(2alpha) on Day 12 to induce luteolysis. Control cows were injected only with prostaglandin. Frequent blood samples were taken during luteolysis (6 to 14 h after PG administration) for assay of plasma LH, estradiol, progesterone, testosterone and androstenedione. The LH pulse frequency in eCG-treated cows was significantly lower than that in control cows (2.4 +/- 0.4 & 6.4 +/- 0.4 pulses/8 h, respectively; P<0.05), and plasma progesterone (3.4 +/- 0.4 vs 1.8 +/- 0.1 ng/ml, for treated and control heifers, respectively; P<0.05) and estradiol concentrations (25.9 +/- 4.3 & 4.3 +/- 0.4 pg/ml, for treated and control heifers, respectively; P<0.05) were higher compared with those of the controls. No LH pulses were detected in FSH-treated cows, and mean LH concentrations were significantly lower than those in the controls (0.3 +/- 0.1 & 0.8 +/- 0.1, respectively; P<0.05). This suppression of LH was associated with an increase in estradiol (9.5 +/- 1.4 pg/ml; P<0.05 compared with controls) but not in progesterone concentrations (2.1 +/- 0.2 ng/ml; P>0.05 compared to controls). Both superovulatory protocols increased the ovulation rate (21.6 +/- 3.9 and 23.0 +/- 4.2, for eCG and FSH groups, respectively; P>0.05). These data demonstrate that super-ovulatory treatments decrease LH pulse frequency during the follicular phase of the treatment cycle. This could be explained by increased steroid secretion in the eCG-trated heifers but not in FSH-treated animals.     

Influence of perioestrous suprabasal progesterone levels on cycle length,oestrous behaviour and ovulation in heifers

In order to induce suprabasal plasma concentrations of progesterone after luteolysis and to determine their effect on oestrous behaviour and ovulation, heifers subcutaneously received silicone implants containing 2.5 (n = 4), 5 (n = 4), 6 (n = 3), 7.5 (n = 3) or 10 (n = 4) g of progesterone, or an empty implant (controls, n = 5) between days 8 and 25 of the cycle (ovulation designated Day 0). Growth of dominant follicles and time of ovulation were determined by ultrasound, and signs of oestrus were recorded and scored. Blood was collected at 2–4 h intervals from Days 15 to 27 and assayed for progesterone concentration. In all heifers, plasma concentrations of progesterone sharply decreased during Days 16–18. Control heifers had their lowest progesterone levels on Days 20.5 and 21, standing oestrus on Day 19.5 ± 0.4 (mean ± SEM), and ovulated on Day 20.7 ± 0.4. A similar pattern was observed in heifers treated with 2.5 and 5 g progesterone. Heifers treated with 6, 7.5 and 10 g of progesterone showed an extended (P < 0.05) interovulatory interval. Onset of prooestrus and time of maximum expression of signs of oestrus were not significantly different from those in controls. However, there was an absence of standing oestrus in most of the cases, signs of oestrus lasted longer (P < 0.05) and were weaker in intensity when doses increased. In these groups, the lowest progesterone concentrations were attained shortly after implant removal. Some heifers treated with 6 and 7.5 g of progesterone had standing oestrus and post oestrous bleeding as seen in the controls but ovulation occurred from Days 24.5 to 27. When plasma progesterone concentrations were over 1 nmol 1⁻¹, disturbed oestrus and delayed ovulation occurred. The extended period of prooestrus and oestrus and delayed ovulation were similar to that described in cases of repeat breeding. It is suggested that suprabasal plasma concentrations of progesterone, after luteolysis, may lead to asynchrony between onset of oestrus and ovulation and consequently be a cause of repeat breeding in cattle.     

Premature prostaglandin F2alpha secretion causes luteal regression in GnRH-induced short estrous cycles in cyclic dairy heifers

This study aimed to confirm that the luteolysis in normal-cycling dairy heifers seen during short estrous cycles induced with cloprostenol (Clp) and GnRH administered 24h apart is caused by a premature release of prostaglandin F(2alpha) (PGF(2alpha)). A further aim was to study the PGF(2alpha) release pattern more closely to determine whether it resembles the spontaneous release occurring during normal regression of the corpus luteum (CL) or whether PGF(2alpha) is continuously secreted after the induced ovulations, leading to short estrous cycles. Twenty-four Ayrshire heifers were allotted to four equally sized groups. After estrus synchronization with 0.5mg of Clp, a new luteolysis was induced with 0.5mg of Clp on Day 6 (groups T-d6 and C-d6) or Day 7 (groups T-d7 and C-d7) after ovulation. Gonadorelin (0.1mg i.m.) was given to groups T-d6 and T-d7 to induce premature ovulation 24h later. Groups C-d6 and C-d7 served as controls. Ovaries were examined daily by transrectal ultrasonography, while blood samples (for progesterone and 15-ketodihydro-PGF(2alpha) analyses) were obtained via a jugular catheter every 3h, starting from the second Clp treatment and continuing for 9 days postovulation. Unresponsiveness to Clp or anovulation resulted in 4 C-d6 heifers being excluded. Four heifers in group T-d6 and three in group T-d7 had a short estrous cycle of 8-12 days, while all others had a cycle of normal length. Significant elevations in 15-ketodihydro-PGF(2alpha) concentrations with recurrent high peaks coincided with a decrease in progesterone concentration and were detected in all heifers that showed a short estrous cycle, but not in any heifers with normal estrous cycles in groups T and C. In conclusion, a premature release of PGF(2alpha), which closely resembles its release during spontaneous luteolysis, causes luteal regression in these short cycles.     

Effect of stage of the estrous cycle on interval to estrus after PGF(2alpha) in beef cattle

Effect of stage of the estrous cycle at the time of prostaglandin F(2alpha) (PGF(2alpha)) injection on subsequent reproductive events in beef females was studied in four trials involving 194 animals. Cycling animals were given two injections of 25 mg PGF(2alpha) 11 days apart or, in some cases, the interval was altered to allow the second injection to fall on a specific day of the cycle. Day of estrous cycle at time of the second injection was determined by estrous detection. Interval from the second PGF(2alpha) injection to the onset of estrus (interval to estrus) was shorter (P<.01) in heifers than in cows. Both cows and heifers injected on days 5 to 9 (early cycle) had a shorter (P<.01) interval to estrus (estrus = day 0) than did those injected on days 10 to 15 (late cycle). Conception rate was lower (P<.05) for early-cycle heifers than for late-cycle heifers inseminated by appointment at 80 hours. There was no significant difference in conception rate of early-or late-cycle heifers or cows inseminated according to estrous detection or early- or late-cycle cows inseminated at 80 hours. Progesterone concentrations in blood samples collected in heifers at 4-hour intervals after the second PGF(2alpha) injection on either day 7 or day 14 declined linearly (P<.05) through 36 hours. Day of the estrous cycle at PGF(2alpha) injection had no effect on rate of progesterone decline, even though heifers injected on day 7 had a shorter (P<.05) interval to estrus. All animals whose cycle length was not affected by the second PGF(2alpha) injection were treated on days 5 through 8 of the cycle, indicating that PGF(2alpha) was less effective in regressing the corpus luteum between days 4 and 9 of the cycle than later in the cycle.     

Interrelationship between plasma estradiol concentration and oxytocin-induced PGF2alpha release in heifers

The objective of this study was to determine if the increase in responsiveness to oxytocin toward the time of luteolysis was correlated with an increase in plasma estradiol in the cow. Six heifers each had a cannula placed in the jugular vein on Day 14 of the estrous cycle. Then, beginning on Day 15, growth of the largest follicles was determined by ultrasonography, and a blood sample was taken via the cannula for the measurement of progesterone and estradiol by radioimmunoassay (RIA). After the first blood sample, 3 more samples were taken at 10-min intervals, 100 IU oxytocin were injected into the vein, and a further 3 blood samples were taken at 15, 30 and 60 min after injection. The concentration of 13,14-dihydro-15-keto prostaglandin F2alpha (PGFM) was measured in these frequent samplings and was used to determine the ability of oxytocin to stimulate PGF2alpha release from the uterus. This procedure was repeated daily for at least 7 d. The results showed that the response to oxytocin increased before luteolysis and that there was a significant increase in the response to oxytocin (P<0.05) before any changes in plasma estradiol or progesterone were detected. These data show that an increase in estradiol secretion from the ovulatory follicle does not appear to initiate luteolysis.     

Jugular plasma concentrations of 13,14-dihydro-15-keto-prostaglandin F2 alpha in prepubertal beef heifers treated with progestogen then challenged with oxytocin.

Prepubertal Angus crossbred heifers (n = 24) between 8 and 10 mo of age were used to determine if progestogen treatment would enhance jugular concentrations of 13,14-dihydro-15-keto-prostaglandin F2 alpha (PGFM) after oxytocin (OT) injections. Heifers were stratified by age and weight and allotted to randomized treatments in a 2 x 2 factorial arrangement. Heifers were treated with either a norgestomet (NOR) implant (6 mg) for 9 d or no implant (0 mg; BLK). On d 8 of NOR treatment, jugular veins were catheterized and, on d 9, blood samples were collected every 15 min for 165 min. The first four samples were used to determine basal PGFM concentrations (an indirect measure of uterine PGF2 alpha release). After collection of the fourth sample, either OT (100 IU) or saline (0 IU; SAL) was injected via the jugular catheter. After the 165-min sample was collected, NOR implants were removed. Beginning 48 h after implant removal, a second 165- min blood sampling period was initiated. Average progesterone concentrations were less than 1 ng/ml during both bleeding periods. Within treatment, PGFM concentrations were similar between the first and second sampling periods; therefore, data within treatment were combined. Basal PGFM concentrations were higher (P < .01) in NOR-treated than in BLK heifers. Oxytocin did not increase PGFM concentrations in BLK-OT heifers; however, a marked increase in PGFM was detected in the NOR-OT heifers in response to oxytocin. Average PGFM concentration was greatest (P < .0001) in NOR-OT heifers, and PGFM profiles differed (P < .0001) between NOR-OT and each of the other treatment groups. Results from this study indicate that NOR increases basal PGFM and may "condition" the uterus to respond to OT in prepubertal heifers.     

Estrus synchronization in Holstein cows using reduced doses of prostaglandin F(2)alpha

There is evidence that the dose of PGF(2)alpha generally used to synchronize estrus (25 mg) is higher than required to induce luteolysis in cattle. To investigate this, 98 Holstein cows from three farms were assigned at random within farm to be treated with a single dose of 25 mg (n=33), 17.5 mg (n=33) or 10 mg (n=32) of PGF(2)alpha on Day 10+/-0.5 (mean +/- SEM) of the estrous cycle. Statistical analyses were conducted using analyses of variance and Chisquare test. Only 59.3% of the cows treated with 10 mg of PGF(2)alpha were detected in estrus compared with 72.7 and 78.7% of the cows treated with 17.5 and 25.0 mg doses, respectively (P>0.05). There were no differences (P>0.05) in pregnancy rates at the first service (40.0, 66.6 and 50.0% for 25, 17.5 and 10 mg, respectively). Concentrations of progesterone in blood were different (P<0.05) for cows treated with 10 mg compared with those of cows treated with 17.5 or 25 mg of PGF(2)alpha. The pattern of changes in progesterone concentrations between the last two groups was not different, and progesterone concentrations of less than 1 ng/ml of serum were observed within the first 36 h post PGF(2)alpha administration. In cows treated with 10-mg dose of PGF(2)alpha, concentrations of progesterone declined during the first 24 h, however, by the end of the experimental period, they were not different to pretreatment concentrations (treatment x time; P<0.05). It is suggested that reducing the dose of PGF(2)alpha from 25 to 17.5 mg do not affect estrus response or pregnancy rate in Holstein cows.     

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.     

Effects of progesterone and oestradiol-17 beta on oxytocin-induced release of prostaglandin F-2 alpha in heifers

Seven bilaterally ovariectomized heifers were used in 4 experiments and received: (1) saline injections, as control; (2) one injection of oestradiol (3 mg; i.v.); (3) two i.v. injections of oxytocin (100 i.u.) 6 h apart; or (4) one oestradiol injection 30 min after the first oxytocin injection and a second oxytocin injection 6 h later. All experiments were performed without progesterone and then after 7, 14 and 21 days of progesterone treatment. Frequent blood samples were taken for 1 h before and 7 h after the first injection of oxytocin or oestradiol for the measurement of 13,14-dihydro-15-keto-PGF-2 alpha (PGFM) by radioimmunoassay. After 7, 14 and 21 days of progesterone priming, oestradiol caused a significant increase (P less than 0.001) in plasma PGFM after 6 h but not before. After 7, 14 and 21 days of progesterone, there was a significant increase (P less than 0.005) in PGFM after the first oxytocin injection and a similar increase following the second. The oxytocin-induced increase in PGFM after 14 and 21 days of progesterone was significantly higher (P less than 0.001) 6 h after oestradiol injection than before the oestradiol injection. There was no significant effect of oestradiol on the response to oxytocin in animals that received no progesterone or in those animals that received progesterone for only 7 days. These results show that, under the influence of progesterone, oestradiol enhances the oxytocin-induced release of PGF-2 alpha, and suggest a possible synergistic action of these hormones for the induction of luteolysis in heifers.     

Effect of dose of prostaglandin F(2alpha) on steroidogenic components and oligonucleosomes in ovine luteal tissue

To determine whether prostaglandin (PG) F(2alpha) had a dose-dependent effect upon secretion of progesterone, oligonucleosome formation, or loss of luteal weight, ewes on Day 9 or 10 of the estrous cycle were administered 0, 3, 10, or 30 mg PGF(2alpha) per 60 kg BW (i.v.), and luteal tissue was collected 9 and 24 h after injection. All doses of PGF(2alpha) decreased (P < 0. 05) concentrations of progesterone in sera by 9 h; however, in ewes treated with 3 mg PGF(2alpha), concentrations of progesterone were similar to control values at 24 h and higher (P < 0.05) than those in the 10- or 30-mg groups. Concentrations of progesterone in sera over all dose levels were highly correlated to luteal concentrations of mRNA encoding steroidogenic acute regulatory protein (P < 0.001), cytochrome P450 side-chain cleavage (P < 0.02), and 3beta-hydroxysteroid dehydrogenase (P < 0.01). Corpora lutea collected at 24 h from ewes treated with the 10- and 30-mg doses of PGF(2alpha) weighed less (P < 0.05) than those from controls. Oligonucleosomes were not present in luteal tissues from control ewes. Surprisingly, all doses of PGF(2alpha)-induced oligonucleosomes in a majority of animals at 9 h and in a majority of ewes treated with 10 and 30 mg of PGF(2alpha) at 24 h. In conclusion, 3 mg of PGF(2alpha) per 60 kg BW transiently decreased serum concentrations of progesterone and induced oligonucleosome formation, but did not result in reduced luteal weight. The 10- and 30-mg doses of PGF(2alpha) decreased secretion of progesterone and induced oligonucleosome formation and luteolysis.     

Effect of continuous infusion of oxytocin on length of the oestrous cycle and luteolysis in cattle

In Exp. I oxytocin (60 micrograms/100 kg/day) was infused into the jugular vein of 3 heifers on Days 14-22, 15-18 and 16-19 of the oestrous cycle respectively. In Exp. II 5 heifers were infused with 12 micrograms oxytocin/100 kg/day from Day 15 of the oestrous cycle until clear signs of oestrus. Blood samples were taken from the contralateral jugular vein at 2-h intervals from the start of the infusion. The oestrous cycle before and after treatment served as the controls for each animal. Blood samples were taken less frequently during the control cycles. In Exp. III 3 heifers were infused with 12 micrograms oxytocin/100 kg/day for 50 h before expected oestrus and slaughtered 30-40 min after the end of infusion for determination of oxytocin receptor amounts in the endometrium. Three other heifers slaughtered at the same days of the cycle served as controls. Peripheral concentrations of oxytocin during infusion ranged between 155 and 641 pg/ml in Exp. I and 18 and 25 pg/ml in Exp. II. In 4 our of 8 heifers of Exps I and II, one high pulse of 15-keto-13,14-dihydro-prostaglandin F-2 alpha (PGFM) appeared soon after the start of oxytocin infusion followed by some irregular pulses. The first PGFM pulse was accompanied by a transient (10-14 h) decrease of blood progesterone concentration. High regular pulses of PGFM in all heifers examined were measured between Days 17 and 19 during spontaneous luteolysis. No change in length of the oestrous cycle or secretion patterns of progesterone, PGFM and LH was observed.(ABSTRACT TRUNCATED AT 250 WORDS)