ACTH treatment disrupts ovarian IGF-I and steroid hormone production.

Hyper-adrenal activity and increased glucocorticoid hormone release are associated with disruptions in reproductive function and adverse effects on the ovary. The aim of this investigation was to determine whether elevated glucocorticoid hormone levels can influence ovarian IGF-I synthesis and action in vivo. To elevate endogenous glucocorticoid levels, gilts were treated with ACTH during the luteal phase of the oestrous cycle (days 9-13) while the control group received saline. The gilts were subsequently ovariectomized on either day 14 or day 18 of the oestrous cycle. Follicular fluid (FF) was collected from individual follicles; IGF-I and steroid hormone concentrations were determined by radioimmunoassay, and IGF-binding protein (IGFBP) expression was assessed by Western ligand blotting. Granulosa cells were also recovered and placed in culture to determine IGF-I, progesterone (P(4)) and oestradiol-17beta (E(2)) production levels. The cells were cultured in serum-free medium for 5 days and supplemented with: (a) media alone, (b) IGF-I, (c) FSH and androstenedione (A(4)), or (d) IGF-I with FSH and A(4). The FF from ACTH-treated gilts was characterized by elevated (P<0.05) cortisol levels on day 14 and lower (P<0.05) E(2) values on both day 14 and day 18. Lower (P<0.05) IGF-I concentrations were also measured in the FF of ACTH-treated gilts collected on day 18. This altered hormone profile in FF was associated with impaired IGF-I and steroid hormone synthesis by granulosa cells. IGF-stimulated P(4) production (P<0.01) by cells recovered from ACTH-treated gilts on day 14 was lower (P<0.05). By day 18, IGF-I, P(4) and E(2) production by cells from the ACTH group were all significantly (P<0. 05) lower. These results demonstrate that increased glucocorticoid concentrations can disrupt subsequent ovarian IGF-I synthesis and IGF action in vivo and can, potentially, impair follicle maturation.

Evaluation of the effect of GnRH on follicular ovarian cysts in dairy cows using trans-rectal ultrasonography.

The objectives of this study were to evaluate ovarian changes in cows with follicular ovarian cysts following treatment with either GnRH or saline. The parameters determined were the intervals from treatment to observation of a CL and from treatment to disappearance of the cyst, and the association between serum concentrations of LH, FSH and the LH/FSH ratio, before and after treatment, with the test intervals. Thirty-nine cows were identified as having follicular cysts. The GnRH treatment induced a significant increase in LH and the LH/FSH ratio. The gonadotropin response was not associated with the intervals from treatment to CL detection or to disappearance of the cyst. Survival curves for the intervals from treatment to CL detection and cyst disappearance indicate that treatment with GnRH or saline did not yield significantly different results for either parameter. The results question the efficacy of treating cystic ovarian disease with GnRH.

Glucocorticoid influence on porcine granulosa cell IGF-I and steroid hormone production in vitro.

The effect of cortisol on granulosa cell (GC) insulin-like growth factor I (IGF-I) synthesis, and IGF-mediated steroid production was examined at various stages of follicle maturation. Granulosa cells were recovered from gilts on Days 14, 18, and 20 of the estrous cycle, while luteinizing GC were recovered on Day 21, just prior to ovulation. The cells were cultured in serum-free medium with increasing concentrations of cortisol (0, 1, 10, and 100 microg/mL) for 5 d with or without IGF-I stimulation (10 ng/mL). During culture all cells were supplemented with FSH and androstenedione (A4). Cellular IGF-I, progesterone (P4) and estradiol-17beta (E2) production was determined by specific radioimmunoassays (RIA), and cell proliferation was assessed. Granulosa cell IGF-I and steroid hormone synthesis increased (P<0.05) with follicle maturation. Direct exposure to high cortisol concentrations, however, altered both IGF-I synthesis and action. Cortisol treatment lowered (P<0.05) IGF-I production by GC recovered on Days 18, 20, and 21. Furthermore, it reduced (P<0.05) IGF-stimulated P4 synthesis at all stages and decreased (P<0.05) IGF-stimulated E2 synthesis by cells recovered on Day 14. In contrast, cortisol enhanced (P<0.05) FSH-stimulated P4 production by GC collected on Days 14 and 18. The opposing effects on FSH and IGF-I action indicate that cortisol did not promote an overall suppressive effect on cell function, nor did it impair cell proliferation. Hence, these results demonstrate that elevated cortisol concentrations can disrupt both IGF-I synthesis and IGF-mediated actions by porcine GC under in vitro conditions, and that specific disruptions are dependent on the stage of follicle maturation.

Hormonal control of estrous cyclicity and attempted superovulation in wood bison (Bison bison athabascae).

The wood bison (Bison bison athabascae) is a threatened Canadian species that has faced extinction twice in the last 100 yr. Development of assisted reproductive technologies could help ensure the long-term propagation and genetic management of this species. The objectives of this study were to refine estrus synchronization techniques and evaluate superovulatory responses after FSH or eCG administration. In Experiment 1, females were fitted with Syncro-mate B (SMB) implants for 9 d and received an injection of either estradiol valerate (E2V; n = 9) or cloprostenol (PGF; n = 9) at implant insertion (Day-9). In Experiment 2, estrus was synchronized with SMB implants and a PGF injection of Day-9, and superovulation was attempted on Day-2 with either 2500 IU eCG (n = 5) or 400 mg Folltropin-V (n = 5). In each experiment, biosin were examined daily for estrual behavior. Ultrasonography was used during the luteal phase to detect ovulation and assess ovarian status; feces were analyzed by ELISA for immunoreactive progestogens (P) to study ovarian endocrine responses. In Experiment 1, a closer synchrony of estrus was observed between Days 2 to 4 among the PGF-treated (77.8%) than the E2V-treated (66.7%) females. Corpora lutea (CL) were detected in 55% of E2V- and PGF-treated females. In Experiment 2, neither treatment successfully induced superovulation, with only a single female per treatment producing > or = 1 CL. In both experiments, progestogen profiles were similar for each treatment (P < 0.05).

Plasma concentrations of immunoreactive relaxin activity and progesterone in the pregnant Asian elephant (Elephas maximus).

Immunoreactive relaxin activity and progesterone concentrations in weekly plasma samples were measured throughout six pregnancies in five Asian elephants (Elephas maximus). One animal aborted, one delivered a stillborn calf at term and the remaining pregnancies concluded normally with living young. Immunoreactive relaxin activity increased by week 20 of pregnancy to reach a mean peak concentrations of > 6 ng/ml in the second trimester. This was followed by a slow decline beginning approximately at 30 weeks before term. A smaller, secondary relaxin rise was observed during the final eight weeks preceding parturition and circulating concentrations remained above pre-pregnancy baseline values for 72 h after birth. In contrast, progesterone concentrations increased immediately after mating and rose to a maximum of > 2 ng/ml by mid-gestation. As with relaxin, progesterone concentrations began to decline gradually approximately 30 weeks before birth. More frequently collected samples before, during and after parturition revealed a decline to pre-pregnancy levels between 2-5 days before parturition, in contrast to elevated relaxin levels (1 ng/ml) during this time. The aborted pregnancy at week 35 was accompanied by a precipitous decline in both immunoreactive relaxin and progesterone concentrations one week before the visible termination of the pregnancy. The delivery of the term stillborn calf occurred five weeks after both immunoreactive relaxin and progesterone concentrations had declined to pre-pregnancy values. Results suggest that immunoreactive relaxin may be important, along with progesterone, in the maintenance of pregnancy in the Asian elephant as well as playing a role in preparation for birth and possibly to indicate foetal well-being.

A comparison of the adrenal cortical response to ACTH stimulation in Angora and non-Angora goats.

The adrenal cortex is believed to be implicated in the high incidence of abortion in the Angora goat. Stimulation testing with adrenocorticotrophic hormone (ACTH) was used to assess the adrenal cortical function in 5 Angora does from herds with a history of abortion and 5 non-Angora does. An acute test involving a single intramuscular (i.m.) injection of 0.25 mg of synthetic ACTH was given during anoestrus, at mid-oestrus, on day 90 and on day 120 of gestation. Blood samples were collected from the jugular vein at 30 min intervals for 1 h before and 5 h after injection. Cortisol concentrations rose within 30 min and returned to baseline values within 3.5 h. Cortisol production was lower (p < 0.01) in the pregnant state compared to the non-pregnant state in both groups. Production of cortisol was consistently lower (p < 0.05) in the Angora does compared to the non-Angora does during anoestrus and pregnancy and marginally so at mid-oestrus. A chronic stimulation test involving once daily injections of 0.5 mg of a depot form of ACTH i.m. for 7 days commencing on day 90 of pregnancy was also conducted. Cortisol concentrations rose to reach a peak on the third day of treatment in both groups. The values then declined in the Angora does despite continued ACTH treatment, while those for the non-Angora does exhibited a second peak. During and following this treatment, two non-Angora does delivered live kids (day 95, day 120). Out of 7 Angora pregnancies, one Angora doe aborted two dead fetuses at day 116. No significant difference in the cortisol response in the acute test was detected between the animals that aborted and their respective cohorts, but the two non-Angora does that aborted had significantly lower cortisol concentrations during depot ACTH administration. Progesterone and oestradiol levels did not differ between Angora and non-Angora animals during pregnancy or on the test days. The results suggest that the steroidogenic response of the adrenal cortex to ACTH stimulation is significantly less in Angora does with a history of abortion than it is in non-Angora does and support the view that the Angora goat would make a more limited adrenal cortical response to a stressful occurrence during pregnancy.

Immunocytochemical localization of adrenocorticotropic hormone-immunoreactive cells of the pars intermedia in thoroughbreds.

OBJECTIVE: To analyze and characterize adenoma and hyperplasia of the pars intermedia (PI) of Thoroughbred mares. ANIMALS: 165 Thoroughbred mares, without clinical signs of hyperadrenocorticism that had been euthanatized or had died, of causes such as sudden death, colic, pneumonia, or trauma, and were necropsied. Five of those horses, 17 to 25 years old, had a large pituitary gland at necropsy. Eight mares, 5 to 15 years old with normal-size pituitary gland, were selected at random for comparison. PROCEDURE: A morphologic comparison of the pituitary gland between horses with and without tumors of the PI was conducted by use of immunocytochemistry and morphometry. RESULTS: In horses with normal pituitary gland, the PI was supplied by a vast capillary or sinusoidal plexus, which connected that in the pars distalis (PD) with that in the pars nervosa (PN). Cells of the PI stained slightly with ACTH antiserum, but some cells in the border region, which is contiguous to the PD, were strongly ACTH immunoreactive. At necrospy, horses with an enlarged pituitary gland also had adenoma of the pituitary gland involving the PI. Cells of the border region were hypertrophied and stained strongly with ACTH antiserum. The area and number of individual tumor cells of the border region of the PI of horses with adenoma were significantly increased, compared with those in horses with normal pituitary gland. CONCLUSIONS: Cells of the PI-PD border region may secrete substantial quantities of ACTH, owing to stimulation by corticotropin-releasing factor. Adenoma and hyperplasia of the PI in Thoroughbred mares may be associated with hyperadrenocorticism.

Fibronectin concentrations correlate with ovarian follicular size and estradiol values in equine follicular fluid.

The amounts of total protein, albumin, fibronectin, alpha 2-macroglobulin (alpha 2-M), immunoglobulin G, ceruloplasmin and antithrombin were determined in fluids collected from 53 preovulatory equine follicles and compared with the contents of estradiol-17 beta, progesterone and androstenedione, with follicle size and the amounts of the equivalent proteins in normal equine plasma. The concentration of fibronectin and the fibronectin/albumin ratios increased significantly with follicle size and with follicular estradiol levels. The alpha 2-M levels and alpha 2-M/albumin ratios correlated with follicle size but not with hormone content. Both fibronectin and alpha 2-M were present in lower amounts in follicular fluid compared with plasma while the other proteins were present in similar amounts. Among the proteins evaluated, there was a positive correlation between the amount of the protein in the follicular fluid and the molecular weight of the protein.

Preliminary findings of altered follicular activity in Holstein cows with coagulation factor XI deficiency.

Factor XI (F XI) deficiency is an autosomal recessive coagulopathy found in Holstein cattle. Affected animals have a 50% greater prevalence of repeat breeding. Therefore, several parameters describing ovarian function were studied. Daily blood sampling revealed that progesterone concentrations were slower to decline from a peak at day 16 (p < 0.01) to values less than 3 nmol/L in F XI-deficient cows (5.14 +/- 0.69 days (mean +/- SD) versus 4.05 +/- 0.63 days in control animals), resulting in an oestrous cycle length of 24.7 +/- 2.1 days compared to 22.9 +/- 3.0 days, respectively. This was not due to an alteration in the availability of prostaglandin F2 alpha (PGF2 alpha) or oxytocin (OT) involved in luteolysis. No significant differences (p > 0.05) were seen between normal (n = 7) and F XI-deficient (n = 7) cows in the peak values or the area under the curve for the pulse in 13,14-dihydro-15-keto PGF2 alpha in response to OT challenge or in the parameters describing the pulse of ovarian OT secretion after PGF2 alpha injection (n = 7 for each) between days 12 and 14. Ovulatory follicular development was assessed by ultrasound monitoring and plasma 17 beta-oestradiol values at 8-h intervals after a luteolytic injection of cloprostenol (n = 6 for each). Follicular diameter was smaller (p < 0.05) and accompanied by lower peak oestradiol values near the time of ovulation in F XI-deficient cows. The results suggest that the oestrous cycle in F XI-deficient cows is characterized by a slower process of luteolysis that may be associated with smaller follicular development.

Measurements of faecal oestradiol and progesterone in non-pregnant and pregnant domestic and exotic cats.

Faecal samples collected for variable periods from 12 animals and five species of cats were assayed for progesterone and oestradiol content by application of standard radioimmunoassays to aliquots (50 microliters) of methanol extracts (4 ml) of a mixture of 0.5 g sample, 0.5 ml water and 1 g aluminium oxide, following partitioning of the total extract with petroleum ether (3 ml), further dilution of assay aliquots and drying. Recoveries averaged 100 and 72% for oestradiol and progesterone, respectively. Results included increases in progesterone during luteal phases or pregnancies to 7688 ng g-1 (tiger, Panthera tigris), 2594 ng g-1 (lion, P. leo), 3000 ng g-1 (cheetah, Acinonyx jubatus) and 4915 ng g-1 (caracal, Felis caracal). Faecal oestradiol peaks near oestrus included 246 ng g-1 (tiger), 175 ng g-1 (lion) 190 ng g-1 (cheetah), 23 ng g-1 (caracal) and 190 ng g-1 (domestic cat, F. catus).

Serum testosterone concentration in response to exogenous GnRH does not predict service rates or pregnancy rates by yearling, performance tested, crossbred bulls.

Ten bulls with a scrotal circumference of less than 30 cm at the end of growth performance testing, and 10 cohorts of the same age, size and breed type with a scrotal circumference greater than 30 cm were used to evaluate if testosterone response following GnRH administration could be used to test for fertility, for semen quality, and for specific pathologic testicular parenchymal changes. Serum testosterone concentrations were determined immediately before and 2 to 3 hours following intramuscular injection of 250 ug GnRH. Bulls were examined for breeding soundness, then fertility was tested in a breeding trial; testicular histology was assessed by determining the percentage of cross-sections of seminiferous tubules with no spermatocytes. The mean (+/-SEM) post-GnRH serum testosterone concentration for all bulls was 11.71 (+/-0.64) ng/ml. In order to examine for an association, the GnRH response was classified as above or below the mean for resultant serum testosterone concentration. The GnRH response classification was not related to the scrotal circumference, percentage of tubules devoid of spermatocytes, or percentage of progressively motile spermatozoa (P>0.10). The percentage of morphologically normal spermatozoa was significantly higher (P<0.05) in the bulls with a higher than mean testosterone secretion in response to GnRH injection. In the breeding trial, the percentage of heifers bred and the percentage of heifers pregnant (60 days post breeding) were not significantly different (P>0.10) between the 2 classifications of GnRH response. The GnRH response test was related to the percentage of morphologically normal spermatozoa but did not predict fertility of yearling bulls in this study.

Adrenocorticotrophic hormone fails to alter plasma fibrinogen and fibronectin values in calves but does so in rabbits.

The intramuscular administration of adrenocorticotrophic hormone (ACTH) to calves, in either a short-acting form (cosyntrophin) or a longer-acting form (ACTHAR Gel), failed to induce any alteration in circulating fibrinogen or fibronectin values, despite marked elevations in plasma cortisol concentrations. With the longer-acting ACTH, plasma cortisol was elevated for at least 12 h following treatment and induced the expected physiological response of an elevation in blood glucose. In contrast, both forms of ACTH induced marked increases (p < 0.01) in plasma fibrinogen and fibronectin when administered to rabbits. The elevation in the circulating levels of these proteins was first observed 24 h after ACTH administration, by which time plasma corticosteroid values had returned to pre-treatment values. With both ACTH preparations the increases in the circulating levels of these proteins were sustained for at least 96 h. The results suggest that, in cattle, the well-recognized increases in plasma fibrinogen values following stress are not associated with the concomitant increase in plasma cortisol. Further, the results clearly illustrate the marked species differences in the response of acute-phase reactant proteins to elevated glucocorticoids.

Morphometric and steroid hormone changes associated with experimental anovulatory follicles in the sow.

Steroid levels and ovarian follicular morphology were examined in sows on days 19 and 26 (day 5 of next cycle) after injection of adrenocorticotropic hormone (ACTH) or dexamethasone (DXM). Five sows received DXM (30 micrograms/kg bodyweight, intramuscularly) at 12 h intervals from days 9 to 14. Another five sows were given ACTH (2 IU/kg bodyweight, intramuscularly) from day 17 to day 19 or the end of estrus. Five control sows received no treatment. Ovulation occurred only in control sows and progesterone was significantly elevated at day 26. Estradiol values in ovarian vein blood were low but variable on day 19 in DXM- and ACTH-treated animals. Androstenedione values were lower (p less than 0.05) on both days in sows receiving DXM but not in those given ACTH compared to control values on day 19. Morphometric analysis, based on six follicles in each of three sows from each treatment group, indicated that follicular and antral diameters and granulosa cell numbers did not differ for either hormone treatment group on either day compared to those of control sows on day 19. The mitotic index suggested that cell replication continued. However, pyknotic and karyorrhectic nuclei were also seen in the hormone treatment groups. Follicles and oocytes from both DXM- and ACTH-treated sows showed signs of early degenerative changes including disorganization of cumulus cells and large lipid droplets in the cytoplasm of oocytes. Significant differences from control follicles in granulosa cell density and theca interna cell density suggested an association with the altered steroid hormone secretion.

Altered androstenedione to testosterone ratios and LH concentrations during musth in the captive male Asian elephant (Elephas maximus).

Greater concentrations of androstenedione than testosterone were usually present during periods of non-musth in plasma collected weekly for various periods up to 2 years in 8 male Asian elephants (4-35 years of age). For the 6 males that exhibited musth the androstenedione/testosterone ratio shifted greatly in favour of testosterone. The severity of musth was assessed weekly using a scale of 1 to 5 for each of 8 behavioural traits including urine dribbling, temporal gland secretion and aggression. A significant correlation (P less than 0.05) was noted between plasma testosterone concentrations and the musth score value in 5 of 6 musth episodes. Brief shifts in the ratio of two androgens when testosterone predominated (n = 106) were seen during the non-musth period in 3 of the males studied continuously for 2 years. In 82% of these instances, stimuli of a sexual or aggressive nature had occurred in the preceding 48 h (chi 2, P less than 0.01). A heterologous bovine assay was used to measure LH values in plasma collected every 15 min for 12 h. Increases in testosterone concentrations followed pulsatile increases in plasma LH concentrations during 7 non-musth periods in 4 animals. Apart from pulse frequency, increases in the variables describing pulsatile LH secretion were seen in 2 strong musth and 2 mild musth episodes compared to non-musth values. A strong musth, however, was characterized by a much greater increase in pulsatile testosterone secretion than was a mild musth and which may be a function of the duration of musth.

Plasma hormone concentrations after administration of dexamethasone during the middle of the luteal phase in cows.

The effect of glucocorticoids on gonadal steroid and gonadotropic hormone concentrations and subsequent follicular activity in cows undergoing normal estrous cycles was evaluated by administration of dexamethasone (DXM) during the middle of the luteal phase. Seven cows were given physiologic saline solution twice daily from day 13 to day 17 of the estrous cycle (control experiment). During the next estrous cycle, cows were administered DXM (2 mg, IM) twice daily on days 13 through 17. Plasma specimens obtained twice daily throughout the control and DXM-treatment cycles were assayed for progesterone and estradiol concentrations. The appearance of estrus after DXM treatment was delayed until days 23 to 25 in 3 cows and was not seen by day 35 in the other 4 cows, compared with mean (+/- SD) cycle length of 22.4 +/- 3.2 in cows during the control experiment. Progesterone concentration remained significantly (P less than 0.01) high on days 19 to 23, whereas estradiol values failed to increase (P less than 0.05) on days 19 and 20 after treatment with DXM. Blood samples were obtained at 15-minute intervals for 12 hours to compare (by analysis of covariance) the effect of DXM treatment on plasma hormone concentrations on day 15 of each cycle with those of day 10. Compared with values during the control experiment, a significant (P less than 0.05) decrease was observed in the size of the pulses of luteinizing hormone (LH) and estradiol, although the number of pulses of each hormone per 12 hours was not affected when cows were given DXM. Baseline concentrations of LH and estradiol were not altered by type of treatment.(ABSTRACT TRUNCATED AT 250 WORDS)

Fetal fluid steroids and their relationship to gonadal steroid secretion in single and twin bovine fetuses.

Steroid concentrations in the fetal fluids of 153 single and 69 twin bovine pregnancies, ranging in age from 35 to 125 d of gestation, were studied to compare gonadal steroid secretions in vitro with the concentrations found in amniotic and allantoic fluids during the early stages of sex differentiation. Among the steroids measured in fetal fluids, only the testosterone level showed a correlation with the amount secreted by the gonads. Significantly higher concentrations of testosterone were associated with male fetuses than with female fetuses. The concentrations of androstenedione, estradiol and estrone in both fetal fluid compartments were generally correlated with age, reflecting the extra-gonadal source of steroids in these fluids. Androstenedione levels in fetal fluids were significantly higher in twins than in singletons, suggesting that this parameter may be useful for the diagnosis of fetal sex and/or type of pregnancy.

Longitudinal study of haematological and biochemical constituents in blood of the Asian elephant (Elephas maximus).

1. Haematological parameters and biochemical analytes were determined in four elephants over a period of one year. 2. The haematological profile remained constant over time and was similar between animals. 3. Values for biochemical analytes were stable except for alkaline phosphatase, gamma glutamyl transferase and creatinine which rose during musth in male elephants. 4. The association of elevated enzyme levels with increased testosterone concentrations is discussed.

Effect of dexamethasone on gonadotrophin secretion in the gonadectomized sow and boar.

Saline solution or dexamethasone (DXM, 35 micrograms/kg bodyweight) was injected intramuscularly twice daily for four days into five ovariectomized sows and five castrated boars. Blood samples from an indwelling jugular vein catheter were taken at 15 min intervals for 12 h prior to DXM injection and on the fourth day of treatment in order to compare the effect on variables describing the pulsatile secretion of luteinizing hormone (LH) and follicle stimulating hormone (FSH). Dexamethasone treatment caused a decrease (p less than 0.05) in the number of pulsatile episodes of LH secretion in both gonadectomized boars and sows resulting in a significant decrease in mean concentrations. Follicle stimulating hormone secretion parameters appeared to be unaffected by DXM injection in both groups of animals. It is concluded that previously described differences in LH suppression in boars and sows given glucocorticoids may be attributed to the influence of gonadal hormones.

Temporal changes in serum prostaglandin F2alpha and oxytocin in dairy cows with short luteal phases after the first postpartum ovulation.

Luteolysis in the cow depends upon an interaction between prostaglandin F2alpha (PGF2alpha) and oxytocin. The objectives of our study were 1) to determine oxytocin concentrations in postpartum dairy cows and 2) to identify the temporal relationship between oxytocin and PGF2alpha release patterns during luteolysis in normal and abbreviated estrous cycles in the postpartum period. Serum oxytocin and PGF2alpha metabolite (PGFM) concentrations from nine cows which had short estrous cycles (<17 d) were compared with those of six cows which had normal estrous cycles. Serum basal oxytocin concentrations in short estrous cycle cows (23.7 to 31.1 pg/ml) were higher (P<0.05) than those of normal estrous cycle cows (14.6 to 19.8 pg/ml). Oxytocin concentrations increased to peak values in both short and normal cycle cows, during luteolysis. Basal PGFM concentrations (112.2 to 137.4 pg/ml) were higher in cows with short cycle (P<0.05) than in cows with normal cycles (62.9 to 87.5 pg/ml). The increase in PGFM concentrations during luteolysis was significant in both normal cycle and short cycle cows (P<0.05). Increases in serum PGFM concentrations were always associated with increases in serum oxytocin concentrations in normal cycle and short cycle cows and the levels decreased simultaneously before the subsequent estrus. Results support the idea of a positive relationship between PGF2alpha and oxytocin concentration during the estrous cycle as well as a possible synergistic action of these hormones in the induction of luteolysis in dairy cattle.