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The aim of this study was to characterize changes in PGF2alpha secretion in mares with persistent corpora lutea that were induced by administering altrenogest during oestrus. In Expt 1, PGF2alpha secretion was compared among mares undergoing normal oestrous cycles (n=7) and mares undergoing prolonged luteal phases (n=6), using the mean 15-ketodihydro-PGF2alpha (PGFM) plasma concentrations, peak PGFM concentrations and number of PGFM surges each day, from day 12 to day 16 of the luteal phase. In Expt 2, oxytocin-induced PGF2alpha secretion was characterized on days 13 and 16 of the luteal phase in mares undergoing normal oestrous cycles (n=6) and in mares undergoing prolonged luteal phases (n=7) by comparing the oxytocin-induced increase in PGFM concentration and total PGF2alpha secretion. In Expt 1, mean PGFM concentrations, peak PGFM concentrations and number of PGFM surges per day were significantly lower in mares undergoing prolonged luteal phases than in mares undergoing normal luteal phases. In Expt 2, the area under the curve for PGFM ng (90 min)(-1) was similar for both groups on day 13 but was significantly lower on day 16 in mares undergoing prolonged luteal phases than in mares undergoing normal luteal phases. No change in total PGF2alpha secretion was observed between day 13 and 16 for mares undergoing normal luteal phases, but a significant decrease was observed from day 13 to day 16 in mares undergoing prolonged luteal phases. On days 13 and 16, the increase in PGFM concentration 5 min after oxytocin administration was significantly higher in mares undergoing normal luteal phases than in mares undergoing prolonged luteal phases. The increase in PGFM concentration 5 min after oxytocin administration was similar on days 13 and 16 for mares undergoing normal luteal phases, but tended to be less on day 16 in mares undergoing prolonged luteal phases. These results indicate that failure of luteolysis in mares undergoing induced prolonged luteal phases is due to decreased uterine sensitivity to oxytocin stimulation or decreased uterine ability to secrete prostaglandin.

At the onset of equine chorionic gonadotrophin (eCG) secretion, eCG stimulates luteal androgen and oestrogen production. Although eCG concentrations increase exponentially from day 37 to day 60 of gestation and eCG is detectable in maternal serum until about day 120-150 of gestation, luteal androgen and oestrogen production peaks between 5 and 10 days after initial exposure to eCG and then decreases gradually. It is not clear how eCG regulates luteal androgen and oestrogen production. In the present study, the steady-state mRNA expression of 3beta-hydroxysteroid dehydrogenase (3beta-HSD), cytochrome P450 17alpha-hydroxylase/17,20-lyase (P450(17alpha)) and cytochrome P450 aromatase (P450arom) in primary corpora lutea before, during and after eCG secretion was determined by northern blotting. Expression of 3beta-HSD was similar at all the stages examined. Cytochrome P450(17alpha) expression increased at the onset of eCG secretion, decreased between days 42 and 46 of gestation and was constant for the remaining period of eCG secretion. Cytochrome P450arom expression was highest before and after eCG secretion and lowest during periods of peak eCG secretion. The differential expression of P45017alpha and P450arom indicates that production of luteal androgen and oestrogen is regulated by P450(17alpha), activity. The effect of eCG on luteal steroidogenic enzyme mRNA expression appears to be stage-specific.

The effect of the dopamine antagonist sulpiride on FSH secretion and onset of reproductive activity in anoestrous mares under different environmental conditions was investigated. In Expt 1, sulpiride (0.5 mg (-)-sulpiride kg(-1) twice a day) had no affect on FSH pulse frequency, mean FSH concentration, basal FSH concentration or FSH pulse amplitude in anoestrous mares. These data do not support the hypothesis that dopamine inhibits reproductive activity by suppressing GnRH secretion, as it does in other species. In Expt 2, the interval to first ovulation (14.8 +/- 1.9 days; range 12-22 days) in five mares treated with sulpiride (0.5 mg (-)-sulpiride kg(-1) twice a day) housed indoors under extended daylength (16 h light: 8 h dark) was significantly shorter (P < 0.02) than in six untreated mares housed indoors under extended daylength (34.3 +/- 5.5; range 16-52 days and seven untreated mares housed outside under natural photoperiod (73 +/- 10; range 37-107 days). However, if the FSH secretion parameters at the start of treatment are treated as covariants, each has a significant effect (P < 0.05) on the interval to ovulation and sulpiride treatment does not have a significant effect. In Expt 3, the interval to first ovulation was not significantly different in sulpiride-treated (200 mg (-)-sulpiride twice a day) and untreated mares maintained outside under natural photoperiod. These results indicate that sulpiride treatment combined with increased temperature (indoor housing) and stimulatory photoperiod (extended daylength) results in a shorter interval to first ovulation and that a nonstimulatory environment decreases the effect of treatment on the interval to first ovulation. The role of FSH secretion at the time of treatment remains to be determined.

Thirty pregnant mares were assigned to 3 groups: Group 1 (n = 10) mares served as controls; Group 2 (n = 10) mares were treated with altrenogest (44 mg/day) from Day 16 to 80 and Group 3 (n = 10) mares were treated with a luteolytic dose of PGF2 alpha on Day 16 followed by altrenogest (44 mg/day) until Day 80. Concentrations of progesterone and chorionic gonadotrophin (CG) in plasma and oestrogen conjugate (OC) in urine were determined between Days 16 and 80 of gestation. In Group 3, complete luteolysis occurred in all 10 mares following administration of PGF2 alpha. Six of the 10 mares did not have an active CL from Day 16 until after Day 60 of pregnancy (Group 3a); the remaining 4 mares developed a new CL on Days 32, 40, 43 and 49 of pregnancy (Group 3b). In Groups 1 and 2, an increase in oestrogen secretion was observed between (Group 3b). In Groups 1 and 2, an increase in oestrogen secretion was observed between Days 35 and 40. In Group 3a, OC concentrations in the absence of an active CL were significantly lower than in Groups 1 and 2, and oestrogen secretion did not increase between Days 20 and 50. In Group 3b, OC concentrations remained low in the absence of an active CL. After the onset of CG secretion, OC concentrations increased to values similar to Groups 1 and 2 in conjunction with the development of a new CL. The results of this experiment demonstrate that the increase in oestrogen secretion observed between Days 35 and 50 of pregnancy requires the presence of an active CL.(ABSTRACT TRUNCATED AT 250 WORDS)

A direct enzyme immunoassay was developed to measure conjugated oestrogens in the plasma of pregnant mares. The antibody was produced in rabbits using oestrone-3-glucuronide (E1G) conjugated to bovine serum albumin. The enzyme conjugate was E1G conjugated to horseradish peroxidase. A sharp increase in plasma E1G concentrations occurred between Days 35 and 40 of gestation. Values declined slightly to Day 45, remained relatively constant to around Day 70 and rose sharply thereafter. Fetal death before Day 35 had no effect on plasma concentrations of E1G. Fetal death after Day 35 in conjunction with endotoxin-induced regression of the corpus luteum (CL) resulted in a decrease in plasma E1G levels to non-pregnant values within 3-4 days. Endotoxaemia without fetal death between Days 35 and 70 resulted in marked, but transient, decreases in plasma E1G concentrations. Fetal death without CL regression after Day 35 did not produce an immediate decline in plasma E1G concentrations and existing levels were maintained for 10-14 days. These findings indicate that between Days 35 and 70 of pregnancy, plasma E1G concentrations are not directly correlated with fetal viability; they appear to reflect increased oestrogen secretion by the CL under the influence of chorionic gonadotropin. After Day 70, E1G concentrations begin to reflect directly the production of oestrogen by the developing feto-placental unit.

Stallions can experience an increase in testicular temperature from bouts of fever or from injury to the testes. In species other than the horse, increased temperature models have been used to study testicular degeneration. This study was undertaken to examine the effects of increased testicular temperature on spermatogenesis in the stallion as measured by semen evaluation. The results of this investigation demonstrate that increased testicular temperature is associated with significant transitory alterations in the routine semen evaluation of the stallion. The duration of increased testicular temperature affected the degree of change observed. The temporal nature and degree of the change in the semen evaluation is described. Predictive formulae for expected changes in the semen evaluation have been derived.

The infusion of Salmonella typhimurium endotoxin into pregnant mares resulted in a biphasic release pattern of PGF-2 alpha as determined by 15-keto-13,14-dihydro-PGF-2 alpha concentrations. The initial phase of 1 h duration was followed by accentuated release by 2 h after infusion; concentrations reached basal levels by 6 h. In 7 mares at 23, 26, 29, 33, 36, 53 and 55 days of gestation, fetal death occurred between 36 and 120 h after infusion; 12 mares at 46, 51, 56, 59, 65, 71, 73, 85, 103, 138, 283 and 318 days of gestation did not abort after endotoxin infusion. Luteal activity was compromised in all mares by 9 h after infusion. Progesterone concentrations were consistently lower in mares that aborted (1-2 ng/ml) than in those that did not abort. Mares therefore appear to be vulnerable to fetal loss by a clinical syndrome induced by Salmonella typhimurium endotoxin until about 50-60 days of gestation.