The effect of exogenous oxytocin on luteal function in mares

Abstract

Daily injections of 150 units oxytocin administered to 6 mares on Days 4, 5, 6, 7 and 8 after ovulation (Day 0 = ovulation) failed to induced luteolysis as indicated by the maintenance of normal plasma progestagen concentrations and the occurrence of normal ovulatory intervals. Three additional mares were given oestrogen injections 24 h before an injection of oxytocin on Day 7 after ovulation, but this treatment also failed to induce luteolysis since plasma progestagen concentrations were maintained in all three mares. Two mares exhibited normal ovulatory intervals, while the third developed a corpus luteum which persisted for 46 days.

Oxytocin induction of pulses of a prostaglandin metabolite and luteolysis in mares

“…Treatment with OT early in the estrous cycle in cattle is luteolytic [25], but in mares, treatment from Days 1 to 7 with a high dose (200 IU [26]) or Days 4 to 8 (150 IU [27]) did not induce luteolysis or shorten the interovulatory interval (IOI). In this regard, the number of OT receptors in the uterus is minimal on Days 4 to 8 in mares [27].…”

“…In this regard, the number of OT receptors in the uterus is minimal on Days 4 to 8 in mares [27]. Despite the stimulation of PGFM by OT doses of 10 to 25 IU/mare [18,20], chronic administration of OT in high doses (e.g., 60 IU/day on Days 7-14) prolongs the luteal phase at least until Day 30 [28][29][30][31].…”

Relationship between endometrial oxytocin receptors and oxytocin-induced prostaglandin F2 release during the oestrous cycle and early pregnancy in pony mares

“…The effects of exogenous oxytocin on luteal function in mares differ somewhat from those in cows. Injec¬ tions of 150 iu of oxytocin on days 2-8 after ovulation did not have any effect on luteal function in mares, as determined by plasma progesterone and duration of cycle (Neely et al, 1979a), whereas similar treatment in cows causes shortening of the oestrous cycle (Armstrong and Hansel, 1959). However, 20 iu oxytocin injected on days 10-13 after ovulation resulted in PGF2[1 release and luteolysis (Goff et al, 1987).…”

Oxytocin – a peptide serving manifold roles during the reproductive cycle in mares

“…The continuous application of OT beginning at day 8 after ovulation prolonged luteal function in mares, what may be caused by inhibition of the upregulation of OT-receptors, and therefore abolished luteolysis (Stout et al 1999). Daily application of OT to mares at days 4, 5, 6, 7 and 8 of the oestrous cycle had no effect on PGFM concentrations and duration of dioestrus (Neely et al 1979), supporting the hypothesis of downregulated OTreceptors up to day 10 of the cycle. In contrast, Vanderwall et al (1998) could not find synchronous OT and PGFM patterns in blood samples, which were drawn from the cavernous sinus during luteolysis, although they support the positive feedback role of OT on PGF 2α as well.…”

“…Interestingly, dioestrous mares release OT as well, when they are exposed to stallions (Nikolakopoulos et al 2000). While earlier studies stated that OT has no function concerning the luteolytic pathway (Arthur 1975, Neely et al 1979, Betteridge et al (1985) could provoke a rise of the PGF 2α metabolite 13,14-dihydro-15-keto PGF 2α (PGFM) by the application of OT to dioestrous mares. This finding was supported by Goff et al (1987), who obtained maximum PGFM response to OT at the time of expected luteolysis.…”