Effect of constant infusion of oxytocin on luteal lifespan and oxytocin-induced release of prostaglandin F2 alpha in heifers.

Two experiments were conducted to determine whether constant infusion of oxytocin would prolong the luteal phase and inhibit uterine prostaglandin F2 alpha (PGF2 alpha) secretion in heifers. In Experiment 1, twelve heifers, treated with saline (SAL) or oxytocin (OXY) via jugular cannulae infusions (INF) or osmotic minipumps (OMP), were allotted at estrus into four treatment groups (n = 3). Treatments were: SAL-INF, SAL-OMP, OXY-INF and OXY-OMP. Physiological saline or oxytocin was given from Days 10 to 23 (Day 0 = estrus) of the estrous cycle. Method of treatment (jugular cannula infusion or osmotic minipump) had no effect (P greater than 0.05) on estrous cycle length or pattern of secretion of progesterone; therefore, data were pooled. Estrous cycle lengths were extended (P less than 0.01) for heifers which received oxytocin (25.3 +/- 0.4 d) compared to saline (20.5 +/- 0.4 d). Luteolysis did not occur in oxytocin-treated heifers until after treatment ceased. Experiment 2 was designed and conducted identically to Experiment 1 with the addition of a "challenge" injection of oxytocin (100 IU oxytocin, i.v.) given on Day 16 of the estrous cycle. Treatment of heifers with oxytocin extended (P less than 0.05) estrous cycle length by an average of 3 d compared to heifers treated with saline. The "challenge" injection induced (P less than 0.05) secretion of PGF2 alpha (as measured by the stable PGF2 alpha metabolite, 15-keto-13,14-dihydro-PGF2 alpha) in saline-treated but not oxytocin-treated heifers. In both Experiment 1 and 2, serum concentrations of FSH were elevated (P less than 0.05) in oxytocin-treated heifers. No increase was observed for LH or prolactin. The rise in estradiol-17 beta at luteolysis was not affected (P greater than 0.10) by treatment. In summary, constant infusion of oxytocin extended luteal lifespan, prolonged secretion of progesterone, and inhibited oxytocin-induced secretion of PGF2 alpha. Constant infusion of oxytocin did not affect serum concentrations of estradiol-17 beta, LH or prolactin; however, serum concentrations of FSH were elevated during the oxytocin treatment period.