Effect of Pregnancy on Oxytocin-induced Release of Prostaglandin F2α in Heifers

Abstract

The effect of pregnancy on the release of prostaglandin F2 alpha (PGF2 alpha) in response to oxytocin (OT) has been examined. Fourteen cyclic heifers received one intravenous injection of 1 IU OT (n = 6) or 100 IU OT (n = 8) 17, 18, or 19 days (Day 17-19) after the onset of estrus (Day 0). Five of these animals also received 100 IU OT at Days 6 and 13 to determine the effect of OT at different times of the cycle. Frequent blood samples were taken for 60 min before and for 90 min after OT injection for the measurement of 15-keto-13,14-dihydro-PGF2 alpha (PGFM) by radioimmunoassay. The experiment was then repeated using the same animals at Day 17-19 of pregnancy (confirmed by the recovery of an embryo the day after OT injection). Following the injection of 1 IU OT, plasma PGFM reached its peak within 30 min with the increase significantly lower (P less than 0.05) in pregnant (1.13 +/- 0.10-fold) than in nonpregnant animals (2.07 +/- 0.27-fold). However, because only 3 of the 6 cyclic animals showed a response to 1 IU OT, the dose was increased to 100 IU in subsequent experiments. The animals that received 100 IU at Days 6 and 13 had no significant increase in PGFM concentrations (1.18 +/- 0.05-fold and 1.01 +/- 0.04-fold, respectively). At Day 17-19 the increase in plasma PGFM reached its peak 5-15 min after 100 IU OT and the increase was significantly greater in nonpregnant (3.23 +/- 0.17-fold) than in pregnant (1.21 +/- 0.02-fold; P = 0.003) heifers. Six of 11 animals injected at Day 17-19 of the cycle showed a decrease in progesterone (P4) the day after OT administration. These data show that the release of PGF2 alpha in response to OT is suppressed in pregnant animals in vivo, suggesting an antiluteolytic role for the embryo in luteostasis.

Effects of progesterone and oestradiol-17 on oxytocininduced release of prostaglandin F-2 in heifers

“…In the cow, as in the sheep, there is a pulsatile release of oxytocin, presumably from the corpus luteum (Walters et al, 1984), which appears to be responsible for the pulsatile release of PGF-2a at the time of luteolysis (Flint & Sheldrick, 1983;Vighio & Liptrap, 1986). The uterus is responsive to oxytocin stimulation at the end, but not during the middle, of the luteal phase of non-pregnant heifers (Lafrance & Goff, 1985). This increasing sensitivity to oxytocin may therefore be important for the timing of luteolysis.…”

“…The regression of the corpus luteum in the cyclic cow is now thought to result from an interaction between oxytocin, released by the corpus luteum (and perhaps the posterior pituitary), and the uterine endometrium resulting in the pulsatile release of the luteolysin prostaglandin (PG) F-2a (Ginther et al, 1967;Milvae & Hansel, 1980;Schams et al, 1983;Wathes, 1984;Oyedipe et al, 1984; Lafrance & Goff, 1985). Furthermore, there is evidence that follicular oestrogens are also involved in the luteolytic process.…”

Transforming growth factor-β1 inhibits luteinization and promotes apoptosis in bovine granulosa cells

“…Cross-reaction of A 4 and E 1 with the E 2 assay was !0.1% (Bélanger et al 1980). P 4 was measured in duplicate as described (Lafrance & Goff 1985). Intra-and inter-assay coefficients of variation were !15% for both assays.…”

Role of transforming growth factor-β1 in gene expression and activity of estradiol and progesterone-generating enzymes in FSH-stimulated bovine granulosa cells

“…Cross-reaction of A 4 and E 1 with the E 2 assay was less than 0.1% (Bélanger et al 1980). P 4 was measured in duplicate as described (Lafrance & Goff 1985). Intra-and inter-assay coefficients of variation were less than 15% for both assays.…”