Summary. The secretion of prostaglandin (PG) F-2α in response to intravenous injection of 100 i.u. oxytocin on Day 18 after oestrus was determined by measuring jugular venous concentrations of 13,14-dihydro-15-keto PGF-2α (PGFM) in 7 pregnant, 6 cyclic and 2 inseminated non-pregnant heifers. Two other heifers received i.v. saline (controls). The immediate responses of pregnant heifers were smaller than in non-pregnant animals (P < 0·05), as were baseline concentrations in the post-response period (P < 0·05). Endometrial oxytocin receptor concentrations were higher in non-pregnant than pregnant heifers (P < 0·05), but PGFM response to oxytocin challenge was not correlated with oxytocin receptor concentration. Oxytocin receptor concentrations on Day 18 were positively correlated with those of plasma oestradiol on Day 17 (P < 0·01) and inversely with plasma progesterone concentrations on Day 18 (P < 0·01). These findings confirm that PGF-2α secretion in response to oxytocin challenge is attenuated in pregnant animals on the 18th day after oestrus and that, while the prevailing steroid environment is of importance in inducing oxytocin receptor activity, the secretion of PGF-2α is not subsequently limited by oxytocin receptor numbers.
The quantities of PGE-2, PGFM and PGF-2α recovered in uterine flushings taken from heifers on Day 18 were greater in pregnant than other animals (P < 0·01, P < 0·05, P < 0·001), respectively). Intrauterine concentrations of PGF-2α and PGFM were not correlated with the plasma PGFM responses.
Keywords: cattle; early pregnancy; oxytocin; PGF-2α