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In sheep, the presence of an embryo in utero on the 12th to 13th day after oestrus prevents luteolysis. These studies investigated whether platelet-activating factor (PAF) could exert an antiluteolytic function, either alone or in combination with interferon. The intrauterine administration of 250 micrograms PAF per horn per day, administered through indwelling cannulae into the uterus as injections twice a day (n = 12) or by continuous infusion (n = 4) failed to extend luteal function compared with controls (n = 8). When indwelling cannulae were used to administer (i) 125 micrograms PAF per uterine horn, as a bolus infusion twice a day (n = 5), (ii) continuous infusion of 500 micrograms bovine recombinant alpha 1-interferon each day (brIFN, n = 5), (iii) 125 micrograms PAF per horn twice a day, plus 500 micrograms brIFN per day (n = 8), or (iv) vehicle (n = 5), the luteal phase was significantly longer in co-infused (iii) than in control (iv) animals. These findings indicate that pharmacological doses of PAF may act synergistically with interferons to prevent luteolysis.

Concentrations of oxytocin were measured in corpora lutea obtained from heifers throughout the oestrous cycle and first 30 days of pregnancy. Values were low during the first 3 days of the cycle (less than 250 ng/g tissue), increasing to 1312 ng/g by Day 4. Values then further increased up to a maximum of 2344 ng/g on Day 12. Concentrations were similar in cyclic and pregnant animals throughout the midluteal phase and were maintained at approximately 1500 ng/g until the 18th (cyclic cows) or 19th (pregnant cows) day after oestrus, when they were again low. Values subsequently remained less than 250 ng/g in pregnant cattle. Concentrations of oxytocin in jugular venous plasma of cyclic (n = 5) and pregnant (n = 4) cows were measured in samples collected every 15 min for 8 h on Days 14, 16, 18 and 19 after oestrus. There were no significant differences in mean concentrations (range: 2.5-4.7 pg/ml) or in the number, frequency or area under the curve of episodes between either cyclic and pregnant animals, or between days. Mean basal concentrations were higher on Day 16 than on Day 14 (P less than 0.05), values on Days 18 and 19 being intermediate. These findings suggest that the corpus luteum contains a finite amount of releasable oxytocin, which is exhausted by Day 18-19 after oestrus, whether or not pregnancy occurs, and that there is no further accumulation of oxytocin in the animal during early pregnancy. The contribution of luteal oxytocin to jugular venous concentrations appears to be less than in sheep, in which values in the jugular vein closely parallel those within the corpus luteum.

Binding of [3H]oxytocin to uterine subcellular preparations ('oxytocin receptor concentrations') was measured in uterine tissue of heifers and multiparous dairy cows at various stages of the oestrous cycle and during early pregnancy. A method for the assay of ovine uterine oxytocin receptors was optimized for use on bovine tissue. Oxytocin receptor concentrations were increased in cyclic animals around the period of luteolysis and oestrus, rising on Day 15 in endometrium and on Day 17 in myometrium while pregnant animals showed no comparable rise. Receptor concentrations then declined on Day 3 after oestrus in myometrium and on Day 5 in endometrium. Some cyclic animals did not show the expected rise in receptors in the late luteal phase; these animals had abnormally high progesterone concentrations for this stage of the cycle. In animals slaughtered on Day 18 after oestrus and/or insemination which had low oxytocin receptor levels, plasma progesterone concentrations were consistently high; while all animals showing the late luteal phase elevation in receptor values had low progesterone concentrations. Oxytocin receptor and progesterone concentrations were negatively correlated (P less than 0.05). These data support the hypothesis that oxytocin receptor level is a key factor in the process of luteolysis in cattle and that in pregnancy there is suppression of uterine oxytocin receptor at the expected time of luteolysis. We suggest that uterine oxytocin receptor levels are partly controlled by circulating steroid hormones and are suppressed during early pregnancy.

The secretion of prostaglandin (PG) F-2 alpha 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-2 alpha (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 less than 0.05), as were baseline concentrations in the post-response period (P less than 0.05). Endometrial oxytocin receptor concentrations were higher in nonpregnant than pregnant heifers (P less than 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 less than 0.01) and inversely with plasma progesterone concentrations on Day 18 (P less than 0.01). These findings confirm that PGF-2 alpha 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-2 alpha is not subsequently limited by oxytocin receptor numbers. The quantities of PGE-2, PGFM and PGF-2 alpha recovered in uterine flushings taken from heifers on Day 18 were greater in pregnant than other animals (P less than 0.01, P less than 0.05, P less than 0.001, respectively). Intrauterine concentrations of PGF-2 alpha and PGFM were not correlated with the plasma PGFM responses.