PGF2 alpha-induced milk ejection in ewes having cyclic or pregnant corpora lutea

The production of luteal oxytocin in ewes, resulting from the intrajugular injection of 200 micrograms of PGF2 alpha, could be determined by the increase in intramammary pressure. This simple indirect method of measuring the activity of the corpus luteum enabled easy detection of renewed post-partum ovulation or the onset of pregnancy. The response was monitored every two days between Days 0 and 25, then every 4 days between Days 25 and 59 in: 9 cyclic ewes (group B); 9 cyclic ewes treated with three daily intramuscular injections of 25 mg of Trilostane, a steroid synthesis inhibitor, between Days 7 and 25 (group A); 11 pregnant ewes (group C). Progesterone levels were determined each day from blood sampled in the jugular vein. Trilostane produced a decrease in plasma progesterone, not a total suppression (fig. 3), but did not significantly modify the intramammary pressure variations resulting from PGF2 alpha injections. These were identical in both cyclic and pregnant ewes during the first 15 days: they increased from D0 to D7 and decreased between D12 and D15 (fig. 4). After D15, the increase in intramammary pressure progressively weakened and became 0 at D17 in the cyclic ewes, whereas in the pregnant animals there was a renewed increase in intramammary pressure until D20; this regressed progressively afterwards and disappeared towards D45. This transitory, renewed activity between D15 and D20 might be an indirect or direct result of the message delivered by the embryo to maintain the corpus luteum. Several hypotheses are discussed with a view to explaining this phenomenon.