Between prepartum luteolysis and onset of expulsion.

In the cow the foetal endocrine signals that initiate the calving process result in prepartum luteolysis. Withdrawal of progesterone (P4) action is a prerequisite for a normal calving. The rather abrupt declining influence of P4 is followed by a cascade of physiological processes in the myometrium and cervix. This contribution will focus on some of these events. Like in many other species, the myometrium in cows is not completely inactivated during pregnancy. So-called contractures have been registered during the final weeks of gestation and their EMG-characteristics in cows show a low frequency (on average: 13.6 per day) and long duration (on average 12.1 min). They are not evenly spread over the day because they occur less frequently when the cows are disturbed for feeding or cleaning their stables. Contractures affect several foetal functions. In the cow these contractures disappear during a period of about 8-9h when maternal plasma P4 levels are rapidly declining before calving. There is experimental evidence that this temporary inhibition is associated with prepartal luteal regression. The cause of this inhibition is still unknown. Because nitrous oxide inhibits smooth muscle cells and evidence in laboratory animals indicates that expression of the inducible form of nitrous oxide (iNOS) is downregulated in myometrium, but upregulated in the cervix around the onset of parturition, we started to investigate the role of this enzyme in bovine tissues around calving. By means of a RT-PCR technique, we obtained a first indication that iNOS is hardly expressed in the myometrium during calving, while expression was clearly detected at day 4 after calving. Analysis of prepartum en periparturient biopsies from myometrium and cervix with quantitative PCR is still underway. In six pregnant cows, provided with uterine EMG-electrodes and with ultrasonic crystals implanted on the caudal cervical rim to measure cervical dilatation, calving was induced with an injection of prostaglandin (PG) F2alpha. While maternal plasma P4 levels had significantly declined within 8h after PG treatment, the myometrium escaped from temporary inhibition with the development of a parturient contractility pattern on average at 13.5h after injection. However, it was only at 28 h after PG treatment that the first sustained increase of the opening of the vaginal ostium of the cervix was measured.