The temporal relationship between changes in cervical dilatation, uterine electromyographic (EMG) activity, and maternal plasma concentrations of estradiol 17β (E2), progesterone (P4), and 13,14-dihydro-15-keto-prostaglandin-F2α (PGFM), was investigated in six parturient cows. Calving was induced with a single injection of a synthetic analogue of prostaglandin F2α (PG) on Day 274 of gestation. Cervical dilatation was measured continuously by measuring the transit time between two implanted ultrasound crystals while at the same time uterine EMG activity was measured through two silver electrodes sutured on the myometrial surface until the expulsive stage of calving had been reached. In blood samples collected at 4-h intervals, starting at the moment of PG injection, the mean plasma E2 concentration gradually increased and was significantly elevated at 28 h after PG injection. At 4 h after PG treatment, the mean P4 concentration had dropped significantly and continued to decrease until a value of around 1 ng/ml was reached, where it stayed until the onset of expulsion. Mean plasma PGFM concentrations increased steadily after PG injection, reaching significantly elevated concentrations at 20 h after treatment. In the five cows that delivered calves in anterior positions, uterine EMG activity, expressed as root mean square (RMS in μV), started to increase at a mean interval (± SD) of 13.1 ± 3.7 h following PG treatment. The increase in EMG activity was significantly correlated with changes in plasma PGFM concentrations. In these cows, dilatation of the caudal cervix started after a mean (± SD) interval of 28.5 ± 1.5 h following PG treatment and dilatation progressed at a mean (± SD) rate of 2.25 ± 0.24 cm/h. In one cow with a calf in the posterior position, uterine EMG activity and dilatation started at 15.8 h and 31.8 h, respectively, after induction of calving. We conclude that a predictable sequence of physiological changes occurs around induction of calving, which allows specific timing of future studies on cellular and biochemical changes within the cervix during parturition.
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