Flumethason-induced calving is preceded by a period of myometrial inhibition during luteolysis

The temporal relationship among changes of the concentrations of the 13,14-dihydro-15-keto metabolite of prostaglandin F2 alpha (PGFM), estrone (E1) and estrone sulphate (E1S) in maternal arterial plasma (MP) and amniotic fluid (AF), the prepartum progesterone (P4) decline in MP, and the evolution of uterine electromyographic (EMG) activity was investigated in 6 cows. Calving was induced by a single i.m. injection of 5 mg flumethason on Day 270 of gestation. The period under investigation was subdivided into four consecutive periods: Period 1 covered the last 2 days before flumethason treatment; Period 2 (mean +/- SEM duration: 16.1 +/- 2.5 h), Period 3 (8.8 +/- 1.1 h), and Period 4 (13.0 +/- 1.5 h) together included the interval between injection and the onset of the expulsive stage of induced parturition. Each was defined by its pattern of uterine EMG activity. During Periods 1 and 2, this activity occurred in long episodes (2-20 min; contractures) at a similar mean (+/- SEM) frequency (0.51 +/- 0.14/h and 0.42 +/- 0.07/h, respectively). No significant differences in hormonal concentrations in MP and AF between these two periods were detected. During Period 3, contractures nearly disappeared (freq: 0.09 +/- 0.05/h), and in MP mean P4 levels were significantly lower and PGFM levels were significantly higher than before. Mean PGFM concentrations in AF were not significantly changed during Period 3. Finally, during Period 4, EMG activity reappeared and a parturient EMG pattern gradually evolved in the presence of a further significant decline of P4 levels and significant increase of PGFM concentrations in MP.