Relationship between prostaglandin-induced luteolysis and temporary inhibition of myometrial activity in late pregnant cows with ear implants containing progestagen

Abstract

The influence of luteolysis on myometrial activity in late pregnant cows was studied by measuring electromyographic (EMG) activity and concentrations of progesterone and 13, 14-dihydro-15-keto-prostaglandin F(2α) (PGFM) in maternal arterial plasma of five cows. Application of two ear implants containing progestagen at day 261 of gestation was followed by an injection of a luteolytic dose of a prostaglandin F(2α) (PGF(2α)) analogue (PG) on day 264. Calving was initiated by removal of the implants and a single injection of 5 mg flumethason on day 270. All calves were born alive at a mean interval of 36 h. After 4 h of hyperactivity immediately following the injection of PG, myometrial activity was almost completely absent for 20 h; before luteolysis the total duration of EMG activity was 7.21 ± 0.31 min h-1 (mean ± SEM), during the 4h after PG 23.61 ± 3.40 min h-1, and during inhibition 2.70 ± 0.87 min h-1. After the last period, EMG activity recurred and reached values of 14.52 ± 3.07 and 11.17 ± 2.87 min h-1, on days 265 and 269, respectively. Concentrations of progesterone in maternal plasma decreased from 4.17 to 1.14ng ml-1 within 12h after PG, whereas concentrations of PGFM in plasma remained low, varying from 15 to 30 pg ml-1. After removal of the implants and injection of flumethason on day 270, EMG activity increased without an inhibitory phase and reached maximum values around the time of expulsion of the calves. PGFM concentrations gradually increased to 40pg ml-1 at 16h after removal of the implants and injection of flumethason; there was then a steep increase to mean concentrations of 200 pg ml-1 during expulsion of the calf. These results indicate that a factor, released during luteolysis, exerts an inhibitory effect on the myometrium.