RELATIONSHIP BETWEEN PROSTAGLANDIN-INDUCED LUTEOLYSIS AND TEMPORARY INHIBITION OF MYOMETRIAL ACTIVITY IN LATE PREGNANT COWS WITH EAR IMPLANTS CONTAINING PROGESTAGEN

The influence of luteolysis on myometrial activity in late pregnant co ws was studied by measuring electromyographic (EMG) activity and conce ntrations of progesterone and 13,14-dihydro-15-keto-prostaglandin F2al pha (PGFM) in maternal arterial plasma of five cows. Application of tw o ear implants containing progestagen at day 261 of gestation was foll owed by an injection of a luteolytic dose of a prostaglandin F2alpha ( PGF2alpha) 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 h yperactivity immediately following the injection of PG, myometrial act ivity was almost completely absent for 20 h; before luteolysis the tot al duration of EMG activity was 7.21 +/- 0.31 min h-1 (mean +/- SEM), during the 4 h after PG 23.61 +/- 3.40 min h-1, and during inhibition 2.70 +/- 0.8 7 min h-1. After the last period, EMG activity recurred a nd reached values of 14.52 +/- 3.07 and 11.17 +/- 2.87 min h-1, on day s 265 and 269, respectively. Concentrations of progesterone in materna l plasma decreased from 4.17 to 1. 14 ng ml-1 within 12 h after PG, wh ereas concentrations of PGFM in plasma remained low, varying from 15 t o 30 pg ml-1. After removal of the implants and injection of flumethas on on day 270, EMG activity increased without an inhibitory phase and reached maximum values around the time of expulsion of the calves. PGF M concentrations gradually increased to 40 pg ml-1 at 16 h after remov al of the implants and injection of flumethason; there was then a stee p increase to mean concentrations of 200 pg ml-1 during expulsion of t he calf. These results indicate that a factor, released during luteoly sis, exerts an inhibitory effect on the myometrium.