Effect of progesterone withdrawal in sheep during late pregnancy.

The concentration of progesterone in the peripheral plasma of seven sheep during late pregnancy was reduced by injection of an inhibitor of 3 beta-hydroxysteroid dehydrogenase activity. Concentrations of progesterone were 10.0 +/- 1.0 (S.E.M) ng/ml (n = 6) before injection of the inhibitor, fell to 1.39 +/- 0.40 ng/ml (n = 6) 30 min after injection, and remained within this lowered range for 6 h after injection. By 20-24h and 30-35h after injection progesterone concentrations had recovered to 4.63 +/- 0.94 and 14.07 +/-4.17 ng/ml respectively (n = 6). Six out of seven ewes delivered prematurely 32.5 +/- 2.9h after injection. Delivery appeared to be normal, and was associated with increasing concentrations of 13, 14-dihydro-15-oxo prostaglandin F2 alpha in peripheral plasma. Concentrations of oestradiol-17 beta in peripheral plasma were slightly raised immediately before delivery, at which time progesterone concentrations were within the preinjection range. These data suggest that progesterone withdrawal is one mechanism that initiates increased prostaglandin F2 alpha secretion in the pregnant sheep.