Progesterone production and clearance in cyclic ewes passively immunized against progesterone

The effects of passive immunization of ewes against progesterone on plasma progesterone concentrations and on the metabolic clearance rate (MCR) and production rate (PR) of progesterone were investigated. Three treatment groups were studied: 1) nonimmunized controls, 2) ewes passively immunized with antiprogesterone serum, and 3) immunized progestagen-treated ewes, treated concomitantly with anti-serum and with a synthetic progestagen that is not bound by the antiserum. Progesterone levels in the immunized ewes reached a maximum of 27.7+/-4.8 nmol/l and were significantly higher (P<0.05) than in the nonimmunized controls (9.2+/-1.1 mol/l) or the immunized progestagen-treated ewes (15.6+/-1.6 nmol/l). Mean progesterone MCR in the immunized ewes was 1.6+/-0.5 and 2.1+/-0.3 liter/min on Days 7 and 13 of the estrous cycle, respectively, compared with 0.8+/-0.2 and 1.4+/-0.3 liter/min, respectively, in nonimmunized controls. The progesterone production rate in the immunized ewes was significantly higher than in nonimmunized controls, and reached 12.0+/-2.2 and 19.7+/-1.6 nmol/min on Days 7 and 13 of the estrous cycle, respectively, compared with 4.6+/-0.6 and 10.0+/-2.5 nmol/min in nonimmunized controls (P<0.03 for both comparisons). Treatment with progestagen had no significant effect on progesterone MCR or PR of immunized ewes. The LH pulse frequency on Days 10 to 11 of the cycle was 0.7+/-0.3, 1.8+/-0.3 and 0.0+/-0.0 pulses/6 h in the control, immunized and immunized progestagen-treated groups, respectively (P<0.05). It is concluded that the increased plasma progesterone levels in the immunized ewes are the result of an increased progesterone production rate, which may have been induced by an increase in gonadotrophin secretion or by a direct effect of the anti-progesterone serum on the ovary.