Independence of ovarian progesterone secretion rate from arterial progesterone concentrations in the pregnant rat

We have sought to determine whether the rate of ovarian progesterone secretion in pregnant rats is inversely related to the arterial plasma progesterone concentrations. For this purpose, rates of ovarian progesterone secretion were measured on day 16 of pregnancy in seven progesterone-treated and eight untreated rats. Treated rats received once-daily s.c. injections of 63.6 mumol progesterone in peanut oil on days 13 to 16. In a separate experiment, this treatment was found to produce a relatively stable fivefold increase in plasma progesterone concentrations. The rate of ovarian blood flow was increased in treated animals (mean +/- S.E.M.; treated, 0.63 +/- 0.08 ml/min; untreated, 0.43 +/- 0.08 ml/min) but the progesterone secretion rate was unchanged (treated, 1.13 +/- 0.20 mumol/day per ovary; untreated, 1.05 +/- 0.15 mumol/day per ovary). The stability of the progesterone secretion rate in the face of a fivefold increase in plasma progesterone concentration implies a lack of negative feedback from progesterone in plasma in the regulation of ovarian progesterone secretion.