Changes in ovarian blood flow and secretion of progesterone and 20 alpha-hydroxypregn-4-en-3-one on day 16 and the morning and afternoon of day 22 of pregnancy in the rat

During late pregnancy in rats, ovarian secretion of progesterone decreases and that of its reduced metabolite, 20 alpha-hydroxypregn-4-en-3-one (20 alpha-OHP), increases. The present study was undertaken to determine whether changes in ovarian blood flow are consistent with changes in progestin secretion. Rats (n = 5 per group) were examined on Day 16, the time of maximal progesterone secretion, and in the morning (AM) and afternoon (PM) of Day 22, the day prior to parturition. Ovarian blood flow was monitored continuously for 60 to 80 min, and serial samples of arterial and ovarian venous blood were obtained at 20-min intervals for determination of ovarian secretion rates of progesterone and 20 alpha-OHP. Ovarian blood flow increased from 0.38 +/- 0.04 ml/min (mean +/- SEM) on Day 16, to 0.77 +/- 0.05 and 0.78 +/- 0.04 ml/min on Day 22 AM and PM, respectively, whereas the secretion of progesterone decreased from 26.9 +/- 4.0 to 4.5 +/- 1.0 and 3.2 +/- 0.3 micrograms/h per ovary. The secretion of 20 alpha-OHP was similar on Day 16 and Day 22 AM (5.6 +/- 1.7 and 5.4 +/- 1.3 micrograms/h per ovary) but then increased to 18.9 +/- 1.2 micrograms/h per ovary by Day 22 PM. Thus the amount of total progestins secreted per unit rate of blood flow relative to that on Day 16 (100%) fell to 15% and 34% on the morning and afternoon of Day 22, respectively. Clearly, the relative changes in ovarian progestin secretion and blood flow in the rat near term to not conform to patterns observed at luteal regression in some other species.(ABSTRACT TRUNCATED AT 250 WORDS)