Luteal blood flow and plasma steroids in rats with corpora lutea of different ages

Ovarian and luteal blood flow rates were measured at different stages of luteal development in anaesthetized rats using 15 +/- 5 micron radioactive microspheres. Ovulations were induced by injection of 8 IU of PMSG at 28 days of age. Steroid concentrations in peripheral plasma were determined using radioimmunoassays. The highest luteal blood flow was found in the youngest corpora lutea and decreased gradually with luteal age. Luteal flow on days 13 and 14 was significantly lower than that of days 2, 4, and 6. Ovarian stromal blood flow remained unchanged throughout the lifespan of the lupus luteum. Peripheral progesterone concentrations rose significantly on days 4, 6, and 8 with a maximum on day 8, as compared to day 2, and fell significantly after day 8. The levels of 20 alpha-OH-progesterone increased sharply on day 10, indicating that functional luteolysis. The progesterone levels fell significantly between day 8 and 10 without a concomitant significant decline in luteal blood flow. Since a fall in progesterone production seem sto precede a drop in luteal blood flow, the initiation of luteolysis does not appear to be caused by a vascular mechanism.