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Activities of the steroidogenic enzymes involved in functional luteolysis and PGF2 alpha induced luteolysis were determined in PMSG-hCG primed immature rats to elucidate the luteolytic effects of PGF2 alpha. Plasma progesterone (P4), plasma 20 alpha dihydroprogesterone (20 alpha OHP4), in vitro production of pregnenolone (P5) from endogenous cholesterol in ovarian mitochondria (8,000 X g pellet; Mt), activities of 3 beta hydroxysteroid dehydrogenase (3 beta HSD) in ovarian microsome (105,000 X g pellet; Ms) and 20 alpha hydroxysteroid dehydrogenase (20 alpha HSD) in ovarian cytosol(105,000 X g supernatant; Sup) were determined. For the investigation of intramitochondrial transport of cholesterol, measurement of free cholesterol (FCh) of Mt and the Lineweaver-Burk plotting for cholesterol side-chain cleavage enzyme (CSCC), prepared by osmotic shock and sonication of Mt, were carried out. Functional life span of the rat corpus luteum was estimated as 12 days post hCG treatment from plasma P4. As plasma P4 decreased, concomitant increase in plasma 20 alpha OHP4 was observed. In vitro production of P5 in Mt correlated well with plasma P4 levels, indicating cholesterol side-chain cleavage reaction is the rate limiting step involved in ovarian steroidogenesis. In contrast, both values of Km and Vmax of CSCC did not change between day 7, on which the highest value of plasma P4 observed, and day 12, on which functional luteolysis was ascertained from plasma P4. Both FCh of Mt and the activity of 3 beta HSD in Ms remained unchanged during the functional life of the corpus luteum. The activity of 20 alpha HSD in Sup increased from day 10 post hCG treatment in accordance with plasma 20 alpha OHP4. These results indicate that 1) decrease in availability of FCh to CSCC within mitochondria and 2) increase in catabolism of P4 into inactive progestin, 20 alpha OHP4, may play key roles in the functional luteolysis. To compare the events observed in functional luteolysis with PGF2 alpha induced luteolysis, the animals were treated either with PGF2 alpha (1 mg/rat; s.c.), cycloheximide (5 mg/rat; i.p.; CX), or vehicle on day 7 post hCG treatment, and sacrificed 1 hour later for the analyses mentioned above. Both plasma P4 and in vitro production of P5 in Mt decreased significantly with PGF2 alpha or CX treatment, whereas the FCh of Mt and the activity of CSCC remained unchanged with PGF2 alpha treatment. Both plasma 20 alpha OHP4 and 20 alpha HSD in Sup increased with PGF2 alpha treatment. However, CX suppressed both of them.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of Prostaglandin F<SUB>2</SUB>α on Steroidogenesis of the PMSG-hCG Primed Rat Ovary in Luteolysis