REGULATION OF MESSENGER-RIBONUCLEIC-ACID ENCODING 3-BETA-HYDROXYSTEROID DEHYDROGENASE DELTA(5)-DELTA(4)-ISOMERASE IN THE OVINE CORPUS-LUTEUM

Three experiments were conducted to determine how steady-state levels of mRNA encoding 3beta-hydroxysteroid dehydrogenase/DELTA5-DELTA4 isom erase (3beta-HSD) in the ovine corpus luteum vary 1) between the two s teroidogenic luteal cell types, 2) during the estrous cycle, and 3) du ring prostaglandin F2alpha (PGF2alpha)-induced luteolysis. In the firs t experiment, RNA (10 mug) was isolated from purified preparations (n = 4) of large or small steroidogenic luteal cells. Large luteal cells contained 42% more (p < 0.05) message for 3beta-HSD per microgram RNA than did small luteal cells, while the amount of mRNA for tubulin did not differ between the two types of luteal cells. To determine whether luteal levels of mRNA for 3beta-HSD differ during the estrous cycle, corpora lutea were collected from cycling ewes (n = 3/day) on Days 3, 6, 9, 12, and 15 postestrus. Levels of mRNA for 3beta-HSD were similar on Days 3, 6, 9, and 12 but were lower (p < 0.05) on Day 15 postestru s, while levels of mRNA for tubulin were unchanged. in the final exper iment, ewes were treated on Day 10 postestrus with two injections of P GF2alpha (5 mg each) or saline (control) at a 4-h interval. Corpora lu tea were collected from ewes (n = 4/treatment) 1 h or 8 h after the se cond injection of PGF2alpha or 8 h after the second saline injection. Levels of mRNA for 3beta-HSD were decreased by > 80% (p < 0.05) at bot h 1 h and 8 h after the second injection of PGF2alpha compared to leve ls in controls, while mRNA for tubulin remained constant. Serum levels of progesterone had decreased by only 28% and luteal weight and proge sterone content were unchanged 1 h after the second injection of PGF2a lpha. The rapid decrease in mRNA for 3beta-HSD suggests that PGF2alpha perhaps decreases the rate of synthesis and probably increases the ra te of degradation of this message.