Prostaglandin F-2 alpha induces a rapid decline in progesterone productionand steroidogenic acute regulatory protein expression in isolated rat corpus luteum without altering messenger ribonucleic acid expression

With interest in steroidogenic acute regulatory protein (StAR) involvement in the luteolytic process, we studied changes in serum progesterone levels and the concomitant expression of StAR mRNA and protein (37-, 32-, and 30-k Da forms) in postovulatory Day 7 corpora lutea (CL) isolated from rats 1 h after injection with prostaglandin F-2 alpha (PCF2 alpha, n = 6) or saline (n = 6), Serum progesterone levels were determined by RIA, StAR and beta-ac tin mRNA expression by Northern analysis, and StAR and beta-actin protein e xpression by Western analysis. Adrenal, brain, and spleen from control anim als were used as positive and negative controls for StAR expression. Scanni ng optical densitometry measurements were standardized by dividing the sign al strength from each StAR autoradiogram lane by that from the correspondin g beta-actin autoradiogram lane. ANOVA was used for significance testing, w ith alpha set at 0.05. The 37-, 32-, and 30-kDa forms of StAR protein were expressed in all adrenal samples, whereas only the 37- and 30-kDa forms wer e found in CL, Serum progesterone levels and expression of the 30-kDa and 3 7-kDa forms of the StAR protein in CL were all found to be significantly lo wer in the PGF(2 alpha)-treated than the saline-treated group. StAR mRNA ex pression was not significantly different in the saline- and PGF(2 alpha)-tr eated rats. The rapid decline in StAR protein expression that accompanies P GF(2 alpha) induced luteolysis, therefore, does not result from significant decline in mRNA expression.