Changes in Follicular Steroidogenic Enzymes following the Preovulatory Surge of Gonadotropins and Experimentally-Induced Atresia1

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Abstract
Atresia that is induced experimentally and the preovulatory surge of gonadotropins stimulate similar changes in follicular steroidogenesis in the rat, i.e., both enhance production of progesterone and reduce production of androgen and 17 beta-estradiol. In this study, mature cycling rats were either stimulated with human chorionic gonadotropin (hCG) or atresia was induced by blocking the proestrous surge of gonadotropins through the use of pentobarbitone or hypophysectomy. Changes in activity of C17,20-lyase (lyase) and 20 alpha-hydroxysteroid dehydrogenase (20 alpha SDH) were estimated from homogenates of 10-15 Graafian follicles by evaluating conversion of precursors to products that were separated and quantified by high performance liquid chromatography (HPLC). Within 3 h of administration to proestrous rats, hCG reduced follicular lyase activity (pmole androstenedione produced per mg protein during 30 min incubation) from (mean +/- SEM) 221.3 +/- 24.2 to 120.2 +/- 30.4, and to 8.5 +/- 0.1 after 9 h. By contrast, 20 alpha SDH activity increased somewhat after hCG stimulation. Similar changes were observed after follicular atresia was induced, with hypophysectomy causing the most striking changes. Lyase was reduced to 60% within 6 h after the operation, and to 2% within 24 h. Activity of 20 alpha SDH was doubled within 6 h of hypophysectomy and remained high even 24 h later. Thus, in preovulatory rat follicles, luteinizing hormone (LH)/hCG reduces lyase activity and similar changes occur in such follicles undergoing atresia. There was no clear correlation between 20 alpha SDH and lyase activities; our results did not support the argument that 20 alpha SDH products regulate lyase following the ovulatory stimulus and atresia.(ABSTRACT TRUNCATED AT 250 WORDS)