The course of steroid release by intact ovarian follicles of Atlantic salmon (Salmo salar) incubated in vitro with and without gonadotrophin.

Intact ovarian follicles were dissected from a single ovary and groups of follicles incubated in balanced saline at 6 degrees with and without a pacific salmon gonadotrophin (GTH) preparation. Aliquots of the incubation media were withdrawn at intervals, the steroids were extracted with solvent, and 17 alpha, 20 beta-dihydroxy-4-pregnen-3-one (17 alpha 20 beta P), 17 alpha-hydroxyprogesterone, progesterone, androstenedione, testosterone, and oestradiol were separated chromatographically and measured by radioimmunoassay. The main findings were (1) germinal vesicle breakdown commenced on Day 8 under the influence of GTH, (2) after an induction period of about 2 days there was a dramatic and continuing release of 17 alpha 20 beta P in the presence of GTH, (3) GTH promoted a moderate release of 17 alpha-hydroxyprogesterone, (4) GTH stimulated testosterone release up to Day 6 but thereafter levels fell, reflecting either absorption or catabolism of the released steroid, (5) GTH induced a precipitous release of androstenedione during the first few hours of incubation, and (6) progesterone and oestradiol release were little affected by GTH. Steroid concentrations were also measured in ova stripped from naturally ovulating wild fish and the relative amounts were shown to bear some similarity to those released into the incubation medium in the presence of GTH. The routes of biosynthesis and functions of these steroids are considered in light of the results.