Steroid release from separated theca and granulosa layers of Atlantic salmon (Salmo salar) ovarian follicles: the effects of a purified salmon gonadotrophin preparation.

Theca and granulosa layers were removed from ovarian follicles of mature Atlantic salmon (Salmo salar) and were separately incubated under sterile conditions with and without a partially purified salmon gonadotrophin preparation (GTH). Aliquots of the incubation media were removed at intervals and analysed for the steroids 17α, 20β-dihydroxy-4-pregnen-3-one (17α20βP), 17α-hydroxyprogesterone, progesterone, androstenedione, testosterone and oestradiol. The biosynthesis of C19 and C21 steroids was very largely restricted to the thecal tissue and was markedly stimulated in the presence of GTH. Androstenedione (max 65 ng/ml) and testosterone (max 14 ng/ml) were released from the earliest stages of incubation whereas the release of 17α-hydroxyprogesterone (max 51 ng/ml) and progesterone (max 5.5 ng/ml) commenced only after a lengthy induction period. A trace (1.0 ng/ml) of 17α20βP was produced by the theca in the presence of GTH but oestradiol was not detected. The granulosa preparations released levels of 17α-hydroxyprogesterone and androstenedione only marginally above the detection limits (ca 0.7 ng/ml) and there was little stimulation of output with GTH. Oestradiol (max 4 ng/ml) was released only in the presence of GTH. 17α20βP, progesterone and testosterone were not detected as products of this tissue. These results, together with those derived earlier from incubations of complete follicles support the view that the synthesis of 17α20βP is essentially a two-cell process in which 17α-hydroxyprogesterone produced in the theca is subject to the action of steroid 20β-hydroxysteroid dehydrogenase in the granulosa. The temporal pattern of release of steroids in these and earlier experiments is considered in relation to mechanisms of steroid biosynthesis and to their possible roles in oocyte final maturation.