IN-VIVO AND IN-VITRO EFFECTS OF 17-ALPHA,20-BETA-DIHYDROXY-4-PREGNENE-S-ONE ON TESTICULAR ANDROGENS IN ATLANTIC SALMON, SALMO-SALAR, MATUREPARR

Mature Atlantic salmon (Salmo salar) male parr were implanted in June and August with Silastic capsules filled with the progestin 17 alpha,2 0 beta-dihydroxy-4-pregnene-3-one (17,20 beta-P) or with empty capsule s. Experiments were terminated in September, when natural androgen lev els are at their highest. Radioimmunoassay (RIA) showed that treatment raised plasma levels of 17,20 beta-P, whereas the content of gonadotr opic hormones (GTH I and II) in the pituitary and plasma levels of the androgens testosterone (T) and 11-ketotestosterone (11-KT) were not i nfluenced. Testicular fragments from mature salmon parr were incubated with different levels of 17,20 beta-P in combination with T or 17 alp ha-hydroxy-4-pregnene-3-one as precursors. A very high level of 17,20 beta-P (3,000 ng/ml) suppressed 11-KT production, whereas 3, 30, or 30 0 ng/ml were without effect. Thus, in these experiments physiological levels of 17,20 beta-P did not suppress androgen 11-KT production or a lter pituitary gonadotropin levels, suggesting that some other mechani sm is responsible for the natural decline in androgen levels when 17,2 0 beta-P levels rise at spawning. (C) 1997 Wiley-Liss, Inc.