In vivo and in vitro effects of 17α,20β-dihydroxy-4-pregnene-3-one on testicular androgens in atlantic salmon, <TOGGLE>Salmo salar</TOGGLE>, mature parr

Mature Atlantic salmon (Salmo salar) male parr were implanted in June and August with Silastic capsules filled with the progestin 17α,20β-dihydroxy-4-pregnene-3-one (17,20β-P) or with empty capsules. Experiments were terminated in September, when natural androgen levels are at their highest. Radioimmunoassay (RIA) showed that treatment raised plasma levels of 17,20β-P, whereas the content of gonadotropic hormones (GTH I and II) in the pituitary and plasma levels of the androgens testosterone (T) and 11-ketotestosterone (11-KT) were not influenced. Testicular fragments from mature salmon parr were incubated with different levels of 17,20β-P in combination with T or 17α-hydroxy-4-pregnene-3-one as precursors. A very high level of 17,20β-P (3,000 ng/ml) suppressed 11-KT production, whereas 3, 30, or 300 ng/ml were without effect. Thus, in these experiments physiological levels of 17,20β-P did not suppress androgen 11-KT production or alter pituitary gonadotropin levels, suggesting that some other mechanism is responsible for the natural decline in androgen levels when 17,20β-P levels rise at spawning. J. Exp. Zool. 277:66–71, 1997. &#169; 1997 Wiley-Liss, Inc.