Relationship Between Seasonal Plasma Estradiol-17β and Testosterone Levels and In Vitro Production by Ovarian Follicles of Amago Salmon (Oncorhynchus rhodurus)

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Abstract
Plasma estradiol-17.beta. and testosterone levels were assessed by radioimmunoassay during the sexual maturation of female amago salmon (O. rhodurus). Estradiol-17.beta. levels gradually increased during vitellogenesis (O. rhodurus). Estradiol-17.beta. levels gradually increased during vitellogenesis (June to Sept.), reached a peak in Sept. (about 16 ng/ml) and rapidly decreased in mature and ovulated fish (about 3-4 ng/ml) in Oct. The seasonal pattern of plasma testosterone levels lagged behind and followed that of estradiol-17.beta. during vitellogenesis, but levels remained high in mature and ovulated fish (90-110 ng/ml). Estradiol-17.beta. levels and the gonadosomatic index (GSI) values correlated well during vitellogenesis: GSI values showed a linear increase, and reached a peak (29.9 .+-. 1.4) in Oct. Values were extremely low in ovulated fish (1.2 .+-. 0.2). In vitro production of estradiol-17.beta. and testosterone by ovarian follicles in response to partially purified chinook salmon gonadotropin (SG-G100) was examined monthly using 18 h incubations. Throughout the vitellogenic period SG-G100 stimulated both estradiol-17.beta. and testosterone production: the steroidogenic response of follicles increased from June (about 2 ng/ml estradiol-17.beta.; 0.1 ng/ml testosterone) to Sept. (about 10 and 14 ng/ml, respectively). In Oct. full-grown immature follicles which could be induced to mature in vitro by hormone treatment produced large amounts of testosterone (about 130 ng/ml) but not estradiol-17.beta.. Postovulatory follicles also produced testosterone but the values were low (10 ng/ml) compared with full-grown immature follicles. Very low levels of estradiol-17.beta. were produced by postovulatory follicles. These results are discussed in relation to other studies on steroidogenesis in the amago salmon, with particular emphasis on the physiological role of testosterone in reproduction in female teleosts.
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