Seasonal variation in specific plasma- and target-tissue binding of androgens, relative to plasma steroid levels, in the brown trout, Salmo trutta L

Abstract

The circulating levels of the plasma androgens, testosterone and 11-ketotestosterone, and their specific binding to skin cytosol, skin nuclear extract, and plasma were determined in mature male and immature male and female brown trout during a single spawning cycle. 11-Ketotestosterone was not bound by any of the fractions examined whereas testosterone was bound with high affinity to plasma ( k D = 32.6 n M), skin cytosol ( k D = 16.9 n M), and skin nuclear extract (k D = 2.6 n M). The binding capacity of each fraction varied independently with time. In mature male fish an increase in specific binding of testosterone to nuclear extract, from 77 to 269 fmol mg −1 protein, occurred between September and November, coincident with peak androgen levels. Following the spawning period and the decline in androgen levels, nuclear-binding capacity in mature fish dropped to a level similar to that of immature fish by June. Nuclear binding in immature fish remained in the range 25–75 fmol mg −1 protein throughout. Plasma-binding capacity of both mature and immature fish declined during the spawning period, from 190 to 125 n M in mature fish and from 360 to 125 n M in immature fish. Plasma-binding capacity in both mature and immature fish increased following spawning to reach levels of 340 n M (mature) and 250 n M (immature). Little change was observed in cytosol-binding capacity of either mature or immature fish. The results suggest that androgen-induced structural changes in the integument are predominantly testosterone stimulated, are initiated by an increase in the concentration of a specific testosterone-binding protein within the nucleus, may be potentiated by a drop in plasma testosterone-binding capacity, and that a cytosol-binding protein of intermediate affinity for testosterone may maintain a high intracellular concentration of steroid.