The effect of temperature on testicular steroid production in the rainbow trout, Salmo gairdneri, in vivo and in vitro.

The effect of temperature on steroidogenesis in the male rainbow trout has been studied both in vitro using endogenous precursors under gonadotrophin stimulation and in vivo in fish held for 2 weeks at three different temperatures. In vitro, the optimum temperature for formation of testosterone and its 11-oxygenated derivatives was 10 degrees, whereas glucuronide formation showed an optimum at 18 degrees. In vivo, plasma levels of testosterone and 11-keto-testosterone were significantly higher at 6 than at 17 degrees, whereas glucuronide levels showed no significant difference. Milt was obtained only from fish held at 6 and 12 degrees. The optimum temperature for free steroid formation in response to gonadotrophin stimulus is shown to be dependent upon glucuronyl transferase content, and its progressive increase during the reproductive cycle may provide a mechanism for the inhibition of free steroid synthesis and hence spermiation at elevated temperatures where gamete survival is poor.