Temperature regulation of ovarian steroid production in the common carp, Cyprinus carpio L., in vivo and in vitro.

The effect of temperature on ovarian steroid production in the common carp, Cyprinus carpio L., has been studied in vitro with exogenous and endogenous precursors, and in fish held at three different temperatures in vivo. With radioactive testosterone as substrate, the major metabolite was testosterone glucuronide, but androstenedione and 5 alpha-androstane-3 beta,17 beta-diol were also identified. 5 alpha-Androstane-3 beta,7 alpha,17 beta-triol was tentatively identified and two other polar metabolites were isolated, one of which was convertible to this triol. A significant increase in production of most metabolites occurred between 20 and 24 degrees. Production of estradiol and testosterone from endogenous substrate under gonadotrophin stimulation in vitro showed a marked temperature dependence, but the response was closely related to ovarian maturity. Stage 4-5 ovaries produced testosterone, while late Stage 3 tissue produced only estradiol. Neither steroid was produced in significant quantities by less mature ovaries. The results indicate that the "switch off" of ovarian aromatase activity at the end of vitellogenesis is actuated by an ovarian rather than by a pituitary factor. Secretion of testosterone and estradiol showed a very significant change with temperature with the optimum at 24-29 degrees. Profiles for individual fish show that this optimal range is extremely narrow, particularly for estradiol, where secretion may increase as much as twentyfold over 5 degrees. The results in general correlate well with 24 degrees as the most favourable temperature for reproduction in carp. Plasma concentrations of testosterone and estradiol closely paralleled the in vitro secretion rates of these hormones. Plasma testosterone levels were greatest in the most mature fish, whereas plasma estradiol was significantly higher in late Stage 3 fish than in those of greater or lesser ovarian maturity. More Stage 4 and 5 fish were found in the group held at 24 degrees than at 20 or 29 degrees for 4 weeks, but all groups contained a high proportion of early Stage 3 fish.