Steroid secretion of rainbow trout testis in vitro: Variation during the reproductive cycle

Testicular tissue collected at different stages of gonadal development was incubated with a pituitary extract (PE) from mature salmon. Three androgens (11-ketotestosterone, OT; 11β-hydroxytestosterone, OHT; and testosterone, T) and 17α,20β-dihydroxyprogesterone (17-20βP) were quantified by radioimmunoassay in incubation media. OHT and OT were secreted in larger quantities than T and 17-20βP. The PE dose that evoked a half-maximal response (ED₅₀), the ratios of maximum stimulated vs baseline secretion, and total testicular steroid output all changed during the reproductive cycle. Androgen secretion in response to PE was low in immature and spent fish, both in terms of ED₅₀ and the ratio of maximum stimulated vs baseline secretion. This ratio increased in testes showing the first signs of maturation and remained elevated during rapid testicular growth, before reaching maximum values at full maturity. The lowest ED₅₀ values were found at the end of spermatogenesis and during the peak spawning period. 17-20βP secretion could not be stimulated noticeably until the fish had entered the spawning period and, as opposed to androgens, remained stimulable in spent fish. ED₅₀ values for 17-20βP ranged, without showing clear-cut variations, above those calculated for androgens. The changes in PE reactivity and steroid secretion capacity during the reproductive cycle are likely to contribute to the changes in circulating steroid concentrations and may allow modulations of testicular steroid production without large changes in circulating GTH levels.