Plasma and urine levels of C18, C19 and C21 steroids in an asynchronous fish, the tilapia Oreochromis mossambicus (Teleostei, Cichlidae).

Female and male tilapia, Oreochromis mossambicus, were treated with luteinizing hormone-releasing hormone analogue (LH-RHa) and pimozide (PIM) or with human chorionic gonadotropin (hCG) to stimulate gonadal development and sexual maturation. Plasma (both sexes) and urine (males) samples were collected periodically for steroid analysis by radioimmunoassay. Plasma levels of estradiol-17 beta (3-6 ng/ml) and testosterone, higher in female (up to 25 ng/ml) than in male (6-13 ng/ml; P < 0.05), were in the range of those established in other tilapia species. Plasma levels of the established teleost oocyte maturation-inducing steroids (MIS), that is 17 alpha,20 beta-dihydroxy-4-pregnen-3-one (17,20 beta-P) and 17 alpha,20 beta, 21-trihydroxy-4-pregnen-3-one (17,20 beta,21-P) were low (1-9 ng/ml) and were not different between treated and control fishes at 8, 12, 16, 24, 48, 72 and 96 hr after injection. Furthermore, in male O. mossambicus, 17,20 beta,21-P was undetectable. Plasma levels of 3 alpha,17 alpha,21-trihydroxy-5 beta-pregnan-20-one (3,17,21-P-5 beta) were very high in both sexes (up to 700 ng/ ml), mostly in hormone-treated groups, whose levels were higher than controls (P < 0.05). Urine levels of conjugated 17,20 beta,21-P (glucuronides and sulphates) were not detectable, but those of 17, 20 beta-P (up to 25 ng/ ml) and 3,17,21-P-5 beta (up to 1 microgram/ml) were higher than free 17,20 beta-P and 3,17,21-P-5 beta measured in the plasma of the same animals (P < 0.05). Both LH-RHa + PIM and hCG induced sexual maturation of O. mossambicus (histological data); nevertheless, during that period all measured steroids, either in plasma or urine, almost did not fluctuate. Thus, this study does not make any comment about the MIS of tilapia. Nevertheless, the high levels of conjugated 3,17,21-P-5 beta and 17,20 beta-P in urine suggest a probable pheromone role for these steroids in this species.