Species variation of gonadotropic activity in an in vitro assay measuring androgen formation by carp ( Cyprinus carpio) testis with special reference to bioassay of piscine gonadotropins

The objectives of this study were to establish a suitable and validated in vitro bioassay of piscine gonadotropins (GTHs) by using a carp testis androgen production system and to compare the androgenic responses in such an assay to gonadotropins from various vertebrate species. The testes from mature carp with gonadosomatic indices of 8–30% were used. Androgen production was first compared with respect to methods for preparation of the carp testis (sliced, minced, homogenized, and collagenase-dispersed testis preparations). The time course of androgen formation, the effects of xanthine and theophylline, and other factors on androgen production also were investigated. Theophylline was more effective than xanthine in potentiation of gonadotropin-evoked androgen formation by carp testis. The testis preparations were incubated in medium 199 (pH 7.40) containing 2 m M theophylline with shaking at 100 cycles/min at 25°C for 4 hr. Homogenized testis preparations had limited ability for androgen production, while sliced, minced, and minced-collagenase-dispersed testis preparations were highly responsive to gonadotropins for androgen production. The minced testis preparation, utilizing 100 mg/ml incubation medium per vial, was chosen as the standard incubation procedure in this study. The minced testis androgen production assay was highly sensitive to gonadotropins from several piscine species (silver carp, common carp, and salmon), and all these GTHs produced parallel dose-related androgen production curves. Mammalian GTHs were also capable of promoting androgen formation by carp testis, but they were much less potent than were piscine GTHs. Pregnant mares' serum gonadotropin (PMSG) was more effective than human chorionic gonadotropin (hCG) in evoking carp testis androgen production. This contrasts with the situation in a similar in vitro mammalian testis androgen assay where hCG is more potent than PMSG. The carp testis androgen production system established in the present study is by far the most sensitive in vitro bioassay currently available for measurement of piscine gonadotropins. This system not only provides a sensitive and convenient bioassay for purified GTH's from various piscine species, but is also useful for phylogenetic comparison of GTH molecules or testicular GTH receptors in various vertebrate classes, and for speculation concerning molecular evolution of the gonadotropic systems.