Steroidogenesis during induced oocyte maturation and ovulation in the African catfish,Clarias gariepinus

Abstract

Changes in ovarian steroidogenesis accompanying oocyte maturation and ovulation were studied in the African catfish,Clarias gariepinus. Laboratory-reared females with postvitellogenic ovaries were treated with pimozide and LHRH-analogue. The plasma gonadotropin levels were determined by means of a homologous radioimmunoassay, the condition of the ovaries was studied by histological examination of the follicles, and the steroidogenetic capacity of the ovaries was analyzed byin vitro incubation of tissue fragments for 3 h with [(3)H]-pregnenolone and [(3)H]androstenedione as precursors. Data were collected at regular intervals between 0 and 16 h after pimozide-LHRH analogue administration.Until 4 h after the beginning of the experiments the plasma gonadotropin levels did not rise above 25 ng/ml, the ovaries remained in the stage of postvitellogenesis, and testosterone was the main end product of steroidogenesis. Four hours later the gonadotropin concentration in the blood had risen to more than 150 ng/ml, and the ovaries had entered the stage of germinal vesicle migration. At the same time steroidogenesis shifted towards the production of 17α,20β-dihydroxy-4-pregnen-3-one, 5β-pregnane-3α, 17α-diol-20-one, 5β-pregnane-3α,6α,17α-triol-20-one, 5β-pregnane-3α,17α,20β-triol and 5β-pregnane-3α,6α,17α,20β-tetrol. During the subsequent stage of germinal vesicle breakdown the plasma gonadotropin level remained high, and the synthesis of the C21-steroids showed a further increase. Simultaneously, the production of some C19-steroid glucuronides was enhanced. The preovulation and especially the postovulation stages were accompanied by a gradual decrease in steroidogenic capacity of the ovaries, even though the plasma gonadotropin level remained high. It is concluded that the prematuration surge of gonadotropin influences the activity of enzymes involved in steroidogenesis, leading to a reduced C17,20-lyase and to an augmented activity of the enzymes 20β-hydroxysteroid dehydrogenase (HSD), 5β-reductase, 3α-HSD, 6α-hydroxylase and UDP-glucuronosyltransferase. During ovulation the activity of all steroidogenic enzymes, including such key enzymes as 3β-HSD and 17α-hydroxylase, gradually decreases.Not only 17α,20β-dihydroxy-4-pregnen-3-one, but also the 5β-reduced pregnanes may be involved in inducing oocyte maturation and/or ovulation. The very polar triol and tetrol products may function, together with the steroid glucuronides as sex pheromones.