Evidence for an involvement of vitellogenin in the steroidogenic activity of rainbow trout (Oncorhynchus mykiss) vitellogenic oocytes.

In vivo and in vitro the concentration of vitellogenin (VTG) inside the oocyte can alter VTG production by the liver, modulating the synthesis of 17beta-estradiol (E(2)) by the ovary. To gain a greater insight into this mechanism, the in vitro production of free and conjugated testosterone (T), E(2), and androstenedione (A) by rainbow trout oocytes from the early and middle vitellogenic stage was measured by radioimmunoassay. There was a decreased E(2) production that was greater in September (40%) than October (30%), by the oocytes incubated with the vitellogenic fraction. The production of E(2) conjugated as glucuronide was lower than sulfate (P < 0.05), but similar in control and VTG-incubated oocytes. Levels of free T increased from September to October, and conjugates were both produced at low levels, and no differences were detected between control and incubated VTG oocytes. The decreased synthesis of E(2) by oocytes incubated with VTG was not followed by an increase in the amount of T or conjugated E(2), because there were no differences under the two circumstances. However, there was a reduced synthesis of A with oocytes producing low levels of E(2). These results suggest that the presence of high levels of VTG in the oocyte suppresses the synthesis of A and E(2), affecting the activities of the enzymes C17,20 lyase and aromatase and probably interfering with the heme protein cytochrome P450 which in the ovary catalyses C 17,20 lyase (P450 c17) and aromatase (P450 arom). Copyright 2000 Academic Press.