Effect of Actinomycin D and Cycloheximide on Gonadotropin‐Induced 17α, 20β‐Dihydroxy‐4‐pregnen‐3‐one Production by Intact Ovarian Follicles and Granulosa Cells of the Amago Salmon, Oncorhynchus rhodurus *

The effect of actinomycin D and cycloheximide on gonadotropin (partially purified chum salmon gonadotropin, SGA)-induced 17α, 20β-dihydroxy-4-pregnen-3-one (17α, 20β-diOHprog, a maturation-inducing steroid in amago salmon) production was examined in intact ovarian follicles and granulosa cells of postvitellogenic amago salmon, Oncorhynchus rhodurus. Both actinomycin D and cycloheximide blocked gonadotropin-induced 17α, 20β-diOHprog production by intact follicles. In contrast, gonadotropin-induced 17α-hydroxyprogesterone production by intact follicles was not abolished by actinomycin D, but was abolished by cycloheximide, suggesting that postvitellogenic amago salmon ovarian follicles already contain the RNAs necessary for the synthesis of 17α-hydroxyprogesterone. In isolated granulosa cells, chum salmon gonadotropin was able to stimulate 17α, 20β-diOHprog production only when a precursor, 17α-hydroxyprogesterone was provided in the incubation medium, indicating that gonadotropin acts directly on granulosa cells to enhance the activity of 20β-hydroxysteroid dehyrogenase (20β-HSD). Total inhibition of 20β-HSD enhancement in granulosa cells, judged by 17α, 20β-diOHprog production, was achieved when actinomycin D was added between 1 hr before the start of incubation with 17α-hydroxyprogesterone and gonadotropin to 6 hr after. With cycloheximide total inhibition was observed when added in the period of 1 hr before to 9 hr after the start of the incubation. These results suggest that chum salmon gonadotropin acts on granulosa cells to enhance the de novo synthesis of 20β-HSD by a mechanism involving RNA synthesis.