Plasma 17α,20β-dihydroxy-4-pregnen-3-one levels during sexual maturation of amago salmon ( Oncorhynchus rhodurus): Correlation with plasma gonadotropin and in vitro production by ovarian follicles

Abstract

Plasma gonadotropin (GtH) and 17α,20β-dihydroxy-4-pregnen-3-one (17α,20β-diOH-prog) levels were assessed by radioimmunoassay during the sexual maturation of female amago salmon ( Oncorhynchus rhodurus). Both GtH and 17α,20β-diOHprog levels were low in vitellogenic females (June to September) and in those with full-grown immature oocytes and were elevated in mature and ovulated females to 40 ng/ml for GtH and 50–70 ng/ml for 17α,20β-diOHprog. In vitro production of 17α,20β-diOHprog by ovarian follicles and its stimulation by partially purified chinook salmon gonadotropin (SG-G100) was examined monthly using 18 hr incubations. In June and July, levels were too low to detect (less than 30 pg/ml) in media from all treatment groups. In August and September, SG-G100 at a concentration of 1 μg/ml stimulated low levels of production (0.2–0.3 ng/ml). Full-grown, immature follicles in October showed a dose-response production of 17α,20β-diOHprog, avaraging over 10 ng/ml when incubated with 1 μg/ml SG-G100. Postovulatory follicles (1–2 days after ovulation) produced large amounts of 17α,20β-diOHprog, averaging over 100 ng/ml with 1 μg/ml SG-G100. These results are discussed in relation to previous work on amago salmon and other species and together indicate that 17α,20β-diOHprog is the major steroid responsible for oocyte maturation in amago salmon, produced as the follicular mediator of gonadotropin.