17 alpha,20 beta-dihydroxy-4-pregnen-3-one: plasma levels during sexual maturation and in vitro production by the testes of amago salmon (Oncorhynchus rhodurus) and rainbow trout (Salmo gairdneri).

The present study investigated seasonal changes in plasma 17 alpha,20 beta-dihydroxy-4-pregnen-3-one (17 alpha,20 beta-diOHprog) levels in precociously mature and fully grown mature male amago salmon (Oncorhynchus rhodurus) and in vitro 17 alpha,20 beta-diOHprog production by the testes of amago salmon and rainbow trout (Salmo gairdneri) in response to chum salmon gonadotropin. Plasma 17 alpha,20 beta-diOHprog levels were very low from June to September, and rapidly increased in October at the beginning of the spawning season in both precociously mature and fully grown mature male amago salmon. High levels of 17 alpha,20 beta-diOHprog were maintained during the period of active spermiation in precociously mature males, and quickly declined in mid-November. Incubation of testicular fragments from spermiating rainbow trout with chum salmon gonadotropin or 17 alpha-hydroxyprogesterone resulted in a highly significant increase in 17 alpha,20 beta-diOHprog levels in the incubation medium. The chum salmon gonadotropin was also effective in stimulating 17 alpha,20 beta-diOHprog production by testicular fragments obtained from amago salmon after the breeding season. These findings are discussed in relation to the possible involvement of 17 alpha,20 beta-diOHprog in spermiation of salmonids.