UPTAKE AND CLEARANCE OF EXOGENOUS ESTRADIOL-17-BETA AND TESTOSTERONE DURING THE EARLY DEVELOPMENT OF COHO SALMON (ONCORHYNCHUS-KISUTCH), INCLUDING EGGS, ALEVINS AND FRY

The uptake and clearance of estradiol-17beta (E2) and testosterone (T) were examined during the initial stages of development of coho salmon (Oncorhynchus kisutch), including eyed-eggs, newly hatched alevins an d first feeding fry. Radiolabeled steroids were administered through t he water in tracer amounts with or without their nonradioactive form a t 400 mug l-1. Regardless of developmental stage, saturation levels we re invariably attained earlier for T than for E2, thus resulting in a higher incorporation of E2. However, both steroids had similar clearan ce patterns. Uptake and clearance was clearly stage-dependent, being f astest in fry, intermediate in alevins and slowest in eggs. Furthermor e, combined uptake and clearance patterns showed that exposure to ster oid was also higher for E2 than for T and stage-dependent, but always markedly highest in alevins. Subsequently, based on the observed elimi nation of the estrogen, a double immersion in E2 at 400 mug l-1, admin istered 2 days apart to maximize exposure during the alevin stage, was assayed for its effect on sex reversal and found to induce the produc tion of 100% females. We suggest that the yolk, which is present in su bstantial amounts during the initial stages of development in salmonid s, can retain the exogenously administered liposoluble steroids, thus providing developing embryos with an extended supply of, and exposure to, these steroids well after the treatment is finished. Together, the se findings help to explain the previously observed high effectiveness of sex steroids administered during early development in regulating g onadal differentiation in salmonids, the higher effectiveness of E2 co mpared to T, and clarify the localization of the most sensitive period to the action of exogenous steroids at the alevin stage in the coho s almon.