Insulin binding to liver plasma membranes of coho salmon during smoltification.

Smoltification of coho salmon (Oncorhynchus kisutch) is accompanied by characteristic changes in plasma insulin levels, with peak values attained at the beginning of the transformation period (the parr to transitional stage). To relate these changes to responsiveness of target tissues a study was made of binding of homologous insulin to liver plasma membranes in coho salmon undergoing the parr-to-smolt transformation. In general, parameters of specific insulin binding to the preparations of liver plasma membranes were similar to same parameters of insulin binding in other teleost fishes investigated so far and in mammals. The total binding capacity of insulin, a value that reflects the total number of binding sites, was higher in smolts than in parrs. The number of high affinity, low capacity binding sites in the liver membranes steadily increased as smoltification progressed, reached a maximum in early smolts, and declined thereafter. Another increase in the numbers of high affinity binding sites was observed in smolts transferred from fresh water to seawater. The specific binding of insulin in salmon at particular developmental stages was probably related to, and determined by, both the quantity of binding sites and their relative affinities. In smolts the highest levels of specific binding of insulin to the liver plasma membranes were coincident with the low levels of insulin in the peripheral blood that passed through the liver.