BECKMAN, B.R.*; SHIMIZU, M; GADBERRY, B; COOPER, K; National Marine Fisheries Service: Feeding rate, growth and plasma IGF-I and 41 kDa IGFBP levels in coho salmon.

Insulin-like growth factor-I (IGF-I) is one of the primary endocrine mediators of growth in vertebrates. The actions of IGF-I are extensively modified by a family of IGF binding proteins (IGFBPs) in mammalian systems. The presence of IGFBPs has also been established in piscine models. In mammalian systems IGFBP3 appears to mediate the anabolic role of IGF-I. Shimizu et al. (this meeting) have identified a 41 kDa IGFBP in coho salmon that reacts to physiological perturbations in a similar manner as mammalian IGFBP3. However, the physiological roles of the IGFBPs in fish has not been fully established. Here we describe an experiment that examined the result of fluctuating feeding rates on IGF-I and 41 kDa IGFBP levels over two week intervals. Approximately 300 yearling coho salmon were individually tagged and placed in four 1.3m circular tanks and fed a maintenance ration for four weeks. Subsequently, fish in two tanks were fed at a 2% body weight/day ration with the remainder were fed 1%. After four weeks feeding rates were again bifurcated, with one tank from each ration level shifted to a 0.5% body weight/day ration for four additional weeks. Finally, all tanks were restored to their original ration level for a further two weeks. All fish were measured at a two week interval. A sub-sample of 12 fish/tank was obtained at a two week interval, plasma collected, and plasma levels of IGF-I and 41 kDa IGFBP were measured by RIA. Both plasma IGF-I and 41 kDa IGFBP responded to changes in feeding rate, decreasing with decreasing feed and increasing with increasing feed. Moreover, plasma IGF-I and 41 kDa IGFBP were both significantly correlated with growth rate and each other over each two week interval.