Insulin and insulin-like growth factor-I (IGF-I) binding in fish red muscle: regulation by high insulin levels

Abstract

Insulin and IGF-I binding to semi-purified red muscle receptors was characterized in brown trout, Salmo trutta and the common carp, Cyprinus carpio. The yield of glycoprotein obtained after semipurification of receptors with WGA-agarose affinity chromatography in μg g −1 initial tissue was 210.6±21 μg g −1 in trout and 108.5±2.5 μg g −1 in carp. IGF-I specific binding (4.72±0.64%/10 μg glycoprotein) was 4–5-times higher than insulin binding (1.04±0.12%/10 μg glycoprotein) in trout red muscle. This difference in binding was due to a higher number and a greater affinity of the IGF-I ( K d, 0.21±0.03 nM) compared with the insulin ( K d, 0.67±0.06 nM) receptors in this tissue. Carp red muscle IGF-I binding (9.14±0.55%/10 μg glycoprotein) surpassed insulin binding (2.59±0.094%/10 μg glycoprotein) mainly because of a greater affinity of the IGF-I ( K d, 0.092±0.027 nM) compared with the insulin ( K d, 0.1515±0.0285 nM) receptor. IGF-I and insulin binding in carp red muscle were higher than in trout, as a consequence of a higher affinity of carp red muscle receptors. Arginine injection provoked acute hyperinsulinemia in both trout (23.3±1.01 ng ml −1) and carp (24.3±1.34 ng ml −1). Specific binding of insulin and IGF-I to the red muscle decreased 4 h after injection. In trout, a decrease of insulin and IGF-I binding of 47.0% and 63.3%, respectively was observed compared with controls; in carp, these values were 44.0% and 45.0%. The number of insulin and IGF-I receptors decreased (42–55%) but affinities did not change suggesting that receptor down-regulation is a consequence of high insulin levels.