INSULIN AND INSULIN-LIKE GROWTH-FACTOR-I (IGF-I) BINDING IN FISH RED MUSCLE - REGULATION BY HIGH INSULIN LEVELS

Insulin and IGF-I binding to semi-purified red muscle receptors was ch aracterized 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 mu g g(-1) initi al tissue was 210.6+/-21 mu g g(-1) in trout and 108.5+/-2.5 mu g g(-1 ) in carp. IGF-I specific binding (4.72+/-0.64%/10 mu g glycoprotein) was 4-5-times higher than insulin binding (1.04+/-0.12%/10 mu g glycop rotein) in trout red muscle. This difference in binding was due to a h igher 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 tis sue. Carp red muscle IGF-I binding (9.14+/-0.55%/10 mu g glycoprotein) surpassed insulin binding (2.59+/-0.094%/10 mu g glycoprotein) mainly because of a greater affinity of the IGF-I (K-d, 0.092+/-0.027 nM) co mpared 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 con sequence of a higher affinity of carp red muscle receptors. Arginine i njection 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%, respective ly 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-regul ation is a consequence of high insulin levels. (C) 1997 Elsevier Scien ce B.V.