UP-REGULATION OF INSULIN BINDING IN FISH SKELETAL-MUSCLE BY HIGH INSULIN LEVELS

The effects of insulin titres on regulation of receptor binding were studied in several fish species. Insulin receptors were semi-purified by affinity chromatography (WGA-agarose) from skeletal muscle of carp, brown trout and rainbow trout that had been subjected to increases in insulinemia produced either by arginine injection, food administration, or adaptation to an experimental diet (extruded diet with high-digestibility carbohydrates). Arginine injection provoked acute hyperinsulinemia in both carp and trout. Specific binding of insulin to the skeletal muscle was significantly increased 3 h after injection (from 5.8 +/- 0.3 to 9.6 +/- 0.9%/10 mu g protein in carp and from 0.8 +/- 0.2 to 1.5 +/- 0.4%/10 mu g in trout). The same effect was observed in carp liver preparations (from 6.0 +/- 0.75 to 9.9 +/- 1.25%/10 mu g). No alterations in tyrosine kinase activity of the receptors were detected in either carp or trout preparations: basal activities of the receptors were maintained (3100 +/- 200 fmol P/fmol receptors/30 min and 3700 +/- 400 fmol P/fmol receptors/30 min, in carp and trout, respectively), as were the percentage of stimulation over basal levels obtained by incubation with insulin (227 +/- 25% and 160 +/- 10% respectively). Food ingestion raised plasma insulin levels more steadily. Specific binding also increased in skeletal muscle preparations, especially in carp (from 5.7 +/- 0.3 to 11 +/- 1.7%/10 mu g at 4 h and 10 +/- 0.7%/10 mu g at 8 h). Tyrosine kinase activity was maintained without significant changes. Rainbow trout adapted for 2 months to an extruded diet presented higher insulin titres and higher glycogen reserves in liver and muscle. Insulin binding to skeletal muscle preparations was also significantly increased (from 0.36 +/- 0.02 to 0.77 +/- 0.1%/10 mu g), as was tyrosine kinase activity (from 132 +/- 4% to 156 +/- 6%, without alterations in the basal activity). Results showed that fish can respond to both acute and maintained increases in insulinemia by increasing the number of insulin receptors. Tyrosine kinase activity, in contrast, is only modified after long-term adaptation.