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

The effects of insulin titres on regulation of receptor binding were s tudied in several fish species. Insulin receptors were semi-purified b y 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-diges tibility carbohydrates). Arginine injection provoked acute hyperinsuli nemia in both carp and trout. Specific binding of insulin to the skele tal 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 al terations in tyrosine kinase activity of the receptors were detected i n 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 i ncubation with insulin (227 +/- 25% and 160 +/- 10% respectively). Foo d ingestion raised plasma insulin levels more steadily. Specific bindi ng 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 prese nted higher insulin titres and higher glycogen reserves in liver and m uscle. Insulin binding to skeletal muscle preparations was also signif icantly increased (from 0.36 +/- 0.02 to 0.77 +/- 0.1%/10 mu g), as wa s tyrosine kinase activity (from 132 +/- 4% to 156 +/- 6%, without alt erations in the basal activity). Results showed that fish can respond to both acute and maintained increases in insulinemia by increasing th e number of insulin receptors. Tyrosine kinase activity, in contrast, is only modified after long-term adaptation.