ACTIVATION OF LIVER AND MUSCLE INSULIN-RECEPTOR TYROSINE KINASE-ACTIVITY DURING INVIVO INSULIN ADMINISTRATION IN RATS

We have studied autophosphorylation and tyrosine kinase activity of the insulin receptor purified from liver and muscle of fasted rats before and after infusion of insulin (100 mU/h) during a 2.5 h glucose clamp. Recovery of insulin receptors and insulin binding to the solubilised receptors was unaffected by the glucose clamp. Autophosphorylation of the insulin receptor β subunit was increased in liver receptors prepared from rats at the end of the glucose clamp compared to rats in the basal state both in the absence of insulin in vitro (109% increase, p<0.001) and after in vitro stimulation with 10-7 mol/l insulin (clamped vs fasted; 96% increase, p<0.001). Insulin (10-7 mol/l) stimulated autophosphorylation was also increased in muscle receptor preparations from clamped rats compared with rats in the basal state (58% increase, p<0.05). In both liver and muscle receptors, the clamp increased the amount of [32P]-phosphate incorporated into the β subunit without changing the sensitivity of the insulin stimulation. HPLC analysis of the tryptic phosphopeptides derived from the β subunit after insulin stimulated autophosphorylation of liver receptors revealed an increase of 32P in all phosphorylation sites without any change in the overall pattern. Tyrosine kinase activity of liver and muscle insulin receptors from clamped rats was also increased approximately twofold (p<0.05) when analysed using a synthetic substrate (poly Glu4 Tyr1). Our results support the notion that the insulin receptor exists in an active and inactive form, and that elevated plasma insulin concentrations increases the proportion of active receptors. © 1990 Springer-Verlag.