In vivo regulation of protein-serine kinases by insulin in skeletal muscle of fructose-hypertensive rats

The effects of tail-vein insulin injection (2 U/kg) on the regulation of protein-serine kinases in hindlimb skeletal muscle were investigated in hyperinsulinemic hypertensive fructose-fed (FF) animals that had been fasted overnight. Basal protein kinase B (PKB) activity was elevated about twofold in FF rats and was not further stimulated by insulin. Phosphatidylinositol 3-kinase (PI3K), which lies upstream of PKB, was increased similar to 3.5-fold within 2-5 min by insulin in control rats. Basal and insulin-activated PI3K activities were further enhanced up to 2-fold and 1.3-fold, respectively, in FF rats. The 70-kDa S6 kinase (S6K) was stimulated about twofold by insulin in control rats. Both basal and insulin-stimulated S6K activity was further enhanced up to 1.5-fold and 3.5-fold, respectively, in FF rats. In control rats, insulin caused a 40-50% reduction of the phosphotransferase activity of the beta-isoform of glycogen synthase kinase 3 (GSK-3 beta), which is a PKB target in vitro. Basal GSK-3 beta activity was decreased by similar to 40% in FF rats and remained unchanged after insulin treatment. In summary, 1) the PI3K --> PKB --> S6K pathway was upregulated under basal conditions, and 2) insulin stimulation of PI3K and S6K activities was enhanced, but both PKB and GSK-3 were refractory to the effects of insulin in FF rats.