The mechanism responsible for the diminished activation of glycogen synthase (GS) in diabetic myotubes remains unclear, but may involve increased activity and/or expression of glycogen synthase kinase-3 (GSK-3). In myotubes established from type 2 diabetic and healthy control subjects we determined GS activity ratio, protein expression, and activity of GSK-3alpha and beta under basal and insulin-stimulated conditions when precultured in increasing insulin concentrations. In myotubes precultured at low insulin concentrations acute insulin stimulation increased GS activity more in control than in diabetic subjects, whereas the corresponding GSK-3alpha but not GSK-3beta activity was significantly reduced by acute insulin treatment in both groups. However, in myotubes precultured at high insulin concentrations the effect of insulin on GS and GSK-3alpha activity was blunted in both groups. The protein expression of GSK-3alpha or beta was unaffected. In conclusion, myotubes with a primary defect in GS activity express insulin responsive GSK-3alpha, suggesting that failure of insulin to decrease GS phosphorylation involves abnormal activity of another kinase or phosphatase. (C) 2004 Elsevier Inc. All rights reserved.