MOLECULAR ANALYSIS OF INSULIN-RESISTANCE IN ISOLATED VENTRICULAR CARDIOMYOCYTES OF OBESE ZUCKER RATS

Isolated ventricular cardiomyocytes obtained from lean and genetically (fa/fa) obese Zucker rats were used to correlate alterations of insul in-induced glucose transport activation and GLUT-4 translocation to po ssible defects of the insulin signaling cascade. Maximal stimulation w ith insulin was found to produce an unaltered translocation of GLUT-4 to the plasma membrane (4.2- and 3.7-fold increase for lean and obese rats, respectively). However, a largely reduced sensitivity of 3-O-met hylglucose transport could be detected in obese rats at physiological doses of insulin (completely unresponsive at 8 x 10(-11) M compared wi th 3-fold stimulation of glucose transport in lean controls). Tyrosine phosphorylation of the insulin receptor beta-subunit and the insulin receptor substrate 1 (IRS-1) was stimulated identically in cardiomyocy tes from both lean and obese rats. Labeling of cells with [P-33]orthop hosphate revealed a marked increase in the serine and/or threonine pho sphorylation of IRS-1 in the obese group (370% of lean controls), with a concomitant reduction in IRS-1 abundance (30-40%). The reduced sens itivity of glucose transport at 8 x 10(-11) M insulin was then found t o correlate to a completely blunted response of IRS-1-associated phosp hatidylinositol S-kinase activity in cardiomyocytes from obese rats. T hose data show that cardiac insulin resistance of obesity involves def ective insulin signaling at low concentrations of the hormone, whereas GLUT-4 translocation is fully operative in the isolated cell. It is s uggested that hyperphosphorylation of IRS-1 may significantly contribu te to the pathogenesis of insulin resistance in the heart.