Effects of wortmannin on insulin- and ischemia-induced stimulation of GLUT4 translocation and FDG uptake in perfused rat hearts.

OBJECTIVE Myocardial glucose transport is enhanced by hormonal and other stimuli such as ischemia and hypoxia which induce glucose transporter 4 (GLUT4) translocation. Whether insulin and ischemia share a common signaling mechanism is not yet known. This study investigated whether phosphatidylinositol 3-kinase (PI3K), a signaling intermediate of the insulin-responsible pathway, also participates in the ischemia-induced stimulation of glucose. METHODS Isolated Langendorff-perfused Sprague-Dawley rat hearts were subjected to 100 nmol/l insulin or 15 min of no-flow ischemia with/without 1 mumol/l wortmannin, an inhibitor of PI3K. After perfusion, relative subcellular glucose transporter GLUT4 distribution was assessed by membrane fractionation and immunoblotting and compared to controls. Uptake kinetics of the glucose analog [18F]fluoro-deoxyglucose (FDG) were also studied during perfusion of rat hearts. RESULTS GLUT4 translocation to the plasma membrane (PM) was increased by insulin 1.8-fold and by ischemia 2.4-fold (P < 0.05). FDG uptake was increased by insulin 6.0-fold and by ischemia 6.2-fold (P < 0.05). Wortmannin 1 mumol/l inhibited insulin-mediated translocation of GLUT4 and increase in FDG uptake completely. However, it did not show any effect on ischemia-stimulated GLUT4 translocation or on ischemia-induced increase in FDG utilization. A significant correlation was found between relative GLUT4 translocation and FDG uptake in hearts of the insulin series (r = 0.9, P < 0.05) and of the ischemia series (r = 0.8, P < 0.05). CONCLUSIONS Our results demonstrate that wortmannin did not inhibit ischemia-induced stimulation of myocardial glucose transport, supporting the hypothesis of different signaling pathways for ischemia and insulin.