Abstract 277: Acute Cardiac Insulin Resistance During Cardiopulmonary Bypass in Mini-pigs: Role of Hexosamine Biosynthesis and Protein O-Glycosylation

Abstract

Our previous study as well as others has demonstrated a strong positive correlation between cardiac insulin signaling and myocardial function in ischemic/reperfused hearts. This study was designed to determine whether cardiac insulin signaling is impaired during cardiopulmonary bypass (CPB) and the possible mechanisms involved. Twelve male mini-pigs were anesthetized and subjected to CPB for 30 min. Blood samples and left ventricle biopsies were taken at pre-bypass (control), aortic cross clamp (AXC) 5 min and AXC release 120 min. Compared with sham-operated group, both blood glucose and insulin levels went up at 120 min after AXC release. Glucose uptake in heart dropped significantly as measured by attenuated coronary arterio-venous glucose difference and reduced cardiac 18 F-fluorodeoxyglucose uptake by Positron Emission Tomography imaging. Furthermore, myocardial insulin signaling was blunted as manifested by decreased phosphorylation of IRS-1, Akt and GSK-3beta (P<0.01, n=4). Interestingly, these changes were associated with a substantial increase of GFAT activity (the rate-limiting enzyme for hexosamine biosynthesis) and a significant increase in protein O-GlcNAcylation in cardiac tissues (P<0.05, n=6). Moreover, pretreatment with Alloxan, an inhibitor of O-GlcNAc-transferase, blocked both the impaired GSK-3beta phosphorylation and the increase in O-GlcNAcylation. Enhancement of cardiac insulin signaling with glucose-insulin-potassium before AXC decreased protein O-GlcNAcylation and ameliorated myocardial dysfunction after CPB. These data indicate the existence of acute myocardial insulin resistance during CPB which is attributable to elevated hexosamine biosynthesis and protein O-glycosylation.