Abstract 3507: Cardioprotection by Glycogen Synthase Kinase-3 Inhibition: Role of Reduced Proton Production From Glucose Metabolism and Attenuation of Calcium Overload
Glycogen synthase kinase-3 (GSK) regulates different signaling pathways affecting glycogen metabolism, protein synthesis, mitosis and apoptosis. Inhibition of GSK limits cardiac ischemia-reperfusion injury, but mechanisms are not clearly defined. This study tested the hypothesis that acute GSK inhibition will stimulate glycogen synthesis, repartition glucose away from glycolysis, reduce proton (H+) production from uncoupled glucose metabolism and limit intracellular Ca2+ ([Ca2+]i) overload. Isolated working rat hearts were perfused with Krebs solution containing 100 μU/ml insulin, and 1.2 mM palmitate and 11 mM [3H/14C]glucose as energy substrates. The selective GSK inhibitor, SB-216763 (SB, 3 μM), given either before global ischemia or at the onset of reperfusion, improved recovery of LV work compared with vehicle-treated hearts (63.6±7.4%, n=6, P<0.01 and 66.9±7.3%, n=6, P<0.01 vs. 29.8±7.3%, n=10, respectively). During reperfusion, SB increased glycogen synthesis (by 118%, P<0.01) and reduced glycolysis and H+ production by 62% (P<0.05) and 70% (P<0.05). Measurement of [Ca2+]i by rapid acquisition indo-1 fluorescence imaging showed that SB reduced diastolic [Ca2+]i overload both during ischemia (by 42%, P<<med>0.0005) as well as during reperfusion (by 61%, P<0.005,). SB had no effects on LV work or glucose metabolism in aerobically-perfused glycogen-replete hearts (n=7). However, in hearts (n=7) depleted of glycogen by substrate-free perfusion to a level similar to that measured at the onset of reperfusion, SB accelerated glycogen synthesis (by 40%, P<0.05) and reduced glycolysis (by 31%, P<0.005) and H+ production (by 38%, P<0.005) independent of changes in LV work. This confirms that SB-induced stimulation of glycogen synthesis and inhibition of glycolysis during reperfusion are a cause of improved Ca2+ homeostasis and recovery of LV work and are not simply a consequence of improved mechanical function. Our study indicates that stimulation of glycogen synthesis and subsequent reduction in H+ production by GSK-3 inhibition is an early and upstream event that lessens Ca2+ overload during ischemia and early reperfusion which may ultimately contribute to previously observed reductions in mitochondrial dysfunction and cell death.