LAUSR

Introduction: Insulin stimulates glucose oxidation, an effect which is associated with simulating pyruvate dehydrogenase (PDH). However, how the insulin signal is transduced from the cell membrane to the mitochondria to stimulate PDH is not known. Protein kinase B (Akt), protein kinase C-delta (PKCδ) and glycogen synthase kinase-3beta (GSK-3β) are main components of the cytosolic insulin signalling pathway and it has been suggested that they can be translocated to the mitochondria following insulin receptor activation in noncardiac tissue. Therefore, we investigated whether any of these kinases has a role mediated cardiac insulin-stimulated glucose oxidation in the heart. Methods and Results: Male and female C57BL/6 mice were anesthetized and hearts were collected and perfused in the isolated working heart mode. Hearts were perfused with [5- 3 H] glucose and [U- 14 C] glucose to simultaneously measure glycolysis and glucose oxidation rates, respectively, in the presence and absence of insulin. Insulin enhanced the phosphorylation and translocation of Akt Ser473 , PKCδ Tyr311 and GSK-3β Ser9 to the cardiac mitochondria along with enhancing PDH activity by decreasing its phosphorylation. Pharmacological inhibition of Akt using AktiVIII completely abolish the stimulatory effect of insulin on cardiac glucose oxidation rates (354 ± 145 vs 2307 ± 185 nmol. g dry wt -1 . min -1 in vehicle-treated hearts, p<0.05). However, pharmacological inhibition of either PKCδ or GSK-3β using Bisindolylmaleimide I or 3F8, respectively, did not have a significant effect on insulin-stimulated glucose oxidation rates. None of the pharmacological inhibitors had any significant effect on cardiac glycolysis. Conclusion: Insulin mediates its stimulatory effect on cardiac glucose oxidation via a mechanism which involve the activation and translocation of Akt to the mitochondria.