Differential Regulation of Glycolysis in Neonatal Versus Adult Cardiac Myocytes: The Role of Hexokinase Isoforms

Abstract

In cardiac cells there is a dramatic switch at birth in metabolism from glycolysis to fatty acid utilization. This switch is paralleled by a switch between the two isoforms of hexokinase, HK1 and HK2, which phosphorylate glucose as the first step in glycolysis. HKI predominates in neonatal cardiac myocytes (NRVM) and HKII in adult cardiac myocytes. Using real time optical methods in isolated cells to study the function and localization of HKs, we found that NRVMs have a much higher glycolytic activity than adult cells. This increased activity is accompanied by stronger HK's interaction with mitochondria as judged by fusion of HK's with fluorescent proteins. In both cell types, all HKI is bound to mitochondria, while HKII distributes between the mitochondria and cytosol. Removal of extracellular glucose displaces HKII from mitochondria in adult cells, but not in NRVMs. The glycolytic inhibitor iodoacetate, which raises intracellular G6P levels, displaced HKII from the mitochondria in both adult and NRVMs but had no effect on HKI distribution. Based on these results, we attribute elevated glycolytic activity of NRVMs to their preferential expression HKI, whose intrinsically higher binding affinity for mitochondria directs glucose and G6P to glycolysis instead of glycogen formation. This in turn lowers G6P levels, facilitating the interaction of HKII with mitochondria. Since HK binding to mitochondria facilitates cardioprotection, this factor may also account, at least in part, for the greater resistance of NRVMs to ischemia/reperfusion injury compared to adult cardiac myocytes.