Compartmentation of hexokinase in rat heart. A critical factor for tracer kinetic analysis of myocardial glucose metabolism.

Abstract

Radiolabeled analogues of 2-deoxyglucose are widely used to trace glucose metabolism in cell cultures, whole organs, and intact animals, although kinetic differences in transport and phosphorylation between these compounds and glucose exist. The present studies were undertaken to determine the effects of insulin stimulation on the phosphorylation of 2-deoxyglucose compared to glucose in the intact, saline-perfused working rat heart. Rates of glucose utilization determined from tritiated glucose differed from rates estimated from the accumulation of I 'CI2-deoxyglucose in a nonconstant manner when comparing rates in the absence or presence of physiologic levels of insulin (13 pU/ml). The fraction of monophosphorylated hexoses that was accounted for by ['4C]2-deoxyglucose 6-phosphate was dramatically decreased in hearts perfused in the presence of insulin. Additionally, hexokinase activity associated with the mitochondrial fraction of tissue extracts was increased in hearts stimulated by insulin. While this redistribution of hexokinase to the mitochondria did not affect the apparent affinity constant for glucose, hexokinase bound to mitochondria exhibited an 8.5-fold decrease in the affinity for 2-deoxyglucose when compared with hexokinase present in the cytosolic fraction. The findings are consistent with an insulin-mediated preferential uptake and phosphorylation of glucose compared to deoxyglucose. The results also imply that the redistribution of hexokinase and the differential effect of insulin on its affinity for tracer and tracee are responsible for changes in the "lumped constant" (i.e., the correction factor used to equate 2-deoxyglucose to glucose uptake). These changes must be taken into account when regional myocardial glucose metabolism is assessed by the 2-deoxyglucose method. (J.

Cardiac and skeletal muscle insulin resistance in patients with coronary heart disease. A study with positron emission tomography.

“…The value of the lumped constant has been measured in the hearts of rabbits (22)(23)(24) and dogs (25), and had been found to be constant over a range of insulin and glucose concentrations as well as during anoxia. However, other authors have reported variability of the lumped constant in perfused rat hearts with the addition of insulin (26,27), and have recommended that PET studies of FDG uptake in the heart should be performed under controlled metabolic conditions. For this reason, the studies here reported have been performed during steady-state conditions of hyperinsulinemia and euglycemia as realized by the insulin clamp technique.…”

Glucose for the Heart

“…Hexokinase is present in the cytosolic fraction of the cell but also binds to the outer mitochondrial membrane. 23 Binding lowers the K m for glucose and increases hexokinase activity, 24 although the K m for 2-deoxyglucose remains nearly 10-fold higher than that for glucose. 24 This attachment also suppresses inhibition of hexokinase by glucose 6-phosphate.…”

“…23 Binding lowers the K m for glucose and increases hexokinase activity, 24 although the K m for 2-deoxyglucose remains nearly 10-fold higher than that for glucose. 24 This attachment also suppresses inhibition of hexokinase by glucose 6-phosphate. 23 Insulin shifts the control strength of glucose uptake from glucose transport to phosphorylation.…”

Effects of Insulin on Subcellular Localization of Hexokinase II in Human Skeletal Musclein Vivo1

“…This could place hexokinase in closer apposition to the supply of glucose from glucose transporters in the membrane and could potentially increase the rate of glucose phosphorylation and uptake. In heart muscle, which primarily expresses HKII, insulin has been reported to redistribute hexokinase activity to the mitochondria (23). These investigators propose that this redistribution is important in insulin stimulation of glucose uptake.…”