Fructose-2,6-bisphosphate, a potent stimulator of phosphofructokinase, is increased by high exogenous glucose perfusion

Relative importance of enhanced glucose uptake versus attenuation of long-chain acyl carnitines in protecting ischemic myocardium

BACKGROUND

A number of experimental studies have shown that increasing glucose use or decreasing accumulation of long-chain acyl carnitines (LCAC) protect ischemic hearts.

METHODS

To evaluate the relative importance of these two strategies in protecting ischemic myocardium, isolated rat hearts (n = 6 in each group) were paced at 300 bpm and subjected to 50 min of low-flow ischemia followed by 60 min of reperfusion. Buffer contained 0.4 m mol/l albumin, 0.4 m mol/l palmitate, and 70 mU/l insulin, and either normal glucose (5 m mol/l) (CON), high glucose (10 m mol/l total) (HG, known to increase glucose use), 5 m mol/l glucose and niacin (10 micromol/l) (NIA, known to increase glucose use and decrease LCAC) or carnitine (10 m mol/l) (CAR, known to increase glucose use and decrease LCAC). Separate groups of hearts were perfused in the presence of 10 micromol/l cytochalasin-B (CB), an inhibitor of insulin-sensitive glucose transporters.

RESULTS

Ischemic injury, as assessed by creatine kinase (CK) release was diminished by an average of 50% in HG, NIA, and CAR hearts, and the percentage recovery of left ventricular (LV) function with reperfusion was enhanced by approximately 20% compared with CON hearts (P < 0.05 for each comparison). Cytochalasin-B abolished all of the salutary effects. Long-chain acyl carnitines levels were higher in HG hearts compared with NIA- and CAR-treated hearts ( P < 0.05), but ischemic protection and functional recovery was greater in HG hearts.

CONCLUSIONS

The data support the adjunctive use of agents that promote glucose uptake during ischemia and suggest that increasing glucose use is more important than decreasing LCAC in the protection against ischemic injury or in the recovery of contractile function.

Protection of ischemic hearts by high glucose is mediated, in part, by GLUT-4

Metabolic interventions that promote glucose use during ischemia have been shown to protect ischemic myocardium and improve functional recovery on reperfusion. We evaluated whether the cardioprotection afforded by high glucose during low-flow ischemia is associated with changes in the sarcolemmal content of glucose transporters, specifically GLUT-4. Isolated rat hearts were paced at 300 beats/min and perfused under normal glucose (5 mM) or high glucose (10 mM) conditions in buffer containing 0.4 mM albumin, 0.4 mM palmitate, and 70 mU/l insulin and subjected to 50 min of low-flow ischemia and 60 min of reperfusion. To determine the importance of insulin-sensitive glucose transporters in mediating cardioprotection, a separate group of hearts were perfused in the presence of cytochalasin B (10 microM), a preferential inhibitor of insulin-sensitive glucose transporters. Ischemic contracture during low-flow ischemia and creatine kinase release on reperfusion was decreased, and the percent recovery of left ventricular function with reperfusion was enhanced in hearts perfused with high glucose (P < 0.03). Hearts perfused with high glucose exhibited increased GLUT-4 protein expression in the sarcolemmal membrane compared with control hearts under baseline conditions, and these changes were additive with low-flow ischemia. In addition, high glucose did not affect the baseline distribution of sarcolemmal GLUT-1 and blunted any changes with low-flow ischemia. These salutary effects were abolished when glucose transporters are blocked with cytochalasin B. These data demonstrate that protection of ischemic myocardium by high glucose is associated with increased sarcolemmal content of the insulin-sensitive GLUT-4 and suggest a target for the protection of jeopardized myocardium.