Role of glucose metabolism in the recovery of postischemic LV mechanical function: effects of insulin and other metabolic modulators

DRD4 Mitigates Myocardial Ischemia/Reperfusion Injury in Association With PI3K/AKT Mediated Glucose Metabolism

Ischemia-reperfusion (I/R) could cause heart irreversible damage, which is tightly combined with glucose metabolism disorder. It is demonstrated that GLUT4 (glucose transporter 4) translocation is critical for glucose metabolism in the cardiomyocytes under I/R injury. Moreover, DRD4 (dopamine receptor D4) modulate glucose metabolism, and protect neurocytes from anoxia/reoxygenation (A/R) injury. Thus, DRD4 might regulate myocardial I/R injury in association with GLUT4-mediated glucose metabolism. However, the effects and mechanisms are largely unknown. In the present study, the effect of DRD4 in heart I/R injury were studied ex vivo and in vitro. For I/R injury ex vivo, DRD4 agonist (PD168077) was perfused by Langendorff system in the isolated rat heart. DRD4 activated by PD168077 improved cardiac function in the I/R-injured heart as determined by the left ventricular developed pressure (LVDP), +dp/dt, and left ventricular end diastolic pressure (LVEDP), and reduced heart damage evidenced by infarct size, the release of troponin T (TNT) and lactate dehydrogenase (LDH). DRD4 activation diminished I/R injury induced apoptosis and enhanced cell viability impaired by I/R injury in cardiomyocyte, showed by TUNEL staining, flow cytometer and CCK8 assay. Furthermore, DRD4 activation did not change total GULT4 protein expression level but increased the membrane GULT4 localization determined by western blot. In terms of mechanism, DRD4 activation increased pPI3K/p-AKT but not the total PI3K/AKT during anoxia/reoxygenation (A/R) injury in vitro. Interestingly, PI3K inhibitor, Wortmannin, blocked PI3K/AKT pathway and depleted the membrane GULT4, and further promoted apoptosis showed by TUNEL staining, flow cytometer, western blot of cleaved caspase 3, BAX and BCL2 expression. Thus, DRD4 activation exerted a protective effect against I/R injury by promoting GLUT4 translocation depended on PI3K/AKT pathway, which enhanced the ability of glucose uptake, and ultimately reduced the apoptosis in cardiomyocytes.

Hearts lacking plasma membrane KATP channels display changes in basal aerobic metabolic substrate preference and AMPK activity

Cardiac ATP-sensitive K+ (KATP) channels couple changes in cellular metabolism to membrane excitability and are activated during metabolic stress, although under basal aerobic conditions, KATP channels are thought to be predominately closed. Despite intense research into the roles of KATP channels during metabolic stress, their contribution to aerobic basal cardiac metabolism has not been previously investigated. Hearts from Kir6.2+/+ and Kir6.2−/− mice were perfused in working mode, and rates of glycolysis, fatty acid oxidation, and glucose oxidation were measured. Changes in activation/expression of proteins regulating metabolism were probed by Western blot analysis. Despite cardiac mechanical function and metabolic efficiency being similar in both groups, hearts from Kir6.2−/− mice displayed an approximately twofold increase in fatty acid oxidation and a 0.45-fold reduction in glycolytic rates but similar glucose oxidation rates compared with hearts from Kir6.2+/+ mice. Kir6.2−/− hearts also possessed elevated levels of activated AMP-activated protein kinase (AMPK), higher glycogen content, and reduced mitochondrial density. Moreover, activation of AMPK by isoproterenol or diazoxide was significantly blunted in Kir6.2−/− hearts. These data indicate that KATP channel ablation alters aerobic basal cardiac metabolism. The observed increase in fatty acid oxidation and decreased glycolysis before any metabolic insult may contribute to the poor recovery observed in Kir6.2−/− hearts in response to exercise or ischemia-reperfusion injury. Therefore, KATP channels may play an important role in the regulation of cardiac metabolism through AMPK signaling.

NEW & NOTEWORTHY In this study, we show that genetic ablation of plasma membrane ATP-sensitive K+ channels results in pronounced changes in cardiac metabolic substrate preference and AMP-activated protein kinase activity. These results suggest that ATP-sensitive K+ channels may play a novel role in regulating metabolism in addition to their well-documented effects on ionic homeostasis during periods of stress.

Enhancement of Glucose Metabolism via PGC-1α Participates in the Cardioprotection of Chronic Intermittent Hypobaric Hypoxia

Background and Aims: Previous studies demonstrated that energy metabolism disturbance impairs cardiac function and chronic intermittent hypobaric hypoxia (CIHH) protects heart against ischemia/reperfusion injury. The present study aimed to test the hypothesis that CIHH protects the heart against ischemia/reperfusion (I/R) injury via improvement of cardiac glucose metabolism.

Methods: Male Sprague-Dawley rats received CIHH treatment simulating 5000-m altitude for 28 days, 6 h per day in a hypobaric chamber or no treatment (control). Body weight, fasting blood glucose, blood lipid and glucose tolerance were measured. The left ventricular function of isolated hearts was evaluated during 30 min of ischemia and 60 min of reperfusion using Langendorff method. The mRNA and protein expression involved in cardiac energy metabolism was determined using quantitative PCR and Western blot techniques.

Results: 1. There was no difference of body weight, fast blood glucose, blood lipid and glucose tolerance between control and CIHH rats under baseline condition (p > 0.05). 2. The recovery of left ventricular function after I/R was improved significantly in CIHH rats compared to control rats (p < 0.05). 3. The expression of cardiac GLUT4 and PGC-1α was increased but PDK4 gene expression was decreased by CIHH treatment at both mRNA and protein level. Also p-AMPK/AMPK ratio was increased in CIHH rats (p < 0.05).

Conclusion: CIHH ameliorates I/R injury through improving cardiac glucose metabolism via upregulation of GLUT4, p-AMPK, and PGC-1α expressions, but downregulation of cardiacPDK4 expression.

Tolerance to ischaemic injury in remodelled mouse hearts: less ischaemic glycogenolysis and preserved metabolic efficiency

AIMS

Post-infarction remodelled failing hearts have reduced metabolic efficiency. Paradoxically, they have increased tolerance to further ischaemic injury. This study was designed to investigate the metabolic mechanisms that may contribute to this phenomenon and to examine the relationship between ischaemic tolerance and metabolic efficiency during post-ischaemic reperfusion.

METHODS AND RESULTS

Male C57BL/6 mice were subjected to coronary artery ligation (CAL) or SHAM surgery. After 4 weeks, in vivo mechanical function was assessed by echocardiography, and then isolated working hearts were perfused in this sequence: 45 min aerobic, 15 min global no-flow ischaemia, and 30 min aerobic reperfusion. Left ventricular (LV) function, metabolic rates, and metabolic efficiency were measured. Relative to SHAM, both in vivo and in vitro CAL hearts had depressed cardiac function under aerobic conditions (45 and 36%, respectively), but they had a greater recovery of LV function during post-ischaemic reperfusion (67 vs. 49%, P < 0.05). While metabolic efficiency (LV work per ATP produced) was 50% lower during reperfusion of SHAM hearts, metabolic efficiency in CAL hearts did not decrease. During ischaemia, glycogenolysis was 28% lower in CAL hearts, indicative of lower ischaemic proton production. There were no differences in mitochondrial abundance, calcium handling proteins, or key metabolic enzymes.

CONCLUSION

Compared with SHAM, remodelled CAL hearts are more tolerant to ischaemic injury and undergo no further deterioration of metabolic efficiency during reperfusion. Less glycogen utilization in CAL hearts during ischaemia may contribute to increased ischaemic tolerance by limiting ischaemic proton production that may improve ion homeostasis during early reperfusion.

Propofol (Diprivan®) and Intralipid® exacerbate insulin resistance in type-2 diabetic hearts by impairing GLUT4 trafficking

BACKGROUND

The IV anesthetic, propofol, when administered as fat emulsion-based formulation (Diprivan) promotes insulin resistance, but the direct effects of propofol and its solvent, Intralipid, on cardiac insulin resistance are unknown.

METHODS

Hearts of healthy and type-2 diabetic rats (generated by fructose feeding) were aerobically perfused for 60 minutes with 10 μM propofol in the formulation of Diprivan or an equivalent concentration of its solvent Intralipid (25 μM) ± insulin (100 mU•L). Glucose uptake, glycolysis, and glycogen metabolism were measured using [H]glucose. Activation of Akt, GSK3β, AMPK, ERK1/2, p38MAPK, S6K1, JNK, protein kinase Cθ (PKCθ), and protein kinase CCβII (PKCβII) was determined using immunoblotting. GLUT4 trafficking and phosphorylations of insulin receptor substrate-1 (IRS-1) at Ser307(h312), Ser1100(h1101), and Tyr608(hTyr612) were measured. Mass spectrometry was used to determine acylcarnitines, phospholipids, and sphingolipids.

RESULTS

Diprivan and Intralipid reduced insulin-induced glucose uptake and redirected glucose to glycogen stores in diabetic hearts. Reduced glucose uptake was accompanied by lower GLUT4 trafficking to the sarcolemma. Diprivan and Intralipid inactivated GSK3β but activated AMPK and ERK1/2 in diabetic hearts. Only Diprivan increased phosphorylation of Akt(Ser473/Thr308) and translocated PKCθ and PKCβII to the sarcolemma in healthy hearts, whereas it activated S6K1 and p38MAPK and translocated PKCβII in diabetic hearts. Furthermore, only Diprivan phosphorylated IRS-1 at Ser1100(h1101) in healthy and diabetic hearts. JNK expression, phosphorylation of Ser307(h312) of IRS-1, and PKCθ expression and translocation were increased, whereas GLUT4 expression was reduced in insulin-treated diabetic hearts. Phosphatidylglycerol, phosphatidylethanolamine, and C18-sphingolipids accumulated in Diprivan-perfused and Intralipid-perfused diabetic hearts.

CONCLUSIONS

Propofol and Intralipid promote insulin resistance predominantly in type-2 diabetic hearts.

AMPK signalling and the control of substrate use in the heart

All mammalian cells rely on adenosine triphosphate (ATP) to maintain function and for survival. The heart has the highest basal ATP demand of any organ due to the necessity for continuous contraction. As such, the ability of the cardiomyocyte to monitor cellular energy status and adapt the supply of substrates to match the energy demand is crucial. One important serine/threonine protein kinase that monitors cellular energy status in the heart is adenosine monophosphate activated protein kinase (AMPK). AMPK is also a key enzyme that controls multiple catabolic and anabolic biochemical pathways in the heart and indirectly plays a crucial role in regulating cardiac function in both physiological and pathophysiological conditions. Herein, we review the involvement of AMPK in myocardial fatty acid and glucose transport and utilization, as it relates to basal cardiac function. We also assess the literature amassed on cardiac AMPK and discuss the controversies surrounding the role of AMPK in physiological and pathophysiological processes in the heart. The work reviewed herein also emphasizes areas that require further investigation for the purpose of eventually translating this information into improved patient care.

Adenosine-mediated inhibition of 5'-AMP-activated protein kinase and p38 mitogen-activated protein kinase during reperfusion enhances recovery of left ventricular mechanical function

Attenuation of excessive rates of myocardial glycolysis limits proton production and Ca(2+) overload during reperfusion and improves recovery of post-ischemic left ventricular (LV) function. In order to elucidate mechanisms underlying glycolytic inhibition by adenosine (ADO), this study tested the hypothesis that the beneficial effects of ADO are due to Ser/Thr protein phosphatase (PP)-mediated inhibition of 5'-AMP-activated protein kinase (AMPK) and phosphofructokinase-2 (PFK-2). In isolated perfused working rat hearts subjected to global ischemia (GI) and reperfusion, ADO (500μmol/l), added 5min prior to the onset of GI and present throughout reperfusion, inhibits glycolysis and proton production during reperfusion and improves post-ischemic LV work. These metabolic effects of ADO are also evident during aerobic perfusion. Assays of glycolytic intermediates show that ADO-induced glycolytic inhibition occurs at the step catalyzed by PFK-1, an effect mediated by reduced activation of PFK-2 by AMPK. The PP1 and PP2A inhibitors, cantharidin (5μmol/l) or okadaic acid (0.1μmol/l), added 10min prior to ADO prevent ADO-induced inhibition of glycolysis and AMPK, as well as ADO-induced cardioprotection. ADO also inhibits p38 MAPK phosphorylation during reperfusion in a cantharidin-sensitive manner, and pharmacological inhibition of p38 MAPK (by SB202190, 10μmol/l) during reperfusion also reduces glycolysis and is cardioprotective. These results indicate that attenuation of glycolysis during reperfusion and cardioprotection can be achieved by inhibition of the stress kinases, AMPK and p38 MAPK.

Insulin and the heart

The main role of insulin in the heart under physiological conditions is obviously the regulation of substrate utilization. Indeed, insulin promotes glucose uptake and its utilization via glycolysis. Insulin, promoting glucose as the main cardiac energy substrate, reduces myocardial O(2) consumption and increases cardiac efficiency. Moreover, insulin seems to augment cardiomyocyte contraction, while it affects favorably myocardial relaxation, increases ribosomal biogenesis and protein synthesis, stimulates vascular endothelial growth factor (VEGF) and thereby angiogenesis, suppresses apoptosis, promotes cell survival and finally ameliorates both myocardial microcirculation and coronary artery resistance, leading to increased blood perfusion of myocardium. Thus, insulin acts directly on heart muscle, and this action is mediated principally through PKB/Akt signal pathway. Under pathological conditions, such as type 2 diabetes, myocardial ischaemia, and cardiac hypertrophy, insulin signal transduction pathways and action are clearly modified. In this review we summarize the evidence that the heart is an important target of insulin action and that elimination of these actions is important in disease states.

Targeting fatty acid and carbohydrate oxidation--a novel therapeutic intervention in the ischemic and failing heart

Cardiac ischemia and its consequences including heart failure, which itself has emerged as the leading cause of morbidity and mortality in developed countries are accompanied by complex alterations in myocardial energy substrate metabolism. In contrast to the normal heart, where fatty acid and glucose metabolism are tightly regulated, the dynamic relationship between fatty acid β-oxidation and glucose oxidation is perturbed in ischemic and ischemic-reperfused hearts, as well as in the failing heart. These metabolic alterations negatively impact both cardiac efficiency and function. Specifically there is an increased reliance on glycolysis during ischemia and fatty acid β-oxidation during reperfusion following ischemia as sources of adenosine triphosphate (ATP) production. Depending on the severity of heart failure, the contribution of overall myocardial oxidative metabolism (fatty acid β-oxidation and glucose oxidation) to adenosine triphosphate production can be depressed, while that of glycolysis can be increased. Nonetheless, the balance between fatty acid β-oxidation and glucose oxidation is amenable to pharmacological intervention at multiple levels of each metabolic pathway. This review will focus on the pathways of cardiac fatty acid and glucose metabolism, and the metabolic phenotypes of ischemic and ischemic/reperfused hearts, as well as the metabolic phenotype of the failing heart. Furthermore, as energy substrate metabolism has emerged as a novel therapeutic intervention in these cardiac pathologies, this review will describe the mechanistic bases and rationale for the use of pharmacological agents that modify energy substrate metabolism to improve cardiac function in the ischemic and failing heart. This article is part of a Special Issue entitled: Mitochondria and Cardioprotection.

Cardioprotection by GSK-3 inhibition: role of enhanced glycogen synthesis and attenuation of calcium overload

AIMS

Glycogen synthase kinase-3 (GSK-3) is a multi-functional kinase that regulates signalling pathways affecting glycogen metabolism, protein synthesis, mitosis, and apoptosis. GSK-3 inhibition limits cardiac ischaemia-reperfusion (IR) injury, but mechanisms are not clearly defined. This study tested the hypothesis that acute GSK-3 inhibition stimulates glycogen synthesis, repartitions glucose away from glycolysis, reduces proton (H+) production from glucose metabolism, and attenuates intracellular Ca2+ (Ca2+(i)) overload.

METHODS AND RESULTS

In isolated perfused working rat hearts subjected to global ischaemia and reperfusion, the selective GSK-3 inhibitor, SB-216763 (SB, 3 micromol/L), when added either prior to ischaemia or at the onset of reperfusion, improved recovery of left-ventricular (LV) work. SB increased glycogen synthesis during reperfusion while glycolysis and H+ production were reduced. Rates of glucose and palmitate oxidation were improved by SB. Measurement of Ca2+(i) concentration by rapid acquisition indo-1 fluorescence imaging showed that SB, when added either prior to ischaemia or at the onset of reperfusion, reduced diastolic Ca2+(i) overload during reperfusion. In aerobic hearts depleted of glycogen by substrate-free perfusion to a level similar to that measured at the onset of reperfusion, SB accelerated glycogen synthesis and reduced glycolysis and H+ production independent of changes in LV work.

CONCLUSION

Our study indicates that reduction in H+ production by GSK-3 inhibition is an early and upstream event that lessens Ca2+(i) overload during ischaemia and early reperfusion independent of LV work which enhances the recovery of post-ischaemic LV function and that may ultimately contribute to previously observed reductions in cell death and infarction.