Insulin-like action of catecholamines and Ca2+ to stimulate glucose transport and GLUT4 translocation in perfused rat heart

Dynamics of training and acute exercise-induced shifts in muscular glucose transporter (GLUT) 4, 8, and 12 expression in locomotion versus posture muscles in healthy horses

Important changes in glucose transporter (GLUT) expression should be expected if the glucose influx plays a pivotal role in fuelling or connecting metabolic pathways that are upregulated in response to exercise. The aim was to assess GLUT4, 8, and 12 dynamics in response to training and acute exercise. Methods: Sixteen untrained Standardbred mares (3-4 year) performed an incremental SET at the start and end of 8 weeks harness training. M. pectoralis (PM) and M. vastus lateralis (VL) muscle biopsies were taken before and after each SET, allowing for comparing rest and acute samples in untrained (UT) and trained (T) condition using Western Blot for GLUT quantification and Image Pro v.10 for Blot analysis. Data were normalized against GAPDH. Basal GLUT-levels of PM versus VL were analysed with the Wilcoxon matched-pairs signed rank test. The effect of acute exercise or training was assessed using the Friedman test with a post hoc Dunn's. Results: Basal GLUT4 and GLUT12 protein expression were significantly higher in the VL compared to the PM (PGLUT4 = 0.031 and PGLUT12 = 0.002). Training had no effect on basal GLUT4 expression, neither in the VL (p > 0.9999), nor the PM (p > 0.9999). However, acute exercise in trained condition significantly decreased GLUT4 expression in the VL (p = 0.0148). Neither training nor acute exercise significantly changed total GLUT8 protein expression. Training significantly decreased total GLUT12 protein expression in rest biopsies, only visible in the VL (p = 0.0359). This decrease was even more prominent in the VL after acute exercise in trained condition (PVL = 0.0025). Conclusion: The important changes seen in GLUT12 expression downregulation, both in response to training and acute exercise in the horse, the downregulation of GLUT4 expression after acute exercise in trained condition and the lack of differential shifts in GLUT8 expression in any of the studied conditions, questions the importance of glucose as substrate to fuel training and exercise in healthy horses. These findings encourage to further explore alternative fuels for their involvement in equine muscular energetics.

Stress hyperglycemia, cardiac glucotoxicity, and critically ill patient outcomes current clinical and pathophysiological evidence

Stress hyperglycemia is a transient increase in blood glucose during acute physiological stress in the absence of glucose homeostasis dysfunction. Its's presence has been described in critically ill patients who are subject to many physiological insults. In this regard, hyperglycemia and impaired glucose tolerance are also frequent in patients who are admitted to the intensive care unit for heart failure and cardiogenic shock. The hyperglycemia observed at the beginning of these cardiac disorders appears to be related to a variety of stress mechanisms. The release of major stress and steroid hormones, catecholamine overload, and glucagon all participate in generating a state of insulin resistance with increased hepatic glucose output and glycogen breakdown. In fact, the observed pathophysiological response, which appears to regulate a stress situation, is harmful because it induces mitochondrial impairment, oxidative stress-related injury to cells, endothelial damage, and dysfunction of several cellular channels. Paradigms are now being challenged by growing evidence of a phenomenon called glucotoxicity, providing an explanation for the benefits of lowering glucose levels with insulin therapy in these patients. In the present review, the authors present the data published on cardiac glucotoxicity and discuss the benefits of lowering plasma glucose to improve heart function and to positively affect the course of critical illness.

Diabetes induced decreases in PKA signaling in cardiomyocytes: The role of insulin

The cAMP-dependent protein kinase (PKA) signaling pathway is the primary means by which the heart regulates moment-to-moment changes in contractility and metabolism. We have previously found that PKA signaling is dysfunctional in the diabetic heart, yet the underlying mechanisms are not fully understood. The objective of this study was to determine if decreased insulin signaling contributes to a dysfunctional PKA response. To do so, we isolated adult cardiomyocytes (ACMs) from wild type and Akita type 1 diabetic mice. ACMs were cultured in the presence or absence of insulin and PKA signaling was visualized by immunofluorescence microscopy using an antibody that recognizes proteins specifically phosphorylated by PKA. We found significant decreases in proteins phosphorylated by PKA in wild type ACMs cultured in the absence of insulin. PKA substrate phosphorylation was decreased in Akita ACMs, as compared to wild type, and unresponsive to the effects of insulin. The decrease in PKA signaling was observed regardless of whether the kinase was stimulated with a beta-agonist, a cell-permeable cAMP analog, or with phosphodiesterase inhibitors. PKA content was unaffected, suggesting that the decrease in PKA signaling may be occurring by the loss of specific PKA substrates. Phospho-specific antibodies were used to discern which potential substrates may be sensitive to the loss of insulin. Contractile proteins were phosphorylated similarly in wild type and Akita ACMs regardless of insulin. However, phosphorylation of the glycolytic regulator, PFK-2, was significantly decreased in an insulin-dependent manner in wild type ACMs and in an insulin-independent manner in Akita ACMs. These results demonstrate a defect in PKA activation in the diabetic heart, mediated in part by deficient insulin signaling, that results in an abnormal activation of a primary metabolic regulator.

Glucose transporters in cardiovascular system in health and disease

Glucose transporters are essential for the heart to sustain its function. Due to its nature as a high energy-consuming organ, the heart needs to catabolize a huge quantity of metabolic substrates. For optimized energy production, the healthy heart constantly switches between various metabolites in accordance with substrate availability and hormonal status. This metabolic flexibility is essential for the maintenance of cardiac function. Glucose is part of the main substrates catabolized by the heart and its use is fine-tuned via complex molecular mechanisms that include the regulation of the glucose transporters GLUTs, mainly GLUT4 and GLUT1. Besides GLUTs, glucose can also be transported by cotransporters of the sodium-glucose cotransporter (SGLT) (SLC5 gene) family, in which SGLT1 and SMIT1 were shown to be expressed in the heart. This SGLT-mediated uptake does not seem to be directly linked to energy production but is rather associated with intracellular signalling triggering important processes such as the production of reactive oxygen species. Glucose transport is markedly affected in cardiac diseases such as cardiac hypertrophy, diabetic cardiomyopathy and heart failure. These alterations are not only fingerprints of these diseases but are involved in their onset and progression. The present review will depict the importance of glucose transport in healthy and diseased heart, as well as proposed therapies targeting glucose transporters.

Energy Metabolism in Exercise-Induced Physiologic Cardiac Hypertrophy

Physiologic hypertrophy of the heart preserves or enhances systolic function without interstitial fibrosis or cell death. As a unique form of physiological stress, regular exercise training can trigger the adaptation of cardiac muscle to cause physiological hypertrophy, partly due to its ability to improve cardiac metabolism. In heart failure (HF), cardiac dysfunction is closely associated with early initiation of maladaptive metabolic remodeling. A large amount of clinical and experimental evidence shows that metabolic homeostasis plays an important role in exercise training, which is conducive to the treatment and recovery of cardiovascular diseases. Potential mechanistic targets for modulation of cardiac metabolism have become a hot topic at present. Thus, exploring the energy metabolism mechanism in exercise-induced physiologic cardiac hypertrophy may produce new therapeutic targets, which will be helpful to design novel effective strategies. In this review, we summarize the changes of myocardial metabolism (fatty acid metabolism, carbohydrate metabolism, and mitochondrial adaptation), metabolically-related signaling molecules, and probable regulatory mechanism of energy metabolism during exercise-induced physiological cardiac hypertrophy.

Changes in Myocardial Metabolism Preceding Sudden Cardiac Death

Heart disease is widely recognized as a major cause of death worldwide and is the leading cause of mortality in the United States. Centuries of research have focused on defining mechanistic alterations that drive cardiac pathogenesis, yet sudden cardiac death (SCD) remains a common unpredictable event that claims lives in every age group. The heart supplies blood to all tissues while maintaining a constant electrical and hormonal feedback communication with other parts of the body. As such, recent research has focused on understanding how myocardial electrical and structural properties are altered by cardiac metabolism and the various signaling pathways associated with it. The importance of cardiac metabolism in maintaining myocardial function, or lack thereof, is exemplified by shifts in cardiac substrate preference during normal development and various pathological conditions. For instance, a shift from fatty acid (FA) oxidation to oxygen-sparing glycolytic energy production has been reported in many types of cardiac pathologies. Compounded by an uncoupling of glycolysis and glucose oxidation this leads to accumulation of undesirable levels of intermediate metabolites. The resulting accumulation of intermediary metabolites impacts cardiac mitochondrial function and dysregulates metabolic pathways through several mechanisms, which will be reviewed here. Importantly, reversal of metabolic maladaptation has been shown to elicit positive therapeutic effects, limiting cardiac remodeling and at least partially restoring contractile efficiency. Therein, the underlying metabolic adaptations in an array of pathological conditions as well as recently discovered downstream effects of various substrate utilization provide guidance for future therapeutic targeting. Here, we will review recent data on alterations in substrate utilization in the healthy and diseased heart, metabolic pathways governing cardiac pathogenesis, mitochondrial function in the diseased myocardium, and potential metabolism-based therapeutic interventions in disease.

Acute and Chronic Effects of Protein Kinase-D Signaling on Cardiac Energy Metabolism

Protein kinase-D (PKD) is increasingly recognized as a key regulatory signaling hub in cardiac glucose uptake and also a major player in the development of hypertrophy. Glucose is one of the predominant energy substrates for the heart to support contraction. However, a cardiac substrate switch toward glucose over-usage is associated with the development of cardiac hypertrophy. Hence, regulation of PKD activity must be strictly coordinated. This review provides mechanistic insights into the acute and chronic regulatory functions of PKD signaling in the healthy and hypertrophied heart. First an overview of the activation pathways of PKD1, the most abundant isoform in the heart, is provided. Then the various regulatory roles of the PKD isoforms in the heart in relation to cardiac glucose and fatty acid metabolism, contraction, morphology, function, and the development of cardiac hypertrophy are described. Finally, these findings are integrated and the possibility of targeting this kinase as a novel strategy to combat cardiac diseases is discussed.

Glucose Transporters in Cardiac Metabolism and Hypertrophy

The heart is adapted to utilize all classes of substrates to meet the high-energy demand, and it tightly regulates its substrate utilization in response to environmental changes. Although fatty acids are known as the predominant fuel for the adult heart at resting stage, the heart switches its substrate preference toward glucose during stress conditions such as ischemia and pathological hypertrophy. Notably, increasing evidence suggests that the loss of metabolic flexibility associated with increased reliance on glucose utilization contribute to the development of cardiac dysfunction. The changes in glucose metabolism in hypertrophied hearts include altered glucose transport and increased glycolysis. Despite the role of glucose as an energy source, changes in other nonenergy producing pathways related to glucose metabolism, such as hexosamine biosynthetic pathway and pentose phosphate pathway, are also observed in the diseased hearts. This article summarizes the current knowledge regarding the regulation of glucose transporter expression and translocation in the heart during physiological and pathological conditions. It also discusses the signaling mechanisms governing glucose uptake in cardiomyocytes, as well as the changes of cardiac glucose metabolism under disease conditions.

5'-AMP-activated protein kinase is inactivated by adrenergic signalling in adult cardiac myocytes

In adult rat cardiac myocytes adrenaline decreased AMPK (AMP-activated protein kinase) activity with a half-time of approximately 4 min, decreased phosphorylation of AMPK (α-Thr172) and decreased phosphorylation of ACC (acetyl-CoA carboxylase). Inactivation of AMPK by adrenaline was through both α1- and β-ARs (adrenergic receptors), but did not involve cAMP or calcium signalling, was not blocked by the PKC (protein kinase C) inhibitor BIM I (bisindoylmaleimide I), by the ERK (extracellular-signal-regulated kinase) cascade inhibitor U0126 or by PTX (pertussis toxin). Adrenaline caused no measurable change in LKB1 activity. Adrenaline decreased AMPK activity through a process that was distinct from AMPK inactivation in response to insulin or PMA. Neither adrenaline nor PMA altered the myocyte AMP:ATP ratio although the adrenaline effect was attenuated by oligomycin and by AICAR (5-amino-4-imidazolecarboxamide-1-β-D-ribofuranoside), agents that mimic 'metabolic stress'. Inactivation of AMPK by adrenaline was abolished by 1 μM okadaic acid suggesting that activation of PP2A (phosphoprotein phosphatase 2A) might mediate the adrenaline effect. However, no change in PP2A activity was detected in myocyte extracts. Adrenaline increased phosphorylation of the AMPK β-subunit in vitro but there was no detectable change in vivo in phosphorylation of previously identified AMPK sites (β-Ser24, β-Ser108 or β-Ser182) suggesting that another site(s) is targeted.

Green tea polyphenols improve cardiac muscle mRNA and protein levels of signal pathways related to insulin and lipid metabolism and inflammation in insulin-resistant rats

Epidemiological studies indicate that the consumption of green tea polyphenols (GTP) may reduce the risk of coronary artery disease. To explore the underlying mechanisms of action at the molecular level, we examined the effects of GTP on the cardiac mRNA and protein levels of genes involved in insulin and lipid metabolism and inflammation. In rats fed a high-fructose diet, supplementation with GTP (200 mg/kg BW daily dissolved in distilled water) for 6 wk, reduced systemic blood glucose, plasma insulin, retinol-binding protein 4, soluble CD36, cholesterol, triglycerides, free fatty acids and LDL-C levels, as well as the pro-inflammatory cytokines, tumor necrosis factor-alpha (TNF-alpha) and IL-6. GTP did not affect food intake, bodyweight and heart weight. In the myocardium, GTP also increased the insulin receptor (Ir), insulin receptor substrate 1 and 2 (Irs1 and Irs2), phosphoinositide-3-kinase (Pi3k), v-akt murine thymoma viral oncogene homolog 1 (Akt1), glucose transporter 1 and 4 (Glut1 and Glut4) and glycogen synthase 1 (Gys1) expression but inhibited phosphatase and tensin homolog deleted on chromosome ten (Pten) expression and decreased glycogen synthase kinase 3beta (Gsk3beta) mRNA expression. The sterol regulatory element-binding protein-1c (Srebp1c) mRNA, microsomal triglyceride transfer protein (Mttp) mRNA and protein, Cd36 mRNA and cluster of differentiation 36 protein levels were decreased and peroxisome proliferator-activated receptor (Ppar)gamma mRNA levels were increased. GTP also decreased the inflammatory factors: Tnf, Il1b and Il6 mRNA levels, and enhanced the anti-inflammatory protein, zinc-finger protein, protein and mRNA expression. In summary, consumption of GTP ameliorated the detrimental effects of high-fructose diet on insulin signaling, lipid metabolism and inflammation in the cardiac muscle of rats.

Impaired insulin-signaling in hypertrophied hearts contributes to ischemic injury

Despite increased glucose utilization by hypertrophied myocardium, these hearts exhibit a slower rate of glucose uptake (GU). We hypothesized that, in hypertrophied myocardium, a defect of the insulin-responsive glucose transporter is responsible for impaired GU and metabolism during ischemia, contributing to post-ischemic myocardial dysfunction. In a rabbit model of pressure-overload hypertrophy, GU ((31)P NMR spectroscopy) and total/phosphorylated insulin-signaling intermediates were assayed: insulin-receptor, insulin-receptor-substrate-1 (IRS-1), phosphatidylinositol-3-kinase (PI3-k), GLUT-4 translocation and contractile function in an isolated heart ischemia/reperfusion model. Total protein was not different between hypertrophied and control hearts. Phosphorylation of IRS-1 and PI3-k activity was significantly lower in hypertrophy during ischemia. GU was impaired pre-ischemia in hypertrophy, remained lower during early reperfusion, and was associated with impaired recovery of contractile function. In conclusion, a defect in IRS-1 phosphorylation and PI3-k activation in hypertrophied hearts restricts insulin-mediated GLUT-4 translocation and ischemia, a known stimulus of GLUT-4 translocation, does not compensate for this defect.

Physiological role of AMP-activated protein kinase in the heart: graded activation during exercise

AMP-activated protein kinase (AMPK) is emerging as a key signaling pathway that modulates cellular metabolic processes. In skeletal muscle, AMPK is activated during exercise. Increased myocardial substrate metabolism during exercise could be explained by AMPK activation. Although AMPK is known to be activated during myocardial ischemia, it remains uncertain whether AMPK is activated in response to the physiological increases in cardiac work associated with exercise. Therefore, we evaluated cardiac AMPK activity in rats at rest and after 10 min of treadmill running at moderate (15% grade, 16 m/min) or high (15% grade, 32 m/min) intensity. Total AMPK activity in the heart increased in proportion to exercise intensity (P < 0.05). AMPK activity associated with the alpha2-catalytic subunit increased 2.8 +/- 0.4-fold (P < 0.02 vs. rest) and 4.5 +/- 0.6-fold (P < 0.001 vs. rest) with moderate- and high-intensity exercise, respectively. AMPK activity associated with the alpha1-subunit increased to a lesser extent. Phosphorylation of the Thr172-regulatory site on AMPK alpha-catalytic subunits increased during exercise (P < 0.001). There was no increase in Akt phosphorylation during exercise. The changes in AMPK activity during exercise were associated with physiological AMPK effects (GLUT4 translocation to the sarcolemma and ACC phosphorylation). Thus cardiac AMPK activity increases progressively with exercise intensity, supporting the hypothesis that AMPK has a physiological role in the heart.

The role of calcium in the regulation of glucose uptake in isolated working rat heart

Catecholamines or ischemia may increase myocardial glucose uptake by an increase in intracellular calcium. We tested the hypothesis that increasing or decreasing extracellular calcium supply would change glucose uptake. Hearts were perfused for 60 min at a physiological workload with Krebs-Henseleit buffer containing glucose (5 mM) and oleate (0.4 mM; bound to 1% BSA). Calcium concentration was 2.5 mM. In group A (control; n = 12), insulin (1 mU/ml) was added at 30 min. In Group B (n = 7), the calcium concentration was increased to 5.0 and 7.5 mM at 20 min and 40 min, respectively. In Group C (n = 7), verapamil was added at 20 min (0.25 microM) and 40 min (1.0 microM) to decrease calcium influx. In group D (n = 7), EDTA was added at 20 min (0.5 mM) and at 40 min (1.5 mM) to decrease the free extracellular calcium. Glucose uptake was measured by 3H2O production from [2-3H]glucose and cardiac work was measured simultaneously. Cardiac power in group B was 8.24 +/- 0.60 mW at 2.5 mM calcium, 9.45 +/- 0.50 mW at 5 mM calcium and 7.99 +/- 0.99 mW at 7.5 mM calcium (n.s.). The addition of verapamil decreased contractile function in a dose-dependent manner (8.50 +/- 0.74 vs. 3.11 +/- 0.84 vs. 1.48 +/- 0.39 mW, p < 0.01) suggesting that verapamil decreased cytosolic calcium concentration. A similar dose-dependent reduction in contractile performance was observed in the EDTA group (8.44 +/- 0.81 vs. 7.42 +/- 0.96 vs. 4.03 +/- 1.32 mW, p < 0.01). Glucose uptake was 1.35 +/- 0.11 micromol/min/g dry weight under control conditions. Glucose uptake increased threefold with the addition of insulin. Increasing extracellular [Ca2+] did not affect glucose uptake. Decreasing Ca2+ availability showed a trend towards a decrease in glucose uptake (n.s.), which was minor compared to the decrease in contractile function. We conclude that extracellular calcium does not regulate glucose uptake in the isolated working rat heart in the presence of glucose and fatty acids as substrates. The trend of decreased glucose uptake when calcium supply was limited may be due to dramatically reduced energy demand and not directly due to changes in calcium.

Troglitazone enhances glucose uptake induced by alpha-adrenoceptor stimulation via phosphatidylinositol 3-kinase in rat heart

1. Thiazolidinedione-derived agents have been reported to act as insulin sensitizers by augmenting insulin-dependent stimulation of phosphatidylinositol 3-kinase (PI3K) activity in a specific manner. It has been suggested that alpha-adrenoceptor stimulation mediates glucose uptake through PI3K in the heart. 2. To elucidate whether the thiazolidinedione-derived agent troglitazone (TRO) affects glucose uptake induced by alpha-adrenoceptor stimulation through PI3K, the rate of glucose uptake was quantified from the rate of accumulation of sugar phosphate (d[SP]/dt) using [(31)P] nuclear magnetic resonance spectroscopy after substitution of glucose with 2-deoxyglucose in rat perfused heart. Hearts were stimulated with 100 micromol/L phenylephrine plus 10 micromol/L propranolol (alpha-adrenoceptor stimulation), or 1 micromol/L isoproterenol plus 10 micromol/L phentolamine (beta-adrenoceptor stimulation). 3. The d[SP]/dt in the alpha- and beta-adrenoceptor-stimulated groups (0.45 +/- 0.06 and 0.42 +/- 0.04 micromol/min per g, respectively) was higher than that of the control group (0.27 +/- 0.02 micromol/min per g; P < 0.01). The addition of 2 microg/mL troglitazone to alpha-adrenoceptor stimulation augmented d[SP]/dt (0.72 +/- 0.08 micromol/min per g; P < 0.05 vs the alpha-adrenoceptor-stimulated group), which was effectively blocked by 3 micromol/L wortmannin (0.35 +/- 0.06 micromol/min per g; P < 0.01 vs troglitazone + alpha-adrenoceptor stimulation group). However, addition of troglitazone to beta-adrenoceptor stimulation did not alter d[SP]/dt (0.33 +/- 0.02 micromol/min per g; P = NS vs the beta-adrenoceptor-stimulated group). 4. These results indicate that troglitazone acutely enhances alpha-adrenoceptor stimulation on glucose uptake through a PI3K-dependent pathway, thus possibly improving glucose utilization in a catecholamine-released state.

The effects of insulin and beta-adrenergic stimulation on glucose transport, glut 4 and PKB activation in the myocardium of lean and obese non-insulin dependent diabetes mellitus rats

Glucose uptake, glut 4 translocation and activation of protein kinase B were measured in Langendorff perfused hearts from (i) Wistar control, (ii) lean, neonatal Streptozotocin induced (Stz) and (iii) Zucker (fa/fa) obese diabetic rats of 10-12 weeks old. Hearts were subjected to stimulation with insulin, isoproterenol (beta-adrenergic agonist) or a combination of insulin and isoproterenol, during the perfusion protocol. Basal myocardial glucose uptake was impaired in both diabetic models, but could be stimulated significantly by insulin. In the Zucker rats, the time-course of insulin action was delayed. Insulin and beta-stimulation of glucose uptake were not additive. Evaluation of sarcolemmal membranes from these hearts showed that the affinity of glut 4 was significantly lower in the Zucker but not in the Stz hearts while a reduced affinity found with a combination of insulin and beta-stimulation in control hearts, was absent in both diabetic models. Total membrane lysates were analyzed for glut 4 expression while an intracellular component was generated to quantify translocation on stimulation as well as activity of protein kinase B (PKB). At this age, the neonatal Streptozotocin induced diabetic animals presented with more faulty regulation concerning adrenergic stimulated effects on elements of this signal transduction pathway while the Zucker fa/fa animals showed larger deviations in insulin stimulated effects. The overall response of the Zucker myocardium was poorer than that of the Stz group. No significant modulation of beta-adrenergic signaling on insulin stimulated glucose uptake was found. The PI-3-kinase inhibitor wortmannin, could abolish glucose uptake as well as PKB activation elicited by both insulin and isoproterenol.

Protecting the myocardium from ischemic injury: a critical role for alpha(1)-adrenoreceptors?

Ischemic preconditioning (IPC) refers to the ability of short periods of ischemia to make the myocardium more resistant to a subsequent ischemic insult. It is the most powerful form of endogenous protection against myocardial infarction and has been demonstrated in all species evaluated to date. However, the cellular mechanisms that drive IPC remain poorly understood. This hypothesis describes an important role for alpha(1)-adrenoreceptors in mediating IPC and discusses the underlying mechanisms by which this is likely achieved. alpha(1)-Adrenoreceptors are present in the myocardium of all mammalian species, and several lines of evidence suggest that they play an important role in mediating IPC. During periods of myocardial hypoxia/ischemia, cardiomyocytes have to rely solely on anaerobic glycolysis for energy production; for this, the cells have to depend on increased glucose entry inside the cell as well as increased glycolysis. Stimulation of alpha(1)-adrenoreceptors increases glucose transport inside the cardiomyocytes by translocating glucose transporter (GLUT)-1 and GLUT-4 from the cytoplasm to the plasma membrane, enhances glycogenolysis by activating phosphorylase kinase, increases the rate of glycolysis by activating the enzyme phosphofructokinase, reduces intracellular acidity produced during excessive glycolysis by activating the Na(+)/H(+) exchanger, and inhibits apoptosis by increasing the levels of the antiapoptotic protein Bcl-2. Myocardial ischemia produces an increase in the expression of alpha(1)-adrenoreceptors in cardiomyocytes, as well as increases the levels of its agonist norepinephrine by several fold. During ischemic states, upregulation of alpha(1)-adrenoreceptors and increase in norepinephrine release could be a powerful adaptive mechanism that drives IPC. An understanding into the role of alpha(1)-adrenoreceptors in mediating IPC could not only point to newer treatments for limiting myocardial damage during myocardial infarction or heart surgery, but could also help in avoiding the use of alpha(1)-antagonists in patients with ischemic heart disease.

Insulin-stimulated glucose transport is dependent on Golgi function in isolated working rat heart

Insulin and epinephrine stimulate glucose uptake through distinct mechanisms. We tested the hypothesis that the golgi apparatus is involved in insulin-stimulated but not epinephrine-stimulated glucose transport and phosphorylation.

METHODS

We perfused isolated working rat hearts with Krebs-Henseleit buffer containing [2-(3)H]glucose (5 mmol/l, 0.05 microCi/ml) and Na-oleate (0.4 mmol/l). In the absence or presence of the inhibitor of golgi function, brefeldin A (30 micromol/l), either insulin (1 mU/ml), epinephrine (1 micromol/l), or phenylephrine (100 micromol/l) plus propranolol (10 micromol/l, selective alpha -adrenergic stimulation) were added to the perfusate.

RESULTS

Cardiac power was stable in all groups (between 8.56+/-0.61 and 10.4+/-1.11 mW) and increased (34%) with addition of epinephrine, but not with selective alpha -adrenergic stimulation. Insulin, epinephrine, and selective alpha -receptor stimulation increased glucose transport and phosphorylation (micromol/min/g dry wt, basal: 1.19+/-0.13, insulin: 3.89+/-0.36, epinephrine: 3.46+/-0.27, alpha -stimulation: 4.08+/-0.40). Brefeldin A increased basal glucose transport and phosphorylation and blunted insulin-stimulated but not epinephrine-stimulated glucose transport and phosphorylation. Selective alpha -stimulated glucose transport and phosphorylation was also blunted by brefeldin A.

CONCLUSIONS

Both insulin and alpha -adrenergic stimulation result in glucose transporter translocation from a pool that requires golgi function. Stimulation with epinephrine results in glucose transporter translocation from a pool that does not require golgi function. The stimulating effects of the alpha -adrenergic pathway on glucose transport and phosphorylation are independent of changes in cardiac performance.

The trimeric GTP-binding protein (G(q)/G(11)) alpha subunit is required for insulin-stimulated GLUT4 translocation in 3T3L1 adipocytes

To investigate the potential role of trimeric GTP-binding proteins regulating GLUT4 translocation in adipocytes, wild type and constitutively active G(q) (G(q)/Q209L), G(i) (G(i)/Q205L), and G(s) (G(s)/Q227L) alpha subunit mutants were expressed in 3T3L1 adipocytes. Although expression of neither the wild type nor G(i)/Q205L and G(s)/Q227L alpha subunit mutants had any effect on the basal or insulin-stimulated translocation of a co-expressed GLUT4-enhanced green fluorescent protein (EGFP) fusion protein, expression of G(q)/Q209L resulted in GLUT4-EGFP translocation in the absence of insulin. In contrast, microinjection of an inhibitory G(q)/G(11) alpha subunit-specific antibody but not a G(i) or G(s) alpha subunit antibody prevented insulin-stimulated endogenous GLUT4 translocation. Consistent with a required role for GTP-bound G(q)/G(11), expression of the regulators of G protein signaling (RGS4 and RGS16) also attenuated insulin-stimulated GLUT4-EGFP translocation. To assess the relationship between G(q)/G(11) function with the phosphatidylinositol 3-kinase dependent pathway, expression of a dominant-interfering p85 regulatory subunit, as well as wortmannin treatment inhibited insulin-stimulated but not G(q)/Q209L-stimulated GLUT4-EGFP translocation. Furthermore, G(q)/Q209L did not induce the in vivo accumulation of phosphatidylinositol-3,4,5-trisphosphate (PIP(3)), whereas expression of the RGS proteins did not prevent the insulin-stimulated accumulation of PIP(3). Together, these data demonstrate that insulin stimulation of GLUT4 translocation requires at least two independent signal transduction pathways, one mediated through the phosphatidylinositol 3-kinase and another through the trimeric GTP-binding proteins G(q) and/or G(11).

Validation of a competitive enzyme-linked immunosorbent assay for measuring the insulin-regulatable glucose transporter

A novel competitive enzyme-linked immunosorbent assay (ELISA) for measuring the insulin-regulatable glucose transporter (GLUT4) has been developed in our laboratory. The purpose of the present study was to verify the competitive ELISA assay. Towards this goal, a time-course study was conducted in control Wistar rat hearts to determine the points at which insulin stimulated an initial and a maximal GLUT4 translocation. Plasma and intracellular membrane fractions were purified from heart ventricles isolated from rats either in the basal state or injected with insulin. GLUT4 content in the membrane fractions was quantified with both the competitive ELISA method and also with enhanced chemiluminescence (ECL) Western blot. It was shown that after insulin injection plasma membrane GLUT4 level increased about 60% at 15 min and intracellular GLUT4 decreased about 40-50% at 5 min and remained at this level throughout the remaining 25 min by both methods. In conclusion, the data from this study demonstrate that the ELISA assay is reliable as verified by the Western blot method.

Contribution of alpha-adrenergic and beta-adrenergic stimulation to ischemia-induced glucose transporter (GLUT) 4 and GLUT1 translocation in the isolated perfused rat heart

The intracellular signaling mechanism of the ischemia-stimulated glucose transporter (GLUT) translocation in the heart is not yet characterized. It has been suggested that catecholamines released during ischemia may be involved in this pathway. The purpose of this study was to evaluate the contribution of alpha-adrenoceptors and beta-adrenoceptors to ischemia-mediated GLUT4 and GLUT1 translocation in the isolated, Langendorff-perfused rat heart. Additionally, GLUT translocation was studied in response to catecholamine stimulation with phenylephrine (Phy) and isoproterenol (Iso). The results were compared with myocardial uptake of glucose analogue [18F]fluorodeoxyglucose (FDG). Subcellular analysis of GLUT4 and GLUT1 protein on plasma membrane vesicles (PM) and intracellular membrane vesicles (IM) using membrane preparation and immunoblotting revealed that alpha- and beta-receptor agonists stimulated GLUT4 translocation from IM to PM (2.5-fold for Phy and 2.1-fold for Iso, P<0.05 versus control), which was completely inhibited by phentolamine (Phe) and propranolol (Pro), respectively. Plasmalemmal GLUT1 moderately rose after Iso exposure, and this was prevented by Pro. In contrast, ischemia-stimulated GLUT4 translocation (2.2-fold, P<0.05 versus control) was only inhibited by alpha-adrenergic antagonist Phe but not by beta-adrenergic antagonist Pro. Similarly, Phe but not Pro inhibited ischemia-stimulated GLUT1 translocation. GLUT data were confirmed by FDG uptake monitored using bismuth germanate detectors. The catecholamine-stimulated FDG uptake (6.9-fold for Phy and 8.9-fold for Iso) was significantly inhibited by Phe and Pro; however, only Phe but not Pro significantly reduced the ischemia-induced 2.5-fold increase in FDG uptake (P<0.05 versus ischemia). This study suggests that alpha-adrenoceptor stimulation may play a role in the ischemia-mediated increase in glucose transporter trafficking leading to the stimulation of FDG uptake in the isolated, perfused rat heart, whereas beta-adrenergic activation does not participate in this signaling pathway.

Alpha1-adrenergic hypothesis for pulmonary hypertension

Pulmonary hypertension (PH) is a chronic and disabling condition that affects the pulmonary vasculature. Once PH is diagnosed, the prognosis is generally poor with a rapid downhill course. PH management is largely empirical because the underlying pathophysiologic mechanisms that are responsible for the excessive vasoconstrictor and vascular smooth muscle proliferative responses are poorly understood. Based on new information concerning the role of adrenergic receptors in regulating various cellular functions, a new perspective on the genesis of PH has emerged, along with a unifying hypothesis for the role of alpha1-adrenergic receptors present in the pulmonary vasculature as the major contributor to the pathophysiologic changes associated with PH. Adrenergic receptors that are present on vascular smooth muscle cells regulate vascular tone and growth. The alpha1-adrenergic receptors that are present on the small- and medium-sized pulmonary arteries have a unique and greatly enhanced affinity and activity to alpha1-adrenergic agonists. Under physiologic conditions, this helps in regulating vascular tone and maintains an adequate ventilation/perfusion matching. However, the excessive stimulation of alpha1-adrenergic receptors produces not only smooth muscle contraction but also proliferation and growth. The conditions that produce an increase in alpha1-adrenoreceptor gene synthesis, density, and activity (such as hypoxia or changes in vessel wall pressure) or increase the levels of its agonists (such as norepinephrine, appetite suppressants, or cocaine) greatly enhance pulmonary vascular smooth muscle contractile and proliferative responses and lead to the development of PH. An understanding of the role played by these receptors in the pathophysiology of PH would not only help to avoid the use of alpha1-agonists for appetite suppression and other disease states, but also would help in developing new drugs to block these receptors. A further understanding of the alpha1-adrenoreceptor subtypes present in the pulmonary vasculature, the factors that regulate their expression, and their intracellular signaling pathways would help researchers to devise newer therapeutic strategies and, hopefully, to find a cure for this crippling condition.

alpha-adrenergic stimulation mediates glucose uptake through phosphatidylinositol 3-kinase in rat heart

We examined whether insulin and catecholamines share common pathways for their stimulating effects on glucose uptake. We perfused isolated working rat hearts with Krebs-Henseleit buffer containing [2-3H]glucose (5 mmol/L, 0.05 microCi/mL) and sodium oleate (0.4 mmol/L). In the absence or presence of the phosphatidylinositol 3-kinase (PI3-K) inhibitor wortmannin (3 micromol/L), we added insulin (1 mU/mL), epinephrine (1 micromol/L), phenylephrine (100 micromol/L) plus propranolol (10 micromol/L, selective alpha-adrenergic stimulation), or isoproterenol (1 micromol/L) plus phentolamine (10 micromol/L, selective beta-adrenergic stimulation) to the perfusate. Cardiac power was found to be stable in all groups (between 8.07+/-0.68 and 10.7+/-0. 88 mW) and increased (25% to 47%) with addition of epinephrine, but not with selective alpha- and beta-adrenergic stimulation. Insulin and epinephrine, as well as selective alpha- and beta-receptor stimulation, increased glucose uptake (the following values are in micromol/[min. g dry weight]: basal, 1.19+/-0.13; insulin, 3.89+/-0.36; epinephrine, 3.46+/-0.27; alpha-stimulation, 4.08+/-0.40; and beta-stimulation, 3.72+/-0.34). Wortmannin completely inhibited insulin-stimulated and selective alpha-stimulated glucose uptake, but it did not affect the epinephrine-stimulated or selective beta-stimulated glucose uptake. Sequential addition of insulin and epinephrine or insulin and alpha-selective stimulation showed additive effects on glucose uptake in both cases. Wortmannin further blocked the effects of insulin on glycogen synthesis. We conclude that alpha-adrenergic stimulation mediates glucose uptake in rat heart through a PI3-K-dependent pathway. However, the additive effects of alpha-adrenergic stimulation and insulin suggest 2 different isoforms of PI3-K, compartmentation of PI3-K, potentiation, or inhibition by wortmannin of another intermediate of the alpha-adrenergic signaling cascade. The stimulating effects of both the alpha- and the beta-adrenergic pathways on glucose uptake are independent of changes in cardiac performance.

Glucose delivery is a major determinant of glucose utilisation in the ischemic myocardium with a residual coronary flow

BACKGROUND

Experimental data from isolated rat hearts suggest that glycolysis in severe myocardial ischemia is inhibited by accumulation of glycolytic metabolites. In contrast, positron emission tomography (PET) in patients with myocardial ischemia records a 'mismatch' between the decreased coronary flow in viable ischemic tissue and an increased fluorodeoxyglucose (18FDG) signal. To resolve this contradiction, we investigated glucose uptake at very low coronary flows in the isolated rat heart model.

METHODS

Rates of glucose uptake were measured in the isolated Langendorff-perfused Wistar rat heart, at control (12-15 ml/g wet wt/min) and low coronary flows (0.1, 0.2 and 0.5 ml/g wet wt/min) and at a range of glucose concentrations (2.75, 5.5, 11 and 22 mM).

RESULTS

The steady-state rate of glucose uptake versus glucose concentration could be described by a double rectangular hyperbola at each coronary flow. Glucose uptake fell to levels significantly below control at low coronary flows. However, the extraction of glucose (glucose uptake as % of glucose delivered) rose sharply, from 1% at control coronary flows, to 25-30% at low coronary flows. Crossover analysis of glycolytic intermediates in freeze-clamped tissue indicated little inhibition at any specific site, although phosphofructokinase activity was restricted when glycolytic substrate availability was high. Insulin and preconditioning both increased glucose uptake with 11 mM glucose, possibly by increasing membrane transporter density and thus increasing glucose delivery to the cytosol.

CONCLUSIONS

Despite the reduction in absolute glucose uptake at low coronary flow-rates, the extraction of glucose was greatly increased, possibly following GLUT4 translocation. Delivery of glucose to the glycolytic pathway appears to be a major controlling site of glycolysis in low-flow ischemia. Downstream regulation is then distributed along the pathway with no one site exerting greater inhibition than reduced glucose delivery.

Glucose and glycogen utilisation in myocardial ischemia--changes in metabolism and consequences for the myocyte

Experimentally, enhanced glycolytic flux has been shown to confer many benefits to the ischemic heart, including maintenance of membrane activity, inhibition of contracture, reduced arrhythmias, and improved functional recovery. While at moderate low coronary flows, the benefits of glycolysis appear extensive, the controversy arises at very low flow rates, in the absence of flow; or when glycolytic substrate may be present in excess, such that high glucose concentrations with or without insulin overload the cell with deleterious metabolites. Under conditions of total global ischemia, glycogen is the only substrate for glycolytic flux. Glycogenolysis may only be protective until the accumulation of metabolites (lactate, H+, NADH, sugar phosphates and Pi ) outweighs the benefit of the ATP produced. The possible deleterious effects associated with increased glycolysis cannot be ignored, and may explain some of the controversial findings reported in the literature. However, an optimal balance between the rate of ATP production and rate of accumulation of metabolites (determined by the glycolytic flux rate and the rate of coronary washout), may ensure optimal recovery. In addition, the effects of glucose utilisation must be distinguished from those of glycogen, differences which may be explained by functional compartmentation within the cell.

Inhibition of myocardial glucose uptake by cGMP

Guanosine 3',5'-cyclic monophosphate (cGMP), a second messenger of nitric oxide (NO), regulates myocardial contractility. It is not known whether this effect is accompanied by a change in heart metabolism. We report here the effects of 8-bromoguanosine 3',5'-cyclic monophosphate (8-BrcGMP), a cGMP analog, on regulatory steps of glucose metabolism in isolated working rat hearts perfused with glucose as the substrate. When glucose uptake was stimulated by increasing the workload, addition of the cGMP analog totally suppressed this stimulation and accelerated net glycogen breakdown. 8-BrcGMP did not affect pyruvate dehydrogenase activity but activated acetyl-CoA carboxylase, the enzyme that produces malonyl-CoA, an inhibitor of long-chain fatty acid oxidation. To test whether glucose metabolism could also be affected by altering the intracellular concentration of cGMP, we perfused hearts with NG-nitro-L-arginine methyl ester (L-NAME), an inhibitor of NO synthase, or with S-nitroso-N-acetylpenicillamine (SNAP), a NO donor. Perfusion with L-NAME decreased cGMP and increased glucose uptake by 30%, whereas perfusion with SNAP resulted in opposite effects. None of these conditions affected adenosine 3',5'-cyclic monophosphate concentration. Limitation of glucose uptake by SNAP or 8-BrcGMP decreased heart work, and this was reversed by adding alternative oxidizable substrates (pyruvate, beta-hydroxybutyrate) together with glucose. Therefore, increased NO production decreases myocardial glucose utilization and limits heart work. This effect is mediated by cGMP, which is thus endowed with both physiological and metabolic properties.

Cyclic AMP suppresses the inhibition of glycolysis by alternative oxidizable substrates in the heart

In normoxic conditions, myocardial glucose utilization is inhibited when alternative oxidizable substrates are available. In this work we show that this inhibition is relieved in the presence of cAMP, and we studied the mechanism of this effect. Working rat hearts were perfused with 5.5 mM glucose alone (controls) or together with 5 mM lactate, 5 mM beta-hydroxybutyrate, or 1 mM palmitate. The effects of 0.1 mM chlorophenylthio-cAMP (CPT-cAMP), a cAMP analogue, were studied in each group. Glucose uptake, flux through 6-phosphofructo-1-kinase, and pyruvate dehydrogenase activity were inhibited in hearts perfused with alternative substrates, and addition of CPT-cAMP completely relieved the inhibition. The mechanism by which CPT-cAMP induced a preferential utilization of glucose was related to an increased glucose uptake and glycolysis, and to an activation of phosphorylase, pyruvate dehydrogenase, and 6-phosphofructo-2-kinase, the enzyme responsible for the synthesis of fructose 2,6-bisphosphate, the well-known stimulator of 6-phosphofructo-1-kinase. In vitro phosphorylation of 6-phosphofructo-2-kinase by cAMP-dependent protein kinase increased the Vmax of the enzyme and decreased its sensitivity to the inhibitor citrate. Therefore, in hearts perfused with various oxidizable substrates, cAMP induces a preferential utilization of glucose by a concerted stimulation of glucose transport, glycolysis, glycogen breakdown, and glucose oxidation.

Quantitative methods for measuring the insulin-regulatable glucose transporter (Glut4)

This review article describes various quantitation methods for the insulin-regulatable glucose transporter (Glut4). Several methods including reconstituted glucose transport, cytochalasin B binding assays, immunocytochemistry, immunoblots, ELISA, and the more recently developed exofacial labels are discussed. Since Glut4 translocates from an intracellular compartment to the plasma membrane in response to the action of insulin, it is of particular interest to measure Glut4 changes in the membrane fractions. Hence, the measurement of Glut4 commonly involves the isolation of cell membranes using subcellular fractionation in combination with one of the quantitation methods. The limitations of each quantitation method due to the use of subcellular fractionation are discussed in this article. As well, the advantages and disadvantages in terms of isoform specificity and technical difficulties of each method are presented.

Molecular mechanisms of contraction-induced translocation of GLUT4 in isolated cardiomyocytes

Isolated adult rat ventricular cardiomyocytes were used to investigate the effects of contractile activity on 3-O-methylglucose transport on the translocation of the insulin-responsive glucose transporter GLUT4, and the possible activation of intermediates of the insulin signaling cascade. When elicited by field stimulation, contraction at 1 Hz did not significantly affect the adenosine triphosphate (ATP) content of cardiac cells, even after 60 min. At 5 Hz, a stable ATP level was observed until 15 minutes with a rapid decline at later time points. Stimulation of cardiomyocytes at 5 Hz for 5 minutes induced a 2-3 fold increase of 3-O-methylglucose transport with no additional stimulation in the presence of insulin (10[-7] M). Subcellular fractionation and immunoblotting analysis of GLUT4 distribution indicated that both contraction and insulin induced an identical increase (8-9-fold) of GLUT4 in the plasma membrane with a concomitant decrease (one third) in the microsomal fraction. Treatment of cardiomyocytes with wortmannin produced a complete inhibition of insulin- and contraction-induced glucose uptake. However, immunoprecipitation of insulin receptor substrate-1 (IRS-1) showed that the p85 regulatory subunit of phosphatidylinositol-3 kinase did not associate with IRS-1 upon contraction but with a marked stimulated association in response to insulin. These data suggest the existence of identical insulin- and contraction-recruitable GLUT4 pool. Contraction-induced signaling may use a limited part of the insulin-signaling cascade, possibly involving IRS-2. We further suggest that insulin resistance at the level of IRS-1 will not affect contraction-regulated glucose uptake by the heart.

Regulation of glucose transport, and glucose transporters expression and trafficking in the heart: studies in cardiac myocytes

Cardiac muscle is characterized by a high rate of glucose consumption. In the absence of insulin, glucose transport into cardiomyocytes limits the rate of glucose utilization and therefore it is important to understand the regulation of glucose transporters. Cardiac muscle cells express 2 distinct glucose transporters, GLUT4 and GLUT1; although GLUT4 is quantitatively the more important glucose transporter expressed in heart, GLUT1 is also expressed at a substantial level. In isolated rat cardiomyocytes, insulin acutely stimulates glucose transport and translocates both GLUT4 and GLUT1 from an intracellular site to the cell surface. Recent evidence indicates the existence of at least 2 distinct intracellular membrane populations enriched in GLUT4 with a different protein composition. Elucidation of the intracellular location of these 2 GLUT4 vesicle pools in cardiac myocytes, their role in GLUT4 trafficking, and their relation to insulin-induced GLUT4 translocation needs to be addressed.

Low-flow ischemia leads to translocation of canine heart GLUT-4 and GLUT-1 glucose transporters to the sarcolemma in vivo

BACKGROUND

Myocardial ischemia increases heart glucose utilization in vivo. However, whether low-flow ischemia leads to the translocation of glucose transporter (GLUT)-4 and/or GLUT-1 to the sarcolemma in vivo is unknown.

METHODS AND RESULTS

In a canine model, we evaluated myocardial glucose metabolism in vivo and the distribution of GLUT-4 and GLUT-1 by use of immunoblotting of sarcolemma and intracellular membranes and immunofluorescence localization with confocal microscopy. In vivo glucose extraction increased fivefold (P < .001) and was associated with net lactate release in the ischemic region. Ischemia led to an increase in the sarcolemma content of both GLUT-4 (15 +/- 2% to 30 +/- 3%, P < .02) and GLUT-1 (41 +/- 4% to 58 +/- 3%, P < .03) compared with the nonischemic region and to a parallel decrease in their intracellular contents. Immunofluorescence demonstrated the presence of both GLUT-4 and GLUT-1 on cardiac myocytes. GLUT-1 had a more prominent cell surface pattern than GLUT-4, which was primarily intracellular in the nonischemic region. However, significant GLUT-4 surface labeling was found in the ischemic region.

CONCLUSIONS

Translocation of the insulin-responsive GLUT-4 transporter from an intracellular storage pool to the sarcolemma occurs in vivo during acute low-flow ischemia. GLUT-1 is also present in an intracellular storage pool from which it undergoes translocation to the sarcolemma in response to ischemia. These results indicate that both GLUT-1 and GLUT-4 are important in ischemia-mediated myocardial glucose uptake in vivo.

Signals mediating stimulation of cardiomyocyte glucose transport by the alpha-adrenergic agonist phenylephrine

Phenylephrine, a potent stimulator of cardiomyocyte glucose transport (GT), caused a rapid rise in cytosolic Ca2+ by 30%. Agents inducing a similar Ca2+ response did not stimulate (angiotension II, vasopressin) or inhibited GT by 20% (elevated extracellular Ca2+). Stimulation of GT by phorbol myristate acetate was additive to both phases of phenylephrine's effect (4 min, 60 min). Phenylephrine had no influence on the adenosine 3', 5'-cyclic monophosphate (cAMP) and guanosine 3',5'-cyclic monophosphate (cGMP) levels. Agents raising cAMP (isoproterenol) or cGMP (e.g., nitroprusside) did not stimulate GT. Wortmannin (inhibitor of 1-phosphatidylinositol 3-kinase) suppressed the action of insulin on GT but not that of phenylephrine. In contrast, the Na+/H+ exchange inhibitor amiloride (which blocks phenylephrine-induced cytosolic alkalinization or even lowers cellular pH) depressed the effect of phenylephrine by 50%, whereas insulin-stimulated GT was little affected. However, raising extracellular pH up to 8.4 failed to increase GT. Lowering pH to 6.8 decreased phenylephrine's effect by 40% whereas insulin-dependent GT was not significantly altered. Clorgyline, tranylcypromine (monoamine oxidase inhibitors), and added catalase suppressed the slow phase of phenylephrine's action, whereas amiloride also affected the fast phase. We conclude that 1) stimulation of cardiomyocyte GT by phenylephrine does not involve cAMP, cGMP, or 1-phosphatidylinositol 3-kinase; 2) protein kinase C activation cannot explain the full extent of stimulation; 3) Ca2+ release or cytosolic alkalinization may be required but is not sufficient to trigger phenylephrine's action, and 4) the slow phase of stimulation is mediated by the monoamine oxidase-dependent degradation of phenylephrine and by the resulting H2O2 formation.

Contraction-independent effects of catecholamines on glucose transport in isolated rat cardiomyocytes

The effects of catecholamines on glucose transport were studied in noncontracting isolated rat cardiomyocytes. alpha-Adrenergic treatment (phenylephrine, or norepinephrine + propranolol) led to an approximately fourfold stimulation of glucose transport in basal cells (no insulin). The effect of phenylephrine was suppressed by the alpha 2-antagonist yohimbine or the beta-antagonist propranolol. The beta-adrenergic agonist isoproterenol partially counteracted the action of phenylephrine (but not that of insulin). Phenylephrine increased glucose transport in two phases with apparent half times of 3.2 and 13.0 min, respectively. Correspondingly, different EC50 values were found after 10 and 45 min on phenylephrine addition (5.0 +/- 1.9 vs. 31.6 +/- 9.6 microM, respectively). Maximal stimulation by phenylephrine was at least partially additive to that of insulin and of other stimulators of glucose transport (e.g., H2O2, vanadate, lithium). Phenylephrine significantly increased the level of cell surface glucose carriers GLUT-1 (1.54-fold) and GLUT-4 (1.78-fold), as assessed by using the specific photolabel 2-N-[4-(1-azi-2,2,2-trifluoroethyl)benzoyl]- 1,3-bis(D-mannos-4-yloxy)propyl-2-amine. In conclusion, catecholamines stimulate cardiomyocyte glucose transport through alpha 1-adrenergic receptors independently or downstream of a contraction-evoked stimulus. This effect is at least partially explained by a recruitment of glucose transporters to the cell surface. The mechanism(s) and/or signals involved differ from those triggered by insulin and insulinomimetic agents.

Acute stimulation of glucose transport by histamine in cardiac microvascular endothelial cells

The purpose of the present work was to study the acute regulation of glucose uptake in cultured cardiac endothelial cells (CEC). Two types of potential stimuli were considered: (1) agents that are known to acutely stimulate glucose transport (i.e., within minutes) in fat and muscle tissues and (2) agents that influence endothelial cell function. Among the former agents, neither insulin, nor catecholamines (adrenaline, dopamine, phenylephrine), nor serotonin affected the rate of glucose transport in CEC, while SH-group reagents (phenylarsine oxide, diamide or menadione) were inhibitory. Among the factors of the second group that were tested (heparin, ADP, histamine, bradykinin), histamine was found to stimulate glucose transport in CEC by 10-50%. This effect was concentration-dependent (with an EC50 value approximately equal to 12 microM) and reached a maximum within 5 min upon histamine addition. This stimulation of glucose transport was suppressed by pyrilamine (100 nM), a specific H1-receptor antagonist, but not by cimetidine (100 microM), a H2-selective antagonist. Northern blot and Western blot analysis of CEC extracts revealed the presence of the ubiquitous glucose transporter isoform GLUT1 mRNA and protein, but not of the 'insulin-regulatable' isoform GLUT4. In conclusion, this is the first report on an acute stimulation of glucose transport in cardiac endothelial cells, in particular, and in an insulin-unresponsive cell type, in general. The effect of histamine is most likely mediated by H1-receptors and cannot be accounted for by a recruitment of GLUT4.

Fundamental limitations of [18F]2-deoxy-2-fluoro-D-glucose for assessing myocardial glucose uptake

BACKGROUND

The glucose tracer analog [18F]2-deoxy-2-fluoro-D-glucose (FDG) is widely used for assessing regional myocardial glucose metabolism in vivo. The reproducibility of this method has recently been questioned because of a discordant affinity of hexokinase for its substrates glucose and 2-deoxyglucose. We therefore compared rates of glucose utilization simultaneously with tissue time-activity curves of FDG uptake before and after changes in the physiological environment of the heart.

METHODS AND RESULTS

Isolated working rat hearts were perfused for 60 minutes with recirculating Krebs buffer containing glucose (10 mmol/L), FDG (1 microCi/mL), [2-3H]glucose (0.05 microCi/mL), and [U-14C]2-deoxyglucose (2-DG; 0.025 microCi/mL). Myocardial glucose uptake was measured by tracer ([2-3H]glucose) and tracer analog methods (FDG and 2-DG) before and after the addition of either insulin (1 mU/mL), epinephrine (1 mumol/L), lactate (40 mmol/L), or D,L-beta-hydroxybutyrate (40 mmol/L) at 30 minutes of perfusion and after acute changes in cardiac workload. Under steady-state conditions, myocardial rates of glucose utilization as measured by tritiated water (3H2O) production from metabolism of [2-3H]glucose, FDG uptake, and 2-DG retention were linearly related. The addition of competing substrates decreased glucose utilization immediately. The addition of insulin increased the rate of glucose utilization as measured by the glucose tracer but not as measured by the tracer analogs. The ratio of 3H2O release/myocardial FDG uptake increased by 111% after the addition of insulin, by 428% after the addition of lactate, and by 232% after the addition of beta-hydroxybutyrate. Epinephrine increased rates of glucose utilization and contractile performance, whereas there was no increase in glucose uptake with a comparable increase in workload alone. There was no change in the relation between the glucose tracer and the tracer analog either with epinephrine or with acute changes in workload.

CONCLUSIONS

The uptake and retention of FDG in heart muscle is linearly related to glucose utilization only under steady-state conditions. Addition of insulin or of competing substrates changes the relation between uptake of the glucose tracer and FDG. These observations preclude the determination of absolute rates of myocardial glucose uptake by the tracer analog method under non-steady-state conditions.

Translocation of two glucose transporters in heart: effects of rotenone, uncouplers, workload, palmitate, insulin and anoxia

Our previous studies on the acute regulation of glucose transport in perfused rat hearts were extended to explore further the mechanism of regulation by anoxia; to test the effects of palmitate, a transport inhibitor; and to compare the translocation of two glucose transporter isoforms (GLUT1 and GLUT4). Following heart perfusions under various conditions, glucose transporters in intracellular membranes were quantitated by reconstitution of transport activity and by Western blotting. Rotenone stimulated glucose uptake and decreased the intracellular contents of glucose transporters. This indicates that it activates glucose transport via net outward translocation, similarly to anoxia. However, two uncouplers of oxidative phosphorylation produced little or no effect. Increased workload (which stimulates glucose transport) reduced the intracellular contents of transporters, while palmitate increased the contents, indicating that these factors cause net translocation from or to the intracellular pool, respectively. Relative changes in GLUT1 were similar to those in GLUT4 for most factors tested. A plot of changes in total intracellular transporter content vs. changes in glucose uptake was roughly linear, with a slope of -0.18. This indicates that translocation accounts for most of the changes in glucose transport, and the basal pool of intracellular transporters is five times as large as the plasma membrane pool.

Effect of glucose on pHin and [Ca2+]in in NIH-3T3 cells transfected with the yeast P-type H(+)-ATPase

NIH-3T3 cells transfected with yeast H(+)-ATPases (RN1a cells) are tumorigenic (Perona and Serrano, 1988, Nature, 334:438). We have previously shown that RN1a cells maintain a chronically high intracellular pH (pHin) under physiological conditions. We have also shown that RN1a cells are serum-independent for growth, maintain a higher intracellular Ca2+ ([Ca2+]in), and glycolyze more rapidly than their non-transformed counterparts (Gillies et al., Proc. Natl. Acad. Sci., 1990, 87:7414; Gillies et al., Cell. Physiol. Biochem., 1992, 2:159). The present study was aimed to understand the interrelationships between glycolysis, pHin, and [Ca2+]in in RN1a cells and their non-transformed counterparts, NIH-3T3 cells. Our data show that the higher rate of glycolysis observed in RN1a cells is due to the presence of low affinity glucose transporters. Consequently, the higher rate of glycolysis is exacerbated at high glucose concentration in RN1a cells. Moreover, the maximal velocity (Vmax) for glucose utilization is up to sixfold higher in RN1a cells than in the NIH-3T3 cells, suggesting that the number of glucose transporters is higher in RN1a than NIH-3T3 cells. Glucose addition to NIH-3T3 cells results in modest decreases in both pHin and [Ca2+]in. In contrast, RN1a cells respond to glucose with a large decrease in pHin, followed by a large decrease in [Ca2+]in. The decrease in [Ca2+]in observed upon glucose addition is likely due to activation of Ca(2+)-ATPase by glycolysis, since the Ca2+ decrease is abolished by the Ca2+ ATPase inhibitors thapsigargin and cyclopiazonic acid. Glucose addition to ATP-depleted cells results in a decrease in [Ca2+]in, suggesting that ATP furnished by glycolysis is utilized by this pump.

Ischemia induces translocation of the insulin-responsive glucose transporter GLUT4 to the plasma membrane of cardiac myocytes

BACKGROUND

Acute myocardial ischemia is accompanied by an increase in glucose uptake and metabolism, which appears to be important in protecting myocardial cells from irreversible ischemic injury. Because insulin augments myocardial glucose uptake by inducing the translocation of glucose transporters from an intracellular compartment to the plasma membrane, we hypothesized that acute ischemia would trigger a similar translocation.

METHODS AND RESULTS

We used a subcellular fractionation method to separate intracellular membrane and plasma membranes from control, ischemic, and hypoxic Langendorff-isolated perfused rat hearts and determined the expression of the major myocardial glucose transporter, GLUT4, in these separated membrane fractions. We found that translocation of GLUT4 molecules occurred in ischemic, hypoxic, and insulin-treated hearts and in hearts that underwent ischemia plus insulin treatment. The percentages of GLUT4 molecules present on the plasma membrane in the different conditions were as follows: control, 18.0 +/- 2.8%; ischemia, 41.3 +/- 9.4%; hypoxia, 31.1 +/- 2.9%; insulin, 61.1 +/- 2.6%; and ischemia plus insulin, 66.8 +/- 5.7%. Among the statistically significant differences in these values were the difference between control and ischemia and the difference between ischemia alone and insulin plus ischemia.

CONCLUSIONS

Ischemia causes substantial translocation of GLUT4 molecules to the plasma membrane of cardiac myocytes. A combination of insulin plus ischemia stimulates an even greater degree of GLUT4 translocation. GLUT4 translocation is likely to mediate at least part of the increased glucose uptake of ischemic myocardium and may be a mechanism for the cardioprotective effect of insulin during acute myocardial ischemia.

Contraction-induced translocation of the glucose transporter Glut4 in isolated ventricular cardiomyocytes

Field stimulation of isolated adult ventricular cardiomyocytes was used to study the effect of contractile activity on 3-O-methylglucose transport and the subcellular distribution of Glut4. Cells contracting at a frequency of 1 Hz for 30 min exhibited unaltered basal and insulin-stimulated rates of glucose transport when compared to resting cells. However, at 5 Hz 3-O-methylglucose transport increased to 224% of control after 5 min. Under these conditions insulin was unable to produce a significant additional stimulation of glucose transport. Immunoblotting with an anti-Glut4 polyclonal antibody showed that both insulin and contraction (5 Hz) increased the amount of Glut4 in a plasma membrane fraction by about 8-fold with a parallel decrease in an intracellular membrane fraction by 60-65%. These data suggest the existence of an identical insulin- and contraction-recruitable Glut4 transporter pool in cardiomyocytes.