In-vivo glucose uptake and glucose transporter proteins GLUT1 and GLUT3 in brain tissue from streptozotocin-diabetic rats

Brain energetics and glucose transport in metabolic diseases: role in neurodegeneration

OBJECTIVES

Neurons and glial cells are the main functional and structural elements of the brain, and the former depends on the latter for their nutritional, functional and structural organization, as well as for their energy maintenance.

METHODS

Glucose is the main metabolic source that fulfills energetic demands, either by direct anaplerosis or through its conversion to metabolic intermediates. Development of some neurodegenerative diseases have been related with modifications in the expression and/or function of glial glucose transporters, which might cause physiological and/or pathological disturbances of brain metabolism. In the present contribution, we summarized the experimental findings that describe the exquisite adjustment in expression and function of glial glucose transporters from physiologic to pathologic metabolism, and its relevance to neurodegenerative diseases.

RESULTS

A exhaustive literature review was done in order to gain insight into the role of brain energetics in neurodegenerative disease. This study made evident a critical involvement of glucose transporters and thus brain energetics in the development of neurodegenerative diseases.

DISCUSSION

An exquisite adjustment in the expression and function of glial glucose transporters from physiologic to pathologic metabolism is a biochemical signature of neurodegenerative diseases.

Imaging diabetic cardiomyopathy in a type 1 diabetic rat model using 18F-FEPPA PET

OBJECTIVE

Diabetic patients often experience chronic inflammation and fibrosis in their cardiac tissues, highlighting the pressing need for the development of sensitive diagnostic methods for longitudinal assessment of diabetic cardiomyopathy. This study aims to evaluate the significance of an inflammatory marker known as translocator protein (TSPO) in a positron emission tomography (PET) protocol for longitudinally monitoring cardiac dysfunction in a diabetic animal model. Additionally, we compared the commonly used radiotracer, 18F-fluoro-2-deoxy-d-glucose (18F-FDG).

METHODS

Fourteen 7-week-old female Sprague-Dawley rats were used in this study. Longitudinal PET experiments were conducted using 18F-N-2-(2-fluoroethoxy)benzyl)-N-(4-phenoxypyridin-3-yl)acetamide (18F-FEPPA) (n = 3), the TSPO radiotracer, and 18F-FDG (n = 3), both before and after the onset of diabetes. Histological and immunohistochemical staining assays were also conducted in both the control (n = 4) and diabetes (n = 4) groups.

RESULTS

Results indicated a significant increase in cardiac tissue uptake of 18F-FEPPA after the onset of diabetes (P < 0.05), aligning with elevated TSPO levels observed in diabetic animals according to histological data. Conversely, the uptake of 18F-FDG in cardiac tissue significantly decreased after the onset of diabetes (P < 0.05).

CONCLUSION

These findings suggest that 18F-FEPPA can function as a sensitive probe for detecting chronic inflammation and fibrosis in the cardiac tissues of diabetic animals.

The effect of estrogen therapy on cerebral metabolism in diabetic female rats

The impact of estrogen on brain function, especially in individuals with diabetes, remains uncertain. This study aims to compare cerebral glucose metabolism levels in intact rats, ovariectomized (OVX) rats, and 17β-estradiol (E2)-treated OVX diabetic female rats. Sixteen rats were administered a single intraperitoneal injection of 70 mg/kg streptozotocin (STZ) to induce diabetes (intact, n = 6; OVX, n = 6; OVX+E2-treated, n = 4). Additionally, 18 rats received an equivalent solvent dose via intraperitoneal injection (intact, n = 6; OVX, n = 6; OVX+E2-treated, n = 6). After 4 weeks of STZ or solvent administration, positron emission tomography scans with 18F-fluorodeoxyglucose (18F-FDG) injection were employed to assess cerebral glucose metabolism. The diabetic rats exhibited substantial reductions in 18F-FDG uptake across all brain regions (all P < 0.01), in contrast to the control rats. Moreover, intact and OVX + E2-treated diabetic female rats displayed more pronounced decreases in cerebral glucose metabolism in the amygdala and hippocampus compared to OVX diabetic female rats (P < 0.05). These findings suggest that diabetes creates an environment wherein estrogen exacerbates neuropathology and intensifies neuronal activity.

Cognitive dysfunction in diabetes: abnormal glucose metabolic regulation in the brain

Cognitive dysfunction is increasingly recognized as a complication and comorbidity of diabetes, supported by evidence of abnormal brain structure and function. Although few mechanistic metabolic studies have shown clear pathophysiological links between diabetes and cognitive dysfunction, there are several plausible ways in which this connection may occur. Since, brain functions require a constant supply of glucose as an energy source, the brain may be more susceptible to abnormalities in glucose metabolism. Glucose metabolic abnormalities under diabetic conditions may play an important role in cognitive dysfunction by affecting glucose transport and reducing glucose metabolism. These changes, along with oxidative stress, inflammation, mitochondrial dysfunction, and other factors, can affect synaptic transmission, neural plasticity, and ultimately lead to impaired neuronal and cognitive function. Insulin signal triggers intracellular signal transduction that regulates glucose transport and metabolism. Insulin resistance, one hallmark of diabetes, has also been linked with impaired cerebral glucose metabolism in the brain. In this review, we conclude that glucose metabolic abnormalities play a critical role in the pathophysiological alterations underlying diabetic cognitive dysfunction (DCD), which is associated with multiple pathogenic factors such as oxidative stress, mitochondrial dysfunction, inflammation, and others. Brain insulin resistance is highly emphasized and characterized as an important pathogenic mechanism in the DCD.

Glucose transporter 3 in neuronal glucose metabolism: Health and diseases

Neurons obtain glucose from extracellular environment for energy production mainly depending on glucose transporter 3 (GLUT3). GLUT3 uptakes glucose with high affinity and great transport capacity, and is important for neuronal energy metabolism. This review summarized the role of neuronal GLUT3 in brain metabolism, function and development under both physiological conditions and in diseases, aiming to provide insights into neuronal glucose metabolism and its effect on brain. GLUT3 stabilizes neuronal glucose uptake and utilization, influences brain development and function, and ameliorates aging-related manifestations. Neuronal GLUT3 is regulated by synaptic activity, hormones, nutrition, insulin and insulin-like growth factor 1 in physiological conditions, and is also upregulated by hypoxia-ischemia. GLUT3-related neuronal glucose and energy metabolism is possibly involved in the pathogenesis, pathophysiological mechanism, progression or prognosis of brain diseases, including Alzheimer's disease, Huntington's disease, attention-deficit/hyperactivity disorder and epilepsy. GLUT3 may be a promising therapeutic target of these diseases. This review also briefly discussed the role of other glucose transporters in neuronal glucose metabolism, which work together with GLUT3 to sustain and stabilize glucose and energy supply for neurons. Deficiency in these glucose transporters may also participate in brain diseases, especially GLUT1 and GLUT4.

Metabolic Transporters in the Peripheral Nerve-What, Where, and Why?

Cellular metabolism is critical not only for cell survival, but also for cell fate, function, and intercellular communication. There are several different metabolic transporters expressed in the peripheral nervous system, and they each play important roles in maintaining cellular energy. The major source of energy in the peripheral nervous system is glucose, and glucose transporters 1 and 3 are expressed and allow blood glucose to be imported and utilized by peripheral nerves. There is also increasing evidence that other sources of energy, particularly monocarboxylates such as lactate that are transported primarily by monocarboxylate transporters 1 and 2 in peripheral nerves, can be efficiently utilized by peripheral nerves. Finally, emerging evidence supports an important role for connexins and possibly pannexins in the supply and regulation of metabolic energy. In this review, we will first define these critical metabolic transporter subtypes and then examine their localization in the peripheral nervous system. We will subsequently discuss the evidence, which comes both from experiments in animal models and observations from human diseases, supporting critical roles played by these metabolic transporters in the peripheral nervous system. Despite progress made in understanding the function of these transporters, many questions and some discrepancies remain, and these will also be addressed throughout this review. Peripheral nerve metabolism is fundamentally important and renewed interest in these pathways should help to answer many of these questions and potentially provide new treatments for neurologic diseases that are partly, or completely, caused by disruption of metabolism.

Glucose transporters in brain in health and disease

Energy demand of neurons in brain that is covered by glucose supply from the blood is ensured by glucose transporters in capillaries and brain cells. In brain, the facilitative diffusion glucose transporters GLUT1-6 and GLUT8, and the Na⁺-d-glucose cotransporters SGLT1 are expressed. The glucose transporters mediate uptake of d-glucose across the blood-brain barrier and delivery of d-glucose to astrocytes and neurons. They are critically involved in regulatory adaptations to varying energy demands in response to differing neuronal activities and glucose supply. In this review, a comprehensive overview about verified and proposed roles of cerebral glucose transporters during health and diseases is presented. Our current knowledge is mainly based on experiments performed in rodents. First, the functional properties of human glucose transporters expressed in brain and their cerebral locations are described. Thereafter, proposed physiological functions of GLUT1, GLUT2, GLUT3, GLUT4, and SGLT1 for energy supply to neurons, glucose sensing, central regulation of glucohomeostasis, and feeding behavior are compiled, and their roles in learning and memory formation are discussed. In addition, diseases are described in which functional changes of cerebral glucose transporters are relevant. These are GLUT1 deficiency syndrome (GLUT1-SD), diabetes mellitus, Alzheimer’s disease (AD), stroke, and traumatic brain injury (TBI). GLUT1-SD is caused by defect mutations in GLUT1. Diabetes and AD are associated with changed expression of glucose transporters in brain, and transporter-related energy deficiency of neurons may contribute to pathogenesis of AD. Stroke and TBI are associated with changes of glucose transporter expression that influence clinical outcome.

Impact of Caffeine Consumption on Type 2 Diabetes-Induced Spatial Memory Impairment and Neurochemical Alterations in the Hippocampus

Diabetes affects the morphology and plasticity of the hippocampus, and leads to learning and memory deficits. Caffeine has been proposed to prevent memory impairment upon multiple chronic disorders with neurological involvement. We tested whether long-term caffeine consumption prevents type 2 diabetes (T2D)-induced spatial memory impairment and hippocampal alterations, including synaptic degeneration, astrogliosis, and metabolic modifications. Control Wistar rats and Goto-Kakizaki (GK) rats that develop T2D were treated with caffeine (1 g/L in drinking water) for 4 months. Spatial memory was evaluated in a Y-maze. Hippocampal metabolic profile and glucose homeostasis were investigated by ¹H magnetic resonance spectroscopy. The density of neuronal, synaptic, and glial-specific markers was evaluated by Western blot analysis. GK rats displayed reduced Y-maze spontaneous alternation and a lower amplitude of hippocampal long-term potentiation when compared to controls, suggesting impaired hippocampal-dependent spatial memory. Diabetes did not impact the relation of hippocampal to plasma glucose concentrations, but altered the neurochemical profile of the hippocampus, such as increased in levels of the osmolites taurine (P < 0.001) and myo-inositol (P < 0.05). The diabetic hippocampus showed decreased density of the presynaptic proteins synaptophysin (P < 0.05) and SNAP25 (P < 0.05), suggesting synaptic degeneration, and increased GFAP (P < 0.001) and vimentin (P < 0.05) immunoreactivities that are indicative of astrogliosis. The effects of caffeine intake on hippocampal metabolism added to those of T2D, namely reducing myo-inositol levels (P < 0.001) and further increasing taurine levels (P < 0.05). Caffeine prevented T2D-induced alterations of GFAP, vimentin and SNAP25, and improved memory deficits. We conclude that caffeine consumption has beneficial effects counteracting alterations in the hippocampus of GK rats, leading to the improvement of T2D-associated memory impairment.

Inward Glucose Transfer Accounts for Insulin-Dependent Increase in Brain Glucose Metabolism Associated with Diet-Induced Obesity

OBJECTIVE

There is a general agreement that there are changes in brain metabolism in insulin-resistant individuals during conditions of hyperinsulinemia. However, the impact on obesity is unclear, and the metabolic constants underlying these modifications are unknown. The aim of this study was to evaluate these changes in a large animal model of diet-induced obesity.

METHODS

Twenty adult miniature pigs were fed with either an obesogenic diet or a regular diet for 5 months. At that time, fat deposition was evaluated using computed tomography scanning, and ¹⁸ fluorodeoxyglucose positron emission tomography images were acquired dynamically both in the fasted state and during a euglycemic-hyperinsulinemic clamp. Glucose uptake rates and pixel-wise modeled brain volumes were calculated together with brain connectivity.

RESULTS

Whole-body insulin sensitivity was reduced by more than 50% in the obesity group. During insulin stimulation, whole-brain insulin-induced increased glucose uptake was unaltered in lean animals but increased markedly in the animals with obesity. The increased glucose uptake reflected an increase in the inward transfer without changes in phosphorylation or outward brain transport. Connectivity was increased in the animals with obesity CONCLUSIONS: Diet-induced obesity is associated with an increase in insulin-stimulated brain glucose uptake as a consequence of a larger inward transfer. These changes occurred together with an increased connectivity in reference to regions associated with memory recollection.

The role of glucose transporters in brain disease: diabetes and Alzheimer’s Disease

The occurrence of altered brain glucose metabolism has long been suggested in both diabetes and Alzheimer’s diseases. However, the preceding mechanism to altered glucose metabolism has not been well understood. Glucose enters the brain via glucose transporters primarily present at the blood-brain barrier. Any changes in glucose transporter function and expression dramatically affects brain glucose homeostasis and function. In the brains of both diabetic and Alzheimer’s disease patients, changes in glucose transporter function and expression have been observed, but a possible link between the altered glucose transporter function and disease progress is missing. Future recognition of the role of new glucose transporter isoforms in the brain may provide a better understanding of brain glucose metabolism in normal and disease states. Elucidation of clinical pathological mechanisms related to glucose transport and metabolism may provide common links to the etiology of these two diseases. Considering these facts, in this review we provide a current understanding of the vital roles of a variety of glucose transporters in the normal, diabetic and Alzheimer’s disease brain.

GTPase ARFRP1 is essential for normal hepatic glycogen storage and insulin-like growth factor 1 secretion

The GTPase ADP-ribosylation factor-related protein 1 (ARFRP1) is located at the trans-Golgi compartment and regulates the recruitment of Arf-like 1 (ARL1) and its effector golgin-245 to this compartment. Here, we show that liver-specific knockout of Arfrp1 in the mouse (Arfrp1(liv-/-)) resulted in early growth retardation, which was associated with reduced hepatic insulin-like growth factor 1 (IGF1) secretion. Accordingly, suppression of Arfrp1 in primary hepatocytes resulted in a significant reduction of IGF1 release. However, the hepatic secretion of IGF-binding protein 2 (IGFBP2) was not affected in the absence of ARFRP1. In addition, Arfrp1(liv-/-) mice exhibited decreased glucose transport into the liver, leading to a 50% reduction of glycogen stores as well as a marked retardation of glycogen storage after fasting and refeeding. These abnormalities in glucose metabolism were attributable to reduced protein levels and intracellular retention of the glucose transporter GLUT2 in Arfrp1(liv-/-) livers. As a consequence of impaired glucose uptake into the liver, the expression levels of carbohydrate response element binding protein (ChREBP), a transcription factor regulated by glucose concentration, and its target genes (glucokinase and pyruvate kinase) were markedly reduced. Our data indicate that ARFRP1 in the liver is involved in the regulation of IGF1 secretion and GLUT2 sorting and is thereby essential for normal growth and glycogen storage.

Longitudinal evaluation of memory performance and peripheral neuropathy in the Ins2C96Y Akita mice

Ins2C96Y Akita mice represent a model of spontaneous early-onset diabetes mellitus, expressing a mutant non-functional isoform of insulin. These mice are characterized by a reduced number of pancreatic beta cells resulting in hypoinsulinemia and hyperglycemia. We obtained longitudinal measures of morning fasting blood glucose levels and gait performance. Sciatic nerve electrophysiology was also performed and the performance of these mice on spatial memory tasks was measured longitudinally. We observed a progressive increase in fasting blood glucose levels that was proportionally associated with increased gait disturbances. Diabetes induced a decrease in the sensory nerve conduction velocity up to the age of 40 weeks. Glucose transporter (GLUT) 3 levels were reduced in the hippocampus of the aged Ins2C96Y Akita mice. However, we failed to detect any significant deficits during reference, reversal or probe tests in the Morris water maze or in a spontaneous alternation task up to the age of 34 weeks old. We found that, up to the age of 34 weeks old, uncontrolled hyperglycemia produced peripheral neuropathy and reduced hippocampal GLUT3 levels in the absence of any effect on spatial memory processing.

Glucose transporter expression in the central nervous system: relationship to synaptic function

The family of facilitative glucose transporter (GLUT) proteins is responsible for the entry of glucose into cells throughout the periphery and the brain. The expression, regulation and activity of GLUTs play an essential role in neuronal homeostasis, since glucose represents the primary energy source for the brain. Brain GLUTs exhibit both cell type and region specific localizations suggesting that the transport of glucose across the blood-brain barrier is tightly regulated and compartmentalized. As seen in the periphery, insulin-sensitive GLUTs are expressed in the brain and therefore may participate in the central actions of insulin. The aim of this review will be to discuss the localization of GLUTs expressed in the central nervous system (CNS), with a special emphasis upon the recently identified GLUT isoforms. In addition, we will discuss the regulation, activity and insulin-stimulated trafficking of GLUTs in the CNS, especially in relation to the centrally mediated actions of insulin and glucose.

Moderate hypoglycemia aggravates effects of hypoxia in hippocampal slices from diabetic rats

We recorded the effects of hypoxia combined with relative hypoglycemia on pre- and post-synaptic potentials in the CA1 area of slices from 4-month-old control and diabetic (streptozotocin-treated) Wistar rats. In experiments on slices kept in 10 or 4 mM glucose (at 33 degrees C), hypoxia was applied until the pre-synaptic afferent volley disappeared--after 12-13 min in most slices, but much earlier (5+/-0.8 min) in diabetic slices kept in 4 mM glucose. When oxygenation was resumed, the afferent volley returned in all slices, for an overall mean recovery of 86.5% (+/-8.8%). Field post-synaptic potentials were fully blocked within 2-3 min of the onset of hypoxia. After the end of hypoxia, they failed to reappear in some slices: overall, their recovery varied between 62 and 68% in control slices, as well as in diabetic slices kept in 10 mM glucose; but recovery was very poor in diabetic slices kept in 4 mM glucose (only 15+/-0.94%). In the latter, hypoxic injury discharges occurred earlier (4.2+/-0.68 min vs. 6.5-8 min for other groups). We conclude that diabetes appears to make hippocampal slices more prone to irreversible loss of synaptic function and early block of axonal conduction when temporary hypoxia is combined with moderate hypoglycemia.

Effect of endogenous beta-hydroxybutyrate on glucose metabolism in the diabetic rabbit brain: a (13)C-magnetic resonance spectroscopy study of [U-(13)C]glucose metabolites

The neurological consequences of diabetes mellitus have recently been receiving greater attention in both clinical and experimental settings. The deleterious effect of hyperglycemia and altered oxidative substrate availability on the diabetic brain is the subject of many studies. The aim of the present study was to examine the effect of the altered metabolic environment, namely, hyperglycemia and hyperketonemia, on glucose metabolism in the diabetic brain. More specifically, we examined the effect of diabetes on the glucose flux via the pyruvate dehydrogenase (PDH) and pyruvate carboxylase (PC) pathways and subsequent metabolism in the tricarboxylic acid cycles in neurons and glia. To this end, [U-(13)C]glucose was infused into the circulation of alloxan-induced diabetic young adult rabbits, and the [(13)C]glucose metabolites were subsequently studied in brain extracts by (13)C-NMR. Significantly elevated brain glucose levels were found. In the hyperketonemic rabbits, elevated cerebral levels of beta-hydroxybutyrate (beta-HBA) were found. Alterations in the labeling patterns of glutamine in the hyperketonemic group lead to the conclusion that the elevated beta-HBA levels inhibit glucose metabolism, mostly in glia. This results in accumulation of glucose in the diabetic brain. In addition, altered levels of glutamine, glutamate, and GABA were also attributed to the effect of beta-HBA on brain metabolism. The possible role of these metabolic perturbations in causing neurological damage remains to be investigated.

Glucose transporters and glucose utilization in rat brain after acute ethanol administration

In the normal adult brain, glucose provides 90% of the energy requirements as well as substrate for nucleic acid and lipid synthesis. In the present study, effects of ethanol on glucose transporters (GLUT) and glucose utilization were examined in rat brain. Male Sprague-Dawley rats weighing 250-300 gms were given either ethanol 3 gm/kg BW or saline i.p. 4 hrs prior to the animal sacrifice and removal of the cerebral cortical tissue. The cortical plasma membranes analyzed by cytochalasin B binding assay showed a decrease in GLUT number but not in GLUT affinity in the ethanol treated rats as compared to the control rats. The estimated Ro values were 70 +/- 8.9 Vs 91 +/- 8.9 pmoles/mg protein (p < 0.05 N=4) and the estimated Kd values were 0.37 +/- 0.03 and 0.28 +/- 0.05 microM (p: NS) in ethanol and control experiments respectively. Immunoblots of purified cerebral plasma membranes and low density microsomal fraction showed 17% and 71% decrease for GLUTI and 54% and 21% (p<0.05 or less; n=6) for GLUT3 respectively in ethanol treated rats than in control animals. Immunofluoresence studies also showed reduction of GLUT1 immunoreactively in choroid plexus and cortical microvessels of ethanol treated rats as compared to control rats. The effect of ethanol on regional cerebral metabolic rates for glucose (CMR(Glc)) was studied using [6-(14)C] glucose and showed statistically insignificant decrease in brain glucose utilization. These data suggest that ethanol in-vivo decrease GLUT number and protein content in rat cerebral cortex.

Neurological changes induced by stress in streptozotocin diabetic rats

Previous studies from our laboratory demonstrated that chronic stress produces molecular, morphological, and ultrastructural changes in the rat hippocampus that are accompanied by cognitive deficits. Glucocorticoid impairment of glucose utilization is proposed as a causative factor involved in stress-induced changes. Current studies have examined the neurological changes induced by stress in rats with a preexisting strain upon their homeostatic load--namely, in streptozotocin (stz)-diabetic rats. Administration of stz (70 mg/kg, i.v.) produced diabetic symptoms such as weight loss, polyuria, polydipsia, hyperglycemia, and neuroendocrine dysfunction. Morphological analysis of hippocampal neurons revealed that diabetes alone produced dendritic atrophy of CA3 pyramidal neurons, an effect potentiated by 7 days of restraint stress. Analysis of genes critical to neuronal homeostasis revealed that glucose transporter 3 (GLUT3) mRNA and protein levels were specifically increased in the hippocampus of diabetic rats, while stress had no effect upon GLUT3 expression. Insulin-like growth factor (IGF) receptor expression was also increased in the hippocampus of diabetic rats subjected to stress. In spite of the activation of these adaptive mechanisms, diabetic rats subjected to stress also had signs of neuronal damage and oxidative damage. Collectively, these results suggest that the hippocampus of diabetic rats is extremely susceptible to additional stressful events, which in turn can lead to irreversible hippocampal damage.

Regulation of GLUT-3 glucose transporter in the hippocampus of diabetic rats subjected to stress

Previous studies from our laboratory have demonstrated that chronic stress produces molecular, morphological, and ultrastructural changes in the rat hippocampus that are accompanied by cognitive deficits. Glucocorticoid attenuation of glucose utilization is proposed to be one of the causative factors involved in stress-induced changes in the hippocampus, producing an energy-compromised environment that may make hippocampal neuronal populations more vulnerable to neurotoxic insults. Similarly, diabetes potentiates neuronal damage in acute neurotoxic events, such as ischemia and stroke. Accordingly, the current study examined the regulation of the neuron-specific glucose transporter, GLUT-3, in the hippocampus of streptozotocin-induced diabetic rats subjected to restraint stress. Diabetes leads to significant increases in GLUT-3 mRNA and protein expression in the hippocampus, increases that are not affected by stress. Collectively, these results suggest that streptozotocin-induced increases in GLUT-3 mRNA and protein expression in the hippocampus may represent a compensatory mechanism to increase glucose utilization during diabetes and also suggest that modulation of GLUT-3 expression is not responsible for glucocorticoid impairment of glucose utilization.

Glycemia-lowering effect of cobalt chloride in the diabetic rat: increased GLUT1 mRNA expression

We have recently shown that expression of the GLUT1 glucose transporter isoform is augmented in cells exposed to cobalt chloride [Co(II)], an agent that stimulates the expression of hypoxia-responsive genes (Behrooz, A., Ismail-Beigi, F., 1997. J. Biol. Chem. 272, 5555-5562.). Here, we examine the effect of Co(II) on glycemia and tissue GLUT1 mRNA content of normal and diabetic rats. The addition of 2 mM Co(II) in the drinking water reduced the glycemia of streptozotocin-induced diabetic rats by day 3 from 32.3 +/- 2.1 to 21.0 +/- 1.9 mM (non-fasting). Co(II) resulted in no change in serum insulin levels of normal or diabetic rats. Treatment with 4 mM Co(II) was more effective than 2 mM Co(II) in reducing the glycemia of diabetic rats, while 6 mM Co(II) was associated with severe toxicity. GLUT1 mRNA content increased significantly in ventricular myocardium, renal cortex, skeletal muscle, cerebrum and liver of normal and diabetic rats treated with 2 mM cobalt chloride (ranging from 1.3- to 2.9-fold in the different tissues). It is concluded that: (1) treatment with Co(II) decreases the glycemia of diabetic rats, and (2) the glycemia-lowering effect of Co(II) is associated with, and may be mediated by, enhanced expression of GLUT1 mRNA.

Glucose metabolism and left ventricular dysfunction are normalized by insulin and islet transplantation in mild diabetes in the rat

The aim of the present experimental study in the rat heart was to assess cardiac performance and metabolism in mild diabetes of 2 months' duration (postprandial blood sugar levels of 307 +/- 101 mg/dl and nearly normal fasting blood glucose of 102 +/- 40 mg/dl) using the working rat heart model at physiological workload with a perfusion time of 60 min. We also compared the effect of two forms of therapy for diabetes, islet transplantation and insulin therapy (s.c.), after 2 months. A 36% reduction in glucose utilization is metabolically characteristic for the diabetic heart, mainly caused by a 55% reduced glucose uptake (P < 0.001), but also by a nearly twofold increased lactate and pyruvate production (P < 0.001). This reduced carbohydrate metabolism is accompanied by a 37% reduction of oxygen uptake (P < 0.001) as well as a significant reduction in myocardial ATP and CP levels (P < 0.001), resulting in a significantly reduced cardiac output (P < 0.001). Moreover, the balance of energy reveals that the diabetic heart obtains 46% of its energy requirements for 1 h from endogenous glycogen, whereas the control heart obtains 91% of its energy needs (i.e. preferentially) from exogenous glucose (only 9% from endogenous glycogen). Both investigated therapeutic interventions led to a complete reversibility of the hemodynamic and metabolic alterations, indicating that the cause of diabetic cardiomyopathy in this model of mild and short-term diabetes is due to a defect in cardiac carbohydrate metabolism, which is correctable by insulin administration.

The influence of thioctic acid on metabolism and function of the diabetic heart

Streptozotocin-diabetes as a model for insulin-deficient Type 1 diabetes leads to cardiomyopathy, characterized by a 50% reduced glucose uptake (P < 0.001) and increased lactate and pyruvate levels (P < 0.001), i.e. a reduced glucose utilization by the heart. As thioctic acid (TA) has favourable effects on glucose metabolism, the influence of this drug at two different doses (0.1 mg/ml and 0.5 mg/ml, added to the perfusion medium) was investigated in the heart after 2 weeks of diabetes, using the working rat heart model at physiological workload about 45 min. TA at high doses led to a normalization of glucose uptake (P < 0.001) and glucose utilization, and consequently to a normalization of oxygen uptake (P < 0.001), myocardial ATP levels (P < 0.001) as well as cardiac output (P < 0.05). Whereas a low dose of TA resulted in a normalization of lactate and pyruvate production (P < 0.001), neither a normalization of glucose utilization nor of cardiac output was achieved by this low dosage. Additionally, TA improved at both doses utilization of endogenous glycogen in the diabetic heart (P < 0.001), the latter here already delivering 45% of the utilized glucose. TA acts especially by increasing glucose uptake, glycogen breakdown and glucose oxidation. Thus, metabolic and hemodynamic sequelae of insulin-deficiency in the heart can be corrected by TA. Due to its anti-diabetic effects on cardiac metabolism, TA could be considered an adjuvant therapy in diabetic cardiomyopathy.