Myocardial infarction and heart failure in the db/db diabetic mouse

Clinical studies have reported that the incidence and severity of myocardial infarction is significantly greater in diabetics compared with nondiabetics after correction for all other risk factors. The majority of studies investigating the pathophysiology of myocardial ischemia-reperfusion injury have focused on otherwise healthy animals. At present, there is a paucity of experimental investigations on the pathophysiology of heart failure in diabetic animals. We hypothesized that the severity of myocardial reperfusion injury and the development of congestive heart failure would be markedly enhanced in the db/db diabetic mouse. Accordingly, we studied the effects of varying durations of in vivo myocardial ischemia and reperfusion on the incidence of heart failure in db/db diabetic mice. Nondiabetic and db/db diabetic mice (10 wk of age) were subjected to 30, 45, or 60 min of left coronary artery occlusion and 28 days of reperfusion. Survival at 24 h of reperfusion was 100% in nondiabetic mice subjected to 30 min of myocardial ischemia and 88% in nondiabetic mice subjected to 45 min of myocardial ischemia. In contrast, survival was 53% in db/db diabetic mice subjected to 30 min of myocardial ischemia and 44% in db/db mice after 45 min of myocardial ischemia. Prolonged survival in nondiabetic mice was not significantly attenuated when compared during the 28-day follow-up period with all groups experiencing >90% survival. Prolonged survival was significantly decreased in db/db mice after both 30 and 45 min of myocardial ischemia compared with sham controls. Furthermore, we observed a significant degree or left ventricular dilatation, cardiac hypertrophy, and cardiac contractile dysfunction in db/db mice subjected to 45 min of myocardial ischemia and 28 days reperfusion. In nondiabetic mice subjected to 45 min of myocardial ischemia, we failed to observe any changes in left ventricular dimensions or fractional shortening. These studies provide a feasible experimental model system for the investigation of heart failure secondary to acute myocardial infarction in the db/db diabetic mouse.     

Hepatic glycogen metabolism in the db/db mouse

Summary Knowledge of the metabolic changes that occur in insulin-resistant type 2 diabetes is relatively lacking compared to insulin-deficient type 1 diabetes. This paper summarizes the importance of the C57BL/KsJ-db/db mouse as a model of type 2 diabetes, and illustrates the effects that insulin-deficient and insulin-resistant states have on hepatic glycogen metabolism. A longitudinal study of db/db mice of ages 2–15 weeks revealed that significant changes in certain parameters of hepatic glycogen metabolism occur during this period. The liver glycogen levels were similar between diabetic and control mice. However, glycogen particles from db/db mice were on average smaller in mass and had shorter exterior and interior chain lengths. Total phosphorylase and phosphorylase a activities were elevated in the genetically diabetic mice. This was primarily due to an increase in the amount of enzymic protein apparently the result of a decreased rate of degradation. It was not possible to find a consistent alteration in glycogen synthase activity in the db/db mice. Glycogen synthase and phosphorylase from diabetic liver revealed some changes in kinetic properties in the form of a decrease in V_max, and altered sensitivity to inhibitors like ATP. The altered glycogen structure in db/db mice may have contributed to changes in the activities and properties of glycogen synthase and phosphorylase. The exact role played by hormones (insulin and glucagon) in these changes is not clear but further studies should reveal their contributions. The db/db mouse provides a good model for type 2 diabetes and for fluctuating insulin and glucagon ratios. Its use should clarify the regulation of hepatic glycogen metabolism and other metabolic processes known to be controlled by these hormones. The other animal models of type 2 diabetes, ob/ob mouse and fatty Zucker (fa/fa) rat, show similar impairment of hepatic glycogen metabolism. The concentrations of glycogen metabolizing enzymes are high and in vitro studies indicate enhanced rate of glycogen synthesis and breakdown. However, streptozotocin-induced diabetic animals and BB rats which resemble insulin-deficient type 1 diabetes are characterized by decreased glycogen turnover as a result of reduction in the levels of glycogen metabolizing enzymes.     

4-Thiaproline reduces heart lipid peroxidation and collagen accumulation in the diabetic db/db mouse

Summary Collagen accumulation is a main pathological feature of diabetic cardiomyopathy. The underlying mechanisms seem to be increased cross linking by reactive carbonyles. The purpose of the study was to decrease the collagen content of total ventricular tissue by the oral administration of thiaproline, which could reduce collagen due to its functions as a proline analogue, blocking collagen production and as a free oxygen radical scavenger, blocking reactive carbonyles and oxygen species and subsequently collagen cross linking.Thiaproline was administered to genetically diabetic db/db mice and compared to untreated animals. Total ventricular collagen as expressed by hydroxyproline was significantly lower in the treated group (means 0.23 micromoles/10 tissue in the treated vs 0.35 micromoles/100 mg tissue in the untreated group, p < 0.001). Significantly more collagen could be eluted in the treated group (p < 0.001) and carboxymethyllysine was significantly reduced in the treated group (p < 0.001). Di-tyrosine and glycemic control did not differ between the groups. Glutathione was significantly increased in the TP treated experimental group (p < 0.001) and lipid peroxidation products were significantly decreased (means 0.221 absorbance in the treated group versus 0.321 absorbance in the untreated diabetic group) correlating with total ventricular collagen content (r = 0.87, p < 0.01).We conclude that thiaproline reduced total ventricular collagen content by inhibiting collagen cross linking as reflected by increased solubility of collagen and expressed by higher elution quantity of collagen. Thiaproline, and/or its metabolites induced increase of heart glutathione which may well have been scavenging reactive carbonyles derived from lipid peroxidation and advanced stage nonenzymatic glycosylation as shown by decreased total ventricular carboxy-methyllysine and lipid peroxidation products paralleling reduced heart collagen content.It remains to be shown that the successful reduction of heart collagen by thiaproline is paralleled by improved functional properties.     

Thymic dysfunction in the mutant diabetic (db/db) mouse

Thymic function has been explored in genetically diabetic homozygous C57BL/KsJ (db/db) mice by evaluating their serum thymic factor (FTS) levels with a rosette assay. As previously reported for other autoimmune mice (NZB or MRL/I mice), the age-dependent decline of FTS levels was significantly accelerated in diabetic mice when compared to heterozygous littermates. Furthermore, FTS inhibitory molecules were detected in db/db mouse sera (as early as 10 wk of age) as evaluated by their ability to absorb in vitro the activity of synthetic FTS in the rosette assay, and in vivo for their capacity to induce the disappearance of endogenous FTS when injected into normal mice. These inhibitors were shown to be immunoglobulins. Histologically, the thymus presented an accelerated involution starting with a cortical lymphocytic depletion and an increased number of Hassall's corpuscles. Ultrastructural studies showed alterations in thymic epithelial cells, mainly represented by an increasing number of cytoplasmic vacuoles. By means of indirect immunofluorescence with anti-FTS monoclonal antibodies, it was shown that the number of FTS+ cells was reduced in db/db mouse thymuses: at the age of 22 wk, diabetic mice had 10 times fewer FTS+ cells than heterozygotes of the same age. Taken together, these results indicate important abnormalities in the thymus of diabetic mice. It is possible that the associated lymphocyte dysfunction plays a role in the pathogenesis of the autoimmune disease presented by db/db mice.     

Recent insights into diabetic renal injury from the db/db mouse model of type 2 diabetic nephropathy

The db/db mouse is the most widely used animal model of type 2 diabetic nephropathy. Recent studies have utilized genetic backcrossing with transgenic mouse strains to create novel db/db strains that either lack or overexpress specific genes. These novel strains [ICAM-1-/-, CCL2-/-, MKK3-/-, osteopontin-/-, plasminogen activator inhibitor-1 (PAI-1)-/-, endothelial nitric oxide synthase-/-, SOD-Tg, rCAT-Tg] have provided valuable insights into the molecular mechanisms which promote diabetic renal injury. In addition, surgical removal of one kidney has been shown to accelerate injury in the remaining kidney of diabetic db/db mice. A number of novel therapeutic agents have also been tested in db/db mice, including inhibitors of inflammation (chemokine receptor antagonists, anti-CCL2 RNA aptamer, anti-c-fms antibody); oxidative stress (oxykine, biliverdin); the renin-angiotensin-aldosterone system (aliskiren, angiotensin-converting enzyme inhibitors, angiotensin receptor blockers, eplerenone); advanced glycation end products (AGE; pyridoxamine, alagebrium, soluble AGE receptor); angiogenesis (NM-3, anti-CXCL12 RNA aptamer, soluble Flt-1); lipid accumulation (statins, farnesoid X receptor agonists, Omacor); intracellular signaling pathways (PKC-β or JNK inhibitors); and fibrosis [transforming growth factor (TGF)-β antibody, TGF-βR kinase inhibitor, soluble betaglycan, SMP-534, CTGF-antisense oligonucleotide, mutant PAI-1, pirfenidone], which have identified potential therapeutic targets for clinical translation. This review summarizes the advances in knowledge gained from studies in genetically modified db/db mice and treatment of db/db mice with novel therapeutic agents.     

Carnosine enhances diabetic wound healing in the db/db mouse model of type 2 diabetes

Diabetes mellitus (DM) is a progressive disorder with severe late complications. Normal wound healing involves a series of complex and well-orchestrated molecular events dictated by multiple factors. In diabetes, wound healing is grossly impaired due to defective, and dysregulated cellular and molecular events at all phases of wound healing resulting in chronic wounds that fail to heal. Carnosine, a dipeptide of alanine and histidine and an endogenous antioxidant is documented to accelerate healing of wounds and ulcers. However, not much is known about its role in wound healing in diabetes. Therefore, we studied the effect of carnosine in wound healing in db/db mice, a mice model of Type 2 DM. Six millimeter circular wounds were made in db/db mice and analyzed for wound healing every other day. Carnosine (100?mg/kg) was injected (I.P.) every day and also applied locally. Treatment with carnosine enhanced wound healing significantly, and wound tissue analysis showed increased expression of growth factors and cytokines genes involved in wound healing. In vitro studies with human dermal fibroblasts and microvascular-endothelial cells showed that carnosine increases cell viability in presence of high glucose. These effects, in addition to its known role as an antioxidant and a precursor for histamine synthesis, provide evidence for a possible therapeutic use of carnosine in diabetic wound healing.     

Astaxanthin protects beta-cells against glucose toxicity in diabetic db/db mice

Oxidative stress induced by hyperglycemia possibly causes the dysfunction of pancreatic beta-cells and various forms of tissue damage in patients with diabetes mellitus. Astaxanthin, a carotenoid of marine microalgae, is reported as a strong anti-oxidant inhibiting lipid peroxidation and scavenging reactive oxygen species. The aim of the present study was to examine whether astaxanthin can elicit beneficial effects on the progressive destruction of pancreatic beta-cells in db/db mice--a well-known obese model of type 2 diabetes. We used diabetic C57BL/KsJ-db/db mice and db/m for the control. Astaxanthin treatment was started at 6 weeks of age and its effects were evaluated at 10, 14, and 18 weeks of age by non-fasting blood glucose levels, intraperitoneal glucose tolerance test including insulin secretion, and beta-cell histology. The non-fasting blood glucose level in db/db mice was significantly higher than that of db/m mice, and the higher level of blood glucose in db/db mice was significantly decreased after treatment with astaxanthin. The ability of islet cells to secrete insulin, as determined by the intraperitoneal glucose tolerance test, was preserved in the astaxanthin-treated group. Histology of the pancreas revealed no significant differences in the beta-cell mass between astaxanthin-treated and -untreated db/db mice. In conclusion, these results indicate that astaxanthin can exert beneficial effects in diabetes, with preservation of beta-cell function. This finding suggests that anti-oxidants may be potentially useful for reducing glucose toxicity.     

Urinary chiro-inositol and myo-inositol excretion is elevated in the diabetic db/db mouse and streptozotocin diabetic rat

Inositol phosphoglycan molecules containing either D-chiro-inositol or myo-inositol have been isolated from various mammalian tissues and are putative mediators of insulin action. Urinary excretion of inositols appears to be altered in diabetes mellitus; however, the relationships with different types of diabetes are unclear. The objective of this study was to determine the urinary excretion of chiro- and myo-inositol in diabetic animal models, including streptozotocin (STZ) rats, db/db mice, and fa/fa Zucker rats. In STZ rats (type 1 diabetes), 12-hr urinary excretion of chiro-inositol was elevated 336-fold and myo-inositol excretion was elevated 47-fold compared with their nondiabetic counterparts. When corrected for creatinine, chiro-inositol excretion was 259-fold higher and myo-inositol excretion was 36-fold higher in STZ rats than in normal rats. The same pattern was observed in db/db mice (type 2 diabetes), where 12-hr urinary chiro-inositol excretion was elevated 247-fold compared with normal mice. When corrected for creatinine, chiro-inositol excretion was 2455-fold higher and urinary myo-inositol excretion was elevated 8.5-fold in db/db mice compared with normal mice. The fa/fa Zucker rats (impaired glucose tolerance) had a pattern of urinary inositol excretion that was similar to the nondiabetic animals (lean Zucker rats, C57BL/6 mice, and Sprague-Dawley rats). In summary, urinary chiro-inositol and myo-inositol excretion was elevated in animal models of type 1 and type 2 diabetes mellitus, concomitant with hyperglycemia and glucosuria.     

Treatment of type 2 diabetic db/db mice with a novel PPARgamma agonist improves cardiac metabolism but not contractile function

Hearts from insulin-resistant type 2 diabetic db/db mice exhibit features of a diabetic cardiomyopathy with altered metabolism of exogenous substrates and reduced contractile performance. Therefore, the effect of chronic oral administration of 2-(2-(4-phenoxy-2-propylphenoxy)ethyl)indole-5-acetic acid (COOH), a novel ligand for peroxisome proliferator-activated receptor-gamma that produces insulin sensitization, to db/db mice (30 mg/kg for 6 wk) on cardiac function was assessed. COOH treatment reduced blood glucose from 27 mM in untreated db/db mice to a normal level of 10 mM. Insulin-stimulated glucose uptake was enhanced in cardiomyocytes from COOH-treated db/db hearts. Working perfused hearts from COOH-treated db/db mice demonstrated metabolic changes with enhanced glucose oxidation and decreased palmitate oxidation. However, COOH treatment did not improve contractile performance assessed with ex vivo perfused hearts and in vivo by echocardiography. The reduced outward K+ currents in diabetic cardiomyocytes were still attenuated after COOH. Metabolic changes in COOH-treated db/db hearts are most likely indirect, secondary to changes in supply of exogenous substrates in vivo and insulin sensitization.     

Echocardiographic assessment of cardiac function in diabetic db/db and transgenic db/db-hGLUT4 mice

Control db/+ and diabetic db/db mice at 6 and 12 wk of age were subjected to echocardiography to determine whether contractile function was reduced in vivo and restored in transgenic db/db-human glucose transporter 4 (hGLUT4) mice (12 wk old) in which cardiac metabolism has been normalized. Systolic function was unchanged in 6-wk-old db/db mice, but fractional shortening and velocity of circumferential fiber shortening were reduced in 12-wk-old db/db mice (43.8 +/- 2.1% and 8.3 +/- 0.5 circs/s, respectively) relative to db/+ control mice (59.5 +/- 2.3% and 11.8 +/- 0.4 circs/s, respectively). Doppler flow measurements were unchanged in 6-wk-old db/db mice. The ratio of E and A transmitral flows was reduced from 3.56 +/- 0.29 in db/+ mice to 2.40 +/- 0.20 in 12-wk-old db/db mice, indicating diastolic dysfunction. Thus a diabetic cardiomyopathy with systolic and diastolic dysfunction was evident in 12-wk-old diabetic db/db mice. Cardiac function was normalized in transgenic db/db-hGLUT4 mice, indicating that altered cardiac metabolism can produce contractile dysfunction in diabetic db/db hearts.     

Overexpression of angiopoietin-2 impairs myocardial angiogenesis and exacerbates cardiac fibrosis in the diabetic db/db mouse model

The angiopoietins/Tie-2 system is essential for the maintenance of vascular integrity and angiogenesis. The functional role of angiopoietin-2 (Ang-2) in the regulation of angiogenesis is dependent on other growth factors such as VEGF and a given physiopathological conditions. This study investigates the potential role of Ang-2 in myocardial angiogenesis and fibrosis formation in the diabetic db/db mouse. Diabetic db/db mice received intramyocardial administration of either adenovirus Ang-2 (Ad-CMV-Ang-2) or Ad-β-gal. The levels of Tie-2, VEGF, caspase-3, Wnt7b, fibroblast-specific protein-1 (FSP-1), and adhesion molecules (ICAM-1 and VCAM-1) expression were measured. Apoptosis, capillary density, and cardiac fibrosis were also analyzed in the db/db mouse hearts. Overexpression of Ang-2 suppressed Tie-2 and VEGF expression in db/db mouse hearts together with significant upregulation of Wnt7b expression. Overexpression of Ang-2 also sensitizes ICAM-1 and VCAM-1 expression in db/db mouse hearts. Immunohistochemical analysis revealed that overexpression of Ang-2 resulted in a gradual apoptosis as well as interstitial fibrosis formation, these leading to a significant loss of capillary density. Data from these studies were confirmed in cultured mouse heart microvascular endothelial cells (MHMEC) exposed to excessive Ang-2. Exposure of MHMEC to Ang-2 resulted in increased caspase-3 activity and endothelial apoptosis. Knockdown of Ang-2 attenuated high glucose-induced endothelial cell apoptosis. Further, counterbalance of Ang-2 by overexpression of Ang-1 reversed loss of capillary density and fibrosis formation in db/db mouse hearts. Our data demonstrate that Ang-2 increases endothelial apoptosis, sensitizes myocardial microvascular inflammation, and promotes cardiac fibrosis and thus contributes to loss of capillary density in diabetic diseases.     

Association between endothelin-1 and collagen deposition in db/db diabetic mouse kidneys

Endothelin-1 has been implicated in diabetic kidney injury, but there are few firm data establishing the temporal and spatial expression of kidney endothelin-1 in diabetes. We performed an immunohistochemical and histopathological analysis to determine endothelin-1 peptide expression in the kidneys of diabetic db/db mice and non-diabetic db/m controls. Diabetic mice were studied at 8 weeks, before histological damage is evident, and again at 16 weeks, when significant glomerular injury has occurred. Urinary endothelin-1 was 6.2- and 3.6-fold higher in 8- and 16-week diabetic mice compared to age-matched controls (P<0.01 db/db vs. db/m). Compared to non-diabetic kidneys, immunoreactive endothelin-1 was first elevated 2.5-fold (P=0.02) in the tubulointerstitial compartment at 8-week and remained high (3.8-fold, P<0.01) at 16 weeks. In contrast, glomerular endothelin-1 was elevated 3.2-fold (P=0.03) only in 16-week diabetic mice. Glomerular and tubulointerstitial endothelin-1 were unchanged in 8- and 16-week non-diabetic mice. Elevated endothelin-1 in diabetic mice associated temporally and spatially with collagen deposition, especially in the tubulointerstitial compartment. The localization of kidney endothelin-1 is consistent with a role for this peptide in renal fibrogenesis. These results also highlight the potential role of ET-1 in the pathogenesis of early tubulointerstitial changes in diabetes.     

The LXR ligand T0901317 induces severe lipogenesis in the db/db diabetic mouse

Liver X receptor (LXR) ligands are currently being evaluated as potential therapeutic agents for the treatment of low HDL. The LXR ligand T0901317 elevates ATP binding cassette transporter A1 (ABCA1) and HDL levels in animal models and induces moderate lipogenesis through upregulation of sterol regulatory element binding protein 1c (SREBP1c). Because insulin may also regulate lipogenesis through SREBP1c and fatty acid synthase (FAS), we investigated the effect of an LXR ligand in hyperinsulinemic mice. Administration of T0901317 to male db/db mice for 12 days resulted in a more severe hypertriacylglycerolemia and hepatic triacylglycerol accumulation than observed in nondiabetic mice. The LXR target genes ABCA1, SREBP1c, FAS, and stearoyl-CoA desaturase 1 were upregulated by T0901317 treatment in both diabetic db/db and nondiabetic C57BLKS mice. Changes in lipogenic gene expression were independent of mouse strain, indicating that the severe lipogenesis observed in LXR ligand-treated db/db mice was not due to additive effects of insulin on lipogenic gene expression. Phosphoenolpyruvate carboxykinase expression was suppressed, suggesting that a shift from gluconeogenesis toward lipogenesis could partially explain our observations in db/db mice. Our data suggest that LXR ligands that have effects on both fatty acid and carbohydrate metabolism should be carefully evaluated in obesity, insulin, and leptin resistance.     

Hepatic adrenergic receptors in the genetically diabetic C57 BL/KsJ (db/db) mouse

1. At 30 weeks of age, homozygote diabetic C57 BL KsJ (db/db) mice were grossly obese, lethargic and displayed moderate hair loss relative to heterozygote control C 57 BL KsJ (db/+) mice. 2. In diabetic mice, compared to control, the total body weights, liver weight: body weight ratios, and blood glucose levels were increased 2.3 fold, 20% and 3.1 fold, respectively. 3. Analysis of plasma membranes isolated from control and diabetic mouse liver established that comparable purity levels were achieved since relative specific activities of the plasma membrane markers 5'-nucleotidase and gamma-glutamyltranspeptidase were similar: 10.2 and 11.4 fold with respect to 5'-nucleotidase in control and diabetic states respectively; and 8.0 and 8.3 fold with respect to gamma-glutamyltranspeptidase in control and diabetic states respectively. 4. A select effect of diabetes on gamma-glutamyltranspepetidase, however, was observed. The activity of this enzyme was found to be reduced 16% in diabetic liver compared to control liver. 5. Assessment of [3H]prazosin and [3H]dihydrolalprenolol binding to mouse liver plasma membranes indicated that although there was no difference in beta-adrenergic receptor binding in control and diabetic states, alpha 1-adrenergic receptor binding was found to be reduced 43% in diabetic mouse liver plasma membranes. 6. Scatchard analyses of kinetic studies indicate that the reduction is a reflection of decreases in alpha 1-adrenergic receptor numbers with no change in alpha 1 receptor affinity in the diabetic state: since for diabetic and control liver plasma membranes, Kd values were 3.41 +/- 0.02 nM and 3.40 +/- 0.01 nM respectively; and Bmax were 650.12 +/- 16.44 fmol mg-1 and 380.76 +/- 12.92 fmol mg-1, respectively.     

Prevention of the metabolic effects of 2-tetradecylglycidate by octanoic acid in the genetically diabetic mouse (db/db)

2-Tetradecylglycidate is a specific inhibitor of the enzyme carnitine palmitoyl transferase, the rate-limiting step in long chain fatty acid oxidation. We previously showed that chronic administration of TDGA to genetically diabetic mice caused a dose-dependent decrease in blood glucose, retarded the development of renal immunopathologic lesions, and resulted in significant cardiomegaly. The present study was designed to evaluate whether all the observed consequences of chronic TDGA administration resulted from inhibition of long chain fatty acid oxidation or whether the drug exerted other nonspecific effects. To circumvent the effects of LCFAO inhibition, diabetic mice were dosed with TDGA and given a diet containing 9% octanoic acid. Octanoic acid is a medium chain fatty acid, whose oxidation is not dependent on the carnitine transferase system and is not inhibited by TDGA. Administration of the octanoate diet to diabetics receiving TDGA abrogated all the drug effects, including lowering of blood glucose and prevention of renal immunopathology. Cardiomegaly, a consequence of increased protein accretion associated with TDGA dosing, did not occur in the octanoate-fed animals. These results indicate that all the actions of TDGA are mediated via its inhibitory effects on long chain fatty acid oxidation. The cardiac changes resulting from chronic TDGA administration suggest that long chain fatty acid oxidation and its relationship with myocardial energetics may exert a regulatory role on protein synthesis in the myocardium.     

Metabolic effects of insulin on cardiomyocytes from control and diabetic db/db mouse hearts

Diabetic db/db mice exhibit profound insulin resistance in vivo, but the specific degree of cardiac insensitivity to insulin has not been assessed. Therefore, the effect of insulin on cardiomyocytes from db/db hearts was assessed by measuring two metabolic responses (deoxyglucose uptake and fatty acid oxidation) and the phosphorylation of two enzymes in the insulin-signaling cascade [Akt and AMP-activated protein kinase (AMPK)]. Maximal insulin-stimulated deoxyglucose transport was reduced to 58 and 40% of control in cardiomyocytes from db/db mice at two ages (6 and 12 wk). Insulin-stimulated deoxyglucose uptake was also reduced in myocytes from transgenic db/db mice overexpressing the insulin-sensitive glucose transporter (db/db-hGLUT4). Treatment of db/db mice for 1 wk with an insulin-sensitizing peroxisome proliferator-activated receptor-gamma agonist (COOH) completely normalized insulin-stimulated deoxyglucose uptake. Insulin had no direct effect on palmitate oxidation by either control or db/db cardiomyocytes, but the combination of insulin and glucose reduced palmitate oxidation, likely an indirect effect secondary to increased glucose uptake. Insulin had no effect on AMPK phosphorylation from either control or db/db cardiomyocytes. Insulin increased the phosphorylation of Akt in all cardiomyocyte preparations (control, db/db, COOH-treated db/db) to the same extent. Thus insulin has selective metabolic actions in mouse cardiomyocytes; deoxyglucose uptake and Akt phosphorylation are increased, but fatty acid oxidation and AMPK phosphorylation are unchanged. Insulin resistance in db/db cardiomyocytes is manifested by reduced insulin-stimulated deoxyglucose uptake.     

Tolbutamide alters glucose transport and metabolism in the embryonic mouse heart

BACKGROUND: Tolbutamide is a sulfonylurea oral hypoglycemic agent widely used for the treatment of non insulin-dependent diabetes mellitus. Tolbutamide produces dysmorphogenesis in rodent embryos and becomes concentrated in the embryonic heart after maternal oral dosing. Tolbutamide increases glucose metabolism in extra-pancreatic adult tissues, but this has not previously been examined in embryonic heart. METHODS: CD-1 mouse embryos were exposed on GD 9.5 to tolbutamide (0, 100, 250, or 500 microg/ml) for 6, 12, or 24 hr in whole-embryo culture. Isolated hearts were evaluated for (3)H-2DG uptake and conversion of (14)C-glucose to (14)C-lactate. Glut-1, HKI, and GRP78 protein levels were determined by Western analysis, and Glut-1 mRNA was measured by RT-PCR. RESULTS: Cardiac (3)H-2DG uptake increased after exposure to 500 microg/ml tolbutamide for 6 hr, and 100, 250, or 500 microg/ml tolbutamide for 24 hr, compared to controls. Glycolysis increased after exposure to 500 microg/ml tolbutamide for 6 or 24 hr compared to controls. Glut-1 protein levels increased in hearts exposed to 500 microg/ml tolbutamide for 12 or 24 hr, and Glut-1 mRNA increased in hearts exposed to 500 microg/ml tolbutamide for 24 hr compared to controls. HKI protein levels increased in hearts exposed to 500 microg/ml tolbutamide for 6 hr, but not 12 or 24 hr. There was no effect on GRP78 protein levels in hearts exposed to tolbutamide for 6, 12, or 24 hr. CONCLUSIONS: Tolbutamide stimulates glucose uptake and metabolism in the embryonic heart, as occurs in adult extra-pancreatic tissues. Glut-1 and HKI, but not GRP78, are likely involved in tolbutamide-induced cardiac dysmorphogenesis.     

Altered metabolism causes cardiac dysfunction in perfused hearts from diabetic (db/db) mice

Contractile function and substrate metabolism were characterized in perfused hearts from genetically diabetic C57BL/KsJ-lepr(db)/lepr(db) (db/db) mice and their non-diabetic lean littermates. Contractility was assessed in working hearts by measuring left ventricular pressures and cardiac power. Rates of glycolysis, glucose oxidation, and fatty acid oxidation were measured using radiolabeled substrates ([5-(3)H]glucose, [U-(14)C]glucose, and [9,10-(3)H]palmitate) in the perfusate. Contractile dysfunction in db/db hearts was evident, with increased left ventricular end diastolic pressure and decreased left ventricular developed pressure, cardiac output, and cardiac power. The rate of glycolysis from exogenous glucose in diabetic hearts was 48% of control, whereas glucose oxidation was depressed to only 16% of control. In contrast, palmitate oxidation was increased twofold in db/db hearts. The hypothesis that altered metabolism plays a causative role in diabetes-induced contractile dysfunction was tested using perfused hearts from transgenic db/db mice that overexpress GLUT-4 glucose transporters. Both glucose metabolism and palmitate metabolism were normalized in hearts from db/db-human insulin-regulatable glucose transporter (hGLUT-4) hearts, as was contractile function. These findings strongly support a causative role of impaired metabolism in the cardiomyopathy observed in db/db diabetic hearts.