Sympathetic nervous dysregulation in the absence of systolic left ventricular dysfunction in a rat model of insulin resistance with hyperglycemia.

BACKGROUND: Diabetes mellitus is strongly associated with cardiovascular dysfunction, derived in part from impairment of sympathetic nervous system signaling. Glucose, insulin, and non-esterified fatty acids are potent stimulants of sympathetic activity and norepinephrine (NE) release. We hypothesized that sustained hyperglycemia in the high fat diet-fed streptozotocin (STZ) rat model of sustained hyperglycemia with insulin resistance would exhibit progressive sympathetic nervous dysfunction in parallel with deteriorating myocardial systolic and/or diastolic function. METHODS: Cardiac sympathetic nervous integrity was investigated in vivo via biodistribution of the positron emission tomography radiotracer and NE analogue [11C]meta-hydroxyephedrine ([11C]HED). Cardiac systolic and diastolic function was evaluated by echocardiography. Plasma and cardiac NE levels and NE reuptake transporter (NET) expression were evaluated as correlative measurements. RESULTS: The animal model displays insulin resistance, sustained hyperglycemia, and progressive hypoinsulinemia. After 8 weeks of persistent hyperglycemia, there was a significant 13-25% reduction in [11C]HED retention in myocardium of STZ-treated hyperglycemic but not euglycemic rats as compared to controls. There was a parallel 17% reduction in immunoblot density for NE reuptake transporter, a 1.2 fold and 2.5 fold elevation of cardiac and plasma NE respectively, and no change in sympathetic nerve density. No change in ejection fraction or fractional area change was detected by echocardiography. Reduced heart rate, prolonged mitral valve deceleration time, and elevated transmitral early to atrial flow velocity ratio measured by pulse-wave Doppler in hyperglycemic rats suggest diastolic impairment of the left ventricle. CONCLUSIONS: Taken together, these data suggest that sustained hyperglycemia is associated with elevated myocardial NE content and dysregulation of sympathetic nervous system signaling in the absence of systolic impairment.

Tracer-determined glucose fluxes in health and type 2 diabetes: basal conditions.

The role of increases in basal glucose production (EGP) in the pathogenesis of hyperglycaemia in type 2 diabetes (DM2) has been controversial. It is proposed here that the differences arose from: (i) different patient populations at different stages in the evolution of the disease, (ii) a non-steady state due to diurnal variations in EGP, and measurements at different times of day, and (iii) differences in experimental techniques: tracers, priming strategies and methods of calculation. Methodologically we show that (i) non-steady-state methods and (ii) a one-compartment model with volume of distribution estimated from tracer data are necessary in DM2. Studies with sufficient data demonstrated diurnal variations in EGP, with the highest rates in the morning, normalizing by late afternoon. Metabolic clearance rate of glucose (MCR) remained constant. Long-standing DM2 demonstrated increases in glycaemia and relative decreases in morning EGP, probably feedback-induced. A falling MCR, partly secondary to glucotoxicity, likely induced the rise in baseline hyperglycaemia.

Metformin and its liver targets in the treatment of type 2 diabetes.

Although a number of assessments disagree, the preponderance of the evidence indicates that the major therapeutic action of metformin in type 2 diabetes (DM2) is on the liver, and glucose production (EGP) in particular. At the level of this organ, the actions of metformin can be characterized as pleiotropic. The major questions addressed here are therefore: (i) the methodological aspects of the determination of glucose fluxes: when glucose production is not found to be elevated in type 2 diabetes, it is not surprising that little action of metformin on this flux is found. The issues of populations examined, experimental protocols, and quantitative methods of flux determination are important in answering this question. Early morning EGP is increased and constitutes a valid target for metformin. (ii) the multiple targets of metformin: metformin acts at a number of sites and interacts with metabolites and hormones. Some of these actions may be expressed at different doses. Although their net effect is therapeutic, not all are oriented towards lowering hyperglycemia, perhaps explaining the more modest effect of this drug than could be anticipated from individual actions. Sites of metformin action can therefore be considered as a compilation of valid therapeutic targets in DM2. Gluconeogenesis, glycogenolysis and glycogen synthesis can be altered by metformin, although in vivo, this also depends on the methodology. Component processes from substrate supply and liver uptake, through a number of glucogenic enzymes, as well as glycogen synthase and phosphorylase have all been shown to be affected. (iii) unifying concepts: reported actions of metformin on the mitochondrial respiratory chain, free fatty acid metabolism, AMP-activated protein kinase, and on membrane proteins directly may all explain subsets of actions that are seen, providing more integrated targets for consideration in the therapy of DM2.

Endogenous glucose production in type 2 diabetes: basal and postprandial. Role of diurnal rhythms.

Glycemia in type 2 diabetes is characterized by a nonsteady but stable diurnal cycle. This leads to morning fasting hyperglycemia. It arises from an underlying circadian pattern in endogenous glucose production because the metabolic clearance rate of glucose is decreased but constant. Therefore, it is important to use appropriate nonsteady tracer methods to measure this rate even under basal conditions. Postprandially, in diabetes, the endogenous glucose production continues to decrease, with only minor deviations from the slope of the basal curve. This suggests a decoupling of endogenous glucose production from the regulatory factors (insulin, glucose) that prevail under normal circumstances. As the duration of diabetes increases, metabolic clearance of glucose continues to deteriorate. This may be partially compensated by a decrease in glucose production. This rate remains, however, inappropriate because its impact on glycemia does not decline.

KATP channel-deficient pancreatic beta-cells are streptozotocin resistant because of lower GLUT2 activity.

In wild-type mice, a single injection of streptozotocin (STZ, 200 mg/kg body wt) caused within 4 days severe hyperglycemia, hypoinsulinemia, significant glucose intolerance, loss of body weight, and the disappearance of pancreatic beta-cells. However, in ATP-sensitive K(+) channel (K(ATP) channel)-deficient mice (Kir6.2(-/-) mice), STZ had none of these effects. Exposing isolated pancreatic islets to STZ caused severe damage in wild-type but not in Kir6.2(-/-) islets. Following a single injection, plasma STZ levels were slightly less in Kir6.2(-/-) mice than in wild-type mice. Despite the difference in plasma STZ, wild-type and Kir6.2(-/-) liver accumulated the same amount of STZ, whereas Kir6.2(-/-) pancreas accumulated 4.1-fold less STZ than wild-type pancreas. Kir6.2(-/-) isolated pancreatic islets also transported less glucose than wild-type ones. Quantification of glucose transporter 2 (GLUT2) protein content by Western blot using an antibody with an epitope in the extracellular loop showed no significant difference in GLUT2 content between wild-type and Kir6.2(-/-) pancreatic islets. However, visualization by immunofluorescence with the same antibody gave rise to 32% less fluorescence in Kir6.2(-/-) pancreatic islets. The fluorescence intensity using another antibody, with an epitope in the COOH terminus, was 5.6 times less in Kir6.2(-/-) than in wild-type pancreatic islets. We conclude that 1) Kir6.2(-/-) mice are STZ resistant because of a decrease in STZ transport by GLUT2 in pancreatic beta-cells and 2) the decreased transport is due to a downregulation of GLUT2 activity involving an effect at the COOH terminus.