Myocardial glucose uptake in patients with the m.3243A > G mutation in mitochondrial DNA.

Mitochondrial mutations impair glucose oxidation and increase glucose uptake in cell cultures and lead to cardiomyopathy in patients. Here we characterize cardiac glucose uptake in 14 patients with the m.3243A > G mutation in mitochondrial DNA. The 14 patients with m.3243A > G and 13 controls were similar in age, physical activity and body mass index. Ten patients had diabetes. Left ventricular glucose uptake per tissue mass (LVGU) was measured with 2-[(18) F]fluoro-2-deoxyglucose positron emission tomography during euglycemic hyperinsulinemia. Cardiac morphology and function were assessed with magnetic resonance imaging. We found that the LVGU was 25% lower in the patients than that in the controls (P = 0.029). LVGU was inversely correlated with mutation heteroplasmy, glycated haemoglobin and fasting lactate in patients. The seven patients with mutation heteroplasmy ≥ 49% had 44% lower LVGU than the seven patients with heteroplasmy < 49%. This difference remained significant after adjustment for concurrent free fatty acid concentration or glycated haemoglobin or glucose uptake in skeletal muscle or all (p < 0.048 [All]). Patients with m.3243A > G had a lower stroke volume and a higher heart rate than the controls, whereas cardiac output and work were similar. Myocardial glucose uptake is not increased but decreased with a threshold effect pattern in patients with the m.3243A > G mutation. The glucose hypometabolism adds to the impaired cardiac energetics and likely contributes to the progression of the mitochondrial cardiomyopathy.

Dorsal striatum and its limbic connectivity mediate abnormal anticipatory reward processing in obesity.

Obesity is characterized by an imbalance in the brain circuits promoting reward seeking and those governing cognitive control. Here we show that the dorsal caudate nucleus and its connections with amygdala, insula and prefrontal cortex contribute to abnormal reward processing in obesity. We measured regional brain glucose uptake in morbidly obese (n = 19) and normal weighted (n = 16) subjects with 2-[¹8F]fluoro-2-deoxyglucose ([¹8F]FDG) positron emission tomography (PET) during euglycemic hyperinsulinemia and with functional magnetic resonance imaging (fMRI) while anticipatory food reward was induced by repeated presentations of appetizing and bland food pictures. First, we found that glucose uptake rate in the dorsal caudate nucleus was higher in obese than in normal-weight subjects. Second, obese subjects showed increased hemodynamic responses in the caudate nucleus while viewing appetizing versus bland foods in fMRI. The caudate also showed elevated task-related functional connectivity with amygdala and insula in the obese versus normal-weight subjects. Finally, obese subjects had smaller responses to appetizing versus bland foods in the dorsolateral and orbitofrontal cortices than did normal-weight subjects, and failure to activate the dorsolateral prefrontal cortex was correlated with high glucose metabolism in the dorsal caudate nucleus. These findings suggest that enhanced sensitivity to external food cues in obesity may involve abnormal stimulus-response learning and incentive motivation subserved by the dorsal caudate nucleus, which in turn may be due to abnormally high input from the amygdala and insula and dysfunctional inhibitory control by the frontal cortical regions. These functional changes in the responsiveness and interconnectivity of the reward circuit could be a critical mechanism to explain overeating in obesity.

Mitochondrial diabetes is associated with insulin resistance in subcutaneous adipose tissue but not with increased liver fat content.

We recently showed that patients with mitochondrial diabetes are insulin resistant in skeletal muscle before the decline in insulin secretion is observed. In this study, we further evaluate whether insulin resistance is associated with increased ectopic fat accumulation and altered adipose and hepatic tissue insulin sensitivity. We studied 15 nonobese patients with the m.3243A > G mutation. Five were without diabetes (group 1), three had newly diagnosed diabetes (group 2), and seven had previously diagnosed diabetes (group 3). Thirteen healthy volunteers of similar age and body mass index (BMI) served as controls. Insulin-stimulated glucose uptake was measured with positron emission tomography using 2- [(18)F]-fluoro-2-deoxyglucose during euglycemic hyperinsulinemia. Fat masses and liver fat content were measured with magnetic resonance imaging and spectroscopy. Compared with controls, insulin-stimulated glucose uptake in adipose tissue was decreased by ∼50% in all groups with the m.3243A > G mutation. In addition, fat masses were not different, but insulin-mediated suppression of lipolysis and adiponectin metabolism were blunted in patients with the m.3243A > G mutation. Hepatic fat content was normal (<5.6%) in 80% of patients and significantly elevated in one case only. Hepatic glucose metabolism in patients with m.3243A > G did not differ from that of controls. In conclusion, m.3243A > G mutation affects subcutaneous adipose tissue metabolism. This seems to occur before aberrant liver metabolism, if any, can be observed or before beta-cell failure results in mitochondrial diabetes.

Comparison of exogenous adenosine and voluntary exercise on human skeletal muscle perfusion and perfusion heterogeneity.

Adenosine is a widely used pharmacological agent to induce a "high-flow" control condition to study the mechanisms of exercise hyperemia, but it is not known how well an adenosine infusion depicts exercise-induced hyperemia, especially in terms of blood flow distribution at the capillary level in human muscle. Additionally, it remains to be determined what proportion of the adenosine-induced flow elevation is specifically directed to muscle only. In the present study, we measured thigh muscle capillary nutritive blood flow in nine healthy young men using PET at rest and during the femoral artery infusion of adenosine (1 mg min(-1) l thigh volume(-1)), which has previously been shown to induce a maximal whole thigh blood flow of approximately 8 l/min. This response was compared with the blood flow induced by moderate- to high-intensity one-leg dynamic knee extension exercise. Adenosine increased muscle blood flow on average to 40 +/- 7 ml x min(-1) x 100 g muscle(-1) with an aggregate value of 2.3 +/- 0.6 l/min for the whole thigh musculature. Adenosine also induced a substantial change in blood flow distribution within individuals. Muscle blood flow during the adenosine infusion was comparable with blood flow in moderate- to high-intensity exercise (36 +/- 9 ml x min(-1) x 100 g muscle(-1)), but flow heterogeneity was significantly higher during the adenosine infusion than during voluntary exercise. In conclusion, a substantial part of the flow increase in the whole limb blood flow induced by a high-dose adenosine infusion is conducted through the physiological non-nutritive shunt in muscle and/or also through tissues of the limb other than muscle. Additionally, an intra-arterial adenosine infusion does not mimic exercise hyperemia, especially in terms of muscle capillary flow heterogeneity, while the often-observed exercise-induced changes in capillary blood flow heterogeneity likely reflect true changes in nutritive flow linked to muscle fiber and vascular unit recruitment.

Cerebral oxygen and glucose metabolism in patients with mitochondrial m.3243A>G mutation.

The m.3243A>G mutation is the most common pathogenic mutation in mitochondrial DNA. It leads to defective oxidative phosphorylation, decreased oxygen consumption and increased glucose utilization and lactate production in vitro. However, oxygen and glucose metabolism has not been studied in the brain of patients harbouring the m.3243A>G mutation. Therefore, 14 patients with the m.3243A>G mutation, not experiencing acute stroke-like episodes and 14 age-matched controls underwent positron emission tomography using 2-[(18)F]fluoro-2-deoxyglucose, [(15)O]H(2)O and [(15)O]O(2) as the tracers during normoglycaemia. The metabolic rate of oxygen and glucose were determined using a quantitative region of interest analysis. Metabolites in unaffected periventricular tissue were measured using magnetic resonance spectroscopy. We found that the cerebral metabolic rate of oxygen was decreased by 26% (range 18%-29%) in the grey as well as the white matter of patients with the m.3243A>G mutation. A decrease in the metabolic rate of glucose was found with predilection to the posterior part of the brain. No major changes were detected in cerebral blood flow or the number of white matter lesions. Our results show that the m.3243A>G mutation leads to a global decrease in oxygen consumption in the grey matter including areas where no other signs of disease were present.

m.3243A>G mutation in mitochondrial DNA leads to decreased insulin sensitivity in skeletal muscle and to progressive beta-cell dysfunction.

OBJECTIVE: To study insulin sensitivity and perfusion in skeletal muscle together with the beta-cell function in subjects with the m.3243A>G mutation in mitochondrial DNA, the most common cause of mitochondrial diabetes. RESEARCH DESIGN AND METHODS: We measured skeletal muscle glucose uptake and perfusion using positron emission tomography and 2-[18F]fluoro-2-deoxyglucose and [15O]H2O during euglycemic hyperinsulinemia in 15 patients with m.3243A>G. These patients included five subjects with no diabetes as defined by the oral glucose tolerance test (OGTT) (group 1), three with GHb <6.1% and newly found diabetes by OGTT (group 2), and seven with a previously diagnosed diabetes (group 3). Control subjects consisted of 13 healthy individuals who were similar to the carriers of m.3243A>G with respect to age and physical activity. Beta-cell function was assessed using the OGTT and subsequent mathematical modeling. RESULTS: Skeletal muscle glucose uptake was significantly lower in groups 1, 2, and 3 than in the control subjects. The glucose sensitivity of beta-cells in group 1 patients was similar to that of the control subjects, whereas in group 2 and 3 patients, the glucose sensitivity was significantly lower. The insulin secretion parameters correlated strongly with the proportion of m.3243A>G mutation in muscle. CONCLUSIONS: Our findings show that subjects with m.3243A>G are insulin resistant in skeletal muscle even when beta-cell function is not markedly impaired or glucose control compromised. We suggest that both the skeletal muscle insulin sensitivity and the beta-cell function are affected before the onset of the mitochondrial diabetes caused by the m.3243A>G mutation.

Maturation of cultured hippocampal slices results in increased excitability in granule cells.

The preparation of hippocampal slices results in loss of input neurons to dentate granule cells, which leads to the reorganization of their axons, the mossy fibers, and alters their functional properties in long-term cultures, but its temporal aspects in the immature hippocampus are not known. In this study, we have focused on the early phase of this plastic reorganization process by analyzing granule cell function with field potential and whole cell recordings during the in vitro maturation of hippocampal slices (from 1 to 17 days in vitro, prepared from 6 to 7-day-old rats), and their morphology using extracellular biocytin labelling technique. Acute slices from postnatal 14-22-day-old rats were analyzed to detect any differences in the functional properties of granule cells in these two preparations. In field potential recordings, small synaptically-evoked responses were detected at 2 days in vitro, and their amplitude increased during the culture time. Whole cell voltage clamp recordings revealed intensive spontaneous excitatory postsynaptic currents, and the susceptibility to stimulus-evoked bursting increased with culture time. In acutely prepared slices, neither synaptically-evoked responses in field potential recordings nor any bursting in whole cell recordings were detected. The excitatory activity was under the inhibitory control of gamma-aminobutyric acid type A receptor. Extracellularily applied biocytin labelled dentate granule cells, and revealed sprouting and aberrant targeting of mossy fibers in cultured slices. Our results suggest that reorganization of granule cell axons takes place during the early in vitro maturation of hippocampal slices, and contributes to their increased excitatory activity resembling that in the epileptic hippocampus. Cultured immature hippocampal slices could thus serve as an additional in vitro model to elucidate mechanisms of synaptic plasticity and cellular reactivity in response to external damage in the developing hippocampus.