Overexpression of UCP3 decreases mitochondrial efficiency in mouse skeletal muscle in vivo.

Uncoupling protein-3 (UCP3) is a mitochondrial transmembrane protein highly expressed in the muscle that has been implicated in regulating the efficiency of mitochondrial oxidative phosphorylation. Increasing UCP3 expression in skeletal muscle enhances proton leak across the inner mitochondrial membrane and increases oxygen consumption in isolated mitochondria, but its precise function in vivo has yet to be fully elucidated. To examine whether muscle-specific overexpression of UCP3 modulates muscle mitochondrial oxidation in vivo, rates of ATP synthesis were assessed by P magnetic resonance spectroscopy (MRS), and rates of mitochondrial oxidative metabolism were measured by assessing the rate of [2- C]acetate incorporation into muscle [4- C]-, [3- C]-glutamate, and [4- C]-glutamine by high-resolution C/ H MRS.

Dissociation of Muscle Insulin Resistance from Alterations in Mitochondrial Substrate Preference.

Alterations in muscle mitochondrial substrate preference have been postulated to play a major role in the pathogenesis of muscle insulin resistance. In order to examine this hypothesis, we assessed the ratio of mitochondrial pyruvate oxidation (V) to rates of mitochondrial citrate synthase flux (V) in muscle. Contrary to this hypothesis, we found that high-fat-diet (HFD)-fed insulin-resistant rats did not manifest altered muscle substrate preference (V/V) in soleus or quadriceps muscles in the fasting state.

Reduced cognitive function, increased blood-brain-barrier transport and inflammatory responses, and altered brain metabolites in LDLr -/-and C57BL/6 mice fed a western diet.

Recent work suggests that diet affects brain metabolism thereby impacting cognitive function. Our objective was to determine if a western diet altered brain metabolism, increased blood-brain barrier (BBB) transport and inflammation, and induced cognitive impairment in C57BL/6 (WT) mice and low-density lipoprotein receptor null (LDLr -/-) mice, a model of hyperlipidemia and cognitive decline. We show that a western diet and LDLr -/- moderately influence cognitive processes as assessed by Y-maze and radial arm water maze.

Assessment of Hepatic Mitochondrial Oxidation and Pyruvate Cycling in NAFLD by (13)C Magnetic Resonance Spectroscopy.

Non-alcoholic fatty liver disease (NAFLD) is the most common chronic liver disease, and there is great interest in understanding the potential role of alterations in mitochondrial metabolism in its pathogenesis. To address this question, we assessed rates of hepatic mitochondrial oxidation in subjects with and without NAFLD by monitoring the rate of (13)C labeling in hepatic [5-(13)C]glutamate and [1-(13)C]glutamate by (13)C MRS during an infusion of [1-(13)C]acetate. We found that rates of hepatic mitochondrial oxidation were similar between NAFLD and control subjects.

Hypophosphatemia promotes lower rates of muscle ATP synthesis.

Hypophosphatemia can lead to muscle weakness and respiratory and heart failure, but the mechanism is unknown. To address this question, we noninvasively assessed rates of muscle ATP synthesis in hypophosphatemic mice by using in vivo saturation transfer [P]-magnetic resonance spectroscopy. By using this approach, we found that basal and insulin-stimulated rates of muscle ATP synthetic flux (V) and plasma inorganic phosphate (P) were reduced by 50% in mice with diet-induced hypophosphatemia as well as in sodium-dependent P transporter solute carrier family 34, member 1 (NaPi2a)-knockout (NaPi2a) mice compared with their wild-type littermate controls.

Direct assessment of hepatic mitochondrial oxidative and anaplerotic fluxes in humans using dynamic 13C magnetic resonance spectroscopy.

Despite the central role of the liver in the regulation of glucose and lipid metabolism, there are currently no methods to directly assess hepatic oxidative metabolism in humans in vivo. By using a new (13)C-labeling strategy in combination with (13)C magnetic resonance spectroscopy, we show that rates of mitochondrial oxidation and anaplerosis in human liver can be directly determined noninvasively. Using this approach, we found the mean rates of hepatic tricarboxylic acid (TCA) cycle flux (VTCA) and anaplerotic flux (VANA) to be 0.

High-intensity interval training increases in vivo oxidative capacity with no effect on P(i)→ATP rate in resting human muscle.

Mitochondrial ATP production is vital for meeting cellular energy demand at rest and during periods of high ATP turnover. We hypothesized that high-intensity interval training (HIT) would increase ATP flux in resting muscle (VPi→ATP) in response to a single bout of exercise, whereas changes in the capacity for oxidative ATP production (Vmax) would require repeated bouts. Eight untrained men (27 ± 4 yr; peak oxygen uptake = 36 ± 4 ml·kg(-1)·min(-1)) performed six sessions of HIT (4-6 × 30-s bouts of all-out cycling with 4-min recovery).

³¹P-magnetization transfer magnetic resonance spectroscopy measurements of in vivo metabolism.

Magnetic resonance spectroscopy offers a broad range of noninvasive analytical methods for investigating metabolism in vivo. Of these, the magnetization-transfer (MT) techniques permit the estimation of the unidirectional fluxes associated with metabolic exchange reactions. Phosphorus (³¹P) MT measurements can be used to examine the bioenergetic reactions of the creatine-kinase system and the ATP synthesis/hydrolysis cycle.

Regulation of hepatic fat and glucose oxidation in rats with lipid-induced hepatic insulin resistance.

Unlabelled: Pyruvate dehydrogenase plays a critical role in the regulation of hepatic glucose and fatty acid oxidation; however, surprisingly little is known about its regulation in vivo. In this study we examined the individual effects of insulin and substrate availability on the regulation of pyruvate dehydrogenase flux (V(PDH) ) to tricarboxylic acid flux (V(TCA) ) in livers of awake rats with lipid-induced hepatic insulin resistance. V(PDH) /V(TCA) flux was estimated from the [4-(13) C]glutamate/[3-(13) C]alanine enrichments in liver extracts and assessed under conditions of fasting and during a hyperinsulinemic-euglycemic clamp, whereas the effects of increased plasma glucose concentration on V(PDH) /V(TCA) flux was assessed during a hyperglycemic clamp in conjunction with infusions of somatostatin and insulin to maintain basal concentrations of insulin.

Intracellular energetics and critical PO2 in resting ischemic human skeletal muscle in vivo.

During ischemia and some types of muscular contractions, oxygen tension (Po(2)) declines to the point that mitochondrial ATP synthesis becomes limited by oxygen availability. Although this critical Po(2) has been determined in animal tissue in vitro and in situ, there remains controversy concerning potential disparities between values measured in vivo and ex vivo. To address this issue, we used concurrent heteronuclear magnetic resonance spectroscopy (MRS) to determine the critical intracellular Po(2) in resting human skeletal muscle in vivo.

Assessment of in vivo mitochondrial metabolism by magnetic resonance spectroscopy.

Magnetic resonance spectroscopy (MRS), a companion technique to the more familiar MRI scan, has emerged as a powerful technique for studying metabolism noninvasively in a variety of tissues. In this article, we review two techniques that we have developed which take advantage of the unique characteristics of (31)P and (13)C MRS to investigate two distinct parameters of muscle mitochondrial metabolism; ATP production can be estimated by using the (31)P saturation-transfer technique, and oxidation via the TCA cycle can be modeled from (13)C MRS data obtained during the metabolism of a (13)C-labeled substrate. We will also examine applications of the techniques to investigate how these parameters of muscle mitochondrial metabolism are modulated in insulin resistant and endurance trained individuals.

Intramyocellular oxygenation during ischemic muscle contractions in vivo.

There is some evidence that the fall in intramyocellular oxygen content during ischemic contractions is less than during ischemia alone. We used proton magnetic resonance spectroscopy to determine whether peak deoxy-myoglobin (dMb) obtained during ischemic ankle dorsiflexion contractions attained the maximal dMb level observed during a separate trial of ischemia alone (resting max). In six healthy young men, the rate of myoglobin desaturation was rapid at the onset of ischemic contractions and then slowed as contractions continued, attaining only 75 +/- 3.

Paradoxical effects of increased expression of PGC-1alpha on muscle mitochondrial function and insulin-stimulated muscle glucose metabolism.

Peroxisome proliferator-activated receptor-gamma coactivator (PGC)-1alpha has been shown to play critical roles in regulating mitochondria biogenesis, respiration, and muscle oxidative phenotype. Furthermore, reductions in the expression of PGC-1alpha in muscle have been implicated in the pathogenesis of type 2 diabetes. To determine the effect of increased muscle-specific PGC-1alpha expression on muscle mitochondrial function and glucose and lipid metabolism in vivo, we examined body composition, energy balance, and liver and muscle insulin sensitivity by hyperinsulinemic-euglycemic clamp studies and muscle energetics by using (31)P magnetic resonance spectroscopy in transgenic mice.

Increased substrate oxidation and mitochondrial uncoupling in skeletal muscle of endurance-trained individuals.

Endurance exercise training is accompanied by physiological changes that improve muscle function and performance. Several studies have demonstrated that markers of mitochondrial capacity are elevated, however, these studies tend to be performed ex vivo under conditions that yield maximal enzyme activities or in vivo but monitoring the response to exercise. Therefore, it is unclear whether basal mitochondrial metabolism is affected by exercise training.

Contraction frequency modulates muscle fatigue and the rate of myoglobin desaturation during incremental contractions in humans.

The metabolic cost of force production, and therefore the demand for oxygen, increases with intensity and frequency of contraction. This study investigated the interaction between fatigue and oxygenation, as reflected by deoxymyoglobin (dMb), during slow and rapid rhythmic isometric contractions having the same duty cycles and relative force-time integrals (FTIs). We used 1H magnetic resonance spectroscopy and measures of dorsiflexor muscle force to compare dMb and fatigue (fall of maximal voluntary force, MVC) in 11 healthy adults (29 +/- 7 y) during 16 min of slow (4 s contraction, 6 s relaxation) and rapid (1.

Impaired mitochondrial substrate oxidation in muscle of insulin-resistant offspring of type 2 diabetic patients.

Insulin resistance is the best predictor for the development of diabetes in offspring of type 2 diabetic patients, but the mechanism responsible for it remains unknown. Recent studies have demonstrated increased intramyocellular lipid, decreased mitochondrial ATP synthesis, and decreased mitochondrial density in the muscle of lean, insulin-resistant offspring of type 2 diabetic patients. These data suggest an important role for mitochondrial dysfunction in the pathogenesis of type 2 diabetes.

In vivo ATP production during free-flow and ischaemic muscle contractions in humans.

The aim of this study was to determine how ATP synthesis and contractility in vivo are altered by ischaemia in working human skeletal muscle. The hypotheses were: (1) glycolytic flux would be higher during ischaemic (ISC) compared to free-flow (FF) muscle contractions, in compensation for reduced oxidative ATP synthesis, and (2) ischaemic muscle fatigue would be related to the accumulation of inhibitory metabolic by-products rather than to the phosphorylation potential ([ATP]/[ADP][P(i)]) of the muscle. Twelve healthy adults (6 men, 6 women) performed six intermittent maximal isometric contractions of the ankle dorsiflexors (12 s contract, 12 s relax), once with intact blood flow and once with local ischaemia by thigh cuff inflation to 220 Torr.

Age-related changes in ATP-producing pathways in human skeletal muscle in vivo.

Energy for muscle contractions is supplied by ATP generated from 1) the net hydrolysis of phosphocreatine (PCr) through the creatine kinase reaction, 2) oxidative phosphorylation, and 3) anaerobic glycolysis. The effect of old age on these pathways is unclear. The purpose of this study was to examine whether age may affect ATP synthesis rates from these pathways during maximal voluntary isometric contractions (MVIC).