AMPK and ACC phosphorylation: effect of leptin, muscle fibre type and obesity

Leptin stimulates fatty acid oxidation via the phosphorylation of AMPK (AMP-activated protein kinase) and ACC (acetyl-CoA carboxylase). Obesity is associated with resistance to the effects of leptin. We determined the action of leptin on AMPKalpha and ACCbeta phosphorylation and lipid metabolism in soleus (SOL) and extensor digitorum longus (EDL) muscles from lean and obese Wistar rats after 1 and 100 nM leptin. Both leptin doses stimulated phosphorylation of AMPKalpha and ACCbeta (P<or=0.05) only in EDL muscles from lean animals. Malonyl-CoA levels were decreased in EDL muscles from lean animals after 1 and 100 nM leptin and significantly after 100 nM leptin in obese animals (P<or=0.05). Long-chain fatty acyl-CoA concentrations were decreased in EDL muscles from both phenotypes after 100 nM leptin. AMPK activation by leptin occurred independently of energy-related metabolites. These data demonstrate that the leptin effect on AMPKalpha and ACCbeta is muscle fibre type dependent and fails in diet-induced obesity.

Substrate channelling in a creatine kinase system of rat skeletal muscle under various pH conditions

The aim of this study was to evaluate myofibrillar creatine kinase (CK) activity and to quantify the substrate channelling of ATP between CK and myosin ATPase under different pH conditions within the integrity of myofibrils. A pure myofibrillar fraction was prepared using differential centrifugation. The homogeneity of the preparation and the purity of the fraction were confirmed microscopically and by enzymatic assays for contaminant enzyme activities. The specific activity of myofibrillar CK reached 584 +/- 33 nmol PCr min(-1) mg(-1) at pH 6.75. Two methods were used to detect CK activity: (1) measurement of direct ATP production, and (2) measurement of PCr consumption. This method of evaluation has been tested in experiments with isolated creatine kinase. No discrepancy in CK activity between the methods was observed in the pH range tested (6.0-7.5). However, the same procedures resulted in a significant discrepancy between the amounts of reacted PCr and produced ATP within the pure myofibrillar fraction. This discrepancy represents the portion of ATP produced by the CK reaction, which is preferentially channelled to the myosin ATPase before diffusing into the bulk solution. The maximum evaluated difference reached 42.3 % at pH 6.95. The substrate channelling between myofibrillar-bound CK and myosin ATPase was evaluated under various pH levels within the physiological range and it reached a maximum value in a slightly acidic environment. These results suggest that ATP/ADP flux control by the CK system is more important at lower pH, corresponding to the physiological state of muscle fatigue.

Creatine kinase reaction in skinned rat psoas muscle fibers and their myofibrils

The aim of this study was to evaluate myofibrillar creatine kinase (EC 2.7.3.2) activity on the background of the effect of substrate channeling by myosin ATPase and to compare it with creatine kinase (CK) activity of whole skinned fibers. In order to assess CK activity, skinned fibers were prepared from the rat psoas major muscles defined by light microscopy. The activity in permeabilized fibers after treatment with saponin, Triton X-100 and Ca(2+)-free medium reached 2.80, 6.97 and 3.32 micromol ATP min(-1) mg(-1) protein, respectively, when a coupled enzyme assay system with external hexokinase and glucose-6-phosphate dehydrogenase was used. Transmission electron microscopy (TEM) revealed a possible interference among activities of sarcolemmal, sarcoplasmic, myofibrillar and mitochondrial CK from persisting structures. For evaluation of the myofibrillar CK itself, a pure myofibrillar fraction was prepared. Fraction purity was confirmed by TEM and by enzymatic assays for marker enzymes. Two procedures, i.e. the coupled enzyme assay and the evaluation of phosphocreatine (PCr) concentration before and after the CK reaction, were used for measurement of CK activity in this fraction. The procedures resulted in 3.2 nmol ATP min(-1) mg(-1) protein and 7.6 nmol PCr min(-1) mg(-1) protein, respectively. These alternative approaches revealed a discrepancy between the reacting portions of PCr by more than 50 %, which provides information about the size of the effect, generally described as substrate channeling.

Energy metabolism of rat skeletal muscle modulated by the rate of perfusion flow

In order to advance our understanding of the phenomenon of flow-induced increases in the metabolism of the relaxed muscle, the metabolic rate of the isolated rat gracilis muscle was investigated at 28 degrees C in vitro. The muscle was perfused with cell-free Krebs-Henseleit bicarbonate buffer containing 5% bovine serum albumin and 5 mM glucose, saturated with a gas mixture of 95% O2 and 5% CO2 and simultaneously superfused with a medium saturated with with a low O2 gas mixture (1% O2, 5% CO2 and 94% N2). Two different perfusion flow rates (0.054 and 0.100 ml min-1) have been used. Their influence on oxygen consumption and lactate production has been measured. After a 100 min perfusion period, the muscle was freeze-clamped and analysed for ATP, phosphocreatine, creatine, lactate, pyruvate, inorganic phosphate and glycogen content. The energy state of the cell and the proportions of glycolytic and mitochondrial fluxes of ATP synthesis were evaluated. During perfusion at the low flow rate of 0.054 ml min-1, the oxygen uptake was 45 +/- 9 nmol min-1 (g wet wt)-1, accompanied by a dominance of anaerobic glycolytic synthesis of ATP over mitochondrial ATP synthesis, even though the total delivery of oxygen to muscle was three times higher than oxygen consumption. Increasing the perfusion flow rate to 0.100 ml min-1 increased the oxygen uptake to 120 +/- 6 nmol min-1 (g wet wt)-1, thus leading to a prevalence of mitochondrial ATP synthesis over glycolytic ATP synthesis. The inner stores of glycogen served as the main substrate of energy metabolism and the role of exogenous substrates in the flow-stimulated increase of oxygen uptake was negligible. The increase in perfusion rate also enhanced the energy state of the muscle fibres, which was expressed either as the creatine charge or as the value of the change of Gibbs free energy of ATP hydrolysis. Data indicate that the change of perfusion flow rate per se, apart from oxygen and exogenous substrate supply, elicits changes in the regulation of energy metabolism within non-contracting skeletal muscle under open microcirculation.

Muscle ATP synthesis and utilisation, balanced during flow-induced increase of respiration

To investigate the control of cell energetic metabolism, creatine charge, ATP/ADP ratio and oxygen consumption (as indicators of an energetic status, the balance between ATP synthesis and degradation and the aerobic ATP turnover, respectively) were evaluated in the rat gracilis muscle, perfused-superfused in vitro. During the perfusion rate of 70 microl/min the ATP/ADP ratio, as well as the creatine charge are kept at the in vivo level. With the decrease of the rate toward 54 microl/min (of an abundant oxygen delivery), the values of both parameters are lower than levels in vivo. With the increase of the rate up to 100 microl/min, both parameters are kept at the in vivo level, when respiration increases by 125%. The data demonstrate the 'unmatched' control of ATP utilisation and synthesis steady rates during the low perfusion rate; during the increasing steady ATP turnover following the increased perfusion rate, the two fluxes are strikingly 'matched', i.e. precisely balanced.

Brown fat is essential for cold-induced thermogenesis but not for obesity resistance in aP2-Ucp mice

The role of brown adipose tissue in total energy balance and cold-induced thermogenesis was studied. Mice expressing mitochondrial uncoupling protein 1 (UCP-1) from the fat-specific aP2 gene promoter (heterozygous and homozygous aP2-Ucp transgenic mice) and their nontransgenic C57BL6/J littermates were used. The transgenic animals are resistant to obesity induced by a high-fat diet, presumably due to ectopic synthesis of UCP-1 in white fat. These animals exhibited atrophy of brown adipose tissue, as indicated by smaller size of brown fat and reduction of its total UCP-1 and DNA contents. Norepinephrine-induced respiration (measured in pentobarbital sodium-anesthetized animals) was decreased proportionally to the dosage of the transgene, and the homozygous (but not heterozygous) transgenic mice exhibited a reduction in their capacity to maintain body temperature in the cold. Our results indicate that the role of brown fat in cold-induced thermogenesis cannot be substituted by increased energy expenditure in other tissues.