Muscle fiber contractile type influences the regulation of mitochondrial function.

Mitochondrial respiratory rates and regulation by phosphate acceptors were studied on permeabilized fiber bundles differing in their myosin heavy chain profiles. The acceptor control ratio, an indicator of oxidation to phosphorylation coupling, and mitochondrial K(m) for ADP were the highest in type I, intermediate in mixed IIa/IIx and the lowest in IIx and predominantly IIb fiber bundles. A functional coupling between mitochondrial creatine kinase and oxidative phosphorylation occurred in type I and IIa/IIx fiber bundles, exclusively. Our study suggests that mitochondrial functioning in fast IIa fibers is closer to that of the slow/I than fast IIx or IIb fibers.

Unusual metabolic characteristics in skeletal muscles of transgenic rabbits for human lipoprotein lipase.

BACKGROUND: The lipoprotein lipase (LPL) hydrolyses circulating triacylglycerol-rich lipoproteins. Thereby, LPL acts as a metabolic gate-keeper for fatty acids partitioning between adipose tissue for storage and skeletal muscle primarily for energy use. Transgenic mice that markedly over-express LPL exclusively in muscle, show increases not only in LPL activity, but also in oxidative enzyme activities and in number of mitochondria, together with an impaired glucose tolerance. However, the role of LPL in intracellular nutrient pathways remains uncertain. To examine differences in muscle nutrient uptake and fatty acid oxidative pattern, transgenic rabbits harboring a DNA fragment of the human LPL gene (hLPL) and their wild-type littermates were compared for two muscles of different metabolic type, and for perirenal fat. RESULTS: Analyses of skeletal muscles and adipose tissue showed the expression of the hLPL DNA fragment in tissues of the hLPL group only. Unexpectedly, the activity level of LPL in both tissues was similar in the two groups. Nevertheless, mitochondrial fatty acid oxidation rate, measured ex vivo using [1-(14C)]oleate as substrate, was lower in hLPL rabbits than in wild-type rabbits for the two muscles under study. Both insulin-sensitive glucose transporter GLUT4 and muscle fatty acid binding protein (H-FABP) contents were higher in hLPL rabbits than in wild-type littermates for the pure oxidative semimembranosus proprius muscle, but differences between groups did not reach significance when considering the fast-twitch glycolytic longissimus muscle. Variations in both glucose uptake potential, intra-cytoplasmic binding of fatty acids, and lipid oxidation rate observed in hLPL rabbits compared with their wild-type littermates, were not followed by any modifications in tissue lipid content, body fat, and plasma levels in energy-yielding metabolites. CONCLUSIONS: Expression of intracellular binding proteins for both fatty acids and glucose, and their following oxidation rates in skeletal muscles of hLPL rabbits were not fully consistent with the physiology rules. The modifications observed in muscle metabolic properties might not be directly associated with any LPL-linked pathways, but resulted likely of transgene random insertion into rabbit organism close to any regulatory genes. Our findings enlighten the risks for undesirable phenotypic modifications in micro-injected animals and difficulties of biotechnology in mammals larger than mice.

Age-related changes in glucose utilization and fatty acid oxidation in a muscle-specific manner during rabbit growth.

The optimal utilization of energy substrates in muscle fibers is of primary importance for muscle contraction and whole body physiology. This study aimed to investigate the age-related changes in some indicators of glucose catabolism and fatty acid oxidation in muscles of growing rabbits. Longissimus lumborum (fast-twitch, LL) and semimembranosus proprius (slow-twitch, SMP) muscles were collected at 10 or 20 weeks of age ( n=6 per age). Glucose transporter GLUT4 content was investigated by immunoblot assay. Activity levels of five enzymes were measured: lactate dehydrogenase (LDH) and phosphofructokinase (PFK) for glycolysis; citrate synthase (CS), isocitrate dehydrogenase (ICDH) and -3-hydroxyacyl-coenzyme A dehydrogenase (HAD) for oxidation. Mitochondrial and peroxisomal oxidation rates were assessed on fresh homogenates using [1-14C]-oleate as substrate. At both ages, mitochondrial and peroxisomal oxidations rates, as well as activities of oxidative enzymes were higher in SMP than in LL. In both muscles, the apparent rate of fatty acid oxidation by the mitochondria did not differ between the two ages. However, a decrease in the activities of the three oxidative enzymes was observed in LL, whereas activities of CS and HAD and peroxisomal oxidation rate of oleate increased between the two ages in SMP muscle. In both muscles, LDH activity increased between 10 and 20 weeks, without variations in glucose uptake (GLUT4 transporter content) and in the first step of glucose utilization (PFK activity). In conclusion, mitochondrial oxidation rate of fatty acids and activities of selected mitochondrial enzymes were largely unrelated. Moreover, regulation of energy metabolism with advancing age differed between muscle types.

Age-related relationships between muscle fat content and metabolic traits in growing rabbits.

This study was aimed at ascribing muscle fat accretion in growing rabbits to changes in several extra-muscular and intra-muscular metabolic pathways. At 10 wk or 20 wk of age (n = 8 per group), tissue lipid content and metabolic indicators of nutrient anabolic or catabolic pathways were simultaneously assessed in the liver, perirenal fat, the heart and the Longissimus lumborum (LL) muscle, together with plasma concentrations in energy-yielding metabolites. Lipid content significantly increased with age (P < or = 0.01) in the glycolytic LL muscle (+67%) and the oxidative heart (+30%). In the former muscle, it was statistically correlated (r2 = 0.68; P < 0.01) to the changes in the orientation of muscle metabolism towards an enhanced lipogenic capacity and a depressed capacity for fatty acid transport and nutrient oxidation, and to indications of lower availability in plasma glucose and triglycerides. In the heart, age-related fat accretion was positively associated (r2 = 0.48, P < 0.01) to intrinsic metabolic changes towards an enhanced lipogenic capacity, together with a lower availability in plasma glucose. Variables representative of cardiac catabolic capacity tended to be negatively correlated to fat content in the heart (r2 = 0.15, P = 0.07). In growing rabbits, muscle fat content variation was proven to result from a reciprocal balance between catabolic and anabolic fatty acid fluxes, rather than to be assigned to one specific energy metabolic pathway.

Mitochondrial and peroxisomal fatty acid oxidation capacities increase in the skeletal muscles of young pigs during early postnatal development but are not affected by cold stress.

In pigs, the optimal utilization of energy substrates within muscle fibers is a prerequisite of the utmost importance for successful adaptation to extra-uterine life. In the present work we demonstrate that fatty acid (FA) oxidative capacities increased within the first five days of life in piglet skeletal muscle. Mitochondrial FA oxidation capacities increased more in the rhomboideus oxidative than in the longissimus lumborum glycolytic muscle (+114% vs. +62%, P < 0.001). The apparent rate of fatty acid degradation by peroxisomes represents 30 to 40% of total FA oxidation capacities and increased by about 170% (P < 0.001) with age in both muscles. The postnatal enhancement of skeletal muscle oxidative capacities was further supported by a rise in acid-soluble and long-chain acylcamitine tissue levels (+67%, P < 0.01), and plasma levels of albumin (+160%, P < 0.001). Cold stress had no effect on mitochondrial and peroxisomal FA oxidation but greatly enhanced (+61%, P < 0.05) the circulating levels of non-esterified fatty acids at five days of life.

Effect of age and cold exposure on morphofunctional characteristics of skeletal muscle in neonatal pigs.

Muscular changes accompanying and/or promoting the rapid postnatal improvement of the thermogenic efficiency of shivering were investigated in piglets. Animals were obtained at birth or killed after 5 days at thermoneutrality (34-30 degrees C) or in the cold (24-15 degrees C), to stimulate intense shivering thermogenesis. Fast-twitch-glycolytic (longissimus lumborum) and slow-twitch-oxidative (rhomboid) muscles were prepared for electron microscopic examination and chemical measurements. Muscle-specific changes in energy stores and metabolism were observed after birth, including the switch from glycogen to lipids and variation of the lactate/pyruvate ratio corresponding to the progressive acquisition of the metabolic type of the mature muscles. There was major age-related and/or cold-induced development of the structures involved in excitation-contraction coupling (triadic profiles, +80% in the cold), oxidative metabolism (number of lipid droplets, +81% with age in the cold; number of mitochondria, +29% with age or cold; surface of mitochondrial inner membranes, +18% with age and +32% in the cold) and contraction potential (myofibril volume, +62% with age). In contrast, neither age nor cold affected capillary volume density and capillary-to-fibre ratio. The observed changes reflect the immaturity and remarkable plasticity of piglet skeletal muscle and are likely to underlie its enhanced capacity for shivering thermogenesis after birth.