Intersubject differences in the effect of acidosis on phosphocreatine recovery kinetics in muscle after exercise are due to differences in proton efflux rates

31P magnetic resonance spectroscopy provides the possibility of obtaining bioenergetic data during skeletal muscle exercise and recovery. The time constant of phosphocreatine (PCr) recovery (tPCr) has been used as a measure of mitochondrial function. However, cytosolic pH has a strong influence on the kinetics of PCr recovery, and it has been suggested that tPCr should be normalized for end-exercise pH. A general correction can only be applied if there are no intersubject differences in the pH dependence of tPCr. We investigated the pH dependence of tPCr on a subject-by-subject basis. Furthermore, we determined the kinetics of proton efflux at the start of recovery. Intracellular acidosis slowed PCr recovery, and the pH dependence of tPCr differed among subjects, ranging from -33.0 to -75.3 s/pH unit. The slope of the relation between tPCr and end-exercise pH was positively correlated with both the proton efflux rate and the apparent proton efflux rate constant, indicating that subjects with a smaller pH dependence of tPCr have a higher proton efflux rate. Our study implies that simply correcting tPCr for end-exercise pH is not adequate, in particular when comparing patients and control subjects, because certain disorders are characterized by altered proton efflux from muscle fibers. Copyright © 2007 the American Physiological Society

Regional variations in intramyocellular lipid concentration correlate with muscle fiber type distribution in rat tibialis anterior muscle

1H MR spectroscopy (MRS) has proved to be a valuable noninvasive tool to measure intramyocellular lipids (IMCL) in research focused on insulin resistance and type II diabetes in both humans and rodents. An important determinant of IMCL is the muscle fiber type, since oxidative type I fibers can contain up to three times more MCL than glycolytic type II muscle fibers. Because these different muscle fiber types are inhomogeneously distributed in rodent muscle, in the present study we investigated the distribution of IMCL within the rat tibialis anterior muscle (TA) in vivo using single-voxel 1H MRS along with the muscle fiber distribution in the TA ex vivo determined from immunohistological assays. IMCL levels in the TA differed by up to a factor of 3 depending on the position of the voxel. The distribution of IMCL over the TA cross section was not random, but emerged in a pattern similar to the distribution of the predominantly oxidative muscle fiber types. Dietary interventions, such as high-fat feeding and 15 hr of fasting, did not significantly change this typical fiber type-dependent pattern of IMCL content. These results stress the importance of voxel positioning when single-voxel 1H MRS is used to study IMCL in rodent muscle. © 2006 Wiley-Liss, Inc

Intersubject differences in the effect of acidosis on phosphocreatine recovery kinetics in muscle after exercise are due to differences in proton efflux rates

31P magnetic resonance spectroscopy provides the possibility of obtaining bioenergetic data during skeletal muscle exercise and recovery. The time constant of phosphocreatine (PCr) recovery (tPCr) has been used as a measure of mitochondrial function. However, cytosolic pH has a strong influence on the kinetics of PCr recovery, and it has been suggested that tPCr should be normalized for end-exercise pH. A general correction can only be applied if there are no intersubject differences in the pH dependence of tPCr. We investigated the pH dependence of tPCr on a subject-by-subject basis. Furthermore, we determined the kinetics of proton efflux at the start of recovery. Intracellular acidosis slowed PCr recovery, and the pH dependence of tPCr differed among subjects, ranging from -33.0 to -75.3 s/pH unit. The slope of the relation between tPCr and end-exercise pH was positively correlated with both the proton efflux rate and the apparent proton efflux rate constant, indicating that subjects with a smaller pH dependence of tPCr have a higher proton efflux rate. Our study implies that simply correcting tPCr for end-exercise pH is not adequate, in particular when comparing patients and control subjects, because certain disorders are characterized by altered proton efflux from muscle fibers. Copyright © 2007 the American Physiological Society

Early or advanced stage type 2 diabetes is not accompanied by in vivo skeletal muscle mitochondrial dysfunction

Objective: Several lines of evidence support a potential role of skeletal muscle mitochondrial dysfunction in the pathogenesis of insulin resistance and/or type 2 diabetes. However, it remains to be established whether mitochondrial dysfunction represents either cause or consequence of the disease. We examined in vivo skeletal muscle mitochondrial function in early and advanced stages of type 2 diabetes, with the aim to gain insight in the proposed role of mitochondrial dysfunction in the aetiology of insulin resistance and/or type 2 diabetes. Methods: Ten long-standing, insulin-treated type 2 diabetes patients, 11 subjects with impaired fasting glucose, impaired glucose tolerance and/ or recently diagnosed type 2 diabetes, and 12 healthy, normoglycaemic controls, matched for age and body composition and with low habitual physical activity levels were studied. In vivo mitochondrial function of the vastus lateralis muscle was evaluated from post-exercise phosphocreatine (PCr) recovery kinetics using 31P magnetic resonance spectroscopy (MRS). Intramyocellular lipid (IMCL) content was assessed in the same muscle using single-voxel 1H MRS. Results: IMCL content tended to be higher in the type 2 diabetes patients when compared with normoglycaemic controls (P=0.06). The 31P MRS parameters for mitochondrial function, i.e. PCr and ADP recovery time constants and maximum aerobic capacity, did not differ between groups. Conclusions: The finding that in vivo skeletal muscle oxidative capacity does not differ between long-standing, insulin-treated type 2 diabetes patients, subjects with early stage type 2 diabetes and sedentary, normoglycaemic controls suggests that mitochondrial dysfunction does not necessarily represent either cause or consequence of insulin resistance and/or type 2 diabetes. © 2008 Society of the European Journal of Endocrinology

Increased mitochondrial content rescues in vivo muscle oxidative capacity in long-term high-fat-diet-fed rats

Mitochondria are thought to play a crucial role in the etiology of muscle insulin resistance (IR). The aim of this study was to gain more insight into the timing and nature of mitochondrial adaptations during the development of high-fat-diet (HFD)-induced IR. Adult Wistar rats were fed HFD or normal chow for 2.5 and 25 wk. Intramyocellular lipids (IMCLs) were quantified in vivo using 1H magnetic resonance spectroscopy (MRS). Muscle oxidative capacity was assessed in vivo using 31P MRS and in vitro by measuring mitochondrial DNA copy number and oxygen consumption in isolated mitochondria. MRS in tibialis anterior muscle revealed 3.3-fold higher IMCL content and 1.2-fold increased oxidative capacity after 2.5 wk of HFD feeding. The latter result could be fully accounted for by increased mitochondrial content. After 25 wk of HFD, maximal ADP-stimulated oxygen consumption in isolated mitochondria oxidizing pyruvate plus malate remained unaffected, while IMCL and mitochondrial content had further increased compared to controls (5.1-fold and 1.4-fold, respectively). Interestingly, in vivo oxidative capacity at this time point was identical to controls. These results show that skeletal muscle in HFD-induced IR accompanied by IMCL accumulation requires a progressively larger mitochondrial pool size to maintain normal oxidative capacity in vivo.—Van den Broek, N. M. A., Ciapaite, J., De Feyter, H. M. M. L., Houten, S. M., Wanders, R. J. A., Jeneson, J. A. L., Nicolay, K., Prompers, J. J. Increased mitochondrial content rescues in vivo muscle oxidative capacity in long-term high-fat-diet-fed rats

Exercise training improves glycemic control in long-standing insulin-treated type 2 diabetic patients

Regular exercise represents an effective strategy to prevent and/or treat type 2 diabetes ( 1 , 2 ). However, the clinical benefits of exercise intervention in a vastly expanding group of long-standing insulin-treated type 2 diabetic patients with comorbidities are less evident. As these patients generally experience muscle weakness ( 3 – 6 ), cardiovascular comorbidities ( 7 – 10 ), and/or exercise intolerance ( 3 , 11 – 13 ), it has proven difficult or even impossible for them to adhere to an intense endurance exercise training regimen ( 14 , 15 ). In the present study, we investigated the feasibility and benefits of a low-impact exercise intervention program, combining both endurance and resistance-type exercise, in long-standing insulin-treated type 2 diabetic patients with a high cardiovascular risk profile. We assessed the impact of 5 months of exercise training on glycemic control, body composition, workload capacity, and whole-body as well as skeletal muscle oxidative capacity