Maximum rate of oxygen consumption (VO_2max in nmol mm⁻³ s⁻¹) of muscle preparations at physiological temperature from various species plotted against the cross-sectional area (in μm²) of the muscle cells in the preparation. A hyperbola was fitted through all data points. The fit hardly deviates from the Hill-type diffusion model shown in the text and is described by the function VO_2max = constant CSA⁻¹. The value of the constant is calculated as the mean of the products VO_2max and CSA for each muscle fiber type and approximates 0.4 pmol mm⁻¹ s⁻¹. Inset cross sections stained for succinate dehydrogenase activity. From left to right: right ventricular wall of normal rat myocardium, rat extensor digitorum longus muscle, human vastus lateralis muscle, iliofibularis muscle of Xenopus laevis; scale bar 100 μm. ratMCT right ventricular rat cardiomyocytes of a monocrotaline-induced pulmonary hypertensive rat, humanCHF vastus lateralis muscle of human chronic heart failure patients. Figure adapted from (Bekedam et al. 2003; Van Der Laarse et al. 1998)

Interactions of signaling pathways and their stimuli involved in turnover of structural muscle (i.e., contractile) protein and protein associated with high oxidative metabolism. In response to contractile activity, calcium increases intracellularly through stretch-activated Ca²⁺ channels and from calcium stores. In addition, growth factors and cytokines are secreted in the extracellular matrix by the muscle fibers where they can bind receptors and activate signaling pathways. The type of contractile activity or mechanical loading combined with the balance between cellular energy (AMP:ATP) and oxygen levels (i.e., ROS production) determine fiber type-specific activation of pathways and thus whether contractile protein is gained or lost (for details see text). AAs amino acids, AMPK AMP-activated protein kinase, bFGF basic fibroblast growth factor, ECM extracellular matrix, FAK focal adhesion kinase, FOXO Forkhead box transcription factors O subfamily, GF + CK growth factors and cytokines, HGF hepatocyte growth factor, IL interleukin, IGF-I insulin-like growth factor-I, MAFbx muscle atrophy F-box, MAPK mitogen-activated protein kinases, MGF mechano-growth factor, mTOR mammalian target of rapamycin, MuRF muscle ring finger, NF-κB nuclear factor kappa-B, PGC-1α peroxisome proliferator-activated receptor γ coactivator-1α, PI3K phosphatidylinositol-3 kinase, PLD phospholipase D, p70S6K 70-kDa ribosomal protein S6 kinase, ROS reactive oxygen species, SC satellite cells, SR sarcoplasmatic reticulum, SRF serum response factor, TNF-α tumor necrosis factor-α, Vps34 vacuolar protein sorting mutant 34

Schematic diagram representing the regulation of muscle fiber size and oxidative metabolism. The type of contractile activity, ranging from sustained low-level activity to short high-level activity, combined with the cellular energy (AMP:ATP) and oxygen (ROS concentration) status determine the rate of synthesis and degradation of both contractile and metabolic protein. The balance between synthesis and degradation reflects the rate of protein turnover and determines whether contractile or metabolic protein is gained or lost, thereby regulating the fiber size. Solid lines are dominant processes in high oxidative fibers, whereas dashed lines are dominant in low oxidative fibers. The plus and minus signs reflect the effects of the different processes (e.g., synthesis, degradation, hypoxia) on fiber size and mitochondrial density (for details see text)

Differences in mRNA concentrations of glyceralde-3 phosphate dehydrogenase (GAPDH), 18S RNA, α-skeletal actin (α-sk actin), muscle ring finger-1 (MuRF1) and muscle atrophy F-box (MAFbx) in high oxidative rat soleus (SO) and low oxidative extensor digitorum longus (EDL) muscles (n = 6) (for methods see supplementary section I). a Total RNA (μg) normalized to muscle tissue weight (mg) showed that SO contains 2.3-fold more total RNA per milligram muscle tissue compared to EDL (p < 0.001). b mRNA normalized to total RNA relative to EDL. No differences between SO and EDL were found for any marker, except for GAPDH that showed 2.7-fold lower expression in SO (p < 0.001). c mRNA normalized to muscle tissue weight relative to EDL. GAPDH showed no significant difference between SO and EDL, whereas 18S RNA (2.1-fold), α-sk actin (2.3-fold), MuRF1 (2.1-fold) and MAFbx (1.8-fold) were all higher in SO (p < 0.05). Asterisks indicate significant difference compared to EDL. Systematic comparison of oxidative capacity and fiber cross-sectional area (CSA) from various hind limb muscles in the rat (Armstrong and Phelps 1984) show that SO predominantly (~90%) consists of slow contracting fibers with high oxidative capacity, whereas EDL contains largely (~60%) fast contracting glycolytic fibers with the lowest oxidative capacity. Within EDL, the high oxidative fibers show significantly smaller CSA than low oxidative fibers and also high oxidative fibers in SO show smaller CSA compared to the low oxidative fibers in EDL (Armstrong and Phelps 1984; Nakatani et al. 1999). Based on these observations and the data from Figs. 1 and 2, it can be concluded that high oxidative fibers are generally smaller and also contain more 18S RNA, α-sk actin-, MuRF1- and MAFbx-mRNA, compared to low oxidative fibers. The literature on rat SO and EDL fiber type composition does not unambiguously show that high oxidative fibers have smaller CSA compared to low oxidative fibers (Deveci et al. 2001; Kupa et al. 1995; Torrejais et al. 1999). The inconsistencies in CSA data of the latter studies compared to other studies with a more systematic approach (Armstrong and Phelps 1984; Nakatani et al. 1999) may be related to age, gender, muscle region or the effect of treatment. In addition, the differences in CSA between high and low oxidative fibers were not always tested for statistical significance. This impairs comparison of these studies, largely because classification of the muscle fiber type highly depends on the reaction intensity of the staining in different fiber cross sections and therefore may yield considerable variation in the estimation of fiber populations

Differences in mRNA concentrations of insulin-like growth factor-I (IGF-1, all isoforms) and myostatin in rat soleus (SO) and extensor digitorum longus (EDL) muscles (n = 6) (for methods see supplementary section I). a mRNA normalized to total RNA relative to EDL. No differences between SO and EDL were found for IGF-1, but myostatin mRNA was 6.5-fold lower in SO (p < 0.001). b mRNA normalized to muscle tissue weight relative to EDL. IGF-1 expression was 2.5-fold higher in SO, whereas myostatin was 2.8-fold lower in SO (p < 0.002). Asterisks indicate significant difference compared to EDL