Contractile properties of muscle fibers from the deep and superficial digital flexors of horses

Creatine Kinase Equilibration and ΔGATP over an Extended Range of Physiological Conditions: Implications for Cellular Energetics, Signaling, and Muscle Performance

In this report, we establish a straightforward method for estimating the equilibrium constant for the creatine kinase reaction (CK Keq″) over wide but physiologically and experimentally relevant ranges of pH, Mg2+ and temperature. Our empirical formula for CK Keq″ is based on experimental measurements. It can be used to estimate [ADP] when [ADP] is below the resolution of experimental measurements, a typical situation because [ADP] is on the order of micromolar concentrations in living cells and may be much lower in many in vitro experiments. Accurate prediction of [ADP] is essential for in vivo studies of cellular energetics and metabolism and for in vitro studies of ATP-dependent enzyme function under near-physiological conditions. With [ADP], we were able to obtain improved estimates of ΔGATP, necessitating the reinvestigation of previously reported ADP- and ΔGATP-dependent processes. Application to actomyosin force generation in muscle provides support for the hypothesis that, when [Pi] varies and pH is not altered, the maximum Ca2+-activated isometric force depends on ΔGATP in both living and permeabilized muscle preparations. Further analysis of the pH studies introduces a novel hypothesis around the role of submicromolar ADP in force generation.

From fibre to function: are we accurately representing muscle architecture and performance?

The size and arrangement of fibres play a determinate role in the kinetic and energetic performance of muscles. Extrapolations between fibre architecture and performance underpin our understanding of how muscles function and how they are adapted to power specific motions within and across species. Here we provide a synopsis of how this 'fibre to function' paradigm has been applied to understand muscle design, performance and adaptation in animals. Our review highlights the widespread application of the fibre to function paradigm across a diverse breadth of biological disciplines but also reveals a potential and highly prevalent limitation running through past studies. Specifically, we find that quantification of muscle architectural properties is almost universally based on an extremely small number of fibre measurements. Despite the volume of research into muscle properties, across a diverse breadth of research disciplines, the fundamental assumption that a small proportion of fibre measurements can accurately represent the architectural properties of a muscle has never been quantitatively tested. Subsequently, we use a combination of medical imaging, statistical analysis, and physics-based computer simulation to address this issue for the first time. By combining diffusion tensor imaging (DTI) and deterministic fibre tractography we generated a large number of fibre measurements (>3000) rapidly for individual human lower limb muscles. Through statistical subsampling simulations of these measurements, we demonstrate that analysing a small number of fibres (n < 25) typically used in previous studies may lead to extremely large errors in the characterisation of overall muscle architectural properties such as mean fibre length and physiological cross-sectional area. Through dynamic musculoskeletal simulations of human walking and jumping, we demonstrate that recovered errors in fibre architecture characterisation have significant implications for quantitative predictions of in-vivo dynamics and muscle fibre function within a species. Furthermore, by applying data-subsampling simulations to comparisons of muscle function in humans and chimpanzees, we demonstrate that error magnitudes significantly impact both qualitative and quantitative assessment of muscle specialisation, potentially generating highly erroneous conclusions about the absolute and relative adaption of muscles across species and evolutionary transitions. Our findings have profound implications for how a broad diversity of research fields quantify muscle architecture and interpret muscle function.

Mandibular muscle troponin of the Florida carpenter ant Camponotus floridanus: extending our insights into invertebrate Ca2+ regulation

Ants use their mandibles for a variety of functions and behaviors. We investigated mandibular muscle structure and function from major workers of the Florida carpenter ant Camponotus floridanus: force-pCa relation and velocity of unloaded shortening of single, permeabilized fibres, primary sequences of troponin subunits (TnC, TnI and TnT) from a mandibular muscle cDNA library, and muscle fibre ultrastructure. From the mechanical measurements, we found Ca²⁺-sensitivity of isometric force was markedly shifted rightward compared with vertebrate striated muscle. From the troponin sequence results, we identified features that could explain the rightward shift of Ca²⁺-activation: the N-helix of TnC is effectively absent and three of the four EF-hands of TnC (sites I, II and III) do not adhere to canonical sequence rules for divalent cation binding; two alternatively spliced isoforms of TnI were identified with the alternatively spliced exon occurring in the region of the IT-arm α-helical coiled-coil, and the N-terminal extension of TnI may be involved in modulation of regulation, as in mammalian cardiac muscle; and TnT has a Glu-rich C-terminus. In addition, a structural homology model was built of C. floridanus troponin on the thin filament. From analysis of electron micrographs, we found thick filaments are almost as long as the 6.8 μm sarcomeres, have diameter of ~ 16 nm, and typical center-to-center spacing of ~ 46 nm. These results have implications for the mechanisms by which mandibular muscle fibres perform such a variety of functions, and how the structure of the troponin complex aids in these tasks.

A Horse of a Different Color?: Tensile Strength and Elasticity of Sloth Flexor Tendons

Tendons must be able to withstand the tensile forces generated by muscles to provide support while avoiding failure. The properties of tendons in mammal limbs must therefore be appropriate to accommodate a range of locomotor habits and posture. Tendon collagen composition provides resistance to loading that contributes to tissue strength which could, however, be modified to not exclusively confer large strength and stiffness for elastic energy storage/recovery. For example, sloths are nearly obligate suspenders and cannot run, and due to their combined low metabolic rate, body temperature, and rate of digestion, they have an extreme need to conserve energy. It is possible that sloths have a tendon "suspensory apparatus" functionally analogous to that in upright ungulates, thus allowing for largely passive support of their body weight below-branch, while concurrently minimizing muscle contractile energy expenditure. The digital flexor tendons from the fore- and hindlimbs of two-toed (Choloepus hoffmanni) and three-toed (Bradypus variegatus) sloths were loaded in tension until failure to test this hypothesis. Overall, tensile strength and elastic (Young's) modulus of sloth tendons were low, and these material properties were remarkably similar to those of equine suspensory "ligaments." The results also help explain previous findings in sloths showing relatively low levels of muscle activation in the digital flexors during postural suspension and suspensory walking.

Coming to grips with life upside down: how myosin fiber type and metabolic properties of sloth hindlimb muscles contribute to suspensory function

Sloths exhibit almost obligatory suspensory locomotion and posture. These behaviors require both strength and fatigue resistance, although we previously found muscle fiber type characteristics in the forelimbs of sloths that belied these initial expectations. Based on locomotor roles of the forelimbs versus hindlimbs in propulsion and braking, respectively, sloth hindlimb musculature should be adapted for force production and energy savings by a near homogeneous expression of slow myosin heavy chain (MHC) fibers. This hypothesis was tested by determining MHC fiber type (%) distribution and energy metabolism in the hindlimbs of three-toed (B. variegatus, N = 5) and two-toed (C. hoffmanni, N = 3) sloths. A primary expression of the slow MHC-1 isoform was found in the hindlimbs of both species. Slow MHC fiber type (%) was significantly greater in the flexors of B. variegatus, whereas expression of fast MHC-2A fibers was significantly greater in the extensors of C. hoffmannni. MHC-1 fibers were largest in cross-sectional area (CSA) and comprised the greatest %CSA in each muscle sampled from both species. Enzyme assays showed elevated activity for anaerobic enzymes (CK and LDH) compared with low-to-moderate activity for aerobic enzymes (3-HAD and CS), and only CK activity was related to body size. These findings emphasize a joint stabilization role by the hindlimbs during suspension, especially in smaller three-toed sloths, and suggest that larger two-toed sloths could have muscles further modified for greater power output and/or prolonged arboreal maneuvering. Moreover, modifications to muscle metabolism rather than MHC expression may be more reflective of functional adaptation in sloth limbs.

Keep calm and hang on: EMG activation in the forelimb musculature of three-toed sloths (Bradypus variegatus)

Sloths exhibit below branch locomotion whereby their limbs are loaded in tension to support the body weight. Suspensory behaviors require both strength and fatigue resistance from the limb flexors; however, skeletal muscle mass of sloths is reduced compared with other arboreal mammals. Although suspensory locomotion demands that muscles are active to counteract the pull of gravity, it is possible that sloths minimize muscle activation and/or selectively recruit slow motor units to maintain support, thus indicating neuromuscular specializations to conserve energy. Electromyography (EMG) was evaluated in a sample of three-toed sloths (Bradypus variegatus; N=6) to test this hypothesis. EMG was recorded at 2000 Hz via fine-wire electrodes implanted into two suites of four muscles in the left forelimb while sloths performed suspensory hanging (SH), suspensory walking (SW) and vertical climbing (VC). All muscles were minimally active for SH. During SW and VC, sloths moved slowly (duty factor: 0.83) and activation patterns were consistent between behaviors; the flexors were activated early and for a large percentage of limb contact, whereas the extensors were activated for shorter burst durations on average and showed biphasic (contact and swing) activity. Muscle activities were maximal for the elbow flexors and lowest for the carpal/digital flexors, and overall activity was significantly greater for SW and VC compared with SH. Wavelet analysis indicated high mean EMG frequencies from the myoelectric intensity spectra coupled with low burst intensities for SH, although the opposite pattern occurred for SW and VC, with the shoulder flexors and elbow flexor, m. brachioradialis, having extremely low mean EMG frequencies that are consistent with recruitment of slow fibers. Collectively, these findings support the hypothesis and suggest that sloths may selectively recruit smaller, fast motor units for suspensory postures but have the ability to offset the cost of force production by recruitment of large, slow motor units during locomotion.

Ontogeny of effective mechanical advantage in eastern cottontail rabbits (Sylvilagus floridanus)

Juvenile animals must survive in the same environment as adults despite smaller sizes, immature musculoskeletal tissues, general ecological naïveté and other limits of performance. Developmental changes in muscle leverage could constitute one mechanism to promote increased performance in juveniles despite ontogenetic limitations. We tested this hypothesis using a holistic dataset on growth and locomotor development in wild eastern cottontail rabbits (Sylvilagus floridanus) to examine ontogenetic changes in hindlimb muscle effective mechanical advantage (EMA). EMA is a dimensionless index of muscle leverage, equal to the quotient of average muscle lever length and the load arm length of the ground reaction force (GRF), effectively representing the magnitude of output force arising from a given muscle force. We found that EMA at the hip and ankle joints, as well as overall hindlimb EMA, significantly declined across ontogeny in S. floridanus, whereas EMA at the knee joint remained unchanged. Ontogenetic decreases in EMA were due to isometric scaling of muscle lever arm lengths alongside positive ontogenetic allometry of GRF load arm lengths - which in turn was primarily related to positive allometry of hindlimb segment lengths. Greater EMA limits the estimated volume of hindlimb extensor muscle that has to be activated in young rabbits, likely mitigating the energetic cost of locomotion and saving metabolic resources for other physiological functions, such as growth and tissue differentiation. An additional examination of limb growth allometry across a diverse sample of mammalian taxa suggests that ontogenetic decreases in limb joint EMA may be a common mammalian trend.

Ontogenetic allometry and architectural properties of the paravertebral and hindlimb musculature in Eastern cottontail rabbits (Sylvilagus floridanus): functional implications for developmental changes in locomotor performance

Due to small body size, an immature musculoskeletal system, and other growth-related limits on performance, juvenile mammals frequently experience a greater risk of predation than their adult counterparts. As a result, behaviorally precocious juveniles are hypothesized to exhibit musculoskeletal advantages that permit them to accelerate rapidly and evade predation. This hypothesis was tested through detailed quantitative evaluation of muscle growth in wild Eastern cottontail rabbits (Sylvilagus floridanus). Cottontail rabbits experience high rates of mortality during the first year of life, suggesting that selection might act to improve performance in growing juveniles. Therefore, it was predicted that muscle properties associated with force and power capacity should be enhanced in juvenile rabbits to facilitate enhanced locomotor performance. We quantified muscle architecture from 24 paravertebral and hindlimb muscles across ontogeny in a sample of n = 29 rabbits and evaluated the body mass scaling of muscle mass (MM), physiological cross-sectional area (PCSA), isometric force (Fmax ), and instantaneous power (Pinst ), along with several dimensionless architectural indices. In contrast to our hypothesis, MM and PCSA for most muscles change with positive allometry during growth by scaling at M b 1.3 and M b 1.1 , respectively, whereas Fmax and Pinst generally scale indistinguishably from isometry, as do the architectural indices tested. However, scaling patterns indicate that the digital flexors and ankle extensors of juvenile S. floridanus have greater capacities for force and power, respectively, than those in adults, suggesting these muscle properties may be a part of several compensatory features that promote enhanced acceleration performance in young rabbits. Overall, our study implies that body size constraints place larger, more mature rabbits at a disadvantage during acceleration, and that adults must develop hypertrophied muscles in order to maintain mechanical similarity in force and power capacities across development. These findings challenge the accepted understanding that juvenile animals are at a performance detriment relative to adults. Instead, for prey-predator interactions necessitating short intervals of high force and power generation relative to body mass, as demonstrated by rapid acceleration of cottontail rabbits fleeing predators, it may be the adults that struggle to keep pace with juveniles.

Cheap labor: myosin fiber type expression and enzyme activity in the forelimb musculature of sloths (Pilosa: Xenarthra)

Sloths are canopy-dwelling inhabitants of American neotropical rainforests that exhibit suspensory behaviors. These abilities require both strength and muscular endurance to hang for extended periods of time; however, the skeletal muscle mass of sloths is reduced, thus requiring modifications to muscle architecture and leverage for large joint torque. We hypothesize that intrinsic muscle properties are also modified for fatigue resistance and predict a heterogeneous expression of slow/fast myosin heavy chain (MHC) fibers that utilize oxidative metabolic pathways for economic force production. MHC fiber type distribution and energy metabolism in the forelimb muscles of three-toed ( Bradypus variegatus, n = 5) and two-toed ( Choloepus hoffmanni, n = 4) sloths were evaluated using SDS-PAGE, immunohistochemistry, and enzyme activity assays. The results partially support our hypothesis by a primary expression of the slow MHC-1 isoform as well as moderate expression of fast MHC-2A fibers, whereas few hybrid MHC-1/2A fibers were found in both species. MHC-1 fibers were larger in cross-sectional area (CSA) than MHC-2A fibers and comprised the greatest percentage of CSA in each muscle sampled. Enzyme assays showed elevated activity for the anaerobic enzymes creatine kinase and lactate dehydrogenase compared with low activity for aerobic markers citrate synthase and 3-hydroxyacetyl CoA dehydrogenase. These findings suggest that sloth forelimb muscles may rely heavily on rapid ATP resynthesis pathways, and lactate accumulation may be beneficial. The intrinsic properties observed match well with suspensory requirements, and these modifications may have further evolved in unison with low metabolism and slow movement patterns as means to systemically conserve energy. NEW & NOTEWORTHY Myosin heavy chain (MHC) fiber type and fiber metabolic properties were evaluated to understand the ability of sloths to remain suspended for extended periods without muscle fatigue. Broad distributions of large, slow MHC-1 fibers as well as small, fast MHC-2A fibers are expressed in sloth forelimbs, but muscle metabolism is generally not correlated with myosin fiber type or body size. Sloth muscles rely on rapid, anaerobic pathways to resist fatigue and sustain force production.

Fiber type composition of epaxial muscles is geared toward facilitating rapid spinal extension in the leaper Galago senegalensis

OBJECTIVES

We hypothesized that the vertical leaper Galago senegalensis will have epaxial extensor muscles with a fast fiber phenotype to facilitate rapid spinal extension during leaping in comparison to the slow-moving quadruped Nycticebus coucang. To test this, we determined the percentage of fiber cross-sectional area (%CSA) devoted to Type 2 fibers in epaxial muscles of G. senegalensis compared to those of N. coucang.

MATERIALS AND METHODS

Immunohistochemistry was used to identify Type 1, Type 2, and hybrid fibers in iliocostalis, longissimus, and multifidus muscles of G. senegalensis (n = 3) and N. coucang (n = 3). Serial muscle sections were used to estimate and compare proportions, cross-sectional areas (CSAs), and %CSAs of Type 1, Type 2, and hybrid fibers between species.

RESULTS

Epaxial muscles of G. senegalensis were comprised predominantly of Type 2 fibers with large CSAs (%CSA range ≈ 83-94%; range of mean CSA = 1,218-1,586 μm2 ). N. coucang epaxial muscles were comprised predominantly Type 1 fibers with large CSAs (%CSA range ≈ 69-77%; range of mean CSA = 983-1,220 μm2 ).

DISCUSSION

The predominance of Type 2 fibers in G. senegalensis epaxial muscles facilitates rapid muscle excursion and spinal extension during leaping, and is consistent with their relatively long muscle fibers. The predominance of Type 1 fibers in N. coucang epaxial muscles may aid in maintaining stable postures during bridging and cantilevering behaviors characteristic of slow-climbing. These histochemical characteristics highlight the major divergent locomotor repertoires of G. senegalensis and N. coucang.

Role of cardiac troponin I carboxy terminal mobile domain and linker sequence in regulating cardiac contraction

Inhibition of striated muscle contraction at resting Ca(2+) depends on the C-terminal half of troponin I (TnI) in thin filaments. Much focus has been on a short inhibitory peptide (Ip) sequence within TnI, but structural studies and identification of disease-associated mutations broadened emphasis to include a larger mobile domain (Md) sequence at the C-terminus of TnI. For Md to function effectively in muscle relaxation, tight mechanical coupling to troponin's core-and thus tropomyosin-is presumably needed. We generated recombinant, human cardiac troponins containing one of two TnI constructs: either an 8-amino acid linker between Md and the rest of troponin (cTnILink8), or an Md deletion (cTnI1-163). Motility assays revealed that Ca(2+)-sensitivity of reconstituted thin filament sliding was markedly increased with cTnILink8 (∼0.9 pCa unit leftward shift of speed-pCa relation compared to WT), and increased further when Md was missing entirely (∼1.4 pCa unit shift). Cardiac Tn's ability to turn off filament sliding at diastolic Ca(2+) was mostly (61%), but not completely eliminated with cTnI1-163. TnI's Md is required for full inhibition of unloaded filament sliding, although other portions of troponin-presumably including Ip-are also necessary. We also confirm that TnI's Md is not responsible for superactivation of actomyosin cycling by troponin.

The functional significance of the last 5 residues of the C-terminus of cardiac troponin I

The C-terminal region of cardiac troponin I (cTnI) is known to be important in cardiac function, as removal of the last 17 C-terminal residues of human cTnI has been associated with myocardial stunning. To investigate the C-terminal region of cTnI, three C-terminal deletion mutations in human cTnI were generated: Δ1 (deletion of residue 210), Δ3 (deletion of residues 208-210), and Δ5 (deletion of residues 206-210). Mammalian two-hybrid studies showed that the interactions between cTnI mutants and cardiac troponin C (cTnC) or cardiac troponin T (cTnT) were impaired in Δ3 and Δ5 mutants when compared to wild-type cTnI. Troponin complexes containing 2-[4'-(iodoacetamido) anilino] naphthalene-6-sulfonic acid (IAANS) labeled cTnC showed that the troponin complex containing cTnI Δ5 had a small increase in Ca(2+) affinity (P < 0.05); while the cTnI Δ1- and Δ3 troponin complexes showed no difference in Ca(2+) affinity when compared to wild-type troponin. In vitro motility assays showed that all truncation mutants had increased Ca(2+) dependent motility relative to wild-type cTnI. These results suggest that the last 5 C-terminal residues of cTnI influence the binding of cTnI with cTnC and cTnT and affect the Ca(2+) dependence of filament sliding, and demonstrate the importance of this region of cTnI.

Frequency dependence of power and its implications for contractile function of muscle fibers from the digital flexors of horses

The digital flexors of horses must produce high force to support the body weight during running, and a need for these muscles to generate power is likely limited during locomotion over level ground. Measurements of power output from horse muscle fibers close to physiological temperatures, and when cyclic strain is imposed, will help to better understand the in vivo performance of the muscles as power absorbers and generators. Skinned fibers from the deep (DDF) and superficial (SDF) digital flexors, and the soleus (SOL) underwent sinusoidal oscillations in length over a range of frequencies (0.5–16 Hz) and strain amplitudes (0.01–0.06) under maximum activation (pCa 5) at 30°C. Results were analyzed using both workloop and Nyquist plot analyses to determine the ability of the fibers to absorb or generate power and the frequency dependence of those abilities. Power absorption was dominant at most cycling frequencies and strain amplitudes in fibers from all three muscles. However, small amounts of power were generated (0.002–0.05 Wkg⁻¹) at 0.01 strain by all three muscles at relatively slow cycling frequencies: DDF (4–7 Hz), SDF (4–5 Hz) and SOL (0.5–1 Hz). Nyquist analysis, reflecting the influence of cross‐bridge kinetics on power generation, corroborated these results. The similar capacity for power generation by DDF and SDF versus lower for SOL, and the faster frequency at which this power was realized in DDF and SDF fibers, are largely explained by the fast myosin heavy chain isoform content in each muscle. Contractile function of DDF and SDF as power absorbers and generators, respectively, during locomotion may therefore be more dependent on their fiber architectural arrangement than on the physiological properties of their muscle fibers.

Equine digital flexor muscles fibers have a relatively large capacity for energy absorption. This physiological property of their muscle fibers may be important to the function of these specialized distal limb muscles during locomotion.