Motivated by the need for an analytical tool that can be used routinely to analyze data collected from isolated, detergent-skinned cardiac muscle fibers, we developed a mathematical model for representing the force response to step changes in muscle length (i.e., quick stretch and release). Our proposed model is reasonably simple, consisting of only five parameters representing: (1) the rate constant by which length change–induced distortion of elastic elements is dissipated; (2) the stiffness of the muscle fiber; (3) the amplitude of length-mediated recruitment of stiffness elements; (4) the rate constant by which this length-mediated recruitment takes place; and (5) the magnitude of the nonlinear interaction term by which distortion of elastic elements affects the number of recruited stiffness elements. Fitting this model to a family of force recordings representing responses to eight amplitudes of step length change (±2.0% baseline muscle length in 0.5% increments) enabled four things: (1) reproduction of all the identifiable features seen in a family of force responses to both positive and negative length changes; (2) close fitting of all records from the whole family of these responses with very little residual error; (3) estimation of all five model parameters with a great degree of certainty; and (4) importantly, ready discrimination between cardiac muscle fibers with different contractile regulatory proteins but showing only subtly different contractile function. We recommend this mathematical model as an analytic tool for routine use in studies of cardiac muscle fiber contractile function. Such model-based analysis gives novel insight to the contractile behavior of cardiac muscle fibers, and it is useful for characterizing the mechanistic effects that alterations of cardiac contractile proteins have on cardiac contractile function.

Nebulin is a giant filamentous F-actin-binding protein (ϳ800 kDa) that binds along the thin filament of the skeletal muscle sarcomere. Nebulin is one of the least well understood major muscle proteins. Although nebulin is usually viewed as a structural protein, here we investigated whether nebulin plays a role in muscle contraction by using skinned muscle fiber bundles from a nebulin knock-out (NEB KO) mouse model. We measured force-pCa (؊log[Ca 2؉ ]) and force-ATPase relations, as well as the rate of tension re-development (k tr ) in tibialis cranialis muscle fibers. To rule out any alterations in troponin (Tn) isoform expression and/or status of Tn phosphorylation, we studied fiber bundles that had been reconstituted with bacterially expressed fast skeletal muscle recombinant Tn. We also performed a detailed analysis of myosin heavy chain, myosin light chain, and myosin light chain 2 phosphorylation, which showed no significant differences between wild type and NEB KO. Our mechanical studies revealed that NEB KO fibers had increased tension cost (5.9 versus 4.4 pmol millinewtons ؊1 mm ؊1 s ؊1 ) and reductions in k tr (4.7 versus 7.3 s ؊1 ), calcium sensitivity (pCa 50 5.74 versus 5.90), and cooperativity of activation (n H 3.64 versus 4.38). Our findings indicate the following: 1) in skeletal muscle nebulin increases thin filament activation, and 2) through altering cross-bridge cycling kinetics, nebulin increases force and efficiency of contraction. These novel properties of nebulin add a new level of understanding of skeletal muscle function and provide a mechanism for the severe muscle weakness in patients with nebulin-based nemaline myopathy.Muscle contraction is based on cyclic interactions between the myosin cross-bridges that are part of the thick filaments and actin, the major protein of the thin filament (1). The thin filament contains troponin C (TnC), 2 troponin I (TnI), and troponin T (TnT), which together make up the troponin (Tn) complex. The Tn complex and tropomyosin (Tm) together participate in the calcium-dependent regulation of thin filament activation (2). In addition to Tn and Tm, thin filaments of vertebrate skeletal muscle also contain nebulin (3). Nebulin is a giant filamentous protein that spans the entire thin filament, with its C terminus anchored in the Z-disk and its N-terminal region located near the thin filament pointed end (4, 5). The majority of the nebulin sequence is composed of ϳ35-amino acid modules, with the central module 9 (M9) and module 162 (M162) modules arranged into seven-module super-repeats (6 -8). This arrangement enables a single nebulin module to interact with a single actin monomer, and each nebulin super-repeat to associate with a single Tm⅐Tn complex (8 -10). Previous work has shown that nebulin plays structural roles (11,12), and this work is focused on the role of nebulin in regulating muscle contraction.Evidence for the role of nebulin in regulating thin filament activation has been obtained in the following: 1) in in vitro studies that showed that nebul...

Chandra M, Tschirgi ML, Ford SJ, Slinker BK, Campbell KB. Interaction between myosin heavy chain and troponin isoforms modulate cardiac myofiber contractile dynamics. Am J Physiol Regul Integr Comp Physiol 293: R1595-R1607, 2007. First published July 11, 2007; doi:10.1152/ajpregu.00157.2007.-Coordinated expression of species-specific myosin heavy chain (MHC) and troponin (Tn) isoforms may bring about a dynamic complementarity to match muscle contraction speed with species-specific heart rates. Contractile system function and dynamic force-length measurements were made in muscle fibers from mouse and rat hearts and in muscle fibers after reconstitution with either recombinant homologous Tn or orthologous Tn. The rate constants of length-mediated cross-bridge (XB) recruitment (b) and tension redevelopment (k tr) of mouse fibers were significantly faster than those of rat fibers. Both the tension cost (ATPase/tension) and rate constant of length-mediated XB distortion (c) were higher in the mouse than in the rat. Thus the mouse fiber was faster in all dynamic and functional aspects than the rat fiber. Mouse Tn significantly increased b and k tr in rat fibers; conversely, rat Tn significantly decreased b and k tr in mouse fibers. Thus the lengthmediated recruitment of force-bearing XB occurs much more rapidly in the presence of mouse Tn than in the presence of rat Tn, demonstrating that the speed of XB recruitment is regulated by Tn. There was a significant interaction between Tn and MHC such that changes in either Tn or MHC affected the speed of XB recruitment. Our data demonstrate that the dynamics of myocardial contraction are different in the mouse and rat hearts because of sequence heterogeneity in MHC and Tn. At the myofilament level, coordinated expression of complementary regulatory contractile proteins produces a functional dynamic phenotype that allows the cardiovascular systems to function effectively at different heart rates. myofiber dynamics; contraction speed; heart rate THERE IS SUBSTANTIAL PROTEIN sequence heterogeneity among orthologous cardiac myosin heavy chain (MHC) and troponin (Tn) isoforms across different animal species (30). This sequence heterogeneity in regulatory contractile proteins significantly affects myofilament dynamics, as assessed by the force response to muscle length change in constantly activated cardiac myofibers, which exhibits two clearly separable processes (3,5,20,30,35): 1) a relatively fast force dynamic associated with myosin cross-bridge (XB) distortion and 2) a relatively slow force dynamic associated with recruitment of additional XB into force-bearing states. The dynamics of XB distortion are principally determined by the enzymatic kinetics of MHC, and the dynamics of XB recruitment are affected greatly by cooperative interactions between Tn actions and XB cycling kinetics (3, 5, 6, 30).Our group (9) recently showed that differences in troponin T (TnT), a subunit of the Tn regulatory protein complex, affected the slow XB recruitment dynamic (9), whereas a shift f...

• A handheld MSOT probe enables real-time molecular imaging of the breast. • MSOT of healthy controls provides a reproducible reference for pathology identification. • MSOT parameters allows for differentiation of invasive carcinoma and healthy tissue.

Imaging dynamics at different temporal and spatial scales is essential for understanding the biological complexity of living organisms, disease state and progression. Optoacoustic imaging has been shown to offer exclusive applicability across multiple scales with excellent optical contrast and high resolution in deep-tissue observations. Yet, efficient visualization of multi-scale dynamics remained difficult with state-of-the-art systems due to inefficient trade-offs between image acquisition time and effective field of view. Herein, we introduce the spiral volumetric optoacoustic tomography technique that provides spectrally enriched high-resolution contrast across multiple spatiotemporal scales. In vivo experiments in mice demonstrate a wide range of dynamic imaging capabilities, from three-dimensional high-frame-rate visualization of moving organs and contrast agent kinetics in selected areas to whole-body longitudinal studies with unprecedented image quality. The newly introduced paradigm shift in imaging of multi-scale dynamics adds to the multifarious advantages provided by the optoacoustic technology for structural, functional and molecular imaging.