We have previously reported chronic low-intensity interval exercise training attenuates fibrosis, impaired cardiac mitochondrial function, and coronary vascular dysfunction in miniature swine with left ventricular (LV) hypertrophy (Emter CA, Baines CP. Am J Physiol Heart Circ Physiol 299: H1348-H1356, 2010; Emter CA, et al. Am J Physiol Heart Circ Physiol 301: H1687-H1694, 2011). The purpose of this study was to test two hypotheses: 1) chronic low-intensity interval training preserves normal myocardial oxygen supply/demand balance; and 2) training-dependent attenuation of LV fibrotic remodeling improves diastolic function in aortic-banded sedentary, exercise-trained (HF-TR), and control sedentary male Yucatan miniature swine displaying symptoms of heart failure with preserved ejection fraction. Pressure-volume loops, coronary blood flow, and two-dimensional speckle tracking ultrasound were utilized in vivo under conditions of increasing peripheral mean arterial pressure and β-adrenergic stimulation 6 mo postsurgery to evaluate cardiac function. Normal diastolic function in HF-TR animals was characterized by prevention of increased time constant of isovolumic relaxation, normal LV untwisting rate, and enhanced apical circumferential and radial strain rate. Reduced fibrosis, normal matrix metalloproteinase-2 and tissue inhibitors of metalloproteinase-4 mRNA expression, and increased collagen III isoform mRNA levels (P < 0.05) accompanied improved diastolic function following chronic training. Exercise-dependent improvements in coronary blood flow for a given myocardial oxygen consumption (P < 0.05) and cardiac efficiency (stroke work to myocardial oxygen consumption, P < 0.05) were associated with preserved contractile reserve. LV hypertrophy in HF-TR animals was associated with increased activation of Akt and preservation of activated JNK/SAPK. In conclusion, chronic low-intensity interval exercise training attenuates diastolic impairment by promoting compliant extracellular matrix fibrotic components and preserving extracellular matrix regulatory mechanisms, preserves myocardial oxygen balance, and promotes a physiological molecular hypertrophic signaling phenotype in a large animal model resembling heart failure with preserved ejection fraction.

Dystrophin mechanically links the costameric cytoskeleton and sarcolemma, yet dystrophin-deficient muscle exhibits abnormalities in cell signaling, gene expression, and contractile function that are not clearly understood. We generated new antibodies specific for cytoplasmic ␥-actin and confirmed that ␥-actin most predominantly localized to the sarcolemma and in a faint reticular lattice within normal muscle cells. However, we observed that ␥-actin levels were increased 10-fold at the sarcolemma and within the cytoplasm of striated muscle cells from dystrophin-deficient mdx mice. Transgenic overexpression of the dystrophin homologue utrophin, or functional dystrophin constructs in mdx muscle, restored ␥-actin to normal levels, whereas ␥-actin remained elevated in mdx muscle expressing nonfunctional dystrophin constructs. We conclude that increased cytoplasmic ␥-actin in dystrophin-deficient muscle may be a compensatory response to fortify the weakened costameric lattice through recruitment of parallel mechanical linkages. However, the presence of excessive myoplasmic ␥-actin may also contribute to altered cell signaling or gene expression in dystrophin-deficient muscle.costamere ͉ muscular dystrophy ͉ sarcolemma D uchenne muscular dystrophy is caused by mutations in the gene encoding dystrophin (1). Dystrophin functions as part of a large complex of sarcolemmal proteins including dystroglycans, sarcoglycans, dystrobrevins, syntrophins, and sarcospan (2, 3). This dystrophin-glycoprotein complex is thought to link the actin-based costameric cytoskeleton with the extracellular matrix and mechanically stabilize the sarcolemma against shear stresses imposed during muscle activity (4). When dystrophin is absent the link between the costamere and sarcolemma is disrupted, resulting in compromised sarcolemma integrity (4).Dystrophin is anchored to the sarcolemma primarily through direct interaction with ␤-dystroglycan (5, 6). Regarding its interaction with costameres, the amino-terminal calponin homology domain and a cluster of basic spectrin repeats within the middle rod domain of dystrophin bind directly to actin filaments (7-10). Cytoplasmic ␥-actin filaments are retained in a costameric pattern on sarcolemma peeled from single myofibers of normal mouse muscle but are absent from all sarcolemma of dystrophin-deficient mdx muscle (11). Thus, dystrophin is necessary for a mechanically strong link between the sarcolemma and costameric actin filaments.Here we peeled sarcolemma from several lines of transgenic mdx mice expressing deletion constructs of dystrophin, and we report that either actin binding domain is sufficient to retain costameric actin on peeled sarcolemma. We generated new polyclonal antibodies (pAbs) and mAbs to cytoplasmic ␥-actin, and we demonstrate that ␥-actin levels are elevated 10-fold in dystrophin-deficient striated muscle. We hypothesize that elevated ␥-actin levels contribute to a compensatory remodeling of the dystrophin-deficient costameric cytoskeleton. Our results also provide the basis for s...

The Frank-Starling relationship provides beat-to-beat regulation of ventricular function by matching ventricular input and output. This review addresses the subcellular mechanisms by which the ventricle adjusts its output (i.e. stroke volume) by changes in end-diastolic volume. The subcellular processes are placed in the context of the four phases of the cardiac cycle with emphasis on the sarcomeric properties that mediate the number of force-generating cross-bridges recruited during pressure development. Additional mechanistic insight is provided regarding the factors that regulate myocyte loaded shortening speeds, which are paramount for dictating ejection volume. Emphasis is placed on the interplay between cross-bridge-induced cooperative activation of the thin filament and cooperative deactivation of the thin filament induced by muscle shortening. The balance of these two properties seems to determine systolic haemodynamics, and how this balance is modulated by sarcomere length, in part, underlies the Frank-Starling relationship.

The Frank-Starling relationship of the heart yields increased stroke volume with greater end-diastolic volume, and this relationship is steeper after beta-adrenergic stimulation. The underlying basis for the Frank-Starling mechanism involves length-dependent changes in both Ca(2+) sensitivity of myofibrillar force and power output. In this study, we tested the hypothesis that PKA-induced phosphorylation of myofibrillar proteins would increase the length dependence of myofibrillar power output, which would provide a myofibrillar basis to, in part, explain the steeper Frank-Starling relations after beta-adrenergic stimulation. For these experiments, adult rat left ventricles were mechanically disrupted, permeabilized cardiac myocyte preparations were attached between a force transducer and position motor, and the length dependence of loaded shortening and power output were measured before and after treatment with PKA. PKA increased the phosphorylation of myosin binding protein C and cardiac troponin I, as assessed by autoradiography. In terms of myocyte mechanics, PKA decreased the Ca(2+) sensitivity of force and increased loaded shortening and power output at all relative loads when the myocyte preparations were at long sarcomere length ( approximately 2.30 mum). PKA had less of an effect on loaded shortening and power output at short sarcomere length ( approximately 2.0 mum). These changes resulted in a greater length dependence of myocyte power output after PKA treatment; peak normalized power output increased approximately 20% with length before PKA and approximately 40% after PKA. These results suggest that PKA-induced phosphorylation of myofibrillar proteins explains, in part, the steeper ventricular function curves (i.e., Frank-Starling relationship) after beta-adrenergic stimulation of the left ventricle.

Key points• An important mechanism in beat-to-beat optimization of heart performance is matching ventricular output with end-diastolic volume, which is known as the Frank-Starling Relationship.• The cellular basis for this regulation involves myofilament length-tension relationships.• We previously showed two populations of length-tension relationships in mammalian left ventricular cardiac myocytes, one steep like fast-twitch skeletal muscle fibres and the other shallow like slow-twitch skeletal muscle fibres, and cardiac myocytes with shallow length-tension relationships shift to steep relationships by protein kinase A-induced myofilament phosphorylation.• The current study investigated the molecular and amino acid residue mechanisms that control length-tension relationships.• The single muscle cell experiments demonstrated that cardiac troponin I phosphorylation at serines 23/24 control length-tension relationships in striated muscle.• This study provides: (i) a mechanism to explain a length dependence of force generation in striated muscle; and (ii) an important target to potentially treat heart disease associated with compromised Frank-Starling relationships.Abstract According to the Frank-Starling relationship, greater end-diastolic volume increases ventricular output. The Frank-Starling relationship is based, in part, on the length-tension relationship in cardiac myocytes. Recently, we identified a dichotomy in the steepness of length-tension relationships in mammalian cardiac myocytes that was dependent upon protein kinase A (PKA)-induced myofibrillar phosphorylation. Because PKA has multiple myofibrillar substrates including titin, myosin-binding protein-C and cardiac troponin I (cTnI), we sought to define if phosphorylation of one of these molecules could control length-tension relationships. We focused on cTnI as troponin can be exchanged in permeabilized striated muscle cell preparations, and tested the hypothesis that phosphorylation of cTnI modulates length dependence of force generation. For these experiments, we exchanged unphosphorylated recombinant cTn into either a rat cardiac myocyte preparation or a skinned slow-twitch skeletal muscle fibre. In all cases unphosphorylated cTn yielded a shallow length-tension relationship, which was shifted to a steep relationship after PKA treatment. Furthermore, exchange with cTn having cTnI serines 23/24 mutated to aspartic acids to mimic phosphorylation always shifted a shallow length-tension relationship to a steep relationship. Overall, these results indicate that phosphorylation of cTnI serines 23/24 is a key regulator of length dependence of force generation in striated muscle.