Work and oxygen consumption of isolated right ventricular papillary muscle in experimental pulmonary hypertension

Right-sided myocardial mechanical efficiency (work output/metabolic energy input) in pulmonary hypertension can be severely reduced. We determined the contribution of intrinsic myocardial determinants of efficiency using papillary muscle preparations from monocrotaline-induced pulmonary hypertensive (MCT-PH) rats. The hypothesis was tested that efficiency is reduced by mitochondrial dysfunction in addition to increased activation heat reported previously. Right ventricular (RV) muscle preparations were subjected to 5 Hz sinusoidal length changes at 37°C. Work and suprabasal oxygen consumption (VO2) were measured before and after cross-bridge inhibition by blebbistatin. Cytosolic cytochrome c concentration, myocyte cross-sectional area, proton permeability of the inner mitochondrial membrane (PP IMM), and monoamine oxidase (MAO)-A and glucose 6-phosphate dehydrogenase (G-6-PDH) activities and phosphatidylglycerol (PG) and cardiolipin (CL) contents were determined. Mechanical efficiency ranged from 23 to 11% in control (n = 6) and from 22 to 1% in MCT-PH (n = 15) and correlated with work (r2 = 0.68, P < 0.0001) but not with VO2 (r2 = 0.004, P = 0.7919). VO2 for cross-bridge cycling was proportional to work (r2 = 0.56, P = 0.0005). Blebbistatin-resistant VO2 (r2 = 0.32, P = 0.0167) and IMM PP (r2 = 0.36, P = 0.0110) correlated inversely with efficiency. Together, these variables explained the variance of efficiency (coefficient of multiple determination R2 = 0.79, P = 0.0001). Cytosolic cytochrome c correlated inversely with work (r2 = 0.28, P = 0.0391), but not with efficiency (r2 = 0.20, P = 0.0867). G-6-PDH, MAO-A and PG/CL increased in the RV wall of MCT-PH but did not correlate with efficiency. Reduced myocardial efficiency in MCT-PH is due to activation processes and mitochondrial dysfunction. The variance of work and the ratio of activation heat reported previously and blebbistatin-resistant VO2 are discussed.

Investigation of the molecular motor of muscle: from generating life in a test tube to myosin structure over beers

This article traces 60 years of investigation of the molecular motor of skeletal muscle from the 1940s through the 1990s. It started with the discovery that myosin interaction with actin in the presence of ATP caused shortening of threads of actin and myosin. In 1957, structures protruding from myosin filaments were seen for the first time and called “cross bridges.” A combination of techniques led to the proposal in 1969 of the “swinging-tilting cross bridge” model of contraction. In the early 1980s, a major problem arose when it was shown that a probe attached to the cross bridges did not move during contraction. A spectacular breakthrough came when it was discovered that only the cross bridge was required to support movement in an in vitro motility assay. Next it was determined that single myosin molecules caused the movement of actin filaments in 10-nm steps. The atomic structure of the cross bridge was published in 1993, and this discovery supercharged the muscle field. The cross bridge contained a globular head or motor domain that bound actin and ATP. But the most striking feature was the long tail of the cross bridge surrounded by two subunits of the myosin molecule. This structure suggested that the tail might act as a lever arm magnifying head movement. Consistent with this proposal, genetic techniques that lengthened the lever arm resulted in larger myosin steps. Thus the molecular motor of muscle operated not by the tilting of the globular head of myosin but by tilting of the lever arm generating the driving force for contraction.

Cross-bridge mechanics estimated from skeletal muscles’ work-loop responses to impacts in legged locomotion

Legged locomotion has evolved as the most common form of terrestrial locomotion. When the leg makes contact with a solid surface, muscles absorb some of the shock-wave accelerations (impacts) that propagate through the body. We built a custom-made frame to which we fixated a rat (Rattus norvegicus, Wistar) muscle (m. gastrocnemius medialis and lateralis: GAS) for emulating an impact. We found that the fibre material of the muscle dissipates between 3.5 and $$23\,\upmu \hbox {J}$$ 23 μ J ranging from fresh, fully active to passive muscle material, respectively. Accordingly, the corresponding dissipated energy in a half-sarcomere ranges between 10.4 and $$68\,z\hbox {J}$$ 68 z J , respectively. At maximum activity, a single cross-bridge would, thus, dissipate 0.6% of the mechanical work available per ATP split per impact, and up to 16% energy in common, submaximal, activities. We also found the cross-bridge stiffness as low as $$2.2\,\hbox {pN}\,\hbox {nm}^{-1}$$ 2.2 pN nm - 1 , which can be explained by the Coulomb-actuating cross-bridge part dominating the sarcomere stiffness. Results of the study provide a deeper understanding of contractile dynamics during early ground contact in bouncy gait.

Abstract P338: Myopeptide Improves Contractile Mechanics In A Mouse Model Of Heart Failure Expressing Phosphoablated Cardiac Myosin Binding Protein-C

Rationale: Normal heart function depends on cardiac myosin binding protein-C (cMyBP-C) phosphorylation. Its decrease is associated with heart failure (HF) by inhibiting actomyosin interactions. In absence of cMyBP-C phosphorylation, the protein is bound to myosin S2, but released when phosphorylated, allowing myosin to form cross-bridges with actin. Challenging cMyBP-C/myosin S2 interaction by myopeptide (the first 126 amino acids of myosin S2) could promote actomyosin interaction in vitro , but its ability to improve contractility in HF remains untested. Objective: To test contractile function in skinned papillary fibers of a cMyBP-C dephosphorylated mouse model using myopeptide. Methods and Results: To mimic constitutive phosphoablation, a knock-in mouse model was established to express cMyBP-C in which serines 273, 282 and 302 were mutated to alanine (cMyBP-C AAA ). Western blotting revealed 50% and 100% of cMyBP-C AAA in het and homo mouse hearts, respectively. Echocardiography showed a decreased percentage of ejection fraction (28%, p<0.01) and fractional shortening (30%, p< 0.05) in both het and homo cMyBP-C AAA mice at 3 months of age, compared to knock-in negative controls. These mice also developed diastolic dysfunction with elevated ratio of E/A and E/e’ waves. Next, pCa-force measurements using skinned papillary fibers determined that maximal force (F max ) and rate of cross-bridge formation ( k tr ) were decreased in the cMyBP-C AAA groups, compared to the control. However, administration of dose-dependent myopeptide increased F max and k tr in wild-type and cMyBP-C AAA permeabilized skinned papillary fibers without affecting myofilament Ca 2+ sensitivity. Conclusions: Myopeptide can increase contractile force and rate of cross-bridge formation by releasing cMyBP-C/myosin S2 and promoting actomyosin formation of cross-bridges, thus validating its therapeutic potential.

Effect of Active Lengthening and Shortening on Small-Angle X-ray Reflections in Skinned Skeletal Muscle Fibres

Our purpose was to use small-angle X-ray diffraction to investigate the structural changes within sarcomeres at steady-state isometric contraction following active lengthening and shortening, compared to purely isometric contractions performed at the same final lengths. We examined force, stiffness, and the 1,0 and 1,1 equatorial and M3 and M6 meridional reflections in skinned rabbit psoas bundles, at steady-state isometric contraction following active lengthening to a sarcomere length of 3.0 µm (15.4% initial bundle length at 7.7% bundle length/s), and active shortening to a sarcomere length of 2.6 µm (15.4% bundle length at 7.7% bundle length/s), and during purely isometric reference contractions at the corresponding sarcomere lengths. Compared to the reference contraction, the isometric contraction after active lengthening was associated with an increase in force (i.e., residual force enhancement) and M3 spacing, no change in stiffness and the intensity ratio I1,1/I1,0, and decreased lattice spacing and M3 intensity. Compared to the reference contraction, the isometric contraction after active shortening resulted in decreased force, stiffness, I1,1/I1,0, M3 and M6 spacings, and M3 intensity. This suggests that residual force enhancement is achieved without an increase in the proportion of attached cross-bridges, and that force depression is accompanied by a decrease in the proportion of attached cross-bridges. Furthermore, the steady-state isometric contraction following active lengthening and shortening is accompanied by an increase in cross-bridge dispersion and/or a change in the cross-bridge conformation compared to the reference contractions.

Cardiac mechanical efficiency is preserved in primary cardiac hypertrophy despite impaired mechanical function

Increased heart size is a major risk factor for heart failure and premature mortality. Although abnormal heart growth subsequent to hypertension often accompanies disturbances in mechano-energetics and cardiac efficiency, it remains uncertain whether hypertrophy is their primary driver. In this study, we aimed to investigate the direct association between cardiac hypertrophy and cardiac mechano-energetics using isolated left-ventricular trabeculae from a rat model of primary cardiac hypertrophy and its control. We evaluated energy expenditure (heat output) and mechanical performance (force length work production) simultaneously at a range of preloads and afterloads in a microcalorimeter, we determined energy expenditure related to cross-bridge cycling and Ca2+ cycling (activation heat), and we quantified energy efficiency. Rats with cardiac hypertrophy exhibited increased cardiomyocyte length and width. Their trabeculae showed mechanical impairment, evidenced by lower force production, extent and kinetics of shortening, and work output. Lower force was associated with lower energy expenditure related to Ca2+ cycling and to cross-bridge cycling. However, despite these changes, both mechanical and cross-bridge energy efficiency were unchanged. Our results show that cardiac hypertrophy is associated with impaired contractile performance and with preservation of energy efficiency. These findings provide direction for future investigations targeting metabolic and Ca2+ disturbances underlying cardiac mechanical and energetic impairment in primary cardiac hypertrophy.

Rotating Cooling Performance of Two-Pass Rectangular Channels With Cross Bridge and Oval-Shaped Dimple

In order to adapt to the high-efficiency and low-resistance performances required by the new generation of gas turbine, a new type of two-pass rectangular channel with cross bridge and oval-shaped dimple structure is proposed for internal cooling of blade mid chord region. Firstly, the flow structure, heat transfer and friction characteristics of the novel channels under stationary and rotating conditions are numerically analyzed and compared in detail. Then the effects of cross bridge type/layout and dimple dimension/arrangement on the cooling performance are discussed. And the coupling mechanism of cross bridge, turning bend, oval-shaped dimple and rotation effect is revealed. The results show that the introduction of the cross bridge enables the coolant flow into the second pass in a distributed manner, which weakens the flow aggregation and extrusion in the tip turning bend region, thus the flow structure is optimized. Although the heat transfer is slightly weakened, the friction factor is reduced by 66.3% and 51.4%, and the overall thermal performance is improved by 16.7% and 11.6% (different cases) at most, for stationary and rotating conditions, respectively. The oval-shaped dimple achieves local heat transfer enhancement by controlling flow separation and reattachment. Furthermore, the optimized cross bridge type/layout and dimple dimension/arrangement are also obtained. This research will provide important reference data for the development of high-efficiency mid chord cooling technology for gas turbine blade.

Myosin cross-bridge kinetics slow at longer muscle lengths during isometric contractions in intact soleus from mice

Muscle contraction results from force-generating cross-bridge interactions between myosin and actin. Cross-bridge cycling kinetics underlie fundamental contractile properties, such as active force production and energy utilization. Factors that influence cross-bridge kinetics at the molecular level propagate through the sarcomeres, cells and tissue to modulate whole-muscle function. Conversely, movement and changes in the muscle length can influence cross-bridge kinetics on the molecular level. Reduced, single-molecule and single-fibre experiments have shown that increasing the strain on cross-bridges may slow their cycling rate and prolong their attachment duration. However, whether these strain-dependent cycling mechanisms persist in the intact muscle tissue, which encompasses more complex organization and passive elements, remains unclear. To investigate this multi-scale relationship, we adapted traditional step-stretch protocols for use with mouse soleus muscle during isometric tetanic contractions, enabling novel estimates of length-dependent cross-bridge kinetics in the intact skeletal muscle. Compared to rates at the optimal muscle length ( L o ), we found that cross-bridge detachment rates increased by approximately 20% at 90% of L o (shorter) and decreased by approximately 20% at 110% of L o (longer). These data indicate that cross-bridge kinetics vary with whole-muscle length during intact, isometric contraction, which could intrinsically modulate force generation and energetics, and suggests a multi-scale feedback pathway between whole-muscle function and cross-bridge activity.