Mechanics, energetics, and crossbridge kinetics of rabbit diaphragm during congestive heart failure

Thermodynamic bifurcation in anoxic heart: A far-from-equilibrium dissipative structure

Thermodynamic consequences of a three-hour long anoxia were investigated on the isolated mammalian rat myocardium. The anoxic heart operated in a far-from-equilibrium manner as attested by the non-linearity between the thermodynamic force and the thermodynamic flow. When subjected to slight fluctuations due to anoxia, the open far-from-equilibrium cardiac system presented a thermodynamic bifurcation at ~ 60 minutes of anoxia. The bifurcation was characterized by a sudden change of direction in the bifurcation diagram of a one-dimensional nonlinear differential equation with one parameter and occurred at a non-hyperbolic fixed point at which moment the heart lost its thermodynamic stability. The parameter of the differential equation was the single force of the myosin molecular motor. These results helped to reflect a self-organized process and the occurrence of a dissipative structure. This offers valuable insights into our understanding of myocardial protection and could be of considerable interest, especially for heart transplants where the recipient must benefit from the donor's heart in the shortest possible time.

Friction in Myocardial Anoxia Leads to Negative Excess Entropy Production, Self-Organization, and Dissipative Structures

Contraction of the heart is caused by actin filaments sliding along myosin filaments. This generates a frictional force inducing wear of the contractile apparatus. We postulated that this process could be exacerbated when the heart was submitted to severe anoxia. Anoxia induced dramatic abnormalities in the molecular properties of actin-myosin crossbridges. We applied the formalism of far-from-equilibrium thermodynamics to the left ventricular papillary muscles (LVPMs) of mammalian rat hearts which had been subjected to a prolonged anoxia (3 h). We showed that when subjected to prolonged anoxia, the heart operated far-from-equilibrium as evidenced by the non-linearity between thermodynamic force (F/T: Frictional force/Kelvin temperature) and thermodynamic flow (v0: myofilament sliding velocity). The rate of entropy production (EPR) was the product of (F/T) and v0. The excess entropy production (EEP) was equal to ∂δ2S∂t = ∂FTδvo; (S: entropy). The tribological system remained stable when EEP was positive and became unstable when EEP became negative, thus characterizing instability of the system and reflecting the occurrence of self-organization and possibly dissipative structures. After 3 h anoxia, re-oxygenation induced significant reversibility. About 20% of the myosin heads did not recover despite re-oxygenation. These results may be of importance in the context of heart transplantation where the delay between the time of sampling from the donor and the time of the graft installation in the recipient should be as short as possible.

Mechanical and Thermodynamic Properties of Non-Muscle Contractile Tissues: The Myofibroblast and the Molecular Motor Non-Muscle Myosin Type IIA

Myofibroblasts are contractile cells found in multiple tissues. They are physiological cells as in the human placenta and can be obtained from bone marrow mesenchymal stem cells after differentiation by transforming growth factor-β (TGF-β). They are also found in the stroma of cancerous tissues and can be located in non-muscle contractile tissues. When stimulated by an electric current or after exposure to KCl, these tissues contract. They relax either by lowering the intracellular Ca2+ concentration (by means of isosorbide dinitrate or sildenafil) or by inhibiting actin-myosin interactions (by means of 2,3-butanedione monoxime or blebbistatin). Their shortening velocity and their developed tension are dramatically low compared to those of muscles. Like sarcomeric and smooth muscles, they obey Frank-Starling's law and exhibit the Hill hyperbolic tension-velocity relationship. The molecular motor of the myofibroblast is the non-muscle myosin type IIA (NMIIA). Its essential characteristic is the extreme slowness of its molecular kinetics. In contrast, NMIIA develops a unitary force similar to that of muscle myosins. From a thermodynamic point of view, non-muscle contractile tissues containing NMIIA operate extremely close to equilibrium in a linear stationary mode.

STZ-diabetic rat heart maintains developed tension amplitude by increasing sarcomere length and crossbridge density

New Findings

What is the central question of this study?

In the papillary muscle from type I diabetic rats, does diabetes‐associated altered ventricular function result from changes of acto‐myosin interactions and are these modifications attributable to a possible sarcomere rearrangement?

What is the main finding and its importance?

For the first time, we showed that type‐I diabetes altered sarcomeric ultrastructure, as seen by transmission electron microscopy, consistent with physiological parameters. The diabetic condition induced slower timing parameters, which is compatible with a diastolic dysfunction. At the sarcomeric level, augmented β‐myosin heavy chain content and increased sarcomere length and crossbridges' number preserve myocardial stroke and could concur to maintain the ejection fraction.

Abstract

We investigated whether diabetes‐associated altered ventricular function, in a type I diabetes animal model, results from a modification of acto‐myosin interactions, through the in vitro recording of left papillary muscle mechanical parameters and examination of sarcomere morphology by transmission electron microscopy (TEM). Experiments were performed on streptozotocin‐induced diabetic and age‐matched control female Wistar rats. Mechanical isometric and isotonic indexes and timing parameters were determined. Using Huxley's equations, we calculated mechanics, kinetics and energetics of myosin crossbridges. Sarcomere length and A‐band length were measured on TEM images. Type I and III collagen and β‐myosin heavy chain (MHC) expression were determined by immunoblotting. No variation in resting and developed tension or maximum extent of shortening was evident between groups, but diabetic rats showed lower maximum shortening velocity and prolonged timing parameters. Compared to controls, diabetics also displayed a higher number of crossbridges with lower unitary force. Moreover, no change in type I and III collagen was associated to diabetes, but pathological rats showed a two‐fold enhancement of β‐MHC content and longer sarcomeres and A‐band, detected by ultrastructural morphometry. Overall, these data address whether a preserved systolic function accompanied by an altered diastolic phase results from a recruitment of super‐relaxed myosin heads or the phosphorylation of the regulatory light chain site in myosin. Although the early signs of diabetic cardiomyopathy were well expressed, the striking finding of our study was that, in diabetics, sarcomere modification may be a possible compensatory mechanism that preserves systolic function.

Diaphragm abnormalities in heart failure and aging: mechanisms and integration of cardiovascular and respiratory pathophysiology

Inspiratory function is essential for alveolar ventilation and expulsive behaviors that promote airway clearance (e.g., coughing and sneezing). Current evidence demonstrates that inspiratory dysfunction occurs during healthy aging and is accentuated by chronic heart failure (CHF). This inspiratory dysfunction contributes to key aspects of CHF and aging cardiovascular and pulmonary pathophysiology including: (1) impaired airway clearance and predisposition to pneumonia; (2) inability to sustain ventilation during physical activity; (3) shallow breathing pattern that limits alveolar ventilation and gas exchange; and (4) sympathetic activation that causes cardiac arrhythmias and tissue vasoconstriction. The diaphragm is the primary inspiratory muscle; hence, its neuromuscular integrity is a main determinant of the adequacy of inspiratory function. Mechanistic work within animal and cellular models has revealed specific factors that may be responsible for diaphragm neuromuscular abnormalities in CHF and aging. These include phrenic nerve and neuromuscular junction alterations as well as intrinsic myocyte abnormalities, such as changes in the quantity and quality of contractile proteins, accelerated fiber atrophy, and shifts in fiber type distribution. CHF, aging, or CHF in the presence of aging disturbs the dynamics of circulating factors (e.g., cytokines and angiotensin II) and cell signaling involving sphingolipids, reactive oxygen species, and proteolytic pathways, thus leading to the previously listed abnormalities. Exercise-based rehabilitation combined with pharmacological therapies targeting the pathways reviewed herein hold promise to treat diaphragm abnormalities and inspiratory muscle dysfunction in CHF and aging.

Combined XIL-6R and urocortin-2 treatment restores MDX diaphragm muscle force

INTRODUCTION

Duchenne muscular dystrophy (DMD) is characterized by progressive muscle degeneration leading to immobility, respiratory failure, and premature death. As chronic inflammation and stress are implicated in DMD pathology, the efficacy of an anti-inflammatory and anti-stress intervention strategy in ameliorating diaphragm dysfunction was investigated.

METHODS

Diaphragm muscle contractile function was compared in wild-type and dystrophin-deficient mdx mice treated with saline, anti-interleukin-6 receptor antibodies (xIL-6R), the corticotrophin-releasing factor receptor 2 (CRFR2) agonist, urocortin 2, or both xIL-6R and urocortin 2.

RESULTS

Combined treatment with xIL-6R and urocortin 2 rescued impaired force in mdx diaphragms. Mechanical work production and muscle shortening was also improved by combined drug treatment.

DISCUSSION

Treatment which neutralizes peripheral IL-6 signaling and stimulates CRFR2 recovers force-generating capacity and the ability to perform mechanical work in mdx diaphragm muscle. These findings may be important in the search for therapeutic targets in DMD. Muscle Nerve 56: E134-E140, 2017.

Diaphragm Abnormalities in Patients with End-Stage Heart Failure: NADPH Oxidase Upregulation and Protein Oxidation

Patients with heart failure (HF) have diaphragm abnormalities that contribute to disease morbidity and mortality. Studies in animals suggest that reactive oxygen species (ROS) cause diaphragm abnormalities in HF. However, the effects of HF on ROS sources, antioxidant enzymes, and protein oxidation in the diaphragm of humans is unknown. NAD(P)H oxidase, especially the Nox2 isoform, is an important source of ROS in the diaphragm. Our main hypothesis was that diaphragm from patients with HF have heightened Nox2 expression and p47^phox phosphorylation (marker of enzyme activation) that is associated with elevated protein oxidation. We collected diaphragm biopsies from patients with HF and brain-dead organ donors (controls). Diaphragm mRNA levels of Nox2 subunits were increased 2.5–4.6-fold over controls (p < 0.05). Patients also had increased protein levels of Nox2 subunits (p47^phox, p22^phox, and p67^phox) and total p47^phox phosphorylation, while phospho-to-total p47^phox levels were unchanged. The antioxidant enzyme catalase was increased in patients, whereas glutathione peroxidase and superoxide dismutases were unchanged. Among markers of protein oxidation, carbonyls were increased by ~40% (p < 0.05) and 4-hydroxynonenal and 3-nitrotyrosines were unchanged in patients with HF. Overall, our findings suggest that Nox2 is an important source of ROS in the diaphragm of patients with HF and increases in levels of antioxidant enzymes are not sufficient to maintain normal redox homeostasis. The net outcome is elevated diaphragm protein oxidation that has been shown to cause weakness in animals.

Skeletal Muscle Changes in Chronic Cardiac Disease and Failure

Peak exercise performance in healthy man is limited not only by pulmonary or skeletal muscle function but also by cardiac function. Thus, abnormalities in cardiac function will have a major impact on exercise performance. Many cardiac diseases affect exercise performance and indeed for some cardiac conditions such as atherosclerotic heart disease, exercise testing is frequently used not only to measure functional capacity but also to make a diagnosis of heart disease, evaluate the efficacy of treatment, and predict prognosis. Early in the course of cardiac diseases, exercise performance will be minimally affected but with disease progression impairment in exercise capacity will become apparent. Ejection fraction, that is, the percent of blood volume ejected with each cardiac cycle is often used as a measure of cardiac performance but frequently there is a dissociation between the ejection fraction and exercise capacity in patients with heart disease. How abnormalities in cardiac function impacts the muscles, vasculature, and lungs to impact exercise performance will here be reviewed. The focus of this work will be on patients with systolic heart failure as the incidence and prevalence of heart failure is reaching epidemic proportions and heart failure is the end result of many other chronic cardiac diseases. The prognostic role of exercise and benefits of exercise training will also be discussed.

Effects of acute respiratory and metabolic acidosis on diaphragm muscle obtained from rats

BACKGROUND

Acute respiratory acidosis is associated with alterations in diaphragm performance. The authors compared the effects of respiratory acidosis and metabolic acidosis in the rat diaphragm in vitro.

METHODS

Diaphragmatic strips were stimulated in vitro, and mechanical and energetic variables were measured, cross-bridge kinetics calculated, and the effects of fatigue evaluated. An extracellular pH of 7.00 was obtained by increasing carbon dioxide tension (from 25 to 104 mmHg) in the respiratory acidosis group (n = 12) or lowering bicarbonate concentration (from 24.5 to 5.5 mM) in the metabolic acidosis group (n = 12) and the results compared with a control group (n = 12, pH = 7.40) after 20-min exposure.

RESULTS

Respiratory acidosis induced a significant decrease in maximum shortening velocity (-33%, P < 0.001), active isometric force (-36%, P < 0.001), and peak power output (-59%, P < 0.001), slowed relaxation, and decreased the number of cross-bridges (-35%, P < 0.001) but not the force per cross-bridge, and impaired recovery from fatigue. Respiratory acidosis impaired more relaxation than contraction, as shown by impairment in contraction-relaxation coupling under isotonic (-26%, P < 0.001) or isometric (-44%, P < 0.001) conditions. In contrast, no significant differences in diaphragmatic contraction, relaxation, or contraction-relaxation coupling were observed in the metabolic acidosis group.

CONCLUSIONS

In rat diaphragm, acute (20 min) respiratory acidosis induced a marked decrease in the diaphragm contractility, which was not observed in metabolic acidosis.

Comparative statistical mechanics of myosin molecular motors in rat heart, diaphragm and tracheal smooth muscle

PURPOSE

Statistical mechanics establishes a link between microscopic properties of matter and its bulk properties. A. Huxley's equations (1957) [1] provide the necessary phenomenological formalism to use statistical mechanics.

METHODS

We compared statistical mechanics in rat diaphragm in tetanus (tet; n=10) and twitch (tw; n=12) modes, in heart in twitch mode (n=20), and in tracheal smooth muscle in tetanus mode (TSM; n=10). This powerful tool makes it possible to determine: (i) statistical entropy (S) which is related to the dispersal of energy and represents a measure of the degree of disorder in muscular system; (ii) thermodynamic force A/T (chemical affinity A and temperature T); (iii) thermodynamic flow (υ); (iv) entropy production rate (A/T×υ), which quantifies irreversible chemical processes generated by myosin crossbridge (CB) molecular motors.

RESULTS

All muscles studied operated near equilibrium, i.e., A<<2500J/mol and in a stationary linear regime, i.e., A/T varied linearly with υ. The heart operated farther from equilibrium than both diaphragm (tet and tw) and TSM, as attested by its high entropy production rate. S was of the same order of magnitude in heart and TSM but lower in diaphragm (tet and tw).

CONCLUSION

CB kinetics derived from A. Huxley's equations conferred a characteristic profile in terms of statistical mechanics on each muscle type. All studied muscles differed in terms of statistical entropy, chemical affinity, and entropy production rate. Stimulation mode (tet and tw) modulated CB kinetics and statistical mechanics. All muscle types operated near equilibrium and in a stationary linear regime.

Respiratory muscle training improves hemodynamics, autonomic function, baroreceptor sensitivity, and respiratory mechanics in rats with heart failure

Respiratory muscle training (RMT) improves functional capacity in chronic heart-failure (HF) patients, but the basis for this improvement remains unclear. We evaluate the effects of RMT on the hemodynamic and autonomic function, arterial baroreflex sensitivity (BRS), and respiratory mechanics in rats with HF. Rats were assigned to one of four groups: sedentary sham (n = 8), trained sham (n = 8), sedentary HF (n = 8), or trained HF (n = 8). Trained animals underwent a RMT protocol (30 min/day, 5 day/wk, 6 wk of breathing through a resistor), whereas sedentary animals did not. In HF rats, RMT had significant effects on several parameters. It reduced left ventricular (LV) end-diastolic pressure (P < 0.01), increased LV systolic pressure (P < 0.01), and reduced right ventricular hypertrophy (P < 0.01) and pulmonary (P < 0.001) and hepatic (P < 0.001) congestion. It also decreased resting heart rate (HR; P < 0.05), indicating a decrease in the sympathetic and an increase in the vagal modulation of HR. There was also an increase in baroreflex gain (P < 0.05). The respiratory system resistance was reduced (P < 0.001), which was associated with the reduction in tissue resistance after RMT (P < 0.01). The respiratory system and tissue elastance (Est) were also reduced by RMT (P < 0.01 and P < 0.05, respectively). Additionally, the quasistatic Est was reduced after RMT (P < 0.01). These findings show that a 6-wk RMT protocol in HF rats promotes an improvement in hemodynamic function, sympathetic and vagal heart modulation, arterial BRS, and respiratory mechanics, all of which are benefits associated with improvements in cardiopulmonary interaction.

Upregulation of PPARbeta/delta is associated with structural and functional changes in the type I diabetes rat diaphragm

BACKGROUND

Diabetes mellitus is associated with alterations in peripheral striated muscles and cardiomyopathy. We examined diaphragmatic function and fiber composition and identified the role of peroxisome proliferator-activated receptors (PPAR alpha and beta/delta) as a factor involved in diaphragm muscle plasticity in response to type I diabetes.

METHODOLOGY/PRINCIPAL FINDINGS

Streptozotocin-treated rats were studied after 8 weeks and compared with their controls. Diaphragmatic strips were stimulated in vitro and mechanical and energetic variables were measured, cross bridge kinetics assessed, and the effects of fatigue and hypoxia evaluated. Morphometry, myosin heavy chain isoforms, PPAR alpha and beta/delta gene and protein expression were also assessed. Diabetes induced a decrease in maximum velocity of shortening (-14%, P<0.05) associated with a decrease in myosin ATPase activity (-49%, P<0.05), and an increase in force (+20%, P<0.05) associated with an increase in the number of cross bridges (+14%, P<0.05). These modifications were in agreement with a shift towards slow myosin heavy chain fibers and were associated with an upregulation of PPARbeta/delta (+314% increase in gene and +190% increase in protein expression, P<0.05). In addition, greater resistances to fatigue and hypoxia were observed in diabetic rats.

CONCLUSIONS/SIGNIFICANCE

Type I diabetes induced complex mechanical and energetic changes in the rat diaphragm and was associated with an up-regulation of PPARbeta/delta that could improve resistance to fatigue and hypoxia and favour the shift towards slow myosin heavy chain isoforms.

Enhancement of shortening velocity, power, and acto-myosin crossbridge (CB) kinetics following long-term treatment with propionyl-L-carnitine, coenzyme Q10, and omega-3 fatty acids in BIO TO-2 cardiomyopathic Syrian hamsters papillary muscle

Impaired functions of myocardial muscle cells in human and animals, is a primary defect associated with idiopathic dilated cardiomyopathy (DCM). The pathophysiological mechanisms implicated in the DCM are yet to be clarified and an effective therapy is still not available. The BIO TO-2 cardiomyopathic Syrian Hamsters (CMSHs) represent an animal model of idiopathic DCM. The aim of this study was to investigate the effect of long-term treatment (2 months) with propionyl-L-carnitine (PLC), coenzyme Q(10), omega-3 fatty acids and a combination of these three agents (formulation HS12607) on mechanical properties and acto-myosin crossbridges (CBs) kinetics of left ventricular (LV) papillary muscle from control and treated 10 month old BIO TO-2 CMSHs. Isometric and isotonic contractile properties of isolated papillary muscle from control and treated CMSHs were investigated, and acto-myosin CB number, force and kinetics were calculated using Huxley's equations. Mechanical parameter values were higher in treated than in control hamsters, particularly when substances were administered together in a coformulation (HS12607). Compared to control, HS12607-treated papillary muscles showed a significant increase of maximum peak isometric tension (P(o)) (30.06 +/- 4.91 vs. 19.74 +/- 5.00 mN/mm(2)), maximum extent of muscle shortening (0.13 +/- 0.03 vs. 0.07 +/- 0.02 L/L(max)), maximum unloaded shortening velocity (1.18 +/- 0.24 vs. 0.53 +/- 0.13 L/L(max) s(-1)) and maximum peak of power output (5.52 +/- 1.61 vs. 1.58 +/- 0.83). The curvature of the hyperbolic force-velocity relationships did not differ between control and treated hamsters. When compared to controls, acto-myosin CB number increased in treated hamsters [(6.67 +/- 1.91) 10(10)/mm(2) vs. (3.55 +/- 2.08) 10(10)/mm(2)], whereas the unitary force of single CB was similar in control and treated animals. The peak value of the rate constant for CB attachment (f(1)) and detachment (g(2)) was higher in treated animals when compared to control. (93.87 +/- 25.82 vs.47.28 +/- 10.88 s(-1) and 214.40 +/- 44.64 vs. 95.56 +/- 23.49 s(-1), respectively). In conclusion, the present study illustrates that supplementation with PLC, CoQ(10) and omega-3 fatty acids improved motor parameters, energetic, and CB kinetics of BIO TO-2 CMSH papillary muscle indicating that these naturally occurring substances may be a valid adjuvant to conventional therapy in DCM.

Use of arginine-glycine-aspartic acid adhesion peptides coupled with a new collagen scaffold to engineer a myocardium-like tissue graft

BACKGROUND

Cardiac tissue engineering might be useful in treatment of diseased myocardium or cardiac malformations. The creation of functional, biocompatible contractile tissues, however, remains challenging. We hypothesized that coupling of arginine-glycine-aspartic acid-serine (RGD+) adhesion peptides would improve cardiomyocyte viability and differentiation and contractile performance of collagen-cell scaffolds.

METHODS

Clinically approved collagen scaffolds were functionalized with RGD+ cells and seeded with cardiomyocytes. Contractile performance, cardiomyocyte viability and differentiation were analyzed at days 1 and 8 and/or after culture for 1 month.

RESULTS

The method used for the RGD+ cell-collagen scaffold coupling enabled the following features: high coupling yields and complete washout of excess reagent and by-products with no need for chromatography; spectroscopic quantification of RGD+ coupling; a spacer arm of 36 A, a length reported as optimal for RGD+-peptide presentation and favorable for integrin-receptor clustering and subsequent activation. Isotonic and isometric mechanical parameters, either spontaneous or electrostimulated, exhibited good performance in RGD+ constructs. Cell number and viability was increased in RGD+ scaffolds, and we saw good organization of cell contractile apparatus with occurrence of cross-striation.

CONCLUSIONS

We report a novel method of engineering a highly effective collagen-cell scaffold based on RGD+ peptides cross-linked to a clinically approved collagen matrix. The main advantages were cell contractile performance, cardiomyocyte viability and differentiation.

Impaired isotonic contractility and structural abnormalities in the diaphragm of congestive heart failure rats

BACKGROUND

Metabolic alterations and decreased isometric force generation have been demonstrated in different animal models for congestive heart failure (CHF). However, as few morphological examinations have been performed on the CHF diaphragm, it is unknown if structural abnormalities comprise a substrate for diaphragm dysfunction in CHF. Therefore, we investigated CHF diaphragm isometric and isotonic contractility together with the presence of structural abnormalities.

METHODS

Isometric twitch (P(t)) and maximal (P(o)) force, shortening velocity and power generation were determined in diaphragm bundles from rats with CHF, induced by myocardial infarction, and sham-operated rats. Immunofluorescence staining of myosin and sarcolemmal components fibronectin, laminin and dystrophin was performed on diaphragm cryosections. Electron microscopy was used to study the ultrastructure of diaphragm fibres.

RESULTS

P(t) and P(o) were respectively approximately 30% and approximately 20% lower in CHF diaphragm bundles than sham. Maximal shortening velocity was reduced by approximately 20% and maximal power generation by approximately 35%. Structural abnormalities were frequently observed in CHF diaphragm fibres and were mainly marked by focal degradation of sarcomeric constituents and expansion of intermyofibrillar spaces with swollen and degenerated mitochondria. Immunofluorescence microscopy showed reduced staining intensities of myosin in CHF diaphragm fibres compared to sham. No differences were found regarding the distribution of fibronectin, laminin and dystrophin, indicating an intact sarcolemma in both groups.

CONCLUSION

This study demonstrates impaired isometric and isotonic contractility together with structural abnormalities in the CHF diaphragm. The sarcolemma of CHF diaphragm fibres appeared to be intact, excluding a role for sarcolemmal injuries in the development of CHF diaphragm dysfunction.

Cardiac Myosin-binding protein C modulates the tuning of the molecular motor in the heart

Cardiac myosin binding protein C (cMyBP-C) is an important regulator of cardiac contractility. Its precise effect on myosin cross-bridges (CBs) remains unclear. Using a cMyBP-C(-/-) mouse model, we determined how cMyBP-C modulates the cyclic interaction of CBs with actin. From papillary muscle mechanics, CB characteristics were provided using A. F. Huxley's equations. The probability of myosin being weakly bound to actin was higher in cMyBP-C(-/-) than in cMyBP-C(+/+). However, the number of CBs in strongly bound, high-force generated state and the force generated per CB were lower in cMyBP-C(-/-). Overall CB cycling and the velocity of CB tilting were accelerated in cMyBP-C(-/-). Taking advantage of the presence of cMyBP-C in cMyBP-C(+/+) myosin solution but not in cMyBP-C(-/-), we also analyzed the effects of cMyBP-C on the myosin-based sliding velocity of actin filaments. At baseline, sliding velocity and the relative isometric CB force, as determined by the amount of alpha-actinin required to arrest thin filament motility, were lower in cMyBP-C(-/-) than in cMyBP-C(+/+). cAMP-dependent protein kinase-mediated cMyBP-C phosphorylation further increased the force produced by CBs. We conclude that cMyBP-C prevents inefficient, weak binding of the myosin CB to actin and has a critical effect on the power-stroke step of the myosin molecular motor.

Minimum alveolar concentration of halogenated volatile anaesthetics in left ventricular hypertrophy and congestive heart failure in rats

BACKGROUND

Although many physiological and pathological conditions affect minimal alveolar concentration (MAC), there are no reliable data on the MAC for halogenated anaesthetics during left ventricular hypertrophy (LVH) and congestive heart failure (CHF). The aim of this experimental study was to determine the MAC values of halothane, isoflurane, and sevoflurane in rats, at early and later stages of cardiomyopathic hypertrophy.

METHODS

LVH was induced by ascending aortic stenosis in 3-4-week-old rats. LVH and CHF in each animal were assessed weekly by echocardiography. MAC of halothane, isoflurane, and sevoflurane was determined using the tail-clamp technique in spontaneously breathing rats from each group. Response vs no-response data were analysed using logistic regression analysis. Data are medians (95% confidence interval).

RESULTS

The MAC of halothane [1.30% (1.26-1.34)], isoflurane [1.52% (1.48-1.57)], and sevoflurane [2.93% (2.78-3.07)] in rats with LVH was not different from sham-operated rats [respectively, 1.23% (1.20-1.26), 1.52% (1.47-1.56), and 2.90% (2.79-3.00)]. Conversely, the MAC of halothane [1.44 (1.39-1.50)] and isoflurane [1.74 (1.69-1.78)], but not sevoflurane [2.99 (2.93-3.06)], was significantly increased in rats with CHF.

CONCLUSIONS

MAC values for halothane, isoflurane, and sevoflurane were unchanged in rats with pressure-induced overload LVH. Conversely, the MAC for halothane and isoflurane, but not sevoflurane, was significantly increased in rats with CHF.

Oxidative stress of myosin contributes to skeletal muscle dysfunction in rats with chronic heart failure

Intrinsic muscle abnormalities affecting skeletal muscle are often reported during chronic heart failure (CHF). Because myosin is the molecular motor of force generation, we sought to determine whether its dysfunction contributes to skeletal muscle weakness in CHF and, if so, to identify the underlying causative factors. Severe CHF was induced in rats by aortic stenosis. In diaphragm and soleus muscles, we investigated in vitro mechanical performance, myosin-based actin filament motility, myosin heavy (MHC) and light (MLC) chain isoform compositions, MLC integrity, caspase-3 activation, and oxidative damage. Diaphragm and soleus muscles from CHF exhibited depressed mechanical performance. Myosin sliding velocities were 16 and 20% slower in CHF than in sham in diaphragm (1.9 ± 0.1 vs. 1.6 ± 0.1 μm/s) and soleus (0.6 ± 0.1 vs. 0.5 ± 0.1 μm/s), respectively (each P < 0.05). The ratio of slow-to-fast myosin isoform did not differ between sham and CHF. Immunoblots with anti-MLC antibodies did not detect the presence of protein fragments, and no activation of caspase-3 was evidenced. Immunolabeling revealed oxidative damage in CHF muscles, and MHC was the main oxidized protein. Lipid peroxidation and expression of oxidized MHC were significantly higher in CHF than in shams. In vitro myosin exposure to increasing ONOO−concentrations was associated with an increasing amount of oxidized MHC and a reduced myosin velocity. These data provide experimental evidence that intrinsic myosin dysfunction occurs in CHF and may be related to oxidative damage to myosin.

Statistical mechanics of myosin molecular motors in skeletal muscles

Statistical mechanics provides the link between microscopic properties of matter and its bulk properties. The grand canonical ensemble formalism was applied to contracting rat skeletal muscles, the soleus (SOL, n = 30) and the extensor digitalis longus (EDL, n = 30). Huxley's equations were used to calculate force (pi) per single crossbridge (CB), probabilities of six steps of the CB cycle, and peak muscle efficiency (Eff(max)). SOL and EDL were shown to be in near-equilibrium (CB cycle affinity 2.5 kJ/mol) and stationary state (linearity between CB cycle affinity and myosin ATPase rate). The molecular partition function (z) was higher in EDL (1.126+/-0.005) than in SOL (1.050+/-0.003). Both pi and Eff(max) were lower in EDL (8.3+/-0.1 pN and 38.1+/-0.2%, respectively) than in SOL (9.2+/-0.1 pN and 42.3+/-0.2%, respectively). The most populated step of the CB cycle was the last detached state (D3) (probability P(D3): 0.890+/-0.004 in EDL and 0.953+/-0.002 in SOL). In each muscle group, both pi and Eff(max) linearly decreased with z and statistical entropy and increased with P(D3). We concluded that statistical mechanics and Huxley's formalism provided a powerful combination for establishing an analytical link between chemomechanical properties of CBs, molecular partition function and statistical entropy.

Changes in crossbridge mechanical properties in diabetic rat cardiomyopathy

Diabetes mellitus is associated with an increased risk of heart failure, resulting from a specific cardiomyopathy independent of coronary atherosclerosis. It is not yet established whether altered myocardial function is related to changes in molecular mechanics of myosin. Accordingly, we investigated the total number, single force and kinetics of myosin crossbridges (CB) in a rat model of streptozotocin-induced diabetic cardiomyopathy. Experiments were conducted on left ventricular papillary muscles from male diabetic (D) Wistar (n = 16) and age-matched control (C) rats (n = 15). Mechanical indices including the maximum unloaded shortening velocity V(max) and the maximum total isometric tension normalized per cross-sectional area TF(max) were determined. Using A. F. Huxley's equations, we calculated the total cycling CB number per mm(2) Psi, the elementary force per single CB Pi, the maximum values of the rate constant for CB attachment f(1) and detachment g(1) and g(2), and the turnover rate of myosin ATPase per site k(cat). The D rats exhibited a 25% decrease in TF(max) and a 34% decrease in V(max) as compared to C. This contractile dysfunction was associated with a significant reduction in Psi (9.0 +/- 1.6 in D versus 11.4 +/- 1.9 10(9)mm(-2) in C, P < 0.001) without significant change in Pi (6.1 +/- 0.8 in D versus 6.3 +/- 0.9 pN in C, NS). In the 2 groups, TF(max) correlated positively with Psi (r = 0.76, P < 0.001 and r = 0.64, P < 0.01, in D and C respectively) but no relationship was found between TF(max) and Pi. As compared to C, D showed lower values of f(1), g(1) and g(2), and a slower turnover rate of myosin ATPase. Thus, present data suggested that the cardiac contractile impairment observed in streptozotocin-induced diabetic rat cardiomyopathy was mainly related to a decrease in active CB total number and CB kinetics alterations without significant change in CB single force.

Mechanical properties of tracheal smooth muscle are impaired in the rabbit with experimental cardiac pressure overload

STUDY OBJECTIVES/DESIGN: Impaired function of striated and arterial smooth muscle is known to occur in humans and animals with various forms of cardiac diseases, but limited information is available on the mechanical behavior of airway smooth muscle. We tested the hypothesis that the baseline mechanical properties of tracheal smooth muscle (TSM) were impaired at an early stage of cardiac overload.

ANIMALS

We used a model of cardiac hypertrophy induced by surgical abdominal aortic stenosis (AS) in adult rabbits. Twelve animals with AS and 8 sham-operated control rabbits were studied 12 weeks after surgery. In rabbits with AS, the heart weight/body weight ratio was higher than in control rabbits (2.36 +/- 0.43 g/kg vs 1.98 +/- 0.20 g/kg, p < 0.05) [mean +/- SD], attesting to moderate cardiac hypertrophy. No clinical signs of congestive heart failure were observed.

MEASUREMENTS

Isolated TSM strips were electrically stimulated at 37 degrees C, 2.5 mM [Ca(2+)](0), against 8 to 10 load levels, from zero load to full isometry. Force-velocity relationship was elicited using the conventional afterloaded isotonic method.

RESULTS

Peak isometric tension was lower in rabbits with AS than in control rabbits (25 +/- 11 mN/mm(2) vs 34 +/- 14 mN/mm(2), p < 0.05), whereas maximum unloaded shortening velocity, maximum extent of muscle shortening, and relaxation parameters did not differ between groups. The curvature of the force-velocity relationship (which reflects the myothermal economy of force generation) and peak mechanical efficiency were lower in rabbits with AS than in control rabbits.

CONCLUSIONS

These results indicate that the contraction of isolated rabbit TSM was less powerful and less economical in cardiac hypertrophy, attesting to early impairment of the mechanical properties of TSM during cardiac overload.

Effects of halogenated anaesthetics on diaphragmatic actin-myosin cross-bridge kinetics

BACKGROUND

The effects of halogenated anaesthetics on cross-bridge (CB) kinetics are unclear. As halogenated anaesthetics do not markedly modify the intracellular calcium transient in the diaphragm, we used an isolated rat diaphragm preparation to assess the effects of halothane and isoflurane on CB kinetics.

METHODS

The effects of halothane and isoflurane (1 and 2 minimum alveolar concentration (MAC)) on rat diaphragm muscle strips were studied in vitro (Krebs-Henseleit solution, 29 degrees C, oxygen 95%/carbon dioxide 5%) in tetanus mode (50 Hz). From the force-velocity curve and using A. F. Huxley's equations, we determined the main mechanical and energetic variables and calculated CB kinetics.

RESULTS

At 1 and 2 MAC, isoflurane and halothane induced no significant inotropic effects. Whatever the concentrations tested, halothane and isoflurane did not significantly modify the CB number, the elementary force per CB, the attachment and detachment constants, the duration of the CB cycle and mean CB velocity.

CONCLUSION

In the rat diaphragm at therapeutic concentrations, halogenated anaesthetics do not significantly modify CB mechanical and kinetic properties.

Severe muscle dysfunction precedes collagen tissue proliferation in mdx mouse diaphragm

After extensive necrosis, progressive diaphragm muscle weakness in the mdx mouse is thought to reflect progressive replacement of contractile tissue by fibrosis. However, little has been documented on diaphragm muscle performance at the stage at which necrosis and fibrosis are limited. Diaphragm morphometric characteristics, muscle performance, and cross-bridge (CB) properties were investigated in 6-wk-old control (C) and mdx mice. Compared with C, maximum tetanic tension and shortening velocity were 37 and 32% lower, respectively, in mdx mice (each P < 0.05). The total number of active CB per millimeter squared (13.0 +/- 1.2 vs. 18.4 +/- 1.7 x 10(9)/mm(2), P < 0.05) and the CB elementary force (8.0 +/- 0.2 vs. 9.0 +/- 0.1 pN, P < 0.01) were lower in mdx than in C. The time cycle duration was lower in mdx than in C (127 +/- 18 vs. 267 +/- 61 ms, P < 0.05). Percentages of fiber necrosis represented 2.8 +/- 0.6% of the total muscle fibers, and collagen surface area occupied 3.6 +/- 0.7% in mdx diaphragm. Our results pointed to severe muscular dysfunction in mdx mouse diaphragm, despite limited necrotic and fibrotic lesions.

Effects of postnatal maturation on energetics and cross-bridge properties in rat diaphragm

The effects of maturation on cross-bridge (CB) properties were studied in rat diaphragm strips obtained at postnatal days 3, 10, and 17 and in adults (10-12 wk old). Calculations of muscle energetics and characteristics of CBs were determined from standard Huxley equations. Maturation did not change the curvature of the force-velocity relationship or the peak of mechanical efficiency. There was a significant increase in the total number of CBs per cross-sectional area (m) with aging but not in single CB force. The turnover rate of myosin ATPase increased, the duration of the CB cycle decreased, and the velocity of CBs decreased significantly only after the first week postpartum. There was a linear relationship between maximum total force and m (r = 0.969, P < 0.001), and between maximum unloaded shortening velocity and m (r = 0.728, P < 0.001). When this study in the rat and previous study in the hamster are compared, it appears that there are few species differences in the postnatal maturation process of the diaphragm.

Velocity of actomyosin sliding in vitro is reduced in dystrophic mouse diaphragm

It has recently been suggested that dystrophin deficiency in mdx diaphragm muscle is associated with quantitative changes in the myosin molecular motor. In vitro motility assays were used to study the kinetics of actomyosin interactions between purified actin filaments and myosin molecules. Monomeric myosin was obtained from the diaphragm and limb (semitendinosus) muscles of 9-mo-old male mdx (mdx) and age-matched control mice. The sliding velocity (vo, microm/s) of fluorescent-labeled actin filaments moving over a myosin-coated surface (40 microg/ml) was measured. In diaphragm, vo was significantly slower in mdx than in control mice (1.2 +/- 0.1 microm s(-1) versus 1.9 +/- 0.1 microm s(-1), p < 0.001). Conversely, there was no significant difference in vo between control and mdx semitendinous muscles (2.4 +/- 0.1 microm s(-1) versus 2.5 +/- 0.1 micro(-1)). As compared with control mice, mdx diaphragm exhibited a shift from IIX-MHC to IIA-MHC (p < 0.001) and a reduction in IIB-MHC (p < 0.01). Semitendinous muscle from control and mdx mice contained almost exclusively type IIB MHC. Our results are in good agreement with the proposal that myosin is altered in dystrophic mouse diaphragm.

Myosin cross-bridge kinetics in airway smooth muscle: a comparative study of humans, rats, and rabbits

To analyze the kinetics and unitary force of cross bridges (CBs) in airway smooth muscle (ASM), we proposed a new formalism of Huxley's equations adapted to nonsarcomeric muscles (Huxley AF. Prog Biophys Biophys Chem 7: 255-318, 1957). These equations were applied to ASM from rabbits, rats, and humans (n = 12/group). We tested the hypothesis that species differences in whole ASM mechanics were related to differences in CB mechanics. We calculated the total CB number per square millimeter at peak isometric tension (Psi x10(9)), CB unitary force (Pi), and the rate constants for CB attachment (f(1)) and detachment (g(1) and g(2)). Total tension, Psi, and Pi were significantly higher in rabbits than in humans and rats. Values of Pi were 8.6 +/- 0.1 pN in rabbits, 7.6 +/- 0.3 pN in humans, and 7.7 +/- 0.2 pN in rats. Values of Psi were 4.0 +/- 0.5 in rabbits, 1.2 +/- 0.1 in humans, and 1.9 +/- 0.2 in rats; f(1) was lower in humans than in rabbits and rats; g(2) was higher in rabbits than in rats and in rats than in humans. In conclusion, ASM mechanical behavior of different species was characterized by specific CB kinetics and CB unitary force.

Myosin cross bridges in skeletal muscles: "rower" molecular motors

Different classes of molecular motors, "rowers" and "porters," have been proposed to describe the chemomechanical transduction of energy. Rowers work in large assemblies and spend a large percentage of time detached from their lattice substrate. Porters behave in the opposite way. We calculated the number of myosin II cross bridges (CB) and the probabilities of attached and detached states in a minimal four-state model in slow (soleus) and fast (diaphragm) mouse skeletal muscles. In both muscles, we found that the probability of CB being detached was approximately 98% and the number of working CB was higher than 10(9)/mm(2). We concluded that muscular myosin II motors were classified in the category of rowers. Moreover, attachment time was higher than time stroke and time for ADP release. The duration of the transition from detached to attached states represented the rate-limiting step of the overall attached time. Thus diaphragm and soleus myosins belong to subtype 1 rowers.

Dual influence of disease and increased load on diaphragm muscle in heart failure

We have recently shown that mitochondrial function and energy metabolism are altered in the myocardium as well as in slow and fast locomotor muscles of rats subjected to prolonged congestive heart failure (CHF) suggesting a generalized metabolic myopathy in heart failure. Here, we investigate whether the diaphragm of CHF animals, which experiences both increased work and the general systemic influence of heart failure, will also be susceptible to altered energy metabolism. Biopsies were obtained from the costal diaphragm of failing rats 8 months after aortic banding. A marked increase in type I and type IIa myosin heavy chains at the expense of types IIx and IIb, suggests an adaptation towards a slower phenotype. Glycolytic enzymes decreased in CHF diaphragm with an increase in the H:M lactate dehydrogenase isoenzyme ratio. These results suggest a reorientation of the diaphragm muscle towards a slow, fatigue-resistant phenotype. However, maximal oxidative capacity assessed in saponin-permeabilized fibers in the presence of ADP was considerably reduced in CHF diaphragm (7.7+/-0.4 v 11.8+/-0.7 micromol O2/min/g dry weight in sham P<0.001), suggesting an alteration in oxidative phosphorylation. Furthermore, ADP sensitivity of CHF mitochondria was significantly increased (apparent Km for ADP 308+/-21 v 945+/-106 microM in sham P<0.001), whereas sensitivity to ADP in the presence of creatine was comparable (Km 79+/-12 v 90+/-11 microM in sham). In heart failure, therefore, the diaphragm muscle seems to adapt towards a more slow and economical contraction as a result of increased workload, but this adaptation is limited by the disease-induced altered mitochondrial function.

Gene-environment interactions and the response to exercise

Many of the symptoms of heart failure (breathlessness and fatigue) are not primarily due to reduced cardiac output, but relate to an impairment of peripheral muscle performance and metabolic efficiency. With regular training it is possible to increase skeletal muscle performance through improvements in muscle efficiency. Recent data suggest that such improvements may be modulated by local tissue renin-angiotensin systems and, in particular, by the local activity of angiotensin-converting enzyme (ACE). These findings might explain the remarkable benefits of ACE inhibition in the treatment of heart failure.

Myosin molecular motor dysfunction in dystrophic mouse diaphragm

Cross-bridge properties and myosin heavy chain (MHC) composition were investigated in isolated diaphragm from 6-mo-old control (n = 12) and mdx (n = 12) mice. Compared with control, peak tetanic tension fell by 50% in mdx mice (P < 0.001). The total number of cross bridges per square millimeter (x10(9)), the elementary force per cross bridge, and the peak mechanical efficiency were lower in mdx than in control mice (each P < 0.001). The duration of the cycle and the rate constant for cross-bridge detachment were significantly lower in mdx than in control mice. In the overall population, there was a linear relationship between peak tetanic tension and either total number of cross bridges per square millimeter or elementary force per cross bridge (r = 0.996 and r = 0.667, respectively, each P < 0.001). The mdx mice presented a higher proportion of type IIA MHC (P < 0.001) than control mice and a reduction in type IIX MHC (P < 0.001) and slow myosin isoforms (P < 0.01) compared with control mice. We concluded that, in mdx mice, impaired diaphragm strength was associated with qualitative and quantitative changes in myosin molecular motors. It is proposed that reduced force generated per cross bridge contributed to diaphragm weakness in mdx mice.

Relaxation of diaphragm muscle

Relaxation is the process by which, after contraction, the muscle actively returns to its initial conditions of length and load. In rhythmically active muscles such as diaphragm, relaxation is of physiological importance because diaphragm must return to a relatively constant resting position at the end of each contraction-relaxation cycle. Rapid and complete relaxation of the diaphragm is likely to play an important role in adaptation to changes in respiratory load and breathing frequency. Regulation of diaphragm relaxation at the molecular and cellular levels involves Ca(2+) removal from the myofilaments, active Ca(2+) pumping by the sarcoplasmic reticulum (SR), and decrease in the number of working cross bridges. The relative contribution of these mechanisms mainly depends on sarcomere length, muscle tension, and the intrinsic contractile function. Increased capacity of SR to take up Ca(2+) can arise from increased density of active SR pumping sites or in slow-twitch fibers from phosphorylation of phospholamban, whereas impaired coupling between ATP hydrolysis and Ca(2+) transport into the SR or intracellular acidosis reduces SR Ca(2+) pump activity. In experimental conditions of decreased contractile performance, slowed, enhanced, or unchanged relaxation rates have been reported in vitro. In vivo, a slowing in the rate of decline of the respiratory pressure is generally considered an early reliable index of respiratory muscle fatigue. Impaired relaxation rate may, in turn, favor mismatch between blood flow and metabolic demand, especially at high breathing frequencies.

Impaired load dependence of diaphragm relaxation during congestive heart failure in the rabbit

The load dependence (LD) of relaxation was studied in the diaphragm of rabbits with congestive heart failure (CHF). CHF (n = 15) was induced by combined chronic volume and pressure overload. Aortic insufficiency was induced by forcing a catheter through the aortic sigmoid valves, followed 3 wk later by abdominal aortic stenosis. Six weeks after the first intervention, animals developed CHF. Sham-operated animals served as controls (C; n = 12). Diaphragm mechanics were studied in vitro on isolated strips, at 22 degrees C, in isotonic and isometric loading conditions. Contractility was lower in the CHF group, as reflected by lower total tension: 1.11 +/- 0.10 in CHF vs. 2.38 +/- 0.15 N/cm(2) in C in twitch (P < 0.001) and 2.46 +/- 0.22 in CHF vs. 4.90 +/- 0.25 N. cm(-2) in C in tetanus (P < 0.001). The index LD was used to quantify the load dependence of relaxation: LD is <1 in load-dependent muscles and tends toward 1 in load-independent muscles. LD was significantly higher in CHF than in C rabbits, in both twitch (0.99 +/- 0.01 vs. 0.75 +/- 0.03; P < 0. 001) and tetanus (0.95 +/- 0.02 vs. 0.84 +/- 0.02; P < 0.001). In the CHF rabbits' diaphragm, the fall in total tension was linearly related to the fall in load dependence of relaxation. The decrease in load dependence of relaxation in CHF animals suggests sarcoplasmic reticulum abnormalities. Impairment of the sarcoplasmic reticulum may also partly account for the decrease in contractile performance of diaphragm in CHF animals.