Effect of phosphate and temperature on force exerted by white muscle fibres from dogfish.

Effects of Pi (inorganic phosphate) are relevant to the in vivo function of muscle because Pi is one of the products of ATP hydrolysis by actomyosin and by the sarcoplasmic reticulum Ca pump. We have measured the Pi sensitivity of force produced by permeabilized muscle fibres from dogfish (Scyliorhinus canicula) and rabbit. The activation conditions for dogfish fibres were crucial: fibres activated from the relaxed state at 5, 12, and 20°C were sensitive to Pi, whereas fibres activated from rigor at 12°C were insensitive to Pi in the range 5-25 mmol l. Rabbit fibres activated from rigor were sensitive to Pi. Pi sensitivity of force produced by dogfish fibres activated from the relaxed state was greater below normal body temperature (12°C for dogfish) in agreement with what is known for other species. The force-temperature relationship for dogfish fibres (intact and permeabilized fibres activated from relaxed) showed that at 12°C, normal body temperature, the force was near to its maximum value

Decreased creatine kinase is linked to diastolic dysfunction in rats with right heart failure induced by pulmonary artery hypertension

Our objective was to investigate the role of creatine kinase in the contractile dysfunction of right ventricular failure caused by pulmonary artery hypertension. Pulmonary artery hypertension and right ventricular failure were induced in rats by monocrotaline and compared to saline-injected control animals. In vivo right ventricular diastolic pressure-volume relationships were measured in anesthetized animals; diastolic force-length relationships in single enzymatically dissociated myocytes and myocardial creatine kinase levels by Western blot. We observed diastolic dysfunction in right ventricular failure indicated by significantly steeper diastolic pressure-volume relationships in vivo and diastolic force-length relationships in single myocytes. There was a significant reduction in creatine kinase protein expression in failing right ventricle. Dysfunction also manifested as a shorter diastolic sarcomere length in failing myocytes. This was associated with a Ca2+-independent mechanism that was sensitive to cross-bridge cycling inhibition. In saponin-skinned failing myocytes, addition of exogenous creatine kinase significantly lengthened sarcomeres, while in intact healthy myocytes, inhibition of creatine kinase significantly shortened sarcomeres. Creatine kinase inhibition also changed the relatively flat contraction amplitude-stimulation frequency relationship of healthy myocytes into a steeply negative, failing phenotype. Decreased creatine kinase expression leads to diastolic dysfunction, we propose this is via local reduction in ATP:ADP ratio and thus to Ca2+-independent force production and diastolic sarcomere shortening. Creatine kinase inhibition also mimics a definitive characteristic of heart failure, the inability to respond to increased demand. Novel therapies for pulmonary artery hypertension are needed. Our data suggest cardiac energetics would be a potential ventricular therapeutic target

Preserved cross-bridge kinetics in human hypertrophic cardiomyopathy patients with MYBPC3 mutations

Mutations in the MYBPC3 gene, encoding cardiac myosin binding protein C (cMyBP-C) are frequent causes of hypertrophic cardiomyopathy (HCM). Previously, we have presented evidence for reduced cMyBP-C expression (haploinsufficiency), in patients with a truncation mutation in MYBPC3. In mice, lacking cMyBP-C cross-bridge kinetics was accelerated. In this study, we investigated whether cross-bridge kinetics was altered in myectomy samples from HCM patients harboring heterozygous MYBPC3 mutations (MYBPC3(mut)). Isometric force and the rate of force redevelopment (k (tr)) at different activating Ca2+ concentrations were measured in mechanically isolated Triton-permeabilized cardiomyocytes from MYBPC3(mut) (n = 18) and donor (n = 7) tissue. Furthermore, the stretch activation response of cardiomyocytes was measured in tissue from eight MYBPC3(mut) patients and five donors to assess the rate of initial force relaxation (k (1)) and the rate and magnitude of the transient increase in force (k (2) and P (3), respectively) after a rapid stretch. Maximal force development of the cardiomyocytes was reduced in MYBPC3(mut) (24.5 +/- 2.3 kN/m(2)) compared to donor (34.9 +/- 1.6 kN/m(2)). The rates of force redevelopment in MYBPC3(mut) and donor over a range of Ca2+ concentrations were similar (k (tr) at maximal activation: 0.63 +/- 0.03 and 0.75 +/- 0.09 s(-1), respectively). Moreover, the stretch activation parameters did not differ significantly between MYBPC3(mut) and donor (k (1): 8.5 +/- 0.5 and 8.8 +/- 0.4 s(-1); k (2): 0.77 +/- 0.06 and 0.74 +/- 0.09 s(-1); P (3): 0.08 +/- 0.01 and 0.09 +/- 0.01, respectively). Incubation with protein kinase A accelerated k (1) in MYBPC3(mut) and donor to a similar extent. Our experiments indicate that, at the cMyBP-C expression levels in this patient group (63 +/- 6 % relative to donors), cross-bridge kinetics are preserved and that the depressed maximal force development is not explained by perturbation of cross-bridge kinetics

Myocardial contraction is 5-fold more economical in ventricular than in atrial human tissue

Cardiac energetics and performance depend on the expression level of the fast (α-) and slow (β-) myosin heavy chain (MHC) isoform. In ventricular tissue, the β-MHC isoform predominates, whereas in atrial tissue a variable mixture of α- and β-MHC is found. In several cardiac diseases, the slow isoform is upregulated; however, the functional implications of this transition in human myocardium are largely unknown. The aim of this study was to determine the relation between contractile properties and MHC isoform composition in healthy human myocardium using the diversity in atrial tissue. Isometric force production and ATP consumption were measured in chemically skinned atrial trabeculae and ventricular muscle strips, and rate of force redevelopment was studied using single cardiomyocytes. MHC isoform composition was determined by one-dimensional SDS-gel electrophoresis. Force development in ventricular tissue was about 5-fold more economical, but nine times slower, than in atrial tissue. Significant linear correlations were found between MHC isoform composition, ATP consumption and rate of force redevelopment. These results clearly indicate that even a minor shift in MHC isoform expression has considerable impact on cardiac performance in human tissue

Prevention of Myofilament Dysfunction by beta-Blocker Therapy in Postinfarct Remodeling

Background-Myofilament contractility of individual cardiomyocytes is depressed in remote noninfarcted myocardium and contributes to global left ventricular pump dysfunction after myocardial infarction (MI). Here, we investigated whether beta-blocker therapy could restore myofilament contractility. Methods and Results-In pigs with a MI induced by ligation of the left circumflex coronary artery, beta-blocker therapy (bisoprolol, MI+beta) was initiated on the first day after MI. Remote left ventricular subendocardial biopsies were taken 3 weeks after sham or MI surgery. Isometric force was measured in single permeabilized cardiomyocytes. Maximal force (F-max) was lower, whereas Ca2+ sensitivity was higher in untreated MI compared with sham (both P<0.05). The difference in Ca2+ sensitivity was abolished by treatment of cells with the beta-adrenergic kinase, protein kinase A. beta-blocker therapy partially reversed F-max and Ca2+ sensitivity to sham values and significantly reduced passive force. Despite the lower myofilament Ca2+ sensitivity in MI+beta compared with untreated myocardium, the protein kinase A induced reduction in Ca2+ sensitivity was largest in cardiomyocytes from myocardium treated with beta-blockers. Phosphorylation of beta-adrenergic target proteins (myosin binding protein C and troponin I) did not differ among groups, whereas myosin light chain 2 phosphorylation was reduced in MI, which coincided with increased expression of protein phosphatase 1. beta-blockade fully restored the latter alterations and significantly reduced expression of protein phosphatase 2a. Conclusions-beta-blockade reversed myofilament dysfunction and enhanced myofilament responsiveness to protein kinase A in remote myocardium after MI. These effects likely contribute to the beneficial effects of beta-blockade on global left ventricular function after MI. (Circ Heart Fail. 2009;2:233-242.