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Objective: The expression of contractile isoforms changes during various pathological conditions but little is known about the consequences of these changes for the mechanical properties in human ventricular muscle. We investigated the feasibility of simultaneous determination of protein composition and isometric force development in single cardiac myocytes from human ventricular muscle tissue obtained from small biopsies taken during open heart surgery. Methods: Small biopsies of about 3 mg wet weight were taken during open heart surgery from patients with aortic valve stenosis. These biopsies were divided in two parts. One part (~ 2 mg) was used for mechanical isolation of single myocytes and subsequent force measurement while the remaining part was used, in aliquots of I μg dry weight, for protein analysis by polyacrylamide gel electrophoresis. The myocytes were attached with silicon glue to a sensitive force transducer and a piezoelectric motor, mounted on an inverted microscope and permeabilized by means of Triton X-100. Force development was studied at various free calcium concentrations. Results: From all biopsies, myocytes could be obtained and the composition of contractile proteins could be determined. The average isometric force (±s.e.m.) at saturating calcium concentration obtained on 20 myocytes from 5 patients amounted to 51 ± 8 kN/m2. Force was half maximal at a calcium concentration of 2.47 ± 0.10 μM. Conclusion: These measurements indicate that it is possible to study the correlation between mechanical properties and protein composition in small biopsies from human ventricular muscle.

Cardiac energetics and performance depend on the expression level of the fast (alpha-) and slow (beta-) myosin heavy chain (MHC) isoform. In ventricular tissue, the beta-MHC isoform predominates, whereas in atrial tissue a variable mixture of alpha- and beta-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.

Background — During ischemia, the intracellular calcium and inorganic phosphate (P i ) concentrations rise and pH falls. We investigated the effects of these changes on force development in donor and failing human hearts to determine if altered contractile protein composition during heart failure changes the myocardial response to Ca 2+ , P i , and pH. Methods and Results — Isometric force was studied in mechanically isolated Triton-skinned single myocytes from left ventricular myocardium. Force declined with added P i to 0.33±0.02 of the control force (pH 7.1, 0 mmol/L P i ) at 30 mmol/L P i and increased with pH from 0.64±0.03 at pH 6.2 to 1.27±0.02 at pH 7.4. Force dependency on P i and pH did not differ between donor and failing hearts. Incubation of myocytes in a P i -containing activating solution caused a potentiation of force, which was larger at submaximal than at maximal [Ca 2+ ]. Ca 2+ sensitivity of force was similar in donor hearts and hearts with moderate cardiac disease, but in end-stage failing myocardium it was significantly increased. The degree of myosin light chain 2 phosphorylation was significantly decreased in end-stage failing compared with donor myocardium, resulting in an inverse correlation between Ca 2+ responsiveness of force and myosin light chain 2 phosphorylation. Conclusions — Our results indicate that contractile protein alterations in human end-stage heart failure alter Ca 2+ responsiveness of force but do not affect the force-generating capacity of the cross-bridges or its P i and pH dependence. In end-stage failing myocardium, the reduction in force by changes in pH and [P i ] at submaximal [Ca 2+ ] may even be less than in donor hearts because of the increased Ca 2+ responsiveness.