Abstract

Objective: Phosphorylation of the myosin light chain 2 (MLC-2) isoform expressed as a percentage of total MLC-2 was decreased in failing (21.1±2.0%) compared to donor (31.9±4.8%) hearts. To assess the functional implications of this change, we compared the effects of MLC-2 dephosphorylation on force development in failing and non-failing (donor) human hearts. Methods: Cooperative effects in isometric force and rate of force redevelopment (K_tr) were studied in single Triton-skinned human cardiomyocytes at various [Ca²⁺] before and after protein phosphatase-1 (PP-1) incubation. Results: Maximum force and K_tr values did not differ between failing and donor hearts, but Ca²⁺-sensitivity of force (pCa₅₀) was significantly higher in failing myocardium (ΔpCa₅₀=0.17). K_tr decreased with decreasing [Ca²⁺], although this decrease was less in failing than in donor hearts. Incubation of the myocytes with PP-1 (0.5 U/ml; 60 min) decreased pCa₅₀ to a larger extent in failing (0.20 pCa units) than in donor cardiomyocytes (0.10 pCa units). A decrease in absolute K_tr values was found after PP-1 in failing and donor myocytes, while the shape of the K_tr–Ca²⁺ relationships remained unaltered. Conclusions: Surprisingly, the contractile response to MLC-2 dephosphorylation is enhanced in failing hearts, despite the reduced level of basal MLC-2 phosphorylation. The enhanced response to MLC-2 dephosphorylation in failing myocytes might result from differences in basal phosphorylation of other thin and thick filament proteins between donor and failing hearts. Regulation of Ca²⁺-sensitivity via MLC-2 phosphorylation may be a potential compensatory mechanism to reverse the detrimental effects of increased Ca²⁺-sensitivity and impaired Ca²⁺-handling on diastolic function in human heart failure.