Oxidation of Myofilament Protein Sulfhydryl Groups Reduces the Contractile Force and Its Ca 2+ Sensitivity in Human Cardiomyocytes

This study sought to characterize the relation between the oxidation of protein sulfhydryl (SH) groups and Ca²⁺-activated force production in the human myocardium. Triton-permeabilized left ventricular cardiomyocytes from donor hearts were exposed to an oxidative (2,2′-dithiodipyridine, DTDP) agent in vitro, and the changes in isometric force, its Ca²⁺ sensitivity, the cross-bridge–sensitive rate constant of force redevelopment at saturating [Ca²⁺] (k _tr,max), and protein SH oxidation were monitored. DTDP (0.1–10 mM for 2 min) oxidized the myocardial proteins and diminished the Ca²⁺-activated force with different concentration dependences (EC_50,SH = 0.17 ± 0.02 mM and EC_50,force = 2.46 ± 0.22 mM; mean ± SEM). The application of 2.5 mM DTDP decreased the maximal Ca²⁺-activated force (to 64%), its Ca²⁺ sensitivity (ΔpCa₅₀ = 0.22 ± 0.02), and the steepness of the Ca²⁺-force relation (n _Hill, from 2.01 ± 0.08 to 1.76 ± 0.08). These changes were paralleled by reductions in the free SH content of the proteins (to 15%) and in k _tr,max (to 75%). SH-specific labeling identified SH oxidation of myosin light chain 1 and actin at DTDP concentrations at which the changes in the contractile parameters occurred. Our data suggest that SH oxidation in selected myofilament proteins diminishes the Ca²⁺-activated force and its Ca²⁺ sensitivity through an impaired Ca²⁺ regulation of the actin–myosin cycle in the human heart.