Effects of calcium, inorganic phosphate, and pH on isometric force in single skinned cardiomyocytes from donor and failing human hearts

Background-During ischemia, the intracellular calcium and inorganic phospha te (P-i) concentrations rise and pH falls. We investigated the effects of t hese changes on force development in donor and failing human hearts to dete rmine if altered contractile protein composition during heart failure chang es the myocardial response to Ca2+, P-i, and pH. Methods and Results-Isometric force was studied in mechanically isolated Tr iton-skinned single myocytes from left ventricular myocardium. Force declin ed 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 activ ating solution caused a potentiation of force, which was larger at submaxim al than at maximal [Ca2+]. Ca2+ sensitivity of force was similar in donor h earts and hearts with moderate cardiac disease, but in end-stage failing my ocardium 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 Ca2+ res ponsiveness of force and myosin light chain 2 phosphorylation. Conclusions-Our results indicate that contractile protein alterations in hu man end-stage heart failure alter Ca2+ responsiveness of force but do not a ffect 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 chan ges in pH and [P-i] at submaximal [Ca2+] may even be less than in donor hea rts because of the increased Ca2+ responsiveness.