Frequency dependent force generation correlates with sarcoplasmic calcium ATPase activity in human myocardium

Objective: In congestive heart failure both a decreased function of the sarcoplasmic Ca2+-ATPase and a negative force-frequency relationship have been shown. This study aimed to investigate a possible relationship between frequency potentiation, sarcoplasmic Ca2+-ATPase activity, and SERCA2 protein expression in human myocardium. Methods: Frequency potentiation was studied in electrically stimulated, isometric, left ventricular papillary muscle strip preparations (37 degrees C, 0.5 - 3.0 Hz) from terminally failing (NYHA IV; n = 5, dilated cardiomyopathy) and nonfailing (donor hearts, n = 5) human myocardium. In the identical samples the Ca2+-ATPase activity (NADH coupled assay) and the protein expression of sarcoplasmic Ca2+-ATPase (SERCA2), phospholamban, and calsequestrin (western blot) were determined. The frequency dependent change in the force of contraction and V-max of the Ca2+-ATPase activity and the protein expression of SERCA2 were correlated with each other. Results: In terminally failing myocardium the force-frequency relationship was negative (2.0 Hz vs. 0.5 Hz: -0.2 +/- 0.1 Delta mN) contrasting a positive rate dependent potentiation of force in nonfailing tissue (2.0 Hz vs. 0.5 Hz: +0.8 +/- 0.2 Delta mN; p < 0.01). In failing myocardium the corresponding maximal sarcoplasmic Ca2+-ATPase activity (V-max) was reduced significantly compared to nonfailing myocardium (174 +/- 24 vs. 296 +/- 31 nmol ATP/mg.min, p < 0.01). The protein expression of SERCA2, phospholamban, and calsequestrin remained unchanged in failing myocardium. The maximal Ca2+-ATPase activity significantly correlated with the frequency dependent change in force of contraction (2 Hz vs. 0.5 Hz: r = 0.88, p = 0.001; 3 Hz vs. 0.5 Hz: r = 0.84, p = 0.004). No correlation between protein expression of SERCA2 and Ca2+-ATPase activity or change in force of contraction was observed. Conclusion: Due to a significant correlation between sarcoplasmic Ca2+-ATPase activity and frequency potentiation, the negative rate dependent force potentiation in human heart failure could be at least in part be attributed to decreased function of the sarcoplasmic Ca2+-ATPase.