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

Abstract

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°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 ΔmN) contrasting a positive rate dependent potentiation of force in nonfailing tissue (2.0 Hz vs. 0.5 Hz: +0.8±0.2 ΔmN; 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.