Rate Dependence of [Na + ] i and Contractility in Nonfailing and Failing Human Myocardium

Abstract

Background — In the failing human heart, altered Ca 2+ homeostasis causes contractile dysfunction. Because Ca 2+ and Na + homeostasis are intimately linked through the Na + /Ca 2+ exchanger, we compared the regulation of [Na + ] i in nonfailing (NF) and failing human myocardium. Methods and Results — [Na + ] i was measured in SBFI-loaded muscle strips. At slow pacing rates (0.25 Hz, 37°C), isometric force was similar in NF (n=6) and failing (n=12) myocardium (6.4±1.2 versus 7.2±1.9 mN/mm 2 ), but [Na + ] i and diastolic force were greater in failing (22.1±2.6 mmol/L and 15.6±3.2 mN/mm 2 ) than in NF (15.9±3.1 mmol/L and 3.50±0.55 mN/mm 2 ; P <0.05) myocardium. In NF hearts, increasing stimulation rates resulted in a parallel increase in force and [Na + ] i without changes in diastolic tension. At 2.0 Hz, force increased to 136±17% of the basal value ( P <0.05), and [Na + ] i to 20.5±4.2 mmol/L ( P <0.05). In contrast, in failing myocardium, force declined to 45±3%, whereas [Na + ] i increased to 27.4±3.2 mmol/L (both P <0.05), in association with significant elevations in diastolic tension. [Na + ] i was higher in failing than in NF myocardium at every stimulation rate. [Na + ] i predicted in myocytes from Na + pipette -contraction relations was 8.0 mmol/L in NF (n=9) and 12.1 mmol/L in failing (n=57; P <0.05) myocardium at 0.25 Hz. Reverse-mode Na + /Ca 2+ exchange induced significant Ca 2+ influx in failing but not NF myocytes, compatible with higher [Na + ] i in failing myocytes. Conclusions — Na + i homeostasis is altered in failing human myocardium. At slow heart rates, the higher [Na + ] i in failing myocardium appears to enhance Ca 2+ influx through Na + /Ca 2+ exchange and maintain sarcoplasmic reticulum Ca 2+ load and force development. At faster rates, failing myocytes with high [Na + ] i cannot further increase sarcoplasmic reticulum Ca 2+ load and are prone to diastolic Ca 2+ overload.