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1 Medicine, Massachusetts General Hospital, Boston, Massachusetts, United States; Medicine/Cardiology, Massachussetts General Hospital/Cardiovascular Research Center, 149 13th Street, Charlestown, Massachusetts, 02129, United States
2 Medicine, University Düsseldorf, Dusseldorf, Germany
3 Medicine, University of Maryland, Baltimore, Maryland, United States
4 Medicine, University of Calgary, Calgary, Canada
5 Medicine, Johns Hopkins University, Baltimore, Maryland, United States
6 Division of Cardiology, Department of Medicine, Johns Hopkins Medical Institutions, Baltimore, Maryland, United States
7 Division of Cardiology, Johns Hopkins Univ. School of Medicine, Baltimore, Maryland, United States
8 Cardiology, University of Maryland, Baltimore, Maryland, United States
9 Medicine, University of Kentucky, Lexington, Kentucky, United States
* To whom correspondence should be addressed. E-mail: gtomasel{at}jhmi.edu.
Myocytes from the failing myocardium exhibit depressed and prolonged Ca2+ transients ([Ca2+]i transients) that are, in part, responsible for contractile dysfunction and unstable repolarization. In order to better understand the molecular basis of the aberrant Ca2+ handling in heart failure (HF), we studied the rabbit pacing tachycardia HF model. Induction of HF was associated with AP duration prolongation that was especially pronounced at low stimulation frequencies. The L-type calcium channel current (ICa,L) density (-0.964±0.172 vs. -0.745±0.128 pA/pF at +10 mV) and NCX (2.1±0.8 vs. 2.3±0.8 pA/pF at +30 mV) currents were not different in myocytes from control and failing hearts. The amplitude of peak [Ca2+]i was depressed (at +10 mV, 0.72±0.07 µM and 0.56±0.04 µM in normal and failing hearts correspondingly, p<0.05) with slowed rates of decay and reduced Ca2+ spark amplitudes (p<0.0001) in myocytes isolated from failing compared to control hearts. Inhibition of sarcoplasmic reticulum Ca2+-ATPase (SERCA2a) revealed a greater reliance on NCX to remove cytosolic Ca2+ in myocytes isolated from failing compared to control hearts (p<0.05). The mRNA levels of the 
1C subunit, ryanodine receptor (RyR) and NCX were unchanged from controls, while SERCA2a and phospholamban (PLB) were significantly down regulated in the failing compared with the control hearts (p<0.05). 1C protein levels were unchanged, RyR, SERCA2a and PLB were significantly down regulated (p<0.05), while NCX protein was significantly up regulated (p<0.05). These results support a prominent role for the sarcoplasmic reticulum in the pathogenesis of HF, in which abnormal SR Ca2+ uptake and release synergistically contribute to the the depressed [Ca2+]i and the altered action potential profile phenotype.
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