Cellular and molecular determinants of altered Ca2+ handling in the failing rabbit heart:: primary defects in SR Ca2+ uptake and release mechanisms

Myocytes from the failing myocardium exhibit depressed and prolonged intracellular Ca2+ concentration ([ Ca2+](i)) transients that are, in part, responsible for contractile dysfunction and unstable repolarization. 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 action potential ( AP) duration prolongation that was especially pronounced at low stimulation frequencies. L- type calcium channel current ( I-Ca,I- L) density ( - 0.964 +/- 0.172 vs. -0.745 +/- 0.128 pA/ pF at + 10 mV) and Na+/Ca2+ exchanger ( NCX) currents ( 2.1 +/- 0.8 vs. 2.3 +/- 0.8 pA/ pF at +30 mV) 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 and 0.56 +/- 0.04 mu M in normal and failing hearts, respectively; P < 0.05), with slowed rates of decay and reduced Ca2+ spark amplitudes ( P < 0.0001) in myocytes isolated from failing vs. control hearts. Inhibition of sarco( endo) plasmic reticulum Ca2+-ATPase ( SERCA) 2a revealed a greater reliance on NCX to remove cytosolic Ca2+ in myocytes isolated from failing vs. control hearts ( P < 0.05). mRNA levels of the alpha(1C)-subunit, ryanodine receptor ( RyR), and NCX were unchanged from controls, while SERCA2a and phospholamban ( PLB) were significantly downregulated in failing vs. control hearts ( P < 0.05). alpha(1C) protein levels were unchanged, RyR, SERCA2a, and PLB were significantly downregulated ( P < 0.05), while NCX protein was significantly upregulated ( P < 0.05). These results support a prominent role for the sarcoplasmic reticulum ( SR) in the pathogenesis of HF, in which abnormal SR Ca2+ uptake and release synergistically contribute to the depressed [ Ca2+](i) and the altered AP profile phenotype.