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,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 μM 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 -α_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). -α_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.