Reduced sarcoplasmic reticulum Ca²⁺ uptake and increased Na⁺–Ca²⁺ exchanger expression in left ventricle myocardium of dogs with progression of heart failure

Abstract

Alteration of intracellular Ca²⁺ homeostasis in failing cardiomyocytes is associated with changes in regulatory proteins located in the sarcoplasmic reticulum (SR) and sarcolemma, which participate in Ca²⁺ fluxes across the membrane during the cardiac cycle. These regulatory proteins include Ca²⁺-ATPase (SERCA 2A), phospholamban (PLB), ryanodine-sensitive Ca²⁺ release channels (RR), and the sarcolemmal Na⁺–Ca²⁺ exchanger (NCX). Although their status is known in failed myocardium, it is poorly understood during the progression of heart failure (HF), particularly in large animals. We studied the left ventricular (LV) myocardium of six dogs with moderate HF and six with severe HF produced by multiple intracoronary microembolizations, compared with six normal dogs (NL). Oxalate-dependent SR Ca²⁺ uptake and expression of SERCA 2A, PLB, phosphorylated PLB at serine 16 (PLB-Ser) and threonine 17 (PLB-Thr), RR, and NCX were determined. Percent LV ejection fraction declined by 47% compared with NL (34.1% ± 1% vs 64% ± 2%) in dogs with moderate HF (HF-2W) 2 weeks after the last embolization and by 42% (20.5% ± 1% vs 34.1% ± 1%) in dogs with severe HF (HF-4M) at 4 months compared with HF-2W. Left ventricular pressure during isovolumic contraction (+dP/dt, mmHg/s) and relaxation (−dP/dt, mmHg/s) was significantly reduced in severe compared with moderate HF. Oxalate-dependent SR Ca²⁺ uptake (nmol ⁴⁵Ca²⁺ accumulated/min per milligram noncollagen protein) declined by 25% (21.3 ± 1 vs 28.5 ± 2) in HF-2W and 49% in HF-4M. Protein expression of SERCA 2A and PLB decreased by 67% and 35%, respectively, in HF-2W compared with NL, whereas SERCA 2A expression increased by 167% and PLB decreased by 40% in HF-4M compared with HF-2W. However, SERCA 2A protein was still significantly lower in HF-4M compared with NL. PLB-Ser and PLB-Thr increased significantly in HF-2W but decreased in HF-4M compared with NL. Similar changes in mRNA encoding PLB and SERCA 2A were observed in dogs with moderate and severe HF. The RR protein level declined in dogs with moderate and severe HF, whereas NCX protein did not change with moderate HF but increased with sever HF. These results suggest that the regulatory proteins responsible for Ca²⁺ uptake, Ca²⁺ release, and Na⁺–Ca²⁺ exchange are critically associated with the deterioration of LV function during the progression of HF.