Mechanisms of Altered Excitation-Contraction Coupling in Canine Tachycardia-Induced Heart Failure, I

Abstract

—Pacing-induced heart failure in the dog recapitulates many of the electrophysiological and hemodynamic abnormalities of the human disease; however, the mechanisms underlying altered Ca²⁺ handling have not been investigated in this model. We now show that left ventricular midmyocardial myocytes isolated from dogs subjected to 3 to 4 weeks of rapid pacing have prolonged action potentials and Ca²⁺ transients with reduced peaks, but durations ≈3-fold longer than controls. To discriminate between action potential effects on Ca²⁺ kinetics and direct changes in Ca²⁺ regulatory processes, voltage-clamp steps were used to examine the time constant for cytosolic Ca²⁺ removal (τ_Ca). τ_Ca was prolonged by just 35% in myocytes from failing hearts after fixed voltage steps in physiological solutions (τ_Ca control, 216±25 ms, n=17; τ_Ca failing, 292±23 ms, n=22; P+/Ca²⁺ exchange was eliminated (τ_Ca control, 282±30 ms, n=13; τ_Ca failing, 576±83 ms, n=11; P2+ uptake and a greater dependence on Na⁺/Ca²⁺ exchange for cytosolic Ca²⁺ removal was confirmed by inhibiting SR Ca²⁺ ATPase with cyclopiazonic acid, which slowed Ca²⁺ removal more in control than in failing myocytes. β-Adrenergic stimulation of SR Ca²⁺ uptake in cells from failing hearts sufficed only to accelerate τ_Ca to the range of unstimulated controls. Protein levels of SERCA2a, phospholamban, and Na⁺/Ca²⁺ exchanger revealed a pattern of changes qualitatively similar to the functional measurements; SERCA2a and phospholamban were both reduced in failing hearts by 28%, and Na⁺/Ca²⁺ exchange protein was increased 104% relative to controls. Thus, SR Ca²⁺ uptake is markedly downregulated in failing hearts, but this defect is partially compensated by enhanced Na⁺/Ca²⁺ exchange. The alterations are similar to those reported in human heart failure, which reinforces the utility of the pacing-induced dog model as a surrogate for the human disease.