Partial Inhibition of Sodium/Calcium Exchange Restores Cellular Calcium Handling in Canine Heart Failure

Abstract

Abstract-Sodium/calcium (Na ϩ /Ca 2ϩ ) exchange (NCX) overexpression is common to human heart failure and heart failure in many animal models, but its specific contribution to the cellular Ca 2ϩ ([Ca 2ϩ ] i ) handling deficit is unclear. Here, we investigate the effects of exchange inhibitory peptide (XIP) on Ca 2ϩ handling in myocytes isolated from canine tachycardic pacing-induced failing hearts. Whole-cell patch-clamped left ventricular myocytes from failing hearts (F) showed a 52% decrease in steady-state sarcoplasmic reticulum (SR) Ca 2ϩ load and a 44% reduction in the amplitude of the [Ca 2ϩ ] i transient, as compared with myocytes from normal hearts (N). Intracellular application of XIP (30 mol/L) normalized the [Ca 2ϩ ] i transient amplitude in F (3.86-fold increase), concomitant with a similar increase in SR Ca 2ϩ load. The degree of NCX inhibition at this concentration of XIP was Ϸ27% and was selective for NCX: L-type Ca 2ϩ currents and plasmalemmal Ca 2ϩ pumps were not affected. XIP also indirectly improved the rate of [Ca 2ϩ ] i removal at steady-state, secondary to Ca 2ϩ -dependent activation of SR Ca 2ϩ uptake. The findings indicate that in the failing heart cell, NCX inhibition can improve SR Ca 2ϩ load by shifting the balance of Ca 2ϩ fluxes away from trans-sarcolemmal efflux toward SR accumulation. Hence, inhibition of the Ca 2ϩ efflux mode of the exchanger could potentially be an effective therapeutic strategy for improving contractility in congestive heart failure. (Ca 2ϩ ) handling is a key factor in the pathophysiology of heart failure. A typical failing heart cell shows a decrease in the ability of the internal stores (the sarcoplasmic reticulum [SR]) to load with Ca 2ϩ , and an increase in Ca 2ϩ extrusion from the cell by the sodium/ calcium exchanger (NCX). NCX overexpression is a component of altered Ca 2ϩ handling in human 1 and animal models, 2,3 but it is unclear whether it is compensatory or contributes to dysfunction. One widely held hypothesis is that NCX overexpression compensates for decreased Ca 2ϩ reuptake into the SR by increasing Ca 2ϩ extrusion from the cell, 4,5 which improves relaxation (positive lusitropic) but at the cost of a further depletion of SR Ca 2ϩ stores (negative inotropic). Further complicating the issue is the observation that NCX overexpression is also found in hypercontractile models with no SR dysfunction. 6 We studied the effect of partially correcting the NCX overexpression (by applying an exchange inhibitory peptide [XIP]) in a canine model of heart failure. Partial inhibition of NCX normalized both SR Ca 2ϩ release and re-uptake, arguing for a critical role for NCX overexpression in the Ca 2ϩ handling deficit as well as for its potential as a therapeutic target. Materials and MethodsThese experiments were performed using a canine tachycardic pacing-induced heart failure model. We, 2,7,8 and others, 9 have previously demonstrated that this animal model reproduces a remarkable number of features of the human disease. Induction of heart failu...

Excitation-contraction coupling and mitochondrial energetics

“…Another concept is to inhibit forward mode I NCX to increase SR Ca 2+ load [91], however, this has not yet been tested in heart failure models in vivo. A traditional pharmacological approach to increase cytosolic Ca 2+ transients is to inhibit Na + /K + -ATPase with digitalis, which increases [Na + ] i and thus, I NCX -mediated Ca 2+ influx.…”

“…Decreased SR Ca 2+ -ATPase activity is partly compensated by increased expression and activity of the NCX [16,65,93,146,178,190], since pronounced forward mode I NCX may maintain diastolic function [85] by removing Ca 2+ to the extracellular space. On the other hand, this pronounced forward mode I NCX may further aggravate SR Ca 2+ depletion, since inhibition of I NCX with an inhibitor peptide (XIP [118]) restored SR Ca 2+ load in failing myocytes [91].…”

“…Interestingly, the increase of SR Ca 2+ -ATPase expression was associated with an improvement of the PCr/ATP ratio in rats with heart failure after aortic banding [52], supporting the hypothesis that reconstitution of EC coupling may improve energetics. However, a specific pharmacological drug for such an intervention is currently not available.Another concept is to inhibit forward mode I NCX to increase SR Ca 2+ load [91], however, thishas not yet been tested in heart failure models in vivo. A traditional pharmacological approach to increase cytosolic Ca 2+ transients is to inhibit Na + /K + -ATPase with digitalis, which increases [Na + ] i and thus, I NCX -mediated Ca 2+ influx.…”

Transcriptomic profiling of the canine tachycardia-induced heart failure model: global comparison to human and murine heart failure

“…Although the canine tachycardia HF model has been used to characterize pathophysiological changes in HF and to explore the roles of individual genes/ proteins in pathogenesis of HF [9][10][11][12][13][14], genome-wide expression analyses to identify pathways/ networks that underlie the molecular genesis of the HF phenotype in this model have not been performed.…”

Regulation of the Na+/Ca2+ Exchanger by Pyridine Nucleotide Redox Potential in Ventricular Myocytes

“…NCX is up-regulated at mRNA, protein, and functional levels in various heart failure models (8 -11). Although increased NCX can have a positive inotropic effect because of more Ca 2ϩ influx during the action potential plateau (12) and compensates for impaired SR Ca 2ϩ ATPase Ca 2ϩ removal (13), inhibition of NCX activity can have beneficial effects on heart failure by enhancing the SR Ca 2ϩ load (10,14,15). NCX consists of nine transmembrane segments and a large intracellular loop that is essential for regulation of its activity by a large variety of factors, such as Ca 2ϩ , phosphatidylinositol 4,5-bisphosphate, and reactive oxygen species (ROS) (16 -18).…”