Transmural Heterogeneity of Na ⁺ –Ca ²⁺ Exchange

Abstract

Spatial electrical heterogeneity has a profound effect on normal cardiac electrophysiology and genesis of cardiac arrhythmias in diseased hearts. The Na ؉ -Ca 2؉ exchanger (NCX) is a key linker, through Ca 2؉ signaling, between contractility and arrhythmias. Here we characterize the differential transmural expression of NCX in normal and rapid pacing-induced failing canine hearts. Significant transmural heterogeneity of NCX was present in normal hearts, as NCX current density measured at ؉80 mV was significantly (P<0.05) greater in epicardial (EPI) (5.49 pA/pF) than mid-myocardial (MID) (2.84 pA/pF) and endocardial (ENDO) (2.21 pA/pF) cells. Interestingly, heart failure caused a selective increase in NCX current density (P<0.05) limited to ENDO (by 202%) and MID (by 76%) but not EPI myocytes (P‫؍‬not significant). The differences in functional expression were associated with changes in both mRNA and protein levels. The normal EPI layer exhibited the greatest NCX mRNA and protein levels compared with MID and ENDO layers, whereas the ENDO layer underwent the most pronounced increase in mRNA (by 185%) and protein (by 207%) levels in heart failure. The transmural NCX gradient, from EPI (greatest) to ENDO (least), is disrupted in heart failure. A selective upregulation of NCX expression in MID and ENDO in heart failure markedly redirects the orientation of the transmural functional gradient of NCX and may lead to enhanced vulnerability to cardiac arrhythmias. S udden death is prevalent in patients with heart failure (HF). 1 Abnormalities in functional expression of calcium-handling proteins are prominent in HF and constitute a crucial link between arrhythmias and decreased contraction. The Na ϩ -Ca 2ϩ exchanger (NCX) plays important roles in both contractile dysfunction and arrhythmogenesis in HF. 2 Prolongation of action potential duration (APD) is a hallmark of failing myocytes, which is arrhythmogenic and contributes to the increased incidence of sudden death in patients with HF. More importantly, transmural dispersion of repolarization (TDR) has a crucial role in arrhythmogenesis. 1 In failing hearts, downregulation of K ϩ currents and upregulation of NCX have been reported (see review by Tomaselli and Zipes 1 ). The transmural homogenenous downregulation of K ϩ currents 3 does not suffice to account for the exaggerated TDR characteristic of HF. We characterized the differential transmural expression of NCX in control and failing hearts. The alterations in transmural NCX heterogeneity in failing hearts highlight a potential substrate for the production of lethal arrhythmias in HF. Materials and MethodsReal-time quantitative RT-PCR, Western blotting, and whole-cell patch clamp were used to measure the NCX mRNA, immunoreactive protein levels, and current, respectively. An expanded Materials and Methods can be found in the online data supplement available at http://circres.ahajournals.org. Figure 1A illustrates representative traces of Ni 2ϩ -sensitive current in a normal MID cell. To confirm that the Ni 2ϩ -se...

Cellular mechanisms of arrhythmogenic cardiac alternans

“…Zygmunt et al (2000) reported an unequal distribution of NCX across the canine left ventricular wall, where peak reverse mode current is greatest in midmyocardial cells and weakest in endocardial cells. Xiong et al (2005) found greater NCX mRNA and protein in the epicardium compared to the midmyocardium and endocardium in normal canine tissue. In a separate series of experiments in guinea pig, we found that alternans prone regions (endocardium) express significantly less SERCA2a and RyR compared to alternans resistant regions (epicardium), suggesting a molecular basis for cellular alternans and specifically implicating calcium cycling proteins responsible for SR reuptake and release.…”

Cellular and subcellular alternans in the canine left ventricle

“…Wan et al (36) demonstrated that myocytes isolated from guinea pig LV ENDO develop Ca 2ϩ alternans at longer BCL than EPI cells and that this difference is not related to any difference in APD. There is also growing evidence that there are differences in expression of Ca 2ϩ -cycling proteins in the three ventricular cell types (24,37,39). These transmural differences in Ca 2ϩ -cycling protein expression may explain the greater sensitivity of ENDO and MID cells to alternans (24).…”

Controlled Release of Matrix Metalloproteinase-1 Plasmid DNA Prevents Left Ventricular Remodeling in Chronic Myocardial Infarction of Rats

“…20 As shown in Figure 5, the expression of NCX1 protein and membrane current through the NCX1 (INCX) were enhanced in the MI group. In the MI+MMP-1 group, INCX was significantly reduced to a similar level as in the Normal group, but no significant decrease in mRNA encoding NCX1 was found.…”