cardiomyocyte-specific mineralocorticoid receptor–dependent signaling contributes to electromechanical vulnerability in acute ischemia–reperfusion via a mechanism involving Ca/calmodulin protein kinase II activation in association with upstream alteration in expression regulation

M ineralocorticoid receptor (MR) antagonists are beneficial in heart failure and can reduce the incidence of sudden cardiac death, although side effects (including hyperkalemia) limit their widespread use. Establishing tissue-specific roles for MR signaling is critical to develop selective target therapies to achieve the beneficial effects of MR blockade without del-eterious side effects. MR signaling involvement in systemic fluid homeo-stasis is well established. Emerging evidence identifies an important role of MR in pathophysiologic regulation of cardiac function. Clinical and experimental studies demonstrate that mineralocorticoid excess is associated with cardiac inflammation, oxidative stress, fibrosis, and hypertrophy— responses characterized by cardiomyocyte gene expression shifts. In vitro and ex vivo studies provide evidence for MR modulation of cardiomyocyte electromechanical function, although a clear mechanistic understanding has not yet been resolved. Aldosterone has been found to increase contrac-tility in isolated hearts and shortening in cardiomyocytes (via a mechanism involving intracellular pH modulation). Consistent with this positive inotropic effect, evidence for aldosterone-mediated augmentation of cardiomyo-cyte calcium (Ca 2+) current density has been observed. 6–8 In contrast, other cardiomyocyte studies have demonstrated no change in either Ca 2+ signaling or contractility with sodium–hydrogen exchanger-1 (NHE-1) activation. 9 Cardiomyocyte-specific MR overexpression is linked with cardiac arrhythmia in the absence of fibrotic or inflamma-tory responses, suggesting direct cardiomyocyte involvement in MR-mediated arrhythmogenesis. 10 Collectively these data show MR capacity to regulate cardiomyocyte operation via multiple mechanisms, but an understanding of how Abstract—Loss of mineralocorticoid receptor signaling selectively in cardiomyocytes can ameliorate cardiac fibrotic and inflammatory responses caused by excess mineralocorticoids. The aim of this study was to characterize the role of cardiomyocyte mineralocorticoid receptor signaling in ischemia–reperfusion injury and recovery and to identify a role of mineralocorticoid receptor modulation of cardiac function. Wild-type and cardiomyocyte mineralocorticoid receptor knockout mice (8 weeks) were uninephrectomized and maintained on (1) high salt (0.9% NaCl, 0.4% KCl) or (2) high salt plus deoxycorticosterone pellet (0.3 mg/d, 0.9% NaCl, 0.4% KCl). After 8 weeks of treatment, hearts were isolated and subjected to 20 minutes of global ischemia plus 45 minutes of reperfusion. Mineralocorticoid excess increased peak contracture during ischemia regardless of genotype. Recovery of left ventricular developed pressure and rates of contraction and relaxation post ischemia–reperfusion were greater in knockout versus wild-type hearts. The incidence of arrhythmic activity during early reperfusion was significantly higher in wild-type than in knockout hearts. Levels of autophosphorylated Ca 2+ /calmodulin protein kinase II (Thr287) were elevated in …