The ERG1 gene encodes a family of potassium channels. Mutations in human ERG1 lead to defects in cardiac repolarization, referred to as the long QT syndrome. Through homologous recombination in mouse embryonic stem cells the ERG1 B potassium channel transcript was eliminated while the ERG1 A transcript was maintained. Heterologous expression of ERG1 isoforms had previously indicated that the deactivation time course of ERG1 B is 10-fold more rapid than that of ERG1 A. In day-18 fetal ؉/؉ myocytes, I Kr exhibited two time constants of deactivation (3,933 ؎ 404 and 350 ؎ 19 ms at ؊50 mV), whereas in age-matched ERG1 B ؊/؊ mice the rapid component was absent. Biexponential deactivation rates (2,039 ؎ 268 and 163 ؎ 43 ms at ؊50 mV) were also observed in adult ؉/؉ myocytes. In adult ERG1 B ؊/؊ myocytes no I Kr was detected. Electrocardiogram intervals were similar in ؉/؉ and ؊/؊ mice. However, adult ؊/؊ mice manifested abrupt spontaneous episodes of sinus bradycardia (>100 ms of slowing) in 6 out of 21 mice. This phenomenon was never observed in ؉/؉ mice (0 out of 16). We conclude that ERG1 B is necessary for I Kr expression in the surface membrane of adult myocytes. Knockout of ERG1 B predisposes mice to episodic sinus bradycardia.

Overexpression of calcineurin in transgenic mouse heart results in massive cardiac hypertrophy followed by sudden death. Sudden deaths are caused by abrupt transitions from sinus rhythm to heart block (asystole) in calcineurin-overexpressing (CN) mice. Preliminary studies showed decreased maximum change in potential over time (dV/dt(max)) of phase 0 of the action potential. Accordingly, the hypothesis was tested that decreased activity of the sodium channel contributes to heart block. Profound decreases in activity of sodium currents (I(Na)) paralleled the changes in action potential characteristics. Progressive age-dependent decreases were observed such that at 42-50 days of life little sodium channel function existed. However, this was not paralleled by decreased protein expression as assessed by immunocytochemistry or by Western blot. Since calcineurin can interact with the ryanodine receptor, we assessed whether chronic in vitro treatment with BAPTA-AM, thapsigargin, and ryanodine could rescue the decrease of I(Na). All of these treatments rescued I(Na) to levels indistinguishable from wild type. The nonspecific PKC inhibitor bisindolylmaleimide I also rescued the decrease of I(Na). To assess whether decreased sodium channel activity contributes to sudden death in vivo, the response to encainide (20 mg/kg) was assessed: 6 of 10 young CN mice died because of asystole, whereas 0 of 10 wild-type mice died (P < 0.01). Moreover, encainide produced exaggerated prolongation of the QRS width in sinus beats before the heart block. Catecholamine tone appears necessary to support life in older CN mice because propranolol (1 mg/kg) triggered asystolic death in five of six CN mice. We conclude that decrease in sodium channel activity is in the common final pathway to asystole in CN mice.

HJ. iNOS in cardiac myocytes plays a critical role in death in a murine model of hypertrophy induced by calcineurin. Am J Physiol Heart Circ Physiol 295: H1122-H1131, 2008. First published July 11, 2008 doi:10.1152/ajpheart.00386.2008.-Transgenic overexpression of calcineurin (CN/Tg) in mouse cardiac myocytes results in hypertrophy followed by dilation, dysfunction, and sudden death. Nitric oxide (NO) produced via inducible NO synthase (iNOS) has been implicated in cardiac injury. Since calcineurin regulates iNOS expression, and since phenotypes of mice overexpressing iNOS are similar to CN/Tg, we hypothesized that iNOS is pathogenically involved in cardiac phenotypes of CN/Tg mice. CN/Tg mice had increased serum and cardiac iNOS levels. When CN/Tg-iNOS Ϫ/Ϫ and CN/Tg mice were compared, some phenotypes were similar: extent of hypertrophy and fibrosis. However, CN/Tg-iNOS Ϫ/Ϫ mice had improved systolic performance (P Ͻ 0.001) and less heart block (P Ͻ 0.0001); larger sodium current density and lower serum TNF-␣ levels (P Ͻ 0.03); and less apoptosis (P Ͻ 0.01) resulting in improved survival (P Ͻ 0.0003). To define tissue origins of iNOS production, chimeric lines were generated. Bone marrow (BM) from wild-type or iNOS Ϫ/Ϫ mice was transplanted into CN/Tg mice. iNOS deficiency restricted to BMderived cells was not protective. Calcineurin activates the local production of NO by iNOS in cardiac myocytes, which significantly contributes to sudden death, heart block, left ventricular dilation, and impaired systolic performance in this murine model of cardiac hypertrophy induced by the overexpression of calcineurin. heart block; sodium current; sudden death; inducible nitric oxide synthase; transgenic overexpression of calcineurin TRANSGENIC OVEREXPRESSION of constitutively active calcineurin (CN/Tg) in cardiac myocytes results in profound concentric hypertrophy followed by ventricular dilatation, interstitial fibrosis, heart failure, and eventually sudden cardiac death (13,26,32,33,40). The sudden deaths relate to ion channelopathies, which are downstream consequences of abnormalities of calcium homeostasis (17) and an inflammatory process (6, 10, 27, 36).Previous studies indicate that inducible nitric oxide (NO) synthase (iNOS) is a downstream target of calcineurin (12,19,36,44,45). The regulation of the iNOS gene promoter by calcineurin has been reported (36). LPS-induced iNOS expression in the heart was abrogated by the pharmacological inhibition of calcineurin (36). Similarly, LPS induced the expression of iNOS in wild-type (WT) hearts but not in the calcineurin A␤ knockout hearts (36). Reciprocally, the overexpression of constitutively active calcineurin in isolated cardiomyocytes caused the dephosphorylation and nuclear accumulation of the transcription factor family originally defined as nuclear factor of activated T-cells (NFAT)c1 and induced strong iNOS expression (36). In addition, chromatin immunoprecipitation confirmed the calcineurin-dependent binding of NFATc1 to the iNOS promoter (36). These data are co...

The prolonged ventricular refractoriness, seen only in calcineurin hearts, impinges on Wenckebach cycle length resulting in heart block and is associated with propensity to ventricular tachycardia.