. Atrial contractile dysfunction, fibrosis, and arrhythmias in a mouse model of cardiomyopathy secondary to cardiac-specific overexpression of tumor necrosis factor-␣. Am J Physiol Heart Circ Physiol 289: H1456 -H1467, 2005. First published May 27, 2005; doi:10.1152/ajpheart.00733.2004.-Transgenic mice overexpressing the inflammatory cytokine TNF-␣ in the heart develop a progressive heart failure syndrome characterized by biventricular dilatation, decreased ejection fraction, decreased survival compared with non-transgenic littermates, and earlier pathology in males. TNF-␣ mice (TNF1.6) develop atrial arrhythmias on ambulatory telemetry monitoring that worsen with age and are more severe in males. We performed in vivo electrophysiological testing in transgenic and control mice, ex vivo optical mapping of voltage in the atria of isolated perfused TNF1.6 hearts, and in vitro studies on isolated atrial muscle and cells to study the mechanisms that lead to the spontaneous arrhythmias. Programmed stimulation induces atrial arrhythmias (n ϭ 8/32) in TNF1.6 but not in control mice (n ϭ 0/37), with a higher inducibility in males. In the isolated perfused hearts, programmed stimulation with single extra beats elicits reentrant atrial arrhythmias (n ϭ 6/6) in TNF1.6 but not control hearts due to slow heterogeneous conduction of the premature beats. Lowering extracellular Ca 2ϩ normalizes conduction and prevents the arrhythmias. Atrial muscle and cells from TNF1.6 compared with control mice exhibit increased collagen deposition, decreased contractile function, and abnormal systolic and diastolic Ca 2ϩ handling. Thus abnormalities in action potential propagation and Ca 2ϩ handling contribute to the initiation of atrial arrhythmias in this mouse model of heart failure. heart failure; atrium; atrial fibrillation; cytokines INFLAMMATORY CYTOKINES, including TNF-␣, are increased in the serum and hearts of patients with congestive heart failure (CHF) and may contribute to the pathophysiology of the disease (18,24,25). We recently engineered mice that overexpress TNF-␣ in the heart under the control of the ␣-myosin heavy chain promoter (TNF1.6 mice) and develop a cardiomyopathy characterized by biventricular dilatation, decreased left ventricular ejection fraction, ventricular arrhythmias, and decreased survival compared with nontransgenic littermates (28). Most of the mice exhibit symptoms of CHF before death (tachypnea, cyanosis, and ascites) and evidence of decompensated heart failure at autopsy (pleural effusions, hepatic congestion, and severe atrial and ventricular dilatation).We have previously used optical mapping of voltage and calcium in ventricles from the TNF1.6 mice to show prolongation of action potential duration (APD), prolongation of calcium transient duration, and elevated diastolic and depressed systolic calcium (33). Premature beats had depressed action potential amplitude and slowed conduction velocities that contributed to initiation of reentrant arrhythmias. Lowering extracellular calcium reversed the abnor...

Mice that overexpress the inflammatory cytokine tumor necrosis factor-alpha in the heart (TNF mice) develop heart failure characterized by atrial and ventricular dilatation, decreased ejection fraction, atrial and ventricular arrhythmias, and increased mortality (males > females). Abnormalities in Ca2+ handling, prolonged action potential duration (APD), calcium alternans, and reentrant atrial and ventricular arrhythmias were previously observed with the use of optical mapping of perfused hearts from TNF mice. We therefore tested whether altered voltage-gated outward K+ and/or inward Ca2+ currents contribute to the altered action potential characteristics and the increased vulnerability to arrhythmias. Whole cell voltage-clamp recordings of K+ currents from left ventricular myocytes of TNF mice revealed an approximately 50% decrease in the rapidly activating, rapidly inactivating transient outward K+ current Ito and in the rapidly activating, slowly inactivating delayed rectifier current IK,slow1, an approximately 25% decrease in the rapidly activating, slowly inactivating delayed rectifier current IK,slow2, and no significant change in the steady-state current Iss compared with controls. Peak amplitudes and inactivation kinetics of the L-type Ca2+ current ICa,L were not altered. Western blot analyses revealed a reduction in the proteins underlying Kv4.2, Kv4.3, and Kv1.5. Thus decreased K+ channel expression is largely responsible for the prolonged APD in the TNF mice and may, along with abnormalities in Ca2+ handling, contribute to arrhythmias.

Chronic systemic hypoxia (SH) enhances myocardial ischemic tolerance in mammals. We studied the delayed cardioprotection caused by acute SH and associated signaling mechanism. Conscious adult male mice were exposed to one or two cycles of hypoxia (H; 10% O(2)) or normoxia (21% O(2)) for various durations (30 min, 2 h, 4 h) followed by 24 h of reoxygenation. Hearts were isolated 24 h later and subjected to ischemia-reperfusion in a Langendorff model. Infarct size was reduced in mice pretreated with one (H4h) or two cycles (H4hx2) of 4 h SH compared with normoxia mice (P < 0.05), which was abolished by an inducible nitric oxide synthase (NOS2) inhibitor (S-methylisothiourea, 3 mg/kg) given before SH or ischemia. H4hx2 also failed to reduce infarct size in NOS2 knockout mice. Cyclooxygenase-2 (COX-2) inhibitor (NS-398, 10 mg/kg) did not block the protection given either before H4hx2 or ischemia. A two- to three fold increase in myocardial NOS2 expression was observed in H4h, H2hx2, and H4hx2 (P < 0.05), whereas endothelial NOS (NOS3) or COX-2 remained unchanged. We conclude that acute SH induces delayed cardioprotection, which is triggered and mediated by NOS2, but not by NOS3 or COX-2.

No abstract

In the present study, we have established the biological effects during 8 weeks of (i) caloric restriction (Cal) and (ii) simultaneous administration of Cal plus 2 hr daily immobilization stress using male Sprague-Dawley rats. Animals were divided into three equal groups: (i) ad libitum fed, (ii) 30% restriction of food intake of the ad libitum diet, and (iii) 30% restriction of food intake plus 2 hr daily immobilization stress. Caloric-restricted animals gained only 30% of the total body weight of the unrestricted animals but received 70% of the food of those rats. Cal animals showed a significant loss in their relative liver and thymus weight and a significant gain in their relative adrenal and testis weight as compared to the control animals. Cal animals had almost 2-fold higher levels of plasma corticosterone levels with a dramatic decrease in the total glucocorticoid receptor (GR) levels in the liver, thymus, heart, and testis as compared to ad libitum fed control animals. Interestingly, Cal animals showed higher levels of lipid peroxidation in both the liver and heart, indicating increased oxidative activities in these tissues when compared with the control animals. In addition, Cal animals had increased heat shock protein 70 (HSP 70) content in the testis. Surprisingly, hardly any significant differences were observed in either total body weight gain, organ weights, Plasma corticosterone levels, or lipid peroxidation between Cal animals and Cal plus immobilization-stressed animals. The results obtained suggest that (i) several stress-related responses such as inhibition of total body weight gain, increased adrenal weight, decreased thymus weight, increased plasma corticosterone, and lipid peroxidation levels in the liver and heart are associated with Cal, but (ii) no additional effects were observed on the parameters that were measured when two stress regimens were given simultaneously, suggesting that animals subjected to two stress regimens can protect themselves by controlling their stress-related thresholds of response through adaptation.