Hyperglycemia and hypoglycemia both can cause prolongation of the Q-T interval and ventricular arrhythmias. Here we studied modulation of human ether-à -gogo-related gene (HERG) K ؉ channel, the major molecular component of delayed rectifier K ؉ current responsible for cardiac repolarization, by glucose in HEK293 cells using whole-cell patch clamp techniques. We found that both hyperglycemia (extracellular glucose concentration Glucose, the primary end product of the digestion of glycogen, is essential for maintaining life activities in organisms. As a major source of metabolic fuel, degradation of glucose via glycolysis and subsequent oxidative phosphorylation generates high energy phosphates to power the biological processes in the cell. Yet, through an exquisitely complex network of control mechanisms, the rate of glucose metabolism is only as great as needed by the organisms. Moreover, glucose also has other regulatory effects on many cellular functions. Either inadequate or excessive glucose can be harmful to the living system. Therefore, the blood glucose level is dynamically controlled. However, under pathological conditions like diabetes, glucose cannot be efficiently utilized, and the blood glucose level rises. When the blood level of glucose is maintained higher than 7 mM, it is considered as hyperglycemia. Diabetes therapy, on the other hand, can lead to an overly low level of blood glucose, which is referred to as hypoglycemia when the level falls below 3 mM.Either hypoglycemia or hyperglycemia can have deleterious effects on the cells. One common feature of electrophysiological alterations caused by both hypoglycemia and hyperglycemia in the heart is prolongation of Q-T interval and the associated ventricular arrhythmias that are presumably responsible for sudden cardiac death in diabetic patients (1-10). However, the ionic mechanisms by which hyperglycemia and hypoglycemia prolong Q-T interval remained unclear, which is at least a part of the reasons why diabetic patients die of mainly cardiac complications.The human either-à -go-go-related gene (HERG) 1 encodes the rapid component of delayed rectifier K ϩ current in the heart, which is the major repolarizing current in the plateau voltage range of cardiac action potentials. HERG K ϩ channels are susceptible to genetic defects and environmental cues, with the consequence being depression of HERG function in most situations (9). Indeed, most of the cases of long Q-T syndrome are ascribed to dysfunction of HERG channels, particularly that induced by therapeutic drugs (13). It is conceivable that HERG alteration might also be involved in the Q-T prolongation induced by hyperglycemia and hypoglycemia. This thought prompted us to carry out a series of experiments to study the effects of glucose on HERG K ϩ channels and the potential mechanisms.
Congestive heart failure (CHF) is associated with susceptibility to lethal arrhythmias and typically increases levels of tumor necrosis factor-␣ (TNF-␣) and its receptor, TNFR1. CHF down-regulates rapid delayed-rectifier K ؉ current (I Kr ) and delays cardiac repolarization. We studied the effects of TNF-␣ on cloned HERG K ؉ channel (human ether-a-go-go-related gene) in HEK293 cells and native I Kr in canine cardiomyocytes with whole-cell patch clamp techniques. TNF-␣ consistently and reversibly decreased HERG current (I HERG ). Effects of TNF-␣ were concentration-dependent, increased with longer incubation period, and occurred at clinically relevant concentrations. TNF-␣ had similar inhibitory effects on I Kr and markedly prolonged action potential duration (APD) in canine cardiomyocytes. Immunoblotting analysis demonstrated that HERG protein level was slightly higher in canine hearts with tachypacing-induced CHF than in healthy hearts, and TNF-␣ slightly increased HERG protein level in CHF but not in healthy hearts. In cells pretreated with the inhibitory anti-TNFR1 antibody, TNF-␣ lost its ability to suppress I HERG , indicating a requirement of TNFR1 activation for HERG suppression. Vitamin E or MnTBAP (Mn(III) tetrakis(4-benzoic acid) porphyrin chloride), a superoxide dismutase mimic) prevented, whereas the superoxide anion generating system xanthine/xanthine oxidase mimicked, TNF-␣-induced I HERG depression. TNF-␣ caused robust increases in intracellular reactive oxygen species, and vitamin E and MnTBAP abolished the increases, in both HEK293 cells and canine ventricular myocytes. We conclude that the TNF-␣/TNFR1 system impairs HERG/I Kr function mainly by stimulating reactive oxygen species, particularly superoxide anion, but not by altering HERG expression; the effect may contribute to APD prolongation by TNF-␣ and may be a novel mechanism for electrophysiological abnormalities and sudden death in CHF.
Abnormal QT prolongation (QT-P) in diabetic patients has become a nonnegligible clinical problem and has attracted increasing attention from basic scientists, because it increases the risk of lethal ventricular arrhythmias. Correction of QT-P may be an important measure in minimizing sudden cardiac death in diabetic patients. Here we report the efficacy of insulin in preventing QT-P and the associated arrhythmias and the mechanisms underlying the effects in a rabbit model of type 1 insulin-dependent diabetes mellitus (IDDM). The heart rate-corrected QT (QTc) interval and action potential duration were considerably prolonged, with frequent ventricular tachycardias. The rapid delayed rectifier K+ current (IKr) was markedly reduced in IDDM hearts, and hyperglycemia depressed the function of the human ether-a-go-go-related gene (HERG), which conducts IKr. The impairment was primarily ascribed to the enhanced oxidative damage to the myocardium, as indicated by the increased intracellular level of reactive oxygen species and simultaneously decreased endogenous antioxidant reserve and by the increased lipid peroxidation and protein oxidation. Moreover, IDDM or hyperglycemia resulted in downregulation of HERG protein level. Insulin restored the depressed IKr/HERG and prevented QTc/action potential duration prolongation and the associated arrhythmias, and the beneficial actions of insulin are partially due to its antioxidant ability. Our study represents the first documentation of oxidative stress as the major metabolic mechanism for HERG K+ dysfunction, which causes diabetic QT-P, and suggests IKr/HERG as a potential therapeutic target for treatment of the disorder.
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