Hideaki Kanki

Allelic variants in long-QT disease genes in patients with drug-associated torsades de pointes.

Background—DNA variants appearing to predispose to drug-associated “acquired” long-QT syndrome (aLQTS) have been reported in congenital long-QT disease genes. However, the incidence of these genetic risk factors has not been systematically evaluated in a large set of patients with aLQTS. We have previously identified functionally important DNA variants in genes encoding K+ channel ancillary subunits in 11% of an aLQTS cohort. Methods and Results—The coding regions of the genes encoding the pore-forming channel proteins KvLQT1, HERG, and SCN5A were screened in (1) the same aLQTS cohort (n=92) and (2) controls, drawn from patients tolerating QT-prolonging drugs (n=67) and cross sections of the Middle Tennessee (n=71) and US populations (n=90). The frequency of three common nonsynonymous coding region polymorphisms was no different between aLQTS and control subjects, as follows: 24% versus 19% for H558R (SCN5A), 3% versus 3% for R34C (SCN5A), and 14% versus 14% for K897T (HERG). Missense mutations (absent in controls) were identified in 5 of 92 patients. KvLQT1 and HERG mutations (one each) reduced K+ currents in vitro, consistent with the idea that they augment risk for aLQTS. However, three SCN5A variants did not alter INa, which argues that they played no role in the aLQTS phenotype. Conclusions—DNA variants in the coding regions of congenital long-QT disease genes predisposing to aLQTS can be identified in ≈10% to 15% of affected subjects, predominantly in genes encoding ancillary subunits.

Atrial Fibrillation in KCNE1-Null Mice

Although atrial fibrillation is the most common serious cardiac arrhythmia, the fundamental molecular pathways remain undefined. Mutations in KCNQ1, one component of a sympathetically activated cardiac potassium channel complex, cause familial atrial fibrillation, although the mechanisms in vivo are unknown. We show here that mice with deletion of the KCNQ1 protein partner KCNE1 have spontaneous episodes of atrial fibrillation despite normal atrial size and structure. Isoproterenol abolishes these abnormalities, but vagomimetic interventions have no effect. Whereas loss of KCNE1 function prolongs ventricular action potentials in humans, KCNE1−/− mice displayed unexpectedly shortened atrial action potentials, and multiple potential mechanisms were identified: (1) K+ currents (total and those sensitive to the KCNQ1 blocker chromanol 293B) were significantly increased in atrial cells from KCNE1−/− mice compared with controls, and (2) when CHO cells expressing KCNQ1 and KCNE1 were pulsed very rapidly (at rates comparable to the normal mouse heart and to human atrial fibrillation), the sigmoidicity of IKs activation prevented current accumulation, whereas cells expressing KCNQ1 alone displayed marked current accumulation at these very rapid rates. Thus, KCNE1 deletion in mice unexpectedly leads to increased outward current in atrial myocytes, shortens atrial action potentials, and enhances susceptibility to atrial fibrillation.

Phosphorylation of the IKs channel complex inhibits drug block: novel mechanism underlying variable antiarrhythmic drug actions.

Background—IKs, an important repolarizing current in heart, is an antiarrhythmic drug target and is markedly increased by activation of protein kinase A (PKA; eg, by &bgr;-adrenergic stimulation). Because &bgr;-adrenergic stimulation is a frequent trigger of arrhythmias, we hypothesized that PKA stimulation inhibits drug block. Methods and Results—CHO cells were transfected with KCNQ1 cDNA (encoding the pore-forming subunit) with or without the ancillary subunit KCNE1. IC50 for quinidine block of basal IKs was 5.8±1.2 &mgr;mol/L, versus 19.9±3.2 &mgr;mol/L (P <0.01) for PKA-stimulated current. A similar >3-fold shift was apparent in the absence of KCNE1 and with the IKs-specific blocker chromanol 293B. The first current recorded after channels were held at rest and exposed to the drug was reduced ≈40%, and further depolarizations increased the block with a time constant (&tgr;) of 181±27 seconds. By contrast, PKA-stimulated channels displayed a <5% first-pulse block and much slower block development (&tgr;=405±85 seconds). Alanine substitution at 3 potential PKA target sites (S27, S468, and T470) generated an IKs that did not increase with PKA stimulation; this mutant retained wild-type drug sensitivity that was unaffected by PKA. Conclusions—Activation of this key intracellular signaling pathway blunts drug block. The onset of block and the data with the PKA-resistant mutant support the concept that phosphorylation of the KCNQ1 subunit directly modulates drug access to a binding site on the channel. These data identify a novel mechanism for modulation of drug-channel interactions that may be especially important during &bgr;-adrenergic stimulation.

Determination of refractory periods and conduction velocity during atrial fibrillation using atrial capture in dogs: Direct assessment of the wavelength and its modulation by a sodium channel blocker, pilsicainide

OBJECTIVES The purposes of this study were to measure the atrial refractory period and the conduction velocity (CV) during atrial fibrillation (AF) and to explore the antiarrhythmic mechanism of a sodium channel blocker, pilsicainide, during AF. BACKGROUND Sodium channel blockers not only decrease the CV, but also prolong the atrial refractory period, particularly during rapid excitation. Because these effects on the wavelength are counteractive and rate dependent, it is critical to measure these parameters during AF. METHODS In eight dogs, after AF was induced under vagal stimulation, a single extra-stimulus was repeatedly introduced from the left atrium and its capture was statistically determined for each coupling interval. The local CV was also measured during constant capture of the fibrillating atrium by rapid pacing. The same procedure was repeated after pilsicainide administration. RESULTS Pilsicainide significantly increased the mode of AF intervals from 81 +/- 10 to 107 +/- 16 ms (p < 0.01). While the CV was decreased from 0.9 +/- 0.1 to 0.7 +/- 0.1 m/s (p < 0.02), the effective refractory period during AF was increased from 69 +/- 11 ms to 99 +/- 17 ms (p < 0.01). As a result, the wavelength was significantly increased by pilsicainide from 6.6 +/- 0.9 to 7.6 +/- 1.2 cm (p < 0.05). CONCLUSIONS During AF, whereas the sodium channel blocker pilsicainide decreases CV, it lengthens the wavelength by increasing the refractory period, an action that is likely to contribute to the drugs ability to terminate the arrhythmia. The direct measurement of refractoriness and CV during AF may provide new insights into the determinations of the arrhythmia and antiarrhythmic drug action.

Polymorphisms in Beta-Adrenergic Receptor Genes in the Acquired Long QT Syndrome

Adrenergic Receptor Polymorphism and Arrhythmia. Introduction: Sympathetic activation is a trigger for life‐threatening arrhythmias in many patients with the congenital long QT syndrome (LQTS), and an increase in heart rate has been reported just prior to torsades de pointes in patients with drug‐associated (acquired) LQTS (aLQTS). We compared the frequencies of five recognized nonsynonymous coding region polymorphisms in genes encoding the β1‐adrenergic and β2‐adrenergic receptors (AR) in 93 patients with aLQTS and 3 control groups: an ethically diverse set of individuals from middle Tennessee (n = 71), a subset of the Polymorphism Discovery Resource obtained from National Human Genome Research Institute (n = 89), and patients who tolerated QT‐prolonging drugs without aLQTS (non‐aLQTS group; n = 66).

Postrepolarization Refractoriness as a Potential Anti–Atrial Fibrillation Mechanism of Pilsicainide, a Pure Sodium Channel Blocker with Slow Recovery Kinetics

The antifibrillatory effect of pilsicainide, a sodium channel blocker with slow recovery kinetics, was investigated in a canine model of atrial fibrillation. Prolonging the atrial effective refractory period is an important mechanism for pharmacological termination of atrial fibrillation. However, the effectiveness of potassium channel blockers has been questioned because of their reverse-use–dependent property. In eight open-chest dogs, the duration of the atrial endocardial monophasic action potential and the atrial effective refractory period were determined using a Franz catheter. Conduction velocity was obtained from a 96-channel mapping electrode at multiple cycle lengths. Inducibility of sustained atrial fibrillation (>30 minutes) was confirmed by atrial burst pacing during bilateral vagal stimulation, and local fibrillation cycle lengths were measured. Five minutes after restarting fibrillation, pilsicainide (0.6 mg/kg + 0.04 mg/kg/min) was administered. After fibrillation was terminated, measurements were repeated. Pilsicainide successfully terminated atrial fibrillation in 7 of 8 dogs after the median time of 5.1 minutes. The conduction velocity decreased significantly. Although pilsicainide did not affect monophasic action potential duration, it caused use-dependent prolongation of the atrial effective refractory period (P < 0.05), creating postrepolarization refractoriness. Accordingly, pilsicainide prolonged the atrial fibrillation cycle length from 80.6 to 113.8 ms (P < 0.05) before termination of fibrillation. Sodium channel blockers with slow recovery kinetics can prolong the atrial effective refractory period without affecting monophasic action potential duration. Unlike potassium channel blockers, these sodium channel blockers maintain postrepolarization refract

Recovery of electrophysiological parameters after conversion of atrial fibrillation

We investigated the recovery of electrophysiological parameters from electrical remodeling after conversion of chronic lone atrial fibrillation in humans. Clinical studies have shown that the longer atrial fibrillation lasts, the more difficult it becomes to maintain the sinus rhythm after cardioversion. To explore the effects of the duration of atrial fibrillation on changes of electrophysiological parameters after conversion, we determined the atrial effective refractory period and P wave duration during right atrial pacing at 1 and 24 h after electrical cardioversion in 15 patients with chronic lone atrial fibrillation (median duration, 6 months). By 24 h after cardioversion, the effective refractory period at a pacing cycle length of 600 ms increased from 225+/-19 to 254+/-27 ms. However, the P wave duration did not decrease significantly 24 h after conversion. As the duration of atrial fibrillation became longer, the prolongation of effective refractory period was more delayed (P<0. 001, r=0.82), and the shortening of P wave duration was significantly smaller within 24 h after cardioversion (P<0. 001, r=0.67). After cardioversion of chronic lone atrial fibrillation, the recovery of shortened atrial refractoriness and prolonged intraatrial conduction time is dependent on the duration of preexisting atrial fibrillation.

COMPARISON OF NERVE GROWTH FACTOR MRNA EXPRESSION IN CARDIAC AND SKELETAL MUSCLE IN STREPTOZOTOCIN-INDUCED DIABETIC MICE

To address the role of nerve growth factor (NGF) in diabetes mellitus (DM)-induced cardiac autonomic neuropathy, we quantitated and compared the expression of NGF mRNA in the cardiac and the skeletal muscle in experimental DM mice with the RT-PCR-HPLC method, which we have developed previously, using a NGF deletion mutant RNA as an internal standard. DM was induced in ICR mice via intraperitoneal injection of streptozotocin. RT-PCR was performed using total RNA extracted from left ventricle and soleus muscle, and the levels of NGF mRNA were quantitated by HPLC analysis. NGF mRNA content of the cardiac muscle was 17-fold higher than the skeletal muscles in control mice. NGF mRNA content of the cardiac muscle in diabetic mice at 6 weeks was 4.0-fold higher than that in the control mice, while that of the skeletal muscle in diabetic mice was not different from the controls. These results indicated that the DM-induced increase in NGF mRNA content was higher in cardiac muscle than skeletal muscle, and that NGF might play an important role in cardiac autonomic neuropathy.

Differential effects of N-acetylcysteine on nitroglycerin- and nicorandil-induced vasodilation in human coronary circulation

We investigated the role of the availability of sulfhydryl groups during vasodilation of the human coronary circulation induced by nitroglycerin and nicorandil. In patients with normal coronary arteries (n = 29) or with coronary artery disease (CAD; n = 26), coronary blood flow (CBF) and epicardial coronary artery diameter after intracoronary administration of 50 microg nitroglycerin or 0.5 mg nicorandil were measured, before and after the intravenous infusion of saline or 100 mg/kg of N-acetylcysteine (NAC). In normal subjects, saline infusion did not alter the nitroglycerin- and nicorandil-induced vasodilation in large epicardial coronary artery. In contrast, NAC potentiated both nitroglycerin- and nicorandil-induced vasodilation. In patients with CAD, nitroglycerin and nicorandil induced less dilation than in normal subjects. NAC augmented the nitroglycerin- and nicorandil-induced vasodilation in the small epicardial coronary artery, but not in the large epicardial segments. In both groups, NAC potentiated the increase in CBF in response to nitroglycerin. However, NAC had no effects on the CBF response to nicorandil. Sulfhydryl availability is at least one determinant of the in vivo responsiveness to nitroglycerin of conductance and resistance vessels in normal human coronary circulation. In patients with CAD, external augmentation of sulfhydryl availability did not affect the depressed response to nitroglycerin in the large epicardial coronary artery. Although nicorandil acts as an NO donor, similar to nitroglycerin, in dilating the epicardial coronary artery, other effects, such as the opening of K(ATP) channel, play a more important role in the nicorandil-induced vasodilation of resistance vessels.

Electropharmacologic effects of pilsicainide, a pure sodium channel blocker, on the remodeled atrium subjected to chronic rapid pacing.

Clinical experience suggests that sodium channel blockers are effective in converting atrial fibrillation of recent onset but not chronic atrial fibrillation. We investigated changes in the electrophysiologic effects of pilsicainide, a pure sodium channel blocker, on the canine atrium during chronic rapid pacing (400/min). Three pairs of bipolar electrodes were sutured to the right atrial appendage in six dogs. Five days later, rapid atrial pacing was started after baseline measurements of the effective refractory period (ERP), the intra-atrial conduction velocity, the atrial wavelength, and the inducibility of atrial fibrillation. These studies were repeated at 2, 7, and 14 days of pacing, both before and after pilsicainide administration. Before pacing, pilsicainide increased ERP more than it decreased conduction velocity, causing an increase of wavelength, particularly at faster rates. However, this use-dependent prolongation of ERP disappeared after 2 days of pacing. Thus, pilsicainide failed to prolong ERP during chronic pacing, allowing progressive shortening of wavelength in the remodeled atrium. The effect of sodium channel blockers on atrial refractoriness may decline as rapid atrial excitation persists, limiting the usefulness of these agents for the treatment of chronic atrial fibrillation.