A revised view of cardiac sodium channel “blockade” in the long-QT syndrome
Nicholas G. Kambouris, … , Gordon F. Tomaselli, Jeffrey R. Balser
Nicholas G. Kambouris, … , Gordon F. Tomaselli, Jeffrey R. Balser
Published April 15, 2000
Citation Information: J Clin Invest. 2000;105(8):1133-1140. https://doi.org/10.1172/JCI9212.
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A revised view of cardiac sodium channel “blockade” in the long-QT syndrome

Abstract

Mutations in SCN5A, encoding the cardiac sodium (Na) channel, are linked to a form of the congenital long-QT syndrome (LQT3) that provokes lethal ventricular arrhythmias. These autosomal dominant mutations disrupt Na channel function, inhibiting channel inactivation, thereby causing a sustained ionic current that delays cardiac repolarization. Sodium channel–blocking antiarrhythmics, such as lidocaine, potently inhibit this pathologic Na current (INa) and are being evaluated in patients with LQT3. The mechanism underlying this effect is unknown, although high-affinity “block” of the open Na channel pore has been proposed. Here we report that a recently identified LQT3 mutation (R1623Q) imparts unusual lidocaine sensitivity to the Na channel that is attributable to its altered functional behavior. Studies of lidocaine on individual R1623Q single-channel openings indicate that the open-time distribution is not changed, indicating the drug does not block the open pore as proposed previously. Rather, the mutant channels have a propensity to inactivate without ever opening (“closed-state inactivation”), and lidocaine augments this gating behavior. An allosteric gating model incorporating closed-state inactivation recapitulates the effects of lidocaine on pathologic INa. These findings explain the unusual drug sensitivity of R1623Q and provide a general and unanticipated mechanism for understanding how Na channel–blocking agents may suppress the pathologic, sustained Na current induced by LQT3 mutations.

Authors

Nicholas G. Kambouris, H. Bradley Nuss, David C. Johns, Eduardo Marbán, Gordon F. Tomaselli, Jeffrey R. Balser

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Unitary currents from stably transfected HEK cells. Cell-attached patche...
Unitary currents from stably transfected HEK cells. Cell-attached patches were repeatedly depolarized to –40 mV for 150 milliseconds at either 10-second (no drug) or 20-second (200 μM lidocaine) intervals (holding potential, –100 or –110 mV). Sweeps with channel openings were selected to emphasize the kinetic character of openings and reopenings and do not reflect the overall probability of opening in the conditions indicated. The patches shown contained ≤ 4 active channels. Shown beneath each column of single-channel records are representative open-time histograms recorded under the same conditions. Each histogram includes data from 150–200 depolarizing sweeps. The histogram ordinates were normalized for visual comparison, and an exponential function (y = A1e–t/τ1 + A2e–t/τ2) was fitted to the data (solid line) to determine the dwell times. Parameters derived from histograms synthesized from individual patches are summarized in Table 1.