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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Article

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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Figure 4

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Time-dependent recovery of availability at –80 mV. HEK cells expressing ...
Time-dependent recovery of availability at –80 mV. HEK cells expressing either WT (a)or mutant (b) Na channels were subjected to the voltage-clamp protocol shown in the inset in the presence or absence of lidocaine. Data are plotted on a logarithmic scale to highlight the individual kinetic components of recovery. Fractional recovery was assessed as the ratio of peak INa during the second, relative to first, pulse (P2/P1) and is normalized to the 1,000-millisecond data point to eliminate differences in tonic block at –80 mV (per Figure 3b). The mean data were fitted to the biexponential function y = A1(1 – e–t/τ1) + A2(1 – e–t/τ2), where A1, τ1 are the amplitude and time constant of the prominent, rapid recovery component, and A2, τ2 reflect a smaller, slow-recovery component. Fitted parameters were as follows: WT (n = 8): A1 = 0.79, τ1 = 31 ms, A2 = 0.21, τ2 = 169 ms; WT + lidocaine (n = 8): A1 = 0.80, τ1 = 23 ms, A2 = 0.20, τ2 = 85 ms; R1623Q (n = 12): A1 = 0.82, τ1 = 9 ms, A2 = 0.18, τ2 = 100 ms; R1623Q + lidocaine (n = 4): A1 = 0.64, τ1 = 14 ms, A2 = 0.36, τ2 = 128 ms. Lidocaine delayed the recovery of R1623Q channels, but did not slow wild-type recovery.