On the Molecular Nature of the Lidocaine Receptor of Cardiac Na ⁺ Channels

Abstract

The mechanism of inhibition of Na+ channels by lidocaine has been suggested to involve low-affinity binding to rested states and high-affinity binding to the inactivated state of the channel, implying either multiple receptor sites or allosteric modulation of receptor affinity. Alternatively, the lidocaine receptor may be guarded by the channel gates. To test these distinct hypotheses, inhibition of Na+ channels by lidocaine was studied by voltage-clamp methods in both native and heterologous expression systems. Native Na+ channels were studied in guinea pig ventricular myocytes, and recombinant human heart Na+ channels were expressed in Xenopus laevis oocytes. Fast inactivation was eliminated by mutating three amino acids (isoleucine, phenylalanine, and methionine) in the III-IV interdomain to glutamines or by enzymatic digestion with alpha-chymotrypsin. In channels with intact fast inactivation, lidocaine block developed with a time constant of 589 +/- 42 ms (n = 7) at membrane potentials between -50 and +20 mV, as measured by use of twin pulse protocols. The IC50 was 36 +/- 1.8 mumol/L. Control channels inactivated within 20 ms, and slow inactivation developed much later (time constant of slow inactivation, 6.2 +/- 0.36 s). The major component of block developed long after activated and open channels were no longer available for drug binding. Control channels recovered fully from inactivation in < 50 ms at -120 mV (time constant, 11 +/- 0.5 ms; n = 50).(ABSTRACT TRUNCATED AT 250 WORDS)

Local anesthetics as effectors of allosteric gating. Lidocaine effects on inactivation-deficient rat skeletal muscle Na channels.

“…We replaced a single phenylalanine residue in the III-IV linker of the rat skeletal muscle Na channel ( 1) 1 with glutamine (F1304Q) (13) and evaluated the effect of lidocaine on these inactivation-disabled channels when expressed in Xenopus oocytes. In contrast to earlier reports with fast inactivation entirely eliminated (6,14), lidocaine enhanced the decay of current flowing through F1304Q channels, essentially restoring the wild-type phenotype. We implemented a contemporary scheme for Na channel gating that incorporates allosteric coupling between activation and fast inactivation (16).…”

“…Gating current measurements support the view that inactivation and QX-314 block are functionally linked (12). In the human cardiac Na channel, lidocaine-induced use dependence is reduced when fast inactivation is eliminated by mutation of a triplet of hydrophobic III-IV linker residues (13, 14). Experiments with long-chain alkylammonium compounds further suggest that local anesthetic molecules may interact directly with the inactivation mechanism (15).…”

A revised view of cardiac sodium channel “blockade” in the long-QT syndrome

“…Wildtype ( Figure 1a, part i) and R1623Q ( Figure 1a, part ii) I Na is shown before and during exposure to 200 µM lidocaine. As shown previously, a major feature of the drug-free R1623Q phenotype is slowed decay of I Na (22, 23), a feature that differs from LQT3 mutations in or near the III-IV linker (10,11,15). Figure 1a illustrates that lidocaine has little effect on the rate of wild-type I Na decay, but speeds the decay of R1623Q I Na , partly correcting the abnormal phenotype (22).…”

“…This pathologic, sustained I Na delays cellular repolarization, prolonging the electrocardiographic QT interval and predisposing patients to polymorphic ventricular tachycardia. Lidocaine binds to amino acids positioned on the intracellular side of the aqueous, ion-conducting pore (12)(13)(14), near (and perhaps overlapping) the inactivation gating domains (15). It is apparent that lidocaine and its analogues reduce the pathologic current through Na channels with SCN5A mutations (10,16), and this may underlie the beneficial clinical effect of these agents in patients with LQT3 disorders (17).…”

Probing kinetic drug binding mechanism in voltage-gated sodium ion channel: open state versus inactive state blockers

“…This behavior of lidocaine is also consistent with the experiments done earlier as was previously reported that lidocaine was more effective in late component of Na C current than peak current in DKPQ channels expressed in mamalian cells. 45,46,47,48,49 Lidocaine preferentially blocked late over peak current and the blockade was equally effective in all 3 channels having mutations N1325S, R1644H and DKPQ expressed in Xenopus oocytes. 50 Lidocaine inhibits dispersed reopening in single channels without affecting mean open times.…”