Fast lidocaine block of cardiac and skeletal muscle sodium channels: one site with two routes of access

Abstract

We have studied the block by lidocaine and its quaternary derivative, QX-314, of single, batrachotoxin (BTX)-activated cardiac and skeletal muscle sodium channels incorporated into planar lipid bilayers. Lidocaine and QX-314, applied to the intracellular side, appear to induce incompletely resolved, rapid transitions between the open and the blocked state of BTX-activated sodium channels from both heart and skeletal muscle. We used amplitude distribution analysis (Yellen, G. 1984. J. Gen. Physiol. 84:157-186.) to estimate the rate constants for block and unblock. Block by lidocaine and QX-314 from the cytoplasmic side exhibits rate constants with similar voltage dependence. The blocking rate increases with depolarization, and the unblocking rate increases with hyperpolarization. Fast lidocaine block was virtually identical for sodium channels from skeletal (rat, sheep) and cardiac (beef, sheep) muscle. Lidocaine block from the extracellular side occurred at similar concentrations. However, for externally applied lidocaine, the blocking rate was voltage-independent, and was proportional to concentration of the uncharged, rather than the charged, form of the drug. In contrast, unblocking rates for internally and externally applied lidocaine were identical in magnitude and voltage dependence. Our kinetic data suggest that lidocaine, coming from the acqueous phase on the cytoplasmic side in the charged form, associates and dissociates freely with the fast block effector site, whereas external lidocaine, in the uncharged form, approaches the same site via a direct, hydrophobic path.

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

“…A logarithmic transformation of the blocked and unblocked current amplitudes (Fig. 5 B, right) allowed estimation of an electrical distance to the binding site (␦) of 0.68 from the cytoplasmic face, and an affinity at 0 mV (K d0 ) of 877 M. This low affinity for the open pore differs from estimates of lidocaine affinity for the inactivated state (K d ‫ف‬ 1-12 M) (4, 9, 46), but is consistent with other analyses of fast voltage-dependent block of the open channel (47). Rapid intrapore block reduces the unitary current amplitude and scales the ensemble average current to some degree, but would not accelerate the macroscopic current decay.…”

“…3), apparently restoring wild-type inactivation gating characteristics. The IC 50 for lidocaine block of FQ plateau current (74 M) approached the lidocaine K d values recently reported for steady state block of wild-type channels fully inactivated by long depolarizing prepulses (12 M) (46), and differed markedly from the low ‫ف(‬ mM) affinities reported for open-pore lidocaine block (47).…”

Evidence for a direct interaction between internal tetra-alkylammonium cations and the inactivation gate of cardiac sodium channels.

“…The simplest explanation for this effect is that Na ions can occupy a site in the pore and inhibit TPeA binding. Similar tram-side effects have been observed for other cytoplasmic blockers of Na channels, such as the quaternary lidocaine derivative QX-314 (Cahalan and Almers, 1979b;Gingrich, Beardsley, and Yue, 1993;Zamponi, Doyle, and French, 1993), and strychnine (Shapiro, 1977). The mechanism of the antagonism of TPeA binding by external Na could be due either to a competitive occupancy of the same site, or to a destabilization (i.e., knock off) of the TPeA when a Na ion occupies an adjacent site within the pore.…”

Novel Molecular Determinants in the Pore Region of Sodium Channels Regulate Local Anesthetic Binding

“…Quaternary ammonium local anesthetic binding remains voltage-dependent in channels that have had inactivation gating removed chemically (Gingrich et al, 1993) or genetically consistent with drug binding in the channel pore. The fractional electrical distance for local anesthetic drug binding in the pore of the Na ϩ channel has been predicted to lie ϳ30% into the electrical field from the extracellular surface (Strichartz, 1973;Gingrich et al, 1993;Zamponi et al, 1993a;Yamagishi et al, 1997). This places the binding site near the selectivity filter (Heinemann et al, 1992;Chiamvimonvat et al, 1996;Favre et al, 1996;Yamagishi et al, 1997).…”