Tryptophan substitution of a putative D4S6 gating hinge alters slow inactivation in cardiac sodium channels.

Voltage-gated Na(+) channels display rapid activation gating (opening) as well as fast and slow inactivation gating (closing) during depolarization. We substituted residue S1759 (serine), a putative D4S6 gating hinge of human cardiac hNav1.5 Na(+) channels with A (alanine), D (aspartate), K (lysine), L (leucine), P (proline), and W (tryptophan). Significant shifts in gating parameters for activation and steady-state fast inactivation were observed in A-, D-, K-, and W-substituted mutant Na(+) channels. No gating shifts occurred in the L-substituted mutant, whereas the P-substituted mutant did not yield sufficient Na(+) currents. Wild-type, A-, D-, and L-substituted mutant Na(+) channels showed little or no slow inactivation with a 10-s conditioning pulse ranging from -180 to 0 mV. Unexpectedly, W- and K-substituted mutant Na(+) channels displayed profound maximal slow inactivation around -100 mV ( approximately 85% and approximately 70%, respectively). However, slow inactivation was progressively reversed in magnitude from -70 to 0 mV. This regression was minimized in inactivation-deficient hNav1.5-S1759W/L409C/A410W Na(+) channels, indicating that the intracellular fast-inactivation gate caused such a reversal. Our data suggest that the hNav1.5-S1759 residue plays a critical role in slow inactivation. Possible mechanisms for S1759 involvement in slow inactivation and for antagonism between fast and slow inactivation are discussed.

State-dependent block of voltage-gated Na+ channels by amitriptyline via the local anesthetic receptor and its implication for neuropathic pain.

Amitriptyline is a tricyclic antidepressant, which also alleviates various pain syndromes at its therapeutic plasma concentration (0.36-0.90 microM). Accumulated evidence suggests that such efficacy may be due to block of voltage-gated Na(+) channels. The Na(+) channel alpha-subunit protein consists of four homologous domains (D1-D4), each with six transmembrane segments (S1-S6). The aims of this study were to locate the amitriptyline receptor in the Na(+) channel alpha-subunit and to compare the amitriptyline affinity in open, inactivated, and resting states of the Na(+) channel. Wild-type and mutant rat skeletal muscle alpha-subunit Na(+) channels were expressed in human embryonic kidney cells and assayed under whole-cell voltage clamp conditions. Our results indicate that the amitriptyline receptor overlaps with the local anesthetic receptor to a great extent in Na(+) channels. Residues N434 (at D1-S6), L1280 (D3-S6), and F1579 (D4-S6) may jointly form parts of the amitriptyline/local anesthetic receptor, with residue L1280 being most critical for amitriptyline binding. Open-channel block by amitriptyline was assessed in inactivation-deficient Na(+) channels and compared with the resting- and inactivated-channel block in wild-type channels. The open-channel block by amitriptyline has the highest affinity, with a 50% inhibitory concentration (IC(50)) of 0.26 microM. The inactivated-channel block by amitriptyline had a weaker affinity (0.51 microM), whereas the resting-channel displayed the weakest affinity (33 microM). We hypothesize that selective block of both persistent late openings and the inactivated state of neuronal Na(+) channel isoforms by amitriptyline also occurs at its therapeutic concentration and likely contributes to its efficacy in pain syndromes.

Mexiletine block of wild-type and inactivation-deficient human skeletal muscle hNav1.4 Na+ channels.

Mexiletine is a class 1b antiarrhythmic drug used for ventricular arrhythmias but is also found to be effective for paramyotonia congenita, potassium-aggravated myotonia, long QT-3 syndrome, and neuropathic pain. This drug elicits tonic block of Na(+) channels when cells are stimulated infrequently and produces additional use-dependent block during repetitive pulses. We examined the state-dependent block by mexiletine in human skeletal muscle hNav1.4 wild-type and inactivation-deficient mutant Na(+) channels (hNav1.4-L443C/A444W) expressed in HEK293t cells with a beta1 subunit. The 50% inhibitory concentrations (IC(50)) for the inactivated-state block and the resting-state block of wild-type Na(+) channels by mexiletine were measured as 67.8 +/- 7.0 microm and 431.2 +/- 9.4 microm, respectively (n= 5). In contrast, the IC(50) for the block of open inactivation-deficient mutant channels at +30 mV by mexiletine was 3.3 +/- 0.1 microm (n= 5), which was within the therapeutic plasma concentration range (2.8-11 microm). Estimated on- and off-rates for the open-state block by mexiletine at +30 mV were 10.4 microm(-1) s(-1) and 54.4 s(-1), respectively. Use-dependent block by mexiletine was greater in inactivation-deficient mutant channels than in wild-type channels during repetitive pulses. Furthermore, the IC(50) values for the block of persistent late hNav1.4 currents in chloramine-T-pretreated cells by mexiletine was 7.5 +/- 0.8 microm (n= 5) at +30 mV. Our results together support the hypothesis that the in vivo efficacy of mexiletine is primarily due to the open-channel block of persistent late Na(+) currents, which may arise during various pathological conditions.

State-dependent block of wild-type and inactivation-deficient Na+ channels by flecainide.

The antiarrhythmic agent flecainide appears beneficial for painful congenital myotonia and LQT-3/DeltaKPQ syndrome. Both diseases manifest small but persistent late Na+ currents in skeletal or cardiac myocytes. Flecainide may therefore block late Na+ currents for its efficacy. To investigate this possibility, we characterized state-dependent block of flecainide in wild-type and inactivation-deficient rNav1.4 muscle Na+ channels (L435W/L437C/A438W) expressed with beta1 subunits in Hek293t cells. The flecainide-resting block at -140 mV was weak for wild-type Na+ channels, with an estimated 50% inhibitory concentration (IC50) of 365 micro M when the cell was not stimulated for 1,000 s. At 100 micro M flecainide, brief monitoring pulses of +30 mV applied at frequencies as low as 1 per 60 s, however, produced an approximately 70% use-dependent block of peak Na+ currents. Recovery from this use-dependent block followed an exponential function, with a time constant over 225 s at -140 mV. Inactivated wild-type Na+ channels interacted with flecainide also slowly at -50 mV, with a time constant of 7.9 s. In contrast, flecainide blocked the open state of inactivation-deficient Na+ channels potently as revealed by its rapid time-dependent block of late Na+ currents. The IC50 for flecainide open-channel block at +30 mV was 0.61 micro M, right within the therapeutic plasma concentration range; on-rate and off-rate constants were 14.9 micro M-1s-1 and 12.2 s-1, respectively. Upon repolarization to -140 mV, flecainide block of inactivation-deficient Na+ channels recovered, with a time constant of 11.2 s, which was approximately 20-fold faster than that of wild-type counterparts. We conclude that flecainide directly blocks persistent late Na+ currents with a high affinity. The fast-inactivation gate, probably via its S6 docking site, may further stabilize the flecainide-receptor complex in wild-type Na+ channels.

Tryptophan scanning of D1S6 and D4S6 C-termini in voltage-gated sodium channels.

Recent reports suggest that four S6 C-termini may jointly close the voltage-gated cation channel at the cytoplasmic side, probably as an inverted teepee structure. In this study we substituted individually a total of 18 residues at D1S6 and D4S6 C-terminal ends of the rNav1.4 Na(+) channel alpha-subunit with tryptophan (W) and examined their corresponding gating properties when expressed in Hek293t cells along with beta1 subunit. Several W-mutants displayed significant changes in activation, fast inactivation, and/or slow inactivation gating. In particular, five S6 W-mutants showed incomplete fast inactivation with noninactivating maintained currents present. Cysteine (C) substitutions of these five residues resulted in two mutants with slightly more maintained currents. Multiple substitutions at these five positions yielded two mutants (L437C/A438W, L435W/L437C/A438W) that exhibited phenotypes with minimal fast inactivation. Unexpectedly, such inactivation-deficient mutants expressed Na(+) currents as well as did the wild-type. Furthermore, all mutants with impaired fast inactivation exhibited an enhanced slow inactivation phenotype. Implications of these results will be discussed in terms of indirect allosteric modulations via amino acid substitutions and/or a direct involvement of S6 C-termini in Na(+) channel gating.