Molecular determinants of state-dependent block of Na+ channels by local anesthetics

A single amino acid change makes a rat neuronal sodium channel highly sensitive to pyrethroid insecticides

Two amino acid substitutions in a housefly sodium channel, L1014F in domain IIS6 and M918T in the IIS4-S5 linker, have been identified in kdr and super-kdr pyrethroid-resistant phenotypes, respectively. Unlike their native insect counterparts, mammalian sodium channels are only weakly sensitive to pyrethroids. Do the sodium channels of mammal and pyrethroid-resistant housefly share similar structural characteristics that account for their low pyrethroid sensitivities? We report here that substitution of isoleucine for methionine at position 874 (equivalent to the super-kdr site 918 in the housefly) in the rat IIA alpha-subunit causes a 100-fold increase in sensitivity.

Chronic urticaria serum induces histamine release, leukotriene production, and basophil CD63 surface expression--inhibitory effects ofanti-inflammatory drugs

BACKGROUND

A role of potential histamine-releasing autoantibodies against the high-affinity IgE receptor on the surface of basophils and mast cells is discussed in the pathogenesis of chronic urticaria. This so-called autoimmune urticaria may be diagnosed by a positive intracutaneous autologous serum skin test, which is found in about 30% of patients with chronic urticaria.

OBJECTIVE

Our purpose was, first, to compare the effect of complement-inactivated sera of 20 patients with chronic urticaria and positive autologous serum skin tests, 20 patients with chronic urticaria and negative skin tests, and 20 control subjects without chronic urticaria (10 atopic and 10 nonatopic subjects) and, second, to analyze the effect of anti-inflammatory drugs on the serum activity.

METHODS

The following assay systems were used: release of histamine in whole blood samples, surface expression of the activation marker CD63 on basophils, and sulfidoleukotriene de novo production in leukocyte suspensions. Whole blood, basophils, and leukocyte suspensions were obtained from a nonatopic and an atopic donor.

RESULTS

Sera of patients with autologous serum skin test positive chronic urticaria resulted not only in significantly increased histamine release compared with skin test-negative chronic urticaria sera but also in a significant higher induction of basophil CD63 surface expression and sulfidoleukotriene de novo production. However, serum activity was neither characteristic for chronic urticaria nor for chronic urticaria with a positive autologous serum skin test. Preincubation with dapsone, chloroquine, and lidocaine dose dependently resulted in a significant reduction of all histamine release, CD63 expression, and sulfidoleukotriene production. In addition, mizolastine was able to inhibit serum-induced sulfidoleukotriene production.

CONCLUSION

Further studies investigating the in vivo effect of these drugs will have to clarify their role in the management of the subset of patients with chronic urticaria demonstrating serum-induced inflammatory effects.

Activation of Drosophila sodium channels promotes modification by deltamethrin. Reductions in affinity caused by knock-down resistance mutations

kdr and super-kdr are mutations in houseflies and other insects that confer 30- and 500-fold resistance to the pyrethroid deltamethrin. They correspond to single (L1014F) and double (L1014F+M918T) mutations in segment IIS6 and linker II(S4-S5) of Na channels. We expressed Drosophila para Na channels with and without these mutations and characterized their modification by deltamethrin. All wild-type channels can be modified by <10 nM deltamethrin, but high affinity binding requires channel opening: (a) modification is promoted more by trains of brief depolarizations than by a single long depolarization, (b) the voltage dependence of modification parallels that of channel opening, and (c) modification is promoted by toxin II from Anemonia sulcata, which slows inactivation. The mutations reduce channel opening by enhancing closed-state inactivation. In addition, these mutations reduce the affinity for open channels by 20- and 100-fold, respectively. Deltamethrin inhibits channel closing and the mutations reduce the time that channels remain open once drug has bound. The super-kdr mutations effectively reduce the number of deltamethrin binding sites per channel from two to one. Thus, the mutations reduce both the potency and efficacy of insecticide action.

A critical residue for isoform difference in tetrodotoxin affinity is a molecular determinant of the external access path for local anesthetics in the cardiac sodium channel

Membrane-impermeant quaternary derivatives of lidocaine (QX222 and QX314) block cardiac Na(+) channels when applied from either side of the membrane, but they block neuronal and skeletal muscle channels poorly from the outside. To find the molecular determinants of the cardiac external QX access path, mutations of adult rat skeletal muscle (micro1) and rat heart (rH1) Na(+) channels were studied by two-electrode voltage clamp in Xenopus oocytes. Mutating the micro1 domain I P-loop Y401, which is the critical residue for isoform differences in tetrodotoxin block, to the heart sequence (Y401C) allowed outside QX222 block, but its mutation to brain type (Y401F) showed little block. mu1-Y401C accelerated recovery from block by internal QX222. Block by external QX222 in mu1-Y401C was diminished by chemical modification with methanethiosulfonate ethylammonium (MTSEA) to the outer vestibule or by a double mutant (mu1-Y401C/F1579A), which altered the putative local anesthetic binding site. The reverse mutation in heart rH1-C374Y reduced outside QX314 block and slowed dissociation of internal QX222. Mutation of mu1-C1572 in IVS6 to Thr, the cardiac isoform residue (C1572T), allowed external QX222 block, and accelerated recovery from internal QX222 block, as reported. Blocking efficacy of outside QX222 in mu1-Y401C was more than that in mu1-C1572T, and the double mutant (mu1-Y401C/C1572T) accelerated internal QX recovery more than mu1-Y401C or mu1-C1572T alone. We conclude that the isoform-specific residue (Tyr/Phe/Cys) in the P-loop of domain I plays an important role in drug access as well as in tetrodotoxin binding. Isoform-specific residues in the IP-loop and IVS6 determine outside drug access to an internal binding site.

A point mutation in domain 4-segment 6 of the skeletal muscle sodium channel produces an atypical inactivation state

We compared wild-type rat skeletal muscle NaChs (micro1) and a mutant NaCh (Y1586K) that has a single amino acid substitution, lysine (K) for tyrosine (Y), at position 1586 in the S6 transmembrane segment of domain 4. In Y1586K, macroscopic current decay is faster, the V(1/2) of the activation curve is shifted in the depolarized direction, and the fast-inactivation curve is less steep compared with mu1. After an 8-ms depolarization pulse, Y1586K recovers from inactivation much more slowly than mu1. The recovery is double exponential, suggesting recovery from two inactivation states. Varying the depolarization protocols isolates entry into an additional, "atypical" inactivation state in Y1586K that is distinct from typical fast or slow inactivation. Substitution of positively charged arginine (R) at Y1586 produces an inactivation phenotype similar to that of Y1586K. Substitution by negatively charged aspartic acid (D) or uncharged alanine (A) at Y1586 produces an inactivation phenotype similar to mu1. Our results suggest that the positive charge of lysine (K) produces the atypical inactivation state in Y1586K. We propose that a conformational change during depolarization alters the relative position of the 1586K residue in the D4-S6 segment and that atypical inactivation in Y1586K occurs via an electrostatic interaction in or near the inner pore region.

Thermal destabilization of transmembrane proteins by local anaesthetics

Local anaesthetics, in addition to anaesthesia, induce the synthesis of heat shock proteins (HSPs), sensitize cells to hyperthermia, and increase the aggregation of nuclear proteins during heat shock. Anaesthetics are membrane active agents, and anaesthesia appears to be due to altered ion channel activity; however, the direct effect of heat shock is protein denaturation. These observations suggest that local anaesthetics may sensitize cells to hyperthermia by interacting with and destabilizing membrane proteins such that protein denaturation is increased. It is shown, using differential scanning calorimetry (DSC), that the local anaesthetics procaine, lidocaine, tetracaine and dibucaine destabilize the transmembrane domains of the Ca2+ -ATPase of sarcoplasmic reticulum and the band III anion transporter of red blood cells. The transmembrane domain of the Ca2+ -ATPase has a transition temperature (Tm) of denaturation of 61 degrees C which is decreased, for example, to 53 degrees C by 15 mM lidocaine. The degree of destabilization (deltaTm) by each anaesthetic is proportional to the lipid to water partition coefficient, and the increased sensitization by anaesthetics with larger partition coefficients and at higher pH suggests that the uncharged forms of the anaesthetics are responsible for destabilization. A Hill analysis of deltaTm for the Ca2+ -ATPase as a function of the concentration of anaesthetic in water gives dissociation constants (Kd) on the order of 10(-4) M, if binding occurs directly from the aqueous phase. This demonstrates moderate affinity binding. However, dissociation constants of 1-3 M are obtained, if binding occurs through the lipid phase, which demonstrates low affinity binding. Thus, the interaction of local anaesthetics with the Ca2+ -ATPase may be moderately specific or non-specific depending on the mechanism of interaction. The observation that local anaesthetics also destabilize the transmembrane domain of the band III protein of erythrocytes suggests that destabilization of transmembrane proteins is a general property of anaesthetics, which is at least in part a mechanism of sensitization to hyperthermia.

Homologation of mexiletine alkyl chain and stereoselective blockade of skeletal muscle sodium channels

The optical isomers (-)-(S)- and (+)-(R)-3-(2, 6-dimethylphenoxy)-2-methyl-1-propanamine (Me2), homologues of the antiarrhythmic and antimyotonic drug mexiletine (Mex), were synthesized and assayed as new potential antimyotonic agents. As observed with Mex, Me2 exhibits an enantioselective behaviour. Tests carried out on sodium currents of single muscle fibres of Rana esculenta demonstrated that (-)-(S)- and (+)-(R)-Me2 were less potent than Mex in producing tonic block, but showed a higher use-dependent block. (-)-(S)-Me2 and (-)-(R)-Mex were also used to study the excitability of muscle fibres of myotonic ADR mice, a phenotype of a recessive form of low G(Cl) myotonia. (-)-(S)-Me2 reduced spontaneous discharges and after discharges better than (-)-(R)-Mex in agreement with the use-dependent block of sodium currents.

Different effects of mexiletine on two mutant sodium channels causing paramyotonia congenita and hyperkalemic periodic paralysis

Effects of the antiarrhythmic and antimyotonic drug mexiletine were studied on two sodium channel mutants causing paramyotonia congenita (R1448H) and an overlap paramyotonic and hyperkalemic paralytic syndrome (M1360V). Channels were expressed in human embryonic kidney cells and studied electrophysiologically, using the whole-cell patch-clamp technique. Compared to the wild-type, channel, both mutants showed alterations of inactivation, i.e. slower inactivation, left shift of steady-state inactivation and faster recovery from inactivation. Mexiletine caused a significantly larger use-dependent block of the R1448H mutant when compared to M1360V and wild-type channels. This can be explained by a prolonged recovery from mexiletine block as observed for R1448H channels, since the affinity of mexiletine for the inactivated state was similar for all three clones. The use-dependent block of sodium channels by mexiletine reduces repetitive series of action potentials and therefore improves muscle stiffness in myotonic patients. The enhanced use-dependent block as seen with R1448H may explain the extraordinary therapeutic efficacy of mexiletine in most patients with paramyotonia congenita.

Beta-estradiol acutely potentiates the depression of cardiac excitability by lidocaine and bupivacaine

Pregnancy is known to increase myocardial susceptibility to bupivacaine-induced cardiovascular collapse, and prolonged pretreatment of rabbits with high doses of progesterone potentiates bupivacaine's depression of the maximal rate of increase (Vmax) of the cardiac action potential. Short-term effects of progesterone are not detected in vitro, but other steroids elevated during pregnancy might be acutely active in this model. These experiments tested whether acute exposure to beta-estradiol potentiates local anesthetic/antiarrhythmic depression of Vmax and conduction velocity in rabbit cardiac tissue in vitro. Standard intracellular microelectrodes were used to measure electrophysiologic changes produced by beta-estradiol, local anesthetics, or both in dissected segments of heart containing the Purkinje fiber and ventricular muscle cells from ovariectomized rabbits. In tissues preincubated in beta-estradiol (3.3 nM), addition of bupivacaine (10.4 microM), or lidocaine (85.4 and 129 microM) decreased Vmax significantly more than in steroid-free Tyrode's (p<0.001). Alone, beta-estradiol had no effect on Vmax and depression of Vmax by the nonanesthetic Na+ channel blocker tetrodotoxin (TTX, 3 microM) was not potentiated by beta-estradiol. In preparations initially exposed to bupivacaine for 30 min, subsequent addition of beta-estradiol decreased Vmax further within 10 min (p<0.05). Bupivacaine's greater depression of Vmax at higher frequencies (3 Hz) was exaggerated by beta-estradiol. However, the rate-dependent slowing of conduction by bupivacaine was lessened or even reversed by beta-estradiol addition. Such rapid physiologic changes cannot be due to genomic actions by the hormone that take hours to manifest. Nor is the potentiation due to a general decrease in membrane excitability because the comparable inhibition by TTX is insensitive to estradiol. Because beta-estradiol potentiates the inhibition of myocardial excitability, but alleviates the slowing of impulse conduction between the Purkinje fiber and ventricular muscle produced by local anesthetics, the hormone must produce changes in more than one ionic conductance. Both pregnancy and conditions that abnormally alter levels of steroid hormones have ramifications for local anesthetic-induced cardiotoxicity and antiarrhythmic pharmacotherapeutics.

The impact of recent ion channel science on the development and use of antiarrhythmic drugs

In the past 20 years in the basic laboratory, tools have been developed to further our understanding of the mechanism of arrhythmias and of the effect of compounds on these or their substrates. Patch clamp studies have better defined the cardiac channels. A new classification of antiarrhythmic drugs was devised, coined the Sicilian Gambit. Drugs, aimed at blocking specific channels, are now being developed. With the cloning of channels, information about their molecular structure became available. One can begin to understand the molecular determinants of antiarrhythmic drug action on specific ion channels, and how this is modulated by effectors. Molecular and electrophysiologic techniques also have been used to collect information about the way disease states affect the cardiac channels, and how this can alter the response to ion channel blockers. This has proceeded utilizing a few technologies. Diseased heart cells from patients, from animal models of disease, or from transgenic mice are studied to examine the change in current expression and channel protein quantity in different stages of disease. Hopefully this new information will make drug therapy more target-oriented.

Increased hindrance on the chiral carbon atom of mexiletine enhances the block of rat skeletal muscle Na+ channels in a model of myotonia induced by ATX

1 The antiarrhythmic drug mexiletine (Mex) is also used against myotonia. Searching for a more efficient drug, a new compound (Me5) was synthesized substituting the methyl group on the chiral carbon atom of Mex by an isopropyl group. Effects of Me5 on Na+ channels were compared to those of Mex in rat skeletal muscle fibres using the cell-attached patch clamp method. 2 Me5 (10 microM) reduced the maximal sodium current (INa) by 29.7+/-4.4 % (n=6) at a frequency of stimulation of 0.3 Hz and 65.7+/-4.4 % (n=6) at 1 Hz. At same concentration (10 microM), Mex was incapable of producing any effect (n=3). Me5 also shifted the steady-state inactivation curves by -7. 9+/-0.9 mV (n=6) at 0.3 Hz and -12.2+/-1.0 mV (n=6) at 1 Hz. 3 In the presence of sea anemone toxin II (ATX; 5 microM), INa decayed more slowly and no longer to zero, providing a model of sodium channel myotonia. The effects of Me5 on peak INa were similar whatever ATX was present or not. Interestingly, Me5 did not modify the INa decay time constant nor the steady-state INa to peak INa ratio. 4 Analysis of ATX-induced late Na+ channel activity shows that Me5 did not affect mean open times and single-channel conductance, thus excluding open channel block property. 5 These results indicate that increasing hindrance on the chiral atom of Mex increases drug potency on wild-type and ATX-induced noninactivating INa and that Me5 might improve the prophylaxis of myotonia.

Voltage-dependent block of normal and mutant muscle sodium channels by 4-Chloro-m-Cresol

1 The effects of 4-Chloro-m-Cresol (4-CmC) were examined on heterologously expressed wild type (WT), Paramyotonia Congenita (R1448H) and Hyperkalemic Periodic Paralysis (M1360V) mutant alpha-subunits of human muscle sodium channels. 2 Block of rested sodium channels caused by 4-CmC was concentration-dependent with an ECR50 of 0.40 mM in WT, 0.45 mM in R1448H and 0.49 mM in M1360V. 3 Inactivation significantly promoted 4-CmC-induced sodium channel block in all clones indicated by 4-CmC-induced shifts of steady-state availability curves, reflecting a higher proportion of channel block at depolarized membrane potentials. Channel block was almost complete (>90%) at concentrations close to the ECR50 (0.5 mM) on application of an inactivating prepulse before the test pulse. 4 4-CmC accelerated the current decay following depolarization and prolonged recovery from inactivation in all clones. Of these, R1448H, the mutant which displayed severely impaired inactivation in the controls, responded to 4-CmC with the most pronounced acceleration of inactivation. Control experiments revealed enhanced recovery from inactivation in the mutants, which was restored to normal in 0.1 mM 4-CmC. 5 4-CmC induced no additional frequency-dependent block. 6 Our results clearly demonstrate that 4-CmC is as effective as lidocaine (Fan et al., 1996) in blocking muscle sodium channels. Low concentrations of the compound (</=ECR50) were able to restore pathologically accelerated recovery from inactivation and impaired inactivation in the mutants to the WT value.

Voltage-gated ion channels and hereditary disease

By the introduction of technological advancement in methods of structural analysis, electronics, and recombinant DNA techniques, research in physiology has become molecular. Additionally, focus of interest has been moving away from classical physiology to become increasingly centered on mechanisms of disease. A wonderful example for this development, as evident by this review, is the field of ion channel research which would not be nearly as advanced had it not been for human diseases to clarify. It is for this reason that structure-function relationships and ion channel electrophysiology cannot be separated from the genetic and clinical description of ion channelopathies. Unique among reviews of this topic is that all known human hereditary diseases of voltage-gated ion channels are described covering various fields of medicine such as neurology (nocturnal frontal lobe epilepsy, benign neonatal convulsions, episodic ataxia, hemiplegic migraine, deafness, stationary night blindness), nephrology (X-linked recessive nephrolithiasis, Bartter), myology (hypokalemic and hyperkalemic periodic paralysis, myotonia congenita, paramyotonia, malignant hyperthermia), cardiology (LQT syndrome), and interesting parallels in mechanisms of disease emphasized. Likewise, all types of voltage-gated ion channels for cations (sodium, calcium, and potassium channels) and anions (chloride channels) are described together with all knowledge about pharmacology, structure, expression, isoforms, and encoding genes.

Point mutations at N434 in D1-S6 of mu1 Na(+) channels modulate binding affinity and stereoselectivity of local anesthetic enantiomers

Voltage-gated Na(+) channels are the primary targets of local anesthetics (LAs). Amino acid residues in domain 4, transmembrane segment 6 (D4-S6) form part of the LA binding site. LAs inhibit binding of the neurotoxin batrachotoxin (BTX). Parts of the BTX binding site are located in D1-S6 and D4-S6. The affinity of BTX-resistant Na(+) channels mutated in D1-S6 (mu1-N434K, mu1-N437K) toward several LAs is significantly decreased. We have studied how residue mu1-N434 influences LA binding. By using site-directed mutagenesis, we created mutations at mu1-N434 that vary the hydrophobicity, aromaticity, polarity, and charge and investigated their influence on state-dependent binding and stereoselectivity of bupivacaine. Wild-type and mutant channels were transiently expressed in human embryonic kidney 293t cells and investigated under whole-cell voltage-clamp. For resting channels, bupivacaine enantiomers showed a higher potency in all mutant channels compared with wild-type channels. These changes were not well correlated with the physical properties of the substituted residues. Stereoselectivity was small and almost unchanged. In inactivated channels, the potency of bupivacaine was increased in mutations containing a quadrupole of an aromatic group (mu1-N434F, mu1-N434W, mu1-N434Y), a polar group (mu1-N434C), or a negative charge (mu1-N434D) and was decreased in a mutation containing a positive charge (mu1-N434K). In mutation mu1-N434R, containing the positively charged arginine, the potency of S(-)-bupivacaine was selectively decreased, resulting in a stereoselectivity (stereopotency ratio) of 3. Similar results were observed with cocaine but not with RAC 109 enantiomers. We propose that in inactivated channels, residue mu1-N434 interacts directly with the positively charged moiety of LAs and that D1-S6 and D4-S6 form a domain-interface site for binding of BTX and LAs in close proximity.

Local anesthetic anchoring to cardiac sodium channels. Implications into tissue-selective drug targeting

Local anesthetics inhibit Na+ channels in a variety of tissues, leading to potentially serious side effects when used clinically. We have created a series of novel local anesthetics by connecting benzocaine (BZ) to the sulfhydryl-reactive group methanethiosulfonate (MTS) via variable-length polyethylether linkers (L) (MTS-LX-BZ [X represents 0, 3, 6, or 9]). The application of MTS-LX-BZ agents modified native rat cardiac as well as heterologously expressed human heart (hH1) and rat skeletal muscle (rSkM1) Na+ channels in a manner resembling that of free BZ. Like BZ, the effects of MTS-LX-BZ on rSkM1 channels were completely reversible. In contrast, MTS-LX-BZ modification of heart and mutant rSkM1 channels, containing a pore cysteine at the equivalent location as cardiac Na+ channels (ie, Y401C), persisted after drug washout unless treated with DTT, which suggests anchoring to the pore via a disulfide bond. Anchored MTS-LX-BZ competitively reduced the affinity of cardiac Na+ channels for lidocaine but had minimal effects on mutant channels with disrupted local anesthetic modification properties. These results establish that anchored MTS-LX-BZ compounds interact with the local anesthetic binding site (LABS). Variation in the linker length altered the potency of channel modification by the anchored drugs, thus providing information on the spatial relationship between the anchoring site and the LABS. Our observations demonstrate that local anesthetics can be anchored to the extracellular pore cysteine in cardiac Na+ channels and dynamically interact with the intracellular LABS. These results suggest that nonselective agents, such as local anesthetics, might be made more selective by linking these agents to target-specific anchors.

The effect of deep pore mutations on the action of phenylalkylamines on the Kv1.3 potassium channel

We investigated the action of the phenylalkylamines verapamil and N-methyl-verapamil on the Kv1.3 potassium channel using the whole-cell configuration of the patch-clamp technique. Our goal was to identify their binding as a prerequisite for using the phenylalkylamines as small, well-defined molecular probes, not only to expand the structural findings made with peptide toxins or by crystallization, but also to use them as lead compounds for the generation of more potent and therefore more specific K+ channel modulators. Competition experiments with charybdotoxin, known to interact with external residues of Kv1.3, showed no interaction with verapamil. The internal application of quarternary N-methyl-verapamil in combination with verapamil suggested competition for the same internal binding site. Verapamil affinity was decreased 6 fold by a mutation (M395V) in a region of the internal pore which forms part of the internal tetraethylammonium (TEA+) binding site, although mutations at neighbouring residues (T396 and T397) were without effect. Modification of C-type inactivation by mutations in the internal pore suggest that this region participates in the inactivation process. The action of phenylalkylamines and local anaesthetics on L-type Ca2+ channels and Na channels, respectively, and verapamil on Kv1.3 indicate very similar blocking mechanisms. This might allow the use of these compounds as molecular probes to map the internal vestibule of all three channel types.

Batrachotoxin-resistant Na+ channels derived from point mutations in transmembrane segment D4-S6

Local anesthetics (LAs) block voltage-gated Na+ channels in excitable cells, whereas batrachotoxin (BTX) keeps these channels open persistently. Previous work delimited the LA receptor within the D4-S6 segment of the Na+ channel alpha-subunit, whereas the putative BTX receptor was found within the D1-S6. We mutated residues at D4-S6 critical for LA binding to determine whether such mutations modulate the BTX phenotype in rat skeletal muscle Na+ channels (mu1/rSkm1). We show that mu1-F1579K and mu1-N1584K channels become completely resistant to 5 microM BTX. In contrast, mu1-Y1586K channels remain BTX-sensitive; their fast and slow inactivation is eliminated by BTX after repetitive depolarization. Furthermore, we demonstrate that cocaine elicits a profound time-dependent block after channel activation, consistent with preferential LA binding to BTX-modified open channels. We propose that channel opening promotes better exposure of receptor sites for binding with BTX and LAs, possibly by widening the bordering area around D1-S6, D4-S6, and the pore region. The BTX receptor is probably located at the interface of D1-S6 and D4-S6 segments adjacent to the LA receptor. These two S6 segments may appose too closely to bind BTX and LAs simultaneously when the channel is in its resting closed state.

Molecular analysis of the Na+ channel blocking actions of the novel class I anti-arrhythmic agent RSD 921

RSD 921 is a novel, structurally unique, class I Na+ channel blocking drug under development as a local anaesthetic agent and possibly for the treatment of cardiac arrhythmias. The effects of RSD 921 on wild-type heart, skeletal muscle, neuronal and non-inactivating IFMQ3 mutant neuronal Na+ channels expressed in Xenopus laevis oocytes were examined using a two-electrode voltage clamp. RSD 921 produced similarly potent tonic block of all three wild-type channel isoforms, with EC50 values between 35 and 47 microM, whereas the EC50 for block of the IFMQ3 mutant channel was 110+5.5 microM. Block of Na+ channels by RSD 921 was concentration and use-dependent, with marked frequency-dependent block of heart channels and mild frequency-dependent block of skeletal muscle, wild-type neuronal and IFMQ3 mutant channels. RSD 921 produced a minimal hyperpolarizing shift in the steady-state voltage-dependence of inactivation of all three wild-type channel isoforms. Open channel block of the IFMQ3 mutant channel was best fit with a first order blocking scheme with k(on) equal to 0.11+/-0.012x10(6) M(-1) s(-1) and k(off) equal to 12.5+/-2.5 s(-1), resulting in KD of 117+/-31 microM. Recovery from open channel block occurred with a time constant of 14+/-2.7 s(-1). These results suggest that RSD 921 preferentially interacts with the open state of the Na+ channel, and that the drug may produce potent local anaesthetic or anti-arrhythmic action under conditions of shortened action potentials, such as during anoxia or ischaemia.

Effect of mexiletine on sea anemone toxin-induced non-inactivating sodium channels of rat skeletal muscle: a model of sodium channel myotonia

The sea anemone toxin ATX II impairs skeletal muscle sodium channel inactivation, mimicking the persistent inward current observed in patients suffering from sodium channel myotonia. Mexiletine has beneficial effects on myotonia. To verify the efficiency of the drug on persistent inward current, we investigated the effect of 50 microM R(-)-mexiletine on sodium channels in cell-attached patches of rat skeletal muscle fibres, in the absence or presence of 2 microM ATX II. With the toxin, a proportion of channels displayed remarkable abnormal activity lasting the entire depolarisation, which resulted in a persistent inward current that represented up to 2.0% of the peak current. Mexiletine reduced by 75% the peak current elicited by depolarisation from -100 to -20 mV. This was due to the reduction by 60% of the maximal available peak current Imax and to the negative shift by -7 mV of steady-state inactivation. Mexiletine also greatly decreased the late current, but the effect was limited to 60% of reduction, comparable to that on Imax. Therefore mexiletine was able to block the ATX II-modified sodium channels, inhibiting the myotonia-producing persistent inward current.

Structural and electrophysiological effects of local anesthetics and of low temperature on myelinated nerves: implication of the lipid chains in nerve excitability

X-ray scattering and electrophysiological experiments performed on toad sciatic nerves as a function of the exposure to either low temperature or tetracaine yielded the following results: (i) the main structural effect is to thicken the individual membranes, thus to stiffen the acyl chains and increase the repeat distance of the one-dimensional lattice, phenomena that are typical of lipid-containing systems with disordered chains; (ii) the electrophysiological effect is to decrease the amplitude and velocity of the compound action potential; (iii) the structural and physiological effects of the two agents are practically identical. Since the structural and the electrophysiological parameters have different origins in the nerves (the structure regards the myelin sheath, the electrical signals originate at the nodes of Ranvier) it is inferred that tetracaine and low temperature exert similar effects on the membranes of both the myelin sheath and the nodes of Ranvier. Also, since local anesthetics act by inhibiting the Na+ channels, these observations suggest that the acyl chain conformation modulates the channel function and thus the generation of action potential.

Effects of a novel cardioprotective drug, JTV-519, on membrane currents of guinea pig ventricular myocytes

We investigated effects of a novel cardioprotective drug, JTV-519 (4-[3-(4-benzylpiperidin-1-yl)propionyl]-7-methoxy-2,3,4,5-tetrahy dro-1,4-benzothiazepine monohydrochloride) on membrane currents of guinea pig ventricular myocytes by whole-cell voltage and current clamp methods. The fast Na+ current (iNa) was activated by ramp pulses from various holding potentials of -90, -80 or -60 mV to 10 mV with various intervals. At 0.2 Hz, JTV-519 inhibited iNa in a concentration-dependent manner with an IC50 of approximately 1.2 and 2 microM at the holding potential of -60 and -90 mM, respectively, implicating a voltage-dependent block. Increasing the pulse frequency from 1 to 2 or 3.3 Hz in the presence of 1 microM JTV-519 shortened the time-course and increased the level of iNa block, indicating a frequency-dependent block. The time-course of iNa blocking by JTV-519 was slower than that of lidocaine and similar to that of quinidine. Ca2+ current (iCa) and the inwardly rectifying K+ current (iK1) were also inhibited by JTV-519. JTV-519 decreased the duration and the height of the plateau of the action potential. We conclude that JTV-519 has frequency- and voltage-dependent blocking effects on iNa as well as inhibition of iCa and iK1.

The molecular and ionic specificity of antiarrhythmic drug actions

Virtually all clinical antiarrhythmic agents act by reducing ion channel conductance, with sodium (Na+), potassium (K+), and calcium (Ca++) channels the primary targets. Na+ channel blockers increase the risk of ischemic ventricular fibrillation and are relatively contraindicated in the presence of active coronary heart disease. Ca++ channel blockers suppress AV nodal conduction and are used to terminate reentrant supraventricular arrhythmias and control the ventricular response to atrial fibrillation. K+ channels constitute the most diverse group of cardiac ion channels. They are the primary targets of Class III antiarrhythmic drugs, the category of such agents presently undergoing the most active development. The rapid delayed rectifier, IKr, plays a key role in repolarization of all cardiac tissues and is the most common (and often only) target of action potential-prolonging drugs. Unfortunately, because of the ubiquity of IKr and the reverse use-dependent action potential prolongation that results from blocking it, IKr blockers are likely to cause torsades de pointes ventricular proarrhythmia. K+ channel blockers, such as amiodarone and azimilide, that affect the slow delayed rectifier IKs as well as IKr, appear to produce a more desirable rate-dependent profile of Class III action. Recently, much has been learned about the molecular basis of K+ channels based on their role in the congenital long QT syndrome. The availability of molecular clones that encode many of the channels in the human heart allows for the rapid screening of many potential new drugs, making possible the development of "designer" antiarrhythmic drugs with specific profiles of channel-blocking selectivity.

Block of brain sodium channels by peptide mimetics of the isoleucine, phenylalanine, and methionine (IFM) motif from the inactivation gate

Inactivation of sodium channels is thought to be mediated by an inactivation gate formed by the intracellular loop connecting domains III and IV. A hydrophobic motif containing the amino acid sequence isoleucine, phenylalanine, and methionine (IFM) is required for the inactivation process. Peptides containing the IFM motif, when applied to the cytoplasmic side of these channels, produce two types of block: fast block, which resembles the inactivation process, and slow, use-dependent block stimulated by strong depolarizing pulses. Fast block by the peptide ac-KIFMK-NH2, measured on sodium channels whose inactivation was slowed by the alpha-scorpion toxin from Leiurus quinquestriatus (LqTx), was reversed with a time constant of 0.9 ms upon repolarization. In contrast, control and LqTx-modified sodium channels were slower to recover from use-dependent block. For fast block, linear peptides of three to six amino acid residues containing the IFM motif and two positive charges were more effective than peptides with one positive charge, whereas uncharged IFM peptides were ineffective. Substitution of the IFM residues in the peptide ac-KIFMK-NH2 with smaller, less hydrophobic residues prevented fast block. The positively charged tripeptide IFM-NH2 did not cause appreciable fast block, but the divalent cation IFM-NH(CH2)2NH2 was as effective as the pentapeptide ac-KIFMK-NH2. The constrained peptide cyclic KIFMK containing two positive charges did not cause fast block. These results indicate that the position of the positive charges is unimportant, but flexibility or conformation of the IFM-containing peptide is important to allow fast block. Slow, use-dependent block was observed with IFM-containing peptides of three to six residues having one or two positive charges, but not with dipeptides or phenylalanine-amide. In contrast to its lack of fast block, cyclic KIFMK was an effective use-dependent blocker. Substitutions of amino acid residues in the tripeptide IFM-NH2 showed that large hydrophobic residues are preferred in all three positions for slow, use-dependent block. However, substitution of the large hydrophobic residue diphenylalanine or the constrained residues phenylglycine or tetrahydroisoquinoline for phe decreased potency, suggesting that this phe residue must be able to enter a restricted hydrophobic pocket during the binding of IFM peptides. Together, the results on fast block and slow, use-dependent block indicate that IFM peptides form two distinct complexes of different stability and structural specificity with receptor site(s) on the sodium channel. It is proposed that fast block represents binding of these peptides to the inactivation gate receptor, while slow, use-dependent block represents deeper binding of the IFM peptides in the pore.

Positive symptoms in multiple sclerosis: their treatment with sodium channel blockers, lidocaine and mexiletine

Patients with multiple sclerosis (MS) often show positive symptoms of painful tonic seizure and dysesthesia as well as negative symptoms of paralysis and hypesthesia. Positive manifestation is paroxysmal and/or persistent. These are considered to be mediated by ectopic impulses generated at the site of demyelination, whereas negative symptoms are caused by conduction block. Conduction block at a demyelinated segment should reduce positive symptoms, but worsen negative ones. As reported previously, lidocaine, an Na channel blocker unmasks silent negative symptoms presumably by further reducing the action current in demyelinated portions and blocking conduction. Furthermore, because it blocks Na channels in a voltage- and frequency dependent manner, fibers that mediate positive symptoms are preferentially blocked. We administered lidocaine to 30 MS patients with positive symptoms. Lidocaine (mean plasma level, 2.4 pg/ml) almost completely abolished the paroxysmal manifestation of painful tonic seizures, neuralgic attacks, paroxysmal itching, and Lhermitte's sign. It also markedly alleviated persistent symptoms, but less so than paroxysmal symptoms. Similar effects were obtained with orally-administered mexiletine (300-400 mg/day), a derivative of lidocaine, but to a lesser extent. Na channel blockers have a dual effect on symptoms in MS, depending on whether symptoms are positive or negative. The mechanism that produces positive symptoms and the effects of the drugs on these symptoms are discussed.

State-dependent cocaine block of sodium channel isoforms, chimeras, and channels coexpressed with the beta1 subunit

Cocaine block of human cardiac (hH1) and rat skeletal (mu1) muscle sodium channels was examined under whole-cell voltage clamp in transiently transfected HEK293t cells. Low affinity block of resting mu1 and hH1 channels at -180 mV was the same, and high affinity block of inactivated channels at -70 mV was the same. Cocaine block of hH1 channels was greater than block of mu1 channels at voltages between -120 mV and -90 mV, suggesting that greater steady-state inactivation of hH1 channels in this voltage range makes them more susceptible to cocaine block. We induced shifts in the voltage dependence of steady-state inactivation at mu1 and hH1 channels by constructing mu1/hH1 channel chimeras or by coexpressing the wild-type channels with the rat brain beta1 subunit. In contrast to several previous reports, coexpression of the rat brain beta1 subunit with mu1 or hH1 produced large positive shifts in steady-state inactivation. Shifts in the voltage dependence of steady-state inactivation elicited linear shifts in steady-state cocaine block, yet these manipulations did not affect the cocaine affinity of resting or inactivated channels. These data, as well as simulations used to predict block, indicate that state-dependent cocaine block depends on both steady-state inactivation and channel activation, although inactivation appears to have the predominant role.

The importance of voltage-dependent sodium channels in cerebral ischaemia

Strategies for the treatment of thromboembolic stroke are based on restoring the blood flow as soon as possible and protecting the neurons from the deleterious consequences of cerebral ischaemia. Interest has focused on blockers of voltage-dependent Na+ channels as potential neuroprotective agents because they prevent neuronal death in various experimental models of cerebral ischaemia and act cytoprotectively in models of white matter damage. Although several Na+ blockers are currently being tested in various phases of clinical development, most of these agents are relatively weak and unspecific. I therefore consider it worthwhile to search for molecules which specifically block voltage-dependent Na+ channels for the treatment of cerebral ischaemia.

The position of the fast-inactivation gate during lidocaine block of voltage-gated Na+ channels

Lidocaine produces voltage- and use-dependent inhibition of voltage-gated Na+ channels through preferential binding to channel conformations that are normally populated at depolarized potentials and by slowing the rate of Na+ channel repriming after depolarizations. It has been proposed that the fast-inactivation mechanism plays a crucial role in these processes. However, the precise role of fast inactivation in lidocaine action has been difficult to probe because gating of drug-bound channels does not involve changes in ionic current. For that reason, we employed a conformational marker for the fast-inactivation gate, the reactivity of a cysteine substituted at phenylalanine 1304 in the rat adult skeletal muscle sodium channel alpha subunit (rSkM1) with [2-(trimethylammonium)ethyl]methanethiosulfonate (MTS-ET), to determine the position of the fast-inactivation gate during lidocaine block. We found that lidocaine does not compete with fast-inactivation. Rather, it favors closure of the fast-inactivation gate in a voltage-dependent manner, causing a hyperpolarizing shift in the voltage dependence of site 1304 accessibility that parallels a shift in the steady state availability curve measured for ionic currents. More significantly, we found that the lidocaine-induced slowing of sodium channel repriming does not result from a slowing of recovery of the fast-inactivation gate, and thus that use-dependent block does not involve an accumulation of fast-inactivated channels. Based on these data, we propose a model in which transitions along the activation pathway, rather than transitions to inactivated states, play a crucial role in the mechanism of lidocaine action.

Molecular and kinetic determinants of local anaesthetic action on sodium channels

(1) Local anaesthetics (LA) rely for their clinical actions on state-dependent inhibition of voltage-dependent sodium channels. (2) Single, batrachoxin-modified sodium channels in planar lipid bilayers allow direct observation of drug-channel interactions. Two modes of inhibition of single-channel current are observed: fast block of the open channels and prolongation of a long-lived closed state, some of whose properties resemble those of the inactivated state of unmodified channels. (3) Analogues of different parts of the LA molecule separately mimic each blocking mode: amines--fast block, and water-soluble aromatics--closed state prolongation. (4) Interaction between a mu-conotoxin derivative and diethylammonium indicate an intrapore site of fast, open-state block. (5) Site-directed mutagenesis studies suggest that hydrophobic residues in transmembrane segment 6 of repeat domain 4 of sodium channels are critical for both LA binding and stabilization of the inactivated state.

Interaction of batrachotoxin with the local anesthetic receptor site in transmembrane segment IVS6 of the voltage-gated sodium channel

The voltage-gated sodium channel is the site of action of more than six classes of neurotoxins and drugs that alter its function by interaction with distinct, allosterically coupled receptor sites. Batrachotoxin (BTX) is a steroidal alkaloid that binds to neurotoxin receptor site 2 and causes persistent activation. BTX binding is inhibited allosterically by local anesthetics. We have investigated the interaction of BTX with amino acid residues I1760, F1764, and Y1771, which form part of local anesthetic receptor site in transmembrane segment IVS6 of type IIA sodium channels. Alanine substitution for F1764 (mutant F1764A) reduces tritiated BTX-A-20-alpha-benzoate binding affinity, causing a 60-fold increase in Kd. Alanine substitution for I1760, which is adjacent to F1764 in the predicted IVS6 transmembrane alpha helix, causes only a 4-fold increase in Kd. In contrast, mutant Y1771A shows no change in BTX binding affinity. For wild-type and mutant Y1771A, BTX shifted the voltage for half-maximal activation approximately 40 mV in the hyperpolarizing direction and increased the percentage of noninactivating sodium current to approximately 60%. In contrast, these BTX effects were eliminated completely for the F1764A mutant and were reduced substantially for mutant I1760A. Our data suggest that the BTX receptor site shares overlapping but nonidentical molecular determinants with the local anesthetic receptor site in transmembrane segment IVS6 as well as having unique molecular determinants in transmembrane segment IS6, as demonstrated in previous work. Evidently, BTX conforms to a domain-interface allosteric model of ligand binding and action, as previously proposed for calcium agonist and antagonist drugs acting on L-type calcium channels.

Ca2+-stimulated mitochondrial reactive oxygen species generation and permeability transition are inhibited by dibucaine or Mg2+

Mitochondrial swelling and membrane protein thiol oxidation associated with mitochondrial permeability transition induced by Ca2+ and t-butyl hydroperoxide or inorganic phosphate, but not 4, 4'-diisothiocyanatostilbene-2,2'-disulfonic acid or phenylarsine oxide, are inhibited by the local anesthetic dibucaine. Dibucaine promotes an inhibition of the Ca2+-induced increase in mitochondrial H2O2 generation measured by the oxidation of scopoletin in the presence of horseradish peroxidase. This decrease in mitochondrial H2O2 generation may be attributed to the reduction of Ca2+ binding to the membrane induced by dibucaine, as assessed by measuring 45Ca2+ binding to the mitochondrial membrane. Mg2+ also inhibited Ca2+ binding to the mitochondrial membrane, mitochondrial swelling, membrane protein thiol oxidation, and H2O2 generation induced by Ca2+. Together, these results demonstrate that the mechanism by which dibucaine and Mg2+ inhibit mitochondrial permeability transition is related to the decrease in reactive oxygen species generation induced by Ca2+-promoted alterations of inner mitochondrial membrane properties.

Mechanistic link between lidocaine block and inactivation probed by outer pore mutations in the rat micro1 skeletal muscle sodium channel

1. Mutations that disrupt Na+ channel fast inactivation attenuate lidocaine (lignocaine)-induced use dependence; however, the pharmacological role of slower inactivation processes remains unclear. In Xenopus oocytes, tryptophan substitution in the outer pore of the rat skeletal muscle channel (micro1-W402) alters partitioning among fast- and slow-inactivated states. We therefore examined the effects of W402 mutations on lidocaine block. 2. Recovery from inactivation exhibited three kinetic components (IF, fast; IM, intermediate; IS, slow). The effects of W402A and W402S on IF and IS differed, but both mutants (with or without beta1 subunit coexpression) decreased the amplitude of IM. In wild-type channels, lidocaine imposed a delayed recovery component with intermediate kinetics, and use-dependent block was attenuated in both W402A and W402S. 3. To examine the pharmacological role of IS relative to IM, drug-exposed beta1-coexpressed channels were subjected to 2 min depolarizations. Lidocaine had no effect on sodium current (INa) after a 1 s hyperpolarization interval that allowed recovery from IM but not IS, suggesting that lidocaine affinity for IS is low. 4. Both W402 mutations reduced occupancy of IM in drug-free conditions, and also induced resistance to use-dependent block. We propose that lidocaine-induced use dependence may involve an allosteric conformational change in the outer pore.

Delimiting the binding site for quaternary ammonium lidocaine derivatives in the acetylcholine receptor channel

The triethylammonium QX-314 and the trimethylammonium QX-222 are lidocaine derivatives that act as open-channel blockers of the acetylcholine (ACh) receptor. When bound, these blockers should occlude some of the residues lining the channel. Eight residues in the second membrane-spanning segment (M2) of the mouse-muscle alpha subunit were mutated one at a time to cysteine and expressed together with wild-type beta, gamma, and delta subunits in Xenopus oocytes. The rate constant for the reaction of each substituted cysteine with 2-aminoethyl methanethiosulfonate (MTSEA) was determined from the time course of the irreversible effect of MTSEA on the ACh-induced current. The reactions were carried out in the presence and absence of ACh and in the presence and absence of QX-314 and QX-222. These blockers had no effect on the reactions in the absence of ACh. In the presence of ACh, both blockers retarded the reaction of extracellularly applied MTSEA with cysteine substituted for residues from alphaVal255, one third of the distance in from the extracellular end of M2, to alphaGlu241, flanking the intracellular end of M2, but not with cysteine substituted for alphaLeu258 or alphaGlu262, at the extracellular end of M2. The reactions of MTSEA with cysteines substituted for alphaLeu258 and alphaGlu262 were considerably faster in the presence of ACh than in its absence. That QX-314 and QX-222 did not protect alphaL258C and alphaE262C against reaction with MTSEA in the presence of ACh implies that protection of the other residues was due to occlusion of the channel and not to the promotion of a less reactive state from a remote site. Given the 12-A overall length of the blockers and the alpha-helical conformation of M2 in the open state, the binding site for both blockers extends from alphaVal255 down to alphaSer248.

Antiarrhythmic drugs and cardiac ion channels: mechanisms of action

In this review a description and an analysis are given of the interaction of antiarrhythmic drugs with their molecular target, i.e. ion channels and receptors. Our approach is based on the concept of vulnerable parameter, i.e. the electrophysiological property which plays a crucial role in the genesis of arrhythmias. To prevent or stop the arrhythmia a drug should modify the vulnerable parameter by its action on channel or receptor targets. In the first part, general aspects of the interaction between drugs channel molecules are considered. Drug binding depends on the state of the channel: rested, activated pre-open, activated open, or inactivated state. The change in channel behaviour with state is presented in the framework of the modulated-receptor hypothesis. Not only inhibition but also stimulation can be the result of drug binding. In the second part a detailed and systematic description and an analysis are given of the interaction of drugs with specific channels (Na+, Ca2+, K+, "pacemaker") and non-channel receptors. Emphasis is given to the type of state-dependent block involved (rested, activated and inactivated state block) and the change in channel kinetics. These properties vary and determine the voltage- and frequency-dependence of the change in ionic current. Finally, the question is asked as to whether the available drugs by their action on channels and receptors modify the vulnerable parameter in the desired way to stop or prevent arrhythmias.

Changes in morphology of human skin fibroblasts induced by local anaesthetics: role of actomyosin contraction

Local anaesthetics block action potentials in the membranes of excitable cells but their effects on non-excitable cells are less well known. Some local anaesthetics are applied directly onto the skin, and for this reason the effect of procaine (p-aminobenzoic acid diethylamino-etyl ester hydrochloride) and tetracaine (4-[butylamino]benzoic acid 2-[dimethylamino]ethyl ester) upon the morphology and cytoskeleton organisation of human skin fibroblasts was investigated. The time lapse video recording of fibroblasts cultured in serum-enriched medium revealed that the cells rapidly change shape after the addition of the anaesthetic. These effects were fully reversible. The microscopic observations were confirmed by quantitative analysis of projected cell area and cell shape parameters. Local anaesthetics significantly changed the actin cytoskeleton organisation, inducing total disappearance of stress fibres. Serum-starvation or myosin light chain kinase inhibitors, KT 5926 inhibitor (8R*,9S*,11S*)-(-)-9-hydroxy-9-methoxycarbonyl-8-methyl-14-n-propoxy-2,3 ,9, 10-tetrahydro-8,11-epoxy,1H,8H,11H-2,7b,11a-triazadibenzo[a,g]c ycloocta[cde] trinden-1-one or wortmannin, which induce the 'relaxed' morphology of the cells, prevent both the anaesthetic-induced changes in cell shape and the disassembly of stress fibres. Together, the observations suggest that local anaesthetics affect the actomyosin system, inducing contraction.

Fundamental properties of local anesthetics: half-maximal blocking concentrations for tonic block of Na+ and K+ channels in peripheral nerve

Local anesthetics suppress excitability by interfering with ion channel function. Ensheathment of peripheral nerve fibers, however, impedes diffusion of drugs to the ion channels and may influence the evaluation of local anesthetic potencies. Investigating ion channels in excised membrane patches avoids these diffusion barriers. We investigated the effect of local anesthetics with voltage-dependent Na+ and K+ channels in enzymatically dissociated sciatic nerve fibers of Xenopus laevis using the patch clamp method. The outside-out configuration was chosen to apply drugs to the external face of the membrane. Local anesthetics reversibly blocked the transient Na+ inward current, as well as the steady-state K+ outward current. Half-maximal tonic inhibiting concentrations (IC50), as obtained from concentration-effect curves for Na+ current block were: tetracaine 0.7 microM, etidocaine 18 microM, bupivacaine 27 microM, procaine 60 microM, mepivacaine 149 microM, and lidocaine 204 microM. The values for voltage-dependent K+ current block were: bupivacaine 92 microM, etidocaine 176 microM, tetracaine 946 microM, lidocaine 1118 microM, mepivacaine 2305 microM, and procaine 6302 microM. Correlation of potencies with octanol:buffer partition coefficients (logP0) revealed that ester-bound local anesthetics were more potent in blocking Na+ channels than amide drugs. Within these groups, lipophilicity governed local anesthetic potency. We conclude that local anesthetic action on peripheral nerve ion channels is mediated via lipophilic drug-channel interactions.

IMPLICATIONS

Half-maximal blocking concentrations of commonly used local anesthetics for Na+ and K+ channel block were determined on small membrane patches of peripheral nerve fibers. Because drugs can directly diffuse to the ion channel in this model, these data result from direct interactions of the drugs with ion channels.

The action of local anaesthetics on the compound action potential is altered by the nature of the permeant ion in frog nerve

Compound action potentials were recorded from the isolated frog sciatic nerve using either sodium or lithium as the permeant ion, and an assessment of the action of local anaesthetics was made. The compound action potentials evoked from the nerve were not different in terms of their mean amplitude or time to peak whether recorded with sodium or lithium as the permeant ion. The local anaesthetics tested, procaine, lignocaine and benzocaine, were more potent, as measured by their IC50 values, by 2.3, 2.1 and 1.8 times, respectively, when lithium rather than sodium was used as the permeant ion. The sensitivity of the nerves to tetrodotoxin was not significantly different whether sodium or lithium was used as the permeant ion. The slope of the concentration inhibition curves was not significantly altered in the presence of sodium or lithium for any of the compounds tested. These results are consistent with the idea that the binding site for local anaesthetics is intimately associated with the pore region of the channel and that the nature of the permeant ion can alter the interaction of the drugs with the sodium channel. However, since this is not a common feature of all compounds which block sodium channels by interacting at the pore, it may help refine the existing structural models of sodium channels.

Carboxyl groups at the membrane interface as molecular targets for local anesthetics

The interaction of the tertiary amine drugs chlorpromazine and dibucaine in their cationic form with carboxyl groups at the membrane surface is studied at concentrations relevant to anesthesia. Spin-labeled stearic acid is used both to provide the carboxyl groups and to monitor binding and ionization behavior in egg lecithin liposomes. Membrane anesthetic concentrations are spectrophotometrically obtained. They are shown to determine the drug influence on carboxyl groups at the membrane surface, independently of aqueous concentrations. The intramembrane association constants (related to the usual aqueous phase ones through the partition coefficient) of the drugs with fatty acids are determined. The same value (10(2) M-1) is obtained for both drugs, suggesting that it is approximately the same for all tertiary amine local anesthetics. pH titrations of anesthetic-treated spin-labeled membranes are performed. The observed shifts in the fatty acid pK are higher than can be produced assuming uniform distribution of the drug in the membrane surface, implying that there is an increased affinity of local anesthetics for superficial carboxyl. This affinity could account for the resting block of voltage-gated Na+ channels. Under these considerations, local anesthetic binding sites at voltage-gated Na+ channels and at sarcoplasmic reticulum Ca(2+)-ATPase are proposed.

Local anesthetic block of batrachotoxin-resistant muscle Na+ channels

Local anesthetics (LAs) are noncompetitive antagonists of batrachotoxin (BTX) in voltage-gated Na+ channels. The putative LA receptor has been delineated within the transmembrane segment S6 in domain IV of voltage-gated Na+ channels, whereas the putative BTX receptor is within segment S6 in domain I. In this study, we created BTX-resistant muscle Na+ channels at segment I-S6 (micro1-N434K, micro1-L437K) to test whether these residues modulate LA binding. These mutant channels were expressed in transiently transfected human embryonic kidney 293T cells, and their sensitivity to lidocaine, QX-314, etidocaine, and benzocaine was assayed under whole-cell, voltage-clamp conditions. Our results show that LA binding in BTX-resistant micro1 Na+ channels was reduced significantly. At -100 mV holding potential, the reduction in LA affinity was maximal for QX-314 (by 17-fold) and much less for neutral benzocaine (by 2-fold). Furthermore, this reduction was residue specific; substitution of positively charged lysine with negatively charged aspartic acid (micro1-N434D) restored or even enhanced the LA affinity. We conclude that micro1-N434K and micro1-L437K residues located near the middle of the I-S6 segment of Na+ channels can reduce the LA binding affinity without BTX. Thus, this reduction of the LA affinity by point mutations at the BTX binding site is not caused by gating changes induced by BTX alone. We surmise that the BTX receptor and the LA receptor within segments I-S6 and IV-S6, respectively, may align near or within the Na+ permeation pathway.

Molecular basis of drug interaction with L-type Ca2+ channels

Different types of voltage-gated Ca2+ channels exist in the plasma membrane of electrically excitable cells. By controlling depolarization-induced Ca2+ entry into cells they serve important physiological functions, such as excitation-contraction coupling, neurotransmitter and hormone secretion, and neuronal plasticity. Their function is fine-tuned by a variety of modulators, such as enzymes and G-proteins. Block of so-called L-type Ca2+ channels by drugs is exploited as a therapeutic principle to treat cardiovascular disorders, such as hypertension. More recently, block of so-called non-L-type Ca2+ channels was found to exert therapeutic effects in the treatment of severe pain and ischemic stroke. As the subunits of different Ca2+ channel types have been cloned, the modulatory sites for enzymes, G-proteins, and drugs can now be determined using molecular engineering and heterologous expression. Here we summarize recent work that has allowed us to determine the sites of action of L-type Ca2+ channel modulators. Together with previous biochemical, electrophysiological, and drug binding data these results provide exciting insight into the molecular pharmacology of this voltage-gated Ca2+ channel family.

Effects of the diuretic agent indapamide on Na+, transient outward, and delayed rectifier currents in canine atrial myocytes

The diuretic agent indapamide has been reported to block the slow component of the delayed rectifier K+ current (IKs) without altering the rapid component (IKr) or the inward rectifier current and has been used as a pharmacological probe for IKs; however, the effects of indapamide on Na+ (INa), L-type Ca2+ (ICa), and transient outward K+ (Ito) currents have not been determined. We applied tight-seal, whole-cell, patch-clamp techniques to assess the effects of indapamide on INa, Ito, ICa, and IKs in canine atrial myocytes. Indapamide inhibited INa, Ito, and IKs in a concentration-dependent and reversible way, without altering ICa. Block increased with depolarization, with the 50% blocking concentration (EC50) decreasing from 129 +/- 26 micromol/L (at -60 mV) to 79 +/- 17 micromol/L (at -10 mV) for INa, from 174 +/- 19 micromol/L (at + 10 mV) to 98 +/- 7 micromol/L (at +60 mV) for Ito and from 148 +/- 28 micromol/L (at +10 mV) to 86 +/- 18 micromol/L (at +60 mV) for IKs. Significant inhibition was seen at concentrations as low as 10 micromol/L for all 3 currents. In addition, indapamide effectively inhibited the ultrarapid delayed rectifier current in a voltage-independent way, with an EC50 of 138 +/- 7 micromol/L at +10 mV. Standard microelectrode experiments showed the effects of indapamide on the action potential to be consistent with the ionic actions seen. We conclude that in addition to its well-recognized IKs-blocking action, indapamide also inhibits INa and Ito effectively and with similar potency. Thus, indapamide is not a reliable pharmacological probe with which to study the specific effects of IKs blockade, and INa and Ito block may contribute to the potential profile of cardiac actions of the compound.

Identification and preclinical testing of novel antiepileptic compounds

Procedures for identifying novel antiepileptic drugs (AEDs) are changing and need to change more. Widespread reliance on two primary screens has led to the identification of novel compounds that resemble either phenytoin (suppressing high-frequency repetitive firing in cultured neurons and prolonging inactivation of voltage-dependent sodium channels identified by the maximal electroshock test) or benzodiazepines (potentiating the inhibitory effect of gamma-aminobutyric acid (GABA), identified by the threshold pentylenetetrazol test). Advances in molecular neurobiology have identified specific molecular targets (subunits of ion channels, neurotransmitter receptors, and transporters) and have made them available in a form permitting high-throughput screening. AEDs can be designed to interact with specific sites on the target molecules. Alternatively, the molecular screens can be used to identify active components in natural products, including folk remedies. Preclinical in vivo screens can be improved by using animals with genetic or acquired epilepsies that have similar modifications in the properties of the target molecules as do human epilepsy syndromes. Future work is likely to define molecular targets for AEDs that will block or reverse chronic epileptogenesis.

Structure and function of voltage-gated sodium channels

1. Sodium channels mediate fast depolarization and conduct electrical impulses throughout nerve, muscle and heart. This paper reviews the links between sodium channel structure and function. 2. Sodium channels have a modular architecture, with distinct regions for the pore and the gates. The separation is far from absolute, however, with extensive interaction among the various parts of the channel. 3. At a molecular level, sodium channels are not static: they move extensively in the course of gating and ion translocation. 4. Sodium channels bind local anaesthetics and various toxins. In some cases, the relevant sites have been partially identified. 5. Sodium channels are subject to regulation at the levels of transcription, subunit interaction and post-translational modification (notably glycosylation and phosphorylation).

Lidocaine and its Analogues Inhibit IL-5-Mediated Survival and Activation of Human Eosinophils

Eosinophils and cytokines active on eosinophils, especially IL-5, are believed to be critically involved in chronic allergic diseases. IL-5 activates eosinophils and enhances their survival in vitro by delaying apoptosis. In this study, we found that lidocaine and six analogues blunt responses of eosinophils to IL-5. Lidocaine and its derivatives inhibit IL-5-mediated eosinophil survival in a concentration-dependent manner (IC50 = 110 μM for 30 pg/ml IL-5). At suboptimal lidocaine concentrations, the eosinophil survival response to IL-5 shifts and more IL-5 is required to maintain survival. The inhibitory effect requires at least 24-h exposure of eosinophils to lidocaine, and the protein kinase C activator, PMA, completely reverses the inhibition. A multiparameter flow-cytometric analysis shows that lidocaine hastens the apoptosis of eosinophils normally delayed by IL-5. Lidocaine does not affect IL-5R expression or IL-5-induced protein tyrosine phosphorylation. Lidocaine also inhibits eosinophil survival mediated by IL-3 or granulocyte-macrophage CSF, although less potently than that mediated by IL-5. Furthermore, lidocaine inhibits eosinophil superoxide production stimulated by IL-5, granulocyte-macrophage CSF, or IL-3, but not that stimulated by platelet-activating factor, immobilized IgG, or PMA. Lidocaine and its derivatives show novel immunomodulatory properties and are able to blunt eosinophil responses to cytokines in addition to their local anesthetic or antiarrhythmic properties. Thus, lidocaine and its derivatives may represent a new class of therapeutic agents to treat patients with allergic diseases.

Sites of volatile anesthetic action on kainate (Glutamate receptor 6) receptors

Molecular mechanisms of anesthetic action on neurotransmitter receptors are poorly understood. The major excitatory neurotransmitter in the central nervous system is glutamate, and recent studies found that volatile anesthetics inhibit the function of the alpha-amino-3-hydroxyisoxazolepropionic acid subtype of glutamate receptors (e.g. glutamate receptor 3 (GluR3)), but enhance kainate (GluR6) receptor function. We used this dissimilar pharmacology to identify sites of anesthetic action on the kainate GluR6 receptor by constructing chimeric GluR3/GluR6 receptors. Results with chimeric receptors implicated a transmembrane region (TM4) of GluR6 in the action of halothane. Site-directed mutagenesis subsequently showed that a specific amino acid, glycine 819 in TM4, is important for enhancement of receptor function by halothane (0. 2-2 mM). Mutations of Gly-819 also markedly decreased the response to isoflurane (0.2-2 mM), enflurane (0.2-2 mM), and 1-chloro-1,2, 2-trifluorocyclobutane (0.2-2 mM). The nonanesthetics 1, 2-dichlorohexafluorocyclobutane and 2,3-dichlorooctafluorobutane had no effect on the functions of either wild-type GluR6 or receptors mutated at Gly-819. Ethanol and pentobarbital inhibited the function of both wild-type and mutant receptors. These results suggest that a specific amino acid, Gly-819, is critical for the action of volatile anesthetics, but not of ethanol or pentobarbital, on the GluR6 receptor.