Ion permeation in normal and batrachotoxin-modified Na+ channels in the squid giant axon

Na+ permeation through normal and batrachotoxin (BTX)-modified squid axon Na+ channels was characterized. Unmodified and toxin-modified Na+ channels were studied simultaneously in outside-out membrane patches using the cut-open axon technique. Current-voltage relations for both normal and BTX-modified channels were measured over a wide range of Na+ concentrations and voltages. Channel conductance as a function of Na+ concentration curves showed that within the range 0.015-1 M Na+ the normal channel conductance is 1.7-2.5-fold larger than the BTX-modified conductance. These relations cannot be fitted by a simple Langmuir isotherm. Channel conductance at low concentrations was larger than expected from a Michaelis-Menten behavior. The deviations from the simple case were accounted for by fixed negative charges located in the vicinity of the channel entrances. Fixed negative charges near the pore mouths would have the effect of increasing the local Na+ concentration. The results are discussed in terms of energy profiles with three barriers and two sites, taking into consideration the effect of the fixed negative charges. Either single- or multi-ion pore models can account for all the permeation data obtained in both symmetric and asymmetric conditions. In a temperature range of 5-15 degrees C, the estimated Q10 for the conductance of the BTX-modified Na+ channel was 1.53. BTX appears not to change the Na+ channel ion selectively (for the conditions used) or the surface charge located near the channel entrances.     

Binding of benzocaine in batrachotoxin-modified Na+ channels. State- dependent interactions

Hille (1977. Journal of General Physiology. 69:497-515) first proposed a modulated receptor hypothesis (MRH) to explain the action of benzocaine in voltage-gated Na+ channels. Using the MRH as a framework, we examined benzocaine binding in batrachotoxin (BTX)-modified Na+ channels under voltage-clamp conditions using either step or ramp command signals. We found that benzocaine binding is strongly voltage dependent. At -70 mV, the concentration of benzocaine that inhibits 50% of BTX-modified Na+ currents in GH3 cells (IC50) is 0.2 mM, whereas at +50 mV, the IC50 is 1.3 mM. Dose-response curves indicate that only one molecule of benzocaine is required to bind with one BTX-modified Na+ channel at -70 mV, whereas approximately two molecules are needed at +50 mV. Upon treatment with the inactivation modifier chloramine-T, the binding affinity of benzocaine is reduced significantly at -70 mV, probably as a result of the removal of the inactivated state of BTX- modified Na+ channels. The same treatment, however, enhances the binding affinity of cocaine near this voltage. External Na+ ions appear to have little effect on benzocaine binding, although they do affect cocaine binding. We conclude that two mechanisms underlie the action of local anesthetics in BTX-modified Na+ channels. Unlike open-channel blockers such as cocaine and bupivacaine, neutral benzocaine binds preferentially with BTX-modified Na+ channels in a closed state. Furthermore, benzocaine can be modified chemically so that it behaves like an open-channel blocker. This compound also elicits a use- dependent block in unmodified Na+ channels after repetitive depolarizations, whereas benzocaine does not. The implications of these findings for the MRH theory will be discussed.     

Charged tetracaine as an inactivation enhancer in batrachotoxin-modified Na+ channels.

Two distinct types of local anesthetics (LAs) have previously been found to block batrachotoxin (BTX)-modified Na+ channels: type 1 LAs such as cocaine and bupivacaine interact preferentially with open channels, whereas type 2 LAs, such as benzocaine and tricaine, with inactivated channels. Herein, we describe our studies of a third type of LA, represented by tetracaine as a dual blocker that binds strongly with closed channels but also binds to a lesser extent with open channels when the membrane is depolarized. Enhanced inactivation of BTX-modified Na+ channels by tetracaine was determined by steady-state inactivation measurement and by the dose-response curve. The 50% inhibitory concentration (IC50) was estimated to be 5.2 microM at -70 mV, where steady-state inactivation was maximal, with a Hill coefficient of 0.98 suggesting that one tetracaine molecule binds with one inactivated channel. Tetracaine also interacted efficiently with Na+ channels when the membrane was depolarized; the IC50 was estimated to be 39.5 microM at +50 mV with a Hill coefficient of 0.94. Unexpectedly, charged tetracaine was found to be the primary active form in the blocking of inactivated channels. In addition, external Na+ ions appeared to antagonize the tetracaine block of inactivated channels. Consistent with these results, N-butyl tetracaine quaternary ammonium, a permanently charged tetracaine derivative, remained a strong inactivation enhancer. Another derivative of tetracaine, 2-(di-methylamino) ethyl benzoate, which lacked a 4-butylamino functional group on the phenyl ring, elicited block that was approximately 100-fold weaker than that of tetracaine. We surmise that 1) the binding site for inactivation enhancers is within the Na+ permeation pathway, 2) external Na+ ions antagonize the block of inactivation enhancers by electrostatic repulsion, 3) the 4-butylamino functional group on the phenyl ring is critical for block and for the enhancement of inactivation, and 4) there are probably overlapping binding sites for both inactivation enhancers and open-channel blockers within the Na+ pore.     

Inactivation of batrachotoxin-modified Na+ channels in GH3 cells. Characterization and pharmacological modification

Batrachotoxin (BTX)-modified Na+ currents were characterized in GH3 cells with a reversed Na+ gradient under whole-cell voltage clamp conditions. BTX shifts the threshold of Na+ channel activation by approximately 40 mV in the hyperpolarizing direction and nearly eliminates the declining phase of Na+ currents at all voltages, suggesting that Na+ channel inactivation is removed. Paradoxically, the steady-state inactivation (h infinity) of BTX-modified Na+ channels as determined by a two-pulse protocol shows that inactivation is still present and occurs maximally near -70 mV. About 45% of BTX-modified Na+ channels are inactivated at this voltage. The development of inactivation follows a sum of two exponential functions with tau d(fast) = 10 ms and tau d(slow) = 125 ms at -70 mV. Recovery from inactivation can be achieved after hyperpolarizing the membrane to voltages more negative than -120 mV. The time course of recovery is best described by a sum of two exponentials with tau r(fast) = 6.0 ms and tau r(slow) = 240 ms at -170 mV. After reaching a minimum at -70 mV, the h infinity curve of BTX-modified Na+ channels turns upward to reach a constant plateau value of approximately 0.9 at voltages above 0 mV. Evidently, the inactivated, BTX-modified Na+ channels can be forced open at more positive potentials. The reopening kinetics of the inactivated channels follows a single exponential with a time constant of 160 ms at +50 mV. Both chloramine-T (at 0.5 mM) and alpha-scorpion toxin (at 200 nM) diminish the inactivation of BTX-modified Na+ channels. In contrast, benzocaine at 1 mM drastically enhances the inactivation of BTX-modified Na+ channels. The h infinity curve reaches minimum of less than 0.1 at -70 mV, indicating that benzocaine binds preferentially with inactivated, BTX-modified Na+ channels. Together, these results imply that BTX-modified Na+ channels are governed by an inactivation process.     

Binding affinity and stereoselectivity of local anesthetics in single batrachotoxin-activated Na+ channels

Several local anesthetics (LA) have been previously shown to block muscle batrachotoxin (BTX)-activated Na+ channels in planar bilayers. The mean dwell time of different LA drugs, however, varies widely, from less than 10 ms to longer than several seconds. In this study, we have examined the structural determinants that govern the dwell time, the binding affinity, and the stereoselectivity of LA drugs using cocaine and bupivacaine homologues, RAC compounds, and their available stereoisomers. Our results from the structure-activity experiments reveal that (a) there are two apparent hydrophobic binding domains present in the LA binding site; one interacts with the aromatic moiety of the LA drugs, and the other interacts with the alkyl group attached to the tertiary amine of the LA drugs; (b) the LA mean dwell time and the binding affinity are largely determined by the hydrophobic interactions; (c) the LA binding site is highly stereoselective, with a difference in KD values over 50- and 6-fold for (+/-) cocaine and (+/-) bupivacaine, respectively; (d) the cocaine stereoselectivity is comparable among muscle, brain, and heart BTX-activated Na+ channels; and finally and most unexpectedly, (e) the stereoselectivity of LA drugs in BTX-activated Na+ channels appears greatly different from that reported in normal Na+ channels. Possible explanations for this difference are discussed.     

Effects of benzocaine on the kinetics of normal and batrachotoxin-modified Na channels in frog node of Ranvier.

The effects of benzocaine (0.5-1 mM) on normal Na currents, and on Na current and gating charge movement (Q) of batrachotoxin (BTX)-modified Na channels were analyzed in voltage-clamped frog node of Ranvier. Without BTX treatment the decay of Na current during pulses to between -40 and 0 mV could be decomposed into two exponential components both in the absence and in the presence of benzocaine. Benzocaine did not significantly alter the inactivation time constant of either component, but reduced both their amplitudes. The amplitude of the slow inactivating component was more decreased by benzocaine than the amplitude of the fast one, leading to an apparently faster decline of the overall Na current. After removal of Na inactivation and charge movement immobilization by BTX, benzocaine decreased the amplitude of INa with no change in time course. INa, QON, and QOFF were all reduced by the same factor. The results suggest that the rate of reaction of benzocaine with its receptor is slow compared to the rates of channel activation and inactivation. The differential effects of benzocaine on the two components of Na current inactivation in normal channels can be explained assuming two types of channel with different rates of inactivation and different affinities for the drug.     

Gating kinetics of batrachotoxin-modified Na+ channels in the squid giant axon. Voltage and temperature effects.

The gating kinetics of batrachotoxin-modified Na+ channels were studied in outside-out patches of axolemma from the squid giant axon by means of the cut-open axon technique. Single channel kinetics were characterized at different membrane voltages and temperatures. The probability of channel opening (Po) as a function of voltage was well described by a Boltzmann distribution with an equivalent number of gating particles of 3.58. The voltage at which the channel was open 50% of the time was a function of [Na+] and temperature. A decrease in the internal [Na+] induced a shift to the right of the Po vs. V curve, suggesting the presence of an integral negative fixed charge near the activation gate. An increase in temperature decreased Po, indicating a stabilization of the closed configuration of the channel and also a decrease in entropy upon channel opening. Probability density analysis of dwell times in the closed and open states of the channel at 0 degrees C revealed the presence of three closed and three open states. The slowest open kinetic component constituted only a small fraction of the total number of transitions and became negligible at voltages greater than -65 mV. Adjacent interval analysis showed that there is no correlation in the duration of successive open and closed events. Consistent with this analysis, maximum likelihood estimation of the rate constants for nine different single-channel models produced a preferred model (model 1) having a linear sequence of closed states and two open states emerging from the last closed state. The effect of temperature on the rate constants of model 1 was studied. An increase in temperature increased all rate constants; the shift in Po would be the result of an increase in the closing rates predominant over the change in the opening rates. The temperature study also provided the basis for building an energy diagram for the transitions between channel states.     

Modeling ion permeation through batrachotoxin-modified Na+ channels from rat skeletal muscle with a multi-ion pore.

The mechanism of ion permeation through Na+ channels that have been modified by batrachotoxin (BTX) and inserted into planar bilayers has been generally described by models based on single-ion occupancy, with or without an influence of negative surface charge, depending on the tissue source. For native Na+ channels there is evidence suggestive of a multi-ion conduction mechanism. To explore the question of ion occupancy, we have reexamined permeation of Na+, Li+, and K+ through BTX-modified Na+ channels from rat skeletal muscle. Single-channel current-voltage (I-V) behavior was studied in neutral lipid bilayers in the presence of symmetrical Na+ concentrations ranging from 0.5 to 3,000 mM. The dependence of unitary current on the mole fraction of Na+ was also examined in symmetrical mixtures of Na(+)-Li+ and Na(+)-K+ at a constant total ionic strength of 206 and 2,006 mM. The dependence of unitary conductance on symmetrical Na+ concentration does not exhibit Michaelis-Menten behavior characteristic of single-ion occupancy but can be simulated by an Eyring-type model with three barriers and two sites (3B2S) that includes double occupancy and ion-ion repulsion. Best-fit energy barrier profiles for Na+, Li+, and K+ were obtained by nonlinear curve fitting of I-V data using the 3B2S model. The Na(+)-Li+ and Na(+)-K+ mole-fraction experiments do not exhibit an anomalous mole-fraction effect. However, the 3B2S model is able to account for the biphasic dependence of unitary conductance on symmetrical [Na+] that is suggestive of multiple occupancy and the monotonic dependence of unitary current on the mole fraction of Na+ that is compatible with single or multiple occupancy. The best-fit 3B2S barrier profiles also successfully predict bi-ionic reversal potentials for Na(+)-Li+ and Na(+)-K+ in both orientations across the channel. Our experimental and modeling results reconcile the dual personality of ion permeation through Na+ channels, which can display features of single or multiple occupancy under various conditions. To a first approximation, the 3B2S model developed for this channel does not require corrections for vestibule surface charge. However, if negative surface charges of the protein do influence conduction, the conductance behavior in the limit of low [Na+] does not correspond to a Gouy-Chapman model of planar surface charge.     

The properties of batrachotoxin-modified cardiac Na channels, including state-dependent block by tetrodotoxin

Batrachotoxin (BTX) modification and tetrodotoxin (TTX) block of BTX-modified Na channels were studied in single cardiac cells of neonatal rats using the whole-cell patch-clamp recording technique. The properties of BTX-modified Na channels in heart are qualitatively similar to those in nerve. However, quantitative differences do exist between the modified channels of these two tissues. In the heart, the shift of the conductance-voltage curve for the modified channel was less pronounced, the maximal activation rate constant, (tau m)max, of modified channels was considerably slower, and the slow inactivation of the BTX-modified cardiac Na channels was only partially abolished. TTX blocked BTX-modified mammalian cardiac Na channels and the block decreased over the potential range of -80 to -40 mV. The apparent dissociation constant of TTX changed from 0.23 microM at -50 mV to 0.69 microM at 0 mV. No further reduction of block was observed at potentials greater than -40 mV. This is the potential range over which gating from closed to open states occurred. These results were explained by assuming that TTX has a higher affinity for closed BTX-modified channels than for open modified channels. Hence, the TTX-binding rate constants are considered to be state dependent rather than voltage dependent. This differs from the voltage dependence of TTX block reported for BTX-modified Na channels from membrane vesicles incorporated into lipid bilayers and from amphibian node of Ranvier.     

Ion conduction in substates of the batrachotoxin-modified Na+ channel from toad skeletal muscle.

Batrachotoxin-modified Na+ channels from toad muscle were inserted into planar lipid bilayers composed of neutral phospholipids. Single-channel conductances were measured for [Na+] ranging between 0.4 mM and 3 M. When membrane preparations were made in the absence of protease inhibitors, two open conductance states were identified: a fully open state (16.6 pS in 200 mM symmetrical NaCl) and a substate that was 71% of the full conductance. The substate was predominant at [Na+] > 65 mM, whereas the presence of the fully open state was predominant at [Na+] < 15 mM. Addition of protease inhibitors during membrane preparation stabilized the fully open state over the full range of [Na+] studied. In symmetrical Na+ solutions and in biionic conditions, the ratio of amplitudes remained constant and the two open states exhibited the same permeability ratios of PLi/PNa and PCs/PNa. The current-voltage relations for both states showed inward rectification only at [Na+] < 10 mM, suggesting the presence of asymmetric negative charge densities at both channel entrances, with higher charge density in the external side. An energy barrier profile that includes double ion occupancy and asymmetric charge densities at the channel entrances was required to fit the conductance-[Na+] relations and to account for the rectification seen at low [Na+]. Energy barrier profiles differing only in the energy peaks can give account of the differences between both conductance states. Estimation of the surface charge density at the channel entrances is very dependent on the ion occupancy used and the range of [Na+] tested. Independent evidence for the existence of a charged external vestibule was obtained at low external [Na+] by identical reduction of the outward current induced by micromolar additions of Mg2+ and Ba2+.     

Na+ and K+ channels: history and structure

In this perspective, we discuss the physiological roles of Na and K channels, emphasizing the importance of the K channel for cellular homeostasis in animal cells and of Na and K channels for cellular signaling. We consider the structural basis of Na and K channel gating in light of recent structural and electrophysiological findings.     

Structure and function of amiloride-sensitive Na+ channels

A new molecular biological epoch in amiloride-sensitive Na⁺ channel physiology has begun. With the application of these new techniques, undoubtedly a plethora of new information and new questions will be forthcoming. First and foremost, however, is the question of how many discrete amiloride-sensitive Na⁺ channels exist. This question is important not only for elucidating structure-function relationships, but also for developing strategies for pharmacological or, ultimately, genetic intervention in such diseases as obstructive nephropathy, Liddle's syndrome, or salt-sensitive hypertension where amiloride-sensitive Na⁺ channel dysfunction has been implicated [17, 62].Epithelia Na⁺ channels purified from kidney are multimeric. However, it is not yet clear which subunits are regulatory and which participate directly as a part of the Na⁺ conducting core and what is the nature of the gate. The combination of electrophysiologic techniques such as patch clamp and the ability to study reconstituted channels in planar lipid bilayers along with molecular biology techniques to potentially manipulate the individual subunits should provide the answers to questions that have puzzled physiologists for decades. It seems clear that the robust versatility of the channel in responding to a wide range of differing and potentially synergistic regulatory inputs must be a function of its multimeric structure and relation to the cytoskeleton. Multiple mechanisms of regulation imply multiple regulatory sites. This hypothesis has been validated by the demonstration that enzymatic carboxyl methylation and phosphorylation have both individual and synergistic effects on the purified channel in planar lipid bilayers.     

Extracellular Zn2+ activates epithelial Na+ channels by eliminating Na+ self-inhibition

Inhibition of epithelial Na(+) channel (ENaC) activity by high concentrations of extracellular Na(+) is referred to as Na(+) self-inhibition. We investigated the effects of external Zn(2+) on whole cell Na(+) currents and on the Na(+) self-inhibition response in Xenopus oocytes expressing mouse alphabetagamma ENaC. Na(+) self-inhibition was examined by analyzing inward current decay from a peak current to a steady-state current following a fast switching of a low Na(+) (1 mm) bath solution to a high Na(+) (110 mm) solution. Our results indicate that external Zn(2+) rapidly and reversibly activates ENaC in a dose-dependent manner with an estimated EC(50) of 2 microm. External Zn(2+) in the high Na(+) bath also prevents or reverses Na(+) self-inhibition with similar affinity. Zn(2+) activation is dependent on extracellular Na(+) concentration and is absent in ENaCs containing gammaH239 mutations that eliminate Na(+) self-inhibition and in alphaS580Cbetagamma following covalent modification by a sulfhydryl-reactive reagent that locks the channels in a fully open state. In contrast, external Ni(2+) inhibition of ENaC currents appears to be additive to Na(+) self-inhibition when Ni(2+) is present in the high Na(+) bath. Pretreatment of oocytes with Ni(2+) in a low Na(+) bath also prevents the current decay following a switch to a high Na(+) bath but rendered the currents below the control steady-state level measured in the absence of Ni(2+) pretreatment. Our results suggest that external Zn(2+) activates ENaC by relieving the channel from Na(+) self-inhibition, and that external Ni(2+) mimics or masks Na(+) self-inhibition.     

Extracellular histidine residues crucial for Na+ self-inhibition of epithelial Na+ channels

Epithelial Na(+) channels (ENaC) participate in the regulation of extracellular fluid volume homeostasis and blood pressure. Channel activity is regulated by both extracellular and intracellular Na(+). The down-regulation of ENaC activity by external Na(+) is referred to as Na(+) self-inhibition. We investigated the structural determinants of Na(+) self-inhibition by expressing wild-type or mutant ENaCs in Xenopus oocytes and analyzing changes in whole-cell Na(+) currents following a rapid increase of bath Na(+) concentration. Our results indicated that wild-type mouse alphabetagammaENaC has intrinsic Na(+) self-inhibition similar to that reported for human, rat, and Xenopus ENaCs. Mutations at His(239) (gammaH239R, gammaH239D, and gammaH239C) in the extracellular loop of the gammaENaC subunit prevented Na(+) self-inhibition whereas mutations of the corresponding His(282) in alphaENaC (alphaH282D, alphaH282R, alphaH282W, and alphaH282C) significantly enhanced Na(+) self-inhibition. These results suggest that these two histidine residues within the extracellular loops are crucial structural determinants for Na(+) self-inhibition.     

Point mutations in alpha-subunit of human cardiac Na+ channels alter Na+ current kinetics

Dietary polyunsaturated fatty acids (PUFAs) prevent ischemia-induced fatal cardiac arrhythmias in animals and probably in humans. This action results from inhibition of ion currents for Na+, Ca2+, and possibly other ions. To extend understanding of this protection we are seeking a possible binding site for the PUFAs on the alpha-subunit of the human cardiac Na+ channel, hH1alpha, transiently expressed in HEK293t cells. Three mutated single amino acid substitutions with lysine were made in the alpha-subunit at Domain 4-Segment 6 (D4-S6) for F1760, Y1767 and at D1-S6 for N406. These are in the putative sites of binding of local anesthetics and batrachotoxin, respectively. The mutants F1760K, Y1767K, and N406K, separately and to different extents, affected the current density, the steady-state inactivation potential, accelerated inactivation, delayed recovery from inactivation, and affected voltage-dependent block, but did not affect activation of the hH1alpha. It is essential to learn that single point mutations in D1-S6 and D4-S6 alone significantly modify the kinetics of human cardiac hH1alpha Na+ currents. The effects of PUFAs on these mutant channels will be the subject of subsequent reports.     

Gating in iodate-modified single cardiac Na+ channels

Summary Elementary Na⁺ currents were recorded at 19°C during 220-msec lasting step depolarizations in cell-attached and inside-out patches from cultured neonatal rat cardiocytes in order to study the modifying influence of iodate, bromate and glutaraldehyde on single cardiac Na⁺ channels.Iodate (10 mmol/liter) removed Na⁺ inactivation and caused repetitive, burst-like channel activity after treating the cytoplasmic channel surface. In contrast to normal Na⁺ channels under control conditions, iodate-modified Na⁺ channels attain two conducting states, a short-lasting one with a voltage-independent lifetime close to 1 msec and, likewise tested between –50 and +10 mV, a long-lasting one being apparently exponentially dependent on voltage. Channel modification by bromate (10 mmol/liter) and glutaraldehyde (0.5 mmol/liter) also included the occurrence of two open states. Also, burst duration depended apparently exponentially on voltage and increased when shifting the membrane in the positive direction, but there was no evidence for two bursting states. Chemically modified Na⁺ channels retain an apparently normal unitary conductance (12.8±0.5 pS). Of the two substates observed, one of them is remarkable in that it is mostly attained from full-state openings and is very short living in nature; the voltage-independent lifetime was close to 2 msec. Despite removal of inactivation, open probability progressively declined during membrane depolarization. The underlying deactivation process is strongly voltage sensitive but, in contrast to slow Na⁺ inactivation, responds to a voltage shift in the positive direction with a retardation in kinetics. Chemically modified Na⁺ channels exhibit a characteristic bursting state much shorter than in DPI-modified Na⁺ channels, a difference not consistent with the hypothesis of common kinetic properties in noninactivating Na⁺ channels.     

Na+ permeation and block of hERG potassium channels

The inactivation gating of hERG channels is important for the channel function and drug-channel interaction. Whereas hERG channels are highly selective for K+, we have found that inactivated hERG channels allow Na+ to permeate in the absence of K+. This provides a new way to directly monitor and investigate hERG inactivation. By using whole cell patch clamp method with an internal solution containing 135 mM Na+ and an external solution containing 135 mM NMG+, we recorded a robust Na+ current through hERG channels expressed in HEK 293 cells. Kinetic analyses of the hERG Na+ and K+ currents indicate that the channel experiences at least two states during the inactivation process, an initial fast, less stable state followed by a slow, more stable state. The Na+ current reflects Na+ ions permeating through the fast inactivated state but not through the slow inactivated state or open state. Thus the hERG Na+ current displayed a slow inactivation as the channels travel from the less stable, fast inactivated state into the more stable, slow inactivated state. Removal of fast inactivation by the S631A mutation abolished the Na+ current. Moreover, acceleration of fast inactivation by mutations T623A, F627Y, and S641A did not affect the hERG Na+ current, but greatly diminished the hERG K+ current. We also found that external Na+ potently blocked the hERG outward Na+ current with an IC50 of 3.5 mM. Mutations in the channel pore and S6 regions, such as S624A, F627Y, and S641A, abolished the inhibitory effects of external Na+ on the hERG Na+ current. Na+ permeation and blockade of hERG channels provide novel ways to extend our understanding of the hERG gating mechanisms.