Papaverine reduces the sodium permeability of the apical membrane and the Potassium permeability of the basolateral membrane in isolated frog skin

Summary The effect of papaverine, an inhibitor of the phosphodiesterase responsible for breakdown of cAMP, on the transepithelial sodium transport across the isolated frog skin was investigated.Serosal addition of papaverine caused initially an increase in the short-circuit current (SCC), a doubling of the cellular cAMP content and a depolarization of the intracellular potential under SCC conditions (V _scc).The initial increase in the SCC was followed by a pronounced decrease both in the SCC and in the natriferic action of antidiuretic hormone (ADH), but papaverine had no inhibitory effect on the ability of ADH to increase the cellular cAMP content. As SCC declines, no hyperpolarization was observed.The I/V relationship across the apical membrane during the inhibitory phase, revealed that papaverine reduces the sodium permeability of the apical membrane (P _Na ^a )as well as intracellular sodium concentration. These observations and the previously noted effect of papaverine on V _scc indicates that papaverine must have an effect on the cellular Cl or K permeability.The basolateral Na,K,2Cl cotransporter was blocked with bumetanide, which should bring the cellular chloride in equilibrium. Bumetanide had no effect on basal SCC and V _scc. When papaverine was added to skins preincubated with bumetanide, the effect of papaverine on SCC and V _scc was unchanged. Therefore, the depolarization of V _scc, observed during the papaverine induced inhibition of the SCC, must be due to a reduction in the cellular K permeability.In conclusion, it is suggested that papaverine reduces the sodium permeability of the apical membrane and the potassium permeability of the basolateral membrane of the frog skin epithelium.     

pH-Dependent electrical properties and buffer permeability of theNecturus renal proximal tubule cell

Summary Necturus kidneys were perfused with Tris-buffered solutions at three different pH values, i.e. 7.5, 6.0 and 9.0. A significant drop in fluid absorption occurred at pH 6.0, whereas pH 9.0 did not increase volume flow significantly. When acute unilateral, i.e. either in the lumen or the peritubular capillaries, and bilateral pH changes were elicited in both directions from 7.5 to 9.0 at a constant Tris-butyrate buffer concentration, both peritubular membrane potential differenceV ₁ and transepithelial potential differenceV ₃ hyperpolarized, independently of the side where the change in pH was brought about. Acid perfusions at pH 6.0 caused a similar response but of opposite sign. Analysis of the potential changes shows that pH influences not only the electromotive force and resistance of the homolateral membrane, but also the electrical properties of the paracellular path. Interference of pH with Na, Cl or K conductance was assessed. Any appreciable role for sodium or chloride was excluded, whereas the potassium transference number (t _K) of the peritubular membrane increased 16% in alkaline pH. However, this increase accounts only for 19 to 36% of the observed hyperpolarization. Since changes in Tris-butyrate buffer concentration at constant pH do not affect V₁ or V₃ considerably, the hyperpolarization in pH 9 cannot be explained by an elevation in internal pH only, or by a Tris-H⁺ ion diffusion potential only. The role of the permeability of the buffers: bicarbonate, butyrate and phosphate, in determining electrical membrane parameters was evaluated. Transport numbers of the buffer anions ranked as follows:t _HCO3>t _butyrate>t _phosphate. It is concluded that modulation of membrane potential by extracellular pH is mediated primarily by a change in peritubular cell membranet _K and additionally by membrane currents carried by buffer anions.     

Effect of conditioning polarization of soma of giant neurons of mollusks on the mechanism of action-potential generation

To ascertain the properties of an excitable membrane of the soma of giant neurons of mollusks, experiments were carried out to study the effect of conditioning shift of the membrane potential on the mechanism of action-potential generation. The effect of conditioning was assessed from changes in the action-potential curve and its first derivative, as well as from the curve of transmembrane currents under voltage clamp conditions. It was found that a change in membrane potential evokes at least two reactions which have opposite effects on the mechanism of generation of action potentials. These reactions evidently have different time characteristics. One of these does not differ notably from the reaction recorded for other excitable structures, and is manifested in the activation (with hyperpolarization) or inactivation (with depolarization) of the mechanism generating action potentials. The other reaction contributes either to an increase (with depolarization) or a decrease (with hyperpolarization) in the efficiency of this mechanism. Conditioning polarization also has a marked effect on the system responsible for repolarization of the membrane during generation of action potentials. This effect is manifested in a change in the reaction of this system to tetraethylammonium ions. The specific membrane systems sustaining excitability and reacting to changes in the strength of the membrane's electrical field were found to be very inert. After a shift in the potential to a given stable level a rearrangement, lasting sometimes tens of seconds, takes place in the membrane.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 2, No. 1, pp. 91–99, January–February, 1970.     

Effects of batrachotoxin on membrane potential and conductance of squid giant axons

The effects of batrachotoxin (BTX) on the membrane potential and conductances of squid giant axons have been studied by means of intracellular microelectrode recording, internal perfusion, and voltage clamp techniques. BTX (550–1100 nM) caused a marked and irreversible depolarization of the nerve membrane, the membrane potential being eventually reversed in polarity by as much as 15 mv. The depolarization progressed more rapidly with internal application than with external application of BTX to the axon. External application of tetrodotoxin (1000 nM) completely restored the BTX depolarization. Removal or drastic reduction of external sodium caused a hyperpolarization of the BTX-poisoned membrane. However, no change in the resting membrane potential occurred when BTX was applied in the absence of sodium ions in both external and internal phases. These observations demonstrate that BTX specifically increases the resting sodium permeability of the squid axon membrane. Despite such an increase in resting sodium permeability, the BTX-poisoned membrane was still capable of undergoing a large sodium permeability increase of normal magnitude upon depolarizing stimulation provided that the membrane potential was brought back to the original or higher level. The possibility that a single sodium channel is operative for both the resting sodium, permeability and the sodium permeability increase upon stimulation is discussed.     

Analysis of models for crustacean stretch receptors

 The mechanisms underlying the diverse responses to step current stimuli of models [Edman et al. (1987) J Physiol (Lond) 384: 649–669] of lobster slowly adapting stretch receptor organs (SAO) and fast-adapting stretch receptor organs (FAO) are analyzed. In response to a step current, the models display three distinct types of firing reflecting the level of adaptation to the stimulation. Low-amplitude currents evoke transient firing containing one to several action potentials before the system stabilizes to a resting state. Conversely, high-amplitude stimulations induce a high frequency transient burst that can last several seconds before the model returns to its quiescent state. In the SAO model, the transition between the two regimes is characterized by a sustained pacemaker firing at an intermediate stimulation amplitude. The FAO model does not exhibit such a maintained firing; rather, the duration of the transient firing increases at first with the stimulus intensity, goes through a maximum and then decreases at larger intensities. Both models comprise seven variables representing the membrane potential, the sodium fast activation, fast inactivation, slow inactivation, the potassium fast activation, slow inactivation gating variables, and the intra cellular sodium concentration. To elucidate the mechanisms of the firing adaptations, the seven-variable model for the lobster stretch receptor neuron is first reduced to a three-dimensional system by regrouping variables with similar time scales. More precisely, we substituted the membrane potential V for the sodium fast activation equivalent potential V _m , the potassium fast inactivation V _n for the sodium fast inactivation V _h , and the sodium slow inactivation V _l for the potassium slow inactivation V _r . Comparison of the responses of the reduced models to those of the original models revealed that the main behaviors of the system were preserved in the reduction process. We classified the different types of responses of the reduced SAO and FAO models to constant current stimulation. We analyzed the transient and stationary responses of the reduced models by constructing bifurcation diagrams representing the qualitatively distinct dynamics of the models and the transitions between them. These revealed that (1) the transient firings prior to reaching the stationary state can be accounted for by the sodium slow inactivation evolving more slowly than the other two variables, so that the changes during the transient firings reflect the bifurcations that the two-dimensional system undergoes when the sodium slow inactivation, considered as a parameter, is varied; and (2) the stationary behaviors of the models are captured by the standard bifurcations of a two-dimensional system formed by the membrane potential and the potassium fast inactivation. We found that each type of firing and the transitions between them is due to the interplay between essentially three variables: two fast ones accounting for the action potential generation and the post-discharge refractoriness, and a third slow one representing the adaptation. Received: 28 February 2000 / Accepted in revised form: 4 October 2000     

Effect of monoidoacetic acid on electrophysiological properties of snail giant neurons

Monoiodoacetic acid (MIA) causes depolarization and a decrease in the amplitude of the action potential and resistance of the membrane, and also leads to significant changes in the potassium and sodium concentrations in the neurons of the subesophageal ganglia. All these changes are more marked with a decrease in pH of the Ringer's solution containing the inhibitor. During the action of acid Ringer's solution without an inhibitor the electrophysiological changes in the neurons develop more slowly and to a lesser degree and they are easily reversible. It is concluded that the electrophysiological changes induced in neurons by MIA are due not only to inhibition of active ion transport but also to changes in the ionic permeability of the membrane and, in particular, to an increase in sodium permeability.A. A. Bogomolets' Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 4, No. 1, pp. 97–104, January–February 1972.     

Changes in the neuronal membrane potential of the mollusk Planorbarius corneus under the action of cardiac glycosides

In experiments on isolated neurones from the gastropod mollusc P. corneus, strophantin and digoxin in low concentrations produce slow hyperpolarization, in higher ones--depolarization; at concentrations about 1 mM, hyperpolarization was more evident. In all cases, the decrease in membrane resistance was observed. Presumably, membrane permeability for potassium ions increases. During application of the drugs in concentrations 10-100 microM, hyperpolarization may be masked by depolarization due to block of Na,K-pump. Higher concentrations, increasing potassium permeability of the membrane, may result in substitution of depolarization by hyperpolarization.     

Glutamine uptake by a sodium-dependent secondary transport system inCorynebacterium glutamicum

Corynebacterium glutamicum took up glutamine by a sodium-dependent secondary transport system. Both the membrane potential and the sodium gradient were driving forces. Glutamine uptake showed Michaelis-Menten kinetics, with aK _m of 36 μM and aV _max of 12.5 nmol min⁻¹ (mg dry weight)⁻¹ at pH 7. Despite a pH optimum in the alkaline range around pH 9, it was shown that uncharged glutamine is the transported species. The affinity for the cotransported sodium was relatively low; an apparentK _m of 1.4 mM was determined. Among various substrates tested, only asparagine, when added in 50-fold excess, led to an inhibition of glutamine transport. It was concluded that glutamine uptake occurs via a specific transport system in symport with at least one sodium ion.     

Depolarization of the neuronal membrane caused by Co-transport of taurine and sodium

C 1300 neuroblastoma cells were cultured and used to study the effect of sodium dependent taurine transport on the membrane potential. Measuring net accumulation of taurine and the depolarization caused by externally applied taurine, we found both processes become active at an external concentration of taurine of 1 mM or more. Net accumulation had K_m of 13 mM and a V_max of 126 nmol × mg of protein^–1×min^–1. The taurine induced depolarization of the neuroblastoma cell was parallelled by a 25 per cent decrease in its membrane impedance. The transport of taurine, the depolarization caused by taurine and the effect of taurine on the membrane impedance, all, had a similar dependence on the external sodium concentration. Our results on the depolarizing cotransport between taurine and sodium at the neuronal membrane, may illustrate an additional mechanism for the control of the electrical activity of neuronal cells.     

H-pumping driven by the plasma membrane ATPase in membrane vesicles from radish: stimulation by fusicoccin

The effect of fusicoccin on Mg:ATP-dependent H⁺-pumping in microsomal vesicles from 24-hour-old radish (Raphanus sativus L.) seedlings was investigated by measuring the initial rate of decrease in the absorbance of the ΔpH probe acridine orange. Fusicoccin stimulated Mg:ATP-dependent H⁺-pumping when the pH of the assay medium was in the range 7.0 to 7.6 while no effect of fusicoccin was detected between pH 6.6 and pH 6.0. Both basal and fusicoccin-stimulated H⁺-pumping were completely inhibited by vanadate and almost unaffected by nitrate. Fusicoccin did not change membrane permeability to protons and fusicoccin-induced stimulation of Mg:ATP-dependent H⁺-pumping was not affected by changes in the buffer capacity of the incubation medium. Deacetylfusicoccin stimulated H⁺-pumping as much as fusicoccin, while the physiologically inactive derivative 8-oxo-9-epideacetylfusicoccin did not. Stimulation of H⁺-pumping was saturated by 100 nanomolar fusicoccin. These data indicate that fusicoccin activates the plasma membrane H⁺-ATPase by acting at the membrane level independently of the involvement of other cell components. The percent stimulation by fusicoccin was the same at all ATP concentrations tested (0.5-5.0 millimolar), thus suggesting that with fusicoccin there is an increase in V_max of the plasma membrane H⁺-ATPase rather than a decrease in its apparent K_m for Mg:ATP.     

Inhibition of sodium currents in frog ranvier node treated with local anesthetics Role of slow sodium inactivation

Effects of different local anesthetics of sodium permeability were studied in single nerve fibres of frog by the method of voltage clamp. Inhibition of sodium current by externally applied tertiary anesthetics, procaine and trimecaine, was the sum of a potentially independent block (reduced $\text{P}{}_{\mathrm{rmNa}}$) and slow sodium inactivation with time constants ranging from tens to hundreds of ms depending on membrane potential (at room temperature). Externally applied uncharged benzocaine produced a potentially independent block only. According to dose-response curves both processes are one-to-one reactions. In the case of trimecaine equilibrium constant the reaction responsible for reduction of $\text{P}{}_{\mathrm{<mtext>Na</mtext>}}$ is about 0.3 mM, while that for slow inactivation is more than ten times less (0.02 mM). Increasing pH from 5.6 to 8.5 markedly accelerated the slow inactivation process at all potential values. Divalent cations Ca²⁺ and Ni²⁺ shifted the steady-state slow inactivation curve along the potential axis and simultaneously reduced slow inactivation at the saturation level. Permanently charged quaternary trimecaine was ineffective when applied externally. Internally applied tertiary anesthetics and quaternary trimecaine as well as externally applied quaternary derivative of lidocaine QX-572 produced a progressively irreversible block enhanced by depolarization and inhibition reversibly increased by repetitive short-term depolarization (frequency-dependent inhibition). Inhibition of sodium currents by repetitive stimulation observed also in the case of externally applied tertiary anesthetics is due mainly to slow inactivation. The data suggests the existence of several types of receptor sites through which local anesthetics exert their blocking action on sodium permeability.     

Mechanism of trace hyperpolarization of the action potential of snail giant neurons]

Depolarization current decreases and hyperpolarization current increases the amplitude of tracing hyperpolarization of the neuron action potential. Calcium-defficient solution supresses the tracing depolarization, and turns the rhythmical activity of the neuron into the flashlike one. An increase of outer concentration of potassium ions decreases the tracing depolarization. The latter is suppressed completely when the membrane behaves as a potassium electrode. The suppressing effect of the increase of potassium outer concentration on tracing hyperpolarization decreases with a decrease of calcium ions content in the medium. When an active release of sodium ions from the cell is inhibited with DNP and substitution of sodium ions by lithium ions the tracing hyperpolarization of the action potential is suppressed. The tracing hyperpolarization is also suppressed during the shunting of the electrogenic effect of potassium pump with the outcoming current of chlorine ions. It is suggested that the tracing hyperpolarization of the single action potential is due to the calcium-dependent fraction of electrogenic release of sodium ions from the cell.     

Understanding LTP in pain pathways

Background

Small neurons of the dorsal root ganglion (DRG) express five of the nine known voltage-gated sodium channels. Each channel has unique biophysical characteristics which determine how it contributes to the generation of action potentials (AP). To better understand how AP amplitude is maintained in nociceptive DRG neurons and their centrally projecting axons, which are subjected to depolarization within the dorsal horn, we investigated the dependence of AP amplitude on membrane potential, and how that dependence is altered by the presence or absence of sodium channel Na_v1.8.

Results

In small neurons cultured from wild type (WT) adult mouse DRG, AP amplitude decreases as the membrane potential is depolarized from -90 mV to -30 mV. The decrease in amplitude is best fit by two Boltzmann equations, having V_1/2 values of -73 and -37 mV. These values are similar to the V_1/2 values for steady-state fast inactivation of tetrodotoxin-sensitive (TTX-s) sodium channels, and the tetrodotoxin-resistant (TTX-r) Na_v1.8 sodium channel, respectively. Addition of TTX eliminates the more hyperpolarized V_1/2 component and leads to increasing AP amplitude for holding potentials of -90 to -60 mV. This increase is substantially reduced by the addition of potassium channel blockers. In neurons from Na_v1.8(-/-) mice, the voltage-dependent decrease in AP amplitude is characterized by a single Boltzmann equation with a V_1/2 value of -55 mV, suggesting a shift in the steady-state fast inactivation properties of TTX-s sodium channels. Transfection of Na_v1.8(-/-) DRG neurons with DNA encoding Na_v1.8 results in a membrane potential-dependent decrease in AP amplitude that recapitulates WT properties.

Conclusion

We conclude that the presence of Na_v1.8 allows AP amplitude to be maintained in DRG neurons and their centrally projecting axons even when depolarized within the dorsal horn.     

Ionic mechanisms for the antiarrhythmic action of cinnamophilin in rat heart

We investigated the electrophysiological effect and antiarrhythmic potential of cinnamophilin (Cinn), a thromboxane A₂ antagonist isolated fromCinnamomum philippinense, on rat cardiac tissues. Action potential and ionic currents in single rat ventricular cells were examined by current clamp or voltage clamp in a whole-cell configuration. In 9 episodes of ischemia-reperfusion arrhythmia, 10 µM Cinn converted 6 of them to normal sinus rhythm. Cinn suppressed the maximal rate of rise of the action potential upstroke (V_max) and prolonged the action potential duration at 50% repolarization (APD₅₀). Voltage clamp study showed that the suppression of V_max by Cinn was associated with an inhibition of sodium inward current (I_Na, IC₅₀=10.0 ± 0.4 µM). At 30 µM, V_1/2 for the steady-state inactivation curve of I_Na was shifted from –84.1 ± 0.2 to –93.0 ± 0.5 mV. Cinn also reduced calcium inward current (I_Ca) dose-dependently with an IC₅₀ value of 9.5 ± 0.3 µM. Cinn (10 µM) reduced the I_Ca with a negative shift of V_1/2 for the steady-state inactivation curve of I_Ca from –32.2 ± 0.3 to –50.7 ± 0.4 mV. The prolongation of APD₅₀ was associated with an inhibition of the integral of potassium outward current with IC₅₀ values between 4.8 and 7.1 µM. At 10 µM, Cinn reduced I_Na without a negative shift of its voltage-dependent steady-state inactivation curves. The inhibition of transient outward current (I_to) by Cinn (3–30 µM) was associated with an acceleration of its time constant of inactivation and negative shift of its potential-dependent steady-state inactivation curves. The equilibrium dissociation constant (K_d) of Cinn to inhibit open state I_to channels, as calculated from the time constant of developing block, was 18.3 µM. The time constant of recovery of I_to from inactivation state was unaffected by Cinn. The rate constant for the relief from the depolarization-dependent block of I_to was calculated to be 23.9 ms. As compared with its effect on I_to, Cinn exerted about half the potency to block I_Na and I_Ca. These results indicate that the inhibition of I_Na, I_Ca and I_to may contribute to the antiarrhythmic activity of Cinn against ischemia-reperfusion arrhythmia.     

Influence of the State of TTX-Resistant Sodium Channels on Electrical Activity of a Nociceptive Sensory Fiber: A Model Study

On a model of a thin (C-type) primary afferent fiber, we examined one of the hypotheses related to the phenomenon of initiation of long-lasting tonic discharges in nociceptive afferents. In the membrane of a region corresponding to the free peripheral terminal of the modeled nociceptive C fiber, there were sodium channels of three types (channels of rapidly inactivating TTX-sensitive current and TTX-resistant channels of two types, Na_V1.8/SNS/PN3 and Na_V1.9/NaN/SNS2). As is known, TTX-resistant sodium currents promote the development of long-lasting trains of action potentials, APs, where the duration of tonic discharges exceeds by orders of magnitude the duration of short stimuli inducing such discharges. Such trains, when transmitted to the spinal cord, are interpreted as pain signals. Using the model, we obtained the time course of changes in the membrane potential in the distal and proximal segments of the nerve fiber and values of the densities of inward and outward TTX-resistant sodium currents through channels Na_V1.9/NaN/SNS2 and Na_V1.8/SNS/PN3 in the norm and in a state mimicking the action of inflammation factors. Results of modeling demonstrated that TTX-resistant sodium currents provide intensification of slow components in the generated APs (plateau afterdepolarization). Having a higher inactivation threshold, these currents are inactivated more slowly and recover more rapidly after inactivation, as compared with the currents through TTX-sensitive sodium channels. Such behavior presupposes a considerable role of the TTX-resistant currents in facilitation of transmission of nociceptive signals under conditions of neuropathic pain characterized by excessive “upregulation” of the respective channels. It can be concluded that expression of TTX-resistant sodium channels in nociceptive sensory neurons possessing primary afferent C fibers, the presence of these channels in the membranes of peripheral terminals of the above fibers, and modification of biophysical properties of such channels under conditions of action of inflammation mediators, when taken together, create substantial prerequisites for initiation of anomalous long-lasting AP trains in the above peripheral terminals and, therefore, for transmission of such signals to the CNS. Such a situation appears to be a key electrophysiological phenomenon responsible for generation of neuropathic and inflammation-related pain.     

Active extrusion of potassium in the yeast Saccharomyces cerevisiae induced by low concentrations of trifluoperazine

Trifluoperazine (TFP), the antipsychotic drug, induces substantial K+ efflux, membrane hyperpolarization and inhibition of H+-ATPase in the yeast Saccharomyces cerevisiae. Investigations on the mechanism of these effects revealed two different processes observed at different incubation conditions. At an acidic pH of 4.5 and an alkaline pH of 7.5, K+ efflux was accompanied by substantial proton influx which led to intracellular acidification and dissipation of delta psi formed by cation efflux. The results indicated nonspecific changes in membrane permeability. Similar results were also observed when cells were incubated at pH 5.5-6.0 with higher concentrations of TFP (above 75 microM). On the other hand, low concentrations of TFP (30-50 microM) at pH 5.5-6.0 caused marked membrane hyperpolarization and K+ efflux unaccompanied by the efflux of other cations and by H+ influx. Our experiments indicate that under these conditions K+ efflux was an active process. (1) K+ efflux proceeded only in the presence of a metabolic substrate and was inhibited by metabolic inhibitors. (2) When 0.3-0.9 mM-KCl was present in the medium at pH 6.0, the concentration of K+ within the cells (measured at the end of the incubation with TFP) was much lower than the theoretical concentration of Kin+ if the distribution of K+ between medium and cell water was at equilibrium (at zero electrochemical gradient). (3) Valinomycin decreased the net K+ efflux and decreased the membrane hyperpolarization induced by TFP, probably by increasing the flux of K+ into the cells along its electrochemical gradient. (4) Conditions which led to active K+ efflux also led to a marked decrease in cellular ATP level. The results indicate that under a specific set of conditions TFP induces translocation of K+ against its electrochemical gradient.     

Effect of procaine on electrical activity of the ranvier node in solutions with high and low pH

The effect of procaine on generation of the action potential and its derivative in solutions with different pH values was studied in experiments on single Ranvier nodes. Minimal concentrations of procaine depressing the action potential were increased in solutions with low pH and reduced in solutions with high pH. The calculated concentrations of the basic and cationic forms of procaine changed regularly: With an increase in pH of the medium the basic decreased and the cationic increased. Excitability of the membrane (the number of sodium channels capable of excitation) did not change regularly in accordance with a change in pH of the medium: It fell on both a decrease and an increase in the pH of the solution. It was concluded from the results that the two forms of procaine interact with the membrane, but with different effectiveness.A. V. Vishnevskii Institute of Surgery, Academy of Medical Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 8, No. 2, pp. 161–167, March–April, 1974.     

Induction of permeability change and restoration of membrane permeability barrier in transformed cell cultures

Transformed 3T3 cells incubated with ATP at an alkaline pH become permeable to phosphorylated compounds. The increase in membrane permeability can be induced by incubation with ATP at a neutral pH but only if sodium fluoride is present. Fluoride is not necessary for activation of the permeability change in these cultures at the alkaline pH. The effect of fluoride is very rapid, and sodium fluoride by itself does not alter membrane permeability. The alteration of membrane permeability by ATP in 3T6 cells is reversible; the permeability barrier is restored by switching to neutral buffer in the presence or absence of divalent cations. The restoration of the membrane permeability barrier is prevented by fluoride, and by ATP itself; this action of ATP is specific and no other nucleoside triphosphates or chelating agents produce this effect. Untransformed 3T3 cells do not exhibit any appreciable change in permeability as a result of ATP treatment either in the presence or absence of fluoride. These results are consistent with the presence on the transformed cell surface of an ATP-requiring protein kinase and a fluoride-inhibitable protein phosphatase, which would be involved in the control of membrane permeability.     

Structural Pharmacology of Voltage-Gated Sodium Channels

Voltage-gated sodium (Na_V) channels initiate and propagate action potentials in excitable tissues to mediate key physiological processes including heart contraction and nervous system function. Accordingly, Na_V channels are major targets for drugs, toxins and disease-causing mutations. Recent breakthroughs in cryo-electron microscopy have led to the visualization of human Na_V1.1, Na_V1.2, Na_V1.4, Na_V1.5 and Na_V1.7 channel subtypes at high-resolution. These landmark studies have greatly advanced our structural understanding of channel architecture, ion selectivity, voltage-sensing, electromechanical coupling, fast inactivation, and the molecular basis underlying Na_V channelopathies. Na_V channel structures have also been increasingly determined in complex with toxin and small molecule modulators that target either the pore module or voltage sensor domains. These structural studies have provided new insights into the mechanisms of pharmacological action and opportunities for subtype-selective Na_V channel drug design. This review will highlight the structural pharmacology of human Na_V channels as well as the potential use of engineered and chimeric channels in future drug discovery efforts.