Interaction of nonylguanidine with the sodium channel

A peptide segment critical for sodium channel inactivation functions as an inactivation gate in a potassium channel

The short cytoplasmic peptide segment connecting domains III and IV of voltage-gated sodium channels (III-IV linker) is essential for fast inactivation. To test the functional similarity between the III-IV linker and the potassium channel inactivation particle, we attached the III-IV linker to the amino terminus of a noninactivating potassium channel. This chimeric channel inactivated rapidly and displayed biophysical properties similar to Shaker A-type potassium channels. Recovery from inactivation in the chimeric channels was accelerated by high external potassium, consistent with the idea that potassium ions passing through the channel displaced the III-IV linker inactivation particle. A mutation that completely abolishes fast inactivation in rat brain sodium channels also completely abolished inactivation in the chimera. These results demonstrate that the sodium channel III-IV linker can function as a fast inactivation gate and suggest a functional relationship between the fast inactivation processes of sodium and potassium channels.

Model ion channels: gramicidin and alamethicin

We have discussed in some detail a variety of experimental studies which were designed to elucidate the conformational and dynamic properties of gramicidin and alamethicin. Although the behavior of these peptides is by no means fully characterized, these studies have already permitted aspects of ion channel activity to be understood in molecular terms. Studies with gramicidin in a variety of organic solutions have revealed conformational heterogeneity of this peptide; at least five major isomers exist, several of which have been characterized in detail using NMR spectroscopy and X-ray crystallography. When added to lipid membranes gramicidin undergoes a further conformational conversion. Although the conformation of gramicidin in membranes is not as well characterized as the solution conformation(s) and an X-ray structure is not yet available, detailed data, particularly from solid-state NMR studies, continue to become available and a right-handed beta 6.3 helical conformation of the peptide backbone is now generally accepted. Two of these beta 6.3 helices joined at their N-termini are believed to form the conducting channel. The conformational behavior of the side-chains of gramicidin in the membrane-bound form is not well established and several NMR, CD, fluorescence and theoretical studies are now focussed on this. Although the side-chains do not directly contact the permeating ions, they can have distinct effects on conductance and selectivity by altering the electrostatic environment sensed by the ion. The dynamics of both side-chain and backbone conformations of gramicidin appear critical to a detailed understanding of the ion transport process in this channel. As the description of the membrane-bound conformation of gramicidin becomes more detailed, simulations of ion transport using computational methods are likely to improve and will further our understanding of the processes of ion transport. As well as internal motion of the backbone and side-chains, gramicidin undergoes rotational and translational motion in the plane of the membrane. These motions do not appear to be essential for the process of ion transport but can affect channel lifetime since lifetime is determined by the rate of association and dissociation of gramicidin monomers. Gramicidin-membrane interactions are also likely to be involved in the frequency of occurrence of channel subconductance states, the frequency of channel flickering and fundamentally in the stability of the membrane-bound gramicidin conformation. Alamethicin forms channels in membranes which are strongly voltage-dependent. The molecular origin of voltage-dependent conductances has been a fundamental problem in biophysics for many years.(ABSTRACT TRUNCATED AT 400 WORDS)

Small iminium ions block gramicidin channels in lipid bilayers

Guanidinium and acetamidinium, when added to the bathing solution in concentrations of approximately 0.1M, cause brief blocks in the single channel potassium currents from channels formed in planar lipid bilayers by gramicidin A. Single channel lifetimes are not affected indicating that the channel structure is not modified by the blockers. Guanidinium block durations and interblock times are approximately exponential in distribution. Block frequencies increase with guanidinium concentration whereas block durations are unaffected. Increases in membrane potential cause an increase in block frequency as expected for a positively charged blocker but a decrease in block duration suggesting that the block is relieved when the blocker passes through the channel. At low pH, urea, formamide, and acetamide cause similar blocks suggesting that the protonated species of these molecules also block. Arginine and several amines do not block. This indicates that only iminium ions which are small enough to enter the channel can cause blocks in gramicidin channels.

Biophysical and molecular mechanisms of Shaker potassium channel inactivation

The potassium channels encoded by the Drosophila Shaker gene activate and inactivate rapidly when the membrane potential becomes more positive. Site-directed mutagenesis and single-channel patch-clamp recording were used to explore the molecular transitions that underlie inactivation in Shaker potassium channels expressed in Xenopus oocytes. A region near the amino terminus with an important role in inactivation has now been identified. The results suggest a model where this region forms a cytoplasmic domain that interacts with the open channel to cause inactivation.

Burst kinetics of sodium channels which lack fast inactivation in mouse neuroblastoma cells

1. The kinetics of the slow inactivation process of Na+ channels were examined by recording single-channel currents from cultured neuroblastoma cells. 2. In order to directly examine slow inactivation, fast inactivation was first removed irreversibly by briefly exposing the internal surface of excised membranes to papain. Following treatment, the time constant for the inactivation of averaged membrane Na+ current increased by over two orders of magnitude, while the open time of individual channels increased by a factor of three. The two effects are consistent with the idea that papain can selectively remove fast inactivation of Na+ channels. 3. In the absence of fast inactivation, Na+ channels continued to open during maintained depolarization of the membrane to potentials less negative than -60 mV. Under these conditions, the opening occurred in bursts 50 ms to hundreds of milliseconds long, followed by silent periods lasting many seconds. The average burst length was found to be equal to the time constant of the decline in average evoked current measured at the same potential, indicating that a burst was terminated by entry of the channel into the slow inactivated state. 4. Histograms of open times revealed two populations of open states at any potential. Bursts could also be classified as either short or long bursts. Bursts appeared to be due to the gating of a single channel, and long bursts contained both types of open states, suggesting that a Na+ channel could have more than one open state. 5. The kinetics of bursts of Na+ channels were voltage dependent. As the membrane was depolarized, the burst length, interval between bursts, and open time all increased. Although the probability of an open channel during a burst increased to almost 1.0 with depolarization, any channel was open less than 0.5% of the time when measured throughout the depolarization. The increase in burst duration with depolarization would occur if the rate of slow inactivation is faster from closed states of the channel than from open states. 6. Records of membrane current evoked by a series of step depolarizations were clustered into those with openings of Na+ channels and those without openings. Records in which a channel did not inactivate during the depolarization were less likely to lead to hibernation, suggesting that this phenomenon is caused by the slow inactivation process.

Effects of saxitoxin analogues and ligand competition on sodium currents of squid axons

Saxitoxin (STX) and several STX analogues from dinoflagellates (genus Protogonyaulax) block sodium conductance in squid giant axons with variable potencies. Toxins, analyzed under voltage clamp, are 21-sulfosaxitoxin, 21-sulfosaxitoxin 11 alpha-hydroxysulfate, 21-sulfosaxitoxin 11 beta-hydroxysulfate, (B1, C1, C2, respectively) and gonyautoxins 2 and 3. The potency sequence for the toxins examined is STX greater than gonyautoxin 3 greater than B1 greater than C2 greater than gonyautoxin 2 much greater than C1. Guanidine, when substituted for sodium in external seawater, reduced the potency of STX to block inward current but did not affect tetrodotoxin activity. Methylguanidine also reduced the ability of STX to block outward sodium current. Inhibitory constants for guanidine and methylguanidine were 116 and 187 mM, respectively. Competition can be explained by binding at or near the toxin binding site but not by surface potential alteration.

Block of sodium channels by internal mono- and divalent guanidinium analogues. Modulation by sodium ion concentration

We have investigated the block of squid axon sodium channels by mono- and divalent guanidinium analogues. The action of these compounds on steady state sodium currents was independent of the presence or absence of the normal inactivation process. Block by both mono- and divalent analogues was voltage-dependent, but was a steeper function of potential for divalent molecules. The voltage-dependence could not, in general, be reproduced by a simple model based on Boltzmann's equation. Inhibition of steady state currents by guanidinium ions with 50 mM internal sodium was reasonably well described by a 1:1 drug/channel binding function. Increasing the internal sodium ion concentration increased both the degree and voltage-dependence of current inhibition. This is in sharp contrast to the decrease in inactivation caused by internal sodium. Changes in the external sodium concentration had very little effect on drug block. These results are consistent with a model of the sodium channel as a multi-ion pore. Only a small increase in block can be produced by increased internal sodium in a three-barrier two-site model, but a four-barrier three-site model can reproduce these experimental findings. The implications of these results for physical models of inactivation are discussed.

Histrionicotoxin and alkylguanidine interactions with the solubilized and membrane-bound muscarinic acetylcholine receptor from porcine atria

The interaction of alkylguanidines and decahydrohistrionicotoxin with the membrane-bound and solubilized muscarinic acetylcholine receptor (mAcChR) from porcine atria was described. Alkylguanidines with alkyl chain lengths from one to ten carbons displaced L-[3H]quinuclidinyl benzilate (L-[3H]QNB) competitively from a single class of sites for the membrane-bound mAcChR. From a plot of -1n Ki versus alkyl carbon chain number, a value of -(473 +/- 30) cal/mol was estimated as the energetic contribution per methylene group to the total binding energy. The binding of alkylguanidines to the digitonin/cholate solubilized mAcChR was complex in nature resulting in titration curves that did not obey the law of mass action for simple competitive inhibition at higher alkyl carbon numbers and a sigmoidal plot of -1n Ki versus carbon number. Decahydrohistrionicotoxin bound in a competitive manner versus L-[3H]QNB to both the membrane-bound (Ki = (6.9 +/- 1.4) X 10(-6) M) and the solubilized (Ki = (1.5 +/- 0.3) X 10(-5) M) preparations.

Solvent substitution as a probe of channel gating in Myxicola. Effects of D2O on kinetic properties of drugs that occlude channels

The effects of solvent substitution on the steady-state and kinetic properties of drugs (gallamine triethiodide) and ions (nonyltriethylammonium and Ba++) known to occlude Na+ and K+ channels have been examined and compared with the effects of D2O on unmodified channels. In general, we observed large isotope effects on the kinetics of occlusion at temperatures of 5 degrees C, but only minor effects at 15 degrees C, consistent with processes involving significant solvent interaction. Steady-state behavior was not affected. In the case of gallamine, where a dual effect on INa is evident, although both processes were D2O sensitive, only the occlusion phase had a significant temperature dependence.

Gallamine triethiodide-induced modifications of sodium conductance in Myxicola giant axons

1. Internal gallamine triethiodide (Flaxedil) modifies Na+ channel kinetics in Myxicola axons but does not alter K+ conductance. The drug has no effect externally. 2. Gallamine initially increases the leakage-conductance, but this effect completely reverses within 30 min despite the maintained presence of drug. 3. During step depolarizations to membrane potentials less than -10 mV, gallamine slows the rate of Na+ inactivation, but all channels which have opened can still inactivate. During depolarizations to more positive potentials, gallamine-modified Na+ currents show a biphasic decline, and at VM greater than -10 mV, Na+ inactivation is incomplete as evidenced by the large Na+ tail currents which follow pulses sufficiently long to have allowed complete inactivation of normal Na+ channels. The tail currents are slower than normal Na+ tails, and exhibit a pronounced hook. With gallamine, the fraction of Na+ channels which do not inactivate increases sigmoidally over the range 0 mV to +80 mV. 4. For VM greater than ENa, gallamine almost completely blocks outward Na+ currents. The block is determined by the direction of Na+ current, rather than the absolute membrane potential. 5. Gallamine has no effect upon the rate of Na+ channel activation, the maximum Na+ conductance, the steady-state Na+ inactivation curve, or the rate of development or removal of inactivation by prepulses. 6. Gallamine eliminated physiological immobilization of intramembrane charge movements (QOFF and QON) and does not itself induce immobilization. Thus, in the presence of gallamine, QOFF following long pulses is the same as QOFF following short pulses.

Block and inactivation of sodium channels in nerve by amino acid derivatives. I. Dependence on voltage and sodium concentration

The side chain of arginine, n-propylguanidinium (nPG), reversibly decreases peak sodium conductance and increases the speed of sodium current decay, when perfused internally. Effects are voltage dependent and are more pronounced at high depolarizations. Results are also dependent on the sodium concentration gradient. Both the decline in peak conductance and the speeding of inactivation are greater if the sodium concentration gradient is reversed from the normal. The decrease in peak sodium current is too large to be due solely to the faster decay kinetics. The difference is not due to a change in slow inactivation of the channel. Sodium current inactivation has also been studied with a double pulse procedure. Results show that at - 70 mV, nPG leaves sodium channels rapidly (less than 500 microseconds) in normal sodium gradient, but more slowly (greater than 1 ms) in reversed sodium gradient. Several structural analogs of nPG have been tested. Shortening the alkyl chain weakens effects significantly. Arginine itself, which contains extra charged groups, is also less effective. n-Propylammonium is active but with an apparent affinity only one-fifth that of arginine. We conclude that nPG acts within the sodium channel, and has at least two modes of action.