Inactivation of monazomycin-induced voltage-dependent conductance in thin lipid membranes. II. Inactivation produced by monazomycin transport through the membrane

At sufficiently large conductances, the voltage-dependent conductance induced in thin lipid membranes by monazomycin undergoes inactivation. This is a consequence of depletion of monazomycin from the membrane solution interface, as monazomycin crosses the membrane to the opposite (trans) side from which it was added. The flux of monazomycin is directly proportional to the monazomycin-induced conductance; at a given conductance it is independent of monazomycin concentration. We conclude that when monazomycin channels break up, some or all of the molecules making up a channel are deposited on the trans side. We present a model for the monazomycin channel: approximately five molecules, each spanning the membrane with its NH3+ on the trans side and an uncharged hydrophilic (probably sugar) group anchored to the cis side, form an aqueous channel lined by--OH groups. The voltage dependence arises from the flipping by the electrical field of molecules lying parallel to the cis surface into the "spanned state;" the subsequent aggregation of these molecules into channels is, to a first approximation, voltage independent. The channel breakup that deposits monomers on the trans side involves the collapsing of the channel in such a way that the uncharged hydrophilic groups remain in contact with the water in the channel as they close the channel from behind. We also discuss the possibility that inactivation of sodium channels in nerve involves the movement from one side of the membrane to the other of the molecules (or molecule) forming the channel.     

Inactivation of the alamethicin-induced conductance caused by quaternary ammonium ions and local anesthetics

Long alkyl chain quaternary ammonium ions (QA), the local anesthetics (LA) tetracaine and lidocaine, imipramine, and pancuronium cause inactivation of the alamethicin-induced conductance in lipid bilayer membranes. The alamethicin-induced conductance undergoes inactivation only when these amphipathic compounds are added to the side containing alamethicin. The concentration of QA required to cause a given amount of inactivation depends on the length of the hydrocarbon chain and follows the sequence C9 greater than C10 greater than C12 greater than C16. LA and imipramine, in contrast to QA or pancuronium, are able to promote appreciable inactivation only if the pH of the alamethicin-free side is equal to or lower than the pK of these compounds. The membrane permeability to QA, LA, or imipramine is directly proportional to the alamethicin-induced conductance and is larger than the one for potassium. The observed steady state and time-course of the inactivation are well described by a model similar to that proposed by Heyer et al. (1976. J. Gen. Physiol. 67:703--729) and extended for any value of the diffuse double layer potential and for LA and imipramine. In this model QA, LA, or imipramine are able to permeate through the membrane only when the alamethicin-induced conductance is turned on. The amphipathic compounds then bind to the other membrane surface, changing the transmembrane potential and turning the conductance off. For a given concentration of QA, LA, or imipramine the extent of inactivation depends on two factors: first, the binding characteristics of these compounds to the membrane surface and second, their ability to permeate through the membrane when the alamethicin-induced conductance is turned on. The several possible mechanisms of permeation of the amphipathic molecules tested are discussed.     

Fluctuation and relaxation analysis of monazomycin-induced conductance in black lipid membranes

Summary Fluctuation and relaxation analyses were performed on monazomycin-induced conductance of lipid bilayer membranes. With both methods a slow (sec) and a fast (msec) current component are apparent; however, the amplitude of the slow, voltage-dependent process is greater than that of the fast component in the step relaxation experiment and less in the fluctuation experiment. The fluctuation analysis showed principally a rapid voltage-dependent process which appears to be related to the multistate character of the conducting channel. The experimental results are interpreted in terms of a simplified kinetic model which is used to calculate relaxation and noise amplitudes.     

The kinetics of monazomycin-induced voltage-dependent conductance. I. Proof of the validity of an empirical rate equation

Monazomycin (a positively-charged, polyene-like antibiotic) induces a strongly voltage-dependent conductance in thin lipid membranes when added to one of the bathing solutions. We show here that the kinetics of conductance changes after a step of membrane potential are only superficially similar to the kinetics of the potassium gating system of squid giant axons, in that the beginning of conductance increases are growth functions of the time, as opposed to power functions of the time. We find that the rate constant (reciprocal of the time constant) of the growth varies with the approximately 2.6 power of the monazomycin concentration. The rate constant also varies exponentially with membrane potential such that an e-fold change is associated with a 10-11 mV change of membrane potential. We show that solutions of a simple differential equation are able to reproduce the actual conductance changes almost exactly. In the accompanying paper (Muller and Peskin. 1981. J. Gen. Physiol. 78:201-229), we derive the differential equation from a molecular model and use the theoretical equation so obtained to investigate the gating current of this system and to predict an interesting form of memory.     

The nature of the voltage-dependent conductance induced by alamethicin in black lipid membranes

Summary Alamethicin induces a conductance in black lipid films which increases exponentially with voltage. At low conductance the increase occurs in discrete steps which form a pattern of five levels, the second and third being most likely. The conductance of each level is directly proportional to salt concentration, inversely proportional to solution viscosity, and nearly independent of voltage.The probability distribution of the five steps is not a function of voltage, but as the voltage is increased, more levels begin to appear. These can be explained as super-positions of the original five, both in position and relative probability.This suggests that the five levels are associated with a physical entity which we call a pore. This point of view is confirmed by the following measurements. The kinetic response of the current to a voltage step is first order, and shows an exponential increase in rate of pore formation and an exponential decrease in rate of pore disappearance with voltage. If these rates are statistical, the number of pores should fluctuate about a voltage-dependent mean. High conductance current fluctuations are too large to be explained by fluctuation in the number of pores alone. But if fluctuations among the five levels are included, the magnitude of the fluctuations at high conductance is accurately predicted.Alamethicin adsorbs reversibly to the membrane surface, and the conductance at a fixed voltage depends on the ninth power of alamethicin concentration and on the fourth power of salt concentration, in the aqueous phase. In our bacterial phosphatidyl ethanolamine membranes, alamethicin added to one side of the membrane produces elevated conductance only when the voltage on that side is increased.On leave of absence from the Facultad de Ciencias, Universidad de Chile, Santiago de Chile.     

Inactivation of potassium current in squid axon by a variety of quaternary ammonium ions

The characteristics of potassium channel block by a diverse group of quaternary ammonium (QA) ions was examined in squid axons. Altering the size and nature of the head and/or tail groups of the QA ions applied internally produced only quantitative differences in the potassium current block. Although their entry rate is diminished, compounds with head groups as large as 11 X 12 A are capable of occluding the channel, whereas the smallest QA ions, with head groups approximately 5 X 6 A, are not potent blockers. When one or three terminal hydrogens of the head group were replaced by hydroxyl moieties, the compound's blocking ability was diminished, suggesting that QA binding is not improved by hydrogen bonding at these positions. QA ions bound to their site within the potassium channel with 1:1 stoichiometry, and the site is perhaps 20% or more of the distance through the membrane electric field. Raising external potassium concentration did not alter the steady-state or kinetic features of the QA block of outward potassium currents; however, increasing temperature or adding Ba2+ internally increased the rate of decay of the QA-blocked currents. From the structure-function analysis of the QA ions, projections concerning both the architecture of the potassium channel's inner mouth and the significance of various chemical constituents of the ions were made. The potassium channel may now be pictured as having a wider mouth (up to 11 X 12 A) extending to the QA binding site and then narrowing quickly to the region of channel selectivity. Important alterations that improve the blocking ability of the compounds include: (a) lengthening the alkyl hydrocarbon tail group (up to 10 carbon), (b) lengthening a second hydrocarbon chain of the head group (e.g., decyldimethylphenylammonium bromide [C10DM phi]), and (c) adding a carbonyl moiety to the tail (e.g., ambutonium).     

Energetics of permeation of thin lipid membranes by ions

The energy of an ion in a thin hydrocarbon membrane relative to its energy in a bulk aqueous phase is considered in terms of the electrostatic and surface components that may be expected to be involved. Except when diffusion activation energies are large compared to partition free energies, the latter will control permeation rate and the state of an ion having the lowest partition energy will be critical for its permeability. This minimum is found when an ion is surrounded with a thin layer of water. All ions of the same charge will tend to be at their lowest state in a sphere of water of the same size. It is concluded, therefore, that all ions of a given charge will have about the same permeability in lipid membranes.     

Non-mediated zero voltage conductance of hydrophobic ions through bilayer lipid membranes.

A charge pulse technique applied to the study of charge transfer at metal-solution interfaces has been used to determine the capacity and the conductance of a membrane bilayer at both zero time and zero voltage. The transport of hydrophobic ions across a glycerol-monooleate bilayer (tetraphenyl borate, picrate, dipicrylamine and tetraphenyl arsonium) has been investigated by this method. A theoretical approach to the problem has been proposed based on one analogous to that used for the compact double layer at metallic electrodes.     

The kinetics of monazomycin-induced voltage-dependent conductance. II. Theory and a demonstration of a form of memory

The empirical differential equation that describes the kinetics of monazomycin-induced voltage-dependent conductance is derived using a standard chemical kinetic formulation and the assumption that monazomycin entry into and exit from the membrane is autocatalytic. The predicted form of gating currents is shown and numerical calculations for this process are made using a range of values for two unmeasured variables. A form of "memory" is then demonstrated, along with the ability of the theoretical equation to explain the nature of the memory.     

Non-mediated zero voltage conductance of hydrophobic ions through bilayer lipid membranes

A charge pulse technique applied to the study of charge transfer at metal-solution interfaces has been used to determine the capacity and the conductance of a membrane bilayer at both zero time and zero voltage. The transport of hydrophobic ion across a glycerol-monooleate bilayer (tetraphenyl borate, picrate, dipicrylamine and tetraphenyl arsonium) has been investigated by this method. A theoretical approach to the problem has been proposed based on one analogous to that used for the compact double layer at metallic electrodes.     

Dipole moment of alamethicin as related to voltage-dependent conductance in lipid bilayers.

The dipole moment of alamethicin, which produces voltage-dependent conductance in lipid-bilayer membranes, was measured in mixed solvents of ethanol and dioxane. The value of the dipole moment was found to increase from 40 to 75 DU (Debye units), as the concentration of ethanol increased from 0 (pure dioxane) to 40%. The relaxation frequency of alamethicin also changes from 10 to 40 MHz, depending upon the concentration of ethanol in mixed solvents. The length of alamethicin was calculated by using the relaxation time and was found to range from approximately 40 to 20 A. The dipole moment was independently calculated from voltage-dependent conductance and compared with the measured value. The calculated value was found to be larger than the value of direct measurements, indicating that several alamethicin molecules are required to form a conducting pore and that their dipole moments are oriented parallel to each other.     

Block of outward current in cardiac purkinje fibers by injection of quaternary ammonium ions

We have studied the effects of iontophoretic injection of the quaternary ammonium compounds tetraethylammonium (TEA) and tetrabutylammonium (TBA) in cardiac purkinje fibers. We find that TBA(+) is a more effective blocker than TEA(+), but injection of either compound reduces the time-dependent outward plateau currents, transient outward current (I(to)), and the delayed rectifier (I(x)). Our findings provide evidence that these outward cardiac currents are carried by channels that in some respects are pharmacologically similar to squid axon potassium channels. We demonstrate that this procedure is a new tool that can be useful in the analysis of membrane currents in the heart.     

Substitution of extracellular sodium ions blocks the voltage-dependent decrease of input conductance evoked by L-aspartate

The response of cultured spinal cord neurones to L-aspartic acid consists of membrane depolarization accompanied by an apparent voltage-dependent reduction in input conductance (Gm). Depolarizations evoked by this amino acid and the decrease in Gm were reduced or eliminated when cultured spinal cord neurones were superperfused with solutions where Na was substituted with choline or Tris. Responses to L-aspartate recovered rapidly upon return to the control (Na-containing solution). The voltage-dependent component of the response to L-aspartate is dependent upon the presence of external Na.