No abstract

The terminology has been modified slightly from the original in that the subscript n has been added to the constant of relative sensitivity, Kn; and the subscripts A and B have been omitted (although they will be written explicitly whenever needed for clarity). Thus, the present K. (or KnAB) corresponds to the original KAB; and n (or nAB) corresponds to the original nAB. 2 One must, of course, calculate any association of such ions as Ag+ and NH4+ which occurs at high pH. GEORGE EISENMAN Cation Selective Glass Electrodes 261

Phospholipid bilayer membranes were bathed in a decimolar solution of monovalent ions, and the conductance produced by neutral carriers of these monovalent cations and anions was used to assess the electric potential at the surface of the membrane. When the bilayers were formed from a neutral lipid, phosphatidylethanolamine, the addition of alkaline earth cations produced no detectable surface potential, indicating that little or no binding occurs to the polar head group with these ions. When the bilayers were formed from a negatively charged lipid, phosphatidylserine, the addition of Sr and Ba decreased the magnitude of the surface potential as predicted by the theory of the diffuse double layer. In particular, the potential decreased 27 mv for a 10-fold increase in concentration in the millimolar-decimolar range. A 10-fold increase in the Ca or Mg concentration also produced a 27 mv decrease in potential in this region, which was again due to screening, but it was necessary to invoke some specific binding to account for the observation that these cations were effective at a lower concentration than Ba or Sr. It is suggested that the ability of the alkaline earth cations to shift the conductancevoltage curves of a nerve along the voltage axis by 20-26 my for a 10-fold increase in concentration may be due to essentially a screening rather than a binding phenomenon.

The environments of metal ions (Li', Na (Fig. 1). The value of Aou for a group (14, 15) characterizes its affinity for water (Ao-< 0) or lack ofit (Ao > 0). The distributions of Fig. 1 show that the atomic groups in a sphere immediately around Zn2+ and Ca2l ions in several proteins are hydrophilic (Aor < 0) and in the spherical region farther from the ion are hydrophobic (Ao > 0). At still greater values of r, (Ao) approaches 0.007 kcal mol1.A-2, the typical average value for a protein.

Open-channel ion permeation properties were investigated for Ca++-activated K+ (CaK) channels in solutions of K+ and its analogues T1+, Rb+, and NH4+. Single CaK channels were inserted into planar lipid bilayers composed of neutral phospholipids, and open-channel current-voltage (I-V) relations were measured in symmetrical and asymmetrical solutions of each of these individual ions. For all concentrations studied, the zero-voltage conductance falls in the sequence K+ greater than T1+ greater than NH4+ greater than Rb+. The shape of the I-V curve in symmetrical solutions of a single permeant ion is non-ohmic and is species-dependent. The I-V shape is sublinear for K+ and T1+ and superlinear for Rb+ and NH4+. As judged by reversal potentials under bi-ionic conditions with K+ on one side of the bilayer and the test cation on the other, the permeability sequence is T1+ greater than K+ greater than Rb+ greater than NH4+ at 300 mM, which differs from the conductance sequence. Symmetrical mixtures of K+ or NH4+ with Rb+ show a striking anomalous mole fraction behavior, i.e., a minimum in single-channel conductance when the composition of a two-ion mixture is varied at constant total ion concentration. This result is incompatible with present models that consider the CaK channel a single-ion pore. In total, the results show that the CaK channel finely discriminates among K+-like ions, exhibiting different energy profiles among these species, and that several such ions can reside simultaneously within the conduction pathway.

An abrupt loss of effectiveness of the presumed carriers, nonactin and valinomycin, in mediating ion conductance occurred at the same temperature as the membrane fluidity, judged visually, was lost. By contrast, the effects of the presumed channel-former, gramicidin, were the same on solid and liquid membranes. Taken together, these findings imply that freezing the membrane primarily reduces the mobility of these antibiotics with little effect on their solubility.

Abstract. Bilayer membranes, formed from various phospholipids, were studied to assess the influence of the charge of the polar head groups on the membrane conductance mediated by neutral "carriers" of cations and anions. The surface charge of an amphoteric lipid, phosphatidyl ethanolamine, was altered by varying the pH, and the surface charge of several lipids was screened by increasing the ionic strength of the solution with impermeant monovalent and divalent electrolytes. The-surface charge should be a key parameter in defining the membrane conductance for a variety of permeation mechanisms; conductance measurements in the presence of carriers may be used to estimate the potential difference, due to surface charge, between the interior of the bilayer and the bulk aqueous phase. The large changes in conductance observed upon varying the surface charge density and the ionic strength agree with those predicted by the Gouy-Chapman theory for an aqueous diffuse double layer. Explicit expressions for the dependence of the membrane conductance on the concentrations of the carrier, the permeant ion, the surface charge density, and the ionic strength are presented.With the development of the artificial phospholipid bilayer membrane,1 2 the relationship between phospholipid composition and permeability properties of the membrane has become accessible to experimental study. The significance of the fluidity of the membrane for solute permeation has been shown.3 Moreover, surface potential measurements on phospholipid monolayers and studies of the electrophoretic mobility of phospholipid dispersions have demonstrated that the presence of a charged polar head group produces a substantial potential at the lipid-solution interface. Such a potential should influence the concentration of ions at the interface, and hence, the permeability properties of the membrane. Studies on phospholipid vesicles4 5 and bilayers6 7 have indeed shown that the anion to cation permselectivity is influenced by the charge of the membrane. Furthermore, the conductance of a bilayer due to the iodide ion or certain weak acids is affected by the charge on the polar head group of the phospholipid.8 9 The present study examines theoretically and experimentally how certain molecules which enhance the membrane conductance, probably by acting as carriers10-12 of particular cations and anions, may be used as "probes" to distinguish between effects of charge and of other variables, such as the fluidity of the bilayer interior and the lipid solubility of the complex.

Single channel patch-clamp techniques were used to study nicotinic acetylcholine receptors in cultured rat myotubes . The single channel conductance in pure cesium and sodium levels off at high concentrations, as if a binding site within the channel were saturating . The conductances at very low concentrations, however, are larger than predicted by the simplest one-site transport model fitted to the high-concentration data . At low concentrations, the current-voltage relations are inwardly rectifying, but they become more ohmic if a small amount of divalent cations is added externally . Magnesium and barium are good permeants that have rather high affinities for the channel. Upon adding low millimolar concentrations of these divalent cations externally to a membrane bathed in pure cesium, the inward current carried by cesium is decreased. As more divalent cations are added, the inward-going currents continued to decrease and the divalent cation replaces cesium as the main current carrier. The ion transport data are described by considering the size, shape, and possible net charge of the channel. In that way, even the complex features of transport are explained in a realistic physical framework. The results are consistent with the channel having long, wide, multiply occupied vestibules that serve as transition zones to the short, selective, singly occupied narrow region of the channel. A small amount of net negative charge within the pore could produce concentration-dependent potentials that provide a simple explanation for the more complicated aspects of the permeation properties .