The dielectric constant of phospholipid bilayers and the permeability of membranes to ions.

The Born charging equation predicts that the permeability of a phospholipid bilayer membrane to ions should depend markedly on the dielectric constant of the membrane. Increasing the dielectric constant of an artificial bilayer increases its permeability to perchlorate or thiocyanate by a factor of 1000, to a value comparable to that of mitochondrial membranes.

Divalent ions and the surface potential of charged phospholipid membranes.

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 conductance-voltage curves of a nerve along the voltage axis by 20-26 mv for a 10-fold increase in concentration may be due to essentially a screening rather than a binding phenomenon.

The activities and concentrations of sodium and potassium in toad oocytes.

1. The activity of potassium, a(K), in the cytoplasm of oocytes from the toad, Bufo bufo, as measured by potassium-sensitive glass micro-electrodes, was 82 mM. The concentration of potassium, C(K), in oocytes from the same ovaries, as determined by flame photometric analysis, was 113 mM. The ratio a(K)/C(K) = 0.73 does not differ significantly from the measured activity coefficient of the normal Ringer bathing solution, which is 0.75.2. The activity of sodium, a(Na), in the cytoplasm of toad oocytes, as measured by sodium-sensitive glass micro-electrodes, was 9.3 mM. The concentration of sodium, C(Na), in oocytes from the same ovaries, as determined by flame photometric analysis, was 25.8 mM. The value of the a(Na)/C(Na) ratio in the cells, 0.36, is only about half the value of either the a(K)/C(K) ratio in the cells or the activity coefficient of sodium in the normal Ringer bathing solution. This implies that about half the sodium in the cell is sequestered in some manner, such that it is unavailable to affect a cation-sensitive micro-electrode.3. When the oocytes were bathed for 5 hr in a sodium-free, lithium-substituted, Ringer solution the a(Na)/C(Na) ratio decreased to 0.06-0.10. This drop in the a(Na)/C(Na) ratio implies that the sodium available to the cation-sensitive micro-electrode can leave the cell much faster than the sequestered sodium.

The effects of a cyclic polyether on the electrical properties of phospholipid bilayer membranes.

The cyclic polyether XXXII, a neutral, lipid soluble molecule, produces large increases in the conductance of bilayer membranes formed from a variety of lipids. The conductance increases linearly with the concentration of alkali metal cation but with the square, and at higher concentrations the cube, of the polyether concentration. This implies that two or three polyether molecules combine with a single cation to carry it across the membrane. In the presence of XXXII the bilayer is permeable solely to cations and the membrane potential is described by an equation of the Goldman-Hodgkin-Katz type. The permeability ratios determined from potential measurements are independent of salt concentration, decrease in the sequence Cs>Rb>K>NH4>Na>Li(1.0,0.25, 0.15, 0.075, 0.007, 0.0013) and are equal to the conductance ratios at low (e.g. 10(-3) M) salt concentration. At higher salt concentrations, the permeability and conductance ratios are not equal and maxima in the conductancevs. salt concentration curves are observed. Both these phenomena are postulated to be caused by the formation of relatively impermeant 1ν1 polyether cation complexes in the aqueous phase. The 1ν1 aqueous association constants deduced from bilayer measurements decrease in the sequence K>Rb>Na>NH4>Cs>Li (120, 34, 26, 19, 12, 4 liters per mole) and agree quantitatively with the literature values for the more water soluble polyether XXXI, which lacks only thet-butyl groups of XXXII.

Surface charge and the conductance of phospholipid membranes.

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.

IONIC PROBES OF MEMBRANE STRUCTURES.

In this paper we have examined the possibility of identifying those membrane structural variables (polar head groups and the nature of hydrocarbon tails) that modulate membrane ionic permeability. Altering the bilayer lipid composition produces variations in physical parameters (surface potential, partition coefficient, and mobility) governing the conductance mediated by neutral carriers of anions and cations. Specifically, the effects of the charged polar head groups are shown to be understandable in terms of the surface potential they produce through the formation of a diffuse double layer, whereas the effects of the viscosity may be demonstrated by "freezing" the membrane. The effects of membrane composition on membrane conductance are illustrated by a third, less well understood, example of how cholesterol alters bilayer conductances. The results indicate the possibility of using positive and negative permeant species as probes of membrane structures.