Chloride transport in porous lipid bilayer membranes

Abstract

This paper describes dissipative Cl− transport in "porous" lipid bilayer membranes, i.e., cholesterol-containing membranes exposed to 1-3 × 10-7 M amphotericin B. PDCl (cm. s-1), the diffusional permeability coefficient for C-, estimated from unidirectional 86Cl- fluxes at zero volume flow, varied linearly with the membrane conductance (Gm, Ω-1 ·cm-2) when the contributions of unstirred layers to the resistance to tracer diffusion were relatively small with respect to the membranes; in 0.05 M NaCl, PDCI was 1.36 × 10-4 cm·s-1 when Gm was 0.02 Ω-1 · cm-2. Net chloride fluxes were measured either in the presence of imposed concentration gradients or electrical potential differences. Under both sets of conditions: the values of PDCI computed from zero volume flow experiments described net chloride fluxes; the net chloride fluxes accounted for - 90-95 % of the membrane current density; and, the chloride flux ratio conformed to the Ussing independence relationship. Thus, it is likely that Cl− traversed aqueous pores in these anion-permselective membranes via a simple diffusion process. The zero current membrane potentials measured when the aqueous phases contained asymmetrical NaCl solutions could be expressed in terms of the Goldman-Hodgkin-Katz constant field equation, assuming that the PDNa/PDCI ratio was 0.05. In symmetrical salt solutions, the current-voltage properties of these membranes were linear; in asymmetrical NaCl solutions, the membranes exhibited electrical rectification consistent with constant-field theory. It seems likely that the space charge density in these porous membranes is sufficiently low that the potential gradient within the membranes is approximately linear; and, that the pores are not electrically neutral, presumably because the Debye length within the membrane phase approximates the membrane thickness. © 1973, Rockefeller University Press., All rights reserved.