Chloride Transport in Porous Lipid Bilayer Membranes

This paper describes dissipative Cl(-) transport in "porous" lipid bilayer membranes, i.e., cholesterol-containing membranes exposed to 1–3 x 10(-7) M amphotericin B. P (DCl) (cm·s(-1)), the diffusional permeability coefficient for Cl(-), estimated from unidirectional (36)Cl(-) 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, P (DCl) was 1.36 x 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 P (DCl) 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 P (DNa)/P (DCl) 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.