Na⁺, K⁺ and Cl⁻ selectivity of the permeability pathways induced through sterol-containing membrane vesicles by amphotericin B and other polyene antibiotics

Abstract

Membrane diffusion potentials induced by amphotericin B (AmB), amphotericin B methyl ester (AmE), N-fructosyl AmB (N FruAmB) and vacidin, an aromatic polyene antibiotic, in ergosterol- or cholesterol-containing egg yolk phosphatidylcholine large unilamellar vesicles (LUV), were measured in various media, in order to determine the relative selectivity of Na⁺, K⁺, Cl⁻ and other ions in these environments. Changes in the membrane potential were followed by fluorescence changes of 3,3′-dipropylthiadicarbocyanine (diS-C₃-(5)). Subtle changes in intercationic selectivity were monitored by measuring biionic potentials, using the fluorescent pH sensitive probe pyranine. In all the cases studied, the intereationic selectivity of the permeability pathways induced by the four antibiotics was weak compared to that of specific biological channels, though distinct differences were noted. With AmB the selectivity appeared to be concentration dependent. Above 5 × 10⁻⁷ M, the sequence determined for sterol-free small unilamellar vesicles (SUV) and cholesterol-containing SUV and LUV, Na⁺ > K⁺ > Rb⁺ ≥ Cs⁺ > Li⁺ (sulfate salts), corresponded closely to Eisenman selectivity sequence number VII. At 5 × 10⁻⁷ M and below the selectivity switched from Na⁺ > K⁺ to K⁺ > Na ⁺. In contrast, Li⁺ was the most permeant ion for AmB channels in the presence of ergosterol. The selectivity between Na⁺ or K⁺ vs. Cl⁻ varied with the antibiotic. It was very strong with vacidin at concentrations below 5 × 10⁻⁷ M, smaller with AmB, nil with AmE and N FruAmB. The selectivities observed were antibiotic, concentration and time de pendent, which confirms the existence of different types of channels.