No abstract

Electrical relaxation studies have been made on lecithin bilayer membranes of varying chain length and degree of unsaturation, in the presence of dipicrylamine. Results obtained are generally consistent with a model for the transport of hydrophobic ions previously proposed by Ketterer, Neumcke, and Läuger (J. Membrane Biol. 5:225, 1971). This medel visualizes as three distinct steps the interfacial absorption, translocation, and desorption of ions. Measurements at high electric field yield directly the density of ions absorbed to the membrane-solution interface. Variation of temperature has permitted determination of activation enthalpies for the translocation step which are consistent with the assumption of an electrostatic barrier in the hydrocarbon core of the membrane. The change of enthalpy upon absorption of ions is, however, found to be negligible, the process being driven instead by an increase of entropy. It is suggested that this increase may be due to the destruction, upon absorption, of a highly ordered water structure which surrounds the hydrophic ion in the aqueous phase. Finally, it is shown that a decrease of transient membrane conductance observed at high concentration of hydrophobic ions, previously interpreted in terms of interfacial saturation, must instead by attributed to a more complex effect equivalent to a reduction of membrane fluidity.

We report here the first observations of the effects of elevated hydrostatic pressure on the kinetics of bilayer membrane conductance induced by the pore-forming antibiotic, alamethicin. Bacterial phosphatidylethanolamine-squalene bilayer membranes were formed by the apposition of lipid monolayers in a vessel capable of sustaining hydrostatic pressures in the range, 0.1-100 MPa (1-1,000 atm). Principal observations were (a) the lifetimes of discrete conductance states were lengthened with increasing pressure, (b) both the onset and decay of alamethicin conductance accompanying application and removal of supra-threshold voltage pulses were slowed with increasing pressure, (c) the onset of alamethicin conductance at elevated pressure became distinctly sigmoidal, suggesting an electrically silent intermediate state of channel assembly, (d) the magnitudes of the discrete conductance levels observed did not change with pressure, and, (e) the voltage threshold for the onset of alamethicin conductance was not altered by pressure. Apparent activation volumes for both the formation and decay of conducting states were positive and of comparable magnitude, namely, approximately 100 A3/event. Observation d indicates that channel geometry and the kinetics of ion transport through open channels were not affected by pressure in the range employed. The remaining observations indicate that, while the relative positions of free-energy minima characterizing individual conducting states at a given voltage were not modified by pressure, the heights of intervening potential maxima were increased by its application.

No abstract

Thin lipid (optically black) membranes were made from sheep red cell lipids dissolved in n-decane. The flux of Br across these membranes was measured by the use of tracer 82Br. The unidirectional flux of Br (in 50-100 m NaBr) was 1-3 X 10 -2 mole/cm 2 sec. This flux is more than 1000 times the flux predicted from the membrane electrical resistance (> 108 ohmcm 2 ) and the transference number for Br-(0.2-0.3), which was estimated from measurements of the zero current potential difference. The Br flux was not affected by changes in the potential difference imposed across the membrane (4-60 mv) or by the ionic strength of the bathing solutions. However, the addition of a reducing agent, sodium thiosulfate (10 -3 M), to the NaBr solution bathing the membrane caused a 90 % reduction in the Br flux. The inhibiting effect of S203 suggests that the Br flux is due chiefly to traces of Br 2 in NaBr solutions. As expected, the addition of Br 2 to the NaBr solutions greatly stimulated the Br flux. However, at constant Br 2 concentration, the Br flux was also stimulated by increasing the Br-concentration, in spite of the fact that the membrane was virtually impermeable to Br-. Finally, the Br flux appeared to saturate at high Br 2 concentrations, and the saturation value was roughly proportional to the Br-concentration. These results can be explained by a model which assumes that Br crosses the membrane only as Br 2 but that rapid equilibration of Br between Br 2 and Br-occurs in the unstirred layers of aqueous solution bathing the two sides of the membrane. A consequence of the model is that Br-"facilitates" the diffusion of Br across the unstirred layers.