Faster-than-anticipated Na(+)/Cl(-) diffusion across lipid bilayers in vesicles

Area of Science:

• Biochemistry
• Biophysics
• Membrane Biology

Background:

• Electrochemical potential gradients across lipid membranes are vital for cellular functions like signal transduction and energy generation.
• Measuring the specific contribution of individual ion diffusion to membrane potential is challenging due to the presence of multiple ion types with varying permeabilities.

Purpose of the Study:

• To elucidate the electrodiffusion dynamics of protons (H+) across lipid bilayers.
• To quantify the influence of proton movement on membrane potential and permeability.

Main Methods:

• Utilized a pH-sensitive fluorophore to monitor lumenal pH changes in vesicles following stepwise alterations in external pH.
• Compared pH dynamics in vesicles containing the gramicidin ion channel versus intact vesicles.

Main Results:

• Vesicles with gramicidin exhibited rapid lumenal pH equilibration to the external pH.
• Intact vesicles displayed a biphasic pH response: an initial rapid change (~1 min) followed by a prolonged slow change (~24 h).
• The Goldman-Hodgkin-Katz model provided a quantitative interpretation, estimating Na+ and Cl- permeability at ~10^-8 cm/s, significantly higher than previously reported.

Conclusions:

• The study provides a novel method for assessing ion permeability across lipid bilayers.
• A significantly higher membrane permeability to Na+ and Cl- was inferred, prompting further investigation into underlying mechanisms.
• Electrodiffusion of H+ plays a critical role in membrane potential dynamics, with intact vesicles exhibiting complex, slow regulatory processes.