pH-dependent lipid packing, membrane permeability and fusion in phosphatidylcholine vesicles.

We have studied the rate of membrane fusion, the lipid dynamics and order and the membrane permeability of phosphatidylcholine vesicles as a function of pH. Acidification induced very different effects depending on the state of the bilayer. In liquid-crystalline bilayers, acidification decreased the rate of membrane fusion, the acyl chain motion and disorder and the rate of K+ release, whereas in solid bilayers acidification increased the rate of membrane fusion, the lipid acyl chain disorder and the rate of K+ release.

Activation of latent K+ uniport in mitochondria treated with the ionophore A23187.

Addition of A23187 plus EDTA to energized mitochondria in KCl medium determines a rapid osmotic swelling due to K+ uptake. The swelling is fully reversed by uncoupler, is stimulated by quinine, and is accompanied by membrane depolarization and increased rate of respiration. A23187-treated mitochondria passively swell in K+ thiocyanate at neutral pH, under conditions where the H+-K+ antiporter appears to be silent.