The passive
proton permeability (
Pnet) of apical membrane vesicles from rabbit parietal cells (gastric) and duodenal and renal cortical brush-border membranes (BBM) was determined by
acridine orange fluorescence quenching. Values of
Pnet were found to be gastric (4 x 10(-4) cm/s) less than duodenal (10(-3) cm/s) much less than renal (10(-2) cm/s). Arrhenius plots of the temperature profile of
proton permeation of gastric vesicles was linear, whereas that of duodenal BBM displayed a discontinuity at 30-33 degrees C.
Alcohols (octyl, benzyl, ethyl) increased
Pnet in a concentration-dependent manner, with efficacy related to their oil-water partition coefficients. In a parallel series of experiments, structural parameters of the vesicle
membrane lipids (fluidity) were monitored from both the steady-state and time-resolved fluorescence anisotropy of
diphenylhexatriene. Fluidity of the membranes was unrelated to
Pnet (renal congruent to duodenal less than gastric). Gastric vesicles demonstrated a linear Arrhenius plot of temperature dependence for fluidity, whereas duodenal BBM demonstrated a discontinuity. Membrane fluidity of gastric and duodenal vesicles was increased by
alcohols, with the same potency as for
Pnet, and these two variables were significantly correlated after perturbation with
alcohols. Thus the fluidity of the
lipid bilayer is not the major determinant of
Pnet, but alteration of its structural parameters, as reflected by fluidity, produces parallel changes in
Pnet.