title: Amiloride sensitivity of proton-conductive pathways in gastric and intestinal apical membrane vesicles.
abstract: Passive proton permeability of gastrointestinal apical membrane vesicles was determined. The nature of the pathways for proton permeation was investigated using amiloride. The rate of proton permeation (kH+) was determined by addition of vesicles (pHi = 6.5) to a pH 8.0 solution containing acridine orange. The rate of recovery of acridine orange fluorescence after quenching by the acidic vesicles ranged from 4 x 10(-3) (gastric parietal cell stimulation-associated vesicles; SAV) and 5 x 10(-3) (duodenal brush-border membrane vesicles; dBBMV) to 11 x 10(-3) sec-1 (ileal BBMV; iBBMV). Amiloride, 0.03 and 0.1 mM, significantly reduced the rate of proton permeation in dBBMV and iBBMV, but not gastric SAV. The decreases in kH+ were proportionately greater in iBBMV as compared with dBBMV. The presence of Na+/H+ exchange was demonstrated in both dBBMV and iBBMV by proton-driven (pHi less than pHo) 22Na+ uptake. Evidence was also sought for the conductive nature of pathways for proton permeation. Intravesicular acidification, again determined by quenching of acridine orange fluorescence, was observed during imposition of K(+)-diffusion potential ([K+]i much much greater than [K+]o). In dBBMV and iBBMV, intravesicular acidification was enhanced in the presence of the K(+)-ionophore valinomycin, indicating that the native K+ permeability is rate limiting. In the presence of valinomycin, the K(+)-diffusion potential drove BBMV intravesicular acidification to levels close to the electrochemical potential. In gastric SAV, acidification was not limited by the K+ permeability. Valinomycin was without effect, but the K+/H+ ionophore nigericin enhanced acidification in gastric SAV, illustrating the low proton permeability of these membranes.(ABSTRACT TRUNCATED AT 250 WORDS)
authors.affiliation: Department of Physiological Sciences, University of Newcastle upon Tyne, Medical School, England.
authors.forename: J M
authors.initials: JM
authors.lastname: Wilkes
authors.forename: B H
authors.initials: BH
authors.lastname: Hirst
mesh_terms: D000584:Amiloride / Q000494:pharmacology*; D000818:Animals; D002462:Cell Membrane / Q000187:drug effects / Q000378:metabolism*; D002463:Cell Membrane Permeability / Q000187:drug effects; D004058:Diffusion; D004553:Electric Conductivity; D005753:Gastric Mucosa / Q000187:drug effects / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D007413:Intestinal Mucosa / Q000187:drug effects / Q000378:metabolism*; D007700:Kinetics; D008564:Membrane Potentials; D011522:Protons; D011817:Rabbits; D012964:Sodium / Q000378:metabolism
chemical_list: D011522:Protons; D000584:Amiloride; D012964:Sodium