ISSN:
1432-1424
Keywords:
amiloride
;
brush-border membranes
;
intestine
;
Na+/H+ antiport
;
parietal cell
;
proton conductance
Source:
Springer Online Journal Archives 1860-2000
Topics:
Biology
,
Chemistry and Pharmacology
Notes:
Summary 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 (k H + was determined by addition of vesicles (pH i = 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 × 10−3 (gastric parietal cell stimulation-associated vesicles; SAV) and 5 × 10−3 (duodenal brush-border membrane vesicles; dBBMV) to 11 × 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 k H + 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 (pH i 〈 pH o ) 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 ≫ [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. Amiloride, 0.03–1 mm, resulted in concentration-dependent reductions of K+-diffusion potential-driven acidification in dBBMV and iBBMV but not in gastric SAV. These data demonstrate that proton permeation in the three membrane types is rheogenic. The sensitivity of the proton-conductive pathways in intestinal BBMV to high concentrations of amiloride correlated with the presence of the Na+/H+ antiport and indicates that this transmembrane protein may represent a pathway for proton permeation.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00231910
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