Sodium-proton exchange in human ileal brush-border membrane vesicles.
This study examines the characteristics of Na+ and H+ transport as well as Na+-H+ exchange in human ileal brush-border membrane vesicles from organ donor intestine. 22Na+ uptake into vesicles and the fluorescence quenching of Acridine orange were employed to measure Na+ and H+ transport, respectively. Concentrative uptake of 22Na+ (4-fold overshoot above equilibrium) was observed under conditions of an outward proton gradient (pHi 5.5; pHo 7.5). Voltage-clamping (Ki+ = Ko+ + valinomycin) reduced the uptake of 22Na+ by 40-50% indicating the presence of Na+ conductance. Dissipation of the Acridine orange fluorescence quench in ileal vesicles with a preformed pH gradient (pHi 5.5; pHo 7.5) was accelerated by either external Na+ or voltage-clamping in the absence of Na+. The effects of Na+ and voltage-clamping were additive under the above conditions. In the absence of a pH gradient, Acridine orange quenching was induced by intravesicular Na+ as well as an interior negative K+ diffusion potential. In voltage-clamped BBMV, pH-driven Na+ uptake was inhibited by amiloride (Ki = 140 microM). The initial rate of pH-driven Na+ uptake was saturable and conformed to Michaelis-Menten kinetics with apparent Km and Vmax values of 27 +/- 1 mM and 47 +/- 1 nmol.(mg protein)-1.(3 s)-1, respectively. Li+ and NH4+, but not Cs+, K+, Rb+ or choline+ inhibited pH gradient-driven 22Na+ uptake. The results demonstrate in human ileal brush-border membrane vesicles the presence of an Na+/H+ exchanger and conductive transport pathways for Na+ and H+.
Ramaswamy
K
K
Gastroenterology Section, Zablocki Veterans Administration Medical Center, Milwaukee, WI 53295.
Harig
J M
JM
Kleinman
J G
JG
Harris
M S
MS
Barry
J A
JA
D000584:Amiloride / Q000494:pharmacology; D001692:Biological Transport / Q000187:drug effects; D002352:Carrier Proteins / Q000378:metabolism*; D002414:Cations, Monovalent / Q000494:pharmacology; D006801:Humans; D006863:Hydrogen-Ion Concentration; D007082:Ileum / Q000378:metabolism*; D066298:In Vitro Techniques; D007700:Kinetics; D008564:Membrane Potentials; D008871:Microvilli / Q000378:metabolism; D011188:Potassium / Q000378:metabolism; D012964:Sodium / Q000378:metabolism*; D017923:Sodium-Hydrogen Exchangers
D002352:Carrier Proteins; D002414:Cations, Monovalent; D017923:Sodium-Hydrogen Exchangers; D000584:Amiloride; D012964:Sodium; D011188:Potassium