Kinetically distinct Na+-H+ exchangers exist on the apical and basolateral membranes of rabbit ileal enterocytes. The apical Na+-H+ exchanger appears to function in electroneutral NaCl transport, whereas the basolateral Na+-H+ exchanger may function in homeostatic intravesicular pH (pH(i)) regulation and volume regulation. This study is designed to determine the presence and characteristics of the Na+-H+ exchanger in basolateral membrane vesicles (BLMV) prepared from jejunal tissues of human organ donors. A well-validated Percoll-gradient technique was used to prepare BLMV. An outwardly directed H+ gradient [pH(i)/extravesicular pH (pH(o)) = 5.2/7.5] resulted in a Na+ uptake overshoot (1.45 ± 0.21 nmol/mg protein) 2.5-fold above equilibrium values (0.59 ± 0.13 nmol/mg protein). Na+ uptake at equilibrium represented transport into an osmotically sensitive intravesicular space as validated by an osmolality study. Na+ uptake represented an electroneutral process, as shown by studies in which negative membrane potentials were induced by K+ and the ionophore valinomycin. Na+ uptake was linear for the first 15 s of transport as depicted by y = 0.042x + 0.002, r2 = 0.98. Dixon plot analysis of amiloride sensitivity revealed an ID50 value for amiloride of 29 μM (fourfold lower than ID50 for brush- border Na+-H+ exchanger). Kinetic studies of amiloride-sensitive Na+ uptake revealed a maximal velocity = 1.53 ± 0.19 nmol · mg protein-1 · 5 s-1 and Michaelis constant = 9.83 ± 3.5 mM. By varying pH(i) a sigmoidal effect of internal H+ on Na+ uptake was noted consistent with an internal modifier site for protons. To confirm this finding, the effect of pH(i) on Na+ efflux and Na+-Na+ exchange was studied. Na+ efflux and Na+-Na+ exchange were greater with pH(i) of 6.5 compared with 7.4. This study suggests for the first time the presence of a Na+-H+ exchanger in human jejunal BLMV that differs in its amiloride sensitivity from the Na+-H+ exchanger found on human jejunal brush-border membranes. This basolateral- membrane Na+-H+ exchanger does possess an internal modifier site.
|Original language||English (US)|
|Journal||American Journal of Physiology - Gastrointestinal and Liver Physiology|
|Issue number||1 27-1|
|State||Published - 1993|
- sodium-hydrogen exchange
ASJC Scopus subject areas
- Physiology (medical)