Rj. Mahajan et al., CHLORIDE TRANSPORT IN HUMAN PROXIMAL COLONIC APICAL MEMBRANE-VESICLES, Biochimica et biophysica acta. Biomembranes, 1280(1), 1996, pp. 12-18
The mechanism(s) of Cl- transport across the human colonic apical memb
ranes are not well understood. Apical membrane vesicles (AMV) purified
from organ donor proximal colonic mucosa and a rapid millipore filtra
tion technique were utilized to study Cl-36(-) uptake into these vesic
les. Outwardly directed OH- and HCO3- gradient stimulated Cl- uptake i
nto these vesicles demonstrating a transient accumulation over equilib
rium uptake. Voltage clamping the membrane potential of the vesicles o
r making them inside positive with K+/valinomycin failed to influence
chloride uptake, indicating that the conductive Cl- uptake pathway is
minimal in proximal colonic AMV. Anion exchange inhibitors, DIDS and S
ITS (1 mM) inhibited OH- and HCO3- stimulated Cl-36(-) uptake by appro
ximate to 60%. Furosemide also demonstrated a small but significant in
hibition of chloride uptake. Amiloride, bumetanide and acetazolamide (
1 mM) failed to inhibit Cl-36 uptake. HCO3- and pH gradient stimulated
36 Cl- uptake exhibited saturation kinetics with an apparent K-m for
chloride of 4.0 +/- 0.7 mM and a V-max of 17.8 +/- 3.9 nmol/mg per min
. Bromide, chloride, nitrate and acetate (50 mM each) inhibited 5 mM C
l-36 uptake. Inwardly directed gradients of Na+, K+, or Na+ and K+ did
not stimulate Cl-36(-) uptake into these vesicles, indicating that up
take of Na+ and Cl- in human proximal colonic AMV does not involve Na-
Cl or Na-K-2Cl cotransport. The above findings indicate that chloride
transport in human proximal colonic AMV involves an electroneutral Cl-
-HCO3- (OH-) exchange process, In view of the previous demonstration o
f a Na+-H+ antiporter in these vesicles, dual ion exchange mechanism o
f Na+-H+ and Cl--HCO3- in apical membrane domain of human colonocytes
is postulated to be the primary mechanism for NaCl absorption in the h
uman proximal colon.