Chloride transport in human proximal colonic apical membrane vesicles.
The mechanism(s) of Cl- transport across the human colonic apical membranes are not well understood. Apical membrane vesicles (AMV) purified from organ donor proximal colonic mucosa and a rapid millipore filtration technique were utilized to study 36Cl- uptake into these vesicles. Outwardly directed OH- and HCO3- gradient stimulated 36Cl- uptake into these vesicles demonstrating a transient accumulation over equilibrium uptake. Voltage clamping the membrane potential of the vesicles or 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 SITS (1 mM) inhibited OH- and HCO3- stimulated 36Cl- uptake by approximately 60%. Furosemide also demonstrated a small but significant inhibition of chloride uptake. Amiloride, bumetanide and acetazolamide (1 mM) failed to inhibit 36Cl uptake. HCO3- and pH gradient stimulated 36Cl- uptake exhibited saturation kinetics with an apparent Km for chloride of 4.0 +/- 0.7 mM and Vmax of 17.8 +/- 3.9 nmol/mg per min. Bromide, chloride, nitrate and acetate (50 mM each) inhibited 5 mM 36Cl uptake. Inwardly directed gradients of Na+, K+, or Na+ and K+ did not stimulate 36Cl- uptake into these vesicles, indicating that uptake 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 of Na+-H+ antiporter in these vesicles, dual ion exchange mechanism of Na+-H+ and Cl-HCO3- in apical membrane domain of human colonocytes is postulated to be the primary mechanism for NaCl absorption in the human proximal colon.
Mahajan
R J
RJ
University of Illinois-Michael Reese Hospital, Chicago, IL, USA.
Baldwin
M L
ML
Harig
J M
JM
Ramaswamy
K
K
Dudeja
P K
PK
D000328:Adult; D001639:Bicarbonates / Q000378:metabolism; D001692:Biological Transport / Q000187:drug effects; D002462:Cell Membrane / Q000187:drug effects / Q000378:metabolism*; D002712:Chlorides / Q000378:metabolism*; D002713:Chlorine; D006801:Humans; D007413:Intestinal Mucosa / Q000378:metabolism*; D007700:Kinetics; D011188:Potassium / Q000494:pharmacology; D011868:Radioisotopes; D014019:Tissue Donors; D014634:Valinomycin / Q000494:pharmacology
D001639:Bicarbonates; D002712:Chlorides; D011868:Radioisotopes; D014634:Valinomycin; D002713:Chlorine; D011188:Potassium