title: Ion pathways in renal brush border membranes.
abstract: The absorbance change of the weak base dye probe, Acridine orange, was used to monitor alterations of pH gradients across renal brush border membrane vesicles. The presence of Na+/H+ or Li+/H+ exchange was demonstrated by diluting Na2SO4 or Li2SO4 loaded vesicles into Na+-or Li+-free solutions, which caused dye uptake. About 20% of the uptake was abolished by lipid permeable cations such as valinomycin-K+ or tetraphenylphosphonium, indicating perhaps the presence of a finite Na+ conductance smaller than electroneutral Na+/H+ exchange. The protonophore tetrachlorosalicylanilide raised the rate of dye uptake under these conditions, hence the presence of an Na+ conductance greater than the H+ conductance was suggested. K+ gradients also induced changes of pH, at about 10% of the Na+ or Li+ rate. Partial inhibition (21%) was seen with 0.1 mM amiloride indicating that K+ was a low affinity substrate for the Na+/H+ exchange. Acceleration both by tetrachlorosalicylanilide (2-fold) and valinomycin (4-fold) suggested the presence of 2 classes of vesicles, those with high and those with low K+ conductance. The larger magnitude of the valinomycin dependent signal suggested that 75% of the vesicles has a low K+ conductance. Inward Cl- gradients also induced acidification, partially inhibited by the presence of tetraphenylphosphonium, and accelerated by tetrachlorosalicylanilide. Thus both a Cl- conductance greater than the H+ conductance and a Cl-/OH- exchange were present. The rate of Na+/H+ exchange was amiloride sensitive with a pH optimum of 6.5 and an apparent Km for Na+ or Li+ of about 10 mM and an EA of 14.3 kcal per mol. A 61-fold Na2SO4 gradient resulted in a pH gradient of 1.64 units which increased to 1.8 with gramicidin. An equivalent NaCl gradient gave a much lower delta pH even in the presence of gramicidin showing that the H+ and Cl- pathways could alter the effect of the Na+/H+ exchange.
mesh_terms: D000165:Acridine Orange; D000818:Animals; D001693:Biological Transport, Active; D002462:Cell Membrane / Q000378:metabolism*; D002712:Chlorides / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D007477:Ions; D007672:Kidney Cortex / Q000378:metabolism*; D007700:Kinetics; D008871:Microvilli / Q000378:metabolism*; D008954:Models, Biological; D011817:Rabbits; D012964:Sodium / Q000378:metabolism
chemical_list: D002712:Chlorides; D007477:Ions; D012964:Sodium; D000165:Acridine Orange