title: Functional roles of apical membrane Na+/H+ exchange in rat medullary thick ascending limb.
abstract: The medullary thick ascending limb (MTAL) of the rat actively absorbs both HCO3- and ammonium. The roles of apical membranes Na+/H+ exchange in these processes and in determining steady-state intracellular pH (pHi) were examined in MTAL perfused in vitro with solutions containing 146 mM Na+ and 25 mM HCO3- (pH 7.4). Addition of 1 mM amiloride or 50 microM ethylisopropylamiloride (EIPA) to the lumen decreased HCO3- absorption (JHCO3) from 10.6 +/- 0.5 to 2.3 +/- 0.3 pmol.min-1.mm-1 (P < 0.001) and pHi from 7.10 +/- 0.02 to 6.86 +/- 0.03 (P < 0.001). The combination of lumen Na+ replacement plus amiloride abolished JHCO3. Chronic metabolic acidosis (CMA) caused a 32% increase in JHCO3 that was inhibited by luminal amiloride. Addition of 4 mM NH4Cl to perfusate and bath markedly decreased pHi (from 7.10 to 6.70) but did not stimulate luminal H+ secretion as assessed by HCO3- absorption. With 4 mM NH4Cl in perfusate and bath, luminal addition of amiloride decreased pHi from 6.70 +/- 0.06 to 6.50 +/- 0.05 (P < 0.005) but had no effect on net ammonium absorption. These results demonstrate that 1) apical membrane Na+/H+ exchange mediates virtually all of HCO3- absorption and is an important determinant of steady-state pHi in the MTAL; 2) the adaptive increase in HCO3- absorption in CMA is mediated by an increase in apical membrane Na+/H+ exchange; 3) ammonium markedly acidifies the cells but does not stimulate luminal acidification, suggesting that pHi is not a predominant influence on apical Na+/H+ exchange activity and that H+ generated in the cells as the result of transcellular ammonium absorption is extruded across the basolateral membrane; and 4) apical membrane Na+/H+ exchange is not important for ammonium absorption.
authors.affiliation: Department of Medicine, University of Texas Medical Branch, Galveston 77555, USA.
authors.forename: D W
authors.initials: DW
authors.lastname: Good
authors.forename: B A
authors.initials: BA
authors.lastname: Watts
mesh_terms: D000042:Absorption / Q000187:drug effects; D000138:Acidosis / Q000378:metabolism; D000584:Amiloride / Q000494:pharmacology; D000818:Animals; D001639:Bicarbonates / Q000378:metabolism; D002462:Cell Membrane / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D007425:Intracellular Membranes / Q000378:metabolism; D007679:Kidney Medulla; D008138:Loop of Henle / Q000187:drug effects / Q000378:metabolism*; D008297:Male; D000644:Quaternary Ammonium Compounds / Q000494:pharmacology; D051381:Rats; D017207:Rats, Sprague-Dawley; D012964:Sodium / Q000494:pharmacology; D017923:Sodium-Hydrogen Exchangers / Q000378:metabolism*
chemical_list: D001639:Bicarbonates; D000644:Quaternary Ammonium Compounds; D017923:Sodium-Hydrogen Exchangers; D000584:Amiloride; D012964:Sodium