Functional roles of apical membrane Na+/H+ exchange in rat medullary thick ascending limb

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Abstract

The medullary thick ascending limb (MTAL) of the rat actively absorbs both HCO3 and ammonium. The roles of apical membrane 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 μM 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 < 6.001) 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 7) apical membrane Na+/H+ exchange mediates virtually all of HCO3+ absorption and is an important determinant of steadystate pH; 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.

Original languageEnglish (US)
Pages (from-to)F691-F699
JournalAmerican Journal of Physiology
Volume270
Issue number4 PART 2
StatePublished - Dec 1 1996

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Keywords

  • Amiloride
  • Ammonium transport
  • Bicarbonate absorption
  • Intracellular ph
  • Renal acid secretion

ASJC Scopus subject areas

  • Physiology (medical)

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