Effects of ammonium on intracellular pH in rat medullary thick ascending limb: mechanisms of apical membrane NH4+ transport.
The renal medullary thick ascending limb (MTAL) actively reabsorbs ammonium ions. To examine the effects of NH4+ transport on intracellular pH (pHi) and the mechanisms of apical membrane NH4+ transport, MTALs from rats were isolated and perfused in vitro with 25 mM HCO3(-)-buffered solutions (pH 7.4). pHi was monitored using the fluorescent dye BCECF. In the absence of NH4+, the mean pHi was 7.16. Luminal addition of 20 mM NH4+ caused a rapid intracellular acidification (dpHi/dt = 11.1 U/min) and reduced the steady state pHi to 6.67 (delta pHi = 0.5 U), indicating that apical NH4+ entry was more rapid than entry of NH3. Luminal furosemide (10(-4) M) reduced the initial rate of cell acidification by 70% and the fall in steady state pHi by 35%. The residual acidification observed with furosemide was inhibited by luminal barium (12 mM), indicating that apical NH4+ entry occurred via both furosemide (Na(+)-NH4(+)-2Cl- cotransport) and barium-sensitive pathways. The role of these pathways in NH4+ absorption was assessed under symmetric ammonium conditions. With 4 mM NH4+ in perfusate and bath, mean steady state pHi was 6.61 and net ammonium absorption was 12 pmol/min/mm. Addition of furosemide to the lumen abolished net ammonium absorption and caused pHi to increase abruptly (dpHi/dt = 0.8 U/min) to 7.0. Increasing luminal [K+] from 4 to 25 mM caused a similar, rapid cell alkalinization. The pronounced cell alkalinization observed with furosemide or increasing [K+] was not observed in the absence of NH4+. In symmetric 4 mM NH4+ solutions, addition of barium to the lumen caused a slow intracellular alkalinization and reduced net ammonium absorption only by 14%.


CONCLUSIONS
(a) ammonium transport is a critical determinant of pHi in the MTAL, with NH4+ absorption markedly acidifying the cells and maneuvers that inhibit apical NH4+ uptake (furosemide or elevation of luminal [K+]) causing intracellular alkalinization; (b) most or all of transcellular ammonium absorption is mediated by apical membrane Na(+)-NH4(+)-2Cl- cotransport; (c) NH4+ also permeates a barium-sensitive apical membrane transport pathway (presumably apical membrane K+ channels) but this pathway does not contribute significantly to ammonium absorption under physiologic (symmetric ammonium) conditions.
Department of Internal Medicine, University of Texas Medical Branch, Galveston 77555-0562.
B A
BA
Watts
D W
DW
Good
D000818:Animals; D001464:Barium / Q000494:pharmacology; D002462:Cell Membrane / Q000378:metabolism; D002712:Chlorides / Q000378:metabolism; D005665:Furosemide / Q000494:pharmacology; D006863:Hydrogen-Ion Concentration; D017136:Ion Transport; D007679:Kidney Medulla; D007684:Kidney Tubules / Q000166:cytology / Q000378:metabolism*; D008297:Male; D011188:Potassium / Q000378:metabolism; D015221:Potassium Channels / Q000378:metabolism; D000644:Quaternary Ammonium Compounds / Q000378:metabolism* / Q000494:pharmacology; D051381:Rats; D017207:Rats, Sprague-Dawley
D002712:Chlorides; D015221:Potassium Channels; D000644:Quaternary Ammonium Compounds; D001464:Barium; D005665:Furosemide; D011188:Potassium