title: Determination of NH4+/NH3 fluxes across apical membrane of macula densa cells: a quantitative analysis.
abstract: Luminal addition of 20 mM NH4+ produced a rapid acidification of rabbit macula densa (MD) cells from 7.50 +/- 0.06 to 6.91 +/- 0.05 at an initial rate of 0.071 +/- 0.008 pH unit/s. In the luminal presence of 5 microM bumetanide, 5 mM Ba2+ or both, the acidification rate was reduced by 57%, 35% and 93% of control levels. In contrast, intracellular pH (pHi) recovery after removing luminal NH4+ was unaffected by bumetanide and Ba2+ but was sensitive to 1 mM luminal amiloride (71% inhibition). The bumetanide-sensitive acidification rate represents most certainly the NH4+ flux mediated by the apical Na+:K+ (NH4+):2Cl- cotransporter, but the Ba(2+)-sensitive portion does not seem to be associated with the apical K+ channels previously characterized by us. The effects of NH4+ entry across the apical membrane were simulated using a simple model involving five adjustable parameters: apical and basolateral permeabilities for NH4+ and NH3 and a parameter describing a pH-regulating mechanism. The model shows that the apical membrane of MD cells is much more permeable to NH3 than it is to NH4+ and, under control conditions, the apical NH4+ flux appears surprisingly high (11-20 mM/s) and challenges the notion that MD cells present a low intensity of ionic transport.
authors.affiliation: Groupe de Recherche en Transport Membranaire, Université de Montréal, Quebec, Canada.
authors.forename: M A
authors.initials: MA
authors.lastname: Laamarti
authors.forename: J Y
authors.initials: JY
authors.lastname: Lapointe
mesh_terms: D000584:Amiloride / Q000494:pharmacology; D000641:Ammonia / Q000378:metabolism*; D000818:Animals; D001464:Barium / Q000494:pharmacology; D002352:Carrier Proteins / Q000378:metabolism; D002462:Cell Membrane / Q000187:drug effects / Q000502:physiology; D005452:Fluoresceins; D005456:Fluorescent Dyes; D006863:Hydrogen-Ion Concentration; D066298:In Vitro Techniques; D007606:Juxtaglomerular Apparatus / Q000166:cytology / Q000502:physiology*; D007686:Kidney Tubules, Distal / Q000166:cytology / Q000502:physiology*; D007700:Kinetics; D015221:Potassium Channels / Q000502:physiology; D000644:Quaternary Ammonium Compounds / Q000378:metabolism*; D011817:Rabbits; D028021:Sodium-Potassium-Chloride Symporters
chemical_list: D002352:Carrier Proteins; D005452:Fluoresceins; D005456:Fluorescent Dyes; D015221:Potassium Channels; D000644:Quaternary Ammonium Compounds; D028021:Sodium-Potassium-Chloride Symporters; D001464:Barium; D000641:Ammonia; D000584:Amiloride; C043829:2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein