DETERMINATION OF NH4+ NH3 FLUXES ACROSS APICAL MEMBRANE OF MACULA DENSA CELLS - A QUANTITATIVE-ANALYSIS/

Luminal addition of 20 mM NH4+ produced a rapid acidification of rabbi t macula densa (MD) cells from 7.50 +/- 0.06 to 6.91 +/- 0.05 at an in itial rate of 0.071 +/- 0.008 pH unit/s. In the luminal presence of 5 mu M bumetanide, 5 mM Ba2+ or both, the acidification rate was reduced by 57%, 35% and 93% of control levels. In contrast, intracellular pH (pH(i)) recovery after removing luminal NH4+ was unaffected by bumetan ide and Ba2+ but was sensitive to 1 mM luminal amiloride (71% inhibiti on). The bumetanide-sensitive acidification rate represents most certa inly the NH4+ flux mediated by the apical Na+:K+ (NH4+):2Cl(-) cotrans porter, but the Ba2+-sensitive portio does not see to be associated wi th 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 pe rmeabilities for NH4+ and NH3 and a parameter describing a pH-regulati ng 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 condi tions, 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.