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Am J Physiol Cell Physiol 274: C883-C891, 1998;
0363-6143/98 $5.00
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Vol. 274, Issue 4, C883-C891, April 1998

Intracellular acidification induced by passive and active transport of ammonium ions in astrocytes

Tavarekere N. Nagaraja and Neville Brookes

Department of Pharmacology and Experimental Therapeutics, University of Maryland School of Medicine, Baltimore, Maryland 21202

We describe an unconventional response of intracellular pH to NH4Cl in mouse cerebral astrocytes. Rapid alkalinization reversed abruptly to be replaced by an intense sustained acidification in the continued presence of NH4Cl. We hypothesize that high-velocity NH+4 influx persisted after the distribution of ammonia attained steady state. From the initial rate of acidification elicited by 1 mM NH4Cl in bicarbonate-buffered solution, we estimate that NH+4 entered at a velocity of at least 31.5 nmol · min-1 · mg protein-1. This rate increased with NH4Cl concentration, not saturating at up to 20 mM NH4Cl. Acidification was attenuated by raising or lowering extracellular K+ concentration. Ba2+ (50 µM) inhibited the acidification rate by 80.6%, suggesting inwardly rectifying K+ channels as the primary NH+4 entry pathway. Acidification was 10-fold slower in rat hippocampal astrocytes, consistent with the difference reported for K+ flux in vitro. The combination of Ba2+ and bumetanide prevented net acidification by 1 mM NH4Cl, identifying the Na+-K+-2Cl- cotransporter as a second NH+4 entry route. NH+4 entry via K+ transport pathways could impact "buffering" of ammonia by astrocytes and could initiate the elevation of extracellular K+ concentration and astrocyte swelling observed in acute hyperammonemia.

potassium channels; sodium-potassium-chloride cotransport; hyperammonemia; intracellular pH; mouse; rat


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