Characterisation of Na+ × H+ exchange in isolated bovine articular chondrocytes

We have previously shown that intracellular pH (pH,) in the articular chondrocyte is regulated by a single mechanism, Na+ Hexchange. HCO3-dependent mechanisms play no role, possibly because of the low numbers of anions in the extracellular matrix, and the uptake of SO,2' and Zn2 by modified anion exchangers. Since pH, can modify matrix metabolism, regulation of Nax Hexchange will have important effects on cartilage integrity. We have therefore performed further experiments to characterise the properties of the chondrocyte Nax Hexchange. Recovery from ammonium-induced intracellular acidosis is 'smart', showing allosteric modification with increasing levels of intracellular acidity, sensitive to amiloride (K0.5 = SμM) and is stimulated by cell shrinkage, exposure to serum, and by application of hydrostatic pressure, seemingly by changes to Nax Hexchange phosphorylation state. These properties are consistent with the NHE-1 isoform of Nax Hexchange. However, using antibodies directed against Nax Hproteins we find that both NHE-1 and NHE-3 are present in these cells. Funded by The Wellcome Trust, UK