Extracellular acidosis suppresses endothelial function by inhibiting store-operated Ca2+ entry via non-selective cation channels

Aims Hypoxia, ischaemia, and exogenous chemicals can induce extracellular and intracellular acidosis, but it is not clear which of these types of acidosis affects endothelial cell function. The synthesis and release of endothelium-derived relaxing factors (EDRFs) are linked to an increase in cytosolic Ca2+ concentration, and we therefore examined the effects of extracellular and intracellular acidosis on Ca2+ responses and EDRF production in cultured porcine aortic endothelial cells. Methods and results Cytosolic pH (pHi) and Ca2+ were measured using fluorescent dyes, BCECM/AM (pH-indicator) and fura-2/AM (Ca2+-indicator), respectively. EDRFs, nitric oxide (NO) and prostaglandin I2 (PGI2) were assessed using DAF-FM/DA (NO-indicator dye) fluorometry and 6-keto PGF1α enzyme immunoassay, respectively. HEPES buffers titrated to pH 6.4, 6.9, and 7.4 were used to alter extracellular pH (pHo), and propionate (20 mmol/L) was applied to cause intracellular acidosis. Extracellular acidosis strongly suppressed bradykinin (BK, 10 nmol/L)- and thapsigargin (TG, 1 µmol/L)-induced Ca2+ responses by 30 and 23% at pHo 6.9, and by 80 and 97% at pHo 6.4, respectively. During the examinations, there were no significant differences in pHi among the three groups at pHo 7.4, 6.9, and 6.4. Extracellular acidosis also inhibited BK-stimulated PGI2 production by 55% at pHo 6.9 and by 77% at pHo 6.4, and NO production by 38% at pHo 6.9 and by 91% at pHo 6.4. The suppressive effects of extracellular acidosis on Ca2+ responses and NO production were reversible. Propionate changed pHi from 7.3 to 6.9, without altering pHo (7.4). Intracellular acidosis had no effect on BK- and TG-induced Ca2+ responses or NO production. Conclusion These results indicate that extracellular, but not intracellular, acidosis causes endothelial dysfunction by inhibiting store-operated Ca2+ entry, so helping to clarify the vascular pathophysiology of conditions such as ischaemia, hypoxia, acidosis, and ischaemia-reperfusion.