Extracellular acidosis and very low [Na ] inhibit NBCn1- and NHE1-mediated net acid extrusion from mouse vascular smooth muscle cells.

AIM

The electroneutral Na , HCO3- cotransporter NBCn1 and Na /H exchanger NHE1 regulate acid-base balance in vascular smooth muscle cells (VSMCs) and modify artery function and structure. Pathological conditions - notably ischaemia - can dramatically perturb intracellular (i) and extracellular (o) pH and [Na ]. We examined effects of low [Na ] and pH on NBCn1 and NHE1 activity in VSMCs of small arteries.

METHODS

We measured pH by 2',7'-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein-based fluorescence microscopy of mouse mesenteric arteries and induced intracellular acidification by NH4+ prepulse technique.

RESULTS

NBCn1 activity - defined as Na -dependent, amiloride-insensitive net base uptake with CO /HCO3- present - was inhibited equally when pH decreased from 7.4 (22 mm HCO3-/5% CO ) by metabolic (pH 7.1/11 mm HCO3-: 22 ± 8%; pH 6.8/5.5 mm HCO3-: 61 ± 7%) or respiratory (pH 7.1/10% CO : 35 ± 11%; pH 6.8/20% CO : 56 ± 7%) acidosis. Extracellular acidosis more prominently inhibited NHE1 activity - defined as Na -dependent net acid extrusion without CO /HCO3- present - at both pH 7.1 (45 ± 9%) and 6.8 (85 ± 5%). Independently of pH , lowering [Na ] from 140 to 70 mm reduced NBCn1 and NHE1 activity <20% whereas transport activities declined markedly (25-50%) when [Na ] was reduced to 35 mm. Steady-state pH decreased more during respiratory (ΔpH /ΔpH  = 71 ± 4%) than metabolic (ΔpH /ΔpH  = 30 ± 7%) acidosis.

CONCLUSION

Extracellular acidification inhibits NBCn1 and NHE1 activity in VSMCs. NBCn1 is equivalently inhibited when pCO is raised or [HCO3-] decreased. Lowering [Na ] inhibits NBCn1 and NHE1 markedly only below the typical physiological and pathophysiological range. We propose that inhibition of Na -dependent net acid extrusion at low pH protects against cellular Na overload at the cost of intracellular acidification.