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

Acta Physiologica

Published online on


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+]o and pHo on NBCn1 and NHE1 activity in VSMCs of small arteries. Methods We measured pHi 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 CO2/HCO3− present – was inhibited equally when pHo decreased from 7.4 (22 mm HCO3−/5% CO2) by metabolic (pHo 7.1/11 mm HCO3−: 22 ± 8%; pHo 6.8/5.5 mm HCO3−: 61 ± 7%) or respiratory (pHo 7.1/10% CO2: 35 ± 11%; pHo 6.8/20% CO2: 56 ± 7%) acidosis. Extracellular acidosis more prominently inhibited NHE1 activity – defined as Na+‐dependent net acid extrusion without CO2/HCO3− present – at both pHo 7.1 (45 ± 9%) and 6.8 (85 ± 5%). Independently of pHo, lowering [Na+]o from 140 to 70 mm reduced NBCn1 and NHE1 activity <20% whereas transport activities declined markedly (25–50%) when [Na+]o was reduced to 35 mm. Steady‐state pHi decreased more during respiratory (ΔpHi/ΔpHo = 71 ± 4%) than metabolic (ΔpHi/ΔpHo = 30 ± 7%) acidosis. Conclusion Extracellular acidification inhibits NBCn1 and NHE1 activity in VSMCs. NBCn1 is equivalently inhibited when pCO2 is raised or [HCO3−]o decreased. Lowering [Na+]o 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 pHo protects against cellular Na+ overload at the cost of intracellular acidification.