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.
Keywords: Na+, HCO3− cotransport; Na+/H+ exchange; acid-base transport; ischaemia; resistance arteries; vascular smooth muscle cells.
© 2017 Scandinavian Physiological Society. Published by John Wiley & Sons Ltd.