Extracellular acidosis and very low [Na+] inhibit NBCn1- and NHE1-mediated net acid extrusion from mouse vascular smooth muscle cells.
OBJECTIVE
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+]oand pHoon NBCn1 and NHE1 activity in VSMCs of small arteries.


METHODS
We measured pHiby 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 pHodecreased from 7.4 (22 mm HCO3-/5% CO2) by metabolic (pHo7.1/11 mm HCO3-: 22 ± 8%; pHo6.8/5.5 mm HCO3-: 61 ± 7%) or respiratory (pHo7.1/10% CO2: 35 ± 11%; pHo6.8/20% CO2: 56 ± 7%) acidosis. Extracellular acidosis more prominently inhibited NHE1 activity - defined as Na+-dependent net acid extrusion without CO2/HCO3- present - at both pHo7.1 (45 ± 9%) and 6.8 (85 ± 5%). Independently of pHo, lowering [Na+]ofrom 140 to 70 mm reduced NBCn1 and NHE1 activity <20% whereas transport activities declined markedly (25-50%) when [Na+]owas reduced to 35 mm. Steady-state pHidecreased more during respiratory (ΔpHi/ΔpHo= 71 ± 4%) than metabolic (ΔpHi/ΔpHo= 30 ± 7%) acidosis.


CONCLUSIONS
Extracellular acidification inhibits NBCn1 and NHE1 activity in VSMCs. NBCn1 is equivalently inhibited when pCO2is raised or [HCO3-]odecreased. Lowering [Na+]oinhibits NBCn1 and NHE1 markedly only below the typical physiological and pathophysiological range. We propose that inhibition of Na+-dependent net acid extrusion at low pHoprotects against cellular Na+overload at the cost of intracellular acidification.
Department of Biomedicine, Aarhus University, Aarhus, Denmark.
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L
Bonde
Department of Biomedicine, Aarhus University, Aarhus, Denmark.
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http://orcid.org/0000-0002-5078-9279
E
Boedtkjer
D000138:Acidosis; D000818:Animals; D001693:Biological Transport, Active; D002478:Cells, Cultured; D006863:Hydrogen-Ion Concentration; D008297:Male; D051379:Mice; D009131:Muscle, Smooth, Vascular / Q000378:metabolism*; D032389:Myocytes, Smooth Muscle / Q000502:physiology*; D012964:Sodium / Q000097:blood*; D027982:Sodium-Bicarbonate Symporters / Q000235:genetics / Q000378:metabolism*; D000074082:Sodium-Hydrogen Exchanger 1 / Q000235:genetics / Q000378:metabolism*
C498904:Slc4a7 protein, mouse; C490997:Slc9a1 protein, mouse; D027982:Sodium-Bicarbonate Symporters; D000074082:Sodium-Hydrogen Exchanger 1; D012964:Sodium
Na+, HCO3− cotransport; Na+/H+ exchange; acid-base transport; ischaemia; resistance arteries; vascular smooth muscle cells