Bonde L, Boedtkjer E
Department of Biomedicine, Aarhus University, Aarhus, Denmark.
Acta Physiol (Oxf). 2017 Oct;221(2):129-141. doi: 10.1111/apha.12877. Epub 2017 Apr 25.
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.
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.
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.
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.
电中性钠-碳酸氢根共转运体NBCn1和钠/氢交换体NHE1调节血管平滑肌细胞（VSMC）的酸碱平衡，并改变动脉的功能和结构。病理状况——尤其是缺血——可显著扰乱细胞内（i）和细胞外（o）的pH值及[Na⁺]。我们研究了低[Na⁺]和pH值对小动脉VSMC中NBCn1和NHE1活性的影响。
我们通过基于2',7'-双（2-羧乙基）-5-（和-6）-羧基荧光素的荧光显微镜测量小鼠肠系膜动脉的pH值，并通过NH₄⁺预脉冲技术诱导细胞内酸化。
NBCn1活性——定义为在存在CO₂/HCO₃⁻时依赖钠、对氨氯地平不敏感的净碱摄取——在pH值从7.4（22 mmol HCO₃⁻/5% CO₂）通过代谢性（pH 7.1/11 mmol HCO₃⁻：22±8%；pH 6.8/5.5 mmol HCO₃⁻：61±7%）或呼吸性（pH 7.1/10% CO₂：35±11%；pH 6.8/20% CO₂：56±7%）酸中毒降低时受到同等程度的抑制。细胞外酸中毒在pH 7.1（45±9%）和6.8（85±5%）时更显著地抑制NHE1活性——定义为在不存在CO₂/HCO₃⁻时依赖钠的净酸排出。与pH值无关，将[Na⁺]从140 mmol/L降至70 mmol/L时，NBCn1和NHE1活性降低<20%，而当[Na⁺]降至35 mmol/L时，转运活性显著下降（25 - 50%）。呼吸性酸中毒期间稳态pH值下降幅度（ΔpHₒ/ΔpHᵢ = 71±4%）大于代谢性酸中毒（ΔpHₒ/ΔpHᵢ = 30±7%）。
细胞外酸化抑制VSMC中NBCn1和NHE1活性。当pCO₂升高或[HCO₃⁻]降低时，NBCn1受到同等程度的抑制。降低[Na⁺]仅在低于典型生理和病理生理范围时才显著抑制NBCn1和NHE1。我们提出，在低pH值时抑制依赖钠的净酸排出以细胞内酸化为代价保护细胞免受钠过载。
