Department of Physiology, Anatomy & Genetics, University of Oxford, Sherrington Building, Parks Road, Oxford OX1 3PT, UK.
Department of Physics, Clarendon Laboratory, University of Oxford, Parks Road, Oxford OX1 3PU, UK.
Cardiovasc Res. 2022 Nov 10;118(14):2946-2959. doi: 10.1093/cvr/cvab364.
In cardiomyocytes, acute disturbances to intracellular pH (pHi) are promptly corrected by a system of finely tuned sarcolemmal acid-base transporters. However, these fluxes become thermodynamically re-balanced in acidic environments, which inadvertently causes their set-point pHi to fall outside the physiological range. It is unclear whether an adaptive mechanism exists to correct this thermodynamic challenge, and return pHi to normal.
Following left ventricle cryo-damage, a diffuse pattern of low extracellular pH (pHe) was detected by acid-sensing pHLIP. Despite this, pHi measured in the beating heart (13C NMR) was normal. Myocytes had adapted to their acidic environment by reducing Cl-/HCO3- exchange (CBE)-dependent acid-loading and increasing Na+/H+ exchange (NHE1)-dependent acid-extrusion, as measured by fluorescence (cSNARF1). The outcome of this adaptation on pHi is revealed as a cytoplasmic alkalinization when cells are superfused at physiological pHe. Conversely, mice given oral bicarbonate (to improve systemic buffering) had reduced myocardial NHE1 expression, consistent with a needs-dependent expression of pHi-regulatory transporters. The response to sustained acidity could be replicated in vitro using neonatal ventricular myocytes incubated at low pHe for 48 h. The adaptive increase in NHE1 and decrease in CBE activities was linked to Slc9a1 (NHE1) up-regulation and Slc4a2 (AE2) down-regulation. This response was triggered by intracellular H+ ions because it persisted in the absence of CO2/HCO3- and became ablated when acidic incubation media had lower chloride, a solution manoeuvre that reduces the extent of pHi-decrease. Pharmacological inhibition of FAK-family non-receptor kinases, previously characterized as pH-sensors, ablated this pHi autoregulation. In support of a pHi-sensing role, FAK protein Pyk2 (auto)phosphorylation was reduced within minutes of exposure to acidity, ahead of adaptive changes to pHi control.
Cardiomyocytes fine-tune the expression of pHi-regulators so that pHi is at least 7.0. This autoregulatory feedback mechanism defines physiological pHi and protects it during pHe vulnerabilities.
在心肌细胞中，细胞内 pH（pHi）的急性紊乱会被精细调节的肌膜酸碱转运体系统迅速纠正。然而，在酸性环境中，这些通量会在热力学上重新达到平衡，这会导致它们的设定 pHi 落在生理范围之外。目前尚不清楚是否存在一种适应性机制来纠正这种热力学挑战，并使 pHi 恢复正常。
左心室冷冻损伤后，通过酸感应 pHLIP 检测到细胞外 pH（pHe）的弥漫性降低。尽管如此，通过 13C NMR 测量的搏动心脏中的 pHi 是正常的。心肌细胞通过减少 Cl--HCO3-交换（CBE）依赖性酸加载和增加 Na+/H+交换（NHE1）依赖性酸外排来适应酸性环境，这可以通过荧光（cSNARF1）来测量。当细胞在生理 pHe 下灌流时，这种适应对 pHi 的影响表现为细胞质碱化。相反，给予口服碳酸氢盐（以改善全身缓冲）的小鼠心肌 NHE1 表达减少，这与 pH 调节转运体的需求依赖性表达一致。在体外，通过将新生心室肌细胞在低 pHe 下孵育 48 小时，可以复制对持续酸性的反应。NHE1 活性的适应性增加和 CBE 活性的降低与 Slc9a1（NHE1）的上调和 Slc4a2（AE2）的下调有关。这种反应是由细胞内 H+离子触发的，因为它在没有 CO2/HCO3-的情况下仍然存在，并且当酸性孵育培养基中氯离子较低时，这种反应就会消失，氯离子的这种溶液操作降低了 pHi 降低的程度。先前被表征为 pH 传感器的 FAK 家族非受体激酶的药理学抑制消除了这种 pH 自调节。为了支持 pH 感应作用，FAK 蛋白 Pyk2（自身）磷酸化在暴露于酸性环境后的几分钟内就减少了，这先于对 pHi 控制的适应性变化。
心肌细胞精细调节 pHi 调节剂的表达，使 pHi 至少为 7.0。这种自动调节反馈机制定义了生理 pHi，并在 pHe 脆弱性期间保护它。
