The pH in both the extracellular space and the cytoplasm of neural cells is the subject of tight control by numerous buffering systems. The extracellular pH (pHo) varies between 7.1 and 7.3, whereas intracellular pH in both neurones and glia lies in a range of 6.8 to 7.5. This means that the concentration of free protons, H+, is quite low, being somewhere around 50 nM in the extracellular milieu and 30-160 nM in the cytosol of neural cells.
Maintenance of extracellular pH is physiologically important, as even small fluctuations of pHo may significantly affect synaptic transmission and neuronal excitability. Lowering of pH below 7.0, for example, almost completely inhibits NMDA receptors; in addition, acidification of the extracellular space can activate proton-sensitive cationic channels (known as ASICs - Acid-Sensitive Ion Channels) present in many types of neurones. Neurones and neuronal terminals are the main source of protons in the brain. Neurones, as the main consumers of energy, produce CO2, which is an end product of oxidative metabolism. The CO2, by reacting with water, produces protons (CO2+H2O^H2CO3^HCO-+H+). Furthermore, protons are released in the course of synaptic transmission, as synaptic vesicles are acidic (with pH ~5.6). These changes, at least in part, are counterbalanced by bicarbonate and proton transporters present in astroglial cells; particularly important is the Na+ /HCO- cotransporter (NBC), which can operate in both directions, either supplying or removing HCO- from the extracellular space.
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