In vivo and in vitro Control of Acid-Base Regulation of Brain Cells During Ischemic and Selective Acidic Exposure
The three-compartment model of brain acid-base regulation postulates that under circumstances of changing function or disease, hydrogen ion concentrations may differ considerably in the interstitial space (ISS), the neurons and the glial cells. During hyperglycemia plus profound ischemia, for example, direct measurements by microelectrodes followed by intracellular HRP staining show that intraglial pH can fall transiently as low as 3.9, although more often the nadir drops to the 4.5–5.5 range. Concurrently, ISS-pH and, by calculation, neuronal pH falls to and remains constant (but not necessarily the same) at pH 6.2. By contrast, during spreading depression, ISS and intraglial pH at first move rapidly and transiently in opposite directions, ISS [H+] rising, intraglial falling. These two then gradually stabilize, whereas neuronal pH remains substantially more steady and near normal, shifting only minimally from resting baseline levels over several minutes’ time. Similar but less pronounced effects follow direct electrical stimulation. The net change represents complex biophysical transmembrane and buffering mechanisms that appear to guard neuronal homeostasis.
Studies carried out on embryonic rat forebrain neurons and glia show that these cells have considerably different vulnerabilities to extracellular acidity depending on the anionic nature of the acid in the bathing medium. In cultures to which HCI was added to the medium, neurons and neuronal processes almost all survived ten minute exposures to pH 3.8, whereas glial cells succumbed after ten minute exposures at pH not lower than 4.2. Both types of cells, however, showed much greater vulnerability to lactic acidification in the media; neither neurons nor glia survived exposure for ten minutes at pH 4.8 or for sixty minutes at the lesser degree of acidity of 5.2. Measurements of intracellular pH in cultured mammalian neurons using the fluorescent dye BCECF demonstrated a rapid fall in intracellular pH from 7.18 to 6.80 when 20 mM lactate was added to pH — clamped extracellular medium held at a pH of 7.35. By contrast, when the pHo in the medium fell from 7.35 to 6.65 in the presence of 20 mM lactate, pHi fell to 6.48 and failed to recover unless pHo was quickly restored to 7.35 and lactate was removed from the medium. The findings indicate that CNS cell membranes are substantially more permeable to lactic acid compared to unorganic acids and that the vulnerability relates to the greater capacity of the former to induce a profound and rapid intracellular acidification.
KeywordsCerebral acid-base regulation glial neuronal pH lactacidosis
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