ACID-BASE HOMEOSTASIS IN BRAIN INJURY

长期以来，人们一直认为酸碱平衡的紊乱与疾病有关

Disturbances in acid-base homeostasis have long been implicated in the pathophysiology of ischemic and traumatic brain injury. The deleterious effects of acidosis have received particular attention. Numerous studies indicate that excessive production of lactic acid is particularly harmful to astrocytes. Although the basis of this susceptibility is not understood, studies of astrocyte pH regulation suggest that acid transport mechanisms of these cells are implicated in these pathophysiological responses. Of particular interest are voltage-dependent acid secretory mechanisms which are activated by glial depolarization. A principal aim of this proposal is to elucidate the role of these astrocytic regulatory processes in acute injuries to the CNS. Astrocyte pH regulation will be investigated with pH microelectrodes, optical recording methods and whole cell patch clamp techniques. The intracellular pH studies will be performed at the level of the whole animal, brain slice and isolated cell, capitalizing on the unique experimental advantages offered by each preparation. Neurons, by contrast, can be protected by acidosis in the injury setting, since external hydrogen ions block NMDA receptor-mediated activity. However, spreading depression and others forms of excessive excitory synaptic activity, are associated with extracellular alkalinization, capable of relieving the H+ block of the NMDA receptor. The magnitude of these pH shifts depends upon the speed of buffering, which is governed by the extracellular activity of carbonic anhydrase. These studies will determine the role of this enzyme in ischemic and traumatic injury to the CNS. Experiments will employ recently-developed microelectrode techniques, which will permit the first real-time determination of pH, bicarbonate and carbon dioxide in injured brain. The microelectrode studies will be conducted in anesthetized rats, in models of cardiac arrest (complete cortical ischemia), stroke (middle cerebral artery occlusion), and spinal cord injury. These experiments will provide a complete description of extracellular acid base status in these afflictions. In conjunction with these measurements, we will determine whether the kinetics of H+ buffering can be enhanced by treatment with buffers or by modulation of carbonic anhydrase activity. Experiments will be extended to the study of spreading depression, and hypoxia, in order to determine the role of extracellular buffering in the manifestation of these pathologies. These studies capitalize upon recent conceptual and technological developments in the study of brain pH regulation. The broad goal of this work is to extend our progress in basic research to the pathophysiological setting. Our efforts will provide insights and understanding into how elemental regulatory processes affect injuries arising from cardiac arrest, stroke, and CNS trauma.