POTASSIUM AND SODIUM-TRANSPORT AND PH REGULATION

The excitatory and metabolic events in nervous tissue lead to localized increases in extracellular potassium (K+) and intracellular hydrogen (H+) and calcium (Ca2+) ion concentrations. Even more pronounced increases are seen under pathological conditions and may interfere with the maintenance of cellular function and structure. Most presentations on the second day focussed on these processes and the mechanisms for the clearance of K+, H+, and Ca2+ from intra- or extra-cellular compartments. The essential role of glial cells was a returning theme. Extracellular K+ is transported into cells by the Na-K pump and by two other processes, Na-K-Cl2 cotransport and spatial buffering, which both depend on the operation of the Na-K pump. The clearance of H+ from the cytoplasm and into the extracellular space is mediated by Na+ gradient dependent processes, Na+/H+ antiport, Cl-/HCO3- exchange, and Na+ -HCO3- cotransport. Also the clearance of cytoplasmic Ca2+ is to a large extent mediated by a Na+ gradient dependent process, the Na+/Ca2+ antiport. There is a wide divergence between the rates of Na-K pump mediated K+ influx measured in various cultures of glial and neuronal cells. There is a considerable need for systematic comparison between the functional capacity and the concentration of Na - K pumps in cell cultures and intact nervous tissue. It has not yet been ascertained whether K+ transport as measured in cultured astrocytes is representative for K+ transport in the in situ functioning astrocyte. In astrocytes, glutamate was shown to elicit a rapid intercellular propagation of a rise in cytoplasmic Ca2+. In neurons, the elevation of cytoplasmic Ca2+ during exposure to hypoxia, ischemia, or excitatory amino acids was described and related to cell damage. Cytoplasmic Ca2+ may be cleared by Na+/Ca2+ antiport as well as by a Ca2+-activated ATPase in the plasma membrane. The total concentration of this enzyme has not been quantified, and little is known about its energy requirements.