title: Sodium uptake and membrane excitation in Paramecium.
abstract: Although the phenotypes of many membrane-excitation mutants of Paramecium are best expressed in Na+-containing solutions, little is known about the role of Na+ in membrane excitation in Paramecium. By measuring 22Na fluxes, we have shown that: (a) The total cellular Na+ content is equivalent to a cytoplasmic concentration of 3--4 mM, if the Na+ concentration is uniform throughout the cell. (b) The kinetics of Na+ uptake can be divided into a saturable Na+ uptake with an apparent Km = 0.15 mM and a nonsaturable Na+ uptake seen at higher Na+ concentrations up to 20 mM. (c) The rate of Na+ uptake in high Na+ solutions is correlated with the duration of backward swimming and membrane excitation in wild type Paramecium and the mutants fast-2 and paranoiac. (d) Na+ uptake is inhibited at 4 degrees C. From these results, we postulate that Na+ uptake is faster when the membrane is depolarized than when it is at the resting potential level.
mesh_terms: D000818:Animals; D002462:Cell Membrane / Q000502:physiology*; D003080:Cold Temperature; D003593:Cytoplasm / Q000378:metabolism; D004594:Electrophysiology; D009068:Movement; D009154:Mutation; D010247:Paramecium / Q000235:genetics / Q000378:metabolism* / Q000502:physiology; D012964:Sodium / Q000378:metabolism*
chemical_list: D012964:Sodium