Sodium uptake and membrane excitation in Paramecium.

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Abstract
Although the phenotypes of many membrane-excitation mutants of P. tetraurelia are best expressed in Na+-containing solutions, little is known about the role of Na+ in membrane excitation in Paramecium. Based on measuring 22Na fluxes, the total cellular Na+ content is equivalent to a cytoplasmic concentration of 3-4 mM, if the Na+ concentration is uniform throughout the cell. 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. 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. Na+ uptake is inhibited at 4.degree. C. Na+ uptake may be faster when the membrane is depolarized than when it is at the resting potential level.