The Journal of Cell Biology, Vol 81, 374-381, Copyright © 1979 by The Rockefeller University Press
Sodium uptake and membrane excitation in Paramecium
HG Hansma
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.
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