ABSTRACT
Regulation of sodium and potassium by Paramecium was studied by measuring fluxes of 22Na and 86Rb respectively. Fluxes of these tracers had the following similarities. (1) Steady-state turnover showed approximately single-pool kinetics with 50% turnover in 
h. (2) Efflux of both cations showed a specific exchange diffusion when the medium contained the corresponding non-radioactive cation. (3) The net efflux of both cations was slow in the absence of exchange diffusion.
Fluxes showed the following differences. (1) The cells had 2 to 4 times as much K+Rb as Na. Therefore, (2) the K+Rb fluxes at steady state were 2–4 times larger than the Na fluxes, since the fractional turnover was the same for both cations. (3) Membrane excitation by Ba2+ stimulated the efflux of Rb but not Na. The efflux of 86Rb during membrane excitation appeared to represent the outward or delayed rectification that repolarizes the membranes after an action potential, since it was absent in a pawn mutant that is lacking in membrane excitation.
