Lithium fluxes in Paramecium and their relationship to chemoresponse

Paramecia respond to environmental stimuli by altering swimming behavior to disperse from or accumulate in the vicinity of the stimulus. We have found, using the T-maze assay, that treatment of paramecia with LiCl in a time- and concentration-dependent manner modifies the normal response to folate, acetate, and lactate from attraction to no response or even repulsion. Responses to NH4Cl were unaffected and to cAMP were variably affected by LiCl. Cells incubated in the presence of K+, or both Na+ and K+, but not Na+ alone reliably recovered attraction to folate. Treatment of cells with 4 mM LiCl for 1 h dramatically slowed swimming speed from about 1 mm/s in NaCl or KCl (control) to 0.18 mm/s in LiCl. Li-treated cells subsequently incubated in 4 mM NaCl, KCl or sequentially in KCl and NaCl for a total of 20 min increased their swimming speed to 0.35, 0.45 and 0.67 mm/s, respectively. Paramecia readily took up Li+ in Na(+)- and K(+)-free media reaching intracellular concentrations of 5-10 mM in 10 mM extracellular Li+. Efflux of intracellular Li+ was stimulated 35% by extracellular 10 mM NaCl and 185% by 10 mM KCl over 10 mM choline chloride. Incubation of cells in 10 mM LiCl for 1 h inhibited the rate of Ca2+ efflux by 44% compared to cells in 10 mM NaCl. This may relate to the mechanism by which Li+ perturbs chemoresponse. A mutant with defects in Ca homeostasis responds normally to NH4Cl, but not to any of the stimuli that are affected by LiCl.