Variation in selectivity of univalent cations in slime moldPhysarum polycephalum caused by reception of polyvalent cations

Summary Specificity of reception on 11 electrolytes in the slime moldPhysarum polycephalum was investigated in the presence of polyvalent cations in media. Membrane potential and motive force of tactic movement were examined with the aid of the double chamber method, and the zeta potential at the membrane surface of the slime mold was measured by electrophoretic mobility. The results obtained are summarized as follows: (1) The presence of polyvalent cations (e.g., Ca²⁺, Mg²⁺, Sr²⁺, Ba²⁺, La³⁺, Th⁴⁺) in medium led to an increase in threshold concentration,C _th , determined from the potential measurements for Na- or Li-salts, and to a decrease inC _th for K-, Rb-, or NH₄-salts,C _th for 11 electrolytes changed discontinuously when the concentration of polyvalent cations in medium exceeded their respective thresholds. (2) TheC _th determined from chemotaxis agreed with that from the potential response both in the presence and absence of polyvalent cations. (3) Sequence of selectivity of univalent cations varied extensively in the presence of polyvalent cations. (4) Changes in the zeta potential induced by NaCl reception agreed with those in the membrane potential even in the presence of Ca²⁺ in medium. (5) TheC _th for reception of NaCl changed sharply at about 12 °C in the presence of polyvalent cations, while that for KCl was independent of the temperature.Conformational changes in surface membrane of the slime mold in response to reception of polyvalent cations were then discussed in relation to the discrimination of univalent cations.