Effect of sugars on salt reception in true slime mold Physarum polycephalum. Physicochemical interpretation of interaction between salt and sugar receptions

Interaction between salt and sugar receptions in plasmodium of Physarum polycephalum was studied by using double-chamber method. Effect of sugars on salt reception was evaluated by measuring membrane potential and the motive force of tactic movement of the slime mold, where salt concentration in one compartment was increased successively with a fixed sugar concentration. Results are summarized as follows: (1) The presence of D-glucose, D-mannose, D-maltose, or sucrose in medium led to increase of the threshold concentration Cth, for salts (chlorides and nitrates of Li, Na, K), whereas D-ribose decreased the threshold for salt reception. D-galactose showed no appreciable effect on Cth of every salt species examined. No change in Cth for salt reception was observed until concentration of sugars exceeded their respective thresholds. (2) Double logarithmic plots of Cth for salts against sugar concentration followed different straight lines for different cations, whose slopes being closely correlated with the effects of lyotropic number of anions in the absence of sugars. (3) Plots of log Cth against the reciprocal of the absolute temperature, 1/T, gave linear relations, and the slopes of the straight line became small with increase of sugar concentration above their respective thresholds. Experimental results obtained here suggest that the structure of water at the interface of cell membrane plays an indispensable role in the interaction between salt and sugar receptions.