[Influence of pH on uptake and release of sodium by Chlorella].

The Na metabolism of Chlorella fusca at high pH values was investigated by radiotracer methods. The Na influx into Chlorella is strongly (20-30 fold) enhanced in 5·10(-3)M NaOH (pH 11,4). It then approaches the rate of the influx of K (labelled with (86)Rb). The rise of the Na influx begins at pH 8.3 and has a point of inflection at pH 8.7. The cells remain undamaged.Na uptake from 5·10(-3)M NaOH leads to a steady state with a 10-fold Na accumulation. The cells then contain 20-30 times more Na than in a neutral medium. The Na efflux remains proportional to the Na concentration in the cell. The Na uptake at high pH is energy-dependent, has saturation kinetics (K M =2·10(-3) mol/l) and is competitively inhibited by K. During Na uptake at high pH an equal amount of K is released from the cells. It is assumed that the enhancement of Na influx in NaOH is due to the fact that the selectivity for K of the membrane-bound K transport system is lowered under the alkaline conditions. Possibly an SH group is involved in this alteration.Neutralisation after Na accumulation from NaOH is followed by energy-dependent net Na efflux. An active Na efflux pump with first order kinetics is postulated. This pump can exchange Na against K and H. It is stimulated by external K, but can work without it.