Der Einfluß des pH-Werts auf Aufnahme und Abgabe von Natrium durch Chlorella

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Abstract
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 5·10-3M NaOH (pH 11,4). It then approaches the rate of the influx of K (labelled with 86Rb). 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 5·10-3M 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.