Sodium transport in Na+-rich Chlorella cells

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Abstract
The rate of Na+/Na+ exchange as measured with 24Na+ in Na+-rich cells of Chlorella pyrenoidosa is governed by a single rate constant and saturates with increasing external Na+ concentration. The K mvalue for this process is 0.8 mM Na+ and the maximum rate of exchange in illuminated cells is about 5 pmoles cm-2 sec-1. These values contrast with a K mof 0.18 mM K+ and maximum rate of about 17 pmoles K+·cm-2·sec-1 for net K+ influx. Although the Na+/Na+ exchange was only slightly sensitive to light it was inhibited by the uncouplers CCCP and DNP and by the energy transfer inhibitor DCCD. This inhibition of the rate of Na+/Na+ exchange was not accompanied by a loss of internal Na+. Both the effect of external K+ on 24Na+ influx into Na+-rich cells and the inhibition of net K+ uptake by the presence of external Na+ indicates that Na+/Na+ and K+/Na+ exchanges share the same carrier and that the external site of this carrier has a three to four times higher affinity for K+ over Na+.