The rate of Na+/Na+exchange as measured with24Na+in Na+-rich cells ofChlorella pyrenoidosais governed by a single rate constant and saturates with increasing external Na+concentration. TheKmvalue for this process is 0.8 mM Na+and the maximum rate of exchange in illuminated cells is about 5 pmoles cm-2sec-1. These values contrast with aKmof 0.18 mM K+and maximum rate of about 17 pmoles K+·cm-2·sec-1for 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+on24Na+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
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