The relationship between adenine-nucleotide levels and metabolism-dependent membrane potential was studied in cells of Nitellopsis obtusa. Effects of ADP and AMP in the presence of ATP on electrogenic pump activity were measured in the dark, using the continuous perfusion method. Both ADP and AMP acte as competitive inhibitors for ATP, the Ki value for either compound being about 0.4 mM. The role of ADP and AMP as regulating factors for the electrogenic pump was investigated under various metabolic conditions. Application of N2 gas in the dark caused a significant membrane depolarization amounting to 90 mV, but cytoplasmic streaming and membrane excitability were not affected. Under anoxia, the ATP level decreased from 1.6 to 0.5 mM; ADP increased but only slightly, and AMP increased greatly. However, the time course of changes in the adenine nucleotides was not concurrent with that of the membrane-potential changes, thus, the adenine-nucleotide level changes cannot fully account for the N2-elicited depolarization. Under light, although the membrane hyperpolarized, no significant changes in the adenine-nucleotide levels were observed. Therefore, the light-induced membrane hyperpolarization cannot be explained solely by changes in adenine-nucleotide levels.
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