Light-induced membrane hyperpolarization and adenine nucleotide levels in perfused characean cells

This study examines the relationship between light-induced membrane hyperpolarization and changes in adenine nucleotide levels in tonoplast-free characean cells. When cells were perfused with a medium containing 1 mM ATP in the dark, the plasma membrane depolarized, the cytosolic ATP level decreased, and the ADP and AMP levels increased. Under light, the membrane hyperpolarized, the ATP level increased, and the ADP and AMP levels decreased. These changes in the adenine-nucleotide levels could partially explain the membrane hyperpolarization. When cells were perfused with a medium containing an ATP-regenerating system consisting of phosphoenolpyruvate and pyruvate kinase, the membrane potential remained in the hyperpolarized state, the ATP level remained at a high level and no light-induced hyperpolarization was observed. The intracellular adenine nucleotide levels were also controlled by continuous perfusion. The membrane potential was determined only by the adenine nucleotide levels of perfusion media, irrespective of the light condition.Chloroplast-free Nitellopsis cells into which isolated Pisum chloroplasts were introduced also showed light-induced membrane hyperpolarization. Pretreatment of chloroplasts with dicyclohexylcarbodiimide (DCCD) completely abolished the hyperpolarization with parallel inhibition of photophosphorylation. These results strongly suggest that changes in adenine nucleotide levels caused by photophosphorylation are responsible for light-induced membrane hyperpolarization in perfused cells.