Summary: Using indirect methods based on uptake of [H]tetraphenylphosphonium cation and [C]benzoic acid by cells of the fungus we found that the illumination-induced transient hyperpolarization of the plasma membrane is followed immediately by a rapid temporary decrease in intracellular pH. Hyperpolarization and intracellular acidification were completely suppressed by 150 mM-KCl and by the K-ionophore valinomycin. The light-induced acidification of the cytoplasm was not observed in the presence of the cytochrome respiratory chain inhibitors antimycin A and mucidin. Based on these results, we hypothesize that the hyperpolarization of the cells is the consequence of an efflux of K through a light-activated K-channel in the plasma membrane. The loss of positive charge in the cytoplasm caused by this efflux of cations is counterbalanced by H originating from the light-activated mitochondrial respiratory chain.


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