We describe the existence of a potassium ion transport mechanism in the mitochondrial inner membrane of a lower eukaryotic organism Acanthamoeba castellanii. We found that substances known to modulate potassium channel activity influenced the bioenergetics of A. castellanii mitochondria. In isolated mitochondria, the rate of resting respiration is increased by about 10% in response to potassium channel openers, i.e. diazoxide and BMS-191095, during succinate-, malate- or NADH-sustained respiration. This effect is strictly dependent on the presence of potassium ions in an incubation medium and is reversed by glibenclamide (a potassium channel blocker). Diazoxide and BMS-191095 also cause a slight but statistically significant depolarization of mitochondrial membrane potential (measured with a TPP⁺-specific electrode), regardless of the respiratory substrate used. The resulting steady state value of membrane potential is restored after treatment with glibenclamide or 1 mM ATP. Additionally, the electrophysiological properties of potassium channels present in the A. castellanii inner mitochondrial membrane are described in the reconstituted system, using black lipid membranes. Our results suggest that an ATP-sensitive potassium channel similar to that of mammalian mitochondria is present in A. castellanii mitochondria.

This work was supported by grant from the Polish Ministry of Education and Science (PBZ-MIN-012/P04/06). The authors thank Prof. Adam Szewczyk and his co-workers from the Laboratory of Intracellular Ion Channels, Nencki Institute of Experimental Biology, Warsaw, Poland - for performing electrophysiological experiments.

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