1607-6729/05/0304- © 2005 Pleiades Publishing, Inc. 0097 Doklady Biochemistry and Biophysics, Vol. 401, 2005, pp. 97–103. Translated from Doklady Akademii Nauk, Vol. 401, No. 1, 2005, pp. 105–111. Original Russian Text Copyright © 2005 by Korotkov, Nikitina, Glazunov, Yagodina. It is well known that the inner mitochondrial mem- brane has low permeability for monovalent cations and protons. This property of the mitochondrial membrane allows mitochondria to stabilize ä + concentration in the matrix and, as a consequence, to stabilize their volume at a certain level which is necessary to maintain their structural and functional integrity during different bio- chemical processes [1]. Under physiological condi- tions, ä + concentration in the matrix is maintained con- stant by the mechanisms that prevent both mitochon- drial swelling (due to K + /H + exchanger) and excessive mitochondrial shrinkage, which appears during depo- larization of the inner mitochondrial membrane as a result of increased rate of ATP synthesis. The latter is achieved by induction of K + uniport during the gating of K ATP -dependent channels [1]. A regulated (K ATP - dependent channels) [1, 2] and unregulated [2] compo- nents are defined in the mechanism of voltage-gated electrophoretic K + uniport. However, energy dissipa- tion on the inner mitochondrial membrane, caused by maintenance of ionic homeostasis, is not significant and does not exceed 5% of the total energy consumption of mitochondria [1]. Understanding the processes of elec- trophoretic K + transport in mitochondria was achieved using the ionophore valinomycin, which increases the permeability of the inner mitochondrial membrane for the monovalent cations K + , Rb + , and Cs + [2–6]. Another important approach to studying the mechanisms of K + transport in mitochondria is to use its analogue Tl + , whose ionic radius is similar to that of K + [7]. A consid- erable swelling of nonenergized mitochondria in the presence of TlNO 3 showed that the inner mitochondrial membrane has a high permeability for Tl + [7]. Nonen- ergized mitochondria swell in the medium containing 20–40 mM thallium acetate, as in experiments with sodium acetate. This led to the assumption about the existence of Tl + /H + exchanger in the inner mitochon- drial membrane, which is analogous to Na + /H + exchanger [7]. Active swelling of energized mitochon- dria in thallium acetate led us to conclude that Tl + , sim- ilar to K + [1, 2, 5], is transported into mitochondria by electrophoretic uniport [4, 5, 7, 8]. Nonactin is a Tl + ionophore that increases Tl + accumulation by energized mitochondria [4, 9, 10]. In comparison with valinomy- cin, nonactin is a much stronger stimulator of swelling of energized mitochondria in Tl + -acetate [5, 9, 10]. In a medium containing both 86 Rb + and 204 Tl + , valinomycin stimulated mitochondrial accumulation of Rb + , whereas nonactin accelerated Tl + -uptake into the mito- chondrial matrix, which was due to a weak complex- ation of nonactin with Rb + and valinomycin with Tl + [5, 9, 10]. The concentrations of monovalent cations used in the isotop experiments (10 –8 –10 –5 M) are very far from the physiological K + concentrations in the cella which are approximately 130–140 mM. The effects of nonactin and valinomycin on mitochondria at high thallium concentrations were studied using the swelling procedure. In the medium containing TlNO 3 , valinomycin had a weak effect on the swelling of non- energized mitochondria, in which almost all K + in the matrix was substituted with Na + to exclude the effect of diffusion potential [9, 10]. The problem about influence of Tl + in the presence of nonactin on rat liver mitochondria has not been well studied. Hence, we investigated the accumulation of 204 Tl + in the presence of different concentrations of nonactin and the effect of Tl + –nonactin complex on 137 Cs + accumulation by energized mitochondria in the presence of valinomycin. Mitochondrial transport of Tl + was also studied using mitochodrial swelling in the media containing TlNO 3 or Tl-acetate. On the other hand, the effect of thallium acetate and nonactin on the enzymes containing SH-groups was assessed by the effect of these compounds on the mitochondrial respi- ration and the activity of monoamine oxidase. For the first time, it was shown that an increase in the nonactin concentration induced an increase in the accumulation coefficients (r) of 204 Tl + in mitochondria. It was found that the accumulation coefficients of 137 Cs + in the pres- ence of valinomycin considerably decreased in the presence of nonactin and Tl + . In the experiments with nonactin, passive swelling of mitochondria in TlNO 3 - containing medium was accelerated and their shrinkage Effect of Thallium Ions on Isolated Mitochondria of Rat Liver in the Presence of Nonactin S. M. Korotkov, E. R. Nikitina, V. V. Glazunov, and O. V. Yagodina Presented by Academician V.L. Sviderskii July 12, 2004 Received July 12, 2004 Sechenov Institute of Evolutionary Physiology and Biochemistry, Russian Academy of Sciences, pr. Morisa Toreza 44, St. Petersburg, 194223 Russia BIOCHEMISTRY, BIOPHYSICS, AND MOLECULAR BIOLOGY