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Nonrespiring rat-liver mitochondria swell in media containing high concentrations of thallous nitrate, indicating passive penetration of TI+ This swelling could be further stimulated by 10 nM or more nonactin while even 1μM valinomycin was without effect. Nonactin was also much more potent than valinomycin in stimulating swelling of respiring mitochondria in the presence of thallous acetate. It is evident that nonactin acts as a potent lonophore of TI + able to promote both the passive and energized uptake of TI + in mitochondria. The distribution of TI+, present in trace concentrations below 1 mM, was measured during energisation by respiration both in the presence and absence of ionophores. Respiration induced net uptake of TI+ only in the presence of lonophores, though TI + as a permeant cation was expected to sense respiration-induced changes in the membrane potential. The data may be interpreted as indicating that no transmembrane potential is formed upon energisatiom, but localized fields, which are able to interact with the lipophilic ionophore complexes of TI +, but not with the hydrophilic cation Tl+ This interpretation is valid only if thermodynamic equilibrium has been reached. [ABSTRACT FROM AUTHOR]