Uptake of Thallous Ions by Mitochondria is Stimulated by Nonactin but not by Respiration Alone

Abstract

Nonrespiring rat-liver mitochondria swell in media containing high concentrations of thallous nitrate, indicating passive penetration of Tl'. This swelling could be further stimulated by 10 nM or more nonactin while even 1 pM 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 ionophore of T1' able to promote both the passive and energized uptake of T1' 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 T1+ only in the presence of ionophores, though T1+ 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 energisation, 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.

Increase in the toxic effects of Tl+ on isolated rat liver mitochondria in the presence of nonactin

“…Nevertheless, despite their increased permeability, the energized mitochondria were rapidly contracted to the initial volume even in the medium with 50 mM TlNO 3 and nonactin ( Figure 1B), which agrees well with the minimal inhibition of DNP-stimulated respiration in the medium with 50 mM Tl + and nonactin ( Figure 3C). Swelling of energized mitochondria increased in the series with 10-50 mM Tl acetate medium (Figure 2A), which was earlier recorded only with 25 mM Tl acetate medium [4,7]. It is known that nonactin, a cyclic ionophore, has a high chemical affinity to Tl + ions [4] and accelerates influx of Tl + in the mitochondrial matrix [10].…”

“…However, it was recently reported that the K + /H [4,36,37], it can be suggested that both release of Tl + from energized mitochondria swollen in the TlNO 3 medium ( Figure 1A) and swelling of nonenergized mitochondria in the Tl acetate medium (Figure 2A) are likely to occur via the mitochondrial K + /H + exchanger, but not a special Tl + /H + exchanger as postulated by Saris et al [3]. On the other hand, the faster swelling of nonenergized mitochondria in the medium with nonactin and 50-75 mM of TlNO 3 ( Figure 1B) or 50 mM of Tl acetate ( Figure 2B) and some retardation in the contraction of energized mitochondria swollen in the medium with TlNO 3 and nonactin ( Figure 1B) are likely to be due to an increase of permeability of the inner mitochondrial membrane to Tl + ions in the presence of nonactin [4,10]. Nevertheless, despite their increased permeability, the energized mitochondria were rapidly contracted to the initial volume even in the medium with 50 mM TlNO 3 and nonactin ( Figure 1B), which agrees well with the minimal inhibition of DNP-stimulated respiration in the medium with 50 mM Tl + and nonactin ( Figure 3C).…”

“…The study of Na-loaded mitochondria was carried out to eliminate the action of potassium diffusion potential after the addition of valinomycin in the medium. Uptake of 204 Tl + increased essentially after injection of nonactin in the medium ( Figure 4A) owing to a high affinity of nonactin to Tl + ions [4]. The combined effect of nonactin and valinomycin on the uptake of 204 Tl + by Na-loaded mitochondria is shown in Figure 4B.…”

“…It is known that swelling of mitochondria in the hypoosmotic (160 mOsm) medium enables revealing the involvement of latent mechanisms (K + uniporter and K + /H + antiporter) in K + [1,6] and Tl + transport [3,4] across the inner mitochondrial membrane. Nonenergized mitochondria swelled weakly in the medium containing 5-25 mM TlNO 3 ( Figure 1A).…”

“…It was found that swelling of energized rat liver mitochondria in the medium containing 25 mM Tl acetate was accelerated in the presence of nonactin [4] or of valinomycin [4,7]. In the medium with 40 mM TlNO 3 , nonactin to a greater extent than valinomycin accelerated the swelling of nonenergized rat liver mitochondria with matrix K + depleted by Na + to exclude the action of potassium diffusion potential [4]. Electron micrograph studies of hepatocytes of rats exposed to chronic thallium intoxication [13,14] and of isolated mitochondria [15] showed that the general toxic effect of Tl + consisted in disruption of intracellular membranes and swelling of mitochondria.…”