Mechanism of mitochondrial transport of thallous ions

Abstract

Rat liver mitochondria were found to swell under nonenergized conditions when suspended in media containing 30-40 mM TINO3. Respiration on succinate caused a rapid contraction of mitochondria swollen under nonenergized conditions. In the presence of thallous acetate, there was a rapid initial swelling under nonenergized conditions until a plateau was reached; respiration on succinate then caused a further swelling. Trace amounts of 204Tl (less than 100 microM) equilibrated fairly rapidly across the mitochondrial membrane. The influx of Tl+ was able to promote the decay not only of a valinomycin-induced K+-diffusion potential but also of respiration-generated fields in the inner membrane in accordance with the electrophoretic nature of Tl+ movement. Efflux of Tl+ showed a half-time of about 10 sec at 20 degrees C and was not affected appreciably by the energy state. Efflux was retarded by Mg2+ and by lowering the temperature. The data indicate that Tl+ when present at high concentrations, 30mM or more, distributes across the mitochondrial inner membrane both in response to electrical fields and to delta pH. In energized mitochondria the uptake of Tl+ would occur electrophoretically, while Tl+/H+ exchange would constitute a leak. In the presence of NO3-, the movements of Tl+ are determined by that of NO3-, indicating short-range coupling of electrical forces. At low concentrations of Tl+, 5 mM or less, there was no indication of a Tl+/H+ exchange, which appears to be induced by high concentrations of Tl+.

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

“…When nonrespiring mitochondria under corresponding conditions are suspended in media containing thallous acetate, there is a fast initial swelling until a plateau has been reached, and respiration will then cause further swelling [9]. The rate of this energized swelling, Fig.2, trace 1, was found to be somewhat stimulated by 10 n M valinomycin, trace 2, and strongly increased by 10 n M nonactin, trace 3.…”

“…Nonrespiring mitochondria swell when suspended in media containing high concentrations of thallous nitrate [9]. Fig.…”

“…It was therefore of interest to study the effect of energisation on the distribution of the hydrophilic TI' alone and as its lipophilic nonactin-complex. In the former case Tl', which is able to move electrophoretically [8,9], would be expected to become distributed at equilibrium across the membrane according to the Nernst equation, and its distribution might then indicate the transmembrane potential. In the latter case TI' would be transported as a lipophilic cation, able to interact with the Stern layer at the boundary between the membrane and the bulk water phase [2].…”

“…4B). Valinomycin seems to cause a response of T1' as to a diffusion potential both in nonrespiring [8] and respiring a valinomycin-T1' complex [9], even though such a complex may be formed at high concentrations of T1' under suitable experimental conditions (Fig. 2) [I I].…”

“…Due to its electrophoretic movement, TI+ at low concentrations is able to abolish the signal of safranine (one of the lipophilic probes of the membrane potential [7]) formed by the valinomycininduced efflux of K f [S] or by respiration [9]. TI+ was found to be distributed according to the membrane potential in erythrocytes [lo].…”