Factors affecting the relative magnitudes of the ouabain-sensitive and the ouabian-insensitive fluxes of thallium ion in erythrocytes

A maximal rate of the ouabain-sensitive ²⁰⁴Tl influx in human erythrocytes can be attained at trace concentrations of Tl⁺ in Mg²⁺ isotonic media free of K⁺ and Na⁺. The maximal influx of Tl⁺ from isotonic Mg(NO₃)₂ at 20°C and pH 7.4 was $\text{0.45}\text{mM}\text{· 1}{}^{\mathrm{?1}}\text{· h}{}^{\mathrm{?1}}$ with a $\text{K}{}_{\mathrm{<mtext>m</mtext>}}$ of 0.025 mM. In contrast to the active influx of Tl⁺, the passive Tl⁺ fluxes were neither saturated nor influenced by external cations in the range of concentrations of Tl⁺ and K⁺ studied. The rate constants of Tl⁺ passive fluxes in human and cat erythrocytes can be related to pH by the equation $\text{log}\text{k}{}_{\mathrm{<mtext>in(out)</mtext>}}\text{= –A + B ·}\text{pH}$, where $\text{A}$ and $\text{B}$ are empirical constants for particular conditions. The apparent activation energy was 16 and 11 kcal/mol in sulphate and nitrate media, respectively. Tl⁺ and the alkali metal cations seem to overcome a common barrier in the erythrocyte membrane. Nevertheless, the rate of the passive penetration of Tl⁺ is about two orders of magnitude faster than those of K⁺ or Rb⁺. An extra non-Coulombic interaction between Tl⁺ and membrane ligands appears to be involved providing an accumulation of Tl⁺ somewhere in the vicinity of the membrane barrier and increasing the diffusion fluxes of Tl⁺ in both directions.