Studies on the Crystalline Lens

XVIII. Kinetics of Thallium (Tl⁺) Transport in Relation to That of the Alkali Metal Cations

¹ Institute of Biological Sciences, Oakland University, Rochester, Mich., Department of Pathology, University of Colorado Medical Center, Denver, Colo.

²⁰⁴Tl accumulates rapidly in cultured rabbit lenses reaching concentration levels greatly in excess of those present in the bathing media, thus suggesting that it enters the lens by active transport. The parameters describing kinetics of thallium transport were evaluated by fitting theoretical curves to experimental data showing the movement of ²⁰⁴Tl into and out of cultured rabbit lenses in the presence of various concentrations of nonlabeled thallium. Curves were calculated on the assumption that movement occurs in accordance with a theoretical model of a pump-leak system which states that active transport into the lens involves a carrier-mediated system and passive transport dependent on diffusional flux along both electrical and chemical gradients. Values of Km and Vmax for the carrier system are 0.15 mM and 0.375 µmoles per hour per lens, respectively. The coefficient for active transport Kp, for the pump, under physiologic conditions, is 2.25 hours^-1 and for the leak Kd, 0.105 hour^-1, both about three times that for potassium. Theoretical curves calculated from the aforementioned transfer coefficients for thallium, and found previously for potassium, rubidium, and cesium, adequately describe both inhibition of ²⁰⁴Tl transport by varying concentrations of the alkali cations and inhibition of transport of alkali cations by different levels of thallium, thus providing strong evidence for the identity of the carrier system responsible for the transport of all four cations. The apparent affinity of thallium for the carrier is approximately seven times that of potassium and rubidium and almost 25 times that for cesium, whereas the magnitude of Vmax is about one quarter that for the alkali cations. Ki for ouabain for the thallium transport system is the same as that of potassium and not significantly higher than that observed elsewhere for the inactivation of Na-K ATPase and the uptake of rubidium. This is further evidence of the similarity between the carrier for thallium and the alkali cations and is in accord with the belief that the active transport system is dependent upon the action of the enzyme Na-K ATPase. The permeability of the lens membranes to thallium, relative to the alkali cations, is unrelated to ion size, suggesting that, like the latter, permeability is dependent upon absorptive rather than frictional forces. Toxicity of thallium is manifest by opacity which develops in all lenses that have accumulated thallium to levels of about 30 mM.

Key Words: lens • thallium • alkali cations • cation transport • kinetics of transport • permeability • diffusion • Na-K ATPase • carrier systems

Submitted on August 5, 1971
Accepted on September 28, 1971