The Movement of Thallium Ions in Muscle

Measurements have been made of the fluxes of thallous ions (Tl⁺) across the membrane of frog sartorius muscle fibers. These show that at an external concentration of 74 µM the influx is about 270 x 10^-15 moles/cm.² sec., while the efflux from a muscle with an internal concentration equal to the above is 5 x 10^-15 moles/cm.² sec. The efflux is increased of the order of 300-fold during a muscle twitch, and Tl⁺ reach a steady-state distribution between fiber water and Ringer solution that is very close to the corresponding ratio for K⁺. High concentrations of Tl⁺ depolarize the membrane about 58 mv. for a tenfold increase in external concentration. The results obtained are consistent with the view that the muscle fiber membrane cannot distinguish between the toxic heavy metal Tl⁺ and K⁺, provided that the concentrations of the former ion are kept low. High concentrations of Tl⁺, if allowed to act for an appreciable period of time, lead to irreversible damage to muscle.