Regulation of 109Cd2+ Uptake into Isolated Neurohypophysial Peptidergic Nerve Terminals

Cadmium (109Cd2+) uptake was studied in a preparation of isolated neurohypophysial nerve terminals. By use of a filter separation method, together with a permeabilizing agent (Triton X-100), two cellular Cd2+ pools have been distinguished. The uptake into the intraterminal pool was governed mainly by a process that displayed saturable kinetics, with a Vmax of 0.15 nmol of Cd2+/mg of protein/min and a Km of 0.18 mM, consistent with a transport system. The superficially bound Cd2+ pool (Triton insensitive), which represented 30-50% of the total Cd2+ bound to the cellular system, was very sensitive to the ionic composition of the incubation medium. Reducing the extracellular Ca2+ or Na+ concentration caused a significant increase in the size of the Triton-insensitive Cd2+ pool. Whereas Na+ did not affect Cd2+ uptake, Ca2+ induced a small, but significant, increase of Cd2+ uptake into the terminals. It is concluded that there is a significant intraterminal uptake of Cd2+, which could explain several physiological effects of this ion.