Effects of Divalent Cations on Outward Potassium Currents in Leech Retzius Nerve Cells

The present study examines the effects of divalent metals, cadmium (Cd²⁺) and manganese (Mn²⁺), on the outward potassium currents of Retzius cells in the hirudinid leeches Haemopis sanguisuga using conventional two-microelectrode voltageclamp techniques. The outward potassium current is activated by depolarization and plays an important role in determining both the neuronal excitability and action potential duration. A strong inhibition of the fast current and a clear reduction in the late currents of the outward current with 1 mM Cd²⁺ were obtained, which indicated that both components are sensitive to this metal. Complete blockage of the fast and partial reduction of the slow outward currents was observed after adding 1 mM Mn²⁺ to the extracellular fluid. These data show that the outward K⁺ current in leech Retzius nerve cells comprises at least two components: a voltage-dependent K⁺ current and a Ca²⁺- activated K⁺ current. These observations also indicate that Cd²⁺ is more eff ective than Mn²⁺ in blocking ion fl ow through these channels and that suppressing Ca²⁺-activated K⁺ outward currents can prolong the action potential in nerve cells.