Properties of a Ca2+ activated K+ conductance in Helix neurones investigated by intracellular Ca2+ ionophoresis.

1. Membrane current responses produced by intracellular ionophoretic injection of Ca2+ ions into voltage clamped Helix neurones were investigated. 2. The Ca2+ activated current depends on the intensity and duration of the Ca2+ injection as well as on the membrane potential. The current-voltage relation is non-linear and shows strong outward rectification. 3. The Ca2+ activated current reverses near the equilibrium potential for K+ ions and the associated current fluctuations disappear at the K+ reversal potential, which indicates a single current source, probably activation of K+ channels. 4. The Ca2+ activated K+ conductance is voltage dependent and changes e-fold per 31 mV membrane depolarization. 5. Extracellular tetraethylammonium, gallamine and quinidine block the Ca2+ activated K4 current. Extracellular 4-aminopyridine has no blocking effect. 6. Sustained injection of high amounts of Ca2+ irreversibly suppresses the outward current. 7. The K+ outward current evoked by repeated Ca2+ injection exhibits summation and facilitation.