The calcium action potential and a prolonged calcium dependent after‐hyperpolarization in mouse neuroblastoma cells.

1. Action potentials elicited in solutions with elevated [Ca2+] (1.8‐40 mM) have been studied in differentiated cells of mouse neuroblastoma clone N1E‐115 in tissue culture. 2. The action potential in high [Ca2+] solutions containing eithr Na+ or Tris is followed by a prolonged after‐hyperpolarization (a.h.p.) lasting 0.5‐4 sec. The a.h.p. reverses sign between ‐75 and ‐85 mV. 3. Externally applied tetraethylammonium (TEA, 15 mM) increases the Ca2+ spike overshoot, prolongs the falling phase and enhances the a.h.p. duration. The a.h.p. is inhibited by Ca2+ antagonists such as La3+, Co2+ and Mn2+. 4. After replacement of Ca2+ by Ba+ or Sr2+ (20mM) action potentials can still be elicited in Na+‐free solution, but no a.h.p. is observed. 5. Increasing [Ca2+] from 1.8 up to 20 mM results in an increased capability of neuroblastoma cells to fire repetitively and in a consistent reduction of the firing rate from about 4‐10 sec‐1 to 0.5‐1.8 sec‐1. 6. It is concluded that Ca2+ entry during the action potential activates a TEA‐resistant K+ conductance which gives rise to the prolonged a.h.p. Data from repetitively firing cells are consistent with the view that the a.h.p. plays a role in the regulation of low‐frequency firing.