Voltage-clamp studies of a slow inward current in bullfrog sympathetic ganglion cells.

Voltage-dependent inward membrane currents of bullfrog sympathetic ganglion cells were analyzed with the voltage clamp method. Two distinct inward currents, a tetrodotoxin (TTX)-sensitive fast inward current (INa) and a TTX-insensitive slow inward current (Isi), were recorded in Ringer solution. The Isi was markedly depressed by removal of external Ca2+ and by the addition of Mn2+, Co2+ or D-600. This suggests that the Isi is, almost exclusively, due to Ca2+. These results indicate that under physiological conditions a Ca2+ influx is induced during initiation of action potentials in these ganglion cells.