Calcium Component to Action Potentials in Rat Pars Intermedia Cells

1. The ionic dependence of the action potential of rat pars intermedia cells was investigated by using intracellular recording techniques. 2. In the presence of tetrodotoxin (TTX, 5 x 10(-6)M), the action potentials evoked by passing depolarizing current through the recording electrode were abolished, confirming that they are mainly dependent on Na; however, when tetraethylammonium (TEA, 10 mM) was added to the TTX-containing solution the imposed depolarizations triggered all-or-none regenerative potentials indicative of involvement of another ion. 3. There TTX-insensitive regenerative potentials persisted when the cells were perifused with Na-free solution but were severely reduced or abolished by Ca-free solution. This suggests that the ion producing these potentials is Ca. 4. These Ca action potentials were suppressed by Ni, Co and Mn in concentrations that did not suppress the "Na spikes' recorded in the absence of TTX and TEA. 5. Sr and Ba could substitute for Ca in maintaining the action potentials recorded in the presence of TTX. These ions also prolonged the duration of these action potentials. 6. The demonstration of a Ca component to the predominantly Na-dependent action potentials of pars intermedia cells heightens the possibility that these action potentials participate in the regulation of secretion.