Electric activity of rat anterior pituitary cells in vitro.

Regenerative responses were seen in most cells either after cessation of an inward current pulse or during an outward current pulse. Two cell groups were distinguished electrophysiologically. Type I cells showed action potentials with maximum rate of rise of 21.2+/-9.0 V/sec (mean+/-S.D., n=19), while type II cells generated small graded depolarising responses with maximum rate of rise less than 3 V/s. The resting potentials of type I and II cells were 31.8+/-14.9 mV (n=19) and 41.7+/-9.8 mV (n=31), respectively. The steady-state current/voltage relationship was linear for both cell types when the membrane potential was more negative than -60 mV. An outward rectification appeared when the membrane potential was more positive than -40 mV. The input resistance was smaller in type I cells (274+/-212 Momega, n=19) than in type II cells (1 112+/-456 Momega, n=16). Even in Na-free solution regenerative responses were observed in most cells. When the Ca2+ concentration was increased tenfold to 24 mM, the maximum rate of rise of the off-response increased from 1.9+/-0.8 V/s (n=11) to 5.7+/-2.12 V/s (n=5). All-or-none action potentials could be evoked in this Ca2+ rich solution. Action potentials of similar maximum rate of rise could be evoked after replacing 24 mM Ca2+ with isomolar Sr2+. Prolonged action potentials were seen after subsitution of Ca2+ for Ba2+. It is concluded that action potentials in most anterior pituitary cells have a Ca component, which in type I cells is additional to a Na component.