Electrical activity of rat anterior pituitary cells in vitro

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Abstract
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 MΩ, n=19) than in type II cells (1 112 ± 456 MΩ, 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 substitution 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.