Action potentials and membrane io pituitary cells (Ca2+ spike/Na+ spike/Ca2+-dependent K+ channel/corticotropin//4

The electrophysiological properties of the mouse anterior pituitary cell line AtT-20/D16-16 were investigated with intracellular and patch-clamp techniques. Clonal AtT-20/D16-16 cells were found to be electrically excitable, with most cells ex- hibiting spontaneous bursting action potentials. The mean burst rates varied from 1.4 Hz at -55 mV to 8.2 Hz at -25 mV, showing an approximately linear frequency-current relationship in the low current range. The bursts consisted of one to several fast Na+ spikes superimposed on a slow pacemaker potential, followed by a Ca2+ spike and a Ca2+-sensitive afterhyperpolarization. Removal of either Na+ or Ca2+ from the bathing medium led to cessation of spontaneous activity and the appearance of arrhythmic firing pat- terns. Single channel recordings revealed the presence of Ca2+- dependent K+ channels with unitary conductances of = 130 pS in physiological medium. These channels were activated by both in- tracellular Ca2+ and membrane depolarization. Addition of nor- epinephrine (10 /iM) led to increases in burst frequency and 3- endorphin secretion mediated by activation of ,3-adrenergic re-