A ROLE FOR A BACKGROUND SODIUM CURRENT IN SPONTANEOUS ACTION-POTENTIALS AND SECRETION FROM RAT LACTOTROPHS

We have used the perforated-patch variation of whole cell patch-clamp techniques, measurements of cytosolic calcium with use of fura 2, and secretion measurements with use of the reverse-hemolytic plaque assay to address the role of depolarizing background currents in maintaining spontaneous action potentials and spontaneous secretion from rat lact otrophs in primary culture. Replacement of bath sodium with tris(hydro xymethyl)aminomethane or N-methyl-D-glucamine caused a dramatic hyperp olarization of the cells, a cessation of spontaneous action potentials , and an increase in input resistance of cells. Tetrodotoxin had no ef fect on spontaneous action potentials, and removal of bath calcium sto pped spiking but did not hyperpolarize the cells. The hyperpolarizatio n in response to removal of bath sodium was associated with a decrease in cytosolic calcium levels. Finally, removal of bath sodium caused a decrease in spontaneous secretion of prolactin from lactotrophs. Thes e data suggest that a background sodium current is essential to drive the membrane to threshold for firing spontaneous calcium-dependent act ion potentials in lactotrophs. This, in turn, results in elevated intr acellular calcium, which supports spontaneous secretion of prolactin f rom these cells.