BK CHANNELS IN INTACT CLONAL RAT PITUITARY-CELLS ARE ACTIVATED BY PHYSIOLOGICAL ELEVATIONS OF THE CYTOSOLIC CA2+ CONCENTRATION AT THE NORMAL RESTING POTENTIAL

Activation of large conductance Ca2+-activated K+ channels (BK channel s) in intact clonal rat pituitary cells (GH(4) cells) was investigated using the cell-attached patch-clamp configuration, This method preven ts loss of intracellular factors which might influence channel activit y. BK channels are generally considered to be inactive at the resting membrane potential in excitable cells. However, at the resting potenti al (0 mV pipette potential), 40% of the cell-attached patches displaye d spontaneously active BK channels, which remained active even at 20 m V hyperpolarization. The peptide thyroliberin (TRH) elevates the cytos olic Ca2+ concentration ([Ca2+](i)) in GH cells by IP3-induced release of Ca2+ from intracellular stores. This rise in [Ca2+](i) occurs conc omitantly with membrane hyperpolarization. TRH stimulation caused acti vation of BK channels in nine out of 30 silent cell-attached patches, and caused enhanced channel activity in seven out. of 29 cell-attached patches containing spontaneously active BK channels. The Ca2+ ionopho re ionomycin activated silent BK channels in three out of 10 cell-atta ched patches, and increased the activity of spontaneously active BK ch annels in seven out of 16 cell-attached patches. The pipette potential was clamped to 0 mV in all these experiments. We conclude that the BK channels in GH(4) cells may be active at the resting membrane potenti al and more negative membrane potentials. The channels may also be act ivated further by physiological elevations of [Ca2+](i) in the same po tential range. Our results point towards new possible physiological ro les for the BK channels in GH(4) cells. This is in agreement with the emerging picture of BK channels highly sensitive to [Ca2+](i) in a wid e variety of cell types.