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 channels) in intact clonal rat pituitary cells (GH(4) cells) was investigated using the cell-attached patch-clamp configuration, This method prevents loss of intracellular factors which might influence channel activity. BK channels are generally considered to be inactive at the resting membrane potential in excitable cells. However, at the resting potential (0 mV pipette potential), 40% of the cell-attached patches displayed spontaneously active BK channels, which remained active even at 20 mV hyperpolarization. The peptide thyroliberin (TRH) elevates the cytosolic Ca2+ concentration ([Ca2+](i)) in GH cells by IP3-induced release of Ca2+ from intracellular stores. This rise in [Ca2+](i) occurs concomitantly with membrane hyperpolarization. TRH stimulation caused activation 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+ ionophore ionomycin activated silent BK channels in three out of 10 cell-attached patches, and increased the activity of spontaneously active BK channels 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 potential and more negative membrane potentials. The channels may also be activated further by physiological elevations of [Ca2+](i) in the same potential range. Our results point towards new possible physiological roles 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 wide variety of cell types.