Characterization of ATP-sensitive potassium channels functionally expressed in pituitary GH₃ cells

ATP-sensitive K⁺ (K(ATP)) channels have been characterized in pituitary GH₃ cells with the aid of the patch-clamp technique. In the cell-attached configuration, the presence of diazoxide (100 μM) revealed the presence of glibenclamide-sensitive K(ATP) channel exhibiting a unitary conductance of 74 pS. Metabolic inhibition induced by 2,4-dinitrophenol (1 mM) or sodium cyanide (300 μM) increased K(ATP) channel activity, while nicorandil (100 μM) had no effect on it. In the inside-out configuration, Mg-ATP applied intracellularly suppressed the activity of K(ATP) channels in a concentration-dependent manner with an IC₅₀ value of 30 μM. The activation of phospholipase A₂ caused by mellitin (1 μM) was found to enhance K(ATP) channel activity and further application of aristolochic acid (30 μM) reduced the mellitin-induced increase in channel activity. The challenging of cells with 4,4'-dithiodipyridine (100 μM) also induced K(ATP) channel activity. Diazoxide, mellitin and 4,4'-dithiodipyridine activated the K(ATP) channels that exhibited similar channel-opening kinetics. In addition, under current-clamp conditions, the application of diazoxide (100 μM) hyperpolarized the membrane potential and reduced the firing rate of spontaneous action potentials. The present study clearly indicates that K(ATP) channels similar to those seen in pancreatic β cells are functionally expressed in GH₃ cells. In addition to the presence of Ca²⁺-activated K⁺ channels, K(ATP) channels found in these cells could thus play an important role in controlling hormonal release by regulating the membrane potential.