Unresponsiveness to glibenclamide during chronic treatment induced by reduction of ATP-sensitive K+ channel activity

The insulin response to the sulfonylurea glibenclamide was markedly impaire d in pancreatic beta-cell line MING cells with chronic glibenclamide treatm ent (MIN6-Glib), The intracellular calcium concentration increased only sli ghtly in response to glibenclamide in MIN6-Glib, While the properties of th e voltage-dependent calcium channels were not altered, the conductance of t he K-ATP channels, the primary target of glibenclamide, was significantly r educed in MIN6-Glib, The ATP-sensitive K+ (K-ATP) channels in MIN6 cells co mprise inwardly rectifying K+ channel member Kir6.2 subunits and sulfonylur ea receptor (SUR) 1 subunits, MIN6 cells have both high- and low-affinity b inding sites for glibenclamide, The binding affinities at these two sites w ere unchanged, but the maximum binding capacities at both sites were simila rly increased by chronic glibenclamide treatment. Both SUR1 and Kir6.2 mRNA levels were not altered, but SUR1 protein was rather increased in MIN6-Gli b. In addition, electron microscopic examination revealed a majority of the SUR1 to be present in a cluster near the plasma membrane in control MING, while it tends to be distributed in the cytoplasm in MIN6-Glib, These data suggest that chronic glibenclamide treatment causes the defect in acute gli benclamide-induced insulin secretion by reducing the number of functional K -ATP channels on the plasma membrane of the beta-cells.