ATP-sensitive K+ conductance in pancreatic zymogen granules: block by glyburide and activation by diazoxide

The properties of transporters (or channels) for monovalent cations in the membrane of isolated pancreatic zymogen granules were characterized with an assay measuring bulk cation influx driven by a proton diffusion potential. The proton diffusion potential was generated by suspending granules in an isotonic monovalent cation/acetate solution and increasing the proton conductance of the membrane with a protonophore. Monovalent cation conductance had the sequence Rb+ > K+ > NA+ > Cs+ > LI+ > N-methyl glucamine+. The conductance could be inhibited by Ca2+, Mg2+, Ba2+, and pharmacological agents such as quinine, quinidine, glyburide and tolbutamide, but not by 5 mM tetra-ethyl ammonium or 5 mM 4-aminopyridine, when applied to the cytosolic surface of the granule membrane. Over 50% of K+ conductance could be inhibited by millimolar concentrations of ATP or MgATP. The inhibition by MgATP, but not by ATP itself, was reversed by the K+ channel opener diazoxide. The inhibitory effect is probably by a noncovalent interaction since it could be mimicked by nonhydrolyzable analogs of ATP and by ADP. The reversal of MgATP inhibition by diazoxide may be mediated by phosphorylation since it was not affected by dilution, and was blocked by the protein kinase inhibitor H7. The properties of the K+ conductance of pancreatic zymogen granule membranes are similar to those of ATP-sensitive K+ channels found in the plasma membrane of insulin-secreting islet cells, neurons, muscle, and renal cells.