Involvement of calmodulin in glucagon-like peptide 1(7-36) amide-induced inhibition of the ATP-sensitive K+ channel in mouse pancreatic beta-cells

The present investigation was designed to examine whether calmodulin is inv olved in the inhibition of the ATP-sensitive K+ (K-ATP) channel by glucagon -like peptide 1(7-36) amide (GLP-1) in mouse pancreatic beta -cells. Membra ne potential, single channel and whole-cell currents through the K-ATP chan nels, and intracellular free Ca2+ concentration ([Ca2+](i)) were measured i n single mouse pancreatic beta -cells. Whole-cell patch-clamp experiments w ith amphotericin-perforated patches revealed that membrane conductance at a round the resting potential is predominantly supplied by the K-ATP channels in mouse pancreatic beta -cells. The addition of 20 nM GLP-1 in the presen ce of 5 mM glucose significantly reduced the membrane K-ATP conductance, ac companied by membrane depolarization and the generation of electrical activ ity. A calmodulin inhibitor N-(6-aminohexyl)-5-chloro-1-naphthalenes (W-7, 20 muM I) completely reversed the inhibitory actions of GLP-1 on the membra ne K-ATP conductance and resultant membrane depolarization. Cell-attached p atch recordings confirmed the inhibition of the K-ATP channel activity by 2 0 nM GLP-1 and its restoration by 20 muM W-7 or 10 muM calmidazolium at the single channel level. Bath application of 20 muM W-7 also consistently abo lished the GLP-1-evoked increase in [Ca2+](i) in the presence of 5 mM gluco se. These results strongly suggest that the mechanisms by which GLP-1 inhib its the K-ATP channel activity accompanied by the initiation of electrical activity in mouse pancreatic beta -cells include a calmodulin-dependent mec hanism in addition to the well-documented activation of the cyclic AMP-prot ein kinase A system.