Glucagon induces suppression of ATP-sensitive K⁺ channel activity through a Ca²⁺/calmodulin-dependent pathway in mouse pancreatic β-cells

Glucagon is known to increase intracellular cAMP levels and enhance glucose-induced electrical activity and insulin secretion in pancreatic β- cell perfused with Krebs-Ringer bicarbonate solution. The present experiments were aimed at evaluation of the hypothesis that changes in β-cells ATP- sensitive K⁺ (K((ATP))) channel activity are involved in the glucagon- induced enhancement of electrical activity. Channel activity was recorded using the cell-attached configuration of the patch-clamp technique. Addition of glucagon (2.9 x 10^-7 M) in the presence of 11.1 mM glucose caused closure of K((ATP)) channels followed by an increase in the frequency of biphasic current transients (action currents) due to action potential generation in the cell. Three calmodulin-antagonists (W-7, chlorpromazine, and trifluoperazine) restored with similar efficacy K((ATP)) channel activity in cells being exposed to glucagon. At 2.8 mM glucose, glucagon did not affect K((ATP)) channel activity until Ca²⁺ was released from Nitr-5 by flash photolysis, at which point channel activity was transiently suppressed. Similar effects were seen when db-cAMP was used instead of glucagon. These results support the view that glucagon and other cAMP-generating agonists enhance glucose-induced β-cell electrical activity through a Ca²⁺/calmodulin dependent-closure of K((ATP)) channels.