ATP-sensitive K⁺ channel mediates the zinc switch-off signal for glucagon response during glucose deprivation

OBJECTIVE - The intraislet insulin hypothesis proposes that glucagon secretion during hypoglycemia is triggered by a decrease in intraislet insulin secretion. A more recent hypothesis based on in vivo data from hypoglycemic rats is that it is the decrease in zinc cosecreted with insulin from β-cells, rather than the decrease in insulin itself, that signals glucagon secretion from α-cells during hypoglycemia. These studies were designed to determine whether closure of the α-cell ATP-sensitive K⁺ channel (K _ATP channel) is the mechanism through which the zinc switch-off signal triggers glucagon secretion during glucose deprivation. RESEARCH DESIGN AND METHODS - All studies were performed using perifused isolated islets. RESULTS - In control experiments, the expected glucagon response to an endogenous insulin switch-off signal during glucose deprivation was observed in wild-type mouse islets. In experiments with streptozotocin-treated wild-type islets, a glucagon response to an exogenous zinc switch-off signal was observed during glucose deprivation. However, this glucagon response to the zinc switch-off signal during glucose deprivation was not seen in the presence of nifedipine, diazoxide, or tolbutamide or if K_ATP channel knockout mouse islets were used. All islets had intact glucagon responses to epinephrine. CONCLUSIONS - These data demonstrate that closure of K_ATP channels and consequent opening of calcium channels is the mechanism through which the zinc switch-off signal triggers glucagon secretion during glucose deprivation.