Subthreshold α 2-adrenergic activation with clonidine counteracts GLP-1 potentiation of glucose-stimulated insulin secretion. A standard insulin radioimmunoassay was employed to examine the insulin secretory response of cells treated with the subthreshold concentration of clonidine, the ED50 concentration of GLP-1 and their combinations following a stepwise stimulation with glucose from 3 to 11 mM for 30 min. Treatment with 11 mM glucose caused a significant insulin secretion as compared with that with 3 mM glucose (, ). Cells incubated with clonidine at the subthreshold concentration 3 nM and control cells displayed similar insulin secretory responses to 11 mM glucose (, ). However, cells treated with GLP-1 at the ED50 concentration 0.1 nM displayed significantly enhanced insulin secretion in comparison with untreated cells following 11 mM glucose stimulation (, ). Furthermore, cells exposed to the ED50 concentration of GLP-1 plus the subthreshold concentration of clonidine exhibited significantly less insulin secretion than cells treated with the ED50 concentration of GLP-1 alone following 11 mM glucose stimulation (, ). Control cells and cells treated either with the ED50 concentration of GLP-1 plus the subthreshold concentration of clonidine or the subthreshold concentration of clonidine alone released similar amounts of insulin in response to 11 mM glucose (, ). versus 3 mM glucose-treated group, versus 11 mM glucose-treated group. versus the ED50 concentration of GLP-1 plus the subthreshold concentration of clonidine group subjected to 11 mM glucose incubation.
