The incretin GLP-1 concentration-dependently potentiates glucose-stimulated insulin secretion from INS-1 cells. A stepwise increase in glucose concentration from 3 to 11 mM for 30 min was used to induce static insulin secretion from cells exposed to different concentrations of GLP-1. Released insulin was measured by a standard insulin radioimmunoassay. 11 mM glucose incubation (closed circle at the far left) caused a significant insulin secretion from cells preincubated with 3 mM glucose (, versus 3 mM glucose-treated group represented by an open circle). GLP-1 at concentrations ranging from 0.0001 to 1000 nM potentiated glucose-stimulated insulin secretion in a concentration-dependent manner. GLP-1 at concentrations of 0.1 nM and higher significantly enhanced insulin release from cells stimulated with 11 mM glucose (, versus 11 mM glucose-treated group). The subthreshold and ED50 concentration of GLP-1 were estimated to be 0.01 and 0.1 nM, respectively. versus 11 mM glucose-treated group.
