Rebaudioside A directly stimulates insulin secretion from pancreatic beta cells: a glucose-dependent action via inhibition of ATP-sensitive K+-channels.

Recently, we showed that rebaudioside A potently stimulates the insulin secretion from isolated mouse islets in a dose-, glucose- and Ca²⁺-dependent manner. Little is known about the mechanisms underlying the insulinotropic action of rebaudioside A. The aim of this study was to define the signalling system by which, rebaudioside A acts. Isolated mouse islets were used in the cAMP[¹²⁵I] scintillation proximity assay to measure total cAMP level, and in a luminometric method to measure intracellular ATP and ADP concentrations. Conventional and permeabilized whole-cell configuration of the patch-clamp technique was used to verify the effect of rebaudioside A on ATP-sensitive K⁺-channels from dispersed single β cells from isolated mouse islets. Insulin was measured by radioimmunoassay from insulinoma MIN6 cells. In the presence of 16.7 mM glucose, the addition of the maximally effective concentration of rebaudioside A (10⁻⁹ M) increased the ATP/ADP ratio significantly, while it did not change the intracellular cAMP level. Rebaudioside A (10⁻⁹ M) and stevioside (10⁻⁶ M) reduced the ATP-sensitive potassium channel (K_ATP) conductance in a glucose-dependent manner. Moreover, rebaudioside A stimulated the insulin secretion from MIN6 cells in a dose- and glucose-dependent manner. In conclusion, the insulinotropic effect of rebaudioside A is mediated via inhibition of ATP-sensitive K⁺-channels and requires the presence of high glucose. The inhibition of ATP-sensitive K⁺-channels is probably induced by changes in the ATP/ADP ratio. The results indicate that rebaudioside A may offer a distinct therapeutic advantage over sulphonylureas because of less risk of causing hypoglycaemia. [ABSTRACT FROM AUTHOR]