A L. extract promotes insulin secretion via a K channel dependent pathway in INS-1 pancreatic β-cells.

BACKGROUND/OBJECTIVE: This study was designed to investigate how a L. extract (POE) stimulates insulin secretion in INS-1 pancreatic β-cells.

MATERIALS/METHOD: INS-1 pancreatic β-cells were incubated in the presence of various glucose concentrations: 1.1 or 5.6, 16.7 mM glucose. The cells were treated with insulin secretagogues or insulin secretion inhibitor for insulin secretion assay using an insulin ELISA kit. In order to quantify intracellular influx of Ca caused by POE treatment, the effect of POE on intracellular Ca in INS-1 pancreatic β-cells was examined using Fluo-2 AM dye.

RESULTS

POE at 10 to 200 µg/mL significantly increased insulin secretion dose-dependently as compared to the control. Experiments at three glucose concentrations (1.1, 5.6, and 16.7 mM) confirmed that POE significantly stimulated insulin secretion on its own as well as in a glucose-dependent manner. POE also exerted synergistic effects on insulin secretion with secretagogues, such as L-alanine, 3-isobutyl-1-methylxanthine, and especially tolbutamide, and at a depolarizing concentration of KCl. The insulin secretion caused by POE was significantly attenuated by treatment with diazoxide, an opener of the K channel (blocking insulin secretion) and by verapamil (a Ca channel blocker). The insulinotropic effect of POE was not observed under Ca-free conditions in INS-1 pancreatic β-cells. When the cells were preincubated with a Ca fluorescent dye, Fluo-2 (acetoxymethyl ester), the cells treated with POE showed changes in fluorescence in red, green, and blue tones, indicating a significant increase in intracellular Ca, which closely correlated with increases in the levels of insulin secretion.

CONCLUSIONS

These findings indicate that POE stimulates insulin secretion via a K channel-dependent pathway in INS-1 pancreatic β-cells.