A Portulaca oleracea L. extract promotes insulin secretion via a K⁺(ATP) channel dependent pathway in INS-1 pancreatic β-cells

ABSTRACT

BACKGROUND/OBJECTIVE: This study was designed to investigate how a Portulaca oleracea 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 Ca2+ caused by POE treatment, the effect of POE on intracellular Ca2+ 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+ ATP channel (blocking insulin secretion) and by verapamil (a Ca2+ channel blocker). The insulinotropic effect of POE was not observed under Ca2+-free conditions in INS-1 pancreatic β-cells. When the cells were preincubated with a Ca2+ 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 Ca2+, which closely correlated with increases in the levels of insulin secretion. CONCLUSIONS: These findings indicate that POE stimulates insulin secretion via a K+ ATP channel-dependent pathway in INS-1 pancreatic β-cells.