RESUMO
AIMS: Steviol glycosides are the sweet components extracted from medicinal plant Stevia rebaudiana Bertoni, which have antihyperglycaemic effects. Steviol glucuronide (SVG) is the metabolite excreted in human urine after oral administration of steviol glycosides. We aimed to clarify whether SVG exerts direct insulin stimulation from pancreatic islets and to explore its mode of action. MATERIALS AND METHODS: Insulin secretion was measured after 60 minutes static incubation of isolated mouse islets with (a) 10-9-10-5 mol/L SVG at 16.7 mmol/L glucose and (b) 10-7 mol/L SVG at 3.3-16.7 mmol/L glucose. Islets were perifused with 3.3 or 16.7 mmol/L glucose in the presence or absence of 10-7 mol/L SVG. Gene transcription was measured after 72 hours incubation in the presence or absence of 10-7 mol/L SVG. RESULTS: SVG dose-dependently increased insulin secretion from mouse islets with 10-7 mol/L exerting the maximum effect in the presence of 16.7 mmol/L glucose (P < .001). The insulinotropic effect of SVG was critically dependent on the prevailing glucose concentration, and SVG (10-7 mol/L) enhanced insulin secretion at or above 11.1 mmol/L glucose (P < .001) and showed no effect at lower glucose concentrations. During perifusion of islets, SVG (10-7 mol/L) had a long-acting and apparently reversible insulinotropic effect in the presence of 16.7 mmol/L glucose (P < .05). Gene-transcript levels of B2m and Gcgr were markedly altered. CONCLUSION: This is the first report to demonstrate that SVG stimulates insulin secretion in a dose- and glucose-dependent manner from isolated mouse islets of Langerhans. SVG may be the main active metabolite after oral intake of steviol glycosides.
RESUMO
Isosteviol (ISV), a diterpene molecule, is an isomer of the backbone structure of a group of substances with proven antidiabetic capabilities. The aim of this study was to investigate if ISV elicits dynamic insulin release from pancreatic islets and concomitantly is able to ameliorate gluco-, lipo-, and aminoacidotoxicity in clonal ß-cell line (INS-1E) in relation to cell viability and insulin secretion. Isolated mice islets placed into perifusion chambers were perifused with 3.3 mM and 16.7 mM glucose with/without 10-7 M ISV. INS-1E cells were incubated for 72 h with either 30 mM glucose, 1 mM palmitate or 10 mM leucine with or without 10-7 M ISV. Cell viability was evaluated with a Cytotoxic Fluoro-test and insulin secretion was measured in Krebs-Ringer Buffer at 3.3 mM and 16.7 mM glucose. In the presence of 3.3 mM glucose, 10-7 M ISV did not change basal insulin secretion from perifused islets. However, at a high glucose level of 16.7 mM, 10-7 M ISV elicited a 2.5-fold increase (-ISV: 109.92 ± 18.64 ng/mL vs. +ISV: 280.15 ± 34.97 ng/mL; p < 0.01). After 72 h gluco-, lipo-, or aminoacidotoxicity in INS-1E cells, ISV treatment did not significantly affect cell viability (glucotoxicity, -ISV: 19.23 ± 0.83%, +ISV: 18.41 ± 0.90%; lipotoxicity, -ISV: 70.46 ± 3.15%, +ISV: 65.38 ± 2.81%; aminoacidotoxicity: -ISV: 8.12 ± 0.63%; +ISV: 7.75 ± 0.38%, all nonsignificant). ISV did not improve impaired insulin secretion (glucotoxicity, -ISV: 52.22 ± 2.90 ng/mL, +ISV: 47.24 ± 3.61 ng/mL; lipotoxicity, -ISV: 19.94 ± 4.10 ng/mL, +ISV: 22.12 ± 3.94 ng/mL; aminoacidotoxicity: -ISV: 32.13 ± 1.00 ng/mL; +ISV: 30.61 ± 1.54 ng/mL, all nonsignificant). In conclusion, ISV acutely stimulates insulin secretion at high but not at low glucose concentrations. However, ISV did not counteract cell viability or cell dysfunction during gluco-, lipo-, or aminoacidotoxicity in INS-1E cells.
