Stimulation of insulin secretion by an aqueous extract of Gymnema sylvestre: role of intracellular calcium
H Asare-Anane, GC Huang, SA Amiel, PM Jones & SJ Persaud
Plant-derived extracts have long been used to treat Type 2 diabetes mellitus (T2DM), but their effects have not been rigorously investigated at the cellular level in vitro. The aims of the current study were to determine the effects of a virgin isolate of Gymnema sylvestre (GS), designated OSA, on pancreatic β-cell function.
Materials and Methods
Cell viability was measured by Trypan blue exclusion tests, [Ca2+]i was determined by single cell microfluorimetry and insulin secretion was measured by radioimmunoassay.
OSA (0.06–2 mg/ml) caused a dose-dependent increase in insulin secretion from MIN6 insulin-secreting cells at both 2mM and 20mM glucose, with statistically significant (P<0.05) effects at ≥0.125 mg/ml. Concentrations of OSA≥0.5 mg/ml decreased cell viability, as assessed by an increase in cellular Trypan blue uptake, so all future experiments were performed with 0.125 mg/ml OSA. The stimulatory effects of OSA on insulin secretion were significantly reduced in the absence of extracellular Ca2+, indicating that at least part of the effect of OSA was dependent on Ca2+ influx. OSA (0.125 mg/ml) stimulated a rapid, transient and reversible elevation in [Ca2+]i in Fura 2-loaded MIN6 cells. OSA significantly stimulated insulin secretion from isolated human islets of Langerhans in perifusion experiments. It caused a rapid increase in insulin secretion at 2mM glucose (peak: 266±54% basal; plateau: 166±12% basal, n=3), and this was readily reversible indicating that insulin release was not secondary to membrane damage. 0.125 mg/ml OSA also potentiated insulin secretion from human islets at 20 mM glucose (rapid 4.4-fold increase over the 20 mM glucose-induced plateau phase).
OSA, a water-soluble extract of GS, caused reversible increases in intracellular calcium and insulin secretion in mouse and human β-cells when used at a concentration (0.125 mg/ml) that did not compromise cell viability. These data suggest that extracts derived from GS may be useful as therapeutic agents for the stimulation of insulin secretion in individuals with T2DM.