title: Investigation of intracellular signalling cascades mediating stimulatory effect of a Gymnema sylvestre extract on insulin secretion from isolated mouse and human islets of Langerhans.
abstract: OBJECTIVE
  Traditional plant-based remedies such as Gymnema sylvestre (GS) extracts have been used to treat diabetes mellitus for many centuries. We have shown previously that a novel GS extract, OSA®, has a direct effect on insulin secretion but its mode of action has not been studied in detail Thus this study investigated the possible underlying mechanism(s) by which OSA® exerts its action.
  
  
  METHODS
  The effects of OSA® on [Ca(2+)]i and K(+) conductances were assessed by Ca(2+) microfluorimetry and electrophysiology in dispersed mouse islets and MIN6 β-cells, respectively. Isolated mouse (from 20 to 25 mice) and human (from 3 donors) islets, and MIN6 β-cells, were used to investigate whether the stimulatory effect of OSA® on insulin secretion was dependent on the presence of extracellular calcium and protein kinase activation.
  
  
  RESULTS
  OSA ®-induced insulin secretion from mouse islets and MIN6 β-cells was inhibited by nifedipine, a voltage-gated Ca(2+) channel blocker, and by the removal of extracellular Ca(2+), respectively. OSA® did not affect the activities of KATP channels or voltage-dependent K(+) channels in MIN6 β-cells but it caused an increase in intracellular Ca(2+) ([Ca(2+)]i) concentrations in Fura-2-loaded mouse islet cells. The insulin secretagogue effect of OSA® was dependent, in part, on protein kinase activation since incubating mouse or human islets with staurosporine, a general protein kinase inhibitor, resulted in partial inhibition of OSA®-induced insulin secretion. Experiments using permeabilized, Ca(2+)-clamped MIN6 β-cells revealed a Ca(2+)-independent component action of OSA® at a late stage in the stimulus-response coupling pathway. OSA®-induced insulin secretion was unexpectedly associated with a decrease in intracellular cAMP levels.
  
  
  CONCLUSIONS
  These data indicate that the GS isolate OSA® stimulates insulin secretion from mouse and human islets in vitro, at least in part as a consequence of Ca(2+) influx and protein kinase activation.
authors: Diabetes Research Group, Division of Diabetes and Nutritional Sciences, King's College London, London, UK.
authors: A
authors: A
authors: Al-Romaiyan
authors: B
authors: B
authors: Liu
authors: R
authors: R
authors: Docherty
authors: G-C
authors: GC
authors: Huang
authors: S
authors: S
authors: Amiel
authors: S J
authors: SJ
authors: Persaud
authors: P M
authors: PM
authors: Jones
mesh_terms: D000818:Animals; D002118:Calcium / Q000378:metabolism; D002460:Cell Line; D031104:Gymnema sylvestre; D006801:Humans; D007328:Insulin / Q000378:metabolism*; D000078790:Insulin Secretion; D052139:Intracellular Calcium-Sensing Proteins / Q000187:drug effects / Q000378:metabolism*; D007515:Islets of Langerhans / Q000187:drug effects / Q000378:metabolism*; D051379:Mice; D008517:Phytotherapy / Q000379:methods; D010936:Plant Extracts / Q000737:chemistry / Q000494:pharmacology*; D028321:Plant Preparations / Q000737:chemistry / Q000494:pharmacology*; D011494:Protein Kinases / Q000187:drug effects / Q000378:metabolism*
chemical_list: D007328:Insulin; D052139:Intracellular Calcium-Sensing Proteins; D010936:Plant Extracts; D028321:Plant Preparations; D011494:Protein Kinases; D002118:Calcium