The Effects And Mechanisms Of Geniposide On Insulin Secretion In Pancreatic Islets

Objective:Geniposide,an iridoid glycoside,separated from the fruit of Gardenia jasminoides Ellis,has hepatoprotective,antioxidant,anti-inflammatory and antidiabetic effects.This study is to explore whether geniposide has anti-diabetic effect and the potential mechanisms on insulin secretion in rat pancreatic islets and beta cells.Methods:(1)Pancreas and its surrounding areas of male Sprague-Dawley(SD)rats were fully exposed.Then,Collagenase P(1 mg / ml)was injected into the pancreas through the common bile duct.After 11 minutes of digestion in 37 ℃ water bath,islets could be obtained by density gradient centrifugation.When we put an appropriate amount of islets into dispase Ⅱ for 5 minutes for digestion,single islet cells could be acquired.(2)The islets were cultured and selected before measured,and the effects of geniposide and other drugs on insulin secretion and cAMP content were test by radioimmunoassay.(3)Patch-clamp technique was used to record potassium channels,calcium channel currents and action potentials.(4)The effect of geniposide or other drugs on intracellular calcium concentration was detected by ion fluorescence imaging system and fluorescence microscope.Results:(1)Geniposide(1 、 10 、 100 μ mol / L)could promote insulin secretion in a dose-dependent manner in the presence of 8.3mmol/L glucose,but not in low glucose(2.8mmol/L),and a maximal increase was observed when islets were exposed to geniposide at a concentration of 10 μ mol/L.(2)The effect of geniposide on insulin secretion was not affected by 2.8mmol/L glucose,but the increase of insulin secretion was observed under 8.3mmol/L and 16.7mmol/L glucose condition.(3)The potentiation of insulin secretion induced by geniposide was blocked by GLP-1R antagonist(exendin(9-39),100nmol/L),AC inhibitor(SQ22536,10μ mol/L)and protein kinase A(PKA)inhibitor(H89,1 μ mol/L).(4)Geniposide apparently elevated cAMP levels under8.3mmol/L glucose conditions but this effect was eliminated in the presence of exendin(9-39)or SQ 22536,although exendin(9-39)or SQ 22536 alone caused no change.(5)The current-voltage relationship curve obtained in patch-clamp experiments showed that geniposide could significantly inhibit the potassium current and reached a maximum inhibitory effect at 10μmol / L(GP,73.40 ± 6.62 p A/p F vs.Control,139.19±12.94 p A/p F).However,the inhibitory effect on Kv currents by geniposide was reversed when β cells were treated with exendin(9-39)or H89 and KT5720(100nmol/L).In addition,geniposide could prolong the action potential duration(APD)of β cells(GP,59.13 ± 3.76 ms vs.Control,28.96 ± 3.02 ms),and activated voltage-dependent calcium channels(GP,-9.23 ±0.42 p A/p F vs.Control,-5.94 ± 0.19 p A/p F,0m V).(6)In the calcium ion imaging experiment,we saw that geniposide(10μmol/L)significantly increased intracellular calcium concentration,exendin(9-39)can significantly eliminate this effect.Conclusions:These favorable findings indicate that geniposide indeed potentiated insulin secretion from rat pancreatic islets,and its insulinotropic effect might be mediated through GLP-1R/cAMP/PKA signaling cascade and ion channels.Geniposide or its analogs may be potential agents for the treatment of type 2 diabetes mellitus.