Study On Intestinal Absorption Mechanism Of Gastrodin And Gastrodigenin

Objectives:Gastrodin and gastrodigenin are derived from Gastrodia elata Blume(Tianma in Chinese),a famous traditional Chinese herbal medicine.In China,Tianma has been used for centuries in the treatment of brain disorders,such as headache,vertigo,insomnia,migraine,neuralgia,neurasthenia,and epilepsy.This herb contains several phenolic compounds including gastrodin(p-hydroxymethylphenyl-?-D-glucopyranoside),4-hydroxybenzylalcohol,4-hydroxybenzaldehyde,and protocatechuic aldehyde.Among these compounds,gastrodin is the most active constituent.Gastrodin has many pharmacological functions,including sedative,hypnosis,anticonvulsant,antiepileptic,analgesic,neuroprotection,and anti-inflammatory.Gastrodin has been widely used in the treatment of central nervous system diseases through oral administration or injection.Gastrodin is available in the market in the form of tablets,capsules,and injections.Gastrodin is a highly water-soluble phenolic glucoside with poor liposolubility.Its saturation solubility in water is approximately 0.45 g·mL-1,and the log P value determined in n-octanol/water system is-1.99.Gastrodin can be absorbed sufficiently and rapidly in the intestine.The oral bioavailability of gastrodin in rats is as high as approximately 80%,and the peak time(Tmax)in the human body is as short as 0.8 h.Drugs are absorbed through different paracellular and transcellular pathways in the gastrointestinal tract.In paracellular drug transport,drugs are transported through the tight junctions or modified tight junctions between intestinal epithelial cells induced by a modulator or during inflammatory diseases.In fact,very few drugs can be absorbed via the paracellular pathway without the help of a penetration enhancer.For example,mannitol is readily soluble in water,and its molecular weight is lower than that of gastrodin(182 vs 286).However,mannitol dose not easily pass through the membrane;thus,it is usually used to validate the integrity of the caco-2 cell monolayer model.Transcellular pathways describe drug transport through both apical and basolateral cellular membranes,as well as through the mucus in the gastrointestinal tract.Transcellular pathways include passive diffusion and carrier-mediated transport.Generally,highly hydrophilic compounds cannot pass easily through the lipid soluble membrane.The mediated carrier is always involved in the intestinal transport,if such compound is well absorbed.Gastrodin is a phenolic glucoside composed of 4-hydroxybenzylalcohol and glucose.Glucose transporter is possibly involved in the intestinal absorption of gastrodin.Thus,the intestinal absorption process may be saturable,and may be affected by glucose and the glucose transporter's inhibitor.The present study aims to investigate whether some glucose transporters mediate intestinal absorption of gastrodin using in vitro,in situ and in vivo intestinal absorption models.Gastrodigenin is also derived from Gastrodia elata Blume.It is the effective part of gastrodin.Gastrodigenin can enter the brain more easily than gastrodin because of its good transmembrane ability.In recent years,most of the researches about gastrodi genin are on its pharmacological effects,while the intestinal absorption about it has n ot been reported in the literature.In the present study,a rat everted gut sac model and intestinal perfusion model were conducted to study the absorption characteristics of gastrodigenin,providing a useful basis for the development of oral preparations of ga strodigenin.Methods and Results1?In vitro studies using the rat everted gut sac modelThe absorption kinetics of gastrodin in the rat everted gut sacs was measured according to the rat everted gut sac model.The sacs were incubated with the oxygenated Ringer's solutions containing 10,50,100,or 200?g· mL-1 gastrodin.The concentrations of gastrodin in the samples were determined via ultra performance liquid chromatography(UPLC).Then the accumulated absorption and apparent permeability were calculated.The absorption of gastrodin in rat everted gut sacs was concentration-dependent in low concentration.However,evident saturable absorption processes were observed in the four intestinal segments at>100 ?g · mL-1 gastrodin concentrations.The intestinal absorption rates of gastrodin displayed regioselectivity:duodenum ?jejunum>ileum>colon.Results indicate that some transporters may be involved in the intestinal absorption of gastrodin.2.In situ studies using the perfused rat intestinal modelSingle-pass intestinal perfusion was operated according to a previous method with minor modifications.Glucose transporter inhibitors(phlorizin and phloretin)were respectively added to the perfusate to investigate their effects on the intestinal absorption of gastrodin.The outlet concentrations of gastrodin in the perfusate were determined via UPLC.In situ intestinal perfusion studies showed that the absorption of gastrodin(50 and 200?g·mL-1)in the four intestinal segments was significantly inhibited by glucose.The inhibition effect was strongest in the ileum,and the value of Peff decreased to approximately 20%.Similar results showed that the absorption rate of gastrodin in the colon was far lower than that in the other intestinal segments,and the Peff at high concentration(200 ?g · mL-1)was the same as that at low concentration(50?g · mL-1).The Peff values of gastrodin in the duodenum,jejunum and ileum were higher than 1.0.This result indicates that gastrodin could be well absorbed in the intestine via some glucose transporters.Phlorizin(0.2 mM),a sodium-dependent glucose transporter 1(SGLT1)inhibitor,could significantly inhibit the absorption of gastrodin in the perfused rat intestines.Peff decreased to approximately 30%in the duodenum,jejunum,and nearly 10%in the ileum.However,equal concentrations of phloretin,a facilitated glucose transporter 2(GLUT2)inhibitor,showed no significant effect.The intestinal absorption of gastrodin was mainly affected by SGLT1.The inhibition effects of 0.2 and 0.4 mM phlorizin were obviously stronger than that of 0.1 mM phlorizin,however,no significant difference between the two concentrations was observed.The inhibitory effect of phlorizin was not concentration dependent and showed saturation at high concentrations.3.In vivo pharmacokinetic studies in ratsAll Male Sprague-Dawley rats were divided randomly into three groups of five rats each:normal(intragastrically administered with 200 mg·kg-1 gastrodin),low-dose glucose(intragastrically administered with 200 mg·kg-1 gastrodin and 400 mg · kg-1 glucose),high-dose glucose(intragastrically administered with 200 mg · kg-1 gastrodin and 800 mg · kg-1 glucose)and the diabetic group(five diabetic rats)was administered with 200 mg ·kg-1 of gastrodin.In vivo pharmacokinetic studies confirmed the competitive inhibition effect of glucose.Tmax of gastrodin was delayed from 28 min to 40 min and 72 min,when coadministered with two and four times the dose of glucose,respectively.Cmax of the high-dose glucose group was only half of that in the normal group because the absorption rate was obviously retarded.However,the AUC showed no significant difference among the three groups.Previous reports showed that higher expressions of SGLT occurred in the small intestine of streptozotocin-induced diabetic rats.The absorption kinetics of gastrodin after oral administration of 200 mg/kg was also studied in diabetic rats.Compared with the normal rats,the absorption rate of gastrodin in the diabetic rats became faster,resulting in shorter Tmax(20 min)in diabetic rats versus 28 min in normal rats.Thus,high SGLT1 expression could enhance the rapid intestinal absorption of gastrodin.4.Intestinal absorption characteristics of gastrodigenin in ratsIn vitro everted gut sac model and in situ rat single-pass intestinal perfusion model were used to evaluate the absorption characteristics of gastrodigenin in the different intestinal segments.The concentrations of gastrodigenin in the samples were determined by UPLC method,and the relevant absorption parameters were calculated.In the everted gut sac tests,no significant difference of absorption among the four segments was observed.A positive correlation was found between drug concentration and the accumulated absorption amount(Q).At the concentration of 400?g · mL-1,the Qs of gastrodigenin in the duodenum,jejunum,ileum and colon were 224.33?g,225.81?g,233.18pg and 189.25 ?g,respectively.The in situ rat single-pass intestinal perfusion tests showed that there was also no significant difference of absorption among the four segments.The absorption rates(A)of gastrodigenin in the duodenum,jejunum,ileum and colon were 45.8%,48.39%,47.00%and 54.35%,respectively.ConclusionsThe present study is the first to demonstrate the intestinal absorption mechanism of gastrodin.Both in vitro and in vivo studies have confirmed that gastrodin can be well absorbed in the intestine via SGLT1.Therefore,gastrodin,a highly water-soluble compound,has good bioavailability via oral administration.In addition,the intestinal absorption of gastrodin can be competitively inhibited by glucose,which may reduce the therapeutic efficacy of gastrodin.Therefore,taking gastrodin with glucose-containing food should be avoided.Gastrodigenin can be well absorbed via passive diffusion in the intestine.The absorption rates of gastrodigenin in the different intestinal segments show no regioselectivity.