Indian Journal of Experimental Biology Vol. 52, June 2014, pp. 623-629 Insulin secreting and α-glucosidase inhibitory activity of hexane extract of Annona squamosa Linn. in streptozotocin (STZ) induced diabetic rats Ranjana & Yamini B Tripathi* Department of Medicinal Chemistry, Institute of Medical Sciences, Banaras Hindu University, Varanasi 221 005, India Received 1 July 2013, revised 13 January 2014 The hexane extract of A. squamosa (ASHE) in 100 and 400 mg/kg body weight dose raised the insulin level when compared with Glimepiride (1 mg/kg) and also inhibited α-glucosidase activity when compared with Acarbose (10 mg/kg) in streptozotocin induced diabetic rats. The ASHE significantly reduced peak blood glucose (Gp30) and area under curve (AUC) in diabetic rats in oral glucose (OGTT) and oral sucrose (OSTT) tolerance test, but there was more reduction of Gp30 value than AUC in OSTT. Thus, it can be suggested that the ASHE, has hypoglycemic role at 2 levels, i.e. it acts as secretagogue and also inhibits the intestinal enzymes, responsible for glucose metabolism. Keywords: α-glucosidase, Annona squamosa, Diabetes, Herbal, Postprandial hyperglycemia Diabetes mellitus is characterized by hyperglycemia together with biochemical alterations of glucose and lipid metabolism1. Current drugs used for the treatment of diabetes include a few groups of chemical compounds such as the drug biguanide (metformin), which although antihyperglycemic, does not affect hypoglycemia in the normal subjects2 and involves extra pancreatic mechanisms3,4. Then, there are the sulfonylureas (glibenclamide, tolbutamide etc.), which are insulin secretagogues, and lastly, the new class of drugs known as thiazolidinedions (rosiglitazone, pioglitazone) which enhances sensitivity to insulin in target tissues5. At present, α-glucosidase inhibitors are the most common oral agents used to decrease postprandial hyperglycemia. It reduces the rate of digestion and absorption of ingested carbohydrates. Drugs like acarbose and miglitol are already in clinical use but they are associated with side effects like abdominal distention, flatulence, meteorism and possibly diarrhea6,7. In addition, numerous studies have been carried out to isolate effective and safe α-glucosidase inhibitors from natural products, including plant materials, as alternative hypoglycemic agents for diabetes that can be used in addition to conventional treatments8-10. —————— *Correspondent author Telephone: 0091-542-2366577, 6702171, 2307547 Fax: 0091-542-2366577, 2368174 E-mail: [email protected]; [email protected] Therefore, search of newer agents is in demand. In this process, standardized herbal extracts may be of high benefit because it is a natural cocktail of many secondary metabolites, which may be active at various sites. Here, we propose to investigate the role of herbal formulation, because of their natural cocktail of variety of secondary metabolites and possibility of multi-targeted mode of action. The Annona squamosa Linn. (AS) is a shrub and cultivated by horticulturists for its fruits, known as caster apple. Reports indicate the hypoglycemic property in its leaves11-14 but the mechanism of action is not very clear. Besides hypoglycemic activity, its leaf extract also possesses anti-oxidant11&15, anti-fertility, anti-tumor16,17, antihyperthyroidism18 and hepato-protective activities19 etc. The secondary metabolites of AS-leaf-extract include variety of steroids, alkaloids, saponins, terpenes, tannins, phenolic substances, volatile oil and mucilage 20,21. The present communication reports the hypoglycemic effect of leaf of AS on streptozotocin (STZ) induced diabetic rats with especial reference to inhibitory response on intestinal α-glucosidase activity. Based on earlier observations, it was found that non-polar hexane fraction was most active therefore its inhibitory study was undertaken in detail by using in vivo and in vitro experimental models. Materials and Methods Materials—Streptozotocin (STZ) (Sigma Chemical Co (St. Louis, MO, USA). Acarbose and Glimepiride 624 INDIAN J EXP BIOL, JUNE 2014 (Bayer pharmaceuticals and Aventis Pharma respectively), glucose assay kit (Accurex), maltose (E. Merck Pvt. Ltd., Mumbai (India), glucose and sucrose (Hi-Media Laboratories Pvt. Ltd., Mumbai (India) were used. All other reagents were of analytical grade. Rats of CF strain were purchased from the central animal house of the Institute. The animals taken under this study were 8-10 weeks old male 5, weighing 100-150 g body weight. The experimental protocol was approved by Animal Ethics Committee of the Institute. All animals were kept on a 12 h dark and light schedule and fed standard laboratory chow ad libtum. Preparation of plant extract—The leaves of Annona squamosa Linn. (AS) were purchased and their authenticity was reconfirmed on pharmacognostical parameters by Department of Dravyaguna, Faculty of Ayurveda, BHU. The voucher specimen of leaves was preserved in the department, wide reference no MC/YBT/1-2009. These leaves were washed, air dried under shade, and pulverized in a mechanical grinder. The 3 extracts namely, hexane (ASHE), methanol (ASME) and water decoctions (ASWD) were separately prepared. The course powder was separately extracted with hexane and methanol in continuous soxhlet extractor for 20 h. The solvent free extracts were prepared by distillation and desiccated until constant weight was attained. The water extract was prepared by boiling weighed amount of dried leaves in distilled water (16 times, w/v) and when the water volume was reduced to ¼, it was filtered and dried at 40 °C in hot air oven. Induction of diabetes by streptozotocin (STZ)—The overnight fasting rats were given intra-peritoneal injection of STZ (50mg/kg, dissolved in chilled citrate buffer (pH 4.5) and after 3 days, blood glucose was measured. The rats having blood glucose, higher than 200 mg/dL, were considered as diabetic for experiments22. The blood was collected from tail of each rat in anesthetic condition. Blood glucose was measured using a GOD/POD method and serum insulin was measured by Immulite 1000 (Siemens) solid-phase, two-site chemiluminescent immunometric assay. Assay of α-glucosidase—The tissue homogenate prepared from small intestine of rats was used as enzyme source. A small pieces (2-3 mm) of smallintestine was taken out in pre-cooled phosphate buffer saline (PBS), thoroughly cleaned, dried on blotting paper, weighed and then homogenized in glass Teflon homogenizer. The homogenate was centrifuged at 5,000 g for 30 min and its supernatant was used as enzyme source. Final volume of supernatant was maintained to 20% (w/v). Spectrophotometric assay method23 was used with slight modification. Tissue homogenate (40 µL) was mixed with 80 µL of test/standard/drug vector and incubated for 15 min at 37 °C. Thereafter, 280 µL maltose (37 mM) was added and further incubated for 30 min. Finally, the reaction was stopped by putting the tubes in boiling water for 10 min. The tubes were centrifuged and glucose concentration was assessed in the supernatant by GOD/POD method based kit. The change in activity was expressed as: Inhibition (%) = (Absorbance Control - Absorbance Test/ Absorbance Control) ×100 In vivo studies : Hypoglycemic screening of different extracts of AS leaves in STZ induced diabetic rats—Rats were randomly divided into 6 groups of 6 rats in each. The 1st and 2nd groups were normal and experimental control and treated with 20% Tween 20 @ 5 mL/kg body weight. The 3rd, 4th and 5th groups were treated with ASHE, ASME and ASWD at 200 mg/kg body weight respectively. The 6th group was treated with Glimepiride (1 mg/kg body weight). After 2 h of oral administration of all extracts and standard drug, blood glucose was measured by GOD/POD method. The hypoglycemic activity of ASHE on 7 days treatment in STZ induced diabetic rats was carried out in different doses. Glimepiride and Acarbose were used as positive control. After 7 days, blood insulin and blood glucose were estimated in all grpous. The designs of groups are given below: Groups Normal control Experimental diabetic control ASHE ASHE Glimepiride (sulphonyl urea group) Acarbose (α-glucosidase inhibitor) Treatment plan 20% Tween 20 @ 5mL/kg body weight 20% Tween 20 @ 5mL/kg body weight 100 mg/kg body weight 400 mg/kg body weight 1 mg/kg body weight 10 mg/kg body weight The effect of ASHE on oral carbohydrate challenge was carried out using glucose and sucrose separately24,25. The animals were randomly divided into different groups for various treatments and to RANJANA & TRIPATHI : ANNONA SQUAMOSA & STREPTOTOCIN INDUCED DIABETES each rat glucose/sucrose were orally given at 4 g/kg dose. At time intervals of 30 min, blood glucose was estimated up to 2 h. Peak glucose value was ascertained at 30 min of glucose administration (Gp30). The concentration verses blood glucose was plotted against time. The formula for AUC determination is as follows: AUC (mg) = (BG0+BG30/2*30) + (BG30+BG60/2*30) + (BG60+BG120/2*60) where BG0, BG30, BG60, BG120 represented the blood glucose at 0, 30, 60 and 120 min. respectively. Effect of ASHE on oral glucose tolerance test (OGTT)—The rats were fasted overnight for 18 h but had free access to water. The rats were divided into following 6 groups of 6 rats each. Group 1, was treated orally with 100 mg/kg body weight of ASHE (R1), rats of treatment group 2 were treated with 400 mg/kg body weight ASHE (R2). The rats of group 3 were treated with Acarbose at 10 mg/kg body weight (R3). In the normal and experimental control groups (R4) only drug vector were orally given (20% Tween 20, 5 ml/kg. After 30 min, Glucose was orally given to rats at 4 g/kg body weight and blood was collected from tail vein at 0 min (before glucose administration) and at 30, 60, and 120 min after glucose administration and blood glucose was measured by GOD/POD method. The effect of ASHE on oral sucrose tolerance test (OSTT) was carried out in the same manner as described above in case of OGTT, but here sucrose was given at the dose of 4 g/kg body weight in place of glucose. Effect of AS hexane fraction on intestinal αglucosidase activity—The effect of ASHE on αglucosidase activity under in vivo condition was carried out in normal and diabetic rats. They were 625 randomly divided in to various groups of 6 each and treated with ASHE/standard/drug-vector for 7 days. Finally all animals were sacrificed under anesthesia to collect small intestine for assay of α-glucosidase activity as mentioned above. In vitro study: Inhibition profile of ASHE on semi-purified αglucosidase enzyme—For assessment of the specific inhibitory activity of hexane fraction on α-glucosidase activity, an in vitro study was performed. Tissue homogenate was prepared from the rat intestine, collected from normal rats. It was used as enzyme source and incubated with different concentrations of ASHE/acarbose/drug vehicle. Thereafter maltose solution was added and activity of α glucosidase was assessed as described above. Statistical analysis—The data have been expressed as mean±SD. Pearson’s correlation analysis (SPSS 16 for Windows, SPSS Inc.) was used to test for the significance of the relationship between the concentration and percentage inhibition. In order to evaluate dose dependent effect of the extract, compared with the experimental control was excluded and data analyzed using 2- way analysis of variance and post hoc multiple comparison (Bonferroni test). Results Hypoglycemic effect of different extracts of AS leaves (2 h study)—All the 3 extracts (ASHE, ASME and ASWD) showed significant hypoglycemic activity in STZ induced diabetic rats at dose of 200 mg/kg body weight, but the hexane extract (ASHE) showed the best potency (Table 1). The lowering of raised blood glucose level was up to 14.49±3.80%, when compared with diabetic experimental control group, which blood glucose was 295.16±3.98 mg/dL. However in similar conditions, Table 1—Hypoglycemic activity of different extracts of AS leaves [Values are mean±SD from 6 animals in each group] Groups Blood glucose (mg/dL) Inhibition in blood glucose level (mg/dL) (%) 0 min 2h Normal rats (drug vector) 73.69±2.32 74.06±2.64 Diabetic rats (drug vector) 294.67±5.26 295.16±3.98 ASHE (200 mg/kg bw) 351.49±6.29 300.54±1.80 14.49±3.80 350.26±3.62 322.40±1.60 7.95±2.90 ASME (200 mg/kg bw) ASWD (200 mg/kg bw) 287±2.45 257±3.60 10.45±3.62 Glimepiride (1 mg/kg bw) 246±4.60 208±2.76 15.44±4.18 ASHE = hexane extract of AS leaves; ASME = methanolic extract of AS leaves; ASWD = water decoction of AS leaves; P values are significant at *< 0.05 and highly significant at **< 0.01 INDIAN J EXP BIOL, JUNE 2014 626 Glimepiride (1 mg/kg body weight) showed hypoglycemic response in range of 15.44±4.18%. Hypoglycemic response of ASHE in STZ induced diabetic rats (7 days)—The ASHE treatment for 7 days showed significant rise in blood insulin along with decrease in blood glucose level. In STZ treated experimental control rats, the blood glucose level was raised to 386.93±2.60 mg/dL when compared to normal control value i.e. 115±3.54. The oral treatment of ASHE showed dose dependent decline in blood glucose. It was 41.18±2.46% at 100 mg/kg and 78.10±1.57% with ASHE 400 mg/kg, when compared experimental diabetic control group. The standard drug Acarbose also showed the potent reduction (61.66%) at 10 mg/kg in glucose level. On the other hand, the serum insulin level declined to 8.56±1.42 µU/mL in experimental diabetic control rats and it was found to be significantly raised in ASHE treated rats to 11.58±1.80 at 100 mg/kg and 16.26 µU/mL in 400 mg/kg. In similar conditions, Glimepiride showed 17.36 µU/mL of rise in insulin level at dose of 1 mg/kg. In normal control animals, the insulin value was 22.4 µU/mL (Table 2). Oral glucose tolerance test (OGTT)/Oral sucrose tolerance test (OSTT)—The earlier experiment indicated the role of ASHE on insulin release in diabetic rats, but its role on intestinal enzyme was not clear. Thus is effect on OGTT and OSTT was assessed. The results were expressed in terms of peak glucose value at 30 min (Gp30) and AUC for blood glucose change recorded up to 120 min of oral glucose/sucrose administration. The Gp30 in normal rats was 111.57±2.70 mg/dL and in experimental diabetic control rats it was 368.10±1.31 mg/dL (Table 3 and Fig. 1). Effect of AS hexane fraction on intestinal αglucosidase activity—To further clarify the role of ASHE on intestinal enzymes, responsible for carbohydrate metabolism, the activity of α-glucosidase was determined in small intestine of rats treated with ASHE. At 100 mg dose, the reduction in activity was 57.06±1.44% and at 400 mg dose this reduction was in range of 75.69±1.7%. The inhibitory response was concentration dependent. In similar conditions the Acarbose treatment (1 mg/kg body weight) showed reduction of 53.60±1.45% (Table 4). Table 2—Effect of ASHE on blood-glucose and blood-insulin level after 7 days of oral treatment to STZ induced diabetic rats [Values are mean±SD from 6 animals in each group] Groups Glucose level (1st day) Glucose level (7th day) Glucose level (% inhibition) Normal control 96.38±2.80 115±4.53 Diabetic control 342.44±3.70 386.93±2.67 ASHE(100 mg/kg bw) 289.06±3.60 170.00±1.80* 41.18±2.46** ASHE (400 mg/kg bw) 361.73±1.70 79.41±1.56** 78.04±1.57** Acarbose (10 mg/kg bw) 383.46±3.98 147.82±2.80** 61.45±1.52** Glimepiride (1 mg/kg) 293.89±2.80 159.10±3.20** 45.86±2.52** ASHE = hexane extract of AS leaves; P values are significant at *< 0.05 and highly significant at **< 0.01 Serum insulin (µU/mL) 22.46±2.45 8.56±1.42 11.58±1.80 16.26±1.20** 10.50±1.94 17.36±0.90** Table 3—Effect of ASHE on peak glucose value at 30 min and AUC during OGTT and OSTT in STZ induced diabetic rats. [Values are mean±SD from 6 animals in each group] Groups OGTT Blood glucose at 0 min (mg/dL) Blood glucose at 30 min (GP30) (mg/dL) ASHE 100 260±2.30 389.72±4.34c,f mg/kg (R1) (49.90±1.03%)* ASHE 400 262.78±2.84 367.10±3.24a,f mg/kg (R2) (38.33±0.92%)* Acarbose 10 245.40±0.93 343.34±1.13c mg/kg (R3) (39.91±0.86%)* Diabetic 222.48±0.722 368.10±1 .31 Control (R4) (65.66±0 .3%)* Normal 79.57±1.42 111.57±2.70 control (39.74±1.30%)* P values: a,d <0.05 ; b,e <0.01 ; c,f <0.001 a,b,c: compared change after glucose loading in diabetic rats OSTT AUC(mg) 40216±104.00c,d Blood glucose at Blood glucose at 30 0 min (mg/dL) min (GP30) (mg/dL) AUC(mg) 393.30 ±2.46 c,f 38550±194.66 c,f (21.80±0.14%)a 39151±193.41c,f 292.52±2.30 342.75 ±2.56 c,f 38411 ±262.98 a,f (17.16±0.43%)* 38191±66.41a 282.45±2.05 317.46±1.94 c 36326 ±308.94 c (12.38±0.29%)* 41901±110.20 223.35±2.45 357.92±2.39 44032 ±302.09 (60.26±0.88%)* 12582±184 82.53±2.54 118.71 ±2.58 14490±308.58 (43.85±1.38%)* with the diabetic control; d,e,f : compared with the Acarbose group *% unit 322.86±2.24 RANJANA & TRIPATHI : ANNONA SQUAMOSA & STREPTOTOCIN INDUCED DIABETES 627 Table 5—Effect of ASHE on α- glucosidase activity under in vitro condition in a semi-purified rat intestinal enzyme preparation [Values are mean±SD from 6 animals in each group] Concentration (mg/mL) Inhibition (%) 1.0 12.40±1.1* 3.0 20.4±1.00* 5.0 27.50±0.59* 7.0 34.50±0.96** 10.0 64.35±0.84** 15.0 66.805±0.34** ASHE: IC50 9.157 mg/mL Acarbose: IC50 0.049 mg/mL Results are expressed in terms of % inhibition in comparison to experimental control (Pearson’s correlation analysis) ASHE = hexane extract of AS leaves; P values are significant at * < 0.05 and highly significant at **< 0.01 Fig. 1—Effect of hexane extract of A. squamosa on oral glucose tolerance test (a) and oral sucrose tolerance test (b) in diabetic rats [R1=ASHE 100 mg/kg, R2=400 mg/kg, R3=Acarbose 10 mg/kg, R4=diabetic control] Table 4—Effect of ASHE activity on intestinal α-glucosidase after oral treatment for 7 days in STZ induced diabetic rats [Values are mean±SD from 6 animals in each group] Concentration (mg/kg) Inhibition in intestinal α-glucosidase (%) ASHE (100) ASHE (400) Acarbose (10) Glimepiride (1) 57.06±1.44** 75.69±1.7** 53.60±1.45** 22.18±1.24 Results are expressed in terms of % inhibition in comparison to experimental control (Pearson’s correlation analysis) ASHE = hexane extract of AS leaves; P values are significant at * < 0.05 and highly significant at **< 0.01 The role of ASHE on intestinal α- glucosidase activity under in vitro condition—Finally, in order to assess the director inhibitory role of ASHE on intestinal α-glucosidase activity, an in vitro study was carried out, by using semi purified enzyme preparation from normal rat intestine. Both ASHE and acarbose significantly inhibited the α-glucosidase activity. The response was concentration (1.0 mg - 10 mg/mL) dependent, varied from 57.02–12.40%. IC50 ASHE was 9.157 (mg/mL). In similar conditions acarbose showed IC50 at 0.049 mg/mL (Table 5). Discussion The highest activity of ASHE, followed with ASWD and ASME, could be due to presence of more active secondary metabolites in the non-polar region. Higher activity in water decoction than methanol extract may be due to higher concentration of nonpolar phytomolecules due to their low melting point. Earlier reports have also indicated the hypoglycemic response of methanolic and water extracts of AS leaves11-14 but for the first time, present communication reports α- glucosidase inhibition with hexane fraction. When ASHE was tested for 7 days treatment in STZ induced diabetic rats, there was reduction in blood glucose along with rise in blood insulin level. This indicates towards its action as secretogague, by targeting pancreas as it’s site of action. The response was similar to Glimepiride as reported26,27. Various other plant extracts have also shown similar secretogague activity27. The data related to OGTT and OSTT indicate that in case of ASHE and Acarbose, the Gp30 value is higher in OGTT as compared to OSTT. This difference indicates that ASHE might be interfering with the metabolism of sucrose, similar to Acarbose28. It is known that sucrose, being a disaccharide, first gets metabolized to monosaccharides by intestinal hydrolases before absorption and α-glucosidase and α-amylase are the important enzymes involved in the digestion of carbohydrates 29. Interestingly, the AUC data of OGTT and OSTT showed different results than the corresponding Gp30 values. In both the cases, the AUC did not show significant differences as compared to differences in 628 INDIAN J EXP BIOL, JUNE 2014 GP30 in OGTT and OSTT. The AUC of OGTT and OSTT showed significant change in ASHE treated rats in comparison to diabetic control rats. However the peak glucose value GP30 recorded after 30 min of oral administration of glucose and sucrose showed significant difference. In sucrose treatment, δ was 17.16±0.43% for ASHE 400 mg/kg where in glucose fed rat it was 38.33±0.92% this indicate the ASHE is delaying in attainment of glycemic peak value in sucrose fed rats. It is indirectly suggesting the inhibition of intestinal enzyme responsible for digestion of starch (sucrase). This suggests the additional property of ASHE towards hypoglycemic property i.e its action as secretogague and also as inhibitor of intestinal enzymes, responsible for carbohydrate metabolism. Other reports have also indicated such reduction in AUC with reference to acarbose and other herbal drugs28,30. This could be due to presence of polyphenolic compounds in the extracts, which binds to the protein and inhibits its activity. When the activity of α-glucosidase in the intestinal homogenate, isolated from the ASHE treated rats for 7 days in different doses of 100 and 400 mg/kg was tested, a significant inhibition was noted in its activity. Acarbose was used as positive control, which also showed similar α-glucosidase inhibitory effect. So the ASHE extract acting on 2 level i.e it raised the insulin level (Table 2) in diabetic rats when compared with Glimepiride. The Glimepiride is a drug of sulphonyl urea group and raises the insulin level to 17.36±0.90 µU/mL, without any significant inhibition of α-glucosidase activity. It was only in range of 22.18±1.24% while in similar conditions, Acarbose, inhibited α-glucosidase activity to tune of 61.66±1.52%. Interestingly, acarbose did not raise the insulin level and it remained at 10.50±1.94 µU/mL (Table 2). However, the hexane extract of Annona squamosa leaves showed both properties together. It raised the insulin level (11.58±1.80 at 100 and 16.26±1.20 µU/mL at 400 mg/kg dose respectively) and also inhibited the activity of α-glucosidase (41.18±2.46% at 100 and 78.10±1.57% at 400 mg/kg, respectively). Thus it could be suggested that hypoglycemic response of leaves of Annona squamosa has additional property than the existing standard drugs such as Glimepiride and Acarbose. The in vitro studies further supported the inhibitory role of ASHE on α-glucosidase activity. On incubation of ASHE extract with enzyme preparation there was concentration dependent inhibition in activity, which could be due to the interaction of secondary metabolites of ASHE with α-glucosidase protein. Acarbose showed significant inhibitory effect also. Therefore, the retardation and delay of carbohydrate absorption with a plant-based α-glucosidase inhibitor offers a prospective therapeutic approach for the management of type 2 diabetes mellitus and borderline patient. 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