Antihyperglycemic effect of ethanolic extract from onion peel in streptozotocin-induced diabetic rats

Abstract

Quercetin is widely consumed flavonoids found in many vegetables such as onion. This study was performed to examine the antihyperglycemic effect of ethanolic onion peel extract, which contains high concentration of quercetin. In order to determine the concentration of quercetin in ethanolic onion peel extract, we analyzed ethanolic onion peel extract using high performance liquid chromatography. The ethanolic onion peel extract had approximately 104.5mg/g quercetin. So primarily we aimed the comparison of the antihyperglycemic effect in pure quercetin compound and equivalent quercetin of ethanolic onion peel extract. Secondly, the antihyperglycemic effect of ethanolic onion peel extract in dose dependent manner was investigated. Antihyperglycemic effect of the ethanolic onion peel extract was measured in streptozotocin-induced diabetic rats. To induce hyperglycemia, 8 week-old male Sprague-Dawley rats (n=38) were divided into two groups. Control group (n=10) was fed control diet (16.9% energy from fat) and experimental group (n=28) was fed high fat diet (41.2% energy from fat) for 3 weeks and intraperitoneal (i.p.) injected 40mg/kg B.W. streptozotocin at 2 weeks. In order to confirm the hyperglycemia induction, the fasting blood glucose (FBG) was measured at one week after injection. FBG of experimental group was significantly higher compared to control (p<0.05). After then experimental group was randomly divided into 4 groups based on FBG levels and fed 0% or 0.5% or 1.0% ethanolic onion peel extract or 0.1% pure quercetin compound diet for 8 weeks. The FBG was measured once a week during experimental period and oral glucose tolerance test (OGTT) was performed at seven weeks after treatment. Although fasting blood glucose level and blood HbA1c were not different among treatment groups, Incremental Area Under the Curve (IAUC) was decreased in the ethanolic onion peel extract group dose dependently. At the highest concentration of ethanolic onion peel extract, the IAUC was significantly decreased (p<0.05) although not statistically significant in equivalent pure quercetin group. The glycogen concentration in the liver was significantly increased in the both of pure quercetin and 1.0% ethanolic onion peel extract groups (p<0.05). In the muscle, the glycogen concentration was significantly increased in the 1.0% ethanolic onion peel extract group and it also increased in the pure quercetin group although not statistically significant. Insulin concentration in plasma and HOMA-IR were not changed significantly with the intake of pure quercetin or ethanolic onion peel extract. However, the plasma C-peptide and free fatty acid (FFA) concentrations were reduced by ethanolic onion peel extract in dose-dependent manner. It was significantly decreased with intake the 1.0% ethanolic onion peel extract (p<0.05) although not changed with the intake of pure quercetin compound. In addition, the plasma nitric oxide (NO) concentration was decreased with intake of ethanolic onion peel extract and statistically significant at the highest concentration (p<0.05). The pure quercetin compound also decreased plasma NO concentration but it was not statistically significant. These results indicate that ethanolic onion peel extract show antihyperglycemic effect by improving insulin resistance and reducing oxidative stress factor such as plasma FFA and NO. In conclusion, ingestion of ethanolic onion peel extract has antihyperglycemic effect in streptozotocin-induced diabetic rats. This study suggests that ethanolic onion peel extract can be available for enhancing diabetic condition as a functional food.;이 연구에서는 quercetin을 함유한 양파껍질주정추출물의 항당뇨 효과를 알아보기 위한 실험을 진행하였다. 우선 양파껍질주정추출물의 quercetin 농도를 알아보기 위해 고성능액체크로마토그래피(HPLC)를 이용하여 분석하였는데, 양파껍질주정추출물에는 약 104.58mg/g의 quercetin이 있었다. 따라서 첫번째로 pure quercetin compound와 동량의 quercetin이 포함된 양파껍질주정추출물의 항당뇨 효과를 비교하고, 다음으로 양파껍질주정추출물이 dose-dependent하게 항당뇨 효과를 보이는지 조사하는 것을 목적으로 하였다. 양파껍질주정추출물의 항당뇨 효과는 streptozotocin으로 유도된 당뇨 쥐를 이용하여 알아보았다. 당뇨를 유발하기 위해, 8주령 수컷 SD rat을 두 그룹으로 나누었다. 대조군 (n=10)은 정상식이 (에너지의 16.9%를 지방으로 한 식이)를 먹이고, 실험군 (n=28)은 고지방식이 (에너지의 41.2%를 지방으로 한 식이)를 3주 동안 먹이고 2주째에 40mg/체중1kg의 streptozotocin을 복강 투여하였다. 고혈당이 유도되었는지 확인하기 위해 복강 투여를 하고 일주일 후에 공복혈당을 측정하였다. 실험군의 공복혈당이 대조군에 비해 유의적으로 높았다(p<0.05). 그 후 실험군은 공복혈당 수준에 따라 무작위로 4 그룹으로 나누었다. 실험기간 동안 공복혈당(FBG)을 매주 측정하였고, 경구 당부하 검사(OGTT)는 treatment 7주차에 수행하였다. 공복혈당과 당화혈색소(HbA1c)는 그룹 간에 차이가 없었지만, 경구 당부하 검사에서 양의 포도당 반응 면적 (IAUC)은 양파껍질주정추출물 그룹에서 dose-dependent하게 감소하였다. 가장 높은 농도의 양파껍질주정추출물에서 IAUC가 유의적으로 감소하였으나(p<0.05), 동량의 pure quercetin 그룹에서는 유의적이지 않았다. 간의 글리코겐 농도는 pure quercetin 그룹과 양파껍질주정추출물 그룹에서 모두 유의적으로 증가하였다(p<0.05). 근육에서는 글리코겐 농도가 1.0% 양파껍질주정추출물 그룹에서 유의적으로 증가하였고, pure quercetin 그룹에서도 증가하였으나 통계적으로 유의적이지는 않았다. 혈장 인슐린 농도와 HOMA-IR은 pure quercetin이나 양파껍질주정추출물의 섭취에 의해 유의적으로 변하지 않았지만, 혈장 C-peptide와 유리지방산(FFA)의 농도는 양파껍질주정추출물에 의해 dose-dependent하게 감소하였다. 이는 1.0% 양파껍질주정추출물의 섭취에 의해 유의적으로 감소하였으나(p<0.05), pure quercetin compound 섭취에 의한 변화는 없었다. 그리고 혈장 일산화질소(NO)의 농도 또한 양파껍질주정추출물 섭취에 의해 감소하였고 가장 높은 농도에서 통계적으로 유의적이었다(p<0.05). Pure quercetin compound 또한 혈장 NO 농도를 감소시켰지만 통계적으로 유의적이지는 않았다. 이러한 결과들로 보아 양파껍질주정추출물은 인슐린 저항성을 개선시키고 유리지방산이나 일산화질소와 같은 산화적 스트레스 요인을 감소시킴으로써 항당뇨 효과를 보인다. 결론적으로 양파껍질주정추출물의 섭취는 streptozotocin으로 유발된 당뇨 쥐에서 항당뇨 효과를 가진다. 이 연구는 양파껍질주정추출물이 기능성 식품으로써 당뇨 질환을 개선시킬 수 있다고 본다.