Title : Grapefruit peel-derived 5,7- dimethoxycoumarin enhances insulin release and stimulates extrapancreatic secretion of amylin in wistar rats
Abstract:
Coumarins are known to have antidiabetic properties. They are equally known to have direct effects on the cardiovascular system. However, their insulin and amylin-releasing effects have not been thoroughly studied. Insulin and amylin are hormones co-secreted from beta cells of the pancreas to control plasma glucose levels. While insulin is secreted exclusively from beta cells of pancreas and is the major driver of glucose into the cells, amylin is known to be equally secreted in clinically insignificant quantities from the extrapancreatic (gastrointestinal) tissues, and regulates satiety, gastric emptying and absorption of glucose, release of digestive enzymes and postprandial release of glucagon from the pancreas. This study investigated the effect of 5,7-dimethoxycoumarin (Citropten) fractions in grapefruit peel on insulin and amylin secretions in normal male Wistar rats.
Methods: The study was carried out at the Department of Pharmacology and Therapeutics, University of Nigeria Enugu Campus. Methanol extract from grapefruit peels was fractionated using vacuum-assisted liquid chromatography with n-hexane, ethyl acetate, and methanol. Gas Chromatography Mass Spectrometry analysis reported ethyl acetate fraction with the highest concentration (86%) of 5,7-dimethoxycoumarin. 2-hour Intraperitoneal Glucose Tolerance Test (IPGTT) and oral glucose tolerance test (OGTT) were performed on groups containing 5 rats each, receiving: 1) negative control, 1 ml of sterile water 2) positive control, 0.2 mg/kg glimepiride, 3) ethyl acetate fraction containing 20 mg/kg 5,7-dimethoxycoumarin, 4) methanol fraction containing 20 mg/kg 5,7-dimethoxycoumarin.
Results: Ethyl acetate purest fraction containing 20 mg/kg 5,7-dimethoxycoumarin had comparable (p>0.05) plasma glucose control on IPGTT as obtained with glimepiride, with indirect insulin secretion effect unlike direct-acting glimepiride. OGTT confirmed oral antidiabetic efficacy of 5,7-dimethoxycoumarin with p<0.01 compared to glimepiride, and plasma glucose standard error of mean (SEM) of 0.5mmol/l compared to glimepiride SEM of 3mmo/l. The plasma glucose confidence intervals for the ethyl acetate purest fraction of 5,7-dimethoxycoumarin on OGTT were 4.5-6.1mmol/l at 90% confidence level, 4.3-6.3mmo/l at 95% confidence level and 4.1-6.5mmo/l at 99% confidence level. Both ethyl acetate (p=0.015) and methanol (p=0.09) fractions of 5,7-dimethoxycoumarin induced clinically significant extrapancreatic amylin secretion 26.65pg/ml and 28.29pg/ml respectively, 2 times that of the positive control (14.25pg/ml) and 3 times that of the negative control (9.43pgml).
Conclusion: 5,7-dimethoxycoumarin will find special applications in the management of obesity and people with diabetes having chronic complications. Since the overall plasma glucose regulation is as a result of synergy between insulin and amylin, attention should be shifted from insulin-based to amylin-based therapy. There is need to focus on natural compounds that stimulate clinically significant extrapancreatic amylin secretion when taken orally, especially in patients with type 1 diabetes who have unregulated glucagon activities due to amylin deficiency, and who are equally insulin dependent.
Keywords: Amylin, 5,7-Dimethoxycoumarin, Extrapancreatic, Grapefruit Peel – derived, Insulin
