IMPACT OF MELATONIN ON SOME INDICATORS OF CARBOHYDRATE METABOLISM IN LIVER OF RATS WITH DEXAMETHASONE-INDUCED DIABETES

Abstract

One frequently employed experimental model, particularly in pre-clinical studies examining the hypoglycemic effects of potential antidiabetic medications, is the metasteroid diabetes model induced by the prolonged administration of glucocorticoids to animals. This experimental study aimed to elucidate the effects of exogenous melatonin (10 mg/kg) on glycogen content and the activity of key enzymes—pyruvate kinase (PK) [EC 2.7.1.40], lactate dehydrogenase (LDH) [EC 1.1.1.27], glucose-6-phosphate dehydrogenase (G-6-PDH) [EC 1.1.1.49], and glucose-6-phosphatase (G-6-P-ase) [EC 3.1.3.9]—in the livers of rats with dexamethasone-induced diabetes. Materials and Methods. The experiments were performed on 44 male 18-month-old white non-linear rats, divided into three groups: 1) control (intact rats), 2) rats with dexamethasone-induced diabetes, 3) rats that amid the progression of dexamethasone-induced diabetes, underwent daily oral administration of melatonin (Sigma, USA) in a dose of 10 mg/kg. Dexamethasone diabetes was modeled by subcutaneous injection of dexamethasone (injection solution 4 mg/ml, KRKA, Slovenia) at a dose of 0.125 mg/kg body weight daily for 13 days (O.V. Stefanov, 2001). Decapitation of animals was carried out in accordance with the norms of the "European Convention for the Protection of Vertebrate Animals Used for Experimental and Other Scientific Purposes" (Strasbourg, 1986). Glucose content in blood from the tail vein of rats, taken on the 14th day of fasting before decapitation of the animals, was determined using a portable glucometer (One Touch Ultra Easy, Life Scan, USA). The content of glycogen and the activity of the studied enzymes of carbohydrate metabolism in the liver were determined according to the generally accepted, previously described methods. A 5% homogenate was prepared from the cold-isolated rat liver in a chilled 50 mM Tris-HCI buffer (pH=7.4) to study the activities of pyruvate kinase, lactate dehydrogenase, glucose-6-phosphate dehydrogenase and glucose-6-phosphatase in the cytosolic fraction. The reliability of the difference between the obtained indicators was assessed using the parametric Student's t-test (for normal distribution) and the non-parametric Mann-Whitney U-test (for non-normal distribution). Differences were considered probable at p≤0.05. Results and discussion. According to our results, in the liver of diabetic rats that did not receive any means of correction of carbohydrate metabolism disorders, the glycogen content was 33% lower than in intact animals. The activities of enzymes such as pyruvate kinase and glucose-6-phosphate dehydrogenase were also reduced in the liver of rats with impaired glucose tolerance by 31.6 and 21.5%, respectively, compared to intact animals, indicating inhibition of glucose oxidation pathways, both at the level glycolysis (decrease of pyruvate kinase), as well as at the level of the oxidative stage of the pentose-phosphate pathway of glucose-6-phosphate oxidation. At the same time, the activity of lactate dehydrogenase and especially glucose-6-phosphatase in the liver of rats with diabetes by 19.5 and 56%, respectively, exceeded the indicators of animals of the control group, which demonstrates the increased activity, intensity of glycogenolysis and gluconeogenesis under conditions of insulin resistance, because glucose-6-phosphatase is the terminal enzyme of these processes. Regarding the investigated parameters of carbohydrate metabolism, both the glucose content in the blood of rats, as well as the glycogen content and the activity of all studied enzymes in the liver of rats that were injected with melatonin against the background of the development of diabetes, did not reliably differ from the parameters of intact animals, which confirms the assumption of the probable hypoglycemic effect of melatonin against the background of the development of diabetes. Conclusions: The daily two-week use of melatonin (10 mg/kg), against the background of the development of dexamethasone diabetes in rats, contributes to the normalization of certain indicators of carbohydrate metabolism in the liver of animals.