The role of melatonin in cardioprotection : an investigation into the mechanisms involved in glucose homeostasis, microvascular endothelial function and mitochondrial function in normal and insulin resistant states

[摘要] ENGLISH ABSTRACT: Introduction:The cardioprotective actions of the hormone melatonin against myocardial ischaemiareperfusioninjury (IRI) are well-established. It has recently been shown to prevent the harmfuleffects of hyperphagia-induced obesity on the susceptibility of the heart to IRI as well as manyof the harmful effects of obesity and insulin resistance. However, the exact mechanismwhereby it exerts its beneficial action is still unknown.The aims of this study were to determine the effects of relatively short-term melatonintreatment in a rat model of diet-induced obesity on: (i) biometric and metabolic parameters,lipid peroxidation, myocardial IRI and intracellular signalling (ii) mitochondrial oxidativephosphorylation function (iii) cardiomyocyte glucose uptake and intracellular signalling. Inaddition, the effects of acute melatonin treatment of cardiac microvascular endothelial cells(CMEC) were determined on cell viability, nitric oxide production (NO), TNF- -induceddysfunction and intracellular signalling.Material and Methods:Male Wistar rats were randomly allocated to two groups for 20 weeks feeding with eitherstandard rat chow or a high calorie diet. Each group was subdivided into 3 groups receivingeither water throughout or melatonin (4mg/kg/day, in the drinking water) for the last 6 or 3weeks of the experimental programme. Hearts, perfused in the working mode, were subjectedto ischaemia/reperfusion and infarct size determined. Mitochondria and cardiomyocytes wereisolated according to standard techniques and oxidative function and glucose uptakerespectively determined. CMEC NO production and cell viability were quantified by FACSanalysis of the fluorescent probes, DAF-2/DA and propidium iodide/Annexin V respectively.Intracellular signalling was evaluated using Western blot and appropriate antibodies.Results:The high-calorie diet caused significant increases in body weight gain, visceral adiposity,fasting blood glucose, serum insulin, triglycerides, HOMA-IR index and a concomitant reduction in serum adiponectin levels as well as larger myocardial infarct sizes after exposureto IRI compared to the control, indicating increased susceptibility to damage. Three as well assix weeks of melatonin administration to obese and insulin resistant rats reduced serum insulinlevels and the HOMA-IR index. Myocardial infarct size was reduced in both control and dietgroups. These effects were associated with increased activation of baseline myocardial STAT-3 and the RISK pathway during reperfusion.The diet had no effect on the oxidative phosphorylation capacity of mitochondria, isolated fromnon-perfused hearts (baseline), but melatonin administration for 6 weeks induced a reductionin state 3 respiration rate; mitochondria isolated from diet hearts subjected to global ischaemia,exhibited an attenuated oxidative phosphorylation process which was improved by melatonintreatment.Melatonin in vitro enhanced cardiomycyte insulin stimulated glucose uptake of normal youngrats but not of insulin resistant rats. In vivo melatonin treatment for 6 weeks increased basal(in diet group) and insulin stimulated glucose uptake in both control and diet groups.Melatonin (1nM) in vitro caused a significant reduction in necrosis and apoptosis of culturedCMEC, associated with a decrease in nitric oxide availability and eNOS activation and aconcomitant increase in PKB/Akt, p38MAPK and AMPK activation. The harmful effects of TNF-treatment on signalling in CMEC could be prevented by co-treatment with melatonin.Conclusions:The results suggest that short-term melatonin treatment was able to significantly attenuate thediet-induced increased myocardial susceptibility to ischaemia/reperfusion damage. It may alsoimprove cardiac glucose homeostasis and mitochondrial oxidative phosphorylation in an insulinresistant state. Melatonin in vitro protects CMEC against apoptosis and necrosis and reducesnitric oxide availability. These beneficial effects of melatonin may ultimately be due to its antioxidantcapacity or receptor-mediated actions, but this remains to be established.

[发布机构] Stellenbosch University