Effect of melatonin on myocardial susceptibility to ischaemia and reperfusion damage in a rat model of high-fat diet-induced obesity

Kaskar, Rafee'ah (2015-12)

Thesis (MScMedSc)--Stellenbosch University, 2015.

Thesis

ENGLISH ABSTRACT: Obesity has reached epidemic proportions worldwide and is currently a serious health problem. It is associated with metabolic abnormalities, oxidative stress, hypertension, insulin resistance and an increased disposition for the development of cardiovascular disease. Elucidation of the pathophysiological mechanisms underlying obesity and its relationship with metabolic and cardiovascular diseases is essential for prevention and management of these disorders. Melatonin, the pineal gland hormone, is a powerful antioxidant and has been shown to protect the myocardium against ischaemia/reperfusion (I/R) injury. Long- as well as shortterm melatonin treatment also reversed several of the harmful effects of obesity in an animal model of hyperphagia-induced obesity (DIO). However, its effects on myocardial I/R injury and intracellular signalling in obesity induced by a high fat diet (HFD) are still unknown. Aims of study: (i) To evaluate the ability of a high fat diet (HFD) to induce obesity in rats. Apart from evaluating its effects on the biometric parameters and resistance to ischaemia/reperfusion injury (as indicated by infarct size in regional ischaemia and functional recovery after global ischaemia), special attention will be given on the interplay between adiponectin, AMPK, leptin, and FFA in this model. (ii) To evaluate the effect of daily oral administration of melatonin to rats on the HFD as well as their littermate controls, on the parameters listed above as well as on the development of obesity. In this study melatonin will be administered from the onset of the feeding of the high fat diet. Methods: Male Wistar rats were divided into 4 groups: (i) control rats (receiving normal rat chow) (C); (ii) control rats receiving melatonin (CM); (iii) obese rats (receiving HFD) (HFD); (iv) obese rats receiving melatonin (HM). Animals were kept on the diet for 16 weeks and melatonin treatment (10mg/kg/day, added to the drinking water) started at the onset of the feeding. Following feeding and treatment, the animals were grouped into fasted/ non-fasted of which biometric parameters were recorded and blood collected at the time of sacrifice for metabolic and biochemical assays. Hearts were perfused in the working mode for evaluation of myocardial function and infarct size determination after exposure to 35min regional ischaemia/60min reperfusion. For study of intracellular signaling, hearts were perfused in the working mode, subjected to 20min global ischaemia/10min reperfusion and freeze-clamped for Western blotting. Plasma leptin, adiponectin, free fatty acid, triglycerides, total cholesterol, phospholipids, conjugated dienes and thiobarbituric reactive substances (TBARS) levels were determined. Several kinases were investigated including, the RISK (reperfusion injury salvage kinase) (PKB/Akt and ERK p44/42) and SAFE (survivor activating factor enhancement) (STAT-3) pathways, AMPK and JNK under baseline conditions or following 10 min reperfusion. In addition, expression of UCP-3 and PGC1-α was determined. Results: Significant increases in body weight, visceral fat, blood glucose, insulin, HOMA index and leptin and a reduction in adiponectin levels were observed in the fasted high fat diet (HFD) group when compared with controls (C). Significant increases in free fatty acid and triglyceride levels were also noted the HFD group while other serum lipid parameters, including TBARS, remained unchanged. No differences in functional recovery during reperfusion or infarct size after exposure to 35 min regional ischaemia, as well as functional recovery during reperfusion after 20 min global ischaemia were observed between the control and HFD groups. Baseline and 10 min reperfusion data were similar for the RISK and SAFE pathway kinases for the control vs HFD groups. The HFD also had no effect on the expression and phosphorylation of myocardial AMPK and JNK, as well as on the expression of UCP-3 and PGC1-α, when compared to the controls. Treatment with melatonin significantly reduced body weight, visceral fat, blood glucose, HOMA index and serum leptin levels in HFD treated groups, while having no effect on the lipid profile. Although melatonin significantly reduced infarct size in both control [% of area at risk: 20.59 ± 2.29 (CM) vs 38.08 ± 2.77 (C)] and high-fat diet groups [% of area at risk: 11.43 ± 2.94 (HM) vs 38.06 ± 3.59 (H)], it was without effect on myocardial functional recovery during reperfusion. Melatonin had no effect on the intracellular signaling pathways studied. Conclusions: The HFD proved to be a useful model of diet-induced obesity with a more pronounced impact on biometric and metabolic changes compared to the DIO model. Long-term melatonin treatment successfully prevented the development of metabolic abnormalities associated with the high fat diet and obesity as well as significantly reduced myocardial infarct size. The mechanisms involved in melatonin-induced cardioprotection in obesity have not been fully elucidated in this study and require further investigation. However, the anti-obesogenic and cardioprotective properties of melatonin were very significant indeed and support the suggestion of this hormone as a potential tool in the treatment of obesity and associated cardiovascular complications.

AFRIKAANSE OPSOMMING: Inleiding: Vetsug (obesiteit) het wêreldwyd epidemiese afmetings aangeneem en word tans as ‘n ‘n ernstige gesondheidsprobleem beskou. Vetsug word geassosieer met metaboliese afwykings, oksidatiewe stres, hipertensie, insulienweerstandigheid en is‘n belangrike risikofaktor vir die ontwikkeling van kardiovaskulêre siekte. Ten spyte hiervan, het onlangse studies ‘n gunstige effek van vetsug op die uitkomste van miokardiale infarksie in pasiënte gerapporteer, die sg obesiteitsparadoks. Kennis van die patofisiologiese meganismes onderliggend aan vetsug en die ontstaan van metaboliese afwykinge en hartsiekte is noodsaaklik vir die voorkoming en behandeling van hierdie toestande. Melatonien, die hormoon afgeskei deur die pineaalklier, is ‘n kragtige antioksidant en vry radikaal opruimer. Dit is voorheen aangetoon dat dit die hart teen iskemie/herperfusie (I/H) besering kan beskerm en sommige van die skadelike gevolge van vetsug in diermodelle kan omkeer. Die effek van melatonien op miokardiale I/H besering en intrasellulêre seintransduksie prosesse in vetsug geïduseer deur ‘n hoë vet dieet is egter nog onbekend. Doelstellings: (i) Die ontwikkeling en karakterisering van ‘n nuwe model van vetsug en insulienweerstandigheid geïnduseer deur 'n hoë vet dieet (HVD) en die evaluering van die effek daarvan op miokardiale I/H besering en die gepaardgaande intrasellulêre seintransduksieprosesse; (ii) Bepaling van die effek van daaglikse toediening van melatonien aan rotte op die HVD sowel as aan kontroles op ‘n standard dieet, op die ontwikkeling van dieet-geïnduseerde metaboliese veranderinge, miokardiale infarktgrootte en funksionele herstel na koronêre arterie afbinding, sowel as intrasellulêre seintransduksie. Metodiek: Vier groepe van manlike Wistar rotte is bestudeer: (i) kontrole rotte (op‘n standaard dieet) (K); (ii) kontrole rotte op ‘n standard dieet plus melatonien (KM); (iii) dieetrotte (op‘n HVD); (iv) HVD rotte wat melatonien ontvang (HM). Die HVD en melatonien (10mg/kg/dag in die drinkwater) is vir 16 weke toegedien. Na die periode van behandeling, is die diere in vastende en nie-vastende groepe verdeel, die biometriese parameters genoteer en bloedmonsters vir metaboliese en biochemiese bepalings versamel, tydens verwydering van die harte. Harte is geperfuseer volgens die werkhartmodel vir bepaling van miokardiale funksie en infarktgrootte na blootstelling aan 35min streeksiskemie. Vir evaluering van intrasellulêre seintransduksie, is geperfuseerde werkende rotharte blootgestel aan 15min globale iskemie/10 min herperfusie en gevriesklamp vir latere analises volgens die Western kladtegniek.hart. Serum leptien, adiponektien, vryvetsure, trigliseried, totale cholesterol, fosfolipiede, gekonjugeerde diene en tiobarbituursuur reaktiewe stowwe (TBARS) is bepaal. Met gebruik van Western kladtegniek, is die aktivering en/of uitdrukking van die RISK (PKB/ Akt en ERK p44/42) en SAFE (STAT-3) seintransduksiepaaie, AMPK, JNK, UCP-3 en PGC1-α, onder basislyn toestande of na 10 min herperfusie bestudeer. Resultate:‘n Beduidende toename in liggaamsgewig, visserale vet, die HOMA indeks, insulien en leptien vlakke is in die HVD groep waargeneem vergeleke met die kontrole (K) rotte. Adiponektien vlakke was laer in die HVD groep. Die HVD groep is ook gekenmerk deur ‘n beduidende styging in serum vryvetsuur en trigliseried vlakke, terwyl die ander lipied parameters, insluitende die TBARS vlakke, onveranderd was. Infarktgrootte en funksionele herstel tydens herperfusie na blootstelling aan 35 min streeksiskemie, asook funksionele herstel tydens herperfusie na 20 min globale iskemie het nie verskil tussen harte van die kontrole en HVD rotte nie. Aktivering van PKB/Akt, ERK p44/p42, STAT3, AMPK en JNK by basislyn en na 10 min herperfusie was soortgelyk in die kontrole en HFD groepe. Die HVD het ook geen effek op die uitdrukking van UCP-3 en PGC1-α in vergelyking met die kontrole gehad nie. Behandeling met melatonien het die liggaamsgewig, visserale vet, bloedglukose, HOMA indeks en serum leptien vlakke in die HVD groepe statisties beduidend verlaag, terwyl dit geen invloed op die lipiedprofiel gehad het nie. Melatonien behandeling het die miokardiale infarktgrootte beduidend en tot dieselfde mate verminder in beide kontrole [20.59 ± 2.29 (KM) vs 38.08 ± 2.77% (K)] en HVD groepe [11.43 ± 2.94 (HM) vs 38.06 ± 3.59% (HVD)]. Geen verskille is egter tussen die funksionele herstel gedurende herperfusie van die behandelde en onbehandelde kontrole en HVD groepe waargeneem nie. Melatonien het ook geen uitwerking op die intrasellulêre seintransduksiepaaie gehad nie. Gevolgtrekkings: Die resultate het getoon dat die HFD 'n goeie model van dieetgeïnduseerde vetsug en insulien weerstandigheid ontlok, met 'n meer uitgesproke impak op biometriese en metaboliese veranderinge as die voorheen gebruikte hoë-sukrose dieet. Langtermyn melatonien- behandeling het die ontwikkeling van metaboliese abnormaliteite geassosieer met die HVD, voorkom, asook miokardiale infarktgrootte na koronêre afbinding beduidend verminder. Die meganismes betrokke in melatonien-geïnduseerde miokardiale beskerming moet egter in meer detail ondersoek word. Die resultate verkry steun die voorstel dat melatonientoediening voordelig sal wees in die behandeling van vetsug en sy kardiovaskulêre komplikasies.

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