| Background:Insulin resistance means that physiological level of insulin does not play the corresponding biological effect. In recent years, with raising standard of living and changes of life style, the incidence of type 2 diabetes increases. It can be caused by environmental and genetic factors. Of them,, chronic ethanol feeding and/or high fat diet play important roles in insulin sensitivity. Ethanol is the most abused drugs used at present. Long term's consumption may be one of the causes leading to insulin resistance. However, epidemiological data concerning ethanol consumption to glucose metabolism or insulin sensitivity are contradictory. For example, it was shown that insulin sensitive is higher in regular moderate ethanol intaked drinkers than that in abstainers. On the other hand, both continuous and high intake of alcohol decreased insulin sensitivity, which could lead to insulin resistance. Taking these reports into consideration, it seems that the effect of alcohol intake on insulin sensitivity remain obscure. It suggests that chronic feeding ethanol is as a independent factor to the impairment of insulin sensitivity.Epidemiological data and experimental researches indicate that high-fat diet is also a single risk factor leading to insulin resistance and type 2 diabetes. Although some researches have demonstrated the effects of high-fat exposure on insulin resistance in several insulin target organs, including skeletal muscles, liver, adipocytes etc, little research has done on heart muscle and so the precise mechanism is not fully eludated yet.To date, both in vivo and in vitro studies have demonstrated that ethanol and/or high-fat induce insulin sensitive drop accompanied with the decline in several molecules expression of insulin signal pathwaysuch as insulin receptor (IR), insulin receptor substrate (IRS) and Glucose transporter 4 (GLUT4), which is proposed as possible contributor to insulin resistance. However, it has been reported that normal glucose intake occurred in those mice with knoctout IR. Thus, insulin signaling pathway only can not well explained the decreased insulin sensitivity induced by ethanol and/or high fat diet.AMPK, AMP-activated protein kinase, is a key regulator of intracellular fatty acid metabolism. Stresses such as exercise, hypoxia, or prolonged starvation could activate AMPK via increasing intracellular AMP/ATP ratio. Besides regulating fatty acid oxidation, AMPK has been proved recently as a regulator of insulin signaling pathway. Recently, a few studies have addressed that AMPK is necessary for GLUT4 regulating glucose uptake. Noticeable, the regulation by AMPK is through a common insulin-independent manner. It is unclear whether AMPK also participates in the pathophysiological process of high-fat and ethanol inducing insulin resistance.Further studies demonstrate that the effect of AMPK on GLUT4 is mediated by myocyte enhancer factor 2 (MEF2), a muscle-specific regulator of GLUT4 gene transcription. The information of published studies suggested that phosphorylated-AMPK upregualtes MEF2 gene transcription leading to an increase in MEF2 protein level, subsequently enhancing its binding activity with GLUT4 promoter to impel GLUT4 gene transcription.Although some researches have reported that ethanol and/or high fat diet can decrease AMPK's expression in skeletal muscle and hepatic cells, there are lacking research in heart muscle.Objective:①effects of chronic ethanol feeding and/or high fat diet on insulin sensitivity of rats atbody level.②effects of ethanol and/or high fat diet on the expression of insulin receptor, insulin receptor substrate 1, insulin receptor substrate 2 and GLUT4 in rat heart muscle.③effects of ethanol and high fat die on AMPK and MEF2 in rat heart muscle.Methods:①Animal feeding: Seventy-two male Wistar rats, divided into six groups, received either distilled water (C, control group) or edible ethanol, which was administered by gastric tube with a single daily dose: 5 g.kg-1 (H, high dose group), 2.5 g.kg-1 (M, middle dose group ) and 0.5 g.kg-1 (L, little dose group), or high fat diet (group HF, ) or ethanol (5 g·kg-1) and high fat diet (group HF+E). In HF group and HF + E group, rats received a high-fat diet, in which 59% calories was provided by fatty acid (mainly lard). The total feeding time was 22 weeks.② Evaluation of insulin sensitivity. Fasting plasma glucose levels were monitored during fed period. At the end of 22 weeks, fasting serum insulin levels as well as visceral fat contents were measured and oral glucose tolerance test (OGTT) was performed, and calculate the HOMA-IR, repectively.③ Effects of ethanol and/or high-fat diet on classical insulin signal pathway: the mRNA levels of IR, IRS1, IRS2 and GLUT4 of rat heart muscle were measured using RT-PCR method, the protein level of GLUT4 was detected by western blotting method.④Effects of ethanol and/or high-fat diet on expression of AMPK signaling pathway: the mRNA levels of AMPK alpha 1 and alpha 2 subunits were measured using RT-PCR; Protein levels of total-and phospho-AMPK alpha subunit were detected using Western blot method.⑤The mRNA and protein levels of MEF2 in the hearts of rats after ethanol and/or high-fat treated were measured by RT-PCR and western blotting method, respectively.Results:①Evaluation of insulin sensitivity at body level: with being treated for 22 weeks, for fasting plasma glucose, fasting serum insulin and 2 h plasma glucose levels after oral glucose load, we failed to detect the significant differences between control and each treated group. But the levels of fasting serum insulin increased both in group HF and group HF+E(P<0.01)②Effects of ethanol and/or high fat diet on classical insulin signal pathway of rat heart muscle: except for IR, the mRNA levels of IRS1, IRS2, GLUT4, AMPKal, AMPKa2, MEF2A and MEF2D decreased in all treated groups relative to the pair-rats.③Effects of ethanol and/or high fat diet on the expression of protein level of GLUT4, AMPKα and MEF2 of rats' heart muscle: Compared with the controls, ethanol and high fat caused decreases in those items above in all groups.Conclusions:①Chronic ethanol and/or high fat consumption could impair insulin sensitivity in rat heart muscle, at least partly via decreasing the expression of GLUT4. ②Ethanol and high fat induced the decline in IR, IRS1 and IRS2 expression, which partly contributed to the decreased expression of GLUT4.③Chronic ethanol and high fat exposure decreased the expression of GLUT4 through , at least, AMPK/MEF2.④chronic Ethanol feeding mainly impaired AMPK signaling pathway, while high fat diet impairment focused on insulin ignaling pathway .
|
PDF Full Text Request