| BackgroundThe prevalence of diabetes mellitus is increasing rapidly, which has become a major problem of public health. Obesity is closely related to the etiology and pathogenesis of type 2 diabetes, and the disturbance of lipid homeostasis is perhaps the link. McGarry's hypothesis of"diabetes mellipitus"might be an explanation of the relationship between obesity and Type 2 diabetes, which renewed the concept of type 2 diabetes. There has been a great deal of literature on the theory of"lipotoxicity", and in fact the idea that abnormalities in free fatty acid (FFA) metabolism may be involved in the etiology of type 2 diabetes was proposed in 1963, when Randle and colleagues introduced the new concept of a glucose-fatty acid cycle. Later other researchers suggested that increased plasma FFA inhibit glucose-stimulated insulin secretion (GSIS), and intracellular deposition of triglyceride (TG) in pancreatic beta cell cause apoptosis and impair regeneration. Furthermore, not only does excessive TG deposit in pancreatic beta cell, but also in skeletal muscle and hepatocyte. These excessive cellular deposition of TG can impair the normal function of these organs, leading to insulin resistance and beta cell dysfunction, which are regarded as the two major pathogenetic factors of T2DM. And adipocytokines which come from hypertrophic adipose cell also lead to insulin resistance.The impaired FFA oxidation in nonadipose tissue cause cytosolic triglyceride accumulation in nonadipose tissues, lead to insulin resistance and beta cell dysfunction [4].The increase TG in hepatocyte inhibit liver inulin and insulin receptor binding, induce hyperinsulinemia and impaired glucose tolerance which cause liver IR[5]. The TG accumulate in skeletal muscle inhibit glucose uptake and utilization which induce muscle IR[5]. It has been proposed that the increase diacylglycerol and PKC activity is relate to skeletal muscle IR the increased insulin receptor and insulin receptor substrate - 1 phosphorylation impair intracellular signal transduction which is also related to skeletal muscle IR[6],. Intracellular deposition of triglyceride (TG) in pancreatic beta cell inhibit glucose-stimulated insulin secretion (GSIS), cause apoptosis and impair regeneration[7].It is proposed that the mechanism maybe the increased FFA heighten acyl-CoA ,active NF-κB by produce cermaide ,which increase NO and free radicle that induce the apoptosis ofβcell. The excessive cellular deposition of TG in nonadipose tissues can lead to insulin resistance and beta cell dysfunction, which may induce T2DM. But the mechanism of this dysregulation remains to be determined. But others view that the abnormalities of FFA can induce cell dysfunction of glucose uptake, transport, phosphorylation, gluconeogenesis, oxidation , which inhibit glucose uptake and utilization and induce insulin resistance. Due to the important role that lipotoxicity plays in pathogenesis of T2DM,we studied the elevated FFA flux from an expanded adipose tissue to nonadipose tissues and observe the excessive cellular deposition of TG in nonadipose tissues in SD rats with high-fat diet. And study the relationship between FFA,cellular deposition of TG in nonadipose tissue and the early stage of IR.AIMRats were fed with the high-fat diet during the test, blood samples are taken from rat, then plasma FFA, glucose and insulin are determined. To explore the accumulation of TG in liver,muscle and pancreas in high-fat-diet induced insulin resistance rat model and the relationship between insulin resistance and the accumulation of TG in liver,muscle and pancreas.METHODSMale rats (n=60)were individually housed and were randomly assigned to two dietary groups: high fat diet (n=30) and normal diet(30).And each group were randomly divided in three groups,2 ,4 and 6 weeks groups (n=10). Diet and water were both unlimited at all times. During the study, high fat diet group and normal diet were kill every two weeks, body weight were measured every two week. The liver, muscle and pancreas sample of rat in each group were taken for oil red O staining after experiment .Blood samples were taken from all rats of the high fat diet or normal diet group. Then the blood samples were taken from the rats by tail bleeding and collected in tubes every two weeks, then the samples were centrifuged and the separated plasma was immediately assayed for glucose, the remaining plasma was frozen and stored at -70°C for later measurement of FFA, insulin.The concentrations of insulin were measured by using a RIA-kit. FFA were measured with a NEFA-C kit. Compare FFA, FINS, insulin resistance index of each group ,and observe the intracellular TG accumulation in liver ,muscle and pancreas, investigate the relationship between FFA, the cytosolic TG accumulation in liver ,muscle and pancreas and insulin resistance.RESULTS1. FINS,glucose and insulin resistance index During the study, There was no difference in glucose concentration between high-fat diet group(5.14±0.41,5.20±0.47,5.32±0.54)and normal-diet group(5.12±0.25,5.14±0.31,5.12±0.46, P >0.05). There was no difference in insulin concentration between high-fat diet rats and normal diet on the 2th week (13.60±2.07 vs12.02±1.37, P >0.05),but significant difference was found on the 4th week(25.05±2.95 vs 12.57±1.20,P <0.01) and the 6th week(31.96±2.57 vs 12.64±1.35, P <0.01). There was no difference in insulin resistance between high-fat diet rats and normal diet on the 2th week(3.12±0.63 vs 2.73±0.35,P >0.05),but significant difference was found on the 4th week(5.81±0.97 vs 2.87±0.29,P <0.01)and the 6th week(7.56±1.12 vs 2.86±0.34, P <0.01).2. On the 4th week , the plasma FFA concentrations were increased only in high-fat diet rats(0.68±0.03 vs 0.60±0.04,P <0.01).On the 6th week , the plasma FFA concentrations were increased only in high-fat diet rats(0.74±0.04 vs 0.62±0.05, P <0.01).There was a significant association between FFA and FINS(r=0.617,P <0.01)in high fat diet rats ;and also significant association between FFA and insulin resistancein high fat diet rats(r=0.694,P <0.01.3. Oil red O staining and computer semi-quantitative analysis During the study, there were significant increases in TG in liver and in muscle than that in the control group on the six week,there was no difference in the accumulation of TG in pancreas. There was no difference in liver intracellular TG accumulation between high-fat diet rats and normal diet on the 2th week(151.0±5.20 vs 153.10±4.16,P >0.05),but there was significant difference on the 4th week(113.75±6.52 vs 140.65±5.12,P <0.01) and on the 6th week(99.50±4.67 vs 128.5±3.96,P <0.01). There was no difference in muscle cytosolic TG accumulation between high-fat diet rats and normal diet on the 2th week(154.05±3.52 vs 155.00±5.6,P >0.05),but significant difference on the 4th week (131.92±3.42 vs 139.39±2.3,P <0.01)and on the 6th week(127.60±3.10 vs 134.70±2.98,P <0.01). There was no difference in pancreas cytosolic TG accumulation between high-fat diet rats and normal diet rats(P >0.05).4.There was a significant association between liver cytosolic TG accumulation and FINS(r=-0.882,P <0.01); between muscle cytosolic TG accumulation and FINS(r=-0.891,P <0.01)in high fat diet rats .5.There was a significant association between liver cytosolic TG accumulation and FFA(r=-0.753,P <0.01); between muscle cytosolic TG accumulation and FFA(r=-0.736,P <0.01)in high fat diet rats .CONCLUSIONS1. The high-fat diet can increase fasting plasma FFA and fasting plasma insulin significantly after 4 week in SD rats.This hint that the increasing fasting plasma FFA is significantly associate with insulin resistance in high- fat-diet SD rats.2. During the study, there were significant increases in the accumulation of TG in liver and muscle in the high-fat-diet SD rats on the 4th and 6th week; and there was significant association between the accumulation of TG in liver,muscle and the fasting plasma FFA and fasting plasma insulin.These results hint that the accumulation of TG in liver and muscle maybe participant insulin resistance emergence on the onset stage of high fat diet in SD rats. However, there was no difference in the accumulation of TG in pancreas between high-fat diet and normal diet on the 4 th and 6th week,these results hint that the accumulation of TG in pancreas maybe not participant insulin resistance emergence on the onset stage of high fat diet in SD rats.
|
PDF Full Text Request