Role of insulin resistance in nucleus tractus solitarii on central cardiovascular regulation in rats
Insulin resistance was thought as the major etiology of hypertension of the metabolic syndrome. Both human and animal studies revealed sympathetic overactivity were present in the metabolic syndrome. Nowadays, most of the studies that examined the etiologies of hypertension of metabolic syndrome were focused on the pathophysiologic effects of insulin resistance on the peripheral vessels. However, there was no study ever examined the insulin resistance in cardiovascular regulatory centers of central nervous system or the pathogenesis of sympathetic overactivity in metabolic syndrome. Our previous study demonstrated that insulin plays a cardiovascular regulatory role in the nucleus tractus solitarii (NTS), one of the cardiovascular regulatory centers in the brain stem. We also demonstrated that the cardiovascular regulatory effects of insulin in the NTS were accomplished through activating PI3K-PKB/Akt-NO signaling pathways. Recently, increases in oxidative stress could raise the incidence rate of diabetes mellitus and cardiovascular diseases had been reported. Besides, it has been reported that there were marked increases in reactive oxidative species (ROS) in various hypertension animal models. It was also reported that elevation of ROS in various tissues may activate the mitogen-activated protein kinase (MAPK) superfamily. Activated MAPKs may phosphorylate insulin receptor substrate 1 (IRS1) on the serine 307 residue. It has been reported that IRS1S307 phosphorylation would inhibit normal insulin signal transduction. The aims of this thesis were to investigate whether the neuronal cells in the NTS would develop insulin resistance in the metabolic syndrome rats, whether development of insulin resistance in the NTS cause hypertension in the metabolic syndrome rats, which signaling molecule in insulin signaling pathway is the key molecule that cause insulin resistance in the NTS, and what the pathogenesis of insulin resistance is in the NTS of metabolic syndrome rats. In the pioneer study, Wistar-Kyoto (WKY) rats were fed with 10% fructose water as their drinking water for 8 weeks. Another group of fructose-fed WKY rats were fed with insulin sensitizer, rosiglitazone, since the 5th week. Blood pressure was measured by tail vein sphygmomanometer every week and venous blood were draw to measure blood sugar and insulin level every other week. Thereafter, all the rats enrolled in this study were fed with 10% fructose water with/without rosiglitazone for 2-3 weeks. My results demonstrated the blood pressure of fructose-fed WKY rats was significantly elevated after 2-week fructose feeding. But at the same time, HOMA-IR did not elevated, which indicated the insulin resistance in the peripheral did not develop yet. Interestingly, at the same time, endogenous insulin in the NTS was significantly elevated in the fructose-fed group. The cardiovascular responses of insulin in the NTS were diminished in the fructose-fed group. While in the rosiglitazone-treated group, the blood pressure and endogenous insulin in the NTS were decreased the baseline level. The cardiovascular responses of insulin in the NTS were restored in the rosiglitazone-treated group. These results indicated insulin resistance do develop in the NTS of fructose-fed rats, and the neuronal insulin resistance in the NTS can induce hypertension. The immunoblotting results demonstrated the phosphorylation of IRS1S307 was significantly elevated in the fructose-fed rats. While the phosphorylation of its downstream molecules, such as AktS473 and eNOSS1177, were significantly decreased as compared with the control group. In the NTS of rosiglitazone-treated group, the phosphorylation of IRS1S307 was decreased, and the phosphorylation of AktS473 and eNOSS1177 were restored. These results indicated that the underline pathogenesis of insulin resistance in the NTS was phosphorylation on the inhibitory serine residue of IRS1, which interfered with the normal insulin signal transduction in the NTS. Increases in ROS in the NTS of fructose-fed rats were demonstrated in the DHE histostaining. Phosphorylation of p38MAPK in the NTS of fructose-fed rats was also detected by immunoblotting. In the NTS of Tempol-treated fructose-fed rats, the phosphorylation of p38MAPK reduced and the nitric oxide production elevated to the basal level. Blood pressure decreased significantly when p38MAPK inhibitor, SB203680, was microinjected into the NTS of fructose-fed rats. These results indicated the pathogenesis of insulin resistance in the NTS is increases in ROS in the NTS, which activate p38MAPK and then phosphorylate IRS1S307. In conclusion, the neuronal cells in the NTS may develop insulin resistance in fructose-fed rats, and the neuronal insulin resistance in the NTS contributes to the hypertension of metabolic syndrome. The mechanism of insulin resistance in the NTS is phosphorylation on the serine 307 residue of IRS1, which interfere with insulin signaling and subsequent NO production in the NTS. The pathogenesis of IRS1S307 phosphorylation is activated p38MAPK which in turn is activated by ROS in the NTS.
Advisor:Ching-Jiunn Tseng; Wen-Chun Hung; Hsiao, Michael; Pei-Jung Lu
School:National Sun Yat-Sen University
School Location:China - Taiwan
Source Type:Master's Thesis
Keywords:hypertension nucleus tractus solitarii insulin resistance
Date of Publication:07/23/2007