Impaired mitochondria-dependent vasodilation in cerebral arteries of Zucker obese rats with insulin resistance

Abstract

Mitochondria affect cerebrovascular tone by activation of mitochondrial ATP-sensitive K+ (K ATP) channels and generation of reactive oxygen species (ROS). Insulin resistance accompanying obesity causes mitochondrial dysfunction, but the consequences on the cerebral circulation have not been fully identified. We evaluated the mitochondrial effects of diazoxide, a putative mitochondrial K ATP channel activator, on cerebral arteries of Zucker obese (ZO) rats with insulin resistance and lean (ZL) controls. Diameter measurements showed diminished diazoxide-induced vasodilation in ZO compared with ZL rats. Maximal relaxation was 38 +/- 3% in ZL vs. 21 +/- 4% in ZO rats (P < 0.05). Iberiotoxin, a Ca2+-activated K+ channel inhibitor, or manganese(III) tetrakis(4-benzoic acid)porphyrin chloride, an SOD mimetic, or endothelial denudation diminished vasodilation to diazoxide, implicating Ca2+-activated K+ channels, ROS, and endothelial factors in vasodilation. Inhibition of nitric oxide synthase (NOS) in ZL rats diminished diazoxide-induced vasodilation in intact arteries, but vasodilation was unaffected in endothelium-denuded arteries. In contrast, NOS inhibition in ZO rats enhanced vasodilation in endothelium-denuded arteries, but intact arteries were unaffected, suggesting that activity of endothelial NOS was abolished, whereas factors derived from nonendothelial NOS promoted vasoconstriction. Fluorescence microscopy showed decreased mitochondrial depolarization, ROS production, and nitric oxide generation in response to diazoxide in ZO arteries. Protein and mRNA measurements revealed increased expression of endothelial NOS and SODs in ZO arteries. Thus, cerebrovascular dilation to mitochondria-derived factors involves integration of endothelial and smooth muscle mechanisms. Furthermore, mitochondria-mediated vasodilation was diminished in ZO rats due to impaired mitochondrial K(ATP) channel activation, diminished mitochondrial ROS generation, increased ROS scavenging, and abnormal NOS activity.

Insulin-Induced Generation of Reactive Oxygen Species and Uncoupling of Nitric Oxide Synthase Underlie the Cerebrovascular Insulin Resistance in Obese Rats

“…We previously observed that NOmediated vasodilation was diminished in ZO arteries despite the evidence of increased eNOS and iNOS expression compared with ZL arteries (Erdos et al, 2004;Katakam et al, 2009b). Moreover, a reduced phosphorylated NOS protein in response to insulin, consistent with reduced NO-mediated vasodilation, was observed in ZO arteries compared with ZL arteries.…”

“…However, the significant distinction displayed by insulin in inducing NOS uncoupling in ZO arteries compared with many vasodilators we previously studied (Erdos et al, 2004;Katakam et al, 2005Katakam et al, , 2009b assures the specificity of insulin's action in cerebrovascular IR. Although it has been reported that hyperleptinemia has been associated with increased eNOS expression and uncoupling (Korda et al, 2008); hyperleptinemia has not been shown to directly stimulate NOS uncoupling.…”

“…For example, previous studies in our laboratory showed that inhibition of NOS diminished vasodilation in cerebral arteries of ZL arteries in response to two mechanistically different vasodilators, acetylcholine and diazoxide (Erdos et al, 2004;Katakam et al, 2009b). Unlike insulin, neither acetylcholine nor diazoxide improved vasodilation to insulin in ZO arteries in the presence of NOS inhibitors, suggesting that insulin displays unique ability to promote NOS uncoupling in IR (Erdos et al, 2004;Katakam et al, 2009b). In addition, we observed the ability to modulate expression of GTP-CH enzyme and activity of NOS isoforms unique to insulin.…”

“…Recently, we identified insulin receptors in rat cerebral arteries and have reported the cerebrovascular actions of insulin (Katakam et al, 2009b). Cerebral arteries are subjected to hyperinsulinemia in IR and T2DM; however, the impact of insulin on cerebral vasoreactivity in IR has never been examined.…”

Reactive oxygen species, reactive nitrogen species and antioxidants in etiopathogenesis of diabetes mellitus type-2

“…It is a cardinal feature in DMT-2, present in about 75-80% patients and almost a universal feature in obese diabetics. The sequence of events are given in Fig 11. It is also present in other settings such as metabolic syndrome, cancer, cardiovascular diseases, burn trauma, cachexia, starvation, pregnancy conditions, but why IR occurs in so many contexts is poorly understood except in case of DMT-2 and obesity (95)(96)(97). The expression of IR differs in different conditions.…”