Diet-Induced Obesity Impairs Endothelium-Derived Hyperpolarization via Altered Potassium Channel Signaling Mechanisms

BackgroundThe vascular endothelium plays a critical role in the control of blood flow. Altered endothelium-mediated vasodilator and vasoconstrictor mechanisms underlie key aspects of cardiovascular disease, including those in obesity. Whilst the mechanism of nitric oxide (NO)-mediated vasodilation has been extensively studied in obesity, little is known about the impact of obesity on vasodilation to the endothelium-derived hyperpolarization (EDH) mechanism; which predominates in smaller resistance vessels and is characterized in this study. Methodology/Principal FindingsMembrane potential, vessel diameter and luminal pressure were recorded in 4th order mesenteric arteries with pressure-induced myogenic tone, in control and diet-induced obese rats. Obesity, reflecting that of human dietary etiology, was induced with a cafeteria-style diet (∼30 kJ, fat) over 16–20 weeks. Age and sexed matched controls received standard chow (∼12 kJ, fat). Channel protein distribution, expression and vessel morphology were determined using immunohistochemistry, Western blotting and ultrastructural techniques. In control and obese rat vessels, acetylcholine-mediated EDH was abolished by small and intermediate conductance calcium-activated potassium channel (SKCa/IKCa) inhibition; with such activity being impaired in obesity. SKCa-IKCa activation with cyclohexyl-[2-(3,5-dimethyl-pyrazol-1-yl)-6-methyl-pyrimidin-4-yl]-amine (CyPPA) and 1-ethyl-2-benzimidazolinone (1-EBIO), respectively, hyperpolarized and relaxed vessels from control and obese rats. IKCa-mediated EDH contribution was increased in obesity, and associated with altered IKCa distribution and elevated expression. In contrast, the SKCa-dependent-EDH component was reduced in obesity. Inward-rectifying potassium channel (Kir) and Na+/K+-ATPase inhibition by barium/ouabain, respectively, attenuated and abolished EDH in arteries from control and obese rats, respectively; reflecting differential Kir expression and distribution. Although changes in medial properties occurred, obesity had no effect on myoendothelial gap junction density. Conclusion/SignificanceIn obese rats, vasodilation to EDH is impaired due to changes in the underlying potassium channel signaling mechanisms. Whilst myoendothelial gap junction density is unchanged in arteries of obese compared to control, increased IKCa and Na+/K+-ATPase, and decreased Kir underlie changes in the EDH mechanism.背景 血管内皮在血流调控中发挥关键作用。内皮介导的血管舒张与收缩机制异常，是包括肥胖相关心血管疾病在内的多种心血管疾病的核心病理基础。尽管肥胖状态下一氧化氮（nitric oxide, NO）介导的血管舒张机制已得到广泛研究，但目前对肥胖如何影响内皮源性超极化（endothelium-derived hyperpolarization, EDH）机制介导的血管舒张尚不清楚；而EDH是小阻力血管中的主要舒张机制，亦是本研究的核心表征对象。 材料与方法及主要结果 本研究以具有压力诱导肌源性张力的4级肠系膜动脉为实验对象，记录对照组与饮食诱导肥胖大鼠的血管膜电位、直径及管腔压力。本研究采用自助餐厅式高脂饲料（脂肪供能约30 kJ）连续喂养16~20周，构建模拟人类饮食源性肥胖的大鼠模型；同周龄、性别匹配的对照组大鼠则饲喂标准维持饲料（脂肪供能约12 kJ）。通过免疫组织化学、蛋白质印迹（Western blotting）及超微结构检测技术，本研究分析了血管通道蛋白的分布、表达水平及血管形态学特征。 在对照组与肥胖大鼠的血管中，乙酰胆碱介导的EDH可被小电导钙激活钾通道（small and intermediate conductance calcium-activated potassium channel, SKCa/IKCa）抑制剂完全阻断，且该通路活性在肥胖状态下受损。分别使用环己基-[2-(3,5-二甲基吡唑-1-基)-6-甲基嘧啶-4-基]-胺（CyPPA）与1-乙基-2-苯并咪唑啉酮（1-EBIO）激活SKCa与IKCa，可分别使对照组及肥胖大鼠的血管发生超极化并舒张。肥胖状态下，IKCa介导的EDH贡献占比升高，且这一变化与IKCa的分布改变及表达上调相关；与之相反，肥胖状态下SKCa依赖性EDH组分的功能显著降低。内向整流钾通道（inward-rectifying potassium channel, Kir）与Na+/K+-ATP酶，分别由钡剂与哇巴因阻断后，可分别减弱对照组大鼠动脉的EDH，并完全消除肥胖大鼠动脉的EDH，这一差异反映了Kir表达与分布的状态改变。尽管血管中膜的特性发生了改变，但肥胖并未影响肌内皮缝隙连接的密度。 结论与意义 肥胖大鼠的EDH介导血管舒张功能受损，其根源在于钾通道信号通路的改变。与对照组相比，肥胖大鼠动脉的肌内皮缝隙连接密度并无变化，但IKCa与Na+/K+-ATPase表达上调、Kir表达下调，共同构成了EDH机制异常的病理基础。