Low frequency intermittent hypoxia reduces endothelium-dependent contraction in C57 mice with diet-induced obesity

Abstract

INTRODUCTION: Obstructive sleep apnoea (OSA) is associated with elevated systemic oxidative stress and inflammation, resulting in endothelial dysfunction. Obesity, a major risk factor for the development and severity of OSA, also modulates contractile and relaxing responses in the blood vessels. OBJECTIVE: To study the impact of intermittent hypoxia (IH), a hallmark feature of OSA, on endothelium-dependent contraction and relaxation in lean and obese mice. METHODS: Male C57BL/6N mice were fed with normal (Std) or high-fat high-cholesterol (HFHC) diet for 6 weeks before exposing to normoxia (Air) or chronic IH (IH) treatment (10 hypoxic events/hour [O2 10% at nadir; 6 h/d]) for 6 weeks. All mice were then sacrificed and endothelial function was assessed ex vivo, using the wire myograph device. Quiescent carotid arteries were used to evaluate endothelium-dependent contraction to acetylcholine, in the presence of L-nitro-arginine methyl ester (L-NAME; a non-selective inhibitor of nitric oxide synthase). To investigate the role of oxidative stress in mediating the endothelium-dependent contraction, half of the carotid arterial rings were pre-incubated with apocynin, an inhibitor for superoxide-generating enzyme, NADPH oxidase, together with L-NAME, before acetylcholine-induced contraction. Pre-contracted aortae were used to assess the endothelium-dependent relaxation to acetylcholine. RESULTS: In the presence of L-NAME, cumulative addition of acetylcholine induced a basal level of endothelium-dependent contraction in carotid artery of Std+Air group. The contraction was significantly increased when the mice were fed with HFHC diet (HFHC+Air) in comparison to Std+Air group. However, treatment with apocynin significantly abolished the augmented response in HFHC+Air. After IH exposure, the HFHC-induced endothelium-dependent contraction was significantly reduced in comparison to HFHC+Air group. There was comparable level of endothelium-dependent contraction between Std+Air and Std+IH groups. On the contrary, treatment with HFHC diet, IH, or their combination did not affect the endothelium-dependent relaxation in aorta compared to Std+Air. CONCLUSION: The data suggest the involvement of superoxide in HFHC-induced endothelium-dependent contraction in carotid artery without affecting the endothelium-dependent relaxation in obese mice. The reduction of HFHC-induced endothelium–dependent contraction on low-frequency IH exposure suggests a defensive mechanism on endothelial function.