Impaired Atrial M 2 -Cholinoceptor Function in Obesity-Related Hypertension

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Abstract
—The aim of this study was to investigate the activity of the parasympathetic limb of the baroreflex arch in a canine model of obesity-related hypertension. Twelve male beagle dogs were randomized into 2 groups. Six dogs were fed with normal canine food and 6 were submitted to a 10-week high-fat diet (HFD). We have evaluated the consequences of HFD on heart rate (HR) and blood pressure (BP) circadian cycles and methylscopolamine dose-response curves. Binding of [ 3 H]-AF-DX 384 and adenylyl cyclase activity were investigated to determine the density and functionality of M 2 -cholinoceptors on right atrial membranes from control and HFD dogs. HFD induced a significant increase in body weight (15±1 vs 12±1 kg), systolic BP (161±5 vs 145±4 mm Hg), diastolic BP (92±3 vs 79±2 mm Hg), and HR (96±4 vs 81±3 bpm). Circadian rhythms of HR and BP observed in the baseline period were abolished after 9 weeks of HFD. After propranolol (1 mg/kg) pretreatment, the dose of methylscopolamine able to induce 50% maximum tachycardia was significantly increased after 9 weeks of HFD (7.4±0.3 vs 4.7±0.1 μg/kg). In the control group, the experimental period failed to modify these parameters. The numbers of M 2 -cholinoceptors measured in right atrial membranes were significantly lower in HFD than in control groups (54±6 vs 27±6 fmol/mg protein). The ability of carbachol to inhibit isoproterenol-stimulated adenylyl cyclase activity was significantly lower in HFD than in control groups (IC 50 =47±12 vs 6.4±1.4 μmol/L). However, the basal activity of adenylyl cyclase was unchanged by HFD. HFD decreases M 2 -cholinoceptor number and function in cardiomyocytes. This could explain the abolition of circadian rhythm of HR and the changes in chronotropic effect brought about by methylscopolamine.