Endogenous Acetylcholine Contributes to Flow-Mediated Dilation in Humans

Abstract

Background & Objective. Flow-mediated dilation (FMD) is a crucial function of the vascular endothelium wherein intraluminal shear stress stimulates endothelium-dependent vasodilation mediated primarily by nitric oxide. Brachial artery FMD (baFMD) is widely evaluated in clinical research as an index of endothelial function, and impaired baFMD is an independent predictor of cardiovascular risk. Despite the physiological and clinical relevance of FMD, the mechanisms by which shear stress is transduced to endothelium-dependent vasodilation (i.e., upstream of nitric oxide production) remain unclear. baFMD corresponds closely with sensitivity to the endothelium-dependent vasodilator acetylcholine (ACh), and preclinical models suggest ACh is released from endothelial cells in response to shear stress. Therefore, the present study tested the hypothesis that activation of muscarinic ACh receptors facilitates baFMD in humans. Methods. baFMD was evaluated using Doppler ultrasound in healthy participants (n=11; 5F/6M; 23±2 y; means ± SE) under control conditions and following local inhibition of muscarinic ACh receptors via intra-arterial atropine infused proximal to the measurement site. baFMD was assessed in response to transient and sustained shear stimuli in separate trials: 1) reactive hyperemia following 5 min of arterial cuff occlusion (RH-FMD) and 2) steady-state rhythmic handgrip exercise at graded intensities (10, 15, 20, & 25% maximal voluntary contraction, MVC) to produce step increases in shear rate (EX-FMD). Blockade effcacy was confirmed via a dose-response to intra-arterial ACh infusion pre and post atropine. Results. Baseline brachial artery diameter and forearm blood flow were similar in all trials (all P>0.05). Atropine reduced peak brachial artery dilation following cuff release (RH-FMD, Δ diameter, atropine: 4.6±0.8 vs. control: 6.5±0.8%, P=0.04) and increased the shear stimulus (shear rate area under the curve, SRAUC: atropine, 29±3 vs. control: 21±3 s−1×103, P=0.01) such that the percent change in brachial diameter was diminished by 35±16% when normalized to SRAUC (baFMD:SRAUC ratio, atropine: 1.7±0.3 vs. control: 3.9±1.0, P=0.02). Similarly, in the EX-FMD trial, atropine reduced brachial artery diameter at all exercise intensities (25% MVC, percent change in diameter: atropine, 10.8±1.4 vs. control, 15.5±1.7%, P=0.04) despite a greater shear stimulus (shear rate: atropine, 10.9±1.1 vs. control, 9.3±0.9 s−1×102, P=0.01). Thus, atropine reduced the slope describing the relationship between brachial artery vasodilation (percent change) and the change in shear rate by 38±8% across all exercise intensities (slope, atropine: 1.3±0.1 vs. control: 2.4±0.4% Δ×s×10−2, P=0.007). As expected, atropine reduced brachial artery vasodilation in response to upstream infusion of ACh (1.5 μg ACh/dl forearm volume/min, Δ diameter, atropine: 2±1 vs. control: 23±4%, P=0.001). Conclusions. The results suggest endogenous ACh facilitates baFMD in response to both transient and sustained shear stimuli, which highlights a novel physiological role of endogenous ACh in vascular regulation in humans and provides insight regarding the basic mechanisms of flow-mediated vasodilation. NIH R01 HL119337 and F31 HL142240. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.