1-Adrenoreceptor activity does not explain lower morning endothelial-dependent, flow-mediated dilation in humans

Research Institute for Sport and Exercise Sciences, Liverpool John Moores University, United Kingdom.
AJP Regulatory Integrative and Comparative Physiology (Impact Factor: 3.11). 03/2011; 300(6):R1437-42. DOI: 10.1152/ajpregu.00042.2011
Source: PubMed


Early morning reduction in endothelium-dependent, flow-mediated dilation (FMD) may contribute to the high incidence of sudden cardiac death at this time of day. The mechanisms underpinning diurnal variation in FMD are unclear, but potentially relate to a circadian rhythm in sympathetic nerve activity. We hypothesized that blockade of α(1)-mediated sympathetic nerve activity would act to attenuate the diurnal variation in FMD. In a randomized and placebo-controlled design, we measured brachial artery FMD in 12 participants (mean age = 26 yr, SD = 3) at 0600 and 1600 after ingestion of an α(1)-blocker (prazosin, 1 mg/20 kg body mass) or placebo. Arterial diameter and shear rate were assessed using edge-detection software. Heart rate and blood pressure were also measured. Data were analyzed using linear mixed modeling. Following placebo, FMD was 8 ± 2% in the morning compared with 10 ± 3% in the afternoon (P = 0.04). Blockade with prazosin led to a slight but nonsignificant increase in morning FMD (P = 0.24) and a significant (P = 0.04) decrease in afternoon FMD, resulting in no diurnal variation (P = 0.20). Shear rate did not differ in the morning or afternoon under either condition (P > 0.23). Blood pressure was lower following prazosin compared with placebo (P < 0.02), an effect that was similar at both times of day (P > 0.34). Heart rate and norepinephrine levels were higher in the afternoon following prazosin. These data indicate that α(1)-adrenoreceptor activity does not explain lower morning endothelium-dependent FMD.

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    • "Few studies have looked at vitamin D administration in healthy young people and very few have looked at micro vascular effects of vitamin D. For example, in a study of blood flow in the fingertip (micro vascular function) vitamin D administration increased brachial artery flow mediated dilation but not fingertip blood flow [28]. But brachial artery dilation is mediated by shear receptors which activate through microvilli and a prostaglandin mediated process including the release of nitric oxide [44–46]. The local response to occlusion is not mediated by prostaglandins or nitric oxide [47]. "
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