Pharmacologic versus flow-mediated assessments of peripheral vascular endothelial vasodilatory function in humans

W have a long-standing interest in the effects of primary aging on cardiovascular function, including vascular endothelial control. In clinical investigations of humans, 2 approaches primarily have been used to study peripheral vascular endothelial vasodilatory responsiveness: intrabrachial artery infusion of endothelium-dependent vasodilators such as acetylcholine with measurement of whole forearm blood flow using venous occlusion plethysmography, and ultrasound-derived brachial artery dilation following sustained occlusion of blood flow to the forearm. Recently, we noticed that the results of previous investigations using these 2 techniques reported different findings related to the effects of aging. Specifically, advancing age was strongly and inversely related to vasodilatory capacity (r 0.86) as determined by acetylcholine infusion, whereas no such correlation with age was observed (r 0.11) using brachial artery reactive hyperemia. These discordant findings suggested that these 2 procedures may not be providing similar information regarding the ability of the vascular endothelium to produce vasodilation upon stimulation among individual subjects and/or populations. Accordingly, the aim of the present study was to determine the relation between peripheral vascular endothelial vasodilatory responsiveness assessed by these 2 techniques. • • • Experiments were performed on 44 healthy men aged 20 to 79 years (Table 1). All subjects were normotensive ( 140/90 mm Hg) and free of overt, chronic diseases as assessed by medical history, physical examination, and complete blood chemistry and hematologic evaluation. Candidates who smoked, were taking medications, or had significant intimamedia thickening, plaque formation, and/or other characteristics of atherosclerosis were excluded. Studies were performed after a 12-hour overnight fast in the supine position under quiet resting conditions. All procedures were reviewed and approved by the Human Research Committee of the University of Colorado at Boulder. All subjects gave their written informed consent to participate. The 2 procedures for assessing vascular endothelial vasodilatory function were performed on separate days. The procedure for the brachial artery acetylcholine infusions has been described previously. Briefly, a 1.5-cm, 20-gauge catheter was inserted into the brachial artery of the nondominant arm under local anesthesia. Drug infusion rates were normalized per 100 ml of tissue and infused at 5 ml/min by a syringe pump. After measuring baseline blood flow for 5 minutes, endothelium-dependent vasodilation was assessed from the responses to incremental doses of acetylcholine (IOLAB Pharmaceuticals, Claremont, California) at rates of 1, 2, 4, 8, and 16 g/100 ml forearm tissue/min. Forearm blood flow was measured in both the experimental and contralateral forearms using strain-gauge venous occlusion plethysmography (D.E. Hokanson, Bellevue, Washington) with both forearms positioned above heart level. Endotheliumdependent vasodilation was expressed as absolute changes above baseline levels during acetylcholine infusion. Ultrasound measurements of brachial artery dilation during reactive hyperemia were performed according to the method originally described by Celermajer et al. An 2-cm segment of brachial artery was located 3 to 5 cm above the antecubital crease. After obtaining baseline brachial diameter, a blood pressure cuff placed on the forearm distal to the measurement site was inflated to 250 mm Hg. The cuff was deflated after 5 minutes and the brachial artery was scanned continuously for 2 minutes. The brachial artery diameter was measured from the images derived from an ultrasound machine (Toshiba SSH-140, Tochigi, Japan) equipped with a high-resolution (7.5 MHz) linFrom the Human Cardiovascular Research Laboratory, Department of Kinesiology and Applied Physiology, University of Colorado at Boulder, Boulder, Colorado; and Divisions of Cardiology and Geriatric Medicine, Department of Medicine, University of Colorado Health Sciences Center, Denver, Colorado. This study was supported by Awards AG00847, AG16071, AG13038, AG06537, and HL03840 from the National Institutes of Health, Bethesda, Maryland; by Award 9960234Z from the American Heart Association, Dallas, Texas; and by an American Diabetes Association Clinical Research Award, Alexandria, Virginia. Dr. Tanaka’s address is: Department of Kinesiology and Applied Physiology, University of Colorado at Boulder, Boulder, Colorado 80309-0354. E-mail: tanakah@colorado. edu. Manuscript received March 30, 2001; revised manuscript received and accepted June 26, 2001. TABLE 1 Selected Characteristics of the Study Subjects