Effect of Systemic Nitric Oxide Synthase Inhibition on Arterial Stiffness in Humans

411 Hypertens Res Vol.30 (2007) No.5 p.411-415 Original Article Effect of Systemic Nitric Oxide Synthase Inhibition on Arterial Stiffness in Humans Jun SUGAWARA1),2), Hidehiko KOMINE1), Koichiro HAYASHI1), Mutsuko YOSHIZAWA1), Takashi YOKOI1), Takeshi OTSUKI3), Nobutake SHIMOJO4), Takashi MIYAUCHI4), Seiji MAEDA3), and Hirofumi TANAKA2) Stiffening of large elastic arteries impairs the buffering function of the arterial system and contributes to cardiovascular disease. The aim of this study was to determine whether endothelium-derived nitric oxide (NO) modulates the stiffness of large elastic arteries in humans. Seven apparently healthy adults (60 ± 3 years, 2 males and 5 females) underwent systemic α-adrenergic blockade (phentolamine) and systemic NO synthase inhibition using N G-monomethyl-L-arginine (L-NMMA) in sequence. Phentolamine was given first to isolate contribution of NO to arterial stiffness by preventing reflex changes in sympathetic tone that result from systemic NO synthase inhibition, and also to compare arterial stiffness at a similar mean arterial pressure. Mean arterial blood pressure decreased (p < 0.05) after phentolamine infusion but returned to baseline levels after L-NMMA infusion. The carotid β-stiffness index (via simultaneous ultrasound and applanation tonometry on the common carotid artery) did not change after the restraint of systemicα-adrenergic nerve activity (9.8±1.2 vs. 9.1 ± 1.1 U) but increased (p < 0.05) after NO synthase inhibition (12.6 ± 2.0 U). These results suggest that NO appears to modulate central arterial stiffness in humans. (Hypertens Res 2007; 30: 411–415) Key Words: arterial compliance, endothelium, sympathetic nervous system Introduction Large elastic arteries in cardiothoracic and carotid regions expand and recoil during cardiac contraction and relaxation, thereby damping fluctuations in arterial pressure and blood flow (1). In sedentary humans, the stiffness of arteries in the cardiothoracic region increases with advancing age (2–4). The stiffening of large elastic arteries impairs the buffering function of the arterial system and leads to cardiovascular dysfunction via elevated systolic blood pressure, augmented left ventricular afterload, decreased coronary blood flow, and blunting of arterial baroreflex sensitivity (5–7). Arterial stiffness is modulated by vasoconstrictor tone exerted by the smooth muscle cells. Nitric oxide (NO) bioavailability is thought to be one of the most potent modulators of smooth muscle tone. While animal studies have consistently demonstrated that NO synthase inhibition increases measures of arterial stiffness (8, 9), human studies remain controversial. Although local intra-arterial infusion of drugs that inhibit NO release is an ideal protocol with which to examine this issue, such a procedure is difficult to perform in the human aorta. As such, most studies have used systemic infusion of N G-monomethyl-L-arginine (L-NMMA) (10, 11). However, these studies are difficult to interpret because the changes in arterial stiffness indices were accompanied by From the 1)Institute for Human Science and Biomedical Engineering, National Institute of Advanced Industrial Science and Technology, Tsukuba, Japan; 2)Cardiovascular Aging Research Laboratory, Department of Kinesiology and Health Education, University of Texas, Austin, USA; and 3)Center for Advanced Research Alliance and 4)Cardiovascular Division, Institute of Clinical Medicine, University of Tsukuba, Tsukuba, Japan. This study was supported by a grant from the National Institutes of Health (AG20966, to H.T.) and by Grants-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan (18300215 and 18650186, to S.M.). Address for Reprints: Jun Sugawara, Ph.D., Department of Kinesiology and Health Education, University of Texas at Austin, 1 University Station, D3700, Austin, TX 78712–1204, USA. E-mail: Received September 7, 2006; Accepted in revised form December 27, 2006. 412 Hypertens Res Vol. 30, No. 5 (2007) Table 1. Selected Physiological Characteristics of Subject Variables Mean±SEM Range Height (cm) Body mass (kg) Body mass index (kg m−2) Body fat (%) VO2 max (mL kg−1 min−1) Plasma glucose (mmol L−1) Plasma total cholesterol (mmol L−1) Plasma HDL cholesterol (mmol L−1) Plasma LDL cholesterol (mmol L−1) Plasma triglyceride (mmol L−1) 161±2 60±4 22.9±1.2 28.4±2.0 28.8±2.3 5.1±0.1 5.8±0.3 1.6±0.1 3.6±0.2 1.3±0.3 153–172 42–72 17.9–26.9 21.5–34.7 18.7–35.8 4.5–5.7 4.7–7.0 1.4–2.2 2.7–4.6 0.5–2.4 VO2 max, maximal oxygen consumption; HDL, high-density lipoprotein; LDL, low-density lipoprotein. increases in mean arterial blood pressure as well as reflex reductions in sympathetic vasoconstrictor tone that could independently affect arterial stiffness. Indeed, Stewart et al. (11) concluded that the effects of systemic infusion of a NO blocker on pulse wave velocity (PWV) could be explained primarily by changes in mean arterial blood pressure. Another issue that makes the interpretation of these studies difficult is the use of indirect measures of arterial stiffness that are influenced by factors other than arterial stiffness. For example, increases in the augmentation index previously observed after L-NMMA (10, 11) may have been caused by increases in systemic vascular resistance rather than increases in arterial stiffness, as the augmentation index reflects not only arterial stiffness but also wave reflection (12). Accordingly, the aim of the present study was to test the hypothesis that endothelium-derived NO modulates the stiffness of large elastic arteries in humans. To account for reflex changes in sympathetic tone as well as increases in mean arterial blood pressure that result from systemic NO synthase inhibition, we administered a systemic α-adrenergic blocker (phentolamine) prior to the infusion of the NO synthase inhibitor. Furthermore, we directly measured arterial stiffness (βstiffness index) by using a combination of ultrasound imaging with simultaneous applanation tonometry. The β-stiffness index was chosen for the present study because it is an index of arterial stiffness that minimizes effect of distending pressure on arterial stiffness (13). Methods Subjects Seven apparently healthy sedentary adults (60 ± 3 years, 2 men and 5 postmenopausal [i.e., estrogen-deficient] women) were studied (Table 1). None of the subjects were current smokers or were currently taking any medications including hormone replacement therapy. All subjects performed an incremental cycling exercise test to assess their cardiovascu- lar disease risk and to obtain maximal oxygen consumption as previously described (14). None of the subjects had any apparent cardiovascular disease as assessed by medical history and physical examination. All potential risks and procedures of the study were explained to the subjects, who gave their written informed consent to participate in the study. The stu (...truncated)