Higher blood flow and circulating NO products offset high-altitude hypoxia among Tibetans S. C. Erzurum* † , S. Ghosh*, A. J. Janocha*, W. Xu*, S. Bauer ‡§ , N. S. Bryan ‡§ , J. Tejero*, C. Hemann ¶ , R. Hille ¶ , D. J. Stuehr*, M. Feelisch ‡ , and C. M. Beall** †† Departments of *Pathobiology and † Pulmonary, Allergy, and Critical Care, Cleveland Clinic, Cleveland, OH 44195; ‡ Whitaker Cardiovascular Institute, Boston University School of Medicine, Boston, MA 02118; § Institute of Molecular Medicine, University of Texas–Houston Health Science Center, Houston, TX 77030; ¶ Department of Molecular and Cellular Biochemistry, Ohio State University, Columbus, OH 43210;  Department of Experimental Medicine and Integrative Biology, University of Warwick, Coventry CV4 7AL, United Kingdom; and **Department of Anthropology, Case Western Reserve University, Cleveland, OH 44106 Edited by Louis J. Ignarro, University of California School of Medicine, Los Angeles, CA, and approved September 18, 2007 (received for review August 9, 2007) The low barometric pressure at high altitude causes lower arterial oxygen content among Tibetan highlanders, who maintain normal levels of oxygen use as indicated by basal and maximal oxygen consumption levels that are consistent with sea level predictions. This study tested the hypothesis that Tibetans resident at 4,200 m offset physiological hypoxia and achieve normal oxygen delivery by means of higher blood flow enabled by higher levels of bioactive forms of NO, the main endothelial factor regulating blood flow and vascular resistance. The natural experimental study design compared Tibetans at 4,200 m and U.S. residents at 206 m. Eighty-eight Tibetan and 50 U.S. resident volunteers (18 –56 years of age, healthy, nonsmoking, nonhypertensive, not pregnant, with normal pulmonary function) participated. Forearm blood flow, an indicator of systemic blood flow, was measured noninvasively by using plethysmography at rest, after breathing supplemental ox- ygen, and after exercise. The Tibetans had more than double the forearm blood flow of low-altitude residents, resulting in greater than sea level oxygen delivery to tissues. In comparison to sea level controls, Tibetans had >10-fold-higher circulating concentrations of bioactive NO products, including plasma and red blood cell nitrate and nitroso proteins and plasma nitrite, but lower concen- trations of iron nitrosyl complexes (HbFe II NO) in red blood cells. This suggests that NO production is increased and that metabolic pathways controlling formation of NO products are regulated differently among Tibetans. These findings shift attention from the traditional focus on pulmonary and hematological systems to vas- cular factors contributing to adaptation to high-altitude hypoxia. circulation | endothelium T he low barometric pressure at high altitude causes lower arterial oxygen content among Tibetan highlanders, who maintain normal levels of oxygen use as indicated by basal and maximal oxygen consumption levels that are consistent with sea level predictions (1–3). Hypothetically, the unavoidably low supply of oxygen in the air and the blood could be offset by increasing blood f low to improve oxygen delivery. Blood f low is determined by numbers, length, and diameter of blood vessels that in turn are largely determined directly or indirectly by levels of NO, a potent vasodilator synthesized in the endothelial cells lining the vessels (4–7). Tibetans have high levels of NO synthesis in the lungs (8), and pulmonary blood flow correlated with NO in a sample studied at 4,200 m (8, 9). This suggests the hypothesis that Tibetan highlanders offset hypoxia with higher systemic blood flow and higher levels of circulating, biologically active metabolites of NO. After synthesis by the endothelium, NO rapidly undergoes reaction in the blood to form products that have circulatory and metabolic effects, including nitrite, nitrate, nitrosothiol proteins (proteins containing NO-cysteine covalent bonds), and -nitrosyl hemoglobin (HbFe II NO), in which NO occupies the heme binding site for oxygen in hemo- globin (5, 10–13). A sample of 88 Tibetans at 4,200 m had forearm blood f low more than double that of a sample of 50 sea level residents at 206 m and circulating concentrations of bio- logically active forms of NO 10-fold higher. These results highlight blood f low and its regulation as central components of Tibetans’ adaptation to high-altitude hypoxia. Results Arterial Oxygen Content, Delivery, and Forearm Blood Flow. Eighty- eight Tibetan native residents at 4,200 m and 50 U.S. sea level residents at 206 m (all healthy, normotensive, nonsmoking, non- pregnant volunteers, 18 –55 years of age) participated in this natural experiment (Table 1). Tibetans were shorter and lighter and had lower arterial oxygen saturation and content. Tibetan men and women had higher forearm blood flow as compared with the sea level group (Fig. 1A and Table 2). Sea level blood flow rates were in the previously reported range (10, 14, 15). Forearm blood flow did not correlate with age, body mass index, arterial oxygen content, or blood pressure in either sample (all P  0.05). Importantly, Tibetans had greater forearm blood f low and yet maintained lower vascular resistance as compared with those at sea level (Table 2). As a consequence of the greater tissue blood flow and higher hemoglobin concentration, Tibetans delivered more than two times more oxygen to the capillary beds of the forearm despite lower arterial oxygen content as compared with sea level (Fig. 1 B–D). Effects of Oxygen Supplementation and Exercise on Forearm Blood Flow. Experiments designed to investigate blood flow regulation tested for the presence of hypoxic vasodilation and exercise-induced vasodilation. First, the presence of a hypoxia-induced vasodilation was determined by oxygen supplementation. Experimental relief from hypoxia by inhalation of 50% oxygen caused Tibetans to achieve oxygen saturations 98% and caused a small reduction of forearm blood flow and systolic blood pressure among Tibetan women, but not men (Table 2). Diastolic blood pressure was not affected by oxygen breathing, but Tibetans experienced a 16% decline in pulse with oxygen breathing (pulse while breathing supplemental oxygen: Tibetan men, 62  2; Tibetan women, 66  1 beats per minute). These findings suggest modest systemic hypoxic vasodilation and tachycardia; however, even after relief of hypoxia by supplemental oxygen, forearm blood flow of the Tibetans remained double that of sea level controls (Table 2). Experimen- tally increasing oxygen demand with 5 min of forearm exercise Author contributions: S.G. and A.J.J. contributed equally to this work; S.C.E., A.J.J., D.J.S., M.F., and C.M.B. designed research; S.G., A.J.J., W.X., S.B., N.S.B., J.T., C.H., R.H., M.F., and C.M.B. performed research; S.C.E., S.G., A.J.J., W.X., D.J.S., M.F., and C.M.B. analyzed data; and S.C.E., S.G., A.J.J., W.X., D.J.S., M.F., and C.M.B. wrote the paper. The authors declare no conflict of interest. This article is a PNAS Direct Submission. †† To whom correspondence should be addressed at: Case Western Reserve University, 238 Mather Memorial Building, Cleveland, OH 44106-7125. E-mail: cynthia.beall@case.edu. © 2007 by The National Academy of Sciences of the USA www.pnas.orgcgidoi10.1073pnas.0707462104 PNAS | November 6, 2007 | vol. 104 | no. 45 | 17593–17598 MEDICAL SCIENCES ANTHROPOLOGY