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‡ʈ, 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; ʈDepartment 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 forearm blood flow more than double that of a sample of 50 sea oxygen content among Tibetan highlanders, who maintain normal level residents at 206 m and circulating concentrations of bio- levels of oxygen use as indicated by basal and maximal oxygen logically active forms of NO Ͼ10-fold higher. These results consumption levels that are consistent with sea level predictions. highlight blood flow and its regulation as central components of This study tested the hypothesis that Tibetans resident at 4,200 m Tibetans’ adaptation to high-altitude hypoxia. offset physiological hypoxia and achieve normal oxygen delivery by means of higher blood flow enabled by higher levels of Results bioactive forms of NO, the main endothelial factor regulating Arterial Oxygen Content, Delivery, and Forearm Blood Flow. Eighty- blood flow and vascular resistance. The natural experimental study eight Tibetan native residents at 4,200 m and 50 U.S. sea level design compared Tibetans at 4,200 m and U.S. residents at 206 m. residents at 206 m (all healthy, normotensive, nonsmoking, non- Eighty-eight Tibetan and 50 U.S. resident volunteers (18–56 years pregnant volunteers, 18–55 years of age) participated in this natural of age, healthy, nonsmoking, nonhypertensive, not pregnant, with experiment (Table 1). Tibetans were shorter and lighter and had normal pulmonary function) participated. Forearm blood flow, an lower arterial oxygen saturation and content. Tibetan men and indicator of systemic blood flow, was measured noninvasively by women had higher forearm blood flow as compared with the sea using plethysmography at rest, after breathing supplemental ox- level group (Fig. 1A and Table 2). Sea level blood flow rates were ygen, and after exercise. The Tibetans had more than double the in the previously reported range (10, 14, 15). Forearm blood flow forearm blood flow of low-altitude residents, resulting in greater did not correlate with age, body mass index, arterial oxygen content, than sea level oxygen delivery to tissues. In comparison to sea level or blood pressure in either sample (all P Ͼ 0.05). Importantly, controls, Tibetans had >10-fold-higher circulating concentrations Tibetans had greater forearm blood flow and yet maintained lower of bioactive NO products, including plasma and red blood cell vascular resistance as compared with those at sea level (Table 2). nitrate and nitroso proteins and plasma nitrite, but lower concen- As a consequence of the greater tissue blood flow and higher trations of iron nitrosyl complexes (HbFeIINO) in red blood cells. hemoglobin concentration, Tibetans delivered more than two times This suggests that NO production is increased and that metabolic more oxygen to the capillary beds of the forearm despite lower pathways controlling formation of NO products are regulated arterial oxygen content as compared with sea level (Fig. 1 B–D). differently among Tibetans. These findings shift attention from the traditional focus on pulmonary and hematological systems to vas- Effects of Oxygen Supplementation and Exercise on Forearm Blood cular factors contributing to adaptation to high-altitude hypoxia. Flow. Experiments designed to investigate blood flow regulation tested for the presence of hypoxic vasodilation and exercise-induced circulation ͉ endothelium vasodilation. First, the presence of a hypoxia-induced vasodilation MEDICAL SCIENCES was determined by oxygen supplementation. Experimental relief he low barometric pressure at high altitude causes lower from hypoxia by inhalation of 50% oxygen caused Tibetans to Ն Tarterial oxygen content among Tibetan highlanders, who achieve oxygen saturations 98% and caused a small reduction of maintain normal levels of oxygen use as indicated by basal and forearm blood flow and systolic blood pressure among Tibetan maximal oxygen consumption levels that are consistent with sea women, but not men (Table 2). Diastolic blood pressure was not level predictions (1–3). Hypothetically, the unavoidably low affected by oxygen breathing, but Tibetans experienced a 16% supply of oxygen in the air and the blood could be offset by decline in pulse with oxygen breathing (pulse while breathing ANTHROPOLOGY Ϯ Ϯ increasing blood flow to improve oxygen delivery. Blood flow is supplemental oxygen: Tibetan men, 62 2; Tibetan women, 66 determined by numbers, length, and diameter of blood vessels 1 beats per minute). These findings suggest modest systemic hypoxic that in turn are largely determined directly or indirectly by levels vasodilation and tachycardia; however, even after relief of hypoxia of NO, a potent vasodilator synthesized in the endothelial cells by supplemental oxygen, forearm blood flow of the Tibetans lining the vessels (4–7). Tibetans have high levels of NO remained double that of sea level controls (Table 2). Experimen- synthesis in the lungs (8), and pulmonary blood flow correlated tally increasing oxygen demand with 5 min of forearm exercise with NO in a sample studied at 4,200 m (8, 9). This suggests the hypothesis that Tibetan highlanders offset hypoxia with higher Author contributions: S.G. and A.J.J. contributed equally to this work; S.C.E., A.J.J., D.J.S., systemic blood flow and higher levels of circulating, biologically 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 active metabolites of NO. After synthesis by the endothelium, 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; NO rapidly undergoes reaction in the blood to form products and S.C.E., S.G., A.J.J., W.X., D.J.S., M.F., and C.M.B. wrote the paper. that have circulatory and metabolic effects, including nitrite, The authors declare no conflict of interest. nitrate, nitrosothiol proteins (proteins containing NO-cysteine This article is a PNAS Direct Submission. II covalent bonds), and ␣-nitrosyl hemoglobin (HbFe NO), in ††To whom correspondence should be addressed at: Case Western Reserve University, 238 which NO occupies the heme binding site for oxygen in hemo- Mather Memorial Building, Cleveland, OH 44106-7125. E-mail: [email protected]. globin (5, 10–13). A sample of 88 Tibetans at 4,200 m had © 2007 by The National Academy of Sciences of the USA www.pnas.org͞cgi͞doi͞10.1073͞pnas.0707462104 PNAS ͉ November 6, 2007 ͉ vol. 104 ͉ no. 45 ͉ 17593–17598 Downloaded by guest on September 23, 2021 (Table 1. Tibetan (4,200 m) and U.S. sea level (206 m) sample characteristics (mean ؎ SEM Arterial O2 Systolic Diastolic Mean O2 saturation, content,* ml Pulse, blood blood arterial † Age, Weight, Hemoglobin, %of of O2/g of beats per pressure, pressure, pressure, Subjects n years Height, m kg g/dl hemoglobin hemoglobin minute mm Hg mm Hg mm Hg Tibetan 25 32 Ϯ 2 1.62 Ϯ 0.01 47 Ϯ 1 16.5 Ϯ 0.3 83.7 Ϯ 0.7 18.2 Ϯ 0.2 74 Ϯ 2 113 Ϯ 277Ϯ 289Ϯ 1 males Tibetan 63 30 Ϯ 1 1.54 Ϯ 0.05 44.5 Ϯ 0.5 14.6 Ϯ 0.1 85.2 Ϯ 0.5 17.4 Ϯ 0.2 79 Ϯ 1 113 Ϯ 176Ϯ 189Ϯ 1 females U.S. 23 35 Ϯ 2 1.78 Ϯ 0.01 86 Ϯ 2 15.5 Ϯ 0.2 96.9 Ϯ 0.2 20.8 Ϯ 0.3 70 Ϯ 2 127 Ϯ 276Ϯ 193Ϯ 1 males U.S. 27 38 Ϯ 2 1.64 Ϯ 0.01 68 Ϯ 3 13.3 Ϯ 0.2 98.1 Ϯ 0.2 19.3 Ϯ 0.4 72 Ϯ 1 114 Ϯ 269Ϯ 184Ϯ 1 females P value‡ 0.2 Ͻ0.001 Ͻ0.001 0.01 Ͻ0.001 Ͻ0.001 0.13 Ͻ0.001 0.7 0.06 P value§ 0.001 Ͻ0.001 Ͻ0.001 Ͻ0.001 Ͻ0.001 Ͻ0.001 Ͻ0.001 0.9 Ͻ0.001 0.004 *Estimated as [hemoglobin in g/dl ϫ % oxygen saturation ϫ 1.39 ml of O2 per gram of hemoglobin]. †Mean arterial pressure was calculated as [2 ϫ (diastolic blood pressure) ϩ systolic blood pressure]/3. ‡Comparison of Tibetan males (n ϭ 25) and U.S. males (n ϭ 23). §Comparison of Tibetan females (n ϭ 63) and U.S. females (n ϭ 27). increased forearm blood flow of participants at both altitudes; from Tibetan and sea level samples initially by an amperometric NO however, Tibetans had much greater flow increase than sea level sensor method after release of NO from nitrite or from total nitrite individuals (Table 2). Taken together, the response to relief of and nitrate. Tibetan men and women had average plasma nitrite of hypoxia and the response to the increased demands of exercise 4.8 Ϯ 1.4 ␮M and 11 Ϯ 2 ␮M, respectively, whereas sea level men demonstrate that blood flow of Tibetans is actively regulated in the and women had undetectable levels (Fig.