The Journal of Clinical Endocrinology & Metabolism Vol. 89, No. 9 4388-4390
Copyright © 2004 by The Endocrine Society
Increase in 6-Hydroxymelatonin Excretion in Humans during Ascent to High Altitudes
Herwig Frisch,
Franz Waldhauser,
Thomas Waldhör,
Andrea Müllner-Eidenböck,
Pritam Neupane and
Katharina Schweitzer
Department of Pediatrics (H.F., F.W., K.S.); Cancer Research Institute, Department of Epidemiology (T.W.); and Department of Ophthalmology (A.M.-E.), University of Vienna, 1090 Vienna, Austria; and Emergency and Outpatient Department, Patan Hospital (P.N.), Lalitpur, Nepal
Address all correspondence and requests for reprints to: Dr. Herwig Frisch, Department of Pediatrics, University Hospital Vienna, Waehringer Guertel 18-20, 1090 Vienna, Austria. E-mail: herwig.frisch{at}meduniwien.ac.at.
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Abstract
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Melatonin (MLT), the pineal gland hormone involved in the regulation of circadian rhythms, shows characteristic diurnal variation. Its physiological role in humans is not clear. Exposure to high altitudes may disrupt the circadian rhythm and lead to various endocrine changes. MLT in humans has not been studied under these conditions. Urinary 6-hydroxy-MLT sulfate (aMT6s) excretion was analyzed during the day (0700–2200 h) and night (2200–0700 h) phases. A cohort of 33 healthy volunteers, aged 19–65 yr, was studied during an ascent to a high altitude in the Himalayas on three occasions (at a lower altitude, at 3400 m, and after reaching maximal altitudes of 5600–6100 m). aMT6s excretion during the daytime remained unchanged during exposure to high altitudes. As expected, nocturnal values were higher than diurnal values at each point in time. However, there was a significant increase in nocturnal MLT excretion after the ascent to high altitudes. Ascent to high altitudes is associated with increased nocturnal excretion of aMT6s. The mechanism and physiological significance of this MLT increase are unclear.
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Introduction
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MELATONIN (MLT), THE pineal gland hormone, is secreted in a characteristic circadian rhythm, with its main production occurring during the dark phase. This endogenous rhythm is driven by the suprachiasmatic nuclei, which regulate the activity of N-acetyltransferase (NAT), the rate-limiting enzyme in MLT biosynthesis. In addition, NAT activity is immediately suppressed by light affecting the retina through a monosynaptic pathway (1). An increase in MLT after strenuous physical exercise has been reported in man, but the results were inconsistent. Recently, an increase in MLT was found in rats that had been exposed to hypobaric hypoxia (2). During ascents to high altitudes, adaptation mechanisms take place, and changes in various hormone parameters, such as thyroid hormones, GH, prolactin, and cortisol, have been reported in man (3). However, there are no data on MLT in humans during ascents to high altitudes. In the present study the effect of exposure to high altitude on MLT secretion was investigated by analyzing urinary 6-hydroxymelatonin sulfate (aMT6s) excretion in healthy volunteers. To our knowledge, this is the first report on MLT in humans after exposure to high altitudes.
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Subjects and Methods
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In May 2002, the high altitude research project Silver Pyramid 2002 took place in the Solu Khumbu region of the Himalayas of Nepal. Thirty-three Caucasians (21 males), aged 41.8 ± 12.4 yr (mean ± SD;range, 19–65 yr), took part in the study. None of them was an athlete, but all were healthy and in good physical condition. They had not been at high altitudes during the 6 months before the study and were not on any medication or dietary program. Examinations took place on three occasions, 2 months before the journey, when a preexamination (A) was performed at a low altitude (Herxheim, Germany; 120 m altitude); after arrival in Kathmandu/Nepal (1400 m), the subjects stayed there for 3 d and were then airlifted to Lukla (2800 m). After 2 d of trekking, they reached Namche Bazaar (3440 m), where the second examination was performed (B). During the next 10 d, maximal altitudes between 5600 and 6100 m were reached, and the third examination (C) was performed at the high altitude laboratory Silver Pyramid at 5050 m.
The daily trekking time ranged from 4–6 h. The subjects carried only a light backpack, no one had symptoms of physical exhaustion, and there was a full food supply three times a day. There were no serious health problems during the study period; however, five volunteers did not take part in the last examination because they had descended the mountain due to symptoms of mild acute mountain sickness.
The complete urine output was collected on three occasions (A, B, and C) during the day (0700–2200 h) and night (2200–0700 h), the volume was measured, and an aliquot was frozen and kept at –20 C until it was assayed. Subjects slept with lights off in a dark room or tent from 2200–0700 h (using only a flashlight if necessary) and moved freely outdoors in bright light during the day, with the constraint of collecting urine in a bottle. Sunrise and sunset at preexamination (A) were at 0630 and 1830 h, respectively; in Nepal at examinations B and C, they were at 0520 and 1840 h, respectively. At the same points in time, blood samples were drawn for other examinations, including lactate levels. Results were also compared with data of an independent control group, 53 healthy subjects (22 males), aged 22–73 yr, who were analyzed in the same laboratory (Waldhauser, F., unpublished observations).
The study was approved by the ethical committee of the Medical Faculty, University of Vienna. Informed consent was obtained from the subjects.
Assay
aMT6s concentrations in urine were assayed with a commercially available RIA kit (Stockgrand Ltd., Guildford, UK) using a method described by J. Arendt and co-workers (4, 5). Performance criteria for this assay in our laboratory were reported previously (6).
Statistical analysis
The statistical analysis was performed using SAS (SAS/STAT User’s Guide, version 8, 1999, SAS Institute, Inc., Cary, NC). Descriptions of the data were made at the 25th, 50th, and 75th percentiles because of asymmetry, and data were analyzed by paired Wilcoxon test.
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Results
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The urinary excretion of aMT6s is reported in Fig. 1
. Values are expressed as micrograms excreted per collection period, i.e. day (0700–2200 h) and night (2200–0700 h) phases.
During the daytime, aMT6s excretion was low and showed no significant changes during the ascent. As expected, nocturnal aMT6s excretion was significantly higher than daytime excretion at all three examinations. However, nocturnal excretion increased while the subjects were climbing to high altitudes and was significantly higher at examinations B (P = 0.0001) and C (P = 0.0001) than at A. Data at A were not different from those obtained from the controls (not significant). The five subjects who developed symptoms of acute mountain sickness and could not be analyzed at C had significantly lower diurnal (P = 0.034), but not nocturnal, MLT excretion at B.
There was no age dependency of nocturnal aMT6s excretion at A (P = 0.51), B (P = 0.17), or C (P = 0.21).
Serum lactate levels were 22.0 ± 4.0 mg/dl (A), 23.6 ± 4.9 (B), and 20.1 ± 7.1 mg/dl (C) and did not change during the observation period (mean ± SD; not significant by Wilcoxon test).
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Discussion
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We found a significant increase in nocturnal urinary aMT6s excretion after exposure to high altitudes. MLT is secreted largely during the night, with the nocturnal secretion phase starting between 2100 and 2300 h, and the morning decrease occurring between 0700 and 0900 h (7). Sixty to 80% of MLT is metabolized in the liver to aMT6s, which is passed into the urine (8). The evaluation of nocturnal urinary aMT6s excretion is therefore a reliable and robust parameter for the analysis of MLT secretion (5, 6).
Nocturnal MLT secretion is high in early childhood and very low in senescence, but is relatively constant in the age group between 20 and 65 yr represented by our subjects (9). We found no age dependency of MLT excretion in our subjects and therefore combined the data from the whole group.
This increase in MLT during an ascent to high altitudes is noteworthy, because to date there is no reliable way to stimulate MLT in humans to analyze pineal function. In fact, a considerable number of compounds have been investigated with respect to an effect on MLT secretion, but not one has been identified as a reliable stimulating agent (10). The administration of various neuroleptics led to an increase in serum MLT concentrations, but it was shown that this effect was due to inhibition of MLT metabolism (11) rather than to pineal MLT stimulation. In an attempt to increase the bioavailability of orally given MLT in depressive patients, the administration of fluvoxamine, an inhibitor of cytochrome P450 enzymes, was shown to slow down MLT degradation (12). Prolonged hypoxia is a potent stimulator of catecholamines (13), which we also confirmed in our study (data not shown). Because the pineal is controlled by norepinephrine, it was postulated that catecholamines may increase serum MLT levels. It has been shown that L-dopa, a norepinephrine precursor, increases MLT synthesis in rats (14), but is without effect on MLT secretion in humans (15, 16). Increased catecholamines can therefore not be the cause of elevated MLT levels at high altitudes.
The effect of physical exercise on MLT secretion has been addressed in several studies. Increased serum MLT levels were found after strenuous exercise, such as long distance running (17, 18), or in athletes (19). In highly trained professional cyclists, urinary aMT6s excretion increased significantly during the day after each race, but there was an overall decrease in morning and evening values throughout the 3-wk racing period (20). These data indicate overall exhaustion of aMT6s excretion, which might be explained by insufficient time for recovery of the pineal gland after bouts of strenuous exercise. The increase in evening values in this study may be due to the late sampling time, when light intensity is decreasing. It should be mentioned that all studies analyzing the effects of exercise on MLT were performed during the daytime, when environmental light will not allow a reliable or comparable analysis of MLT secretion. No conclusions regarding the secretory capacity can be drawn, because MLT was determined only in individual serum or spontaneous urine samples. In a recent review it was concluded that regardless of the intensity, exercise during the daytime has no consistent acute effect on MLT secretion (21, 22). However, our subjects were not athletes, they were not on a constant training program, and the physical exercise performed during the trekking in Nepal was of rather moderate intensity. They carried only light backpacks, and the maximal duration of daily walking was not more than 6 h. Lactate levels did not increase, thus indicating fully aerobic activity. In conclusion, it may be stated that physical exercise does not appear to be responsible for the MLT increase in our study.
The release of opioids, which were increased in long distance runners, has been discussed as a factor responsible for the rise in plasma MLT. However, administration of naltrexone, an opioid receptor antagonist, had no effect on the rise in MLT, which does not support the hypothesis of opioids being involved in a running-induced increase in MLT (23).
In a recent study, adult rats were exposed to an equivalent of 8000 m for 2 h in an altitude chamber, which resulted in a steady increase in plasma MLT levels, peaking on d 7 after exposure. Electron microscopy of the pineal gland showed that at the same time the mitochondrial numbers, lipid droplets and granular endoplasmatic reticulum in the pinealocytes were increased (2), which indicates increased synthesis of MLT. Plasma MLT levels returned to the normal range thereafter, and the ultrastructure of the pinealocytes was comparable to that in control animals after 14 and 21 d. This observation of high altitude exposure in rats leading to structural changes in the pineal gland and an increase in MLT production is in agreement with our observation. Evaluation of urinary aMT6s in our study reflects the amount of MLT that was produced and secreted by the pineal during the respective phases and thus allows the conclusion that there was, in fact, an increase in MLT secretion on exposure to high altitudes.
MLT may play a role in immunoregulation, because a protective effect in models of local inflammation has been shown by inhibiting nitrous oxide production (24). In addition, the damage resulting from inflammation was decreased by MLT, which reduced the up-regulation of proinflammatory cytokines (25). In rats given exogenous MLT, the expression of CR3 receptors and major histocompatibility complex class II antigens in macrophages was up-regulated (26). Exposure to high altitudes was recently seen as a factor associated with oxidative stress and free radical damage to the blood-brain barrier. These changes were discussed as causal factors for the development of acute mountain sickness (27).
Sleep disorders are a common occurrence at high altitudes, showing a deterioration in the quality of sleep. The duration of light sleep is increased at the expense of deep sleep (stage III, IV, and rapid eye movement). In addition, there are increasing phases of periodic breathing resulting in frequent awakenings (28). It is well known that MLT plays an important physiological role as a pacemaker in the circadian rhythm system (29). Whether the increase in MLT may be seen as a means to counterbalance the disturbed sleep-wake cycle at high altitudes or the development of acute mountain sickness (30) remains speculative.
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Acknowledgments
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We express our gratitude to the Ev-K2-CNR committee (Bergamo, Italy) and RONAST (Kathmandu, Nepal) for being allowed to use the pyramid laboratory-observatory free of charge. We are grateful to the probands for their cooperation under the demanding conditions of this field study. We thank the kind people of Nepal, without whose help this project would not have been possible.
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Footnotes
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This work was supported by Pfizer Corp. Austria (formerly Pharmacia Austria) and Grandis Biotech (Germany).
Abbreviations: aMT6s, 6-Hydroxy-MLT sulfate; MLT, melatonin; NAT, N-acetyltransferase.
Received December 24, 2003.
Accepted June 11, 2004.
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References
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- Brezinski A 1997 Melatonin in humans. N Engl J Med 336:186–195[Free Full Text]
- Kaur C, Srinivasan KN, Singh J, Peng CM, Ling EA 2002 Plasma melatonin, pinealocyte morphology, and surface receptors/antigen expression on macrophages/microglia in the pineal gland following a high-altitude exposure. J Neurosci Res 67:533–543[CrossRef][Medline]
- Ramirez G, Herrera R, Pineda D, Bittle PA, Rabb HA, Bercu BB 1995 The effects of high altitude on hypothalamic-pituitary secretory dynamics in men. Clin Endocrinol (Oxf) 43:11–18[Medline]
- Arendt J, Bojkowski C, Franey C, Wright J, Marks V 1985 Immunoassay of 6-hydroxymelatonin sulfate in human plasma and urine: abolition of the urinary 24-hour rhythm with atenolol. J Clin Endocrinol Metab 60:1166–1173[Abstract]
- Bojkowski CJ, Arendt J, Shih MC, Markey SP 1987 Melatonin secretion in humans assessed by measuring its metabolite, 6-sulfatoxymelatonin. Clin Chem 33:1343–1348[Abstract/Free Full Text]
- Kovacs J, Brodner W, Kirchlechner V, Arif T, Waldhauser F 2000 Measurement of urinary melatonin: a useful tool for monitoring serum melatonin after its oral administration. J Clin Endocrinol Metab 85:666–670[Abstract/Free Full Text]
- Trinchard Lugan I, Waldhauser F 1989 The short-term secretion pattern of human serum melatonin indicates apulsatile hormone release. J Clin Endocrinol Metab 69:663–669[Abstract]
- Kveder S, McIsaac WM 1961 The metabolism of melatonin (N-acetyl-5-methoxytryptamine) and 5-methoxytryptamine. J Biol Chem 236:3214[Free Full Text]
- Waldhauser F, Weiszenbacher G, Tatzer E, Gisinger B, Waldhauser M, Schemper M, Frisch H 1988 Alterations in nocturnal serum melatonin levels in humans with growth and aging. J Clin Endocrinol Metab 66:648–652[Abstract]
- Touitou Y, Bogdan A, Claustrat B, Touitou C 1987 Drugs affecting melatonin secretion in man. In: Trentini G, De Gaetani C, Pevet P, eds. Fundamentals and clinics in pineal research. New York: Raven Press; 349–356
- Garde E, Micic S, Knudsen K, Angelo HR, Wulf MC 1994 8-Methoxypsoralen increases daytime plasma melatonin levels in humans through inhibition of metabolism. Photochem Photobiol 60:475–480[Medline]
- Haertter S, Wang X, Weigmann H, Friedberg T, Arand M, Oesch F, Hiemke C 2001 Differential effects of fluvoxamine and other antidepressants on the biotransformation of melatonin. J Clin Psychopharmacol 21:167–174[CrossRef][Medline]
- Rostrup M 1998 Catecholamines, hypoxia and high altitude. Acta Physiol Scand 162:389–399[CrossRef][Medline]
- Lynch HJ, Eng JP, Wurtman RJ 1973 Control of pineal indole biosynthesis by changes in sympathetic tone caused by factors other than environmental lighting. Proc Natl Acad Sci USA 70:1704–1707[Abstract/Free Full Text]
- Vaughan GM, McDonald SD, Jordan RM, Allen JP, Bell R, Stevens EA 1979 Melatonin, pituitary function and stress in humans. Psychoneuroendocrinology 4:351–362[CrossRef][Medline]
- Wetterberg L 1978 Melatonin in humans physiological and clinical studies. J Neural Transm 13(Suppl):289–310
- Ronkainen H, Vakkuri O, Kauppila A 1986 Effects of physical exercise on the serum concentration of melatonin in female runners. Acta Obstet Gynecol Scand 65:827–829[Medline]
- Strassmann RJ, Appenzeller O, Lewy AJ, Qualls CR, Peake GT 1989 Increase in plasma melatonin, ß-endorphin, and cortisol after a 28.5-mile mountain race: relationship to performance and lack of effect of naltrexone. J Clin Endocrinol Metab 69:540–545[Abstract]
- Laughlin GA, Loucks AB, Yen SSC 1991 Marked augmentation of nocturnal melatonin secretion in amenorrheic athletes, but not in cycling athletes: unaltered by opioidergic or dopaminergic blockade. J Clin Endocrinol Metab 73:1321–1326[Abstract]
- Lucia A, Diaz B, Hoyos J, Fernandez C, Villa G, Bandres F, Chicharro JL 2001 Hormone levels of world class cyclists during the Tour of Spain stage race. Br J Sports Med 35:424–430[Abstract/Free Full Text]
- Buxton OM, L’Hermite-Baleriaux M, Hirschfeld U, Van Cauter E 1997 Acute and delayed effects of exercise on human melatonin secretion. J Biol Rhythm 12:568–574
- Elias AN, Wilson AF, Pandian MR, Rojas FJ, Kayaleh R, Stone SC, James N 1993 Melatonin and gonadotropin secretion after acute exercise in physically active males. Eur J Appl Physiol 66:357–361[CrossRef]
- Appenzeller O, Wood SC 1992 Peptides and exercise at high and low altitudes. Int J Sports Med 13:135–140
- Cuzzocrea S, Costantino G, Gitto E, Fulia F, Serraino I, Cordaro S, Barberi I, De Sarro A, Caputi AP 2000 Protective effects of melatonin in ischemic brain injury. J Pineal Res 29:217–227[CrossRef][Medline]
- Reiter RJ, Calvo JR, Karbownik M, Qi W, Tan DX 2000 Melatonin and its relation to the immune system and inflammation. Ann NY Acad Sci 917:376–386[Abstract/Free Full Text]
- Kaur C, Ling EA 1999 Effects of melatonin on macrophages/microglia in postnatal rat brain. J Pineal Res 26:158–168[Medline]
- Askew EW 2002 Work at high altitude and oxidative stress: antioxidant nutrients. Toxicology 180:107–119[CrossRef][Medline]
- Wickramasinghe H, Anholm JD 1999 Sleep and breathing at high altitude. Sleep Breath 3:89–102[CrossRef][Medline]
- Arendt J 2003 Importance and relevance of melatonin to human biological rhythms. J Neuroendocrinol 15:427–431[CrossRef][Medline]
- Roach RC, Loeppky JA, Icenogle MV 1996 Acute mountain sickness: increased severity during simulated high altitude compared with normobaric hypoxia. J Appl Physiol 81:1908–1910[Abstract/Free Full Text]
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Abstract
Introduction
, Diurnal; 
, nocturnal.