Recognition of thrombosis as a complication of exposure to high altitude has stimulated interest in rheological changes resulting from hypobaric hypoxia. Previous studies of platelet counts at high altitude have yielded conflicting results and have not been studied in conjunction with potential mediating cytokines. We studied the effects of high-altitude exposure on platelet numbers, thrombopoietin (tpo) and erythropoietin (epo) levels in man. A group of 28 volunteers from the Bolivian Airforce stationed at Santa Cruz (600 m altitude) were studied 48 h and 1 week after their ascent to La Paz (3600 m). In addition 105 volunteers based at Santa Cruz for at least 1 year were compared with 175 age- and sex-matched residents at El Alto (4200 m). Platelet counts were measured immediately after sampling and serum samples assayed for tpo and epo. In the ascending group, mean platelet counts were 251×109, 367×109 and 398×109/1 at 600 m and following 48 h and 1 week at 3600 m respectively. Mean tpo levels were 132.5, 76 and 92 pg/ml with epo values of 2.98, 11.6 and 7.9 mIU/ml respectively. In the resident populations mean platelet counts were 271×109/l in the low- and 471×109/1 in the high-altitude groups. Mean tpo and epo levels measured 69.3 pg/ml and 4.5 mIU/ml respectively at 600 m and 58.5 pg/ml and 5.1 mIU/ml at 4200 m. In conclusion we have demonstrated a significant and sustained elevation in platelet numbers within 48 h of ascent to high altitude. Our findings do not support a role for tpo as a mediator of the increased platelet count. However, these data do not discount epo as a potential candidate.
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Acknowledgements Thanks are due to the following groups and individuals without whose help this study would not have been possible: The Bolivian Airforce and commanders Colonel Abasto at La Paz and Colonel Perez at Santa Cruz; Wing Commander Gradwell and RAF N. Luffenham for use of the hypobaric chamber; Dr. Enrique Vargas and the Instituto Boliviano de Biologica de Altura, La Paz; Alpha Laboratories and Carl Sorace for the supply of a platelet aggregometer and associated support; Gamm-ador, UK, for the supply of a Becton Dickinson QBC Autoread analyser; Amgen for assistance with ELISA kits. We are also grateful for support from a private endowment to the University of Liverpool. This project complied with appropriate local guidelines. John G. Hudson and Angela L. Bowen contributed equally to this project. Professor Donald Heath died peacefully on 10th February 1997. This study was the last to be inspired by his long fascination with adaptation to hypobaric hypoxia.