We observed here that serum EPO increased after the first day by 78% in continuous and by 71% in intermittent normobaric hypoxia corresponding to the altitude of 2500 m. This increase seems to be greater than reported earlier at moderate altitude (3,18) and after short-term intermittent normobaric hypoxia (19,32), evidently due to differences between hypoxic stresses and rates of hypoxic exposures. After the initial peak, serum EPO decreased slightly but remained elevated (P < 0.05) during all days under both continuous and intermittent normobaric hypoxia. It has been documented that circulating levels of erythropoietin increase in relation to the severity of ambient hypoxia and duration of the hypoxic stimulus (1,7,19). Under exposures to both high and moderate altitudes serum erythropoietin has been shown to peak from day 1 to day 5 after the ascent and to decline thereafter toward baseline levels (1,3,8,19). Several additional factors facilitate physiological responses to acute hypoxia, such as the rate of shift from normoxic to hypoxic conditions (12). Repeated abrupt changes from normoxic to hypoxic conditions are probably the feature in this experiment that could explain the delayed stimulation of erythropoietin during the intermittent hypoxic exposures. Acid-base balance and work performed under hypoxia also influence the secretion of erythropoietin at hypoxia (9,23,32). Neither maximal nor submaximal exercise has an immediate effect on erythropoietin secretion at normoxia or hypoxia. Submaximal exercise has been reported to cause a late-onset rise in serum erythropoietin. This is in contrast to maximal exercise that seemed to hamper erythropoietin secretion or even to decrease serum erythropoietin (32). In the present study, the light work performed under continuous hypoxia should have further stimulated erythropoietin secretion compared with intermittent exposure, where the subjects worked during daytime in normoxic conditions but were rather sedentary under hypoxic exposure. The results in serum erythropoietin measurements did, however, show quite different figures. Serum erythropoietin peaked after the first night up to a significantly higher level than the prestudy value or the values obtained during consequent measurements during continuous hypoxia. All the measurements of serum erythropoietin under intermittent exposure were statistically significantly higher than the prestudy value but were more evenly distributed, so that no statistical differences were observed between measurements during the intermittent exposure.
Concentration of red cell 2,3-DPG increased on the first day in continued hypoxia and remained elevated (P < 0.05) throughout the exposure. Under intermittent hypoxia, 2,3-DPG increased significantly on the third day and remained elevated (P < 0.05) during the rest of the exposure. These results are in accordance with earlier findings (18,25). Mairbäurl et al. (25) studied training male students at 2300 m and observed a significantly higher rise in 2,3-DPG in the exercising group than in controls. The greater physical activity under continuous hypoxia could partly account for the more rapid rise of 2,3-DPG under continuous hypoxia in this study. On the other hand, the coincidence of the rise in 2,3-DPG with increased erythropoietin secretion is in accordance with the earlier findings that the increase of red cell 2,3-DPG is one of the first reactions to acute hypoxia and reflects the severity of hypoxic stress (20,21,24). The reticulocyte number increased steadily during exposures to both continuous and intermittent hypoxia and reached in both cases a significant rise (P < 0.05) on days 5 and 7. The late increase in reticulocyte number only during the last 3 d of the 7-d hypoxic exposure explains why no significant changes in Hct or Hb were attained.
This is the first experiment where changes in serum soluble transferrin receptor prompted by ambient hypoxia have been studied. In the present study, the serum transferrin receptor showed a significant increase on days 5 and 7 (P < 0.05) both in continuous and intermittent exposures to normobaric hypoxia. The same magnitude in erythropoietin stimulation and rise of serum transferrin receptor indicates equal erythropoietic response during both continuous and intermittent normobaric hypoxia. The level of serum transferrin receptor in normal subjects reflects the rate of erythropoiesis except in iron deficiency, where an unproportional increase from 2- to 15-fold is observed (2,15). It has been suggested that normal iron stores may not always be sufficient for the highly accelerated erythropoiesis after administration of recombinant human erythropoietin (r-HuEPO), even in case of simultaneous oral iron supplementation (5,31). Studies with r-HuEPO has shown a pronounced increase in Hct and a 2–3 fold rise in serum transferrin receptor, which indicates a far more accelerated erythropoiesis than in our study, where Hb and Hct values did not show any significant changes during the 7-d study period. Normal values of serum ferritin and serum transferrin receptor before hypoxic interventions in our subjects indicate normal reticuloendothelial iron stores. Additionally, keeping in mind that our hypoxic stress was moderate, it can be assumed that iron delivery was sufficient to provide a normal erythropoietic response (13,14). This is further supported by the coincidence of the reticulocyte response and the rise in serum transferrin receptor, which thus seems to reflect an increased rate of erythropoiesis.
Erythrocythemia induced by exogenous erythropoietin or blood transfusion has been shown to benefit exercise performance both at sea level (10,11,33,34) and under hypoxia (28), which is the main reason why altitude training is so generally incorporated in endurance training. However, studies of the effects of training at moderate altitude on physical performance have yielded variable results (35) even with improved hematological parameters (16,18). This is probably due to the deleterious effects of a reduced training intensity at altitude on the neuromuscular system, which has been proposed to oppose or even override the benefits in oxygen transport gained by the acclimatization process (22,30,35). Our results are novel and important from the training point of view indicating that daily intermittent exposures to normobaric hypoxia at sea level can yield the same hematological effects as living at the corresponding altitude. It should be considered that S-EPO, S-transferrin receptor concentration, and reticulocyte number are indirect measures of red cell production. Interestingly, our findings support those of a very recent study, where the same increase in the red cell number was observed in subjects living and training at 2500 m as in those living at 2500 m and training at 1250 m (22). It should be considered, however, that their subjects were affected by a rather severe hypoxia during the training sessions at 1250 m, with oxyhemoglobin saturations down <80% (22).
We did not measure red cell mass in this 7-d experiment because a detectable rise in red cell mass in moderate hypoxia would take a considerably longer time and should be established in the future studies (4). Twelve hours of daily intermittent hypoxia was thought to be the maximum dose that could easily be incorporated into training. The level of induced hypoxia of 2500 m was suggested to be sufficient for erythropoietic stimulation but still well tolerated by the participants. The effect of other levels of altitude and duration should certainly also be studied further.
The suggestion that living at normobaric hypoxia as described in the present paper and training at sea level provides an optimal setting for enhancing performance in endurance exercise remains to be established.
This study was supported by the Finnish Olympic Committee and the Ministry of Education (DNRO 145/722/96). We would like to thank Ms. Liisa Tiusanen, Ms. Maria Uusimäki, Ms. Seija Linnaluoto, Mr. Kari Kajaus, and Mr. Teuvo Ylikoski for their expert assistance.
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Chair: L. Bruce Gladden