INTRODUCTION

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THE EFFECT OF HYPOXIC VS. NORMOXIC training on maximal exercise capacity has received considerable attention in sports circles; however, there have been relatively few well-controlled studies on the subject. Aerobic exercise training increases maximal O₂ consumption (O_2 max) by increasing the rate at which O₂ is supplied to the exercising muscles, largely through an increase in cardiac output secondary to the increase in stroke volume (4, 27), and by improving the extraction of O₂ by the contracting muscles (25). Because artificially increasing hematocrit increases O_2 max (2, 5), it is possible that polycythemia induced by acclimatization to hypoxia could also lead to improved maximal exercise performance. However, acclimatization to hypoxia results in other cardiovascular and respiratory changes that also influence the O₂ transport system. These include pulmonary hypertension and right ventricular hypertrophy (21, 23), decreased chronotropic response to catecholamines in the presence of an elevated sympathetic drive (24), as well as a decrease in maximal heart rate (22) and cardiac output (7), all of which may offset the positive effects of polycythemia on exercise performance. Because the extent to which these changes occur depends on the severity and duration of hypoxia, it is difficult to predict whether the positive features of acclimatization will outweigh the negative ones in a given set of conditions and therefore whether hypoxic training will confer an advantage over normoxic training. In addition, the hypoxia-induced decrease in exercise capacity may make it difficult, depending on the severity of hypoxia, to maintain the same absolute training intensity as in normoxia, which further complicates the interpretation of the results.

The objective of these experiments was to determine the effect of living and training in hypoxia on maximal exercise capacity, using a design that would allow an assessment of the effects of acclimatization, training, and training plus acclimatization while maintaining the same absolute training intensity as normoxic trained controls. We selected a moderate level of hypoxia at which absolute training intensity could be maintained in normoxic and hypoxic conditions and which would adequately stimulate polycythemia (19). We hypothesized that, if absolute training intensity was the same, living and training in moderate hypoxia (MHx) would result in a larger increase in O_2 max than living and training in normoxia.

The studies were carried out in rats, an animal frequently employed in exercise and altitude studies that shares with humans several features of acclimatization to hypoxia (6-8). The preparation allowed for characterization of the effects of hypoxia and training on the various steps of the O₂ cascade from the atmosphere to the tissues. Both normoxic and hypoxic groups were trained at equal absolute work rates such that the increase in O₂ requirements associated with exercise training would be comparable in both groups, independent of the prevailing PO₂.

	METHODS

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Animal model and training protocol. Seven-week-old male Sprague Dawley rats were randomly assigned to live in normoxia [inspired PO₂ (PI_O2) = 147 Torr] or in MHx (PI_O2 = 110 Torr). Each group was then subdivided into sedentary and trained subgroups. This division resulted in four experimental groups with 16 animals in each group: normoxic sedentary (NS), normoxic trained in normoxia (NTN), hypoxic sedentary (HS), and hypoxic trained in hypoxia (HTH). All four groups were housed in the same room with hypoxic groups placed in hypobaric chambers set to MHx. Ambient temperature was 22.5°C with a 12:12-h light-dark cycle. Training lasted 10 wk and was performed in an eight-lane treadmill through which the appropriate O₂-N₂ gas mixtures could be circulated: NTN trained at PI_O2 = 147 Torr and HTH at PI_O2 = 110 Torr. Training intensity started at ~80% of the O_2 max of normoxic sedentary animals and was increased gradually over 6 wk until it reached 30 m/min on a 10° incline, 1 h/day, 5 days/wk. This training intensity was maintained for the last 4 wk. Absolute training intensity was the same for both NTN and HTH. We chose to train both groups at equal absolute work rates because the O₂ requirements are the same in both conditions. However, because O_2 max is reduced in MHx, the training intensity relative to the corresponding O_2 max was higher in HTH than in NTN. Thus, whereas the absolute work rate and O₂ requirements were the same for both trained groups, the relative work rate of HTH was ~10% higher than that of NTN.

Maximal exercise protocol. Once the training protocol was completed, the animals were anesthetized with pentobarbital sodium (40 mg/kg ip); a PE-50 catheter was placed in the left carotid artery, and a PE-10 catheter was placed into the main pulmonary artery with the help of an introducer guide catheter. Adequate positioning of the pulmonary artery catheter was determined by the blood pressure tracing and verified at autopsy the day after the experiment. The catheters were tunneled subcutaneously, exteriorized at the back of the neck, cut at a length of 2 in., and flame sealed.

Gas exchange and O₂ transport determinations. The box enclosing the treadmill was airtight except for the in- and outflow ports, which are independent of one another. PI_O2 was maintained at 147 Torr by mixing O₂ and N₂. Flow of the gas mixture entering the treadmill box was maintained constant at 20 liters ATPS/min by use of a Cameron Instruments precision gas flow mixer. Inflowing and outflowing O₂ concentrations and outflowing CO₂ concentration (inflowing gas was CO₂ free) were measured continuously and simultaneously by use of an Applied Electrochemistry O₂ analyzer and a Columbus Instruments CO₂ analyzer, respectively. The O₂ and CO₂ analyzers were calibrated with gas mixtures measured with a precision of ±0.005%. The output of the O₂ and CO₂ meters was fed into a computer to provide determination of O₂, CO₂, and respiratory exchange ratio (R) every 5 s. O₂ and CO₂ (expressed in ml STPD · min¹ · kg¹) were calculated from the inflowing and outflowing O₂ concentration difference, the outflowing CO₂ concentration and the outflowing gas flow.

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	RESULTS

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Each group was composed of 16 animals. Only the data from the animals that achieved O_2 max as defined above are presented. The number of animals in each group is listed in Table 1. Acclimatization to hypoxia in the sedentary rats did not result in a significant increase in O_2 max (Table 1, NS vs. HS). This occurred in spite of significant increases in Hb concentration and Ca_O2 of ~5% in HS over NS (Table 1). Training, on the other hand, resulted in the expected increase in O_2 max, in both the normoxic (Table 1, NS vs. NTN) and the hypoxic groups (Table 1, HS vs. HTH). The increase in O_2 max in both groups was mediated in part though an increase in the rate of O₂ delivery to the tissues (O_2 max; Table 2, NS vs. NTN and HS vs. HTH). The improvement in O_2 max resulted from an increase in _max, due exclusively to increases in maximal stroke volume (SV_max, Table 2). Training also increased the rate of tissue O₂ extraction. Both O₂ ER and O_2 max/P_O2, a composite parameter that reflects the diffusional O₂ conductance at the tissue level (10, 26), were significantly higher in HTH than in HS (Table 1). In the normoxic group, training resulted in a significant increase in O_2 max/P_O2 only (Table 1, NS vs. NTN). Blood lactate levels during maximal exercise were lower in HS and HTH than in NS and NTN, respectively (Table 1). This is in agreement with previous observations on the effect of acclimatization lowering blood lactate levels during maximal exercise (1). Exercise training resulted in similar maximal exercise blood lactate levels as those in sedentary rats (Table 1, NS vs. NTN and HS vs. HTH). This is also in agreement with previous observations of higher work output for similar blood lactate concentrations in trained vs. untrained subjects (11). Although training effectively enhanced maximal exercise capacity, the effect was the same for both normoxic trained and hypoxic trained animals, i.e., O_2 max increased to the same extent in NTN and in HTH (Table 1). As it occurred in the hypoxic sedentary animals, HTH had a small (~5%) but significant increase in Ca_O2 (Table 1, NTN vs. HTH); however, this was not effectively translated into a higher O_2 max. Other than the effects on Ca_O2 and blood lactate, there were no other significant differences in the O₂ transport system between NTN and HTH.

As expected, rats acclimatized to hypoxia developed pulmonary hypertension (Fig. 1, NS vs. HS and NTN vs. HTH); however, the degree of hypertension was significantly attenuated with hypoxic training (Fig. 1, HS vs. HTH). Because the Ppa-to- ratios (Ppa/; Table 2) parallel the changes in Ppa from rest to maximal exercise in all four groups, the pulmonary hypertension present in the acclimatized groups is not due to a higher blood flow. Therefore, it can be concluded that the increase in Ppa is due to an increase in pulmonary vascular resistance. Acclimatization to MHx significantly increased Ppa/ by 35%, both at rest and during maximal exercise (Table 2, NS vs. HS), whereas acclimatization to and training in MHx resulted in a 5% increase in Ppa/ at rest and a 17% increase during maximal exercise (Table 2, NS vs. HTH).

View larger version (20K): [in this window] [in a new window]   Fig. 1.   Pulmonary arterial pressure (Ppa) plotted as a function of cardiac output () at rest and during maximal exercise. NS and HS, normoxic and hypoxic sedentary control groups, respectively; NTN, normoxic group trained in normoxia; HTH, hypoxic group trained in moderate hypoxia. Values are means ± SE. Horizontal and vertical bars represent 1 SE on either side of the mean.

	DISCUSSION

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The main findings of this study are: 1) acclimatization to and training at equal absolute work rates at the level of hypoxia used in the present experiments do not improve maximal aerobic capacity above that obtained with normoxic training and 2) training in hypoxia moderates hypoxic pulmonary hypertension.

Acclimatization to MHx in sedentary rats did not result in significant changes in O_2 max. This result is similar to those obtained after acclimatization to more severe levels of hypoxia, which have shown that O_2 max in rats (7) and humans (3) changes little, if at all, after acclimatization. One major factor for the modest effect of acclimatization to severe hypoxia on O_2 max is the reduction in _max, which offsets the increase in Ca_O2 and prevents substantial increases in the rate of O₂ delivery to contracting muscles. The decrease in _max is in part due to a reduction in maximal heart rate (HR_max; Refs. 6, 7, 22). This results from a diminished chronotropic response to catecholamines as a consequence of the downregulation of myocardial -adrenoceptors (14). The role of reduced _max in limiting O_2 max after acclimatization was demonstrated by the observation in acclimatized rats that an increase in _max, produced by increasing HR_max by atrial pacing, was translated into a proportionate increase in O_2 max (6). The present study shows that, in contrast to more severe hypoxia, acclimatization to MHx does not lead to significant changes in _max, HR_max, or SV_max. Nevertheless, the fact remains that the increase in Ca_O2 of HS was not translated into an increase in O_2 max. Other things being equal, the increase in Ca_O2 should have been translated into a proportionate increase in O_2 max, as is seen, for example, when hematocrit is artificially increased by red blood cell infusion (2). It is possible that the effect of increased blood O₂ content was offset by an aggregate of small decreases in O₂ conductances caused by acclimatization: average _max was 2% lower (Table 2), O₂ ER 3% lower (Table 1), and O_2 max/P_O2 10% lower (Table 1) in HS than in NS. Although the individual differences in these variables were not statistically significant, their combined effect may have offset the effect of the increase in Ca_O2 on O_2 max. The lower O₂ ER and O_2 max/P_O2 values of HS suggest a negative effect of chronic hypoxia on the efficacy of capillary-to-tissue O₂ transfer, a possibility that has been raised before (9, 18).

The PI_O2 selected for these experiments represents a level of hypoxia sufficient to elicit a polycythemic response (Table 1) while also allowing hypoxic trained rats to maintain the same absolute training intensity as normoxic trained animals. In preliminary experiments, we determined that exercise at this PI_O2 resulted in a reduction of O_2 max of ~10% below the normoxic value of sedentary rats. This relatively small effect allowed us to maintain the absolute training intensity in both groups at ~80% of the O_2 max of sedentary normoxic rats. Equal absolute training intensities have the same O₂ requirements independent of the environmental PO₂ and eliminate training intensity and O₂ flux levels as confounding factors.

The exercise training protocol was effective in increasing maximal exercise capacity: O_2 max increased by ~13% in both HTH and NTN above their respective sedentary controls. However, there was no significant difference in O_2 max between NTN and HTH. The increase in O_2 max in the trained groups was mediated in part by an increase in the rate of O₂ delivery to the tissues, as indicated by a 7-9% increase in O_2 max in both NTN and HTH above their corresponding sedentary controls (Table 2). In each case, the factor responsible for the training-induced increase in O_2 max was an increase in SV_max (Table 2), indicating that the mechanisms by which exercise training increases the maximal rate of blood O₂ convection are similar in both the hypoxic and normoxic trained groups.

Exercise training also influenced O₂ extraction by the tissues, although the evidence for this was more clear in HTH than in NTN. The efficacy of O₂ extraction by the tissues was evaluated by the changes in Ca_O2  C_O2, O₂ ER, and the ratio O_2 max/P_O2. Training in hypoxia significantly increased all three indexes above the levels observed in the sedentary rats (Table 1, HS vs. HTH). On the other hand, O_2 max/P_O2 was the only variable of tissue O₂ extraction that was significantly increased by normoxic training (Table 1, NS vs. NTN). The apparently larger effect of hypoxic training on O₂ extraction may be due to the relatively smaller values of the O₂ extraction indexes in HS mentioned above. The data suggest that acclimatization to hypoxia lowers O₂ ER in sedentary rats and that training in hypoxia increases O₂ ER more than normoxic training. Although the absolute training intensity was the same, training intensity relative to O_2 max was higher in HTH, because hypoxia of this magnitude lowers O_2 max by ~10%. It is possible that the higher relative training intensity created a more severe hypoxic condition in the exercising muscles of HTH and thus produced a stronger stimulus for the mechanisms responsible for changes in tissue O₂ extraction.

The practice of "living high and training low" has been shown to improve running performance over living and training in normoxia because it elicits polycythemia while maintaining normoxic training levels (17, 19). The present studies show that if normoxic training intensity is maintained in hypoxia, there is no improvement on maximal exercise capacity above that achieved by living and training in normoxia. The apparently larger effect of hypoxic training on O₂ ER does not lead to an improvement of this parameter over NTN but simply offsets the negative effect of acclimatization on O₂ ER. The lack of improvement in O_2 max of HTH over NTN implies that hypoxic training may oppose other possible beneficial aspects associated with acclimatization.

Both HS and HTH rats developed pulmonary hypertension; however, Ppa was significantly lower in the trained than in the sedentary rats both at rest and during maximal exercise (Table 2). Because Ppa was measured in normoxia, it is clear that the Ppa values observed under these conditions do not include a component of hypoxic pulmonary vasoconstriction (HPV). Accordingly, the most likely sources for the differences in Ppa between HS and HTH are due to an effect of exercise training on the extent of pulmonary vascular remodeling and/or in the balance of pulmonary vasodilators and vasoconstrictors. Chronic exercise training in normoxic conditions moderates HPV (15) and the response to vasoconstrictors in rats (16). A less intense HPV could result in a lower stimulus for pulmonary vascular remodeling and could therefore be responsible for the moderation of pulmonary hypertension in HTH during normoxic conditions. Additionally, it is possible that exercise training modifies the pulmonary vascular response to vasodilators or vasoconstrictors. Studies in pulmonary arterial rings suggest that chronic exercise training leads to increased endothelium-dependent vasodilation and reduced production of prostanoid vasoconstrictors (13). However, this appears to be restricted only to coronary artery-ligated animals (12). It was suggested that the different effects of exercise training on pulmonary vasoreactivity between coronary artery-ligated and normal animals could result from larger increases in vascular shear stress secondary to abnormal cardiac function in the coronary artery-ligated animals, thus leading to upregulation of endothelial NOS gene expression (12, 20). Whether a similar mechanism operates during hypoxic exercise training in the rat is not clear from the present studies and should be the subject of further research.

In summary, acclimatization to and training in MHx increase maximal exercise capacity to the same level observed after a training protocol of equal absolute intensity under normoxic conditions. The potential advantage conferred by the increase in Ca_O2 was not translated into a significant increase in O_2 max because of the aggregate effect of small offsetting factors. Training in hypoxia, on the other hand, resulted in a moderation of the hypoxic pulmonary hypertension, suggesting that chronic exercise training may assist in the acclimatization process by reducing pulmonary hypertension.