Exercise training in hypobaric hypoxia has been widely accepted as a useful method of enhancing the endurance performance of athletes, and a considerable amount of experimental evidence has accumulated regarding the efficacy of this training method [1-4]. Under hypobaric hypoxia, atmospheric pressure decreases; thus, the partial pressure of oxygen in the atmosphere decreases so that oxygen available in the same volume of air decreases, thereby inducing hypoxia [5,6]. Exercise under hypoxia induces lower arterial oxygen saturation and red blood cell deformability, whereas carbohydrate substrate mobilization and blood lactate levels during exercise are augmented [5,7-9]. In general, these physiological responses during exercise under hypoxia improve athletic performance and promote peripheral adaptation in the muscles [10]. However, the mechanism for these adaptive phenomena is not yet fully understood.

Hypoxia makes the energy supply more dependent on the glycolytic system during exercise by reducing the systemic oxygen supply, and these changes lead to metabolic acidosis through increased ATP synthesis by anaerobic metabolic processes, an increase of hydrogen ions, and a decrease in pH [11,12]. Exercise-induced accumulation of lactate and hydrogen ions reduces exercise performance [12,13]. Once hydrogen ions are produced in the muscles through exercise, they are buffered primarily by the bicarbonate (HCO_3-) buffering system in the bloodstream [10]. Hypoxia rapidly increases minute ventilation and induces increased carbon dioxide removal, which relates to respiratory alkalosis [12,14]. Exercise in a hypoxic environment causes complex changes in the acid-base balance by HCO_3- and is simultaneously affected in terms of both metabolic acidosis induced by exercise and respiratory alkalosis induced by hypoxia [14]. Therefore, the exercise-induced kinetics of blood HCO_3- under hypoxia reflect the homeostasis of the acid-base balance in blood [10].

In addition to the acid-base balance, the ion balance is critical to muscle excitation, contraction, metabolism, and thus for muscle function during exercise [15]. The accumulation of extracellular potassium ions (K⁺) reduces the excitability of active muscles and lowers exercise tolerance [16,17]. Increased muscle Na⁺-K⁺ pump concentration via exercise training is usually associated with a reduced rise in plasma K⁺ during exercise [17]. Exercise under hypoxia is more dependent on anaerobic metabolism, with a concomitant increase in K⁺ during exercise in the blood, than exercise under normoxia [5,7]. These metabolic responses during exercise under hypoxia lead to acid-base responses, such as a concomitant release of K⁺ from the cell into extracellular fluid by increasing the opening of the muscular ATP-sensitive K⁺ channels [18,19]. In other words, exercise under hypoxia may augment exercise-induced alterations of acid-base and ion balances compared with exercise under normoxia. Lühker et al. [14] examined the differences in acid-base and ion balance during exercise under severe hypoxia (12% F_iO₂, equivalent to an altitude of 4500 m) and normoxia. They found a significant difference in pH and blood lactate levels during a 200 W cycle ergometer exercise between hypoxia and normoxia. However, Sumi et al. [10] compared the effects of acidbase and ion balance in response to high-intensity interval exercise under moderate hypoxia (14.5% F_iO₂, equivalent to an altitude of 3000 m) in endurance athletes. They reported that exercise under hypoxia did not elicit a decrease in blood pH or increase in K⁺ levels compared to an equivalent level of exercise under normoxia. The difference between these studies is the result of the difference in hypoxic conditions and exercise intensity.

Therefore, it is important to examine the differences in acid-base and ion balance during exercise under various hypoxic conditions. The purpose of this study was to evaluate the acid-base and ion balance at rest and after exercise in healthy males under normoxia, moderate hypoxia, and severe hypoxia. We hypothesized that exercise under moderate and severe hypoxia would induce greater exerciseinduced alterations of acid-base and ion balances compared with exercise under normoxia.

According to our hypothesis, exercise under moderate and severe hypoxia would induce greater differences in glucose, lactate, S_CO₂, pH, and HCO_3- levels in capillary blood compared with exercise under normoxia. In addition, lactate, S_CO₂, and HCO_3- levels showed greater changes in the SH trial than the MH trial. However, there were no great differences in Na⁺, K⁺, and Ca²⁺ levels in MH and SH trials compared with the N trial.

Hypoxia decreases the systemic oxygen-carrying capacity and peripheral oxygen-utilizing capacity, making it more dependent on glycolysis; these changes induce an increase in blood glucose levels, blood lactate levels, and respiratory exchange ratio during exercise [8,23]. Exposure to and exercise under hypoxia induce a decrease in oxygen saturation in the arterial blood, resulting in an augmented energy supply from the glycolytic system, increasing the blood lactate level and production of hydrogen ions (H+) [9,10,24]. The augmented metabolic stress stimulates the opening of ATP-sensitive K⁺ channels in the muscle, subsequently increasing K⁺ outflow to the extracellular fluid in the active muscle [24,25]. Additionally, proton production is further increased during exercise under hypoxia; protons are then released into the bloodstream, and the blood is buffered by the HCO_3- buffer system [14,15]. Moreover, the acid-base response by the HCO_3- buffer system is caused by an increase in ventilation during exercise [12]. Lühker et al. [14] investigated the differences in acid-base and ion balance during exercise under severe hypoxia (12% F_iO₂, equivalent to an altitude of 4500 m) and normoxia, and hypoxia showed a significant decrease in pH and increase in blood lactate levels during a 200 W cycle ergometer exercise compared to normoxia. In our study, SH showed a greater blood glucose level after exercise compared to the MH and N trials, and exercise-induced blood lactate level elevations were significantly increased as the hypoxic conditions became more severe; SH > MH > N trials. In addition, the MH and SH trials showed a lower S_CO₂, pH, and HCO_3- than the N trial after exercise. These findings were consistent with those of Sumi et al. [10] and Woorons et al [26].

Regarding the ion balance, our initial hypothesis was that the MH and SH trials would augment the exercise-induced blood K⁺ elevation compared with the N trial. Because lower S_CO₂ and pH increases the opening of K_ATP channels, we believed that K⁺ would efflux from the working muscle into the bloodstream [24,27]. The increased accumulation of extracellular K⁺ during exercise under hypoxia is manifested by the enhanced release of K⁺ in the working muscle and decreased reuptake of K⁺ during exercise [24]. Street et al. [18] verified that extracellular K⁺ accumulation and metabolic alkalosis were strongly correlated; their findings seem reasonable, considering the significant increase in K⁺ and Ca²⁺ induced by exercise in all trials (SH, MH, and N) in our study.

However, our results did not show a significant difference in K⁺ after exercise between SH and MH trials and the N trial. Although the exercise type and intensity differed, results reported by Sumi et al. [10] and Sumi et al. [24] regarding ion balance were similar to those of our study. Sumi et al. [10] investigated the effects of acid-base and ion balance in response to high-intensity interval exercise under moderate hypoxia (14.5% F_iO₂) and normoxia in endurance athletes. They reported that exercise under hypoxia did not elicit a decrease in blood pH and increase in K⁺ levels compared with an equivalent level of exercise under normoxia. The difference between these previous studies is due to the difference in hypoxic conditions and exercise intensity. Additionally, Sumi et al. [24] investigated the acid-base balance and K⁺ kinetics in response to exercise (relative exercise intensity) under moderate hypoxia (14.5% F_iO₂) among endurance athletes (age: 20.7 ± 0.9 years, height: 172.5 ± 2.2 cm, body mass: 61.6 ± 2.8 kg). They reported that endurance exercise under moderate hypoxia elicited a decline in venous pH; however, it did not induce an increase in blood K⁺ levels compared with normoxia. These two previous studies suggest that exercise (high-intensity interval exercise and endurance exercise) under moderate hypoxia cause greater metabolic stress and similar exercise-induced elevation of blood K⁺ compared with the same exercise under normoxia, despite lower exercise absolute load. These findings were confirmed in our study examining the effects of exercise under moderate and severe hypoxia on the acid-base and ion balance in healthy males compared with exercise under normoxia. In other words, our study confirmed that severe hypoxic conditions induced a greater metabolic response during exercise, without affecting the ion balance, such as K⁺, Na⁺, and Ca²⁺ kinetics, during exercise compared to normoxia.

In conclusion, the MH and SH trials induced greater differences in glucose, lactate, S_CO₂, pH, and HCO_3- levels in capillary blood compared to the N trial. Additionally, lactate, S_CO₂, and HCO_3- levels showed greater changes in the SH than in the MH trial. However, there were no significant differences in Na⁺, K⁺, and Ca²⁺ levels in MH and SH trials compared with the N trial.