The re-establishment of the normal blood lactate response to exercise in humans after prolonged acclimatization to altitude

Abstract

1. One to five weeks of chronic exposure to hypoxia has been shown to reduce peak blood lactate concentration compared to acute exposure to hypoxia during exercise, the high altitude 'lactate paradox'. However, we hypothesize that a sufficiently long exposure to hypoxia would result in a blood lactate and net lactate release from the active leg to an extent similar to that observed in acute hypoxia, independent of work intensity. 2. Six Danish lowlanders (25-26 years) were studied during graded incremental bicycle exercise under four conditions: at sea level breathing either ambient air (0 m normoxia) or a low-oxygen gas mixture (10 % O(2) in N(2), 0 m acute hypoxia) and after 9 weeks of acclimatization to 5260 m breathing either ambient air (5260 m chronic hypoxia) or a normoxic gas mixture (47 % O(2) in N(2), 5260 m acute normoxia). In addition, one-leg knee-extensor exercise was performed during 5260 m chronic hypoxia and 5260 m acute normoxia. 3. During incremental bicycle exercise, the arterial lactate concentrations were similar at sub-maximal work at 0 m acute hypoxia and 5260 m chronic hypoxia but higher compared to both 0 m normoxia and 5260 m acute normoxia. However, peak lactate concentration was similar under all conditions (10.0 +/- 1.3, 10.7 +/- 2.0, 10.9 +/- 2.3 and 11.0 +/- 1.0 mmol l(-1)) at 0 m normoxia, 0 m acute hypoxia, 5260 m chronic hypoxia and 5260 m acute normoxia, respectively. Despite a similar lactate concentration at sub-maximal and maximal workload, the net lactate release from the leg was lower during 0 m acute hypoxia (peak 8.4 +/- 1.6 mmol min(-1)) than at 5260 m chronic hypoxia (peak 12.8 +/- 2.2 mmol min(-1)). The same was observed for 0 m normoxia (peak 8.9 +/- 2.0 mmol min(-1)) compared to 5260 m acute normoxia (peak 12.6 +/- 3.6 mmol min(-1)). Exercise after acclimatization with a small muscle mass (one-leg knee-extensor) elicited similar lactate concentrations (peak 4.4 +/- 0.2 vs. 3.9 +/- 0.3 mmol l(-1)) and net lactate release (peak 16.4 +/- 1.8 vs. 14.3 mmol l(-1)) from the active leg at 5260 m chronic hypoxia and 5260 m acute normoxia. 4. In conclusion, in lowlanders acclimatized for 9 weeks to an altitude of 5260 m, the arterial lactate concentration was similar at 0 m acute hypoxia and 5260 m chronic hypoxia. The net lactate release from the active leg was higher at 5260 m chronic hypoxia compared to 0 m acute hypoxia, implying an enhanced lactate utilization with prolonged acclimatization to altitude. The present study clearly shows the absence of a lactate paradox in lowlanders sufficiently acclimatized to altitude.

Figure 1. Arterial lactate concentration and net lactate release from the active leg during incremental bicycle exercise (A) and incremental one-leg knee-extensor exercise (B)

* Significant difference between 5260 m chronic hypoxia vs. 0 m acute hypoxia and 5260 m acute normoxia vs. 0 m normoxia. § Significant difference between 5260 m chronic hypoxia vs. 0 m normoxia and 0 m acute hypoxia vs. 0 m normoxia. # Significant difference between 5260 m acute normoxia vs. 5260 m chronic hypoxia. Values are means ±s.e.m. for six subjects with incremental bicycle exercise and five subjects with incremental one-leg knee-extensor exercise.

Figure 2. Arterial noradrenaline and adrenaline concentrations at the moment of exhaustion for incremental bicycle exercise (A) and one-leg knee-extensor exercise (B)

For details see Fig. 1.

Figure 3. Arterial pH during incremental bicycle exercise (A) and one-leg knee-extensor exercise (B)

For details see Fig. 1.

Figure 6. Relationship between net lactate release and net proton release during incremental bicycle exercise (A) and incremental one leg-knee-extensor exercise (B)

For details see Fig. 1.

Figure 4. Arterial bicarbonate concentration and net bicarbonate release from the active leg during incremental bicycle exercise (A) and incremental one-leg knee-extensor exercise (B)

For details see Fig. 1.

Figure 5. Arterial base deficit and calculated leg net proton release during incremental bicycle exercise (A) and incremental one-leg knee-extensor exercise (B)

For details see Fig. 1.

Figure 7. Schematic presentation of muscle proton transport

CA, carbonic anhydrase; A, weak acid.