多段階運動負荷時における血中乳酸濃度の三相性推移の意義

The significance of tri-phagic change of blood lactate level during multigraded exercise

　運動負荷試験において血中乳酸濃度の測定を行った研究報告がこれまで数多くなされてきたが，とくに多段階運動負荷試験においてはいわゆるanaerobic thresholdの概念に基づいて血中乳酸濃度の上昇開始を決定することが重視されてきた^1,2）。著者らは多段階運動負荷時の血中乳酸濃度の推移を詳細に検討したところ酸素消費量の増大に対する血中乳酸濃度の変化はanaerobic thresholdを境とした上昇を示すだけではなく三相性の推移を呈することを認めた。この現象の運動時エネルギー代謝と循環調節における意義を考察し報告する。

　The measurement of blood lactate level during maximal treadmill exercise test with one minute incremental protocol was studied in 10 subjects of sedentary middle-aged men. The tri-phagic increase in serial blood lactate with increasing oxygen consumption (VO₂) was found. Initially no significant change (phase I), then gradual increasing (phase II) and finally rapid increasing (phase III) of the lactate level were noted. The V0₂ at the onset of phase II was 14.0±2.1 ml/ min･kg (amounting to 38. 2±5.7% of VO₂max), the VO₂ at the onset of phase III was 27.6±3.7ml/min･kg (amounting to 75.5±9.6% of VO₂max) and the maximal oxygen consumption (VO₂max) was 36.7±3.3ml/min･kg. The VO₂ at the onset of phase II which corresponds to so-called anaerobic threshold (AT) was smaller than the reported values of AT determined by ventilatory method (amounting to 50% of VO2max). The ratio of increased lactate level to increased VO2 (⊿lactate/⊿VO₂) was 0.03±0.10 at the phase I, 0.65 ±0.16 at the phase II and 2.92±0.81 at the phase III. It was 4.5 times larger at the phase III than at the phase II. Our results, the very low level of VO₂ at the onset of phase II which is almost one third of VO₂max, may not explain the circulatory disturbance but the metabolic characteristics of skeletal muscle as the genesis of anaerobic threshold. On the other hand, the rapid increase in lactate level of the phase III may suggest the shortage of oxygen supply to the skeletal muscle. Therefore the phase III may indicate the disturbance of circulatory regulation corresponding to the increased energy demand during exercise. It is concluded that the tri-phagic blood lactate level during multigraded exercise may be one of the indicators to obtain the information of metabolic and circulatory regulation during exercise.