Acid?base balance at exercise in normoxia and in chronic hypoxia. Revisiting the "lactate paradox"

Abstract

Transitions between rest and work, in either direction, and heavy exercise loads are characterized by changes of muscle pH depending on the buffer power and capacity of the tissues and on the metabolic processes involved. Among the latter, in chronological sequence: (1). aerobic glycolysis generates sizeable amounts of lactate and H(+) by way of the recently described, extremely fast (20-100 ms) "glycogen shunt" and of the excess of glycolytic pyruvate supply; (2). hydrolysis of phosphocreatine, tightly coupled with that of ATP in the Lohmann reaction, is known to consume protons, a process undergoing reversal during recovery; (3). anaerobic glycolysis sustaining ATP production in supramaximal exercise as well as in conditions of hypoxia and ischemia, is responsible for the accumulation of large amounts of lactic acid (up to 1 mol for the whole body). The handling of metabolic acids, i.e., acid-base regulation, occurs both in blood and in tissues, mainly in muscles which are the main producers and consumers of lactic acid. The role of both blood and muscle bicarbonate and non-bicarbonate buffers as well as that of lactate/H(+) cotransport mechanisms is analyzed in relation to acid-base homeostasis in the course of exercise. A section of the review deals with the analysis of the acid-base state of humans exposed to chronic hypoxia. Particular emphasis is put on anaerobic glycolysis. In this context, the so-called lactate paradox is revisited and interpreted on the basis of the most recent findings on exercise at altitude.

Exercise intolerance at high altitude (5050m): Critical power and W′

“…Although the respiratory alkalosis that develops on immediate HA ascent (e.g. Tables 2 and 3) would be expected to temper the deleterious consequences of developing acidosis in the exercising muscles, subsequent renal compensation compromises the bicarbonate-related buffer capacity (Cerretelli and Samaja, 2003). However, the extent to which whole-body buffer capacity might be affected in our study is uncertain.…”

“…Resting and peak exercise muscle [ATP] and [PCr] levels have been reported to be essentially unaffected by ascent to HA (van Hall et al, 2009). Likewise, maximum blood [L − ] has been reported to be similar to SL values on immediate ascent (despite the lowerV O 2 max ), and although this is widely reported to decline as acclimatization proceeds (the "lactate paradox") (Cerretelli and Samaja, 2003;Reeves et al, 1992;West, 1986), no blunting was evident in arterial [L − ], muscle [L − ] or leg net lactate release during incremental exercise for moderate HA exposure (i.e. two weeks at 4100 m) compared to acute hypoxia at SL (van Hall et al, 2009).…”

Polymorphisms in theHBBgene relate to individual cardiorespiratory adaptation in response to endurance training

“…6 Since haemoglobin is an excellent buffer, an increase in the circulating haemoglobin mass helps improve the blood buffering capacity. 7 A higher blood buffering capacity could facilitate lactate 8 and proton release from the active muscle and increase the capacity to produce energy via the anaerobic pathways.…”

Capillary Supply and Fiber Morphometry in Rat Myocardium after Intermittent Exposure to Hypobaric Hypoxia

“…The most serious effects of high altitude on human physiology are due to the low oxygen partial pressure of the inspired air; consequently, different adjustments are needed to improve the tissue oxygen availability. Hypobaric hypoxia causes an increase in ventilation (see West, 1993), increases in arteriovenous O 2 difference, hemoglobin concentration, and hematocrit (Ferretti et al, 1990a), profound effects on the structure and function of skeletal muscle tissue (Ferretti et al, 1990b;Hoppeler et al, 1990), acid-base alterations with an important incidence in the affinity of hemoglobin to oxygen (Cerretelli and Samaja, 2003), and elevated erythropoietin levels (Eckardt et al, 1989). Moreover, prolonged exposure to hypobaric hypoxia also induces a degree of physical deterioration, which increases with altitude (Kayser, 1994).…”