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POINT-COUNTERPOINT COMMENTS
ACTES Laboratory (EA 3596)
Department of Physiology
University of the French West Indies
Guadeloupe
e-mail: pconnes{at}yahoo.fr
The following letters are in response to Point:Counterpoint series "Lactic acid accumulation is an advantage/disadvantage during muscle activity" that appeared in the April issue (J Appl Physiol 100: 1410–1414, 2006; http://jap.physiol.org/content/vol100/issue4/2006).
To the Editor: Exercise physiologists often focused their attention on only one physiological factor involved in exercise performance. The effect of lactic acidosis on exercise performance may be both advantageous and disadvantageous depending on the physiological factor of performance considered. When muscle is considered, the accumulation of lactic acid seems to contribute to the appearance of fatigue mainly because of an effect of lowered pH on the release of K+ from the contacting muscles (1). But, muscle activity and exercise performance are also dependant on muscle blood flow, the latter being dependant on the perfusion pressure, the vascular tone, and the blood rheological properties (4, 5). Lactate and hydrogen ions are able to modulate blood rheology by either decreasing red blood cell deformability or improving it (3). Because impairment in RBC deformability may adversely affect capillary recruitment and physiological mechanisms that ensure adequate delivery of oxygen to tissue, red blood cell deformability is able to influence exercise performance (4). In addition, change in lactic acid concentrations is involved in the regulation of vascular tone and can favor exercise hyperemia (2). Although the debate concerning the advantageous or disadvantageous effects of lactic acid on muscle activity and exercise performance is complex, the kind of exercise performed could determine the answer because anaerobic performance could be limited by lactic acid accumulation into muscles, whereas aerobic performance, which depends on muscle blood flow, could be also positively affected by the accumulation of lactate and hydrogen ions.
REFERENCES
O2 max is/is not limited by cardiac output and locomotor muscle blood flow." J Appl Physiol 100: 2099, 2006.
Equipe Modélisation des Activités Sportives
Université de Savoie
Le Bourget du Lac, France
e-mail: laurent.messonnier{at}univ-savoie.fr
Unité de Recherche Physiologie et Physiopathologie de l'Exercice et Handicap
Université Jean Monnet
Saint Etienne, France
To the Editor: The etiology of muscle fatigue remains incompletely solved especially in vivo (3). The causes of muscle fatigue vary according to the type, duration, and intensity of exercise and to the physical fitness and health status of the subjects. Nevertheless, an elevated sarcolemmal lactate (and proton) transport capacity constitutes an advantage during muscle activity.
During moderate-intensity exercise, efficient lactate exchanges contribute to the "cell-cell lactate shuttle," i.e., the delivery of lactate for its use as fuel by neighboring or distant active and oxidative muscles fibers or as substrate by other tissues (e.g., for neoglucogenesis by the liver; Ref. 1).
During high-intensity exercise, the lactate exchange ability was positively correlated with the capacity to prolong exercise (4). Whether this correlation between the lactate exchange ability and performance was (directly or indirectly) causal or coincidental, the question is still under debate. Nevertheless, it remains that an elevated lactate exchange ability seems to constitute a protective mechanism against muscle fatigue. Inasmuch as the transport of lactate across the sarcolemma is mediated mainly by the lactate-H+ cotransport via the monocarboxylate transporters MCT1 and MCT4, an elevated lactate transport capacity delays both muscle lactate accumulation and intracellular pH decrease and seems to favor muscle activity. In accordance, previous experiments have shown that metabolic alkalosis enhances the net lactate release rate from the active muscles and work capacity (2, 5) contrary to acidosis that reduces lactate efflux from muscle and exercise tolerance (5).
All this makes an elevated sarcolemmal lactate (and proton) transport capacity an advantage during muscle activity.
REFERENCES
O2max is related to the lactate exchange and removal abilities. Int J Sports Med 23: 433–438, 2002.[CrossRef][Web of Science][Medline]
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