Roles And Mechanisms Of Lactic Acid In Exercises-induced Central Fatigue

Physical fatigue is a complex phenomenon which may relate to both peripheral and central mechanisms. The important role of central nervous system(CNS) in fatigue (central fatigue) has been demonstrated by more and more evidence over the past several decades. The unwillingness to generate and maintain adequate CNS drive to the working muscle is the most liky explanation of central fatigue, but the mechanisms underlying this type of fatigue are not well documented yet. Peripheral fatigue has been well studied and can involve the accumulation of lactic acid, which inspires us if lactic acid is related to central fatigue. However, little attention has been given to the roles of lactic acid on central fatigue. It was found that lactic acid is the major substrate in brain energy metabolism, produced by astrocytes and released in the extracellular space to form a pool readily available for neurons in case of high energy demand. During prolonged or intense exercises, the concentration of lactic acid in brain increases distinctively. But it is unknow whether lactic acid accumulats within the activated brain areas in central fatigue and exert influences on it. So this researches were performed in vivo and in vitro to probe the roles and mechanisms of lactic acid on central fatigue. We hypothesized that central fatigue would be linked with brain lactic acid which would affect the neuronal function through changing the neuronal excellular environment. It is helpful to has profound insight into the central fatigue, simutaneously to provide experimental basis for fatigue precaution and delay. Here are the results.In vivo1 Central fatigue was induced by exhaustive exercises. Rats were divided into 3 groups: control group(C), the exhaustive exercises group training for one time(SE) and the exhaustive exercises group training for 7 days (O) . The changes of weight, Hb, BUN and blood lactic acid were determined to estimate rats exercise abilities. In addition, SEP, 5-HT and DA/5-HT were measured to evaluate central nervous system function. According to the comprehensive analysis of above indexes, the central fatigue model was established. After 1 week training, the weight of O group rats increased slowly whereas BUN and blood lactic acid increased significantly. Both DA and DA/5-HT decreased and the latency of SEP was prolonged. Those indicated exercises-induced central fatigue was established. During central fatigue, cortex lactic acid contents of O and SE groups became elevated, whereas glycogen content was unchanged. The results showed that the accumulation of lactic acid occurred in central fatigue.2 In order to probe the effect of lactic acid on central excitability, the cortex EEG was observed through lactic acid injection in lateral cerebral ventricle. EEG was depressed with the amplitude and frequency decreased significantly, which indicated that lactic acid contributed to the inhibition of central nervous system excitabilityIn vitro1 The survival rate of neurons decreased dose-dependently by lactic acid concentration^ ~ 20 u mol/ml, 24h);ln addition, lactic acid per se (pH was maintaining at 7.35) can also decrease the survival rate of neurons. It is suggested that the mechanism of lactic acid on the survival rate of neurons were complicated. Thereinto, not only the lactate- induced pH decrease but also lactic acid per se may exert influence on it.2 Both the lactate dehydrogenase leakage and cell volume of neurons...