A fadiga no exercício físico é modulada pela neurotransmissão colinérgica nos núcleos ventromediais hipotalâmicos, em ambiente frio

AUTOR(ES)
DATA DE PUBLICAÇÃO

2007

RESUMO

Cholinergic blockade in the ventromedial hypothalamic nucleus (VMH) reduces tail heat loss and induces a higher increase in the heat storage rate during physical exercise, antecipating the onset of fatigue in 37%. Although the evidences indicating the involvement of VMH cholinergic receptors on thermoregulation during the exercise until fatigue, the control of these nucleus in an ambiental condition which the heat production is not different from the heat dissipation to the environment has not been described. In these conditions, the heat variation rate and the activation of temperature sensitive neurons would not be altered in control group and, thus, the fatigue would be related to factors other than the thermoregulatory adjustments. Furthermore, it is unclear if the VMH thermogenic activity depends on ambient temperature and exercise-induced increase in metabolic rate. Therefore, the aim of the study was to determine an ambient temperature where the internal body temperature is not altered during the physical exercise until fatigue and to verify the effect of physical exercise and ambient temperature on thermogenic activity of VMH. Adult male Wistar rats weighting 250-350 g were used. The study was divided in two experimental protocols. During the first experimental protocol, intact rats were submited to treadmill running (20 m/min and 5% inclination) until fatigue at ambient temperatures of 8ºC, 12ºC and 15ºC and with relative humidity controlled at 50%. Internal body temperature reduced during the exercise at 8ºC and increased at 15ºC. During the exposure to 12ºC, internal body temperature was not modificated from the beginning of the exercise until the fatigue point. Tail skin temperature decreased two minutes after the exercise had started and remained reduced until the onset of fatigue at the three ambient temperatures. During the second experimental protocol, animals were fixed to a stereotaxic apparatus and received a bilateral implant of cannulae in the VMH. The rats were submitted to two experimental conditions with an interval of at least two days between the trials. Rats received an injection of either 0,2 µL of 0,15 M saline solution or 2,5x10-2 M methylatropine in the VMH and, immediately after this procedure, the animals were submitted to exercise. Internal body temperature, tail skin temperature and time to fatigue were measured. This protocol was reproduced at different ambients temperatures of 5ºC, 12ºC and 15ºC and with relative humidity always controlled at 50%. Cholinergic blockade in the VMH reduced the time to fatigue at the three ambients temperatures. Internal body temperature reduced during the physical exercise at 5ºC increased under 15ºC and was not modified at 12ºC. Methylatropine injection in the VMH failed to change internal body temperature compared to saline group. Tail skin temperature reduced during the exercise in both groups in all environmental conditions. No differences were observed in tail skin temperature between saline and methylatropine groups at the three ambient temperatures. Taking the above data into account, we concluded that exercising running rats at 12ºC represents an experimental protocol where the internal body temperature does not alter throughtout the exercise until the onset of fatigue. Furthermore, VMH muscarinic cholinoceptors determine the time to fatigue regardless of the ambient temperature in which the physical activity is carried out.

ASSUNTO(S)

receptores colinérgicos decs exercícios físicos aspectos fisiológicos teses. fadiga teses. receptores muscarínicos decs corpo temperatura regulação teses. rato fisiologia teses. homeostase teses.

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