Heat storage rate and acute fatigue in rats

Abstract

Thermal environmental stress can anticipate acute fatigue during exercise at a fixed intensity (%VO 2max ). Controversy exists about whether this anticipation is caused by the absolute internal temperature (T int , ºC), by the heat storage rate (HSR, cal/min) or by both mechanisms. The aim of the present study was to study acute fatigue (total exercise time, TET) during thermal stress by determining T int and HSR from abdominal temperature. Thermal environmental stress was controlled in an environmental chamber and determined as wet bulb globe temperature (ºC), with three environmental temperatures being studied: cold (18ºC), thermoneutral (23.1ºC) or hot (29.4ºC). Six untrained male Wistar rats weighing 260-360 g were used. The animals were submitted to exercise at the same time of day in the three environments and at two treadmill velocities (21 and 24 m/min) until exhaustion. After implantation of a temperature sensor and treadmill adaptation, the animals were submitted to a Latin square experimental design using a 2 x 3 factorial scheme (velocity and environment), with the level of significance set at P<0.05. The results showed that the higher the velocity and the ambient temperature, the lower was the TET, with these two factors being independent. This result indicated that fatigue was independently affected by both the increase in exercise intensity and the thermal environmental stress. Fatigue developed at different T int and HSR showed the best inverse relationship with TET. We conclude that HSR was the main anticipating factor of fatigue. Key wordsIt has been well documented that the total exercise time (TET) is reduced in warmer environments and internal body temperature (T int ) has been considered to be a limiting factor during prolonged physical exercise (1-5). This anticipation of fatigue seems to occur in order to prevent an increased T int from compromising homeostasis. The mechanisms responsible for exercise interruption due to an increase in T int are still not well understood; controversy exists about whether there is a critical absolute T int (in ºC) or whether the heat storage rate (HSR, in cal/min) is as important or even more important than T int in the origin of fatigue. Fuller et al. (6), studying the effect of warm environments on T int in rats during treadmill exercise, reported a shorter TET in animals exercising in warmer environments and similar abdominal and hypothalamic temperatures at the end of exercise at the time of fatigue, and suggested the existence of a critical absolute T int which may lead to the interruption of activity. Walters et al. (7),

The Involvement of Brain Monoamines in the Onset of Hyperthermic Central Fatigue

“…This detail allows for the possibility that reaching a critically high body temperature by itself may not be the main factor in predicting fatigue. In fact, this result is in agreement with the concept that dynamic variables strongly contribute to the reduction of physical performance Hasegawa et al, 2008;Lacerda et al, 2005Lacerda et al, , 2006Leite et al, 2006, L. O. Rodrigues et al, 2003A. G. Rodrigues et al, 2004Soares et al, 2004Soares et al, , 2007Wanner et al, 2007).…”

“…Even though it has been stated that central fatigue coincides with the attainment of fixed high body and brain temperatures (Fuller et al, 1998;Gonzalez-Alonso et al, 1999;Nielsen et al, 1993;Walters et al, 2000), there is evidence that dynamic mechanisms are consistent regulators of feelings of fatigue. In support of this hypothesis, the rates of heat storage and body heating that are associated with changes in neurotransmitter content in thermoregulatory centers emerge as important factors in determining fatigue Hasegawa et al, 2008;Lacerda et al, 2005Lacerda et al, , 2006Leite et al, 2006, L. O. Rodrigues et al, 2003A. G. Rodrigues et al, 2004Soares et al, 2004Soares et al, , 2007Wanner et al, 2007).…”

“…G. Rodrigues et al, 2004Soares et al, 2004Soares et al, , 2007Wanner et al, 2007). There are many findings that differ from the hypothesis that fatigue is predicted by a defined high body temperature Hasegawa et al, 2008;Lacerda et al, 2005Lacerda et al, , 2006Leite et al, 2006, L. O. Rodrigues et al, 2003A. G. Rodrigues et al, 2004Soares et al, 2004Soares et al, , 2007Wanner et al, 2007).…”

“…However, the manipulation of the body temperature increase rate was restricted to a very narrow range and was possibly not large enough to induce noticeable changes in body temperature. Moreover, individual body temperatures respond differently to heat stimuli and depend on many factors, such as dehydration, nutrition, fitness and motivation, as well as exercise intensity and thermal environmental stress (Kay & Marino, 2000;L. O. Rodrigues et al, 2003).…”

Comportamento da produção de espécies reativas de oxigênio em miocárdio de ratos submetidos a treinamento de baixa intensidade em diferentes temperaturas

“…Com uma mesma carga de trabalho, esta prática, em ambientes quentes, promove aumento na freqüência cardíaca e no débito cardíaco, assim como aumento na temperatura interna e cutânea, comparativamente com o mesmo exercício realizado em ambientes frios. Neste sentido, exercícios físicos em ambientes quentes promovem aumento na concentração sanguínea de lactato e existem evidências de depleção mais rápida do glicogênio muscular (9) . Estudos relatam que altas temperaturas teciduais induzem alterações estruturais e funcionais nas proteínas celulares envolvidas no transporte eletrolítico por meio da membrana celular e do retículo sarcoplasmático e durante a respiração mitocondrial, antecipando a fadiga muscular com redução da performance atlética (10)(11) .…”