Exercise activates compensatory thermoregulatory reaction in rats: a modeling study

Abstract

The importance of exercise is increasingly emphasized for maintaining health. However, exercise itself can pose threats to health such as the development of exertional heat shock in warm environments. Therefore, it is important to understand how the thermoregulation system adjusts during exercise and how alterations of this can contribute to heat stroke. To explore this we measured the core body temperature of rats (Tc) running for 15 min on a treadmill at various speeds in two ambient temperatures (Ta = 25°C and 32°C). We assimilated the experimental data into a mathematical model that describes temperature changes in two compartments of the body, representing the muscles and the core. In our model the core body generates heat to maintain normal body temperature, and dissipates it into the environment. The muscles produce additional heat during exercise. According to the estimation of model parameters, at Ta = 25°C, the heat generation in the core was progressively reduced with the increase of the treadmill speed to compensate for a progressive increase in heat production by the muscles. This compensation was ineffective at Ta = 32°C, which resulted in an increased rate of heat accumulation with increasing speed, as opposed to the Ta = 25°C case. Interestingly, placing an animal on a treadmill increased heat production in the muscles even when the treadmill speed was zero. Quantitatively, this "ready-to-run" phenomenon accounted for over half of the heat generation in the muscles observed at maximal treadmill speed. We speculate that this anticipatory response utilizes stress-related circuitry.

Amphetamine enhances endurance by increasing heat dissipation

“…The presence of a “hypothermic” phase is related to the decrease in core heat production in physiologic “anticipation” of/during exercise, which was described by us earlier (Yoo et al. ). Eventually, the body temperatures of rats in all groups started rising.…”

“…The value of η was fixed at 0.125 min −1 based on Yoo et al. (). Heat exchange between the core and environment is proportional to another heat transfer coefficient η a and the difference of temperatures between the core and environment, .…”

Ventromedial hypothalamic melanocortin receptor activation: regulation of activity energy expenditure and skeletal muscle thermogenesis

“…Baseline BAT temperatures were higher than that of muscle and varied similarly across the day (data not shown) and treadmill activity suppressed BAT thermogenesis to baseline levels or below, consistent with mathematical models where thermogenic systems and heat dissipation are altered to compensate for activity‐related heat production (Yoo et al . ).…”

Why is it easier to run in the cold?

“…Figure 1 illustrates the dynamics of heat generations and temperatures in the 2 compartments at the highest sustainable speed at various ambient temperatures with an assumption that initial temperatures in both compartments are the same 1,3 . In our experiments, rats were able to maintain the speed of 18 m/min with zero incline for more than an hour.…”

“…Thermoregulatory system may turn off or suppress metabolic heat production when body temperature increases due to physical activity. To test this hypothesis we conducted a study 1 in which we measured body temperature dynamics in rats running on a treadmill with 4 different speeds and at 2 different ambient temperatures. The first thing we noticed was that before starting to rise, the body temperature dropped slightly during first several minutes of the run when the experiments were performed at normal room temperature (T a = 25°C), and not at the higher ambient temperature we used (T a = 32°C).…”