Temperature Control of Hypertensive Rats during Moderate Exercise in Warm Environment

The control of body temperature in Spontaneously Hypertensive Rat (SHR) subjected to exercise in warm environment was investigated. Male SHR and Wistar rats were submitted to moderate exercise in temperate (25°C) and warm (32°C) environments while body and tail skin temperatures, as well as oxygen consumption, were registered. Total time of exercise, workload performed, mechanical efficiency and heat storage were determined. SHR had increased heat production and body temperature at the end of exercise, reduced mechanical efficiency and increased heat storage (p < 0.05). Furthermore, these rats also showed a more intense and faster increase in body temperature during moderate exercise in the warm environment (p < 0.05). The lower mechanical efficiency seen in SHR was closely correlated with their higher body temperature at the point of fatigue in warm environment (p < 0.05). Our results indicate that SHR exhibit significant differences in body temperature control during moderate exercise in warm environment characterized by increased heat production and heat storage during moderate exercise in warm environment. The combination of these responses result in aggravated hyperthermia linked with lower mechanical efficiency. Key Points The practice of physical exercise in warm environment has gained importance in recent decades mainly because of the progressive increases in environmental temperature; To the best of our knowledge, these is the first study to analyze body temperature control of SHR during moderate exercise in warm environment; SHR showed increased heat production and heat storage that resulted in higher body temperature at the end of exercise; SHR showed reduced mechanical efficiency; These results demonstrate that when exercising in a warm environment the hypertensive rat exhibit differences in temperature control. Key words: Thermoregulation, physical exercise, hypertension, heat stress, exercise performance Introduction The practice of physical exercise, as well as other activities of daily living, in warm environment has gained importance in recent decades mainly because of the progressive increases in environmental temperature, which is directly related to hospitalizations due to cardiovascular disease and heat stroke (Easterling et al., 2000; Ebi et al., 2004; Nybo, 2008). During exercise performed in the heat, hypertensive patients experience an elevated blood pressure response and a greater thermal strain than normotensive individuals (Kenney et al., 1984). Since low-to-moderate exercise is currently prescribed as a non-pharmacological treatment for hypertension (Carneiro-Junior et al., 2013), and warm environment may represent additional cardiovascular stress during physical activity, the study of body temperature (Tb) control in hypertension emerge as a focus of research. The Spontaneously Hypertensive Rat (SHR) has been extensively used to study cardiovascular disease since it is considered a good animal model of human essential or primary hypertension (Folkow, 1987; Okamoto and Aoki, 1963). Both species share pathophysiological cardiovascular features of hypertension such as increased total peripheral resistance and hyperresponsiveness of the sympathoadrenal system to stressful stimuli (Folkow, 1987; Okamoto and Aoki, 1963; Trippodo and Frohlich, 1981). In order to maintain Tb constant at ~37 °C, the balance between heat production and heat loss is accurately coordinated by the central nervous system (Romanovsky et al., 2007; Webb, 1995). During exercise, the inherent increase in heat production in response to greater metabolic rate leads to the activation of heat loss mechanisms to avoid critically high Tb (Leite et al. 2007; Pires et al., 2007). It is well accepted that homoeothermic animal species are able to control Tb efficiently in a range of cool to moderate ambient conditions (Galloway and Maughan, 1997). However, this capacity has been shown to be impaired during exercise conducted in warm environment (Fuller et al., 1998; Walters et al., 2000). In such situation, increased blood flow to maintain the energy demand of active muscles occurs simultaneously with increases of cutaneous blood flow required to meet the demand of Tb regulation (Folkow, 1987; Gonzalez-Alonso et al., 2008). These combined demands for blood flow can result in a competition for the available cardiac output (Rowell, 1974), resulting in increased Tb and reduced physical capacity. In fact, it has been shown that high Tb and increased rates of body heating and heat storage are limiting factors for physical performance during prolonged exercise (Fuller et al., 1998; Lacerda et al., 2005; Leite et al., 2006). Taking into account the relationship between the cardiovascular system and the regulation of Tb, and that hypertension can affect the mechanisms of heat dissipation due to increased peripheral resistance (Kellogg et al., 1998; O’Leary and Wang, 1994; Wright et al., 1978), studies have been conducted to investigate the thermoregulatory ability of SHR (Barney et al., 1999; Berkey et al., 1990). Berkey et al. (1990) measured core temperature of SHR at ambient temperatures of 5, 15, 25, 35 and 40°C. After 1h of exposure to these different ambient temperatures the only ambient temperature at which SHR had higher core temperature than normotensive controls was at 40°C (Berkey et al., 1990). Additionally, Barney et al. (1999) exposed SHRs to ambient temperature of 25 or 37.5°C for 3.5h, reporting that SHR had both higher core temperature and evaporative water loss during heat exposure than normotensive controls (Barney et al., 1999). Such studies support that SHR has a hyperesponse to stress, which leads to higher core temperature in situations like exposure to warm environment (Berkey et al., 1990) and dehydration due to heat stress (Barney et al, 1999). Despite these previous results, there are few studies investigating the control of Tb during exercise in warm environment associated with pathological conditions of the cardiovascular system, i.e., hypertension. Thus, the aim of the present study was to investigate temperature control in SHRs subjected to moderate exercise in warm environment.