Influence of exogenous and endogenous estrogen on thermoregulatory responses to mild heat and the interaction with light
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(2020) 70:56 Marui et al. J Physiol Sci https://doi.org/10.1186/s12576-020-00782-x
Open Access
ORIGINAL PAPER
Influence of exogenous and endogenous estrogen on thermoregulatory responses to mild heat and the interaction with light and dark phases Shuri Marui1, Yuta Masuda1,2, Issei Kato1,2 and Kei Nagashima1*
Abstract The present study aimed to determine the influence of estradiol ( E2) and the interaction with circadian phases on thermoregulatory responses to mild heat in female rats. Heat loss and production during 3-h exposure to the environment at an ambient temperature of 28–34 °C were assessed by measuring abdominal temperature (Tabd), tail skin temperature, and oxygen consumption in ovariectomized rats with and without E 2 replacement (OVX + E2 and OVX, respectively) and in control rats in the proestrus (P) and diestrus (D) phases. In the light phase, Tabd remained unchanged in all groups. Tabd increased in the dark phase, but was lower in the OVX + E2 and P groups than in the OVX and D groups. Oxygen consumption decreased at 34 °C, but to a lesser extent in the OVX + E2 group than in the OVX group. These results suggest that E2 activates thermoregulation in mild heat in the dark phase. Keywords: Female hormones, Circadian rhythm, Autonomic thermoregulation, Core temperature, Hot environment Background Several studies have reported that estradiol ( E2) activates thermoregulatory mechanisms during exposure to heat in female rats [1–5]. Baker et al. [1] demonstrated that, in the extreme heat of 38 °C, the increase in body temperature was greater in ovariectomized (OVX) rats than in OVX rats with E2 replacement (OVX + E2). They suggested that reduced evaporative heat loss in OVX rats is involved in the mechanism. However, Dacks and Rance [2] reported that the increase in body temperature was greater in OVX rats than in OVX + E2 even at an ambient temperature (Ta) of 32.5 °C, a temperature at which evaporative heat loss is less involved in thermoregulation [6]. They also reported that dry heat loss from the tail *Correspondence: k‑[email protected] 1 Body Temperature and Fluid Laboratory, Faculty of Human Sciences, Waseda University, Mikajima 2‑579‑15, Tokorozawa, Saitama 359‑1192, Japan Full list of author information is available at the end of the article
was greater in OVX rats than in OVX + E2 rats. Hosono et al. [3] reported that, at Ta of 32–36 °C, heat-escape behavior did not differ between OVX and OVX + E2 rats. These results suggest that E2 activates neither the heat loss response of the tail nor thermoregulatory behavior in mild heat. Thus, it remains unclear how E 2 attenuates the increase in body temperature in mild heat. Previous studies have reported that acute and chronic heat exposure suppress metabolism with attenuation of thyroid function [7, 8] and/or decreased spontaneous activity [9]. The results suggest that the attenuation of metabolism is part of the thermoregulatory response to heat, although it is unclear whether E2 is involved in the underlying mechanism. Endoth
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