Submitted on April 14, 2003
Accepted on August 15, 2003
Regulation mode of evaporative cooling underlying a strategy of the heat-tolerant FOK rat for enduring ambient heat
FUJIYA FURUYAMA1*, MASATAKA MURAKAMI2, ETSURO TANAKA3, HIDEKI HIDA1, DAISUKE MIYAZAWA4, TAKANORI OIWA1, YOSHIAKI ISOBE1, and HITOO NISHINO1
1 Department of Neurophysiology and Brain Science, Nagoya City University Graduate School of Medical Sciences, Nagoya, Aichi, Japan
2 Department of Molecular Physiology, National Institute of Physiological Sciences, Okazaki, Aichi, Japan
3 Departments of Surgery, Physiology, and Cardiology, and Center for Regenerative Medicine, Tokai University School of Medicine, Isehara, Kanagawa, Japan
4 Department of Preventive Nutraceutical Sciences, Graduate School of Pharmaceutical Sciences, Nagoya City University, Nagoya, Aichi, Japan
* To whom correspondence should be addressed. E-mail: heiza-ff{at}med.nagoya-cu.ac.jp.
Compared to other rat strains, the inbred FOK rat is extremely heat tolerant. This increased heat tolerance is due largely to the animal's enhanced saliva spreading abilities. The aims of the present study were to: 1) quantify the heat tolerance capacity of FOK rats and 2) determine the regulatory mode of the enhanced salivary cooling in these animals. Various strains of rats were acutely exposed to heat. In the heat intolerant strains, saliva spreading was insufficient and the core temperature (Tc) rose rapidly. In contrast, FOK rats maintained an elevated Tc plateau (39.5 ± 0.7°C) for 5-6 hours over a wide range of ambient temperatures (Ta) (37.5 - 42.5°C). In the hot environments the FOK rats secreted copious amounts of saliva and spread it over wider than the entire ventral body surface. The FOK rats had a low Tc threshold for salivation and the salivation rate increased linearly in proportion to the Tc deviation from the threshold. No strain difference or temperature effect was observed in the saliva secretion rate from in vitro submandibular glands perfused by sufficient doses of Ach. These results suggest that: 1) The ability of FOK rats to maintain a moderate steady-state hyperthermia (39.5 ± 0.7°C) over a wide Tarange is enabled by a lowered threshold Tc for salivation and functional negative feedback control of saliva secretion, and 2) Strain differences in ability to endure heat stress are mainly attributable to changes in the thermoregulatory control system rather than to altered secretory abilities of the salivary glands.
