Genetic development of an inbred rat strain with increased resistance adaptation to a hot environment

The purpose of this study was to demonstrate resistance adaptation based on genotypic adaptation and to develop an inbred rat strain with genotypic resistance adaptation to a hot environment. Survival time (ST) at an ambient temperature (Ta) of 42.5 degrees C was determined without appreciable thermal damage. Rats with the longest ST were sibmated over 30 generations and designated FOK. The ST, evaporative water loss, and body water economy increased rapidly during the first 10-15 generations, followed by a more gradual increase. The FOK rat resisted a Ta of 42.5 degrees C for > 5 h; this ability was hereditary. Body size progressively decreased during the early generations. The ST was longer (P < 0.01) and evaporative water loss was significantly higher (P < 0.01) in the FOK rat compared with three other strains. Tail lengths in the FOK rat were slightly shorter than those of three controls (P < 0.01). The heat loss system in the FOK rat may depend on the ability to mobilize and evaporate body fluids efficiently. FOK rats can be used for phenotypic comparisons with other strains, as well as in molecular genetic studies on thermoregulation, osmoregulation, and resistance adaptation to heat using recombinant inbred and recombinant congenic lines.