The effect of heat stress on body development in rats

Rats of both sexes were reared immediately after weaning from the 21st to the 42nd day of life under various conditions of heat, ranging between 23-36°C, and relative humidity of between 20-40% RH. At higher temperatures the rats did not survive. Between 32-34°C, weights of body, liver, kidney and thyroid were depressed. Up to 34°C, the adrenals were hardly affected, but in females they were enlarged at a temperature of 36°C. The tibia test showed the opposite effect, being highly increased in male rats and less so in female rats. This was apparently due to a compensatory anabolic reaction of STH, insulin and testosterone. The female sex organs were significantly stimulated by heat both at unlimited food intake and with paired feeding: vaginal opening was observed up to 5 days earlier, with uterine and ovary weights increased. However, testicle weight and prostate weight and testicle descent, were not significantly changed, whereas seminal vesicle weight increased by 40% obviously due to testosterone release. The above results are explained on the basis of hypothalamus and pituitary hormone interactions mediated by the median eminence which is closely associated with the descending pathways of heat regulation.