Decreased heat tolerance is associated with hypothalam o-pituitary-adrenocortical axis impairment

When rats are exposed to heat, they adapt themselves to the stressor with a wide inter-individual variability. Such differences in heat tolerance may be related to particularities in the hypothalamo-pituitary-adrenocortical (HPA) axis activation. To further this hypothesis, 80 rats instrumented with a telemetric device for abdominal temperature (T(abd)) measurement were separated into two groups. Sixty-eight rats were exposed during 90 min at an ambient temperature of 40 degrees C, and 12 rats to an ambient temperature of 22 degrees C. Heat-exposed rats were then divided into three groups using the a posteriori k-means clustering method according to their T(abd) level at the end of heat exposure. Heat tolerant rats (Tol, n=30) exhibiting the lowest T(abd) showed a slight dehydration, a moderate triglyceride mobilization, but the highest plasma adrenocorticatropic-hormone (ACTH) and corticosterone levels. Conversely, heat exhausted rats (HE, n=14) presented the highest T(abd), a higher degree of dehydration, a greater metabolic imbalance with the lowest plasma triglyceride level and the highest lactate concentration, as well as a lowest plasma corticosterone and ACTH levels. The fact that the proopiomelanocortin (POMC) mRNA content within the pituitary was low despite of a high c-fos mRNA level is also relevant. Current inflammatory processes in HE rats were underlined by lower inhibitory factor kappa B alpha (I kappa B alpha) mRNA and higher tumor necrosis factor-alpha (TNF-alpha) and interieukin-1 beta (IL-1 beta) mRNA. In conclusion, data show that intolerance to heat exposure is associated to an HPA axis impairment, possibly related to changes occurring in the I kappa B alpha and TNF-alpha mRNA levels. (c) 2007 IBRO. Published by Elsevier Ltd. All rights reserved.