MODIFIED RESPONSES OF CIRCULATING CORTISOL, THYROID-HORMONES, AND GLUCOSE TO EXOGENOUS CORTICOTROPIN AND THYROTROPIN-RELEASING-HORMONE IN FOOD-DEPRIVED SHEEP

In a previous experiment, food deprivation was found to suppress the increase of plasma cortisol and thyroid hormones in stressed animals. Because both the hypothalamo-adrenocortical and the thyroid axes are stimulated during stress, we investigated in this study whether a similar pattern of changes occurs in food-deprived sheep following corticotropin (ACTH) or thyrotropin-releasing hormone (TRH) administration. Each hormone was given as a bolus injection on the fifth day of food deprivation. Blood was sampled by venipuncture five times: 0.5 h before and 1, 3, 5, and 9 h after injection of the hormone. The peak of plasma cortisol in food-deprived sheep following ACTH administration exceeded fourfold the corresponding peak in fed animals. This suggests that food deprivation may enhance the sensitivity of the adrenocortical gland to ACTH and/or reduce binding sites for cortisol in target tissues. In fed animals, TRH was without effect on plasma cortisol level, whereas in food-deprived sheep cortisol transiently increased 2.5-fold, suggesting greater permeability of the blood-brain barrier for TRH. In food-deprived animals, plasma T3 was decreased to 22.6% of basal level, and elevated plasma cortisol after ACTH injection was not able to decrease it further. On the other hand, in fed sheep increased plasma cortisol did decrease plasma T3 as much as 4.2-fold. Circulating T4 was not affected by ACTH treatment. The DELTA increase of plasma T3 and T4 following TRH administration was comparable in fed and fasted animals. In contrast, the maximal plasma levels of both hormones were always lower in fasted animals by about 25% due to lowered basal level, suggesting that food deprivation preserves the sensitivity of the hypothalamo-thyroid axis. Administration of ACTH to fed animals increased plasma glucose by 62% above basal level. Food deprivation lowered plasma glucose. A high level of endogenous cortisol following ACTH injection raised plasma glucose only by 19.5%. The results obtained suggest the lesser effectiveness of cortisol in food-deprived animals.