The effect of fasting on the diurnal rhythm of rat acth and corticosterone secretion

Abstract

A tight association of feeding, metabolism, and function of the hypothalamo-pituitary-adrenocortical (HPA) axis is well known. Studies of HPA axis responsivity to various diets and food restriction show that not only fasting but also the cyclic circadian input importantly determine these responses. We therefore studied the response of the rat HPA system to different fasting periods (12, 18, 24, 36, and 48 h), all imposed at 6 PM, by measuring blood ACTH and corticosterone concentration. The corresponding metabolic parameters, e.g., blood glucose and free fatty acid (FFA) levels, were also assessed. The obtained results show that fasting altered the normal diurnal rhythm in levels of blood ACTH and FFA, but not corticosterone and glucose. All applied fasting periods increased HPA system activity in comparison with ad libitum fed rats. However, the most pronounced elevation in the blood ACTH concentration was observed after 18 h and 36 h of fasting, while the CORT increment was the highest 48 h after food restriction began. As expected, the blood glucose concentration decreased under the influence of all fasting periods, being the lowest after 12 h of overnight fasting. In contrast to that, fasted rats exhibited a normal level of serum FFA early in the morning, an elevated level thereafter, and the peak level 48 h after the onset of starvation.