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Food Restriction Enhances Endogenous and Corticotropin-Induced Plasma Elevations of Free but Not Total Corticosterone Throughout Life in Rats

^a Department of Physiology, The University of Texas Health Science Center, San Antonio
^b Aging Research and Education Center, The University of Texas Health Science Center, San Antonio

Eun-Soo Han, Department of Physiology, The University of Texas Health Science Center, San Antonio, TX 78229-3900 E-mail: han{at}uthscsa.edu.

Decison Editor: John Faulkner, PhD

Chronic food restriction (FR), which retards many aging processes, enhances the endogenous diurnal peak of plasma total corticosterone (B) in young rats. Although the FR-dependent enhancement of total B disappears in aged rats, increased levels of the bioavailable fraction, free B, appear to be maintained. In young rats, we previously found that the FR-induced increase in the diurnal peak of total B is associated with increased adrenal response to corticotropin, also know as adrenocorticotropic hormone (ACTH). Here we show that the FR-enhanced adrenal response of total B to ACTH disappears with age but that the enhanced response of free B is maintained. We measured the endogenous diurnal peak and the response to ACTH of total and free B in 10-, 16-, and 22-month-old ad-libitum fed and FR male Fischer 344 rats in the afternoon, when plasma B peaks. At 10 and 16 months, FR rats showed enhanced total plasma B responses to ACTH relative to ad-libitum fed rats, but not at 22 months. By contrast, the response of free B to ACTH was enhanced by FR at all ages. The effect of FR on patterns of endogenous total and free diurnal B in these three age groups paralleled the ACTH-response data. The enhanced adrenocortical response of FR rats to ACTH does not reflect an increased expression of ACTH-receptor (ACTH-R) mRNA, because ACTH-R mRNA/µg adrenal RNA and ACTH-R mRNA/mg adrenal weight did not differ between ad-libitum fed and FR rats at any age.

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