| Summary: | Purpose of study: Strong individual and life-history variation in serum levels of glucocorticoids has been documented in many wildlife species. Less is known about the underlying variation in hypothalamic-pituitary-adrenal axis (HP A) responsiveness and its impacts on metabolism. Procedure: To characterize changes due to a simulated acute stress, 18 free-ranging adult male northern elephant seals were challenged with an intramuscular injection of slow release adrenocorticotropic hormone (ACTH) over 3 sample periods: early in the breeding season, after 70+ days of the breeding fast, and during peak molt. Subjects were blood sampled every 30 minutes for 2 hrs post-injection. Breeding animals were recaptured and sampled at 48 hrs. This was not possible in molting subjects due to animal density and mobility. Findings: In response to the ACTH injection, cortisol increased 4-6 fold in all groups, and remained elevated at 48 hrs in early breeding subjects. ACTH was also a strong secretagogue for aldosterone, causing a 3-8 fold increase in concentration. Cortisol and aldosterone responses did not vary between groups but were significantly correlated within individuals. Endogenous ACTH was not suppressed by the challenge in any study group. The ACTH challenge resulted in elevations in plasma glucose during late breeding and molting. The ACTH challenge suppressed testosterone and thyroid hormone (T3) at 48 hrs in early breeding males. The ACTH challenge increased plasma non-esterified fatty acids only during molting. Conclusion: These data suggest that sensitivity of the HPA axis is maintained but the metabolic impacts of cortisol and feedback inhibition of the axis varies with life history stage. Strong impacts of the challenge on testosterone, thyroid hormone and protein sparing suggest the importance of maintaining low cortisol levels during the breeding fast. These data suggest that metabolic adaptations to extended fasting in northern elephant seals include significant alterations in tissue responses to regulatory ...
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