Endocrine responses to repeated adrenocorticotropic hormone administration in free-ranging elephant

Understanding the physiological response of marine mammals to anthropogenic stressors can inform marine ecosystem conservation strategies. Stress stimulates release of glucocorticoid (GC) hormones, which increase energy substrate availability while suppressing energy-intensive processes. Exposure to repeated stressors can potentially affect an animal’s ability to respond to and recover from subsequent challenges. To assess the endocrine response of a marine mammal to repeated stressors, we administered adrenocorticotropic hormone (ACTH) to free-ranging juvenile northern elephant seals (Mirounga angustirostris; n=7) once daily for four days. ACTH administration induced significant, but transient (\u3c24 h) elevation in circulating cortisol levels (p \u3c 0.0001). These increases did not vary in magnitude between the first ACTH challenge on day 1 and the last challenge on day 4. In contrast, aldosterone levels remained elevated above baseline for at least 24 hours after each ACTH injection (p \u3c 0.001), and responses were greater on day 4 than day 1 (p \u3c 0.01). Total triiodothyronine (tT3) levels were decreased on day 4 relative to day 1 (p \u3c 0.01), while reverse triiodothyronine (rT3) concentrations increased relative to baseline on days 1 and 4 (p \u3c 0.001) in response to ACTH, indicating a suppression of thyroid hormone secretion. There was no effect of ACTH on the sex steroid dehydroepiandrosterone (DHEA). These results suggest that elephant seals are able to mount adrenal responses to multiple ACTH challenges. However, repeated stress results in facilitation of aldosterone secretion and suppression of tT3, which may impact osmoregulation and metabolism. We propose that aldosterone and tT3 are informative additional indicators of repeated stress in marine mammals