Endocrine and immunological responses to adrenocorticotrophic hormone (ACTH) administration in juvenile harbor seals (Phoca vitulina) during winter and summer

There is increasing interest in measuring endocrine and immune parameters in free-ranging seals and sea lions, but there is a lack of understanding in how an acute stress response, often associated with capture and handling, influences these parameters of interest. The main objective of this study was to assess the impact of a simulated stressor on both endocrine and immune parameters. During two seasons, exogenous adrenocorticotrophic hormone (ACTH) was administered to seven female juvenile harbor seals and the response of several hormones (cortisol, aldosterone, total and free thyroxine and total triiodothyronine) and immunological parameters (total and differential leukocyte counts and peripheral blood mononuclear cells (PBMC) proliferation) were assessed. Cortisol peaked at 165 min (winter 203.1 ± 84.7 ng/ml; summer 205.3 ± 65.7 ng/ml) and remained significantly elevated 240 min after ACTH infusion in both seasons. Aldosterone peaked at 90 min (winter 359.3 ± 249.3 pg/ml; summer 294.1 ± 83.7 pg/ml) and remained elevated 240 min after administration of ACTH in both seasons. An increase in circulating total white blood cells was driven primarily by the increase in neutrophils which occurred simultaneously with a decrease in lymphocytes leading to an overall increase in neutrophil to lymphocyte ratio. These findings demonstrate that a simulated stress response in juvenile harbor seals results in a predictable increase in both cortisol and aldosterone concentrations, and were associated with altered immunological parameters.