Plasma cortisol responses to remote adrenocorticotropic hormone (ACTH) infusion in free-ranging red deer ( Cervus elaphus)

Abstract

A remote infusion and blood collection device (Dracpac) was used on free-ranging red deer stags to deliver i.v. a range of ACTH 1–24 doses and collect blood for cortisol determination. In Experiment 1, conducted in September, saline and 1, 4, and 16 IU ACTH/100 kg bodyweight were infused (n = 5–7 per treatment). In Experiment 2 (November), 16 and 64 IU ACTH/100 kg were infused (n = 5 per treatment); and the response of animals to restraint was also assessed (n = 6). Pre-infusion concentrations of plasma cortisol were low (mean 6.5 ± 1.0 ng/ml), and unaffected by infusion of saline. ACTH significantly elevated plasma cortisol concentrations, with mean peak concentrations occurring 20–40 min postinfusion. Duration of the response was dose dependent, ranging from 80–160 min. With increasing doses of ACTH, maximal peak heights plateaued at approximately 40 and 60 ng/ml in September and November, respectively; whereas the areas under the curves tended to increase. The minimum dose of ACTH that resulted in a maximal peak cortisol response was 4 IU. The maximum peak height of the cortisol response to 16 IU tended to be higher ( P < 0.10) in November compared with September, suggesting that adrenal responsiveness may change over this period. The adrenal response to a 16-IU ACTH challenge in Experiment 2 resulted in similar plasma cortisol concentrations to an acute stress event (restraint). Four IU/100 kg ACTH i.v. is recommended as an appropriate dose for ACTH infusion studies in red deer stags. When used in conjunction with the Dracpac technique, it becomes a stress-free, repeatable procedure for assessing aspects of adrenal cortex physiology in free-ranging deer.