Plasma cortisol concentrations were determined from the blood of eight mature female reindeer (Rangifer tarandus tarandus) after an intravenous injection of either saline (control) or 100, 250 or 500 μg of synthetic ACTH. Blood samples were collected at 0, 2, 4, 6, 8, 10, 15, 20, 25, 30, 45, 60, 75, 90, 120, 150, and 180 min after the injections. The aims were to determine the appropriate dose of ACTH for adrenal stimulation tests, to define the dose level of ACTH which elicited a maximal cortisol response and to describe the range of blood cortisol concentrations for reference when evaluating the stress responses of reindeer.
The mean plasma cortisol concentrations (s.e.) at the zero sample times (t0) of the control and the ACTH treatments varied between 93·4 (11·8) and 132·5 (18·1) nmol/l. The total plasma cortisol response (area under curve, AUC, nmol/l × min) increased with increasing dose of ACTH (P < 0·001). The AUC of the control treatment was significantly smaller than of the ACTH treatments (P < 0·001). The highest dose of ACTH (500 μg) gave a significantly bigger AUC than the lowest dose (100 μg) (P = 0·008). The maximal plasma cortisol concentrations (CMAX) were achieved within 60 min of the ACTH injections. The ranges of individual CMAX were 59·0 to 136·8 nmol/l for the control treatment, and 110·0 to 252·0, 152·0 to 247·5 and 135·1 to 257·1 nmol/l for 100, 250 and 500 μg ACTH, respectively. The difference in CMAX was significant between the control treatment and the ACTH treatments (P < 0·001) but not between the different doses of ACTH. The plasma cortisol concentrations at the end of the observation period at t180 were not significantly affected by the ACTH treatment (P > 0·05).
In conclusion, the 100-μg dose of ACTH was sufficient to produce a significant cortisol response compared with the control treatment. Increasing the dose did not increase the maximal response, but tended to elongate the response profile. The blood sampling frequency used in the study was found suitable for detection of the cortisol response in reindeer.