Circadian Rhythm of Cortisol Secretion in Dogs of Different Daily Activities

Abstract

The concentrations of immunoreactive cortisol were monitored over 24 hours in the serum of a total of 23 dogs. The dogs were divided into three groups: (a) control group with no exercise, (b) experimental group, (c) working dogs. Blood was collected from each dog at the intervals of 90-180 min. The cortisol concentrations in different time periods were mutually compared using the Kruskal-Wallis test for the analysis of variance. Circadian pattern was found in the study. Cortisol concentrations found between 10.00 and 13.00 (46.5 17.0 nmol/l) were significantly higher (P < 0.05) than those found between 19.00 and 22.00 (38.9 16.0 nmol/l). The results differed within individual groups. Diurnal rhythms were confirmed in control group but not found in experimental group and in working dogs. In experimental group, other significant (P < 0.05) differences were also found between various time intervals compared to the total pattern but no diurnal rhythm was established. In working dogs no significant differences (P < 0.05) were found between cortisol concentrations in different time intervals. The results suggest that cortisol concentrations in the dog are subject to considerable changes during the day. Depending on the characteristics of day and night activities a diurnal rhythm can be found in some dogs.