The effects of aging on the circadian rhythm of serum cortisol in the dog.

The purpose of this study was to determine if aging affects the circadian rhythm of serum cortisol. Female beagle dogs belonging to three age groups were used: adult (3.3 +/- 0.6 (SD) years), old (12.1 +/- 0.3 years), and puppies (8.4 +/- 0.2 weeks). Blood samples were collected by cephalic or jugular venipuncture at 3-h intervals during three 24-h periods and analyzed for total serum cortisol concentrations by radioimmunoassay. The circadian rhythm was present in the serum cortisol of adult animals, but no significant changes during a 24-h period could be detected in the old animals. No circadian rhythm in serum cortisol was present in the puppies. It is concluded that the circadian rhythm in plasma cortisol is disrupted in old animals and is not yet developed in puppies.