Effect of alternate-day prednisolone administration on hypophyseal-adrenocortical activity in dogs

Objective-To evaluate effect of alternate-day oral administration of predni solone on endogenous plasma ACTH concentration and adrenocortical response to exogenous ACTH in dogs. Animals-12 Beagles. Procedure-Dogs were allotted to 2 groups (group 1, 8 dogs treated with 1 mg of prednisolone/kg of body weight; group 2, 4 dogs given excipient only). During a 30-day period, blood samples were collected for determination of p lasma ACTH and cortisol concentrations before, during, and after treatment with prednisolone. From day 7 to 23, prednisolone or excipient was given on alternate days. Sample collection (48-hour period with 6-hour intervals) w as performed on days 1 ,7, 15, 21, and 28; on other days, sample collection was performed at 24-hour intervals. Pre- and post-ACTH plasma cortisol con centrations were determined on days 3, 9, 17, 23, and 30. Results-A significant difference was detected between treatment and time fo r group 1. Plasma ACTH concentrations significantly decreased for 18 to 24 hours after prednisolone treatment in group-1 dogs. At 24 to 48 hours, ACTH concentrations were numerically higher but not significantly different in group-1 dogs. Post-ACTH plasma cortisol concentration significantly decreas ed after 1 dose of prednisolone and became more profound during the treatme nt period. However, post-ACTH cortisol concentration returned to the refere nce range 1 week after prednisolone administration was discontinued. Conclusions and Clinical Relevance-Single oral administration of 1 mg of pr ednisolone/kg significantly suppressed plasma ACTH concentration in dogs fo r 18 to 24 hours after treatment. Alternate-day treatment did not prevent s uppression, as documented by the response to ACTH.