EFFECTS OF SHORT-TERM AND LONG-TERM ADMINISTRATION OF PHENOBARBITAL ON ENDOGENOUS ACTH CONCENTRATION AND RESULTS OF ACTH STIMULATION TESTS IN DOGS

The effects of short-term phenobarbital administration were evaluated in 6 adult mixed-breed dogs that received phenobarbital (5 mg/kg of bo dy weight, PO, q 12 h) for 8 consecutive weeks. Six additional dogs se wed as untreated controls. At 2-week intervals, endogenous adrenocorti cotropic hormone (ACTH) concentration and cortisol concentration befor e and 2 hours after administration of porcine aqueous ACTH (2.2 IU/hg, IM) were measured. By means of one-way ANOVA, we were not able to det ect a significant (P greater than or equal to 0.05) difference in endo genous ACTH concentration and cortisol concentration before and after exogenous ACTH administration within groups over time or between group s at any time. To evaluate effects of long-term phenobarbital administ ration, sera and plasma were collected from 5 epileptic dogs that had received phenobarbital for > 2 years and had serum phenobarbital conce ntrations > 20 mu g/dl. Endogenous ACTH concentration and cortisol con centration, before and after administration of ACTH, were within estab lished reference ranges for all 5 dogs. Together, these results sugges t that phenobarbital administration alone does not affect endogenous A CTH concentration or response to exogenous ACTH administration in dogs , and that these may be valid screening tests for hyperadrenocorticism in most dogs receiving phenobarbital.