Clonidine effects on disposition of xenobiotics in rats: inhibited elimination of flow‐limited but not extraction‐limited agents

1 The α 2 ‐adrenoceptor agonist, clonidine, reduces the hepatobiliary clearance of the anionic dye, sulphobromophthalein (BSP) in rodents. We now compare the effects of clonidine on BSP elimination with its effects on disposition of compounds which are metabolized by hepatic microsomal mixed function oxidases. 2 BSP, 100 mg kg −1 was administered i.v. to rats at 4h after s.c. saline or clonidine, 0.2 mg kg −1 . Thirty min later, plasma BSP levels were 121.4 ± 2.25 μg ml −1 in saline‐treated rats, while in clonidine‐treated rats they were 631.5 ± 141.0 μg ml −1 . Clonidine raised hepatic BSP levels from 256.0 ± 28.9 μg g −1 tissue to 568.5 ± 86.5 μg g −1 . 3 Acute administration of clonidine (0.2 mg kg −1 s.c.) or repeated clonidine dosing (0.2 mg kg −1 , s.c. twice daily for 10 days) did not affect the disposition of intravenously administered [ 14 C]‐antipyrine (15 mg kg −1 ). 4 Activities of the P450 mixed function oxidase enzymes, aniline hydroxylase and aminopyrine N‐demethylase, were identical in liver microsomes from saline‐treated rats and in microsomes from rats given single or multiple s.c. doses of clonidine (0.2 mg kg −1 ). 5 Addition of clonidine or other 2‐substituted imidazoles at concentrations up to 2 μ m did not affect the activities of aniline hydroxylase or of aminopyrine N‐demethylase in suspensions of rat liver microsomes. Other substituted imidazoles, including cimetidine, clotrimazole and metronidazole, at concentrations of 0.2 μ m or higher, inhibited the activities of these microsomal enzymes. 6 Clonidine slowed BSP elimination, which is probably hepatic blood flow‐limited, but not the extraction‐limited elimination of antipyrine, which is metabolized by hepatic microsomal enzymes.