EPINEPHRINE INCREASES SPINAL-CORD CONCENTRATIONS OF [H-3] CLONIDINE HYDROCHLORIDE IN RABBITS AFTER EPIDURAL INFUSION

Epinephrine is often given with epidurally administered drugs to prolo ng and enhance analgesia, which is partly attributed to alpha-adrenerg ic processes. This investigation evaluates the effect of epinephrine o n the distribution of epidurally administered [H-3]-clonidine hydrochl oride (clonidine HCl) in serum and in the central nervous system. Afte r placing a lumbar epidural catheter via a laminectomy, rabbits were r andomly assigned to receive 26 mu Ci Of clonidine HCl with epinephrine (1:200,000) (n = 5) or without (control; n = 5) for 90 min. During th e administration, which included bolus and slow infusion, blood sample s were collected at 15-min intervals. At the end of the administration , rabbits were perfused with normal saline, leading to exsanguination. Brain and spinal cord tissues were excised for radiometric analysis. In both groups, the concentration of clonidine HCl was greatest in the lumbar cord. Epinephrine further enhanced accumulation of clonidine H Cl into the lumbar cord but did not alter the concentration of clonidi ne HCl in serum, brain, cervical cord, and thoracic cord. We conclude that lumbar administration of epidural clonidine HCl leads to increase d concentrations in tile lumbar cord, which is further enhanced by epi nephrine. The increased spinal cord accumulation of clonidine may be a nother mechanism by which epinephrine improves epidural analgesia.