THE EFFECTS OF EPIDURAL MORPHINE ON CARDIAC AND RENAL SYMPATHETIC-NERVE ACTIVITY IN ALPHA-CHLORALOSE-ANESTHETIZED CATS

Background Epidural morphine yields postoperative pain relief and hemo dynamic stability. However, the effects of epidural morphine on sympat hetic tone are unclear. This study was designed to elucidate the effec ts of epidural morphine on cardiac (CSNA) and renal (RSNA) sympathetic nerve activity by direct measurement in anesthetized cats. Methods: T hirty mongrel cats anesthetized with alpha-chloralose were randomly as signed to one of the following five groups: control(0.2 ml/kg thoracic epidural normal saline; n = 5); thoracic epidural morphine (n = 9); l umbar epidural morphine (n = 6); vagotomized, sinoaortic denervated, t horacic epidural morphine (n = 5); or intravenous morphine (n = 5), Me an arterial pressure (MAP), heart rate (HR), CSNA, and RSNA were measu red 0, 15, 30, 60, 90, and 120 min after saline or morphine (200 mu g/ hg) administration and 15 min after reversal with 200 mu g naloxone gi ven intravenously. Results: In the control group, no changes in measur ed variables were found after either thoracic epidural saline or intra venous naloxone. Thoracic and lumbar epidural morphine both significan tly reduced MAP, HR, CSNA, and RSNA 30 through 120 min after morphine administration (P < 0.05). These changes were reversed by intravenous naloxone. Changes after thoracic epidural morphine administration in v agotomized, baroreceptor-denervated cats were similar to those in inta ct cats. Intravenous morphine produced no significant changes except f or a decrease in MAP, which was reversed by intravenous naloxone. Conc lusion: In contrast to intravenous morphine, thoracic and lumbar epidu ral morphine both inhibited cardiac and renal sympathetic nerve activi ty and consequently reduced MAP and HR in alpha-chloralose anesthetize d cats.