Naloxone potentiates epinephrine's pressor actions in endotoxemic rats

Naloxone's pressor effects in shock may be mediated through antagonism of endogenous opioid inhibition of sympathoadrenal catecholaminergic systems. Since circulating catecholamine levels are not further elevated by naloxone in endotoxemic animals, it is possible that naloxone acts upon opiate receptors to enhance catecholamine actions at the receptor or postreceptor level. To investigate this hypothesis, we sought to determine whether naloxone treatment would augment the pressor actions of exogenous catecholamines (epinephrine) in normal and endotoxemic rats. Naloxone (3 mg/kg intravenous [i.v.] bolus followed by 3 mg/ml/hr infusion) significantly augmented the pressor response to 10, 20, and 50 micrograms/kg of i.v. epinephrine by 69%, 48% and 14%, respectively, in endotoxin (5 mg/kg, LD20, i.v.) -treated rats but not in normal rats. Likewise, the duration of the pressor response to epinephrine was significantly increased by naloxone. These findings suggest that the coadministration of naloxone and epinephrine may be of benefit in the treatment of shock.