(+)NALOXONE POTENTIATES THE INOTROPIC EFFECT OF EPINEPHRINE IN THE ISOLATED DOG HEART

Naloxone potentiates the inotropic effect of circulating catecholamine s in the isolated canine heart. The stereospecificity of this response was evaluated with the aid of the less active (+)enantiomer of naloxo ne. The more common (-)isomer of naloxone increased the contractile re sponse to epinephrine only at the higher dose tested (4 mg). This eff ect of naloxone was not observed at a tentold lower dose (0.4 mg), ind icating a very narrow dose-response range. (+)Naloxone was effective a t the lower dose and was, therefore, equal to or better than (-)naloxo ne in potentiating the inotropic eff ect of epinephrine. When introduc ed afterward, (-)naloxone did not add to the effect of (+)naloxone. Th ese data suggest that naloxone modifies cellular responsiveness to cat echolamines through a nontraditional opiate receptor, through a nonopi ate receptor, or through a nonreceptor mechanism. (C) 1993 Wiley-Liss, Inc.