NALOXONE POTENTIATES INOTROPIC BUT NOT CHRONOTROPIC EFFECTS OF ISOPROTERENOL INVITRO

Tachycardia often limits the usefulness of conventional beta-adrenergic inotropic therapy; therefore, a compound possessing positive inotropic but not chronotropic properties would be useful. These experiments determined the inotropic and chronotropic properties of naloxone in spontaneously contracting guinea pig atria and electrically paced papillary muscles in the presence and absence of isoproterenol. In atria naloxone exhibited no significant intrinsic inotropic properties. In the presence of isoproterenol, naloxone increased contractility in spontaneously contracting atria and electrically paced ventricular tissues by 11% and 79% respectively, without any significant change in contraction rate. Since naloxone potentiated the inotropic but not chronotropic effects of isoproterenol, it may be a useful adjunct to conventional beta-adrenergic therapy where an inotropic but not chronotropic effect is desired.