PRESYNAPTIC EFFECTS OF EPINEPHRINE ON NOREPINEPHRINE RELEASE FROM CARDIAC SYMPATHETIC-NERVES IN DOGS

The possible functional role of tissue epinephrine in the modulation of norepinephrine release from cardiac sympathetic nerve endings in anesthetized dog was investigated. Observations were carried out under control conditions and after a short- (10 min) and long-term (180 min) epinephrine infusion (92 ng.kg-1.min-1). An increase in the stimulation-induced release of norepinephrine after intravenous administration of a selective beta2-agonist (fenoterol, 0.5 mug/kg) indicated the presence of the beta2-facilitatory mechanism. Furthermore, the facilitatory effect of fenoterol was inhibited by intravenous administration of a selective beta2-antagonist (ICI 118551, 1 mg/kg). Short-term epinephrine infusion did not facilitate the stimulation-induced release of norepinephrine, when tissue epinephrine content in the left ventricle was increased 1.5-fold, without, as well as with, alpha2-blockade (yohimbine, 0.3 mg/kg). However, stimulation-induced release of norepinephrine from the myocardium was significantly potentiated in animals in which tissue epinephrine in the left ventricle was greatly increased (5.6-fold) by a prolonged infusion of epinephrine (180 min). It is concluded that a facilitatory mechanism mediated by presynaptic beta2-adrenoceptors is present in cardiac sympathetic nerve endings of the dog. Some of our observations support the hypothesis that this mechanism may be influenced by locally released epinephrine and, thus, by tissue epinephrine content.