Differences in the metabolic fate of noradrenaline released by electrical stimulation or by tyramine

Summary Strips of canine saphenous vein were loaded with ³H-noradrenaline (1.4 M) and perifused with Krebs solution and either subjected to field stimulation or exposed to tyramine 40 M. ³H, noradrenaline and its metabolites were determined in the perifusion fluid. Stimulation caused an increase predominantly in noradrenaline, followed by DOPEG, whereas tyramine released DOPEG in larger amounts than noradrenaline. Tyramine had more sustained effects than stimulation. Cocaine (1.6 M) drastically reduced DOPEG efflux due to stimulation, but had no effects on the pattern of release by tyramine. It is concluded that tyramine releases noradrenaline which is deaminated before it reaches the synaptic gap, whereas after stimulation deamination of the transmitter occurs after re-uptake.Results presented in part to the 8th Annual Meeting of the Portuguese Pharmacological Society (Coimbra, December 1977) and to the 3rd International Symposium on Vascular Neuroeffector Mechanisms (Louvain-Antwerpen, July 24–26, 1978). This work was supported by a grant from Instituto Nacional de Investigação Científica (FmPl)