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

Strips of canine saphenous vein were loaded with 3H-noradrenaline (1.4 micrometer) and perifused with Krebs solution and either subjected to field stimulation or exposed to tyramine 40 micrometer. 3H, noradrenaline and its metabolites were determined in the perifusion fluid. Stimulation caused an increase predominantly in noradrenaline, followed by DOPEG, whereas tyramine release DOPEG in larger amounts than noradrenaline. Tyramine had more sustained effects than stimulation. Cocaine (1.6 micrometer) 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.