A kinetic study of the release of noradrenaline by tyramine

Summary

Dog saphenous vein strips obtained from untreated or reserpine-pretreated animals were incubated with 1,4 μmol/l ³H-(−)noradrenaline for 60 min after inhibition of the noradrenaline-metabolizing enzymes and of extraneuronal uptake. At the end of the incubation period the strips were perifused during 200 min. Some strips were exposed to tyramine from the 100th to the 200th min of perifusion. A compartment analysis of spontaneous or tyramine-induced efflux of ³H-noradrenaline was made. The spontaneous efflux of strips obtained from untreated animals had a long half time (t/2=269 min), and most of the ³H-noradrenaline which accumulated in the strips did not participate in the efflux (“bound fraction”, representing 85% of tissue activity at the 100th min of perifusion). The strips were exposed to five concentrations of tyramine, ranging from 0.49–3,240 μmol/l. At all concentrations, tyramine mobilized only one noradrenaline compartment. The increase of tyramine concentration decreased the “bound fraction”, which became negligible for the highest concentration of tyramine. The half time of ³H-efflux induced by tyramine decreased from 278–106 min when the tyramine concentration was increased from 0.49–40 μmol/l. However, further increases of the concentration of tyramine up to 3240 μmol/l did not result in a significantly lower half time.

Strips obtained from reserpine-pretreated animals were characterized by a low accumulation of ³H-noradrenaline (403 ng/g vs. 1,374 ng/g for strips obtained from unpretreated animals, at the 100th min of perifusion), the efflux had a half time of only 122 min and the “bound fraction” was smaller than in strips obtained from unpretreated animals (“bound fraction” representing 47% of tissue activity at the 100th min of perifusion). Reserpine-pretreated strips were exposed to 40 μmol/l tyramine. Under these experimental conditions there was no significant “bound fraction”. The ³H-efflux induced by tyramine decayed according to a two-compartment model, with half times of 8 and 50 min.

The results support the view that in preparations from unpretreated animals the size of the noradrenaline pool available for release by tyramine is dependent on the concentration of the latter amine. Furthermore, the half time of the efflux evoked by tyramine in strips with intact vesicular stores is dependent on a rate-limiting process, probably of vesicular location.