Release of endogenous noradrenaline from an isolated elastic artery

1. The overflow of noradrenaline following field stimulation of vasoconstrictor nerves has been studied in isolated preparations of the guineapig thoracic aorta.2. Stimulation with trains of 3000 pulses at 5 and 25 pulses.s(-1) resulted in mean overflows of 0.15 and 0.17 (ng.g(-1)).pulse(-1) respectively.3. These overflows were not affected by pre-treatment of the aortae with the inhibitor of neuronal amine re-uptake desmethylimipramine (10(-5)M) or with the inhibitors of extraneuronal amine uptake metanephrine (5 x 10(-4)M) or desoxycorticosterone (2.5 x 10(-5)M).4. In contrast, inhibition of monoamine oxidase and catechol-O-methyl transferase with pargyline (1 x 10(-4)M) and tropolone (2.5 x 10(-4)M) caused the mean overflows to be increased to 0.27 and 0.31 (ng.g(-1)).pulse(-1) in response to stimulation at 5 and 25 pulses.s(-1) respectively. Inhibition of either enzyme alone had little effect.5. It is concluded that in the guinea-pig aorta nervously released noradrenaline is inactivated primarily by enzymic metabolism, while neural re-uptake is insignificant. This situation is the reverse of that existing in the uterine artery of the same species.6. Phenoxybenzamine (10(-5)M) also caused pronounced increases in overflow at both high and low frequencies of stimulation. This could be due to an effect of phenoxybenzamine on release of noradrenaline or on some tissue uptake mechanism distinct from the normal neuronal and extraneuronal pathways.7. Calculations based on the overflow at 5 pulses.s(-1) following total enzyme inhibition indicated that a single stimulating pulse liberated 2.1 x 10(-4) of the total transmitter store.