Evoked release of endogenous norepinephrine in the canine saphenous vein. Inhibition by acetylcholine.

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Abstract
Most of the data concerning adrenergic neurotransmission in the blood vessel wall was obtained from isolated blood vessels previously incubated with radiolabeled norepinephrine [NE]. The tissue content of NE and dopamine was determined in the dog saphenous vein using a radioenzymatic assay; tissue content of dopamine was 30 times less than tissue content of NE and no epinephrine was detected. During superfusion of isolated canine saphenous vein preparations, superfusate samples were collected for subsequent radioenzymatic analysis of NE, epinephrine and dopamine. The basal efflux of endogenous NE declined slightly with time. Nerve stimulation caused frequency-dependent increases in tension, paralleled by increases in efflux of endogenous NE. The changes in tension were correlated significantly with the changes in NE overflow. Acetylcholine at 5 .times. 10-7 M had no effect on basal tension or basal NE overflow. During nerve stimulation at 2 Hz and at 5 Hz, it significantly depressed the contractile response and the evoked overflow of endogenous NE. Neither dopamine nor epinephrine was detected during these superfusion studies. The present experiments introduce the means for measuring endogenous NE overflow in the canine saphenous vein, validate earlier work on this blood vessel and provide direct evidence for the inhibitory effect of acetylcholine on adrenergic neurotransmission in the blood vessel wall.