Evaluation of mechanisms controlling the release and inactivation of the adrenergic transmitter in the rabbit portal vein and vas deferens

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Abstract
1 A method is described for the detection and assay of picogramme quantities of noradrenaline. This involves transferring Krebs solution containing noradrenaline to a cascade system where the catecholamine may be bioassayed on superfused preparations of the rabbit aorta and iliac artery. 2 Electrical field stimulation of the rabbit vas deferens and portal vein caused the release, into the bathing medium, of a material which was identified by pharmacological and chemical tests as noradrenaline. 3 Cocaine (0·3–5 μg/ml) caused a marked increase in noradrenaline output after electrical stimulation of the portal vein and vas deferens. This effect appeared to be maximal at a concentration of 2·4 μg/ml; when the cocaine concentration was increased above 10 μg/ml the noradrenaline output was greatly reduced. 4 Phenoxybenzamine (5 μg/ml) caused a 4–8 times greater increase in noradrenaline output than cocaine; however, the increase in output due to phenoxybenzamine was much smaller in tissues pretreated with cocaine. 5 Corticosterone (20 μg/ml) increased noradrenaline output by 30–40% in untreated vas deferentia, but caused a 300% increase in output in tissues pretreated with cocaine. Cocaine also caused a much greater increase in output in tissues pretreated with corticosterone than in untreated tissues. 6 Treatment with pargyline plus tropolone caused a 100–200% increase in noradrenaline output; this effect was not modified by cocaine, but was abolished when the tissues were pretreated with either phenoxybenzamine or corticosterone. 7 When tissues were stimulated for 240 pulses at 1–16 Hz, the output per pulse of noradrenaline increased linearly with the logarithm of the frequency of stimulation. This relationship between frequency and output was seen in both untreated tissues, and in tissues treated with cocaine, phenoxybenzamine, corticosterone or pargyline plus tropolone. 8 It is concluded that cocaine enhances output by blocking the neuronal reuptake of noradrenaline, and corticosterone by blocking the extraneuronal uptake and subsequent metabolism of noradrenaline. Phenoxybenzamine acts by blocking both neuronal and extraneuronal uptake mechanisms. There appears to be a dynamic balance in the distribution of noradrenaline between the two uptake mechanisms after the release of the transmitter from the nerve endings. 9 It is calculated that more than 90% of the noradrenaline released by nerve stimulation (240 pulses at 2–16 Hz) is inactivated by neuronal and extraneuronal uptake mechanisms. 10 It is calculated that the fraction of the total noradrenaline store that is released by one pulse at 2 Hz is 6·6 × 10−5 in the portal vein and 5·6 × 10−5 in the vas deferens; the corresponding values at 16 Hz were 15·9 × 10−5 and 16·2 × 10−5.