Summary
This study was undertaken in an attempt to explain why in some of the tissues in which noradrenaline and ATP act as co-transmitters the noradrenergic component predominates, while in others the predominant component is purinergic. Four different tissues were used: the epididymal portion of the rat vas deferens and the rabbit ear artery, tissues where the noradrenergic component predominates, and the prostatic portion of the rat vas deferens and the rabbit jejunal artery, where the purinergic component predominates. The noradrenaline content as well as the electrically-evoked release of noradrenaline were determined in all tissues. To determine the evoked release, the tissues were pretreated with pargyline (1 mmol · 1−1) and then exposed to 3H-noradrenaline, washed out and transmurally stimulated (1 Hz). In addition, the influence of inhibition of neuronal uptake by desipramine (40nmo1·1−1) on pre- and postjunctional effects of adrenaline and α-methylnoradrenaline (and/or noradrenaline) was compared.
The noradrenaline content of the tissues averaged: 17.4, 23.2, 3.1, and 4.8 μg·g−1 for the epididymal and the prostatic portions of the rat vas deferens and for the ear and the jejunal arteries of the rabbit, respectively. The fractional electrically-evoked release of 3H-noradrenaline was 2.02 and 2.04 × 10−5 for the epididymal and the prostatic portions of the rat vas deferens, respectively, and 3.33 and 3.26 × 10−5 for the ear and the jejunal arteries of the rabbit, respectively. Desipramine enhanced much more the postjunctional effect of noradrenaline, adrenaline, and α-methylnoradrenaline in the epididymal than in the prostatic portion of the rat vas deferens. Moreover, desipramine similarly enhanced the effects of noradrenaline and adrenaline in the ear artery, while it did not modify the responses to these amines in the jejunal artery. The influence of inhibition of neuronal uptake on the prejunctional effect of sympathomimetic agonists was studied in the rat vas deferens only. Desipramine caused very marked and similar enhancements of the inhibitory effect of the amines in the epididymal and the prostatic portions (72.5 and 93.5 times, respectively, for adrenaline, and 34.7 and 41.4 times, respectively, for α-methylnoradrenaline).
These results are compatible with the hypothesis that postjunctional adrenoceptors are situated closer to the varicosities in the epididymal than in the prostatic portion of the rat vas deferens and also closer in the rabbit ear than in the rabbit jejunal artery. Therefore, the variation in the relative importance of the noradrenergic and the purinergic components may depend on the distance between the sites from where the transmitters are released and the sites where they act.
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Gonçalves, J., Guimarães, S. Influence of neuronal uptake on pre- and postjunctional effects of α-adrenoceptor agonists in tissues with noradrenaline — ATP cotransmission. Naunyn-Schmiedeberg's Arch Pharmacol 344, 532–537 (1991). https://doi.org/10.1007/BF00170648
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DOI: https://doi.org/10.1007/BF00170648