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Subtype determination of presynaptic α2-autoreceptors in the rabbit pulmonary artery and human saphenous vein

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Abstract

The pharmacological properties of the presynaptic α2-autoreceptors mediating inhibition of noradrenaline release were investigated in human saphenous vein and rabbit pulmonary artery. Segments of these blood vessels were incubated with [3H]noradrenaline and subsequently superfused with physiological salt solution containing uptake1 and uptake2 blockers. The potencies of α2-adrenoceptor antagonists in facilitating (pEC40) the electrically (2 Hz) evoked tritium overflow were determined. The order of potency and potency ratios of α2-adrenoceptor antagonists obtained in our experiments were compared with the corresponding order of affinity and affinity ratios from radioligand binding studies in tissues and cells expressing only one of the α2-adrenoceptor subtypes.

In the rabbit pulmonary artery, oxymetazoline was a highly potent agonist at presynaptic α2-adrenoceptors, as reflected by its ability to inhibit at low concentrations the electrically evoked tritium overflow. However, in the human saphenous vein oxymetazoline behaved as a partial agonist, which, in interaction experiments with the α2-adrenoceptor agonist B-HT 920 (2-amino-6-ally,-5,6,7,8-tetrahydro-4H-thiazolo-[4,5-d]-azepine), exhibited high potency in antagonizing the inhibitory effect of the latter drug on tritium overflow. Prazosin given alone at concentrations up to 1 μmol/1 did not affect tritium overflow. The data obtained with oxymetazoline and prazosin make it very improbable that the α2-autoreceptors on the sympathetic nerves in both tissues are of the α2B- or α2c-subtype. In both blood vessels, rauwolscine given alone was highly potent in facilitating the electrically evoked overflow. In agreement with this, rauwolscine exhibited high potency in antagonizing the inhibitory effect of oxymetazoline on tritium overflow in the rabbit pulmonary artery and of B-HT 920 in the human saphenous vein. The ratio phentolamine/rauwolscine calculated from their potencies in increasing the electrically evoked tritium overflow was also used to discriminate between the various α2-adrenoceptor subtypes. Comparison of this potency ratio with the corresponding affinity ratios for α2-adrenergic binding sites on HT 29 cells, human platelets, bovine pineal gland, rat submaxillary gland, and cell lines transfected with the human α2 genes indicates that in the rabbit pulmonary artery and human saphenous vein the pharmacological characteristics of the autoreceptors conform best to those of α2A-adrenoceptors. Finally, in both blood vessels the potencies of the antagonists BDF 6143 (4-chloro-2-(2-imidazolin-2-ylamino)-isoindoline), rauwolscine, corynanthine, phentolamine, idazoxan, SKF 104078 (6-chloro-9-[(3-methyl-2-butenyl) oxyl]-3-methyl-lH-2,3,4,5-tetrahydro-3-benzazepine), and/or tolazoline in facilitating evoked noradrenaline release was determined. The potencies of these drugs which can discriminate between α2A- and α2D-adrenoceptors (but not between these and α2B/2c-adrenoceptors) were correlated significantly with their affinities for α2A, but not α2D, sites in radioligand binding studies.

In conclusion, the present results suggest that the sympathetic nerves of the human saphenous vein and rabbit pulmonary artery are endowed with α2-autoreceptors of the α2A subtype.

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  1. Institut für Pharmakologie und Toxikologie, der Rheinischen Friedrich-Wilhelms-Universität Bonn, Reuterstra\e 2b, D-53113, Bonn, Germany

    G. J. Molderings & M. Göthert

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  1. G. J. Molderings
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  2. M. Göthert
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Molderings, G.J., Göthert, M. Subtype determination of presynaptic α2-autoreceptors in the rabbit pulmonary artery and human saphenous vein. Naunyn-Schmiedeberg's Arch Pharmacol 352, 483–490 (1995). https://doi.org/10.1007/BF00169381

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  • DOI: https://doi.org/10.1007/BF00169381

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