Summary
We studied in vivo responsiveness of venous α1 and α2-adrenoceptors, measuring the diameter changes in superficial veins in response to α-adrenergic agonists and antagonists in healthy human volunteers. The dorsal hand vein technique was used because it permits complete dose-response studies of venous constriction without confounding reflex alterations.
Local infusions of all agonists studied induced dose-dependent contraction of the hand vein; the maximal effects (Emax) were: norepinephrine (88% ± 10%), methox amine (97% ± 5%), phenylephrine (95% ± 6%), clonidine (54% ± 12%), and azepexole (68% ± 26%). Clonidine reduced the norepinephrine-induced venoconstriction by 11% ± 10%. Oral doses of 1 mg prazosin antagonized the venoconstriction induced by norepinephrine, methoxamine, and clonidine, but not by azepexole. Yohimbineantagonism was observed against all agonists studied. Inhibition by yohimbine of clonidine-induced venoconstriction was irreversible over 60–180 min.
Results show that the in vivo effects on veins of α-adrenergic agonists are in good agreement with results from in vitro experiments. Agonists with α1- and α2-adrenoceptor subtype selectivity cause venoconstriction in vivo, but α2-receptor mediated constriction is intrinsically weaker. Clonidine acts as a partial antagonist against norepinephrine, presumably on postsynaptic α2-receptors. At high doses, α2-adrenoceptor subtype selectivity of clonidine and yohimbine appear to be partially lost in vivo.
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Blöchl-Daum, B., Korn, A., Wolzt, M. et al. In vivo studies on alpha-adrenergic receptor subtypes in human veins. Naunyn-Schmiedeberg's Arch Pharmacol 344, 302–307 (1991). https://doi.org/10.1007/BF00183004
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DOI: https://doi.org/10.1007/BF00183004