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Relative pre- and postsynaptic potencies of α-adrenoceptor agonists in the rabbit pulmonary artery

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Summary

The rabbit pulmonary artery contains postsynaptic α-adrenoceptors which mediate smooth muscle contraction; its noradrenergic nerves contain presynaptic α-adrenoceptors which mediate inhibition of the release of the transmitter evoked by nerve impulses. Dose-response curves for the pre- and postsynaptic effects of eight α-receptor agonists were determined on superfused strips of the artery in the presence of cocaine, corticosterone and propranolol.

  1. 1.

    According to the concentrations which caused 20% of the maximal contraction (EC20 post), the postsynaptic rank order of potency was: adrenaline > noradrenaline > oxymetazoline > naphazoline > phenylephrine > tramazoline >α-methylnoradrenaline > methoxamine. The pA2 values of phentolamine against oxymetazoline, phenylephrine, α-methylnoradrenaline and methoxamine were 7.43, 7.48, 7.59 and 7.69, respectively.

  2. 2.

    For the investigation of presynaptic effects, the arteries were preincubated with 3H-noradrenaline. All agonists inhibited the overflow of tritium evoked by transmural sympathetic nerve stimulation. According to the concentrations which reduced the stimulation-induced overflow by 20% (EC20 pre), the rank order of potency was: adrenaline > oxymetazoline > tramazoline > α-methylnoradrenaline > noradrenaline > naphazoline > phenylephrine > methoxamine. 10−5 M phentolamine shifted the presynaptic dose-response curves for noradrenaline and oxymetazoline to the right.

  3. 3.

    The ratio EC20 pre/EC20 post was calculated for each agonist as an index of its relative post- and presynaptic potency. According to the ratios, the agonists were arbitrarily classified into three groups. Group 1 (ratio about 30; preferentially postsynaptic agonists) comprised methoxamine and phenylephrine; group 2 (ratio near 1; similar pre- and postsynaptic potencies) comprised noradrenaline, adrenaline and naphazoline; group 3 (ratio below 0.2; preferentially presynaptic agonists) comprised oxymetazoline, α-methylnoradrenaline and tramazoline (as well as clonidine).

  4. 4.

    Preferentially presynaptic and preferentially postsynaptic agonists had opposite effects on the vasoconstrictor response to nerve stimulation. Methoxamine and phenylephrine either did not change or enhanced, but never reduced, the response. In contrast, oxymetazoline, α-methylnoradrenaline and tramazoline at low concentrations selectively inhibited the response to stimulation at low frequency (0.25–2 Hz).

  5. 5.

    It is concluded that α-adrenoceptor agonists vary widely in their relative pre- and postsynaptic potencies, possibly because of structural differences between pre- and postsynaptic α-receptors. Pre- and postsynaptic components contribute to their overall postsynaptic effect in actively transmitting synapses. The preferential activation of presynaptic α-receptors results in α-adrenergic inhibition of synaptic transmission.

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Authors and Affiliations

  1. Pharmakologisches Institut, Klinikum Essen, Hufelandstraße 55, D-4300, Essen, Germany

    K. Starke, T. Endo & H. D. Taube

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  1. K. Starke
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  2. T. Endo
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  3. H. D. Taube
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Starke, K., Endo, T. & Taube, H.D. Relative pre- and postsynaptic potencies of α-adrenoceptor agonists in the rabbit pulmonary artery. Naunyn-Schmiedeberg's Arch. Pharmacol. 291, 55–78 (1975). https://doi.org/10.1007/BF00510821

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  • DOI: https://doi.org/10.1007/BF00510821

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