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

Abstract

The rabbit pulmonary artery contains postsynaptic alpha-adrenoceptors which meidate smooth muscle contraction; its noradrenergic nerves contain presynaptic alpha-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 alpha-receptor agonists were determined on superfused strips of the artery in the presence of cocaine, corticosterone and propranolo. 1. According to the concentrations which caused 20% of the maximal contraction (EC20 post), the postsynaptic rank order of potency was: adrenaline greater than noradrenaline greater than oxymetazoline greater than naphazoline greater than phenylephrine greater than tramazoline greater than alpha-methylnoradrenaline greater than methoxamine. The pA2 values of phentolamine againstoxymethazoline, phenylephrine, alpha-methylnoradrenaline and methoxamine were 7.43, 7.48, 7.59 and 7.69, respectively. 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 greater than oxymetazoline greater than tramazoline greater than alpha-methylnoradrenaline greater than noradrenaline greater than naphazoline greater than phenylephrine greater than methoxamine. 10(-5) M phentolamine shifted the presynaptic dose-response curves for moradrenaline and oxymethazoline to the right. 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, alpha-methylnoradrenaline and tramazoline (as well as clonidine). 4. Preferentially presynaptic and preferentially postsynaptic agonists had opposite effects on the basoconstrictor response to nerve stimulation. Methoxamine and phenylephrine either did not change or enhanced, but never reduced, the response. In contrast, oxymetazoline, alpha-methylnoradrenaline and tramazoline at low concentrations selectively inhibited the response to stimulation at low frequency (0.25-2Hz). 5. It is concluded that alpha-adrenoceptor agonists vary widely in their relative pre- and postsynaptic potencies, possibly because of structural differences between pre- and postsynaptic alpha-receptors. Pre- and postsynaptic components contribute to their overll postsynaptic effec in actively transmitting synapses. The preferential activation of presynaptic alpha-receptors results in alpha-adrenergic inhibition of synaptic transmission.

α2‐Adrenoceptor agonists enhance responses to certain other vasoconstrictor agonists in the rat tail artery

“…This result is consistent with the findings of Starke et al (1975), using the rabbit pulmonary artery, and Steinsland & Nelson (1975), using the rabbit ear artery; they showed that clonidine inhibited responses to sympathetic nerve stimulation in much lower concentrations than those required to produce vasoconstriction. Both pulmonary and ear arteries from rabbit lack postjunctional a2-adrenoceptors (Cambridge & Davey, 1980;Hieble & Woodward, 1984), so the only a2-adrenoceptors on which clonidine can act are prejunctional, thus reducing the release of noradrenaline.…”

Sixth Gaddum Memorial Lecture National Institute for Medical Research, Mill Hill, January 1977

“…In support of this hypothesis it has been demonstrated that a-adrenoceptor agonists inhibit transmitter release during nerve stimulation (Langer, Enero, Adler-Graschinsky & Stefano, 1972b;Starke, 1972b;Starke, Montel, Gay & Merker, 1974;Starke, Endo & Taube, 1975a;Langer, Dubocovich & Celuch, 1975a). As shown in Table 1 (Starke et al, 1975a).…”

“…The reduction i,a transmitter release obtained by exposure to a-receptor agonists was equally observed in tissues in which the response of the effector organ is mediated by a-receptors (Kirpekar, Furchgott, Wakade & Prat, 1973;Starke et al, 1974;Starke et al, 1975a;Langer et al, 1975a) or by ,-receptors (Starke, 1972b;Langer et al, 1972b;Rand, McCulloch & Story, 1975 (Langer, 1970;De Potter et al, 1971;Farnebo & Hamberger, 1971a;Starke et al, 1971;Enero et al, 1972; Dubocovich Farah & Langer, 1974;Cubeddu et al, 1974;Langer, 1974b; (Enero & Langer, 1973;Cubeddu & Weiner, 1975). Consequently it appears that when the concentration of released noradrenaline in the synaptic cleft falls below a certain threshold, it fails to trigger the presynaptic negative feed-back mechanism that regulates noradrenaline release.…”

Effect of catecholamine-receptor stimulating agents on blood pressure after local application in the nucleus tractus solitarii of the medulla oblongata

“…injection of clonidine in cats (Svensson et al, 1975) or local administration in the locus coeruleus using micro-iontophoresis (Cedarbaum and Aghajanian, 1975). These results may be explained by a selective activation of presynaptic a-receptors by clonidine (Starke et al, 1975).…”

“…The a-mimetic substances investigated, however, did not decrease blood pressure dose-dependently after their microinjection into the A2-region. Neither the a-mimetics with a relatively preferential postsynaptic effect (methoxamine and phenylephrine) or that with a relatively presynaptic effect (oxymetazoline) (Starke et al, 1975;Berthelsen and Pettinger, 1977) had a marked effect. From these data it is not possible to define the catecholaminergic structures in the As-region responsible for the decrease of blood pressure.…”