Anti-Sympathetic Action of Sympathomimetic Amines

Effect of sympathomimetic amines on the blocking action of guanethidine, bretylium and xylocholine

Experiments were carried out in which the adrenergic neurone blocking activity of xylocholine, bretylium and guanethidine was studied by the use of the inhibitory responses of the isolated rabbit ileum to lumbar sympathetic nerve stimulation, and the contractions of the nictitating membrane of the anaesthetized cat in response to stimulation of the cervical sympathetic nerves. In both these preparations, after blockade of the effects of sympathetic nerve stimulation had been produced with xylocholine, bretylium or guanethicdine, the sympathomimetic amines, dexamphetamine, mephentermine, hydroxyamphetamine, ephedrine and phenethylamine, reversed the blockade; if these amines were given first, then the adrenergic neurone blocking agents were ineffective. Tyramine and dopamine were effective on the isolated rabbit ileum but not on the cat's nictitating membrane. Effective antagonism of the adrenergic neurone blocking drugs was also shown by some substances which inhibit mono-amine oxidase but only those which in addition possess sympathomimetic effects. Thus phenelzine, pheniprazine and tranylcypromine were effective whereas iproniazid and nialamide were not. Since xylocholine, bretylium and guanethidine were all antagonized by the same agents, it seems likely that they all produce sympathetic blockade by a similar mechanism. The possibility is discussed that the sympathomimetic amines which antagonize the adrenergic neurone blocking drugs are competing with these substances for the same receptor sites.

Evidence for a competitive antagonism of guanethidine by dexamphetamine

After guanethidine had blocked the response of the cat nictitating membrane to sympathetic nerve stimulation, dexamphetamine restored the responses to all frequencies of stimulation. Dexamphetamine antagonized the sympathetic nerve block by guanethidine in the isolated sympathetically innervated rabbit ileum; the evidence suggests that the antagonism was competitive. Dexamphetamine antagonized the sympathetic nerve block by guanethidine in the isolated hypogastric nerve-vas deferens preparation of the guinea-pig. Doses of dexamphetamine, larger than those required to antagonize the blocking action of guanethidine, abolished the responses of the nictitating membrane, ileum and vas deferens to nerve stimulation. Dexamphetamine did not influence the depletion of noradrenaline by guanethidine in the heart and spleen of rabbits. The hypothesis is advanced that both dexamphetamine and guanethidine act on the store of noradrenaline at sympathetic nerve endings.

TACHYPHYLAXIS TO SOME SYMPATHOMIMETIC AMINES IN RELATION TO MONOAMINE OXIDASE

Tachyphylaxis to the effects of indirectly acting sympathomimetic amines has been studied on the blood pressure of the cat, rabbit and rat, on the cat spleen and nictitating membrane and on the rabbit heart. The pressor responses to tyramine and to phenethylamine declined slowly with repeated injection; the extent of tachyphylaxis induced by these amines depended on the dosage and on the frequency of injection. The pressor responses to alpha-methyltyramine and to dexamphetamine (alpha-methylphenethylamine) declined rapidly with successive injections. The tachyphylaxis induced by one indirectly acting sympathomimetic amine is crossed to others, but not to directly acting amines, such as noradrenaline. In animals treated with nialamide, a drug which inhibits monoamine oxidase, the tachyphylaxis induced by tyramine and by phenethylamine was similar to that produced by their alpha-methyl derivatives in normal animals. Similar results were obtained when the responses to indirectly acting sympathomimetic amines were studied on the cat spleen in situ and on the rabbit heart in vitro. Indirectly acting sympathomimetic amines impaired the responses of the cat nictitating membrane to sympathetic nerve stimulation; this effect was most evident with alpha-methylated amines.

The effect of beta-adrenoreceptor antagonists on the inhibition of pendular movements of rabbit ileum produced by periarterial sympathetic nerve stimulation and some sympathomimetic amines

The effects of propranolol and of the selective beta 1- and beta 2-adrenoreceptor blocking drugs atenolol and ICI 118,551 were determined on the inhibitory responses of isolated segments of rabbit ileum to noradrenaline, isoprenaline and salbutamol and to periarterial sympathetic nerve stimulation. Responses to isoprenaline (0.04-10.24 microM) and salbutamol (1.4-89.6 microM) were blocked by propranolol in concentrations up to 5.0 and 12.8 microM, respectively. Responses to sympathetic nerve stimulation were reduced but responses to noradrenaline (0.03-1.92 microM) were unaffected by propranolol in concentrations up to 10.0 and 5.0 microM, respectively. Atenolol in concentrations up to 30.0 microM blocked responses to isoprenaline (0.04-2.56 microM) but did not affect responses to noradrenaline, salbutamol or sympathetic nerve stimulation in concentrations up to 3.0, 3.0 and 1.0 microM, respectively. However, when responses to noradrenaline and sympathetic nerve stimulation were reduced by phentolamine (1.0 microM), atenolol then produced further reductions. Responses to isoprenaline (0.04-2.56 microM) and salbutamol (1.4-89.6 microM) were blocked by ICI 118,551 in concentrations up to 0.5 microM. Responses to sympathetic nerve stimulation were reduced but responses to noradrenaline were unaffected by ICI 118,551 in concentrations up to 0.01 and 0.3 microM, respectively. Salbutamol (0.1 microM) increased the inhibitory response to sympathetic nerve stimulation and this effect was blocked by ICI 118,551 (0.01 microM). It was concluded that blockade of beta 2-adrenoreceptors, presumably located on sympathetic nerve terminals, decreases the release of transmitter noradrenaline and that blockade of beta 1-adrenoreceptors, presumably located in longitudinal smooth muscle cells, reduces the response to transmitter noradrenaline when alpha-adrenoreceptors are also blocked.

Impairment of sympathetic nerve responses by dopa, dopamine and their alpha-methyl analogues

Dopa and dopamine reduced the response of the nictitating membrane to postganglionic sympathetic nerve stimulation in cats pretreated with the monoamine oxidase inhibitor pargyline. A similar reduction could be obtained with α-methyldopa and α-methyldopamine in untreated cats. The sensitivity of the membrane to injected noradrenaline, α-methylnoradrenaline or adrenaline was not reduced. The possible mechanism of the anti-hypertensive action of monoamine oxidase inhibitors and α-methyldopa is discussed in the light of these findings.