Comparison of bretylium and guanethidine: tolerance, and effects on adrenergic nerve function and responses to sympathomimetic amines

Bretylium depresses the slope of regression lines relating frequency of sympathetic nerve stimulation to magnitude of contractions of the cat nictitating membrane. In contrast, guanethidine and reserpine preferentially abolish responses to low rates of nerve stimulation and cause a roughly parallel shift of the regression lines. The hypersensitivity of the nictitating membranes of cats to intravenous adrenaline or noradrenaline is far greater after a series of small daily doses of bretylium or guanethidine than after single large doses. The maximal sensitivity produced was similar to that after postganglionic sympathetic nerve section and exceeded that produced by ganglion blockade. The development of hypersensitivity to catechol amines is accompanied by some return of responses of the nictitating membranes to sympathetic nerve stimulation despite continued daily administration of bretylium or guanethidine. In cats given bretylium daily, responses to low rates of nerve stimulation become greater than in controls unless the dose of bretylium given subcutaneously is 50 mg/kg or more. When marked hypersensitivity to catechol amines has been produced by giving bretylium or guanethidine daily for 7 or 14 days, the sympathomimetic effects of these compounds are greater. Responses to intravenous dimethylphenylpiperazinium are also increased and the results suggest that even large daily doses of adrenergic neurone blocking agents do not appreciably impair the functioning of the adrenal medulla. The pressor effects of intravenous adrenaline, noradrenaline and dimethylphenylpiperazinium iodide increase less than the corresponding nictitating membrane responses. These results are discussed in relation to tolerance to adrenergic neurone blockade, and differences between the effects of bretylium and guanethidine found in man. Bretylium and guanethidine depress the slopes of the dose-response curves for the pressor and nictitating membrane contracting effects of tyramine. When single doses or a short series of daily doses were given, guanethidine caused more depression of the slopes than did bretylium, but nevertheless large depressions of slope were found after giving bretylium daily for several weeks. The magnitude of the responses can be greater or less than in controls depending on the dose of the sympathomimetic amine, the dose of the adrenergic neurone blocking agent and the duration of its administration. The results suggest that injection of tyramine produces a progressively smaller release of adrenaline or noradrenaline during the daily administration of bretylium (or guanethidine) but that in some test situations this is more than compensated for by the development of hypersensitivity to the catechol amine released. Some corresponding changes in responses to amphetamine and ephedrine are also described.     

Effects of a haloalkylamine on responses to and disposition of sympathomimetic amines

1. The mechanisms by which a haloalkylamine (GD-131) alters the inactivation of and potentiates responses to certain sympathomimetic amines, and the relationship of these actions to the similar effects of cocaine were investigated in rabbit aortic strips. The technique of oil immersion was used to assess rates of amine inactivation.2. Exposures to GD-131, which produced no detectable alpha-adrenergic blockade, markedly slowed the inactivation of noradrenaline. It was concluded that it is unnecessary to postulate a role of adrenergic receptors in the inactivation of catecholamines to account for the reported effects of haloalkylamines on amine output during adrenergic nerve stimulation.3. The reduction in the rate of noradrenaline inactivation produced by moderate exposure to GD-131 was approximately equivalent to that due to inhibition of both monoamine oxidase (MAO) and catechol-O-methyl transferase (COMT). Addition of GD-131 did not further slow inactivation in preparations in which MAO and COMT had been inhibited, but the effects of both GD-131 and of enzyme inhibition on noradrenaline disposition were additive with that of cocaine.4. Cocaine consistently inhibited and GD-131 markedly potentiated responses to tyramine. The augmentation of responses by GD-131 was much greater than could be accounted for by the slight release of endogenous catecholamine by this agent. Thus the principal effect of the haloalkylamine appears not to involve inhibition of nerve cell membrane transport of amine.5. Maximal exposure to GD-131 short of that which produced alpha-adrenergic blockade sometimes slowed the inactivation of noradrenaline as much as did inhibition of both MAO and COMT plus the maximal effect of cocaine.6. These results seem best explained by postulating that GD-131 and other haloalkylamines inhibit the passage of sympathomimetic amines through biological membranes. Passage to sites of enzymatic inactivation, predominantly in non-neuronal tissue, is most readily inhibited. The "cocaine-sensitive mechanism," transport to sites of binding and storage, can also be inhibited, but is considerably less sensitive.7. GD-131 potentiated responses to noradrenaline more than did the maximally effective concentration of cocaine. Cocaine produced very little additional potentiation when added in the presence of GD-131, whereas the latter had a reduced, but still significant effect in the presence of cocaine. Most of the effect of cocaine and at least half of that of GD-131 was due to a common action on effector cells, which is unrelated to any alteration of amine disposition. The balance of the potentiation by GD-131 may be due to inhibition of access of amine to sites of enzymatic inactivation, perhaps involving a reduction in the volume of distribution in intracellular water, and a very small part of the potentiation by cocaine may be secondary to inhibition of transport of amine to sites of binding and storage.8. On the basis of the present observations, it is postulated that a major part of the noradrenaline released by adrenergic nerve activity is involved in the activation of tissue receptors and has its action terminated by movement away from the region of the receptors. A small portion of the mediator is removed by the circulation, some is taken up by adrenergic nerves, but the major part enters non-nervous cells and is distributed in intracellular water. The capacity of this intracellular compartment appears to be limited and enzymatic inactivation is essential to maintain its function. O-methylation is the dominant primary enzymatic process in the inactivation of physiological amounts of noradrenaline, but MAO appears to function "in series" as an effective alternate pathway of disposition.     

Steroid potentiation of responses to sympathomimetic amines in aortic strips

1. Responses to catecholamines (adrenaline, noradrenaline, nordefrine) were enhanced by 17beta-oestradiol, progesterone and desoxycorticosterone in untreated and reserpine pretreated aortic strips. Responses to tyramine, believed mediated via endogenous catecholamines, were enhanced only in untreated strips.2. Responses to sympathomimetic amines lacking the catechol nucleus (phenylephrine, synephrine, methoxamine) were potentiated inconsistently by the steroids and reserpine pretreatment reduced markedly the frequency of potentiated responses.3. Known inhibitors of catechol-O-methyl transferase (tropolone, U-0521, pyrogallol) potentiated responses to catecholamines and abolished the enhancing effects of the steroids-when the steroids were given first, there was no further increase in response to catecholamines on adding inhibitors of catechol-O-methyl transferase.4. Experiments with the oil-immersion technique, to eliminate diffusion of drug from the tissue, indicated that 17beta-oestradiol, progesterone and desoxycorticosterone decreased the rate at which aortic strips inactivated adrenaline by O-methylation.5. It is concluded that 17beta-oestradiol, progesterone and desoxycorticosterone potentiate responses to catecholamines in aortic strips by inhibiting a major mechanism for their inactivation.     

Effects of reserpine on the disposition of sympathomimetic amines in vascular tissue

1. The effects of reserpine pretreatment on the intrinsic inactivation of low concentrations of phenylephrine and noradrenaline in strips of rabbit thoracic aorta were assessed by measuring the rates of relaxation, after oil immersion to prevent loss of active amine by diffusion into the surrounding medium.2. Reserpine pretreatment considerably augmented the amplitude of responses to low concentrations of phenylephrine, noradrenaline and nordefrine (Cobefrine).3. Reserpine pretreatment did not reduce the overall rate of inactivation of either phenylephrine or noradrenaline, but it did appear to decrease the contribution of uptake and storage, measured as an increased effect of enzyme inhibition and a decreased effect of cocaine on the rate of inactivation.4. The role of catechol-O-methyl transferase (COMT), but not that of monoamine oxidase (MAO), in terminating the action of noradrenaline was increased in strips from animals pretreated with reserpine. Thus it appears that interference with intraneuronal storage diverts active amine to inactivation by COMT in vascular tissue, rather than by MAO as has been previously suggested.5. As in preparations not treated with reserpine, inhibition of MAO alone had little effect on the rate of inactivation of noradrenaline, and this enzyme appears to function predominantly as an alternate pathway of little importance as long as COMT activity is unimpaired. Enzymatic processes accounted for about 85 and 70% of the inactivation of a low concentration of noradrenaline in reserpine pretreated and untreated preparations, respectively.6. Cocaine potentiated responses to noradrenaline and phenylephrine as effectively in reserpine pretreated as in untreated preparations, and inhibition of the pathways of enzymatic inactivation did not appreciably decrease the potentiation produced by this agent.7. The present results cannot be explained by the hypothesis that interference with amine inactivation by nerve uptake and storage is responsible for the potentiation of responses to noradrenaline or phenylephrine by either reserpine or cocaine, and emphasize the unrealiability of potentiation as an index of interference with mechanisms involved in terminating the action of sympathomimetic amines.     

Effects of deuterium substitution on the chronotropic responses to some sympathomimetic amines in the isolated rat atria

Summary In spontaneously beating rat atria the potencies for the chronotropic effects of the following deuterated phenylethylamine derivatives were higher than the potencies of the corresponding non-substituted (protio-) amines: ,,d₂--phenylethylamine; ,,,-d₄-p-tyramine; ,,,-d₄-m-tyramine; ,,-d₃-p-octopamine. In contrast, ,,-d₃-noradrenaline and ,,-d₃-m-octopamine were equipotent with the corresponding protio-amines. Experiments performed in atria depleted of endogenous noradrenaline by pretreatment with reserpine and in atria exposed to the monoamine oxidase (MAO) inhibitor pargyline indicated: a. p-octopamine had both direct and indirect effects, but the chronotropic responses to p-octopamine in tissues with normal MAO activity depended mostly on the direct action of the amine; deuterium substitution enhanced the indirect component of action of p-octopamine; b. m-octopamine possessed considerable indirect effects while d₃-m-octopamine behaved as an amine of direct action. The substitution of deuterium for hydrogens in the -carbon of the alkyl-side chain of phenylethylamines decreases the rate of deamination by MAO. Therefore, the results obtained with all the amines, except for m-octopamine and ,,p-d₃-m-octopamine, could be interpreted in terms of the direct, indirect or mixed action of those compounds and/or of the influence that MAO activity has on the chronotropic responses to these amines. The results obtained with protio-and deuterio-m-octopamine suggested that deuterium substitution, either at the - or the -carbon, can alter some other mechanisms in addition to the enzymatic deamination.Career Investigator on leave of absence from the Consejo de Investigaciones Cientificas y Técnicas, ININFA, Junín 956, 5°P, RA-1113 Buenos Aires, Argentina Send offprint requests to S. M. Celuch     

Centrally mediated cardiovascular responses to intracisternal injections of sympathomimetic amines in anesthetized rats.

The cardiovascular effects resulting from intracisternal (i.c.) injections of sympathomimetic amines were studied in alpha-chloralose-urethanized rats. Norepinephrine (0.5-5 mug i.c.) caused a typical rise in blood pressure with no significant change in heart rate and a fall in blood pressure with a bradycardia, which were completely blocked after treatment with phentolamine (10-50 mug i.c.) L-isoproterenol (0.05-0.5 mug i.c.) and trimetoquinol (0.5-3 mug i.c.), a beta-sympathomimetic agent, usually caused a fall in blood pressure with a tachycardia, which was reduced after treatment with propranolol (10-50 mug i.c.), but trimetoquinol was inclined to cause a rise in blood pressure with a tachycardia. Epinephrine (5 mug i.c.) showed both centrally mediated alpha- and beta-sympathomimetic effects. Tyramine (0.5-1 mg i.c.) caused mixed blood pressure responses presumably due to a release of norepinephrine and epinephrine, and these responses were partially blocked after treatment with phentolamine (100 mug i.c.) or propranolol (50 mug i.c.). These observations suggest that both alpha- and beta-sensitive adrenergic zones may exist on the vasomotor center of the pons and medulla in rats, and both norepinephrine and epinephrine might centrally play a physiological role as the neurotransmitters controlling blood pressure in rats.     

The response of an isolated artery to sympathomimetic amines

The rabbit ear central artery preparation responded with contractions to noradrenaline, dopamine, tyramine, octopamine, phenylethylamine, β-phenylethanolamine and periarterial sympathetic nerve stimulation. Noradrenaline, dopamine and nervous stimulation gave monophasic responses. Tyramine and octopamine gave biphasic responses consisting of an initial fast contraction followed by a second contraction of slow onset and long duration. Phenylethylamine and phenylethanolamine also gave biphasic responses, but the second contraction was of quick onset and short duration of action and often merged with the first contraction. Cocaine, superior cervical ganglionectomy and reserpinization reduced the second phase of the contraction to tyramine, octopamine and phenylethylamine. The second phase of the response to phenylethanolamine was reduced by reserpine but not by cocaine or denervation.     

The action of sympathomimetic amines on heart rate in relation to the effect of reserpine

When the heart-lung preparation is made from a dog treated with reserpine, catechol amines such as noradrenaline and isoprenaline have a greater effect on the rate of the heart than they have in a preparation from a normal dog. Other sympathomimetic amines such as tyramine and ephedrine, on the other hand, are found to have lost their action. Since treatment with reserpine has been shown to cause the store of noradrenaline in the heart to disappear, and the infusion of noradrenaline into the preparation made from a reserpine-treated animal restores the action of tyramine, it is concluded that substances like tyramine and ephedrine normally act by liberating noradrenaline from the store, and do not act directly. Cocaine, like reserpine, increases the effect of noradrenaline and decreases the effect of tyramine on the heart rate; it appears to block the release of noradrenaline from the store in the heart.