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.

Animals↗

PubMed1969

Mechanism of cocaine potentiation of responses to amines.

1. Effects of cocaine on the magnitude of responses to several biologically active amines and on their rates of inactivation were studied in strips of rabbit thoracic aorta in vitro.2. Although cocaine both potentiated responses to noradrenaline, adrenaline and phenylephrine and slowed their inactivation, the correlation between these two parameters under various experimental conditions was poor, and in all cases the delay in intrinsic inactivation was inadequate to account for the observed potentiation.3. Potentiation of responses to noradrenaline by cocaine was little decreased in strips stored at 6 degrees C for up to 10 days, although the response to low doses of tyramine was abolished much earlier. Similarly, cocaine clearly potentiated responses to noradrenaline for at least 28 hr at 37 degrees C, at which time responses to noradrenaline alone were markedly decreased.4. Cocaine potentiated responses to phenylephrine as well after 60 as after 10 min exposure to the amine in strips in which all intra-neuronal disposition of this amine had been eliminated by treatment with reserpine and iproniazid.5. Cocaine effectively potentiated responses to histamine, but had only a slight and variable effect on those to 5-hydroxytryptamine (5-HT). It did not alter the tissue inactivation of histamine, but did significantly slow the inactivation of 5-HT.6. Procaine slowed amine inactivation in the same way and to the same extent as did cocaine, but did not potentiate responses or affect the potentiation produced by cocaine added in its presence.7. Cocaine potentiated responses to methoxamine to approximately the same degree as it did those to noradrenaline, although studies by the oil immersion technique clearly demonstrated that the aortic strips were entirely incapable of inactivating methoxamine.8. The observations reported and discussed are incompatible with the hypothesis that cocaine potentiates responses to sympathomimetic amines because it prevents their inactivation by nerve uptake and storage and thus diverts larger amounts of agonist to tissue receptors. It is concluded that potentiation and inhibition of amine inactivation reflect two largely independent actions of cocaine in this vascular smooth muscle preparation, and probably in other organs, and that potentiation is a generally unreliable criterion of the blockade of processes inactivating sympathomimetic amines or of the importance of these processes in terminating the action of the amines.

Amines↗

PubMed1969

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.

Amines↗

PubMed1969