Actions of cocaine and tyramine on the uptake and release of H3-norepinephrine in the heart

Biochemical Assessment of Pheochromocytoma and Paraganglioma

Pheochromocytoma and paraganglioma (PPGL) require prompt consideration and efficient diagnosis and treatment to minimize associated morbidity and mortality. Once considered, appropriate biochemical testing is key to diagnosis. Advances in understanding catecholamine metabolism have clarified why measurements of the O-methylated catecholamine metabolites rather than the catecholamines themselves are important for effective diagnosis. These metabolites, normetanephrine and metanephrine, produced respectively from norepinephrine and epinephrine, can be measured in plasma or urine, with choice according to available methods or presentation of patients. For patients with signs and symptoms of catecholamine excess, either test will invariably establish the diagnosis, whereas the plasma test provides higher sensitivity than urinary metanephrines for patients screened due to an incidentaloma or genetic predisposition, particularly for small tumors or in patients with an asymptomatic presentation. Additional measurements of plasma methoxytyramine can be important for some tumors, such as paragangliomas, and for surveillance of patients at risk of metastatic disease. Avoidance of false-positive test results is best achieved by plasma measurements with appropriate reference intervals and preanalytical precautions, including sampling blood in the fully supine position. Follow-up of positive results, including optimization of preanalytics for repeat tests or whether to proceed directly to anatomic imaging or confirmatory clonidine tests, depends on the test results, which can also suggest likely size, adrenal vs extra-adrenal location, underlying biology, or even metastatic involvement of a suspected tumor. Modern biochemical testing now makes diagnosis of PPGL relatively simple. Integration of artificial intelligence into the process should make it possible to fine-tune these advances.

In-vivo evidence of a role for nitric oxide in regulating the activity of the norepinephrine transporter

We examined the role of nitric oxide (NO) in the regulation of neuronal uptake of norepinephrine (uptake-1) in rats under anesthesia. The effect on systolic blood pressure of two pressor drugs that work by different mechanisms, norepinephrine and angiotensin II, was explored in anesthetized rats under control conditions and after prevention of NO synthesis with Nw-nitro-L-arginine (L-NNA). The results showed that whereas the pressor effects of increasing doses of norepinephrine were potentiated by L-NNA, those of angiotensin II were not affected, which implied that NO was selectively involved in modulating the pressor effect of norepinephrine. To explore the mechanisms involved in this potentiation, we examined the effect of L-NNA on the pressor effect of tyramine, a purely-indirectly-acting sympathomimetic amine which enters nerve terminals thorough uptake 1 and liberates norepinephrine from storage vesicles. Increasing doses of tyramine produced pressor effects which, in contrast to those of norepinephrine, were significantly attenuated by pre-treatment with L-NNA. Similarly, pretreatment with cocaine, the classical inhibitor of uptake 1, significantly decreased the pressor effect of tyramine; however, the response to tyramine was then restored when L-NNA was administered, thus reversing the effect of cocaine. We conclude that NO plays a major role in the adrenergic system by enhancing the activity of uptake 1 in sympathetic nerve terminals. Blockade of uptake 1 by cocaine is also partly dependent on NO. The stimulus for the mobilization of the NO synthase pathway in adrenergic neurons and the subsequent steps involved in modulating uptake 1 deserve further exploration.

On the mechanism of tachyphylaxis to tyramine in the isolated rat heart

Tyramine was shown to release [(3)H]-catecholamines from an isolated rat heart previously perfused with [(3)H]-noradrenaline. With successive injections of tyramine the amount of [(3)H]-catecholamine released fell progressively and there was a parallel decrease in the increment of amplitude and rate of contraction of the heart. Reserpinized hearts were shown to take up less [(3)H]-noradrenaline than normal hearts. Release of radioactivity and loss of responsiveness to tyramine occurred more rapidly in the reserpinized heart. In the same preparation the uptake of [(14)C]-tyramine exceeded the quantity of the noradrenaline released.

The mode of action of tyramine

The repeated administration of tyramine to the isolated perfused guinea-pig heart gradually decreased the positive inotropic response which was paralleled by a decrease in the noradrenaline content of the heart and an increase in the noradrenaline content of the perfusate. Phenethylamine, ephedrine, amphetamine, guanethidine and bretylium were administered to the isolated heart until no further positive inotropic effect was obtained. The absence of an inotropic response with phenethylamine and guanethidine was associated with a decrease in the noradrenaline content of the heart. The absence of an inotropic effect with amphetamine, bretylium and ephedrine was not associated with a decrease in the amount of noradrenaline in the heart.

An action of 5-hydroxytryptamine on adrenaline receptors

Contractions of isolated strips of cat spleen due to 5-hydroxytryptamine, adrenaline, histamine and acetylcholine were antagonized by phenoxybenzamine. Responses to both 5-hydroxytryptamine and adrenaline were not blocked in strips which were protected by a high concentration of either 5-hydroxytryptamine or adrenaline throughout exposure to phenoxybenzamine. The contraction due to a large dose of 5-hydroxytryptamine lasted less than 1 hr even when the drug was still present. Strips thus desensitized to 5-hydroxytryptamine responded normally to acetylcholine and histamine but did not respond to adrenaline. The actions of 5-hydroxytryptamine and adrenaline were blocked by 2-bromolysergic acid diethylamide or by dihydroergotamine. These results indicated that 5-hydroxytryptamine and adrenaline act on the same receptors. Cocaine potentiated the action of adrenaline but inhibited the action of 5-hydroxytryptamine. The sensitivity to 5-hydroxytryptamine of spleen strips from cats treated 24 hr earlier with reserpine was only one-fiftieth of that of normal strips. Cocaine potentiated the action of 5-hydroxytryptamine on strips from reserpine-treated cats. A high concentration of 5-hydroxytryptamine in spleen strips from reserpine-treated cats and in cocaine-treated strips prevented phenoxybenzamine from blocking the actions of adrenaline. The effects of tyramine on spleen strips almost exactly paralleled the effects of 5-hydroxytryptamine. Strips showing tachyphylaxis to tyramine did not respond to 5-hydroxytryptamine. It is concluded that 5-hydroxytryptamine has a dual action, viz., a major action due to release of stored noradrenaline and a minor direct action of adrenaline receptors.

A QUANTITATIVE STUDY OF THE EFFECT OF COCAINE ON THE RESPONSE OF THE CAT NICTITATING MEMBRANE TO NERVE STIMULATION AND TO INJECTED NORADRENALINE

Results are reported of a quantitative study of the potentiating effect of cocaine on the responses of the cat nictitating membrane to intravenously and intra-arterially injected noradrenaline, as well as to different types of sympathetic nerve stimulation. Responses of the membrane to noradrenaline were potentiated more with intravenous than with close-arterial injections. From studies of the responses of the nictitating membrane to various forms of sympathetic nerve stimulation before and after injection of cocaine, conclusions are drawn as to the extent to which the transmitter amine liberated by nerve activity is normally removed and its effect thereby limited in duration and extent. This uptake was greatest at low stimulus frequencies. The mechanism by which cocaine potentiates sympathetic responses is discussed.

Release of [3H]noradrenaline by alpha 1-adrenoceptor agonists

Mouse isolated vas deferens preincubated with [3-H]noradrenaline was superfused and the effect of alpha 1-adrenoceptor agonists was studied on the release of total radioactivity ([3H]noradrenaline + 3H-metabolites) and [3H]noradrenaline. Reverse phase high pressure liquid chromatography (HPLC) combined with scintillation spectrometry was used to separate [3H]noradrenaline from its metabolites. Among the alpha 1-adrenoceptor agonists (1-phenylephrine, ST-587(2-(2(1)-chloro-5-trifluoromethyl phenylamino)-imidazole), (-)-amidephrine, methoxamine, cirazoline and 1-noradrenaline) studied 1-phenylephrine, ST-587 and 1-noradrenaline were capable of releasing 3H-noradrenaline. The effect of noradrenaline was stereospecific. As determined by HPLC combined with scintillation spectrometry the release of total radioactivity in response to 1-noradrenaline is mainly due to [3H]noradrenaline. It is suggested that 1-noradrenaline, 1-phenylephrine, and ST-587 in addition to their direct effect on different receptors they also have indirect action through the release of noradrenaline which might be partly involved in the pharmacological responses. The mechanisms whereby 1-noradrenaline and 1-phenylephrine release noradrenaline would appear to involve a saturable Ca-independent and a cocaine and temperature sensitive process. On the basis of our findings among the alpha 1-adrenoceptor agonists studied (-)-amidephrine, methoxamine and cirazoline is a better choice than 1-phenylephrine or ST-587 for selective stimulation of postjunctional alpha 1-adrenoceptors, they do not release noradrenaline.

Tyrosine administration increases striatal dopamine release in rats with partial nigrostriatal lesions

Partial, unilateral nigrostriatal lesions of varying severity were produced in rats by injecting graded doses of 6-hydroxydopamine into the substantia nigra. Formation of the dopamine metabolites dihydroxyphenylacetic acid and homovanillic acid in each surviving nigrostriatal neuron (estimated by the ratios of dihydroxyphenylacetic acid to dopamine and homovanillic acid to dopamine in the striatum) increased significantly when dopamine concentrations in striata containing lesions had been reduced to 25% or less of control values, but remained unchanged in rats with less severe lesions. These findings suggest that, in rats with severe damage of nigrostriatal dopaminergic neurons, surviving neurons increase their firing rates and accelerate dopamine synthesis and release. In rats that had lesions and enhanced striatal dopamine release, but not in rats with lesser lesions (i.e., which reduced ipsilateral dopamine concentrations by less than 75%), administration of tyrosine (250 mg/kg) caused further significant increases in formation of dihydroxyphenylacetic acid and homovanillic acid. These findings provide further evidence that tyrosine availability can enhance dopamine synthesis in and release from nigrostriatal neurons if the firing rates of these neurons are accelerated.

Platelet monoamine oxidase activity in epilepsy

The platelet monoamine oxidase activity (MAO) of 33 patients with epilepsy was compared with a group of neurological patients and a group of normal control subjects. We found that the MAO activity was increased significantly in the epileptic group when compared with the normal. This was not related to anticonvulsant medication. These results could be explained by the effect of epilepsy or anticonvulsant medication on the maturation of platelets.

The effects of changes in ionic environment and modification of adrenergic function on the vascular responses to sympathomimetic amines

The responses to each of four sympathomimetic amines: noradrenaline 200 ng, octopamine 50 μg, metaraminol 20 μg and tyramine 100 μg were studied in the perfused rat mesentery preparation. Perfusion with Ca2+- and Mg2+-free solutions potentiated the responses to all four amines compared with control responses obtained during normal Krebs perfusion. Under perfusion conditions using either normal or Ca2+- and Mg2+-free Krebs solution, nialamide and reserpine retained their characteristic effects on the responses to each amine. Cocaine and desipramine abolished the responses to tyramine but potentiated those to noradrenaline and metaraminol under all perfusion conditions. The responses to each of the amines were only antagonized by ouabain when Ca2+ ions were present in the perfusion solution. It is concluded that perfusion with Ca2+- and Mg2+-free solution interferes with the normal uptake mechanisms occurring in the adrenergic neuron.

Protection against induction of supersensitivity to catecholamines by cocaine

1. Adrenaline, noradrenaline, isoprenaline, tyramine, phentolamine, pronethalol, histamine and acetylcholine were each tested for their ability to prevent cocaine from causing supersensitivity to catecholamines in cat spleen strips in vitro. A high concentration of one of these drugs was added to the bath 5 min before cocaine hydrochloride (10 mug/ml). The effect on subsequent responses to catecholamines was compared with the effect of cocaine in control strips in the absence of an interfering drug.2. Phentolamine completely abolished the potentiating effect of cocaine. Large doses of adrenaline or noradrenaline reduced, but did not completely prevent, potentiation. Tyramine, isoprenaline, pronethalol, histamine and acetylcholine did not prevent potentiation.3. The ability of these drugs to interfere with potentiation does not correlate well with their ability to interfere with uptake of noradrenaline. Interference with uptake by cocaine is therefore unlikely to account fully for potentiation.

The effect of cocaine and imipramine on tyramine-induced release of noradrenaline-3H from the rat vas deferens in vitro

1. Tyramine 10(-4)M significantly increased release of noradrenaline-7-(3)H (NA-7-(3)H) from rat vas deferens in vitro.2. Neither cocaine 10(-5)M nor imipramine 10(-7)M-10(-6)M significantly reduced tyramine-induced release of NA-7-(3)H.3. Increasing the exposure time to cocaine and imipramine from 10 to 20 min or pre-incubating the tissue with a wide range of NA-7-(3)H concentrations (3.3-333.3 ng/ml.) did not affect the lack of inhibition by cocaine and imipramine.4. It is suggested that the tyramine receptor in rat vas deferens differs from that in other systems and that blockade of tyramine-released noradrenaline at alpha-adrenergic receptors may be the most important mechanism for tyramine antagonism by imipramine-like drugs in this tissue.

The action of tyramine on the dog isolated atrium

It has been confirmed that tyramine has positive inotropic and chronotropic actions on the dog isolated atrium. These responses were incompletely and reversibly inhibited by cocaine, but completely and irreversibly blocked by phenoxybenzamine. Blockade of the atrial beta-receptors by dichloroisoprenaline could be overcome by noradrenaline and by larger doses of tyramine. With the aortic strip of the reserpinized rabbit for assay, tyramine was shown to release a vasoactive material from the dog atrium whose receptors were blocked by dichloroisoprenaline. The use of an antihistamine and an anti-5-hydroxytryptamine agent (cyproheptadine) appeared to exclude the possibility that the effect was due to the release of histamine or 5-hydroxytryptamine from the atrium by tyramine. Further observations of the action of the vasoactive material on the guinea-pig ileum and on the rat fundal strip strongly suggested that the material was a catechol amine. It was concluded that under these conditions tyramine acts by liberating catechol amines from storage sites so that the amines are free to act at receptor sites. The behaviour of the atrium to tyramine in the presence of cocaine or of phenoxybenzamine suggests that the liberation of catechol amines by tyramine differs from the release due to adrenergic nerve stimulation. It is suggested that, after an infusion of tyramine, there is a much slower release of catechol amines than after stimulation of adrenergic nerves.