Rat and human cardiac monoamine oxidase: a comparison with other tissues

Some properties of semicarbazide-sensitive amine oxidases

The semicarbazide-sensitive amine oxidases (SSAOs) comprise a substantial but diffuse group of enzymes separable from classical monoamine oxidase in several respects. Differences in cofactor requirement, molecular weight and subcellular distribution are crucial for such a separation. Differential sensitivity to enzyme inhibitors, characterized by resistance to inhibition by acetylenic MAO inhibitors coupled with sensitivity to semicarbazide and some related compounds are characteristic of these enzymes. SSAO enzymes have been found in the plasma of man, ox, pig and horse, for example as well as in the solid tissues of many species. Extensive studies have so far failed to produce any conclusive evidence to indicate what the precise functions of many of these enzymes may be. Indeed in most cases there is no clear idea as to the nature of the preferred physiological substrate, although many amines with pharmacological activity have been shown to be substrates. The actions of these amines may be potentiated following inhibition of SSAO, but as yet little is known whether or not these actions can be important in vivo. An attempt is made in this review to bring together some of the evidence to see if there are indications for future endeavours.

Myocardial H2O2 production in the unanaesthetized rat. Influence of fasting, myocardial load and inhibition of superoxide dismutase and monoamine oxidase

Myocardial H2O2 production was studied by means of the in vivo administration of aminotriazole (AT), which inactivates the catalase-H2O2 complex compound I. Measurements of the residual catalase activity in male and female rats indicate that beta-oxidation of fatty acids in peroxisomes does not contribute in a substantial way to energy production in response to fasting or to an increased myocardial load, despite previous data on peroxisomes in myocardium. Superoxide dismutase (SOD) inhibition by diethyldithiocarbamate demonstrates that SOD participates in the production of H2O2 in physiological conditions. Such a role was not demonstrated for monoamine oxidase through inhibition by phenelzine.

Deamination of aliphatic amines by monoamine oxidase A and B studied using a bioluminescence technique

Deamination of n-octylamine and n-decylamine has been studied in various tissues using a new bioluminescence technique. Selectivity of n-octylamine and n-decylamine as substrates for monoamine oxidase (MAO) A or B has been determined using both clorgyline and (-)-deprenyl inhibition curves and kinetic parameters. Homogenates of rat brain, liver and heart containing predominantly MAO-A or -B were prepared by preincubation for 60 min with (-)-deprenyl or clorgyline (30 nM), respectively. Human placenta (MAO-A) and platelet (MAO-B) were used as reference tissues containing only one MAO form. In tissues (rat liver, brain) containing both MAO forms in equal proportion, inhibition curve studies showed a preference of both substrates for the B form of the enzyme; however, where MAO-A was the major form (rat heart, human placenta), clorgyline was the more effective inhibitor. In the beef brain cortex n-octylamine showed marked preference for MAO-B, whereas n-decylamine was selective toward-MAO-A. Kinetic studies in general supported the picture of greater selectivity of the aliphatic amine substrates for deamination by MAO-B, as reflected by lower Km values for this enzyme type. However, n-octylamine was more selective for MAO-B than n-decylamine in both kinetic and inhibition curve studies. The deamination of these aliphatic amine substrates cannot be explained only by reference to the binary classification of MAO into types A and B.

Cardiac norepinephrine releasing action of kynuramine, an endogenous diamine derived from L-tryptophan

Kynuramine, an endogenous metabolite of L-tryptophan, was found to function as an indirectly acting sympathomimetic amine in rat atria in vitro. Kynuramine released tritium from atria preloaded with [3H]norepinephrine (NE), an effect which was blocked completely by pretreatment with reserpine or 6-hydroxydopamine. Release by kynuramine was calcium-independent and was potentiated by inhibition of monoamine oxidase but was only partially sensitive (50%) to inhibition by cocaine (10(-4)M). The ability of kynuramine to enter cardiac cells was demonstrated in whole atria by measuring its intracellular deamination rate by monoamine oxidase. Blockade of neuronal uptake (cocaine) and extraneuronal uptake (SKF 550) had no effect upon this measure. It is concluded that knyuramine releases cardiac NE, in part, by a cocaine-sensitive mechanism but that the process operating for the membrane transport of kynuramine in both neuronal and non-neuronal cells remains uncertain. The data are discussed in relation to the possible cardiac consequences of L-tryptophan ingestion in man.

Monoamine oxidase in young and adult rat brain capillaries

Capillaries were isolated from young and adult rat brain and the mitochondrial type A and B monoamine oxidase activities in these endothelial cells were determined. Type A activity was observed to be predominant in both young and adult rats and was increased with maturation while type B activity remained unchanged. The predominance of type A MAO in the rat brain capillaries is consistent with that of the rat aorta and heart tissue.

Monoamine oxidase activities of dissociated cell fractions from rat ventricular muscle

Cell fractions enriched in cardiac muscle cells (myocytes), on the one hand, and in non-myocytes, on the other, were prepared by dissociation of rat ventricular tissue with collagenase. Amine oxidase activities in homogenates of these cell fractions and also in homogenates of the corresponding undissociated ventricular tissue were compared. In addition, the activity of alkaline phosphatase (AP), an enzyme found predominantly associated in the heart with non-myocytes, particularly capillary endothelial cells, was also measured. No significant difference in the activity of MAO-A (assayed with 1 mM 5-hydroxytryptamine) was found between myocyte and non-myocyte fractions. In contrast, the activities of alkaline phosphatase (AP) and also the semicarbazide-sensitive amine oxidase (SSAO), assayed with 1 microM benzylamine (BZ), were both significantly higher in non-myocytes, by several-fold, than in myocyte fractions. Studies of the inhibition by clorgyline of 1 mM BZ metabolism confirmed that both MAO-A and MAO-B can also contribute to BZ oxidation in the rat heart. These experiments indicated different ratios of MAO-A: MAO-B in the various cell fractions. The ratios of the percentage contributions of MAO-A and MAO-B, respectively, to the total metabolism of 1 mM BZ were 78:20 (myocytes), 43:52 (non-myocytes) and 57:32 (undissociated tissue). These results suggest that MAO-B, in addition to AP and SSAO, may be associated preferentially with non-myocyte constituents of the rat heart. Although cardiac myocytes appear to contain predominantly MAO-A, this enzyme form is also localized, with high activity, to the non-myocyte fraction. However, since the non-myocyte fraction is heterogeneous in its cell content, containing vascular components of the coronary microcirculation, as well as other cells of connective tissue origin, the exact cellular localization of the enzyme activities within this fraction has not yet been defined.

Properties of monoamine oxidase in monkey heart

The activities of monoamine oxidase (MAO) in a homogenate and the mitochondrial fraction of monkey (Macaca fascicularis) heart were measured with labelled benzylamine, serotonin (5-HT), tyramine and beta-phenylethylamine (PEA) as substrates. The effects of clorgyline, deprenyl and semicarbazide on MAO were also investigated. Benzylamine deamination was more sensitive to deprenyl than to clorgyline, although it was not completely inhibited by a high concentration of either, and the remaining activity was completely inhibited by pretreatment with semicarbazide. The activities of 5-HT and tyramine were both more sensitive to clorgyline than to deprenyl, and both inhibitors gave single-sigmoidal inhibition curves. Experiments with increasing concentrations of PEA as substrate and clorgyline and deprenyl as inhibitors indicated that at higher concentrations, PEA was a substrate for MAO-A as well as MAO-B in monkey heart. These results suggest that monkey heart mitochondria contain not only MAO-A and MAO-B, but also clorgyline resistant amine oxidase (CRAO). The ratio of MAO-A, MAO-B and CRAO was determined from plots of inhibitions with each substrate. The kinetic constants of MAO in monkey heart were compared with those of MAO in hearts of other animals. The enzymic properties of MAO in monkey heart were discussed on the basis of these results.

Cardiovascular actions of kynuramine and 5-hydroxykynuramine in pithed rats

Kynuramines occur endogenously in brain and peripheral tissues as metabolites of indoleamino acids and indolamines but little is known regarding their possible physiological and/or pharmacological activity. The present study has investigated the effects of kynuramine and 5-hydroxykynuramine on the cardiovascular system of pithed rats and attempted to correlate effects seen on adrenergic and serotonergic receptors with ligand binding experiments done in vitro using rat brain membranes. Kynuramine was found to release cardiac catecholamines and to act as a weak partial agonist on vascular alpha-adrenoceptors. Hydroxylation in the 5-position (5-hydroxykynuramine) did not alter cardiac potency but increased pressor activity 100-fold. Vasopressor responses to 5-hydroxykynuramine were mediated via a dual agonistic action on both postsynaptic alpha 2-adrenoceptors and vascular serotonin2 receptors. These findings were supported by ligand binding studies, in which both kynuramine and 5-hydroxykynuramine showed affinity for cortical [3H]spiroperidol and [3H]clonidine binding sites. Overall, the results show that kynuramines can exert peripheral (and possibly central) actions which may prove to be of physiological and/or pharmacological significance.

Blood pressure and heart rate effects of kynuramine in pithed rats

Kynuramine increased heart rate and blood pressure in pithed rats. Heart rate responses were blocked by beta-adrenoceptor antagonists and reserpine pretreatment. Both of these treatments reduced the pressor responses to kynuramine. Remaining pressor activity was inhibited by phentolamine. The pressor responses to kynuramine were reversed to depressor responses during infusion of norepinephrine but not vasopressin. It is concluded that kynuramine releases cardiac catecholamines and acts as a partial agonist on vascular alpha-adrenoceptors.

A comparison in rat and mouse heart of the ability of several local anesthetics to inhibit monoamine oxidase

Several local anesthetics were examined and compared in rat and mouse heart for MAO inhibitory action. Procaine, chloroprocaine, procainamide and tetracaine were more potent in rat than mouse heart. Piperocaine, cyclomethycaine, hexylcaine and butacaine were somewhat more potent in mouse than rat heart. Propoxycaine and dibucaine were essentially equipotent in the two tissues. Since rat heart possesses primarily type A and mouse heart primarily type B MAO, these results suggest that some local anesthetics have a preferential action on one form of the enzyme while others have a preferential action on the other. Still another group has a non-preferential action.

In vitro and in vivo inhibition by benserazide of clorgyline-resistant amine oxidases in rat cardiovascular tissues

Bernserazide (D,L-serine 2-[2,3,4-trihydroxybenzyl]-hydrazide) as been shown to inhibit the clorgyline-resistant amine oxidase (CRAO) activities which metabolize benzylamine in homogenates of rat aorta, heart and brown adipose tissue. In vitro studies showed a concentration- and time-dependent inhibition of CRAO in heart and aorta which was reversed by dialysis for 18hr. At high concentrations (10(-4)-10(-3)M) benserazide appeared to increase enzyme activity towards and occasionally above control value. These increases became more prominent after long periods of preincubation (especially in the presence of saturating benzylamine concentrations) and remained after dialysis of those homogenates preincubated with benserazide. The administration of benserazide for one or seven days in daily doses of 5-150 mg/kg also inhibited CRAO activity in vivo in a dose-dependent manner, with greater inhibition after seven days treatment. Reversal of inhibition, by dialysis of tissue homogenates from benserazide-treated rats, was much slower than was found with homogenates incubated in vitro with the drug. After benserazide administration to rats, MAO-A activity towards 5-hydroxytryptamine was generally not inhibited, and in fact was significantly increased in some cases. The administration of L-DOPA (250 mg/kg) together with benserazide (40 mg/kg) resulted in a similar degree of CRAO inhibition in aorta and heart to that seen after benserazide alone. These findings are discussed with regard to the use of these drugs in the therapy of Parkinson's Disease, although the paucity of information about the physiological function of CRAO makes the significance of its inhibition by benserazide unclear.

A simple method for screening monoamine oxidase (MAO) inhibitory drugs for type preference

A simple method is described for screening monoamine oxidase (MAO) inhibitors for preferential action on either type A or type B MAO. Kynuramine, a substrate for both types of MAO, was used and the sources of enzyme were rat heart (type A) and mouse heart (type B). The method clearly showed that clorgyline (a preferential type A inhibitor) preferentially inhibited rat heart MAO; whereas pargyline (a preferential type B inhibitor) preferentially inhibited mouse heart MAO. Pheniprazine, an inhibitor without type preference, was equally effective in the two tissues. The method was used to substantiate a report by others that amphetamine has preferential inhibitory action against type A MAO.

An alpha-adrenoceptor inhibitory action of kynuramine

Kynuramine, an endogenously occurring amine, inhibited the vasoconstrictor responses to norepinephrine in the isolated perfused mesenteric arteries of rats and blocked the relaxation of rabbit intestinal smooth muscle by phenylephrine. Similarly, kynuramine reversed clonidine-induced inhibition of the cholinergic twitch response in the guinea pig ileum. These effects, seen with concentration ranging from 4 to 60 microgram/ml, are consistent with an alpha-adrenoceptor inhibitory action of kynuramine at both presynaptic and postsynaptic sites. Kynuramine itself, lacked intrinsic activity on alpha-adrenoceptors and showed a complete lack of affinity for beta-adrenoceptors in the rabbit intestine and guinea pig tracheal chain preparation. None of the effects of kynuramine could be attributed to the formation of 4-hydroxyquinoline, the deaminated metabolite of kynuramine. Additionally, no evidence was obtained for an action on muscarinic receptors or non-specific effects on vascular smooth muscle. However, a slight, transient stimulant action was evident on intestinal smooth muscle. It is concluded that kynuramine inhibits both presynaptic and postsynaptic alpha-adrenoceptors in vitro and it is conceivable that the compound might function as an endogenous inhibitor of alpha-adrenoceptors in vivo.

Selective inhibition by amezinium of intraneuronal monoamine oxidase

Intra- and extraneuronal compartments of rat hearts were selectively labelled by perfusion with 3H-noradrenaline in the presence of corticosterone 87 microM or cocaine 30 microM, respectively. The subsequent outflow of 3H-compounds was examined. As little as 1 nM amezinium diminished the outflow of intraneuronally formed 3H-DOPEG. This effect was antagonized by cocaine. Amezinium 1 microM was necessary to diminish the outflow of extraneuronally formed 3H-DOPEG. This effect was not counteracted by corticosterone. The results indicate that amezinium is both a potent and, at low concentrations, selective inhibitor of intraneuronal MAO.