Effects of clorgyline and (-)deprenyl on the deamination of normetanephrine and noradrenaline in strips and homogenates of the canine saphenous vein

The role of MAO-A and MAO-B in the metabolic degradation of noradrenaline in human arteries

1. Segments of human cystic, gastric and ileocolic arteries were obtained from patients undergoing surgery. 2. Segments of arterial tissues, the noradrenaline content of which ranged between 0.27 and 0.52 microg g(-1), were incubated with 0.1 micromol l(-1) [3H]-noradrenaline for 30 min and the accumulation of the amine as well as the formation of metabolites was measured. 3. In all the arteries, oxidative deamination predominated over O-methylation; the mean values of the deaminated and O-methylated metabolites formed for the three arteries were 247.6 and 82.4 pmol g(-1) tissue, respectively. Dihydroxymandelic acid (DOMA) was the most abundant metabolite. 4. Both clorgyline (a selective MAO-A inhibitor) and selegiline (a selective MAO-B inhibitor) reduced the formation of dihydroxyphenylglycol (DOPEG), DOMA and O-methylated-deaminated metabolites (OMDA), and increased that of normetanephrine (NMN). However, clorgyline depressed the formation of DOPEG more than that of DOMA, while selegiline depressed the formation of DOMA more than that of DOPEG. 5. In conclusion, three major differences distinguish the metabolism of noradrenaline by human arteries from that observed in other species: (1) the large predominance of deamination over O-methylation; (2) the extremely high formation of DOMA; and (3) the relative lack of selectivity of clorgyline and selegiline for MAO-A and B, respectively. Since the arterial vessels used were collected from patients older than 60 years, the morphological changes depending on age may explain the increase in DOMA formation.

The role of monoamine oxidase in the metabolism of exogenous noradrenaline by the human saphenous vein

Human saphenous vein segments were obtained from patients subjected to coronary bypass surgery. As determined by HPLC-ED, the veins had a relatively low content of noradrenaline and high content of the deaminated metabolites, dihydroxyphenylglycol (DOPEG) and dihydroxymandelic acid (DOMA). In vein segments which had been incubated with 3H-noradrenaline (0.1 mumol/l), the oxidative deamination pathway predominated over the O-methylating one. Deamination occurred both at the neuronal and extraneuronal level; DOPEG appearing to be a good index of intraneuronal deamination, whereas DOMA and O-methylated and deaminated metabolites were mainly formed extraneuronally. Both MAO type A and MAO type B selective inhibitors reduced the deamination of noradrenaline; deamination was also found to be partially sensitive to semicarbazide. Inhibition of neuronal uptake or of deamination increased O-methylation. The human saphenous vein thus metabolizes exogenous noradrenaline following a pattern which substantially differs from that shown to occur in various blood vessels from other animal species.

Oxidative deamination of noradrenaline in human blood vessels

Human vascular tissue (saphenous vein and uterine artery) was incubated with tritiated noradrenaline. In both vessels, oxidative deamination predominated over O-methylation. Deamination was due to the action of MAO type A and B, as well as of a semicarbazide-sensitive oxidase. Whereas DOPEG was a good index of intraneuronal deamination, NMN, DOMA and OMDA appeared to be entirely extraneuronal in origin. Extraneuronal deamination plays an important role in human vascular tissue and may contribute to the inactivation of circulating catecholamines.

Time course of the metabolite patterns of intraventricularly injected [3H]noradrenaline in rat brain regions

In the hypothalamus, septum, pons with medulla, and hippocampus regions of rat brain, the level of radioactivity of [3H]noradrenaline and of five of its metabolites were determined up to 6 h after intraventricular injection of the tritiated amine. The following main results were found: In anterior hypothalamus and septum, the [3H]noradrenaline level declined in two phases. Similar turnover curves were obtained for the primary deaminated metabolites, with almost the same final half-lives as for [3H]noradrenaline. The level of the initial methylation product, normetanephrine, also showed a biphasic decline, which did not correspond to that of [3H]noradrenaline but rather was faster throughout the experiment. The final metabolites (i.e., the glycol sulfates) reached maximal levels in hypothalamus and septum earlier than in other regions. Thereafter, their levels declined with almost similar rates in all areas tested, but always faster than the [3H]noradrenaline level. The following conclusions were drawn: In areas rich in catecholaminergic nerve terminals, there seems to be a site, in addition to the vesicular storage pool, that accumulates exogenous noradrenaline and then releases it with relatively short half-lives. The contents of primary deaminated metabolites followed the turnover of [3H]noradrenaline at both sites. Exogenous [3H]noradrenaline seems to be methylated at two extraneuronal sites, which are distinguished by the rates of subsequent deamination. The size of the pool of slowly deaminated [3H]normetanephrine that is formed immediately after [3H]noradrenaline injection determined the apparent turnover of this product throughout the experiment and, thus, like the final metabolites, reflects for several hours the initial degradation of the unstored [3H]noradrenaline, rather than the metabolism of the stored amine.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of a monoamine oxidase A inhibitor, FLA 668 (+) on the adrenergic mechanisms of the dog saphenous vein

FLA 668 (+) [(S)-(+)-4-amino-2, alpha-dimethylethylphenethylamine (+)-hydrogen bitartrate] selectively inhibited MAO type A in homogenates of dog saphenous vein, with a IC50 of 1 mumol/l for MAO A and greater than 1 mmol/l for MAO B. At concentrations of 1 mumol/l and higher, FLA 668(+) progressively decreased the formation of deaminated metabolites by saphenous vein strips incubated with 3H-noradrenaline, with a preferential action on DOPEG formation. Normetanephrine formation increased but this increase did not wholly compensate for the reduction in the formation of deaminated metabolites. FLA 668(+) caused increased reactivity of saphenous vein strips to noradrenaline and markedly reduced the accumulation of 3H-noradrenaline in the incubated tissue; both effects were still evident in the presence of clorgyline. After cocaine, FLA 668(+) caused no further increase in sensitivity. It is concluded that FLA 668(+) is, in the saphenous vein of the dog, a selective inhibitor of MAO type A and that it exerts a cocaine-like effect.

Influence of MAO A and MAO B on the inactivation of noradrenaline in the saphenous vein of the dog

Specific inhibitors of MAO A (clorgyline 0.1 mumol/l) and of MAO B [(-)deprenyl 10 mumol/l] were used in dog saphenous vein strips in order to study the relative influence of the two types of MAO on: 1. Termination of contractile response to exogenous noradrenaline (NA); 2. Metabolism and accumulation of exogenous (3H)-NA; 3. Metabolism of 3H-NA released by electrical stimulation. To study the termination of contractile response to exogenous NA, the oil immersion technique was used to determine the time for half-relaxation. The experiments were performed on strips with or without treatment with cocaine 10 mumol/l (to inhibit neuronal uptake) plus U-0521 100 mumol/l (to inhibit catechol-O-methyl transferase) before and after exposure to MAO inhibitors. Clorgyline, but not (-)deprenyl enhanced significantly the time for half-relaxation of the strips, whether cocaine was present or not. To study the metabolism and accumulation of exogenous 3H-NA, the strips were incubated (with or without preincubation with cocaine) with 3H-NA 0.23 mumol/l and 2.3 mumol/l in the presence and in the absence of MAO inhibitors. The formation of deaminated metabolites was significantly reduced by clorgyline, but not by deprenyl. To study the metabolism of 3H-NA released by electrical stimulation, the strips were incubated with 3H-NA 1.4 mumol/l. In the presence of cocaine and U-0521, field stimulation was applied during two periods of 5 min (10 Hz, 100 V, 2 ms), in the absence or presence of MAO inhibitors. Under these experimental conditions clorgyline, but not deprenyl, abolished DOPEG formation, without affecting the other metabolites.(ABSTRACT TRUNCATED AT 250 WORDS)