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

Effects of clorgyline on the release of noradrenaline from rat vas deferens

Release of endogenous noradrenaline (NA) and functional activity of presynaptic alpha 2-adrenoceptors were measured in isolated rat vas deferens following acute (2 mg/kg) or chronic (21 * 1 mg/kg daily, i.p.) treatment with clorgyline. Noradrenaline tissue content was higher in rats treated chronically with clorgyline than in those treated acutely. In vitro experiments done in the presence of 1 microM desipramine and 0.5 microM yohimbine showed that NA release, elicited by electrical field stimulation, was higher in chronic clorgyline rats than in controls, while no significant difference was found between acute clorgyline and control rats. Yohimbine enhanced evoked release of NA in all treatment groups, provided that desipramine was present in the Krebs solution, and the enhancement was non significantly higher in chronic clorgyline than in acute clorgyline and control rats. Efflux of 3,4-dihydroxyphenylglycol was lower in chronic clorgyline rats than in other groups. No difference was found between the treatment groups in a 1 min [3H]NA uptake into the tissue, nor in the ability of desipramine (1 microM) to block the uptake. The results indicate that following chronic treatment with clorgyline, evoked release of NA increased, and there was no reduction in the ability of the presynaptic alpha 2-adrenoceptor inhibitory mechanism to reduce nerve stimulation induced release of NA.

Clinical pharmacology of moclobemide, a new reversible monoamine oxidase inhibitor

The clinical pharmacology of the new reversible monoamine oxidase (MAO) inhibitor, moclobemide, was examined in three separate studies in healthy male volunteers. In a single oral dose study, moclobemide (25-150 mg) was rapidly absorbed from the gastrointestinal tract and had a relatively short plasma half-life (mean 1.3 h after 150 mg). A decrease in the plasma concentrations of the noradrenaline metabolite 4- hydroxy-3-methoxyphenylglycol (HMPG), however, indicated a longer time to peak pharmacodynamic effect and longer duration of activity. Assay of platelet MAO activity did not reveal any evidence of irreversible inhibition of the B form of the isoenzyme. Single oral doses of moclobemide (150 and 300 mg) significantly lowered the threshold to the cardiovascular effects ('cheese reaction') of intravenous tyramine. However, after repeated administration of 100 mg three times daily for over 2 weeks, moclobemide caused significantly less potentiation than did phenelzine (15 mg three times per day) on the cardiovascular effects of oral tyramine, a clinically more relevant model. The MAO-B inhibitor, selegiline (5 mg once daily), also lowered the oral tyramine threshold significantly. Moclobemide was generally well tolerated by these healthy volunteers.

The actions of 5-hydroxytryptamine receptor agonists and antagonists at pre- and postjunctional level on the canine saphenous vein

The prejunctional and postjunctional 5-HT receptors of the canine saphenous vein were studied. The release of 3H-noradrenaline (3H-NA) from incubated saphenous vein strips was inhibited by 5-hydroxytryptamine (5-HT) in a concentration-dependent way (5-HT concentrations: 0.01, 0.1 and 1.0 mumol.l-1), but not by the selective 5-HT1A agonist 8-hydroxy-dipropylaminotetralin (8-OH-DPAT; 1 and 10 mumol.l-1). The inhibitory effect of 5-HT was antagonized by metitepine and methysergide, but not by yohimbine, (-)-pindolol or ketanserin. In strips preincubated with 5-HT (1.2 mumol.l-1), the fractional release of 3H-NA was slightly reduced (paired experiments). 5-HT and 8-OH-DPAT caused concentration-dependent contractions of the saphenous smooth muscle. A parallel shift of the concentration-response curve for 8-OH-DPAT to the right was caused by metitepine and yohimbine, but not by ketanserin. The contractions caused by 5-HT were antagonized by metitepine and yohimbine (parallel displacement of the curves to the right), as well as by ketanserin and methysergide (with a depression of the upper part of the curve). Blockade of alpha-adrenoceptors (due to prazosin plus a low concentration of yohimbine) also resulted in a weak antagonistic effect. Ketanserin and metitepine displaced the noradrenaline concentration-response curve to the right. We conclude that the saphenous vein of the dog is endowed with prejunctional receptors of the 5-HT1 type which can not be classified as belonging either to the 1A or 1B subtype; and that at the postjunctional level 5-HT1 (possibly of the 1D subtype) and 5-HT2 receptors are present.(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.

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

The influence of specific inhibitors of MAO A (clorgyline) and MAO B [(-)deprenyl] on the metabolism of normetanephrine (NMN) in strips of canine saphenous vein was studied, both in the absence and in the presence of inhibitors of neuronal (cocaine) and extraneuronal (hydrocortisone) uptake. Moreover, the formation of metabolites of noradrenaline and of NMN by saphenous vein homogenates and the influence of clorgyline or (-)deprenyl on this formation are described. Clorgyline reduced to the same degree (by about 70%) the formation of methoxy-hydroxy-phenylglycol (MOPEG) and of vanillylmandelic acid (VMA) in strips incubated with NMN, whereas (-)deprenyl reduced by about 50% the formation of MOPEG and had no effect on VMA production. Hydrocortisone had effects very similar to those of (-)deprenyl. Saphenous vein homogenates (O-methylation inhibited), deaminated both noradrenaline and NMN; clorgyline and (-)deprenyl reduced the formation of metabolites of both noradrenaline and NMN. It is concluded that both MAO A and B are able to deaminate noradrenaline and NMN, but that in the intact tissue the former has no access to MAO B. Even in intact tissues MAO B may play a role in the metabolism (but not in the inactivation) of noradrenaline by deaminating the NMN formed from noradrenaline and giving preferentially origin to MOPEG.