The role of monoamine oxidase in the response of the isolated central artery of the rabbit ear to tyramine

Release of endogenous noradrenaline from an isolated elastic artery

1. The overflow of noradrenaline following field stimulation of vasoconstrictor nerves has been studied in isolated preparations of the guineapig thoracic aorta.2. Stimulation with trains of 3000 pulses at 5 and 25 pulses.s(-1) resulted in mean overflows of 0.15 and 0.17 (ng.g(-1)).pulse(-1) respectively.3. These overflows were not affected by pre-treatment of the aortae with the inhibitor of neuronal amine re-uptake desmethylimipramine (10(-5)M) or with the inhibitors of extraneuronal amine uptake metanephrine (5 x 10(-4)M) or desoxycorticosterone (2.5 x 10(-5)M).4. In contrast, inhibition of monoamine oxidase and catechol-O-methyl transferase with pargyline (1 x 10(-4)M) and tropolone (2.5 x 10(-4)M) caused the mean overflows to be increased to 0.27 and 0.31 (ng.g(-1)).pulse(-1) in response to stimulation at 5 and 25 pulses.s(-1) respectively. Inhibition of either enzyme alone had little effect.5. It is concluded that in the guinea-pig aorta nervously released noradrenaline is inactivated primarily by enzymic metabolism, while neural re-uptake is insignificant. This situation is the reverse of that existing in the uterine artery of the same species.6. Phenoxybenzamine (10(-5)M) also caused pronounced increases in overflow at both high and low frequencies of stimulation. This could be due to an effect of phenoxybenzamine on release of noradrenaline or on some tissue uptake mechanism distinct from the normal neuronal and extraneuronal pathways.7. Calculations based on the overflow at 5 pulses.s(-1) following total enzyme inhibition indicated that a single stimulating pulse liberated 2.1 x 10(-4) of the total transmitter store.

The influence of amine metabolizing enzymes on the pharmacology of tyramine in the isolated perfused mesenteric arterial bed of the rat

1. The pressor response to the infusion of tyramine (Tyr) into the isolated perfused mesenteric arterial bed of the rat has been studied at both a low and a high dose (0.2 and 2.0 mumol) and the effect of monoamine oxidase-A (MAO-A) and semicarbazide-sensitive amine oxidase (SSAO) inhibition was examined. Very little MAO-B activity is found in homogenates of this tissue when Tyr is used as substrate. 2. Inhibition of SSAO by treating rats with 1 mg kg-1 (E)-2-(3',4'-dimethoxyphenyl)-3-fluoroally lamine (MDL 72145) 1 h before dissection, had no significant effect on the maximum pressure attained or the area under the curve (AUC) of the response to both low and high doses of Tyr. Inhibition of MAO-A, by inclusion of 10 microM clorgyline in the perfusing fluid, resulted in no significant potentiation at both low or high doses of Tyr. The inhibition of both these enzymes together substantially increased the AUC of the pressor response. 3. Cocaine (3 microM) significantly potentiated the responses to adrenaline (Ad). At this dose, cocaine significantly reduced the peak height and the AUC of the responses to both doses of Tyr. 4. Inhibition of extraneuronal uptake mechanisms with corticosterone (29 microM) did not potentiate the response to Ad and did not significantly alter the response to Tyr (low dose). 5. The effects of MDL 72145 and clorgyline on the directly acting amine, Ad, were studied. MDL 72145 caused a small but significant increase in the EC50 and in the maximum response to Ad, whilst clorgyline (10 microM) increased the EC50 value slightly and decreased the maximum response.

Determinants of smooth muscle sensitivity

1. Deviation supersensitivity and post-junctional supersensitivity are important determinants in the control of the response of smooth muscle. 2. Post-junctional supersensitivity is a process whereby effector cells have augmented responsiveness as a consequence of an impaired physiological stimulus. Deviation supersensitivity which occurs as a result of inhibition of sites of loss or inactivation of the transmitter substance noradrenaline and its analogues also produces an enhanced response of smooth muscle cell preparations. 3. The enzymes monoamine oxidase and catechol-O-methyl transferase are irreversible site of loss of noradrenaline. Functional impairment of these enzymes results in deviation supersensitivity. 4. The complexities involved in unravelling the contribution of noradrenaline metabolizing enzymes to deviation supersensitivity are highlighted with emphasis upon the pioneering work of Professor de la Lande in this field.

Effect of progesterone on the metabolism of noradrenaline in rabbit uterine endometrium and myometrium

The metabolism of (-)-3H-noradrenaline was examined in the endometrium and the myometrium from rabbits which had received 17 beta-oestradiol, either alone (oestrogen-dominated) or with progesterone (progesterone-dominated). The progesterone treatment resulted in a 2.5-fold increase in 3H-NMN formation in the endometrium, with no change in 3H-DOPEG, 3H-DOMA or 3H-OMDA formation. In the myometrium, progesterone caused a 5-fold increase in 3H-NMN formation and a 2.5-fold increase in 3H-OMDA formation, but did not affect 3H-DOPEG or 3H-DOMA formation. In the progesterone-dominated endometrium, both 3H-NMN and 3H-OMDA formation were strongly inhibited by cocaine 30 mumol/l. When O-methylation was inhibited by a COMT inhibitor, cocaine prevented the resultant increases in deamination of noradrenaline to 3H-DOPEG and in the accumulation of 3H-noradrenaline by the tissue. The 3H-noradrenaline which accumulated in endometria, in which both MAO and COMT were inhibited, was firmly bound; desipramine 3 mumol/l and (+)-amphetamine 10 mumol/l were equieffective with cocaine 30 mumol/l in inhibiting the accumulation. Cocaine 30 mumol/l was without effect on 3H-NMN and 3H-OMDA formation in the progesterone-dominated myometrium, nor did it prevent the increase in 3H-DOPEG formation produced by COMT inhibition. Fluorescent histochemical analysis of the endometrium indicated that the epithelial cells of the endometrial glands were the site of cocaine-sensitive noradrenaline accumulation. It is concluded that progesterone stimulates O-methylation in the endometrium and myometrium in different ways.(ABSTRACT TRUNCATED AT 250 WORDS)

Neuronal deamination of endogenous and exogenous noradrenaline in the mesenteric artery of the spontaneously hypertensive rat

The noradrenaline (NA) content of the mesenteric arteries from spontaneously-hypertensive rats (SHR) are greater than those in arteries from normotensive Kyoto Wistar rats (WKY). The possibility that impaired neuronal monoamine oxidase (MAO) activity in mesenteric arteries from SHR rats was responsible for the differences in NA content was explored. The in-vitro formation of dihydroxyphenylethylene glycol (DOPEG) by intact segments of mesenteric arteries was used as an index of neuronal MAO activity. There were no differences in the production of DOPEG from endogenous NA by arteries from normotensive and hypertensive rats. Moreover, the formation of DOPEG from exogenous NA was similar in arteries from SHR and WKY rats. The neuronal uptake of NA was indistinguishable between mesenteric arteries from SHR and WKY rats. The results argue against an impairment of neuronal MAO in contributing to the enhanced content of NA in the mesenteric artery of the SHR rat.

Effects of nialamide on responses of dog isolated arteries to tyramine and transmural electrical stimulation

In dog mesenteric arteries, nialamide (10(-5) M), a monoamine oxidase inhibitor, potentiated contractile response to tyramine but not to noradrenaline or transmural electrical stimulation (TES). Bretylium, desipramine or prior reserpinization inhibited the potentiating action of nialamide on the tyramine-induced contraction. Contractions induced by octopamine were not reduced by prior reserpinization. In the coronary artery, relaxing responses to tyramine were potentiated but those to noradrenaline and TES were not potentiated by nialamide. In the cerebral artery, nialamide failed to potentiate tyramine-induced contraction. The functional role of intraneuronal monoamine oxidase in sympathomimetic effects is discussed.

The pressor actions of noradrenaline, angiotensin II and saralasin in chronic autonomic failure treated with fludrocortisone

1 Treatment of postural hypotension due to chronic autonomic failure with fludrocortisone increased the pressor sensitivity to intravenous noradrenaline. Fludrocortisone increased the blood pressure in the standing but not the lying position. These effects of fludrocortisone may be the result of increased sensitivity of vascular receptors to noradrenaline. 2 The pressor action of angiotensin II, to which patients were supersensitive, may have involved the stimulation of alpha-adrenoceptors since it was partially antagonised by phentolamine. 3 Saralasin had a marked, paradoxical, pressor effect. This may have been mediated by vascular alpha-adrenoceptors because log dose-response curves of saralasin-induced increases in systolic pressure were shifted to the right in a parallel fashion after phentolamine. 4 Fludrocortisone treatment increased the pressor sensitivity to intravenous saralasin but not to angiotensin-II.

Monoamine oxidase in rat arteries: evidence for different forms and selective localization

1. Two forms of monoamine oxidase activity were differentiated in rat mesenteric and femoral artery by means of substrate and inhibitor specificities: one form deaminated tyramine, 5-hydroxytryptamine and noradrenaline and was highly sensitive to pargyline and clorgyline but resistant towards carbonyl reagents. This form resembled type A monoamine oxidase previously described. The other deaminated tyramine but not 5-hydroxytryptamine or noradrenaline and was inhibited by carbonyl reagents but not by clorgyline or pargyline.2. About one third of the total monoamine oxidase in homogenates of rat mesenteric artery was recovered in a 10(5)g supernatant. Both forms were partially soluble, but relatively less of the type A activity was recovered in the soluble fraction.3. Chemical sympathectomy with 6-hydroxydopamine resulted in a loss of 59% of monoamine oxidase activity in the mesenteric artery. There was a selective loss of type A activity, as revealed by the 70% decrease in 5-hydroxytryptamine deaminating ability and by the marked decrease in clorgyline sensitivity. The second monoamine oxidase species was resistant to 6-hydroxydopamine. The soluble activity was not affected by chemical sympathectomy. Most of the transmitter-specific monoamine oxidase of the arterial wall was localized within the adrenergic nerve endings. Our observations are consistent with the hypothesis that extraneuronal monoamine oxidase plays only a minor role in metabolizing noradrenaline in sympathetically innervated tissues.4. Plasma amine oxidase might originate from the arterial wall since it has similar characteristics to that found in the mesenteric artery.

Resistance of noradrenaline in blood vessels to depletion by 6-hydroxydopamine or immunosympathectomy

1. The degree of the decrease in the noradrenaline concentrations caused by 6-hydroxydopamine or immunosympathectomy was different in different areas of the cardiovascular system.2. In rats or guinea-pigs 6-hydroxydopamine depleted the noradrenaline content of the heart by 90%, of the mesenteric vein by 80% and of the mesenteric artery and aorta by 30-60%. Immunosympathectomy elicited a 70% reduction in the cardiac noradrenaline but only a 50% reduction in the noradrenaline of the blood vessels of the rat.3. The tyrosine hydroxylase activity of the heart, blood vessels, or adrenal glands was not significantly altered 2 weeks after 6-hydroxydopamine. Nor was the monoamine oxidase activity in heart or blood vessels changed.4. The inconsistent ability of both 6-hydroxydopamine and immunosympathectomy to abolish experimental hypertension may be due to the partial persistence of noradrenaline and functional sympathetic nervous system activity in the blood vessels.