Influence of vesicular storage and monoamine oxidase activity on [11C]phenylephrine kinetics: studies in isolated rat heart.
[11C]Phenylephrine (PHEN) is a radiolabeled analogue of norepinephrine that is transported into cardiac sympathetic nerve varicosities by the neuronal norepinephrine transporter and taken up into storage vesicles localized within the nerve varicosities by the vesicular monoamine transporter. PHEN is structurally related to two previously developed sympathetic nerve markers: [11C]-meta-hydroxyephedrine and [11C]epinephrine. To better characterize the neuronal handling of PHEN, particularly its sensitivity to neuronal monoamine oxidase (MAO) activity, kinetic studies in an isolated working rat heart system were performed. METHODS: Radiotracer was administered to the isolated working heart as a 10-min constant infusion followed by a 110-min washout period. Two distinctly different approaches were used to assess the sensitivity of the kinetics of PHEN to MAO activity. In the first approach, oxidation of PHEN by MAO was inhibited at the enzymatic level with the MAO inhibitor pargyline. In the second approach, the two hydrogen atoms on the a-carbon of the side chain of PHEN were replaced with deuterium atoms ([11C](-)-alpha-alpha-dideutero-phenylephrine [D2-PHEN]) to inhibit MAO activity at the tracer level. The importance of vesicular uptake on the kinetics of PHEN and D2-PHEN was assessed by inhibiting vesicular monoamine transporter-mediated storage into vesicles with reserpine. RESULTS: Under control conditions, PHEN initially accumulated into the heart at a rate of 0.72+/-0.15 mL/min/g wet. Inhibition of MAO activity with either pargyline or di-deuterium substitution did not significantly alter this rate. However, MAO inhibition did significantly slow the clearance of radioactivity from the heart during the washout phase of the study. Blocking vesicular uptake with reserpine reduced the initial uptake rates of PHEN and D2-PHEN, as well as greatly accelerated the clearance of radioactivity from the heart during washout. CONCLUSION: These studies indicate that PHEN kinetics are sensitive to neuronal MAO activity. Under normal conditions, efficient vesicular storage of PHEN serves to protect the tracer from rapid metabolism by neuronal MAO. However, it is likely that leakage of PHEN from the storage vesicles and subsequent metabolism by MAO lead to an appreciable clearance of radioactivity from the heart. (+info)
Serotonin metabolism in rat mesangial cells: involvement of a serotonin transporter and monoamine oxidase A.
BACKGROUND: Serotonin is one of the factors regulating mesangial cell proliferation, and convergent evidence supports its involvement in the development of glomerulonephritis. In this study, we identified a serotonin transporter and the amine-degrading enzyme monoamine oxidases (MAOs) in mesangial cells, and we studied their involvement in serotonin degradation. METHODS: MAOs were characterized in membrane preparations and intact mesangial cells by enzyme assay using [14C]5-hydroxytryptamine and [14C]beta-phenylethylamine as specific substrates for MAO-A and MAO-B, respectively, and by Western blot analysis. The expression of a serotonin transporter was determined by [14C]5-hydroxytryptamine uptake experiments and Western blot. Mesangial cell proliferation was measured by BrdU incorporation. RESULTS: Quantitation of the MAO isoforms by enzyme assay and Western blot analysis showed that MAO-A was largely predominant in mesangial cells, accounting for approximately 90% of the total enzyme population. The MAO substrate [14C]serotonin was transported into mesangial cells by a saturable uptake system (Vmax 310 +/- 36 pmol/30 min/mg protein; Km 5.9 +/- 1.4 microM) displaying the pharmacological properties of a serotonin transporter. The expression of a serotonin transporter was confirmed by Western blot analysis. MAO activity measured in intact cells showed that after accumulation into mesangial cells, [14C]serotonin was metabolized by MAO-A. Finally, serotonin-mediated mesangial cell proliferation was significantly increased after irreversible MAO inhibition. CONCLUSIONS: Our results suggest that serotonin concentration and function in glomeruli may be regulated in part by its transport into mesangial cells and degradation by MAO-A. (+info)
Characterization of extracellular dopamine clearance in the medial prefrontal cortex: role of monoamine uptake and monoamine oxidase inhibition.
In vitro rotating disk electrode (RDE) voltammetry and in vivo microdialysis were used to characterize dopamine clearance in the rat medial prefrontal cortex (mPFC). RDE studies indicate that inhibition by cocaine, specific inhibitors of the dopamine transporter (DAT) and norepinephrine transporter (NET), and low Na(+) produced a 50-70% decrease in the velocity of dopamine clearance. Addition of the monoamine (MAO) inhibitors, l-deprenyl, clorgyline, pargyline, or in vivo nialamide produced 30-50% inhibition. Combined effects of uptake inhibitors with l-deprenyl on dopamine clearance were additive (up to 99% inhibition), suggesting that at least two mechanisms may contribute to dopamine clearance. Dopamine measured extracellularly 5 min after exogenous dopamine addition to incubation mixtures revealed that most conditions of DAT/NET inhibition did not produce elevated dopamine levels above controls. Inhibition of MAO produced elevated dopamine levels only after long-term, but not short-term, incubation in vitro. Short-term incubation of l-deprenyl combined with DAT and NET uptake inhibitors increased dopamine above control levels, consistent with more than one mechanism of dopamine clearance. Local infusion of pargyline (100 or 300 microm) into the mPFC or striatum via microdialysis produced more pronounced and immediate increases in mPFC dopamine levels compared with striatum. Furthermore, dopamine elevation in the mPFC was not accompanied by a decrease in the dopamine metabolites, 3,4-dihydroxyphenylacetic acid and homovanillic acid, as found in the striatum. These findings may have revealed a unique mechanism of mPFC dopamine clearance and therefore contribute to the understanding of multiple behaviors that involve mPFC dopamine transmission, such as schizophrenia, drug abuse, and working memory function. (+info)
Effects of acute cocaine treatment on the turnover of 5-hydroxytryptamine in the rat brain.
1. The effects of cocaine (20 mg/kg s.c.) on 5-hydroxytryptamine (5-HT) turnover were examined in rats. 2. In vivo cocaine administration resulted in decreased turnover of 5-HT, as indicated by the decreased accumulation of 5-HT after pargyline administration and the decreased accumulation of 5-hydroxyindoleacetic acid (5-HIAA) following probenecid injection. 3. A time-related decrease in 5-HIAA concentrations and a small fall in 5-HT concentrations in the whole brain were observed following the acute administration of cocaine hydrochloride (20 mg/kg). Tryptophan levels were found to be slightly decreased in the brain. 4. Enhanced reactivity, but neither stereotypy nor hyperthermia, was observed following cocaine injection (20 mg/kg). 5. It is concluded that cocaine inhibits the turnover of brain 5-HT and that this action of cocaine may be responsible for the differences in a number of pharmacological effects between cocaine and amphetamine. (+info)
Inhibition of rat liver microsomal CYP1A2 and CYP2B1 activity by N-(2-heptyl)-N-methyl-propargylamine and by N-(2-heptyl)-propargylamine.
(R)-N-(2-Heptyl)-N-methyl-propargylamine (R-2HMP) and (R)-N-(2-heptyl)-propargylamine (R-2HPA) are analogs of R-deprenyl. R-Deprenyl, a selective monoamine oxidase B inhibitor, is a mechanism-based inactivator of purified CYP2B1. The aim of the present study was to determine whether R-2HMP and R-2HPA behaved like deprenyl with respect to inhibiting cytochrome P450 (CYP450) enzyme activity. The activities of CYP1A2 and CYP1A1 were assessed by measuring the deethylation of 7-ethoxyresorufin by liver microsomes obtained from control and beta-naphthoflavone-treated female Wistar rats, respectively. CYP2B1 activity was assessed by measuring depentylation of 7-pentoxyresorufin by liver microsomes obtained from phenobarbital-treated rats. The activity of CYP1A1 was unaffected by 100 microM concentrations of R-deprenyl, R-2HMP, or R-2HPA. In contrast, the activities of CYP1A2 and CYP2B1 were significantly decreased. In general, the percentage of CYP1A2 activity remaining in the presence of 100 microM of one of these propargylamines ranged from 45 to 56%, whereas 10% or less of CYP2B1 activity remained. No marked differences between the various propargylamines were observed. The IC(50) values for the inhibition of CYP2B1 activity by R-deprenyl, R-2HMP, and R-2HPA were found to be 2.6, 8.5, and 3.6 microM, respectively. The S-enantiomers of deprenyl, 2HMP, and 2HPA also inhibited the activity of microsomal CYP2B1. R-2HMP, R-2HPA, and S-2HPA were found to be mechanism-based inactivators of CYP2B1 activity. The inactivation constants k(inact) and K(I) were found to be as follows: R-deprenyl, 1.3 microM and 0.32 min(-1); R-2HMP, 0.8 microM and 0.08 min(-1); R-2HPA, 0.5 microM and 0.36 min(-1); and S-2HPA, 0.24 microM and 0.18 min(-1). (+info)
Effect of fluphenazine on tissue noradrenaline concentrations and its interaction with pargyline.
Fluphenazine caused a small decrease in the noradrenaline (NA) concentrations of the rat brain, possibly by impairing granular amine storage. The drug diminished the rise in brain and heart NA concentrations induced by pargyline, suggesting that it might posses inhibitory properties on neuronal uptake mechanisms and/or NA synthesis. Fluphenazine abolished conditioned avoidance responses in rats, an effect which was maintained after the concomitant administration of pargyline, when NA concentrations remained high. This suggests that the fluphenazine-induced sedation is not mediated via its effect on brain NA content, but is possibly due to the effect of the drug on NA turnover rates in the brain. (+info)
Oxygen dependency of monoamine oxidase activity in the intact lung.
Hydrogen peroxide generated by monoamine oxidase (MAO)-mediated deamination of biogenic amines has been implicated in cell signaling and oxidative injury. Because the pulmonary endothelium is a site of metabolism of monoamines present in the venous return, this brings into question a role for MAO in hyperoxic lung injury. The objective of this study was to evaluate the O(2) dependency of the MAO reaction in the lung. To this end, we measured the pulmonary venous effluent concentrations of the MAO substrate [(14)C]phenylethylamine and its metabolite [(14)C]phenylacetic acid after the bolus injection of either phenylethylamine or phenylacetic acid into the pulmonary artery of perfused rabbit lungs over a range of PO(2) values from 16 to 518 Torr. The apparent Michaelis constant for O(2) was approximately 18 microM, which is more than an order of magnitude less that measured for purified MAO. The results suggest a minimal influence of high O(2) on MAO activity in the normal lung and demonstrate the importance of measuring reaction kinetics in the intact organ. (+info)
Oxidation products of polyamines induce mitochondrial uncoupling and cytochrome c release.
Spermine is shown to uncouple isolated mitochondria and to trigger the selective release of cytochrome c. Pargyline, an inhibitor of amine oxidase (AO), fully prevented these effects of spermine, which instead were potentiated by exogenous AO. Hydrogen peroxide, an oxidation product of spermine, mimicked the effects of spermine on mitochondria, while the addition of catalase prevented them. Spermidine and putrescine also caused mitochondrial uncoupling and triggered cytochrome c release, with a potency which correlated with the substrate preference of mitochondrial AO. Pargyline protected human lymphoma U937 cells against UVB-induced apoptosis, by reducing AO activity, mitochondrial uncoupling and cytochrome c release. (+info)