A novel G protein-coupled receptor of Schistosoma mansoni (SmGPR-3) is activated by dopamine and is widely expressed in the nervous system

Schistosomes have a well developed nervous system that coordinates virtually every activity of the parasite and therefore is considered to be a promising target for chemotherapeutic intervention. Neurotransmitter receptors, in particular those involved in neuromuscular control, are proven drug targets in other helminths but very few of these receptors have been identified in schistosomes and little is known about their roles in the biology of the worm. Here we describe a novel Schistosoma mansoni G protein-coupled receptor (named SmGPR-3) that was cloned, expressed heterologously and shown to be activated by dopamine, a well established neurotransmitter of the schistosome nervous system. SmGPR-3 belongs to a new clade of "orphan" amine-like receptors that exist in schistosomes but not the mammalian host. Further analysis of the recombinant protein showed that SmGPR-3 can also be activated by other catecholamines, including the dopamine metabolite, epinine, and it has an unusual antagonist profile when compared to mammalian receptors. Confocal immunofluorescence experiments using a specific peptide antibody showed that SmGPR-3 is abundantly expressed in the nervous system of schistosomes, particularly in the main nerve cords and the peripheral innervation of the body wall muscles. In addition, we show that dopamine, epinine and other dopaminergic agents have strong effects on the motility of larval schistosomes in culture. Together, the results suggest that SmGPR-3 is an important neuronal receptor and is probably involved in the control of motor activity in schistosomes. We have conducted a first analysis of the structure of SmGPR-3 by means of homology modeling and virtual ligand-docking simulations. This investigation has identified potentially important differences between SmGPR-3 and host dopamine receptors that could be exploited to develop new, parasite-selective anti-schistosomal drugs.

Kinetic cooperativity of tyrosinase. A general mechanism

Tyrosinase shows kinetic cooperativity in its action on o-diphenols, but not when it acts on monophenols, confirming that the slow step is the hydroxylation of monophenols to o-diphenols. This model can be generalised to a wide range of substrates; for example, type S(A) substrates, which give rise to a stable product as the o-quinone evolves by means of a first or pseudo first order reaction (α-methyl dopa, dopa methyl ester, dopamine, 3,4-dihydroxyphenylpropionic acid, 3,4-dihydroxyphenylacetic acid, α-methyl-tyrosine, tyrosine methyl ester, tyramine, 4-hydroxyphenylpropionic acid and 4-hydroxyphenylacetic acid), type S(B) substrates, which include those whose o-quinone evolves with no clear stoichiometry (catechol, 4-methylcatechol, phenol and p-cresol) and, lastly, type S(C) substrates, which give rise to stable o-quinones (4-tert-butylcatechol/4-tert-butylphenol).

Metabolism is required for the expression of ecstasy-induced cardiotoxicity in vitro

Cardiovascular complications associated with 3,4-methylenedioxymethamphetamine (MDMA, ecstasy) abuse have increasingly been reported. The indirect effect of MDMA mediated by a sustained high level of circulating biogenic amines may contribute to the cardiotoxic effects, but other factors, like the direct toxic effects of MDMA and its metabolites in cardiac cells, remain to be investigated. Thus, the objective of the present in vitro study was to evaluate the potential cardiotoxic effects of MDMA and its major metabolites 3,4-methylenedioxyamphetamine (MDA), N-methyl-alpha-methyldopamine (N-Me-alpha-MeDA), and alpha-methyldopamine (alpha-MeDA) using freshly isolated adult rat cardiomyocytes. The cell suspensions were incubated with these compounds in the final concentrations of 0.1, 0.2, 0.4, 0.8, and 1.6 mM for 4 h. alpha-MeDA, N-Me-alpha-MeDA, and their respective aminochromes (oxidation products) were quantified in cell suspensions by HPLC-DAD. The toxic effects were evaluated at hourly intervals for 4 h by measuring the percentage of cells with normal morphology, glutathione (GSH), and glutathione disulfide (GSSG); intracellular Ca(2+), ATP, and ADP; and the cellular activities of glutathione peroxidase, glutathione reductase, and glutathione-S-transferase. No toxic effects were found after exposure of rat cardiomyocytes to MDMA or MDA at any of the tested concentrations for 4 h. In contrast, their catechol metabolites N-Me-alpha-MeDA and alpha-MeDA induced significant toxicity in rat cardiomyocytes. The toxic effects were characterized by a loss of normal cell morphology, which was preceded by a loss of GSH homeostasis due to conjugation of GSH with N-Me-alpha-MeDA and alpha-MeDA, sustained increase of intracellular Ca(2+) levels, ATP depletion, and decreases in the antioxidant enzyme activities. The oxidation of N-Me-alpha-MeDA and alpha-MeDA into the toxic compounds N-methyl-alpha-methyldopaminochrome and alpha-methyldopaminochrome, respectively, was also verified in cell suspensions incubated with these MDMA metabolites. The results obtained in this study provide evidence that the metabolism of MDMA into N-Me-alpha-MeDA and alpha-MeDA is required for the expression of MDMA-induced cardiotoxicity in vitro, being N-Me-alpha-MeDA the most toxic of the studied metabolites.

Stereochemical analysis of 3,4-methylenedioxymethamphetamine and its main metabolites in human samples including the catechol-type metabolite (3,4-dihydroxymethamphetamine)

3,4-Methylenedioxymethamphetamine (MDMA; "ecstasy") is a designer drug commonly misused in large segments of young populations. MDMA is usually formulated in tablets of its racemate (1:1 mixture of its enantiomers) in doses ranging from 50 to 200 mg. MDMA has an enantioselective metabolism, the (S)-enantiomer being metabolized faster than the (R)-enantiomer. Different pharmacologic properties have been attributed to each enantiomer. The carbon responsible for MDMA chirality is preserved along its metabolic disposition. An analytical method has been developed to determine MDMA enantiomers and those from its major metabolites, 3,4-methylenedioxyamphetamine (MDA), 3,4-dihydroxymeth-amphetamine (HHMA), and 4-hydroxy-3-methoxymethamphet-amine (HMMA). It has been applied to the analysis of plasma and urine samples from healthy recreational users of MDMA who participated voluntarily in a clinical trial and received 100 mg (R,S)-MDMA. HCl orally. (R)/(S) ratios both in plasma (0-48 h) and urine (0-72 h) for MDMA and MDA were >1 and <1, respectively. Ratios corresponding to HHMA and HMMA, close to unity, deviate from theoretical expectations and are most likely explained by the ability of MDMA to autoinhibit its own metabolism. The short elimination half-life of (S)-MDMA (4.8 h) is consistent with the subjective effects and psychomotor performance reported in subjects exposed to MDMA, whereas the much longer half-life of the (R)-enantiomer (14.8 h) correlates with mood and cognitive effects experienced on the next days after MDMA use.

Hepatotoxicity of 3,4-methylenedioxyamphetamine and ?-methyldopamine in isolated rat hepatocytes: formation of glutathione conjugates

The amphetamine designer drugs 3,4-methylenedioxymethamphetamine (MDMA or "ecstasy") and its N-demethylated analogue 3,4-methylenedioxyamphetamine (MDA or "love") have been extensively used as recreational drugs of abuse. MDA itself is a main MDMA metabolite. MDMA abuse in humans has been associated with numerous reports of hepatocellular damage. Although MDMA undergoes extensive hepatic metabolism, the role of metabolites in MDMA-induced hepatotoxicity remains unclear. Thus, the aim of the present study was to evaluate the effects of MDA and alpha-methyldopamine (alpha-MeDA), a major metabolite of MDA, in freshly isolated rat hepatocyte suspensions. The cells were incubated with MDA or alpha-MeDA at final concentrations of 0.1, 0.2, 0.4, 0.8, or 1.6 mM for 3 h. The toxic effects induced following incubation of hepatocyte suspensions with these metabolites were evaluated by measuring cell viability, the extent of lipid peroxidation, levels of glutathione (GSH) and glutathione disulfide (GSSG), the formation of GSH conjugates, and the activities of GSSG reductase (GR), GSH peroxidase (GPX), and GSH S-transferase (GST). MDA induced a concentration- and time-dependent GSH depletion, but had a negligible effect on lipid peroxidation, cell viability, or on the activities of GR, GPX, and GST. In contrast, alpha-MeDA (1.6 mM, 3 h) induced a marked depletion of GSH accompanied by a loss on cell viability, and decreases in GR, GPX and GST activities, although no significant effect on lipid peroxidation was found. For both metabolites, GSH depletion was not accompanied by increases in GSSG levels; rather, 2-(glutathion- S-yl)-alpha-MeDA and 5-(glutathion- S-yl)-alpha-MeDA were identified by HPLC-DAD/EC within cells incubated with MDA or alpha-MeDA. The results provide evidence that one of the early consequences of MDMA metabolism is a disruption of thiol homeostasis, which may result in loss of protein function and the initiation of a cascade of events leading to cellular damage.

Synthesis, in vitro formation, and behavioural effects of glutathione regioisomers of alpha-methyldopamine with relevance to MDA and MDMA (ecstasy)

Administration of 3,4-methylenedioxymethamphetamine (MDMA) or 3,4-methylenedioxyamphetamine (MDA) to rats produces serotonergic nerve terminal degeneration. However, they are not neurotoxic when injected directly into the brain, suggesting the requirement for peripheral metabolism of MDMA to a neurotoxic metabolite. Alpha-methyldopamine (alpha-MeDA) is a major metabolite of MDA. There are indications that a glutathione metabolite of alpha-MeDA and/or 3,4-dihydroxymethamphetamine may be responsible for the neurotoxicity and some of the behavioural effects produced by MDMA and/or MDA. The present study details the synthesis, purification and separation of the 5-(glutathion-S-yl)-alpha-MeDA and 6-(glutathion-S-yl)-alpha-MeDA regioisomers of alpha-MeDA. Incubation of MDA with human liver microsomes demonstrated that production of both glutathione adducts are related to cytochrome P450 2D6 isoform activity. Following intracerebroventricular administration (180 nmol) of either GSH adduct into Dark Agouti or Sprague-Dawley rats only 5-(glutathion-S-yl)-alpha-MeDA produced behavioural effects characterised by hyperactivity, teeth chattering, tremor/trembling, head weaving, splayed posture, clonus and wet dog shakes. Pre-treatment with a dopamine receptor antagonist (haloperidol, 0.25 mg/kg; i.p.) attenuated hyperactivity, teeth chattering, low posture and clonus and potentiated splayed postural effects. These results indicate that MDA can be converted into two glutathione regioisomers by human liver microsomes, but only the 5-(glutathion-S-yl)-alpha-MeDA adduct is behaviourally active in the rat.

Synthesis and capillary electrophoretic analysis of enantiomerically enriched reference standards of MDMA and its main metabolites

Enantiomerically-enriched (S)-3,4-methylenedioxymethamphetamine (MDMA) and its main metabolites (S)-4-hydroxy-3-methoxymethamphetamine (HMMA) and (S)-3,4-dihydroxymethamphetamine (HHMA) were prepared for unequivocal identification of the differential enantioselective metabolism of these compounds as well as for its application in the analysis of biological samples. Capillary electrophoresis with cyclodextrin derivatives and a chemical correlation of (S)-MDMA, (S)-HMMA and (S)-HHMA has been performed to assign the absolute stereochemistry of major isomers in analytical standards enriched with such enantiomers.

The neurotoxic effects of methylenedioxymethamphetamine (MDMA) and its metabolites on rat brain spheroids in culture

Rat whole-brain spheroids were used to assess the intrinsic neurotoxicity of methylenedioxy-methamphetamine (MDMA, Ecstasy) and two of its metabolites, dihydroxymethamphetamine (DHMA) and 6-hydroxy-MDMA (6-OH MDMA). Exposure of brain spheroids to MDMA or the metabolite 6-OH MDMA (up to 500 micromol/L) for 5 days in culture did not alter intracellular levels of glutathione (GSH), glial fibrillary acidic protein (GFAP) or serotonin (5-HT). In contrast, exposure to the metabolite DHMA, which can deplete intracellular thiols, significantly increased GSH levels (up to 170% of control) following exposure to 50 and 100 micromol/L DHMA. There was also a significant reduction in the levels of glial fibrillary acidic protein (GFAP) and GSH by DHMA at the highest concentration tested (500 micromol/L) but there was no effect on 5HT. This may constitute a sublethal neurotoxic compensatory response to DHMA in an attempt to replenish depleted intraneural GSH levels following metabolite exposure. Rat whole-brain spheroids may thus be a useful in vitro model to delineate mechanisms and effects of this class of neurotoxin.

The oxidation of dopamine and epinine by the two forms of monoamine oxidase from rat liver

Information on the "in vitro" oxidation of epinine by monoamine oxidase (MAO) compared to dopamine is very poor. The aim of this work was to study the oxidative deamination of epinine and dopamine by rat liver MAO-A and MAO-B. The contributions of MAO-A and B to the metabolism of dopamine (55% and 45%, respectively) and epinine (70% and 30%, respectively) were similar. The results of this study show that epinine is a substrate for both forms of MAO in rat liver, although the contribution of MAO A to the deamination of this secondary amine appears to be slightly more important than that of MAO B.

Metabolism of 5-(glutathion-S-yl)-alpha-methyldopamine following intracerebroventricular administration to male Sprague-Dawley rats

5-(Glutathion-S-yl)-alpha-methyldopamine [5-(GSyl)-alpha-MeDA] is a putative metabolite of the serotonergic neurotoxicants 3,4-(+/-)-(methylenedioxy)amphetamine and 3,4-(+/-)-(methylenedioxy)methamphetamine. Glutathione (GSH) conjugates of several polyphenols are biologically (re)active. Therefore, as part of our studies on the role of 5-(GSyl)-alpha-MeDA in MDA-mediated neurotoxicity, we determined the regional brain metabolism of 5-(GSyl)-alpha-MeDA (720 nmol) following intracerebroventricular administration to male Sprague-Dawley rats. 5-(GSyl)-alpha-MeDA was rapidly cleared from all brain regions examined, and regional differences in the distribution of gamma-glutamyl transpeptidase (gamma-GT) correlated with the formation of 5-(cystein-S-yl)-alpha-methyldopamine (5-[CYS]-alpha-MeDA). We also observed the formation of 5-(N-acetyl-L-cystein-S-yl)-alpha-MeDA (5-[NAC]-alpha-MeDA) in all brain regions, indicating that the brain has the ability to synthesize mercapturic acids. Peak concentrations of 5-(NAC)-alpha-MeDA were found in the order: hypothalamus > midbrain/diencephalon/telencephalon > pons/medulla > hippocampus > cortex > striatum. In contrast to 5-(GSyl)-alpha-MeDA and 5-(CYS)-alpha-MeDA, 5-(NAC)-alpha-MeDA was eliminated relatively slowly from the brain. Differences were also found in cystein conjugate N-acetyltransferase activity in microsomes prepared from the various brain regions, but little difference was observed in brain cytosolic N-acetyl-L-cysteine conjugate N-deacetylase activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Presence of N-methyldopamine in parkinsonian and normal human brains

N-Methyldopamine (epinine) has been identified for the first time in parkinsonian and normal human brains by gas chromatography-mass spectrometry. N-Methylsalsolinol and N-methylnorsalsolinol, which are analogues of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, which produces parkinsonism in humans, may be synthesized from N-methyldopamine by the Pictet-Spengler condensation reaction as an alternative metabolic pathway.

Effects of ibopamine on exercise-induced increase in norepinephrine in normal men

The effects of 100 mg ibopamine, an orally active aselective dopamine (DA) agonist, on plasma catecholamines was evaluated in 8 healthy men during sympathetic stimulation by graded exercise in a single-blind, placebo-controlled cross-over study. The exercise consisted of progressive cycling activity less than or equal to 90% of the previously determined VO2max. Graded exercise resulted in an increase in systolic and mean blood pressure (SBP, MBP), heart rate, norepinephrine (NE) and epinephrine level, with a decrease in diastolic BP (DBP). The increase in NE was significantly blunted by ibopamine as compared with placebo. No differences for BP, heart rate (HR), or epinephrine between placebo- and ibopamine study day were noted. In previous studies, ibopamine decreased resting plasma NE in patients with congestive heart failure (CHF), whereas plasma NE was not altered by ibopamine in healthy volunteers. This different outcome in both categories might therefore be explained by the absence of substantial sympathetic stimulation in normal humans at rest. Because it is reasonable to assume that the effect of ibopamine on systemic and local hemodynamics is negligible as compared with the effect of exercise in the healthy volunteers, the plasma decrease caused by ibopamine is probably related to stimulation of DA2-receptors. In conclusion, ibopamine blunts the increase of plasma NE during graded exercise in healthy men.

Simultaneous determination of free catecholamines and epinine and estimation of total epinine and dopamine in plasma and urine by high-performance liquid chromatography with fluorimetric detection

Epinine (N-methyldopamine) is the pharmacologically active hydrolysis product of the prodrug ibopamine, which is currently being widely studied for the treatment of congestive heart failure. This paper reports a sensitive and reliable method for the simultaneous determination of free catecholamines and epinine in plasma and urine. The compounds are isolated from plasma or urine by a specific liquid-liquid extraction, derivatized with the selective fluorogenic agent 1,2-diphenylethylenediamine, and quantitated by high-performance liquid chromatography with gradient elution and fluorimetric detection. The limits of detection for the derivatized catecholamines and epinine are 0.3-0.6 pg of injected compound. Intra- and inter-assay coefficients of variation of all four compounds are good (1-8%), as are the accuracy and linearity. A method is also reported for the determination of total dopamine and epinine in plasma and urine based on the same principle. This method, in which deconjugation is accomplished by acid hydrolysis at 95 degrees C, also shows good sensitivity and reproducibility.

Transformation of dopamine and alpha-methyldopamine by NG108-15 cells: formation of thiol adducts

The catecholamines, alpha-methyldopamine (alpha-MeDA) and dopamine (DA), have been implicated in 3,4-(methylenedioxy)amphetamine (MDA) toxicity. The toxicity and metabolic fate of alpha-MeDA, a metabolite of MDA, and DA, a neurotransmitter released by MDA administration, were examined in NG108-15 cells. Both catechols were found to accumulate intracellularly into NG108-15 cells. alpha-MeDA was about 4 times more toxic than DA in the cells. The depletion of glutathione (GSH) by buthionine sulfoximine (BSO) resulted in a drastic increase (10 times) in the alpha-MeDA mediated toxicity while the toxicity of DA was enhanced by 2 times. DA was largely metabolized to dihydroxyphenylacetic acid (DOPAC) and, to a smaller extent, formed an adduct with GSH. alpha-MeDA was primarily metabolized to a GSH adduct. alpha-MeDA was also metabolized to a product which was identified as the cysteinyl adduct. These adducts were identified by HPLC coelution with authentic standards. The GSH and cysteinyl adducts are presumably formed through conjugation of the thiols with intermediary quinone oxidation products of DA and alpha-MeDA. Previous studies indicate that alpha-MeDA is significantly more toxic than DA, especially under conditions of GSH depletion. The results of this study suggest that alpha-MeDA toxicity may occur through cytoplasmic accumulation and oxidation to a reactive quinone species followed by reaction with vital thiol functions or generation of reactive oxygen species. Cytoplasmic DA levels, on the other hand, appear to be significantly lower due to MAO metabolism and vesicular storage, and therefore, DA appears less likely to form conjugates with thiol groups or participate in possible redox cycling.

Assessment of the role of alpha-methylepinine in the neurotoxicity of MDMA

To assess the potential involvement of metabolism of 3,4-methylenedioxymethamphetamine (MDMA) to the catechol alpha-methylepinine in producing serotonergic neurotoxicity, we attempted to correlate aspects of this reaction with the neurotoxicity profile of MDMA. In contrast to the stereoselectivity of S-(+)-MDMA in causing persistent declines in rat brain 5-hydroxyindole levels, no stereochemical component to the metabolic reaction was apparent. Rat liver microsomes generated a significantly greater amount of alpha-methylepinine than did mouse microsomes, but similar amounts of metabolite were produced by brain microsomes from the two species. Formation of alpha-methylepinine by hepatic, but not brain, microsomes was inhibited by SKF 525A and induced by phenobarbital, possibly indicating a tissue specificity in cytochrome P-450-dependent metabolism of MDMA. To directly assess whether alpha-methylepine is a likely mediator of MDMA neurotoxicity, the compound was administered intracerebroventricularly. No persistent declines in biogenic amines or their metabolites were observed one week following treatment. These data suggest that alpha-methylepinine alone is not responsible for the neurotoxic effects of MDMA.

Lung epinephrine synthesis

We studied in vitro and in vivo epinephrine (E) synthesis by rat lung. Nine days after removal of the adrenal medullas, circulating E was reduced to 7% of levels found in sham-operated rats but 30% of lung E remained. Treatment of demedullated rats with 6 hydroxydopamine plus reserpine did not further reduce lung E. In the presence of S-[3H]adenosylmethionine lung homogenates readily N-methylated norepinephrine (NE) to form [3H]E. The rate of E synthesis by lung homogenates was progressively more rapid with increasing NE up to a concentration of 3 mM, above which it declined. The rate of E formation was optimal at an incubation pH of 8 and at temperatures of approximately 55 degrees C. We compared the E-forming enzyme(s) of lung homogenates with those of adrenal and cardiac ventricle. The adrenal contains mainly phenylethanolamine N-methyltransferase (PNMT), which is readily inhibited by SKF 29661 and methylates dopamine (DA) very poorly. Cardiac ventricles contain mainly nonspecific N-methyltransferase (NMT), which is poorly inhibited by SKF 29661 and readily methylates both DA and NE. Lung homogenates were inhibited by SKF 29661 about half as well as adrenal but more than ventricle. We used the rate of E formation from NE as an index of PNMT-like activity and deoxyepinephrine synthesis from DA as an index of NMT-like activity. PNMT and NMT activity in rat lung homogenates were not correlated with each other, displayed different responses to change in temperature, and were affected differently by glucocorticoids.(ABSTRACT TRUNCATED AT 250 WORDS)

Cardiac atria and ventricles contain different inducible adrenaline synthesising enzymes

STUDY OBJECTIVE - The aim of the study was to investigate adrenaline synthesis in atrial and ventricular homogenates. DESIGN - The study involved the use of a new assay which measures the rate at which tissue homogenates convert noradrenaline into adrenaline, or dopamine into N-methyldopamine. This was coupled with a sensitive assay for tissue catecholamines in an investigation of ventricular and atrial homogenates from rats exposed to adrenal demedullation and chemical depletion of cardiac catecholamines. MEASUREMENTS and RESULTS - Atrial and ventricular homogenates from 12 male Sprague-Dawley rats were investigated. Atrial adrenaline forming activity resembled adrenal phenylethanolamine-N-methyltransferase (PNMT) in its relatively high affinity for noradrenaline, substrate specificity for noradrenaline over dopamine, and inhibition by the PNMT inhibitor SKF 29661. Ventricular tissue nonspecifically methylated both noradrenaline and dopamine, and was less inhibited by SKF 29661. Adrenal demedullation induced activity of ventricular adrenaline forming enzyme. CONCLUSIONS - The cardiac atria and ventricles contain different inducible adrenaline forming enzymes. About one third of cardiac adrenaline may be synthesised by the heart itself. The ventricular enzyme can synthesise adrenaline from noradrenaline, and N-methyldopamine from dopamine.

Extraadrenal adrenaline formation by two separate enzymes

Adrenaline (A) is synthesized in the adrenal medullae by the enzyme phenylethanolamine-N-methyltransferase (PNMT). After surgical removal of the adrenal medullae tissue A levels ranged from 22% of control in the heart to 125% of control in the liver. Use of a novel assay to measure tissue A formation revealed that many tissues can synthesize A using PNMT and another enzyme that N-methylates both noradrenaline and dopamine. These enzymes are non-neuronal, inducible and synthesize a major fraction of tissue and urine A.

Metabolism of catecholamine esters by cultured bovine brain microvessel endothelial cells

The metabolism of epinine (N-methyldopamine) and epinine diesters (acetyl, benzoyl, pivaloyl, and isobutyryl) by brain endothelium was investigated using primary cultures of bovine brain microvessel endothelial cells. 3,4-Dihydroxyphenylacetic acid (DOPAC), the product of monoamine oxidase (MAO)-mediated degradation of epinine, was the only metabolite detected by HPLC with electrochemical detection following incubation of the cell monolayers with epinine or its esters. This metabolism could be inhibited by the MAO inhibitors pargyline, clorgyline, and deprenyl, with the system being most sensitive to inhibition by clorgyline. Compared with epinine, incubation of cell monolayers with the diester prodrugs led to increased drug (epinine plus epinine diesters) tissue levels. With the exception of the diacetyl ester, lower levels of DOPAC were observed with the diester prodrugs than with the parent compound. Hydrolysis by serine-dependent esterases appears to be necessary for the subsequent oxidation by MAO. The permeabilities of epinine and the diester prodrugs through endothelial cell monolayers grown on porous supports were related to their lipophilicity and molecular weight.

3- and 4-O-sulfoconjugated and methylated dopamine: highly reduced binding affinity to dopamine D2 receptors in rat striatal membranes

The binding properties of 3- and 4-O-sulfo-conjugated dopamine (DA-3-O-S, DA-4-O-S) as well as 3-O-methylated dopamine (MT) to rat striatal dopamine D2 receptors were investigated. 3H-spiperone was used as a radioligand in the binding studies. In saturation binding experiments (+)butaclamol, which has been reported to bind to dopaminergic D2 and serotoninergic 5HT2 receptors, was used in conjunction with ketanserin and sulpiride, which preferentially label 5HT2 and D2 receptors, respectively, in order to discriminate between 3H-spiperone binding to D2 and to 5HT2 receptors. Under our particular membrane preparation and assay conditions, 3H-spiperone binds to D2 and 5HT2 receptors with a maximal binding capacity (Bmax) of 340 fmol/mg protein in proportions of about 75%:25% with similar dissociation constants KD (35 pmol/l; 43 pmol/l). This result was verified by the biphasic competition curve of ketanserin, which revealed about 20% high (KD = 24 nmol/l) and 80% low (KD = 420 nmol/l) affinity binding sites corresponding to 5HT2 and D2 receptors, respectively. Therefore, all further competition experiments at a tracer concentration of 50 pmol/l were performed in the presence of 0.1 mumol/l ketanserin to mask the 5HT2 receptors. DA competition curves were best fitted assuming two binding sites, with high (KH = 0.12 mumol/l) and low (KL = 18 mumol/l) affinity, present in a ratio of 3:1. The high affinity binding sites were interconvertible by 100 mumol/l guanyl-5-yl imidodiphosphate [Gpp(NH)p], resulting in a homogenous affinity state of DA receptors (KD = 2.8 mumol/l).2+ off

Kinetic study of the transient phase of a chemical reaction system coupled to an enzymatically catalyzed step. Application to the oxidation of epinine by tyrosinase

The present work deals with epinine oxidation by mushroom tyrosinase and sodium metaperiodate. Intermediates produced within short reaction times were characterized by repetitive scanning spectrophotometry and the stoichiometry of the appearance of the respective aminochrome was established. The oxidation pathway from epinine to aminochrome had the following steps: epinine----o-quinone-H+----o-quinone----leukoaminochrome----aminoc hrome. The stoichiometry for the conversion of o-quinone-H+ into the aminochrome of epinine followed the equation: 2 o-quinone-H+----epinine+aminochrome. A transient phase kinetic study has been developed for the system of chemical reactions coupled to an enzymatically catalyzed step, these taking place when epinine is oxidized by mushroom tyrosinase. Rate constants for the implied chemical steps at different temperature and pH values were calculated from analysis of the progress curves of aminochrome accumulation with time. The thermodynamic activation parameters of the chemical steps were also calculated.

Ibopamine (SK&F 100168) pharmacokinetics in relation to the timing of meals

The effect of the timing of a standard meal relative to a single oral dose of 200 mg ibopamine, on the appearance of its pharmacologically active metabolite, epinine, in plasma was investigated in a randomised crossover study in 12 healthy volunteers. After a 12 h fast, ibopamine was administered either in the fasting state (no meal), or 1 h before, 0.5 h before, immediately after, 2 h after or 3 h after a standardised meal. Blood samples taken immediately before and at intervals for 3 h after dosing were analysed for free epinine. Maximum concentration (Cmax), time to Cmax(tmax), and area under the concentration-time curve (AUC) for free epinine in plasma were calculated. When compared with the fasting state, Cmax and AUC0-3h were significantly reduced when ibopamine was given immediately after or 2 h after a meal. AUC was also reduced for ibopamine given 0.5 h before a meal. tmax was significantly delayed when ibopamine was given immediately after, or 2 or 3 h after a meal. Thus, administration of ibopamine with or shortly after a meal reduced the rate and extent of appearance of free epinine in plasma. The clinical significance of reduced epinine levels on acute dosing in the presence of food is unknown.

Analysis of epinine and its metabolites in man after oral administration of its pro-drug ibopamine using high-performance liquid chromatography with electrochemical detection

Ibopamine (N-methyldopamine O,O'-diisobutyrol ester, hydrochloride) is an ester prodrug of epinine. Epinine is a cardiovascular agent used in congestive heart failure because of its dopaminergic and adrenoreceptor agonist properties. Quantitative analytical methods, using high-performance liquid chromatography coupled with electrochemical detection, were developed for the determination of epinine and its known metabolites in biological media. Epinine was extracted from human plasma and urine via an alumina adsorption procedure; this procedure was also used to estimate epinine conjugates after prior enzymatic hydrolysis. Penicillamine was added to the incubation mixture to inhibit isoquinoline production. Urinary dihydroxyphenylacetic acid levels were obtained using the same alumina adsorption procedure, while a separate analytical procedure utilizing a direct high-performance liquid chromatographic analysis of samples was developed for homovanillic acid and its conjugates. Coefficients of variation for all the assays were below 8%. These methods were used to study the pharmacokinetics and metabolic fate of epinine after oral administration of ibopamine to healthy volunteers.

Evaluation of acute hemodynamic effects and pharmacokinetic behaviour of ibopamine in patients with severe heart failure

The aim of the present investigation was to evaluate the acute hemodynamic effects of a single oral dose of 200 mg ibopamine (SB-7505), the 3,4-diisobutyryl ester of N-methyldopamine (epinine) in 11 patients with congestive heart failure (CHF) and to compare the influence of dopamine infusion and oral ibopamine on left ventricular function. Free and conjugated epinine plasma levels were also investigated in these patients and in a group of healthy volunteers to evaluate the correlation between epinine plasma levels and hemodynamic effects. The oral administration of 200 mg ibopamine to patients with CHF, increased cardiac index (+35%) and reduced peripheral and pulmonary vascular resistance (-29% and -26%) without modifying heart rate and systemic or pulmonary arterial pressure. The hemodynamic effect reached its maximum at about 90 min and was still present at 240 min. Ibopamine at a dose of 200 mg elicited similar effects to those observed with dopamine, 2-4 micrograms/kg/min. The pharmacokinetic behaviour of ibopamine in these patients was similar to that observed in a group of healthy volunteers. Free epinine plasma levels peaked at 30-60 min and decreased rapidly, thus showing a shorter time course than the hemodynamic effects.

Synthesis and chemical properties of ibopamine and of related esters of N-substituted dopamines--synthesis of ibopamine metabolites

The therapeutic usefulness of intravenously infused dopamine in congestive heart failure and in shock prompted us to synthesize a wide series of 3,4-O-diesters of dopamine and N-substituted derivatives to obtain an orally active dopamine-like prodrug having adequate absorption and duration of action. The pharmacological results and in particular, the hemodynamic studies in the dog led to the selection of ibopamine, i.e. the 3,4-diisobutyryl ester of N-methyldopamine and to its development as a useful drug for the chronic treatment of congestive heart failure. The choice of ibopamine from among several analogs was also influenced by other favourable properties such as good chemical stability in pharmaceutical formulations and in the biopharmaceutical phases of the absorption, and fast enzymatic activation of the prodrug by plasma and peripheral tissue esterases; the latter property appeared desirable to avoid any accumulation in the central nervous system and consequent undesired side effects. The isomeric mixture of 3-O- and 4-O-isobutyrates of N-methyldopamine as well as the main conjugated metabolites, i.e. the 3-O- and 4-O-sulphate and 4-O-beta-glucuronide of N-methyldopamine were synthesized as analytical references in metabolic studies and for the investigation on their pharmacokinetic and pharmacological properties. Dopamine O-sulphates were also prepared using the methods developed for the corresponding N-methyl derivatives.

Ibopamine, an orally active dopamine-like drug: metabolism and pharmacokinetics in dogs

Ibopamine (SB-7505), the 3,4-diisobutyryl ester of N-methyldopamine (epinine), exerts, on oral administration, cardiovascular effects similar to those of intravenously infused dopamine. Plasma levels and urinary excretion of metabolites were investigated in dogs after oral administration of 4 mg/kg of ibopamine hydrochloride. Epinine, which was readily formed from ibopamine by esterases hydrolysis, was present in plasma in free and sulphate-conjugated form. The urinary metabolites after 6 h from the administration amounted to 62% of the dose, as a sum of 37% of epinine 3-O-sulphate, and 15 and 10% of 4-hydroxy-3-methoxyphenylacetic acid and 3,4-dihydroxyphenylacetic acid, respectively, both in free and conjugated form. When the main metabolite, epinine 3-O-sulphate, was administered intravenously it appeared to be excreted in urine without being deconjugated to any detectable extent, while it appeared to be partially deconjugated on oral administration.

Ibopamine kinetics after a single oral dose in healthy volunteers

In order to describe kinetics after single administration and to test dose independence in the therapeutic dose range, ibopamine (SB-7505), the 3,4-diisobutyrylester of N-methyldopamine (epinine), was given orally to six healthy volunteers at multiple dose levels in a cross-over fashion. Doses employed were 50, 100 and 200 mg with a wash-out period of at least three days between doses. Plasma levels were studied after the 100 mg dose, and urinary recoveries of the major metabolites were measured after each dose. After oral intake of ibopamine, both conjugated and free epinine were detectable in plasma at the earliest sampling times (i.e. 5-10 min), with a hybrid absorption half-life of 0.25 h. Peak plasma concentration mean values of total and free epinine were 33 mumol/l and 35 nmol/l, respectively, and mean time to plasma peak concentration was 1.5 and 0.71 h, respectively. 24-h urinary recovery of conjugated epinine, homovanillic acid and dihydroxyphenylacetic acid accounted for about two thirds of the dose, without dose-dependent mechanisms affecting total elimination. Presystemic sulfate conjugation as a potentially saturable metabolic step at higher dose levels is discussed, although evidence was not found of its saturation in the studied dose range.

Activity of ibopamine on some isolated organs

The pharmacological activity of ibopamine (SB-7505), the 3,4-diisobutyryl ester of N-methyldopamine, was compared with that of epinine (N-methyldopamine) and dopamine on some isolated organs. The results of this study show that ibopamine and epinine exerts a positive inotropic effect on cat papillary muscle by stimulating the cardiac beta 1- and alpha-adrenoceptors; a positive chronotropic effect on cat right atrium by stimulating the cardiac beta 1-adrenoceptors; a relaxing effect on guinea-pig trachea by stimulating beta 2-adrenoceptors; a relaxing effect on rabbit splenic artery pretreated with phenoxybenzamine and contracted with PGF2 alpha by stimulating the DA1 receptors; an inhibitory effect on the vasoconstriction of rabbit ear artery induced by electrical stimuli by stimulating the DA2 receptors. Esterase activation, either by organ or plasma enzymes, appeared to be a prerequisite of ibopamine activity. Suppression of this activity by esterase inhibitors confirmed that the pharmacological responses are related to the formation of epinine. The magnitude and rate of response of some isolated organs to ibopamine appeared to be related to the esterase content of the tissue.

Ibopamine, an orally active dopamine-like drug: metabolism and pharmacokinetics in rats

Ibopamine (SB-7505), the 3,4-diisobutyrylester of N-methyldopamine (epinine), was rapidly hydrolyzed to epinine by plasma esterases of rat as well as of other animal species and man. Ibopamine was rapidly and extensively metabolized after oral administration to rat. Plasma levels of free epinine peaked at 30-60 min from the administration; conjugated epinine was present in larger amount, with a maximum at 3 h. Both free and conjugated epinine were still detectable at 6 h, but not at 24 h. Epinine 4-O-glucuronide, 4-hydroxy-3-methoxyphenylacetic acid and 3,4-dihydroxyphenylacetic acid appeared as main urinary metabolites; epinine 3-O-sulphate, epinine 3-O-methylether and its glucuronide, and trace amounts of epinine 4-O-sulphate were also detected.

Metabolism of carbidopa to alpha-methyldopamine and alpha-methylnorepinephrine in rats

Recent observations on the central and peripheral actions of carbidopa (CD) combined with our own results with the compound led us to examine its metabolism and effects on brain catecholamines in rats. CD was found to undergo a two-stage N-deamination process in vivo giving rise to alpha-methyldopa (AMD) and alpha-methyldopamine respectively. Further, beta-hydroxylation yielded alpha-methylnorepinephrine. These metabolic products were demonstrated in rat brain with reductions in norepinephrine and 3-methoxy-4-hydroxyphenylglycol, and little effect on dopamine. These results are consistent with the alpha-2 agonist effects of alpha-methylnorepinephrine. The relative formation of alpha-methyldopamine from CD was about 26% of an equivalent dose of AMD. It is concluded that some of the central effects of CD may be mediated by its metabolism to AMD, which readily crosses the blood-brain barrier. Possible implications of the findings are discussed.

Amine transport into chromaffin ghosts. Kinetic measurements of net uptake of biologically and pharmacologically relevant amines using an on-line amperometric technique

The kinetic parameters for net transport of dopamine, epinephrine, norepinephrine, 5-hydroxytryptamine, S alpha-methyldopamine, R alpha-methyldopamine, and 1R,2S alpha-methylnorepinephrine into highly purified bovine chromaffin ghosts were determined using an on-line amperometric technique. Chromaffin ghosts devoid of endogenous amines were formed from lysis of chromaffin granules under hypotonic conditions, extensive washing of the scattered membranes, followed by resuspension in iso-osmotic media and overnight dialysis. When chromaffin ghosts formed so as to generate and maintain a large delta pH were suspended in 185 mM KCl, 10 mM Hepes at pH 7.0, 37 degrees C, the addition of MgATP resulted in rapid acidification of the intravesicular space, which was maintained at pH 6.0 (+/- 0.1) for over 30 min. Kinetic net amine transport was subsequently measured with a glassy carbon electrode. The initial rates of uptake were found to follow Michaelis-Menten kinetics. Computer based statistical analysis of the data using distribution-free procedures yielded Km (and V) values as follows: in microM (nmol X mg protein-1 X min-1) dopamine, 16.2 (14.0); R-norepinephrine, 32.5 (12.9); R-epinephrine, 35.1 (15.2); 5-hydroxytryptamine, 4.7 (5.1); S alpha-methyldopamine, 17.7 (11.2); R alpha-methyldopamine, 44.2 (9.9); 1R,2S alpha-methylnorepinephrine, 76.5 (12.5). The physiologic and pharmacologic implications of these kinetic parameters are discussed.

Antihypertensive metabolites of alpha-methyldopa

alpha-Methyldopa acts through a metabolite in the central nervous system. Of the three metabolites most prominently considered as potential mediators of alpha-methyldopa hypotension--alpha-methyldopamine, alpha-methylnorepinephrine (MNE), and alpha-methylepinephrine (ME)--ME is the most potent depressor agent following intravenous infusion into rats, following injection into the lateral ventricle, and following injection into the solitary tract nucleus (NTS). The depressor effect of ME in the NTS is attenuated as effectively by timolol as by yohimbine, while the combination of both timolol and yohimbine completely abolishes the pharmacological activity of ME in the NTS. ME is approximately eightfold more potent than MNE in the NTS and also has a greater susceptibility to timolol attenuation.

Changes in brain alpha-adrenergic receptors after alpha-methyldopa administration to spontaneously hypertensive rats

The hypotensive action of methyldopa has been linked to production of the metabolites methyldopamine and methylnorepinephrine in brain. We have studied the effect of long-term (72 hour) intravenous infusions of methyldopa to awake restrained spontaneously hypertensive rats and normotensive Wistar-Kyoto control animals to look for differences in hypotensive effect, differences in concentrations of natural and alpha-methylated catecholamines, and differences in alpha 1 and alpha 2-adrenergic receptor populations. Results described here indicate that hypertensive rats have a greater reduction in blood pressure and a larger increase in hypothalamic and brain stem methylnorepinephrine concentrations than do the normotensive animals. The methylnorepinephrine concentration reached a plateau value in hypothalamus in both strains while pons and medulla showed progressive, dose-related increases in concentration. These regional and strain differences in the metabolism of alpha-methyldopa suggest that the production of methylnorepinephrine in brain stem nuclei is most correlated with the hypotensive action of methyldopa. alpha 2 Agonist binding (p-amino-clonidine) declined in both hypothalamus and brain stem, and the fall was greater in hypertensive than in normotensive rats. alpha 1-Adrenergic receptor binding (prazosin) was increased, again more in hypertensive than in normotensive rats. The down regulation of alpha 2-adrenergic receptors and the up regulation of alpha 1-adrenergic receptors are compatible with increased alpha 2-adrenergic agonist presynaptic inhibition of catecholamine release with resultant postsynaptic alpha 1-adrenergic receptor supersensitivity. Spontaneously hypertensive rats showed greater methylnorepinephrine production, larger up regulation of alpha 1-adrenergic receptors, and greater down regulation of alpha 2-adrenergic receptors than did the normotensive animals; these changes may be physiological markers for the greater antisympathetic action of methyldopa in hypertensive animals.

Interactions of epinephrine, norepinephrine, dopamine and their corresponding alpha-methyl-substituted derivatives with alpha and beta adrenoceptors in the pithed rat

The effects of alpha-methyl substitution of epinephrine, norepinephrine and dopamine were investigated at alpha-1, alpha-2, beta-1 and beta-2 adrenoceptors in the pithed rat. alpha-Methyl substitution of these three phenethylamines variably altered their capacity to elicit alpha adrenoceptor-mediated vasoconstriction, with slightly enhanced potency being observed for alpha-methyl substitution of norepinephrine and dopamine and a marked reduction in potency for alpha-methyl substitution of epinephrine. However, in all instances, alpha-methyl substitution resulted in a higher selectivity for alpha-2 adrenoceptors (over alpha-1 adrenoceptors). Thus, while epinephrine, norepinephrine and dopamine all produced vasoconstriction that was mediated equally by postsynaptic vascular alpha-1 and alpha-2 adrenoceptors, their corresponding alpha-methyl-substituted derivatives produced vasoconstriction exclusively by activation of postsynaptic vascular alpha-2 adrenoceptors. The beta-1 adrenoceptor-mediated chronotropic effects of these phenethylamines were inconsistently affected by alpha-methyl substitution, with an increase in potency being observed for alpha-methyl substitution of norepinephrine and decreases in potency being observed for alpha-methyl substitution of epinephrine and dopamine. In marked contrast, alpha-methyl substitution of epinephrine, norepinephrine and dopamine was associated with consistent and dramatic increases in potency for beta-2 adrenoceptor-mediated vasodepressor activity. These results indicate that alpha-2 and beta-2 adrenoceptors possess the unique ability to recognize and/or accept alpha-methyl substituents on phenethylamines and that this ability is not shared by their respective receptor subtypes, the alpha-1 and beta-1 adrenoceptors. Furthermore, the results show that alpha-methylepinephrine is a potent beta adrenoceptor agonist, with an apparent 500-fold selectivity for beta-2 adrenoceptors over beta-1 adrenoceptors.

Stereochemical requirements of alpha 2-adrenergic receptors for alpha-methyl substituted phenethylamines

The alpha 1- and alpha 2-adrenergic effects of the stereoisomers of alpha-methyldopamine were evaluated in guinea pig aorta and field-stimulated guinea pig ileum, respectively, in order to establish the stereochemical requirements of these receptors for alpha-methyl substituted phenethylamines. The alpha 1-adrenergic receptor did not distinguish between the stereoisomers of alpha-methyldopamine which is in marked contrast to the alpha 2-adrenergic receptor where a dramatic stereochemical preference for the 2S(+)-isomer was observed. In addition, 2R(-)-alpha-methyldopamine displayed no alpha-receptor subtype specificity whereas 2S(+)-alpha-methyldopamine was highly selective (23 fold) for the alpha 2-adrenergic receptor. These results indicate that the alpha 2-adrenergic receptor can recognize and accept methyl substituents at the alpha-carbon atom of phenethylamines when correctly oriented, while the alpha 1-adrenergic receptor cannot. Thus, the alpha-carbon atom is a major determinant of the alpha 2-adrenergic effects of phenethylamines, and plays an important role in determining alpha-receptor subtype specificity. It is hypothesized that the alpha 2-adrenergic receptor (but not alpha 1) has an additional recognition site which will accommodate alpha-substituted phenethylamines.

alpha-Methyldopa, alpha-methyldopamine an alpha-methylnoradrenaline: substrates for the thermolabile form of human platelet phenol sulphotransferase

1 Sulphate conjugation catalyzed by phenol sulphotransferase (PST) is an important pathway in the catabolism of alpha-methyldopa (MD). Variations in PST activity in an easily obtained tissue such as the human platelet might reflect individual differences in the sulphate conjugation of MD in other organs and tissues. There are at least two forms of human platelet PST, a thermolabile form for which dopamine is a substrate and a thermostable form for which low concentrations of phenol can serve as a substrate. 2 MD, alpha-methyldopamine (MDA) and alpha-methylnoradrenaline (MNA) were tested as substrates for human platelet PST. All three were substrates for the thermolabile form of the enzyme and none were substrates for the thermostable form of PST. Apparent Michaelis-Menten (Km) values for MD, MDA and MNA were 5.5, 0.014 and 0.28 mM, respectively. Apparent Km values for 3'-phosphoadenosine-5'-phosphosulphate, the sulphate donor for the reaction, were 0.08, 0.13 and 0.10 microM, respectively, for the three catechol substrates. The pH optima for the reaction were 7.5 for MD and 6.5 for both MDA and MNA. 3 When platelet homogenates from 20 individual subjects were tested, there were significant correlations between PST activities measured with dopamine and those measured with MD, MDA and MNA (r = 0.54, 0.98 and 0.93, P less than 0.02, less than 0.001, and less than 0.001, respectively), but not between activities measured with low concentrations of phenol and those measured with MD, MDA and MNA (r = 0.021, 0.045 and 0.046, respectively). There results were also compatible with the conclusion that MD, MDA and MNA were substrates for the thermolabile form of platelet PST. 4 These observations will make it possible to test the hypothesis that variations in the activity of the thermolabile form of platelet PST may reflect individual differences in the sulphate conjugation of MD, MDA and MNA.

[Effect of S-adenosyl-L-homocysteine of dopamine catabolism]

1.--The administration of SAH to rats, at physiologically active dose on the sleep, does not change the urinary level of MD and NM. On the other hand, the excretion of DA and NA decreases. 2.--In the brain, SAH does not modify neither the concentration of NA and NM in hypothalamus and thalamus, nor the concentration of DA and MD in corpus striatum. 3.--After intracisternally injection of [14C]DA or [3H]NA, SAH increases the level of [14C]MD and [3H]NM. 4.--Contrary to the studies in vitro, where SAH is an inhibitor of COMT, on the rat it does not seem prevent the methylation of DA and NA.

Formation of alpha-methyldopamine ("Catecholamphetamine") from p-hydroxyamphetamine by rat brain microsomes

Amphetamine is a sympathomimetic and psychotropic drug which is extensively metabolized in liver and in brain. One of its major metabolites, p-hydroxyamphetamine, is accumulated by cortical and striatal synaptosomes. In order to learn whether p-hydroxyamphetamine can be further metabolized to a catecholamine, a sensitive radioenzymatic assay was developed which couples the formation of the "catecholamphetamine" to rapid O-methylation by catechol-O-methyltransferase in the presence of [3H]-methyl-S-adenosylmethionine. Rat brain microsomes contain a cytochrome P-450-dependent monooxygenase which synthesizes catecholamphetamine from p-hydroxyamphetamine. The formation of this catechol metabolite may be involved in the development of tolerance in chronic amphetamine use.

alpha-Methyldopamine, a key intermediate in the metabolic disposition of 3,4-methylenedioxyamphetamine in vivo in dog and monkey

Metabolites of 3,4-methylenedioxyamphetamine in the urine of dogs and monkeys were separated by gas-liquid chromatography as their trifluoroacetyl and/or n-butyl ether derivatives and identified by comparison of the chromatographic and mass spectrometric behavior of these derivatives with those of synthetic compounds. The metabolites identified in dog and monkey urine were alpha-methyldopamine, 3-O-methyl-alpha-methyldopamine, and 3,4-dihydroxybenzyl methyl ketone. The monkey urine also contained 3,4-methylenedioxybenzyl methyl ketone and 3,4-methylenedioxybenzoic acid present as a glucuronide and/or sulfate conjugate, whereas the dog urine had 3-methoxy-4-hydroxybenzoic acid present as a conjugate other than glucuronide and sulfate. The phenolic metabolites in both species were present free and as glucuronide and/or sulfate conjugates.

Gas chromatographic-mass spectrometric identification of epinine in rat superior cervical ganglia and formation in vivo

Epinine was identified in rat superior cervical ganglia by a gas chromatographic-mass spectrometric method. The deuterated methyl group of i.v. administered labeled methionine was incorporated into epinine at a slow rate, although epinephrine-CD3 was rapidly formed. These results indicate that epinine found in the ganglia is not a precursor of epinephrine.

Effect of alpha-methyldopa on dopamine synthesis and release in rat striatum in vitro

The effect of alpha-methyldopa and alpha-methyldopamine (alpha-MDA) on the rate of hydroxylation of radioactive tyrosine was studied in striatal slices from rat brain. This was done by measuring the formation of 3-H-H2O as well as the accumulation of 3-H-dopamine (3-H-DA) from L-3, 5-3-H-tyrosine. alpha-Methyldopa, at tissue concentrations similar to those found in vivo after systemic administration, produced a decrease in both 3-H-H2O and 3-H-DA. The marked decrease (91thyldopa injection, also inhibited 3-H-H2O formation. The inhibitory effect of alpha-methyldopa on 3-H-H2O formation was not reduced by the addition of brocresine, which prevents the formation of alpha-MDA. The effects of alpha-methyldopa and alpha-MDA on the release of 3-H-DA that had been taken up into brain slices, was also studied. Although alpha-methyldopa, 1000 muM, did not increase the release of 3H-DA from tissue, alpha-MDA did. However, the latter was more potent in inhibiting 3-H-H2O formation from 3-H-tyrosine than in releasing 3-H-DA. These results, as well as the close similarity between the percent reduction of 3-H-H2O formation and tissue 3-H-DA levels, suggest that alpha-methyldopa decreases tissue levels of dopamine by inhibiting tyrosine hydroxylase activity in DA neurons.