Effects of catechol-O-methyltransferase inhibition on the plasma clearance of noradrenaline and the formation of 3,4-dihydroxyphenylglycol in the rabbit

The role of neuronal and extraneuronal plasma membrane transporters in the inactivation of peripheral catecholamines

Catecholamines are translocated across plasma membranes by transporters that belong to two large families with mainly neuronal or extraneuronal locations. In mammals, neuronal uptake of catecholamines involves the dopamine transporter (DAT) at dopaminergic neurons and the norepinephrine transporter (NET) at noradrenergic neurons. Extraneuronal uptake of catecholamines is mediated by organic cation transporters (OCTs), including the classic corticosterone-sensitive extraneuronal monoamine transporter. Catecholamine transporters function as part of uptake and metabolizing systems primarily responsible for inactivation of transmitter released by neurons. Additionally, the neuronal catecholamine transporters, recycle catecholamines for rerelease, thereby reducing requirements for transmitter synthesis. In a broader sense, catecholamine transporters function as part of integrated systems where catecholamine synthesis, release, uptake, and metabolism are regulated in a coordinated fashion in response to the demands placed on the system. Location is also important to function. Neuronal transporters are essential for rapid termination of the signal in neuronal-effector organ transmission, whereas non-neuronal transporters are more important for limiting the spread of the signal and for clearance of catecholamines from the bloodstream. Besides their presynaptic locations, NET and DAT are also present at several extraneuronal locations, including syncytiotrophoblasts of the placenta and endothelial cells of the lung (NET), stomach and pancreas (DAT). The extraneuronal monoamine transporter shows a broad tissue distribution, whereas the other two non-neuronal catecholamine transporters (OCT1 and OCT2) are mainly localized to the liver, kidney, and intestine. Altered function of peripheral catecholamine transporters may be involved in disturbances of the autonomic nervous system, such as occurs in congestive heart failure and hypernoradrenergic hypertension. Peripheral catecholamine transporters provide important targets for clinical imaging of sympathetic nerves and diagnostic localization and treatment of neuroendocrine tumors, such as neuroblastomas and pheochromocytomas.

New, selective catechol-O-methyltransferase inhibitors as therapeutic agents in Parkinson's disease

Levodopa remains the most effective drug for Parkinson's disease (PD). However, its benefits are limited owing to extensive metabolism by catechol-O-methyltransferase (COMT), especially if levodopa is used in combination with peripheral dopa-decarboxylase inhibitors. A new generation of potent, orally active, selective, and reversible COMT inhibitors has become available recently. Among these, tolcapone and entacapone have been best characterised. Preclinical and clinical studies have shown that COMT inhibitors markedly enhance levodopa availability and prolong its plasma half-life. In recent large clinical trials they proved to be able to ameliorate motor fluctuations, reduce disability, and decrease levodopa requirements in PD patients. The tolerability profiles of entacapone and tolcapone are good. COMT inhibition promises to become an important means of extending the benefits of levodopa therapy in PD.

Pharmacokinetics and hypotensive effect of diltiazem in rabbits: comparison of diltiazem with its major metabolites

To assess the contribution of its metabolites to the antihypertensive effects of diltiazem, a previously established rabbit model has been used to compare the pharmacokinetics and haemodynamic effects of the drug with those of its major metabolites deacetyldiltiazem (M1) and deacetyl-N-monodemethyldiltiazem (M2). Diltiazem, M1 and M2 were administered separately to each animal (n = 5 or 6 per study group) as a single 5 mg kg(-1) intravenous dose. Blood samples, systolic and diastolic blood pressure (SBP and DBP) and heart rate were recorded for each rabbit up to 8 h, and urine samples were collected for 48 h post-dose. Plasma concentrations of diltiazem and its major metabolites were determined by HPLC. The results showed that systemic clearance (CL) and volume of distribution at steady state (Vdss) were smaller for diltiazem than for the metabolites. Diltiazem and the metabolites reduced both SBP and DBP, the effects of diltiazem being most potent. Their effects on heart rate were highly variable and not statistically different between treatment groups (P > 0.05). These results indicate that diltiazem is a more potent hypotensive agent than M1 or M2, possibly because of the higher plasma concentrations secondary to the smaller CL and Vdss of diltiazem compared with the metabolites. The effects of the metabolites might, however, be more sustained.

Pharmacokinetics and hypotensive effect of deacetyl N-monodesmethyl diltiazem (M2) in rabbits after a single intravenous administration

Deacetyl N-monodesmethyl diltiazem (M2) is a major metabolite of the widely used calcium antagonist diltiazem (DTZ). In order to study the pharmacokinetic and haemodynamic effects of this metabolite, M2 was administered as a single 5 mg/kg dose intravenously (i.v.) to New Zealand white rabbits (n = 5) via a marginal ear vein. Blood samples, blood pressure (SBP and DBP), and heart rate (HR) recordings were obtained from each rabbit up to 8 h, and urine samples for 48 h post-dose. Plasma concentrations of M2 were determined by HPLC. The results showed that there were no identifiable basic metabolites which could be quantified and characterized in the plasma. The apparent terminal t1/2 and AUC were 2.8 +/- 0.7 h and 2000 +/- 290 ng x h/ml, respectively. The Cl and Clr of M2 were 38 +/- 4.8 ml/min/kg and 0.57 +/- 0.23 ml/min/kg, respectively. M2 significantly decreased blood pressure (SBP and DBP) for up to 2 h post-dose (P < 0.05), but had no significant effect on the heart rate (P > 0.05). The Emax and EC50 as estimated by the inhibitory sigmoidal Emax model were 15 +/- 7% and 450 +/- 46 ng/ml, respectively, for SBP; 15 +/- 20% and 430 +/- 120 ng/ml for DBP.

The contribution by monoamine oxidase and catechol-O-methyltransferase to the total-body and pulmonary plasma clearance of catecholamines

To study the effects of inhibition of catechol-O-methyltransferase (COMT) and monoamine oxidase (MAO) on the removal of circulating catecholamines, anaesthetized rabbits were infused for 120 min with 3H-labelled noradrenaline, adrenaline and dopamine. Total-body plasma clearances (Cltot) and pulmonary fractional extractions (ERp) of the infused amines and the cardiac output of plasma (CO(p)) were determined under steady-state conditions at the end of each of two consecutive 60-min treatment periods. MAO and COMT were inhibited by treatment with pargyline (40 mg/kg) and tolcapone (3 mg/kg followed by 1.5 mg/kg given every 30 min), respectively. Two groups of animals were studied. Group I involved animals treated with tolcapone throughout and given pargyline at the beginning of the second treatment period. In group II, pargyline was given at the beginning of the first, and the treatment with tolcapone was started at the beginning of the second treatment period. As previous experiments had shown that COMT inhibition alone is without any effect on Cltot of the three catecholamines considered here, the results obtained in the first treatment period of group I can be taken to reflect control results. At the end of the first treatment period, Cltot of noradrenaline, adrenaline and dopamine (expressed as a percentage of CO(p)) was 88%, 85% and 142%, respectively, in group I (COMT inhibition) and 67%, 77% and 115%, respectively, in group II (MAO inhibition; P < 0.05 for the group difference regarding Cltot of noradrenaline and dopamine). MAO inhibition on top of COMT inhibition (group I) lowered Cltot of noradrenaline, adrenaline and dopamine by 23%, 12% and 26%, respectively, and COMT inhibition on top of MAO inhibition (group II) reduced Cltot of these catecholamines by 13%, 20% and 17%, respectively. At the end of the first treatment period, the pulmonary plasma clearance (Clp = ERp x CO(p)) of noradrenaline and dopamine was 13 and 25 ml kg-1 min-1, respectively, in group I and 12 and 28 ml kg-1 min-1, respectively, in group II. Clp of adrenaline did not differ from zero in either group. Clp of noradrenaline and dopamine was reduced by 74% and 70%, respectively, when both enzymes were inhibited in group I and by 70% and 67%, respectively, when both enzymes were inhibited in group II. Hence, inhibition of either MAO or COMT alone had little, if any, effect on the removal of noradrenaline, adrenaline and dopamine on passage through the systemic and pulmonary circulation. Combined inhibition of both MAO and COMT was highly effective in reducing the pulmonary clearance of noradrenaline and dopamine, but produced only minor decreases in the total-body clearance of all three catecholamines.

The neuronal and extraneuronal origins of plasma 3-methoxy-4-hydroxyphenylglycol in rats

3-Methoxy-4-hydroxyphenylglycol (MHPG) is formed by the sequential actions of monoamine oxidase (MAO) and catechol-O-methyltransferase on norepinephrine within extraneuronal tissues or by extraneuronal O-methylation of 3,4-dihydroxyphenylglycol (DHPG) produced intraneuronally from norepinephrine. This study examined the contributions of neuronal and extraneuronal norepinephrine metabolism to formation of MHPG in rats. Spillover of MHPG into plasma (605 +/- 28 pmol/kg per min) was higher than spillover of DHPG (463 +/- 15 pmol/kg per min), norepinephrine (165 +/- 9 pmol/kg per min) and normetanephrine (56 +/- 4 pmol/kg per min). Comparison of MHPG spillover with the increment in normetanephrine spillover after MAO inhibition (81 pmol/kg per min) combined with the spillover of MHPG derived from circulating normetanephrine (21 pmol/kg per min) indicated that only 17% of plasma MHPG is derived from the extraneuronal deamination and O-methylation of norepinephrine; the remaining 83% is from DHPG produced by deamination of norepinephrine within neurons. The production of MHPG from infused DHPG indicated that 30% of the MHPG in plasma is produced by O-methylation of DHPG after entry of DHPG into the bloodstream and 53% (83-30) from metabolism of DHPG before its entry into the bloodstream. Metabolism of circulating norepinephrine made a minor (1-3%) contribution to plasma concentrations of DHPG and MHPG, but a much larger (46%) contribution to plasma normetanephrine. The results provide a comprehensive examination of norepinephrine metabolism by O-methylation and deamination pathways. Since MHPG is the principal norepinephrine metabolite excreted in the rat, the findings also show that total body turnover of norepinephrine is dependent mainly on neuronal metabolism of the transmitter; at rest extraneuronal pathways contribute as little as 15% to norepinephrine turnover.

Plasma normetanephrine for examination of extraneuronal uptake and metabolism of noradrenaline in rats

The importance of neuronal reuptake for terminating the actions of noradrenaline is well established, but the role of extraneuronal uptake is less clear. This study used plasma concentrations of the extraneuronal noradrenaline metabolite, normetanephrine, to estimate rates of extraneuronal removal of noradrenaline in rats. Animals received infusions of 3H-noradrenaline, with and without inhibition of catechol-O-methyltransferase (COMT) and monoamine oxidase (MAO), to examine the extraneuronal removal of noradrenaline and formation of normetanephrine from infused and endogenous noradrenaline. Infusions of 3H-normetanephrine were also carried out to examine the plasma kinetics of normetanephrine before and after inhibition of MAO. Normetanephrine was cleared rapidly from the circulation and had a short plasma halflife (1 min). Spillover of normetanephrine into plasma (79 pmol kg-1 min-1) was a third that of noradrenaline, but increased 2.8-fold after inhibition of MAO; noradrenaline spillover remained unchanged. Combined inhibition of MAO and COMT decreased the plasma clearance of 3H-noradrenaline by 38%, reflecting removal of 3H-noradrenaline by extraneuronal uptake. Division of the rate of extraneuronal removal of 3H-noradrenaline by the specific activity of plasma 3H-normetanephrine during the 3H-noradrenaline infusion indicated that the rate of extraneuronal removal of endogenous noradrenaline was 250 pmol kg-1 min-1; this was close to the spillover of normetanephrine into plasma after inhibition of MAO (219 pmol kg-1 min-1). Forty-five% of plasma normetanephrine was derived from circulating noradrenaline and 55% from noradrenaline before entry into the circulation. Assuming that these proportions reflected the sources of noradrenaline metabolized extraneuronally indicated that the rate of extraneuronal metabolism of noradrenaline before entry into the circulation was 138 pmol kg-1 min-1. Comparison of this with the rates at which noradrenaline was recaptured by sympathetic nerves (2540 pmol kg-1 min-1) or spilled over into plasma (228 pmol kg-1 min-1), indicated that 87% of the noradrenaline released by sympathetic nerves was recaptured, 5% was metabolized extraneuronally and 8% escaped into plasma. Thus, extraneuronal uptake removes much less of the noradrenaline released by sympathetic nerves than neuronal reuptake.

The part played by catechol-O-methyltransferase in the plasma kinetics of 3,4-dihydroxyphenylglycol and 3,4-dihydroxyphenylalanine in the anaesthetized rabbit

The present study, carried out in anaesthetized rabbits, aimed at determining the effects of catechol-O-methytransferase (COMT) inhibition on the plasma kinetics of infused 3,4-dihydroxyphenylglycol (DOPEG) and 3,4-dihydroxyphenylalanine (DOPA) as well as on endogenous plasma noradrenaline, DOPEG, DOPA and 3-methoxy-4-hydroxyphenylglycol (MOPEG). The plasma kinetics of infused MOPEG were also evaluated. To block the function of COMT, 3,4-dihydroxy-4'-methyl-5-nitrobenzophenone (Ro 40-7592) was given intravenously. Dose-finding experiments, in which the drug-induced fall in endogenous plasma MOPEG was used to quantify COMT inhibition, indicated that a Ro 40-7592 dose of 3 mg/kg followed by 1.5 mg/kg every 30 min was sufficient to obtain a virtually complete inhibition of COMT. More than 150 min of COMT inhibition were required for endogenous MOPEG to disappear from plasma, since the plasma half-life of MOPEG was 54 min. COMT inhibition produced marked increases in the plasma levels of endogenous DOPA (1.7-fold) and DOPEG (3.9-fold) and did not alter endogenous plasma noradrenaline. The results concerning the effect of COMT inhibition on the plasma kinetics of infused DOPA and DOPEG were as follows: the plasma clearance of DOPA was not altered, whereas that of DOPEG fell by 41%; the plasma half-life of DOPA increased from 4.9 to 13.0 min and that of DOPEG from 4.8 to 31.0 min; there was an increase in the volume of distribution of DOPA (2 to 3-fold) and DOPEG (4 to 5-fold).(ABSTRACT TRUNCATED AT 250 WORDS)