Protection of neuronal uptake-1 inhibitors in ischemic and anoxic hearts by norepinephrine-dependent and -independent mechanisms

Shengmai injection combined with conventional therapy in treating Adriamycin-related cardiotoxicity: A protocol for systematic review and meta-analysis of randomized controlled trials

BACKGROUND

Tumor is a common and frequently-occurring disease that seriously threatens human health, and is one of the main causes of death. Adriamycin (ADM) is the most commonly used and effective anti-tumor chemotherapeutics in clinical practice, but they can cause severe cardiotoxicity, which obviously limits their clinical application. Shengmai injection is a modern injection form of traditional Chinese medicine widely used for heart failure, myocardial infarction, cardiogenic shock, and cardiotoxicity patients in China. Therefore, we design this systematic review and meta-analysis to assess the effectiveness and safety of Shengmai injection for treating ADM-related cardiotoxicity.

METHODS

We will methodically search PubMed, EMBASE, Cochrane Library, Science Network, China National Knowledge Infrastructure, Wanfang Database, Chinese Journal Database, and China Biomedical Literature Database, in order to include randomized controlled trials which used Shengmai injection in treating ADM-related cardiotoxicity up to September 2020. The search strategies will use the following phrase: "Shengmai injection," "Adriamycin," "doxorubicin," "cardiotoxicity," "cardiomyopathy," "randomized controlled trial." The outcomes included cardiotoxicity rate, echocardiography, electrocardiogram, myocardial enzymes. Two researchers will independently select the study, extract the data and assess the quality by using Stata 14.0 and RevMan 5.3 software. The plan follows the preferred reporting items declared by the systematic review and meta-analysis plan, and the complete systematic review will follow the preferred reporting items for systematic reviews and meta-analysis (PRISMA) statement.

CONCLUSION

The effectiveness and safety of Shengmai injection will be assessed in treating ADM-related cardiotoxicity which can give some evidence for clinical decision making.

TRIAL REGISTRATION NUMBER

INPLASY202090040.

Facilitation of ischaemia-induced ventricular fibrillation by catecholamines is mediated by β1 and β2 agonism in the rat heart in vitro

BACKGROUND AND PURPOSE

Antiarrhythmic β-blockers are used in patients at risk of myocardial ischaemia, but the survival benefit and mechanisms are unclear. We hypothesized that β-blockers do not prevent ventricular fibrillation (VF) but instead inhibit the ability of catecholamines to facilitate ischaemia-induced VF, limiting the scope of their usefulness.

EXPERIMENTAL APPROACH

ECGs were analysed from ischaemic Langendorff-perfused rat hearts perfused with adrenoceptor antagonists and/or exogenous catecholamines (CATs: 313 nM noradrenaline + 75 nM adrenaline) in a blinded and randomized study. Ischaemic zone (IZ) size was deliberately made small or large.

KEY RESULTS

In rat hearts with large IZs, ischaemia-induced VF incidence was high in controls. Atenolol, butoxamine and trimazosin did not affect VF at concentrations with β1 -, β2 - or α1 - adrenoceptor specificity and selectivity (confirmed in separate rat aortae myography experiments). In hearts with small IZs and low baseline incidence of ischaemia-induced VF, CATs, delivered to the uninvolved zone (UZ), increased ischaemia-induced VF incidence. This effect was not mimicked by atrial pacing, hence, not due to sinus tachycardia. However, the CATs-facilitated increase in ischaemia-induced VF was inhibited by atenolol and butoxamine (but not trimazosin), indicative of β1 - and β2 - but not α1 -adrenoceptor involvement (confirmed by immunoblot analysis of downstream phosphoproteins). CATs did not facilitate VF in low-flow globally ischaemic hearts, which have no UZ.

CONCLUSIONS AND IMPLICATIONS

Catecholamines facilitated ischaemia-induced VF when risk was low, acting via β1 - and β2 - adrenoceptors located in the UZ. There was no scope for facilitation when VF risk was high (large IZ), which may explain why β-blockers have equivocal effectiveness in humans.

Acute myocardial infarction in a young bodybuilder taking anabolic androgenic steroids: A case report and critical review of the literature

We describe a case report of a 30-year-old bodybuilder suffering acute myocardial infarction (AMI). He had been taking stanozolol and testosterone for two months. The coronary angiogram showed high thrombotic burden in the left anterior descending artery without underlying atherosclerosis. Few case reports of AMI in athletes taking anabolic androgenic steroids (AASs) have been reported so far. AAS-related AMI is possibly underreported in the medical literature due to the desire of the affected individuals to hide AAS use. Physicians should always consider the possibility of AAS abuse in the context of a young athlete suffering AMI. AASs can predispose to AMI through the acceleration of coronary atherosclerosis. Additionally, thrombosis without underlying atherosclerosis or vasospasm is highly possible to cause AMI in AAS users. Complications after AMI may be more frequent in AAS users.

Anabolic Androgenic Steroid (AAS) related deaths: autoptic, histopathological and toxicological findings

Anabolic androgenic steroids (AASs) represent a large group of synthetic derivatives of testosterone, produced to maximize anabolic effects and minimize the androgenic ones. AAS can be administered orally, parenterally by intramuscular injection and transdermally. Androgens act by binding to the nuclear androgen receptor (AR) in the cytoplasm and then translocate into the nucleus. This binding results in sequential conformational changes of the receptor affecting the interaction between receptor and protein, and receptor and DNA. Skeletal muscle can be considered as the main target tissue for the anabolic effects of AAS, which are mediated by ARs which after exposure to AASs are up-regulated and their number increases with body building. Therefore, AASs determine an increase in muscle size as a consequence of a dose-dependent hypertrophy resulting in an increase of the cross-sectional areas of both type I and type II muscle fibers and myonuclear domains. Moreover, it has been reported that AASs can increase tolerance to exercise by making the muscles more capable to overload therefore shielding them from muscle fiber damage and improving the level of protein synthesis during recovery. Despite some therapeutic use of AASs, there is also wide abuse among athletes especially bodybuilders in order to improve their performances and to increase muscle growth and lean body mass, taking into account the significant anabolic effects of these drugs. The prolonged misuse and abuse of AASs can determine several adverse effects, some of which may be even fatal especially on the cardiovascular system because they may increase the risk of sudden cardiac death (SCD), myocardial infarction, altered serum lipoproteins, and cardiac hypertrophy. The aim of this review is to focus on deaths related to AAS abuse, trying to evaluate the autoptic, histopathological and toxicological findings in order to investigate the pathophysiological mechanism that underlines this type of death, which is still obscure in several aspects. The review of the literature allowed us to identify 19 fatal cases between 1990 and 2012, in which the autopsy excluded in all cases, extracardiac causes of death.

Desipramine prevents cardiac gap junction uncoupling

BACKGROUND AND PURPOSE

Uncoupling of cardiac gap junction channels is an important arrhythmogenic mechanism in ischemia/reperfusion. Antiarrhythmic peptide AAP10 (H-Gly-Ala-Gly-Hyp-Pro-Tyr-CONH(2)) has been shown to prevent acidosis-induced uncoupling and ischemia-related increase in dispersion. Previous structure-effect investigations and subsequent computer modeling studies indicated that the tricyclic antidepressant desipramine may exert similar effects as AAP10.

METHODS

We assessed the binding of (14)C-AAP10 to membranes of rabbit cardiac ventricles and its displacement with desipramine in a classical radioligand binding and competition study. Gap junction currents were measured between isolated pairs of human atrial cardiomyocytes under normal and acidotic (pH 6.3) conditions with or without 1 μmol/l desipramine using dual whole-cell voltage clamp. The effect of 1 μmol/l desipramine was assessed in isolated rabbit hearts (Langendorff technique) undergoing local ischemia by coronary occlusion with 256-channel electrophysiological mapping and subsequent analysis of connexin43 (Cx43) expression, phosphorylation (Western blot), and subcellular localization (immunohistology).

RESULTS

We found saturable (14)C-AAP10 binding to cardiac membranes (K (D), 0.29 ± 0.11 nmol/l; B (max), 42.5 ± 7.2 pmol/mg) which could be displaced by desipramine with a K (D.High) = 0.14 μmol/l and a K (D.Low) = 22 μmol/l. Acidosis reduced the gap junction conductance in human cardiomyocyte pairs from 24.1 ± 4.7 to 11.5 ± 2.5 nS, which could be significantly reversed by desipramine (26.6 ± 4.8 nS). In isolated hearts, ischemia resulted in significantly increased dispersion of activation-recovery intervals, loss of membrane Cx43, and dephosphorylation of Cx43, which all could be prevented by desipramine.

CONCLUSION

Desipramine seems to prevent the uncoupling of cardiac gap junctions and ischemia-related increase in dispersion.

Nandrolone Potentiates Arrhythmogenic Effects of Cardiac Ischemia in the Rat

Anabolic steroid abuse has been associated with thrombosis and arteriosclerosis, both of which predispose to myocardial ischemia and infarction. However, there are reports of sudden cardiac death in the absence of thrombus and atheroma following anabolic steroid use. Although treatment with the commonly abused steroid, nandrolone, has been shown to decrease recovery of systolic function following ischemia in isolated rat hearts, it is unknown whether anabolic steroids can increase the incidence of fatal arrhythmia associated with cardiac ischemia. Anesthetized male Sprague-Dawley rats were administered vehicle or nandrolone (10-160 microg/kg/min iv) 10 min prior to 15-min occlusion of the left anterior descending coronary artery followed by 10-min reperfusion. Nandrolone, in this dose range, did not significantly change heart rate, blood pressure, or cardiac rhythm in the absence of ischemia. However, the fraction of rats surviving ischemia was significantly (p < 0.05) decreased by nandrolone at both 40 and 160 microg/kg/min, while survival time during ischemia was decreased significantly (p < 0.001) by nandrolone 160 microg/kg/min. An increase (p < 0.05) in the duration of ventricular fibrillation was noted at the highest compared to the lowest dose of nandrolone, corresponding to a significant increase in the fraction of rats experiencing ventricular fibrillation (p < 0.01). Nandrolone had no effect on the frequency or duration of ventricular fibrillation or survival time during reperfusion. Although the mechanisms underlying these effects are currently unclear, they indicate that exposure to anabolic steroids in combination with transient reductions in coronary blood flow may explain some reports of sudden cardiac death in anabolic steroid users.

Inhibition of nonexocytotic norepinephrine release by desipramine reduces myocardial infarction size

During myocardial ischemia, a substantial accumulation of norepinephrine occurs in the ischemic zone due to a local nonexocytotic release of norepinephrine. Norepinephrine release is driven by the neuronal monoamine transporter (NET), which reverses its usual transmembrane transport direction. We investigated whether this local accumulation of norepinephrine contributes to irreversible myocardial injury in an in vivo model of myocardial infarction. Male, anaesthetized Wistar rats were subjected to 30 min coronary occlusion and subsequent 120 min reperfusion. Five minutes prior to coronary occlusion, the NET inhibitor desipramine was administered intravenously. Infarct size (IS) was determined by TTC-staining and was related to the area at risk (AAR). The influence of desipramine on cardiac norepinephrine release was investigated in isolated perfused hearts with 30 min of regional ischemia. Norepinephrine was measured in the effluent from the hearts by HPLC and electrochemical detection. Desipramine (0.1–0.8 mg/kg) dose-dependently reduced infarct size (IS/AAR) from 0.54 to 0.21 and suppressed postischemic norepinephrine release from 245 to 108 pg/mL. In summary, the data indicate that nonexocytotic release of norepinephrine in myocardial ischemia exaggerates acute ischemic damage, because suppression of ischemia-induced release of norepinephrine by the tricyclic antidepressant desipramine effectively reduces infarct size in an in vivo model of myocardial ischemia.

Cardiospecific Overexpression of the Prostaglandin EP 3 Receptor Attenuates Ischemia-Induced Myocardial Injury

Background— The generation of prostaglandin E 2 (PGE 2 ) is significantly increased in acute myocardial ischemia and reperfusion. PGE 2 , in addition to other prostaglandins, protects the reperfused ischemic myocardium. It has been hypothesized that this cardioprotection is mediated by E-type prostaglandin receptors of the G i -coupled EP 3 subtype.

Methods and Results— We tested this hypothesis by generating transgenic (tg) mice with cardiospecific overexpression of the EP 3 receptor. According to ligand binding, a 40-fold overexpression of the EP 3 receptor was achieved in membranes prepared from tg hearts compared with wild-type (wt) littermates. In isolated cardiomyocytes from tg mice, the forskolin-induced rise in cAMP was markedly attenuated, indicating coupling of the overexpressed EP 3 receptor to inhibitory G proteins (G i ) with constitutive receptor activity. There was no evidence for EP 3 receptor coupling to G q -mediated protein kinase C signaling. Isolated hearts from tg and wt mice were subjected to 60 minutes of no-flow ischemia and 45 minutes of reperfusion. In tg hearts, ischemic contracture was markedly delayed compared with wt hearts, and the ischemia-induced increase in left ventricular end-diastolic pressure was reduced by 55%. Creatine kinase and lactate dehydrogenase release was significantly decreased by 85% and 73%, respectively, compared with wt hearts.

Conclusions— Constitutive prostaglandin EP 3 receptor signaling exerts a protective effect on cardiomyocytes, which is probably G i mediated and results in a remarkable attenuation of myocardial injury during ischemia and reperfusion. Cardioprotective actions of E-type prostaglandins may be mediated by this receptor subtype.

Left regional cardiac perfusion in vitro with platelet-activating factor, norepinephrine and K+ reveals that ischaemic arrhythmias are caused by independent effects of endogenous "mediators" facilitated by interactions, and moderated by paradoxical antagonism

Various putative drug targets for suppression of ischaemia-induced ventricular fibrillation (VF) have been proposed, but therapeutic success in the suppression of sudden cardiac death (SCD) has been disappointing. Platelet-activating factor (PAF) is a known component of the ischaemic milieu. We examined its arrhythmogenic activity, and its interaction with two other putative mediators, norepinephrine and K(+), using an ischaemia-free in vitro heart bioassay, and a specific PAF antagonist (BN-50739). PAF (0.1-100 nmol) was administered selectively to the left coronary bed of rat isolated hearts using a specially designed catheter. In some hearts, PAF was administered to the left coronary bed during concomitant regional perfusion with norepinephrine and/or K(+). In separate studies, PAF accumulation in the perfused cardiac tissue was evaluated using (3)H-PAF. PAF evoked ventricular arrhythmias concentration-dependently (P<0.05). It also widened QT interval and reduced coronary flow selectively in the PAF-exposed left coronary bed (both P<0.05). Two exposures of hearts to PAF were necessary to evoke the QT and rhythm effects. The PAF-induced arrhythmias and coronary vasoconstriction were partially suppressed by the PAF antagonist BN-50739 (10 microm), although BN-50739 itself widened QT interval. K(+) (8 and 15 mm) unexpectedly antagonised the arrhythmogenic effects of PAF without itself eliciting arrhythmias (P<0.05). Norepinephrine (0.1 microm) had little or no effect on the actions of PAF, while failing to evoke arrhythmias itself. Nevertheless, the combination of 15 mm K(+) and 0.1 microm norepinephrine evoked arrhythmias of a severity similar to arrhythmias evoked by PAF alone, without adding to or diminishing the arrhythmogenic effects of PAF. (3)H-PAF accumulated in the cardiac tissue, with 43+/-5% still present 5 min after bolus administration, accounting for the need for two exposures of the heart to PAF for evocation of arrhythmias. Thus, PAF, by activating specific receptors in the ventricle, can be expected to contribute to arrhythmogenesis during ischaemia. However, its interaction with other components of the ischaemic milieu is complex, and selective block of its actions (or its accumulation) in the ischaemic milieu is alone unlikely to reduce VF/SCD.

Role of Histamine H3 Receptors during Ischemia/Reperfusion in Isolated Rat Hearts

Histamine H3 receptors are involved in regulating the release of norepinephrine (NE), in both central and peripheral nervous systems. We investigated the effect of R-alpha-methylhistamine (R-HA), a selective H3 receptor agonist, and thioperamide (Thiop), a selective H3 receptor antagonist, on ischemia/reperfusion-induced changes in carrier-mediated NE release and cardiac function in isolated rat heart. Hearts were subjected to 40-minute ischemia followed by 30-minute reperfusion. Ischemia/reperfusion evoked massive NE release, which was markedly suppressed by the treatment with desipramine (DMI), a neuronal NE transporter blocker. Ischemia/reperfusion-induced cardiac dysfunction (decreases in left ventricular developed pressure, LVDP, and the first derivative of left ventricular pressure, dP/dt, and a rise in left ventricular end diastolic pressure, LVEDP) was also improved by the DMI treatment. The treatment with R-HA also significantly decreased the excessive NE release induced by the ischemia/reperfusion, improved the recovery of LVDP and dP/dt, and suppressed the rise in LVEDP. Thiop did not affect NE release and cardiac function after the reperfusion. When R-HA was administered concomitantly with Thiop, R-HA failed to attenuate ischemia/reperfusion-induced NE release and cardiac dysfunction. Thus, it seems likely that the ischemia/reperfusion-induced carrier-mediated NE release in rat hearts is negatively regulated by the activation of H3 receptors, probably located on cardiac noradrenergic nerve endings.

Carrier-mediated norepinephrine release and reperfusion arrhythmias induced by protracted ischemia in isolated perfused guinea pig hearts: effect of presynaptic modulation by alpha(2)-adrenoceptor in mild hypothermic ischemia

Yohimbine, an alpha(2)-adrenoceptor antagonist, has been reported to protect hypoxic myocardium and inhibit carrier-mediated norepinephrine (NE) release and reperfusion arrhythmias (ventricular fibrillation; VF) in normothermic ischemia. In heart surgery, mild hypothermic (tepid) cardioplegia has been reported to reduce metabolic demand and permit immediate recovery of cardiac function. Therefore, we determined the effect of yohimbine on NE release and reperfusion arrhythmias in isolated perfused guinea pig hearts of tepid temperature (32 degrees C) ischemia model. Stepwise increase of global ischemia period (20, 40, and 60 min) induced a progressive increase of NE release and duration of VF. Neuronal uptake 1 inhibitor desipramine (100 nM) and Na(+)-H(+) exchanger inhibitor 5-N-ethyl-N-isopropyl-amiloride (10 microM) decreased NE and VF in 60-min hypothermic ischemia. This indicated that NE release induced by protracted tepid ischemia was due to carrier-mediated release. Yohimbine (1 microM) markedly reduced NE release and VF (p < 0.01 versus control) and 5-bromo-N-(4,5-dihydro-1H-imidazol-2-yl)-6-quinoxalinamine [UK 14,304 (UK); 10 microM], an alpha(2)-adrenoceptor agonist, increased NE release and VF (p < 0.01 versus control). Yohimbine (1 microM) prevented the potentiated effect of UK (10 microM) in hypothermia (p < 0.01 versus UK). Our findings indicate that presynaptic reduction of carrier-mediated NE release seems to be one of the most important factors controlling reperfusion arrhythmias, and alpha(2)-adrenoceptor blockade by yohimbine (1 microM) in tepid ischemia may contribute to effective myocardial protection in terms of NE release and reperfusion arrhythmia.

Vagal neurostimulation in patients with coronary artery disease

We tested the hypotheses that (1) progression of coronary artery disease (CAD) increases sympathetic inflow to the heart, thus impairing cardiac blood supply, and (2) reduced sympathetic tone improves cardiac microcirculation and ameliorates severity of anginal symptoms. Electrical irritation of the nerve auricularis--a sensitive ramus of the vagus nerve--provides a central sympatholytic action. Using this technique, we studied the effects of vagal neurostimulation (VNS) on hemodynamics, the content of atrial noradrenergic nerves and the microcirculatory bed of CAD patients. VNS was performed in the preoperative period of CAD patients with severe angina pectoris. The comparison groups consisted of untreated patients with CAD or Wolff-Parkinson-White syndrome. Atrial tissue of patients with this syndrome (n = 6); with effort angina (n = 14); with angina at rest (n = 10); and with severe angina treated with VNS (n = 8) contained the following volume percentages of noradrenergic nerves: 1.7+/-0.1%, 1.3+/-0.3%, 0.5+/-0.1% (p < 0.05 vs. the other groups) and 1.3+/-0.2%, respectively. In these groups, cardiac microcirculatory vessels (diameter, 10-20 microm) had the following densities: 2.7+/-0.2%, 3.4+/-0.2%, 2.0+/-0.4% (p < 0.05 vs. the other groups) and 3.3+/-0.3%, respectively. VNS treatment abolished angina at rest, decreased heart rate and blood pressure. It improved left ventricular ejection fraction from 50+/-1.5% to 58+/-1.0% (p < 0.05), also changing left ventricular diastolic filling. The ratio of time velocity integrals of the early (Ei) to late (Ai) waves increased from 1.07+/-0.12 to 1.65+/-0.17 after VNS (p < 0.05). In electrocardiograms of VNS-treated patients, QRS- and QT-duration were shortened. the PQ-interval did not change, but T-wave configuration improved. In the postoperative period, heart failure occurred in 90% of the control group. vs. 12% in patients treated with VNS (p < 0.05). We conclude that CAD is characterized by overactivity of sympathetic cardiac tone. Vagal stimulation reduced sympathetic inflow to the heart, seemingly via an inhibition of norepinephrine release from sympathetic nerves. VNS' sympatholytic/vagotonic action dilated cardiac microcirculatory vessels and improved left ventricular contractility in patients with severe CAD.

The reverse transport of DA, what physiological significance?

It is well established that midbrain dopamine neurons innervating the striatum, release their neurotransmitter through an exocytotic process triggered by the neural firing and involving a transient calcium entry in the terminals. Long ago, it had been proposed, however, that another mechanism of release could co-exist with classical exocytosis, involving the reverse-transport of the cytosolic amine by the carrier, ordinarily responsible for uptake function. This atypical mode of release could be evoked directly at the preterminal level by multiple environmental endogenous factors involving transient alterations of the sodium gradient. It cannot be excluded that this mode of release participates in the firing-induced release. In contrast with the classical exocytosis of a preformed DA pool, the reverse-transport of DA requires simultaneous alterations of intraterminal amine metabolism including synthesis and displacement from storage compartment. The concept of a reverse-transport of dopamine is coming from the observations that releasing substances, such as amphetamine-related molecules, actually induce this type of transport. A large set of arguments advocates that reverse-transport plays a role in the maintenance of basal extracellular DA concentration in striatum. It was also often evoked in physiopathological situations including ischemia, neurodegenerative processes, etc. The most recent studies suggest that this release could occur mainly outside the synapses, and thus could constitute a major feature in the paracrine transmission, sometimes evoked for DA.

Time course and mechanism of myocardial catecholamine release during transient ischemia in vivo

BACKGROUND

Elevated concentrations of norepinephrine (NE) have been observed in ischemic myocardium. We investigated the magnitude and mechanism of catecholamine release in the myocardial interstitial fluid (MIF) during ischemia and reperfusion in vivo through the use of microdialysis.

METHODS AND RESULTS

In 9 anesthetized pigs, interstitial catecholamine concentrations were measured in the perfusion areas of the left anterior descending coronary artery (LAD) and the left circumflex coronary artery. After stabilization, the LAD was occluded for 60 minutes and reperfused for 150 minutes. During the final 30 minutes, tyramine (154 nmol. kg(-1). min(-1)) was infused into the LAD. During LAD occlusion, MIF NE concentrations in the ischemic region increased progressively from 1. 0+/-0.1 to 524+/-125 nmol/L. MIF concentrations of dopamine and epinephrine rose from 0.4+/-0.1 to 43.9+/-9.5 nmol/L and from <0.2 (detection limit) to 4.7+/-0.7 nmol/L, respectively. Local uptake-1 blockade attenuated release of all 3 catecholamines by >50%. During reperfusion, MIF catecholamine concentrations returned to baseline within 120 minutes. At that time, the tyramine-induced NE release was similar to that seen in nonischemic control animals despite massive infarction. Arterial and MIF catecholamine concentrations in the left circumflex coronary artery region remained unchanged.

CONCLUSIONS

Myocardial ischemia is associated with a pronounced increase of MIF catecholamines, which is at least in part mediated by a reversed neuronal reuptake mechanism. The increase of MIF epinephrine implies a (probably neuronal) cardiac source, whereas the preserved catecholamine response to tyramine in postischemic necrotic myocardium indicates functional integrity of sympathetic nerve terminals.

Catechol-O-methyltransferase activity in CHO cells expressing norepinephrine transporter

1. We examined the existence of catecholamine metabolizing enzymes (catechol-O-methyltransferase, COMT, and monoamine oxidase, MAO) in CHO cells transfected with norepinephrine (NE) transporter (NET) cDNA. 2. NET activity was studied by incubating cells with [3H]-NE (0. 5 microCi ml-1, 20 min) in a Na+ containing medium. Incubation with [3H]-NE lead to [3H] accumulation at 47797+/-4864 d.p.m. per well. Specific inhibitors of NET abolished this uptake. 3. During post-uptake incubation, [3H] leaked rapidly from cells and the extracellular phase comprised 89% of total radioactivity within 40 min. Both [3H] retention and [3H] 'leakage' were largely unaffected by inhibitors for MAO. In contrast, COMT inhibitors, U-0521 and Ro 41-0960, dose-dependently increased intracellular [3H]-NE retention with a maximal increase of 4.5 fold. The EC50 for Ro 41-0960 was 139-times lower than that of U-0521. U-0521 largely inhibited [3H] 'leakage' and doubled the apparent Vmax for [3H]-NE uptake. 4. Addition of U-0521 during uptake incubation increased intracellular NE content by 8 fold. Normetanephrine, the COMT-dependent metabolite of NE, was formed in large quantities during post-uptake incubation. U-0521 significantly inhibited the formation of NMN with an equal preservation of intracellular NE. 5. CHO cells expressing NET possess COMT activity, which is responsible for the metabolism of NE to form lipophilic metabolite normetanephrine. The apparent 'properties' of the NET function expressed in CHO cells changed, after inhibition of COMT, in such a way closer to that described in the native neuronal preparations.