**6. Release of catecholamines**

8 Neuroscience – Dealing with Frontiers

Fig. 3. Catecholamines in axon terminals. The axon terminal has neurotransmitter storage vesicles with catecholamines and cotransmitters. The vesicular monoamine transporter (VMAT) is responsible for the transport of catecholamines to the storage vesicles, maintaining

catecholamines are reuptaken to the presynaptic terminal by noradrenaline transporter (NET), and/or taken to extraneuronal cells by the extraneuronal transporters. The most important extraneuronal transporter for catecholamines is extraneuronal monoamine transporter (EMT). Catecholamines are metabolized by intracellular enzymes. Monoamine oxidase (MAO) is located in the outer membrane of mitochondria in neurons and in extraneuronal cells. Catechol-*O*-methyl transferase (COMT) is located at extraneuronal cells. Both enzymes (COMT and MAO) are the main responsible for the metabolism of catecholamines. The enzymatic process leads to the formation of several metabolites. To exert their actions, the catecholamines and other neurotransmitters in the synaptic cleft bind to different pre- and postsynaptic receptors. Such binding leads to alterations in the postsynaptic cell and activation of intracellular pathways through G proteins. In presynaptic neurons, catecholamines bind to

their cytosolic concentration low. After catecholamine release to the synaptic cleft,

autoreceptors and activate feed-back responses that change their own release*.* 

Depolarization of the axonal terminal triggers the release of catecholamines into the cleft. The content of the vesicles, including enzymes, neurotransmitters, and hormones, is discharged to the exterior through a process termed exocytosis (Figure 3)*.* Triggered exocytosis is the most common cellular mechanism for the release of polar molecules and it involves several mechanisms that include vesicle docking, priming, and fusion. It can be regulated or constitutive. In synaptic transmission, a synchronized mechanism provides the cells with a way for precisely timed release of molecules into the extracellular space (Pocock and Richards, 2006). The full sequence of steps by which the nerve impulse affects the release of transmitters from sympathetic neurons is not yet fully understood.

In the adrenal medulla, the triggering event to exocytosis is the release of acetylcholine by the preganglionic fibres; that acetylcholine binds to nicotinic receptors on chromaffin cells which produce a localized depolarization. In most types of cells, regulated exocytosis is triggered by the increase in cytosolic free calcium (Ca2+) (Meir et al., 1999, Pocock and Richards, 2006). This cytosolic Ca2+ enters the cells from the extracellular medium, through the voltage-gated Ca2+ channels. The voltage-gated Ca2+ channels open following depolarization of the plasma membrane, making the Ca2+ able to diffuse into the cell down its electrochemical gradient. Ca2+ is also mobilized from intracellular stores (mainly the endoplasmic reticulum). As result, the intracellular free Ca2+ increases and it triggers the fusion of secretory vesicles with the plasma membrane. Ca2+-triggered secretion involves the interaction of highly conserved molecular scaffolding proteins and leads to the docking of the vesicles (Aunis, 1998, Meir et al., 1999). As fusion continues, a pore is formed and it connects the extracellular space with the interior of the vesicle thus providing a pathway for vesicle content to diffuse into the extracellular space (Pocock and Richards, 2006) (Figure 3).

Other aspects concerning modulation of catecholamine transmission or secretion will be better addressed in a subsequent section where adrenoceptors and their role in augmenting or diminishing the synaptic release will be focused.

### **7. Neuronal transporters involved in catecholamine reuptake**

Synaptic transmission involves the regulated release of transmitters into the synaptic cleft, where they interact with receptors that subsequently transduce the information. The removal of catecholamines from the cleft usually occurs by uptake either back to the presynaptic terminal or into the postsynaptic cell (Iversen et al., 1967, Axelrod and Kopin, 1969). A number of high-affinity transporters have been identified, namely for dopamine, NA, 5-HT, and several aminoacid transmitters (Masson et al., 1999). These transporters are members of an extensive family that share common structural motifs, particularly the putative 12-transmembrane helices (Bönisch and Brüss, 2006). The transporters show a similar configuration of intracellular and extracellular loops or related regions that contain

Adrenaline and Noradrenaline: Partners and Actors in the Same Play 11

The importance of neuronal catecholamine transporters in maintaining vesicular levels of catecholamines is well illustrated in mutant mice lacking NET. NET is a high-affinity system, relatively selective for NA, with a low maximum rate of uptake, and it is important in maintaining releasable stores of NA (Bönisch and Brüss, 2006). The knockout NET mice showed depletion of NA intraneuronal stores, lack of inhibition of neuronal amine synthesis, protracted clearance, and elevated NA extracellular levels (Wang et al., 1999). Interestingly, in cyclic voltammetry experiments with the knockout NET mice, the rate of NA clearance was decreased by only six fold when compared to the rate of wild-type animals (Xu et al., 2000). The knockout NET mice behave like wild-type animals treated with antidepressant drugs and are hyperresponsive to locomotor stimulation with

In summary, the neuronal reuptake process aims to assure constant and high levels of neurotransmitters in the releasing neuron and low concentrations in the cleft (Eisenhofer, 2001, Bönisch and Brüss, 2006). It works as an integrated part of the neurotransmitter recycling process, since in addition to their *de novo* synthesis, the stores of monoamines in the terminal portions of the neural fibres are also replenished by their active transport back to the terminals. Moreover, the reuptake system contributes to the degradation of catecholamines since the metabolizing enzymes are found intracellularly (Westfall and Westfall, 2006). Furthermore, the reuptake of catecholamines maintains the concentration gradient within the neuronal vesicles. Two distinct neuron carrier-mediated transport systems are involved: one across the axoplasmic membrane from the extracellular fluid to the cytoplasm, NET or dopamine transporter, and the other from the cytoplasm into the storage vesicles, the VMAT-2 (Bloom, 2006). The removal of catecholamines from the cytoplasm into the storage system by VMAT-2 acts as an amplification step for the overall uptake process developed by NET or dopamine transporter (Schuldiner, 1994, Sonders et al., 2005). In a broader sense, catecholamine transporters function as part of an integrated system where catecholamine synthesis, release, uptake, and metabolism are regulated in a coordinated fashion (Eisenhofer, 2001). Therefore, neuronal catecholamine transporters function not only as part of metabolizing systems, but perhaps more importantly, as part of the recycling system, operating in series with VMAT-2 to maintain catecholamine neuronal

Catecholamines are also taken up by extraneuronal transporters. A low affinity, high capacity uptake for NA and ADR in the isolated perfused rat heart was found by Iversen (Iversen, 1963, 1965b,a). Iversen observed that radiolabelled [3H]-NA in the isolated heart entered into at least two intracellular pools with distinct rate constants. In fact, [3H]-NA entered one pool approximately seven-times faster when compared to the other pool (Iversen, 1963). In 1967, Malmfors demonstrated, by fluorescence microscopy, that the myocytes were responsible for an uptake process (Malmfors, 1967). After these discoveries, the neuronal NA uptake system was designated as "uptake-1" and the extraneuronal transport system as "uptake-2" (Iversen, 1965b). The extraneuronal uptake of catecholamines is mediated by organic cation transporters (OCTs), that include the classic corticosterone-sensitive extraneuronal monoamine transporter and traditionally named "uptake-2" that nowadays is called OCT3 (Eisenhofer, 2001, Schömig et al., 2006). Other

psychostimulants (Xu et al., 2000).

stores (Eisenhofer, 2001) (Figure 3).

**8. Catecholamine extraneuronal transporters** 

members of this family are: OCT1 and OCT2.

phosphorylation and glycosylation sites and intracellular located amino- and carboxylterminal residues (Eisenhofer, 2001). Dopamine is cleared by dopamine transporter whilst NA and ADR are cleared by NA transporter (NET) (Figure 3) (Trendelenburg, 1988, Apparsundaram et al., 1997, Eisenhofer, 2001). These plasma membrane transporters have higher substrate specificity than vesicular transporters and may be viewed as targets ("receptors") for several drugs such as cocaine (NET) or fluoxetine (5-HT transporter) (Bönisch and Brüss, 2006, Capela et al., 2008), but also MDMA (both) (Capela et al., 2009). Dopamine transporter and NET exhibit overlapping, yet distinct, substrate selectivity, translocation efficiency, and antagonist sensitivity (Buck and Amara, 1994, Giros et al., 1994, Pifl et al., 1996, Bönisch and Brüss, 2006). Other than the corresponding neurons, NET is also present in adrenal medulla, liver, and placenta, whereas dopamine transporter is present in the stomach, pancreas, and kidneys (Eisenhofer, 2001).

The neuronal NA uptake system was first reported in the spleen and heart (Axelrod et al., 1959, Axelrod et al., 1961). The availability of radiolabelled catecholamines enabled Axelrod and co-workers to examine the fate of these amines after their intravenous administration to laboratory animals (Axelrod et al., 1959, Axelrod et al., 1961). They observed a selective accumulation of radiolabelled ADR and NA in sympathetically innervated organs, which was dependent on intact sympathetic nerve terminals and it was inhibited by cocaine (Axelrod et al., 1959, Axelrod et al., 1961). It was shown that this accumulation occurred through the activity of NET.

The activity of neurotransmitter transporters, like NET and dopamine transporter, is dependent on intracellular sodium ion (Na+) and chloride ion (Cl−) concentrations. The Na+ gradient (extracellular concentration of Na+ is higher) across the plasma membrane is the main driving force, dictating the direction of the transport of neurotransmitters and cosubstrate ions (in these case Na+). The normal direction is inwards (Masson et al., 1999, Sonders et al., 2005, Bönisch and Brüss, 2006). The negative intracellular membrane potential also contributes for the above mentioned driving force and it is mainly created by the potassium ion (K+) gradient (Bönisch and Brüss, 2006).

Transport of NA by NET is saturated and it is characterized by a half-saturation constant or Michaelis constant (Km) of about 0.8 μM; ADR is transported by NET with a Km of 2.8 μM in a maximum velocity two times lower than the transport of NA (Apparsundaram et al., 1997, Bönisch and Brüss, 2006). NET has a high affinity for NA and a somewhat lower affinity for ADR, while isoproterenol is not a substrate for this system (Eisenhofer, 2001, Bönisch and Brüss, 2006). The common structural requirement for uptake by NET is the presence of an ionisable nitrogen not incorporated in the aromatic ring system (Eisenhofer, 2001).

In amphibians, an ADR specific transporter has been identified, with characteristics distinct from those of dopamine transporter or NET (Apparsundaram et al., 1997). In mammals, the presence of an ADR specific transporter was not yet clarified, however *in situ* hybridization studies indicate the absence of both dopamine transporter and NET messenger ribonucleic acid (mRNA) in ADR-synthesizing neurons in the brainstem of adult rats (Lorang et al., 1994). Thus, as ADR is also cleared at these sites, these terminals might have a distinct catecholamine transporter from dopamine transporter or NET; as an alternative, ADR may act as an endocrine regulator that does not require rapid reuptake in those neurons (Lorang et al., 1994, Apparsundaram et al., 1997).

phosphorylation and glycosylation sites and intracellular located amino- and carboxylterminal residues (Eisenhofer, 2001). Dopamine is cleared by dopamine transporter whilst NA and ADR are cleared by NA transporter (NET) (Figure 3) (Trendelenburg, 1988, Apparsundaram et al., 1997, Eisenhofer, 2001). These plasma membrane transporters have higher substrate specificity than vesicular transporters and may be viewed as targets ("receptors") for several drugs such as cocaine (NET) or fluoxetine (5-HT transporter) (Bönisch and Brüss, 2006, Capela et al., 2008), but also MDMA (both) (Capela et al., 2009). Dopamine transporter and NET exhibit overlapping, yet distinct, substrate selectivity, translocation efficiency, and antagonist sensitivity (Buck and Amara, 1994, Giros et al., 1994, Pifl et al., 1996, Bönisch and Brüss, 2006). Other than the corresponding neurons, NET is also present in adrenal medulla, liver, and placenta, whereas dopamine transporter is present in

The neuronal NA uptake system was first reported in the spleen and heart (Axelrod et al., 1959, Axelrod et al., 1961). The availability of radiolabelled catecholamines enabled Axelrod and co-workers to examine the fate of these amines after their intravenous administration to laboratory animals (Axelrod et al., 1959, Axelrod et al., 1961). They observed a selective accumulation of radiolabelled ADR and NA in sympathetically innervated organs, which was dependent on intact sympathetic nerve terminals and it was inhibited by cocaine (Axelrod et al., 1959, Axelrod et al., 1961). It was shown that this accumulation occurred

The activity of neurotransmitter transporters, like NET and dopamine transporter, is dependent on intracellular sodium ion (Na+) and chloride ion (Cl−) concentrations. The Na+ gradient (extracellular concentration of Na+ is higher) across the plasma membrane is the main driving force, dictating the direction of the transport of neurotransmitters and cosubstrate ions (in these case Na+). The normal direction is inwards (Masson et al., 1999, Sonders et al., 2005, Bönisch and Brüss, 2006). The negative intracellular membrane potential also contributes for the above mentioned driving force and it is mainly created by the

Transport of NA by NET is saturated and it is characterized by a half-saturation constant or Michaelis constant (Km) of about 0.8 μM; ADR is transported by NET with a Km of 2.8 μM in a maximum velocity two times lower than the transport of NA (Apparsundaram et al., 1997, Bönisch and Brüss, 2006). NET has a high affinity for NA and a somewhat lower affinity for ADR, while isoproterenol is not a substrate for this system (Eisenhofer, 2001, Bönisch and Brüss, 2006). The common structural requirement for uptake by NET is the presence of an

In amphibians, an ADR specific transporter has been identified, with characteristics distinct from those of dopamine transporter or NET (Apparsundaram et al., 1997). In mammals, the presence of an ADR specific transporter was not yet clarified, however *in situ* hybridization studies indicate the absence of both dopamine transporter and NET messenger ribonucleic acid (mRNA) in ADR-synthesizing neurons in the brainstem of adult rats (Lorang et al., 1994). Thus, as ADR is also cleared at these sites, these terminals might have a distinct catecholamine transporter from dopamine transporter or NET; as an alternative, ADR may act as an endocrine regulator that does not require rapid reuptake in those neurons (Lorang

ionisable nitrogen not incorporated in the aromatic ring system (Eisenhofer, 2001).

the stomach, pancreas, and kidneys (Eisenhofer, 2001).

potassium ion (K+) gradient (Bönisch and Brüss, 2006).

et al., 1994, Apparsundaram et al., 1997).

through the activity of NET.

The importance of neuronal catecholamine transporters in maintaining vesicular levels of catecholamines is well illustrated in mutant mice lacking NET. NET is a high-affinity system, relatively selective for NA, with a low maximum rate of uptake, and it is important in maintaining releasable stores of NA (Bönisch and Brüss, 2006). The knockout NET mice showed depletion of NA intraneuronal stores, lack of inhibition of neuronal amine synthesis, protracted clearance, and elevated NA extracellular levels (Wang et al., 1999). Interestingly, in cyclic voltammetry experiments with the knockout NET mice, the rate of NA clearance was decreased by only six fold when compared to the rate of wild-type animals (Xu et al., 2000). The knockout NET mice behave like wild-type animals treated with antidepressant drugs and are hyperresponsive to locomotor stimulation with psychostimulants (Xu et al., 2000).

In summary, the neuronal reuptake process aims to assure constant and high levels of neurotransmitters in the releasing neuron and low concentrations in the cleft (Eisenhofer, 2001, Bönisch and Brüss, 2006). It works as an integrated part of the neurotransmitter recycling process, since in addition to their *de novo* synthesis, the stores of monoamines in the terminal portions of the neural fibres are also replenished by their active transport back to the terminals. Moreover, the reuptake system contributes to the degradation of catecholamines since the metabolizing enzymes are found intracellularly (Westfall and Westfall, 2006). Furthermore, the reuptake of catecholamines maintains the concentration gradient within the neuronal vesicles. Two distinct neuron carrier-mediated transport systems are involved: one across the axoplasmic membrane from the extracellular fluid to the cytoplasm, NET or dopamine transporter, and the other from the cytoplasm into the storage vesicles, the VMAT-2 (Bloom, 2006). The removal of catecholamines from the cytoplasm into the storage system by VMAT-2 acts as an amplification step for the overall uptake process developed by NET or dopamine transporter (Schuldiner, 1994, Sonders et al., 2005). In a broader sense, catecholamine transporters function as part of an integrated system where catecholamine synthesis, release, uptake, and metabolism are regulated in a coordinated fashion (Eisenhofer, 2001). Therefore, neuronal catecholamine transporters function not only as part of metabolizing systems, but perhaps more importantly, as part of the recycling system, operating in series with VMAT-2 to maintain catecholamine neuronal stores (Eisenhofer, 2001) (Figure 3).

### **8. Catecholamine extraneuronal transporters**

Catecholamines are also taken up by extraneuronal transporters. A low affinity, high capacity uptake for NA and ADR in the isolated perfused rat heart was found by Iversen (Iversen, 1963, 1965b,a). Iversen observed that radiolabelled [3H]-NA in the isolated heart entered into at least two intracellular pools with distinct rate constants. In fact, [3H]-NA entered one pool approximately seven-times faster when compared to the other pool (Iversen, 1963). In 1967, Malmfors demonstrated, by fluorescence microscopy, that the myocytes were responsible for an uptake process (Malmfors, 1967). After these discoveries, the neuronal NA uptake system was designated as "uptake-1" and the extraneuronal transport system as "uptake-2" (Iversen, 1965b). The extraneuronal uptake of catecholamines is mediated by organic cation transporters (OCTs), that include the classic corticosterone-sensitive extraneuronal monoamine transporter and traditionally named "uptake-2" that nowadays is called OCT3 (Eisenhofer, 2001, Schömig et al., 2006). Other members of this family are: OCT1 and OCT2.

Adrenaline and Noradrenaline: Partners and Actors in the Same Play 13

In short, when compared to NET, extraneuronal monoamine transporter exhibits lower affinity (i.e. higher Km) for catecholamines, favours ADR and isoproterenol over NA or dopamine, and shows a higher maximum rate for catecholamine uptake (i.e. higher maximum velocity) (Iversen, 1965a, b, Eisenhofer et al., 1992b, Eisenhofer, 2001, Schömig et al., 2006). Extraneuronal monoamine transporter is not Na+-dependent and displays a completely different profile of pharmacological inhibition (Schömig et al., 2006, Westfall and Westfall, 2006). Thus, in contrast to the importance of NET in the clearance of neuronal released catecholamines, clearance of circulating catecholamines is predominantly made by

The pharmacological actions of NA and ADR are terminated by: (i) reuptake into nerve terminals by NET; (ii) uptake at extraneuronal sites by extraneuronal monoamine transporter, OCT1, or OCT2; and (iii) metabolic transformation. Catecholamines undergo a complex metabolic fate, mediated by several enzymes, including aldehyde reductase, aldose reductase, aldehyde dehydrogenase, alcohol dehydrogenase, catechol-*O*-methyltransferase (COMT), dopamine β-hydroxylase, monoamine oxidase (MAO) type A and B, monoaminepreferring phenolsulfotransferase, and PNMT (Dooley, 1998, Goldstein et al., 2003). *In vivo,* two of the most important enzymes responsible for the metabolic transformation of

MAO was first described as tyramine oxidase by Mary Hare-Bernheim in 1928, since it catalyses the oxidative deamination of tyramine. This enzyme was found to oxidize several monoamines, including catecholamines, i.e. dopamine, NA, and ADR, and also 5-HT. MAO

RCH2NH2 + H2O + O2 → RCHO + NH3 + H2O2 MAO acts on primary amines and also on some secondary and tertiary amines (Nagatsu, 1991). It is located in the outer membrane of mitochondria (Schnaitman et al., 1967) both in neuronal and extraneuronal cells (Trendelenburg, 1988) (Figure 3). It is a flavo-protein, with flavin adenine dinucleotide as cofactor (Kearney et al., 1971). Two forms of MAO exist, i.e. MAO-A and MAO-B (Johnston, 1968). In humans, MAO-A is abundant in the brain, liver, and in the syncytiotrophoblast layer of term placenta, whereas liver, lungs, platelets, lymphocytes, osteocytes that surround the blood vessels, and the intestine are rich in MAO-B (Abell and Kwan, 2001, Nagatsu, 2004). In the human brain, MAO-A is located in the catecholamine-containing regions, with the highest levels being found in *locus ceruleus*. MAO-B is prominent in the dorsal *raphe nuclei*, which are known to have serotonergic neurons. MAO-B is also present in the posterior hypothalamus and in glial cells (Abell and

5-HT, NA, and ADR are preferential substrates for MAO-A, and clorgyline and Ro 41-1049 are MAO-A inhibitors (Johnston, 1968, Kitaichi et al., 2010); MAO-B prefers βphenylethylamine as a substrate and it is inhibited by deprenyl and lazabemide (Knoll et al., 1978, Kitaichi et al., 2010). Dopamine, tyramine, and tryptamine are oxidized with equal affinity by MAO-A and MAO-B (Glover et al., 1977). Pargyline inhibits both MAO-A and

[amine: oxygen oxidoreductase (deaminating)] catalyses the following reaction:

non-neuronal mechanisms (Eisenhofer, 2001).

**9. Metabolism and action termination** 

**9.1 Monoaminoxidase metabolism** 

Kwan, 2001, Nagatsu, 2004).

catecholamines are MAO and COMT (Westfall and Westfall, 2006).

All three OCTs can transport catecholamines in addition to a wide variety of other organic acids, including 5-HT, histamine, choline, spermine, guanidine, and creatinine (Eisenhofer, 2001). The affinity of extraneuronal monoamine transporter for NA is rather low (Km > 0.5 mM), which is compensated by its high capacity (high turnover number). The transport efficiency of OCT1 and OCT2 for dopamine, NA, ADR, and 5-HT is usually low (Schömig et al., 2006).

OCTs share common features. A single positive charge is required to be an OCT substrate, while uncharged, doubly charged or negatively charged substances are not transported by OCTs (Schömig et al., 2006). A decrease in extracellular pH and the depolarization of the cytoplasmatic membrane will reduce the OCT activity. The OCT mediated transport is independent of Na+ and Cl<sup>−</sup> gradients and all three OCTs are transporters, not just channels, as shown by trans-stimulation experiments. OCT1, OCT2 or extraneuronal monoamine transporter may mediate electrogenic uniport of a substrate or electroneutral exchange of two substrates (antiport). The OCTs are relatively resistant to the inhibitors of the neuronal transporters such as desipramine (Eisenhofer et al., 1991, Schömig et al., 2006), but are inhibited by steroids, such as corticosterone, and the *O*-methylated metabolites of catecholamines, like normetanephrine and metanephrine (Eisenhofer, 2001, Costa et al., 2009a, Nissinen and Männistö, 2010). Other compounds, like GF120918, were shown to inhibit extraneuronal monoamine transporter at low concentrations in isolated rat cardiomyocytes (Costa et al., 2009a).

Extraneuronal monoamine transporter (OCT3) is expressed in many but not all tissues. The expression varies greatly between organs and during organ development (e.g. in placenta). Consistently, high expression has been reported for placenta and heart (Eisenhofer, 2001). Other reports indicate high extraneuronal monoamine transporter expression in the area postrema (Haag et al., 2004, Vialou et al., 2004) and very low in the kidneys (Eisenhofer, 2001). Interestingly, extraneuronal monoamine transporter expression has been also suggested to occur in neurons (Kristufek et al., 2002, Shang et al., 2003).

For NA, the uptake by NET is more relevant for signal termination than extraneuronal uptake. It has been estimated that the sympathetic nerves remove approximately 87% of released NA by NET, while 5% is removed by extraneuronal monoamine transporter. The remainder 8% is diffused into the circulation (Eisenhofer, 2001). This data, however, does not necessarily reflect the relative importance of the two processes. The proximity of the neuronal transporters to the location of catecholamine release (when compared to extraneuronal transporters) implies that the transmitter removed by extraneuronal transport has a higher duration and wider range of action. Extraneuronal uptake, therefore, may be particularly important for the removal of neuronal released catecholamines in tissues where high concentrations of adrenergic receptors exist (Eisenhofer, 2001). The clearance of circulating catecholamines is primarily mediated by non neuronal mechanisms, with liver and kidney accounting for over 60% (Eisenhofer, 2001, Westfall and Westfall, 2006). As stated above, most NA released by sympathetic nerves is removed by neuronal reuptake, which is in contrast with that of ADR. ADR when secreted directly into the bloodstream from the adrenal medulla is predominantly inactivated by extraneuronal uptake and metabolism. ADR has lower affinity for neuronal uptake than NA and consequently the neuronal process contributes approximately 50% less for its elimination when compared with NA (Iversen, 1965a). In contrast, ADR is removed by extraneuronal uptake 2-3 times more efficiently than NA (Iversen, 1965b, Eisenhofer et al., 1992b).

In short, when compared to NET, extraneuronal monoamine transporter exhibits lower affinity (i.e. higher Km) for catecholamines, favours ADR and isoproterenol over NA or dopamine, and shows a higher maximum rate for catecholamine uptake (i.e. higher maximum velocity) (Iversen, 1965a, b, Eisenhofer et al., 1992b, Eisenhofer, 2001, Schömig et al., 2006). Extraneuronal monoamine transporter is not Na+-dependent and displays a completely different profile of pharmacological inhibition (Schömig et al., 2006, Westfall and Westfall, 2006). Thus, in contrast to the importance of NET in the clearance of neuronal released catecholamines, clearance of circulating catecholamines is predominantly made by non-neuronal mechanisms (Eisenhofer, 2001).
