Perspectives in Pharmacology Sources and Significance of Plasma Levels of Catechols and Their Metabolites in Humans

Human plasma contains several catechols, including the catecholamines norepinephrine, epinephrine, and dopamine, their precursor, L-3,4-dihydroxyphenylalanine (L-DOPA), and their deaminated metabolites, dihydroxyphenylglycol, the main neuronal metabolite of norepinephrine, and dihydroxyphenylacetic acid, a deaminated metabolite of dopamine. Products of metabolism of catechols include 3-methoxytyrosine (from LDOPA), homovanillic acid and dopamine sulfate (from dopamine), normetanephrine, vanillylmandelic acid, and methoxyhydroxyphenylglycol (from norepinephrine), and metanephrine (from epinephrine). Plasma levels of catechols and their metabolites have related but distinct sources and therefore reflect different functions of catecholamine systems. This article provides an update about plasma levels of catechols and their metabolites and the relevance of those levels to some issues in human health and disease. Near the end of the 19th century, soon after the description of the profound cardiovascular effects of injected adrenal extract and the purification and identification of epinephrine as the vasoactive principal of the adrenal gland, researchers began to develop chemical means to assess activity of what came to be called the sympathoadrenomedullary system. The first chemical method for such measurement was colorimetric, based on the unusual susceptibility of epinephrine to oxidize, forming a brownish compound called “adrenochrome”. Early attempts to measure circulating levels of epinephrine and related compounds chemically failed, mainly because the potency of epinephrine corresponds to very low normal concentrations in the bloodstream. Bioassays such as used by the great American physiologist, Walter B. Cannon were the first to detect successfully epinephrine release into the circulation. Cannon later developed and exploited a preparation based on the magnitude of the increase in heart rate in animals with denervated hearts; abolition of the increase by adrenalectomy confirmed the hormone’s adrenal source. Subsequent chemical methods depended on fluorescence detection (after the trihydroxyindole reaction or ethylenediamine condensation) or radioenzymatic assays (after methylation with S-adenosylmethionine and catechol-Omethyltransferase). Ironically, current sensitive chemical methods using liquid chromatography with electrochemical detection depend on the same catechol oxidation as did the original colorimetric method. For almost the whole of the first half of the last century, epinephrine was the only catecholamine to receive attention. Cannon proposed—erroneously—that epinephrine was not only the main vasoactive hormone released by the adrenal gland but also the chemical messenger released from sympathetic nerves. This fit with his concept of a unitary sympathoadrenomedullary system, which would help maintain homeostasis (a word he coined) during emergencies but would not be necessary in day-to-day life. Fifty years after the discovery of epinephrine, norepinephrine, rather than epinephrine, was finally identified as the main sympathetic neurotransmitter regulating the cardiovascular system in mammals. Although the notion of a single, emergency sympathoadrenomedullary system remains prominent in current research and practice, it is evident that in many situations Article, publication date, and citation information can be found at http://jpet.aspetjournals.org. DOI: 10.1124/jpet.103.049270. ABBREVIATIONS: DOPA, L-3,4-dihydroxyphenylalanine; DOPAC, dihydroxyphenylacetic acid; DHPG, dihydroxyphenylglycol; ADH, alcohol dehydrogenase; COMT, catechol-O-methyltransferase; DBH, dopamine-hydroxylase; MAO, monoamine oxidase; MHPG, methoxyhydroxyphenylglycol; HVA, homovanillic acid; VMA, vanillylmandelic acid; BH4, tetrahydrobiopterin; LAAAD, L-aromatic-amino acid decarboxylase; NE, norepinephrine; PST, phenolsulfotransferase; DA, dopamine. 0022-3565/03/3053-800–811 THE JOURNAL OF PHARMACOLOGY AND EXPERIMENTAL THERAPEUTICS Vol. 305, No. 3 U.S. Government work not protected by U.S. copyright 49270/1069702 JPET 305:800–811, 2003 Printed in U.S.A. 800 at A PE T Jornals on A ril 0, 2017 jpet.asjournals.org D ow nladed from the sympathetic nervous and adrenomedullary hormonal systems are regulated separately and that there is a continuous basal level of sympathetic nervous activity. Human plasma contains six readily detectable catechols, compounds containing two adjacent hydroxyl groups on a benzene ring. The main plasma catechols are the three catecholamines, their precursor, L-3,4-dihydroxyphenylalanine (DOPA; levodopa), and their deaminated metabolites dihydroxyphenylacetic acid (DOPAC) from dopamine and dihydroxyphenylglycol (DHPG, dihydroxyphenylethylene glycol) from norepinephrine. Catecholamines undergo a complex fate, mediated by several enzymes, including aldehyde reductase, aldose reductase, aldehyde dehydrogenase, alcohol dehydrogenase (ADH), catechol-O-methyltransferase (COMT), dopaminehydroxylase (DBH), monoamine oxidase (MAO) types A and B, monoamine-preferring phenolsulfotransferase (SULT1A3 or m-PST), and phenylethanolamine-N-methyltransferase in various combinations. Because these enzymes are expressed differently among tissues, circulating levels of the products have distinctive sources and reflect specific aspects of sympathetic neuronal and adrenomedullary hormonal system functions (Table 1). This brief review provides an update about plasma levels of catechols and their metabolites and illustrates the relevance of those levels to several issues in human health and disease. Separate sections deal with norepinephrine, epinephrine, and dopamine and their metabolites, followed by indices of catecholamine biosynthesis. Sympathetic Noradrenergic Function Plasma Norepinephrine. Norepinephrine in the bloodstream emanates mainly from networks of sympathetic nerves that enmesh blood vessels—especially arterioles— throughout the body and pervade organs such as the heart and kidneys. The caliber of the arterioles determines total peripheral resistance to blood flow. The sympathetic innervation of the smooth muscle cells in arteriolar walls therefore represents a focal point in neural regulation of blood pressure. In the heart, sympathetic nerves form lattice-like networks around myocardial cells and also supply coronary arterial vessels. Because of the close architectural association between the sympathetic nerves and myocardial and arteriolar smooth muscle cells, one might predict an important role of sympathetic nerves in regulation of cardiovascular performance. Only a very small proportion of norepinephrine released from sympathetic nerves reaches the bloodstream unchanged. The main route of inactivation of norepinephrine is by reuptake into the nerve terminals. Under resting conditions, however, most of the norepinephrine produced in sympathetic nerves is metabolized before entry of the transmitter into the interstitial fluid or plasma (Fig. 1). Since plasma norepinephrine is derived from sympathetic nerves, plasma norepinephrine levels have been widely used to indicate sympathetic nervous system activity. The relationship between plasma norepinephrine levels and sympathetic nerve traffic is not simple. The plasma concentration depends on both the rate of release of norepinephrine into the plasma and the rate of removal from the plasma. Thus, a high plasma norepinephrine level does not necessarily indicate a high rate of sympathetic nerve traffic; a decrease in removal from the plasma also can increase plasma norepinephrine levels, without a change in the rate of sympathetic nerve traffic. Second, the sympathetic nervous system consists of myriad nerves distributed throughout the body, and stressors can activate sympathetic nerve traffic heterogeneously to different organs. For blood sampling from humans, most researchers use the antecubital vein. Since sympathetic nervous activity in the forearm and hand arm influences levels of norepinephrine in antecubital venous plasma, those levels may not accurately reflect changes in sympathetic nervous activity elsewhere in the body during stress. Only a small amount of plasma norepinephrine comes from the adrenal gland under resting conditions, but during some stress responses, such as acute glucoprivation, the adrenal contribution to plasma norepinephrine increases. Third, since only a small proportion of norepinephrine released from sympathetic nerve endings actually reaches the circulation unchanged, small variations in efficiency of the cell membrane norepinephrine transporter can markedly alter the amount of norepinephrine reaching the plasma. Fourth, any of several endogenous biochemicals—including norepinephrine itself, by activating presynaptic 2-adrenoceptors—have the potential to modulate release of norepinephrine from the nerve terminals. In clinical studies, 2adrenoceptor stimulation has been shown to inhibit norepinephrine release into the bloodstream in the heart and forearm. Fifth, in some pathological states and in response to a variety of sympathomimetic amines, norepinephrine is released from sympathetic nerve terminals by a nonexocytotic mechanism differing from the calcium-dependent, exocytotic mechanism of release in response to sympathetic nerve traffic. Cardiac ischemic anoxia is an example of such a pathologic state. Increased net leakage of norepinephrine from vesicular storage sites builds up norepinephrine concentrations in the axoplasm, and exit of norepinephrine via the cell membrane norepinephrine transporter can then lead to norepinephrine entry into the interstitial fluid. Sympathomimetic amines such as tyramine and amphetamine increase plasma norepinephrine levels by this nonexocytotic mechanism. While these considerations do not invalidate plasma norepinephrine levels in arm venous blood in diagnosis, assessment of drug effects, or prognosis, it is evident that plasma norepinephrine levels must be interpreted with care, keeping in mind the purpose of the test, the characteristics of the patient, the possible interacting effects of medications, and other factors that can influence the obtained results. Plasma Norepinephrine Kinetics. In virtually all organs, some of released norepinephrine enters the venous drainage. The rate of entry of norepinephrine into the arterial plasma (“total body spillover”) can be measured using a tracer kinetic method, based on dilution of infused [H]norepinephrine by endogenous norepinephrine. Because of [H]norepinephrine extraction from the circulation in the forearm tissues, use of antecubital venous plasma levels of [H]norepinephrine overestimates whole-body norepinephrine clearance. Healthy people release about 0.3 to 0.5 g/ min (1.7–3.0 nmol/min) of norepinephrine into arterial plasma, resulting in a plasma norepinephrine concentration too low to exert hormonal effects. By applying the same tracer dilution principle, one can Autonomic Regulation 801 at A PE T Jornals on A ril 0, 2017 jpet.asjournals.org D ow nladed from