Direct measurement of dopamine O-sulfate in plasma and cerebrospinal fluid

X-ray crystal structure of human dopamine sulfotransferase, SULT1A3. Molecular modeling and quantitative structure-activity relationship analysis demonstrate a molecular basis for sulfotransferase substrate specificity

Humans are one of the few species that produce large amounts of catecholamine sulfates, and they have evolved a specific sulfotransferase, SULT1A3 (M-PST), to catalyze the formation of these conjugates. An orthologous protein has yet to be found in other species. To further our understanding of the molecular basis for the unique substrate selectivity of this enzyme, we have solved the crystal structure of human SULT1A3, complexed with 3'-phosphoadenosine 5'-phosphate (PAP), at 2.5 A resolution and carried out quantitative structure-activity relationship (QSAR) analysis with a series of phenols and catechols. SULT1A3 adopts a similar fold to mouse estrogen sulfotransferase, with a central five-stranded beta-sheet surrounded by alpha-helices. SULT1A3 is a dimer in solution but crystallized with a monomer in the asymmetric unit of the cell, although dimer interfaces were formed by interaction across crystallographic 2-fold axes. QSAR analysis revealed that the enzyme is highly selective for catechols, and catecholamines in particular, and that hydrogen bonding groups and lipophilicity (cLogD) strongly influenced K(m). We also investigated further the role of Glu(146) in SULT1A3 using site-directed mutagenesis and showed that it plays a key role not only in defining selectivity for dopamine but also in preventing many phenolic xenobiotics from binding to the enzyme.

Impairment of metabolic hepatic nerve action by chronic but not acute ethanol intoxication studied in isolated perfused rat liver

BACKGROUND/AIMS

Liver carbohydrate metabolism and blood flow are regulated by hepatic nerves and hormones such as glucagon, insulin or catecholamines. Acute and chronic application of alcohol are known to depress the function of central and peripheral nerves. The extent of inhibition of the autonomic nervous system is not well characterized; thus, the possible impairment of hepatic nerve function by acute and chronic application of ethanol was investigated.

METHODS

Rat livers were perfused simultaneously via both the portal vein and hepatic artery. Hepatic nerves were stimulated electrically for 2 min (20 Hz, 20 V, 2 ms). As a control, noradrenaline (1 microM) was infused into the portal vein for 2 minutes.

RESULTS

During acute application of ethanol in portal concentrations of 50, 150 and 300 mM, which elevated basal glucose release, stimulation of hepatic nerves as well as portal noradrenaline infusion caused the same increase in glucose output and decrease in portal and arterial flow as in controls. Following chronic application of ethanol by feeding rats the Lieber-DeCarli liquid diet containing 5% (v/v) ethanol for 4 and 6 weeks, only nerve stimulation caused a significantly reduced enhancement of glucose output (50%, p < 0.025), whereas portal noradrenaline was as effective as in controls. Noradrenaline overflow was significantly reduced following nerve stimulation.

CONCLUSION

The decrease in nerve stimulation-dependent glucose output and noradrenaline overflow in chronically ethanol fed rats indicates an impaired function of hepatic nerves.

Loss of regulation by sympathetic hepatic nerves of liver metabolism and haemodynamics in chronically streptozotocin-diabetic rats

The consequences of autonomic diabetic neuropathy, a common complication of chronic diabetes mellitus, have been studied mainly with regard to heart and stomach function. Since the autonomic nervous system also regulates liver carbohydrate metabolism and haemodynamics via hepatic nerves, it was the purpose of this study to examine the function of hepatic nerves in chronically diabetic rats. Diabetes was induced by i.p. injection of streptozotocin. Rat livers were perfused via both portal vein and hepatic artery. Hepatic nerves were stimulated for 2 min using a platinum electrode placed around the portal vein and the hepatic artery; in an additional stimulation phase noradrenaline was infused into the portal vein. Stimulation of hepatic nerves as well as portal noradrenaline infusion increased hepatic glucose output and reduced flow in control and in acutely (48-h) diabetic animals, which still had almost normal glycogen content. In addition stimulation also caused an overflow of noradrenaline into the caval vein. However, nerve stimulation neither increased glucose output nor decreased flow in 4-month diabetic rats. In these rats noradrenaline overflow was nearly completely abolished and hepatic glycogen content was markedly depleted. Portal noradrenaline infusion in chronically diabetic rats reduced flow to a similar extent as in controls, yet the increase in glucose output was diminished. The lack of nerve stimulation-dependent glucose output, flow reduction and noradrenaline overflow is indicative of a profound loss of function of hepatic autonomic nerves in chronically diabetic rats.

In patients receiving dopamine infusions for treatment of shock do free radicals convert dopamine to 6-hydroxydopamine?

Free hydroxyl radicals react with dopamine in vitro to produce the neurotoxin 6-hydroxydopamine (6-OHDA). If 6-OHDA were produced in vivo it could destroy central and/or peripheral neurons. Free radicals are produced in patients with cardiovascular or toxic shock. These patients are often treated with intravenous dopamine infusions. Using a newly developed method for measuring 6-OHDA in biological samples, we have examined blood from 15 patients receiving dopamine infusions for treatment of shock. 6-OHDA neither binds to nor elutes from either alumina or ion-exchange resins; therefore we used ODA-silica (Sep-Pak) to prepare samples for HPLC analysis with electrochemical detection. 6-OHDA disappeared rapidly from whole blood or serum at room temperature but was stable at -70 degrees C in an HClO4 extract. Recovery from Sep-Pak was 95% and detection limit 10 nmol/L. We repeated the experiments that showed in vitro hydroxyl radical conversion of dopamine to 5- and 6-OHDA. We detected neither 5- nor 6-OHDA in plasma samples from patients receiving dopamine infusions at 10-28 micrograms kg-1 min-1. Plasma dopamine in these patients was 1-5 mumol/L. We conclude that any 6-OHDA produced by hydroxyl radical attack in vivo is rapidly scavenged by endogenous substances containing sulfhydryl groups.

Signal propagation via gap junctions, a key step in the regulation of liver metabolism by the sympathetic hepatic nerves

Cell-to-cell communication via gap junctions has been proposed to be involved in the metabolic actions of sympathetic liver nerves in the rat. The effects of hepatic nerve stimulation and noradrenaline-, PGF2 alpha- and glucagon infusion on glucose metabolism and perfusion flow were studied in perfused rat liver in the absence and presence of the gap junctional inhibitors, heptanol, carbenoxolone and (4 beta)phorbol 12-myristate 13-acetate (4 beta PMA). (i) Stimulation of the hepatic nerve plexus increased glucose output, decreased flow and caused an overflow of noradrenaline into the hepatic vein. (ii) Heptanol completely inhibited not only the nerve stimulation-dependent metabolic and hemodynamic alterations but also the noradrenaline overflow. Thus the heptanol-dependent inhibitions were caused primarily by a strong impairment of transmitter release. (iii) Carbenoxolone inhibited the effects of neurostimulation on glucose metabolism partially by about 50%, whereas it left perfusion flow and noradrenaline overflow essentially unaltered. (iv) 4 beta PMA reduced the nerve stimulation-dependent enhancement of glucose release by about 80% but the noradrenaline-dependent increase in glucose output only by about 30%; the increase in glucose release by PGF2 alpha and by glucagon remained essentially unaltered. 4 beta PMA reduced the nerve stimulation-dependent decrease in portal flow by about 35% but did not affect the noradrenaline-and PGF2 alpha-elicited alterations, nor did it alter noradrenaline overflow. The results allow the conclusion that gap junctional communication plays a major role in the regulation of hepatic carbohydrate metabolism by sympathetic liver nerves, but not by circulating noradrenaline, PGF2 alpha or glucagon.

Inhibition by noradrenaline and adrenaline of the increase in glucose and lactate output and decrease in flow after sympathetic nerve stimulation in perfused rat liver: possible involvement of protein kinase C

In perfused rat liver stimulation of the hepatic nerve plexuses increased via alpha 1-receptors glucose and lactate output decreased flow and caused an overflow of noradrenaline into the hepatic vein. Infusion of noradrenaline and adrenaline also elicited similar metabolic and hemodynamic alterations via alpha 1-receptors, whereas infusion of isoproterenol via beta 2-receptors enhanced glucose output and slightly reduced lactate release without affecting flow. The influence of circulating catecholamines on the nerve stimulation-dependent changes was investigated. Noradrenaline (100 nmol/L) or adrenaline (40 nmol/L) but not isoproterenol (1 mumol/L), which themselves caused about half-maximal alterations, strongly inhibited the nerve stimulation-induced increase in glucose and lactate output and decrease in flow but had no effect on noradrenaline overflow. The protein kinase C activator (4 beta)phorbol 12-myristate, 13-acetate (100 nmol/L) but not its analog (4 alpha)phorbol 12,13-didecanoate (100 nmol/L) strongly inhibited the metabolic and hemodynamic changes caused by nerve stimulation or noradrenaline infusion. The protein kinase C inhibitor H7 (20 mumol/L) partially prevented the inhibition of the nerve actions by noradrenaline. The results lead us to conclude that noradrenaline and adrenaline inhibited the metabolic and hemodynamic nerve actions by means of a mechanism involving protein kinase C rather than presynaptic alpha-receptors or beta-receptors. The catecholamines apparently increased via alpha 1-receptors inositol 1,4,5-trisphosphate, which in turn enhanced cytosolic Ca2+ and thus altered metabolism and in part hemodynamics, and diacylglycerol, which in turn activated protein kinase C and thus feedback inhibited the signal chain from alpha 1-receptors via G proteins to phospholipase C.

Metabolic and hemodynamic responses of bivascularly perfused rat liver to nerve stimulation, noradrenaline, acetylcholine and glucagon in thioacetamide-induced micronodular cirrhosis

Thioacetamide-induced rat cirrhosis was characterized by single-cell necroses, fibrosis, nodular parenchyma, decrease in parenchymal volume density and an increase in liver weight per body weight so that the total amount of parenchyma was not altered. The glycogen content was normal, and signs of decompensation were not found. Isolated livers were single-pass perfused by way of both the hepatic artery and the portal vein. In the normal livers stimulation of the nerve plexuses around the hepatic artery or portal vein (7.5 Hz; 2 msec) and infusions of noradrenaline (1 mumol/L) by way of either vessel and of acetylcholine (10 mumol/L) by way of the artery only increased glucose output, reduced both portal and arterial flow and increased the intravascular pressures. Glucagon (0.5 nmol/L) augmented glucose release and had no hemodynamic effects. In chronically thioacetamide-injured livers all stimuli caused smaller metabolic alterations per gram of liver weight and decreased portal flow more and arterial flow less with stronger enhancements of intravascular pressures than in the controls. The lowered metabolic responsiveness per gram of cirrhotic liver was largely compensated by the increase in liver weight. Thus despite massive histological alterations and pronounced increases in stimulation-dependent resistances - predominantly in the portal system - cirrhotic rat livers responded in their glucose metabolism to nervous and hormonal stimuli in almost the same manner as normal livers.

Increase in glucose and lactate output and perfusion resistance by stimulation of hepatic nerves in isolated perfused rat liver: role of alpha 1-, alpha 2-, beta 1- and beta 2-receptors

Rat liver was perfused in situ via the portal vein without recirculation: 1) Electrical stimulation of the nerve bundles around hepatic artery and portal vein increased glucose and lactate output, reduced flow and caused an overflow of noradrenaline into the hepatic vein. The alpha-agonist phenylephrine also augmented glucose and lactate output and lowered flow with an ED50 of about 1 microM, while the beta-agonist isoproterenol increased glucose output but reduced lactate output with an ED50 of about 0.2 microM and left flow unaltered. 2) The alpha 1-receptor antagonist prazosin (KI at alpha 1-sites approximately 1 nM, at alpha 2-sites approximately 100 nM) inhibited the nerve stimulation-dependent increase in glucose and lactate output and reduction of flow with an ID50 of about 1 nM, while the alpha 2-receptor antagonist yohimbine (KI at alpha 2-sites approximately 10 nM, at alpha 1-sites approximately 1500 nM) was inhibitory only with an ID50 of about 400 nM. 10 nM prazosin clearly reduced the nerve actions, completely blocked the effects of 1 microM phenylephrine and left the effects of 0.2 microM isoproterenol unaltered. 10 nM yohimbine did not affect the nerve actions nor the effects of phenylephrine or isoproterenol. 3) The beta 1-receptor antagonist metoprolol (KI at beta 1-sites approximately 100 nM, at beta 2-sites approximately 1.2 microM) at 10 microM concentrations did not interfere with the nerve stimulation-dependent increase in glucose and lactate output or the decrease in flow. It did not have any specific alpha-antagonistic influence either on the changes brought about by 1 microM phenylephrine; however, it blocked the beta 2-mediated increase in glucose output by isoproterenol.(ABSTRACT TRUNCATED AT 250 WORDS)

Control of glycogenolysis and blood flow by arterial and portal adrenaline in perfused liver

In isolated liver from fed rats, simultaneously single-pass-perfused via both the hepatic artery (80 mmHg, 30-35% flow) and the portal vein (10 mmHg, 70-65% flow), adrenaline was infused either singly or jointly via the hepatic artery or the portal vein in the absence or presence of the alpha 1-blocker prazosin and the beta 2-blocker butoxamine. It was found that: (1) arterial adrenaline caused increases in glucose and lactate output which were slower in onset, smaller in peak height but longer in duration than did portal adrenaline; (2) arterial adrenaline elicited a much more pronounced decrease in flow and increase in pressure in the ipsilateral vessel than did portal adrenaline, and arterial, but not portal, adrenaline elicited qualitatively similar alterations also in the contralateral vessel; (3) arterial adrenaline caused metabolic changes mainly via alpha 1-receptors, with beta 2-receptors playing a permissive role via haemodynamic alterations, whereas portal adrenaline acted only via alpha 1-receptors; (4) arterial adrenaline decreased arterial flow via alpha 1-receptors counteracted via beta 2-receptors and operated on portal flow as portal adrenaline only via alpha 1-receptors; and (5) arterial adrenaline was extracted to a far greater extent than portal adrenaline. The results indicate that the hepatic artery and the portal vein can function as independent sites of hormonal signal input, which interact by complex, still undefined, mechanisms.

Control of glycogenolysis and blood flow by arterial and portal norepinephrine in perfused liver

In isolated rat liver single pass perfused via both the hepatic artery (80 mmHg, 30% flow) and the portal vein (10 mmHg, 70% flow), norepinephrine (NE) was infused either singly or jointly via the hepatic artery or the portal vein in the absence or presence of the alpha 1-blocker prazosin or the beta 2-blocker butoxamine. Arterial NE caused an increase in glucose output and a shift from lactate uptake to release that was slower in onset and smaller in peak height but longer in duration than the alterations affected by portal NE. The sum of the metabolic changes by arterial and portal NE was not equal to the changes by jointly applied arterial plus portal NE. The metabolic alterations by arterial NE were mediated via alpha 1-receptors, with beta 2-receptors probably having a permissive function, but those by portal NE were transmitted only via alpha 1-receptors. Arterial NE caused a strong decrease in arterial flow and contralaterally also a smaller reduction of portal flow. Portal NE decreased portal flow but did not significantly influence arterial flow. The sum of the alterations in flow by arterial and portal NE was not equal to the changes by jointly applied NE. The hemodynamic alterations in the artery by arterial NE were the results of actions via alpha 1-receptors and counteractions via beta 2-receptors, whereas the changes in the portal vein by arterial NE and portal NE were mediated via alpha 1-receptors. About 65% of arterial and only 30% of portal NE was extracted during a single path. The results indicate that the hepatic artery and the portal vein can function as independent sites of hormonal signal input, which interact by complex but still undefined mechanisms in the regulation of metabolism and hemodynamics.

Differential control of glycogenolysis and flow by arterial and portal acetylcholine in perfused rat liver

The effects of acetylcholine on glucose and lactate balance and on perfusion flow were studied in isolated rat livers perfused simultaneously via the hepatic artery (100 mmHg, 25-35% of flow) and the portal vein (10 mmHg, 75-65% of flow) with a Krebs-Henseleit bicarbonate buffer containing 5 mM-glucose, 2 mM-lactate and 0.2 mM-pyruvate. Arterial acetylcholine (10 microM sinusoidal concentration) caused an increase in glucose and lactate output and a slight decrease in arterial and portal flow. These effects were accompanied by an output of noradrenaline and adrenaline into the hepatic vein. Portal acetylcholine elicited only minor increases in glucose and lactate output, a slight decrease in portal flow and a small increase in arterial flow, and no noradrenaline and adrenaline release. The metabolic and haemodynamic effects of arterial acetylcholine and the output of noradrenaline and adrenaline were strongly inhibited by the muscarinic antagonist atropine (10 microM). The acetylcholine-dependent alterations of metabolism and the output of noradrenaline were not influenced by the alpha 1-blocker prazosin (5 microM), whereas the output of adrenaline was increased. The acetylcholine-dependent metabolic alterations were not inhibited by the beta 2-antagonist butoxamine (10 microM), although the overflow of noradrenaline was nearly completely blocked and the output of adrenaline was slightly decreased. These results allow the conclusion that arterial, but not portal, acetylcholine caused sympathomimetic metabolic effects, without noradrenaline or adrenaline being involved in signal transduction.

Preparation of dopamine 3-O-sulphate and dopamine 4-O-sulphate as reference substances and high-performance liquid chromatographic trace determination

Simple syntheses of the biologically important but hitherto difficult to obtain dopamine sulpho conjugates dopamine 3-O-sulphate (I) and dopamine 4-O-sulphate (II), as analytical reference substances, starting from dopamine hydrochloride are described. A method for the determination of I and II with reversed-phase high-performance liquid chromatographic separation and coulometric detection in human urine together with calibration and current-voltage curves are presented. Detection limits of approximately 100 pg of I or II and unequivocal substance identifications even in very complex substrates such as human urine are reported.

Effects of oral and intravenous administrations of dopamine and L-dopa on plasma levels of two isomers of dopamine sulfate in man

The levels of two isomers of dopamine sulfate, dopamine-3-O-sulfate (DA3S) and dopamine-4-O-sulfate (DA4S), in human plasma were measured by HPLC-fluorometry. The basal plasma levels of DA3S and DA4S in the early morning were 13.8 +/- 1.9 and 3.2 +/- 0.5 pmoles/ml, respectively (means +/- S.E.M.). Oral administrations of dopamine (50 mg/body) and 1-dihydroxyphenylalanine (L-DOPA, 250 mg/body) increased the plasma levels of these dopamine sulfates almost 100-fold to 1807 +/- 266 and 1674 +/- 195 pmoles/ml of DA3S, and 466 +/- 83 and 321 +/- 76 pmoles/ml of DA4S. Intravenous dopamine infusion (5 micrograms/kg/min for 30 min) markedly increased the plasma free dopamine concentration, as expected, but increased the levels of DA3S and DA4S only slightly to 110 +/- 32 and 25 +/- 9 pmoles/ml, respectively. In contrast, intravenous L-DOPA (25 mg/body) resulted in a slight increase of free dopamine followed by marked increases of DA3S and DA4S to 691 +/- 219 and 139 +/- 40 pmoles/ml, respectively. These data indicate that O-sulfation of dopamine, especially 3-O-sulfation, is the main pathway for metabolism of intravenously and orally administered L-DOPA and orally ingested dopamine. This sulfation is suggested to occur in the gut wall.

Analytical techniques for the determination of phenolic amine neurotransmitter conjugates

This paper reviews methods developed in the author's laboratory for the measurement of the two sulfate conjugates of dopamine in biological material. The methods are sensitive enough for most applications relating to dopamine sulfate. Some are applicable to measurement of sulfate conjugates of other phenolic amines.