The bicuculline-like properties of dopamine sulfate in rat brain

To determine whether the convulsive action of intraventricularly injected dopamine sulfate, a dopamine metabolite present in rat brain and human cerebrospinal fluid, could be due to its interaction with GABAergic pathway, we compared the convulsive effect of dopamine sulfate with that of bicuculline in the conscious rat and determined the interaction of dopamine sulfate with [3H] GABA binding and uptake in rat brain tissues. The results showed that the convulsive effects of dopamine sulfate and of bicuculline could be abolished by GABA agonists diazepam and muscimol, but not by DA antagonists haloperidol and metoclopramide. In addition they were additive. Both dopamine 3-O-sulfate and dopamine-4-O-sulfate, like bicuculline, could displace sodium-independent [3H] GABA binding to rat brain synaptic membranes (IC50 = 400 microM) but had no action on GABA uptake. DA sulfate had no effect on [3H] strychnine binding to rat brain homogenates. This evidence together with the structural resemblance between dopamine sulfate and GABA suggested that the convulsive activity of dopamine sulfate may result from its interaction with central GABA receptors.     

Epinephrine, norepinephrine, and dopamine during a 4-day head-down bed rest

Head-down bed rest at an angle of 6 degrees was used as an experimental model to simulate the hemodynamic effects of microgravity, i.e., the shift of fluids from the lower to the upper part of the body. The sympathoadrenal activity during acute (from 0.5 to 10 h) and prolonged (4 days) head-down bed rest was assessed in eight healthy men (24 +/- 1 yr) by measuring epinephrine (E), norepinephrine (NE), dopamine (DA), and methoxylated metabolite levels in their plasma and urine. Catecholamine (CA) and methoxyamine levels were essentially unaltered at any time of bed rest. Maximal changes in plasma were on the second day (D2): NE, 547 +/- 84 vs. 384 +/- 55 pg/ml; DA, 192 +/- 32 vs. 141 +/- 16 pg/ml; NS. After 24 h of bed rest, heart rate decreased from 71 +/- 1 to 63 +/- 3/min (P less than 0.01). Daily dynamic leg exercise [50% maximum O2 uptake (VO2 max)] used as a countermeasure did not alter the pattern of plasma CA during bed rest but resulted in a higher urinary NE excretion during postexercise recovery (+45% on D2; P less than 0.05). The data indicate no evident relationship between sympathoadrenal function and stimulation of cardiopulmonary receptors or neuroendocrine changes induced by central hypervolemia during head-down bed rest.     

Reserpine pretreatment prevents increases in extracellular striatal dopamine following L-DOPA administration in rats with nigrostriatal denervation

The influence of L-DOPA and reserpine on extracellular dopamine (DA) levels in the striatum of intact and dopaminergic denervated rats was studied using the brain microdialysis technique. In intact rats, reserpine (5 mg/kg s.c.) reduced extracellular DA levels to 4% of basal values. L-DOPA (50 mg/kg i.p.) had no effect on extracellular DA levels in reserpine-pretreated rats. In rats with 6-hydroxydopamine-induced lesion of the nigrostriatal dopaminergic system, basal levels of extracellular DA were low but markedly increased by L-DOPA (50 mg/kg i.p.). In 6-hydroxydopamine-lesioned rats, pretreatment with reserpine (5 mg/kg s.c.) diminished L-DOPA (50 mg/kg i.p.)-induced increases in extracellular DA levels to 16% of those obtained in denervated animals not pretreated with reserpine (p<0.01). These results suggest that in the intact striatum, extracellular DA stems mainly from vesicular storage sites and that in the striatum with dopaminergic denervation, a large part of the L-DOPA-derived extracellular DA is also derived from a vesicular pool that is released by an exocytosis mechanism.     

Effects of l-DOPA on the Concentrations of Free and Sulfoconjugated Cathecholamines in Plasma, Cerebrospinal Fluid, Urine, and Central and Peripheral Nervous System Tissues of the Rat

The effects of subcutaneous injection of L-beta-3,4-dihydroxyphenylalamine (L-DOPA) on the concentrations of the catecholamines and catecholamine sulfates in the central and peripheral nervous systems of the rat were studied. The results showed that free 3,4-dihydroxyphenylethylamine (DA, dopamine) increased rapidly and markedly in the hypothalamus and striatum after L-DOPA but DA sulfate did not change. Increased concentrations of DA sulfate were detected in the CSF and in the plasma, where it reached a concentration of 130.8 +/- 12.8 ng/ml at 2 h, seven times the level of free DA (19.1 +/- 2.9 ng/ml). In the kidney the ratio of DA sulfate to free DA was reversed in favor of free DA. Urine samples of L-DOPA-treated rats showed a higher increase of free DA than DA sulfate, but free norepinephrine (NE) and NE sulfate remained unchanged. Concentrations of free DA and free NE in the adrenal glands of L-DOPA-treated rats showed no change. Adrenal DA sulfate and NE sulfate were not detectable in the control and L-DOPA-treated rats, suggesting that the adrenal glands lack the capacity to take up or store catecholamines and their sulfate counterparts from the plasma.     

Dopamine sulfate formation and phenol sulfotransferase activity in dog and human platelets

High performance liquid chromatography with radioactive flow detection was used to examine the accumulation and sulfoconjugation of dopamine by human and dog platelets. Platelets from both species accumulated similar amounts of dopamine from the incubation medium, but only human platelets were found to convert 3H-dopamine to 3-H-dopamine sulfate. This difference between the two species was associated with a relative absence of phenol sulfotransferase activity in dog platelets as compared to human platelets. Dog platelets did not appear to contain an inhibitor of phenol sulfotransferase activity. Despite the apparent difference in the ability of platelets to form dopamine sulfate, conc concentrations of dopamine-3-O-sulfate and dopamine-4-O-sulfate were similar in dog and human plasma. These data suggest that platelets may represent a potential source of at least some of the dopamine sulfate found in human plasma, but not in dog plasma.     

Determination of sulfated biogenic amines: emphasis on dopamine sulfate isomers

Investigations concerning the specific conjugates of dopamine (DA) in the body have been hampered by the lack of specific and sensitive methodology for their detection. Because there is increasing interest in the occurrence and measurement of sulfated metabolites of DA and the other catecholamines (CA) norepinephrine and epinephrine, reliable methods for the detection and quantitation of these compounds in biological samples are needed. This paper reviews recently developed methods for the measurement of the two sulfate conjugates of DA in the body, DA-3-O-sulfate and DA-4-O-sulfate. The methods are sensitive enough for most, if not all, routine applications relating to DA sulfate. With minor modifications, some of the methods should be applicable to measurement of sulfate conjugates of many other electrochemically active compounds, such as other CA or indoles, their metabolites, and some drugs and metabolites.     

Activation of 5-HT(1A) but not 5-HT(1B) receptors attenuates an increase in extracellular dopamine derived from exogenously administered L-DOPA in the striatum with nigrostriatal denervation

In order to determine whether L-DOPA-derived extracellular dopamine (DA) in the striatum with dopaminergic denervation is affected by activation of serotonin autoreceptors (5-HT(1A) and 5-HT(1B) receptors), we applied in vivo brain microdialysis technique to 6-hydroxydopamine-lesioned rats and examined the effects of the selective 5-HT(1A) receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) and the selective 5-HT(1B) receptor agonist CGS-12066 A on L-DOPA-derived extracellular DA levels. Single L-DOPA injection (50 mg/kg i.p.) caused a rapid increase and a following decrease of extracellular DA, with a peak value at 100 min after L-DOPA injection. Pretreatment with both 0.3 mg/kg and 1 mg/kg 8-OH-DPAT (i.p.) significantly attenuated an increase in L-DOPA-derived extracellular DA and the times of peak DA levels were prolonged to 150 min and 225 min after L-DOPA injection, respectively. These 8-OH-DPAT-induced changes in L-DOPA-derived extracellular DA were antagonized by further pretreatment with WAY-100635, a selective 5-HT(1A) antagonist. In contrast, intrastriatal perfusion with the 5-HT(1B) agonist CGS-12066 A (10 nM and 100 nM) did not induce any changes in L-DOPA-derived extracellular DA. Thus, stimulation of 5-HT(1A) but not 5-HT(1B) receptors attenuated an increase in extracellular DA derived from exogenous L-DOPA. These results support the hypothesis that serotonergic neurons are primarily responsible for the storage and release of DA derived from exogenous L-DOPA in the absence of dopaminergic neurons.     

Striatal dopamine metabolism in monoamine oxidase B-deficient mice: a brain dialysis study

We have studied striatal dopamine (DA) metabolism in monoamine oxidase (MAO) B-deficient mice using brain microdialysis. Baseline DA levels were similar in wild-type and knock-out (KO) mice. Administration of a selective MAO A inhibitor, clorgyline (2 mg/kg), increased DA levels and decreased levels of its metabolites in all mice, but a selective MAO B inhibitor, l-deprenyl (1 mg/ kg), had no effect. Administration of 10 and 50 mg/kg L-DOPA, the precursor of DA, increased the levels of DA similarly in wild-type and KO mice. The highest dose of L-DOPA (100 mg/kg) produced a larger increase in DA in KO than wild-type mice. This difference was abolished by pretreating wild-type mice with l-deprenyl. These results suggest that in mice, DA is only metabolized by MAO A under basal conditions and by both MAO A and B at high concentrations. This is in contrast to the rat, where DA is always metabolized by MAO A regardless of concentration.     

Determination of Serotonin and Dopamine Metabolites in Human Brain Microdialysis and Cerebrospinal Fluid Samples by UPLC-MS/MS: Discovery of Intact Glucuronide and Sulfate Conjugates

An UPLC-MS/MS method was developed for the determination of serotonin (5-HT), dopamine (DA), their phase I metabolites 5-HIAA, DOPAC and HVA, and their sulfate and glucuronide conjugates in human brain microdialysis samples obtained from two patients with acute brain injuries, ventricular cerebrospinal fluid (CSF) samples obtained from four patients with obstructive hydrocephalus, and a lumbar CSF sample pooled mainly from patients undergoing spinal anesthesia in preparation for orthopedic surgery. The method was validated by determining the limits of detection and quantification, linearity, repeatability and specificity. The direct method enabled the analysis of the intact phase II metabolites of 5-HT and DA, without hydrolysis of the conjugates. The method also enabled the analysis of the regioisomers of the conjugates, and several intact glucuronide and sulfate conjugates were identified and quantified for the first time in the human brain microdialysis and CSF samples. We were able to show the presence of 5-HIAA sulfate, and that dopamine-3-O-sulfate predominates over dopamine-4-O-sulfate in the human brain. The quantitative results suggest that sulfonation is a more important phase II metabolism pathway than glucuronidation in the human brain.     

Effect of barbitone sodium and thiopental sodium on brain dopamine, noradrenaline, serotonin and 5-hydroxyindoleacetic acid content in Arvicanthis niloticus

The quantitative estimation of total dopamine (DA), noradrenaline (NE), serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) content in the whole brain tissue of normal Nile grass rat, Arvicanthis niloticus, gives and average of 631 +/- 12 ng DA/g, 366 +/- 12 ng NE/g, 617 +/- 15 ng 5-HT/g and 431 +/- 10 ng 5-HIAA/g fresh brain tissue. The effect of barbitone sodium and thiopental sodium on the total DA, NE, 5-HT and 5-HIAA content in the brain tissue of the Nile grass rat, Arvicanthis niloticus, was studied. The total DA, NE, 5-HT and 5-HIAA contents were determined 5 hr after i.p. injection of different doses of barbitone sodium (20, 40 and 80 mg/ml/100 g body wt) and thiopental sodium (5, 10 and 20 mg/ml/100 g body wt). The effect of different time intervals (1, 10, 30 min, 1, 2.5, 5, 8, 16, 24 and 48 hr) on the total brain DA, NE, 5-HT and 5-HIAA content was investigated after i.p. injection of 40 mg of barbitone sodium and 10 mg of thiopental sodium/ml/100 g body wt. Both barbitone sodium and thiopental sodium caused an increase in DA, NE and 5-HT content and a decrease in 5-HIAA content in the brain tissue of Arvicanthis niloticus. The increase in the whole brain contents of DA, NE and 5-HT after the administration of barbitone sodium and thiopental sodium may be due either to inhibition of transmitter release by an action at the monoamine nerve terminal or to effects causing a decrease in nerve impulse flow. On the other hand, the decrease in 5-HIAA may be due to the decrease in the turnover of 5-HT.     

High-performance liquid chromatographic determination of dopamine sulfoconjugates in urine after l-DOPA administration

A procedure was developed for the separation and determination of dopamine-3-O-sulfate (DM-3-S) and dopamine-4-O-sulfate (DM-4-S) in the urine of subjects administered l-DOPA. The method consists of sample preparation using cation- and anion-exchange resins followed by determination of the sulfates by high-performance liquid chromatography. The addition recoveries were 96 ± 2.9% (S.D.) for DM-3-S and 93 ± 3.0% (S.D.) for DM-4-S. Twenty samples could be measured per day. When every 2-h urine specimen from normal subjects was analysed after l-DOPA administration (0.5 g), the maximum excretion of each sulfate was observed in the second 2-h specimen. For the first 6 h 7.5 ± 1.5% (S.D.) of the administered l-DOPA was excreted as DM-O-sulfates. During this time, the ratio of DM-4-S to the DM-O-sulfates was 11.7 ± 0.58% (S.D.).     

Extracellular Concentrations of Dopamine and Metabolites in the Rat Caudate After Oral Administration of a Novel Catechol-O-Methyltransferase Inhibitor Ro 40–7592

The effect of the systemic administration of a novel, orally active, catechol-O-methyltransferase (COMT) inhibitor, Ro 40-7592, on the in vivo extracellular concentrations of dopamine (DA) and its metabolites, dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), was studied by transcerebral microdialysis in the dorsal caudate of freely moving rats. Ro 40-7592 (at doses of 3.0, 7.5, and 30 mg/kg p.o.) elicited a marked and long-lasting reduction of HVA, and at doses of 7.5 and 30 mg/kg, an increase of DOPAC output, but it failed to increase DA output. The administration of L-beta-3,4-dihydroxyphenylalanine (L-DOPA, 20 and 50 mg/kg p.o.) with a DOPA decarboxylase inhibitor (benserazide) increased both HVA and DOPAC output, but failed to modify significantly extracellular DA concentrations in dialysates; in contrast, combined administration of L-DOPA+benserazide with Ro 40-7592 (30 mg/kg p.o.) resulted in a significant increase in DA output. Ro 40-7592 prevented the L-DOPA-induced increase in HVA output and markedly potentiated the increase in DOPAC output. To investigate to what extent the increase in extracellular DA concentrations was related to an exocitotic release, tetrodotoxin (TTX) sensitivity was tested. Addition of TTX to Ringer, although abolishing DA output in the absence of L-DOPA, partially reduced it in the presence of L-DOPA+Ro 40-7592 and even more so after L-DOPA without the COMT inhibitor. The results of the present study suggest that metabolism through COMT regulates extracellular concentrations of DA formed from exogenously administered L-DOPA but not of endogenous DA.(ABSTRACT TRUNCATED AT 250 WORDS)     

Novel multifunctional neuroprotective iron chelator-monoamine oxidase inhibitor drugs for neurodegenerative diseases. In vivo selective brain monoamine oxidase inhibition and prevention of MPTP-induced striatal dopamine depletion

Several multifunctional iron chelators have been synthesized from hydroxyquinoline pharmacophore of the iron chelator, VK-28, possessing the monoamine oxidase (MAO) and neuroprotective N-propargylamine moiety. They have iron chelating potency similar to desferal. M30 is a potent irreversible rat brain mitochondrial MAO-A and -B inhibitor in vitro (IC50, MAO-A, 0.037 +/- 0.02; MAO-B, 0.057 +/- 0.01). Acute (1-5 mg/kg) and chronic [5-10 mg/kg intraperitoneally (i.p.) or orally (p.o.) once daily for 14 days]in vivo studies have shown M30 to be a potent brain selective (striatum, hippocampus and cerebellum) MAO-A and -B inhibitor. It has little effects on the enzyme activities of the liver and small intestine. Its N-desmethylated derivative, M30A is significantly less active. Acute and chronic treatment with M30 results in increased levels of dopamine (DA), serotonin(5-HT), noradrenaline (NA) and decreases in DOPAC (dihydroxyphenylacetic acid), HVA (homovanillic acid) and 5-HIAA (5-hydroxyindole acetic acid) as determined in striatum and hypothalamus. In the mouse MPTP (N-methy-4-phenyl-1,2,3,6-tetrahydropyridine) model of Parkinson's disease (PD) it attenuates the DA depleting action of the neurotoxin and increases striatal levels of DA, 5-HT and NA, while decreasing their metabolites. As DA is equally well metabolized by MAO-A and -B, it is expected that M30 would have a greater DA neurotransmission potentiation in PD than selective MAO-B inhibitors, for which it is being developed, as MAO-B inhibitors do not alter brain dopamine.     

Salt intake and intestinal dopaminergic activity in adult and old Fischer 344 rats

We have earlier shown that the renal dopaminergic system failed to respond to high salt (HS) intake in old (24-month-old) Fisher 344 rats (Hypertension 1999;34:666-672). In the present study, intestinal Na+,K+-ATPase activity and intestinal dopaminergic tonus were evaluated in adult and old Fischer 344 rats during normal salt (NS) and HS intake. Basal intestinal Na+,K+-ATPase activity (nmol Pi/mg protein/min) in adult rats (142+/-6) was higher than in old Fischer 344 rats (105+/-7). HS intake reduced intestinal Na+,K+-ATPase activity by 20% (P<0.05) in adult, but not in old rats. Dopamine (1 microM) failed to inhibit intestinal Na+,K+-ATPase activity in both adult and old Fischer 344 rats (NS and HS diets). In adult animals, co-incubation of pertussis toxin with dopamine (1 microM) produced a significant inhibitory effect in the intestinal Na+,K+-ATPase activity. L-DOPA and dopamine tissue levels in the intestinal mucosa of adult rats were higher (45+/-9 and 38+/-4 pmol/g) than those in old rats (27+/-9 and 14+/-1 pmol/g). HS diet did not change L-DOPA and DA levels in both adult and old rats. DA/L-DOPA tissue ratios, an indirect measure of dopamine synthesis, were higher in old (1.1+/-0.2) than in adult rats (0.6+/-0.1). Aromatic L-amino acid decarboxylase (AADC) activity in the intestinal mucosa of old rats was higher than in adult rats. HS diet increased the AADC activity in adult rats, but not in old rats. It is concluded that intestinal dopaminergic tonus in old Fisher 344 rats is higher than in adult rats and is accompanied by lower basal intestinal Na+,K+-ATPase activity. In old rats, HS diet failed to alter the intestinal dopaminergic tonus or Na+,K+-ATPase activity, whereas in adult rats increases in AADC activity were accompanied by decreases in Na+,K+-ATPase activity. The association between salt intake, increased dopamine formation and inhibition of Na+,K+-ATPase at the intestinal level was not as straightforward as that described in renal tissues.     

Role of high-affinity dopamine uptake and impulse activity in the appearance of extracellular dopamine in striatum after administration of exogenous L-DOPA: studies in intact and 6-hydroxydopamine-treated rats

The differential behavioral and neurochemical effects of exogenous L-DOPA in animals with intact versus dopamine (DA)-denervated striata raise questions regarding the role of DA terminals in the regulation of dopaminergic neurotransmission after administration of exogenous L-DOPA. In vivo microdialysis was used to monitor the effect of exogenous L-DOPA on extracellular DA in intact and DA-denervated striata of awake rats. In intact striatum, a small increase in extracellular DA was observed after administration of L-DOPA (50 mg/kg i.p.) but in DA-denervated striatum a much larger increase in extracellular DA was elicited. Additional experiments assessed the role of high-affinity DA uptake and impulse-dependent neurotransmitter release in the effect of exogenous L-DOPA on extracellular DA in striatum. Pretreatment with GBR-12909 (20 mg/kg i.p.), a selective DA uptake inhibitor, enhanced the ability of L-DOPA to increase extracellular DA in intact striatum. However, in DA-denervated striatum, inhibition of DA uptake did not alter the extracellular DA response to L-DOPA. Impulse-dependent neurotransmitter release was blocked by the infusion of tetrodotoxin (TTX; 1 microM), an inhibitor of fast sodium channels, through the dialysis probe. Application of TTX significantly attenuated the L-DOPA-induced increase in extracellular DA observed in striatum of intact rats pretreated with GBR-12909. In a similar manner, TTX infusion significantly attenuated the increase in extracellular DA typically observed in striatum of 6-OHDA-lesioned rats after the administration of L-DOPA. The present results indicate that DA terminals, via high-affinity uptake, play a crucial role in the clearance of extracellular DA formed from exogenous L-DOPA in intact striatum. This regulatory mechanism is absent in the DA-denervated striatum. In addition, this study has shown that DA synthesized from exogenous L-DOPA primarily is released by an impulse-dependent mechanism in both intact and DA-denervated striatum. The latter result suggests an important role for a nondopaminergic neuronal element in striatum that serves as the primary source of extracellular DA formed from exogenous L-DOPA.     

Effects of dopaminergic agonists on plasma luteinizing hormone-releasing hormone (LRH) and gonadotropins in man

To try to determine the site and mode of action of dopamine (DA) on the secretion of gonadotropins in humans, 4 series of studies were performed in 19 healthy volunteers (7 healthy males, 5 females in follicular phase and 7 postmenopausal females). The intravenous infusion of DA at the rate of 4 micrograms/kg/min decreased plasma LH levels to 75.9 +/- 3.7 (mean +/- SE)% of the initial levels. The oral administration of 500 mg of levodopa (Dopa) also suppressed the concentration of plasma LH to 73.0 +/- 3.5%, but the pretreatment with two doses of 100 mg each of carbidopa (CD), a peripheral dopa decarboxylase inhibitor, attenuated this suppressing effect of Dopa on LH levels (nadir 82.4 +/- 4.1). Plasma FSH were not significantly altered by these drugs. On the other hand, the concentration of endogenous LRH in peripheral blood remained unchanged throughout the studies. The administration of CD alone had no effect on any of these 3 parameters. These results would suggest the direct suppressing effect of DA on pituitary gonadotrophs, although the modulation at the level of the hypothalamus also cannot be ruled out.     

Abnormal dopamine sensitivity in some human prolactinomas

In most of human prolactin (PRL)-secreting adenomas, dopamine and dopamine agonists normally suppress the excessive PRL secretion. Nevertheless, a subpopulation of such patients presents a relative insensitivity to the ergot derivative bromocriptine. Six patients with a macroadenoma (n = 5) or microadenoma (n = 1) were considered resistant to bromocriptine which, at a daily dose of 15-60 mg, did not normalize high plasma PRL levels. Culture studies of these adenoma cells showed that: (1) 10(-8) M bromocriptine produced a 32 +/- 16% inhibition of PRL release versus 65 +/- 12% obtained in the same conditions with normal human pituitary cells; (2) sulpiride (10(-6) M) reversed the inhibitory effects of bromocriptine, and (3) the bacterial endotoxins, cholera toxin (10(-11) M) and pertussis toxin (250 ng/ml), respectively, produced a 45-500% increase and a total abolition of bromocriptine-induced PRL inhibition. These observations and recent data of the literature allow to discuss the possibility of receptor or postreceptor defects in such tumors.     

Parathyroid hormone and calcitonin secretion in man: effect of dopamine agonists and antagonists

This study was undertaken to evaluate the physiological role, if any, of dopamine (DA) in modulating parathyroid hormone (PTH) and calcitonin (CT) secretion in man. Infusion of DA (5 micrograms/kg/min) into 6 normal men, decreased serum immunoreactive prolactin (iPRL) and concomitantly increased serum iPTH to 140 +/- 6.8% of baseline (P less than 0.01) at 30 min, with decline thereafter, despite continuation of the DA infusion. Serum iCT levels did not significantly change. Chlorpromazine (50 mg IM), decreased serum iPTH to 75 +/- 5.4% and 79 +/- 3.7% of baseline (P less than 0.01) at 30 and 60 min, respectively, associated with an increase in iPRL. There was subsequent return of iPTH to baseline even though iPRL remained elevated. iCT levels did not significantly change. These observations would suggest that DA may play a physiological role in iPTH, but not iCT, secretion. However, infusion of more nearly physiological doses of DA (0.02, 0.2, and 2.0 micrograms/kg/min) lowered serum iPRL to levels similar to those after the larger DA dose, but with no concomitant increase in either iPTH or iCT. Also, 1) the DA agonist bromocriptine decreased serum iPRL without modifying iPTH or iCT; 2) the DA precursor, levodopa, and the DA antagonist, metoclopramide, had no effect on serum iPTH or iCT levels. These studies suggest that 1) the transient stimulatory effect of DA on iPTH secretion is pharmacological, and 2) DA does not have a physiological role in secretion of iPTH or iCT in man.     

Free dopamine in dog plasma: lack of relationship with sympathoadrenal activity

To investigate the relationship between dopamine (DA) released into the bloodstream and sympathoadrenal activity, levels of free DA, norepinephrine (NE), and epinephrine (E) in plasma were recorded in four dogs subjected to three tests: treadmill exercise at two work levels [55 and 75% maximal O2 uptake; 15 min], normobaric hypoxia (12% O2; 1 h), combined exercise and hypoxia. Normoxic exercise induced slight nonsignificant decreases in the arterial partial pressure of O2 (PaO2), increases in NE [median values and ranges during submaximal work vs. rest: 1086 (457-1,637) vs. 360 (221-646) pg/ml; P less than 0.01] and E [277 (151-461) vs. 166 (95-257) pg/ml; P less than 0.05], but it failed to alter the DA level. Hypoxia elicited large decreases in PaO2 [hypoxia vs. normoxia: 42.8 (40.3-50.0) vs. 97.6 (83.2-117.6) Torr; P less than 0.01], increases in DA [230 (105-352) vs. 150 (85-229) pg/ml; P less than 0.01] and NE [383 (219-1,165) vs. 358 (210-784) pg/ml; P less than 0.05], but it failed to alter the E level. Combined exercise and hypoxia further increased NE levels but did not alter the DA response to hypoxia alone. The data indicate that free DA in plasma may vary independently of the sympathoadrenal activity.     

Effects of L-dopa and L-tyrosine on release of free and conjugated dopamine, homovanillic acid and dihydroxyphenylacetic acid from slices of rat striatum

Conjugation (presumably with sulfate) is a demonstrable metabolic pathway for 3, 4-dihydroxyphenylethylamine (dopamine, DA) in brain. Studies were done to determine whether conjugation becomes of increased significance in the presence of precursors of DA. The effects of 3, 4-dihydroxyphenylalanine (L-DOPA) and L-tyrosine on the efflux of free and conjugated DA, 3, 4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid from slices from striatum in rats were studied under quiescent conditions and during release evoked by 40 mM K+ or by 5 X 10(-5) M phenylethylamine (PEA). Conjugated DA was present in the basal efflux from striatal slices and the amounts present were increased during evoked release. More conjugated DA was present in superfusate during K+-evoked release than during PEA-evoked release. L-Tyrosine (5 X 10(-4) M or 5 X 10(-5) M) had little effect on the efflux of conjugated DA, but decreased the amounts of free DA released by PEA, and attenuated the increase in DOPAC that occurred during K+-evoked release of transmitter. L-DOPA (5 X 10(-5) M) increased the formation of conjugated DA, but to a lesser extent than that of free DA or of DOPAC. Thus even after the addition of precursors, conjugation remains a minor metabolic pathway for DA relative to O-methylation or oxidative deamination. The data also suggest that conjugation of DA occurs chiefly outside of the dopaminergic neurons in striatum.