Tyce, Gertrude M.

Given the common pathways for uptake and synthesis for dopamine and serotonin, enhanced renal dopamine synthesis in response to increased substrate 3,4-dihydroxyphenylalanine (l-DOPA) is postulated to decrease renal serotonin synthesis. The present study compared the effects of chronic oral administration ofl-DOPA on dopamine and serotonin excretion in vivo, with the effects of enhanced dopamine synthesis per nephron due to adaptation to reduced renal mass (RRM). Four groups of rats were studied: sham-operated rats and rats with RRM in the absence and presence of chronic orall-DOPA.l-DOPA (2 mg ⋅ 100 g body wt−1⋅ day−1) for 6–14 days increased calculated dopamine synthesis per nephron in sham-operated rats from 2.0 ± 0.3 ( n = 7) to 13.6 ± 1.8 pg ⋅ day−1⋅ nephron−1( n = 7, P < 0.05) and in rats with RRM from 6.1 ± 1.3 ( n = 7) to 39.3 ± 5.2 pg ⋅ day−1⋅ nephron−1( n = 7, P < 0.05). Chronic orall-DOPA concomitantly decreased serotonin synthesis per nephron in sham-operated rats (1.6 ± 0.1 to 1.0 ± 0.1 pg ⋅ day−1⋅ nephron−1, n = 7, P < 0.05) and in rats with RRM (5.6 ± 0.9 to 2.6 ± 0.4 pg ⋅ day−1⋅ nephron−1, n = 7, P < 0.05). Both serotonin and dopamine synthesis per nephron were increased in rats with RRM. In conclusion, chronic oral administration ofl-DOPA enhances dopamine excretion and decreases serotonin excretion in normal rats and in rats with reduced renal mass. Both dopamine and serotonin excretions per nephron were elevated by renal mass reduction.

The tissue content and overflow of norepinephrine (NE), epinephrine (Epi), dopamine (DA), Met-enkephalin (Met-Enk), and neuropeptide Y (NPY) from isolated, retrogradely perfused dog adrenal glands were studied. Under resting conditions, approximately 25% of the overflow of autocoids from the glands was Ca2+ dependent; the cholinergic antagonists hexamethonium and atropine had no effects on basal efflux. Stimulation with the nicotinic agonist 1,1-dimethyl-4-phenylpiperazinium iodide (DMPP; 3 or 50 microM) or with the muscarinic agonist pilocarpine (50 microM or 1 mM) evoked releases of autocoids. These releases were blocked or dramatically reduced by appropriate antagonists or by the removal of Ca2+ from the perfusate. Expressed as percentages of tissue stores, the rank order of overflow of autocoids was E approximately DA much greater than NE during resting conditions, DA much greater than E approximately NE during stimulation with 50 microM DMPP, and DA greater than Epi greater than NE during stimulation with 1 mM pilocarpine. These data are consistent with different mechanisms of release for the catecholamines, perhaps from different cell populations. The data support corelease of peptides and catecholamines, although clear pairing of autocoids could not be confirmed.

We tested the hypothesis that dopamine (DA) acts as an autocrine/paracrine regulator of Na(+)-Pi symport in proximal tubules, using opossum kidney (OK) cells as an in vivo model. Both DA and parathyroid hormone (PTH) increased adenosine 3',5'-cyclic monophosphate (cAMP) and inhibited Na(+)-gradient-dependent uptake of 32P but not that of L-[3H]-alanine. Incubation of OK cells with L-dopa, a DA precursor, resulted in accumulation of DA (7.4 nM), a ninefold increase of cAMP in the medium, and an inhibition (-10%) of Na(+)-Pi uptake. Carbidopa, an inhibitor of aromatic-L-amino acid decarboxylase, prevented the formation of DA from L-dopa, the increase in cAMP, and the inhibition of Na(+)-Pi cotransport. Pi-replete OK cells produced more DA (+15%) from L-dopa than Pi-deprived cells; however, the endogenous DA inhibited Na(+)-Pi cotransport both in Pi-deprived and in Pi-replete cells. Thus OK cells can synthesize DA from L-dopa in a quantity sufficient to elicit both the maximum DA-stimulated cAMP accumulation and inhibition of Na(+)-Pi cotransport in the same cell population. Our data, obtained on an in vitro system, support the hypothesis proposing that DA generated in proximal tubular cells can modulate, via cAMP, the Na(+)-Pi symport in the same or adjacent cells. If present in the kidney, this pathway might represent an autocrine/paracrine system that can contribute to regulation of renal Pi homeostasis.

The source and fate of sulfoconjugated catecholamines present in plasma are not known. Sulfated norepinephrine has been shown to overflow together with free norepinephrine from a typical peripheral neuroeffector junction. Some released free norepinephrine is removed via the lymphatic system, but it is not known whether sulfated catecholamines are removed similarly. Serotonin, another biogenic amine, is released from the enterochromaffin cells into portal blood. The aims of the present study were to determine whether conjugated catecholamines and serotonin could be detected in thoracic duct lymph of dogs and to compare concentrations in lymph with those in samples of portal blood collected before and after the dogs were fed. A method is described for the extraction and purification of free and conjugated catecholamines and serotonin from lymph. In lymph and in plasma, there were similar concentrations of free (0.2-0.4 ng/ml) and conjugated (0.1-0.2 ng/ml) norepinephrine and of conjugated dopamine (0.2-1.0 ng/ml). The concentrations of free serotonin were much lower in lymph (0.1-0.2 ng/ml) than in portal blood (100-200 ng/ml). Small amounts of conjugated serotonin (0.1-0.4 ng/ml) were detected in lymph but were not detected in plasma. Feeding caused increases in concentration of conjugated dopamine in lymph and in plasma and small increases in the output of free norepinephrine and free and conjugated serotonin in lymph.

Neuropeptide Y-like immunoreactivity (NPY-LI), norepinephrine (NE), and 3,4-dihydroxyphenylglycol (DOPEG), the intraneuronal metabolite of NE, were measured in superfusate before, during, and after electrical stimulation (ES) of nerves and in the tissue extract of dog pulmonary artery after in vitro superfusion. Vessels were stimulated at 12, 6, or 1 Hz. NE and DOPEG were quantified by high-performance liquid chromatography with electrochemical detection, NPY-LI by radioimmunoassay. The Krebs-Ringer superfusate was maintained at a partial pressure of oxygen of either 472, 100, 50, or 20 mmHg. Mean DOPEG efflux was 0.17, 0.15, 0.13, and 0.07 pmol/min during basal conditions and 0.73, 0.39, 0.30, and 0.15 pmol/min, respectively, during 12-Hz continuous stimulation using these four oxygen pressures. Stimulation-evoked efflux of NPY-LI was 1.5, 1.1, 0.3, and 0.2 fmol/min during 12-Hz stimulation. These studies provide evidence that the production and subsequent efflux of DOPEG into superfusate under resting conditions, during ES, and following ES are oxygen sensitive. Additionally, the efflux of NPY-LI from dog pulmonary artery resulting from high frequencies of ES is oxygen sensitive.

Dihydroxyphenylglycol (DOPEG), the metabolite of norepinephrine (NE) that arises intraneuronally, was measured together with NE in superfusates collected before, during, and after nerve stimulation and in extracts of dog saphenous vein after superfusion and electrical stimulation (ES). Different concentrations of NE in the synaptic clefts were achieved by treating tissues with corticosterone, corticosterone and yohimbine, corticosterone and cocaine, or by omitting drugs from the superfusate. NE and DOPEG were quantitated by liquid chromatography with electrochemical detection. The time courses of NE overflow and DOPEG efflux into superfusate were followed. The amounts of DOPEG in superfusates under basal conditions were two to four times higher than the amounts of NE and progressively increased during ES except in tissues with neuronal uptake inhibited. NE overflow reached a steady state within the first 6 min of ES. Increased NE concentrations in synaptic clefts resulted in increased DOPEG production except where neuronal uptake was inhibited. The increased DOPEG production during ES appears to reflect the increased rate of neuronal uptake, which results in more NE being available for intraneuronal metabolism. No evidence was found that newly formed DOPEG was delayed in leaving the tissue. Thus the increase in DOPEG production that occurs during ES may be useful as an index of neuronal uptake of NE in dog saphenous vein.

BACKGROUND Because acute exercise increases systemic catecholamines, we sought to determine whether exercise training would alter daily or exercise-related catecholamine release and inactivation. METHODS AND RESULTS In 24-hour urine collections, catecholamines and metabolites provided indexes of overall oxidative deamination, sulfation, and O-methylation. Plasma catecholamines, the sulfoconjugates of each, and chromogranin-A were determined at rest and during exercise in 10 well-trained male subjects and nine minimally trained male subjects (maximal oxygen uptake 55.2 and 42.5 ml/kg/min, respectively), and levels of activities of catechol-O-methyltransferase (COMT), monoamine oxidase B (MAO-B), and thermolabile phenolsulfotransferase (TL-PST) were also determined. Plasma-free catecholamines showed minimal differences between the two groups at submaximal exercise (4 minutes) but large differences at maximal exercise, reflecting the different exercise levels attained. Inactivation of plasma catecholamines by sulfation across rest and exercise tended to be greater in the well-trained group, with small increases in both plasma sulfoconjugated dopamine and sulfoconjugated norepinephrine. In the well-trained group, urinary metabolites demonstrated trends toward increased dopamine release (p less than 0.07) and small increases in the daily release of epinephrine and its sulfoconjugated metabolites. Indexes of deamination, sulfoconjugation, and O-methylation, with the exception of a reduced deamination of dopamine and the activities of COMT, MAO-B, and TL-PST were not different in the two groups. CONCLUSIONS Despite considerable differences in the exercise activities per week between well-trained and minimally trained individuals, there were minimal differences in the release and metabolism of catecholamines at rest or during exercise.

The pharmacological and biochemical tests available for the diagnosis of pheochromocytoma were evaluated among 28 patients with proved tumors and 148 other patients in whom the diagnosis was suspected. Of the biochemical tests on urine, determinations of total metanephrines (MN) and of vanilmandelic acid (VMA) were much less subject to false-positive results than was determination of catecholamines (CA). Since urinary MN values changed the most in tumor patients, MN determination is preferred as the current screening test. Of the pharmacological tests, the histamine test continues to be the most valuable, especially when combined with determination of plasma CA concentration. The phentolamine (Regitine) test has not proved to be of any distinct value as a primary diagnostic aid. Any of the chemical or pharmacological tests occasionally may give a false-positive result. The urinary output of excessive amounts of homovanillic acid (HVA) was an indication of the presence of pheochromocytoma with some features histologically similar to other neural crest tumors but was not an indication of malignancy. Reliable diagnosis of pheochromocytoma still must be based on multiple studies in an appropriate clinical setting.