Effect of alpha-human atrial natriuretic peptide on the synthesis of dopamine in the rat kidney

Dopamine and renal function and blood pressure regulation

Dopamine is an important regulator of systemic blood pressure via multiple mechanisms. It affects fluid and electrolyte balance by its actions on renal hemodynamics and epithelial ion and water transport and by regulation of hormones and humoral agents. The kidney synthesizes dopamine from circulating or filtered L-DOPA independently from innervation. The major determinants of the renal tubular synthesis/release of dopamine are probably sodium intake and intracellular sodium. Dopamine exerts its actions via two families of cell surface receptors, D1-like receptors comprising D1R and D5R, and D2-like receptors comprising D2R, D3R, and D4R, and by interactions with other G protein-coupled receptors. D1-like receptors are linked to vasodilation, while the effect of D2-like receptors on the vasculature is variable and probably dependent upon the state of nerve activity. Dopamine secreted into the tubular lumen acts mainly via D1-like receptors in an autocrine/paracrine manner to regulate ion transport in the proximal and distal nephron. These effects are mediated mainly by tubular mechanisms and augmented by hemodynamic mechanisms. The natriuretic effect of D1-like receptors is caused by inhibition of ion transport in the apical and basolateral membranes. D2-like receptors participate in the inhibition of ion transport during conditions of euvolemia and moderate volume expansion. Dopamine also controls ion transport and blood pressure by regulating the production of reactive oxygen species and the inflammatory response. Essential hypertension is associated with abnormalities in dopamine production, receptor number, and/or posttranslational modification.

Atrial natriuretic factor stimulates renal dopamine uptake mediated by natriuretic peptide-type A receptor

To determine the effects of atrial natriuretic factor (ANF) on renal dopamine (DA) metabolism, 3H-DA and 3H-L-DOPA uptake by renal tubular cells was measured in experiments carried out in vitro in Sprague-Dawley rats. The receptor type involved was also analyzed. The results indicate that ANF increased at 30 min, DA uptake in a concentration-response fashion having 10 pM ANF as the threshold concentration. Conversely, the uptake of the precursor L-DOPA was not modified by the peptide. ANF effects were observed in tissues from external and juxtamedullar cortex and inner medulla. On this basis, 100 nM ANF was used to continue the studies in external cortex tissues. DA uptake was characterized as extraneuronal uptake, since 100 microM hydrocortisone blocked ANF-induced increase of DA uptake. Renal DA uptake was decreased at 0 degrees C and in sodium-free medium. The effects of ANF in these conditions were not present, confirming that renal DA uptake is mediated by temperature- and sodium-dependent transporters and that the peptide requires the presence of the ion to exhibit its actions on DA uptake. The biological natriuretic peptide type A receptor (NPR-A) mediates ANF effects, since 100 nM anantin, a specific blocker, reversed ANF-dependent increase of DA uptake. The natriuretic peptide type C receptor (NPR-C) is not involved, since the specific analogous 100 nM 4-23 ANF amide has no effect on renal DA uptake and does not alter the effects of 100 nM ANF. In conclusion, ANF stimulates DA uptake by kidney tubular cells. ANF effects are mediated by NPR-A receptors coupled to guanylate cyclase and cGMP as second messenger. The process involved was characterized as a typical extraneuronal uptake, and characterized as temperature- and sodium-dependent. This mechanism could be related to DA effects on sodium reabsorption and linked to ANF enhanced natriuresis in the kidney. The increment of endogenous DA into tubular cells, as a consequence of increased DA uptake, would permit D1 receptor recruitment and Na+,K+-ATPase activity inhibition, which results in decreased sodium reabsorption and increased natriuresis.

Increased plasma atrial natriuretic factor in catecholamine-producing tumor patients

The aim of this study was to evaluate plasma levels of ANF in patients with catecholamine-secreting tumors with and without hypertension and to relate ANF secretion to levels of plasma and urinary catecholamines and blood pressure. Twenty-one pheochromocytoma (15 with sustained, 6 with paroxysmal hypertension), 6 neuroblastoma (1 hypertensive) patients and 28 aged-matched controls were studied in basal conditions. Plasma and urinary norepinephrine (NE),epinephrine (E), dopamine (DA) and DOPA were determined by HPLC-ED and plasma ANF by RIA. Both neuroblastoma and pheochromocytoma patients had significantly higher plasma ANF levels than controls. Neuroblastomas showed higher ANF concentration than pheochromocytomas. No differences were found in plasma ANF between hypertensive and normotensive patients. Pheochromocytomas with ANF levels within the normal range had plasma and urinary NE and urinary DA and DOPA levels significantly higher than patients with high ANF. Plasma ANF levels were unrelated to systolic or diastolic blood pressure or heart rate. A negative correlation between plasma ANF and urinary DA was found only in the patients groups. In conclusion, plasma ANF was increased in pheochromocytoma and neuroblastoma patients. Our data suggest that the excessive catecholamine secretion is not responsible for the increased ANF secretion in these patients. The significance of the relationships among plasma ANF and urinary and plasma catecholamines requires further investigation.

Apical and basolateral uptake and intracellular fate of dopamine precursor L-dopa in LLC-PK1 cells

The present study was aimed at the uptake of L-3,4-dihydroxyphenylalanine (L-dopa) and its intracellular decarboxylation to dopamine. The accumulation of L-dopa from the apical side in cells cultured in collagen-treated plastic was found to be a saturable process with a Michaelis constant (Km) of 123 +/- 17 microM and a maximal velocity (Vmax) of 6.0 +/- 0.2 nmol.mg protein-1.6 min-1. The uptake of L-dopa applied from either the apical or basal cell borders in cells cultured in polycarbonate filters was also found to be saturable; nonlinear analysis of saturation curves for apical and basal application revealed Km values of 63.8 +/- 17.0 and 42.5 +/- 9.6 microM and Vmax values of 32.0 +/- 5.8 and 26.2 +/- 3.4 nmol.mg protein-1.6 min-1, respectively. Cell monolayers incubated with L-dopa, applied from either the apical or the basal side, in the absence of benserazide, led to the accumulation of newly formed dopamine. The intracellular accumulation of newly formed dopamine was a saturable process with apparent Km values of 20.5 +/- 8.2 and 247.3 +/- 76.8 microM when the substrate was applied from the apical and basal side, respectively. Some of the newly formed dopamine escaped to the extracellular milieu. The basal outward transfer of dopamine was five- to sevenfold of that occurring at the apical side and was uniform over a wide range of concentrations of intracellular dopamine; the apical outward transfer of the amine depended on the intracellular concentration of dopamine and was a nonsaturable process. The apical and basal outward transfers of dopamine were insensitive to cocaine (10 and 30 microM) and GBR-12909 (1 and 3 microM). The accumulation of exogenous dopamine in LLC-PK1 cells was found to be saturable; nonlinear analysis of the saturation curves revealed for the apical and basal application of dopamine a Km of 17.7 +/- 4.3 and 96.0 +/- 28.1 microM and a Vmax of 2.0 +/- 0.1 and 2.2 +/- 0.3 nmol.mg protein-1.6 min-1, respectively. However, both cocaine (10, 30, or 100 microM) and GBR-12909 (1 or 3 microM) were found not to affect the uptake of 100 microM dopamine applied from either the apical or the basal cell border. In conclusion, the data presented here show that LLC-PK1 cells are endowed with considerable aromatic L-amino acid decarboxylase (AADC) activity and transport L-dopa quite efficiently through both the apical and basal cell borders. On the other hand, our observations support the possibility of a basal-to-apical gradient of AADC activity and the possibility that LLC-PK1 cells might constitute an interesting in vitro model for the study of the renal dopaminergic physiology.

Uptake of L-3,4-dihydroxyphenylalanine and dopamine formation in cultured renal epithelial cells

In the presence of benserazide (50 microM), L-3,4-dihydroxyphenylalanine (L-DOPA) was rapidly accumulated in both LLC-PK1 and OK cells; equilibrium was attained at 30 min of incubation. For these LLC-PK1 and OK cells, the analysis revealed a rate constant of inward transport (k(in) in pmol/mg protein/min) of 3.6 +/- 0.4 and 18.1 +/- 0.3 and a rate constant of outward transport (k(out) in pmol/mg protein/min) of 1.0 +/- 0.1 and 5.2 +/- 0.1, respectively. Nonlinear analysis of the saturation curves for LLC-PK1 and OK cells revealed a Km (in microM) of 86 +/- 12 and 14 +/- 4, respectively. The cellular accumulation of the substrate was temperature-dependent and stereoselective. Aromatic L-amino acid decarboxylase (AAAD) activity was determined in cell homogenates; nonlinear analysis of the saturation curves revealed, for LLC-PK1 and OK cells, a Km (in microM) of 1866 +/- 107 and 845 +/- 153 and a Vmax (in nmol/mg protein/15 min) of 4.4 +/- 0.1 and 0.9 +/- 0.1, respectively. In the absence of benserazide, only a limited amount of the L-DOPA taken up was decarboxylated to dopamine in cell monolayers; the Km value (in microM) for decarboxylation of intracellular L-DOPA in LLC-PK1 and OK cells was 61 +/- 14 and 108 +/- 36, respectively. A low amount of newly formed dopamine was found to escape to the apical bathing fluid. This outward transfer of newly formed dopamine was a nonsaturable process up to 300 microM intracellular dopamine. In conclusion, the data presented here show that OK cells are endowed with a more efficient L-DOPA uptake system than LLC-PK1 cells, but the latter are endowed with a significantly higher AAAD activity than OK cells. In both cell lines, intracellular L-DOPA is rapidly converted to dopamine, some of which diffuses out of the cell.

High sodium intake increases the urinary excretion of L-3,4-dihydroxyphenylalanine but fails to alter the urinary excretion of dopamine and amine metabolites in Wistar rats

1. The present study has examined the daily urinary excretion of L-DOPA, dopamine and its metabolites (DOPAC, 3-MT and HVA) during normal salt (NS) and high salt(HS) diets. 2. Daily urinary excretion of L-DOPA, DA, DOPAC, 3-MT and HVA during the 4-day period of NS diet averaged, respectively, 7.6 +/- 0.4, 71 +/- 5, 217 +/- 22, 570 +/- 90 and 1217 +/- 110 nmol/kg/day. The slight increase in the urinary excretion of DA, DOPAC and 3-MT (16% to 42% increase), when rats were fed a HS diet, did not achieve statistical significance. 3. In contrast, the urinary levels of L-DOPA during the HS diet period (11 +/- 1 nmol/kg/day) were found to be significantly higher than during the NS diet period; the maximal increase in the urinary excretion of L-DOPA (93% increase) was observed in the first day and then a progressive decline was observed towards the end of the HS intake period. 4. During the first 5 days of the HS intake period, the urine output of noradrenaline (NA) was found to increase (27% to 83%) and then to progressively decline to baseline values (13.5 +/- 0.7 nmol/ kg/day). Urinary excretion of adrenaline (AD) during the HS intake period was found to increase (72% to 146%); the mean daily urinary excretion of AD during the NS diet period averaged 2.5 +/- 0.4 nmol/ kg/day. NS and DA contents in the kidney of rats on a NS diet were not significantly different from that of rats in a HS diet. 6. It is concluded that long-term HS intake in Wistar rats fail to change the urinary excretion of DA and of its metabolites (DOPAC, 3-MT and HVA). Furthermore, the discrepant profile in the urinary excretion of L-DOPA and DA during HS intake might be related to a reduction in the tubular uptake of the amino acid, rather than reflecting a decrease in its decarboxylation.

Dopamine-sodium relationship in type 2 diabetic patients

Diabetes mellitus is known to be associated with sodium retention. The aim of the present paper was to investigate the possible role of the renal dopaminergic system in the disturbed sodium homeostasis of Type 2 diabetic patients. The urinary dopamine excretion, which represents the local kidney production, was lower in Type 2 diabetic patients as compared to controls and decreased in insulin treated patients as compared to patients treated without insulin. Urinary dopamine excretion correlated positively with sodium excretion in non-insulin treated patients and in controls, but not in insulin treated patients. In contrast to findings in healthy volunteers, an intravenous sodium load failed to increase the dopamine excretion in Type 2 diabetic patients, despite similar increments in sodium excretion. A low-dose dopamine infusion caused significantly lower natriuretic responses in insulin treated Type 2 diabetic patients as compared to controls, but not in non-insulin treated patients. These findings suggest that Type 2 diabetic patients display a derangement of the renal dopaminergic system, which is accentuated by insulin treatment.

Assessment of renal dopaminergic system activity in the nitric oxide-deprived hypertensive rat model

1. The present paper reports changes in the urinary excretion of dopamine, 5-hydroxytryptamine and amine metabolites in nitric oxide deprived hypertensive rats during long-term administration of NG-nitro-L-arginine methyl ester (L-NAME). Aromatic L-amino acid decarboxylase (AAAD) activity in renal tissues and the ability of newly-formed dopamine to leave the cellular compartment where the synthesis of the amine has occurred were also determined. 2. Twenty four hours after exposure to L-NAME, both systolic (SBP) and diastolic (DBP) blood pressure were increased by 20 mmHg; heart rate was slightly decreased. During the next 13 days both SBP and DBP increased progressively reaching 170 +/- 3 and 116 +/- 3 mmHg, respectively. 3. Baseline urinary excretion of L-DOPA, dopamine, 3,4-dihydroxyphenylacetic acid (DOPAC), 3-methoxytyramine (3-MT) and homovanillic acid (HVA) during the 4 day period of stabilization averaged 4.4 +/- 0.5, 13.8 +/- 0.3, 37.4 +/- 0.8, 180.0 +/- 2.7 and 206.1 +/- 6.7 nmol day-1, respectively. The urinary excretion of L-DOPA, dopamine and DOPAC, but not that of 3-MT and HVA, were increased from day 6-8 of L-NAME administration onwards (L-DOPA, up to 13.4 +/- 2.1; dopamine, up to 23.0 +/- 1.6; DOPAC, up to 62.8 +/- 3.7 nmol day-1). Baseline daily urinary excretion of 5-hydroxytryptamine and 5-hydroxyindolacetic acid (5-HIAA) averaged 73.5 +/- 1.1 and 241.7 +/- 5.4 nmol day-1, respectively. During the first week of L-NAME administration, the urinary excretion of both 5-hydroxytryptamine and 5-HIAA did not change significantly; however, as was found with dopamine and DOPAC, changes in the urinary excretion of 5-hydroxytryptamine were evident during the second week of L-NAME administration. 4. In experiments performed on homogenates of isolated renal tubules, the decarboxylation of L-DOPA to dopamine was dependent on the concentration of L-DOPA used (10 to 5000 microM) and saturable at 1000 microM. AAAD activity as determined in homogenates (Vmax, in nmol mg-1 protein h-1; Km in microM) was significantly (P < 0.01) higher in rats given L-NAME for 14 days (Vmax = 25 +/- 2; Km = 72 +/- 10) than in control rats (Vmax = 14 +/- 1; Km = 63 +/- 7), rats given L-NAME for 7 days (Vmax = 15 +/- 1; Km = 69 +/- 5) and rats given L-NAME plus L-arginine (Vmax = 13 +/- 1; Km = 60 +/- 3) for 14 days. 5. A considerable amount of the total dopamine formed from added L-DOPA in kidney slices escaped into the incubation medium. The application of the Michaelis-Menten equation to the net transport of newly-formed dopamine allowed the identification of a saturable (carrier-mediated transfer) and a non-saturable component (diffusion). No significant differences in the diffusional rate of transfer(0.14 +/- 0.02 micro mol-1) were observed between the four experimental groups. However, the saturable outward transfer of dopamine (Vmax, in micromol mg-1 protein h-1; Km in microM) was higher in control animals(Vmax= 2.3 +/- 0.2; Km = 568 +/- 67) than that in rats treated with L-NAME for 14 days (Vmax = 0.8 +/- 0.02;Km = 241 +/- 21), but similar to that observed in rats receiving L-NAME plus L-arginine (Vmax= 2.4+/- 0.2; Km= 618 +/- 61); the saturable dopamine outward rate of transfer in rats given L-NAME for 7days (Vmax = 3.9 +/- 0.2; Km = 1006 +/- 32) was higher than in controls.6. In conclusion, the present studies show that the hypertensive response resulting from the long-term administration of L-NAME is accompanied by an increased urinary excretion of dopamine and 5-hydroxytryptamine, which appears to follow an enhanced activity of renal AAAD. The observation that the increased AAAD activity can be reversed by the administration of L-arginine to L-NAME treated rats favours the view that the adaptational response which results in an enhanced AAAD activity probably involves a decrease in the generation of nitric oxide.

Atrial natriuretic hormone inhibits corticotropin-releasing hormone-induced prolactin release in man

Atrial natriuretic hormone (ANH) is found in heart myocytes, and also in the CNS. The inhibitory action of ANH on the hypothalamic-pituitary-adrenocortical (HPA) system has been established by in vivo and in vitro experiments, and could be of considerable importance: whereas several synergists to corticotropin-releasing hormone (CRH), the key hormone of the HPA system, are characterized in the past, until now ANH seems to be the only peptide which counterbalances the effects of CRH at the pituitary. As well as at the corticotroph, CRH has a stimulatory influence upon the lactotroph in vivo, and like ACTH and corticosteroids prolactin (PRL) is released in response to physical and cognitive challenges. To test the hypothesis of whether ANH also inhibits the CRH-mediated prolactin release a randomized, placebo-controlled, double-blind study in 12 males aged from 25 to 30 years was conducted. With regard to the diurnal variation of the HPA system activity we compared the prolactin release by 100 micrograms hCRH during a 30 min infusion of placebo, 150 micrograms ANH or 3 IU arginine vasopressin in the morning (08:00 h) and evening (19:00 h). Evaluation of morning and evening effects revealed that administration of hCRH led to a prompt rise of plasma PRL concentration. Infusion of ANH resulted in a significantly reduced maximum increment of PRL compared to placebo (0.83 +/- 0.87 vs 2.85 +/- 1.57 ng/ml, mean +/- SD, n = 12, p < .001). In addition, the AUC values were significantly lower under ANH than in the placebo condition. Infusion of AVP did not significantly change the PRL response to CRH vs placebo.(ABSTRACT TRUNCATED AT 250 WORDS)

Dopamine formation, from its immediate precursor 3,4-dihydroxyphenylalanine, along the rat digestive tract

The formation of dopamine, from L-3,4-dihydroxyphenylalanine (L-dopa), in fragments of non-glandular and glandular stomach, duodenum, jejunum, ileum and proximal and distal colon of the rat was examined. The deamination of newly-formed dopamine into 3,4-dihydroxyphenylacetic acid (dopac) was also studied. The synthesis of dopamine in tissues incubated with 500 microM L-dopa for 20 min in conditions of catechol-O-methyltransferase (Comt) inhibition was found to be in the duodenum, jejunum and ileum 2-fold that in the proximal colon, 6-fold that in the glandular stomach and 120-fold that in the non-glandular stomach and distal colon. The formation of dopac in these tissues followed the pattern of amine formation. In the jejunum, the formation of dopamine and dopac was found to be dependent on the concentration of L-dopa (50 to 5000 microM) used. In another set of experiments, it was found that the formation of dopamine in jejunal segments loaded with increasing concentrations of L-dopa (50, 100 and 500 microM) was a time-dependent process. The rate constant (k) of formation of dopamine as a function of time was found to be similar (0.050 +/- 0.005) with either concentration of L-dopa; the rate constant of dopac formation in these experiments was, in contrast, found to be greater at the highest concentrations of L-dopa (100 and 500 microM). Aromatic L-amino acid decarboxylase (Aaad) activity determined in homogenates of the jejunal mucosa was found to be twice that observed in homogenates of the remaining jejunal wall (muscular).(ABSTRACT TRUNCATED AT 250 WORDS)

Renal tubular dopamine outward transfer during Na(+)-H+ exchange activation by alpha 1- and alpha 2-adrenoceptor agonists

1. The present work describes the effects of inhibitors (amiloride and ethylisopropylamiloride) and activators (quinoxaline and phenylephrine) of the Na(+)-H+ exchanger on the outflow of dopamine in rat kidney slices loaded with L-dihydroxyphenylalanine (L-DOPA). 2. Incubation of kidney slices loaded with 50 microM L-DOPA in the presence of increasing concentrations of amiloride or ethylisopropylamiloride (EIPA) resulted in a concentration-dependent decrease in the outflow of newly-formed dopamine; the IC50 value for EIPA was 5.6 +/- 0.3 microM. Phenylephrine and quinoxaline were found to produce a concentration-dependent increase in the outflow of newly-formed dopamine; the EC50 values for the phenylephrine and quinoxaline were, respectively, 0.9 +/- 0.1 and 0.08 +/- 0.01 microM. 3. The facilitatory effect of phenylephrine on the outflow of dopamine was found not to be affected by yohimbine (100 nM), but was abolished by prazosin (1 microM), whereas that of quinoxaline was found to be selectively antagonized by yohimbine (100 nM), but not by prazosin (1 microM); EIPA (10 microM) was also found to abolish the effect of both phenylephrine and quinoxaline. The facilitatory effect of quinoxaline was also found to be reduced by 42-48% and 56-78% by, dibutyryl adenosine cyclic 3',5'-monophosphate (dibutyryl cyclic AMP; 250 microM) and forskolin (10 microM), respectively, but not by the protein kinase C (PKC) inhibitor, (+)-sphingosine (10 microM). In contrast, (+)-sphingosine (10 microM) was found to antagonize markedly (43- 69% reduction) the facilitatory effect of phenylephrine; dibutyrylcyclic AMP (250 microM) and forskolin (10 microM) were also found to reduce significantly the facilitatory effect of phenylephrine, by 42-53% and 44-59% respectively.4. A synergistic effect between alphal- and alpha2-adrenoceptors was observed for submaximal concentrations of quinoxaline (50 nM) and phenylephrine or submaximal concentrations of phenylephrine (0.5 microM) and quinoxaline, but not for maximal effective concentrations of either agonist. Dibutyryl cyclic AMP(250 microM) or forskolin (10 microM) produced a marked decrease (35-85% reduction) of the synergistic effect between phenylephrine and quinoxaline. The addition of phorbol 12,13-dibutyrate (PDBu; 500 nM) was found not to alter the outflow of newly-formed dopamine, but did potentiate (18-42% increase) the facilitatory effect of quinoxaline on the amine outflow. This effect was found to occur for submaximal concentrations of quinoxaline (10, 50 and 100 nM) and found to be antagonized by (+)-sphingosine(10 microM). In contrast, PDBu (500 nM) was found not to potentiate the facilitatory effect of phenylephrine on dopamine outflow.5. In conclusion, inhibition of the Na+-H+ antiport by amiloride and EIPA results in considerable reduction in the outflow of newly-formed dopamine, whereas the activation of this mechanism by both phenylephrine and quinoxaline results in facilitation of the outflow of dopamine; this effect is selectively reversed by alphal- and alpha2-adrenoceptor antagonists and EIPA. The synergistic effect between quinoxaline and phenylephrine may be related to an amplification of a reaction at a given point in the post-receptor transducing pathway.

Kinetic study of the tubular dopamine outward transporter in the rat and dog kidney

1. The present study has determined the kinetic characteristics of the outflow of dopamine of renal origin in slices of rat and dog renal cortex loaded with exogenous L-dihydroxyphenylalanine (L-DOPA 5 to 5000 microM). 2. In both dog and rat renal tissues the production of dopamine was found to be dependent on the concentration of L-DOPA used and reached its maximum at 2500 microM L-DOPA. The decarboxylation of L-DOPA in rat cortical slices (16.4 +/- 2.6 to 1479.2 +/- 85.2 nmol g-1) was 6 fold that in the dog (2.2 +/- 0.4 to 252.1 +/- 21.2 nmol g-1). In the rat kidney a large amount (approximately 50%) of the dopamine (5.2 +/- 0.6 to 743.4 +/- 58.3 nmol g-1) was found to escape into the incubation medium, whereas in dog renal slices the amount of newly-formed dopamine escaping into the incubation medium (0.7 +/- 0.2 to 46.5 +/- 9.3 nmol g-1) was less than 25% of the total amount of the amine formed. 3. The application of the Michaelis-Menten equation to the net transport of newly-formed dopamine has allowed the identification of a saturable (carrier-mediated transfer) and a non-saturable component (diffusion). The Vmax (nmol g-1 15 min-1) and Km (nM) values for the saturable component were, respectively, 340 +/- 41 and 396 +/- 45 in the rat kidney and 112 +/- 16 and 319 +/- 35 in the dog kidney. In both rat and dog renal tissues, the magnitude of the non-saturable component was found to be of minor importance up to a concentration of 250 nmol g-1 of dopamine to be transported. At high concentrations of the amine (greater then 250 nmol g-1), only attainable in rat kidney slices, most of the dopamine was found to leave the compartment where the synthesis did occur through a non-saturable transport system.4. In conclusion, the results presented here show that the outflow of newly-formed dopamine in both dog and rat kidney slices loaded with exogenous L-DOPA follows Michaelis-Menten kinetics with a saturable component and a non-saturable one, the latter assuming particular importance only at higher concentrations of the amine.

Enhanced protein kinase C mediated inhibition of renal dopamine synthesis during high sodium intake

The synthesis of dopamine, from L-beta-3,4-dihydroxyphenylalanine (L-DOPA), in renal tissue of rats on either a normal sodium (NS) or high sodium (HS) diet for 1 week or 6 weeks was examined. Aromatic L-amino acid decarboxylase (AAAD) activity determined in kidney homogenates in "1 week HS" (Vmax = 11.5 +/- 1.6 nmol/mg protein/hr) and "6 weeks HS" rats (Vmax = 10.6 +/- 1.5 nmol/mg protein/hr) was greater (P < 0.02) than that in "NS" rats (Vmax = 7.7 +/- 0.8 nmol/mg protein/hr). Km (microM) values in "NS", in "1 week HS" and "6 weeks HS" rats were similar. The formation of dopamine in kidney slices loaded with 100 microM L-DOPA depended exponentially on the concentration of sodium in the medium (0-160 mM). In kidney slices obtained from "1 week HS" rats the decarboxylation of added L-DOPA was significantly greater (P < 0.01) than that observed in kidney slices obtained from "NS" and "6 weeks HS" rats; the rate constant of formation of dopamine as a function of sodium concentration in the incubation medium was, however, similar in "NS" rats to that in "1 week HS" and "6 weeks HS" rats. Ouabain produced a concentration dependent decrease in the synthesis of dopamine in all three experimental groups; the magnitude of the inhibitory effect of 1.0 mM ouabain was greater in "1 week HS" rats (77% reduction; P < 0.01) than in "NS" rats (59% reduction; P < 0.01) and in "6 weeks HS" rats (23% reduction; P = 0.08). Activation of protein kinase C by phorbol 12,13-dibutyrate (PDBu) and the calcium ionophore A23187 produced a concentration-dependent reduction in the formation of dopamine in rat kidney slices, but not in kidney homogenates; the magnitude of the inhibitory effect was greater in "1 week HS" rats than in "NS" and "6 weeks HS" rats. Submaximal concentrations of PDBu (10 nM) were synergistic with the inhibitory effect of A23187 on the formation of dopamine: again, this effect was more marked in "1 week HS" rats than in "NS" and "6 weeks HS" rats. The effects of PDBu and PDBu plus A23187, but not those of A23187 alone, were antagonized in a concentration-dependent manner by d-sphingosine, a protein kinase C inhibitor. It is concluded that the increased activity of AAAD in renal tissues of rats submitted to HS intake is accompanied in "1 week HS" but not in "6 weeks HS" rats by enhanced inhibition of dopamine formation during protein kinase C activation.

Actin cytoskeleton, tubular sodium and the renal synthesis of dopamine

The present study has examined the effect of colchicine and cytochalasin B, two cytoskeleton disrupter compounds, on the formation of dopamine in slices of rat renal cortex loaded with exogenous L-3,4-dihydroxyphenylalanine (L-DOPA); the deamination of newly formed dopamine into 3,4-dihydroxyphenylacetic acid (DOPAC) was also examined. The accumulation of newly formed dopamine and DOPAC in kidney slices loaded with L-DOPA (10-100 microM) was found to be dependent on the concentration of L-DOPA, being similar in control conditions and in preparations treated with increasing concentrations of colchicine (5, 10 and 50 microM). By contrast, cytochalasin B (5, 10 and 50 microM) was found to produce a concentration-dependent reduction in the formation of dopamine and of its deaminated metabolite DOPAC in kidney slices loaded with L-DOPA (10-100 microM). The inhibitory effect of cytochalasin B on the formation of dopamine was found to be completely abolished in kidney slices pretreated with ouabain (500 microM) or when sodium concentration in the incubation was reduced from 120 to 20 mM. On its own, ouabain (500 microM) was found to reduce the formation of dopamine by 55%; the effect of reducing sodium concentration in the incubation medium to 20 mM was also a significant reduction (53% decrease) in the formation of dopamine. The accumulation of DOPAC did always parallel that of its parent amine. It is concluded that the renal formation of dopamine is dependent on the concentration of sodium in the medium and the integrity of the tubular transport of sodium, namely on the association between actin cytoskeleton and Na+,K(+)-ATPase, appears to be determinant.