Antagonistic actions of renal dopamine and 5-hydroxytryptamine: endogenous 5-hydroxytryptamine, 5-HT1A receptors and antinatriuresis during high sodium intake

Serotonin and blood pressure regulation

5-Hydroxytryptamine (5-HT; serotonin) was discovered more than 60 years ago as a substance isolated from blood. The neural effects of 5-HT have been well investigated and understood, thanks in part to the pharmacological tools available to dissect the serotonergic system and the development of the frequently prescribed selective serotonin-reuptake inhibitors. By contrast, our understanding of the role of 5-HT in the control and modification of blood pressure pales in comparison. Here we focus on the role of 5-HT in systemic blood pressure control. This review provides an in-depth study of the function and pharmacology of 5-HT in those tissues that can modify blood pressure (blood, vasculature, heart, adrenal gland, kidney, brain), with a focus on the autonomic nervous system that includes mechanisms of action and pharmacology of 5-HT within each system. We compare the change in blood pressure produced in different species by short- and long-term administration of 5-HT or selective serotonin receptor agonists. To further our understanding of the mechanisms through which 5-HT modifies blood pressure, we also describe the blood pressure effects of commonly used drugs that modify the actions of 5-HT. The pharmacology and physiological actions of 5-HT in modifying blood pressure are important, given its involvement in circulatory shock, orthostatic hypotension, serotonin syndrome and hypertension.

Substrate-dependent regulation of MAO-A in rat mesangial cells: involvement of dopamine D2-like receptors

In the present study, we investigated the existence of a back-regulation of the catecholamine-degrading enzyme monoamine oxidase (MAO)-A by dopamine in rat renal cells. In proximal tubule cells, MAO-A expression was not modified after dopamine receptor stimulation. In contrast, in mesangial cells, enzyme assay and Western blots showed that MAO activity and protein increased by approximately 80% after 48-h incubation with the D(2)-like receptor agonist bromocriptine and quinpirole but not with the D(1)-like receptor agonist SKF-38393. This effect was prevented by the D(2)-receptor antagonist sulpiride and domperidone. The increase in MAO-A protein was preceded by an augmentation of MAO-A mRNA that was prevented by the transcriptional inhibitor actinomycin D. Bromocriptine effect was mimicked by the PKA inhibitor H89 and inhibited by the PKA activator 8-bromo-cAMP. These results show for the first time the existence of a dopamine-dependent MAO-A regulation involving D(2)-like receptors, inhibition of the cAMP-PKA pathway, and an ex novo enzyme synthesis.

Hormonal-dependent recruitment of Na+,K+-ATPase to the plasmalemma is mediated by PKC beta and modulated by [Na+]i

1. The present study demonstrates that stimulation of hormonal receptors of proximal tubule cells with the serotonin-agonist 8-hydroxy-2-(di-n-propylamino) tetraline (8-OH-DPAT) induces an augmentation of Na(+),K(+)-ATPase activity that results from the recruitment of enzyme molecules to the plasmalemma. 2. Cells expressing the rodent wild-type Na(+),K(+)-ATPase alpha-subunit had the same basal Na(+),K(+)-ATPase activity as cells expressing the alpha-subunit S11A or S18A mutants, but stimulation of Na(+),K(+)-ATPase activity was completely abolished in either mutant. 3. 8-OH-DPAT treatment of OK cells led to PKC(beta)-dependent phosphorylation of the alpha-subunit Ser-11 and Ser-18 residues, and determination of enzyme activity with the S11A and S18A mutants indicated that both residues are essential for the agonist-dependent stimulation of Na(+),K(+)-ATPase activity. 4. When cells were treated with both dopamine and 8-OH-DPAT, an activation of Na(+),K(+)-ATPase was observed at basal intracellular sodium concentration (approximately 9 mM), and this activation was gradually reduced and became a significant inhibition as the concentration of intracellular sodium gradually increased from 9 to 19 mM. Thus, besides the antagonistic effects of dopamine and 8-OH-DPAT, intracellular sodium modulates whether an activation or an inhibition of Na(+),K(+)-ATPase is produced.

Response of jejunal Na+, K+-ATPase to 5-hydroxytryptamine in young and adult rats: effect of fasting and refeeding

The present study is aimed to evaluate the effects of 5-hydroxytryptamine (5-HT) upon jejunal Na+,K+-ATPase in young (20-day-old) and adult (60-day-old) rats, and determine the effect of food intake on the response of the sodium pump to the amine. Basal Na+,K+-ATPase activity in jejunal epithelial cells from young rats was twice that in adult animals and responded to 5-HT with stimulation. In adult rats, fasting reduced by 25% basal jejunal Na+, K+-ATPase activity, whereas in young rats, no such change was observed. The sensitivity of jejunal Na+,K+-ATPase to 5-HT in young fasted rats was similar to that observed in fed animals. The effect of refeeding in young rats was a 2-fold increase in jejunal Na+, K+-ATPase activity, this being accompanied by insensitivity to 5-HT. In adult rats, refeeding was accompanied by an increase in jejunal Na+,K+-ATPase activity. It is concluded that the stimulatory effect of 5-HT upon jejunal Na+,K+-ATPase activity is a phenomenon dependent on both age and type of diet. In young rats, it is the food intake that plays an important role in development of insensitivity of Na+,K+-ATPase to stimulation by 5-HT, while in adult animals fasting or fasting followed by refeeding does not play a major role in regulating its sensitivity to the amine.

Food intake abolishes the response of rat jejunal Na(+),K(+)-ATPase to dopamine

The aim of the present study was to evaluate whether the sensitivity of jejunal Na(+),K(+)-ATPase to inhibition by dopamine (DA) in young rats is related to the type of food (breast milk vs. solid) or reflects a developmental adaptation. When 18-d-old rats were separated from their dams and fed solid food (the same used to feed adult rats) for 2 d, intestinal Na(+),K(+)-ATPase activity was significantly greater than that of breast-fed pups of the same age (20 d) (127 +/- 8 vs. 52 +/- 4 nmol Pi. mg protein(-1). min(-1); P < 0.05). Activity in rats fed solid food was insensitive to inhibition by 1 micromol/L DA. Na(+),K(+)-ATPase activity in 60-d-old rats (117. 4 +/- 4.2 nmol Pi. mg protein(-1). min(-1)) was also higher (P < 0. 05) than in breast-fed rats, and DA (1 micromol/L) did not inhibit enzyme activity. The B(max) value for binding of [(3)H]-Sch 23390 in 20-d-old breast-fed rats did not differ from that in age-matched rats fed a solid food for 2 d and or that in 60-d-old rats. Levels of DA, but not L-3,4-dihydroxyphenylalanine and amine metabolites, in the jejunal mucosa of 20-d-old rats that had eaten solid food for 2 d were 60% lower than in age-matched rats, breast-fed rats, and not different from those in the jejunal mucosa of 60-d-old rats fed the solid food. We conclude that in adult rats, in contrast to in young rats, DA does not inhibit jejunal Na(+),K(+)-ATPase activity, and food intake in young rats plays an important role in the development of the insensitivity of Na(+),K(+)-ATPase activity to DA.

Renal dopamine and sodium homeostasis

During the past decade, it has become evident that dopamine plays an important role in the regulation of fluid and electrolyte balance and blood pressure. Dopamine exerts its actions through two families of dopamine receptors, designated D1-like and D2-like, which are identical in the brain and in peripheral tissues. The two D1-like receptors--D1 and D5 receptors--expressed in mammals are linked to stimulation of adenylyl cyclase. The three D2-like receptors--D2, D3, and D4,--are linked to inhibition of adenylyl cyclase. Dopamine affects fluid and electrolyte balance by regulation of renal excretion of electrolytes and water through actions on renal hemodynamics and tubular epithelial transport and by modulation of the secretion and/or action of vasopressin, renin, aldosterone, catecholamines, and endothelin B receptors (ETB) receptors. It also affects fluid and sodium intake by way of "appetite" centers in the brain and alterations of gastrointestinal tract transport. The production of dopamine in neural and non-neural tissues and the presence of receptors in these tissues suggest that dopamine can act in an autocrine or paracrine fashion. This renal autocrine-paracrine function, which becomes most evident during extracellular fluid volume expansion, is lost in essential hypertension and in some animal models of genetic hypertension. This deficit may be caused by abnormalities in renal dopamine production and polymorphisms or abnormal post-translational modification and regulation of dopamine receptor subtypes.

Serotonin metabolism in rat mesangial cells: involvement of a serotonin transporter and monoamine oxidase A

BACKGROUND

Serotonin is one of the factors regulating mesangial cell proliferation, and convergent evidence supports its involvement in the development of glomerulonephritis. In this study, we identified a serotonin transporter and the amine-degrading enzyme monoamine oxidases (MAOs) in mesangial cells, and we studied their involvement in serotonin degradation.

METHODS

MAOs were characterized in membrane preparations and intact mesangial cells by enzyme assay using [14C]5-hydroxytryptamine and [14C]beta-phenylethylamine as specific substrates for MAO-A and MAO-B, respectively, and by Western blot analysis. The expression of a serotonin transporter was determined by [14C]5-hydroxytryptamine uptake experiments and Western blot. Mesangial cell proliferation was measured by BrdU incorporation.

RESULTS

Quantitation of the MAO isoforms by enzyme assay and Western blot analysis showed that MAO-A was largely predominant in mesangial cells, accounting for approximately 90% of the total enzyme population. The MAO substrate [14C]serotonin was transported into mesangial cells by a saturable uptake system (Vmax 310 +/- 36 pmol/30 min/mg protein; Km 5.9 +/- 1.4 microM) displaying the pharmacological properties of a serotonin transporter. The expression of a serotonin transporter was confirmed by Western blot analysis. MAO activity measured in intact cells showed that after accumulation into mesangial cells, [14C]serotonin was metabolized by MAO-A. Finally, serotonin-mediated mesangial cell proliferation was significantly increased after irreversible MAO inhibition.

CONCLUSIONS

Our results suggest that serotonin concentration and function in glomeruli may be regulated in part by its transport into mesangial cells and degradation by MAO-A.

Caco-2 cells in culture synthesize and degrade dopamine and 5-hydroxytryptamine: a comparison with rat jejunal epithelial cells

To explore the usefulness of Caco-2 cells in the study of intestinal dopaminergic and 5-hydroxytryptaminergic physiology, we have undertaken the study of aromatic L-amino acid decarboxylase (AADC), catechol-O-methyltransferase (COMT) and type A and B monoamine oxidase (MAO-A and MAO-B) activities in these cells using specific substrates. The activity of these enzymes was also evaluated in isolated rat jejunal epithelial cells. The results showed that Vmax values (in nmol mg protein(-1) h(-1)) for AADC, using L-DOPA as the substrate, in rat jejunal epithelial cells (127.3+/-11.4) were found to be 6-fold higher than in Caco-2 cells (22.5+/-2.6). However, Km values in Caco-2 cells (1.24+/-0.37 mM) were similar to those observed in rat jejunal epithelial cells (1.30+/-0.29 mM). Similar results were obtained when AADC activity was evaluated using L-5HTP as substrate; in rat jejunal epithelial cells Vmax values (in nmol mg prot(-1) h(-1)) were found to be 5-fold that in Caco-2 cells (16.3+/-1.0 and 3.0+/-0.2, respectively), and Km values in Caco-2 cells (0.23+/-0.08 mM) were again similar to those observed in rat intestinal epithelial cells (0.09+/-0.03 mM). Caco-2 cells were not able to O-methylate dopamine, in contrast to rat jejunal epithelial cells (Vmax = 8.6+/-0.4 nmol mg protein(-1)(h-1); Km = 516+/-57 microM). Vmax values (in nmol mg protein(-1)(h-1)) for type A and B MAO in Caco-2 cells (19.0+/-0.6 and 5.4+/-0.6, respectively) were found to be significantly lower (P<0.05) than those in rat jejunal epithelial cells (46.9+/-3.1 and 9.6+/-1.2, respectively); however, no significant differences in the Km values were observed between Caco-2 and rat jejunal epithelial cells for both type A and B MAO. In conclusion, Caco-2 cells in culture are endowed with the synthetic and metabolic machinery needed to form and degrade DA and 5-HT, though, no COMT activity could be detected in these cells.

L-3,4-dihydroxyphenylalanine and L-5-hydroxytryptophan share the same transporter in Opossum kidney cells

Opossum kidney (OK) cells, which have the ability to synthesise dopamine and 5-HT, have been used as an in vitro model for the study of renal actions of dopamine and 5-hydroxytryptamine (5-HT). The present study reports on the uptake of their immediate precursors L-3,4-dihydroxyphenylalanine (L-DOPA) and L-5-hydroxytryptophan (L-5-HTP). IC50 values for L-5-HTP (1569 microM) obtained in the presence of a nearly saturating (250 microM) concentration of L-DOPA were 6-fold those obtained when using non-saturating (0.25 and 25 microM) concentrations of L-DOPA (251 and 266). Vmax values (in nmol mg protein-1 6 min-1) for L-DOPA uptake are identical in the absence (13.6) and the presence of 250 microM L-5-HTP (13.3), but K(m) values (microM) are significantly greater (P < 0.05) when L-DOPA uptake was studied in the presence of L-5-HTP (90 vs 1.79). IC50 values for L-DOPA (679 microM) obtained in the presence of a near saturating (250 microM) concentration of L-5-HTP were almost 3-fold those obtained when non-saturating (0.25 and 25 microM) concentrations of L-5-HTP were used (254 and 220). Vmax values (in nmol mg protein-1 6 min-1) for L-5-HTP uptake are identical in the absence (11.2) and the presence of 250 microM L-DOPA (11.7), but K(m) values (microM) are significantly greater (P < 0.05) when L-5-HTP uptake was studied in the presence of L-DOPA (103 vs 220). It is concluded that L-DOPA and L-5-HTP share the same transporter(s) and each compound exerts a competitive type of inhibition upon the other.

The renal monoamine oxidases: pathophysiology and targets for therapeutic intervention

The mitochondrial enzyme monoamine oxidases A and B are among the major metabolic agents for the degradation of the biogenic amines adrenaline, noradrenaline, dopamine and serotonin. The fact that the kidney contains a large amount of monoamine oxidase suggests that the renal effects of biogenic amines might depend in part on the activity of these enzymes.

Antagonistic actions of renal dopamine and 5-hydroxytryptamine: effects of amine precursors on the cell inward transfer and decarboxylation

1. The present work was designed to examine the interference of L-3,4-dihydroxyphenylalanine (L-DOPA) on the cell inward transport of L-5-hydroxytryptophan (L-5-HTP) and on its decarboxylation by aromatic L-amino acid decarboxylase (AAAD) in rat isolated renal tubules. 2. The accumulation of both L-5-HTP and L-DOPA in renal tubules was found to occur through non-saturable and saturable mechanisms. The kinetics of the saturable component L-5-HTP and L-DOPA uptake in renal tubules were as follows: L-5-HTP, Vmax = 24.9 +/- 4.5 nmol mg-1 protein h-1 and Km = 121 (95% confidence limits: 75, 193) microM (n = 5); L-DOPA, Vmax = 58.0 +/- 4.3 nmol mg-1 protein h-1 and Km = 135 (97, 188) microM (n = 5). When the saturation curve of L-5-HTP tubular uptake was performed in the presence of L-DOPA (250 microM), the maximal rate of accumulation of L-5-HTP in renal tubules was found to be markedly (P < 0.01) reduced (Vmax = 10.5 +/- 1.7 nmol mg-1 protein h-1, n = 4); this was accompanied by a significant (P < 0.05) increase in Km values (325 [199, 531] microM, n = 4). 3. L-DOPA (50 to 2000 microM) was found to produce a concentration-dependent decrease (38% to 91% reduction) in the tubular uptake of 5-HTP; the Ki value (in microM) of L-DOPA for inhibition of L-5-HTP uptake was found to be 29.1 (13.8, 61.5) (n = 6). 4. At the highest concentration tested the organic anion inhibitor, probenecid (10 microM) produced no significant (P = 0.09) changes in L-5-HTP and L-DOPA uptake (18% and 22% reduction, respectively). The organic cation inhibitor, cyanine 863 (1-ethyl-2-[1,4-dimethyl-2-phenyl-6-pyrimidinylidene)methyl]-quino linium) produced a potent inhibitory effect on the tubular uptake of L-5-HTP (Ki = 212 [35, 1289] nM, n = 8), being slightly less effective against L-DOPA uptake (Ki = 903 [584, 1396] nM, n = 5). The cyanine derivatives 1,1-diethyl-2,4-cyanine (decynium 24) and 1,1-diethyl-2,2-cyanine (decynium 22) potently inhibited the tubular uptake of both L-5-HTP (Ki = 100 [49, 204] and 120 [26, 561] nM, n = 4-6, respectively) and L-DOPA (Ki = 100 [40, 290] and 415 [157, 1094] nM, n = 5, respectively). 5. The Vmax and Km values for AAAD using L-DOPA as the substrate (Vmax = 479.9 +/- 74.0 nmol mg-1 protein h-1; Km = 2380 [1630, 3476] microM; n = 4) were both found to be significantly (P < 0.01) higher than those observed when using L-5-HTP (Vmax = 81.4 +/- 5.2 nmol mg-1 protein h-1, Km = 97 [87, 107] microM, n = 10). The addition of 5 mM L-DOPA to the incubation medium reduced by 30% (P < 0.02) the maximal rate of decarboxylation of L-5-HTP (Vmax = 56.7 +/- 3.1 nmol mg-1 protein h-1, n = 10) and resulted in a significant (P < 0.05) increase in Km values (249 [228, 270] microM, n = 10). 6. The results presented suggest that L-5-HTP and L-DOPA are using the same transporter (most probably, the organic cation transporter) in order to be taken up into renal tubular cells; L-DOPA exerts a competitive type of inhibition upon the tubular uptake and decarboxylation of L-5-HTP. The decrease in the formation of 5-HT as induced by L-DOPA may also depend on a decrease in the rate of its decarboxylation by AAAD.