Modulation by bradykinin and nitric oxide of angiotensin II-induced apoptosis in a vascular smooth muscle cell phenotype.

There is evidence for a clinical benefit of ACE inhibitors or AT1 antagonists in cardiovascular diseases with deleterious smooth muscle cells (SMC) apoptosis. We have previously shown that angiotensin II (Ang II) induces a phenotype-dependent SMC apoptosis. We asked whether bradykinin (BK) and nitric oxide (NO) could modulate Ang II-induced SMC apoptosis. BK alone did not induce significant apoptosis in either spindle (Sp-SMC) or epithelioid (Ep-SMC) SMC phenotypes cultured in serum reduction, but phenotype-dependently, reduced cell proliferation. Pretreatment with BK partly impaired Ang II-induced reduction of Ep-SMC culture viability and partly prevented apoptotic features. Pretreatment with sodium nitroprusside completely prevented all Ang II-induced deleterious effects in Ep-SMC, i. e. reduction of culture viability, Annexin V binding, nuclear condensation and cell fragmentation. These findings indicate that the BK-NO system may phenotype-dependently modulate SMC survival and in particular may oppose, mostly by NO, Ang II-induction of apoptosis in the Ep-SMC phenotype.

The B1-agonist [des-Arg10]-kallidin activates transcription factor NF-kappaB and induces homologous upregulation of the bradykinin B1-receptor in cultured human lung fibroblasts.

The bradykinin B1-receptor is strongly upregulated under chronic inflammatory conditions. However, the mechanism and reason are not known. Because a better understanding of the mechanism of the upregulation will help in understanding its potential importance in inflammation, we have studied the molecular mechanism of B1-receptor upregulation in cultured human lung fibroblasts (IMR 90) in response to IL-1beta and the B1-agonist [des-Arg10]-kallidin. We show that treatment of human IMR 90 cells by IL-1beta stimulates the expression of both B1-receptor mRNA and protein. The latter was studied by Western blot analysis using antipeptide antibodies directed against the COOH-terminal part of the human B1-receptor. We furthermore report the novel observation that the B1-receptor is upregulated by its own agonist which was completely blocked by the specific B1-antagonist [des-Arg10-Leu9]-kallidin, indicating an upregulation entirely mediated through cell surface B1-receptors. The increased population of B1-receptors was functionally coupled as exemplified by an enhancement of the B1-agonist induced increase in free cytosolic calcium. Upregulation by the B1-agonist was blocked by a specific protein kinase C inhibitor. B1-agonist-induced upregulation was correlated to the induction of transcription factor nuclear factor kappaB (NF-kappaB) which efficiently bound to the NF-kappaB-like sequence located in the promoter region of the human B1-receptor gene. This correlation was further confirmed by reporter gene assays which showed that this NF-kappaB-like sequence, in the B1-receptor promoter context, could contribute to IL-1beta and DLBK-induced B1-receptor transcription activation, and by the effect of NF-kappaB inhibitor pyrrolidinedithiocarbamate which diminished both B1-receptor upregulation and NF-kappaB activation. NF-kappaB is now recognized as a key inflammatory mediator which is activated by the B1-agonist but which is also involved in B1-receptor upregulation.

Compared effects of a low and a high sodium diet on the renal and urinary concentration and activity of kallikrein in normal rats.

Different sodium intakes may affect or alter the urinary excretion of renal kallikrein. We have compared the renal and the urinary effects of sodium depletion and sodium loading with 1% NaCl on total kallikrein, using a direct radio-immunoassay against immunoreactive kallikrein and on active kallikrein assessed by a kininogenase assay with a kinin radio-immunoassay. Sodium depletion resulted in an increase in renal and urinary excretion of both the immunoreactive kallikrein and the kininogenase activity. Sodium loading resulted in a slight but significant decrease in renal tissue immunoreactive kallikrein content without any change in the kininogenase activity, while the urinary excretion of the kininogenase activity was greatly increased and the urinary immunoreactive kallikrein remained steady. This sodium depletion induces consistent effects at the renal and urinary level, resulting probably from a stimulation of biosynthesis. However, during sodium loading, renal and urinary measurements of kallikrein are dissociated. Furthermore, kininogenase activity and immunoreactive kallikrein are not correlated in either of the two studied compartments. Thus sodium loading seems to induce independent effects at the renal and urinary levels, possibly resulting from different mechanisms.

Yohimbine increases submaxillary kallikrein release into the saliva in dogs: evidence for alpha 2-adrenoceptor-mediated inhibition of cholinergic pathways.

1. The effects of the alpha 2-adrenoceptor antagonist, yohimbine (0.5 mg kg-1, i.v.) on basal, sympathetic and parasympathetic stimulation-induced submaxillary kallikrein release were investigated in the anaesthetized dog. Kallikrein was measured by its kininogenase activity before and after trypsin activation which also allowed a study of the proportion of active to total enzyme. 2. Yohimbine induced a rapid, three fold increase in basal kallikrein release correlated with an increase in salivary flow rate which lasted for 60 min following injection. 3. Sectioning the chorda tympani did not affect basal kallikrein release but abolished yohimbine-induced rise in salivary kallikrein secretion. 4. Parasympathetic stimulation alone induced a 3 to 4 fold increase in basal kallikrein release correlated with an increase in salivary flow rate. Yohimbine induced a significant additional increase in parasympathetic-stimulated kallikrein release. 5. When the cervical sympathetic nerve was sectioned the basal kallikrein release decreased by 30 to 40%. 6. Sympathetic stimulation alone also induced a 3 to 4 fold increase in basal kallikrein. This was not correlated with the salivary flow and unaffected by yohimbine. 7. The results indicate that yohimbine increases submaxillary kallikrein release into the saliva by inhibition of presynaptic alpha 2-adrenoceptors located on the chorda tympani nerve endings.

Cyclosporine A-induced increase in glomerular cyclic GMP in rats and the involvement of the endothelinB receptor.

1. A transient two fold increase in the cyclic GMP content was observed in rat freshly isolated glomeruli 6 to 9 h after a single subcutaneous injection of 20 mg kg-1 cyclosporine A (CsA) in conscious animals. 2.In vitro stimulation with endothelin 3 (ET-3) of isolated glomeruli obtained from CsA-untreated rats resulted in a dose-dependent increase in cyclic GMP content. The increase observed with 10 nM ET-3 was similar to that observed in glomeruli isolated 9 h after in vivo CsA administration. 3. The rise in glomerular cyclic GMP content after in vivo CsA injection was prevented by in vivo treatment with L-NAME (10 mg kg-1) or by in vitro calcium deprivation of the incubation medium. 4. The stimulating effects of CsA on glomerular cyclic GMP content were inhibited by in vivo administration of the ETB receptor antagonist BQ-788 (2 mg kg-1) but not by the ETA receptor antagonist BQ-123 (2 mg kg-1). 5. The maximum increase in glomerular cyclic GMP content induced in vitro by acetylcholine (100 microM) and by ET-3 (100 nM) was slightly lower (approximately by 20-25%; P < 0.05) in glomeruli from CsA-treated rats than in glomeruli from untreated rats. In contrast, the maximum increase achieved with 1 microM sodium nitroprusside was similar in both groups. 6. A single subcutaneous injection of CsA did not significantly alter the glomerular mRNA expression of constitutive endothelial NO synthase (eNOS), as evaluated by RT-PCR, whereas the mRNA expression of the inducible NO synthase (iNOS), which follows pretreatment with lipopolysaccharide, was prevented. 7. These results indicate that in vivo administration of a single dose of cyclosporine A transiently increases the cyclic GMP content of freshly isolated glomeruli, and that activation of ETB receptors and stimulation of the NO pathway are involved in this process. Furthermore, a single administration of CsA does not impair eNOS mRNA expression and only slightly reduces NO-dependent glomerular cyclic GMP production.

Bradykinin stimulates production of inositol (1,4,5) trisphosphate in cultured mesangial cells of the rat via a BK2-kinin receptor.

1. Using [125I-Tyr0]-BK, as radiolabelled ligand, and various agonists and antagonists of bradykinin (BK) we identified a single class of specific BK2-binding sites in mesangial cell membranes (Bmax = 73 fmol mg-1 protein and Kd = 3.7 nM). 2. Following the addition of 0.1 microM BK, inositol (1,4,5) trisphosphate (IP3) formation increased within 20 s from a basal level of 64 to a maximal value of 175 pmol mg-1 protein. 3. Incubation in a Ca(2+)-free medium did not change IP3 production but a 5 min preincubation with 1 mM EGTA completely prevented the BK-induced IP3 formation, suggesting that IP3 formation is partly dependent on extracellular calcium. 4. The BK2 antagonist D-Arg-Hyp3-D-Phe7-BK (10 microM) but not the BK1 antagonist (des-Arg9-Leu8-BK) abolished IP3 production in response to 0.1 microM BK. Pretreatment of mesangial cells with pertussis toxin was without effect on BK-induced IP3 formation, whereas phorbol 12-myristate 13-acetate significantly enhanced (by 25%) BK-induced IP3 formation. 5. The present data demonstrate that inositol phosphate breakdown in rat mesangial cells can be mediated via activation of a BK2-kinin receptor and is under negative control of protein-kinase C.

Characterization of a B2-bradykinin receptor in rat renal mesangial cells.

The specific binding of bradykinin (BK) was investigated using membrane fractions from mesangial cells in primary culture, a cloned cell line, and in intact adherent cells with three different radiolabelled BK analogues: 125I-[Tyr0]BK, 125I-[Tyr5]BK and 125I-[Tyr8]BK. The best radioligand was 125I-[Tyr0]BK, and assay conditions were determined to ensure maximal stable binding. Binding appeared to be reversible and not to be inhibited by a wide variety of protease inhibitors including converting enzyme inhibitor and phosphoramidon. The maximum density of binding sites (Bmax) was about 88 +/- 18 fmol/mg protein, which is equivalent to about 6000 sites/cell, and the dissociation constant averaged 2 nM. No significant difference in Bmax was observed between membranes from cells in primary culture and those from cloned cells. Of the BK analogues tested, unmodified BK exhibited the highest inhibition constant (close to 10(-10) M). No displacement of 125I-[Tyr0]BK was observed in the presence of the B1 agonist des-Arg9-BK or several unrelated peptides, including atrial natriuretic factor and angiotensin I and II, whereas 50% inhibition of binding was achieved with the B2 antagonist [D-Arg,Hyp3,D-Phe7]BK (10(-9)M). Addition of BK for 3 min to the incubation medium of cloned mesangial cells induced a dose- and time-dependent increase in PGE2 unlike des-Arg9-BK, which showed no such effect. The secretion was strongly inhibited by prior incubation with the B2 antagonist [D-Arg,Hyp3,D-Phe7]BK. The pharmacological profile of the binding site determined with various BK agonists and antagonists, and the stimulating effect of binding site activation on prostaglandin release strongly suggest that B2-kinin-like receptors are present in rat mesangial cells.

Renal kallikrein excretion as a distal nephrotoxicity marker during cadmium exposure in rats.

Cadmium exposure is known to induce hypertension, but development of hypertension is not universal in exposed animals. However, the cellular uptake of cadmium could also exert renal cytotoxic effects which have been, until now, essentially only studied at the proximal tubule level. Kallikrein is an enzyme synthetized in renal cortex and excreted in the urine in the distal tubule. Therefore, to evaluate the distal renal effect of cadmium, we studied the daily urinary kallikrein excretion (UKE) in conscious unrestrained female Brown Norway rats during long-term chronic exposure to 2 dosages of cadmium given subcutaneously 3 times a week, a low dose (LD): 0.25 mg/kg and a high dose (HD): 1 mg/kg. Neither dose of cadmium was able to induce significant hypertension in the treated animals. HD administration for 24 weeks resulted in a decreased UKE associated with an increase in plasma renin activity and sodium and potassium excretions. LD administration had no significant effect on UKE. Twenty weeks after stopping cadmium administration, a persistent reduction in UKE was still observed; furthermore, the group which had been previously administered a LD of cadmium, now also exhibited a reduced UKE. During this re-examination period in both groups, the UKE reductions were associated with normal systolic blood pressure, glycosuria, natriuresis. Our data show that cadmium administration can influence UKE, plasma renin activity, plasma aldosterone concentration and electrolyte excretion without inducing any variation of blood pressure. This may reflect a nephrotoxic, non-hypertensive effect. Since this effect persisted after stopping cadmium administration, it may indicate a prolonged irreversible nephrotoxic effect at the distal nephron level. Thus, UKE may be a useful non-invasive index to evaluate distal nephrotoxicity.

Effects of cicletanine on vasoactive systems in conscious sinoaortic-denervated dogs.

The present study investigates the antihypertensive action of cicletanine, a new antihypertensive compound with diuretic properties (or placebo), on vasopressor (catecholamines, renin-aldosterone) as well as vasodepressor (prostaglandins, kallikrein-kinin) systems in conscious chronic sinoaortic denervated (SAD) dogs. Cicletanine (10 mg/kg twice a day, per os, for one month) lowered blood pressure and heart rate. The antihypertensive action does not involve an effect on sympathetic tone (since plasma catecholamine levels were unmodified) or on plasma aldosterone levels. By contrast, urinary 6 keto PGF1 or PGE2 levels and kallikrein activity were enhanced. This result indicates that the antihypertensive effect of cicletanine is associated with a stimulation of potential vasodepressor systems (such as prostaglandins or kinins).

Bradykinin-induced inhibition of cell proliferation and tyrosine kinase activity in rat mesangial cells.

The relationship between cell proliferation, protein tyrosine phosphorylation, phosphotyrosine kinase activity and bradykinin receptor activation in rat mesangial cells was investigated. We demonstrated that bradykinin (BK), through the B2 receptor, induced a dose-dependent inhibition of mesangial cell proliferation stimulated by fetal calf serum. We next found that BK induced a dose-dependent inhibition of phospho-tyrosine kinase activity. Treatments with pertussis-toxin, inhibition of phospholipase C and protein kinase C inhibitors and chelation of free cytosolic calcium did not change the bradykinin-induced inhibition of phosphotyrosine kinase. Western blot analysis of phosphotyrosinated proteins demonstrated that BK reduced tyrosine phosphorylation of several proteins among which we identified the 125-focal adhesion kinase. Taken together, these data suggest for the first time that, in proliferating rat mesangial cells, B2 receptor stimulation is able to induce, via a pertussis insensitive pathway, the inhibition of tyrosine kinase activity and mesangial cell proliferation.

Inhibition of cGMP accumulation in mesangial cells by bradykinin and tyrosine kinase inhibitors.

We examined the effect of bradykinin (BK) on the accumulation of cGMP of the mesangial cell (MC), a smooth muscle-like cell of the renal glomerulus. BK caused a time- and concentration dependent reduction of the cGMP concentration. In addition, BK inhibited total protein tyrosine kinase (PTK) activity. Two tyrosine kinase inhibitors (TKI) genistein and tyrphostin also reduced the cGMP concentration. The inhibition of BK and TKI were not additive. The inhibition of PTK by BK, mediated through activation of the B2-receptor, was unaffected by inhibitors of Gi/o proteins, phospholipase C, protein kinase C, cyclooxygenase and Ca2+ release from intracellular stores. Only IBMX a broad spectrum inhibitor of phosphodiesterases (PDE) and 8-methoxymethyl IBMX a specific type-1 PDE inhibitor prevented the inhibitory effects of BK and TKI indicating the involvement of type-1 PDE. In addition, BK had no effect on soluble guanylate cyclase (sGC) and nitric oxide synthase activity. In freshly isolated glomeruli, which represent the physiological environment of MC, BK also reduced the cGMP concentration. Like in MC, the inhibitory effect was suppressed by IBMX. These data demonstrate that BK suppresses a PTK-dependent pathway of cGMP production in rat MC at a level downstream of NO synthase and sGC. It is suggested that BK and TKI inhibitors decrease cGMP levels by preventing tyrosine phosphorylation of type-1 PDE activity, thereby leading to enzyme activation.

Mild renal failure induced by subchronic exposure to molybdenum: urinary kallikrein excretion as a marker of distal tubular effect.

In this study we investigated the effect of two molybdenum (Mo) doses (40 and 80 mg/kg/d) on renal function. Neither dose of Mo was able to induce significant hypertension in treated animals. Subchronic exposure to high doses of Mo resulted in a delay in body weight gain associated with mild renal failure marked by a decrease in glomerular filtration. An increase in diuresis and urinary kallikrein excretion associated with unchanged glycosuria and proximal tubular enzymuria (alanine aminopeptidase and gamma-glutamyl transpeptidase) evoked a preferential mild effect at the distal tubule.

[Evidence for a glomerular receptor for bradykinin in rats. Effect of sodium intake on density and affinity of the receptor]. / Mise en évidence d'un récepteur glomérulaire à la bradykinine chez le rat. Effet de l'apport sodé sur la densité et l'affinité de ce récepteur.

Bradykinin (BK) have been involved in a lot of pharmacological and biological effects including natriuresis, vasodilatation, inflammation and pain mediation. All these potent effects of BK are presumably mediated via one or more specific receptors which have been classified in two types named B1 and B2 receptors. In the kidney, specific binding have been reported successively in cortical epithelial membranes, in renomedullary interstitial cells and in cortical collecting tubules. Furthermore, since a large number of vasoactive compounds have been shown to regulate renal glomerular hemodynamics, we examined the binding of BK in rat glomerular membranes and the effect of variable salt diet on the density (Bmax) and the dissociation constant (KD) of this binding. Incubation of a radiolabeled bradykinin analog, [125I]-Tyr8-BK with a crude membrane preparation obtained from isolated rat glomeruli revealed a time dependent binding. The binding was saturable, reversible and was a linear function of protein membrane concentration. The radiolabeled Tyr8-BK bound to a single class of binding sites with an equilibrium dissociation constant (KD) of 2.75 +/- 0.7 nM and a density (Bmax) of 32.1 +/- 5.2 fmol/mg protein. [125I]-Tyr8-BK binding was reversed by bradykinin (Ki = 0.4 10(-9) M) and by other kinin analogs. However, Des-Arg9-BK had no effect on binding of the radiolabelled BK. These results are consistent with the presence of a B2-kinin like receptor in rat glomeruli. Low salt diet (during one month) and water deprivation (during 4 days) induced a decrease in the density of glomerular BK like receptors respectively (Bmax = 12.45 +/- 1.3 and 13.25 +/- 1.17 fmol/mg protein).(ABSTRACT TRUNCATED AT 250 WORDS)

Indirect inhibition by bradykinin of cyclic AMP generation in isolated rat glomeruli and mesangial cells.

The present study was designed to evaluate the effect of the activation of bradykinin (BK) receptors on intracellular cAMP levels in isolated glomeruli as well as in cultured rat mesangial cells. BK affected basal cAMP content only in the presence of the phosphodiesterase inhibitor isobutylmethylxanthine. Furthermore, BK inhibited forskolin-, prostaglandin E2-, and isoproterenol-stimulated cAMP accumulation, both in the presence and in the absence of isobutylmethylxanthine. The inhibitory effect of BK was independent of stimulation of cAMP degradation by phosphodiesterase. No direct inhibition of the in vitro adenylyl cyclase activity was observed, suggesting a requirement for cytoplasmic constituents. Use of the phospholipase A2 inhibitor mepacrine and treatment with pertussis toxin did not modify the inhibitory effect of BK, indicating that neither the phospholipase A2 pathway nor the inhibitory G protein is involved. The effect of BK was completely prevented by two selective protein kinase C (PKC) inhibitors, staurosporine and bisindolylmaleimide. Furthermore, use of the diacylglycerol analog 1-oleoyl-2-acetyl-rac-glycerol and direct activation of PKC with phorbol-12-myristate-13-acetate mimicked the effect of BK, whereas the biologically inactive phorbol ester 4 alpha-phorbol-12, 13-didecanoate was without effect. Furthermore, down-regulation of PKC by long term pretreatment with phorbol-12-myristate-13-acetate abolished the inhibitory effect of BK on stimulated cAMP levels. These results demonstrate that BK inhibits forskolin-, prostaglandin E2-, and isoproterenol-stimulated cAMP formation through activation of the phospholipase C pathway. The subsequent production of diacylglycerol associated with stimulation of PKC in turn inhibits stimulated cAMP accumulation.

B2-kinin receptor like binding in rat glomerular membranes.

Incubation of a radiolabeled bradykinin analog, [125I]-Tyr8-BK with a crude membrane preparation obtained from isolated rat glomeruli revealed a time dependent binding. The binding was saturable, reversible and was a linear function of protein membrane concentration. The radiolabeled Tyr8-BK bound to a single class of binding sites with an equilibrium dissociation constant (KD) of 3.9 +/_ 0.7 nM and a density (Bmax) of 31 +/- 5 fmol/mg protein. The BK-receptor complex was not affected by angiotensin II or by arginine vasopressin and atrial natriuretic factor. BK binding was reversed by bradykinin (Ki = 0.3 10(-9) M), and by other kinin analogs in the following order of potency: Lys-BK, Met-Lys-BK, Thi5,8-D Phe7-BK. However, Des-Arg9-BK had no effect on binding of the radiolabelled BK. These results are consistent with the presence of a B2-kinin like receptor in rat glomeruli.

Direct radioimmunoassay of rat urinary kallikrein: its application to the determination of active and inactive kallikrein concentration after HPLC analysis.

Rat urinary kallikrein (RUK) was purified to apparent homogeneity by a three-step procedure and antibodies were raised in rabbits. Renal kallikrein exists as an active and an inactive form. A specific radioimmunoassay (RIA) was developed to measure directly the total kallikrein. The antibody used in this radioimmunoassay recognized both forms. No cross reactivity was detected with trypsin, esterase A or human urine. When iodinated rat urinary kallikrein was used, the detection range was between 0.125 and 16 ng with 6.5% intraassay variation and 8.1% between assay variation. Intrarenal kallikrein was measured in renal tissue after homogeneisation and solubilisation. Correlations between this RIA and the kininogenase activity or the amidolytic activity were highly significant. Since kallikrein exists as an inactive precursor the direct measurement of the total immunoreactive protein differs from activities determinations. An HPLC ion exchange system has been developed to separate active and inactive forms directly from urine, with a recovery of 79 +/- 11%. This procedure permits measurement of inactive forms. Rat urine contains as much inactive kallikrein as active kallikrein.

Characterization of two different affinity B2-kinin binding sites in rat glomeruli.

We have recently characterized a bradykinin (BK) receptor in rat renal mesangial cells (1). Activation of this receptor is associated with PGE2 release and IP3 formation suggesting involvement in cell contraction which can be linked to the control of the glomerular filtration rate (2). Whether this mesangial BK receptor is the unique glomerular BK receptor remains to be elucidated. In an attempt to answer to this question, we performed binding studies using decapsulated isolated glomeruli. Scatchard analysis of the binding data obtained with this preparation revealed the presence of two distinct B2-kinin binding sites. However, a consistent difference was observed in both the affinity and the density. We further investigated the pharmacological binding profile after an initial step of solubilization. Several experiments were performed to establish optimal conditions of solubilization. For this, different detergents such as Triton X-100, CHAPS and n-octyl beta-D glucopyranoside were tested at various concentrations, durations and temperatures of incubation. The binding was performed with two different [125I]-Tyr0-BK concentrations (0.5 and 7 nM) with either untreated decapsulated glomeruli or solubilized preparation for 45 minutes at +4 degrees C in the binding buffer containing a mixture of protease inhibitors. The greatest binding was achieved after treating glomeruli with 25 mM n-octyl beta-D glucopyranoside for 60 minutes at +4 degrees C under constant shaking. Two B2-kinin receptors of different affinities were detected. The same binding characteristics were obtained both in the 12,000 x g and 100,000 x g supernatant.(ABSTRACT TRUNCATED AT 250 WORDS)

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.