AT1 receptors in the collecting duct directly modulate the concentration of urine

Mice lacking AT(1) angiotensin receptors have an impaired capacity to concentrate the urine, but the underlying mechanism is unknown. To determine whether direct actions of AT(1) receptors in epithelial cells of the collecting duct regulate water reabsorption, we used Cre-Loxp technology to specifically eliminate AT(1A) receptors from the collecting duct in mice (CD-KOs). Although levels of AT(1A) receptor mRNA in the inner medulla of CD-KO mice were significantly reduced, their kidneys appeared structurally normal. Under basal conditions, plasma and urine osmolalities and urine volumes were similar between CD-KO mice and controls. The increase in urine osmolality in response to water deprivation or vasopressin administration, however, was consistently attenuated in CD-KO mice. Similarly, levels of aquaporin-2 protein in inner and outer medulla after water deprivation were significantly lower in CD-KO mice compared with controls, despite its normal localization to the apical membrane. In summary, these results demonstrate that AT(1A) receptors in epithelial cells of the collecting duct directly modulate aquaporin-2 levels and contribute to the concentration of urine.     

Angiotensin II enhances tubuloglomerular feedback via luminal AT(1) receptors on the macula densa.

BACKGROUND: Recent studies have revealed angiotensin II subtype 1 (AT1) receptors on macula densa cells, raising the possibility that angiotensin II (Ang II) could enhance tubuloglomerular feedback (TGF) by affecting macula densa cell function. We hypothesized that Ang II enhances TGF via activation of AT1 receptors on the luminal membrane of the macula densa. METHODS: Rabbit afferent arterioles and the attached macula densa were simultaneously microperfused in vitro, keeping pressure in the afferent arteriole at 60 mm Hg. RESULTS: The afferent arteriole diameter was measured while the macula densa was perfused with low NaCl (Na+, 5 mmol/L; Cl-, 3 mmol/L) and then with high NaCl (Na+, 79 mmol/L; Cl-, 77 mmol/L) to induce a TGF response. When TGF was induced in the absence of Ang II, the afferent arteriole diameter decreased by 2.4 +/- 0.5 microm (from 17.3 +/- 1.0 to 14.9 +/- 1.2 microm). With Ang II (0.1 nmol/L) present in the lumen of the macula densa, the diameter decreased by 3.8 +/- 0.7 microm (from 17.3 +/- 1.0 to 13.5 +/- 1.2 microm, P < 0.05 vs. TGF with no Ang II, N = 8). To test whether Ang II enhances TGF via activation of AT1 receptors on the luminal membrane of the macula densa, Ang II plus losartan (1 micromol/L) was added to the lumen. Losartan itself did not alter TGF. When TGF was induced in the absence of Ang II and losartan, the afferent arteriole diameter decreased by 2.3 +/- 0.3 microm (from 15.9 +/- 1.0 to 13.6 +/- 1.2 microm). When Ang II and losartan were both present in the macula densa perfusate, the diameter decreased by 2.4 +/- 0.4 microm (from 15.8 +/- 0.9 to 13.4 +/- 0.7 microm, P> 0.8 vs. TGF with no Ang II and losartan, N = 7). We then examined whether AT2 receptors on the macula densa influence the effect of luminal Ang II on TGF. When TGF was induced in the absence of Ang II plus PD 0123319-0121B (1 micromol/L), the afferent arteriole diameter decreased by 2.4 +/- 0.2 microm (from 17.0 +/- 0.9 to 14.6 +/- 0.8 microm). When Ang II and PD 0123319-0121B were both present in the macula densa lumen, the diameter decreased by 3.9 +/- 0.2 microm (from 16.8 +/- 0.9 to 12.9 +/- 0.9 microm, P < 0.001 vs. TGF with no Ang II and PD 0123319-0121B, N = 8). PD 0123319-0121B itself had no effect on TGF. To assure that this effect of Ang II was not due to leakage into the bath, losartan was added to the bath. When TGF was induced in the absence of Ang II with losartan in the bath, the afferent arteriole diameter decreased by 2.8 +/- 0.5 microm (from 19.3 +/- 1.2 to 16.5 +/- 0.8 microm). After Ang II was added to the macula densa perfusate and losartan to the bath, the diameter decreased by 4.0 +/- 0.7 microm (from 18.9 +/- 1.1 to 14.9 +/- 0.5 microm, P < 0.01 vs. TGF with no Ang II in the lumen and losartan in the bath, N = 8). CONCLUSIONS: These results demonstrate that Ang II enhances TGF via activation of AT1 receptors on the luminal membrane of the macula densa.     

Angiotensin II stimulates vacuolar H+ -ATPase activity in renal acid-secretory intercalated cells from the outer medullary collecting duct

Final urinary acidification is mediated by the action of vacuolar H(+)-ATPases expressed in acid-secretory type A intercalated cells (A-IC) in the collecting duct. Angiotensin II (AngII) has profound effects on renal acid-base transport in the proximal tubule, distal tubule, and collecting duct. This study investigated the effects on vacuolar H(+)-ATPase activity in A-IC in freshly isolated mouse outer medullary collecting ducts. AngII (10 nM) stimulated concanamycin-sensitive vacuolar H(+)-ATPase activity in A-IC in freshly isolated mouse outer medullary collecting ducts via AT(1) receptors, which were also detected immunohistochemically in A-IC. AngII increased intracellular Ca(2+) levels transiently. Chelation of intracellular Ca(2+) with BAPTA and depletion of endoplasmic reticulum Ca(2+) stores prevented the stimulatory effect on H(+)-ATPase activity. The effect of AngII on H(+)-ATPase activity was abolished by inhibitors of small G proteins and phospholipase C, by blockers of Ca(2+)-dependent and -independent isoforms of protein kinase C and extracellular signal-regulated kinase 1/2. Disruption of the microtubular network and cleavage of cellubrevin attenuated the stimulation. Finally, AngII failed to stimulate residual vacuolar H(+)-ATPase activity in A-IC from mice that were deficient for the B1 subunit of the vacuolar H(+)-ATPase. Thus, AngII presents a potent stimulus for vacuolar H(+)-ATPase activity in outer medullary collecting duct IC and requires trafficking of stimulatory proteins or vacuolar H(+)-ATPases. The B1 subunit is indispensable for the stimulation by AngII, and its importance for stimulation of vacuolar H(+)-ATPase activity may contribute to the inappropriate urinary acidification that is seen in patients who have distal renal tubular acidosis and mutations in this subunit.     

Angiotensin II subtype AT1 and AT2 receptors regulate microvascular hydraulic permeability via cAMP and cGMP

INTRODUCTION: Angiotensin II receptor subtypes (AT1 and AT2) have been shown to modulate microvascular fluid leak. However, their intracellular signal transduction pathways have not been elucidated. We hypothesized that AT1 activation exerts its permeability-increasing effect by provoking cGMP synthesis and inducing cAMP degradation and that AT2 activation decreases fluid leak by stimulating cAMP synthesis and enhancing cGMP degradation. METHODS: Using a microcannulation technique, hydraulic permeability (Lp) was measured in rat mesenteric venules. The messenger signal transduction of ATI was studied during continuous perfusion with the AT1 agonist, Sar1 plus either 1) a cGMP synthesis inhibitor, LY83583, or 2) an inhibitor of cAMP degradation, Rolipram. Likewise, AT2 signal transduction was studied with the AT2 agonist, CGP42112A, plus either 1) a cAMP synthesis inhibitor, dideoxyadenosine, or 2) an inhibitor of cGMP degradation, Zaprinast. Lp values are represented as mean +/- SEM x 10(-7) cm/s/cm H2O. For each group n = 6. RESULTS: Inhibition of cGMP synthesis blunted the permeability-increasing effect of AT1 agonism and decreased the peak Lp from 4.91 +/- 0.25 to 2.30 +/- 0.10 (P < 0.001). Inhibition of cAMP degradation also reduced the effect of AT1 agonism on peak L(p) from 2.25 +/- 0.22 to 1.30 +/- 0.13 (P < 0.001). Meanwhile, cAMP synthesis inhibition completely blocked the permeability-decreasing effect of AT2 agonism during which Lp increased from a baseline of 0.92 +/- 0.08 to a peak of 4.38 +/- 0.20 (P < 0.001). During inhibition of cGMP degradation, AT2 activation was able to decrease peak Lp from 2.26 +/- 0.15 to 1.46 +/- 0.05 (P < 0.001). CONCLUSIONS: When cGMP synthesis and cAMP degradation were inhibited, the effect on fluid leak by AT1 activation was blunted. Inhibition of cAMP synthesis completely blocked the effect of AT2 activation on fluid leak, while AT2 activation continued to decrease fluid leak despite inhibition of cGMP degradation. The AT1 receptor appears to increase fluid leak by stimulating both cGMP synthesis and cAMP degradation, while the AT2 receptor decreases fluid leak by stimulating cAMP synthesis, but not cGMP degradation.     

Angiotensin II stimulates cellular hypertrophy of LLC-PK1 cells through the AT1 receptor

We have investigated possible growth-promoting effects of angiotensinII (ANG II) in the pig tubular epithelial cell line LLC-PK₁.1O^–6–10^–10 M ANG II inhibited proliferationas measured by [³H]thymidine incorporation. However, the sameconcentration of peptide induced tubular hypertrophy as assessedby increases in de novo protein synthesis, total protein, andRNA content. These effects were mediated through AT₁ receptors.Furthermore, LLC-PK₁. cells exhibited specific binding sitesfor ANG II and expressed mature mRNA for the vascular smoothmuscle AT₁ receptor. ANG II incubation of cells significantlylowered intracellular cAMP, and the hypertrophogenic actionof ANG II was abolished by co-incubation with the relative stableanalogue dibutyryl-cAMP, suggesting the involvement of intracellularnucleotides in the signal-transduction processes leading tohypertrophy. Our results collectively suggest that ANG II isa hypertrophogenic growth factor for LLC-PK₁ cells. Consideringthe importance of tubular hypertrophy in the progression ofrenal failure in many models, strategies to block the actionof ANG II on tubular cells may offer an attractive alternativeway to control deterioration of renal function besides modulationof haemodynamics.     

Angiotensin II activates nuclear transcription factor-kappaB through AT1 and AT2 receptors

BACKGROUND: Recent evidence suggests that angiotensin II (Ang II) induces a variety of proinflammatory mediators including chemokines. Nuclear factor-kappaB (NF-kappaB) activation plays an important role in Ang II-mediated inflammation. The present study investigated which Ang II receptor subtype is involved in NF-kappaB activation. We focused particularly on the Ang II subtype 2 (AT2) receptor because we previously observed that Ang II-induction of the chemokine RANTES in vitro and in vivo is mediated through AT2 receptors.METHODS: AT1 or AT2 receptors were selectively overexpressed in COS7 cells that normally do not express Ang II receptors. In addition, rat glomerular endothelial cells (GER) that express AT1 and AT2 receptors and PC12 cells that exclusively exhibit AT2 receptors were studied also. Ang II-receptor expression was confirmed by Western blots of membrane lysates. NF-kappaB DNA binding in vitro was detected by electrophoretic shift assays. In addition, in vivo transactivation of a reporter gene construct with kappa enhancer coupled to luciferase also was investigated. Expression of the inhibitor of kappaB alpha (IkappaB-alpha) was detected by Western blots.RESULTS: In AT1 or AT2 receptor transfected cells, but not untransfected COS7 cells, 10-7 mol/L Ang II induced NF-kappaB DNA binding in vitro, as detected by electrophoretic shift assays and in vivo transactivation of a reporter gene construct. The AT2 receptor antagonist PD 123319 but not losartan attenuated Ang II-mediated NF-kappaB activation in COS7 cells transfected with AT2 receptors. While Ang II also induced NF-kappaB activation in PC12 cells, this activation was blocked by PD 123319. Finally, stimulation of GERs with Ang II led to the activation of NF-kappaB through both subtypes of Ang II receptors. Nuclear extracts from COS7 cells transfected with AT2 receptors and PC12 cells with NF-kappaB DNA-binding activity consisted of p50/p65 complexes. There was no difference in subunit composition of nuclear proteins from Ang II-stimulated AT1 receptor transfected COS7 cells. An artificial peptide (p-Amino-Phe6-Ang II) with a high affinity for the AT2 receptor also activated NF-kappaB. Ang II-induced activation of NF-kappaB was associated with degradation of IkappaB-alpha in all studied cell lines.CONCLUSIONS: Our results clearly demonstrate in various cell lines that Ang II induces NF-kappaB activation through AT2 receptors. These data may have important therapeutic consequences, because potential Ang II-mediated proinflammatory renal and cardiovascular effects may not be totally antagonized by the currently increased clinical use of AT1 receptor antagonists.     

Angiotensin II, via AT1 and AT2 receptors and NF-kappaB pathway, regulates the inflammatory response in unilateral ureteral obstruction

Inflammatory cell infiltration plays a key role in the onset and progression of renal injury. The NF-kappaB participates in the inflammatory response, regulating many proinflammatory genes. Angiotensin II (Ang II), via AT(1) and AT(2) receptors, activates NF-kappaB. Although the contribution of Ang II to kidney damage progression is already established, the receptor subtype involved in the inflammatory cell recruitment is not clear. For investigating this issue, the unilateral ureteral obstruction (UUO) model was used in mice, blocking Ang II production/receptors and NF-kappaB pathway. Two days after UUO, obstructed kidneys of wild-type mice presented a marked interstitial inflammatory cell infiltration and increased NF-kappaB activity. Treatment with AT(1) or AT(2) antagonists partially decreased NF-kappaB activation, whereas only the AT(2) blockade diminished monocyte infiltration. Obstructed kidneys of AT(1)-knockout mice showed interstitial monocyte infiltration and NF-kappaB activation; both processes were abolished by an AT(2) antagonist, suggesting AT(2)/NF-kappaB involvement in monocyte recruitment. In wild-type mice, only angiotensin-converting enzyme inhibition or combined therapy with AT(1) plus AT(2) antagonists blocked monocyte infiltration, NF-kappaB activation, and upregulation of NF-kappaB-related proinflammatory genes. Therefore, AT(1) and AT(2) blockade is necessary to arrest completely the inflammatory process. Treatment with two different NF-kappaB inhibitors, pirrolidin-dithiocarbamate and parthenolide, diminished monocyte infiltration and gene overexpression. These data show that Ang II, via AT(1) and AT(2) receptors and NF-kappaB pathway, participates in the regulation of renal monocyte recruitment and may provide a rationale to investigate further the role of AT(2) in human kidney diseases.     

Angiotensin III activates nuclear transcription factor-kappaB in cultured mesangial cells mainly via AT(2) receptors: studies with AT(1) receptor-knockout mice

Nuclear factor-kappaB (NF-kappaB) regulates many genes involved in renal pathophysiologic processes. It was previously demonstrated that angiotensin II (AngII) and its amino-terminal degradation product AngIII activate NF-kappaB in mesangial cells. However, which are the Ang receptor subtypes involved in the NF-kappaB pathway and whether these Ang peptides act through the same or different receptors in mesangial cells have not been evaluated. Under the culture conditions used, quiescent rat mesangial cells expressed both AT(1) and AT(2) receptors. To investigate the receptors involved in the NF-kappaB pathway, two different approaches were used, i.e., pharmacologic studies, using specific AT(1) and AT(2) receptor antagonists and agonists, and studies in AT(1) receptor-knockout mice. In cultured rat mesangial cells, both AT(1) and AT(2) receptor antagonists inhibited AngII-induced NF-kappaB DNA binding activity, whereas NF-kappaB activation elicited by AngIII was mainly blocked by the AT(2) receptor antagonist. Similar results were observed for cytosolic IkappaBalpha degradation. An AT(2) receptor agonist also activated NF-kappaB. In AT(1) receptor-knockout murine mesangial cells, AngIII and AngII increased NF-kappaB activity and degraded cytosolic IkappaBalpha; both processes were blocked by the AT(2) receptor antagonist. These data demonstrate that, in mesangial cells, NF-kappaB activation is mediated by AT(1) and AT(2) receptors, suggesting a novel intracellular signaling mechanism for AT(2) receptors in the kidney. Some differences in Ang peptide receptor-mediated responses were also observed. AngII activates NF-kappaB via AT(1) and AT(2) receptors, whereas AngIII acts mainly via AT(2) receptors. These results suggest the potential involvement of the AngIII/AT(2) receptor/NF-kappaB pathway in pathophysiologic processes in the kidney and provide a better understanding of the renin-angiotensin system.     

Compensatory hypertrophy and adaptation in the cortical collecting duct

The cortical collecting duct (CCD) undergoes hypertrophy and functional adaptation following reduction of renal mass. The nature and mechanisms of these changes have been investigated using microperfusion of isolated CCD from rabbit remnant kidneys. By 1 week after reduction of renal mass, tubule hypertrophy and increased sodium transport are fully developed. The transport adaptations are specific or selective, since bicarbonate transport in these CCD is unchanged. Mineralocorticoids may play an important role in the hypertrophy and increased sodium transport, since plasma aldosterone increases early after reduction of renal mass. Also, adrenalectomy abolishes the changes in size and sodium transport, even with supplementation of aldosterone to unstressed physiologic levels. Epidermal growth factor also has immediate effects on CCD sodium transport; however, the direction of the effect is opposite--an inhibition of transport.     

Bellini duct (collecting duct) carcinoma of the kidney

Carcinoma of the collecting ducts, or Bellini carcinoma, is a rare renal tumour and, unlike most renal cell carcinomas, it derives from distal tubules. It displays highly aggressive behaviour and has a poor prognosis. In this study, the authors present three cases which they observed over the past three years.     

Mechanism of iodide transport in the rabbit cortical collecting duct

Background Pendrin, an anion exchanger known to participate in iodide transport in the apical membrane of follicular cells of the thyroid gland, has recently been shown to exist in the apical membrane of the β- and γ-intercalated (β/γ-IC) cells of the cortical collecting duct (CCD). We examined mechanisms of iodide transport in the CCD. Methods Rabbit CCD was perfused in vitro, and lumen-to-bath flux coefficients for both ¹²⁵I⁻ (K_{I (lb)}) and ³⁶Cl⁻ (K_{Cl (lb)}) were measured simultaneously. The intracellular pH (pHi) of β/γ-IC cells in the perfused CCD was measured by microscopic fluorometory, by loading 2′,7′-bis-(2-carboxyethyl)-5(6)-carboxyfluorescein tetraacetoxy methylester (BCECF-AM), a fluorescent marker for pHi. The effects on pHi of the replacement of NaCl with Na cyclamate, NaI, or NaBr in the lumen or bath were observed. Results K_{I (lb)} was comparable to or slightly higher than K_{Cl (lb)}. Both iodide and chloride in the lumen caused self- and cross-inhibitions to both fluxes. The addition of 5-nitro-2-(-3-phenylpropylamino)-benzoate (NPPB), a Cl⁻ channel inhibitor, to the bath significantly reduced K_{Cl (lb)}, but not K_{I (lb)}. Replacement of luminal fluid NaCl with Na cyclamate, NaI, or NaBr caused alkalization of pHi, no change in pHi, and slight acidification of pHi, respectively. Replacement of bath NaCl with Na cyclamate, NaI, or NaBr caused alkalization, alkalization, and acidification of pHi, respectively. Luminal NaI prevented the acidification of pHi caused by bath Na cyclamate. Conclusions The data are consistent with the model that iodide is transported via the Cl⁻/HCO₃⁻ exchanger in the apical membrane of β/γ-IC cells and exits the basolateral membrane via an electroneutral transporter that is distinct from the Cl⁻ channel. We could not, however, identify which type of β/γ-IC cell was mainly responsible.     

Angiotensin II stimulates calcineurin activity in proximal tubule epithelia through AT-1 receptor-mediated tyrosine phosphorylation of the PLC-gamma1 isoform

Angiotensin II (AngII) contributes to the maintenance of extracellular fluid volume by regulating sodium transport in the nephron. In nonepithelial cells, activation of phospholipase C (PLC) by AT-1 receptors stimulates the generation of 1,4,5-trisphosphate (IP(3)) and the release of intracellular calcium. Calcineurin, a serine-threonine phosphatase, is activated by calcium and calmodulin, and both PLC and calcineurin have been linked to sodium transport in the proximal tubule. An examination of whether AngII activates calcineurin in a model of proximal tubule epithelia (LLC-PK1 cells) was performed; AngII increased calcineurin activity within 30 s. An examination of whether AngII activates PLC in proximal tubule epithelia was also performed after first showing that all three families of PLC isoforms are present in LLC-PK1 cells. Application of AngII increased IP(3) generation by 60% within 15 s, which coincided with AngII-induced tyrosine phosphorylation of the PLC-gamma1 isoform also observed at 15 s. AngII-induced tyrosine phosphorylation was blocked by the AT-1 receptor antagonist, Losartan. Subsequently, an inhibitor of tyrosine phosphorylation blocked the AngII-induced activation of calcineurin, as did coincubation with an inhibitor of PLC activity and with an antagonist of the AT-1 receptor. It is therefore concluded that AngII stimulates calcineurin phosphatase activity in proximal tubule epithelial cells through a mechanism involving AT-1 receptor-mediated tyrosine phosphorylation of the PLC isoform.     

The Role of Angiotensin II Receptor 1 (AT1) Blockade in Cisplatin-Induced Nephrotoxicity in Rats: Gender-Related Differences

It is documented that chronic renal diseases are gender related. The protective role of angiotensin II receptor 1 (AT1) blocker losartan against cisplatin (CP)-induced nephrotoxicity was reported in males, but the role of gender is not well known. Six groups of Wistar rats were studied. Rats were divided into two groups of males and females to receive losartan for 9 days plus a single dose of CP (7 mg/kg) at day 3. Two positive control groups of males and females received the same regimen, except that they received saline instead of losartan. The negative control groups received saline instead of CP at day 3 and also saline instead of losartan. The blood samples were obtained, and the kidneys underwent histopathological investigations. All the CP-treated animals lost weight, but losartan promoted weight loss in females (p < 0.05). Coadministration of losartan and CP in females, but not in males, significantly increased the serum levels of blood urea nitrogen and creatinine when compared with the negative and positive control groups (p < 0.05). No significant differences were observed in serum levels of total proteins, magnesium, and nitrite between the groups. Administration of CP increased the kidney tissue damage score (KTDS) and normalized kidney weight (p < 0.05). However, in the presence of AT1 blockade, the KTDS (nonsignificantly) and normalized kidney weight (significantly, p < 0.05) increased in the CP-treated females. Such an observation was not seen in males. Losartan may prevent CP-induced nephrotoxicity in males, but it promotes the CP-induced damage in females, which may be related to the renin–angiotensin system receptors in the kidneys.     

The Role of Angiotensin II Receptor 1 (AT1) Blockade in Cisplatin-Induced Nephrotoxicity in Rats: Gender-Related Differences

It is documented that chronic renal diseases are gender related. The protective role of angiotensin II receptor 1 (AT1) blocker losartan against cisplatin (CP)-induced nephrotoxicity was reported in males, but the role of gender is not well known. Six groups of Wistar rats were studied. Rats were divided into two groups of males and females to receive losartan for 9 days plus a single dose of CP (7 mg/kg) at day 3. Two positive control groups of males and females received the same regimen, except that they received saline instead of losartan. The negative control groups received saline instead of CP at day 3 and also saline instead of losartan. The blood samples were obtained, and the kidneys underwent histopathological investigations. All the CP-treated animals lost weight, but losartan promoted weight loss in females (p < 0.05). Coadministration of losartan and CP in females, but not in males, significantly increased the serum levels of blood urea nitrogen and creatinine when compared with the negative and positive control groups (p < 0.05). No significant differences were observed in serum levels of total proteins, magnesium, and nitrite between the groups. Administration of CP increased the kidney tissue damage score (KTDS) and normalized kidney weight (p < 0.05). However, in the presence of AT1 blockade, the KTDS (nonsignificantly) and normalized kidney weight (significantly, p < 0.05) increased in the CP-treated females. Such an observation was not seen in males. Losartan may prevent CP-induced nephrotoxicity in males, but it promotes the CP-induced damage in females, which may be related to the renin-angiotensin system receptors in the kidneys.     

Angiotensin II stimulates endothelin-1 secretion in cultured rat mesangial cells.

The present study was designed to test two hypotheses: (1) that angiotensin II (Ang II) stimulates endothelin-1 secretion in cultured rat mesangial cells and (2) that atrial and brain natriuretic peptides (ANP and BNP) inhibit the above-mentioned secretion in these cells. Ang II stimulated immunoreactive (ir) endothelin-1 secretion in a concentration-dependent manner between 10(-8) M and 10(-7) M. The protein kinase C (PKC) inhibitors from two chemical classes, H7 and staurosporine, inhibited secretion following such stimulation. The stimulatory effect of Ang II was also abolished in the PKC-depleted cells. Rat ANP(1-28) and rat BNP-45, which are the respective major circulating forms of ANP and BNP in rats, potently inhibited Ang II-stimulated endothelin-1 secretion in a concentration-dependent manner. Inhibition by ANP and BNP of Ang II-stimulated endothelin-1 secretion was paralleled by an increase in the cellular level of cyclic guanosine 5'-monophosphate (GMP). The addition of a cyclic GMP analogue, 8-bromo cyclic GMP, reduced the stimulated endothelin-1 secretion. Rat ANP(5-25) was less effective that rat ANP(1-28) with respect to inhibiting ir-endothelin-1 secretion and increasing cellular cyclic GMP. These findings indicate that Ang II stimulates endothelin-1 secretion in cultured rat mesangial cells by a mechanism probably involving activation of PKC, and that rat ANP and BNP inhibit this stimulated secretion through a cyclic GMP-dependent process.