Characterization of SR 121463A, a highly potent and selective, orally active vasopressin V2 receptor antagonist

Arginine vasopressin receptor antagonists (vaptans): pharmacological tools and potential therapeutic agents

Arginine vasopressin (AVP) attracted attention as a potentially important neurohormonal mediator of the heart failure (HF) syndrome and hyponatremic states in humans because AVP influences renal handling of free water, vasoconstriction and myocyte biology through activation of V₂ and V₁(a) receptors. Current research is exploring V₂- and dual V₁(a)/V₂ receptor antagonism for the treatment of hyponatremia, as well as for the congestion and edema associated with chronic HF, because vasopressin receptor antagonists might offer benefits in comparison with conventional loop diuretics. The purpose of this review is to update the current status of experimental and clinical studies with available vasopressin receptor antagonists (conivaptan and tolvaptan) and their potential role in the treatment of HF and hyponatremia of multiple causes.

Identification, characterization and rescue of a novel vasopressin-2 receptor mutation causing nephrogenic diabetes insipidus

OBJECTIVE

X-linked nephrogenic diabetes insipidus (XNDI), caused by mutations in the V2 vasopressin receptor (V2R), is clinically distinguished from central diabetes insipidus (CDI) by elevated serum vasopressin (AVP) levels and unresponsiveness to 1-desamino-8-d-arginine vasopressin (DDAVP). We report two infants with XNDI, and present the characterization and functional rescue of a novel V2R mutation.

PATIENTS

Two male infants presented with poor growth and hypernatraemia. Both patients had measurable pretreatment serum AVP and polyuria that did not respond to DDAVP, suggesting NDI. However, both also had absent posterior pituitary bright spot on MRI, which is a finding more typical of CDI.

METHODS

The AVPR2 gene encoding V2R was sequenced. The identified novel missense mutation was re-created by site-directed mutagenesis and expressed in HEK293 cells. V2R activity was assessed by the ability of transfected cells to produce cAMP in response to stimulation with DDAVP. Membrane localization of V2R was assessed by fluorescence microscopy.

RESULTS

Patient 1 had a deletion of AVPR2; patient 2 had the novel mutation L57R. In transiently transfected HEK293 cells, DDAVP induced detectable but severely impaired L57R V2R activity compared to cells expressing wild-type V2R. Fluorescence microscopy showed that myc-tagged wild-type V2R localized to the cell membrane while L57R V2R remained intracellular. A nonpeptide V2R chaperone, SR121463, partially rescued L57R V2R function by allowing it to reach the cell membrane.

CONCLUSIONS

L57R V2R has impaired in vitro activity that can be partially improved by treatment with a V2R chaperone. The posterior pituitary hyperintensity may be absent in infants with XNDI.

Cell death and endoplasmic reticulum stress: disease relevance and therapeutic opportunities

The accumulation of unfolded proteins in the endoplasmic reticulum (ER) represents a cellular stress induced by multiple stimuli and pathological conditions. These include hypoxia, oxidative injury, high-fat diet, hypoglycaemia, protein inclusion bodies and viral infection. ER stress triggers an evolutionarily conserved series of signal-transduction events, which constitutes the unfolded protein response. These signalling events aim to ameliorate the accumulation of unfolded proteins in the ER; however, when these events are severe or protracted they can induce cell death. With the increasing recognition of an association between ER stress and human diseases, and with the improved understanding of the diverse underlying molecular mechanisms, novel targets for drug discovery and new strategies for therapeutic intervention are beginning to emerge.

cAMP-dependent chloride secretion mediates tubule enlargement and cyst formation by cultured mammalian collecting duct cells

Polycystic kidney diseases result from disruption of the genetically defined program that controls the size and geometry of renal tubules. Cysts which frequently arise from the collecting duct (CD) result from cell proliferation and fluid secretion. From mCCD(cl1) cells, a differentiated mouse CD cell line, we isolated a clonal subpopulation (mCCD-N21) that retains morphogenetic capacity. When grown in three-dimensional gels, mCCD-N21 cells formed highly organized tubular structures consisting of a palisade of polarized epithelial cells surrounding a cylindrical lumen. Subsequent addition of cAMP-elevating agents (forskolin or cholera toxin) or of membrane-permeable cAMP analogs (CPT-cAMP) resulted in rapid and progressive dilatation of existing tubules, leading to the formation of cystlike structures. When grown on filters, mCCD-N21 cells exhibited a high transepithelial resistance as well as aldosterone- and/or vasopressin-induced amiloride-sensitive and -insensitive current. The latter was in part inhibited by Na(+)-K(+)-2Cl(-) cotransporter (bumetanide) and chloride channel (NPPB) inhibitors. Real-time PCR analysis confirmed the expression of NKCC1, the ubiquitous Na(+)-K(+)-2Cl(-) cotransporter and cystic fibrosis transmembrane regulator (CFTR) in mCCD-N21 cells. Tubule enlargement and cyst formation were prevented by inhibitors of Na(+)-K(+)-2Cl(-) cotransporters (bumetanide or ethacrynic acid) or CFTR (NPPB or CFTR inhibitor-172). These results further support the notion that cAMP signaling plays a key role in renal cyst formation, at least in part by promoting chloride-driven fluid secretion. This new in vitro model of tubule-to-cyst conversion affords a unique opportunity for investigating the molecular mechanisms that govern the architecture of epithelial tubes, as well as for dissecting the pathophysiological processes underlying cystic kidney diseases.

Fluorescent agonists and antagonists for vasopressin/oxytocin G protein-coupled receptors: usefulness in ligand screening assays and receptor studies

Different series of fluorescent agonists and antagonists have been developed and characterized for arginine-vasopressin and oxytocin G protein-coupled receptors. Both cyclic and linear peptide analogs of the neurohypophysial hormones are useful tools for investigating receptor localization and trafficking, analysing receptor structural organization, and developing new receptor-selective high-throughput ligand screening assays.

Pharmacology of vasopressin antagonists

Congestive heart failure (CHF) is characterized by fluid and water retention, which frequently is a therapeutic challenge. Most conventional diuretics act primarily as saluretics, i.e. they inhibit renal tubular electrolyte reabsorption, which due to osmotic pressure promotes excretion of isotonic fluid. Arginine vasopressin (AVP) via the V(1A) receptor vasoconstricts and via the V(2) receptor promotes water reabsorption in the renal collecting duct by inserting aquaporin-2 water channels into the luminal membrane. Novel V(2) receptor antagonists act as powerful aquaretics, i.e. they excrete free water. We review the pharmacology of non-selective V(1A)/V(2) receptor antagonists and selective V(2) receptor antagonists currently in clinical development.

Identification of novel selective V2 receptor non-peptide agonists

Peptides with agonist activity at the vasopressin V(2) receptor are used clinically to treat fluid homeostasis disorders such as polyuria and central diabetes insipidus. Of these peptides, the most commonly used is desmopressin, which displays poor bioavailability as well as potent activity at the V(1b) receptor, with possible stress-related adverse effects. Thus, there is a strong need for the development of small molecule chemistries with selective V(2) receptor agonist activity. Using the functional cell-based assay Receptor Selection and Amplification Technology (R-SAT((R))), a screening effort identified three small molecule chemotypes (AC-94544, AC-88324, and AC-110484) with selective agonist activity at the V(2) receptor. One of these compounds, AC-94544, displayed over 180-fold selectivity at the V(2) receptor compared to related vasopressin and oxytocin receptors and no activity at 28 other G protein-coupled receptors (GPCRs). All three compounds also showed partial agonist activity at the V(2) receptor in a cAMP accumulation assay. In addition, in a rat model of central diabetes insipidus, AC-94544 was able to significantly reduce urine output in a dose-dependent manner. Thus, AC-94544, AC-88324, and AC-110484 represent novel opportunities for the treatment of disorders associated with V(2) receptor agonist deficiency.

Sodium excretion in response to vasopressin and selective vasopressin receptor antagonists

The mechanisms by which arginine vasopressin (AVP) exerts its antidiuretic and pressor effects, via activation of V2 and V1a receptors, respectively, are relatively well understood, but the possible associated effects on sodium handling are a matter of controversy. In this study, normal conscious Wistar rats were acutely administered various doses of AVP, dDAVP (V2 agonist), furosemide, or the following selective non-peptide receptor antagonists SR121463A (V2 antagonist) or SR49059 (V1a antagonist). Urine flow and sodium excretion rates in the next 6 h were compared with basal values obtained on the previous day, after vehicle treatment, using each rat as its own control. The rate of sodium excretion decreased with V2 agonism and increased with V2 antagonism in a dose-dependent manner. However,for comparable increases in urine flow rate, the V2 antagonist induced a natriuresis 7-fold smaller than did furosemide. Vasopressin reduced sodium excretion at 1 mug/kg but increased it at doses >5 umg/kg,an effect that was abolished by the V1a antagonist. Combined V2 and V1a effects of endogenous vasopressin can be predicted to vary largely according to the respective levels of vasopressin in plasma,renal medulla (acting on interstitial cells), and urine (acting on V1a luminal receptors). In the usual range of regulation, antidiuretic effects of vasopressin may be associated with variable sodium retention. Although V2 antagonists are predominantly aquaretic, their possible effects on sodium excretion should not be neglected. In view of their proposed use in several human disorders, the respective influence of selective (V2) or mixed (V1a/V2) receptor antagonists on sodium handling in humans needs reevaluation.

Non-peptide arginine-vasopressin antagonists: the vaptans

Arginine-vasopressin is a hormone that plays an important part in circulatory and water homoeostasis. The three arginine-vasopressin-receptor subtypes--V1a, V1b, and V2--all belong to the large rhodopsin-like G-protein-coupled receptor family. The vaptans are orally and intravenously active non-peptide vasopressin receptor antagonists that are in development. Relcovaptan is a selective V1a-receptor antagonist, which has shown initial positive results in the treatment of Raynaud's disease, dysmenorrhoea, and tocolysis. SSR-149415 is a selective V1b-receptor antagonist, which could have beneficial effects in the treatment of psychiatric disorders. V2-receptor antagonists--mozavaptan, lixivaptan, satavaptan, and tolvaptan--induce a highly hypotonic diuresis without substantially affecting the excretion of electrolytes (by contrast with the effects of diuretics). These drugs are all effective in the treatment of euvolaemic and hypervolaemic hyponatraemia. Conivaptan is a V1a/V2 non-selective vasopressin-receptor antagonist that has been approved by the US Food and Drug Administration as an intravenous infusion for the inhospital treatment of euvolaemic or hypervolaemic hyponatraemia.

Effects of novel vasopressin receptor antagonists on renal function and cardiac hypertrophy in rats with experimental congestive heart failure

Arginine vasopressin (AVP) plays an important role in renal hemodynamic alterations, water retention, and cardiac remodeling in congestive heart failure (CHF). The present study evaluated the acute and chronic effects of vasopressin V(1a) receptor subtype (V(1a)) and vasopressin V(2) receptor subtype (V(2)) antagonists on renal function and cardiac hypertrophy in rats with CHF. The effects of acute administration of SR 49059 [(2S)1-[(2R,3S)-5-chloro-3-(2-chlorophenyl)-1-(3,4-dimethoxybenzene-sulfonyl)-3-hydroxy-2,3-dihydro-1H-indole-2-carbonyl]-pyrrolidine-2-carboxamide)] (0.1 mg/kg) and SR 121463B (1-[4-(N-tert-butylcarbamoyl)-2-methoxybenzenesulfonyl]-5-ethoxy-3-spiro-[4-(2-morpholinoethoxy)cyclohexane]indol-2-one, fumarate; equatorial isomer) (0.3 mg/kg), V(1a) and V(2) antagonists, respectively, on renal function, and of chronic treatment (3.0 mg/kg/day for 7 or 28 days, via osmotic minipumps or p.o.), on water excretion and cardiac hypertrophy were studied in rats with aortocaval fistula and control rats. CHF induction increased plasma AVP (12.8 +/- 2.5 versus 32.2 +/- 8.3 pg/ml, p < 0.05). Intravenous bolus injection of SR 121463B to controls produced dramatic diuretic response (from 5.5 +/- 0.8 to 86.3 +/- 21.9 microl/min; p < 0.01). In contrast, administration of SR 49059 did not affect urine flow. Likewise, administration of SR 121463B, but not SR 49059, to rats with CHF significantly increased urinary flow rate from 20.8 +/- 6.4 to 91.6 +/- 26.5 microl/min (p < 0.01). The diuretic effects of SR 121463B were associated with a significant decline in urinary osmolality and insignificant change of Na+ excretion. In line with its acute effects, chronic administration of SR 121463B to CHF rats increased daily urinary volume 2 to 5-fold throughout the treatment period. Both SR 121463B and SR 49059 significantly reduced heart weight in CHF rats when administered for 4 weeks, but not 1 week. These results suggest that V(2) and V(1a) antagonists improve water balance and cardiac hypertrophy in CHF and might be beneficial for the treatment of water retention and cardiac remodeling in CHF.

[Difference in urine concentration according to gender and ethnicity: possible involvement in the different susceptibility to various renal and cardiovascular diseases]

Men and African-Americans are known to be at greater risk of urolithiasis and cardiovascular and renal diseases than women and Caucasians. Previous studies suggest that the antidiuretic effects of vasopressin and/or a greater urine concentration are associated with the rate of progression of these diseases. The present review addresses possible sex and ethnic-related differences in urine volume and osmolality which could participate in this male and black higher predominance. We reanalyzed 24h-urine data collected previously by different investigators for other purposes. In studies concerning healthy subjects (six studies) or patients with chronic kidney disease or Diabetes mellitus (three studies), men excreted a larger osmolar load than women, with a 15 to 30% higher urinary osmolality (or another index of urine concentration based on the urine/plasma creatinine concentration ratio) and a similar 24h urine volume than in women. In two American studies, African-Americans showed a significantly higher urinary concentration than Caucasians and a lower 24h-urine volume. Sex and ethnic differences in thirst threshold, vasopressin level, or other regulatory mediators may contribute to the higher urinary concentration of men and of African Americans. These differences could play a role in the greater susceptibility of these subjects to these pathologies. New prospective studies should take into account the antidiuretic effects of vasopressin as a potential risk factor in the initiation and progression of cardiovascular and renal diseases.

Treatment of hyponatremia

Hyponatremia is an electrolyte disorder that is defined by a serum sodium concentration of less than 136 mmol/L. Hyponatremia occurs at a high incidence. It is commonly associated with mild to moderate mental impairment. Hypoosmolar hyponatremia occurs in the setting of plasma volume deficiency ("hypovolemia", e. g. after gastrointestinal fluid loss), liver cirrhosis and cardiac failure ("hypervolemic" hyponatremia) and syndrome of inappropriate antidiuretic hormone secretion ("euvolemic" hyponatremia). Excessive antidiuretic hormone and continued fluid intake are the pathogenetic causes of these hyponatremias. Whereas hypovolemic hyponatremia is best corrected by isotonic saline, conventional proposals for euvolemic and hypervolemic hyponatremia consist of the following: fluid restriction, lithium carbonate, demeclocycline, urea and loop diuretic. None of these nonspecific treatments is entirely satisfactory. Recently a new class of pharmacological agents -orally available vasopressin antagonists, collectively called vaptans- have been described. A number of clinical trials using vaptans have been performed already. They showed vaptans to be effective, specific and safe in the treatment of euvolemic and hypervolemic hyponatremia.

Molecular mechanisms of antidiuretic effect of oxytocin

Oxytocin is known to have an antidiuretic effect, but the mechanisms underlying this effect are not completely understood. We infused oxytocin by osmotic minipump into vasopressin-deficient Brattleboro rats for five days and observed marked antidiuresis, increased urine osmolality, and increased solute-free water reabsorption. Administration of oxytocin also significantly increased the protein levels of aquaporin-2 (AQP2), phosphorylated AQP2 (p-AQP2), and AQP3 in the inner medulla and in the outer medulla plus cortex. Immunohistochemistry demonstrated increased AQP2 and p-AQP2 expression and trafficking to the apical plasma membrane of principal cells in the collecting duct, and increased AQP3 expression in the basolateral membrane. These oxytocin-induced effects were blocked by treatment with the vasopressin V2 receptor antagonist SR121463B, but not by treatment with the oxytocin receptor antagonist GW796679X. We conclude that vasopressin V2 receptors mediate the antidiuretic effects of oxytocin, including increased expression and apical trafficking of AQP2, p-AQP2, and increased AQP3 protein expression.

Hormonal control of the renal immune response and antibacterial host defense by arginine vasopressin

Ascending urinary tract infection (UTI) and pyelonephritis caused by uropathogenic Escherichia coli (UPEC) are very common infections that can cause severe kidney damage. Collecting duct cells, the site of hormonally regulated ion transport and water absorption controlled by vasopressin, are the preferential intrarenal site of bacterial adhesion and initiation of inflammatory response. We investigated the effect of the potent V2 receptor (V2R) agonist deamino-8-D-arginine vasopressin (dDAVP) on the activation of the innate immune response using established and primary cultured collecting duct cells and an experimental model of ascending UTI. dDAVP inhibited Toll-like receptor 4–mediated nuclear factor κB activation and chemokine secretion in a V2R-specific manner. The dDAVP-mediated suppression involved activation of protein phosphatase 2A and required an intact cystic fibrosis transmembrane conductance regulator Cl⁻ channel. In vivo infusion of dDAVP induced a marked fall in proinflammatory mediators and neutrophil recruitment, and a dramatic rise in the renal bacterial burden in mice inoculated with UPECs. Conversely, administration of the V2R antagonist SR121463B to UPEC-infected mice stimulated both the local innate response and the antibacterial host defense. These findings evidenced a novel hormonal regulation of innate immune cellular activation and demonstrate that dDAVP is a potent modulator of microbial-induced inflammation in the kidney.

Biological characterization of rodent and human vasopressin V1b receptors using SSR-149415, a nonpeptide V1b receptor ligand

[3H]SSR-149415 is the first tritiated nonpeptide vasopressin V1b receptor (V1bR) antagonist ligand. It was used for studying rodent (mouse, rat, hamster) and human V1bR from native or recombinant origin. Moreover, a close comparison between the human and the mouse V1bR was performed using SSR-149415/[3H]SSR-149415 in binding and functional studies in vitro. [3H]SSR-149415 binding was time-dependent, reversible, and saturable. Scatchard plot analysis gave a single class of high-affinity binding sites with apparent equilibrium dissociation constant ( Kd) ∼1 nM and maximum binding density (Bmax) values from 7,000 to 300,000 sites/cell according to the cell line. In competition experiments, [3H]SSR-149415 binding was stereospecific and dose-dependently displaced by reference peptide and nonpeptide arginine vasopressin (AVP)/OT ligands following a V1b rank order of affinity: SSR-149415 = AVP > dCha > dPen > dPal > dDavp > SSR-126768A > SR-49059 > SSR-149424 > OT > SR-121463B. Species differences between human, rat, mouse, and hamster V1bR were observed. Autoradiography studies with [3H]SSR-149415 on rat and human pituitary showed intense specific labeling confined to corticotroph cells and absence of labeling in the other tissues examined. SSR-149415 potently and stereospecifically antagonized the AVP-induced inositol phosphate production and intracellular Ca2+ increase (EC50 from 1.83 to 3.05 nM) in recombinant cell lines expressing either the mouse or the human V1bR. AVP (10−7 M) exposure of AtT20 cells expressing mouse or human EGFP-tagged V1bR induced their rapid internalization. Preincubation with 10−6 M SSR-149415 counteracted the internalization process. Moreover, recycling of internalized receptors was observed upon 10−6 M SSR-149415 treatment. Thus SSR-149415/[3H]SSR-149415 are unique tools for studying animal and human V1bR.

Effect of YM218, a nonpeptide vasopressin V1A receptor-selective antagonist, on rat mesangial cell hyperplasia and hypertrophy

Mesangial cell growth constitutes a key feature of progressive glomerular injury. Vasopressin (AVP), a potent peptide vasoconstrictor, acts on mesangial cells through the V(1A) receptors, inducing contraction and cell proliferation. This study examined the effects of YM218, a nonpeptide AVP V(1A) receptor-selective antagonist, on the mitogenic and hypertrophic effects of AVP in rat mesangial cells. When added to mesangial cells whose growth was arrested, AVP concentration-dependently induced hyperplasia and hypertrophy. YM218 potently prevented AVP-induced hyperplasia and hypertrophy of these cells. Furthermore, AVP stimulated endothelin (ET)-1 secretion from mesangial cells in a concentration-dependent manner and this effect was potently inhibited by YM218. ET-1 also induced hyperplasia and hypertrophy in mesangial cells and this effect was completely abolished by ET(A) receptor-selective antagonist YM598. In addition, AVP-induced hyperplasia and hypertrophy were partly inhibited by YM598. These results suggest that AVP may modulate mesangial cell growth not only by its direct action but also through the stimulation of ET-1 secretion. YM218 displays high potency in inhibiting the AVP-induced physiologic responses of mesangial cells via the V(1A) receptors and is a potent pharmacologic probe for investigating the physiologic and pathophysiologic roles of AVP in several renal diseases.

PKD1 haploinsufficiency causes a syndrome of inappropriate antidiuresis in mice

Mutations in PKD1 are associated with autosomal dominant polycystic kidney disease. Studies in mouse models suggest that the vasopressin (AVP) V2 receptor (V2R) pathway is involved in renal cyst progression, but potential changes before cystogenesis are unknown. This study used a noncystic mouse model to investigate the effect of Pkd1 haploinsufficiency on water handling and AVP signaling in the collecting duct (CD). In comparison with wild-type littermates, Pkd1(+/-) mice showed inappropriate antidiuresis with higher urine osmolality and lower plasma osmolality at baseline, despite similar renal function and water intake. The Pkd1(+/-) mice had a decreased aquaretic response to both a water load and a selective V2R antagonist, despite similar V2R distribution and affinity. They showed an inappropriate expression of AVP in brain, irrespective of the hypo-osmolality. The cAMP levels in kidney and urine were unchanged, as were the mRNA levels of aquaporin-2 (AQP2), V2R, and cAMP-dependent mediators in kidney. However, the (Ser256) phosphorylated AQP2 was upregulated in Pkd1(+/-) kidneys, with AQP2 recruitment to the apical plasma membrane of CD principal cells. The basal intracellular Ca(2+) concentration was significantly lower in isolated Pkd1(+/-) CD, with downregulated phosphorylated extracellular signal-regulated kinase 1/2 and decreased RhoA activity. Thus, in absence of cystic changes, reduced Pkd1 gene dosage is associated with a syndrome of inappropriate antidiuresis (positive water balance) reflecting decreased intracellular Ca(2+) concentration, decreased activity of RhoA, recruitment of AQP2 in the CD, and inappropriate expression of AVP in the brain. These data give new insights in the potential roles of polycystin-1 in the AVP and Ca(2+) signaling and the trafficking of AQP2 in the CD.

Small-molecule vasopressin-2 receptor antagonist identified by a g-protein coupled receptor "pathway" screen

G-protein-coupled receptors (GPCRs) such as the vasopressin-2 receptor (V(2)R) are an important class of drug targets. We developed an efficient screen for GPCR-induced cAMP elevation using as read-out cAMP activation of cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channels. Fischer rat thyroid cells expressing CFTR and a halide-sensing yellow fluorescent protein (H148Q/I152L) were transfected with V(2)R. Increased cell Cl(-) conductance after agonist-induced cAMP elevation was assayed using a plate reader from cell fluorescence after solution I(-) addition. The Z' factor for the assay was approximately 0.7 with the V(2)R agonist [deamino-Cys1, Val4, d-Arg8]-vasopressin (1 nM) as positive control. Primary screening of 50,000 small molecules yielded a novel, 5-aryl-4-benzoyl-3-hydroxy-1-(2-arylethyl)-2H-pyrrol-2-one class of V(2)R antagonists that are unrelated structurally to known V(2)R antagonists. The most potent compound, V(2)R(inh)-02, which was identified by screening 35 structural analogs, competitively inhibited V(2)R-induced cAMP elevation with K(i) value of approximately 70 nM and fully displaced radiolabeled vasopressin in binding experiments. V(2)R(inh)-02 did not inhibit forskolin or beta(2)-adrenergic receptor-induced cAMP production and was more than 50 times more potent for V(2)R than for V(1a)R. The favorable in vitro properties of the pyrrol-2-one antagonists suggests their potential usefulness in aquaretic applications. The CFTR-linked cAMP assay developed here is applicable for efficient, high-throughput identification of modulators of cAMP-coupled GPCRs.

Ethnic Differences in Urine Concentration: Possible Relationship to Blood Pressure

The mechanisms that account for the susceptibility of black individuals to hypertension and their reduced ability to excrete sodium are poorly understood. Vasopressin administration has been shown in healthy humans to delay sodium excretion along with its antidiuretic action. Black individuals have been reported to have higher vasopressin levels than white individuals. Therefore, this study investigated retrospectively 24-h urine volume (V) and urine concentration index (urine-to-plasma ratio of creatinine concentration), as well as their possible relationships with BP, in a cohort of 141 healthy young black and white individuals (18 to 40 y). Black individuals were found to have a significantly lower V and higher urine concentration than white individuals, especially during daytime. In addition, they exhibited a blunted nocturnal fall in fluid and electrolyte excretion and a higher pulse pressure than white individuals. Higher urine concentration and lower V were associated significantly with higher PP (but not with systolic or diastolic BP) in men. These relations remained significant after adjustment for age, body mass index, and sodium and potassium excretion. These results suggest that an enhanced tendency to concentrate urine may delay the excretion of the daily ingested fluid and sodium and may increase pulse pressure in young normotensive individuals. The higher urine concentration that is observed in black individuals (which could represent an adaptation to better water conservation) may participate in their enhanced susceptibility to hypertension. If these results are confirmed in further studies, then vasopressin V2 receptor antagonists might offer a novel antihypertensive strategy, especially in the black population.

Vaptans and the treatment of water-retaining disorders

Hyponatremia is a frequent and symptomatic electrolyte disorder for which specific treatments have been lacking. Hyponatremia is attributable to nonosmotic vasopressin stimulation and continued increased fluid intake. In the past, peptidic derivatives of arginine vasopressin proved that blockade of vasopressin V-2 receptors served to improve hyponatremia, however, these antagonists had intrinsic agonistic activity, too. In the past decade, random screening of molecules uncovered nonpeptide, orally available vasopressin antagonists without agonistic properties. The agents show competitive binding to the vasopressin V-2 receptor at an affinity comparable with that of arginine vasopressin. Four antagonists have undergone extensive study. Three of these agents--lixivaptan or VPA 985; SR 121 463 B; tolvaptan or OPC 41,061--are specific V-2 antagonists whereas conivaptan or YM 087 is a V-1/V-2 mixed antagonist. In animal and clinical studies all of the agents were able to correct water retention and hyponatremia in a dose-dependent manner. There was no tachyphylaxis, even when the agents were given over many weeks. It is expected that the clinical use of the agents will lead to a major improvement in the treatment of hyponatremia.

Successful long-term treatment of hyponatremia in syndrome of inappropriate antidiuretic hormone secretion with satavaptan (SR121463B), an orally active nonpeptide vasopressin V2-receptor antagonist

The effects of satavaptan (SR121463B), a novel long-acting orally active vasopressin V(2)-receptor antagonist, were investigated in patients with the syndrome of inappropriate antidiuretic hormone secretion (SIADH). In the first part of this randomized, double-blind study, 34 patients first were treated with satavaptan (versus placebo) for up to 5 d and then during 23 d of open-label dosage-adjustment period. In the second part of the study, long-term efficacy and safety of satavaptan was assessed in an open-label trial during at least 12 mo. Mean (+/-SD) serum sodium (SNa) levels before treatment were 127 +/- 2 mmol/L (placebo, n = 8), 125 +/- 6 mmol/L (25 mg, n = 14), and 127 +/- 5 mmol/L (50 mg, n = 12). Responders (patients SNa levels normalized or increased by at least 5 mmol/L from baseline during the double-blind period) were 79% in the 25-mg group (SNa 136 +/- 3 mmol/L; P = 0.006), 83% in the 50-mg group (SNa 140 +/- 6 mmol/L; P = 0.005), and 13% in the placebo group (SNa 130 +/- 5 mmol/L). No drug-related serious adverse events were recorded. During the long-term treatment, 15 of 18 enrolled patients achieved 6 mo and 10 achieved 12 mo of treatment. The SNa response was maintained during this time with a good tolerance. The new oral vasopressin V(2)-receptor antagonist satavaptan adequately corrects mild or moderate hyponatremia in patients with SIADH and has a good safety profile.

Regulation of UT-A1-mediated transepithelial urea flux in MDCK cells

Transepithelial [(14)C]urea fluxes were measured across cultured Madin-Darby canine kidney (MDCK) cells permanently transfected to express the urea transport protein UT-A1. The urea fluxes were typically increased from a basal rate of 2 to 10 and 25 nmol.cm(-2).min(-1) in the presence of vasopressin and forskolin, respectively. Flux activation consisted of a rapid-onset component of small amplitude that leveled off within approximately 10 min and at times even decreased again, followed by a delayed, strong increase over the next 30-40 min. Forskolin activated urea transport through activation of adenylyl cyclase; dideoxyforskolin was inactive. Vasopressin activated urea transport only from the basolateral side and was blocked by OPC-31260, indicating that its action was mediated by basolateral V(2) receptors. In the presence of the phosphodiesterase inhibitor IBMX, vasopressin activated as strongly as forskolin. By itself, IBMX caused a slow increase over 50 min to approximately 5 nmol.cm(-2).min(-1). 8-Bromoadenosine 3',5'-cyclic monophosphate (8-BrcAMP; 300 microM) activated urea flux only when added basolaterally. IBMX augmented the activation by basolateral 8-BrcAMP. Urea flux activation by vasopressin and forskolin were only partially blocked by the protein kinase A inhibitor H-89. Even at concentrations >10 microM, urea flux after 60 min of stimulation was reduced by <50%. The rapid-onset component appeared unaffected by the presence of H-89. These data suggest that activation of transepithelial urea transport across MDCK-UT-A1 cells by forskolin and vasopressin involves cAMP as a second messenger and that it is mediated by one or more signaling pathways separate from and in addition to protein kinase A.

Hyponatremia in cirrhosis: clinical features and management

The presence of dilutional hyponatremia has a poor prognosis for survival in patients with cirrhosis and ascites. Effective and safe treatments are needed to improve prognosis in patients with cirrhosis and dilutional hyponatremia. The initial approach to management includes fluid restriction, low sodium diet, and minimizing the use of diuretics. In addition, the use of hypertonic saline should be avoided in patients with cirrhosis and dilutional hyponatremia. Furthermore, patients should be placed on the top of the list for liver transplantation if they are appropriate candidates. Although V2 arginine vasopressin receptor antagonists that selectively enhance solute-free water excretion in patients with cirrhosis seem very promising, two points must be considered in relation to the available data. First, although the results of phase-2 studies are encouraging, the efficacy and safety of these compounds should be further evaluated. Second, the clinical utility of these agents in cirrhosis has only been assessed in short-term studies. The long-term effects of these drugs remain unknown. Future research with these compounds should not only focus on the effects on serum sodium, but also on treatment and prevention of recurrence of ascites. In addition, the possible beneficial effects of these drugs in the prevention of hepatic encephalopathy would be worth studying.

Hyponatremia in the neurosurgical patient: epidemiology, pathophysiology, diagnosis, and management

OBJECTIVE

Hyponatremia is an important and common electrolyte disorder in critically ill neurosurgical patients that has been reported in association with a number of different primary diagnoses. The correct diagnosis of the pathophysiological cause is vital because it dramatically alters the treatment approach.

METHODS

We review the epidemiology and presentation of patients with hyponatremia, the pathophysiology of the disorder with respect to sodium and fluid balance, and the diagnostic procedures for determining the correct cause.

RESULTS

We then present the various treatment options, including discussion of one of the newest groups of agents, the arginine vasopressin receptor antagonists, currently under study for the treatment of hyponatremia in neurosurgical patients.

CONCLUSION

Hyponatremia is a serious comorbidity in neurosurgical patients that requires particular attention as its treatment varies by cause and its consequences can affect neurological outcome.

Sex difference in urine concentration across differing ages, sodium intake, and level of kidney disease

Men are known to be at greater risk of urolithiasis and cardiovascular and renal diseases than women. Previous studies suggest that greater urine concentration is associated with acceleration of progression of chronic kidney disease (CKD), increased urinary albumin excretion, and delayed renal sodium excretion. The present review addresses possible sex-related differences in urine volume and osmolality (U(osm)) that could participate in this male risk predominance. Because of the scarcity of information, we reanalyzed 24-h urine data collected previously by different investigators for other purposes. In nine studies concerning healthy subjects (6 studies) or patients with CKD or diabetes mellitus, U(osm) (or another index of urine concentration based on the urine/plasma creatinine concentration ratio) was 21-39% higher (i.e., about a 150 mosm/kgH2O difference) in men than in women. Urine volume was not statistically different. Thus, the larger osmolar load of men (related to their higher food intake) is excreted in a more concentrated urine with no difference in urine volume. This sex difference was not influenced by the level of sodium excretion and was still present in CKD patients. Sex differences in thirst threshold, AVP level, and other regulatory mediators may all contribute to the higher male U(osm). Because of the previously demonstrated adverse effects of vasopressin and/or high urine concentrating activity, the greater tendency of men to concentrate urine could participate in their greater susceptibility to urolithiasis and hypertension and to the faster progression towards end-stage renal failure.

Binding and signal transduction characteristics of the nonpeptide vasopressin V1A receptor-selective antagonist YM218 in cultured rat mesangial cells

Vasopressin (AVP) causes mesangial cell contraction, proliferation and hypertrophy. The present study investigated the effects of YM218, a potent, nonpeptide AVP V(1A) receptor-selective antagonist, on rat mesangial cells using binding, signal transduction and cell growth assays. Specific binding of (3)H-AVP to rat mesangial cell plasma membranes was dependent upon time, temperature and membrane protein concentration. Scatchard plot analysis of equilibrium binding data revealed the existence of a single class of high-affinity binding sites with the expected V(1A) receptor profile. YM218 showed high affinity for V(1A) receptors, exhibiting a K(i) value of 0.19 nmol/l. AVP concentration-dependently increased intracellular Ca(2+) ([Ca(2+)](i)) levels, stimulated mitogen-activated protein (MAP) kinase and induced hyperplasia. Conversely, YM218 potently suppressed [Ca(2+)](i) elevation, activation of MAP kinase and hyperplasia induced by AVP. These results indicate that YM218 displays both high affinity for rat mesangial cell V(1A) receptors and high potency in inhibiting AVP-induced signal transduction and growth response. Therefore, YM218 is a useful pharmacologic tool for investigating the physiologic and pathophysiologic roles of AVP in kidney, and may have clinical application in the prevention or regression of mesangial cell growth.

Functional rescue of vasopressin V2 receptor mutants in MDCK cells by pharmacochaperones: relevance to therapy of nephrogenic diabetes insipidus

Intracellular retention of a functional vasopressin V2 receptor (V2R) is a major cause of congenital nephrogenic diabetes insipidus (NDI) and rescue of V2R mutants by nonpeptide antagonists may restore their basolateral membrane (BM) localization and function. However, the criteria for efficient functional rescue of G protein-coupled receptor (GPCR) mutants at clinically feasible antagonist concentrations are unknown. We found that the four nonpeptide antagonists SR49059, OPC31260, OPC41061, and SR121463B induced maturation and rescued the BM expression of eight of nine different V2R mutants, stably expressed in physiologically relevant polarized cells. The extent of maturation and rescued BM expression correlated with the antagonists' concentration and affinity for the V2R. Displacement of the antagonists by AVP and subsequent cAMP generation inversely correlated with the antagonists' affinities for the V2R but is partially influenced by antagonist-specific aspects. Despite limited increases in maturation and cell-surface expression of V2R mutants, the low-affinity SR49059 optimally induced functional rescue at high concentrations, due to its easy displacement by vasopressin. At clinically feasible antagonist concentrations, however, only the high-affinity antagonists OPC31260 and OPC41061 induced functional rescue, as at these concentrations the extent of BM expression became limited. In conclusion, functional rescue of mutant V2Rs at clinically feasible concentrations is most effective with high-affinity antagonists. As OPC31260 and OPC41061 are clinically safe, they are promising candidates to relieve NDI. Moreover, as numerous other diseases are caused by endoplasmic reticulum-retained GPCRs for which cell-permeable antagonists become available, our finding that high-affinity antagonists are superior is anticipated to be important for pharmacotherapy development of these diseases.

Probing the existence of G protein-coupled receptor dimers by positive and negative ligand-dependent cooperative binding

An increasing amount of ligand binding data on G protein-coupled receptors (GPCRs) is not compatible with the prediction of the simple mass action law. This may be related to the propensity of most GPCRs, if not all, to oligomerize. Indeed, one of the consequences of receptor oligomerization could be a possible cross-talk between the protomers, which in turn could lead to negative or positive cooperative ligand binding. We prove here that this can be demonstrated experimentally. Saturation, dissociation, and competition binding experiments were performed on vasopressin and oxytocin receptors expressed in Chinese hamster ovary or COS-7 cells. Linear, concave, and convex Scatchard plots were then obtained, depending on the ligand used. Moreover, some competition curves exhibited an increase of the radiotracer binding for low concentrations of competitors, suggesting a cooperative binding process. These data demonstrate that various vasopressin analogs display either positive or negative cooperative binding. Because positive cooperative binding cannot be explained without considering receptor as multivalent, these binding data support the concept of GPCR dimerization process. The results, which are in good accordance with the predictions of previous mathematical models, suggest that binding experiments can be used to probe the existence of receptor dimers.

Vasopressin receptor antagonists. Therapeutic potential in the management of acute and chronic heart failure

Despite the use of ACE inhibitors and beta-blockers, the morbidity and mortality of patients with chronic heart failure remains quite high. This has stimulated the development of new therapies, many based on the neurohormonal hypothesis. There are now multiple agents being developed for the treatment of heart failure designed to block many of the neurohormones that are increased in these patients. One of the hormones that is increased in chronic heart failure is vasopressin. Vasopressin reduces free water secretion and at high concentrations, causes vasoconstriction in the peripheral vasculature. Antagonists to vasopressin will promote free water excretion (aquaresis) and vasodilatation with a subsequent reduction in afterload. In theory, these agents would be beneficial for both acute exacerbations of heart failure (free water excretion) and chronic heart failure (neurohormonal blockade). We review the potential uses of these antagonists for these two conditions and the promising results of small, hemodynamic trials with the new vasopressin antagonists that have already been performed.

Vasopressin antagonists as aquaretic agents for the treatment of hyponatremia

Hyponatremia is the most frequent electrolyte disorder encountered in hospitalized patients. It is a state of relative water excess due to stimulated arginine vasopressin (AVP) and fluid intake greater than obligatory losses. This kind of hyponatremia occurs in the syndrome of inappropriate antidiuretic hormone secretion, congestive heart failure, and liver cirrhosis. Fluid restriction is the presently recommended treatment for hyponatremia. However, fluid restriction may be very difficult for patients to achieve, is slow to work, and does not allow a graded therapeutic approach. More efficient and specific treatments of hyponatremia are needed. In this respect, pharmacologic research has yielded a number of compounds exhibiting antagonistic qualities at the vasopressin V2 receptor. Among these agents, peptidic derivatives of AVP turned out to have intrinsic antidiuretic properties in vivo when given over days or weeks. The development of such agents for use in patients has not been pursued. However, several promising nonpeptide, vasopressin receptor antagonists have been described; these agents are VPA-985 (lixivaptan), YM-087 (conivaptan), OPC-41061 (tolvaptan), and SR-121463. Prospective, randomized, placebo-controlled trials performed with these agents found that they corrected hyponatremia efficiently and safely. Most of the studies were conducted over a 4- to 28-day period. Long-term studies will be needed in the future to address such issues as the eventual benefit to patients and the effects of vasopressin antagonists on morbidity and mortality of patients with hyponatremia.

A metabolic study of patients with lung cancer and hyponatremia of malignancy

PURPOSE

One-third of patients with lung cancer and hyponatremia have no evidence of ectopic arginine vasopressin (AVP) production and the cause of their hyponatremia is not conclusively established. We sought to distinguish patients with hyponatremia caused by elevated AVP versus those with ectopic atrial natriuretic peptide (ANP) via this detailed metabolic study.

EXPERIMENTAL DESIGN

We enrolled 24 patients recently diagnosed with lung cancer in a metabolic study in which patients were placed on sodium and fluid restriction for 4 days. Serum electrolytes, osmolality, urine electrolytes and osmolality, plasma AVP, ANP, aldosterone, urinary cyclic AMP and cyclic guanosine 3',5'-monophosphate were measured daily and tumor tissue was obtained to measure ectopic hormone production. We attempted to characterize the pathophysiology of hyponatremia caused by ectopic ANP production in patients with small cell lung cancer (SCLC) and to determine its effect on the aldosterone axis.

RESULTS

Seven of the nine patients with SCLC presented with hyponatremia and three had elevated ANP levels at presentation without elevation of AVP. All three patients who presented with hyponatremia and elevated ANP showed a decline in serum sodium following fluid restriction, whereas two patients with SCLC and elevated AVP had normalized serum sodium levels. The combination of hyponatremia and elevated ANP was associated with a persistent natriuresis and inappropriately low aldosterone levels despite sodium restriction, suggesting ANP suppression of the aldosterone axis.

CONCLUSIONS

Management of patients with hyponatremia and SCLC should be guided by the knowledge that some patients with SCLC have ectopic production of ANP as the cause of their hyponatremia.

Hyperosmolality in vivo upregulates aquaporin 2 water channel and Na-K-2Cl co-transporter in Brattleboro rats

There are considerable experimental results that indicate that arginine vasopressin (AVP)-independent factors are involved in urinary concentration. This study examined the role of hyperosmolality in vivo to modulate aquaporin 2 (AQP2) and Na-K-2Cl co-transporter (NKCC2), pivotal factors in urinary concentration, in AVP-deficient Brattleboro (BB) rats. Hyperglycemia with associated hyperosmolality occurred in diabetic BB rats (BBDM). Protein abundance of AQP2 increased and was reversed by insulin in the inner medulla (IM; control 100+/-5%; BBDM 146+/-8%; BBDM+Ins 122+/-9%; P<0.001) and inner stripe of outer medulla (ISOM; control 100+/-4%; BBDM 123+/-8%; BBDM+Ins 93+/-6%; P<0.05). These results were confirmed by immunohistochemistry studies. NKCC2 rose in the ISOM but was not reversed with insulin treatment. For investigation of the role of hyperosmolality in the absence of hyperglycemia on the regulation of the expression of renal AQP and NKCC2, studies were performed with hyperosmolality that was induced by 0.5% NaCl in drinking water in BB rats. Hyperosmolality that was induced by NaCl increased significantly the protein abundance of IM AQP2 (121+/-2 versus 100+/-5%; P<0.01), ISOM AQP2 (135+/-6 versus 100+/-5%; P<0.001), cortex plus outer stripe of outer medulla AQP2 (121+/-4 versus 100+/-1%; P<0.001), ISOM NKCC2 (133+/-1 versus 100+/-4%; P<0.05), and cortex plus outer stripe of outer medulla NKCC2 (142+/-16 versus 100+/-9%; P<0.05). In conclusion, hyperosmolality, secondary to either glucose or NaCl, upregulated renal AQP2 and NKCC2 in vivo in BB rats.

Effects of YM218, a nonpeptide vasopressin V1A receptor-selective antagonist, on vasopressin-induced growth responses in human mesangial cells

Mesangial cells are centrally-located glomerular pericytes with contractile, endocrine, and immunity-regulating functions. These cells are thought to maintain normal glomerular function, since mesangial cell proliferation and extracellular matrix formation are hallmarks of chronic glomerular disease. Vasopressin causes mesangial cell contraction, proliferation and hypertrophy. Consequently, the effects of YM218, a potent, nonpeptide vasopressin V(1A) receptor-selective antagonist, on the growth responses of human mesangial cells to vasopressin were investigated. YM218 showed high affinity for vasopressin V(1A) receptors, exhibiting a K(i) value of 0.18 nM. Vasopressin concentration-dependently increased intracellular Ca(2+) levels and induced hyperplasia and hypertrophy in cultured mesangial cells, YM218 potently inhibited these vasopressin-induced responses. These results clearly show that YM218 has both strong affinity for human mesangial cell vasopressin V(1A) receptors and great potency in inhibiting the vasopressin-induced growth responses of mesangial cells controlled by the vasopressin V(1A) receptors. The hyperplasia and hypertrophy of mesangial cells in vitro caused by vasopressin indicate its possible in vivo role in glomerular disease pathogenesis. Therefore, YM218 is a potent pharmacologic probe to investigate the physiologic and pathophysiologic roles of vasopressin in the development of renal disease.

Pharmacological Chaperone Activity of SR49059 to Functionally Recover Misfolded Mutations of the Vasopressin V1a Receptor

Pharmacological chaperones represent a new class of ligand with the potential to facilitate the delivery of misfolded, but still active, G-protein-coupled receptors to the cell surface. Using transfected HEK 293T cells, treatment with a nonpeptide antagonist, SR49059, dramatically increased ( approximately 60-fold) the surface expression of a misfolded, nonfunctional and intracellularly localized vasopressin V(1a) receptor (V(1a)R) mutant (D148A). This rescue of surface expression (111 +/- 7%) was almost identical to wild type assessed by confocal microscopy and quantitative enzyme-linked immunosorbent assay-based techniques. Recovery was not specific to D148A, since other surface-impaired mutations, D148N and D148E, and wild type were also increased following SR49059 exposure. However, surface delivery was specific to SR49059, since V(1a)R-selective peptide ligands or unrelated ligands were unable to mimic this action, suggesting that SR49059 acts intracellularly. SR49059-mediated surface rescue was time-, mutant-, and concentration-dependent but not directly related to its binding affinity. Maximal recovery was achieved following 12 h of treatment and did not involve de novo receptor synthesis or a consequence of preventing endogenous constitutive activity and/or internalization. Once at the surface, all mutants displayed enhanced signaling ability, and D148A was able to undergo agonist-mediated internalization. SR49059 was not effectively removed from the receptor, since signaling (EC(50)) of both wild type and D148A was reduced approximately 40-fold. This is the first report of a pharmacological chaperone ligand to act on misfolded mutant V(1a) Rs. This work provides an excellent model to understand the mechanistic action of an important new class of drug that may have potential in the treatment of diseases caused by inherited mutations.

Studies on the chemistry of thienoanellated O,N- and S,N-containing heterocycles. Part 30: Synthesis and pharmacological properties of thieno[2,3-b][1,4]thiazines with potential vasopressin receptor antagonistic activity

A series of new nonpeptide vasopressin antagonists with a 6-ethyl-thieno[2,3-b][1,4]thiazine or 6-benzyl-thieno[2,3-b][1,4]thiazine skeleton and structural modifications of the aryl side chain were synthesized in this study. The effects on guinea pig heart and smooth muscle preparations were investigated. In the presence of AVP the compounds showed an antagonistic effect. The compounds did not change spontaneous rate in right atria and exerted a slight but not significant negative inotropic effect in papillary muscles. The relaxing effect on vascular smooth muscle and terminal ileum was far more pronounced. Generally the relaxing effect on terminal ilea was more potent maybe due to difference in V1a receptor density. Our results demonstrate that compounds with an ethyl group in position six on the thienothiazine ring (14, 16, 18 and 22) exerted the most potent relaxing activity in terminal ilea, whereas compounds with a phenyl ring in position six reduced this effect.

High-throughput screening of G protein-coupled receptor antagonists using a bioluminescence resonance energy transfer 1-based beta-arrestin2 recruitment assay

In this study, the authors developed HEK293 cell lines that stably coexpressed optimal amounts of beta-arrestin2-Rluc and VENUS fusions of G protein-coupled receptors (GPCRs) belonging to both class A and class B receptors, which include receptors that interact transiently or stably with beta-arrestins. This allowed the use of a bioluminescence resonance energy transfer (BRET) 1- beta-arrestin2 translocation assay to quantify receptor activation or inhibition. One of the developed cell lines coexpressing CCR5-VENUS and beta-arrestin2- Renilla luciferase was then used for high-throughput screening (HTS) for antagonists of the chemokine receptor CCR5, the primary co-receptor for HIV. A total of 26,000 compounds were screened for inhibition of the agonist-promoted beta-arrestin2 recruitment to CCR5, and 12 compounds were found to specifically inhibit the agonist-induced beta-arrestin2 recruitment to CCR5. Three of the potential hits were further tested using other functional assays, and their abilities to inhibit CCR5 agonist-promoted signaling were confirmed. This is the 1st study describing a BRET1-beta-arrestin recruitment assay in stable mammalian cells and its successful application in HTS for GPCRs antagonists.

Molecular biology of hereditary diabetes insipidus

The identification, characterization, and mutational analysis of three different genes-the arginine vasopressin gene (AVP), the arginine vasopressin receptor 2 gene (AVPR2), and the vasopressin-sensitive water channel gene (aquaporin 2 [AQP2])-provide the basis for understanding of three different hereditary forms of "pure" diabetes insipidus: Neurohypophyseal diabetes insipidus, X-linked nephrogenic diabetes insipidus (NDI), and non-X-linked NDI, respectively. It is clinically useful to distinguish two types of hereditary NDI: A "pure" type characterized by loss of water only and a complex type characterized by loss of water and ions. Patients who have congenital NDI and bear mutations in the AVPR2 or AQP2 genes have a "pure" NDI phenotype with loss of water but normal conservation of sodium, potassium, chloride, and calcium. Patients who bear inactivating mutations in genes (SLC12A1, KCNJ1, CLCNKB, CLCNKA and CLCNKB in combination, or BSND) that encode the membrane proteins of the thick ascending limb of the loop of Henle have a complex polyuro-polydipsic syndrome with loss of water, sodium, chloride, calcium, magnesium, and potassium. These advances provide diagnostic and clinical tools for physicians who care for these patients.

Development and therapeutic indications of orally-active non-peptide vasopressin receptor antagonists

Vasopressin (AVP) and oxytocin (OT) are cyclic nonapeptides whose actions are mediated by the stimulation of specific G-protein-coupled receptors (GPCRs) currently classified into V(1)-vascular (V(1)R), V(2)-renal (V(2)R) and V(3)-pituitary (V(3)R) AVP receptors and OT receptors (OTR). The signal transduction pathways coupled to the different subtypes of AVP/OT receptors are reviewed. The recent cloning of the different members of the AVP/OT family of receptors now allows the extensive characterisation of the molecular determinants involved in agonist and antagonist binding, as well as signal transduction coupling. Potential therapeutic uses of AVP receptor antagonists include: the blockade of V(1)-vascular AVP receptors in arterial hypertension, congestive heart failure (CHF) and peripheral vascular diseases; the blockade of V(2)-renal AVP receptors in the syndrome of inappropriate vasopressin secretion, CHF, liver cirrhosis, nephrotic syndrome and any state of excessive retention of free water and subsequent hyponatraemia; the blockade of V(3)-pituitary AVP receptors in adrenocorticotropin (ACTH)-secreting tumours. The pharmacological and clinical profile of orally-active non-peptide AVP receptor antagonists is reviewed.

Effects of long-term oral treatment with selective vasopressin V2 receptor antagonist (OPC-31260) on adriamycin-induced heart failure in rats

BACKGROUND

In the treatment of heart failure, the effects of therapeutic agents on life prognosis remains unclear. We investigated the effects of long-term oral administration of a nonpeptide, selective, vasopressin V2 receptor antagonist, OPC-31260, on Sprague-Dawley rats that were treated with adriamycin to induce progressive water retention.

METHODS

Intraperitoneal saline was administered to 14 rats as a control (Group 1). A total cumulative dose of 15 mg/kg of adriamycin was administered intraperitoneally in six equal doses over a period of 2 weeks to another 52 rats. Adriamycin-treated rats were further divided into Group 2, which received saline (p.o.), and Group 3, which received 50 mg/kg (p.o.) of V2 antagonist. Oral administration continued every day for 6 weeks. Group 1 rats also received saline (p.o.) for 6 weeks.

RESULTS

The V2 antagonist decreased urine osmolality and increased diuresis of rats in Group 3. Urinary excretion of electrolytes was not increased by the V2 antagonist in Group 3. Serum osmolality was likewise unchanged by the V2 antagonist in Group 3. Plasma concentrations of vasopressin were significantly higher in Group 3 than in the other groups (Group 1, 4.0+/-1.1 pg/ml; Group 2, 4.2+/-1.5 pg/ml; Group 3, 8.5+/-1.0 pg/ml; p<0.05). During the experimental period, survival rate was higher in Group 3 than in Group 2 (Group 1, 100%; Group 2, 59%; Group 3, 83%).

CONCLUSION

Our data show that administration of orally active V2 antagonist did not reduce the survival of adriamycin-treated rats through continuous aquaretic action, despite elevated plasma levels of vasopressin.

Posttranscriptional control of aquaporin-2 abundance by vasopressin in renal collecting duct principal cells

Prevailing expression levels of aquaporin-2 (AQP2) mRNA play a major role in regulating AQP2 protein abundance. Here, we investigated whether AQP2 protein abundance is regulated at a posttranscriptional level as well. The expression levels of both AQP2 mRNA and protein increase in response to arginine vasopressin (AVP) in a concentration- and time-dependent manner in cultured immortalized mouse collecting duct principal cells (mpkCCD(cl4) cells). AVP washout from the medium of AVP-pretreated cells revealed that AQP2 mRNA expression progressively decreased over time, whereas AQP2 protein abundance first increased immediately after AVP washout and then gradually decreased over time. Inversely, increasing AVP concentration led to a time-dependent increase of AQP2 mRNA, whereas AQP2 protein abundance first decreased immediately after AVP supplementation and then gradually increased over time. These transient effects arose from altered V2-receptor activity because they could be abolished by SR-121463B, a specific V2-receptor antagonist. Although cycloheximide administration had no effect on transient alterations of AQP2 protein content, these effects were attenuated by administration of chloroquine, a lysosomal inhibitor, or lactacystin, a proteasomal inhibitor. Short-term inhibition of PKA activity significantly increased AQP2 protein abundance and blunted the transient alterations of AQP2 protein content induced by AVP washout and supplementation. In addition, phosphorylated AQP2 abundance increased immediately after AVP supplementation. These results indicate that in response to AVP AQP2 protein abundance in collecting duct principal cells is principally influenced by AQP2 mRNA content but is additionally regulated by PKA-dependent negative feedback acting on AQP2 protein degradation.

An overview of SSR149415, a selective nonpeptide vasopressin V(1b) receptor antagonist for the treatment of stress-related disorders

Vasopressin (AVP) and corticotropin-releasing factor (CRF) are key mediators in the organism's neuro-adaptive response to stress. Through pituitary and central vasopressin V(1b) receptors, AVP participates in the control of the hypothalamic-pituitary-adrenal axis (HPA) and is involved in various emotional processes. SSR149415 is the first selective, orally active vasopressin V(1b) receptor antagonist yet described. It is a competitive antagonist with nanomolar affinity for animal and human V(1b) receptors and displays a highly selective profile with regard to a large number of receptors or enzymes. In vitro, SSR149415 potently antagonizes functional cellular events associated with V(1b) receptor activation by AVP, such as intracellular Ca(2+) increase or proliferation in various cell systems. Pharmacological studies, performed by measuring ACTH secretion induced by various stimulants such as hormones (AVP or AVP + CRF) or physical stress (restraint or forced swimming stress and dehydration) in conscious rats or mice, confirm the antagonist profile of SSR149415 and its efficacy in normalizing ACTH secretion in vivo. SSR149415 is active by the oral route, at doses from 3 mg/kg, it potentiates CRF effect and displays a long-lasting oral effect in the different models. At 10 mg/kg p.o. its duration of action is longer than 4 h. This molecule also decreases anxiety and exerts marked antidepressant-like activity in several predictive animal models. The anxiolytic effects of SSR149415 have been demonstrated in various Generalized Anxiety Disorders (GAD) models (four-plate, punished drinking, elevated plus-maze, light dark, mouse defense test battery, fear-potentiated startle and social interaction tests). It is as effective as the benzodiazepine diazepam in the acute stress exposure test. SSR149415 has similar efficacy to the reference antidepressant drug, fluoxetine, in acute (forced-swimming) and chronic (chronic mild stress and subordination stress) situations in rodents. SSR149415 also reduces offensive aggression in the resident-intruder model in mice and hamsters. Depending on the model, the minimal effective doses are in the range of 1-10 mg/kg i.p. or 3-10 mg/kg p.o. SSR149415 is devoid of adverse effects on motor activity, sedation, memory or cognitive functions and produces no tachyphylaxis when administered repeatedly. It is well-tolerated in animals and humans and exhibits an adequate ADME profile. Thus, SSR149415 is a new dual anxiolytic/antidepressant compound, which appears to be free of the known side effects of classical anxiolytic/antidepressant drugs. Clinical trials are in progress, they will hopefully demonstrate its therapeutical potential for treating stress-related disorders.