Vasopressin Type Research Articles

The Selective Vasopressin Type 1a Receptor Agonist Selepressin (FE 202158) Blocks Vascular Leak in Ovine Severe Sepsis*

To determine if the selective vasopressin type 1a receptor agonist selepressin (FE 202158) is as effective as the mixed vasopressin type 1a receptor/vasopressin V2 receptor agonist vasopressor hormone arginine vasopressin when used as a titrated first-line vasopressor therapy in an ovine model of Pseudomonas aeruginosa pneumonia-induced severe sepsis. Prospective, randomized, controlled laboratory experiment. University animal research facility. Forty-five chronically instrumented sheep. Sheep were anesthetized, insufflated with cooled cotton smoke via tracheostomy, and P. aeruginosa were instilled into their airways. They were then placed on assisted ventilation, awakened, and resuscitated with lactated Ringer's solution titrated to maintain hematocrit ± 3% from baseline levels. If, despite fluid management, mean arterial pressure fell by more than 10 mm Hg from baseline level, an additional continuous IV infusion of arginine vasopressin or selepressin was titrated to raise and maintain mean arterial pressure within no less than 10 mm Hg from baseline level. Effects of combination treatment of selepressin with the selective vasopressin V2 receptor agonist desmopressin were similarly investigated. In septic sheep, MAP fell by ~30 mm Hg, systemic vascular resistance index decreased by ~50%, and ~7 L of fluid were retained over 24 hours; this fluid accumulation was partially reduced by arginine vasopressin and almost completely blocked by selepressin; and combined infusion of selepressin and desmopressin increased fluid accumulation to levels similar to arginine vasopressin treatment. Resuscitation with the selective vasopressin type 1a receptor agonist selepressin blocked vascular leak more effectively than the mixed vasopressin type 1a receptor/vasopressin V2 receptor agonist arginine vasopressin because of its lack of agonist activity at the vasopressin V2 receptor.

Characterization of the effects of the vasopressin V2 receptor on sweating, fluid balance, and performance during exercise.

A regulatory effect of arginine vasopressin (AVP) on sweat water conservation has been hypothesized but not definitively evaluated. AVP-mediated insertion of sweat and salivary gland aquaporin-5 (AQP5) water channels through activation of the vasopressin type 2 receptor (V2R) remains an attractive, yet unexplored, mechanism that could result in a more concentrated sweat with resultant decreased water loss. Ten runners participated in a double-blind randomized control treadmill trial under three separate pharmacological conditions: a placebo, V2R agonist (0.2 mg desmopressin), or V2R antagonist (30 mg tolvaptan). After a familiarization trial, runners ran for 60 min at 60% of peak speed followed by a performance trial to volitional exhaustion. Outcome variables were collected at three exercise time points: baseline, after the steady-state run, and after the performance run. Body weight losses were <2% across all three trials. Significant pharmacological condition effects were noted for urine osmolality [F = 84.98; P < 0.0001] and urine sodium concentration ([Na(+)]) [F = 38.9; P < 0.0001], which verified both pharmacological activation and inhibition of the V2R at the kidney collecting duct. Plasma osmolality and [Na(+)] demonstrated significant exercise (F = 26.0 and F = 11.1; P < 0.0001) and condition (F = 5.1 and F = 3.8; P < 0.05) effects (osmolality and [Na(+)], respectively). No significant exercise or condition effects were noted for either sweat or salivary [Na(+)]. Significant exercise effects were noted for plasma [AVP] (F = 22.3; P < 0.0001), peak core temperature (F = 103.3; P < 0.0001), percent body weight change (F = 6.3; P = 0.02), plasma volume change (F = 21.8; P < 0.0001), and thirst rating (F = 78.2; P < 0.0001). Performance time was not altered between conditions (P = 0.80). In summary, AVP acting at V2R does not appear to regulate water losses from body fluids other than renal excretion during exercise.

Sclederma of Poria cocos exerts its diuretic effect via suppression of renal aquaporin-2 expression in rats with chronic heart failure

Ethnopharmacological relevanceSclederma of Poria cocos (Hoelen) has been used as a diuretic in traditional Asian medicine. However, the underlying mechanism by which Sclederma of Poria cocos (hoelen) exerts its diuretic effect has not been well identified. The aim of the present study was to evaluate the effects of Sclederma of Poria cocos (hoelen) in rats with chronic heart failure (CHF) induced by acute myocardial infarction and to investigate the underlying mechanisms. Materials and methodsAn aqueous extract of Sclederma of Poria cocos (hoelen) (2.4g/kg/d, 1.2g/kg/d or 0.6g/kg/d) or furosemide (20mg/kg/d) was administered orally to male Sprague-Dawley rats starting on the day of coronary ligation. The urine output of all rats was quantified and collected every day for 1 or 4 weeks. The expression of aquaporin-2 (AQP2) was examined after treatment for 1 or 4 weeks. ResultsUrinary output increased significantly and urinary osmolality decreased after oral administration of Sclederma of Poria cocos (hoelen) for both 1 and 4 weeks. Sclederma of Poria cocos (hoelen) caused less electrolyte disorder than furosemide. Furthermore, Sclederma of Poria cocos (hoelen) reduced the levels of plasma BNP in CHF rats, whereas furosemide had no effect. Importantly, both mRNA and protein expression of AQP2 were down-regulated and urinary excretion of AQP2 was decreased after administration of Sclederma of Poria cocos (hoelen) to CHF rats. Similarly, Sclederma of Poria cocos (hoelen) reduced plasma arginine vasopressin (AVP) level and down-regulated vasopressin type 2 receptor (V2R) mRNA expression. ConclusionsSclederma of Poria cocos (hoelen) exerts its diuretic effect and improves cardiac function in CHF rats via the AVP-V2R-AQP2 axis.

Common α2A and α2C adrenergic receptor polymorphisms do not affect plasma membrane trafficking

Various naturally occurring polymorphic forms of human G protein-coupled receptors (GPCRs) have been identified and linked to diverse pathological diseases, including receptors for vasopressin type 2 (nephrogenic diabetes insipidus) and gonadotropin releasing hormone (hypogonadotropic hypogonadism). In most cases, polymorphic amino acid mutations disrupt protein folding, altering receptor function as well as plasma membrane expression. Other pathological GPCR variants have been found that do not alter receptor function, but instead affect only plasma membrane trafficking (e.g., delta opiate and histamine type 1 receptors). Thus, altered membrane trafficking with retained receptor function may be another mechanism causing polymorphic GPCR dysfunction. Two common human α2A and α2C adrenergic receptor (AR) variants have been identified (α2A N251K and α2C Δ322-325 ARs), but pharmacological analysis of ligand binding and second messenger signaling has not consistently demonstrated altered receptor function. However, possible alterations in plasma membrane trafficking have not been investigated. We utilized a systematic approach previously developed for the study of GPCR trafficking motifs and accessory proteins to assess whether these α2 AR variants affected intracellular trafficking or plasma membrane expression. By combining immunofluorescent microscopy, glycosidic processing analysis, and quantitative fluorescent-activated cell sorting (FACS), we demonstrate that neither variant receptor had altered intracellular localization, glycosylation, nor plasma membrane expression compared to wild-type α2 ARs. Therefore, pathopharmacological properties of α2A N251K and α2C Δ322-325 ARs do not appear to be due to altered receptor pharmacology or plasma membrane trafficking, but may involve interactions with other intracellular signaling cascades or proteins.

The physiological and pathophysiological functions of renal and extrarenal vasopressin V2 receptors

The arginine vasopressin (AVP) type 2 receptor (V2R) is unique among AVP receptor subtypes in signaling through cAMP. Its key function is in the kidneys, facilitating the urine concentrating mechanism through the AVP/V2 type receptor/aquaporin 2 system in the medullary and cortical collecting ducts. Recent clinical and research observations strongly support the existence of an extrarenal V2R. The clinical importance of the extrarenal V2R spans widely from stimulation of coagulation factor in the endothelium to as yet untested potential therapeutic targets. These include V2R-regulated membranous fluid turnover in the inner ear, V2R-regulated mitogensis and apoptosis in certain tumor tissues, and numerous other cell types where the physiological role of V2Rs still requires further research. Here, we review current evidence on the physiological and pathophysiological functions of renal and extrarenal V2Rs. These functions of V2R are important, not only in rare diseases with loss or gain of function of V2R but also in relation to the recent use of nonpeptide V2R antagonists to treat hyponatremia and possibly retard the growth of cysts and development of renal failure in autosomal dominant polycystic kidney disease. The main functions of V2R in principal cells of the collecting duct are water, salt, and urea transport by modifying the trafficking of aquaporin 2, epithelial Na(+) channels, and urea transporters and vasodilation and stimulation of coagulation factor properties, mainly seen with pharmacological doses of 1-desamino-8-D-AVP. The AVPR2 gene is located on the X chromosome, in a region with high probability of escape from inactivation; this may lead to phenotypic sex differences, with females expressing higher levels of transcript than males.

Hereditary Nephrogenic Diabetes Insipidus: Molecular Basis of the Defect and Potential Novel Strategies for Treatment

The antidiuretic hormone vasopressin regulates water reabsorption in the nephron by inducing apical plasma membrane exocytosis of the water channel aquaporin 2 in the kidney collecting duct principal cells. Disruption of this physiological mechanism by genetic alteration of either the vasopressin type 2 receptor gene or the aquaporin 2 gene, results in a rare genetic disorder known as nephrogenic diabetes insipidus, which main hallmark are polyuria and polydipsia. Over the last decades, analysis of patients affected by this disease helped genetists, clinicians, cell and molecular biologists and pharmacologists to better understand the physiology of water reabsorption in the kidney, the molecular basis of the disease and to propose protocols for rapid diagnosis and pharmacological handling of the disease. Much still remains to be done in terms of targeted therapy to make sure that these patients benefit from an improved quality of life. In this article we provide an overview on the most recent strategies under investigation for rescuing the mutated gene products activity or for bypassing defective vasopressin receptor signaling.

Assessment of quality of life during long-term treatment of tolvaptan in refractory heart failure: design and rationale of the AQUA-TLV study.

The vasopressin type 2 receptor antagonist tolvaptan (TLV) has recently become available to treat congestion in in-hospital patients with heart failure (HF). However, there is no evidence confirming the long-term efficacy of TLV. The Assessment of Quality of life during long-term treatment of TLV in refractory heart failure (AQUA-TLV) study is a multicenter, open-labeled, randomized, controlled clinical trial that will enroll 100 patients from 18 hospitals in Japan. Patients with HF assigned to New York Heart Association class III or IV, who had a previous history of hospitalization due to congestive HF during the past 1 year and ongoing symptomatic congestion with baseline urine osmolality > 350 mOsm/L regardless of being prescribed daily furosemide ≥ 60 mg are randomized to the conventional diuretics group (50 patients) and TLV add-on group (50 patients), and their quality of life will be assessed using the Minnesota Living with Heart Failure Questionnaire after 6 months of treatment. This study is the first multicenter prospective randomized study in Japan to evaluate the long-term clinical effectiveness of TLV compared with conventional treatment in patients with congestive HF (UMIN Clinical Trial Registry Number: UMIN 000009604).

Efficacy and Safety of Tolvaptan in Heart Failure Patients With Volume Overload

Loop diuretics are commonly used in heart failure (HF) patients, but they are sometimes associated with insufficient response as well as adverse events. In such diuretics-resistant cases, tolvaptan, a vasopressin type 2 receptor antagonist, shows the improvement of volume overload without electrolyte imbalance. Tolvaptan was launched in Japan in 2010, and a post-marketing surveillance has been performed to evaluate the safety and efficacy of tolvaptan in real-world clinical settings. HF patients with insufficient response to loop diuretics were enrolled: 1,053 to evaluate for efficacy and 1,057 patients for the safety assessment. Decreases in body weight from baseline were 1.0±1.6kg at day 2 and increases in urine volume were 631±1,179ml at day 1 (both P<0.0001 vs. baseline). Congestive symptoms were significantly improved within 14 days. Adverse drug reactions (ADR) were observed in 18.7%, with thirst being the most frequent ADR (10%). Hypernatremia was a complication in 40 patients (3.8%). Predictive factors for the occurrence of hypernatremia were the starting dosage of tolvaptan (15mg/day), baseline serum sodium level (≥142mEq/L) and serum potassium level (<3.8mEq/L) at baseline. In the real-world clinical setting, tolvaptan demonstrated aquaretic efficacy in HF patients with diuretic-resistant volume overload. We recommend a lower dose of tolvaptan in Japanese patients with normonatremia and hypokalemia to prevent hypernatremia.

Fluvastatin Increases AQP2 Urine Excretion in a Dyslipidemic Patient with Nephrogenic Diabetes Insipidus: An In Vivo and In Vitro Study

Objective: Among the pleiotropic effects of statins, we have previously reported that fluvastatin increases the amount of plasma membrane-expressed AQP2 in renal collecting duct cells both in vitro and in vivo, independently of vasopressin. This effect may be of potential clinical significance for the treatment of patients affected by nephrogenic diabetes insipidus forms caused by inactivating mutations of the vasopressin type 2 receptor. Here we report the effect of fluvastatin on AQP2 plasma membrane abundance on an adult male XNDI patient treated with statins. Methods: An adult male NDI patient, carrying an inactivating mutation of the V2R, under conventional treatment to reduce polyuria, was also treated with fluvastatin because of high levels of blood cholesterol. AQP2 plasma membrane expression in the kidney was monitored by measuring urinary excreted AQP2 (u-AQP2), before starting fluvastatin treatment and during a three months follow-up period. The effect of fluvastatin was also tested in vitro in mouse kidney cortical collecting duct MCD4 cells. Results: u-AQP2 increased in a time- and dose-dependent manner after treatment with 40 and 80 mg/day of fluvastatin for 90 days. However, at this drug dosage, increased uAQP2 was not accompanied by reduction of diuresis and increase of urine osmolality. The effect of fluvastatin on AQP2 excretion was confirmed in vitro in cultured renal cells. Conclusions: We first demonstrate that the use of fluvastatin increased AQP2 plasma membrane expression in an NDI dyslipidemic patient. This observation was confirmed by in vitro studies using mouse cultured renal cells treated with fluvastatin. Although a clinical relevant effect of fluvastatin on total diuresis and urine osmolality was not observed at the used dosages, these results suggest further investigation on the possible role of HMG CoA reductase inhibitors to improve the efficacy of the current NDI therapy.

Demeclocycline attenuates hyponatremia by reducing aquaporin-2 expression in the renal inner medulla

Binding of vasopressin to its type 2 receptor in renal collecting ducts induces cAMP signaling, transcription and translocation of aquaporin (AQP)2 water channels to the plasma membrane, and water reabsorption from the prourine. Demeclocycline is currently used to treat hyponatremia in patients with the syndrome of inappropriate antidiuretic hormone secretion (SIADH). Demeclocycline's mechanism of action, which is poorly understood, is studied here. In mouse cortical collecting duct (mpkCCD) cells, which exhibit deamino-8-D-arginine vasopressin (dDAVP)-dependent expression of endogenous AQP2, demeclocycline decreased AQP2 abundance and gene transcription but not its protein stability. Demeclocycline did not affect vasopressin type 2 receptor localization but decreased dDAVP-induced cAMP generation and the abundance of adenylate cyclase 3 and 5/6. The addition of exogenous cAMP partially corrected the demeclocycline effect. As in patients, demeclocycline increased urine volume, decreased urine osmolality, and reverted hyponatremia in an SIADH rat model. AQP2 and adenylate cyclase 5/6 abundances were reduced in the inner medulla but increased in the cortex and outer medulla, in the absence of any sign of toxicity. In conclusion, our in vitro and in vivo data indicate that demeclocycline mainly attenuates hyponatremia in SIADH by reducing adenylate cyclase 5/6 expression and, consequently, cAMP generation, AQP2 gene transcription, and AQP2 abundance in the renal inner medulla, coinciding with a reduced vasopressin escape response in other collecting duct segments.

Congestive heart failure in rats is associated with increased collecting duct vasopressin sensitivity and vasopressin type 2 receptor reexternalization

A number of studies have shown that rats with congestive heart failure (CHF) have increased protein levels of the vasopressin (AVP)-regulated water channel aquaporin-2 (AQP2) even during conditions with unchanged circulating levels of AVP, suggesting an increase in the sensitivity of the AVP type 2 (V2) receptor in experimental CHF. The present study was aimed at investigating AVP signaling in rats with moderate CHF (left ventricular end diastolic pressure >10 mmHg; normal plasma AVP levels) induced by ligation of the left anterior descending coronary artery. Sham-operated rats were used as controls. Western blotting analyses revealed an increased abundance of AQP2 in renal cortex (+33 ± 9% of sham; P < 0.05) and in inner medulla (IM) (+54 ± 15% of sham; P < 0.05) in CHF rats compared with sham-operated controls. Dose-response studies on isolated collecting ducts (CDs) showed an increased accumulation of cAMP in response to AVP in CHF rats compared with controls. V2 receptor surface-binding studies in isolated IMCDs showed a marked and comparable AVP-induced V2 receptor internalization in response to AVP in both CHF and control rats. As expected V2 receptor surface binding remained low after AVP challenge in control rats. In contrast to this, V2 receptor surface binding returned to pre-AVP levels within 30 min in the CHF rats, indicating an obtained recycling ability of the V2 receptor in CHF. Together the results indicate the presence of an increased AVP sensitivity in the CDs from CHF rats, associated with an acquired recycling ability of the V2 receptor.

A large deletion of the AVPR2 gene causing severe nephrogenic diabetes insipidus in a Turkish family

X-linked nephrogenic diabetes insipidus (NDI) is a rare hereditary disease caused by mutations in arginine vasopressin type 2 receptor (AVPR2) and characterized by the production of large amounts of urine and an inability to concentrate urine in response to the antidiuretic hormone vasopressin. We have identified a novel 388 bp deletion starting in intron 1 and ending in exon 2 in the AVPR2 gene in a patient with NDI and in his family. We have revealed that this mutation is a de novo mutation for the mother of the proband patient. Prospective clinical data were collected for all family members. The water deprivation test confirmed the diagnosis of diabetes insipidus. The patient has severe symptoms like deep polyuria nocturia, polydipsia, and fatigue. He was given arginine vasopressin treatment while he was a child. However, he could not get well due to his nephrogenic type of illness. Both of his nephews have the same complains in addition to failure to grow. We have sequenced all exons and intron-exon boundaries of the AVPR2 gene of all family members. The analyses of bioinformatics and comparative genomics of the deletion were done via considering the DNA level damage. AVPR2 gene mutation results in the absence of the three transmembrane domains, two extracellular domains, and one cytoplasmic domain. Three-dimensional protein structure prediction was shown. We concluded that X-linked NDI and severity of illness in this family is caused by a novel 388 bp deletion in the AVPR2 gene that is predicted to truncate the receptor protein, and also this deletion may lead to dysfunctioning in protein activity and inefficient or inadequate binding abilities.

Morphological and functional changes in a new animal model of Ménière's disease

The purpose of this study was to clarify the underlying mechanism of vertiginous attacks in Ménière's disease (MD) while obtaining insight into water homeostasis in the inner ear using a new animal model. We conducted both histopathological and functional assessment of the vestibular system in the guinea-pig. In the first experiment, all animals were maintained 1 or 4 weeks after electrocauterization of the endolymphatic sac of the left ear and were given either saline or desmopressin (vasopressin type 2 receptor agonist). The temporal bones from both ears were harvested and the extent of endolymphatic hydrops was quantitatively assessed. In the second experiment, either 1 or 4 weeks after surgery, animals were assessed for balance disorders and nystagmus after the administration of saline or desmopressin. In the first experiment, the proportion of endolymphatic space in the cochlea and the saccule was significantly greater in ears that survived for 4 weeks after surgery and were given desmopressin compared with other groups. In the second experiment, all animals that underwent surgery and were given desmopressin showed spontaneous nystagmus and balance disorder, whereas all animals that had surgery but without desmopressin administration were asymptomatic. Our animal model induced severe endolymphatic hydrops in the cochlea and the saccule, and showed episodes of balance disorder along with spontaneous nystagmus. These findings suggest that administration of desmopressin can exacerbate endolymphatic hydrops because of acute V2 (vasopressin type 2 receptor)-mediated effects, and, when combined with endolymphathic sac dysfunction, can cause temporary vestibular abnormalities that are similar to the vertiginous attacks in patients with MD.

Noncanonical Control of Vasopressin Receptor Type 2 Signaling by Retromer and Arrestin

The vasopressin type 2 receptor (V2R) is a critical G protein-coupled receptor (GPCR) for vertebrate physiology, including the balance of water and sodium ions. It is unclear how its two native hormones, vasopressin (VP) and oxytocin (OT), both stimulate the same cAMP/PKA pathway yet produce divergent antinatriuretic and antidiuretic effects that are either strong (VP) or weak (OT). Here, we present a new mechanism that differentiates the action of VP and OT on V2R signaling. We found that vasopressin, as opposed to OT, continued to generate cAMP and promote PKA activation for prolonged periods after ligand washout and receptor internalization in endosomes. Contrary to the classical model of arrestin-mediated GPCR desensitization, arrestins bind the VP-V2R complex yet extend rather than shorten the generation of cAMP. Signaling is instead turned off by the endosomal retromer complex. We propose that this mechanism explains how VP sustains water and Na(+) transport in renal collecting duct cells. Together with recent work on the parathyroid hormone receptor, these data support the existence of a novel "noncanonical" regulatory pathway for GPCR activation and response termination, via the sequential action of β-arrestin and the retromer complex.

High-Throughput Screen for Pharmacoperones of the Vasopressin Type 2 Receptor

Pharmacoperone drugs correct the folding of misfolded protein mutants and restore function (i.e., “rescue”) by correcting the routing of (otherwise) misrouted mutants. Assays for pharmacoperones have not been applied to screen large libraries previously. Currently, most pharmacoperones possess intrinsic agonist or antagonist activities since these were identified using high-throughput screens aimed at discovering direct agonists or antagonists. Here we describe an ultra-high-throughput compatible no-wash assay system designed to specifically identify pharmacoperones of the vasopressin type 2 receptor (V2R). Development of such assays is important and novel since useful chemical structures with the ability to control cellular trafficking but lacking intrinsic agonist or antagonist properties have not likely been identified using existing screens. In the described assay, the level of functional human V2R (hV2R) (mutant) present in each test well is quantitated by stimulation with saturating levels of agonist followed by use of a luminescent-based cyclic adenosine monophosphate assay. This allows the assay to identify compounds that increase the trafficking of mutant hV2R[L83Q] in our model system.

Excretion of urinary exosomal AQP2 in rats is regulated by vasopressin and urinary pH

Urinary exosomes are small vesicles secreted into urine from all renal epithelial cell types and known to contain proteins that are involved in renal secretion and reabsorption. Among these proteins, urinary exosomal aquaporin-2 (AQP2) has been suggested to be useful for diagnosis of renal disease. However, the mechanisms underlying the excretion of urinary exosomal AQP2 are largely unknown. In this study, we examined the mechanisms of urinary exosomal AQP2 excretion in vivo, using diuretics including furosemide (FS), an inhibitor of the sodium-potassium-chloride symporter; acetazolamide (ACTZ), an inhibitor of carbonic anhydrase; OPC-31260 (OPC), a vasopressin type 2 receptor antagonist; and NaHCO3, a urinary alkalizing agent. Samples of urine from rats were collected for 2 h just after treatment with each diuretic, and urinary exosomes were isolated by ultracentrifugation. Urinary exosomal AQP2 excretion was dramatically increased by treatment with FS accompanied by urine acidification or with ACTZ accompanied by urine alkalization. Immunohistochemistry showed that apical localization of AQP2 was clearly evident and the plasma vasopressin level was increased after each treatment. Although treatment with OPC alone had no significant effect, coadministration of OPC completely inhibited the FS-induced and partially reduced the ACTZ-induced responses, respectively. Treatment with NaHCO3 increased the excretion of urinary exosomal AQP2 accompanied by urine alkalization. This increased response was partially inhibited by coadministration of OPC. These data suggest that an increased plasma level of vasopressin promoted the excretion of urinary exosomal AQP2 and that urine alkalinization also increased it independently of vasopressin.

Chronic blockade of vasopressin type1a receptor and/or type2 receptor improves both cardiac and renal damage and dysfunction in hypertensive rats

Background: Vasopressin (AVP) type 1a receptor (V1aR) antagonist could theoretically benefit heart failure (HF) patients by decreasing afterload and ameliorating remodeling. However, the conflicting results have been reported. Also, there is a concern that increased AVP may have detrimental effects through V1aR in HF patients treated with oral type 2 receptor (V2R) antagonist, tolvaptan. Therefore, we examined whether the chronic administration of V1a antagonist may exert beneficial effects and also its additive effects to tolvaptan treatment in rat hypertensive HF model. Methods and results: In Dahl salt-sensitive hypertensive rats, not only circulating AVP level, but also myocardial AVP and V1a receptor (V1aR), and renal V1aR and V2R expressions were significantly activated during the transition to HF (18-21 weeks). First, they were chronically treated with vehicle, V1aR antagonist (OPC21268), tolvaptan, or OPC21268 plus tolvaptan (combination) from pre-hypertrophic stage (6 weeks). All three treatments did not affect blood pressure. In OPC21268 and tolvaptan groups, the median survival time was significantly improved from 112 day (vehicle) to 141 day and 160 day, respectively. It was improved to a greater extent with the combined treatment (175 day). Echocardiography showed the improvement of fractional shortening (FS) was observed in all three treatment groups and the suppression of LV hypertrophy (LVH) only in OPC 21268 and combination groups. Furthermore, renal histopathologic damage (glomerulosclerosis and tubulointerstitial fibrosis) was ameliorated, and renal function (creatinine clearance) was also improved in all three treatment groups at HF stage. When these treatments were started at the established LVH phase (11 weeks), the improvement of survival was observed only in tolvaptan group and no additive improvement in the combined treatment. Conclusions: These findings suggest that V1aR blockade may ameliorate the development of LVH and HF in the treatment of hypertensive HF with renal injuries, and have limited effects starting after the establishment of LVH. Also, the chronic administration of both V1a and V2 antagonists exerts further beneficial effects. The underlying mechanism may be related to protection of myocardial and renal damages independent of blood pressure.

Experience of a Vasopressin Receptor Antagonist, Tolvaptan, Under the Unique Indication in Japanese Heart Failure Patients

Tolvaptan is a vasopressin type 2 receptor antagonist. Currently, tolvaptan is indicated for the treatment of severe hyponatremia in most countries, including the United States, but in Japan the indication is broader; it can be used in congestive heart failure patients in addition to standard or optimal diuretic therapy, regardless of their serum sodium levels or systolic function. Our review of this newly introduced drug is based on our experience in Japan. Clinical Pharmacology & Therapeutics (2013); 94 4, 449–451. doi:10.1038/clpt.2013.147

Arginine Vasopressin Enhances Cell Survival via a G Protein–Coupled Receptor Kinase 2/β-Arrestin1/Extracellular-Regulated Kinase 1/2–Dependent Pathway in H9c2 Cells

Circulating levels of arginine vasopressin (AVP) are elevated during hypovolemia and during cardiac stress. AVP activates arginine vasopressin type 1A (V(1A))/Gα(q)-coupled receptors in the heart and vasculature and V(2)/Gα(s)-coupled receptors in the kidney. However, little is known regarding the signaling pathways that influence the effects of V(1A) receptor (V(1A)R) activation during cellular injury. Using hypoxia-reoxygenation (H/R) as a cell injury model, we evaluated cell survival and caspase 3/7 activity in H9c2 myoblasts after treatment with AVP. Pretreatment of H9c2 cells with AVP significantly reduced H/R-induced cell death and caspase 3/7 activity, effects that were blocked via both selective V(1A)R inhibition and mitogen-activated protein kinase (MEK1/2) inhibition. AVP increased extracellular-regulated kinase 1/2 (ERK1/2) phosphorylation in a concentration-dependent manner that was sensitive to MEK1/2 inhibition and V(1A)R inhibition, but not V(1B)R or V(2)R inhibition. Discrete elements of the V(1A)/Gα(q)-protein kinase C (PKC) and V(1A)/G protein-coupled receptor kinase (GRK)/β-arrestin signaling cascades were inhibited to dissect the pathways responsible for the protective effects of V(1A)R signaling: Gα(q) (overexpression of Gq-I-ires-green fluorescent protein), PKC (administration of Ro 31-82425; 2-[8-(aminomethyl)-6,7,8,9-tetrahydropyrido[1,2-a]indol-3-yl]-3-(1-methyl-1H-indol-3-yl)maleimide, HCl, bisindolylmaleimide X, HCl), GRK2 [C-terminal GRK2 peptide overexpression and small interfering RNA (siRNA) knockdown], GRK5 (siRNA knockdown), and β-arrestin1 (siRNA knockdown). These studies demonstrated that both Gα(q)/PKC- and GRK2/β-arrestin1-dependent V(1A)R signaling were capable of inducing ERK1/2 phosphorylation in response to AVP stimulation. However, AVP-mediated protection against H/R was elicited only via GRK2- and β-arrestin1-dependent signaling. These results suggest that activation of the V(1A)R in H9c2 cells mediates protective signaling via a GRK2/β-arrestin1/ERK1/2-dependent mechanism that leads to decreased caspase 3/7 activity and enhanced survival under conditions of ischemic stress.

Active arrestin proteins crystallized

Arrestin proteins regulate cellular signalling cascades initiated by ubiquitous G-protein-coupled receptors. Crystal structures reveal that two arrestins undergo similar structural changes on activation. See Letters p.137 & p.142 Arrestin proteins are negative regulators of G-protein-coupled receptor (GPCR) function and also act as G-protein-independent signalling proteins. Before forming a high-affinity complex, arrestins must be activated, and two papers in this issue of Nature focus on the interaction between GCPRs and activated arrestin at the atomic scale. Yong Ju Kim et al. mimicked the initial activation step by truncating the carboxy terminus of arrestin to produce the naturally occurring splice variant called p44 and determined its crystal structure. This structure provides insight into the role of naturally occurring truncated arrestins in the visual system. Arun Shukla et al. present the structure of non-visual β-arrestin-1 in complex with an antibody fragment (Fab30) and a fully phosphorylated 29-amino-acid C-terminal peptide derived from a GPCR, the arginine vasopressin type 2 receptor. Taken together, these two studies reveal striking conformational changes associated with arrestin activation.