Effect of long-term treatment with selective vasopressin V1 and V2 receptor antagonist on the development of heart failure in rats

Vasopressin actions in the kidney renin angiotensin system and its role in hypertension and renal disease

Vasopressin, also named antidiuretic hormone (ADH), arginine vasopressin (AVP) is the main hormone responsible for water maintenance in the body through the antidiuretic actions in the kidney. The posterior pituitary into the blood releases vasopressin formed in the hypothalamus. Hypothalamic osmotic neurons are responsible to initiate the cascade for AVP actions. The effects of AVP peptide includes activation of V2 receptors which stimulate the formation of cyclic AMP (cAMP) and phosphorylation of water channels aquaporin 2 (AQP2) in the collecting duct. AVP also has vasoconstrictor effects through V1a receptors in the vasculature, while V1b is found in the nervous system. V1a and b receptors increases intracellular Ca²⁺ while activation of V2 receptors of signaling pathways are related to cAMP-dependent phosphorylation in kidney collecting ducts acting in coordination to stimulate water and electrolyte homeostasis. AVP potentiate formation of intratubular angiotensin II (Ang II) through V2 receptors-dependent distal tubular renin formation, contributing to Na+ reabsorption. On the same way, Ang II receptors are able to potentiate the effects of V2-dependent stimulation of AQP2 abundance in the plasma membrane. The role of AVP in hypertension and renal disease has been demonstrated in pathological states with the involvement of V2 receptors in the progression of kidney damage in diabetes and also on the stimulation of intracellular pathways linked to the development of polycystic kidney.

Effects of Long-term Blockade of Vasopressin Receptor Types 1a and 2 on Cardiac and Renal Damage in a Rat Model of Hypertensive Heart Failure

The effects of chronic blockade of vasopressin type 1a receptors (V1aR) and the additive effects of a type 2 receptor (V2R) antagonist on the treatment of hypertension-induced heart failure and renal injury remain to be unknown. In this study, Dahl salt-sensitive hypertensive rats were chronically treated with a vehicle (CONT), a V1aR antagonist (OPC21268; OPC), a V2R antagonist (tolvaptan; TOLV), or a combination of OPC21268 and tolvaptan (OPC/TOLV) from the pre-hypertrophic stage (6 weeks). No treatment altered blood pressure during the study. Significant improvements were seen in median survival for the OPC and TOLV, and the OPC/TOLV showed a further improvement in Kaplan-Meier analysis. Echocardiography showed suppressed left ventricular hypertrophy in the OPC and OPC/TOLV at 11 weeks with improved function in all treatment groups by 17 weeks. In all treatment groups, improvements were seen in the following: myocardial histological changes, creatinine clearance, urinary albumin excretion, and renal histopathologic damage. Also, key mRNA levels were suppressed (eg, endothelin-1 and collagen). In conclusion, chronic V1aR blockade ameliorated disease progression in this rat model, with additive benefits from the combination of V1aR and V2R antagonists. It was associated with protection of both myocardial and renal damage, independent of blood pressure.

Controlled and cardiac-restricted overexpression of the arginine vasopressin V1A receptor causes reversible left ventricular dysfunction through Gαq-mediated cell signaling

BACKGROUND

[Arg8]-vasopressin (AVP) activates 3 G-protein-coupled receptors: V1A, V2, and V1B. The AVP-V1A receptor is the primary AVP receptor in the heart; however, its role in cardiac homeostasis is controversial. To better understand AVP-mediated signaling in the heart, we created a transgenic mouse with controlled overexpression of the V1A receptor.

METHODS AND RESULTS

The V1A receptor transgene was placed under the control of the tetracycline-regulated, cardiac-specific α-myosin heavy chain promoter (V1A-TG). V1A-TG mice had a normal cardiac function phenotype at 10 weeks of age; however, by 24 weeks of age, tetracycline-transactivating factor/V1A-TG mouse hearts had reduced cardiac function, cardiac hypertrophy, and dilatation of the ventricular cavity. Contractile dysfunction was also observed in isolated adult cardiac myocytes. When V1A receptor transgene was induced to be expressed in adult mice (V1A-TG(Ind)), left ventricular dysfunction and dilatation were also seen, albeit at a later time point. Because the V1A receptor mediates cell signaling through Gα(q) protein, we blocked Gα(q) signaling by crossing tetracycline-transactivating factor/V1A mice with transgenic mice that expressed a small inhibitory peptide against Gα(q). Gα(q) blockade abrogated the development of the heart failure phenotype in tetracycline-transactivating factor/V1A-TG mice. The heart failure phenotype could be reversed by administration of doxycycline.

CONCLUSIONS

Our results demonstrate a role for V1A-mediated signaling in the development of heart failure and support a role for V1A blockade in the treatment of patients with elevated levels of vasopressin.

New topics in vasopressin receptors and approach to novel drugs: research and development of conivaptan hydrochloride (YM087), a drug for the treatment of hyponatremia

Hyponatremia is the most common electrolyte disorder in hospitalized patients and is associated with the risk of intractable seizures and death. The effectiveness of conventional therapies for hyponatremia is inconsistent, and the rapid correction of plasma sodium levels is thought to result in the occurrence of neurological complications. Arginine vasopressin (AVP) is the primary regulator of renal electrolyte-free water reabsorption via AVP-receptor type 2 (V2-R), and inappropriate or excessive AVP secretion independent of serum osmolality frequently causes excessive water retention, which is the etiological basis of hyponatremia. Therefore, the use of V2-R antagonists as anti-hyponatremic drugs in the clinical setting is anticipated to be reliable and safe. Conivaptan hydrochloride (YM087) is a novel dual AVP-R antagonist for AVP-R types 1a (V1a) and V2-R. In vitro studies have shown that it possesses high affinity for V1a-R and V2-R without any species differences. It also potently inhibited AVP-induced intracellular signaling through human V2 and V1a receptors with no agonistic activity. Conivaptan hydrochloride improved the plasma sodium concentration and plasma osmolality in hyponatremic rats, and its effectiveness was demonstrated in hyponatremic patients. This drug has been approved for use in the United States, which will bring relief to patients with hyponatremia.

Effects of nonpeptide vasopressin V2 antagonist tolvaptan in rats with heart failure

Similar to other neurohormones that are activated in chronic heart failure (CHF), circulating arginine vasopressin (AVP) is elevated in patients with CHF. The precise role of AVP in the pathophysiology of cardiovascular disease is controversial. AVP is a peptide hormone that contributes to water retention and vasoconstriction in CHF through effects on V(2) and V(1a) receptors, respectively. In the present study, the effect of V(2) receptor (V(2)R) blockade using tolvaptan was assessed in a rat model of myosin-induced experimental autoimmune myocarditis. CHF was elicited in Lewis rats by immunization with porcine cardiac myosin, and 28 days after immunization rats were treated for 28 days with oral tolvaptan (3 or 10mg/(kg day)) or vehicle. CHF was characterized by left ventricular remodeling and impaired systolic and diastolic function. Chronic V(2)R blockade increased urine volume and urinary AVP excretion and decreased urine osmolality but had no natriuretic effect, and as a result caused increases in plasma osmolality and sodium. High doses of tolvaptan markedly elevated electrolyte-free water clearance. V(2)R blockade did not activate the renin-angiotensin system, not influence cardiac remodeling, cardiac function, or survival. The upregulation of aquaporin 2 protein in the kidney of CHF rats was inhibited by the administration of V(2)R antagonist. These results suggest that in a rat model of CHF, AVP plays a major role in water retention through the renal V(2)R.

Recognition and treatment of hyponatremia in acutely ill hospitalized patients

OBJECTIVE

The objective of this paper was to discuss the diagnosis, pathophysiology, and management of hyponatremia among critically ill, hospitalized patients (eg, after surgery or in the intensive care unit).

METHODS

English-language literature published between 1967 and 2006 was searched using several key words (AVP receptor antagonists, hyponatremia, SIADH, conivaptan, tolvaptan, and lixivaptan) and by accessing MEDLINE and ScienceDirect. Meeting abstracts from scientific sessions (American Society of Nephrology Renal Week 2004 and the Endocrine Society's 87th Annual Meeting [2005]) were reviewed. The package insert for conivaptan hydrochloride injection was referenced from . Clinical trials included in this review were randomized and placebo controlled.

RESULTS

Based on the literature we researched, hyponatremia is the most common electrolyte disorder encountered in critical care and is associated with a variety of conditions, including congestive heart failure and the syndrome of inappropriate antidiuretic hormone secretion. Because hyponatremia can arise in hypervolemic, euvolemic, and hypovolemic states, clinicians may not recognize its presence and cause. Incorrect management can lead to significant morbidity and mortality. Physicians need to recognize risk factors and symptoms and use appropriate treatment guidelines for hyponatremia. Traditionally, therapy for hyponatremia has been limited by efficacy and safety concerns. Arginine vasopressin (AVP) receptor antagonists, therapeutic agents that promote aquaresis in patients with hyponatremia by targeting V(1a) receptors in the vascular smooth muscle, V(2) receptors in the kidney, or both, are under development. A dual-receptor antagonist targeting both V(1a) and V(2) receptors is now approved for the treatment of euvolemic hyponatremia in hospitalized patients.

CONCLUSIONS

Hyponatremia, an electrolyte abnormality found in critically ill patients, can be associated with significant morbidity and mortality. AVP receptor antagonists show promise as effective and tolerable treatments for patients with hyponatremia.

Vasopressin antagonists: role in the management of hyponatremia

Hyponatremia is a common electrolyte disorder associated with potentially serious or life-threatening consequences. Serum osmolality and sodium concentration [Na+] are regulated by thirst, the hormone arginine vasopressin (AVP), and renal water and sodium handling. Hyponatremia is frequently caused by dysregulation of AVP, which accompanies disorders of water retention, such as congestive heart failure (CHF) and the syndrome of inappropriate secretion of antidiuretic hormone (SIADH). Clinical trials with AVP receptor antagonists have confirmed the important role of AVP in the pathophysiology of hyponatremia and suggest these agents are efficacious in treating hyponatremia associated with SIADH, cirrhosis, and CHF. Acting directly at AVP receptors in the renal tubules, these agents promote aquaresis - the electrolyte-sparing excretion of free water - in patients with hyponatremia. In clinical trials, AVP receptor antagonists have been shown to increase the serum [Na+] and urine output while decreasing urine osmolality.

Vasopressin Antagonism in Heart Failure

Treatment of chronic heart failure (HF) is based on interference with the renin-angiotensin-aldosterone system and the adrenergic nervous system. Diuretics are used in volume-expanded patients. Insights from clinical trials and registries establish the need to consider correcting both cardiac loading conditions and nonload-related biological factors if HF therapy is to be optimized. Arginine vasopressin (AVP) represents a potentially attractive target for therapy in both acute and chronic HF. Excessive AVP secretion could contribute to both systolic and diastolic wall stress via V1a- and V2-mediated effects on the peripheral vasculature and on water retention. Arginine vasopressin also may directly and adversely affect myocardial function due to the effect of V1a activation on myocardial contractility and cell growth. Last, AVP may contribute to hyponatremia, a powerful predictor of poor outcome in HF. The development of effective nonpeptide antagonists to both the V1a and V2 receptors for AVP now allows for testing the hypotheses that interfering with AVP-mediated signaling could be beneficial in HF. This review summarizes the theoretical rationale for further development of such therapy, reviews the status of current compounds under development, and suggests key issues that need to be addressed as these agents undergo further clinical testing.

Intravenous administration of conivaptan hydrochloride improves cardiac hemodynamics in rats with myocardial infarction-induced congestive heart failure

We investigated the effects of intravenously administered conivaptan hydrochloride, a dual vasopressin V1A and V2 receptor antagonist, on cardiac function in rats with congestive heart failure following myocardial infarction, and compared results with those for the selective vasopressin V2 receptor antagonist SR121463A. Rats were subjected to left coronary artery occlusion to induce myocardial infarction, which in turn led to congestive heart failure. At 4 weeks after coronary occlusion, conivaptan (0.03, 0.1 and 0.3 mg/kg i.v.) dose-dependently increased urine volume and reduced urine osmolality in both myocardial infarction and sham-operated rats. SR121463A (0.3 mg/kg i.v.) also increased urine volume and decreased urine osmolality in myocardial infarction rats, to a degree comparable to that by conivaptan (0.3 mg/kg i.v.). At 6 weeks after surgery, myocardial infarction rats showed increases in right ventricular systolic pressure, right atrial pressure, left ventricular end-diastolic pressure and relative weights of the heart and the lungs, and a decrease in first derivative of left ventricular pressure (dP/dt(max))/left ventricular pressure, showing that congestive heart failure was well established. Conivaptan (0.3 mg/kg i.v.) significantly reduced right ventricular systolic pressure, left ventricular end-diastolic pressure, lung/body weight and right atrial pressure in myocardial infarction rats. Moreover, conivaptan (0.3 mg/kg i.v.) significantly increased dP/dt(max)/left ventricular pressure. SR121463A at a dose of 0.3 mg/kg i.v. significantly decreased left ventricular end-diastolic pressure and right atrial pressure, and tended to decrease right ventricular systolic pressure and relative lung weight in myocardial infarction rats. Although the aquaretic and preload-reducing effects of SR121463A were similar to those of conivaptan, SR121463A failed to improve dP/dt(max)/left ventricular pressure. These results suggest that dual vasopressin V1A and V2 receptor antagonists provide greater benefit than selective vasopressin V2 receptor antagonists in the treatment of congestive heart failure.

Acute myocardial infarction induces hypothalamic cytokine synthesis

The inflammatory milieu of acute myocardial infarction (MI) is theoretically conducive to enhanced cytokine synthesis within the brain. We tested the hypothesis that synthesis of tumor necrosis factor-alpha (TNF-alpha), an indicator of proinflammatory cytokine activity, increases in brain after MI. MI was induced in rats by ligating the left anterior descending coronary artery and confirmed by echocardiography. Plasma and tissue levels of TNF-alpha were measured using ELISA; TNF-alpha mRNA was measured with real-time PCR. Heart, brain, and plasma samples were obtained 0.5, 1, 4, or 24 h or 4 wk after MI. TNF-alpha synthesis increased in the brain, heart, and plasma within minutes to hours after MI and was sustained over the interval tested. Among the brain tissues examined, TNF-alpha increased selectively in hypothalamus. Chronic treatment with pentoxifylline prevented the increases in TNF-alpha in brain, heart, and plasma measured 4 wk after MI. MI-induced cytokine synthesis in the hypothalamus and its prevention by pentoxifylline have important implications in the context of the development of heart failure after MI.

Increased thirst and vasopressin secretion after myocardial infarction in rats

Impaired regulation of salt and water balance in left ventricular dysfunction and heart failure can lead to pulmonary and peripheral edema and hyponatremia. Previous studies of disordered water regulation in heart failure have used models of low cardiac output with normal cardiac function (e.g., inferior vena cava ligation). We investigated thirst and vasopressin (AVP) secretion in a rat myocardial infarction model of chronic left ventricular dysfunction/heart failure in response to a 24-h water deprivation period. Thirst (implied from water drunk), hematocrit, plasma renin activity, and plasma AVP concentrations increased with water deprivation vs. ad libitum water access. Thirst and plasma AVP concentrations were significantly positively correlated with infarct size after 24-h water deprivation but not under ad libitum water access conditions. The mechanism by which this occurs is unclear but could involve increased osmoreceptor sensitivity, altered stimulation of baroreceptors, the renin-angiotensin system, or altered central neural control.

Acute decompensated heart failure: a contemporary approach to pharmacotherapeutic management

Hospital admissions for acute decompensated heart failure (ADHF) have increased precipitously during the past few decades and are projected to continue to increase in the future. To optimize patient outcomes and reduce the costs associated with this disorder, evidenced-based pharmacotherapy is essential. Continuous infusions of loop diuretic therapy rather than bolus dosing may enhance efficacy and reduce the extent of diuretic resistance. Nesiritide is a pharmacologically novel preload and afterload reducer but based on clinical trial evidence should be reserved for those unable to take or with resistance to intravenous nitrate therapy. Catecholamine- and phosphodiesterase-based inotropic therapies are efficacious, but the increased risk of arrhythmogenesis and the potential for negative survival effects limit their use. The experimental agent levosimendan is a positive inotropic agent but does not increase myocyte calcium concentrations as do catecholamines or phosphodiesterase inhibitors. Clinical trial evidence demonstrates a positive survival benefit for levosimendan versus dobutamine.

A vasopressin receptor antagonist (VPA-985) improves serum sodium concentration in patients with hyponatremia: a multicenter, randomized, placebo-controlled trial

Hyponatremia in advanced cirrhosis and ascites or congestive heart failure (CHF) is the result of an inappropriate increase in vasopressin secretion, which acts through activation of specific V(2) receptors in the distal renal nephron to increase water reabsorption. This study investigates the efficacy and safety of 3 different doses of the V(2) receptor antagonist, VPA-985, in correcting hyponatremia over a 7-day inpatient study period. Forty-four hospitalized patients (33 patients with cirrhosis, 6 with CHF, and 5 with syndrome of inappropriate antidiuretic hormone (SIADH) were studied on a constant sodium intake, with VPA doses of 25, 125, and 250 mg twice daily or placebo. Serum sodium measurements were repeated after every daily dose, and the next dose withheld for excessive serum sodium rises. Fluid intake was adjusted according to previous 24-hour urinary outputs. Adverse events were based on clinical signs of dehydration or encephalopathy. VPA-985 produced a significant overall aquaretic response compared with placebo, with significant dose related increases in free water clearance (P <.05) and serum sodium (P <.05), without significant changes in orthostatic blood pressure or serum creatinine levels. Five patients (50%) on 250 mg twice daily had to have medication withheld on multiple occasions. End-of-study plasma vasopressin levels increased significantly in the 2 larger dose groups. In conclusion, VPA-985 appears effective and safe in appropriate doses in correcting abnormal renal water handling and hyponatremia in conditions associated with water retention. Higher doses of VPA-985 may produce significant dehydration and will require close monitoring with their use.

Pharmacological characterization of YM471, a novel potent vasopressin V(1A) and V(2) receptor antagonist

The pharmacologic profile of YM471 ((Z)-4'-[4,4-difluoro-5-[2-(4-dimethylaminopiperidino)-2-oxoethylidene]-2,3,4,5-tetrahydro-1H-1-benzoazepine-1-carbonyl]-2-phenylbenzanilide monohydrochloride), a novel potent vasopressin V(1A) and V(2) receptor antagonist, was investigated using several in vitro and in vivo techniques. YM471 showed high affinity for rat vasopressin V(1A) and V(2) receptors, exhibiting K(i) values of 0.16 and 0.77 nM, respectively. In contrast, YM471 exhibited much lower affinity for rat vasopressin V(1B) and oxytocin receptors, with K(i) values of 10.5 microM and 31.0 nM, respectively. In conscious rats, oral administration of YM471 (0.1-3.0 mg/kg) produced dose-dependent inhibition of the pressor response caused by exogenous vasopressin and increased urine excretion and decreased urine osmolality; this effect lasted more than 8 h. In all biological assays used, YM471 exhibited no agonistic activity. These results demonstrate that YM471 exerts potent and long-lasting antagonistic activity on both vasopressin V(1A) and V(2) receptors, and that this compound may be a useful tool for clarifying the physiologic and pathophysiologic roles of vasopressin and the therapeutic usefulness of the vasopressin receptor antagonist.

Two molecular forms of plasma adrenomedullin during tilt test in healthy subjects

There is accumulating evidence suggesting that adrenomedullin (AM) may participate in the regulation of circulatory homeostasis and pathophysiology of cardiovascular disease. A recent study revealed that two molecular forms of AM, an active form of mature AM (AM-m) and an intermediate inactive form of glycine-extended AM (AM-Gly), circulate in human plasma. The object of the present study was to evaluate the effect of orthostasis on a time course of two molecular forms of plasma AM and to compare them with the behavior of other vasoactive hormones. Twelve healthy male volunteers were studied. The experimental protocol consisted of 20 min of supine rest, tilting at 70 degrees for 20 min, and then 20 min of supine rest. Blood pressure and heart rate were measured every minute. Blood samples were obtained before, at 2 and 18 min during the tilt test, and 2 and 18 min after the test for the measurements of vasoacting hormones and hematocrit. Blood pressure and heart rate were slightly increased earlier during tilting and then remained elevated until the end of the test. The increase in heart rate and blood pressure returned to normal levels early after the tilt test. Plasma epinephrine and norepinephrine significantly increased during the tilt test. These hormones returned to normal levels 18 min after the test. The plasma renin activity, antidiuretic hormone and dopamine were also increased by the end of the tilt test, whereas plasma atrial natriuretic peptide was significantly decreased after the tilt test. Hematocrit increased slightly in the early phase of the tilt test and was further increased by the end of the test. In contrast, plasma AM-Gly or AM-m did not change during the tilt test or the recovery period. Nitric oxide metabolites did not change, either. There were no significant relationships between plasma catecholamines and AM. Plasma brain natriuretic peptide did not change during the tilt test or the recovery period, either. These results suggest that the two molecular forms of AM, AM-m and AM-Gly in plasma, did not respond to the short term tilting stress. These findings may support the hypothesis that plasma AM is secreted in a constitutive manner from the vascular wall.

Treatment of Severe Hyponatremia: Conventional and Novel Aspects

Abstract.Hyponatremia is a frequent electrolyte disorder. A hyponatremia is called acute severe (<115 mM) when the duration has been <36 to 48 h. Such patients often have advanced symptoms as a result of brain edema. Acute severe hyponatremia is a medical emergency. It should be corrected rapidly to approximately 130 mM to prevent permanent brain damage. In contrast, in chronic severe hyponatremia (>4 to 6 d), there is no brain edema and symptoms are usually mild. In such patients, a number of authors have recommended a correction rate <0.5 mM/h to approximately 130 mM to minimize the risk of cerebral myelinolysis. Sometimes it is not possible to diagnose whether a severe hyponatremia is acute or chronic. In such cases, an initial imaging procedure is helpful in deciding whether rapid or slow correction should be prescribed. The modalities of treatment of severe hyponatremia have so far consisted of infusions of hypertonic saline plus fluid restriction. In the near future, vasopressin antagonists will become available. Preliminary experience has already demonstrated their efficiency of inducing a sustained water diuresis and a correction of hyponatremia.

Effects of cardiotrophin-1 on hemodynamics and endocrine function of the heart

Cardiotrophin-1 (CT-1), a member of the interleukin-6 superfamily of cytokines, possesses hypertrophic actions and atrial natriuretic peptide (ANP)-producing activity in vitro. The goal of our study is to elucidate whether CT-1 affects the cardiovascular system in vivo. Intravenous injection of CT-1 (4-100 microg/kg) in conscious rats evoked significant declines in blood pressure and reflex increases in heart rate (HR) in a dose-dependent manner. CT-1 induced no significant change in cardiac output (from 260.7 +/- 11.0 to 264.7 +/- 26.6 ml. min(-1). kg(-1), P = not significant), which was compatible with the results from isolated perfused rat hearts; HR, change in pressure over time, left ventricular developed pressure, and perfusion pressure were unaffected. Northern blot and RT-PCR analyses revealed that CT-1 increased expression of inducible nitric oxide synthase (iNOS) in lung and aorta but not in heart or liver. Pretreatment with aminoguanidine, a specific iNOS inhibitor, inhibited both iNOS mRNA production and the depressor effect of CT-1. Interestingly, CT-1 increased ventricular expression of ANP and brain natriuretic peptide (BNP). The data demonstrate that CT-1 elicits its hypotensive effect via a nitric oxide-dependent mechanism and that CT-1 induces ANP and BNP mRNA expression in vivo.

Aquaretic and hormonal effects of a vasopressin V(2) receptor antagonist after acute and long-term treatment in rats

A single oral administration of 1-[4-(N-tert-butylcarbamoyl)-2-methoxybenzene sulfonyl]-5-ethoxy-3-spiro-[4-(2-morpholinoethoxy)cyclohexane]indo l-2 -one SR121463 (0.3-3 mg/kg), a vasopressin non-peptide V(2) receptor antagonist, to rats induced dose-dependent aquaresis which was accompanied by Na(+), K(+), aldosterone and arginine vasopressin excretion over 6 h after dosing. However, no solute excretion was observed over 24 h. As a result of aquaresis, hemoconcentration and increases in plasma angiotensin II and adenocorticotrophin hormone were seen with 3 mg/kg at 2 h after dosing. Chronic treatment with SR121463 (3 mg/kg/dayx28 days) induced a marked aquaresis associated with aldosterone and vasopressin excretion. After a week of treatment, urine volume and aldosterone excretion were reduced ( approximately 40%) and then stabilised, while urine vasopressin excretion remained almost constant throughout the study. There were no changes in arterial pressure, plasma osmolality, plasma sodium concentration, or in number and affinity of liver vasopressin V(1A) and kidney V(2) receptors 24 h after the last treatment. These results indicate that SR121463 is a potent aquaretic agent and might be useful for the chronic management of water-retaining diseases in humans.

The treatment of hyponatraemia using vasopressin antagonists

Hyponatraemia is a frequent electrolyte disorder. It is primarily attributable to vasopressin excess plus sustained fluid intake. Hyponatraemia causes CNS symptoms, especially during the first 2-4 days; these symptoms are related to brain swelling. Hyponatraemia occurs in the setting of liver cirrhosis and congestive cardiac failure, in which it is related to stimulation by low arterial blood pressure acting through baroreceptors. Hyponatraemia also occurs in the syndrome of inappropriate antidiuretic hormone secretion, usually from neoplasms releasing vasopressin. The conventional treatment of hyponatraemia used to be fluid restriction and treatment of the underlying disorder. This kind of treatment has been unreliable, cumbersome and difficult to comply with for the patient. In the future, effective vasopressin V2 antagonists will become available for clinical use in the treatment of hyponatraemia, and are expected to improve the management of hyponatraemia. Pharmacological characteristics and observations of biological effects of three antagonists are reported in the present article.

Vasopressin receptor antagonism--a therapeutic option in heart failure and hypertension

The precise role of vasopressin in the pathophysiology of cardiovascular disease is controversial, but this peptide hormone is important for several reasons. Firstly, circulating concentrations of vasopressin are elevated in heart failure and some forms of hypertension. Secondly, there is evidence that vasopressin is synthesized not only in the hypophysial-pituitary axis but also in peripheral tissues including the heart where it acts as a paracrine hormone. Thirdly, vasopressin has vasoconstrictor, mitogenic, hyperplastic and renal fluid retaining properties which, by analogy with angiotensin II, may have deleterious effects when present in chronic excess. Finally, the availability of orally active non-peptide vasopressin receptor antagonists allows vasopressin receptor antagonism to be considered as a therapeutic option in cardiovascular disease.

Increased brain angiotensin receptor in rats with chronic high-output heart failure

BACKGROUND

The renin-angiotensin system (RAS) plays a key role in the pathophysiology of chronic heart failure (CHF). In rats, we reported that CHF enhances dipsogenic responses to centrally administered angiotensin I, and central inhibition of the angiotensin-converting enzyme (ACE) prevents cardiac hypertrophy in CHF. This suggests that the brain RAS is activated in CHF. To clarify the mechanism of the central RAS activation in CHF, we examined brain ACE and the angiotensin receptor (AT) among rats with CHF.

METHODS AND RESULTS

We created high-output heart failure in 22 male Sprague-Dawley rats by aortocaval shunt. Four weeks after surgery, we examined ACE mRNA by reverse transcriptase polymerase chain reaction (RT-PCR) and AT by binding autoradiography. ACE mRNA levels were not significantly increased in the subfornical organ (SFO), the hypothalamus, or in the lower brainstem of CHF rats (n = 5) compared with sham-operated rats (SHM) (n = 6). Binding densities for type 1 AT (AT1) in the SFO (P < .05), paraventricular hypothalamic nuclei (P < .05), and solitary tract nuclei (P < .05) were higher in rats with CHF (n = 5) than in SHM rats (n = 6). Thus, in rats with CHF, AT1 expression is increased in brain regions that are closely related to water intake, vasopressin release, and hemodynamic regulation.

CONCLUSIONS

The fact that AT1 expression was upregulated in important brain regions related to body fluid control in CHF rats indicates that the brain is a major site of RAS action in CHF rats and, therefore, a possible target site of ACE-inhibitors in the treatment of CHF.

Selective V2-receptor vasopressin antagonism decreases urinary aquaporin-2 excretion in patients with chronic heart failure

Aquaporin-2 (AQP-2), a water channel located on the apical membrane of collecting duct cells, regulates water reabsorption under the control of vasopressin (AVP). Using an antibody directed to human AQP-2, a quantitative Western blot analysis was performed to determine the collecting duct responsiveness to an oral, nonpeptide, V2 receptor antagonist (VPA-985) in patients with chronic NYHA II and III heart failure. Standards were derived by conjugating the immunizing peptide to maleimide-activated bovine serum albumin and a standard curve was generated for each blot. Quantification of baseline steady-state AQP-2 excretion was done by collecting urine on the day before study drug administration. The next day patients received either placebo or VPA-985 at one of four different doses and urine was collected every 2 h. Thereafter, urinary AQP-2 excretion was calculated as a ratio of the urine flow and was expressed in pmol/h. During baseline, steady-state excretion did not change significantly (T0-T2, 458 +/- 44; T2-T4, 443 +/- 35; T4-T6, 422 +/- 35; T6-T8, 401 +/- 30). Compared to placebo, urinary AQP-2 excretion decreased significantly and in all groups in a dose-dependent manner during VPA-985 administration. The most impressive decrease was observed in the 250-mg group (T0-T2, 89 +/- 5; T2-T4, 50 +/- 18; T4-T6, 43 +/- 22; T6-T8, 42 +/- 23; P < 0.001 during each period compared with baseline and placebo results). VPA-985 significantly increased solute-free water clearance and urine output and significantly decreased urinary osmolality. Urinary AQP-2 excretion correlated best with solute-free water clearance during T0-T2 and T2-T4 collection, but a correlation with urinary osmolality and urinary output was also found during these periods. In conclusion, AQP-2 urinary excretion, as measured by quantitative Western analysis, is a sensitive biologic marker to assess the short-term responsiveness of the collecting duct to a V2 receptor AVP antagonist in chronic heart failure.

Recent advances in the understanding of water metabolism in heart failure

Hyponatremia is common in advanced heart failure and relates to the severity of the disease. Non-osmotic arginine vasopressin (AVP) release and biosynthesis have been shown to be increased during chronic cardiac failure (CHF) and baroreceptors pathways have been demonstrated to play a major role in this non-osmotic stimulation of AVP. Decreased cardiac output unloads the baroreceptors and activates the sympathetic nervous system, thus stimulating AVP through a separate pathway which overrides the osmotic pathway. Besides sympathetic nervous system activation, neurohumoral peptides, such as angiotensin II, endothelins, natriuretic peptides and prostaglandins, could also participate in the non-osmotic AVP activation. The vasoconstrictor effect of AVP has been supported by the decrease systemic vascular resistance during the administration of V1 receptor AVP antagonist in CHF patients. Administration of V2 receptor AVP antagonists corrects the hyponatremia and has been demonstrated to improve survival in animal models of heart failure. Preliminary data in humans with CHF also demonstrate urinary dilution and correction of hyponatremia with orally active non-peptide V2 receptor antagonists. Finally, upregulation of the AVP-regulated water channels, aquaporin-2 (AQP2), located in the collecting duct cells has been shown in experimental heart failure. This AQP2 upregulation can be entirely suppressed by V2 receptor AVP antagonists paralleling the correction of the hyponatremia. Thus, non-osmotic release of AVP in CHF upregulates AQP2 water channels, enhances water reabsorption and causes hyponatremia. The V1, and perhaps the V2, receptor activation may also diminish cardiac function.

Diuretic treatment and diuretic resistance in heart failure

Diuretic therapy decreases capillary wedge pressure and improves New York Heart Association (NYHA) functional class both in acute and chronic heart failure. In advanced symptomatic heart failure, loop diuretics are generally necessary to improve symptoms of congestion. Diuretic resistance in the edematous patient has been defined as a clinical state in which diuretic response is diminished or lost before the therapeutic goal of relief from edema has been reached. The major causes of diuretic resistance are functional renal failure (prerenal azotemia), hyponatremia, altered diuretic pharmacokinetics, and sodium retention caused by counterregulatory mechanisms intended to reestablish the effective arterial blood volume. Therapeutic approaches to combat diuretic resistance include restriction of fluid and sodium intake, use of angiotensin-converting-enzyme (ACE) inhibitors, changes in route (oral, intravenous) and timing (single dose, multiple doses, continuous infusion) of diuretic therapy, and use of diuretic combinations.

Pharmacological characterization of the human vasopressin receptor subtypes stably expressed in Chinese hamster ovary cells

Three subtypes of human (h) arginine vasopressin (AVP) receptors, hV1A, hV1B and hV2, were stably expressed in Chinese hamster ovary (CHO) cells and characterized by [3H]-AVP binding studies. In addition, the coupling of the expressed receptor protein to a variety of signal transduction pathways was investigated. Scatchard analysis of saturation isotherms for the specific binding of [3H]-AVP to membranes, prepared from CHO cells transfected with hV1A, hV1B and hV2 receptors, yielded an apparent equilibrium dissociation constant (Kd) of 0.39, 0.25 and 1.21 nM and a maximum receptor density (Bmax) of 1580 fmol mg(-1) protein, 5230 fmol mg(-1) protein and 7020 fmol mg(-1) protein, respectively. Hill coefficients did not differ significantly from unity, suggesting binding to homogenous, non-interacting receptor populations. Pharmacological characterization of the transfected human AVP receptors was undertaken by measuring the relative ability of nonpeptide AVP receptor antagonists, YM087, OPC-21268, OPC-31260, SR 49059 and SR 121463A, to inhibit binding of [3H]-AVP. At hV1A receptors, the relative order of potency was SR49059>YM087>OPC-31260>SR 121463A> >OPC-21268 and at hV2 receptors, YM087=SR 121463A>OPC-31260>SR 49059> >OPC-21268. In contrast, the relative order of potency, at hV1B receptors, was SR 49059> >SR 121463A=YM087=OPC-31260=OPC-21268. In CHO cells expressing either hV1A or hV1B receptors, AVP caused a concentration-dependent increase in intracellular Ca2+ concentration ([Ca2+]i) with an EC50 value of 1.13 nM and 0.90 nM, respectively. In contrast, stimulation of CHO cells expressing hV2 receptors resulted in an accumulation of cyclic AMP with an EC50 value of 2.22 nM. The potency order of antagonists in inhibiting AVP-induced [Ca2+]i or cyclic AMP response was similar to that observed in radioligand binding assays. In conclusion, we have characterized the pharmacology of human cloned V1A, V1B and V2 receptors and used these to determine the affinity, selectivity and potency of nonpeptide AVP receptor antagonists. Thus they may prove to be a valuable tool in further examination of the physiological and pathophysiological roles of AVP.

The effect of vasopressin V1- and V2-receptor antagonists on hemodynamics in early and late phase after myocardial infarction in rats

This study was designed to assess the effect of a nonpeptide vasopressin V1-receptor antagonist, OPC-21268, and a vasopressin V2-receptor antagonist, OPC-31260, on hemodynamics in the early phase and the late phase after myocardial infarction in rats. In the early phase, OPC-21268 (30 mg/kg/day) or OPC-31260 (30 mg/kg/day) was orally administered from day 1 to day 5 after the operation; and hemodynamics were measured at day 5, in the late phase from 10 weeks to 11 weeks and measured at the end of 11 weeks. In the early phase, OPC-21268 reduced the left ventricular end-diastolic pressure (LVEDP) concomitantly with the reduction in systemic blood pressure, but did not change LVEDP in the late phase. OPC-31260 reduced LVEDP and central venous pressure in both phases. OPC-21268 improved hemodynamics only in the early phase and OPC-31260 improved it in both phases.

Long-term effects of nonpeptide vasopressin V2 antagonist OPC-31260 in heart failure in the rat

The hormone arginine vasopressin (AVP) contributes to water retention and vasoconstriction in congestive heart failure (CHF) through effects at the V2 and V1a receptors, respectively. The effect of long-term V2 receptor (V2R) blockade using OPC-31260 was assessed in a rat model of postinfarction-induced CHF. Rats underwent coronary artery ligation or sham operation and were treated for 6 mo with oral OPC-31260 (10 mg . kg-1 . day-1) or vehicle. CHF was characterized by left ventricular remodeling and impaired systolic function, increased cardiac and lung weight, and elevated plasma atrial natriuretic peptide; plasma AVP and plasma renin activity were not increased. Chronic V2R blockade increased urine volume (P < 0.01) and decreased urine osmolality (P < 0.01) but had no natriuretic effects. V2R blockade did not activate the renin-angiotensin system but increased plasma AVP in CHF (P < 0.01). V2R blockade did not influence cardiac remodeling, cardiac function, or survival. These results suggest that AVP plays a major role in water retention through the renal V2R in a rat model of CHF. V2R blockade using OPC-31260 may represent an alternative to standard diuretic therapy in the management of water retention that characterizes heart failure.

Cardiac production and secretion of adrenomedullin are increased in heart failure

Plasma adrenomedullin (AM) levels are reportedly increased in heart failure, but whether the cardiac production and secretion of AM is increased in heart failure remains unknown. To investigate the sites of production and secretion of AM in heart failure, we measured plasma AM levels and peptide and mRNA levels of AM in various tissues in rats with heart failure. We also examined whether the heart actually secretes AM into the circulation in patients with heart failure. We measured plasma and tissue AM levels by specific radioimmunoassay and AM mRNA by Northern blot analysis in rats with heart failure produced by aortocaval fistula. We also measured plasma AM levels in the coronary sinus and aorta in patients with left ventricular dysfunction before and after rapid right ventricular pacing. The increase in plasma AM levels in heart failure rats correlated with ventricular weight. Tissue AM levels were increased in the heart and lungs but not in the kidneys or adrenals of rats with heart failure. Similarly, tissue AM mRNA levels were also increased in the heart and lungs of heart failure rats. Plasma AM levels were higher in the coronary sinus than in the aorta in patients with left ventricular dysfunction. Rapid right ventricular pacing increased plasma atrial natriuretic peptide but not AM. These results suggest that plasma AM levels are increased in heart failure in proportion to the severity of heart failure and that cardiac production and secretion of AM is increased in heart failure rats. The lung may be another site for increased production of AM in heart failure rats. Human failing heart actually secretes AM into the circulation, and the regulation of AM secretion appears to differ from that of atrial natriuretic peptide.