Blood pressure-lowering effect of an orally active vasopressin V1 receptor antagonist in mineralocorticoid hypertension in the rat

Measuring copeptin, a surrogate for vasopressin in patients with hypertension - Can it identify those who are volume Responsive?

BACKGROUND

Among hypertensive patients, plasma renin activity is lower and the response to diuretic monotherapy greater in volume responsive hypertensive patients. We hypothesized that hormones influencing extracellular volume such as vasopressin / antidiuretic hormone (ADH) might permit the development of a simple test to identify those with volume-related hypertension. Such a test might be of particular benefit to the Black population which is purported to have a higher incidence of volume-related and responsive hypertension. Thus, using copeptin, a surrogate marker for ADH, we studied if there were differences in this hormone between those with and without volume responsive hypertension.

METHODS

Serum copeptin was measured in biobanked blood samples from the Genetic Epidemiology of Responses to Antihypertensives (GERA) I study and analyzed with other variables from the study dataset.

RESULTS

There was no relationship between PRA and copeptin values nor could the response in blood pressure be predicted by the copeptin values. However, baseline copeptin levels were higher in Black than in White subjects (7.5 pmol/L vs 5.4 pmol/L, P < 0.001) while plasma sodium and calculated plasma osmolality were slightly lower in keeping with the concept that Black subjects have more volume-related hypertension. In addition, after hydrochlorothiazide (HCTZ), copeptin was significantly lower in Black (6.2 pmol/L, P = 0.004) but unchanged in White subjects (5.2 pmol/L, P = 0.901) and there were also changes in sodium.

CONCLUSION

The current study suggests differences in ADH physiology between hypertensive Black and White patients. However, the use of copeptin to identify volume responsive patients could not be confirmed.

Evidence for a Prehypertensive Water Dysregulation Affecting the Development of Hypertension: Results of Very Early Treatment of Vasopressin V1 and V2 Antagonism in Spontaneously Hypertensive Rats

In addition to long-term regulation of blood pressure (BP), in the kidney resides the initial trigger for hypertension development due to an altered capacity to excrete sodium and water. Betaine is one of the major organic osmolytes, and its betaine/gamma-aminobutyric acid transporter (BGT-1) expression in the renal medulla relates to interstitial tonicity and urinary osmolality and volume. This study investigated altered water and sodium balance as well as changes in antidiuretic hormone (ADH) activity in female spontaneously hypertensive (SHR) and normotensive Wistar Kyoto (WKY) rats from their 3–5 weeks of age (prehypertensive phase) to SHR’s 28–30 weeks of age (established hypertension-organ damage). Young prehypertensive SHRs showed a reduced daily urine output, an elevated urine osmolarity, and higher immunostaining of tubule BGT-1, alpha-1-Na-K ATPase in the outer medulla vs. age-matched WKY. ADH circulating levels were not different between young prehypertensive SHR and WKY, but the urine aquaporin2 (AQP2)/creatinine ratio and labeling of AQP2 in the collecting duct were increased. At 28–30 weeks, hypertensive SHR with moderate renal failure did not show any difference in urinary osmolarity, urine AQP2/creatinine ratio, tubule BGT-1, and alpha-1-Na-K ATPase as compared with WKY. These results suggest an increased sensitivity to ADH in prehypertensive female SHR. On this basis, a second series of experiments were set to study the role of ADH V1 and V2 receptors in the development of hypertension, and a group of female prehypertensive SHRs were treated from the 25th to 49th day of age with either V1 (OPC21268) or V2 (OPC 41061) receptor antagonists to evaluate the BP time course. OPC 41061-treated SHRs had a delayed development of hypertension for 5 weeks without effect in OPC 21268-treated SHRs. In prehypertensive female SHR, an increased renal ADH sensitivity is crucial for the development of hypertension by favoring a positive water balance. Early treatment with selective V2 antagonism delays future hypertension development in young SHRs.

Induction of hypertension blunts baroreflex inhibition of vasopressin neurons in the rat

Vasopressin secretion from the posterior pituitary gland is determined by action potential discharge of hypothalamic magnocellular neurosecretory cells. Vasopressin is a potent vasoconstrictor, but vasopressin levels are paradoxically elevated in some patients with established hypertension. To determine whether vasopressin neurons are excited in hypertension, extracellular single-unit recordings of vasopressin neurons from urethane-anaesthetized Cyp1a1-Ren2 rats with inducible angiotensin-dependent hypertension were made. The basal firing rate of vasopressin neurons was higher in hypertensive Cyp1a1-Ren2 rats than in non-hypertensive Cyp1a1-Ren2 rats. The increase in firing rate was specific to vasopressin neurons because oxytocin neuron firing rate was unaffected by the induction of hypertension. Intravenous injection of the α1-adrenoreceptor agonist, phenylephrine (2.5 μg/kg), transiently increased mean arterial blood pressure to cause a baroreflex-induced inhibition of heart rate and vasopressin neuron firing rate (by 52 ± 9%) in non-hypertensive rats. By contrast, intravenous phenylephrine did not inhibit vasopressin neurons in hypertensive rats, despite a similar increase in mean arterial blood pressure and inhibition of heart rate. Circulating angiotensin II can excite vasopressin neurons via activation of afferent inputs from the subfornical organ. However, the increase in vasopressin neuron firing rate and the loss of inhibition by intravenous phenylephrine were not blocked by intra-subfornical organ infusion of the angiotensin AT1 receptor antagonist, losartan. It can be concluded that increased vasopressin neuron activity at the onset of hypertension is driven, at least in part, by reduced baroreflex inhibition of vasopressin neurons and that this might exacerbate the increase in blood pressure at the onset of hypertension.

Role of mineralocorticoid action in the brain in salt-sensitive hypertension

1. The mechanisms by which excessive salt causes hypertension involve more than retention of sodium and water by the kidneys and are far from clear. Mineralocorticoids act centrally to increase salt appetite, sympathetic drive and vasopressin release, resulting in hypertension that is prevented by the central infusion of mineralocorticoid receptor (MR) antagonists. The MR has similar affinity for aldosterone and the glucocorticoids corticosterone or cortisol. Specificity is conferred in transport epithelia by the colocalization of the MR with 11β-hydroxysteroid dehydrogenase Type 2. Coexpression also occurs in some neurons, notably those of the nucleus tractus solitarius that are activated by sodium depletion and aldosterone and mediate salt-seeking behaviour. 2. The salt-induced hypertension of the Dahl salt-sensitive rat is mitigated by the central infusion of a mineralocorticoid antagonist even though circulating aldosterone is normal or reduced in salt-sensitive (SS). Contrary to reports that salt appetite in the Dahl salt-sensitive rat is depressed, we found that it is increased compared with that in Spraque-Dawley rats. 3. Extra-adrenal aldosterone synthesis in the brain occurs in minute amounts that could only be relevant locally. Expression of aldosterone synthase mRNA and aldosterone concentrations in the brain of Dahl salt-sensitive rats are increased compared with Spraque-Dawley rats. The central infusion of inhibitors of aldosterone synthesis lowers salt-induced hypertension in the Dahl salt-sensitive rat, suggesting a role for excessive Dahl salt-sensitive synthesis in the brain. Brain MR, particularly those in the paraventricular nuclei, regulate inflammatory processes that are exacerbated by sodium and lead to cardiovascular dysfunction.

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.

Vasopressin and angiotensin receptors of the medial septal area in the control of mean arterial pressure induced by vasopressin

Introduction. Brain arginine 8-vasopressin (AVP), through the V1a- and V2-receptors, is essential for the maintenance of mean arterial pressure (MAP). Central AVP interacts with the components of the renin-angiotensin system, which participate in MAP regulation. This study aimed to determine the effects of V1a-, V2- and V1a/V2-AVP selective antagonists and AT1- and AT2-angiotensin II (Ang II) selective antagonists on the MAP induced by AVP injected into the medial septal area (MSA) of the brain.

Materials and methods. Male Holtzman rats with stainless steel cannulae implanted into the MSA were used in experiments. Direct MAP was recorded in conscious rats.

Results. AVP administration into the MSA caused a prompt and potent pressor response in a dose-dependent fashion. Pretreatment with the V1a- and V2-antagonists reduced, whereas prior injection of the V1a/V2-antagonist induced a decrease in the MAP that remained below the baseline. Both AT 1- and AT2-antagonists elicited a decrease, while simultaneous injections of two antagonists were more effective in decreasing the MAP induced by AVP.

Conclusion. These results indicate there is a synergism between the V1a- and V2-AVP and AT1and AT2-Ang II receptors in the MSA in the regulation of MAP.

Pharmacological characterization of RWJ-676070, a dual vasopressin V(1A)/V(2) receptor antagonist

The dysregulation of arginine vasopressin (AVP) release and activation of vasopressin V(1A) and V(2) receptors may play a role in disease. The in vitro and in vivo pharmacology of RWJ-676070, a potent, balanced antagonist of both the V(1A) and V(2) receptors is described. RWJ-676070 binding and intracellular functional antagonist activity was characterized using cells expressing V(1A), V(1B) or V(2) receptors. Its inhibition of V(1A) receptor-mediated contraction of vascular rings and platelet aggregation was determined. V(2) receptor-medated aquaresis was determined in rats, dogs and monkeys. V(1A) receptor-mediated inhibitory activity was assessed in vivo in a vasopressin-induced hypertension model and in normotensive rats and in two hypertensive rat models. RWJ-676070 inhibited AVP binding to human V(1A) and V(2) receptors (Ki=1 and 14 nM, respectively). RWJ-676070 inhibited V(1A) receptor-induced intracellular calcium mobilization and V(2) receptor-induced cAMP accumulation with Ki values of 14 nM and 13 nM, respectively. The compound was slightly less potent against rat V(1A) receptors. RWJ-676070 inhibited V(1A) receptor-mediated vasoconstriction in rat and dog vascular rings and AVP-induced human platelet aggregation. Dose dependent aquaresis was demonstrated in rats, dogs and monkeys following oral administration. RWJ-676070 inhibited AVP-induced hypertension in rats but had no effect on arterial pressure in normotensive and spontaneously hypertensive rats but did decrease arterial pressure in Dahl, salt-sensitive hypertensive rats. RWJ-676070 is a new, potent antagonist of V(1A) and V(2) receptors that may be useful for treatment of diseases benefiting from balanced inhibition of both V(1A) and V(2) receptors.

Protective effects of estradiol in the brain of rats with genetic or mineralocorticoid-induced hypertension

Abnormalities of hippocampus and hypothalamus are commonly observed in rats with genetic (SHR) or mineralocorticoid/salt-induced hypertension. In the hippocampus, changes include decreased cell proliferation in the dentate gyrus (DG), astrogliosis and decreased neuronal density in the hilus, whereas in the hypothalamus expression of arginine vasopressin (AVP) is markedly elevated. Here, we report that estradiol treatment overturns these abnormalities. We used 16-week-old male SHR with blood pressure (BP) approximately 190 mmHg and their normotensive Wistar-Kyoto (WKY) controls, and male Sprague-Dawley rats made hypertensive by administration of 10mg deoxycorticosterone acetate (DOCA) every other day plus 1% NaCl as drinking fluid for 4 weeks (BP approximately 160 mmHg). Controls received oil vehicle plus 1% NaCl only. Half of the animals in each group were implanted s.c. with a single estradiol benzoate pellet weighing 14 mg for 2 weeks. Estradiol-treated SHR and DOCA-salt rats showed, in comparison to their respective steroid-free groups: (a) enhanced proliferation in the DG measured by bromodeoxyuridine incorporation; (b) decreased number of glial fibrillary acidic protein (GFAP) immunopositive astrocytes; (c) increased density of neurons in the hilus of the DG, and (d) decreased hypothalamic AVP mRNA expression. These results indicate that neuronal and glial alterations of hypertensive models are plastic events reversible by steroid treatment. The estradiol protective effects may be of pharmacological interest to attenuate the consequences of hypertensive encephalopathy.

Effects of V2-receptor antagonist tolvaptan and the loop diuretic furosemide in rats with heart failure

Diuretics are frequently required to treat fluid retention in patients with chronic heart failure (CHF). Unfortunately, they can lead to a decline in renal function, electrolyte depletion, and neurohormonal activation. Arginine vasopressin (AVP) promotes renal water reabsorption via the V(2) receptor (V(2)R) and its levels are increased in CHF. This study was conducted to characterize the diuretic effect of tolvaptan, a non-peptide AVP V(2)R antagonist, and furosemide, a loop diuretic in a rat model of CHF after 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, and furosemide. CHF was characterized by left ventricular remodeling and impaired systolic and diastolic function. Tolvaptan produces a diuresis comparable to furosemide. Unlike tolvaptan, furosemide significantly increased urinary sodium and potassium excretion. Tolvaptan markedly elevated electrolyte-free water clearance (E-CH(2)O) or aquaresis to a positive value and increased urinary AVP excretion. In contrast to tolvaptan, furosemide elevated only electrolyte clearance (E-Cosm) but not E-CH(2)O. The differences in diuretic profile reflected the changes in plasma sodium and hormone levels. Tolvaptan dose dependently elevated plasma sodium concentration, but furosemide tended to decrease it. Furosemide significantly elevated plasma renin activity and aldosterone concentration. On the other hand, tolvaptan did not affect these parameters. Our results suggest that, tolvaptan have a potential medical benefit for the treatment of edematous conditions in CHF by removing excess water from the body without activating the RAAS or causing serum electrolyte imbalances.

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.

Therapeutic potential of vasopressin receptor antagonists

Arginine vasopressin (AVP) is a neuropeptide hormone that plays an important role in circulatory and sodium homeostasis, and regulating serum osmolality. Several clinical conditions have been associated with inappropriately elevated levels of AVP including heart failure, cirrhosis of the liver and the syndrome of inappropriate secretion of antidiuretic hormone. Three receptor subtypes that mediate the actions of AVP have been identified (V(1A), V(2) and V(1B)). Activation of V(1A) receptors located in vascular smooth muscle cells and the myocardium results in vasoconstriction and increased afterload and hypertrophy. The V(2) receptors located primarily in the collecting tubules mediate free water absorption. The V(1B) receptors are located in the anterior pituitary and mediate adrenocorticotropin hormone release. The cardiovascular and renal effects of AVP are mediated primarily by V(1A) and V(2) receptors. Antagonism of V(1A) receptors results in vasodilatation and antagonism of V(2) receptors resulting in aquaresis, an electrolyte-sparing water excretion. Several non-peptide AVP antagonists (vasopressin receptor antagonists [VRAs]) also termed 'vaptans' have been developed and are vigorously being studied primarily for treating conditions characterised by hyponatraemia and fluid overload. Conivaptan is a combined V(1A)/V(2)-receptor antagonist that induces diuresis as well as haemodynamic improvement. It has been shown in clinical trials to correct euvolaemic and hypervolaemic hyponatraemia, and has been approved by the US FDA for the treatment of euvolaemic hyponatraemia as an intravenous infusion. Tolvaptan, a selective V(2)-receptor antagonist, has undergone extensive clinical studies in the treatment of hyponatraemia and heart failure. It has been shown to effectively decrease fluid in volume overloaded patients with heart failure and to correct hyponatraemia. A large outcome study (n = 4133 patients) will define its role in the management of heart failure. Lixivaptan and satavaptan (SR-121463) are other selective V(2)-receptor antagonists being evaluated for the treatment of hyponatraemia. In addition, a potential role for the vaptans in attenuating polyuria in nephrogenic diabetes insipidus and cyst development in polycystic kidney disease is being explored. Ongoing clinical trials should further define the scope of the potential therapeutic role of VRAs.

V1a vasopressin receptors maintain normal blood pressure by regulating circulating blood volume and baroreflex sensitivity

Arginine-vasopressin (AVP) is a hormone that is essential for both osmotic and cardiovascular homeostasis, and exerts important physiological regulation through three distinct receptors, V1a, V1b, and V2. Although AVP is used clinically as a potent vasoconstrictor (V1a receptor-mediated) in patients with circulatory shock, the physiological role of vasopressin V1a receptors in blood pressure (BP) homeostasis is ill-defined. In this study, we investigated the functional roles of the V1a receptor in cardiovascular homeostasis using gene targeting. The basal BP of conscious mutant mice lacking the V1a receptor gene (V1a-/-) was significantly (P < 0.001) lower compared to the wild-type mice (V1a+/+) without a notable change in heart rate. There was no significant alteration in cardiac functions as assessed by echocardiogram in the mutant mice. AVP-induced vasopressor responses were abolished in the mutant mice; rather, AVP caused a decrease in BP, which occurred in part through V2 receptor-mediated release of nitric oxide from the vascular endothelium. Arterial baroreceptor reflexes were markedly impaired in mutant mice, consistent with a loss of V1a receptors in the central area of baroreflex control. Notably, mutant mice showed a significant 9% reduction in circulating blood volume. Furthermore, mutant mice had normal plasma AVP levels and a normal AVP secretory response, but had significantly lower adrenocortical responsiveness to adrenocorticotropic hormone. Taken together, these results indicate that the V1a receptor plays an important role in normal resting arterial BP regulation mainly by its regulation of circulating blood volume and baroreflex sensitivity.

Vasopressin receptor antagonists: pharmacological tools and potential therapeutic agents

The present survey deals with the development and applications of non-peptidergic vasopressin receptor antagonists. The existence of at least three vasopressin receptors (V(1), V(2) and V(3) respectively) is firmly established. V(1)-receptors play a relevant role in the regulation of vascular tone, whereas V(2)-receptors are known to mediate the antidiuretic activity of vasopressin at the level of the renal collecting ducts. The V(3)-receptor appears to be involved in the release of the adreno-corticotropic hormone. Vasopressin receptor antagonists which are peptides have been known for several decades, more recently, both V(1)- and V(2)-receptor blockers which are non-peptidergic have been introduced, as well as agents with affinity for both V(1)- and V(2)-receptor subtypes. A survey of these non-peptidergic antagonists is presented here. Such compounds are useful as pharmacological tools, and they can also be thought of as therapeutic agents as therapeutic agents in cardiovascular and renal diseases. Selective V(1)- and V(2)-receptor antagonists were used to study the interaction between vasopressin receptors and sympathetic neurones. Depending on the experimental model used this interaction can occur at either the pre- or postsynaptic sites. In both cases predominantly V(1)-receptors are involved. A brief survey is given of the potential use of V-receptor antagonists in the drug therapy of syndrome of inappropriate antidiuretic hormone secretion and other water retaining disorders, congestive heart failure and certain forms of hypertension (in particular in the Negroid hypertensive 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.

Enhanced Ca2+ response to AVP in preglomerular vessels from rats with genetic hypertension during different hydration states

Exaggerated arginine vasopressin (AVP)-induced calcium signaling and renal vasoconstriction, characteristic in young spontaneously hypertensive rats (SHR) during euvolemia, are related to greater amounts of V1a receptor mRNA and V1a protein in preglomerular resistance arterioles. The present study determined whether V1a receptor density and calcium signal transduction in the renal vasculature of young SHR is regulated appropriately during physiological changes in hydration state. [3H]AVP ligand binding documented two- to threefold greater density of V1a receptors in euvolemic SHR vs. Wistar-Kyoto (WKY) rats. Parallel changes in V1a receptor density were observed in both strains during chronic water loading (plus approximately 50 fmol/mg) and during dehydration (minus approximately 50 fmol/mg). Affinity was unchanged. Real-time RT-PCR demonstrated that V1a mRNA in preglomerular arterioles was three times greater in euvolemic SHR. Dehydration decreased expression approximately 50% in renal vessels independent of rat strain; water loading increased V1a mRNA. Thus V1a receptor regulation correlated with changes in mRNA in a normal manner in response to chronic changes in AVP concentration, albeit set at a higher level in SHR. In dehydrated animals, AVP increased the cytosolic Ca2+ concentration ([Ca2+]i) by 60 +/- 5 and 112 +/- 13 nM cytosolic Ca2+ in WKY and SHR, respectively (P < 0.01), whereas in hydrated animals the [Ca2+]i increase was 168 +/- 10 and 220 +/- 18 nM, respectively (P < 0.05). In all hydration states, calcium signaling was greater in SHR compared with WKY (P < 0.05). Calcium signaling paralleled changes in the receptor density and mRNA. Mechanisms other than hydration state per se are likely to be responsible for the two- to threefold difference in the V1a receptor density between WKY and SHR in the renal vasculature at the critical age of 6 wk.

Identification of the binding sites of the SR49059 nonpeptide antagonist into the V1a vasopressin receptor using sulfydryl-reactive ligands and cysteine mutants as chemical sensors

To identify the binding site of the human V1a vasopressin receptor for the selective nonpeptide antagonist SR49059, we have developed a site-directed irreversible labeling strategy that combines mutagenesis of the receptor and use of sulfydryl-reactive ligands. Based on a three-dimensional model of the antagonist docked into the receptor, hypothetical ligand-receptor interactions were investigated by replacing the residues potentially involved in the binding of the antagonist into cysteines and designing analogues of SR49059 derivatized with isothiocyanate or alpha-chloroacetamide moieties. The F225C, F308C, and K128C mutants of the V1a receptor were expressed in COS-7 or Chinese hamster ovary cells, and their pharmacological properties toward SR49059 and its sulfydryl-reactive analogues were analyzed. We demonstrated that treatment of the F225C mutant with the isothiocyanate-derivative compound led to dose-dependent inhibition of the residual binding of the radio-labeled antagonist [125I]HO-LVA. This inhibition is probably the consequence of a covalent irreversible chemical modification, which is only possible when close contacts and optimal orientations exist between reactive groups created both on the ligand and the receptor. This result validated the three-dimensional model hypothesis. Thus, we propose that residue Phe225, located in transmembrane domain V, directly participates in the binding of the V1a-selective nonpeptide antagonist SR49059. This conclusion is in complete agreement with all our previous data on the definition of the agonist/antagonist binding to members of the oxytocin/vasopressin receptor family.

Vasopressin induces vascular superoxide via endothelin-1 in mineralocorticoid hypertension

We have recently reported that endothelin-1 (ET-1), which is increased in the arteries of deoxycorticosterone acetate (DOCA)-salt hypertensive rats, stimulates superoxide production. However, the humoral mechanisms responsible for ET-1-induced superoxide formation in low-renin models of hypertension, such as DOCA-salt hypertension, remain undefined. Vasopressin is known to upregulate vascular preproET-1 gene expression in DOCA-salt rats, an effect that is absent in vasopressin-deficient Brattleboro rats treated with DOCA-salt. The present study tested the hypothesis that vasopressin contributes to ET-1-induced vascular superoxide production in DOCA-salt hypertensive rats. Carotid arterial segments of DOCA, sham (uninephrectomized), or normal (untreated) rats were used for the study. In vitro vasopressin treatment of carotid arteries from normal rats for 24 hours, but not 4 hours, increased both ET-1 and superoxide levels. The increase of vasopressin-induced superoxide was reduced by pretreatment of the vessels with ABT627, a selective ETA receptor antagonist ABT627. Vasopressin, ET-1, and superoxide levels were significant elevated in carotid arteries of DOCA-salt rats compared with sham controls. The selective V1-vasopressin receptor antagonist (beta-Mercapto-beta, beta-cyclopentamethylenepropiony1, O-Me-Tyr2, Arg8 vasopressin, ME-AVP), decreased superoxide both in vasopressin-treated vessels of normal rats and in vessels of DOCA-salt rats, with a concomitant reduction of ET-1 content. These results suggest that vasopressin increases vascular superoxide levels by stimulating ET-1 formation in mineralocorticoid hypertension, and that V1-vasopressin receptors play an important role in this process.

Effect of the vasopressin receptor antagonist conivaptan in rats with heart failure following myocardial infarction

Myocardial infarction often induces congestive heart failure accompanied by a significant increase in plasma vasopressin concentration. To delineate the role of vasopressin in the pathogenesis of congestive heart failure, the acute hemodynamic and aquaretic effects of conivaptan (YM087, 4'-(2-methyl-1,4,5,6-tetrahydroimidazo[4,5-d][1]benzoazepine-6-carbonyl)-2-phenylbenzanilide monohydrochloride), a combined vasopressin V(1A) and V(2) receptor antagonist, were assessed in rats with heart failure induced by myocardial infarction. Left coronary artery ligation resulted in decreased left ventricular systolic pressure and first derivatives of left ventricular developed pressure, as well as increased left ventricular end-diastolic pressure, lung and right ventricular weight. Single oral administration of conivaptan (0.3 to 3.0 mg/kg) dose-dependently increased urine volume and decreased urine osmolality in heart failure rats. Furthermore, conivaptan (3.0 mg/kg) attenuated the changes in left ventricular end-diastolic pressure, lung and right ventricular weight induced by heart failure while reducing blood pressure. These results show that vasopressin plays a significant role in elevating vascular tone through vasopressin V(1A) receptors and plays a major role in retaining free water through vasopressin V(2) receptors in this model of congestive heart failure. Additionally, conivaptan, with its dual vasopressin V(1A) and V(2) receptor-inhibiting properties, could exert a beneficial effect on cardiac function in the congestive heart failure rat model.

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.

The hypothalamus and hypertension

Most forms of hypertension are associated with a wide variety of functional changes in the hypothalamus. Alterations in the following substances are discussed: catecholamines, acetylcholine, angiotensin II, natriuretic peptides, vasopressin, nitric oxide, serotonin, GABA, ouabain, neuropeptide Y, opioids, bradykinin, thyrotropin-releasing factor, vasoactive intestinal polypeptide, tachykinins, histamine, and corticotropin-releasing factor. Functional changes in these substances occur throughout the hypothalamus but are particularly prominent rostrally; most lead to an increase in sympathetic nervous activity which is responsible for the rise in arterial pressure. A few appear to be depressor compensatory changes. The majority of the hypothalamic changes begin as the pressure rises and are particularly prominent in the young rat; subsequently they tend to fluctuate and overall to diminish with age. It is proposed that, with the possible exception of the Dahl salt-sensitive rat, the hypothalamic changes associated with hypertension are caused by renal and intrathoracic cardiopulmonary afferent stimulation. Renal afferent stimulation occurs as a result of renal ischemia and trauma as in the reduced renal mass rat. It is suggested that afferents from the chest arise, at least in part, from the observed increase in left auricular pressure which, it is submitted, is due to the associated documented impaired ability to excrete sodium. It is proposed, therefore, that the hypothalamic changes in hypertension are a link in an integrated compensatory natriuretic response to the kidney's impaired ability to excrete sodium.

The basic and clinical pharmacology of nonpeptide vasopressin receptor antagonists

The neurohypophysial hormone arginine vasopressin (AVP) is a cyclic nonpeptide whose actions are mediated by the stimulation of specific G protein--coupled membrane receptors pharmacologically classified into V1-vascular (V1R), V2-renal (V2R) and V3-pituitary (V3R) AVP receptor subtypes. The random screening of chemical compounds and optimization of lead compounds recently resulted in the development of orally active nonpeptide AVP receptor antagonists. Potential therapeutic uses of AVP receptor antagonists include (a) the blockade of V1-vascular AVP receptors in arterial hypertension, congestive heart failure, and peripheral vascular disease; (b) the blockade of V2-renal AVP receptors in the syndrome of inappropriate vasopressin secretion, congestive heart failure, liver cirrhosis, nephrotic syndrome and any state of excessive retention of free water and subsequent dilutional hyponatremia; (c) the blockade of V3-pituitary AVP receptors in adrenocorticotropin-secreting tumors. The pharmacological and clinical profile of orally active nonpeptide vasopressin receptor antagonists is reviewed here.

Role of endothelin and vasopressin in DOCA-salt hypertension

1. The relative roles of endothelin (ET) and vasopressin (AVP) in the regulation of blood pressure (BP), cardiac output (CO) and total peripheral resistance (TPR) were investigated in the early stages (24 - 31 days) of development of hypertension in the conscious deoxycorticosterone acetate (DOCA)-salt hypertensive rat model. 2. BP was recorded with radiotelemetry devices and CO with ultrasonic transit-time probes. TPR was calculated from the BP and CO recordings. The contributions of endogenous ET and AVP were studied by infusing [d(CH(2))(5)(1),O-Me_Tyr(2),Arg(8)]-vasopressin, a V(1)-receptor antagonist, and bosentan, a mixed ET(A)/ET(B) receptor antagonist (Study 1). Vascular responsiveness was estimated from the changes in TPR evoked by i.v. infusions of ET-1 and AVP (Study 2). 3. In study 1, infusion of bosentan reduced TPR and BP dramatically in DOCA-salt hypertensive rats but not in SHAM control rats, and this effect was greater when the AVP system had been blocked. In contrast, the V(1) receptor antagonist alone failed to change TPR and BP in DOCA-salt hypertensive rats. However, subsequent infusion of the V(1) receptor antagonist during the plateau phase of the response in bosentan pretreated DOCA-salt hypertensive rats led to significant decreases in both BP and TPR. 4. In study 2, TPR and BP responses to ET-1, but not AVP, were greater in DOCA-salt rats than in control rats. CO responses to ET-1 or AVP were similar in the two groups. 5. The results suggest that both ET and AVP play a role in the maintenance of TPR and BP; when one system is blocked the other compensates. However, the magnitude of the contribution to the hypertensive state appears greater for ET than for AVP. Enhanced vascular responses to ET appear to contribute to this greater role.

Vasopressin V1a receptor antagonism does not reverse adrenocorticotrophin-induced hypertension in the rat

1. The role of arginine vasopressin (AVP) was examined in adrenocorticotrophin (ACTH)-induced hypertension in Sprague-Dawley rats using the non-peptide AVP V1a receptor antagonist OPC 21268. 2. In an acute study, six rats were pretreated with ACTH for 11 days and direct arterial blood pressure (4 h), plasma osmolality and electrolyte concentrations were measured after OPC 21268 gavage. In a chronic study, 40 rats were randomly divided into four groups: (i) sham injection + sham gavage; (ii) ACTH + sham gavage; (iii) sham injection + OPC 21268; or (iv) ACTH + OPC-21268 for 16 days. Systolic blood pressure (SBP), water intake, urine volume (UV), urine osmolality and electrolytes, food intake, bodyweight and plasma osmolality and electrolyte concentrations were measured. 3. In the acute study, direct mean arterial blood pressure did not change with OPC 21268 (122+/-2 and 120+/-3 mmHg at 0 and 240 min, respectively). 4. In the chronic study, OPC 21268 did not affect ACTH-induced rises in blood pressure (from 125+/-2 (control) to 145+/-5 mmHg (group 4) compared with 122+/-3 (control) to 149+/-5 mmHg (group2)). Water intake and UV increased (from 29+/-2 to 83+/-6 mL/day; and from 5+/-1 to 36+/-5 mL/day, respectively) and the change in bodyweight decreased from 0+/-2 to -107+/-7 g. 5. These results suggest that AVP (at the V1a receptor) does not play a significant role in the maintenance of ACTH-induced hypertension.

Study of V(1)-vascular vasopressin receptor gene microsatellite polymorphisms in human essential hypertension

Vasopressin (AVP) actions on vascular tone and blood pressure are mainly mediated by the V(1)-vascular receptor (V(1)R). We recently reported the structure and functional expression of the human V(1)R cDNA and described the genomic characteristics, tissue expression, chromosomal localization, and regional mapping of the human V(1)R gene, AVPR1A. To test whether the V(1)R is a marker for human essential hypertension, we sequenced the human AVPR1A gene and its 5; upstream region and found several DNA microsatellite motifs. One (GT)(14)-(GA)(13)-(A)(8)microsatellite is located 2983 bp downstream of the transcription start site, within a 2.2 kbp intron interrupting the coding sequence of the receptor. Three other microsatellites are present in the 5; flanking DNA of the AVPR1A gene: a (GT)(25)dinucleotide repeat, a complex (CT)(4)-TT-(CT)(8)-(GT)(24)motif and a (GATA)(14)tetranucleotide repeat located respectively 3956 bp, 3625 bp and 553 bp upstream of the transcription start site. Analysis of these polymorphisms in 79 hypertensive and 86 normotensive subjects for the (GT)(14)-(GA)(13)-(A)(8)and the (GT)(25)motifs revealed a high percentage of heterozygosity but no difference in alleles frequencies between the two groups. A linkage study using the affected sib pair method and the (GT)(25)repeat in 446 hypertensive sib pairs from 282 French Caucasian pedigrees showed no excess of alleles sharing at the AVPR1A locus. No linkage was found in the subgroups of patients with early onset hypertension (diagnosis before age 40) or severe hypertension (diastolic blood pressure >/=100 mmHg or requirement for >/=two medications). These findings suggest that molecular variants of the V(1)R gene are not involved in unselected forms of essential hypertension.

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.

Effects of the nonpeptide V(1) vasopressin receptor antagonist SR49059 in hypertensive patients

We assessed the clinical and pharmacological profile of the orally active V(1) vascular vasopressin (AVP) receptor nonpeptide antagonist SR49059 (SR) during the osmotic stimulation of AVP release in hypertensive patients. In a double-blind crossover-versus-placebo study, 24 untreated stage I or II essential hypertensive patients (12 whites and 12 blacks) received a single 300 mg oral dose of SR 2 hours before the stimulation of AVP secretion with a 5% hypertonic saline infusion. Hemodynamic, humoral, and hormonal parameters were monitored for up to 28 hours after drug administration. SR did not alter blood pressure or heart rate before the saline infusion and did not reduce the blood pressure increment induced by the hypertonic saline infusion. However, the blood pressure peak at the end of the hypertonic saline infusion was slightly lower in the presence of SR (P=0.04). Heart rate was significantly faster between 4 and 6 hours after SR administration (P=0.02). The rise in plasma sodium and osmolality triggered by the saline infusion was not modified by SR, but AVP release was slightly greater in the presence of SR (P<0.0003). AVP-induced aggregation of blood platelets in vitro was significantly reduced by SR, with a peak effect 2 hours after drug administration that coincided with the SR peak plasma concentration. Plasma renin activity and aldosterone before and after the saline infusion were not modified by SR. Urine volume and osmolality were not altered by SR administration. SR effects were similar in the 2 ethnic groups as well as in salt-sensitive versus salt-resistant patients. In a situation of AVP osmotic release and volume expansion in hypertensive patients, a single oral dose of the V(1) vascular AVP receptor nonpeptide antagonist SR49059, which is able to block AVP-induced platelet aggregation, exerts a transient vasodilation effect that is not associated with a sustained blood pressure reduction. SR49059 is a pure V(1) vascular receptor antagonist that is devoid of V(2) renal receptor actions.

Blood pressure and small arteries in DOCA-salt-treated genetically AVP-deficient rats: role of endothelin

Hypertension is associated with structural and mechanical abnormalities of resistance arteries. We have recently reported that vasopressin may be involved in the blood pressure elevation and remodeling of resistance arteries in deoxycorticosterone acetate (DOCA)-salt hypertension, perhaps by modulating vascular endothelin-1 expression. We tested this hypothesis further by examining DOCA-salt hypertension in homozygous vasopressin-deficient Brattleboro (BB) rats in comparison with Long-Evans (LE; control) rats. Mesenteric resistance arteries (lumen <300 microm) were studied on pressurized myographs. After 5 weeks, systolic blood pressure was greater in LE DOCA-salt-treated rats (189+/-5 mm Hg) compared with uniephrectomized (UNx) LE control rats (117+/-4 mm Hg; P<0.01). The increase in blood pressure induced by DOCA-salt treatment was attenuated in vasopressin-deficient rats, such that BB DOCA-salt-treated rats exhibited only a slight elevation of blood pressure (134+/-6 mm Hg) compared with BB-UNx rats (111+/-4 mm Hg; P<0.05). Resistance arteries in LE DOCA-salt-treated rats had a smaller lumen diameter and a larger media width, media cross-sectional area, and media-lumen ratio compared with LE-UNx rats. Isobaric stiffness was unaltered in resistance arteries from LE DOCA-salt-treated rats, despite stiffening of the arterial wall components as indicated by a significant increase in the slope of the media stress-incremental elastic modulus relationship. DOCA-salt treatment in the absence of endogenous vasopressin, ie, in homozygous di/di BB rats, failed to alter vascular structure or wall component stiffness and resulted in a lesser degree of blood pressure elevation. Reverse transcription-polymerase chain reaction analysis revealed that DOCA-salt treatment enhanced endothelin gene expression in LE rats but failed to do so in BB rats. These data indicate that vasopressin plays a critical role in modulating vascular structure and mechanics, as well as blood pressure, in DOCA-salt-induced hypertension. Moreover, these effects of vasopressin are in part mediated by enhancement of endothelin expression.

In vivo and in vitro characterisation of a nonpeptide vasopressin V(1A) and V(2) receptor antagonist (YM087) in the rat

This paper reports the in vitro and in vivo characterisation of a nonpeptide, orally active, vasopressin V(1A) and V(2) receptor antagonist, YM087 (methyl-1,4,5,6-tetrahydroimidazo[4, 5-d][1]benzoazepine-6-carbonyl)-2-phenylbenzanilide monohydrochloride) in the rat. YM087 dose dependently displaced the vasopressin V(1A) receptor antagonist radioligand, 125I-labelled [d(CH(2))(5),sarcosine(7)]vasopressin at vasopressin V(1A) receptors in liver and kidney medulla membranes and caused a concentration dependent displacement of the vasopressin V(2) receptor antagonist radioligand [3H]desGly-NH(2)(9)[d(CH(2))(5), D-Ile(2), Ile(4)]vasopressin at vasopressin V(2) receptors in kidney medulla membranes. In vitro binding kinetic studies showed YM087 acted as a competitive antagonist at liver V(1A) and kidney V(1A) and V(2) vasopressin receptors. Oral administration of YM087 (0.1-3 mg/kg) dose dependently inhibited vasopressin binding to liver V(1A) and kidney V(1A) and V(2) vasopressin receptors over 24 h. Oral YM087 (1-3 mg/kg/day) for 7 days in normotensive rats caused a dose dependent aquaresis with no effect on systolic blood pressure. These results show that YM087 is an orally effective vasopressin V(1A) and V(2) receptor antagonist that may be useful in the treatment of conditions characterised by vasoconstriction and fluid retention such as congestive heart failure.

Pharmacological profile of orally administered YM087, a vasopressin antagonist, in conscious rats

1. YM087 is a newly synthesized non-peptide arginine vasopressin (AVP) antagonist that shows high affinity for both V1A and V2 receptors. In the present study, the V1A and V2 receptor antagonist effects of orally administered YM087 were assessed in conscious rats. 2. In conscious rats, orally administered YM087 (0.1, 0.3 and 1.0 mg/kg) did not affect basal blood pressure, but YM087 dose-dependently inhibited 30 mU/kg, i.v., AVP-induced pressor responses. This inhibition lasted for over 8 h following the oral administration of the highest dose of YM087 (1 mg/kg). 3. In rats deprived of water and food for 16-18 h, oral administration of YM087 (0.1, 0.3, 1 and 3 mg/kg) dose-dependently increased urine volume and reduced urine osmolality, with associated increases in urinary sodium and potassium excretion. However, these increases in electrolyte excretion were lower than those seen at comparable diuretic doses of furosemide (3, 10, 30 and 100 mg/kg, p.o.). 4. Oral administration of YM087 (0.3, 1 and 3 mg/kg) produced a dose-dependent increase in urine volume in rats allowed free access to water, with the diuretic effect peaking 2-4 h post-dosing at all dose levels. The diuretic effect of YM087 was sustained 8-10 h after a dose of 3 mg/kg; this is in contrast with the transient diuresis seen after furosemide (100 mg/kg, p.o.) dosing. 5. The present results demonstrate that YM087 is an orally active AVP antagonist with potent and long-lasting effects. YM087 suppressed V1A receptor-mediated pressor responses to AVP with minimal effects on basal haemodynamics and exerted a diuretic effect without increased electrolyte excretion by inhibiting V2 receptor-mediated water reabsorption.

Association of arginine vasopressin and arterial blood pressure in a population-based sample

BACKGROUND

The role of arginine vasopressin (AVP) in the development and maintenance of arterial hypertension is controversial. Furthermore, it remains unclear whether chronic treatment with antihypertensive agents modulates levels of AVP, with potential secondary effects on vascular tone and fluid homeostasis.

OBJECTIVE

To investigate the relation between plasma AVP and arterial blood pressure in a population-based sample of 534 middle-aged subjects.

RESULTS

Overall, levels of AVP were higher in hypertensive subjects (2.15 +/- 0.26 pg/ml; n = 289) than in normotensive subjects (1.45 +/- 0.15 pg/ml; n = 245; P < 0.05). (Hypertension was defined as systolic blood pressure > 160 mmHg and/or diastolic blood pressure > 95 mmHg, or receiving antihypertensive medication.) In untreated individuals, plasma levels of AVP were found to be correlated with both systolic (r = 0.15, P = 0.002) and diastolic (r = 0.14, P = 0.005) blood pressure. The differences between the lowest and highest quartile of AVP levels were 5.1 mmHg (P = 0.03) and 2.6 mmHg (NS) for systolic and diastolic blood pressure, respectively, after adjustment for age and sex. Moreover, it appeared that the relation between AVP and blood pressure was particularly strong in subjects with low levels of renin (< 10 mU/l; n = 118; systolic blood pressure r = 0.24, P = 0.007; diastolic blood pressure r = 0.19, P = 0.03). Specifically, patients receiving monotherapy with diuretics (n = 39) or beta-blockers (n = 54) displayed elevated plasma levels of AVP (2.93 +/- 0.98 pg/ml and 2.74 +/- 0.74 pg/ml respectively); however, only in patients taking diuretics was this finding apparently independent of confounding variables. Other monotherapies with angiotensin-converting enzyme inhibitors (n = 9) or Ca(2+)-antagonists (n = 19) were not associated with levels of AVP.

CONCLUSION

The data suggest that plasma levels of AVP display a discernible relationship with arterial blood pressure and hypertension, particularly when renin levels are low. In addition, with the exception of diuretics, no modulation of AVP levels is attributable to the intake of antihypertensive agents as it occurs in a population-based sample.

Effect of vasopressin antagonism on structure and mechanics of small arteries and vascular expression of endothelin-1 in deoxycorticosterone acetate salt hypertensive rats

The structural and mechanical properties of small arteries are altered in rat models of hypertension. The precise role of humoral factors in these changes has not been determined. In deoxycorticosterone acetate (DOCA) salt hypertension, endothelin-1 (ET-1) peptide content and gene expression are enhanced in mesenteric resistance arteries. These vessels also present augmented vasoconstrictor responsiveness to vasopressin versus control uninephrectomized rats. To determine whether an interaction exists between vasopressin and ET-1 in the pathogenesis of small-artery structural alterations in DOCA-salt rats, we examined the effect of chronic V1 vasopressin receptor antagonism (OPC-21268, 30 mg/kg BID) on the structure and mechanical properties of mesenteric resistance arteries using a pressure myograph and the effect on preproendothelin-1 (preproET-1) gene expression, determined by Northern blot analysis of preproET-1 mRNA. Tail-cuff systolic pressures were elevated in DOCA-salt (200+/-11 mm Hg) versus uninephrectomized rats (109+/-4 mm Hg) and decreased slightly but significantly by OPC-21268 to 187+/-7 mm Hg (P<0.01). Treatment with DOCA-salt increased vascular media-lumen ratios and media cross-sectional areas and reduced both stress and incremental elastic modulus for a given pressure. However, there was no change in distensibility or incremental elastic modulus versus media stress. OPC-21268 partially attenuated the vascular growth in DOCA-salt rats. PreproET-1 mRNA was increased 2-fold in mesenteric arteries of DOCA-salt rats versus uninephrectomized rats, an effect abrogated by OPC-21268. Thus, DOCA-salt hypertension is associated with altered morphology of the small-arterial wall, without altering stiffness of the arterial wall components. OPC-21268 regressed in part these changes, suggesting the involvement of vasopressin. The concomitant attenuation of enhanced ET-1 expression by OPC-21268 suggests that ET-1 may be involved in mediating in part the vascular effects of vasopressin in DOCA-salt hypertensive rats.

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.

New hormonal blockade strategies in cardiovascular disease

The circulation is controlled by overlapping haemodynamic, structural and neurohumoral mechanisms. Many hormonal vasoactive substances, mostly derived from endothelial cells, are also growth regulators. Although neurohormonal systems are involved in normal physiological compensatory responses they often become maladaptive in conditions such as congestive heart failure. The success of blocking the renin angiotensin system by angiotensin converting enzyme (ACE) inhibitors has led to efforts to block other hormonal systems. Neutral endopeptidase (NEP), the major enzymatic pathway for degradation of natriuretic peptides, has a similar catalytic site to ACE. This has led to compounds that simultaneously inhibit both enzymes. Such dual ACE/NEP inhibitors show promise in experimental hypertension and heart failure. Similar dual NEP/ECE (endothelin converting enzyme) inhibitors are becoming available. The hormone vasopressin has dual actions on the vasculature and the kidney via specific membrane receptors. Specific orally active vasopressin receptor antagonists have been developed and their therapeutic potential in hypertension, heart failure and oedematous states are being explored.

Benzamil blockade of brain Na+ channels averts Na(+)-induced hypertension in rats

To determine the possible involvement of brain amiloride-sensitive Na+ channels in Na(+)-induced hypertension, we investigated the effects of benzamil hydrochloride, a specific blocker of these Na+ channels, on the acute pressor mechanisms of intracerebroventricular infusion of hypertonic NaCl and the continuous pressor mechanisms of Na(+)-induced chronic hypertension, such as deoxycorticosterone acetate-salt hypertensive or stroke-prone spontaneous hypertensive rats, and of non-Na(+)-induced hypertension, such as renovascular hypertensive rats. Intracerebroventricular preinjection with benzamil (1 or 10 nmol/kg) abolished the increase in mean arterial pressure, heart rate, abdominal sympathetic discharge, and plasma vasopressin concentration induced by an acute increase in cerebrospinal Na+ concentrations at intracerebroventricular infusion of 1.5 M hypertonic NaCl. Continuous intracerebroventricular infusion of benzamil (1 or 10 nmol.kg-1.day-1) for 7 days attenuated Na(+)-induced chronic hypertension in both deoxycorticosterone acetate-salt and stroke-prone spontaneous hypertensive rats, accompanied by reduction of urinary excretion of vasopressin and norepinephrine but not in renovascular hypertensive rats. Intravenous infusion of benzamil (10 nmol.kg-1.day-1) for 7 days affected neither arterial pressure nor urinary excretion of vasopressin and norepinephrine in either model of hypertension. Benzamil-blockable brain amiloride-sensitive Na+ channels are expected to function as one of the Na+ receptors in the brain and to be involved in the pressor mechanism of Na(+)-induced hypertension.

Effects of SR 49059, a new orally active and specific vasopressin V1 receptor antagonist, on vasopressin-induced vasoconstriction in humans

We have evaluated the efficacy of SR 49059, a new orally active and specific vasopressin V1 receptor antagonist (arginine-vasopressin [AVP]), in the blockade of the vascular effects of exogenous AVP in healthy subjects. In preliminary experiments, two procedures to measure the V1 vascular effects of AVP were assessed. First, the AVP-induced changes in skin blood flow were investigated by the injection of increasing doses of AVP intradermally, with or without a previous local vasodilation with calcitonin gene-related peptide (CGRP). In a second protocol, AVP was infused intra-arterially, and the changes in radial artery diameter and blood flow were measured. The intradermal injection of AVP caused significant decreases in skin blood flow, and the use of CGRP increased the sensitivity of the method by a factor of 10(2) to 10(3). AVP infused intra-arterially caused dose-dependent decreases in the radial artery diameter and blood flow. In the main study, the potency and efficacy of SR 49059 to block the AVP-induced changes in skin blood flow were assessed in 12 healthy men with a double-blind, triple crossover study design. The subjects were randomized to receive a placebo orally and 30 mg and 300 mg of the antagonist at a 1-week interval. The subjects were then further randomized to evaluate the efficacy of the same doses of the antagonist to block the vasoconstriction of the radial artery induced by an intra-arterial infusion of AVP. SR 49059 inhibits, dose-dependently and significantly, the AVP-induced changes in skin blood flow, with a peak effect occurring between 2 and 6 hours after injection. In addition, the 300-mg dose of SR 49059 completely blocked the vasoconstriction of the radial artery induced by AVP. In conclusion, skin blood-flow measurement, after intradermal injection of AVP on a skin area vasodilated with CGRP, is an effective method to investigate the V1 vascular effect of AVP in humans. SR 49059 is a potent and specific antagonist of V1 receptors, which blocks the AVP-induced vasoconstriction.

Effects of OPC-21268, a vasopressin V1-receptor antagonist, on expression of growth factors from glomeruli in spontaneously hypertensive rats

To assess the chronic in vivo effects of OPC-21268, a vasopressin-V1 receptor antagonist, on renal injury, we investigated the mRNA expressions of platelet-derived growth factor (PDGF) B-chain, transforming growth factor (TGF)-beta1 and proliferating cell nuclear antigen (PCNA) in the glomeruli of spontaneously hypertensive rats (SHR) treated with OPC-21268 for 3 weeks. SHR aged 10 weeks were given 2% NaCl in drinking water for 3 weeks. The OPC group was fed a 0.5% OPC-21268-containing diet for 3 weeks and the control group was given a normal diet. There were no significant changes in the time course of systolic blood pressure, heart rate, urine volume, or urinary sodium, protein and N-acetyl-beta-glucosaminidase (NAG) excretion between the two groups. Serum electrolytes, protein and creatinine levels also did not differ between the groups. The mRNA expressions of PDGF B-chain, TGF-beta1 and PCNA in the glomerulus were examined using reverse transcriptase-polymerase chain reaction (RT-PCR) methods. The mRNA expressions of PDGF B-chain and PCNA among these were significantly suppressed in the OPC group. No significant differences in renal histology including the organ weights were found between the two groups; however, the glomerular size tended to be enlarged in the OPC group. These findings suggest that chronic V1-receptor blockade directly inhibits the glomerular proliferative injury of salt-loaded SHR at the established hypertension stage.

Chronic vasopressin antagonism in two-kidney, one-clip renovascular hypertension

The role of vasopressin (AVP) in the maintenance of hypertension in the rat model of two-kidney, one-clip (2K1C) Goldblatt hypertension was assessed using the nonpeptide orally effective V1a receptor antagonist, OPC-21268. Rats were studied eight weeks after surgery when mean arterial pressure (MAP) was significantly increased in 2K1C rats compared to SHAM operated controls (2K1C 139 +/- 6, SHAM 106 +/- 3, P < 0.01). Neither acute (OPC-21268, 30 mg/kg) nor chronic (OPC-21268, 30 mg/kg twice daily) V1a receptor blockade reduced blood pressure in either 2K1C or SHAM rats. The results of binding kinetic studies confirmed that OPC-21268 was effective at its putative site of action, the V1a receptor in both 2K1C and SHAM rats. These results indicate AVP is not involved in the maintenance of blood pressure in the 2K1C model of renovascular hypertension.

Endothelium-dependent and NO-mediated desensitization to vasopressin in rat aorta

1. The present study was performed to characterize the tachyphylaxis of rat aortae to vasopressin. Isometric tension generated by rat thoracic aorta sliced in 4 mm rings, was recorded. 2. Tension generated by intact rings increased with cumulative additions of vasopressin up to 10 nM (1.51 +/- 0.15 g). After this concentration, most rings lost their tension and relaxed to 1.09 +/- 0.17 g (P < 0.001) despite further addition of vasopressin. This tachyphylaxis was not observed in endothelium-denuded rings (from 2.87 +/- 0.12 g to 2.68 +/- 0.17 g). 3. Repeated administrations of supramaximal concentration (100 nM) of vasopressin confirmed an enhanced desensitization in intact rings, compared to endothelium-denuded rings. No desensitization to phenylephrine was observed in intact or in endothelium-denuded rings. 4. Dose-response curves to a V1 receptor agonist, [Phe2, Ile3, Orn8]-vasopressin, and to a V2 receptor agonist, [deamino-Cys1,D-Arg8]-vasopressin, were performed in intact preparations. An increase in tension, followed by a desensitization was observed with the V1 receptor agonist. In contrast, the V2 receptor agonist did not induce any response. 5. Pretreatment of intact aortic rings with the cyclo-oxygenase inhibitor, diclofenac (1 microM), did not prevent the desensitization to vasopressin. In contrast, NO synthase inhibition with NG-nitro-L-arginine (30 microM) resulted in an attenuated desensitization to vasopressin in intact rings (from 2.46 +/- 0.17 to 2.25 +/- 0.22 g, NS). 6. To confirm the involvement of NO, endothelium-denuded rings were pretreated with sodium nitroprusside (SNP). At a concentration of 10 nM, SNP induced a desensitization to vasopressin comparable with that observed in intact rings. 7. Pretreatment of endothelium-denuded rings with 8-bromo-cyclic GMP (100 microM) reduced maximum contraction to vasopressin without producing any desensitization. In contrast, guanylate cyclase inhibition with either LY 83,583 (10 microM) or methylene blue (10 microM) blocked completely the desensitization of intact rings to vasopressin. 8. The results suggest that the endothelium-dependent tachyphylaxis to vasopressin is due to rapid desensitization and is mediated by NO. However, it is unclear whether this effect of NO involves cyclic GMP.

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

Vasopressin has been implicated in the pathogenesis of heart failure as one of the most potent vasoconstrictors. However, whether the increase in plasma vasopressin levels modifies the pathophysiology of heart failure remains unknown. To investigate the effect of long-term inhibition of vasopressin in the development of heart failure, we administered a selective, orally effective, nonpeptide vasopressin antagonist, the V1 receptor antagonist OPC-21268 (100 mg center dot kg-1 center dot day-1) or a V2 receptor antagonist, OPC-31260 (20 mg center dot kg-1 center dot day-1) to rats with heart failure induced by the creation of an aortocaval fistula (AVF) and to sham-operated rats for 4 weeks, beginning on the first postoperative day. The heart failure in this experiment was characterized by an increase in the weights of the right and left ventricles, the lungs, and the right and left appendage, increase in left ventricular end-diastolic pressure (LVEDP), increase in right ventricular systolic pressure (RVSP), increase in right atrial pressure (RAP), and an increase in the plasma level of atrial natriuretic peptide (ANP) as compared with no change in sham-operated rats. There were no differences in shunt ratio between treated and untreated heart failure groups. Chronic administration of the V2 receptor antagonist OPC-31260 significantly reduced the weight of the right ventricle (1.17 +/- 0.39 vs. 0.90 +/- 0.13 g/kg, p < 0.05), RVSP (53 +/- 18 vs. 39 +/- 4 mm Hg, p < 0.05), LVEDP (11.8 +/- 5.2 vs. 6.5 +/- 2.8 mm Hg, p < 0.05) and the plasma concentrations of ANP (554 +/- 271 vs. 193 +/- 39 pg/ml, p < 0.05) as compared with the values of rats with untreated HF. Chronic treatment with the V1 receptor antagonist OPC-21268 did not alter hemodynamics, organ weights, or hormone concentrations. These results suggest that vasopressin did not contribute mainly to the maintenance of systemic hemodynamics through the V1 receptor in this heart failure model. Vasopressin may play a role, at least in part, in the fluid retention in the development of heart failure through the V2 receptor. OPC-31260 may present a new approach to the treatment of heart failure.

Attenuation of genetic hypertension after short-term vasopressin V1A receptor antagonism

Abnormalities of the vasopressin system are found in genetic hypertension. This study compares the delayed effects of a brief period of vasopressin V1A receptor blockade and angiotensin-converting enzyme inhibition in young female and male spontaneously hypertensive rats (SHR) on the development of hypertension in adult life. In a separate study, the role of vasopressin in the maintenance of blood pressure in adult SHR was assessed. Young SHR received either the nonpeptide vasopressin V1A receptor antagonist OPC-21268, the angiotensin-converting enzyme inhibitor ramipril, or vehicle from 6 to 10 weeks of age. During the treatment period, OPC-21268 and ramipril reduced systolic blood pressure compared with control SHR (P < .001). Blood pressure in male SHR 7 weeks after treatment withdrawal was 178 +/- 1 mm Hg in ramipril-treated, 184 +/- 1 mm Hg in OPC-21268-treated, and 200 +/- 2 mm Hg in control SHR (P < .001). Similar results were seen in female SHR, although both OPC-21268 and ramipril were less effective antihypertensive agents in female compared with male SHR. The sustained attenuation in blood pressure was not associated with significant cardiovascular structural changes (left ventricular-to-body weight ratio, renal weight-to-body weight ratio, mesenteric resistance artery media-to-lumen ratio). Results of vasopressin V1A receptor binding kinetics and plasma renin or aldosterone concentrations did not suggest a lasting effect of OPC-21268 on the vasopressin system or of ramipril on the renin-angiotensin system following treatment withdrawal.(ABSTRACT TRUNCATED AT 250 WORDS)

Beyond ACE inhibition: new developments in drug therapy for hypertension

The growing list of vasoactive substances known to be involved in blood pressure control provides new targets for antihypertensive drugs. Currently under development are alternative strategies for blockade of the renin-angiotensin system (e.g., renin inhibition and angiotensin II receptor antagonism) that may have fewer side effects than angiotensin-converting-enzyme inhibition, and antagonists to other vasocontrictor peptides, such as endothelin and vasopressin. Novel strategies to enhance the effects of endogenous vasodilators, such as natriuretic peptides and nitric oxide, are also being explored.