OPC-21268, a vasopressin V1 antagonist, produces hypotension in spontaneously hypertensive rats

The Role of Arginine-Vasopressin in Stroke and the Potential Use of Arginine-Vasopressin Type 1 Receptor Antagonists in Stroke Therapy: A Narrative Review

Stroke is a life-threatening condition in which accurate diagnoses and timely treatment are critical for successful neurological recovery. The current acute treatment strategies, particularly non-invasive interventions, are limited, thus urging the need for novel therapeutical targets. Arginine vasopressin (AVP) receptor antagonists are emerging as potential targets to treat edema formation and subsequent elevation in intracranial pressure, both significant causes of mortality in acute stroke. Here, we summarize the current knowledge on the mechanisms leading to AVP hyperexcretion in acute stroke and the subsequent secondary neuropathological responses. Furthermore, we discuss the work supporting the predictive value of measuring copeptin, a surrogate marker of AVP in stroke patients, followed by a review of the experimental evidence suggesting AVP receptor antagonists in stroke therapy. As we highlight throughout the narrative, critical gaps in the literature exist and indicate the need for further research to understand better AVP mechanisms in stroke. Likewise, there are advantages and limitations in using copeptin as a prognostic tool, and the translation of findings from experimental animal models to clinical settings has its challenges. Still, monitoring AVP levels and using AVP receptor antagonists as an add-on therapeutic intervention are potential promises in clinical applications to alleviate stroke neurological consequences.

Vasopressin and v1br gene expression is increased in the hypothalamic pvn of borderline hypertensive rats

Vasopressin (VP) is a neurohypophyseal peptide best known for its role in maintaining osmotic and cardiovascular homeostasis. The main sources of VP are the supraoptic and paraventricular (PVN) nuclei of the hypothalamus, which coexpress the vasopressin V1a and V1b receptors (V1aR and V1bR). Here, we investigated the level of expression of VP and VP receptors in the PVN of borderline hypertensive rats (BHRs), a key integrative nucleus for neuroendocrine cardiovascular control. Experiments were performed in male BHRs and Wistar rats (WRs) equipped with a radiotelemetry device for continuous hemodynamic recording under baseline conditions and after saline load without or with stress. Autonomic control of the circulation was evaluated by spectral analysis of blood pressure (BP) and heart rate (HR) variability and baroreceptor reflex sensitivity (BRS) using the sequence method. Plasma VP was determined by radioimmunoassay, and VP, V1aR, and V1bR gene expression was determined by RT-qPCR. Under baseline conditions, BHRs had higher BP, lower HR, and stronger BRS than WRs. BP and HR variability was unchanged. In the PVN, overexpression of the VP and V1bR genes was found, and plasma VP was increased. Saline load downregulated V1bR mRNA expression without affecting VP mRNA expression or plasma VP and BP. Adding stress increased BP, HR, and low-frequency sympathetic spectral markers and decreased plasma VP without altering the level of expression of VP and VP receptors in the PVN. It follows that overexpression of VP and V1bR in the PVN is a characteristic trait of BHRs and that sympathetic hyperactivity underlies stress-induced hypertension.

Vasopressin & Oxytocin in Control of the Cardiovascular System: An Updated Review

Since the discovery of vasopressin (VP) and oxytocin (OT) in 1953, considerable knowledge has been gathered about their roles in cardiovascular homeostasis. Unraveling VP vasoconstrictor properties and V1a receptors in blood vessels generated powerful hemostatic drugs and drugs effective in the treatment of certain forms of circulatory collapse (shock). Recognition of the key role of VP in water balance via renal V2 receptors gave birth to aquaretic drugs found to be useful in advanced stages of congestive heart failure. There are still unexplored actions of VP and OT on the cardiovascular system, both at the periphery and in the brain that may open new venues in treatment of cardiovascular diseases. After a brief overview on VP, OT and their peripheral action on the cardiovascular system, this review focuses on newly discovered hypothalamic mechanisms involved in neurogenic control of the circulation in stress and disease.

Renal vasoconstriction by vasopressin V1a receptors is modulated by nitric oxide, prostanoids, and superoxide but not the ADP ribosyl cyclase CD38

Renal blood flow (RBF) responses to arginine vasopressin (AVP) were tested in anesthetized wild-type (WT) and CD38(-/-) mice that lack the major calcium-mobilizing second messenger cyclic ADP ribose. AVP (3-25 ng) injected intravenously produced dose-dependent decreases in RBF, reaching a maximum of 25 ± 2% below basal RBF in WT and 27 ± 2% in CD38(-/-) mice with 25 ng of AVP. Renal vascular resistance (RVR) increased 75 ± 6% and 78 ± 6% in WT and CD38(-/-) mice. Inhibition of nitric oxide (NO) synthase with nitro-L-arginine methyl ester (L-NAME) increased the maximum RVR response to AVP to 308 ± 76% in WT and 388 ± 81% in CD38(-/-) (P < 0.001 for both). Cyclooxygenase inhibition with indomethacin increased the maximum RVR response to 125 ± 15% in WT and 120 ± 14% in CD38(-/-) mice (P < 0.001, <0.05). Superoxide suppression with tempol inhibited the maximum RVR response to AVP by 38% in both strains (P < 0.005) but was ineffective when administered after L-NAME. The rate of RBF recovery (relaxation) after AVP was slowed by L-NAME and indomethacin (P < 0.001, <0.005) but was unchanged by tempol. All vascular responses to AVP were abolished by an AVP V1a receptor antagonist. A V2 receptor agonist or antagonist had no effect on AVP-induced renal vasoconstriction. Taken together, the results indicate that renal vasoconstriction by AVP in the mouse is strongly buffered by vasodilatory actions of NO and prostanoids. The vasoconstriction depends on V1a receptor activation without involvement of CD38 or concomitant vasodilatation by V2 receptors. The role of superoxide is to enhance the contractile response to AVP, most likely by reducing the availability of NO rather than directly stimulating intracellular contraction signaling pathways.

Vasopressin and oxytocin in control of the cardiovascular system

Vasopressin (VP) and oxytocin (OT) are mainly synthesized in the magnocellular neurons of the paraventricular (PVN) and supraoptic nucleus (SON) of the hypothalamus. Axons from the magnocellular part of the PVN and SON project to neurohypophysis where VP and OT are released in blood to act like hormones. Axons from the parvocellular part of PVN project to extra-hypothalamic brain areas (median eminence, limbic system, brainstem and spinal cord) where VP and OT act like neurotransmitters/modulators. VP and OT act in complementary manner in cardiovascular control, both as hormones and neurotransmitters. While VP conserves water and increases circulating blood volume, OT eliminates sodium. Hyperactivity of VP neurons and quiescence of OT neurons in PVN underlie osmotic adjustment to pregnancy. In most vascular beds VP is a potent vasoconstrictor, more potent than OT, except in the umbilical artery at term. The vasoconstriction by VP and OT is mediated via V1aR. In some vascular beds, i.e. the lungs and the brain, VP and OT produce NO dependent vasodilatation. Peripherally, VP has been found to enhance the sensitivity of the baro-receptor while centrally, VP and OT increase sympathetic outflow, suppresse baro-receptor reflex and enhance respiration. Whilst VP is an important mediator of stress that triggers ACTH release, OT exhibits anti-stress properties. Moreover, VP has been found to contribute considerably to progression of hypertension and heart failure while OT has been found to decrease blood pressure and promote cardiac healing.

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.

Increased expression of ENaC subunits and increased apical targeting of AQP2 in the kidneys of spontaneously hypertensive rats

In models of genetic hypertension, renal tubular dysfunction could be involved in the increased sodium and water reabsorption. However, the molecular basis for the increased renal sodium and water retention remains largely undefined in spontaneously hypertensive rats (SHR). We hypothesized that dysregulation of renal epithelial sodium channels (ENaC), sodium (co)transporters, or aquaporin-2 (AQP2) could be involved in the pathogenesis of hypertension in SHR. Six-week-old or twelve-week-old SHR and corresponding age-matched Wistar-Kyoto control rats (WKY) were studied. In both SHR groups, systolic blood pressure was markedly increased, whereas urine output, creatinine clearance, and urinary sodium excretion were decreased compared with corresponding WKY. Moreover, urine osmolality and urine-to-plasma osmolality ratio were increased compared with WKY. Semiquantitative immunoblotting demonstrated that the protein abundance of β- and γ-subunits of ENaC was increased in the cortex and outer stripe of the outer medulla and inner stripe of the outer medulla (ISOM) in SHR, whereas α-ENaC abundance was increased in ISOM. Immunoperoxidase microscopy confirmed the increased labeling of β-ENaC and γ-ENaC subunits in the late distal convoluted tubule, connecting tubule, and cortical and outer medullary collecting duct segments. In contrast, subcellular localization of α-ENaC, β-ENaC, and γ-ENaC was not changed. Expression of sodium/hydrogen exchanger type 3, bumetanide-sensitive Na-K-2Cl cotransporter, and thiazide-sensitive Na-Cl cotransporter was not altered in SHR. AQP2 levels were increased in the ISOM in SHR, and immunoperoxidase microscopy demonstrated an increased apical labeling of AQP2 in the inner medullary collecting duct in SHR. These results suggest that the increased protein abundance of ENaC subunits as well as the increased apical targeting of AQP2 may contribute to renal sodium and water retention observed during the development of hypertension in SHR.

Effects of Nonpeptide and Selective V1 and V2 Antagonists on Blood Pressure Short-Term Variability in Spontaneously Hypertensive Rats

Effects of V(1) (OPC-21268) and V(2) (OPC-31260) vasopressin antagonists on blood pressure (BP) short-term variability were investigated in adult spontaneously hypertensive rats (SHR) under basal conditions and after the stimulation of vasopressin release by hemorrhage. BP was recorded intra-arterially and sampled at 20 Hz to be analyzed on a personal computer. BP time spectra were calculated on 30 stationary overlapping 2048 point-time series. Spectral power was estimated in total (0.00976 - 3 Hz), very low frequency (VLF: 0.00976 - 0.195 Hz), low frequency (LF: 0.195 - 0.605 Hz), and high frequency (HF: 0.8 - 3 Hz) regions. Under basal conditions a V(1) antagonist (5 mg/kg, i.v.) decreased BP without affecting BP variability, while combined (V(1) + V(2)) blockade or V(2) blockade (1 mg/kg, i.v.) alone did not affect cardiovascular parameters. Mild hemorrhage (5 ml/kg per min) increased HF-BP variability, while moderate (10 ml/kg per min) and massive (15 ml/kg per min) hemorrhage did not affect it. In V(1), but not V(2), antagonist pre-treated SHR HF-BP increased significantly after moderate and massive hemorrhage. V(1) or V(2) antagonist pre-treatment also enhanced VLF-BP variability during massive hemorrhage. Moreover V(1) blockade prevented hemorrhage-induced bradycardia, while V(2) blockade potentiated it. It follows that in adult SHR, vasopressin buffers BP oscillations in HF and VLF frequency domains only in hypovolaemic conditions and that the modulation of the autonomic adjustment of the HR to hemorrhage by vasopressin is preserved.

Are sodium-dependent V1 receptors and sympathetic nerve activations involved in regulation of blood pressure in borderline-hypertensive Hiroshima rats?

Sympathetic nerve activity (SNA) was estimated by the magnitude of depressor response after ganglionic blockade with hexamethonium bromide (C6; 25 mg/kg weight). The depressor effects of C6 were significantly less in borderline-hypertensive Hiroshima rats (BHR) than in deoxycorticosterone acetate (DOCA)-salt hypertensive rats (DOCA rats) or in spontaneously hypertensive rats (SHR), but they were not different in BHR and normotensive control Wistar rats (NCR). After sympatho-inhibition, the depressor effects of a selective vasopressin V1 receptor antagonist (V1A; 10 microg/kg: [d(CH2)5(1), O-Me-Tyr2, Arg8]-vasopressin) were significantly greater in BHR than in DOCA rats, SHR or NCR. In a previous study, we reported that the depressor effects of C6 were significantly less in BHR than in SHR, but after sympatho-inhibition, the depressor effects of V1A were significantly greater in BHR than in SHR (Hypertens Res 2002; 25: 241-248). After high-salt diet loading in the present study (8% salt-containing diet for 10 weeks), the magnitudes of increase in mean arterial pressure in BHR and NCR were almost the same. There was almost no difference in the depressor effects of V1A after sympatho-inhibition between BHR with high-salt intake and BHR without high-salt intake. The depressor effects of an angiotensin-converting enzyme inhibitor, captopril (1 mg/kg), were almost the same between BHR and NCR both before and after sympatho-inhibition. However, these effects were completely inhibited after the high-salt diet. The results show that SNA was within the normal range in BHR and that no further accelerated responsiveness of endogenous vasopressin was observed in BHR after high-salt intake.

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.

Contribution of endogenous vasopressin to regional hemodynamics in borderline-hypertensive Hiroshima rats

Inbred borderline-hypertensive Hiroshima rats (BHR) of the Wistar strain established in our laboratory are characterized by elevated plasma levels of vasopressin (Teranishi et al.: Jpn J Pharmacol 1999; 79: 251-255). To investigate the role of endogenous vasopressin in hypertension in BHR, we assessed the effect of a selective vasopressin V1 receptor antagonist (V1A) on regional hemodynamics using an electromagnetic flowmeter. The basal values of mean arterial pressure, heart rate, and regional resistances of renal and hindquarter vascular beds in conscious BHR were significantly higher than those in normotensive control rats (NCR). Injection of V1A (10 microg/kg) did not appreciably affect the hemodynamics in either animal. Successive administration of V1A after ganglionic blockade with hexamethonium bromide (C6; 25 mg/kg), however, significantly attenuated renal, mesenteric and hindquarter resistances in BHR but not in NCR. The hypotensive effect of V1A after sympathoinhibition was significantly greater in BHR than in both NCR and spontaneously hypertensive rats. However, the hypotensive effect induced by V1A was diminished only in BHR pretreated with a ganglionic blockade followed by injection of captopril (1 mg/kg). These findings indicate that BHR could be used as a new hypertensive model with an abnormality in endogenous vasopressin-mediated vasoconstriction appearing in the presence of angiotensin II after sympathoinhibition.

Effects of nonpeptide V1a and V2 antagonists on blood pressure fast oscillations in conscious rats

This paper describes the effects of vasopressin nonpeptide selective V1a (OPC-21268) and V2 (OPC-31260) antagonists on fast blood pressure (BP) oscillations in conscious non-haemorrhaged and haemorrhaged rats. Equidistant sampling at 20 Hz allowed direct spectral analysis of BP on 30 overlapping 2048 point-time series. In non-haemorrhaged rats, V1a antagonist (5 mg/kg; i.v) reduced BP and low-frequency (LF-BP) component while subsequent administration of V2 antagonist (1 mg/kg; i.v) reversed these changes and enhanced the very low-frequency (VLF-BP) component. In haemorrhaged rats (5-15 ml/kg/min) V2 antagonist pre-treatment enhanced the VLF-BP component during normotensive bleeding, while the V1a antagonist pre-treatment modified BP variability after hypotensive haemorrhage by enhancing the HF-SBP component. The results suggest that under normotensive conditions vasopressin by the stimulation of both V1a and V2 receptors buffers BP variability in the VLF-BP frequency domain. In addition, under hypotensive conditions vasopressin, by the stimulation of V1a receptors buffers the respiration-induced HF-BP oscillation.

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.

Upregulation of V(1) receptors in renal resistance vessels of rats developing genetic hypertension

Previous studies have demonstrated that arginine vasopressin (AVP) produces exaggerated renal vasoconstriction in young spontaneously hypertensive rats (SHR) relative to normotensive rats. The exaggerated renal vascular reactivity does not appear to be due to a primary defect in postreceptor calcium signal transduction. Although the magnitudes of vascular responses differ, the relative proportions of calcium entry and mobilization pathways evoked by AVP in renal resistance vessels are similar in these rat strains. The purpose of the present study was to evaluate possible differences in V(1) mRNA and receptor density and affinity in preglomerular resistance vessels (<50 microm) obtained from young Wistar-Kyoto (WKY) and SHR. Quantitative RT-PCR analysis revealed twofold greater expression of the V(1a) receptor gene in preglomerular arterioles of 7-wk-old SHR compared with WKY. In vitro radiolabeled ligand binding studies were performed under equilibrium conditions on preglomerular resistance arterioles freshly isolated from kidneys of 7-wk-old rats. The results indicate that AVP receptor density (B(max)) is two to three times greater in SHR than in WKY (248 +/- 24 vs. 91 +/- 11 fmol/mg protein, P < 0.001). The affinity does not differ between strains (K(d) = 0.5 nM). Displacement studies yielded similar results for SHR and WKY. Unlabeled AVP completely displaced [(3)H]AVP binding, with an IC(50) of 2.5 x 10(-10) M. Expression of AVP receptor types in afferent arterioles was evaluated using the V(1) receptor agonist, [Phe(2), Ile(3),Org(8)]vasopressin, the V(1) receptor antagonist, [d(CH(2))(5), Tyr(Me)(2), Tyr(NH(2))(9)]Arg(8)-vasopressin, and the V(2) receptor agonist, desamino-[D-Arg(8)]vasopressin. Both the V(1) agonist and antagonist displaced up to 90% of the AVP binding with IC(50) values of 4 x 10(-8) and 8 x 10(-7) M, respectively. The V(2) receptor agonist was a weak inhibitor, displacing less than 15% of AVP binding at a high concentration of 10(-4) M. These results demonstrate that virtually all AVP receptors in the preglomerular arterioles are of the V(1) type. Collectively, our results provide evidence that the enhanced renal reactivity to AVP is mediated by a higher density of V(1) receptors associated with increased gene expression in renal resistance vessels of SHR developing genetic hypertension.

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.

Arginine vasopressin V1-receptor antagonism in an ovine model of acute myocardial infarction

Arginine vasopressin (AVP) has an uncertain role in the pathogenesis of increased myocardial and other regional vascular resistance after cardiac injury. The extreme increase in plasma AVP levels observed in some individuals after myocardial infarction potentially exacerbates already compromised myocardial perfusion. We assessed whether blockade of AVP, by administration of the specific V1-receptor antagonist OPC-21268, can reduce the severity of myocardial injury after embolization in our ovine model of acute myocardial infarction. Embolizations resulted in a pattern of changes in ECG and cardiac enzymes consistent with moderate to severe acute myocardial infarction, resulting in left ventricular dysfunction [left ventricular ejection fraction (LVEF) reduced from 52-53% to 38%]. However, no statistically significant differences were observed between groups in hemodynamic or neurohumoral indices of myocardial damage. By contrast, the hypothalamus-pituitary-adrenal (HPA) response [including plasma AVP (p < 0.01), adrenocorticotropic hormone (ACTH; p < 0.01), and cortisol (p < 0.01)) to embolizations was significantly increased in the sheep infused with OPC-21268. In conclusion, whereas plasma HPA responses differed between the two groups, the similar responses in cardiac enzymes, LVEF, and hemodynamic and neurohumoral factors in both groups does not support a role of V1 receptor-mediated exacerbation of myocardial injury in this model of myocardial infarction. Assessment of different receptor antagonists or examination of other models of cardiac injury may further clarify the role of the markedly increased AVP levels that can occur in myocardial infarction.

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.

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.

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)

Glucocorticoids regulate V1a vasopressin receptor expression by increasing mRNA stability in vascular smooth muscle cells

Enhancement of vascular responsiveness is considered to be one of the major contributing factors observed in glucocorticoid-induced hypertension. We examined the effects of glucocorticoids on V1a arginine vasopressin receptor mRNA and protein levels in vascular smooth muscle cells. Dexamethasone (1 mumol/L) produced a 1.8-fold increase in V1a receptor density without changing its affinity. Steady-state values of V1a receptor mRNA, analyzed by Northern blotting, increased 2.7-fold after a 12-hour exposure to dexamethasone. This effect of dexamethasone was blocked by the glucocorticoid antagonist RU38486 and did not occur in the presence of the protein synthesis inhibitor cycloheximide. The V1a receptor gene transcription rate, determined by nuclear run-off assays, was unchanged in cells treated with dexamethasone for 12 hours. Dexamethasone increased the half-life of V1a receptor mRNA by 2.2-fold. These findings suggest that dexamethasone upregulates the expression of the V1a receptor by increasing mRNA stability rather than by gene transcription and that de novo protein synthesis is involved in this regulation.

Are vasopressin peripheral V1 receptors involved in the development of malignant hypertension and stroke in SHR-SPs?

The aim of this study was to determine whether activation of vasopressin (AVP) peripheral V1 receptors is involved in the development of malignant hypertension, stroke, and end-organ damage in stroke-prone spontaneously hypertensive rats (SHR-SPs). For this purpose, young salt-loaded SHR-SPs were treated orally daily from their 5th to 34th week of age, by a selective AVP V1 receptor antagonist, SR 49059, used in a dose (30 mg/kg) that achieved complete peripheral V1 receptor blockade. Untreated SHR-SPs served as controls. SR 49059 slightly and transiently (8th to 10th week of age) limited the rise in blood pressure, but thereafter systolic blood pressure values were similar in the two groups of SHR-SPs. Stroke-related mortality was not significantly different in SR 49059-treated and in control animals (65% vs 65% at 30 weeks, 65% vs 83% at 34 weeks). SR 49059 did not prevent the increases in fluid intake, diuresis and proteinuria seen in controls. Histological examination of the brain, kidneys and heart revealed that the development of fibrinoid necrosis and arterial thickening was not prevented by SR 49059, nor was that of malignant nephroangiosclerosis and of myocardial infarction and fibrosis. These data strongly suggest that AVP peripheral V1 receptor activation is not involved in the pathological processes that develop in SHR-SPs.

Effects of the non-peptide inhibitor OPC-21268 on oxytocin and vasopressin stimulation of rat and human myometrium

OPC-21268 (1-[-1-[4-(3-acetylaminopropoxy)benzoyl]-piperidyl]-3,4- dihydro-2(1H)-quinolinone), a non-peptide vasopressin V1 receptor antagonist, inhibited oxytocin- and vasopressin-induced contractions of myometrial strips from rats and from full-term pregnant women. Administered intravenously in rats the drug also inhibited uterine contractions caused by infusion of oxytocin. When incubated with purified plasma membranes from rat or human myometrial tissue, OPC-21268 inhibited the specific receptor binding of tritiated oxytocin and vasopressin in a dose-dependent and reversible way.