Molecular pharmacology of V1a vasopressin receptors

The role of vasopressin and the vasopressin type V1a receptor agonist selepressin in septic shock

Septic shock remains one of the major causes of morbidity and mortality in the critically ill. Despite early goal therapy and administration of cathecholaminergic agents, up to 30% of patients succumb to the disease. In this manuscript, we first summarize the standard of care of patients with septic shock and current guidelines. We review the physiologic role of vasopressin and its role in septic shock management. We then review the most up-to-date evidence on the potential role of V1a receptor agonists such as Selepressin, in septic shock. Exciting new trials are being completed in order to elucidate the role of V1a receptor agonists as potential first-line vasopressor alternatives in the therapy of circulatory shock in septic patients.

Influence of thoracic epidural anesthesia on gastric oxygenation during hypothermia and hemorrhage

OBJECTIVE

Hypothermia preserves gastric mucosal microvascular oxygenation (μHbO2) during hemorrhagic shock. Additionally, hypothermia activates the sympathetic nervous system that leads to the release of vasopressin. The aim of this study was to evaluate whether the effect of hypothermia is mediated via the sympathetic nervous system and/or via vasopressin.

METHODS

In prospective and randomized experiments on five anesthetized dogs (foxhounds, cross-over design, 6 groups with n=5 per group) we analyzed the effects of hemorrhage on μHbO2 during mild hypothermia (HT, 34 °C), during additional thoracic epidural anesthesia (HT/TEA) and during additional vasopressin V1 receptor blockade (HT/VB). As control groups, effects of hemorrhage were studied under normothermia alone (NT), during additional thoracic epidural anesthesia (NT/TEA) and during additional vasopressin V1 receptor blockade (NT/VB).

RESULTS

Hemorrhage decreased μHbO2 from 81 ± 3 to 49 ± 8%. In contrast, in the presence of hypothermia, μHbO2 was significantly higher during hemorrhagic shock (from 79 ± 3 to 66 ± 9%) despite a similar decrease in DO2. The effect of hypothermia on μHbO2 was reduced in the presence of thoracic epidural anesthesia or vasopressin receptor blockade.

CONCLUSIONS

Hypothermia preserves μHbO2 during hemorrhagic shock. This effect is partially abolished during thoracic epidural anesthesia or during vasopressin receptor blockade. The sympathetic nervous system and the vasopressin V1 receptor are partially involved in mediating the effect of hypothermia on gastric oxygenation during hemorrhage.

Exploiting uncertainty measures in compounds activity prediction using support vector machines

The great majority of molecular modeling tasks require the construction of a model that is then used to evaluate new compounds. Although various types of these models exist, at some stage, they all use knowledge about the activity of a given group of compounds, and the performance of the models is dependent on the quality of these data. Biological experiments verifying the activity of chemical compounds are often not reproducible; hence, databases containing these results often possess various activity records for a given molecule. In this study, we developed a method that incorporates the uncertainty of biological tests in machine-learning-based experiments using the Support Vector Machine as a classification model. We show that the developed methodology improves the classification effectiveness in the tested conditions.

The selective vasopressin type 1a receptor agonist selepressin (FE 202158) blocks vascular leak in ovine severe sepsis*

OBJECTIVE

To determine if the selective vasopressin type 1a receptor agonist selepressin (FE 202158) is as effective as the mixed vasopressin type 1a receptor/vasopressin V2 receptor agonist vasopressor hormone arginine vasopressin when used as a titrated first-line vasopressor therapy in an ovine model of Pseudomonas aeruginosa pneumonia-induced severe sepsis.

DESIGN

Prospective, randomized, controlled laboratory experiment.

SETTING

University animal research facility.

SUBJECTS

Forty-five chronically instrumented sheep.

INTERVENTIONS

Sheep were anesthetized, insufflated with cooled cotton smoke via tracheostomy, and P. aeruginosa were instilled into their airways. They were then placed on assisted ventilation, awakened, and resuscitated with lactated Ringer's solution titrated to maintain hematocrit ± 3% from baseline levels. If, despite fluid management, mean arterial pressure fell by more than 10 mm Hg from baseline level, an additional continuous IV infusion of arginine vasopressin or selepressin was titrated to raise and maintain mean arterial pressure within no less than 10 mm Hg from baseline level. Effects of combination treatment of selepressin with the selective vasopressin V2 receptor agonist desmopressin were similarly investigated.

MEASUREMENTS AND MAIN RESULTS

In septic sheep, MAP fell by ~30 mm Hg, systemic vascular resistance index decreased by ~50%, and ~7 L of fluid were retained over 24 hours; this fluid accumulation was partially reduced by arginine vasopressin and almost completely blocked by selepressin; and combined infusion of selepressin and desmopressin increased fluid accumulation to levels similar to arginine vasopressin treatment.

CONCLUSIONS

Resuscitation with the selective vasopressin type 1a receptor agonist selepressin blocked vascular leak more effectively than the mixed vasopressin type 1a receptor/vasopressin V2 receptor agonist arginine vasopressin because of its lack of agonist activity at the vasopressin V2 receptor.

Vasopressin V(1A) receptors mediate the increase in gastric mucosal oxygenation during hypercapnia

Hypercapnia (HC) improves systemic oxygen delivery (DO₂) and microvascular hemoglobin oxygenation of the mucosa (μHbO₂). Simultaneously, HC increases plasma levels of vasopressin. Although vasopressin is generally regarded a potent vasoconstrictor particularly in the splanchnic region, its effects on splanchnic microcirculation during HC is unclear. The aim of this study was to evaluate the role of endogenous vasopressin on gastric mucosal oxygenation and hemodynamic variables during physiological (normocapnia) and hypercapnic conditions. Five dogs were repeatedly anesthetized to study the effect of vasopressin V(1A) receptor blockade ([Pmp¹,Tyr(Me)²]-Arg⁸-Vasopressin, 35  μg/kg) on hemodynamic variables and μHbO₂ during normocapnia or HC (end-tidal CO₂ 70  mmHg). In a control group, animals were subjected to HC alone. μHbO₂ was measured by reflectance spectrophotometry, systemic DO₂ was calculated from intermittent blood gas analysis, and cardiac output was measured by transpulmonary thermodilution. Data are presented as mean±s.e.m. for n=5 animals. During HC alone, DO₂ increased from 12±1 to 16±1 ml/kg per min and μHbO₂ from 70±4 to 80±2%. By contrast, additional vasopressin V(1A) receptor blockade abolished the increase in μHbO₂ (80±2 vs. 69±2%) without altering the increase in DO₂ (16±1 vs. 19±2  ml/kg per min). Vasopressin V1A receptor blockade (VB) during normocapnia neither affected DO₂ (13±1 vs. 14±1  ml/kg per min) nor μHbO₂ (75±3 vs. 71±5%). Vasopressin V(1A) receptor blockade abolished the increase in μHbO₂ during HC independent of DO₂. Thus, in contrast to its generally vasoconstrictive properties, the vasopressin V1A receptors seem to mediate the increase in gastric microcirculatory mucosal oxygenation induced by acute HC.

Vasopressin: a novel target for the prevention and retardation of kidney disease?

After several decades during which little attention was paid to vasopressin and/or urine concentration in clinical practice, interest in vasopressin has renewed with the availability of new, potent, orally active vasopressin-receptor antagonists--the vaptans--and with the results of epidemiological studies evaluating copeptin (a surrogate marker of vasopressin) in large population-based cohorts. Several experimental studies in rats and mice had previously shown that vasopressin, acting via vasopressin V2 antidiuretic receptors, contributes to the progression of chronic kidney disease; in particular, to autosomal dominant polycystic kidney disease. New epidemiological studies now suggest a role for vasopressin in the pathogenesis of diabetes mellitus and metabolic disorders via activation of hepatic V1a and/or pancreatic islet V1b receptors. The first part of this Review describes the adverse effects of vasopressin, as revealed by clinical and experimental studies in kidney diseases, hypertension, diabetes and the metabolic syndrome. The second part provides insights into vasopressin physiology and pathophysiology that may be relevant to the understanding of these adverse effects and that are linked to the excretion of concentrated nitrogen wastes and associated hyperfiltration. Collectively, the studies reviewed here suggest that more attention should be given to the vasopressin-thirst-urine concentration axis in clinical investigations and in patient care. Whether selective blockade of the different vasopressin receptors may provide therapeutic benefits beyond their present indication in hyponatraemia requires new clinical trials.

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

BACKGROUND

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

METHODS AND RESULTS

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

CONCLUSIONS

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

New, potent, selective, and short-acting peptidic V1a receptor agonists

[Arg(8)]vasopressin (AVP) produces vasoconstriction via V(1a) receptor (V(1a)R)-mediated vascular smooth muscle cell contraction and is being used to increase blood pressure in septic shock, a form of vasodilatory hypotension. However, AVP also induces V(2) receptor (V(2)R)-mediated antidiuresis, vasodilation, and coagulation factor release, all deleterious in septic shock. The V(1a)R agonist terlipressin (H-Gly(3)[Lys(8)]VP) also lacks selectivity vs the V(2)R and has sizably longer duration of action than AVP, preventing rapid titration of its vasopressor effect in the clinic. We designed and synthesized new short acting V(1a)R selective analogues of general structure [Xaa(2),Ile(3),Yaa(4),Zaa(8)]VP. The most potent and selective compounds in in vitro functional assays (e.g., [Phe(2),Ile(3),Asn(Me(2))(4),Orn(8)]VP (31), [Phe(2),Ile(3),Asn((CH(2))(3)OH)(4),Orn(8)]VP (34), [Phe(2),Ile(3),Hgn(4),Orn(iPr)(8)]VP (45), [Phe(2),Ile(3),Asn(Et)(4),Dab(8)]VP (49), [Thi(2),Ile(3),Orn(iPr)(8)]VP (59), [Cha(2),Ile(3),Asn(4),Orn(iPr)(8)]VP (68)) were tested by intravenous bolus in rats for duration of vasopressive action. Analogues 31, 34, 45, and 49 were as short-acting as AVP. Compound 45, FE 202158, is currently undergoing clinical trials in septic shock.

The vasopressin V1b receptor modulates plasma corticosterone responses to dehydration-induced stress

Vasopressin V1b receptor knockout (V1b⁻/⁻) mice were used to investigate a putative role for the V1b receptor (V1bR) in fluid regulation and in the hypothalamic-neurohypophysial system (HNS) and hypothalamic-pituitary-adrenal (HPA) axis responses to osmotic stress induced by water deprivation (WD). Male wild-type and V1b⁻/⁻ mice were housed in metabolic cages to allow determination of water intake and urine volume and osmolality. When provided with food and water ad lib., spontaneous urine volume and urine osmolality did not differ between genotypes. Similarly, WD for 24 h caused comparable decreases in urine volume and increases in urine osmolality irrespective of genotype. WD resulted in an increase in plasma corticosterone concentration in wild-type animals; however, this WD-induced increase in plasma corticosterone was significantly attenuated in V1b⁻/⁻ mice. Comparable increases in neuronal activation, indicated by increased c-fos mRNA expression, and in vasopressin mRNA expression occurred in both the supraoptic nucleus and paraventricular nucleus (PVN) of wild-type and V1b⁻/⁻ mice following WD; however, the WD-induced decrease in corticotrophin-releasing hormone mRNA expression seen in the PVN of wild-type mice was not observed in the PVN of V1b⁻/⁻ mice. These data suggest that, although the vasopressin V1bR is not required for normal HNS function, it is necessary for a full HPA-axis response to the osmotic stress of WD.

Role of various neurotransmitters in mediating the long-term endocrine consequences of prenatal alcohol exposure

Adult rats and mice born to dams exposed to alcohol (fetal alcohol-exposed [FAE]) exhibit enhanced activity of their hypothalamic-pituitary-adrenal (HPA) axis when exposed to stressors. However, the mechanisms responsible for this phenomenon remain incompletely understood. Here two possibilities are reviewed: one that pertains to nitric oxide (NO), an unstable gas that stimulates the HPA axis; and one that focuses on catecholamines, which also stimulate this axis. Significant alterations were not observed in levels of NO synthase, the enzyme responsible for NO formation, in the paraventricula nucleus (PVN) of FAE rats. However, the stimulatory influence of this gas on the hypothalamic-pituitary-adrenal (HPA) axis was enhanced in these animals, thereby providing a mechanism likely to participate in the neuroendocrine hyperactivity that is the hallmark of this model. It was also recently shown that, while the ability of catecholamines to release adrenocorticotropic hormone (ACTH) was comparable in control rats and rats exposed to alcohol during embryonic development, there was a significant upregulation of the C1 brain-stem region when these latter animals were exposed to mild footshocks. Since this region sends prominent projections to the PVN, its increased activity may participate in the HPA axis hyperactivity observed in FAE offspring. Finally, microarray technology was used to search for potential differences in genes present in the brains of control and FAE mice. When these brains were collected on day 17.5 of embryonic development, several genes were upregulated, while others were downregulated, which may provide potential new candidates that mediate the influence of prenatal alcohol on the HPA axis of adult offspring.

Extracellular loops and ligand binding to a subfamily of Family A G-protein-coupled receptors

GPCRs (G-protein-coupled receptors) are a large family of structurally related proteins which mediate their effects by coupling to G-proteins. The V(1a)R (V(1a) vasopressin receptor) is a member of a family of related GPCRs that are activated by vasopressin {AVP ([Arg(8)]vasopressin)}, OT (oxytocin) and related peptides. These receptors are members of a subfamily of Family A GPCRs called the neurohypophysial peptide hormone receptor family. GPCRs exhibit a conserved tertiary structure comprising a bundle of seven TM (transmembrane) helices linked by alternating ECLs (extracellular loops) and ICLs (intracellular loops). The cluster of TM helices is functionally important for ligand binding, and, furthermore, activation of GPCRs involves movement of these TM helices. Consequently, it might be assumed that the extracellular face of GPCRs is composed of peptide linkers that merely connect important TM helices. However, using a systematic mutagenesis approach and focusing on the N-terminus and the second ECL of the V(1a)R, we have established that these extracellular domains fulfil a range of important roles with respect to GPCR signalling, including agonist binding, ligand selectivity and receptor activation.

Endogenous nitric oxide reduces the efficacy of the endothelin system to maintain blood pressure during high epidural anaesthesia in conscious dogs

BACKGROUND AND OBJECTIVE

During high epidural anaesthesia, endothelin only contributes minimally to blood pressure stabilization. This phenomenon could result from the inhibitory action of nitric oxide on the endothelin system. To clarify this, we studied the interaction between nitric oxide and endothelin during high epidural anaesthesia in conscious dogs, in comparison to the interaction of nitric oxide and vasopressin.

METHODS

Six animals were used in 45 individual experiments randomly arranged as follows: N-omega-nitro-arginine-methylester 0.3-10 mg kg-1 under physiological conditions or during high epidural anaesthesia (lidocaine 1%) and N-omega-nitro-arginine-methylester (l-NAME) 0.3-10 mg kg-1 after preceding endothelin (Tezosentan(R)) or vasopressin (beta-mercapto-beta,beta-cyclo-penta-methylene-propionyl-O-Me-Tyr-Arg-vasopressin) receptor blockade under physiological conditions or during high epidural anaesthesia. During control experiments normal saline was injected either intravenously (n = 5) or into the epidural space (n = 4).

RESULTS

N-omega-nitro-arginine-methylester increased mean arterial pressure dose-dependently in all groups. However, this effect was substantially reduced in the presence of the endothelin receptor antagonist compared to N-omega-nitro-arginine-methylester alone, both under control conditions (7 +/- 3 vs. 21 +/- 3 mmHg; P < 0.05) and during high epidural anaesthesia (17 +/- 3 vs. 30 +/- 1 mmHg; P < 0.05). Blockade of vasopressin showed no similar relationship with N-omega-nitro-arginine-methylester.

CONCLUSIONS

The diminished increase in mean arterial pressure after injection of N-omega-nitro-arginine-methylester only during endothelin receptor blockade indicates that endogenous nitric oxide inhibits the action of endothelin during high epidural anaesthesia and might thus explain the reduced efficacy of endothelin in maintaining blood pressure during high epidural anaesthesia.

Arginine vasopressin inhibits Kir6.1/SUR2B channel and constricts the mesenteric artery via V1a receptor and protein kinase C

Kir6.1/SUR2B channel is the major isoform of K(ATP) channels in the vascular smooth muscle. Genetic disruption of either subunit leads to dysregulation of vascular tone and regional blood flows. To test the hypothesis that the Kir6.1/SUR2B channel is a target molecule of arginine vasopressin (AVP), we performed studies on the cloned Kir6.1/SUR2B channel and cell-endogenous K(ATP) channel in rat mesenteric arteries. The Kir6.1/SUR2B channel was expressed together with V1a receptor in the HEK-293 cell line. Whole cell currents of the transfected HEK cells were activated by K(ATP) channel opener pinacidil and inhibited by K(ATP) channel inhibitor glibenclamide. AVP produced a concentration-dependent inhibition of the pinacidil-activated currents with IC(50) 2.0 nM. The current inhibition was mediated by a suppression of the open-state probability without effect on single-channel conductance. An exposure to 100 nM PMA, a potent PKC activator, inhibited the pinacidil-activated currents, and abolished the channel inhibition by AVP. Such an effect was not seen with inactive phorbol ester. A pretreatment of the cells with selective PKC blocker significantly diminished the inhibitory effect of AVP. In acutely dissociated vascular smooth myocytes, AVP strongly inhibited the cell-endogenous K(ATP) channel. In isolated mesenteric artery rings, AVP produced concentration-dependent vasoconstrictions with EC(50) 6.5 nM. At the maximum effect, pinacidil completely relaxed vasoconstriction in the continuing exposure to AVP. The magnitude of the AVP-induced vasoconstriction was significantly reduced by calphostin-C. These results therefore indicate that the Kir6.1/SUR2B channel is a target molecule of AVP, and the channel inhibition involves G(q)-coupled V1a receptor and PKC.

Systematic analysis of the entire second extracellular loop of the V(1a) vasopressin receptor: key residues, conserved throughout a G-protein-coupled receptor family, identified

The roles of extracellular residues of G-protein-coupled receptors (GPCRs) are not well defined compared with residues in transmembrane helices. Nevertheless, it has been established that extracellular domains of both peptide-GPCRs and amine-GPCRs incorporate functionally important residues. Extracellular loop 2 (ECL2) has attracted particular interest, because the x-ray structure of bovine rhodopsin revealed that ECL2 projects into the binding crevice within the transmembrane bundle. Our study provides the first comprehensive investigation into the role of the individual residues comprising the entire ECL2 domain of a small peptide-GPCR. Using the V(1a) vasopressin receptor, systematic substitution of all of the ECL2 residues by Ala generated 30 mutant receptors that were characterized pharmacologically. The majority of these mutant receptor constructs (24 in total) had essentially wild-type ligand binding and intracellular signaling characteristics, indicating that these residues are not critical for normal receptor function. However, four aromatic residues Phe(189), Trp(206), Phe(209), and Tyr(218) are important for agonist binding and receptor activation and are highly conserved throughout the neurohypophysial hormone subfamily of peptide-GPCRs. Located in the middle of ECL2, juxtaposed to the highly conserved disulfide bond, Trp(206) and Phe(209) project into the binding crevice. Indeed, Phe(209) is part of the Cys-X-X-X-Ar (where Ar is an aromatic residue) motif, which is well conserved in both peptide-GPCRs and amine-GPCRs. In contrast, Phe(189) and Tyr(218), located at the extreme ends of ECL2, may be important for determining the position of the ECL2 cap over the binding crevice. This study provides mechanistic insight into the roles of highly conserved ECL2 residues.

Charged extracellular residues, conserved throughout a G-protein-coupled receptor family, are required for ligand binding, receptor activation, and cell-surface expression

For G-protein-coupled receptors (GPCRs) in general, the roles of extracellular residues are not well defined compared with residues in transmembrane helices (TMs). Nevertheless, extracellular residues are important for various functions in both peptide-GPCRs and amine-GPCRs. In this study, the V(1a) vasopressin receptor was used to systematically investigate the role of extracellular charged residues that are highly conserved throughout a subfamily of peptide-GPCRs, using a combination of mutagenesis and molecular modeling. Of the 13 conserved charged residues identified in the extracellular loops (ECLs), Arg(116) (ECL1), Arg(125) (top of TMIII), and Asp(204) (ECL2) are important for agonist binding and/or receptor activation. Molecular modeling revealed that Arg(125) (and Lys(125)) stabilizes TMIII by interacting with lipid head groups. Charge reversal (Asp(125)) caused re-ordering of the lipids, altered helical packing, and increased solvent penetration of the TM bundle. Interestingly, a negative charge is excluded at this locus in peptide-GPCRs, whereas a positive charge is excluded in amine-GPCRs. This contrasting conserved charge may reflect differences in GPCR binding modes between peptides and amines, with amines needing to access a binding site crevice within the receptor TM bundle, whereas the binding site of peptide-GPCRs includes more extracellular domains. A conserved negative charge at residue 204 (ECL2), juxtaposed to the highly conserved disulfide bond, was essential for agonist binding and signaling. Asp(204) (and Glu(204)) establishes TMIII contacts required for maintaining the beta-hairpin fold of ECL2, which if broken (Ala(204) or Arg(204)) resulted in ECL2 unfolding and receptor dysfunction. This study provides mechanistic insight into the roles of conserved extracellular residues.

Regulation of the avian kidney by arginine vasotocin

Nonapeptides secreted by the neurohypophysis have important roles in vertebrate cardio-fluid homeostasis. In birds, those peptides include mesotocin (the representative of the neutral, or oxytocin-like, nonapeptide family) and vasotocin (the representative of the basic, or vasopressin-like, hormones). The function of mesotocin is not well defined, but it does appear to have osmoregulatory functions. Vasotocin is considered the primary avian antidiuretic hormone. Receptors for AVT in avian kidney-either on renal vasculature or on the tubules-have yet to be localized or identified. However, AVT quite certainly effects antidiuresis via both vascular and tubular mechanisms. The former entail a reduction in the rate of glomerular filtration, apparently via constriction of afferent arterioles. Evidence for the latter (direct tubular action of AVT) has accumulated in recent years and includes enhanced fractional tubular water reabsorption, activation of second messenger (cAMP) in thick ascending limbs and collecting ducts, and modest AVT-stimulated water permeability in collecting ducts associated with expression of aquaporins. The relative importance of the renal vascular vs. tubular actions in birds likely depend on the dose of the hormone, the physiological condition of the animal, and the species of bird.

Analysis of interactions responsible for vasopressin binding to human neurohypophyseal hormone receptors-molecular dynamics study of the activated receptor-vasopressin-G(alpha) systems

Vasopressin (CYFQNCPRG-NH(2), AVP) is a semicyclic endogenous peptide, which exerts a variety of biological effects in mammals. The main physiological roles of AVP are the regulation of water balance and the control of blood pressure and adrenocorticotropin hormone (ACTH) secretion, mediated via three different subtypes of vasopressin receptors: V1a, V1b and V2 receptors (V1aR, V1bR and V2R, respectively). They are the members of the class A, G-protein-coupled receptors (GPCRs). AVP also modulates several behavioral and social functions. In this study, the interactions responsible for AVP binding to vasopressin V1a and V2 receptors versus the closely related oxytocin ([I3,L8]AVP, OT) receptor (OTR) have been investigated. Three-dimensional models of the activated receptors were constructed using multiple sequence alignment, followed by homology modeling using the complex of activated rhodopsin with Gt(alpha) C-terminal peptide of transducin MII-Gt(338-350) prototype as a template. AVP was docked into the receptor-G(alpha) systems. The three lowest-energy pairs of receptor-AVP-G(alpha) (two complexes per each receptor) were selected. The 1-ns unconstrained molecular dynamics (MD) of complexes embedded into the fully hydrated 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylcholine (POPC) lipid bilayer was conducted in the AMBER 7.0 force field. Six relaxed receptor-AVP-G(alpha) models were obtained. The residues responsible for AVP binding to vasopressin receptors have been identified and a different mechanism of AVP binding to V2R than to V1aR has been proposed.

The N-Terminal Juxtamembrane Segment of the V1a Vasopressin Receptor Provides Two Independent Epitopes Required for High-Affinity Agonist Binding and Signaling

It is fundamentally important to define how agonist-receptor interaction differs from antagonist-receptor interaction. The V1a vasopressin receptor (V1aR) is a member of the neurohypophysial hormone subfamily of G protein-coupled receptors. Using alanine-scanning mutagenesis of the N-terminal juxtamembrane segment of the V1aR, we now establish that Glu54 (1.35) is critical for arginine vasopressin binding. The mutant [E54A]V1aR exhibited decreased arginine vasopressin affinity (1700-fold) and disrupted signaling, but antagonist binding was unaffected. Mutation of Glu54 had an almost identical pharmacological effect as mutation of Arg46, raising the possibility that agonist binding required a mutual interaction between Glu54 and Arg46. The role of these two charged residues was investigated by 1) substituting Glu54; 2) inserting additional Glu/Arg in transmembrane helix (TM) 1; 3) repositioning the Glu/Arg in TM1; and 4) characterizing the reciprocal mutant [R46E/E54R]V1aR. We conclude that 1) the positive/negative charges need to be precisely positioned in this N terminus/TM1 segment; and 2) Glu54 and Arg46 function independently, providing two discrete epitopes required for high-affinity agonist binding and signaling. This study explains why Glu and Arg, part of an -R(X3)L/V(X3)E(X3)L- motif, are conserved at these loci throughout this G protein-coupled receptor subfamily and provides molecular insight into key differences between agonist and antagonist binding requirements.

Role of nitric oxide and protein kinase C in the tachyphylaxis to vasopressin in rat aortic rings

The contribution of endothelium-derived mediators and protein kinase C in the tachyphylaxis to arginine vasopressin (AVP) was assessed in the rat aorta. Endothelium-intact (E+) and denuded rings (E-) obtained from the rat thoracic aorta were exposed to three administrations of a supramaximal concentration of AVP (100 nM), lasting 20 min and 45 min apart. N-Omega-nitro-L-arginine (NNLA), a non-selective inhibitor of all isoforms of NO synthase, and AMT, a selective inhibitor for the inducible (iNOS) and neuronal (nNOS) isoforms, diminished the tachyphylaxis to AVP significantly in both E+ and in E- rings. No iNOS could be detected by Western blots in freshly isolated rings or in rings exposed to AVP, despite a strong signal in rings isolated from LPS-treated rats, while nNOS could be constitutively detected. Inhibition of prostaglandins or epoxyeicosatrienoic acids (EETs) synthesis by diclofenac or clotrimazole, respectively, had no effect on tachyphylaxis while combination of these agents diminished tachyphylaxis in E+ only. Combination of NNLA, diclofenac and clotrimazole blocked completely the tachyphylaxis. Inhibition of PKC by either chelerythrine or bisindolylmaleimide I-HCl (BisI) led to a significant diminution of AVP tachyphylaxis only in E-. Activation of PKC with phorbol-12-myristate-13-acetate (PMA) simulated tachyphylaxis to AVP in E- only, effect blocked by the NO donor, SNP. In conclusion, NO produced from constitutive nNOS present in vascular smooth muscle cells participates in tachyphylaxis to AVP. PKC is involved in this tachyphylaxis only in E- rings, the presence of NO probably diminishing the effects of this kinase.

An arginyl in the N-terminus of the V1a vasopressin receptor is part of the conformational switch controlling activation by agonist

Defining how the agonist-receptor interaction differs from that of the antagonist-receptor and understanding the mechanisms of receptor activation are fundamental issues in cell signalling. The V1a vasopressin receptor (V1aR) is a member of a family of related G-protein coupled receptors that are activated by neurohypophysial peptide hormones, including vasopressin (AVP). It has recently been reported that an arginyl in the distal N-terminus of the V1aR is critical for binding agonists but not antagonists. To determine specific features required at this locus to support high affinity agonist binding and second messenger generation, Arg46 was substituted by all other 19 encoded amino acids. Our data establish that there is an absolute requirement for arginyl, as none of the [R46X]V1aR mutant constructs supported high affinity agonist binding and all 19 had defective signalling. In contrast, all of the mutant receptors possessed wildtype binding for both peptide and nonpeptide antagonists. The ratio of Ki to EC50, an indicator of efficacy, was increased for all substitutions. Consequently, although [R46X]V1aR constructs have a lower affinity for agonist, once AVP has bound all 19 are more likely than the wildtype V1aR to become activated. Therefore, in the wildtype V1aR, Arg46 constrains the inactive conformation of the receptor. On binding AVP this constraint is alleviated, promoting the transition to active V1aR. Our findings explain why arginyl is conserved at this locus throughout the evolutionary lineage of the neurohypophysial peptide hormone receptor family of G-protein coupled receptors.

A double-blind randomized trial: prophylactic vasopressin reduces hypotension after cardiopulmonary bypass

BACKGROUND

Inhibition of angiotensin-converting enzyme (ACE) predisposes patients to vasodilatory hypotension after cardiopulmonary bypass (CPB). This hypotension has been correlated with arginine vasopressin deficiency and can be corrected by its replacement. In patients receiving ACE inhibition, we investigated whether initiation of vasopressin before CPB would diminish post-CPB hypotension and catecholamine use by avoiding vasopressin deficiency.

METHODS

Cardiac surgical patients on ACE inhibitor therapy were randomized to receive vasopressin (0.03 U/min) (n = 13) or an equal volume of normal saline (n = 14) starting 20 minutes before CPB.

RESULTS

Vasopressin did not change pre-CPB mean arterial pressure or pulmonary artery pressure. After CPB, the vasopressin group had a lower peak norepinephrine dose than the placebo group (4.6 +/- 2.5 versus 7.3 +/- 3.5 microg/min, p = 0.03), a shorter period on catecholamines (5 +/- 6 versus 11 +/- 7 hours, p = 0.03), fewer hypotensive episodes (1 +/- 1 versus 4 +/- 2, p < 0.01), and a shorter intensive care unit length of stay (1.2 +/- 0.4 versus 2.1 +/- 1.4 days, p = 0.03).

CONCLUSIONS

In this cohort, prophylactic administration of vasopressin, at a dose without a vasopressor effect pre-CPB, reduced post-CPB hypotension and vasoconstrictor requirements, and was associated with a shorter intensive care unit stay.

Vasopressin receptor 1a-mediated negative regulation of B cell receptor signaling

We report here a study of T and B cell development and function in mice with disruption of the vasopressin receptor 1a (v1a) gene. Loss of the v1a receptor caused a shift from IgM(high)/IgD(high) to the more mature IgM(low)/IgD(high) B cells, a significantly greater extent of splenic B cells proliferation in response to anti-IgM stimulation, and enhanced IgG1 and IgG2b production in response to immune challenge with T-dependent antigen. B-1 cells were increased in v1a(-/-) mice. In contrast, T cell differentiation and activation were normal in v1a(-/-) mice. Our data identify a novel function for v1a in the periphery as a negative regulator of B cell receptor (BCR) signaling. These data suggest that in addition to its other stress-related effects, vasopressin may also serve as a counter-regulatory restraint upon the immune system during fight or flight situations.

Use of vasopressin in refractory hypotension in children with vasodilatory shock: five cases and a review of the literature

This article describes case studies of five children treated with vasopressin for refractory hypotension. In addition, physiology and pharmacology of vasopressin are reviewed in a comprehensive survey of the literature from 1966 until the present. In all five children, blood pressure increased immediately after vasopressin administration. The preliminary success of vasopressin for hypotension the setting of vasodilatory shock is promising. This limited use of vasopressin in the setting of refractory hypotension in these patients appears to be safe; the appropriate patient population and dose regimen are not yet determined.

Endogenous endothelin and vasopressin support blood pressure during epidural anesthesia in conscious dogs

We studied whether endogenous endothelin, like endogenous vasopressin, helps to maintain blood pressure during high epidural anesthesia when efferent sympathetic drive is diminished. On different days, six awake dogs underwent each of the following five interventions: blockade of vasopressin V(1a) receptors using [d(CH(2))(5)Tyr(Me(2))]AVP, (40 microg/kg) or endothelin receptors using tezosentan (3 mg/kg followed by 3 mg. kg(-1). h(-1)) with or without epidural anesthesia (1% lidocaine, intraindividual dose did not differ between experiments), and epidural saline (n = 5). The effects of endothelin- or vasopressin-receptor blockade were analyzed (means +/- SEM) and compared by an analysis of variance for repeated measures (paired Student's t-test, alpha-adjusted, P < 0.05). Vasopressin-receptor blockade decreased blood pressure (10 +/- 2 mm Hg) only in the presence of epidural anesthesia, whereas endothelin-receptor blockade reduced blood pressure both in the presence and absence of epidural anesthesia (12 +/- 3 versus 10 +/- 1 mm Hg). During baseline and each intervention, plasma concentrations of vasopressin and big-endothelin were measured and compared by a Wilcoxon's rank sum test; P < 0.05. Vasopressin concentrations increased during epidural anesthesia and after additional endothelin receptor blockade, but big-endothelin concentrations remained unchanged during each intervention. We conclude that vasopressin acts as a reserve system, as it stabilizes blood pressure specifically during epidural anesthesia, whereas the unchanged concentrations of big-endothelin indicate that the endothelin system is not specifically activated to support blood pressure during epidural anesthesia.

IMPLICATIONS

We studied in awake dogs whether endogenous endothelin, like endogenous vasopressin, helps to maintain blood pressure during resting conditions and epidural anesthesia. Only vasopressin was specifically activated to support blood pressure during epidural anesthesia, whereas endothelin supported blood pressure to the same extent during epidural anesthesia and during resting conditions.

Palmitoylation of the vasopressin V1a receptor reveals different conformational requirements for signaling, agonist-induced receptor phosphorylation, and sequestration

In this study, we establish that the V1a vasopressin receptor (V1aR) is palmitoylated, and we show that this modification has an important functional role. Palmitoylation of the V1aR occurs within the Cys371/Cys372 couplet located in the proximal C-terminal tail domain. Substitution of these residues in a [C371G/C372G]V1aR construct effectively disrupted receptor palmitoylation. Our data also indicate an additional palmitoylation site at another locus in the receptor, as yet undefined. [3H]Palmitate incorporation was agonist-sensitive and increased following exposure to [Arg8]vasopressin (AVP). Given the hydrophobic nature of the acyl chain, palmitoylation of the C terminus of G-protein-coupled receptors has been proposed to form an additional intracellular loop. Consequently, palmitoylation/depalmitoylation will have a profound effect on the local conformation of this domain. The V1aR palmitoylation status regulated both phosphorylation and sequestration of the receptor, and furthermore, palmitoylation, phosphorylation, and sequestration were all regulated by AVP. The palmitoylation-defective construct [C371G/C372G]V1aR exhibited decreased phosphorylation compared to wild-type V1aR, under both basal and AVP-stimulated conditions, and was sequestered at a faster rate. In contrast, the binding of four different classes of ligand and intracellular signaling were not affected by palmitoylation. This study therefore establishes that there are different conformational requirements for signaling, agonist-induced phosphorylation, and sequestration of the V1aR.

Design of oxytocin antagonists, which are more selective than atosiban

We report the solid phase synthesis of four pairs of L- and D-thienylalanine (Thi/D-Thi) position two modified analogues of the following four oxytocin (OT) antagonists: des-9-glycinamide [1-(beta-mercapto-beta,beta-pentamethylene propionic acid), 2-O-methyltyrosine, 4-threonine]ornithine-vasotocin (desGly(NH2)9,d (CH2)5[Tyr(Me)2,Thr4]OVT) (A); the Tyr-(NH2)9 analogue of (A), d(CH2)5[Tyr(Me)2,Thr4,Tyr-(NH2)9]OVT (B); the Eda9 analogue (where Eda = ethylenediamine) of (A), d(CH2)5[Tyr(Me)2, Thr4, Eda9]OVT (C); and the retro Tyr10 modified analogue of (C), d(CH2)5[Tyr(Me)2, Thr4, Eda9<--Tyr10]OVT (D). The eight new analogues of A-D are (1) desGly(NH2),d(CH2)5[Thi2,Thr4]OVT, (2) desGly(NH2),d(CH2)5[D-Thi2,Thr4]OVT, (3) d(CH2)5[Thi2, Thr4,Tyr-(NH2)9]OVT, (4) d(CH2)5[D-Thi2,Thr4,Tyr-(NH2)9]OVT (5) d(CH2)5[Thi2,Thr4Eda9]OVT, (6) d(CH2)5[D-Thi2,Thr4,Eda9]OVT, (7) d(CH2) [Thi2,Thr4,Eda9<--Tyr10]OVT, (8) d(CH2),[D-Thi2,Thr4,Eda9<--Tyr10]OVT. We also report the synthesis of (C). Peptides 1-8 and C were evaluated for agonistic and antagonistic activities in in vitro and in vivo OT assays, in in vivo vasopressor (V1a receptor) assays and in in vivo antidiuretic (V2 receptor) assays. None of the eight peptides nor C exhibit oxytocic or vasopressor agonism. Peptides 1-8 are extremely weak V2 agonists (antidiuretic activities range from < 0.0005 to 0.20 U/mg). Peptide C is a weak mixed V2 agonist/antagonist. Peptides 1-8 and C exhibit potent in intro (no Mg2+) OT antagonism (anti-OT pA2 values range from 7.76 to 8.05). Peptides 1-8 are all OT antagonists in vivo (estimated in vivo anti-OT pA2 values range from 6.54-7.19). With anti-V1a pA2 values of approximately 5-5.80, peptides 1-8 exhibit marked reductions in anti-V1a potencies relative to those of the parent peptides A-D (anti-V1a pA2 range from 6.48 to 7.10) and to l-deamino[D-Tyr(Et)2, Thr4]OVT (Atosiban, trade name Tractocile) (anti-V1a pA2-6.14). Atosiban has recently been approved in Europe for clinical use for the prevention of premature labour (Pharm. J. 264(7-100): 871). Peptides 1-8 exhibit striking gains in in vitro anti-OT/anti-V1a selectivities with respect to the parent peptides A, B, C and D and to Atosiban. Peptides 1-8 exhibit anti-OT (in vitro)/anti-V1a selectivities of 450, 525, 550, 450, approximately 1080, 116, 355, 227 respectively. The corresponding values for A-D and Atosiban are 30, 4.2, 4.3, 2.6 and 37. With the exception of peptide 6, the remaining seven peptides exhibit 3-18-fold gains in anti-OT (in vivo)/anti-V1a selectivity with respect to Atosiban, peptides 1-8 exhibit anti-OT (in vivo)/anti-V1a selectivities of 22, approximately 82, approximately 82, 147, approximately 83, 11, 31 and 42. By comparison, Atosiban exhibits an anti-OT (in vivo)/anti-V1a selectivity = 8. With an estimated in vivo anti-OT pA2 value = 7.19+/-0.06, peptide 4 is equipotent with Atosiban (pA2 = 7.05+/-0.05). However, with its significantly reduced anti-vasopressor potency, pA2 = approximately 5, it is approximately 18 times more selective for OT receptors with respect to VP V1a receptors than Atosiban. Since we have shown that V1a antagonism could be an unwanted side-effect in tocolytics, peptide 4 and some of the OT antagonists reported here have advantages over Atosiban and thus may be suitable candidates for evaluation as potential tocolytic agents for the treatment of preterm labour.

Effect of N-glycosylation on ligand binding affinity of rat V1a vasopressin receptor

A rat Vla vasopressin (rVla) receptor has two putative N-glycosylation sites at 14th and 27th amino acid asparagine in the extracellular N-terminus. In the present study, we examined the possible roles of N-glycosylation of the N-terminus in the receptor function. Three point mutants for deglycosylated rVla receptor were generated in which the 14th and/or the 27th asparagine was replaced with glutamine, namely N14Q, N27Q, and N14:27Q, each tagged with an enhanced green fluorescent protein (EGFP) at their C-termini, and transfected to COS-7 or HEK292 cells. The two single mutants and a double mutant have progressively smaller molecular mass compared to the wild-type receptor as determined by immunoblot analysis, indicating that the two sites are effectively glycosylated in vivo. The maximal ligand binding capacities of three mutant receptors were comparable to that of wild-type (17.02 +/- 1.32 pmol/g protein) with modest changes in ligand binding affinities: N27Q and N14:27Q had decreased binding affinities compared to N14Q and wild-type receptors. The reduced binding affinities of the deglycosylated mutants are not likely due to the impaired intracellular transport since their traffickings were indistinguishable from one another. Taken together, these results suggest that the N-glycosylation at the two sites of the N-terminus of rV1a receptor minimally affects the surface expression and trafficking of the receptor.

Identification of the glycosylation sites utilized on the V1a vasopressin receptor and assessment of their role in receptor signalling and expression

Most of the large family of G-protein-coupled receptors (GPCRs) possess putative N-glycosylation sites within their N-termini. However, for the vast majority of GPCRs, it has not been determined which, if any, of the consensus glycosylation sites are actually utilized or what the functional ramifications are of modification by oligosaccharide. The occurrence and function of glycosylation of the V(1a) vasopressin receptor (V(1a)R) has been investigated in this study. Using a combination of translation systems that are either glycosylation-competent or do not support glycosylation, we established that of the four putative N-glycosylation sites at Asn(14), Asn(27), Asn(198) and Asn(333) only the first three sites are actually modified by carbohydrate. This was confirmed by disruption of consensus sites by site-directed mutagenesis, individually and in combination. The V(1a)R is not O-glycosylated. The functionality of a series of glycosylation-defective V(1a)R constructs was characterized after expression in HEK 293T cells. It was found that carbohydrate moieties are not required for the receptor to bind any of the four classes of ligand available, or for intracellular signalling. The glycosylation status of the V(1a)R did, however, regulate the level of total receptor expression and also the abundance of receptor at the cell surface. Furthermore, the nature of this regulation (increased or decreased expression) was dictated by the locus of the oligosaccharide modification. Modification of any one of the consensus sites alone, however, was sufficient for wild-type expression, indicating a redundancy within the glycosylation sites. A role for the carbohydrate in the correct folding or stabilization of the V(1a)R is indicated. Glycosylation is not required, however, for efficient trafficking of the receptor to the cell surface. This study establishes the functional importance of N-glycosylation of the V(1a)R.

Cell cycle-dependent coupling of the vasopressin V1a receptor to different G proteins

Arginine vasopressin (AVP) regulates biological processes by binding to G protein-coupled receptors. In Swiss 3T3 fibroblasts, expressing the V(1a) subtype of vasopressin receptors, AVP mobilizes calcium from intracellular stores. In proliferating cells, the AVP-induced increase in intracellular calcium concentration ([Ca(2+)](i)) was mediated by G proteins of the G(q) family, which are insensitive to pertussis toxin (PTX) pretreatment of the cells. In quiescent cells, the AVP-induced increase in [Ca(2+)](i) was partially PTX-sensitive, suggesting an involvement of G(i) proteins. We confirmed this by photoaffinity labeling of G proteins in Swiss 3T3 cell membranes activated by AVP. In Swiss 3T3 cells arrested in the G(0)/G(1) phase of the cell cycle, the AVP-induced increase in [Ca(2+)](i) was also partially PTX-sensitive but was PTX-insensitive in cells arrested in other phases of the cell cycles. The blocking effect of PTX pretreatment in G(0)/G(1) cells was mimicked by microinjection of antisense oligonucleotides suppressing the expression of the Galpha(i3) subunits. These results were confirmed by microinjection of antibodies directed against the C terminus of G protein alpha-subunits. The data presented indicate that in Swiss 3T3 fibroblasts synchronized in the G(0)/G(1) phase of the cell cycle the V(1a) receptor couples to G(q/11) and G(i3) to activate the phospholipase C-beta, leading to release of intracellular calcium.

Arginine vasopressin in the treatment of 50 patients with postcardiotomy vasodilatory shock

BACKGROUND

The barroreflex-mediated secretion of arginine vasopressin has been found to be defective in a variety of vasodilatory shock states, such as postcardiotomy shock, and administration of the hormone markedly improves vasomotor tone and blood pressure. The high incidence of vasodilatory shock in patients undergoing left ventricular assist device (LVAD) implantation makes this population an ideal model in which to assess the risks and benefits of vasopressin.

METHODS

The medical records of the 102 patients receiving LVADs at Columbia-Presbyterian Medical Center from January 1995 to August 1998 were reviewed. Fifty patients were eligible for study based on a history of arginine vasopressin administration in the operating room or intensive care unit within 24 hours of implantation.

RESULTS

Despite LVAD implantation and the administration of vasopressors, patients were hypotensive with a mean arterial pressure less than 60 mm Hg. The administration of vasopressin (0.09+/-0.05 U/min) increased mean arterial pressure (58+/-13 to 75+/-14 mm Hg; p<0.001) while reducing norepinephrine administration (11.7+/-13 to 7.9+/-6.0 mcg/min; p = 0.023). There was no significant change in LVAD flow. The incidence of compromised regional perfusion was not different between LVAD patients who received vasopressin as compared to hemodynamically stable LVAD patients who did not receive vasopressin.

CONCLUSIONS

We have demonstrated vasopressin at low doses to be a safe and an effective vasopressor in 50 patients with postcardiotomy vasodilatory shock.

Novel strategies for the design of receptor-selective vasopressin analogues: Aib-substitution and retro-inverso transformation

1. We determined the pharmacological profile of novel backbone-modified peptides designed as protease-resistant, selective analogues of AVP. Binding affinities of peptides were determined at both V1A and V2 subtypes of vasopressin receptor (VPR). Biological potencies of selected peptides were tested in pressor and antidiuretic bioassays. 2. Substitution of the achiral alpha-aminoisobutyric acid (Aib) at position 4 or 7 of AVP produced peptides that selectively bound the V2 VPR. Both [Aib4]AVP (140 IU mg-1) and [Aib7]AVP (36 IU mg-1) are selective antidiuretic agonists with little or no activity in uterotonic and pressor assays. 3. [Aib4] and [Aib7] derivatives of the linear V1A-selective antagonist [PhaaDTyr(Et)2Arg6Tyr(NH2)9]AVP bound selectively and with high affinity (Kd 0.51 and 4.1 nM respectively) to the V1A VPR. Bioassays confirmed that these peptides were potent antivasopressor agents (pA2 8.10 and 8.36 respectively). 4. A total retro-inverso strategy was used to prepare protease-resistant mimetics of both AVP and linear V1A-selective antagonists. Cyclic retro-inverso mimetics of AVP did not bind either V1A or V2 VPRs. In contrast, rationally designed retro-inverso mimetics of linear V1A-selective antagonists selectively bound the V1A VPR. 5. Our findings indicate novel methods to improve the pharmacodynamic and pharmacokinetic parameters of neurohypophysial hormone analogues which could be equally applicable to other peptide-receptor systems.

Homologous and heterologous phosphorylation of the vasopressin V1a receptor

The vasopressin V1a receptor undergoes homologous and heterologous desensitizations which can be mimicked by activation of protein kinase C. This suggests that phosphorylation of the V1a receptor may be involved in the desensitization mechanisms. Such a phosphorylation was presently investigated in HEK 293 cells stably transfected with rat vasopressin V1a receptor. Metabolic labelling and immunoprecipitation of epitope-tagged V1a receptor evidenced a 52-kDa band and a 92-kDa band. Glycosidase treatments and immunoblotting experiments suggest that the 52-kDa band corresponds to an immature unprocessed receptor protein, whereas the 92-kDa band would correspond to a highly glycosylated form of the mature V1a receptor. Exposure of the cells to vasopressin induced a selective 32P phosphate incorporation in the 92-kDa form of the receptor. This homologous ligand-induced phosphorylation was dose dependent with maximal phosphate incorporation corresponding to four times the basal level. Stimulation of the endogenous phospholipase C-coupled m3 muscarinic receptor by carbachol-induced heterologous phosphorylation of the V1a receptor whose amplitude was half that of the homologous phosphorylation. This heterologous phosphorylation was associated with a reduced vasopressin-dependent increase in intracellular calcium.

Low-dose vasopressin restores diuresis both in patients with hepatorenal syndrome and in anuric patients with end-stage heart failure

OBJECTIVES

The purpose of this study was to confirm earlier reports that low-dose vasopressin (LDVP) analogues promote urine output in patients with hepatorenal syndrome (HRS) and to check whether this mode of therapy could also be effective in renal shutdown due to nonhepatic conditions.

DESIGN

A prospective, open, interventional study.

SETTING

An intermediate-level (step-down) medical intensive care unit within a general medical ward of a large university-affiliated hospital.

SUBJECTS

Eighteen successive hospitalized patients with HRS (mean age 65 +/- 13 years) and 11 patients with end-stage congestive heart failure (CHF) (mean age 81 +/- 5 years) who failed to restore urine output with conventional treatment (fluids, dopamine, and diuretics) given for at least 24 h.

INTERVENTIONS

The patients received LDVP (1 IU h-1) continuously in addition to the conventional treatment.

MAIN OUTCOME MEASURES

Urine output and creatinine clearance every 24 h.

RESULTS

In the HRS group, before treatment the urine output was 155 +/- 9 mL 24 -1h (mean +/- SD). After treatment with LDVP for 24, 48, and 72 h, urine output improved to 1067 +/- 87, 1020 +/- 501, and 1311 +/- 988 mL 24 -1h, respectively (P < 0.0001 for all measures; two-tailed paired t-test). In the CHF group, before treatment the urine output was 99 +/- 99 mL 24 -1h. After treatment with LDVP for 24, 48, and 72 h, this improved to 1125 +/- 994 mL 24 -1h (P = 0.0028), 1821 +/- 1300 mL 24 -1h (P = 0.004), and 2920 +/- 2423 mL 24 -1h (P = 0.0012), respectively. The improvement in urine output was not accompanied by a parallel improvement in creatinine clearance. The overall outcome did not change, and all patients except two in each group succumbed to their end-stage disease, due to nonrenal causes.

CONCLUSIONS

LDVP is effective in restoring urine output both in HRS and in CHF. This suggests that LDVP affects mechanisms not specifically related to liver disease. LDVP may be useful in critical patients with renal shutdown whilst awaiting liver or heart transplantation.

Visualization of cell surface vasopressin V1a receptors in rat hepatocytes with a fluorescent linear antagonist

To visualize cell surface V1a vasopressin receptors in rat hepatocytes in the absence of receptor-mediated endocytosis, we used a high-affinity fluorescent linear antagonist, Rhm8-PVA. Epifluorescence microscopy (3CCD camera) and fluorescence spectroscopy were used. Rhm8-PVA alone did not stimulate Ca2+ signals and competitively blocked Ca2+ signals (Kinact of 3.0 nM) evoked by arginine vasopressin (vasopressin). When rat hepatocytes were incubated with 10 nM of Rhm8-PVA for 30 min at 4C, the fluorescent antagonist bound to the surface of cells, presumably the plasma membrane. The V1a receptor specificity of Rhm8-PVA binding was confirmed by its displacement by the nonfluorescent antagonist V4253 and by the natural hormone vasopressin at 4C. Prior vasopressin-mediated endocytosis of V1a receptors at 37C abolished binding of the labeled antagonist, whereas in non-preincubated cells, Rhm8-PVA labeled the cell surface of rat hepatocytes. When cells labeled with Rhm8-PVA at 4C were warmed to 37C to initiate receptor-mediated internalization of the fluorescent complex, Rhm8-PVA remained at the cell surface. Incubation temperature at 4C or 37C had little effect on binding of Rhm8-PVA. We conclude that Rhm8-PVA is unable to evoke receptor-mediated endocytosis and can readily be used to visualize cell surface receptors in living cells.

Characterization of G protein-coupled receptors expressed by ECV304 human endothelial cells

The aims of this study were to characterize G protein-coupled receptors endogenously expressed by ECV304 human endothelial cells, and to determine the utility of this transformed cell line as a vehicle for the expression of cloned receptors. Cellular responses to a broad range of agonists were determined by measuring changes in the intracellular content of second messengers (inositol phosphates and cyclic adenosine monophosphate). These studies identified H1 histamine receptors, P2U-purinoceptors and lysophosphatidic acid receptors which are functionally coupled to phosphoinositidase C. G protein-coupled receptors which bind adenosine (A2 receptor), calcitonin, and adrenaline (beta-adrenoceptor), and markedly stimulate adenylyl cyclase, are also endogenously expressed by ECV304. Agonists which did not stimulate ECV304 cells are: angiotensin II, angiotensin1-7, bombesin, bradykinin, desArg9-bradykinin, carbachol, endothelin-1, neurotensin, serotonin, substance K, substance P, thrombin and vasopressin. The rat Via vasopressin receptor was expressed by lipofection in two antibiotic-resistant clonal lines and expression confirmed by measuring agonist-induced changes in inostol phosphate production. We conclude that the ECV304 cell line is a suitable in vitro system to study the signal transduction pathways of some endogenous G protein-coupled receptors known to modulate endothelial function in vivo. ECV304 is also appropriate for the expression and functional characterization of cloned receptor proteins.

The behavioral effect of vasopressin in the ventral hippocampus is antagonized by an oxytocin receptor antagonist

[Arg8]vasopressin improved long-term retrieval processes and relearning in a go-no go visual discrimination task when bilaterally microinjected at a dose of 25 pg/animal into the ventral hippocampus of mice, 10 min prior to the retention session. We had shown that this enhancing effect is antagonized by pretreatment with equal or lower doses (25 pg or 1 ng) of the vasopressin V1 receptor antagonist, (d(CH2)5Tyr(Me)-vasopressin). The present study was an attempt to determine whether the vasopressin V2 receptor antagonist or oxytocin receptor antagonist is as effective as the vasopressin V1 receptor antagonist to block the behavioral effect of vasopressin in the ventral hippocampus. We tested the effect of 25 pg of [d(CH2)5-D-Ile2,Ile4,Arg8]vasopressin, a vasopressin V2 receptor antagonist, and [d(CH2)5,Tyr(Me)2,Thr4,Tyr-NH9(2)]ornithine vasotocin, an oxytocin receptor antagonist, under the same experimental conditions as those used to test the effect of the vasopressin V1 receptor antagonist. The results showed that the vasopressin V2 receptor antagonist microinjected into the ventral hippocampus did not alter the enhancing effect of vasopressin on retrieval and relearning. In contrast, the oxytocin receptor antagonist blocked the vasopressin-enhancing effect on retention processes. We can conclude from the data that both vasopressin V1 receptors and oxytocin receptors seem to be involved in the enhancing effect of vasopressin on memory retention. In contrast, the vasopressin V2 receptors do not seem to be involved in the effect of the peptide.

Effects of the vasopressin V1 agonist [Phe2,Ile3,Orn8]] vasopressin on regional kidney perfusion and renal excretory function in anesthetized rabbits

To test whether renal V1-receptors selectively influence blood flow in the renal medulla, we compared the effects of infusion of [Phe2,Ile3,Orn8]vasopressin (3-30 ng/kg/min) by the intravenous, renal arterial, and renal medullary interstitial routes in anesthetized rabbits. Intravenous [Phe2,Ile3,Orn8]vasopressin (30 ng/kg/min) reduced renal medullary perfusion (MBF) by 36 +/- 5% but did not significantly affect cortical perfusion (CBF). MBF was also reduced with the renal arterial (35 +/- 5%) and renal medullary interstitial (40 +/- 7%) routes but, in contrast to the intravenous infusion, CBF was also reduced, by 21 +/- 3% and 15 +/- 3%, respectively. Urine flow and sodium excretion were increased by [Phe2,Ile3,Orn8]vasopressin, and with direct intrarenal administration, this effect was similar for both the infused (left) and noninfused (right) kidneys. After a 20-min renal medullary interstitial infusion of [3H]norepinephrine, radiolabel concentration was approximately fivefold greater in the left medulla than in the left cortex. We conclude that [Phe2,Ile3,Orn8]vasopressin acts on V1-receptors to alter regional kidney blood flow and tubular salt and water handling. The V1-receptors involved are almost certainly within the kidney itself, but given the contrasting effects of the different infusion routes on MBF and CBF, we cannot exclude the possibility that some of the observed effects of [Phe2,Ile3,Orn8]vasopressin are mediated by activation of extra-renal V1-receptors.

Role of protein kinase C and carboxyl-terminal region in acute desensitization of vasopressin V1a receptor

The role of protein kinase C activation and carboxyl-terminal region in rapid desensitization of the vasopressin V1a receptor was investigated in Xenopus oocytes. Preincubation of the oocytes with vasopressin or with the diacylglycerol analog 1-oleoyl-2-acetyl-sn-glycerol (OAG), or direct injection of active protein kinase C, all blunted the calcium response of the V1a receptor. Truncation of the 51 terminal amino acids (S374STOP) modified neither the intracellular calcium response to vasopressin nor its desensitization by vasopressin or OAG. These data suggest that desensitization of the V1a receptor is mediated by PKC activation and that its carboxyl-terminal domain is not required for signal transduction and rapid desensitization.

Pharmacological characterization of a vasopressin V1 receptor in the isolated human gastric artery

Species-related specific differences in the pharmacological profile of vasopressin V1a receptors have been reported. Thus, the aim of the present study was to identify a vascular preparation of human origin expressing V1a receptors. Vasopressin was found to contract human gastric artery strips without endothelium with high affinity (pEC50 8.9). The maximal effect induced by vasopressin was inversely related to the diameter of the vessel. Oxytocin was found to contract the human gastric artery strips with low potency (pEC50 7.2). Contraction induced by vasopressin was competitively antagonized by the non peptide vasopressin receptor antagonists SR 49059 (pA2 9.2), OPC 21268 (pA2 6.2) and OPC 31260 (pA2 7.1). The order of potency of agonists (vasopressin > > oxytocin) and of antagonists (SR 49059 > > OPC 31260 > OPC 21268) indicate the contraction induced by vasopressin in the isolated human gastric artery is mediated by the V1a receptor type. The present data are similar to those obtained in different preparations expressing the native human V1a receptor as well as to those obtained in cell transfected with this receptor. The human gastric artery is a monoreceptor system of great utility for studying the effects of new drugs interacting with the human V1a receptor.

Autoradiographic localization of vasopressin V1a receptors in the rat kidney using [3H]-SR 49059

Localization and characterization of binding sites of the selective non-peptide vasopressin receptor V1a ligand, [3H]-SR 49059, were investigated in the adult rat kidney by quantitative autoradiography using a fast-detecting radioluminographic phosphor-imaging plate system. [3H]-SR 49059, like the other V1a ligands used, showed a total absence of binding in the papilla, discrete and sparse labeling in the cortex and maximal binding in the outer part of the inner medulla. This labeling seemed to be mainly associated with medullary interstitial cells and vascular elements of the vasa recta. Conversely, [3H]-AVP intensely labeled the V2-enriched medulla-papillary portion of the kidney and, to a lesser extent, the cortical structures. [3H]-SR 49059 binding, quantified in the outer part of the inner medulla in rat kidney sections, was time-dependent, reversible, saturable and a single class of high affinity binding sites (Kd = 1.48 +/- 0.16 nM) was identified. The relative potencies of the reference peptide and non-peptide compounds to inhibit [3H]-SR 49059 binding confirm the V1a nature of the site and the stereospecificity of this binding. Thus, [3H]-SR 49059 allows the mapping and characterization of the V1a receptor population present in the rat kidney. The stability and the highly selective affinity of this non-peptide ligand for rat and human V1a receptors make it a suitable probe for the localization of V1a receptors in organs expressing heterogeneous populations of receptors.

Renal bradykinin and vasopressin receptors: ligand selectivity and classification

We studied the specific binding of radiolabeled bradykinin ([3H]BK) and vasopressin ([3H]AVP) to membrane preparations of bovine and porcine kidney medulla. [3H]BK reversibly labeled a single site (Kd = 1.06 nM) in bovine kidney medulla independently of [Mg2+]. The number of BK receptors in bovine kidney medulla, Bmax = 122 fmol/mg protein, is markedly (2- to 3-fold) higher than that reported in other tissues. Further characterization by ligand binding indicated that the bovine bradykinin receptor was the B2a subtype, pharmacologically related to B2a receptors expressed by human and rabbit tissues. In contrast, the specific binding of [3H]BK, but not [3H]AVP, to porcine kidney medulla (Kd = 0.32 nM, Bmax = 45 fmol/mg) was dependent upon the presence of enzyme inhibitors to prevent the rapid and selective degradation of bradykinin. Interspecies differences were revealed for renal medulla V2 vasopressin receptors with respect to their abundance and their affinity for several V2-selective ligands. In summary, (i) bovine kidney medulla is a convenient source of tissue for studying the B2a bradykinin receptor subtype; (ii) there are significant species-dependent differences in both the abundance of renal medulla B2a and V2 receptors and the ligand selectivity of V2 receptors; and (iii) these findings are significant in relation to the physiological and pathological roles of renal kinins and their interaction with the neurohypophysial peptide hormone system.

Molecular recognition of peptide and non-peptide ligands by the extracellular domains of neurohypophysial hormone receptors

This study was designed to ascertain whether the extracellular loops of vasopressin/oxytocin receptors bind ligands and, if so, to locate the molecular determinants of this ligand-receptor interaction. Ligand-binding studies were employed using a rat liver V1a vasopressin receptor preparation and both peptide and non-peptide receptor ligands. Synthetic peptides corresponding to defined regions of the extracellular surface of the neurohypophysial hormone receptors recognized radioligands. These receptor mimetics inhibited the binding of radioligands to the V1a receptor with apparent affinities (pKi) ranging from 3.1 to 6.75. The same mimetics had no effects on the binding of angiotensin II to the rat AT1 receptor, indicating specificity for V1a receptor ligands. A mimetic peptide (DITYRFRGPDWL) of the first extracellular loop (ECII) of the V1a vasopressin receptor also inhibited vasopressin-stimulated, but not angiotensin II-stimulated, glycogen phosphorylase in isolated rat hepatocytes. In contrast, scrambled ECII mimetics displayed greatly reduced affinity for vasopressin. In addition, the role of peptide side-chain versus main-chain atoms in the binding of ligands by vasopressin receptors was addressed using retro-inverso peptide mimetics. Our findings indicate a precise orientation of the extracellular receptor surface (particularly the ECII domain) which facilitates the initial 'capture' of both peptide and non-peptide ligands. Moreover, the data indicate that the main-chain atoms of both a major binding-site determinant in the first extracellular loop of the receptor and the neurohypophysial hormones contribute significantly to the ligand-receptor interaction. These findings also suggest that soluble receptor-binding domains have therapeutic potential.

Probing the V1a vasopressin receptor binding site with pyroglutamate-substituted linear antagonists

We have synthesized eight analogues of the linear vasopressin antagonist DTyr(Et)2-Phe3-Gln4-Asn5-Arg6-Pro7-Arg8-Tyr(NH2)9 substituted with L-, or D-, pyroglutamate at position-1, Asn or Val at position-4 and Arg or Met at position 6. All of these peptides bound to the V1a vasopressin receptor with affinities ranging 33.6-5, 470 nM. Of this series, only two peptides, [LpGlu1Val4Arg6Tyr(NH2)9]AVP Kd = 48.4 nM and [DpGlu1Val4Arg6Tyr(NH2)9]AVP Kd = 691 nM, bound to the V2 vasopressin receptor. All of the neurohypophysial hormone receptors studied (V1a VPR, V2 VPR and OTR) were found to be stereoselective with respect to the N-terminal pGlu residue. The effect on binding characteristics of L-pGlu1 and D-pGlu1 analogues was dependent on both the sequence of the peptide and on the receptor subtype in question. From these data we found that peptide 5, which has the structure DpGlu-DTyr(Et)-Phe-Val-Asn-Arg-Pro-ARg-Tyr(NH2), exhibited the highest V1a/OTR selectivity reported to date (V1aVPR Kd = 82 nM; OTR no binding at 10 microM). As such, peptide 5 will provide useful leads to the development of ligands with enhanced V1a/OTR selectivity. The binding affinity and hydrophobicity of pyroglutamate-substituted peptides was compared with previously characterized V1a receptor antagonists which contained a range of position-1 substitutions. The hydrophobicity of both cyclic and linear antagonists was markedly increased relative to the agonists AVP and [Phe2Orn8]VT but increased hydrophobicity alone did not exclusively lead to high affinity antagonists. Data presented support the contention that in addition to a general increase in hydrophobicity/lipophilicity, position-1 influences the pharmacophore of vasopressin antagonists by providing molecular determinants for ligand/receptor interaction.