Carboxy terminus of vasopressin required for activity but not binding

Structures of the arginine-vasopressin and oxytocin receptor signaling complexes

Arginine-vasopressin (AVP) and oxytocin (OT) are neurohypophysial hormones which share a high sequence and structure homology. These are two cyclic C-terminally amidated nonapeptides with different residues at position 3 and 8. In mammals, AVP and OT exert their multiple biological functions through a specific G protein-coupled receptor family: four receptors are identified, the V1a, V1b, V2 receptors (V1aR, V1bR and V2R) and the OT receptor (OTR). The chemical structure of AVP and OT was elucidated in the early 1950s. Thanks to X-ray crystallography and cryo-electron microscopy, it took however 70 additional years to determine the three-dimensional structures of the OTR and the V2R in complex with their natural agonist ligands and with different signaling partners, G proteins and β-arrestins. Today, the comparison of the different AVP/OT receptor structures gives structural insights into their orthosteric ligand binding pocket, their molecular mechanisms of activation, and their interfaces with canonical Gs, Gq and β-arrestin proteins. It also paves the way to future rational drug design and therapeutic compound development. Indeed, agonist, antagonist, biased agonist, or pharmacological chaperone analogues of AVP and OT are promising candidates to regulate different physiological functions and treat several pathologies.

A Self-Assembling Lipidic Peptide and Selective Partial V2 Receptor Agonist Inhibits Urine Production

Lipidised analgesic peptide prodrugs self-assemble into peptide nanofibers; with the nanofiber morphology protecting the peptide from plasma degradation and improving therapeutic efficacy. Extending this learning, we hypothesised that a self-assembling lipidized peptide arginine vasopressin (AVP) receptor agonist, that had not been designed as a prodrug, could prove pharmacologically active and control urine production. The only approved AVP receptor agonist, desmopressin is indicated for the treatment of central diabetes insipidus (DI), bedwetting, haemophilia A and von Willebrand disease. Desmopressin is well tolerated by most patients, however adverse effects, such as hyponatraemia and water intoxication necessitate a strict fluid intake, thus motivating the search for alternative DI treatments. Selective V2 receptor agonism is required for anti-DI activity and we hypothesised that our new lipidized peptide (METx) would lead to selective AVP receptor agonism. METx was synthesised and characterised and then tested for activity against the V2, V1a and OT uterine receptors and not tested against the V1b receptor as METx was not expected to cross the blood brain barrier. METx was also tested in vivo in a healthy rat model. METx forms nanofibers and is a partial V2 receptor agonist (determined by measuring MDCK cell line cAMP accumulation), producing 57% of AVP's maximal activity (EC50 = 2.7 nM) and is not a V1a agonist up to a concentration of 1 μM (determined by measuring A7r5 cell line D-myo-inositol-1-phosphate accumulation). METx is a weak OT receptor antagonist, reducing the frequency of OT induced contractions (EC50 = 350 nM) and increasing the OT EC50 from 0.081 nM to 21 nM at a concentration of 600 nM. METx (41 nM) had no effect on spontaneous uterine contractions and METx (100 nM) had no effect on OT induced uterine contractions. Simulated binding studies show that binding avidity to the receptors follows the trend: V2 > OT > V1a. On intravenous injection, a nanoparticle formulation of METx reduced urine production in a healthy rat model in a dose responsive manner, with 40 mg kg^-1 METx resulting in no urine production over 4 hours. The lipidized self-assembling peptide - METx - is a selective competitive V2 receptor agonist and an anti-diuretic.

Vasopressin inactivation: Role of insulin-regulated aminopeptidase

The physiological importance of vasopressin inactivation has long been appreciated, but the mechanisms and potential pathophysiologic roles of this process remain active subjects of research. Human Placental Leucine Aminopeptidase (P-LAP, encoded by the LNPEP gene) is an important determinant of vasopressinase activity during pregnancy and is associated with gestational diabetes insipidus and preeclampsia. Insulin-Regulated Aminopeptidase (IRAP), the rodent homologue of P-LAP, is coregulated with the insulin-responsive glucose transporter, GLUT4, in adipose and muscle cells. Recently, the Tether containing a UBX domain for GLUT4 (TUG) protein was shown to mediate the coordinated regulation of water and glucose homeostasis. TUG sequesters IRAP and GLUT4 intracellularly in the absence of insulin. Insulin and other stimuli cause the proteolytic cleavage of TUG to mobilize these proteins to the cell surface, where IRAP acts to terminate the activity of circulating vasopressin. Intriguingly, genetic variation in LNPEP is associated with the vasopressin response and mortality during sepsis, and increased copeptin, a marker of vasopressin secretion, is associated with cardiovascular and metabolic disease. We propose that in the setting of insulin resistance in muscle, increased cell-surface IRAP and accelerated vasopressin degradation cause a compensatory increase in vasopressin secretion. The increased vasopressin concentrations present at the kidneys then contribute to hypertension in the metabolic syndrome. Further analyses of metabolism and of vasopressin and copeptin may yield novel insights into a unified pathophysiologic mechanism linking insulin resistance and hypertension, and potentially other components of the metabolic syndrome, in humans.

Discovery of Potent, Selective, and Short-Acting Peptidic V2 Receptor Agonists

The vasopressin analogue desmopressin (desamino-d-arginine⁸ vasopressin, dDAVP, 1) is a potent vasopressin 2 (V₂) receptor (V₂R) agonist approved in many countries for the treatment of diabetes insipidus, primary nocturnal enuresis, nocturia, and coagulation disorders. Since 1 is primarily excreted via the kidneys, an age-related decline in kidney function leads to slower elimination, prolonged antidiuresis, and hyponatremia. In search of novel, potent, selective, and short-acting peptidic V₂R agonists, we synthesized a series of C-terminally truncated analogues of [Val⁴]dDAVP, 2, modified in positions 2, 3, and 7 and/or at the disulfide bridge. The peptides were evaluated for in vitro potency at the human V₂ receptor, selectivity versus the related receptors (human vasopressin 1a receptor, human vasopressin 1b receptor, and human oxytocin receptor), and pharmacokinetic profiles in rodents and other higher species. The truncated analogues show excellent potency at the V₂R, increased systemic clearance, and shorter half-life in rats. Two compounds 19 (c(Bua-Cpa-Thi-Val-Asn-Cys)-Pro-Agm) and 38 (c(Bua-Cpa-Thi-Val-Asn-Cys)-Pro-d-Arg-NEt₂) have been selected for clinical development for nocturia.

Subtle modifications to oxytocin produce ligands that retain potency and improved selectivity across species

Oxytocin and vasopressin mediate various physiological functions that are important for osmoregulation, reproduction, cardiovascular function, social behavior, memory, and learning through four G protein-coupled receptors that are also implicated in high-profile disorders. Targeting these receptors is challenging because of the difficulty in obtaining ligands that retain selectivity across rodents and humans for translational studies. We identified a selective and more stable oxytocin receptor (OTR) agonist by subtly modifying the pharmacophore framework of human oxytocin and vasopressin. [Se-Se]-oxytocin-OH displayed similar potency to oxytocin but improved selectivity for OTR, an effect that was retained in mice. Centrally infused [Se-Se]-oxytocin-OH potently reversed social fear in mice, confirming that this action was mediated by OTR and not by V1a or V1b vasopressin receptors. In addition, [Se-Se]-oxytocin-OH produced a more regular contraction pattern than did oxytocin in a preclinical labor induction and augmentation model using myometrial strips from cesarean sections. [Se-Se]-oxytocin-OH had no activity in human cardiomyocytes, indicating a potentially improved safety profile and therapeutic window compared to those of clinically used oxytocin. In conclusion, [Se-Se]-oxytocin-OH is a novel probe for validating OTR as a therapeutic target in various biological systems and is a promising new lead for therapeutic development. Our medicinal chemistry approach may also be applicable to other peptidergic signaling systems with similar selectivity issues.

Development of a human vasopressin V1a-receptor antagonist from an evolutionary-related insect neuropeptide

Characterisation of G protein-coupled receptors (GPCR) relies on the availability of a toolbox of ligands that selectively modulate different functional states of the receptors. To uncover such molecules, we explored a unique strategy for ligand discovery that takes advantage of the evolutionary conservation of the 600-million-year-old oxytocin/vasopressin signalling system. We isolated the insect oxytocin/vasopressin orthologue inotocin from the black garden ant (Lasius niger), identified and cloned its cognate receptor and determined its pharmacological properties on the insect and human oxytocin/vasopressin receptors. Subsequently, we identified a functional dichotomy: inotocin activated the insect inotocin and the human vasopressin V1b receptors, but inhibited the human V1aR. Replacement of Arg8 of inotocin by D-Arg8 led to a potent, stable and competitive V1aR-antagonist ([D-Arg8]-inotocin) with a 3,000-fold binding selectivity for the human V1aR over the other three subtypes, OTR, V1bR and V2R. The Arg8/D-Arg8 ligand-pair was further investigated to gain novel insights into the oxytocin/vasopressin peptide-receptor interaction, which led to the identification of key residues of the receptors that are important for ligand functionality and selectivity. These observations could play an important role for development of oxytocin/vasopressin receptor modulators that would enable clear distinction of the physiological and pathological responses of the individual receptor subtypes.

Vasopressin-receptor antagonists

Despite a crucial role in body fluid homeostasis, elevated vasopressin levels can also be pathological in conditions such as congestive heart failure, liver cirrhosis and the syndrome of inappropriate antidiuretic hormone secretion. The result of elevated vasopressin is renal water retention and hyponatremia, a low serum sodium concentration. Hyponatremia is associated with excess morbidity and mortality. Nonpeptide vasopressin-receptor antagonists represent a new drug class of small molecules that competitively inhibit one or more of the vasopressin receptors. There are three vasopressin receptors in humans, including V1a, V1b and V2. Selective V2- and combined V1a/V2-receptor antagonists have been developed for the treatment of hyponatremia resulting from congestive heart failure, liver cirrhosis and the syndrome of inappropriate antidiuretic hormone secretion. Two nonpeptide vasopressin-receptor antagonists, conivaptan and tolvaptan, have recently been approved by American and European drug authorities for clinical use. This article aims to provide a succinct and clinical update on nonpeptide vasopressin-receptor antagonists, including their mechanism of action, performance in randomized clinical trials and current clinical status.

Impact of the Merrifield solid phase method on the design and synthesis of selective agonists and antagonists of oxytocin and vasopressin: a historical perspective

This tribute to Bruce Merrifield traces the author's fortuitous path in 1964 from Vincent du Vigneaud's laboratory to the laboratory of D. W. Woolley to learn the solid phase method and then to his first faculty position in the Department of Biochemistry, McGill University, Montreal in 1965. It recalls the key roles played from early 1966 to July 1967 by Bruce Merrifield, John Stewart, Arnold Marglin, Herb Takashima, and Vincent du Vigneaud in providing key advice to the author's efforts to use the solid phase method to synthesize oxytocin; while simultaneously the du Vigneaud and Merrifield laboratories were collaborating on the solid phase synthesis of deamino-oxytocin. Both syntheses were published in the same issue of the Journal of American Chemical Society in 1968. Also described is how this breakthrough impacted the author's scientific career: by leading to highly productive collaborative studies, initially with Wilbur H. Sawyer and subsequently with others, on the design and synthesis of selective agonists, antagonists, and radioiodinated ligands for oxytocin and vasopressin receptors. These syntheses were greatly facilitated by the contributions of highly talented graduate students, research technicians, and visiting peptide chemists from Hungary, England, Poland, Bulgaria, and China. Many of these peptides have become very valuable pharmacological tools in studies on the peripheral and central effects of oxytocin and vasopressin: further attesting to the profound impact of the solid phase method as the cornerstone for all the discoveries, which he and his collaborators and coworkers have made over the past 40 years.

Investigation of mechanism of desmopressin binding in vasopressin V2 receptor versus vasopressin V1a and oxytocin receptors: Molecular dynamics simulation of the agonist-bound state in the membrane–aqueous system

The vasopressin V2 receptor (V2R) belongs to the Class A G protein-coupled receptors (GPCRs). V2R is expressed in the renal collecting duct (CD), where it mediates the antidiuretic action of the neurohypophyseal hormone arginine vasopressin (CYFQNCPRG-NH2, AVP). Desmopressin ([1-deamino, 8-D]AVP, dDAVP) is strong selective V2R agonist with negligible pressor and uterotonic activity. In this paper, the interactions responsible for binding of dDAVP to vasopressin V2 receptor versus vasopressin V1a and oxytocin receptors has been examined. Three-dimensional activated models of the receptors were constructed using the multiple sequence alignment and the complex of activated rhodopsin with Gt(alpha) C-terminal peptide of transducin MII-Gt(alpha) (338-350) prototype (Slusarz, R.; Ciarkowski, J. Acta Biochim Pol 2004 51, 129-136) as a template. The 1-ns unconstrained molecular dynamics (MD) of receptor-dDAVP complexes immersed in the fully hydrated 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylcholine (POPC) membrane model was conducted in an Amber 7.0 force field. Highly conserved transmembrane residues have been proposed as being responsible for V2R activation and G protein coupling. Molecular mechanism of the dDAVP binding has been suggested. The internal water molecules involved in an intricate network of the hydrogen bonds inside the receptor cavity have been identified and their role in the stabilization of the agonist-bound state proposed.

Design, synthesis and pharmacological characterization of a potent radioiodinated and photoactivatable peptidic oxytocin antagonist

Using a segment strategy, we have synthesized four iodinated photoactivatable cyclic peptidic ligands of oxytocin, bearing a beta-mercapto-betabeta-cyclopentamethylene propionic group (Pmp) on their N-terminus. All the syntheses were RP-HPLC monitored, and the compounds were HPLC purified. They were characterized by 1H NMR, MALDI-TOF, or FAB mass spectrometries. The affinities of Pmp-Tyr(Me)-Ile-Thr-Asn-Cys-Gly-Orn-Phe(3I,4N3)-NH2 (20), Pmp-Tyr-Ile-Thr-Asn-Cys-Gly-Orn-Phe(3I,4N3)-NH2 (21), Pmp-Tyr(Me)-Ile-Thr-Asn-Cys-Pro-Orn-Phe(3I,4N3)-NH2 (22), and Pmp-Tyr-Ile-Thr-Asn-Cys-Pro-Orn-Phe(3I,4N3)-NH2 (23) were evaluated as inhibition constants (K(i), in nM) for the human oxytocin receptor expressed in Chinese hamster ovary cells by displacement of a radioiodinated disulfide-cyclized antagonist (Elands et al. Eur. J. Pharmacol. 1987, 147, 197-207). The most potent of them, compound 22, was synthesized by another method in order to allow its radiolabeling by 125I. Its dissociation constant (K(d)) for the human oxytocin receptor, directly measured in saturation studies, was 0.25 +/- 0.04 nM, and its antagonist properties were determined by inactivation of phospholipase C, thus obtaining an inactivation constant (K(inact)) of 0.18 +/- 0.02 nM, evaluated by inositol phosphate accumulation. This compound is a very good tool for the mapping of peptidic antagonist binding sites in the human oxytocin receptor.

Vesicle-mediated phosphatidylcholine reapposition to the plasma membrane following hormone-induced phospholipase D activation

Phospholipase D (PLD) activation involved in signal transduction may lead to the hydrolysis of conspicuous amounts of phosphatidylcholine (PC). This study shows that PLD activation significantly alters the plasma membrane (PM) environment and the membrane exchange dynamics. PC-PLD activation in vasopressin (AVP)-stimulated L6 myogenic cells was accompanied by increased exocytosis and decreased membrane fluidity, as shown by transmission EM and fluorescence spectroscopy of trimethylammonium-diphenyl-hexatriene. AVP-induced exocytosis appeared to be brefeldin A-insensitive. PLD inhibition by Zn(2+) and PC de novo synthesis inhibition by hexadecylphosphocholine abolished AVP-induced vesicle traffic. Upon AVP stimulation, metabolically labeled PC decreased in PM, then transiently increased in microsomes, and returned to the prestimulus level in the PM within 5 min, a phenomenon requiring PC neosynthesis and microtubule functionality. Vesicle traffic with similar features was also observed after endothelin-1-induced PC-PLD activation in rat peritubular myoid cells. These results indicate that, in nonsecretory cells, exocytosis coupled to PC de novo synthesis restores PM-PC, conspicuously consumed during PLD-mediated signal transduction.

Discovery and design of novel and selective vasopressin and oxytocin agonists and antagonists: the role of bioassays

Synthetic oxytocin and vasopressin agonists and antagonists have become important tools for research and were instrumental in the identification of the four known receptor subtypes, V1a, V2, V1b (V3) and oxytocin, of these peptide hormones. However, the relative lack of receptor selectivity, particularly of the antagonists, has limited their usefulness as experimental probes and their potential as therapeutic agents. We now present some findings from our continuing studies aimed at the design of more selective oxytocin and vasopressin agonists and antagonists and a structure-activity relationship update on our recently discovered novel hypotensive vasopressin peptides. Bioassays have been, and continue to be, of critical importance in leading to the discovery of the novel agonists, antagonists and hypotensive peptides reported here. This paper highlights three main aspects of these studies. (1) Replacement of the tyrosine2 and/or phenylalanine3 residues in the V2 agonist deamino,[Val4,D-Arg8]arginine-vasopressin (dVDAVP) by thienylalanine resulted in selective V2 agonists with strikingly high potencies. However, the peptide solutions were unstable and lost activity over time. These highly potent V2 agonists, which are devoid of vasopressor activity, are promising leads for improving drugs for treating diabetes insipidus, enuresis and coagulation disorders. (2) Diaminopropionic acid and diaminobutyric acid substitution at position-5 in oxytocin and in V1a antagonists yielded, respectively, the first specific antagonist for the oxytocin receptor, desGly-NH2,d(CH2)5[D-Trp2,Thr4,Dap5]OVT and the first specific antagonist for the vasopressin V1a receptor, d(CH2)5[Tyr(Me)2,Dab5]AVP. The availability of single receptor subtype-specific or selective antagonists will enhance our ability to delineate receptor functions. Utilising these new receptor specific probes, we were able to show that the uterotonic action of vasopressin is mediated principally by oxytocin and not by V1a receptors. (3) Replacement of the phenylalanine3 residue in the V1a/V2/oxytocin antagonist, d(CH2)5[D-Tyr(Et)2,Val4]AVP, with arginine3 yielded the novel, selective, hypotensive vasopressin peptide, d(CH2)5[D-Tyr(Et)2,Arg3,Val4]AVP (Peptide I). Bioassay characterisations of Peptide I show that its vasodepressor action is independent of the peripheral autonomic, bradykinin, nitric oxide and prostaglandin systems and is not mediated by the known classical oxytocin and vasopressin receptors. These findings suggest the existence of a new vasopressin receptor subtype that may be relevant to the vasodilating action of vasopressin in regional vascular beds. Iodinatable hypotensive peptides have been synthesised and could be developed as markers for the putative new receptor. Ongoing structure-activity relationship studies on Peptide I have led to more potent and selective hypotensive peptides for use as new research tools and as leads for the development of a new class of antihypertensive agents.

Synthesis and structure-activity investigation of novel vasopressin hypotensive peptide agonists

We report the solid phase synthesis and vasodepressor potencies of the novel hypotensive peptide [1(-beta-mercapto-beta,beta-pentamethylene propionic acid)-2-O-ethyl-D-tyrosine, 3-arginine, 4-valine] arginine vasopressin, d(CH2)5[D-Tyr(Et)2, Arg3, Val4]AVP (A), its related Lys3 (B), Tyr-NH(9)2 (C), [Lys3, Tyr-NH(9)2 (D) analogs and in a preliminary structure-activity study of positions 2-4 and 7-9, 24 analogs (1-24) of A-C. Peptides 1-6, 9-14 have the following single substituents at positions 2, 3, 4, 8 and 9 in (A): 1, D-Tyr(Me)2; 2, L-Tyr(Et)2; 3, Orn3; 4, N-Me-Arg3; 5, Glu3; 6, Arg4; 9, D-Arg8; 10, Eda9; 11, Arg-NH(9)2; 12, Ala-NH(9)2; 13, desGly9; 14, desGly-NH(9)2. Peptides 15 and 16 are analogs of B which possess the following single modifications: 15, Arg-NH(9)2; 16, desGly9. Peptides 7 and 8 are analogs of (C) with the following single modification: 7, Gln4; 8, Lys8. Peptides 17-24 are analogs of A possessing the following multiple modifications: 17, [Sar7, Eda9]; 18, [Arg7, Eda9]; 19, [Arg7, Eda9<--Tyr10]; 20, [Arg4, Arg-NH(9)2]; 21, [Ile4, desGly9]; 22, [Arg4, desGly9]l; 23, [Arg7, desGly9]; 24, [Arg7, Lys8, desGly9]. All 24 new peptides were evaluated for agonistic and antagonistic activities in in vivo antidiuretic (V2-receptor), vasopressor (V1a-receptor) and in in vitro (no Mg2+) oxytocic (OT-receptor) assays and like the parent peptides (A-D) (Chan et al. Br. J. Pharmacol. 1998; 125: 803-811) were found to exhibit no or negligible activities in these assays. Vasodepressor potencies were determined in anesthetized male rats with baseline mean arterial blood pressure maintained at 110-120 mmHg. The effective dose (ED), in microg 100 g(-1) i.v., required to produce a vasodepressor response of 5 cm2, area under the vasodepressor response curve (AUC) during the 5-min period following the injection of the test peptide, was determined. Therefore, the EDs measure the relative vasodepressor potencies of the hypotensive peptides. The following ED values were obtained for A-D and for peptides 1-24: A, 4.66; B, 5.75; C, 10.56; D, 11.60; 1, approximately 20; 2, approximately 30; 3, 6.78; 4, non-detectable (ND); 5, ND; 6, approximately 32; 7, ND; 8, 8.67; 9, ND; 10, 2.43; 11, 3.54; 12, 10.57; 13, 4.81; 14, ND; 15, 4.47; 16, 9.78; 17, 5.72; 18, 1.10; 19, 1.05; 20, 10.41; 21, 9.13; 22, approximately 33; 23, 3.01; 24, 1.71. A is clearly the most potent of the four original hypotensive peptides A-D. These data provide insights to which modification of A enhance, retain or abolish hypotensive potencies. Six of the new hypotensive peptides are significantly more potent than A. These are peptides 10, 11, 18, 19, 23 and 24. Peptide 19, a radioiodinatable ligand, is ten times more potent than C or D. The Gln4 modification of C and the N-Me-Arg3, Glu3, D-Arg8 and desGly-NH(9)2 modifications of A abolished hypotensive potency. By contrast, the Eda9, Arg-NH(9)2, [Sar7, Eda9], [Arg7, Eda9<- -Tyr10], [Arg7, desGly9], [Arg7, Lys8, desGly9] modifications of A all led to enhancements of hypotensive potency. This initial structure-activity exploration provides useful clues to the design of (a) more potent vasodepressor peptides and (b) high affinity radioiodinatable ligands for the putative AVP vasodilating receptor. Some of the peptides here may be of value as pharmacological tools for studies on the complex cardiovascular actions of AVP and may lead to the development of a new class of anti-hypertensive agents.

Position three in vasopressin antagonist tolerates conformationally restricted and aromatic amino acid substitutions: a striking contrast with vasopressin agonists

We report the solid-phase synthesis and some pharmacological properties of 12 position three modified analogues (peptides 1-12) of the potent non-selective antagonist of the antidiuretic (V2-receptor), vasopressor (V1a-receptor) responses to arginine vasopressin (AVP) and of the uterine contracting (OT-receptor) responses to oxytocin (OT), [1(-beta mercapto-beta,beta-pentamethylenepropionic acid)-2-O-ethyl-D-tyrosine 4-valine] arginine vasopressin [d(CH2)5D-Tyr(Et)2VAVP] (A) and two analogues of (B) (peptides 13,14), the 1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid3 (Tic3) analogue of (A). Peptides 1-12 have the following substituents at position three in (A): (1) Pro; (2) Oic; (3) Atc; (4) D-Atc; (6) D-Phe; (7) Ile; (8) Leu; (9) Tyr; (10) Trp; (11) Hphe; (12) [HO]Tic; Peptide (13) is the Tyr-NH2(9) analogue of (B): Peptide (14) is the D-Cys(6) analogue of (B). All 14 new peptides were evaluated for agonistic and antagonistic activities in in vivo V2 and V1a assays and in vitro (no Mg2+)n oxytocic assays. With the exception of the D-Phe3 peptide (No. 6), which exhibits very weak V2 agonism (approximately 0.0017 U/mg), none of the remaining 13 peptides exhibit any agonistic activities in these assays. In striking contrast to their deleterious effects on agonistic activities in AVP, the Pro3, Oic3, Tyr3 and Hphe3 substitutions in (A) are very well tolerated, leading to excellent retention of V2, V1a and OT antagonistic potencies. All are more potent as V2 antagonists than the Ile3 and Leu3 analogues of (A). The Tyr-NH2(9) and D-Cys(6) substitutions in (B) are also well tolerated. The anti-V2 pA2 values of peptides 1-5 and 7-14 are as follows (1) 7.77 +/- 0.03; (2) 7.41 +/- 0.05; (3) 6.86 +/- 0.02; (4) 5.66 +/- 0.09; (5) approximately 5.2; (7) 7.25 +/- 0.08; (8) 6.82 +/- 0.06; (9) 7.58 +/- 0.05; (10) 7.61 +/- 0.08; (11) 7.59 +/- 0.07; (12) 7.20 +/- 0.05; (13) 7.57 +/- 0.1; (14) 7.52 +/- 0.06. All analogues antagonize the vasopressor responses to AVP, with anti-V1a pA2 values ranging from 5.62 to 7.64, and the in vitro responses to OT, with anti-OT pA2 values ranging from 5.79 to 7.94. With an anti-V2 potency of 7.77 +/- 0.03, the Pro3 analogue of (A) is surprisingly equipotent with (A), (anti-V2 pA2 = 7.81 +/- 0.07). These findings clearly indicate that position three in AVP V2/V1a antagonists, in contrast to position three in AVP agonists, is much more amenable to structural modification than had heretofore been anticipated. Furthermore, the surprising retention of V2 antagonism exhibited by the Pro3, Oic3, Tyr3, Trp3 and Hphe3 analogues of (A), together with the excellent retention of V2 antagonism by the Tyr-NH2(9) and D-Cys6 analogues of (B) are promising new leads to the design of potent and possibly orally active V2 antagonists for use as pharmacological tools and/or as radioiodinatable ligands and for development as potential therapeutic agents for the treatment of the hyponatremia caused by the syndrome of the inappropriate secretion of the antidiuretic hormone (SIADH).

Evidence for circulating vasopressin-like peptides in a case of polyuria

Plasma immunoreactive vasopressin (iAVP) was studied by RIA in a patient suffering from polyuria during chronic treatment with lithium. The combined use of two antisera specific for different regions of the AVP molecule allowed us to detect circulating forms which are modified in the acyclic tripeptide portion. In this lithium-treated patient, iAVP was abnormally low with respect to plasma osmolality. However, iAVP increased during hypertonic saline infusion, probably through an osmosensitive mechanism. A remarkable finding was that contrary to the observations made in healthy subjects and in another patient with diabetes insipidus, iAVP measured with the antiserum specific for the acyclic portion of the AVP molecule was below the values measured with the antiserum specific for the hexapeptide ring. This unusual immunoreactivity profile suggests that the plasma of this polyuric lithium-treated patient contains vasopressin-like peptides which differ from arginine vasopressin in the structure of the C-terminal tripeptide tail.

Distinct hydro-osmotic receptors for the neurohypophysial peptides vasotocin and hydrins in the frog Rana esculenta

The biological properties of vasotocin, hydrin 1 (vasotocinyl-Gly-Lys-Arg) and hydrin 2 (vasotocinyl-Gly), in particular the hydro-osmotic activities on the frog skin, the frog urinary bladder and the frog kidney, have been compared. Hydrins are as active or more active than vasotocin on the first two organs but they are virtually devoid of antidiuretic activity in the rat and the frog, in contrast to vasotocin. It appears that where the oxytocin ring (residues 1-6), present in the three peptides, is necessary for the action on the three organs, the C-terminal amidated group of vasotocin is necessary for the renal receptor but not for the skin and bladder receptors. It is known that amphibians have two types of vasotocin receptors, V1 and V2, homologous to the vascular/hepatic V1 and the renal V2 vasopressin receptors of mammals, respectively. We suggest that adaptation has led to specialization of (at least) two subtypes of hydro-osmotic V2 receptors, the renal subtype on which vasotocin is mainly active for the reabsorption of tubular water, and the skin/bladder subtype on which hydrin 2 is specifically involved in ensuring the rehydration of the animal. Cooperative evolution might have created in anuran Amphibia, on the one hand, two hydro-osmotic peptides, vasotocin and hydrin 2, derived from a single precursor through differential processing; and on the other hand, two corresponding receptors in kidney and skin for internal and external water recovery.

The binding site of neuropeptide vasopressin V1a receptor. Evidence for a major localization within transmembrane regions

To identify receptor functional domains underlying binding of the neurohypophysial hormones vasopressin (AVP) and oxytocin (OT), we have constructed a three-dimensional (3D) model of the V1a vasopressin receptor subtype and docked the endogenous ligand AVP. To verify and to refine the 3D model, residues likely to be involved in agonist binding were selected for site-directed mutagenesis. Our experimental results suggest that AVP, which is characterized by a cyclic structure, could be completely buried into a 15-20-A deep cleft defined by the transmembrane helices of the receptor and interact with amino acids located within this region. Moreover, the AVP-binding site is situated in a position equivalent to that described for the cationic neurotransmitters. Since all mutated residues are highly conserved in AVP and OT receptors, we propose that the same agonist-binding site is shared by all members of this receptor family. In contrast, the affinity for the antagonists tested, including those with a structure closely related to AVP, is not affected by mutations. This indicates a different binding mode for agonists and antagonists in the vasopressin receptor.

Loss of biological activity of arginine vasopressin during its degradation by vasopressinase from pregnancy serum

BACKGROUND AND OBJECTIVE

Degradation of AVP by placental vasopressinase may precipitate gestational diabetes insipidus, which in some cases is accompanied by pre-eclampsia. Abnormally elevated vasopressinase has also been reported in pre-eclampsia without diabetes insipidus. This association between excessive vasopressinase production and pre-eclampsia might be explained if the products of AVP degradation by vasopressinase retained pressor activity even after anti-diuretic activity had been destroyed. Recent evidence indicates that such products may raise blood pressure in rats. The objective of this study was, therefore, to purify vasopressinase and investigate its action on both the V1 and V2 receptor-stimulating activity of AVP.

DESIGN

Vasopressinase was purified from pooled pregnancy serum by ammonium sulphate precipitation, followed by sequential ion exchange, lentil lectin affinity and gel filtration chromatography. Purified enzyme was then used to degrade AVP and the loss of both immunoreactivity and biological activity monitored. Loss of V1 receptor-stimulating activity and V2 receptor-stimulating activity was compared by two-way ANOVA.

PATIENTS

Blood was obtained from healthy women between week 34 and the end of pregnancy. Pooled serum from 20-30 patients was used as starting material for the purification of vasopressinase.

MEASUREMENTS

AVP immunoreactivity was measured by radioimmunoassay, V1 receptor-stimulating activity by a platelet aggregation bioassay, and V2 receptor-stimulating activity by adenylate cyclase stimulation in LLC-PK1 target cells.

RESULTS

Purified vasopressinase was a dimeric protein of molecular weight 330 kDa, which cleaved the synthetic substrate S-benzyl-L-cysteine-4-nitroanilide with a Km of 0.33 mM. Incubation of AVP (0.1 mM) with vasopressinase (0.66 milligrams) at 37 degrees C led to a parallel loss of both AVP immunoreactivity and biological activity. The rates of loss of V1 and V2 receptor mediated activities were not significantly different.

CONCLUSIONS

We report the first direct comparison between the loss of V1 and V2 receptor mediated activities of vasopressinase degraded AVP. There was no significant retention of V1, relative to V2, receptor mediated activity. AVP degradation products are unlikely to be pathogenic in hypertensive pregnancy.

C-terminal amidated peptides: production by the in vitro enzymatic amidation of glycine-extended peptides and the importance of the amide to bioactivity

Peptidylglycine alpha-amidating enzyme (alpha-AE) can be used in an in vitro reaction to convert C-terminal glycine-extended peptides to peptide hormones with a C-terminal amino acid amide. Structure-activity data for 45 bioactive peptides show that the C-terminal amide is required for the full biological activity of most amidated peptide hormones. These data emphasize the role alpha-AE can have in amidated peptide production.

Oxytocin antagonists in preterm labour and delivery

Evidence has been gained that an oxytocin receptor antagonist given by intravenous infusion effectively stops uterine contractions in threatened as well as in actual preterm labour. The findings suggest that the increase of oxytocin receptors is aetiologically important in uncomplicated preterm labour. Oxytocin antagonists could therefore be an attractive alternative to currently used drugs by virtue of their high specificity and lack of serious side-effects. Their use in prophylactic and maintenance therapy may be greater when modified analogues have been developed that allow non-parenteral therapy.

Antagonism of antidiuretic hormone in domestic pigs (Sus scrofa)

1. Morphological and physiological aspects of renal function are shared by humans and swine. SK&F 101926 is a potent antagonist of vasopressin binding to V2 receptors and vasopressin stimulation of adenylate cyclase activity in renomedullary membranes from both species. 2. Unexpectedly, SK&F 101926 proved to be an antidiuretic agonist in humans. Hence, we evaluated SK&F 101926 for antidiuretic agonist and antagonist activities in conscious domestic pigs. 3. During water diuresis (Uosm < 230 mOsm/kg H2O), administration of SK&F 101926 (100 micrograms/kg, i.v.) produced a maximal Uosm of 192 +/- 18 mOsm/kg H2O, a concentration not significantly different from that in vehicle-treated pigs. 4. In hydropenia, SK&F 101926 produced a modest decrease in Uosm, from 945 to 629 mOsm/kg H2O (P < 0.05). 5. In in vitro studies subsequently performed using renomedullary tissue from the same pigs, SK&F 101926 displayed high affinity for V2 receptors (Kbind = 11.8 nM) and high potency to inhibit vasopressin-stimulation of adenylate cyclase (Ki = 3.9 nM). 6. No activity of SK&F 101926 to stimulate adenylate cyclase activity was detected. 7. We conclude that, in spite of its activity in in vitro assays, SK&F 101926 is a weak antidiuretic antagonist in domestic pigs. 8. These results underscore the limited utility of assessments of vasopressin receptor binding and vasopressin-stimulated adenylate cyclase activities in vitro to predict functional antidiuretic activities in vivo.

Fluorescent and biotinylated linear peptides as selective bifunctional ligands for the V1a vasopressin receptor

We have designed and synthesized a linear peptide analogue of arginine vasopressin. This peptide, [1-phenylacetyl, 2-O-methyl-D-tyrosine, 6-arginine, 8-arginine, 9-lysinamide]vasopressin (PhAcALVP), has a lysinamide residue substituted for the more usual glycinamide at position 9. Derivatization of PhAcALVP at the N epsilon-lysyl amino group with N-hydroxysuccinimide esters of aminomethylcoumarin (Mec) and biotin (Btn) produced the bifunctional ligands PhAcAL(Mec)VP and PhAcAL(Btn)VP, respectively. Pharmacological characterization of these peptides revealed that all were high-affinity V1a-selective antagonists. PhAcAL(Btn)VP can simultaneously bind to both the rat liver V1a receptor and avidin conjugates. Using this strategy, we were able to study the distribution of V1a receptors on the surface of the rat mammary tumour cell line, WRK-1. Routine epifluorescent microscopy and confocal image analysis were used to observe the distribution of avidin-Texas-Red associated with receptor-bound PhAcAL(Btn)VP. We conclude that PhAcALVP is a useful precursor for the production of hetero-bifunctional V1a-selective ligands. Both PhAcAL(Mec)VP and PheAcAL(Btn)VP can be used selectively to probe the V1a receptor and will be versatile tools for a variety of histocytochemical applications, including receptor localization and purification.

Identification and characterization of [125I]arginine vasopressin binding sites on human peripheral blood mononuclear cells

Arginine vasopressin (AVP) is a nonapeptide that has been shown to be released from the posterior pituitary during stress. Although noted primarily for its hemodynamic and homeostatic properties, AVP also appears to have an effect on the immune system. It may modulate cellular immunity via its enhancement of the autologous mixed lymphocyte response (AMLR), an effect which we have demonstrated to occur over a wide dose range with a maximum at 10(-7) M. In this study, we examined the binding of [125I]AVP, and AVP analogues to human peripheral blood mononuclear cells (PBMC). AVP inhibited [125I]AVP (0.2 nM) binding on PBMC in a dose-dependent manner with maximal inhibition being reached at 10(-8) M. Specific [125I]AVP binding, as defined as that which could be displaced by 1 x 10(-6) M AVP, was saturable, time-dependent, and linear to cell concentration. Specific binding reached saturation at approximately 1000 pM in 45 minutes. From Scatchard analysis of saturation experiments it appeared to be a homogeneous population of binding sites with KD of approximately 0.5 nM and Bmax of approximately 7.6 fmole/8 x 10(6) cells, corresponding to approximately 527 binding sites/cell. There was a good correlation between AVP binding and cell number. AVP failed to dissociate completely from its binding sites in 60 minutes, perhaps because of the formation of a high-affinity ligand-binding site complex. From competitive binding studies with various AVP antagonists and analogues, it was found that the AVP binding site appeared to be V1-like. AVP binding occurred predominantly on B-cells and macrophages. Having provided evidence for the existence of specific, high affinity, and saturable V1-like AVP binding sites, we suggest a potential modulatory role for AVP in the communication between the neuroendocrine and immune systems.

The effect of arginine vasopressin on the autologous mixed lymphocyte reaction

Arginine vasopressin (AVP) is a nonapeptide that has been shown to be released from the supraoptic and paraventricular nuclei of the hypothalamus during stress. Although noted primarily for its hemodynamic as well as homeostatic properties, AVP also appears to have an effect on the immune system. It may modulate cellular immunity via its enhancement of the autologous mixed lymphocyte response (AMLR), an effect which we have demonstrated to occur over a wide dose range with a maximum at 10(-7) M. The increase in proliferation following a single addition of AVP in a 6-day culture appears to be augmented when the peptide is added daily throughout the same culture period. Enhanced proliferation appears to be a specific response that is influenced by arginine residues in position 8 of this nonapeptide. Having provided evidence for the existence of receptors with moderate affinity for AVP, we suggest a potential modulatory role for AVP in support of the concept of a communication between the neuroendocrine and immune systems. Since various autoimmune conditions may be aggravated by stress, stress-induced release of neuropeptides such as AVP may play an important role in modulating immune regulation of these disease states.

Peptidylglycine alpha-amidating monooxygenase activity and TRH and CRF biosynthesis. Role of copper

Carboxy-terminal amidation of biologically active peptides, an important characteristic of more than half of these substances, occurs during the maturation process of peptide precursors. It is catalyzed by peptidylglycine alpha-amidating monooxygenase (PAM), an enzyme that is copper-dependent. We show here that alterations of copper stores in cultured cells from different origins (pancreas and hypothalamus) affect the immunoreactivity of thyrotropin-releasing hormone (TRH) and corticotropin-releasing factor (CRF) (two alpha-amidated peptides). This suggests that copper can affect neuropeptide biosynthesis and may play a role in the endocrine or central nervous system function.

A unified model for T cell antigen recognition and thymic selection of the T cell repertoire

Positive selection of T cells during thymic differentiation predisposes mature T cells to recognize glycoproteins encoded by "self" alleles of the major histocompatibility complex (MHC) as "restricting elements" for antigen presentation. Yet, negative selection also occurs during thymic differentiation resulting in the clonal deletion of T cells reactive with "self" MHC glycoproteins. Thymic processes of positive and negative selection represent a paradox because the "altered self" view of T cell receptor (TcR) recognition indicates that the same type of TcR-MHC glycoprotein binding interaction mediates both positive and negative selection of the T cell repertoire. Most contemporary models of thymic selection attempt to explain these paradoxical observations by quantitative differences of affinity. That is, TcR interactions with MHC ligands that are of medium affinity lead to positive selection whereas those that are of high affinity lead to negative selection. The purpose of this manuscript is to provide an alternative model of thymic selection based on the efficacy (the ability of a ligand to catalyze receptor mediated biological activity) of TcR-MHC ligand interactions. The "efficacy" model predicts that among those thymocytes exhibiting affinity for self MHC ligands, some clones bind self MHC ligands without efficacy whereas others bind these ligands with efficacy. Immature T cells that bind MHC ligands without efficacy do not undergo TcR mediated activation and thereby escape clonal deletion. Instead, these T cells compete for growth--promoting sites on thymic antigen presenting cells (APC) based upon their clonotypic TcR affinity for self MHC ligands. These T cells experience positive thymic selection and eventually dominate a repertoire of mature T cells predisposed to exhibit non-efficacious binding to "self" MHC ligands. In contrast, immature T cells that exhibit efficacious binding to self MHC ligands are deleted from the T cell repertoire during thymic maturation. By this mechanism, the mature T cell repertoire is selected so that clonotypic T cells are predisposed to bind the very sites on MHC glycoproteins responsible for antigen presentation without risk of autoimmunity. Given the clonotypic diversity of the mature repertoire, complexes of foreign peptides and self MHC glycoproteins would be recognized by clones of the appropriate specificity as highly efficacious ligands. In summary, the "efficacy" model is entirely consistent with the "altered self" concept of T cell antigen recognition and readily accounts for both positive and negative selection of the T cell repertoire.

A selective biotinylated probe for V1a vasopressin receptors

We have designed and synthesized a biotinylated vasopressin antagonist which is a selective probe for studying the V1a subtype of vasopressin receptor. Initially we synthesized the novel vasopressin analogue d(CH2)5Tyr(Me)2LysNH2(9)AVP (ALVP). Biotinamidocaproate was subsequently coupled to the epsilon-amino group of ALVP to generate the novel biotinylated probe d(CH2)5Tyr(Me)2Lys(N epsilon-biotinamido-caproate)NH2(9)AVP (ALBtnVP). Pharmacological characterization of ALVP and ALBtnVP established that both ligands were high affinity antagonists at V1a receptors, and that both displayed marked V1a/V2 selectivity. The observation that receptor-bound ALBtnVP was bi-functional, and thereby able to bind conjugated derivatives of avidin or streptavidin, allowed ALBtnVP to be utilized as a selective probe for V1a receptors. This strategy allowed the visualization of V1a receptors on the surface of WRK-1 cells and hippocampal neurons, by using streptavidin-gold with electron microscopy and fluorescein-avidin with light microscopy. We conclude that ALBtnVP is a useful probe for V1a receptors.

Efficient solution phase synthesis of [1-(beta-mercapto-beta, beta-cyclopentamethylenepropionic acid)- 2-(O-ethyl-D-tyrosine)-4-valine-9-desglycine]arginine vasopressin

A convergent synthesis of the peptide [1-(beta-mercapto-beta, beta-cyclopentamethylenepropionic acid)- 2-(O-ethyl-D-tyrosine)-4-valine-9-desglycine]arginine vasopressin (1), based on the classical solution phase method, was developed. The molecule is assembled by a 3 + 4 coupling via the azide method; then the disulfide bridge is installed by iodine treatment of the bis-acetamidomethyl protected thiols, and the terminal arginine amide added by a 7 + 1 coupling. The method has been used to prepare gram quantities of 1 in more than 98% purity and in 13% yield (based on tetrapeptide intermediate 13) after a single stage purification. The method appears to be particularly suitable for the large scale preparation of 1 and other vasopressin congeners. A novel, albeit low level, transfer of acetamidomethyl group from the sulfur of cysteine to the asparagine amide side-chain was detected following hydrogen chloride treatment of Boc-containing intermediates.

Hypotension induced by vasopressin antagonists in rats: role of mast cell degranulation

SK&F 101926, a synthetic peptide, is a potent antagonist of vasopressin at both the V2 and the V1 receptors. Following intravenous administration of SK&F 101926 (5 mg/kg), mean arterial pressure (MAP) immediately fell 75 mm Hg. Heart rate increased approximately 50 beats/min. Cutaneous flushing and cyanosis appeared approximately 2 to 5 min after the SK&F 101926 administration. Three of the five rats died within 40 min with no improvement in either color or MAP. The two surviving animals slowly recovered from these symptoms. The hypotension and flushing recorded in these studies resembled the effects during hypotensive shock. SK&F 101926 degranulated rat peritoneal mast cells in vitro as measured by the liberation of histamine. Analogs of SK&F 101926 were identified having reduced activity to release histamine from mast cells in vitro. The activity of these analogs to release histamine in vivo was also tested, as reflected by rat paw edema. A positive correlation was found between the potency to produce edema in vivo and the potency to release mast cell histamine in vitro (r = 0.94, p less than 0.05). In addition, compounds that released mast cell histamine and induced rat paw edema also produced hypotension and death when administered intravenously, while analogs which produced minimal histamine release in vitro produced minimal or no cardiovascular changes or lethality in vivo at the same dosages (5 mg/kg). Finally, cyproheptadine (10 mg/kg), an antagonist at both the serotonin and the histamine receptors, blunted the effects of SK&F 101926 on MAP and blocked the lethality. Pretreatment with a combination of histamine (H1 and H2) antagonists provided little protection against the SK&F 101926-induced toxicity. These data indicate that the cardiovascular toxicity of SK&F 101926 (and related peptides) is mediated via the release of autocoids from mast cells. Serotonin appears to play a major role in mediating the cardiovascular toxicity of SK&F 101926.

Identification of an allatostatin from adult Diploptera punctata

A peptide (allatostatin) causing strong and rapid inhibition of juvenile hormone synthesis in vitro by corpora allata from reproductively active females has been isolated from brain/retrocerebral complexes of the cockroach Diploptera punctata. The primary structure of this 13-residue peptide has been determined: Ala-Pro-Ser-Gly-Ala-Gln-Arg-Leu-Tyr-Gly-Phe-Gly-Leu-NH2. Removal of the terminal amide group caused at least a ten thousandfold loss of activity. This neurohormone has no sequence similarity with any other known neuropeptide. Its target in the biosynthetic pathway is located prior to the conversion of farnesol to juvenile hormone.

Experimental uses of neurohypophysial hormone analogs

Structural analogs of neurohypophysial peptides with specific agonistic and antagonistic activities are valuable tools for research on these hormones and their receptors. They may also provide new or improved therapeutic agents.

Vasopressin receptor antagonism in rhesus monkey and man: stereochemical requirements

The vasopressin antidiuretic (V2) antagonist activity of the position 6 stereoisomers of four vasopressin analogs were tested for water diuretic activity in the rhesus monkey and for activity to inhibit vasopressin-stimulated adenylate cyclase activity in rhesus monkey and human renomedullary tissue in vitro. Replacement of the mercapto groups of the cysteine residues with methylene groups resulted in compounds having similar in vitro potencies to their disulfide analogs; however, these 'dicarba' compounds demonstrated more potent aquaretic activity. Position 6 D enantiomers were associated with less vasopressin antagonist activity in vitro in both species. Based upon these studies, the most potent aquaretic structure identified was the dicarba analog SK & F 105494.

Ontogenetic expression of peptidyl-glycine alpha-amidating monooxygenase mRNA in the rat pancreas

Qualitative and quantitative expression of m.RNA coding for Peptidyl-Glycine alpha-Amidating Monooxygenase (PAM) in the developing rat pancreas was investigated by Northern and dot blot hybridization, with a bovine PAM c.DNA probe (0.7 kb fragment). A specific hybridization signal was evidenced for a 3.7 kb m.RNA species. Measurement of PAM m. RNA rate during the rat pancreas ontogenesis revealed a biphasic profile which appeared corelated with that of gastrin and TRH m.RNA respectively. On the other hand, streptozotocin-treatment resulted in a 50% decrease of PAM m.RNA levels.

Glucocorticoid regulation of amidating enzyme in a neoplastic C-cell line

Posttranslational carboxyl-terminal amidation of many peptides is accomplished by peptidylglycine alpha-amidating monooxygenase. We have previously demonstrated that glucocorticoids stimulate production of amidated products by the CA-77 rat medullary thyroid carcinoma cell line. The present investigation was undertaken to determine whether amidation enzyme activity changes in parallel. Enzyme activity, similar to that found in other tissues, was readily detected in cell extracts and conditioned cultured medium. Stimulation with the calcitonin secretagogue calcium increased secretion of enzyme activity and lowered cell extract activity. Treatment of cultures with dexamethasone, but no other steroid, decreased by 50-70% the basal amidation enzyme activity secreted. There was no associated change in cellular activity. The decrease in medium activity was partially reversible and steroid-dose dependent. The glucocorticoid-induced change in medium activity was due to a decreased Vm. These experiments demonstrate that the alpha-amidating activity of the CA-77 cells can be hormonally regulated.

Novel linear antagonists of the antidiuretic (V2) and vasopressor (V1) responses to vasopressin

We report the solid phase synthesis of a series of 16 linear analogues of the cyclic antagonist of the antidiuretic (V2) and the vasopressor (V1) responses to arginine vasopressin (AVP), d(CH2)5[D-Tyr(Et)2, Val4]AVP(A). Peptide 1, the linear precursor of (A), (CH2)5(SH)-CH2-CO-D-Tyr(Et)-Phe-Val-Asn-Cys-Pro-Arg-Gly-NH2 was modified at position six with alpha-L-aminobutyric acid (Abu) to give peptide 2. Further modifications of the Abu6 analogue (No. 2) at position one by substituting cyclohexylacetic acid (Caa), cyclohexylpropionic acid (Cpa), 1-adamantaneacetic acid (Aaa), phenylacetic acid (Phaa), tert.-butylacetic acid (t-Baa), isovaleric acid (Iva), propionic acid (Pa), L-penicillamine (P), tert.-butoxycarbonyl (Boc) or omitting any substituent at this position, and/or in combination with Arg-NH2(9), Ala-NH2(9), D-Arg8-Arg-NH2(9), and desGly9 modifications yielded the remaining 14 peptides. All 16 peptides were examined for agonistic and antagonistic potencies in AVP V2 and V1 assays in rats. Apart from the Cpa analogue and the analogue lacking any substituent in the 1-position, all exhibit substantial V2 and V1 antagonism. A number are as potent as (A) as V2 antagonists. With an anti-V2 pA2 = 8.11 +/- 0.07, Aaa-D-Tyr(Et)-Phe-Val-Asn-Abu-Pro-Arg-Arg-NH2 (No. 6) is as potent as any cyclic AVP V2 antagonist reported to date. The PaI analogue of No. 6 exhibits promising anti-V2/anti-V1 selectivity. These findings prove conclusively that a ring structure is not a requirement for recognition of or for binding to AVP V2 or V1 receptors. This discovery thus offers a promising new approach to the design of peptide and non-peptide antagonists of AVP and perhaps also to other cyclic peptides such as somatostatin, atrial-natriuretic factor, insulin, and the recently discovered endothelin. Some of these linear antagonists may be of value as pharmacological tools and as therapeutic agents.

Acute infectious pneumonia is accompanied by a latent vasopressin-dependent impairment of renal water excretion

The mechanism of hyponatremia associated with pneumonia has not been definitely established. Moreover, renal water excretion was never systematically investigated in cases of pneumonia without hyponatremia. We therefore studied nine consecutive patients breathing spontaneously (nasal oxygen in five), with acute infectious pneumonia and normal plasma sodium concentration. All the patients were previously healthy. Water loads were administered during illness and after recovery. Extracellular fluid volume, arterial blood pressure, PaO2, and PaCO2 were identical during and after pneumonia. By contrast, renal water excretion was markedly impaired during pneumonia and returned to normal values after recovery. This was attested to by a significant decrease in minimum urine osmolality together with significant increases in the percentage of the excreted water load and the maximum free water clearance, after resolution of the pneumonia. Plasma arginine vasopressin values were significantly higher during pneumonia than after recovery despite similar plasma sodium concentrations, both before and after water load. A positive correlation between plasma arginine vasopressin and minimum urine osmolality was found during pneumonia. Thus, impairment in renal water excretion appeared to be due to resetting of the vasopressin osmostat and could not be attributed to any recognized nonosmotic stimulus for vasopressin secretion. On the other hand, these defects varied in severity depending on the extent of the pneumonia and persisted until clearing of alveolar opacities, accounting for their protracted course in some patients. We conclude that water excretion is impaired in most if not in all patients with acute infectious pneumonia (especially if extended), and that the administration of hypotonic solutions should be avoided in these patients.

Properties of vasopressin-activated human platelet high affinity GTPase

Effect of 8-arginine vasopressin (AVP) was examined on human platelet membrane GTPase activity as an index of a G-protein involvement. AVP stimulated a high-affinity GTPase activity in a dose-dependent manner (Kact = 1.1 +/- 0.2 nM). This stimulation was blocked by a V1a antagonist, thus confirming the V1a nature of the platelet AVP receptor. There were important variations among individuals in the AVP-induced stimulation of GTPase activity, that were in relation with the AVP-maximal binding capacity. These data suggest a causal relationship between the binding of AVP to its receptor and transduction elicited by a G-protein, without amplification. In addition, in view of the variable AVP responsiveness observed among individuals, platelet AVP-receptor appears to be subject to regulation.

Flushing and haemodynamic responses to vasopressin peptides in the rhesus monkey

1. The mechanism of the flushing, hypotension and tachycardia associated with i.v. administration of desGlyd(CH2)5D-Tyr(Et)VAVP (SK&F 101926; 25 micrograms kg-1) and the selective V2 antidiuretic agonist, desamino-8-D-arginine vasopressin (dDAVP; 3 micrograms kg-1) was studied in ketamine-anaesthetized rhesus monkeys. 2. The flushing associated with SK&F 101926 was reduced by pretreatment with a mast cell stabilizer and by repeated administration of peptide (within 2-4 weeks). A similar desensitization to dDAVP-associated flushing was observed on repeated administration. 3. Treatment with dDAVP also resulted in reduced SK&F 101926-associated flushing. 4. The hypotension associated with SK&F 101926 was not affected by pretreatment with a mast cell stabilizer. A similar degree of hypotension was observed with repeated administration of either SK&F 101926 or dDAVP. 5. The tachycardia associated with SK&F 101926 was reduced by pretreatment with a mast cell stabilizer or repeated administration of SK&F 101926. Repeated administration of dDAVP, however, resulted in an enhanced tachycardia. 6. Indomethacin (5 mg kg-1 i.v.) did not alter the flushing or the hypotension associated with the administration of either SK&F 101926 or dDAVP, but resulted in an enhanced tachycardia to SK&F 101926. 7. Administration of a selective V1 vasopressor antagonist did not result in flushing, hypotension or tachycardia. 8. It was concluded that the flushing response to vasopressin-like peptides in rhesus monkeys may be due to an action on mast cells, whereas the haemodynamic responses are not, but probably involve direct vasodilator actions.

Radioimmunoassay of desglycinamide-arginine vasopressin and its application in a pharmacokinetic study in the rat

The purpose of this investigation was to develop a sensitive and selective radioimmunoassay for Desglycinamide-Arginine Vasopressin (DGAVP). DGAVP was extracted from rat plasma after protein precipitation, using Sep-Pak C18 cartridges and 50 mM glycine buffer/methanol (10:90) solution. Extraction recovery was 73 +/- 14% (mean +/- S.D.; n = 11) and good linearity was achieved in the concentration range of 0.25-128 pg/tube. Instantaneous tracer addition resulted in a detection limit of 250 fg/tube, whereas 24 hours preincubation and delayed tracer addition resulted in a detection limit of 100 fg/tube. Intra-assay variation ranged between 7.4% and 10.0% depending on the peptide concentration and inter-assay variation was 13.2%. Using this procedure, plasma pharmacokinetics of DGAVP in the rat were determined after IV administration. DGAVP plasma concentration showed a rapid distribution phase (t1/2 = 1.0 +/- 0.2 min) and a somewhat slower elimination phase (t1/2 = 7.2 +/- 2.1 min). High clearance values (CLss = 97 +/- 30 ml.min-1) suggest rapid metabolism by amino- and carboxy-peptidases.

Autoradiographic localization of vasopressin and oxytocin binding sites in rat kidney

The presence of vasopressin receptors of the V1 (vascular) type and of oxytocin receptors in the rat kidney was investigated using an autoradiographical approach. Rat kidney sections were incubated with tritiated vasopressin ([3H]vasopressin, 1.5 nM) or oxytocin ([3H]oxytocin, 3 nM). The ligand selectivity of the [3H]vasopressin binding sites detected was deduced from competition experiments using one selective unlabeled ligand for V2 (antidiuretic) vasopressin receptors (1-deamino-[8-D-arginine]-vasopressin, dDAVP) and one selective unlabeled ligand for V1 receptors (des-glycineamide-[1-(beta-mercapto-beta,beta-cyclopentamethylene propionic acid]-arginine vasopressin, des(Gly(NH2)9d(CH2)5-AVP). Specific and dense [3H]vasopressin labeling was observable in the medullopapillary and cortical portions of the kidney. Specific [3H]vasopressin binding in the cortex was insensitive to the V1-selective ligand, des(Gly(NH2)9d(CH2)5-AVP, but was inhibited by dDAVP. Glomerular structures identified as such by microscopical observation of the kidney sections were specifically labeled with [3H]oxytocin and [125I]-SAR1-angiotensin II but not with [3H]vasopressin. It is concluded that V1 receptors which have been evidenced on mesangial cells in culture are not expressed in a detectable quantity on mesangial cells in situ. The specific [3H]oxytocin binding to glomeruli might reflect the presence on glomerular structures of oxytocin receptors involved in the effects of the hormone on renal hemodynamics, and possibly in some of the effects ascribed to vasopressin.

Potent V2 vasopressin antagonists with structural changes at their C-terminals

A variety of structural changes were made in the C-terminals of four potent antidiuretic (V2) antagonists. The parent analogs were all derivatives of [1-(beta-mercapto-beta,beta-cyclopentamethylenepropionic acid)]arginine-vasopressin, d(CH2)5AVP, namely d(CH2)5[D-Phe2,Ile4]AVP, d(CH2)5[D-Ile2,Ile4]AVP, d(CH2)5[D-Tyr(Et)2, Val4]AVP and d(CH2)5[D-Tyr(Et)2,Ile4]AVP. A number of amino acid amides were substituted for the C-terminal 9-glycinamide without reducing their V2-antagonistic potencies in rats. Many non-amino acid structures were also tolerated at the C-terminals of these antagonists and this end of these peptides can be prolonged without interfering with antagonistic potencies. Such altered V2-antagonists may be useful for the development of radioactive ligands, affinity labels and in affinity columns for studies on antidiuretic receptors. These C-terminal modifications also provide useful information for the further development of potent and specific V2-antagonists which can be valuable pharmacological tools and also promise to become useful clinically for the treatment of excessive water retention.

Reversal of a trimethyltin-induced learning deficit by desglycinamide-8-arginine vasopressin

Trimethyltin (TMT) is an organometal neurotoxin which produces lesions primarily in the limbic system. Selectivity seems to depend upon the dose, but the hippocampus and related entorhinal cortical structures, of importance for learning and memory, are most often described as target sites. We have previously demonstrated that subjects treated with a moderate dose of TMT prior to acquisition sessions, are unable to learn a forward autoshaping task with a 6 sec delay of reinforcement, but are capable of acquiring the same task when no delay of reinforcement is used. These data suggested that the performance deficit is one of learning (i.e. consolidation) rather than of memory (i.e. storage), retrieval, or sensorimotor impairment. To more rigorously test this hypothesis, we determined if performance of a task already learned would be impaired by the neurotoxin. Adult male Long Evans rats were given 10 acquisition sessions of 24 trials, following which TMT (6.0 mg/kg, p.o.) was administered. One month later, these rats performed the lever-touching behavior as well as controls, despite the fact that the same dose of TMT interfered with learning if given one month prior to acquisition sessions, thus confirming our hypothesis. In a second experiment we determined if the peptide analog of vasopressin, desglycinamide-8-arginine vasopressin (DGAVP), could reverse a learning deficit in a population of non-learners. Rats were treated with TMT or water vehicle one month prior to autoshaping. TMT significantly retarded acquisition. After 10 sessions of 12 trials each, non-learners (i.e. rats treated with TMT that failed to associate the lever with delivery of a reinforcer) were administered saline or DGAVP (7.5 micrograms/kg, s.c.) 1 hr before sessions 11-13; treatment was discontinued prior to sessions 14 and 15. Peptide treated subjects showed evidence of acquisition and exhibited higher levels of lever-directed behavior than saline treated nonlearners. Performance was maintained after DGAVP treatment was discontinued, indicating that the learning-enhancing action of DGAVP was not transient or state-dependent.

Receptors mediating the CRH effects of vasopressin and oxytocin

The data highlighted here suggest that the CRH effects of vasopressin and oxytocin are mediated by one and the same hypophysial receptor which has unique pharmacological specificity. The nomenclature for this receptor type is not established; both V3(53) and V1 beta 39 have been proposed. The former proposal is logical if the pharmacology of ligand recognition is emphasized, whereas the latter designation takes into account that transmembrane signalling from V1 receptors occurs via coupling proteins Go and Gi but not Gs. Such issues are best resolved after cDNA cloning of the genes for the receptors: in the meanwhile the working definition V3 seems more convenient. Several studies show that pituitary V3 receptors are regulated by the concentration of vasopressin in hypophysial portal blood and the amount of glucocorticoid hormones in the circulation (see Ref. 9 for review). Work in this area should clarify further the intracellular mechanism of the CRH action of vasopressin, as well as the factors that determine the responsiveness of corticotrophs to various secretagogues. Most recently, it has been shown that vasopressin is a potent thyrotropin-releasing hormone. This finding extends further the growing concept that there is considerable "cross-talk" between the classical neuroendocrine axes. These were previously thought to be separated by the hypothalamic organization of "final common pathways" of neuroendocrine motoneurons in the hypothalamus, each producing a unique neurohormone to regulate a single type of adenohypophysial cell. It seems that the days of the validity of this hypothesis are numbered, and an important task will be to determine the possible physiological significance of the "cross-talk" within the hypothalamo-pituitary unit in the regulation adrenocortical function.