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Abstract: Oxytocin (OT) and vasopressin (AVP) mediate their biological actions by acting on four known receptors: The OT (uterine) and the AVP V1a (vasopressor), V1b (pituitary), V2 (renal) receptors and a fifth putative AVP V1c? (vasodilating) receptor. This presentation will summarize some highlights of the recent progress, in the design and synthesis of selective peptide agonists, antagonists, radioiodinated ligands, fluorescent ligands and bivalent ligands for these receptors. Here we present published and unpublished pharmacological data on the most widely used agonists, antagonists and labelled ligands. The pharmacological properties of promising new selective OT antagonists and V1b agonists are also presented. This review should serve as a useful guide for the selection of the most appropriate ligand for a given study. The current status of non-peptide OT and AVP antagonists and agonists is also summarized. The relative merits of peptide and non-peptide AVP and OT agonists and antagonists as: (1) research tools and (2) therapeutic agents will be evaluated. Many of the receptor selective peptide agonists and antagonists from this and other laboratories are far more widely used as pharmacological tools for studies on the peripheral and central effects of OT and AVP than their non-peptide counterparts. In addition to OT and to a lesser extent AVP (pitressin), a number of OT and AVP analogues; such as carbetocin (OT agonist) dDAVP (desmopressin, V2 agonist), terlipressin (V1a agonist), felypressin (V1a agonist) and atosiban (Tractocile OT antagonist) are also in clinical use. Despite much early promise, no non-peptide V1a or OT antagonists are currently in clinical trials. While a number of orally active non-peptide V2 antagonists (Vaptans); notably, Tolvaptan, Lixivaptan and Satavaptan, are currently in Phase III clinical trials; to date, only the mixed V2/V1a, antagonist Conivaptan (Vaprisol), has been approved by the US FDA for clinical use (by i.v. administration), for the treatment of euvolemic and hypervolemic hyponatremia in hospitalized patients. Promising new non-peptide V1b and OT antagonists, as well as non-peptide V2 and OT agonists are now in pre-clinical development. [Copyright &y& Elsevier]

Abstract: The development of “selective”” drugs targeting oxytocin/vasopressin receptors has enormously progressed since the original synthesis of oxytocin more than 50 years ago. However, several factors still hamper the availability of a rich and complete range of selective agonists and antagonists acting at the different oxytocin/vasopressin receptor subtypes, making the use of these drugs still a daunting task. In this paper we will briefly review the major problems encountered when dealing with oxytocin/vasopressin selective ligands, proving few rules for their correct pharmacological use, in order to avoid common pitfalls. Finally, we will glimpse at new challenges, such us the discovery of coupling selective ligands, which foster the search for new classes of selective compounds. [Copyright &y& Elsevier]

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G-protein-coupled receptors (GPCRs) play a major role in cell-cell communication in the CNS. These proteins oscillate between various inactive and active conformations, the latter being stabilized by agonists. Although mutations can lead to constitutive activity, most of these destabilize inactive conformations, and none lock the receptor in an active state. Moreover, GPCRs are known to form dimers, but the role of each protomer in the activation process remains unclear. Here, we show that the heterodimeric GPCR for the main inhibitory neurotransmitter, the GABA[subB] receptor, can be locked in its active state by introducing two cysteines expected to form a disulphide bridge to maintain the binding domain of the GABA[subB1] subunit in a closed form. This constitutively active receptor cannot be inhibited by antagonists, but its normal functioning, activation by agonists, and inhibition by antagonists can be restored after reduction with dithiothreitol. These data show that the closed state of the binding domain of GABA[subB1] is sufficient to turn ON this heterodimeric receptor and illustrate for the first time that a GPCR can be locked in an active conformation. [ABSTRACT FROM AUTHOR]

The adenylate cyclase enzymes from pig kidney medulla, rat liver and rat brain membranes were solubilized with Triton X-100 into three different states : basal, NaF-activated and guanosine 5'-(β,γ-imino)triphosphate- [GuoPP(NH)P]-activated. The effects of 2-mercaptoethanol and other thiol-protecting reagents on the response of solubilized adenylate cyclase to NaF and GuoPP(NH)P and on its apparent sedimentation constants were examined. In the absence of 2-mercaptoethanol, GuoPP(NH)P produced very little stimulation of the adenylate cyclase activity in 200 000 x g supernatant fractions of detergent-treated membranes. 2-Mercaptoethanol markedly enhanced the response to GuoPP(NH)P and to a lesser extent to NaF. When membranes were solubilized and ultracentrifuged without 2-mercaptoethanol, the adenylate cyclases in all of the three states studied had the same apparent sedimentation coefficient (7.4 S, 7.6 S and 7.9 S for the enzymes from pig kidney, rat liver and rat brain respectively). In the presence of 2-mercaptoethanol the apparent sedimentation constant of the adenylate cyclase solubilized in the basal activity state fell by about 1 S, that of the GuoPP(NH)P-preactivated state remained unchanged and that of the NaF-preactivated state dropped with a concomitant loss in activity. The reduction in size and activity were abolished when NaF was added to the sucrose density gradients. Stimulation by GuoPP(NH)P or NaF of the light form of adenylate cyclase (basal state solubilized in the presence of 2-mercaptoethanol) raised the apparent sedimentation constant, and the values obtained were identical to those found for the preactivated forms of the enzyme. [ABSTRACT FROM AUTHOR]

Specific vasopressin receptors in rat liver membranes were recently characterized [Cantau et al., Journal of Receptors Research, in the press]. They differ from the receptors characterized earlier in kidney membranes as regards coupling with adenylate cyclase and specificity towards vasopressin structural analogues [Rajerison et al. (1974) J. Biol. Chem. 249, 6390–6400; Butlen et al. (1978) Mol. Pharmacol. 14, 1006–1017]. The object of the present work was to see whether these differences reflect a difference in the molecular size of liver and kidney vasopressin receptors. For this purpose, rat liver and kidney membranes were incubated with [³H]vasopressin and solubilized with Triton X-100 (0.3%). The properties of the macromolecular components of soluble extracts to which labelled vasopressin remained bound were observed to resemble those of the intact membrane receptors as regards binding reversibility at 30–37°C and sensitivity to guanyl nucleotides. The hydrodynamic parameters of the soluble hormone-receptor complexes were estimated from Ultrogel column filtration experiments and from sucrose density gradient ultracentrifugation experiments. The following values were obtained for liver and kidney receptors respectively: Stokes' radii: 5.4 and 5.6 nm; standard sedimentation coefficients: 3.7 and 3.7 S; partial specific volumes: 0.75 and 0.78 ml · g-1; molecular weight: 83000 and 80000. These results indicate that the marked functional differences between liver and kidney receptors are not accompanied by appreciable differences in molecular size. [ABSTRACT FROM AUTHOR]

The effects of 17 non-ionic detergents on pig kidney plasma membrane adenylate cyclase were tested (digitonin; detergents from the Triton series: X45, X100, X102, X114, X165, X305, X405, N101; the Brij series: 35, 56, 96 and the Tween series: 20, 40, 60, 80, 85). Membranes (2.66 mg protein/ml) were treated with increasing amounts of detergent for 150 min at 0 °C. The adenylate cyclase activities present in treated membranes and in their corresponding non-sedimentable fractions were measured at 30 °C. Only detergents having hydrophilic lipophilic balance values from 12 to 14 were effective solubilizing agents. For these detergents, the solubilization yield was dependent on their structure. When solubilized adenylate cyclase kept at 0 °C was incubated at 30 °C, the enzyme catalytic activity exponentially decreased towards an equilibrium value with a half-time of 5 min. This temperature-dependent adjustment of enzyme activity was reversible. Its magnitude was dependent on the detergent used. A more progressive but less pronounced effect was observed with non-solubilizing detergents. The micellar form of detergent in the incubation medium was found to be responsible for the appearance of microheterogeneity in the population of solubilized enzyme molecules differing by the substrate accessibility. This effect was suppressed when reducing the detergent concentration in the incubation medium. [ABSTRACT FROM AUTHOR]

We have synthesized and fully characterized by fast-atom-bombardment-mass, NMR and ultraviolet spectroscopies the vasopressin antagonist 3-azidophenylpropionyl-D-Tyr(Me)-Phe-Gln-Asn-Arg-Pro-ArgTyr(3I)-NH2. Easily radioiodinatable just before use, it has a high affinity for the natural rat liver V1a receptor [dissociation constant (Kd) = 54 ± 20 pM; Carnazzi, E., Aumelas, A., Barberis, C., Guillon, G. & Seyer, R. (1994) J. Med. Chem. 37, 1841-1849] and for both the rat vasopressin V1a, receptor expressed in Spodoptera frugiperda 9 cells (Sf9 cells, Kd = 688 ± 35 pM) and in COS-7 cells (Kd = 320 ± 20 pM). This probe labels specifically the V1a receptors in an ultraviolet-dependent manner, and binds covalently to about 12% of the receptors with high stability over several days, even in dissociation or solubilization conditions. SDS/PAGE studies and autoradiographic analyses of the photolabeled receptors reveal a single band (49.5 kDa) and two bands. (63 kDa and 93.6 kDa) for receptor-probe associations obtained in Sf9 and COS-7 cells respectively. These molecular masses are consistent with non-glycosylated and highly glycosylated forms of the receptor, according to each expression system. In rat liver membranes, we have identified apparent molecular masses of about 32, 45 and more than 67 kDa. We finally demonstrated a proteolysis of the receptor that appeared to be Zn2+ and leupeptin sensitive. The high potency of this ligand is promising for the monitoring of the purification of the V1a receptor and for mapping its antagonist-binding site. [ABSTRACT FROM AUTHOR]

After partial hepatectomy, liver mass is restored through a complex regulatory network of cytokines, growth factors, and hormones. In this study, we show that an interplay between hypothalamic vasopressin secretion and a remodeling of Ca&sup2+; signals in the liver contributes significantly to rat liver compensatory growth and bile flow. [ABSTRACT FROM AUTHOR]

In rat portal vein myocytes, α-adrenoceptors activate voltage-dependent Ca channels via a transduction pathway requiring protein kinase C activation mediated by a pertussis-toxin-sensitive G-protein. As revealed by the use of antibodies directed against the different α-subunits expressed in portal vein myocytes, we show that the clonidine-induced stimulation of voltage-dependent Ca channels is mainly mediated by a G-protein. [ABSTRACT FROM AUTHOR]

Abstract: Though ischaemia/reperfusion injury induces renin–angiotensin systemic (RAS) activation and increased heart angiotensin production, the effects of blockade of the two main angiotensin II receptors, AT1 and AT2, are not definitively established. Using a Langendorff heart preparation, effects of Valsartan 10-7M (AT1 receptor blocker), PD 123319 10-7M (AT2 receptor blocker) or both in the presence of a controlled concentration of angiotensin II (10-8M) in order to reproduce systemic RAS activation were studied in adult male Wistar rat hearts submitted to ischaemia/reperfusion. Ischaemia/reperfusion impaired both systolic and diastolic function through a no-reflow phenomenon. Presence of a controlled concentration of angiotensin in the perfusate, enough to produce a significant AT1-induced vasoconstriction before ischaemia, has no relevant influence on ischaemia/reperfusion injury. Only blockade of both AT1 and AT2 receptors significantly improved recovery from ischaemia; better ventricle function paralleled better perfusion. The results suggest that blockade of angiotensin II receptors is cumulative since blockade of AT1 and AT2 receptors is more effective than blockade of just one of them. [Copyright &y& Elsevier]

Abstract: In this study, we identified the multifunctional protein GC1q-R as a novel vasopressin V2 receptor (V2R) interacting protein. For this purpose, we have developed a proteomic approach combining pull-down assays using a cyclic peptide mimicking the third intracellular loop of V2R as a bait and mass spectrometry analyses of proteins isolated from either rat or human kidney tissues or the HEK 293 cell line. Co-immunoprecipitation of GC1q-R with the c-Myc-tagged h-V2R expressed in a HEK cell line confirmed the existence of a specific interaction between GC1q-R and the V2 receptor. Then, construction of a mutant receptor in i3 loop allowed us to identify the i3 loop arginine cluster of the vasopressin V2 receptor as the interacting determinant for GC1q-R interaction. Using purified receptor as a bait and recombinant (74–282) GC1q-R, we demonstrated a direct and specific interaction between these two proteins via the arginine cluster. [Copyright &y& Elsevier]

In this study, we investigated the mechanism by which a peptide mimicking the third cytoplasmic loop of the vasopressin V2 receptor inhibits signaling. This loop was synthesized as a cyclic peptide (i3 cyc) that adopted defined secondary structure in solution. We found that i3 cyc inhibited the adenylyl cyclase activity induced by vasopressin or a nonhydrolyzable analog of GTP, guanosine 5'-O-(3-thio)triphosphate. This peptide also affected the specific binding of [³H]AVP by converting vasopressin binding sites from a high to a low affinity state without any effect on the global maximal binding capacity. The inhibitory actions of i3 cyc could also be observed in the presence of maximally uncoupling concentration of guanosine 5'-O-(3-thio)triphosphate, indicating a direct effect on the receptor itself and not exclusively on the interaction between the Gs protein and the V2 receptor (V2-R). Bioluminescence resonance energy-transfer experiments confirmed this assumption, because i3 cyc induced a significant inhibition of the bioluminescence resonance energy-transfer signal between the Renilla reniformis luciferase and the enhanced yellow fluorescent protein fused V2-R. This suggests that the proper arrangement of the dimer could be an important prerequisite for triggering Gs protein activation. In addition to its effect on the receptor itself, the peptide exerted some of its actions at the G protein level, because it could also inhibit gnanosine 5'-O-(3thio)triphosphate-stimulated AC activity. Taken together, the data demonstrate that a peptide mimicking V2-R third intracellular loop affects both the dimeric structural organization of the receptor and has direct inhibitory action on Gs. [ABSTRACT FROM AUTHOR]

In this study, we characterized the bovine vasopressin V1a, V1b, V2 receptor isoforms and compared their pharmacological properties to those of corresponding rat and human vasopressin receptor subtypes. Specific binding sites of high affinity for vasopressin were found in all bovine tissues tested (kidney, liver and pituitary). Using a large series of recent peptidic and non-peptidic selective vasopressin agonists or antagonists, we demonstrated the presence of vasopressin V2, V1a or V1b receptors in the kidney, liver and pituitary bovine tissues, respectively. This extensive characterization of bovine vasopressin receptor isoforms validates the pharmacological vasopressin receptor classification earlier established for the rat and human species. As expected, the bovine vasopressin receptors look much more like human receptors than rat ones. Interestingly, among the three vasopressin receptor isoforms studied, the vasopressin V1b receptor subtype is the best conserved for the three species studied. [Copyright &y& Elsevier]