Vasopressin receptors

Other potential oxytocia antagonists are being developed using leads from naturally occurring, nonpeptide stmctures, such as an extract from Streptomjces (48). A selective nonpeptide vasopressin receptor antagonist also has been found (49). 

Oxytocin and Vasopressin Receptors. The actions of oxytocin and vasopressin are mediated through their interactions with receptors. Different receptor types as well as different second messenger responses help explain their diverse activities in spite of the hormones stmctural similarities. Thus oxytocin has at least one separate receptor and vasopressin has been shown to have two principal receptor types, and V2. Subclasses of these receptors have been demonstrated, and species differences further compHcate experimental analysis. It is apparent that both oxytocin and receptors function through the GP/1 phosphoHpase C complex (75), while the V2 receptors activate cycHc AMP (76). 

Logically, ADH receptor antagonists, and ADH synthesis and release inhibitors can be effective aquaretics. ADH, 8-arginine vasopressin [113-79-17, is synthesized in the hypothalamus of the brain, and is transported through the supraopticohypophyseal tract to the posterior pituitary where it is stored. Upon sensing an increase of plasma osmolaUty by brain osmoreceptors or a decrease of blood volume or blood pressure detected by the baroreceptors and volume receptors, ADH is released into the blood circulation it activates vasopressin receptors in blood vessels to raise blood pressure, and vasopressin V2 receptors of the nephrons of the kidney to retain water and electrolytes to expand the blood volume. 

Inhibition of V2 vasopressin receptors causes an increase in urine volume primarily by reducing the re-absoiption of water along the collecting duct, an aquaretic effect that is fundamentally different from the natriuretic actions discussed so far. Nevertheless, some of the conditions calling for the use of natriuretic intervention are identical to those in which the administration of a new class of orally active nonpeptide V2 antagonists may be useful (tolvaptan, lixivaptan, and others). 

In some cases, receptor inactivation, e.g., of the V2 vasopressin receptor, is mediated by agonist-induced enzymatic cleavage of the GPCR. This nonendocytic proteolysis is promoted by a plasma membrane-associated metalloprotease. Proteinase-activated receptors (PARs) such as the thrombin receptor also follow a distinctly different pathway. PARs require the enzymatic cleavage of their N terminus, and the newly generated N terminus activates the receptor. Once... 

AVP and OT are cyclic nonapeptides with a disulphide bridge between the cysteine residues 1 and 6, resulting in a six-amino acid ring and a COOH-terminal a-amidated three-residue tail. OT differs only in two amino acids from AVP lie in position 3, which is essential for OT recqrtor (OTR) stimulation and Leu in position 8. AVP has a Phe in position 3 and an Arg in position 8. Arg 8 is essential for acting upon vasopressin receptors (Fig. 1). Lysipressin, found in pigs and some marsupials, has a Lys in position 8 [1]. 

Several nonpeptidic, orally active vasopressin receptor antagonists have been developed. The dual V1A/V2R antagonist conivaptan is used in the treatment of hyponatraemia and could also become useful for diseases such as congestive heart failure, in which increased peripheral resistance and dilutional hyponatremia both are present [4]. Side effects of conivaptan include headache, injection site reactions, vomiting, diarrhoea, constipation and thirst. 

These include nicotinic acetylcholine receptors, neuronal calcium channels, muscle sodium channels, vasopressin receptors, and iV-methyl-D-aspartate (NMDA) receptors. Some general features of the structure, function, and evolution of biologically active peptides isolated from Conus venom are presented. 

Oxytocin and vasopressin are closely related peptides sharing seven out of nine amino acids. Vasopressin is able to bind to all vasopressin and oxytocin receptors with nanomolar affinity, whereas oxytocin has greater affinity for the oxytocin receptor than the vasopressin receptors. The affinity of the peptides for the receptors is shown in Table 7.1 (data from Mouillac et al. [20]). 

The search for an effective non-peptide oxytocin antagonist has become a major goal of a number of pharmaceutical companies because of the poor pharmacokinetic properties and especially the lack of oral bioavailability associated with peptidic antagonists. Early research in this field was dominated by Merck, but in recent years significant research efforts at GlaxoSmithKline and Serono have been published. A number of other companies, notably Sanofi-Aventis, Yamanouchi and Wyeth, have had a major presence in vasopressin receptor research and oxytocin is frequently included in patent claims for the molecules. Occasionally, oxytocin-selective compounds have been reported, usually derived by adaptation of the vasopressin antagonist template. 

A substantial review of the molecular pharmacology of human vasopressin receptors (including oxytocin) and peptidic agonists and antagonists has appeared [135]. 

Preterm labour is the major cause of perinatal morbidity and mortality. Oxytocin antagonists offer an attractive approach to prevention. Chapter 7 reviews three decades of medicinal chemistry in this field. The peptide approach has resulted in valuable injectable products. Selectivity over the related vasopressin receptors and improvement in pharmacokinetic profile have been the key challenges for more recent non-peptide programmes, and these seem likely to yield orally available medicines. 

Figure 4 Schematic representation of the Ca2+-transporting systems affecting cellular calcium homeostasis during hormonal stimulation, oq = oq-adrenergic receptor VP = vasopressin receptor PLC = phospholipase C PI = phosphatidylinositol PIP = phospha-tidylinositol-4-phosphate PIP2 = phosphatidylinositol-4,5-biphosphate IP3 = inositol-1,4,5-triphosphate DG = diacylglycerol PKC = protein kinase C. (Modified from Refs. 125 and 285.)...

Vasopressin also plays an important role in short-term regulation of blood pressure through its action on vascular smooth muscle. This hormone is the most potent known endogenous vasoconstrictor. Two types of vasopressin receptors have been identified V, receptors mediate vasoconstriction... 

Additional applications of BSOCOES and sulfo-BSOCOES include investigations of the cellular and subcellular distribution of the type II vasopressin receptor (Fenton et al., 2007), TNF-alpha (Grinberg et al., 2005), and studying mechanisms in the control of plasmid replication (Das et al., 2005). 

Fenton, R.A. Brond, L., Nielsen, S., and Praetorius, J. (2007) Cellular and subcellular distribution of the type II vasopressin receptor in kidney. Am. J. Physiol. Renal. Physiol. 10.1152/ajprenal.00316.2006. 

Zhu, X. and Wess, J. (1998) Truncated V2 vasopressin receptors as negative regulators of wild-type V2 receptor function. Biochemistry 37, 15773-15784. 

Schulz, A., Grosse, R., Schultz, G., Gudermann, T., and Schoneberg, T. (2000) structural implication for receptor oligomerization from functional reconstitution studies of mutant V2 vasopressin receptors. J. Biol. Chem. 275, 2381-2389. 

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