Synthetic analogs of oxytocin and the vasopressins

Investigation of cytotoxic effect and action mechanism of a synthetic peptide derivative of rabbit cathelicidin against MDA-MB-231 breast cancer cell line

Antimicrobial peptides (AMPs) have sparked significant interest as potential anti-cancer agents, thereby becoming a focal point in pursuing novel cancer-fighting strategies. These peptides possess distinctive properties, underscoring the importance of developing more potent and selectively targeted versions with diverse mechanisms of action against human cancer cells. Such advancements would offer notable advantages compared to existing cancer therapies. This research aimed to examine the toxicity and selectivity of the nrCap18 peptide in both cancer and normal cell lines. Furthermore, the rate of cellular death was assessed using apoptosis and acridine orange/ethidium bromide (AO/EB) double staining at three distinct incubation times. Additionally, the impact of this peptide on the cancer cell cycle and migration was evaluated, and ultimately, the expression of cyclin-dependent kinase 4/6 (CDK4/6) genes was investigated. The results obtained from the study demonstrated significant toxicity and selectivity in cancer cells compared to normal cells. Moreover, a strong progressive increase in cell death was observed over time. Furthermore, the peptide exhibited the ability to halt the progression of cancer cells in the G1 phase of the cell cycle and impede their migration by suppressing the expression of CDK4/6 genes.

Targeted Liposomal Drug Delivery: Overview of the Current Applications and Challenges

In drug development, it is not uncommon that an active substance exhibits efficacy in vitro but lacks the ability to specifically reach its target in vivo. As a result, targeted drug delivery has become a primary focus in the pharmaceutical sciences. Since the approval of Doxil® in 1995, liposomes have emerged as a leading nanoparticle in targeted drug delivery. Their low immunogenicity, high versatility, and well-documented efficacy have led to their clinical use against a wide variety of diseases. That being said, every disease is accompanied by a unique set of physiological conditions, and each liposomal product must be formulated with this consideration. There are a multitude of different targeting techniques for liposomes that can be employed depending on the application. Passive techniques such as PEGylation or the enhanced permeation and retention effect can improve general pharmacokinetics, while active techniques such as conjugating targeting molecules to the liposome surface may bring even further specificity. This review aims to summarize the current strategies for targeted liposomes in the treatment of diseases.

The Current and Promising Oral Delivery Methods for Protein- and Peptide-Based Drugs

Drugs based on peptides and proteins (PPs) have been widely used in medicine, beginning with insulin therapy in patients with diabetes mellitus over a century ago. Although the oral route of drug administration is the preferred one by the vast majority of patients and improves compliance, medications of this kind due to their specific chemical structure are typically delivered parenterally, which ensures optimal bioavailability. In order to overcome issues connected with oral absorption of PPs such as their instability depending on digestive enzymes and pH changes in the gastrointestinal (GI) system on the one hand, but also their limited permeability across physiological barriers (mucus and epithelium) on the other hand, scientists have been strenuously searching for novel delivery methods enabling peptide and protein drugs (PPDs) to be administered enterally. These include utilization of different nanoparticles, transport channels, substances enhancing permeation, chemical modifications, hydrogels, microneedles, microemulsion, proteolytic enzyme inhibitors, and cell-penetrating peptides, all of which are extensively discussed in this review. Furthermore, this article highlights oral PP therapeutics both previously used in therapy and currently available on the medical market.

Cell-free Biosynthesis of Peptidomimetics

A wide variety of peptidomimetics (peptide analogs) possessing innovative biological functions have been brought forth as therapeutic candidates through cell-free protein synthesis (CFPS) systems. A key feature of these peptidomimetic drugs is the use of non-canonical amino acid building blocks with diverse biochemical properties that expand functional diversity. Here, we summarize recent technologies leveraging CFPS platforms to expand the reach of peptidomimetics drugs. We also offer perspectives on engineering the translational machinery that may open new opportunities for expanding genetically encoded chemistry to transform drug discovery practice beyond traditional boundaries.

Therapeutic peptides: current applications and future directions

Peptide drug development has made great progress in the last decade thanks to new production, modification, and analytic technologies. Peptides have been produced and modified using both chemical and biological methods, together with novel design and delivery strategies, which have helped to overcome the inherent drawbacks of peptides and have allowed the continued advancement of this field. A wide variety of natural and modified peptides have been obtained and studied, covering multiple therapeutic areas. This review summarizes the efforts and achievements in peptide drug discovery, production, and modification, and their current applications. We also discuss the value and challenges associated with future developments in therapeutic peptides.

Leu8 and Pro8 oxytocin agonism differs across human, macaque, and marmoset vasopressin 1a receptors

Oxytocin (OXT) is an important neuromodulator of social behaviors via activation of both oxytocin receptors (OXTR) and vasopressin (AVP) 1a receptors (AVPR1a). Marmosets are neotropical primates with a modified OXT ligand (Pro⁸-OXT), and this ligand shows significant coevolution with traits including social monogamy and litter size. Pro⁸-OXT produces more potent and efficacious responses at primate OXTR and stronger behavioral effects than the consensus mammalian OXT ligand (Leu⁸-OXT). Here, we tested whether OXT/AVP ligands show differential levels of crosstalk at primate AVPR1a. We measured binding affinities and Ca²⁺ signaling responses of AVP, Pro⁸-OXT and Leu⁸-OXT at human, macaque, and marmoset AVPR1a. We found that AVP binds with higher affinity than OXT across AVPR1a, and marmoset AVPR1a show a 10-fold lower OXT binding affinity compared to human and macaque AVPR1a. Both Leu⁸-OXT and Pro⁸-OXT produce a less efficacious response than AVP at human AVPR1a and higher efficacious response than AVP at marmoset AVPR1a. These data suggest that OXT might partially antagonize endogenous human AVPR1a signaling and enhance marmoset AVPR1a signaling. These findings aid in further understanding inconsistencies observed following systemic intranasal administration of OXT and provide important insights into taxon-specific differences in nonapeptide ligand/receptor coevolution and behavior.

Binding Characteristics of Two Oxytocin Variants and Vasopressin at Oxytocin Receptors from Four Primate Species with Different Social Behavior Patterns

A clade of New World monkeys (NWMs) exhibits considerable diversity in both oxytocin (OT) ligand and oxytocin receptor (OTR) structure. Most notable is the variant Pro⁸-OT, with proline instead of leucine at the eighth position, resulting in a rigid bend in the peptide backbone. A higher proportion of species that express Pro⁸-OT also engage in biparental care and social monogamy. When marmosets (genus Callithrix), a biparental and monogamous Pro⁸-OT NWM species, are administered the ancestral Leu⁸-OT, there is no change in social behavior compared with saline treatment. However, when Pro⁸-OT is administered, marmosets' sociosexual and prosocial behaviors are altered. The studies here tested the hypothesis that OTR binding affinities and OT-induced intracellular Ca²⁺ potencies would favor the native OT ligand in OTRs from four primate species, each representing a unique combination of ancestral lineage, breeding system, and native OT ligand: humans (Leu⁸-OT, monogamous, apes), macaques (Leu⁸-OT, nonmonogamous, Old World monkey), marmosets (Pro⁸-OT, monogamous, NWM), and titi monkeys (Leu⁸-OT, monogamous, NWM). OTRs were expressed in immortalized Chinese hamster ovary cells and tested for intact-cell binding affinities for Pro⁸-OT, Leu⁸-OT, and arginine vasopressin (AVP), as well as intracellular Ca²⁺ signaling after stimulation with Pro⁸-OT, Leu⁸-OT, and AVP. Contrary to our hypothesis, Pro⁸-OT bound at modestly higher affinities and stimulated calcium signaling at modestly higher potencies compared with Leu⁸-OT in all four primate OTRs. Thus, differences downstream from a ligand-receptor binding event are more likely to explain the different behavioral responses to these two ligands.

Oxytocin and vasopressin agonists and antagonists as research tools and potential therapeutics

We recently reviewed the status of peptide and nonpeptide agonists and antagonists for the V(1a), V(1b) and V(2) receptors for arginine vasopressin (AVP) and the oxytocin receptor for oxytocin (OT). In the present review, we update the status of peptides and nonpeptides as: (i) research tools and (ii) therapeutic agents. We also present our recent findings on the design of fluorescent ligands for V(1b) receptor localisation and for OT receptor dimerisation. We note the exciting discoveries regarding two novel naturally occurring analogues of OT. Recent reports of a selective VP V(1a) agonist and a selective OT agonist point to the continued therapeutic potential of peptides in this field. To date, only two nonpeptides, the V(2) /V(1a) antagonist, conivaptan and the V(2) antagonist tolvaptan have received Food and Drug Administration approval for clinical use. The development of nonpeptide AVP V(1a), V(1b) and V(2) antagonists and OT agonists and antagonists has recently been abandoned by Merck, Sanofi and Pfizer. A promising OT antagonist, Retosiban, developed at Glaxo SmithKline is currently in a Phase II clinical trial for the prevention of premature labour. A number of the nonpeptide ligands that were not successful in clinical trials are proving to be valuable as research tools. Peptide agonists and antagonists continue to be very widely used as research tools in this field. In this regard, we present receptor data on some of the most widely used peptide and nonpeptide ligands, as a guide for their use, especially with regard to receptor selectivity and species differences.

Arginine vasopressin receptor antagonists (vaptans): pharmacological tools and potential therapeutic agents

Arginine vasopressin (AVP) attracted attention as a potentially important neurohormonal mediator of the heart failure (HF) syndrome and hyponatremic states in humans because AVP influences renal handling of free water, vasoconstriction and myocyte biology through activation of V₂ and V₁(a) receptors. Current research is exploring V₂- and dual V₁(a)/V₂ receptor antagonism for the treatment of hyponatremia, as well as for the congestion and edema associated with chronic HF, because vasopressin receptor antagonists might offer benefits in comparison with conventional loop diuretics. The purpose of this review is to update the current status of experimental and clinical studies with available vasopressin receptor antagonists (conivaptan and tolvaptan) and their potential role in the treatment of HF and hyponatremia of multiple causes.

Molecular dynamics simulation of human neurohypophyseal hormone receptors complexed with oxytocin-modeling of an activated state

The neurohypophyseal hormone oxytocin (CYIQNCPLG-NH(2), OT) is involved in the control of labor, secretion of milk and many social and behavioral functions via interaction with its receptors (OTR) located in the uterus, mammary glands and peripheral tissues, respectively. In this paper we propose the interactions responsible for OT binding and selectivity to OTR versus vasopressin ([F3,R8]OT, AVP) receptors: V1aR and V2R, all three belonging to the Class A G protein-coupled receptors (GPCRs). Three-dimensional models of the activated receptors were constructed using a multiple sequence alignment and the activated rhodopsin-transducin (MII-Gt) prototype [Slusarz and Ciarkowski, 2004] as a template. The 1 ns unconstrained molecular dynamics (MD) of three pairs of receptor-OT complexes (two complexes per each receptor) immersed in the fully hydrated 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylcholine (POPC) lipid bilayer was conducted in the AMBER 7.0 force field. The relaxed models of ligand-receptor complexes were used to identify the putative binding sites of OT. The stabilizing interactions with conserved Gln residues in all complexes were identified. The nonconserved hydrophobic residues were proposed as responsible for OTR-OT selectivity and ligand recognition. These results provide guidelines for experimental site-directed mutagenesis and if confirmed, they may be helpful in designing new selective OT analogs with both agonistic or antagonistic properties.

Novel selective hypotensive vasopressin peptides: cardiovascular and structure–activity-relationship studies

Recently, we discovered a series of peripheral acting selective hypotensive vasopressin peptides. Whether these peptides may interact with receptors outside the vasopressin receptor family and affect cardiac function could not be excluded. Accordingly, we tested the effects of these hypotensive vasopressin peptides on blood pressure and heart rate in intact rats and on the heart rate, ventricular contractile force and coronary flow of isolated perfused rat hearts. We found that the hypotensive vasopressin peptides did not modify cardiac function, either in vivo or in vitro. The vasodepressor potency was reduced when assayed in rats with vasopressin-maintained baseline blood pressure, suggesting that vasopressin and the hypotensive peptide compete for a common vasodilating vasopressin receptor in the vasculature. We have now synthesized more potent and radioiodinatable hypotensive peptides that could serve as lead compounds for the development of a radiomarker for the putative vasodilating vasopressin receptor.

Discovery of novel selective hypotensive vasopressin peptides that exhibit little or no functional interactions with known oxytocin/vasopressin receptors

1. Arginine-vasopressin (VP) has both vasoconstricting and vasodilating action. We report here the discovery of four novel selective hypotensive VP analogues: d(CH2)5[D-Tyr(Et)2,Arg3,Val4]AVP; d(CH2)5[D-Tyr(Et)2,Lys3,Val4]AVP and their iodinatable Tyr-NH2(9) analogues. 2. Bioassays in rats for activities characteristic of neurohypophysial peptides showed that the four VP peptides possessed little or no V1a, V2 or oxytocin (OT) receptor agonistic or antagonistic activities. 3. In anaesthetized rats, these peptides (0.05-0.10 mg kg(-1) i.v.) elicited a marked fall in arterial blood pressure. 4. Blockade of cholinoceptors, adrenoceptors and bradykinin B2 receptors, and inhibition of prostaglandin synthesis had little effect on their vasodepressor action. 5. Classical V1a, V2 and OT receptor antagonists did not block the vasodepressor response. 6. L-NAME, 0.2 mg kg(-1) min(-1), markedly suppressed the hypotensive response to ACh but not the vasodepressor response to the hypotensive VP peptides. However, the duration of the vasodepressor response was shortened. Very high doses of L-NAME attenuated both the vasodepressor response and the duration of action. 7. These findings indicate that the vasodepressor action of these VP peptides is independent of the peripheral autonomic, bradykinin and PG systems and is not mediated by the known classical OT/VP receptors. NO does not appear to have an important role in their vasodepressor action. 8. The discovery of these novel VP peptides could lead to the development of new tools for the investigation of the complex cardiovascular actions of VP and the introduction of a new class of hypotensive agents. The two iodinatable hypotensive VP peptides could be radiolabelled as potential markers for the localization of the receptor system involved.

Oxytocin antagonist blocks the vasodepressor but not the vasopressor effect of neurohypophysial peptides in chickens

Cockerels with permanent cannulas in the brachial artery and vein were put into isolated slings. Arterial pressure and heart rate were continuously recorded. Following habituation, tests were initiated. In each cockerel 2 nmol/kg of the tested neurohypophysial peptide (NPs) or analogue was IV injected six times at 6-min intervals. Arginine vasotocin (AVT) caused an immediate vasodepressor (VDP) effect and tachycardia. These subsided within 20-30 s and were followed by a vasopressor (VP) response and bradycardia. On repeated injections of AVT, the VDP response declined and bradycardia intensified. Arginine vasopressin (AVP), oxytocin (OT), and mesotocin (MT) had short-lasting VDP effect in the following order of potency: OT = MT > AVT > AVP. Only AVT and, more effectively, AVP, caused a VP response. The VDP effect of MT and OT declined on repeated injections. When AVT was injected after three injections of MT, it had mostly an immediate VP effect. Although the V1 agonist is VP in chickens, at the dose used the V1 antagonist, [d(CH2)5,O-Me-Tyr2]AVP, had no effect on cardiovascular responses to AVT. Pretreatment with OT antagonist, [d(CH2)5-O-Me-Tyr2-Thr4.Tyr9.Orn8]VT, abolished the VDP effect of all NPs. Thus, MT had no effect on blood pressure, whereas AVP and, more effectively, AVT, had a marked immediate VP action. In chickens the VDP effect of NPs is probably mediated by an OT/MT-like receptor, wherein the peptide's ring structure, shared by AVT, OT, and MT, is important.(ABSTRACT TRUNCATED AT 250 WORDS)

An in situ hybridization and immunohistochemical study of vasotocin neurons in the hypothalamus of water-deprived chickens

The distribution of immunoreactive vasotocin (IR-AVT) and AVT mRNA in the hypothalamus of White Leghorn cocks was determined by immunohistochemistry and in situ hybridization, respectively. In control birds that were provided with water ad lib, AVT mRNA was distributed in the periventricular and lateral regions of the hypothalamus in clusters of neurons that correspond structurally with the mammalian paraventricular (PVN) and supraoptic (SON) nuclei. Although the distribution of AVT, identified by immunohistochemistry of adjacent serial sections within the hypothalamus, was similar to the distribution of AVT mRNA, the possibility that some positive staining was due to mesotocin neurons was not excluded. Water deprivation for 2 and 4 days resulted in both an increase in levels of AVT mRNA per neuron and the number of AVT mRNA-containing cells. Additionally, water deprivation resulted in a decrease in the amount of IR-AVT per neuron. The results indicate that osmotic stimulation increases AVT gene expression not only in individual neurons but also by activating subpopulation of neurons that are not observed in normally hydrated birds.

Proteolytic conversion of oxytocin in vivo after microinjection in the rat hippocampus

Since previous studies in vivo have shown that oxytocin is metabolized by rat synaptic membrane-bound aminopeptidase- and endopeptidase-like enzymes, the proteolytic conversion of oxytocin was studied in vivo after microinjection in the rat hippocampus, a brain area that contains oxytocinergic nerve endings and receptors. Isolation of the formed peptide fragments from the injected brain area after homogenization and adsorption on a Sep-Pak cartridge by high performance liquid chromatography, and their characterization by amino acid analysis, revealed that, when oxytocin (50 nmol in 0.5 microliter) was microinjected in the CA1 field of the rat hippocampus, only the N-terminal fragment oxytocin(1-8) was formed in such amount that could be characterized. The microinjection of [3H-Tyr2]oxytocin (10 pmol) revealed that in addition to oxytocin(1-8), free [3H]tyrosine was formed. Taken together with previous findings showing that C-terminal oxytocin fragments as well oxytocin(1-8) are formed by membrane-bound aminopeptidases and endopeptidases in vitro, respectively, the results suggest that, in addition to aminopeptidases, endopeptidase-like enzymes are involved in the proteolysis of endogenous brain oxytocin.

Effects of N-acetylimidazole on oxytocin binding in bovine mammary tissue

The effects of N-acetylimidazole on specific binding of oxytocin to microsomal fractions of bovine mammary gland were studied. N-acetylimidazole suppressed oxytocin binding, with time and concentration dependence. Decreased oxytocin binding activity appeared to be due to decreased affinity of the hormone for its receptor. Acetylation of oxytocin, rather than of oxytocin receptors, seemed to be responsible for the decreased binding.

Penile erection and yawning induced by oxytocin and related peptides: structure-activity relationship

The potency of several oxytocin-related peptides in inducing penile erection and yawning after injection into a lateral ventricle of male rats was compared. Substitution of two amino acids in the oxytocin molecule or deletion of the C-terminal glycinamide as in des-GlyNH2-oxytocin [oxytocin(1-8)] reduced oxytocin potency in inducing both effects, the rank order being: oxytocin greater than [Thr4,Gly7]-oxytocin congruent to isotocin [( Ser4,Ile8]-oxytocin) greater than vasopressin [( Phe3,Arg8]-oxytocin) greater than des-GlyNH2-oxytocin. Oxytocin's ability to induce penile erection and yawning was abolished by permanent opening of the disulfide bridge by reduction and carboxymethylation. Oxytocin(1-6) and oxytocin(7-9) were also inactive. Penile erection and yawning induced by oxytocin-related peptides were antagonized in a dose-dependent manner by nonapeptide antagonists with a rank order of potency that follows their antioxytocic activity (d[(CH2)5Tyr(Me)Orn8]-vasotocin congruent to [Pen1,Phe(Me)2,Thr4,Orn8]-oxytocin greater than d[(CH2)5Tyr(Me)Arg8]-vasopression). Carboxymethylated oxytocin, oxytocin(1-6), and oxytocin(7-9) were devoid of antagonistic activity. The present results suggest that central oxytocin receptors mediating the expression of penile erection and yawning are structurally related to those present in the uterus and in the mammary gland.

Relationships between renal hemodynamics and plasma levels of arginine vasotocin and mesotocin during hemorrhage in the domestic fowl (Gallus domesticus)

1. Renal tissue blood flow (renal perfusion) and plasma levels of arginine vasotocin (AVT) and mesotocin (MT) were measured in anesthetized chickens before and during hemorrhage. 2. Renal perfusion did not decrease (P less than 0.05) until nearly 50% of the blood volume had been removed. The decrease in renal perfusion was not related to arterial blood pressure but was concomitant with an increase (P less than 0.05) in plasma AVT levels. 3. Renal perfusion during hemorrhage was positively correlated with plasma MT levels by the regression equation: renal perfusion = 0.091 (MT)-1.1459 which was highly significant (P less than 0.001, r2 = 0.95). 4. The results of this study suggest that MT as well as AVT may participate in regulating blood flow in the avian kidney.

Physiological effects of arginine vasotocin and mesotocin in cockerels

1. The effects of continuous infusion of 0.1, 1.0 and 10.0 mU/min/kg body weight of arginine vasotocin (AVT) or mesotocin (MT) on cardiovascular and thermoregulatory responses, on plasma osmolality and ionic composition and on plasma concentrations of AVT, MT, prolactin and aldosterone, were investigated in conscious White Leghorn cockerels. 2. Neither of the peptides, at any dose, affected cardiovascular functions, plasma ions and osmolality. Infusion of MT at the rate of 10 mU/min/kg body weight increased respiratory rate. Both peptides at doses of 1 and 10 mU/min/kg reduced the temperatures of the comb and shank but had no effect on the skin and cloaca. 3. Doses of 0.1 and 1.0 mU MT/min/kg reduced plasma aldosterone and at 10 mU/min/kg increased plasma AVT. At any given dose MT had no effect on plasma prolactin. AVT at 0.1 and 1.0 mU/min/kg of AVT reduced plasma MT. AVT at 1.0 mU/min/kg increased plasma prolactin and at 10 mU/min/kg reduced plasma aldosterone. 4. During saline infusion, plasma MT was positively correlated with plasma AVT and negatively correlated with respiratory rate and cloacal temperature. Plasma AVT showed a positive correlation with plasma MT and aldosterone and a negative correlation with respiratory rate and skin temperature. 5. During saline infusion, there was no significant correlation between cardiovascular functions, or plasma osmolality and ionic composition and plasma MT or AVT. 6. The present study suggests that interrelationships between circulating concentrations of AVT and MT do exist and that AVT affects aldosterone secretion. These neurohypophysical peptides are involved in thermoregulation.

Plasma levels of immunoreactive mesotocin and vasotocin during oviposition in chickens: relationship to oxytocic action of the peptides in vitro and peptide interaction with myometrial membrane binding sites

Plasma concentrations of immunoreactive vasotocin (AVT) and mesotocin (MT) were measured periodically before and subsequent to spontaneous oviposition in conscious chickens. The concentrations of AVT and MT approximately an hour prior to oviposition were 5.2 +/- 1.1 microU/ml and 14.7 +/- 5.1 pg/ml, respectively. Plasma AVT levels increased abruptly at oviposition (25.1 +/- 3.3 microU/ml) and decreased to 5.0 +/- 0.6 microU/ml within 30 min postoviposition. Significant changes in MT were not observed. The data indicate that AVT is selectively released during oviposition. The uterus was removed immediately after oviposition and the oxytocic potencies of several peptides were tested on muscle strips in vitro. The order of oxytocic potencies was AVT greater than or equal to arginine vasopressin (AVP) much greater than MT = pressinoic acid. Partially purified membranes were prepared from separate portions of the uteri used in the oxytocic assay. [3H]arginine8 vasopressin, [3H]AVP, bound to membranes saturably (Bmax = 17 fmol/mg protein) and with high affinity (Kd = 0.7 nM). The rank order of potency of the peptides in displacing [3H]AVP from the binding sites was the same as in the oxytocic assay which suggests that the [3H]AVP binding sites in uterine membranes represent physiological receptors that interact with AVT during oviposition.

[Cardiovascular effect of the antidiuretic hormone arginine vasopressin]

The two major biological actions of vasopressin are antidiuresis and vasoconstriction. The antidiuretic action of low concentrations of vasopressin is well established and concentrations 10 to 100 times above those required for antidiuresis elevate arterial blood pressure. Antidiuresis is mediated by V2-receptors at the kidney, whereas vasopressin constricts arterioles by binding at V1-receptors. Pharmacological effects of specific antagonists of the vasoconstrictor activity of vasopressin (vascular or V1-receptor antagonists) are presented. Low concentrations of vasopressin do have significant hemodynamic effects. Physiological concentrations of vasopressin cause vasoconstriction and elevate systemic vascular resistance. In subjects with intact cardiovascular reflex activity, however, cardiac output falls concomitantly and blood pressure therefore does not change. In animals with baroreceptor deafferentation or in patients with blunted baroreceptor reflexes (autonomic insufficiency) a rise in plasma vasopressin causes vasoconstriction and an increase in blood pressure, because cardiac output does not fall under these conditions. Vasopressin contributes substantially via increase in systemic vascular resistance to maintain blood pressure during water deprivation. During hemorrhage and hypotension vasopressin has a major role to restore blood pressure. In experimental hypertension vasopressin contributes to the development and maintenance of high blood pressure in DOCA, but not in genetic hypertensive rats. The role of vasopressin in human hypertension is not yet clear. Vasopressin in extrahypothalamic areas of the brain affects circulatory regulation by interaction with central cardiovascular control centers. The exact mechanism of how vasopressin is involved in central regulation of blood pressure remains to be established. In contrast to our previous opinion vasopressin is a vasoactive hormone also at low plasma concentrations. Its cardiovascular action is more complex than previously assumed.

Vasopressin receptor subtypes in dorsal hindbrain and renal medulla

We have investigated the ability of a series of synthetic vasopressin analogues and related peptides to compete with (3H)-arginine8 vasopressin for binding sites in rat renal medulla and dorsal hindbrain. In renal medulla, arginine8 vasopressin and deamino arginine8 vasopressin, a selective antidiuretic, were equipotent while two antagonists of the pressor action of arginine vasopressin were less potent. In the dorsal hindbrain, arginine8 vasopressin and the pressor antagonists were more potent than the synthetic antidiuretic. Potency profiles of these and other analogues suggest that the renal medulla and dorsal hindbrain vasopressin receptors represent different subtypes.

Biological action and binding sites for vasopressin on the mesenteric artery from normal and sodium-depleted rats

We have recently demonstrated specific binding for 3H-arginine8-vasopressin (3H-AVP) to high affinity sites on membranes of rat mesenteric arteries. We have now measured the biological activity of this peptide (AVP) and analogues on the perfused rat mesenteric artery. There was a close relationship between the ED50 of agonists or the pA2 of antagonists on the perfused tissue and the relative potency (IC50) of analogues for displacing 3H-AVP from the membrane preparation. The ED50 measured was 67 +/- 7 ng for AVP and 7.2 +/- 1.1 microgram for oxytocin. In sodium-depleted rats we have observed an increase (27%) of the maximal response to AVP with no significant change in ED50 (from 2.8 +/- 1.0 X 10(-8) M to 1.3 +/- 0.2 X 10(-8) M). On the membrane preparation, the number of binding sites for 3H-AVP was increased from 71 +/- 17 fmole/mg protein (Kd 3.5 +/- 0.5 nM) to 115 +/- 10 fmole/mg protein (Kd 4.8 +/- 0.3 nM) in the sodium-depleted rat by comparison to control animals. These results suggest that AVP and its analogues interact in a similar manner in the in vitro perfused rat mesenteric artery and with the membrane receptors isolated from the same tissue. Receptors for AVP are increased in the mesenteric vascular bed by sodium depletion.

Oxytocin: anti-insulin-like effects in isolated fat cells

Oxytocin has both insulin-like and insulin antagonistic actions in fat cells in vitro. The anti-insulin-like effects of oxytocin in dispersed rat fat cells have been studied. The magnitude of the anti-insulin-like activity varies with the metabolic pathway of glucose utilization; oxidation [14CO2 production], 32%; glycogen synthesis (D-[U-14C] glucose incorporation into glycogen), 77%. In addition, direct inhibition of the activation of fat cell glycogen synthase has been shown. These inhibitory effects depend upon an intact disulfide ring, since the ability of N-ethylmaleimide-reacted oxytocin to inhibit insulin-stimulated processes was reduced by more than 90% when compared to the intact molecule.

Role of the carboxamide groups of the asparagine and glycinamide residues of oxytocin. Syntheses and biological properties of [5-beta-cyanoalanine] oxytocin and [9-alpha-aminoacetonitrile] oxytocin

In an attempt to see whether the C=O and the NH2 of CONH2 of asparagine5 and glycinamide9 are both essential for biological activity, [5-beta-cyanoalanine] oxytocin and [9-alpha-aminoacetonitrile] oxytocin have been synthesized. Each of these analogs contains a nitrile group in place of the carboxamide group of Asn5 and GlyNH92 respectively; the nitrile group can simulate the carbonyl portion of the carboxamide, but lacks the hydrogen-bond donating capacity of its NH2 portion. Substitution of a nitrile group produced opposite biological effects in the 5 and the 9 positions; the 5-substituted analog showed very low activities (less than 3% of those of oxytocin) while the 9-substituted analog showed extremely high activities (with an in vivo uterine activity of 906 U/mg almost twice that of oxytocin). The results clearly suggest that the mechanisms of interaction of the carboxamide groups with the receptor sites are different for residues 5 and 9.

Photoaffinity labeling of the antidiuretic hormone receptor

A study to determine the feasibility of photoaffinity labeling the antidiuretic hormone receptor in the toad urinary bladder has been carried out. Two photoactivated derivatives of oxytocin have been synthesized, purified, and characterized chemically and biologically. Photolysis of the toad bladder in the presence of one of these derivatives, 2-nitro-5-azidobenzoylglycyloxytocin, produces a permanent inhibition of the response to native oxytocin. This inhibition can be relieved by protecting the hormone receptor with excess oxytocin during the photolysis. These results suggest that the photolysis-dependent inhibition of the response to native hormone is due to covalent incorporation of the photoaffinity label into the hormone receptor.