Vasopressin analogues with selective pressor activity

Vasopressin and Its Analogues: From Natural Hormones to Multitasking Peptides

Human neurohormone vasopressin (AVP) is synthesized in overlapping regions in the hypothalamus. It is mainly known for its vasoconstricting abilities, and it is responsible for the regulation of plasma osmolality by maintaining fluid homeostasis. Over years, many attempts have been made to modify this hormone and find AVP analogues with different pharmacological profiles that could overcome its limitations. Non-peptide AVP analogues with low molecular weight presented good affinity to AVP receptors. Natural peptide counterparts, found in animals, are successfully applied as therapeutics; for instance, lypressin used in treatment of diabetes insipidus. Synthetic peptide analogues compensate for the shortcomings of AVP. Desmopressin is more resistant to proteolysis and presents mainly antidiuretic effects, while terlipressin is a long-acting AVP analogue and a drug recommended in the treatment of varicose bleeding in patients with liver cirrhosis. Recently published results on diverse applications of AVP analogues in medicinal practice, including potential lypressin, terlipressin and ornipressin in the treatment of SARS-CoV-2, are discussed.

Reinterpreting anomalous competitive binding experiments within G protein-coupled receptor homodimers using a dimer receptor model

An increasing number of G protein-coupled receptors (GPCRs) have been reported to be expressed in the plasma membrane as dimers. Since most ligand binding data are currently fitted by classical equations developed only for monomeric receptors, the interpretation of data could be misleading in the presence of GPCR dimers. On the other hand, the equations developed from dimer receptor models assuming the existence of two orthosteric binding sites within the dimeric molecule offer the possibility to directly calculate macroscopic equilibrium dissociation constants for the two sites, an index of cooperativity (D_C) that reflects the molecular communication within the dimer and, importantly, a constant of radioligand-competitor allosteric interaction (K_DAB) in competitive assays. Here, we provide a practical way to fit competitive binding data that allows the interpretation of apparently anomalous results, such as competition curves that could be either bell-shaped, monophasic or biphasic depending on the assay conditions. The consideration of a radioligand-competitor allosteric interaction allows fitting these curve patterns both under simulation conditions and in real radioligand binding experiments, obtaining competitor affinity parameters closer to the actual values. Our approach is the first that, assuming the formation of receptor homodimers, is able to explain several experimental results previously considered erroneous due to their impossibility to be fitted. We also deduce the radioligand concentration responsible for the conversion of biphasic to monophasic or to bell-shaped curves in competitive radioligand binding assays. In conclusion, bell-shaped curves in competitive binding experiments constitute evidence for GPCR homodimerization.

Selective V(1a) agonism attenuates vascular dysfunction and fluid accumulation in ovine severe sepsis

Vasopressin analogs are used as a supplement to norepinephrine in septic shock. The isolated effects of vasopressin agonists on sepsis-induced vascular dysfunction, however, remain controversial. Because V(2)-receptor stimulation induces vasodilation and procoagulant effects, a higher V(1a)- versus V(2)-receptor selectivity might be advantageous. We therefore hypothesized that a sole, titrated infusion of the selective V(1a)-agonist Phe(2)-Orn(8)-Vasotocin (POV) is more effective than the mixed V(1a)-/V(2)-agonist AVP for the treatment of vascular and cardiopulmonary dysfunction in methicillin resistant staphylococcus aureus pneumonia-induced, ovine sepsis. After the onset of hemodynamic instability, awake, chronically instrumented, mechanically ventilated, and fluid resuscitated sheep were randomly assigned to receive continuous infusions of either POV, AVP, or saline solution (control; each n = 6). AVP and POV were titrated to maintain mean arterial pressure above baseline - 10 mmHg. When compared with that of control animals, AVP and POV reduced neutrophil migration (myeloperoxidase activity, alveolar neutrophils) and plasma levels of nitric oxide, resulting in higher mean arterial pressures and a reduced vascular leakage (net fluid balance, chest and abdominal fluid, pulmonary bloodless wet-to-dry-weight ratio, alveolar and septal edema). Notably, POV stabilized hemodynamics at lower doses than AVP. In addition, POV, but not AVP, reduced myocardial and pulmonary tissue concentrations of 3-nitrotyrosine, VEGF, and angiopoietin-2, thereby leading to an abolishment of cumulative fluid accumulation (POV, 9 ± 15 ml/kg vs. AVP, 110 ± 13 ml/kg vs. control, 213 ± 16 ml/kg; P < 0.001 each) and an attenuated cardiopulmonary dysfunction (left ventricular stroke work index, PaO(2)-to-FiO(2) ratio) versus control animals. Highly selective V(1a)-agonism appears to be superior to unselective vasopressin analogs for the treatment of sepsis-induced vascular dysfunction.

Probing the existence of G protein-coupled receptor dimers by positive and negative ligand-dependent cooperative binding

An increasing amount of ligand binding data on G protein-coupled receptors (GPCRs) is not compatible with the prediction of the simple mass action law. This may be related to the propensity of most GPCRs, if not all, to oligomerize. Indeed, one of the consequences of receptor oligomerization could be a possible cross-talk between the protomers, which in turn could lead to negative or positive cooperative ligand binding. We prove here that this can be demonstrated experimentally. Saturation, dissociation, and competition binding experiments were performed on vasopressin and oxytocin receptors expressed in Chinese hamster ovary or COS-7 cells. Linear, concave, and convex Scatchard plots were then obtained, depending on the ligand used. Moreover, some competition curves exhibited an increase of the radiotracer binding for low concentrations of competitors, suggesting a cooperative binding process. These data demonstrate that various vasopressin analogs display either positive or negative cooperative binding. Because positive cooperative binding cannot be explained without considering receptor as multivalent, these binding data support the concept of GPCR dimerization process. The results, which are in good accordance with the predictions of previous mathematical models, suggest that binding experiments can be used to probe the existence of receptor dimers.

Reversibility of hepatorenal syndrome by prolonged administration of ornipressin and plasma volume expansion

Hepatorenal syndrome is caused by a marked vasoconstriction of the renal circulation. It is suggested that the renal vasoconstriction is related to an overactivity of vasoconstrictor systems secondary to a vasodilation of the arterial circulation that causes a reduction in effective arterial blood volume. To test this hypothesis, 16 cirrhotic patients with hepatorenal syndrome were treated with a combination of ornipressin, a potent vasoconstrictor agent, and plasma volume expansion with albumin to improve effective arterial blood volume. The combined treatment was administered either for 3 or 15 days (8 patients each), and the effects on renal function, vasoactive systems, and systemic hemodynamics were assessed. The 3-day treatment with ornipressin and albumin was associated with a normalization of the overactivity of renin-angiotensin and sympathetic nervous systems, a marked increase in atrial-natriuretic peptide levels, and only a slight improvement in renal function. However, when ornipressin and albumin were administered for 15 days, a remarkable improvement in renal function was observed, with normalization of serum-creatinine concentration, a marked increase in renal plasma flow and glomerular filtration rate, and a persistent suppression in the activity of vasoconstrictor systems. However, 3 of 8 patients on 15-day therapy treatment had to be discontinued because of ischemic complications. In conclusion, the decrease in effective arterial blood volume and the activation of vasoconstrictor systems play a crucial role in the pathogenesis of hepatorenal syndrome. Although the prolonged administration of ornipressin combined with plasma volume expansion reverses hepatorenal syndrome, this treatment should be used with great caution in clinical practice because of the risk of ischemic complications.

Action of vasopressin on hypoglossal motoneurones of the rat: presynaptic and postsynaptic effects

The distribution of vasopressin binding sites in the hypoglossal nucleus of newborn rats was determined using autoradiography on film and a radioiodinated vasopressor antagonist. These sites predominated in the ventromedial and dorsal divisions of the nucleus. The effect of vasopressin on hypoglossal neurones was studied in brainstem slices of newborn animals, using the single-electrode voltage-clamp technique. Vasopressin, at 0.1-0.5 microM, generated a sustained inward current in a majority of neurones, an action which was mediated by V1-type receptors. Antidromic activation or morphological characterization of biocytin-labelled neurones indicate that part of the vasopressin-sensitive cells were motoneurones. When synaptic transmission was blocked by perfusing the preparation with a low-calcium/high-magnesium solution, the average vasopressin current decreased by 65%; and following TTX treatment, the peptide current decreased by 55%. In contrast, in a low-calcium solution, i.e., under conditions of reduced synaptic transmission but of increased neuronal excitability, the vasopressin current was not significantly altered. These results may be interpreted by assuming that the action of vasopressin is in part postsynaptic and in part presynaptic, the latter effect probably depending upon action potential propagation. Current-voltage relations suggest that the postsynaptic effect of vasopressin was due to the induction of a non-inactivating inward current, reversing in polarity at around -15 mV. The data raise the possibility that, in young animals, endogenous vasopressin may modulate the activity of hypoglossal motoneurones.

Labelling of vasopressin and oxytocin receptors from the human uterus

Four labelled ligands, [3H]arginine vasopressin ([3H]AVP), [3H]oxytocin ([3H]OT), [3H]d(CH2)5[Tyr(Me)2]AVP ([3H]VPA), and [125I]d(CH2)5[Tyr(Me)2-Thr4-Orn8-Tyr(NH2)9]OT([125I]OTA] and nine unlabelled analogues exhibiting enhanced selectivity for rat oxytocin (OT) and vasopressin (VP) receptors were used to characterize OT and VP receptors on myometrial membranes from non-pregnant and pregnant human uteri. On membranes from non-pregnant uteri, [3H]AVP, [3H]VPA, and [125I]OTA labelled with high affinity (Kd values: 3.2, 2 and 0.8 nM, respectively) a major and apparently homogeneous population of sites, the ligand selectivity of which resembled that of rat V1a VP receptors. On membranes from pregnant and non-pregnant uteri, [3H]OT labelled a single population of high-affinity sites that could be distinguished from VP receptors on the basis of ligand selectivity. Several analogues (in particular [125I]OTA) that are highly selective for rat OT receptors exhibited a much less pronounced selectivity for human OT receptors. Experiments with [3H]VPA allowed detection of VP receptors on myometrical membranes from pregnant uteri and confirmed that only OT but not VP receptors increase during pregnancy in humans.

Effects of vasopressin and other neuropeptides on rostral medullary sympathoexcitatory neurons 'in vitro'

Neurons with intrinsic pacemaker activity and presumed sympathoexcitatory function were recorded in rat tissue slices within the confines of the rostroventrolateral reticular nucleus (RVL). These cells were excited in dose-dependent fashion by arginine vasopressin (AVP, 10(8)-10(6) M) but not by oxytocin (up to 10(7) M). The effect of AVP was mimicked by the V1-selective agonist [Phe2,Orn8]vasotocin (VT) (1 microM) but not by the V2-agonist [Val4,D-Arg8]vasopressin (VP) (1.9 microM). The effect of AVP (10(-7) M) was completely blocked by SKF 101926 (10(7) M), a non-selective antagonist and by d(CH2)5[Tyr(Me)2]AVP, a V1-selective antagonist but was unaffected by the V2-selective antagonist d(CH2)5[D-Ile2,Ile4,Ala-NH2 9]AVP. These cells were also activated by thyrotropin-releasing hormone (TRH) (10(-7)-10(-6) M), calcitonin gene-related peptide (CGRP) (4 X 10(-8) M), substance P, (10(-6) M), neuropeptide Y (NPY) (10(-8) M) and inhibited by Met-enkephalin (10(-6) M) and morphine (2 mM). Corticotropin-releasing factor (CRF) (10(-7) M) and angiotensin II (10(-6) M) were ineffective. In conclusion, RVL pacemaker neurons have vasopressin receptors reminiscent of the V1 (vascular and pressor) subtype. Their pacemaking activity is modulated by low doses of several other peptides also known to produce large vasomotor effects after introduction into the cerebroventricular space.

Single unit response of suprachiasmatic neurons to arginine vasopressin (AVP) is mediated by a V1-like receptor in the hamster

A large population of local circuit neurons within the suprachiasmatic nucleus (SCN) exhibit immunopositive staining for arginine-vasopressin (AVP). AVP has been reported to be released from SCN neurons in a circadian pattern that peaks during the subjective day. Using an in vitro hypothalamic slice preparation, the present study investigated how SCN neurons respond to AVP over the circadian cycle and whether the response to AVP is mediated by V1- or V2-like AVP receptors. Exposure of the slice to AVP 10(-8) M produced excitatory responses in 51% of the 74 SCN neurons examined. A statistically significant day-night difference in the percentage of SCN units responding to AVP was observed (chi 2 = 15.62; P less than 0.01). During the dark phase 73% were excited by AVP, while during the light phase only 24% had excitatory responses. The threshold concentration of AVP ranged from 10(-9) to 10(-10) M and 10(-7) to 10(-8) M during the dark and light phase, respectively. In a second experiment the effects of selective V1 and V2 agonists and antagonists were determined. A V1, but not a V2 receptor antagonist was found to block the effects of AVP on single unit activity. Similarly, a V1 but not a V2 receptor agonist mimicked the effects of AVP.(ABSTRACT TRUNCATED AT 250 WORDS)

Vasopressin, oxytocin and synthetic analogues: the use of bioassays

Bioassay procedures have been the prerequisite for detection, purification, elucidation of the structure and the synthesis of neurohypophysial hormones. After a review of the history of these bioassays and of the standard preparations including international standards, the current international standards and the bioassay methods prescribed by the pharmacopoeias are described. Some important methodological details are also mentioned.

Electrophysiological and autoradiographical evidence of V1 vasopressin receptors in the lateral septum of the rat brain

Extracellular recordings were obtained from single neurons located in the lateral septum, an area known to receive a vasopressinergic innervation in the rat brain. Approximately half of the neurons tested responded to 8-L-arginine vasopressin (AVP) by a marked increase in firing rate at concentrations greater than 1 nM. The effect of vasopressin was blocked by synthetic structural analogues possessing antagonistic properties on peripheral vasopressin and oxytocin receptors. Oxytocin was much less potent than vasopressin in firing septal neurons, and a selective oxytocic agonist was totally ineffective. The action of vasopressin on neuronal firing was mimicked by the vasopressor agonist [2-phenylalanine,8-ornithine]vasotocin but not by the selective antidiuretic agonist 1-deamino[8-D-arginine]vasopressin. In a parallel study, sites that bind [3H]AVP at low concentration (1.5 nM) were found by in vitro autoradiography in the lateral septum. Adjacent sections were also incubated with 1.5 mM [3H]AVP and, in addition, with 100 nM [2-phenylalanine,8-ornithine]vasotocin or 1-deamino[8-D-arginine]vasopressin--i.e., the same compounds as those used for the electrophysiological study. Results showed that the vasopressor agonist, but not the antidiuretic agonist, displaced [3H]AVP, thus indicating that the vasopressin binding sites detected by autoradiography in the septum were V1 (vasopressor type) rather than V2 (antidiuretic type) receptors. Based on the electrophysiological evidence, we conclude that these receptors, when occupied, lead to increased firing of lateral septal neurons.

The effects of ornipressin and adrenaline on lignocaine nerve blocks

An electrophysiological method measuring nerve conduction following the electrical stimulation of a sciatic nerve branch in anaesthetised rats was used to evaluate under strictly controlled conditions the effects of 1:100,000 adrenaline and of 0.03 IU X ml-1 ornipressin on the speed of onset, depth and duration of lignocaine anaesthesia. It was shown that compared with plain lignocaine, lignocaine with ornipressin produced shorter onset of anaesthesia and improved its depth and duration. Adding adrenaline to lignocaine merely extended its duration of action. Ornipressin, therefore, even at low concentration, can be considered as a viable alternative vasoconstrictor to adrenaline, the use of which is undesirable in some categories of patients.

Characterization of a uterine-type oxytocin receptor in the rat hippocampus

Extracellular recordings were obtained from a class of nonpyramidal neurones in hippocampal slices. Oxytocin applied to the bath at concentrations of 1 nM or greater excited these cells. This effect was reversibly antagonized by a synthetic structural analogue known to block the peripheral endocrine effects of oxytocin. The effect of oxytocin was mimicked by a selective oxytocic agonist and, with less potency, by vasopressin and by other structural analogues. The potencies of oxytocin, vasopressin, and of these analogues in the hippocampus correlated well with their uterotonic activities but not with their vasopressor or antidiuretic activities. The data suggest that a class of hippocampal neurones is endowed with receptors for oxytocin that are similar to those of uterine smooth muscle cells.

DPAVP: a vasopressin analog with selective microvascular and RES actions for the treatment of circulatory shock in rats

The present study indicates that: (a) local administration of low concentrations of an analog of vasopressin, 1-deamino-[2-phenylalanine, 8-arginine]-vasopressin (DPAVP), constricts venules in the rat splanchnic terminal vascular bed of normal animals, unlike that seen for catecholamines; (b) maximal concentrations of DPAVP narrow but do not occlude both arterioles and venules: (c) microscopic muscular venules (31-39 mu i.d.) do not narrow more than 20% in response to the vasopressin analog DPAVP; and (d) terminal arterioles (17-23 mu i.d.) do not narrow more than 50% in response to DPAVP. Systemic administration of DPAVP to rats subjected to hemorrhage or bowel ischemia shock more than doubles survival rates over control rats receiving Ringer solution. Infusion of DPAVP produces a dose-dependent effect on arterial blood pressure, microscopic capacitance vessels, large arterioles and small arteries. In addition, i.v. administration of DPAVP: (a) returns arterial hematocrit towards normal after shock; and (b) regenerates and sustains vasomotion and venular tone, decreases microvascular hyperreactivity characteristic of shock syndromes, restores constricted arteriolar lumen sizes towards normal, predisposes to a splanchnic microbed virtually free of stasis, petechiae and leukocytic sticking, and restores capillary perfusion and outflow to near-normal. Further, DPAVP effectively restores the early reticuloendothelial system (RES) phagocytic depression, characteristic of shock syndromes, to normal; the latter eventuating in RES hyper-phagocytic activity. These findings indicate it is possible to synthesize vasoactive molecules which: (a) exert selective microvascular and RES phagocytic effects; and (b) are highly beneficial in the therapy of low-flow states, at least in rats.

Natriferic and hydrosmotic effects of neurohypophysial peptides and their analogues in augmenting fluid uptake by Bufo melanostictus

1. The effect of intramuscular injection of 8-arginine vasotocin, 8-arginine vasopressin, 8-lysine vasopressin, oxytocin, 8-ornithine oxytocin and 8-ornithine vasopressin on fluid uptake across the skin was studied in the live toad, Bufo melanostictus, bathed either in distilled water or in NaCl solution (0.1 g/100 ml.).2. When the bathing solution was distilled water, 8-arginine vasotocin was the most potent, 0.14 nmole/kg augmenting the rate of fluid uptake by 50%. Compared with it the others had relative potencies of: 8-arginine vasopressin 0.8, 8-lysine vasopressin 0.8 x 10(-3), oxytocin 0.8 x 10(-3), 8-ornithine oxytocin 0.8 x 10(-2), 8-ornithine vasopressin < 1.4 x 10(-4).3. When the bathing solution contained 0.1% NaCl, 8-arginine vasotocin was again the most potent, 0.06 nmole/kg augmenting the rate of fluid uptake by 50%. Compared with it the others had relative potencies of: 8-arginine vasopressin 0.3, 8-lysine vasopressin 0.3 x 10(-3), oxytocin 0.3 x 10(-2), 8-ornithine oxytocin 0.8 x 10(-2), 8-ornithine vasopressin < 0.6 x 10(-4).4. Dose-response curves for each peptide showed that in the case of 8-arginine vasopressin, 8-lysine vasopressin and 8-ornithine vasopressin the augmentation of rate of fluid uptake did not differ in the absence or in the presence of NaCl in the bathing solution; whereas in the case of 8-arginine vasotocin, oxytocin, and 8-ornithine oxytocin the augmentation was greater in the presence of sodium chloride.5. Support has been found for the postulate of a binary action of some neurohypophysial peptides on amphibian skin, arginine in position 8 being correlated with hydrosmotic effect, and isoleucine in position 3 with natriferic effect.