Receptor-specific antagonists of vasopressin and oxytocin. A current perspective

Six Decades of History of Hypertension Research at the University of Toledo: Highlighting Pioneering Contributions in Biochemistry, Genetics, and Host-Microbiota Interactions

PURPOSE OF REVIEW

The study aims to capture the history and lineage of hypertension researchers from the University of Toledo in Ohio and showcase their collective scientific contributions dating from their initial discoveries of the physiology of adrenal and renal systems and genetics regulating blood pressure (BP) to its more contemporary contributions including microbiota and metabolomic links to BP regulation.

RECENT FINDINGS

The University of Toledo College of Medicine and Life Sciences (UTCOMLS), previously known as the Medical College of Ohio, has contributed significantly to our understanding of the etiology of hypertension. Two of the scientists, Patrick Mulrow and John Rapp from UTCOMLS, have been recognized with the highest honor, the Excellence in Hypertension award from the American Heart Association for their pioneering work on the physiology and genetics of hypertension, respectively. More recently, Bina Joe has continued their legacy in the basic sciences by uncovering previously unknown novel links between microbiota and metabolites to the etiology of hypertension, work that has been recognized by the American Heart Association with multiple awards. On the clinical research front, Christopher Cooper and colleagues lead the CORAL trials and contributed importantly to the investigations on renal artery stenosis treatment paradigms. Hypertension research at this institution has not only provided these pioneering insights, but also grown careers of scientists as leaders in academia as University Presidents and Deans of Medical Schools. Through the last decade, the university has expanded its commitment to Hypertension research as evident through the development of the Center for Hypertension and Precision Medicine led by Bina Joe as its founding Director. Hypertension being the top risk factor for cardiovascular diseases, which is the leading cause of human mortality, is an important area of research in multiple international universities. The UTCOMLS is one such university which, for the last 6 decades, has made significant contributions to our current understanding of hypertension. This review is a synthesis of this rich history. Additionally, it also serves as a collection of audio archives by more recent faculty who are also prominent leaders in the field of hypertension research, including John Rapp, Bina Joe, and Christopher Cooper, which are cataloged at Interviews .

Vasopressin inhibits glutamate release via two distinct modes in the brainstem

The hypothalamus coordinates autonomic responses in part through arginine vasopressin (AVP) released in medial nucleus tractus solitarius (NTS). However, the mechanisms and sites of AVP action within NTS pathways are uncertain. In brainstem slices, we activated solitary tract (ST) primary afferents to release glutamate and tested whether AVP modulated synaptic transmission to second-order neurons. NTS neurons were classified as second order by ST synaptic characteristics or the presence of anterograde tracers from peripheral baroreceptor afferents. Stimulus recruitment curves indicated ST-EPSCs on individual neurons were evoked by stimulation of single ST axons. Variance-mean (V-M) analysis of ST-EPSCs in individual neurons revealed uniformly high release probability (p approximately 0.9) from an average of 19 release sites (N) and a quantal size (q) of 34.0 +/- 4.7 pA. In 26 of 49 neurons, AVP inhibited afferent synaptic transmission. In most neurons, AVP reduced ST-EPSC amplitudes (n = 20) by decreasing p to 0.65, whereas q, N, and conduction times were unaffected. The V1a antagonist SR49059 alone decreased ST-EPSC V and increased M, suggesting tonic AVP actions, and blocked exogenous AVP action (n = 4). In other neurons with identical ST release properties, AVP induced synaptic failures and increased conduction time without altering the V-M relationship of successful ST-EPSCs (n = 6). Interestingly, frequency-depressed ST-EPSCs were not affected by AVP. AVP failed to alter holding or voltage-dependent potassium currents. Thus, AVP regulates NTS neurons by two distinct novel and state-dependent mechanisms: one, an analog, graded presynaptic inhibition of terminal glutamate release and the other, a binary, extraterminal block of conducted excitation.

Conformational studies of vasopressin analogues modified with N-methylphenylalanine enantiomers in dimethyl sulfoxide solution

The conformations of [Arg8]vasopressin (AVP) analogues substituted at positions 2 and 3 with N-methylphenylalanine (MePhe) enantiomers were earlier investigated by using nuclear magnetic resonance (NMR) spectroscopy in aqueous solution. A comparison of the results obtained in H2O/D2O (9:1) and DMSO-d6 has shown the structures in the first solution to be more flexible than those in DMSO-d6. This is manifested by a higher percentage of minor conformations in H2O/D2O. The largest differences between the NMR spectra in both solvents were noticed for [MePhe2, D-MePhe3]AVP (II) and [D-Cys1,MePhe2,D-MePhe3]AVP (III). Namely, in the ROESY spectra in aqueous solution, the cis/trans isomerization between MePhe2-DMePhe3 and D-Cys1-MePhe2 for II and III, respectively, is observed, while in DMSO-d6, the appropriate cross peaks indicate isomerization across the Cys6-Pro7 peptide bond. In the case of the remaining peptides, the position of cis/trans isomerization is the same in aqueous solution and in dimethyl sulfoxide. [D-MePhe2,MePhe3]AVP (V) displays low antiuterotonic and antipressor activities, while [D-MePhe2,)]AVP (IV) is a weak but selective blocker of oxytocin (OT) receptors in the uterus. The former shows similar conformational preferences as another antagonist of V1a and OT receptors-namely, [Acc2,D-Arg8]VP (Acc: 1-aminocyclohexane-1-carboxylic acid)-investigated by us. In the case of IV, the cis peptide bond between residues at positions 2 and 3 might be the reason for selectivity.

Gene regulation in the magnocellular hypothalamo-neurohypophysial system

The hypothalamo-neurohypophysial system (HNS) is the major peptidergic neurosecretory system through which the brain controls peripheral physiology. The hormones vasopressin and oxytocin released from the HNS at the neurohypophysis serve homeostatic functions of water balance and reproduction. From a physiological viewpoint, the core question on the HNS has always been, "How is the rate of hormone production controlled?" Despite a clear description of the physiology, anatomy, cell biology, and biochemistry of the HNS gained over the last 100 years, this question has remained largely unanswered. However, recently, significant progress has been made through studies of gene identity and gene expression in the magnocellular neurons (MCNs) that constitute the HNS. These are keys to mechanisms and events that exist in the HNS. This review is an inventory of what we know about genes expressed in the HNS, about the regulation of their expression in response to physiological stimuli, and about their function. Genes relevant to the central question include receptors and signal transduction components that receive and process the message that the organism is in demand of a neurohypophysial hormone. The key players in gene regulatory events, the transcription factors, deserve special attention. They do not only control rates of hormone production at the level of the gene, but also determine the molecular make-up of the cell essential for appropriate development and physiological functioning. Finally, the HNS neurons are equipped with a machinery to produce and secrete hormones in a regulated manner. With the availability of several gene transfer approaches applicable to the HNS, it is anticipated that new insights will be obtained on how the HNS is able to respond to the physiological demands for its hormones.

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.

Identification of residues responsible for the selective binding of peptide antagonists and agonists in the V2 vasopressin receptor

To improve our understanding of the functional architecture of G protein-coupled receptors, we have taken advantage of differences among mammalian species in ligand binding to search for the rat versus human selectivity determinants of the V2 vasopressin receptor and of its peptide ligands. Our data indicate that residue 2 of species-selective peptide antagonists such as d(CH2)5-[D-Ile2,Ile4, Tyr-NH29]arginine vasopressin controls their rat versus human selectivity. For species-selective agonists such as desmopressin, residues 1 and 8 modulate the binding selectivity. Among residues different between rat and human V2 receptors, those localized in the upper part of the human V2 receptor have been substituted with their rat V2 homologs. Pharmacological analysis of mutant receptors revealed that residues 202 and 304 fully control the species selectivity of the discriminating antagonists in an independent and additive manner. A third residue (position 100) is necessary to observe an equivalent phenomenon for the discriminating agonists. The substitution of these three residues does not modify the affinity of the nonselective agonists and antagonists. In conclusion, extracellular loops and the top of the transmembrane domains of V2 vasopressin receptors may provide the molecular basis for peptide ligand-binding species selectivity. Very few residues in these regions may control the binding mode of both agonists and antagonists.

Vasopressin-induced currents in rat neonatal spinal lateral horn neurons are G-protein mediated and involve two conductances

Arginine vasopressin (AVP) receptors are expressed early in the developing spinal cord. To characterize AVP-induced conductances in lower thoracic sympathetic preganglionic (SPN) and other lateral horn neurons, we used patch-clamp recording techniques in neonatal (11-21 days) rat spinal cord slices. Most (90%) of 273 neurons, including all 68 SPNs, responded to AVP with membrane depolarization and/or a V1 receptor-mediated, dose-dependent (0.01-1.0 microM) and tetrodotoxin (TTX)-resistant inward current. A role for G-proteins was indicated by persistence of this inward current after intracellular dialysis with GTP-gamma-S or GMP-PNP, its marked reduction with GDP-beta-S, and significant reduction, but not abolition, after preincubation with pertussis toxin or in the presence of N-ethylmaleimide. Analysis of individual current-voltage (I-V) relationships in 57 cells indicated the presence of two different membrane conductances. In 21 cells, net AVP-induced currents reversed around -103 mV, reflecting reduction in one or more barium-sensitive potassium conductances; in 12 cells, net AVP-induced current reversed around -40 mV and was not significantly sensitive to several potassium channel blockers including barium, tetraethylammonium chloride (TEA), 4-aminopyridine (4AP), cesium, or glibenclamide, suggesting increase in a nonselective cationic conductance that was separate from Ih; in 24 cells where I-V lines shifted in parallel, AVP-induced inward currents were significantly greater and probably involved both conductances. These data indicate that SPNs and a majority of unidentified neonatal lateral horn neurons possess functional G-protein-coupled V1-type vasopressin receptors. The wide distribution of AVP receptors in neonatal spinal lateral column cells suggests a role that may extend beyond involvement in regulation of autonomic nervous system function.

Vasopressin can increase coronary perfusion pressure during human cardiopulmonary resuscitation

OBJECTIVES

To determine the hemodynamic effect of vasopressin on coronary perfusion pressure (CPP) in prolonged human cardiac arrest.

METHODS

A prospective, open-label clinical trial of vasopressin during cardiac resuscitation was performed. Ten patients presenting in cardiac arrest initially received resuscitative measures by emergency physicians according to Advanced Cardiac Life Support (ACLS) guidelines. A central venous catheter for fluid and drug administration and a femoral artery catheter for measurement of CPP (aortic minus right atrial relaxation phase pressures) were placed. When each patient was deemed nonsalvageable, 1.0 mg epinephrine was given and CPP was measured for 5 minutes, followed by a dose of vasopressin (1.0 U/kg). CPP measurements were continued for another 5 minutes.

RESULTS

The mean duration of cardiac arrest (out-of-hospital interval plus duration of ED ACLS) was 39.6 +/- 16.5 min. There was no improvement in CPP after 1.0 mg of epinephrine. Vasopressin administration resulted in a significant increase of CPP in 4 of the 10 patients. Patients responding to vasopressin had a mean increase in CPP of 28.2 +/- 16.4 mm Hg (range: 10-51.5), with these peak increases occurring at 15 seconds to 4 minutes after administration. The increases in the vasopressin levels after administration did not differ between the responders and nonresponders.

CONCLUSIONS

In this human model of prolonged cardiac arrest, 40% of the patients receiving vasopressin had a significant increase in CPP. This pilot study suggests that investigation of earlier use of vasopressin as a therapeutic alternative in the treatment of cardiac arrest is warranted.

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).

A diurnal rhythm of stimulatory input to the hypothalamo-pituitary-adrenal system as revealed by timed intrahypothalamic administration of the vasopressin V1 antagonist

The mammalian suprachiasmatic nuclei (SCN) contain an endogenous pacemaker that generates daily rhythms in behavior and secretion of hormones. We hypothesized that the SCN imposes its circadian rhythm on the rest of the brain via a rhythmic release of its transmitters in its target areas. Previously, we demonstrated a pronounced inhibitory effect of vasopressin (VP), released from SCN terminals in the dorsomedial hypothalamus, on the release of the adrenal hormone corticosterone. In the present study, microdialysis-mediated intracerebral administration of the VP V1-receptor antagonist was used to pursue the study of the mechanisms underlying the circadian control of basal corticosterone release. Using timed administrations of the VP antagonist divided equally over the day/night cycle, we were able to uncover the existence of an additional stimulatory input from the SCN to the hypothalamopituitary-adrenal (HPA) axis. Peak activity of this stimulatory SCN input takes place during the second half of the light period, after the daily peak of VP secretion, with a delay of approximately 4-6 hr. In all likelihood, the inhibitory and stimulatory circadian input via separate mechanisms affects corticosterone release. Together, these two opposing circadian control mechanisms of the HPA axis enable a precise timing of the circadian peak in corticosterone release.

Endothelium-dependent and NO-mediated desensitization to vasopressin in rat aorta

1. The present study was performed to characterize the tachyphylaxis of rat aortae to vasopressin. Isometric tension generated by rat thoracic aorta sliced in 4 mm rings, was recorded. 2. Tension generated by intact rings increased with cumulative additions of vasopressin up to 10 nM (1.51 +/- 0.15 g). After this concentration, most rings lost their tension and relaxed to 1.09 +/- 0.17 g (P < 0.001) despite further addition of vasopressin. This tachyphylaxis was not observed in endothelium-denuded rings (from 2.87 +/- 0.12 g to 2.68 +/- 0.17 g). 3. Repeated administrations of supramaximal concentration (100 nM) of vasopressin confirmed an enhanced desensitization in intact rings, compared to endothelium-denuded rings. No desensitization to phenylephrine was observed in intact or in endothelium-denuded rings. 4. Dose-response curves to a V1 receptor agonist, [Phe2, Ile3, Orn8]-vasopressin, and to a V2 receptor agonist, [deamino-Cys1,D-Arg8]-vasopressin, were performed in intact preparations. An increase in tension, followed by a desensitization was observed with the V1 receptor agonist. In contrast, the V2 receptor agonist did not induce any response. 5. Pretreatment of intact aortic rings with the cyclo-oxygenase inhibitor, diclofenac (1 microM), did not prevent the desensitization to vasopressin. In contrast, NO synthase inhibition with NG-nitro-L-arginine (30 microM) resulted in an attenuated desensitization to vasopressin in intact rings (from 2.46 +/- 0.17 to 2.25 +/- 0.22 g, NS). 6. To confirm the involvement of NO, endothelium-denuded rings were pretreated with sodium nitroprusside (SNP). At a concentration of 10 nM, SNP induced a desensitization to vasopressin comparable with that observed in intact rings. 7. Pretreatment of endothelium-denuded rings with 8-bromo-cyclic GMP (100 microM) reduced maximum contraction to vasopressin without producing any desensitization. In contrast, guanylate cyclase inhibition with either LY 83,583 (10 microM) or methylene blue (10 microM) blocked completely the desensitization of intact rings to vasopressin. 8. The results suggest that the endothelium-dependent tachyphylaxis to vasopressin is due to rapid desensitization and is mediated by NO. However, it is unclear whether this effect of NO involves cyclic GMP.

Design and synthesis of highly selective in vitro and in vivo uterine receptor antagonists of oxytocin: comparisons with Atosiban

We report the solid phase synthesis and some pharmacological properties of seven position two analogues (peptides 1-7) of one of our lead oxytocin antagonists, des-9-glycinamide[1-(beta-mercapto-beta,beta-pentamethylenepropionic+ ++ acid),2-O-methyltyrosine,4-threonine]ornithinevasotocin(desGly+ ++-NH2, d(CH2)5-[Tyr(Me)2,Thr4]OVT) (A). Peptides 1-7 have the following substituents at position two (1) D-Tyr(Me); (2) L-Tyr(Et); (3) D-Tyr(Et); (4) L-Tyr; (5) D-Tyr; (6) D-Phe and (7) D-Trp. These were evaluated for agonistic and antagonistic activities in in vitro and in vivo OT assays, in vivo vasopressor (V1a-receptor) assays and in vivo antidiuretic (V2-receptor) assays. None of the seven peptides exhibits oxytocic or vasopressor agonism. Peptides 1, 2, 4, 6 and 7 are extremely weak V2 agonists (V2 activities range from 0.001 to 0.02 U/mg). Peptides 3 and 5 exhibit weak V2 antagonism (pA2 < 6.0 and < 5.5, respectively). Peptides 1-7 exhibit potent in vitro (no Mg2+) OT antagonism (anti-OT pA2 values range from 7.66 to 8.03). Peptides 1 and 4-7 exhibit potent in vivo OT antagonism. Estimated in vivo anti-OT pA2 values range from 7.06 to 7.79 (peptides 2 and 3 were not tested). With anti-V1a pA2 values of 5.17-6.25 all seven peptides exhibit reduced anti-V1a potencies relative to the parent peptide (A) (anti-V1a pA2 = 6.46). Four of these peptides (4-7) exhibit striking gains in in vitro and in vivo anti-OT/anti-V1a selectivities compared to (A) which has an in vitro selectivity of 30 and an in vivo selectivity of 18. The D-Tyr2 (5), D-Trp2 (7), D-Phe2 (6) and L-Tyr2 (4) analogues of (A) exhibit anti-OT (in vitro)/anti-V1a selectivities = 240, 390, 404 and 540, respectively. The L-Tyr2 (4), D-Trp2 (7), D-Phe2 (6) and D-Tyr2 (5) analogues exhibited anti-OT (in vivo)/anti-V1a selectivities of 72, 80, 88 and 95, respectively. Peptides 4-7 appear to be the most selective peptide OT antagonists reported to date. In this regard it may be noted that they appear to be as or more potent and much more selective than the closely related OT antagonist 1-deamino[D-Tyr(Et)2,Thr4]OVT (Atosiban) which is currently undergoing clinical trial as a potential therapeutic agent for the prevention of premature labor. Atosiban (peptide 8) was resynthesized and pharmacologically evaluated in our laboratories. Atosiban exhibits the following antagonistic potencies. Anti-OT (in vitro, no Mg2+) pA2 = 7.71; anti-OT in vivo pA2 = 7.05; anti-V1a pA2 = 6.14 and anti-V2 pA2 approximately 5.9. Its anti-OT (in vivo)/anti-V1a selectivity is 8. Some of these antagonists may be suitable candidates for evaluation as potential tocolytic agents for use in the treatment of pre-term labor. They could also serve as useful new pharmacological tools for studies on the physiological roles of oxytocin. Finally, the findings presented here provide useful clues for the design of more potent and more selective OT antagonists.