Synthesis and some pharmacological properties of potent and selective antagonists of the vasopressor (V1-receptor) response to arginine-vasopressin

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 .

Differential activation of arginine-vasopressin receptor subtypes in the amygdaloid modulation of anxiety in the rat by arginine-vasopressin

RATIONALE

The amygdala plays a paramount role in the modulation of anxiety and numerous studies have shown that arginine vasopressin (AVP) elicits anxiogenic effects following either its systemic or septal administration.

OBJECTIVES

The aim of this paper was to study the involvement of vasopressinergic neurotransmission in the amygdaloid modulation of unconditioned anxiety and to ascertain whether or not AVP receptor subtypes may have a differential role in this modulation.

METHODS

Anxiety behavior was evaluated both in Shock-Probe Burying Test and Light-Dark Box following the bilateral microinfusion of AVP alone or AVP together with either AVP 1a or AVP 1b receptor antagonists into the central amygdala (CeA).

RESULTS

AVP microinfusion elicited at low (1 ng/side) but not at high doses (10 ng/side) anxiogenic-like responses in the Shock-Probe Burying Test but not in the Light-Dark Box. SSR149415, an AVP 1b antagonist unlike Manning compound, an AVP 1a antagonist, fully prevented AVP effects in the Shock-Probe Burying Test when it was administered simultaneously with AVP. In addition, oxytocin receptor blockade also failed to affect AVP effects. No effects of any AVP antagonist by itself were observed in both anxiety paradigms.

CONCLUSIONS

Our results indicate that AVP 1b receptor contribute to the amygdaloid modulation of anxiety at least in the context of the Shock-Probe Burying Test since no effects were noticed in the Light-Dark Box. It remains to the future to ascertain whether AVP receptor subtypes have indeed differential actions either in the modulation of global or specific features of unconditioned anxiety.

Effect of vasopressin-induced chronic hyponatremia on the regulation of the middle cerebral artery of the rat

Vasopressin (arginine vasopressin, AVP) plays a crucial role in maintaining body fluid homeostasis. Excessive release of vasopressin can lead to hyponatremia. Changes in cerebral circulation during vasopressin-induced chronic hyponatremia are not elucidated. The present study has been designed to investigate the effect of chronic vasopressin-induced hyponatremia on the regulation of the tone of the middle cerebral artery (MCA) of the rat. Chronic hyponatremia was induced in vivo with the help of vasopressin, released continuously from subcutaneously implanted ALZET mini-osmotic pumps, and a liquid diet. After 3.5 days of chronic hyponatremia, the plasma Na⁺ concentration decreased to 119 ± 3 mM. MCAs were isolated and placed in a MOPS-buffered saline solution containing 121 mM Na⁺. Chronic hyponatremia did not affect the response of the MCA to increased intravascular pressure, to the administration of acetylcholine (ACh) and nitric oxide (NO) donor (SNAP, S-nitroso-N-acetyl-DL-penicillamine), and to increased K⁺ concentration, but impaired the response of the MCA to increased extravascular H⁺ concentration. Disturbed response of the MCA to acidosis was associated neither with the impairment of K_ATP channels nor with the activation of vasopressin V₁ receptor. Correction of hyponatremia did not restore the response of the MCA to acidosis. These results indicate that cerebral blood vessels do not fully adapt to prolonged vasopressin-induced hyponatremia.

Chronic hyperosmotic stress converts GABAergic inhibition into excitation in vasopressin and oxytocin neurons in the rat

In mammals, the increased secretion of arginine-vasopressin (AVP) (antidiuretic hormone) and oxytocin (natriuretic hormone) is a key physiological response to hyperosmotic stress. In this study, we examined whether chronic hyperosmotic stress weakens GABA(A) receptor-mediated synaptic inhibition in rat hypothalamic magnocellular neurosecretory cells (MNCs) secreting these hormones. Gramicidin-perforated recordings of MNCs in acute hypothalamic slices prepared from control rats and ones subjected to the chronic hyperosmotic stress revealed that this challenge not only attenuated the GABAergic inhibition but actually converted it into excitation. The hyperosmotic stress caused a profound depolarizing shift in the reversal potential of GABAergic response (E(GABA)) in MNCs. This E(GABA) shift was associated with increased expression of Na(+)-K(+)-2Cl(-) cotransporter 1 (NKCC1) in MNCs and was blocked by the NKCC inhibitor bumetanide as well as by decreasing NKCC activity through a reduction of extracellular sodium. Blocking central oxytocin receptors during the hyperosmotic stress prevented the switch to GABAergic excitation. Finally, intravenous injection of the GABA(A) receptor antagonist bicuculline lowered the plasma levels of AVP and oxytocin in rats under the chronic hyperosmotic stress. We conclude that the GABAergic responses of MNCs switch between inhibition and excitation in response to physiological needs through the regulation of transmembrane Cl(-) gradients.

Impact of the Merrifield solid phase method on the design and synthesis of selective agonists and antagonists of oxytocin and vasopressin: a historical perspective

This tribute to Bruce Merrifield traces the author's fortuitous path in 1964 from Vincent du Vigneaud's laboratory to the laboratory of D. W. Woolley to learn the solid phase method and then to his first faculty position in the Department of Biochemistry, McGill University, Montreal in 1965. It recalls the key roles played from early 1966 to July 1967 by Bruce Merrifield, John Stewart, Arnold Marglin, Herb Takashima, and Vincent du Vigneaud in providing key advice to the author's efforts to use the solid phase method to synthesize oxytocin; while simultaneously the du Vigneaud and Merrifield laboratories were collaborating on the solid phase synthesis of deamino-oxytocin. Both syntheses were published in the same issue of the Journal of American Chemical Society in 1968. Also described is how this breakthrough impacted the author's scientific career: by leading to highly productive collaborative studies, initially with Wilbur H. Sawyer and subsequently with others, on the design and synthesis of selective agonists, antagonists, and radioiodinated ligands for oxytocin and vasopressin receptors. These syntheses were greatly facilitated by the contributions of highly talented graduate students, research technicians, and visiting peptide chemists from Hungary, England, Poland, Bulgaria, and China. Many of these peptides have become very valuable pharmacological tools in studies on the peripheral and central effects of oxytocin and vasopressin: further attesting to the profound impact of the solid phase method as the cornerstone for all the discoveries, which he and his collaborators and coworkers have made over the past 40 years.

Arginine vasopressin increases glutamate release and intracellular Ca2+ concentration in hippocampal and cortical astrocytes through two distinct receptors

Arginine vasopressin (AVP), released from the CNS, plays an important role in regulating several aspects of CNS functions including aggression, anxiety, and cognition. In this study, we report a novel finding that AVP induces glutamate release from astrocytes isolated from the cerebral cortex and hippocampus. We also investigated the types of AVP receptors involved in the AVP-induced increase in glutamate release from astrocytes isolated from the hippocampus and cortex of neonatal rats. We showed that the AVP (0.1-1000 nmol/L) induced increase in glutamate release and [Ca(2+)](i) is brought about by two distinct subtypes of V(1) receptors (V(1a) and V(1b)). Our results suggested that V(1b) receptors are predominantly expressed in astrocytes isolated from the hippocampus and V(1a) receptors are solely expressed in astrocytes isolated from the cerebral cortex of neonatal rats. The results of the western blot analyses confirmed these pharmacological data. In addition, the AVP-induced increase in glutamate did not contribute to an increase in [Ca(2+)](i), as blockade of metabotropic glutamate receptors did not alter the AVP-induced increase in [Ca(2+)](i). In addition, the administration of a phospholipase A(2) inhibitor failed to alter AVP-induced [Ca(2+)](i) increase suggesting the lack of involvement of this enzyme.

Centrally administered interleukin-1 beta sensitizes to the central pressor action of angiotensin II

The objective of the present study was to find out whether brain vasopressin (AVP) and angiotensin II (Ang II) are involved in pressor response to intracerebroventricular (ICV) infusion of interleukin-1 beta (IL-1beta). The experiments were performed on conscious, 12- to 14-week-old Sprague-Dawley rats. Mean arterial blood pressure (MAP) and heart rate (HR) were recorded continuously under baseline conditions and during ICV infusion periods. In the first part of the study, the rats were ICV-infused with one of the following: 0.9% NaCl saline (5 microl/h-control), IL-1beta (100 ng/h), angiotensin AT1 receptor antagonist (losartan 10 microg/h), IL-1beta together with losartan, V1 receptors antagonist (V1ANT), d(CH2)5[Tyr(Me)2,Ala-NH2(9)]AVP, 400 ng/h) or IL-1beta together with V1ANT. Saline infusion did not influence MAP, while administration of IL-1beta elicited a significant but transient increase in MAP. The pressor response to IL-1beta was abolished by losartan but not by V1ANT. On the other hand, combined administration of IL-1beta and V1ANT resulted in increase in HR, which was absent during infusion of IL-1beta alone. In the second part of the study after ICV pretreatment with IL-1beta or 0.9% NaCl (control), the rats received ICV infusion of one of the following: 0.9% NaCl saline, subpressor dose of Ang II (5 ng/15 s) or subpressor dose of AVP (5 ng/15 s). Subpressor doses of Ang II and AVP did not influence MAP and HR in saline-pretreated rats. The same dose of Ang II but not AVP applied in IL-1beta-pretreated rats resulted in a significant increase in MAP. The study provides evidence that IL-1beta through its action in the brain increases sensitivity to central pressor action of Ang II. Additionally, we found that AVP and in particular V1 receptors do not play important role in the central pressor action of IL-1beta, however, they may influence its effect on HR regulation.

Enhanced involvement of brain vasopressin V1 receptors in cardiovascular responses to stress in rats with myocardial infarction

Stress is one of the factors provoking cardiovascular complications. The purpose of the study was to explore the role of vasopressin (VP) in central control of arterial blood pressure and heart rate under resting conditions and during stimulation by an alarming stress (air jet stress) in myocardial infarct-induced cardiac failure. Six groups of male Sprague Dawley (SD) rats were subjected either to sham surgery (sham rats) or to ligation of a left coronary artery (infarcted rats). After 5 weeks both infarcted and sham rats were subjected either to intracerebroventricular infusion of artificial cerebrospinal fluid (aCSF) (sham aCSF and infarcted aCSF), [Arg8]-VP (sham VP and infarcted VP) or VP V1a receptor antagonist (d(CH2)5[Tyr(Me)2Ala-]VP, sham V1ANT and infarcted V1ANT). Air jet stress elicited significantly greater increases in mean arterial blood pressure (MABP) and heart rate in the infarcted aCSF than in the sham aCSF rats. Intracerebroventricular infusion of V1ANT significantly reduced resting MABP and MABP and heart rate increases in response to stress in the infarcted but not in the sham rats. Intracerebroventricular infusion of VP elicited a significant increase in resting MABP in the infarcted VP but not in the sham VP rats. The results provide evidence for enhanced engagement of the brain V1 VP receptors in regulation of resting MABP and in generation of exaggerated cardiovascular responses to air jet stress during the post-infarct state.

Reduced hypothalamic vasopressin secretion underlies attenuated adrenocorticotropin stress responses in pregnant rats

We sought to explain decreased ACTH secretory responses to stress in pregnant rats by investigating hypothalamic CRH and vasopressin secretion and actions on anterior pituitary corticotrophs. In late pregnancy median eminence, CRH content was reduced (by 12%). Anterior pituitary proopiomelanocortin mRNA expression, measured by in situ hybridization but not radioimmunoassayed ACTH content, was also reduced (by 45% on d 21); CRH receptor (CRHR)1 mRNA expression was unaltered in pregnancy, but V1b receptor mRNA expression was reduced (by 19%). ACTH secretory responses, measured in jugular blood, to CRH (200 ng/kg iv) or vasopressin (1.7 microg/kg, iv) were reduced on d 21 vs. virgins (49% and 44%), but the response to combined CRH and vasopressin injection was intact. Either antalarmin (CRHR1 antagonist; 20 mg/kg ip) or dP(Tyr(Me)2),Arg-NH2(9))AVP (V1a/b antagonist; 10 microg/kg, iv) pretreatment reduced the ACTH secretory response to forced swimming (90 sec) in virgin rats (by 57% and 40%), but only antalarmin was effective in pregnant rats (53% decrease). In vitro, measuring ACTH secretion from acutely dispersed anterior pituitary cells showed increased corticotroph sensitivity in pregnancy to CRH and to CRH augmentation by vasopressin, attributable to increased intracellular cAMP action. Hence, in late pregnancy, reduced anterior pituitary CRHR1 or V1b receptor expression did not impair corticotroph responses to CRH or vasopressin. Rather, diminished secretagogue secretion in vivo accounts for reduced action of stress levels of exogenous CRH or vasopressin alone; the late pregnancy attenuated ACTH secretory response to swim stress is deduced to be due to reduced vasopressin release by parvocellular paraventricular nuclei neurones.

Dehydration-induced cross-regulation of apelin and vasopressin immunoreactivity levels in magnocellular hypothalamic neurons

Apelin, a neuropeptide recently identified as the endogenous ligand for the G protein-coupled receptor APJ, is highly concentrated in brain structures involved in the control of body fluid homeostasis including the supraoptic (SON) and paraventricular (PVN) hypothalamic nuclei. To clarify the implication of apelin in the regulation of water balance, we sought to determine whether apelin colocalized with arginine vasopressin (AVP) in the rat SON and PVN. We also investigated the effects of water deprivation on the levels of apelin within these two nuclei by comparison with those of AVP. Using dual immunolabeling confocal microscopy, we found that a large proportion of apelin-immunoreactive neurons colocalized AVP within both the SON and PVN, but that the two peptides were segregated within distinct subcellular compartments inside these cells. Both the number and labeling intensity of magnocellular apelin-immunoreactive cells increased significantly after 24- or 48-h dehydration, whereas the number and labeling density of AVP-immunoreactive neurons significantly decreased. The dehydration-induced increase in apelin immunoreactivity was markedly diminished by central injection of a selective vasopressin-1 receptor antagonist. Conversely, the effect of dehydration was mimicked by a 16-min intracerebroventricular infusion of AVP, again in a vasopressin-1 receptor antagonist-reversible manner. These results provide additional evidence for the involvement of the neuropeptide apelin in the control of body fluid homeostasis. They further suggest that the dehydration-induced release of AVP from magnocellular hypothalamic neurons may be responsible for the observed increase in immunoreactive apelin levels within the same neurons and thus that the release of one peptide may block that of another peptide synthesized in the same cells.

Peritubular AVP regulates bicarbonate reabsorption in cortical distal tubule via V(1) and V(2) receptors

Peritubular arginine vasopressin (AVP) regulates bicarbonate reabsorption in the cortical distal tubule via V(1) and V(2) receptors. The dose-dependent effects of peritubular AVP on net bicarbonate reabsorption (J(HCO)) were evaluated by stationary microperfusion of in vivo early (ED; distal convoluted tubule) and late distal (LD; connecting tubule and initial collecting duct) segments of rat kidney, using double-barreled H(+)-sensitive, ion-exchange resin/reference (1 M KCl) microelectrodes. AVP (10(-11) M) perfused into peritubular capillaries increased J(HCO), compared with basal levels during intact capillary perfusion with blood, in ED and LD segments. AVP (10(-9) M) also increased J(HCO) in both segments, but the effect of AVP (10(-11) M) was significantly higher. A specificV(1)-receptor antagonist alone or with AVP (10(-11) or 10(-9) M) reduced J(HCO) below basal levels. A specific V(2)-receptor antagonist alone or plus AVP (10(-11) M) did not affect J(HCO) but increased AVP (10(-9) M)-mediated stimulation. 8-Bromoadenosine 3',5'-cyclic monophosphate alone reduced J(HCO) below basal levels and also reduced AVP (10(-11) M)-mediated stimulation. (Deamino-Cys(1), D-Arg(8)) vasopressin (a V(2)-selective agonist) also reduced J(HCO) below basal levels. These results show that peritubular AVP stimulates J(HCO) in ED and LD segments via basolateral V(1) receptors and that basolateral V(2) receptors have a dose-dependent inhibitory effect mediated by cAMP. The data also indicate that endogenous AVP stimulates distal J(HCO) via basolateral V(1) receptors.

Central blockade of vasopressin V(1) receptors attenuates postexercise hypotension

We tested the hypothesis that central arginine vasopressin (AVP) mediates postexercise reductions in arterial pressure (AP) and heart rate (HR). To test this hypothesis, nine spontaneously hypertensive rats (SHR) were instrumented with a 22-gauge stainless steel guide cannula in the right lateral cerebral ventricle and with a carotid arterial catheter. After the rats recovered, AP and HR were assessed before and after a single bout of dynamic exercise with the central administration of vehicle or the selective AVP V(1)-receptor antagonist d(CH(3))(5) Tyr(Me)-AVP (AVP-X). AP and HR were significantly decreased below preexercise values with central administration of vehicle [P < 0.05, change (Delta)-21 +/- 4 mmHg and Delta-20 +/- 6 beats/min, respectively]. In sharp contrast, after exercise with central administration of AVP-X, both AP (Delta+8 +/- 5 mmHg) and HR (Delta+24 +/- 9 beats/min) were not significantly different from preexercise values (P > 0.05). Furthermore, AVP-X at rest did not significantly alter AP (181 +/- 11 vs. 178 +/- 11 mmHg, P > 0.05) or HR (328 +/- 24 vs. 331 +/- 22 beats/min, P > 0.05). Thus central blockade of AVP V(1) receptors prevented postexercise reductions in AP and HR. These data suggest that AVP, acting within the central nervous system, mediates postexercise reductions in AP and HR in the SHR.

Upregulation of vasopressin V1A receptor mRNA and protein in vascular smooth muscle cells following cyclosporin A treatment

1. The major side effects of the immunosuppressive drug cyclosporin A (CsA) are hypertension and nephrotoxicity. It is likely that both are caused by local vasoconstriction. 2. We have shown previously that 20 h treatment of rat vascular smooth muscle cells (VSMC) with therapeutically relevant CsA concentrations increased the cellular response to [Arg8]vasopressin (AVP) by increasing about 2 fold the number of vasopressin receptors. 3. Displacement experiments using a specific antagonist of the vasopressin V1A receptor (V1AR) showed that the vasopressin binding sites present in VSMC were exclusively receptors of the V1A subtype. 4. Receptor internalization studies revealed that CsA (10(-6) M) did not significantly alter AVP receptor trafficking. 5. V1AR mRNA was increased by CsA, as measured by quantitative polymerase chain reaction. Time-course studies indicated that the increase in mRNA preceded cell surface expression of the receptor, as measured by hormone binding. 6. A direct effect of CsA on the V1AR promoter was investigated using VSMC transfected with a V1AR promoter-luciferase reporter construct. Surprisingly, CsA did not increase, but rather slightly reduced V1AR promoter activity. This effect was independent of the cyclophilin-calcineurin pathway. 7. Measurement of V1AR mRNA decay in the presence of the transcription inhibitor actinomycin D revealed that CsA increased the half-life of V1AR mRNA about 2 fold. 8. In conclusion, CsA increased the response of VSMC to AVP by upregulating V1AR expression through stabilization of its mRNA. This could be a key mechanism in enhanced vascular responsiveness induced by CsA, causing both hypertension and, via renal vasoconstriction, reduced glomerular filtration.

V(1) receptors in luminal action of vasopressin on distal K(+) secretion

Luminal perfusion with collected proximal fluid increases distal K(+) secretion compared with artificial solutions. Arginine vasopressin (AVP), present in luminal fluid, might be responsible for this observation. K(+) secretion rate (J(K)) was measured by K(+)-sensitive microelectrodes during paired luminal stationary microperfusion with control and AVP-containing 0.5 mM K(+) solutions. J(K) was 1.34 +/- 0.35 (n = 24 tubules) nmol x cm(-2) x s(-1) during perfusion with 10(-9) M AVP, against 0.90+/-0.12 nmol x cm(-2) x s(-1) (n = 21) in control (P<0.02). With 10(-9) M AVP+10(-6) M beta-mercapto-beta-beta-cyclopenta-methylenepropionyl(1), O-Me-Tyr(2)-Arg(8) vasopressin (MCMV), a specific peptide V(1)-receptor antagonist, J(K) was 0.36+/-0.067 against 0.77+/-0.10 (control; n = 9) nmol x cm(-2) x s(-1) (P<0.01). With 10(-6) M MCMV alone, J(K) was 0.37+/-0.04 against a control of 0.62+/-0.06 (n = 19) nmol. cm(-2). s(-1) (P<0.01). A peptide V(2) antagonist had no such effect. In Brattleboro rats, which do not produce endogenous AVP, MCMV had no effect when given alone, although AVP still stimulated J(K). In conclusion, luminal AVP stimulates distal J(K) significantly. The V(1) antagonist MCMV inhibits the effect of AVP but also reduces J(K) when given alone. This suggests that AVP acts luminally via V(1) receptors but also that there appears to be a background effect of endogenous AVP blocked by the antagonist.

Synthesis and structure-activity investigation of novel vasopressin hypotensive peptide agonists

We report the solid phase synthesis and vasodepressor potencies of the novel hypotensive peptide [1(-beta-mercapto-beta,beta-pentamethylene propionic acid)-2-O-ethyl-D-tyrosine, 3-arginine, 4-valine] arginine vasopressin, d(CH2)5[D-Tyr(Et)2, Arg3, Val4]AVP (A), its related Lys3 (B), Tyr-NH(9)2 (C), [Lys3, Tyr-NH(9)2 (D) analogs and in a preliminary structure-activity study of positions 2-4 and 7-9, 24 analogs (1-24) of A-C. Peptides 1-6, 9-14 have the following single substituents at positions 2, 3, 4, 8 and 9 in (A): 1, D-Tyr(Me)2; 2, L-Tyr(Et)2; 3, Orn3; 4, N-Me-Arg3; 5, Glu3; 6, Arg4; 9, D-Arg8; 10, Eda9; 11, Arg-NH(9)2; 12, Ala-NH(9)2; 13, desGly9; 14, desGly-NH(9)2. Peptides 15 and 16 are analogs of B which possess the following single modifications: 15, Arg-NH(9)2; 16, desGly9. Peptides 7 and 8 are analogs of (C) with the following single modification: 7, Gln4; 8, Lys8. Peptides 17-24 are analogs of A possessing the following multiple modifications: 17, [Sar7, Eda9]; 18, [Arg7, Eda9]; 19, [Arg7, Eda9<--Tyr10]; 20, [Arg4, Arg-NH(9)2]; 21, [Ile4, desGly9]; 22, [Arg4, desGly9]l; 23, [Arg7, desGly9]; 24, [Arg7, Lys8, desGly9]. All 24 new peptides were evaluated for agonistic and antagonistic activities in in vivo antidiuretic (V2-receptor), vasopressor (V1a-receptor) and in in vitro (no Mg2+) oxytocic (OT-receptor) assays and like the parent peptides (A-D) (Chan et al. Br. J. Pharmacol. 1998; 125: 803-811) were found to exhibit no or negligible activities in these assays. Vasodepressor potencies were determined in anesthetized male rats with baseline mean arterial blood pressure maintained at 110-120 mmHg. The effective dose (ED), in microg 100 g(-1) i.v., required to produce a vasodepressor response of 5 cm2, area under the vasodepressor response curve (AUC) during the 5-min period following the injection of the test peptide, was determined. Therefore, the EDs measure the relative vasodepressor potencies of the hypotensive peptides. The following ED values were obtained for A-D and for peptides 1-24: A, 4.66; B, 5.75; C, 10.56; D, 11.60; 1, approximately 20; 2, approximately 30; 3, 6.78; 4, non-detectable (ND); 5, ND; 6, approximately 32; 7, ND; 8, 8.67; 9, ND; 10, 2.43; 11, 3.54; 12, 10.57; 13, 4.81; 14, ND; 15, 4.47; 16, 9.78; 17, 5.72; 18, 1.10; 19, 1.05; 20, 10.41; 21, 9.13; 22, approximately 33; 23, 3.01; 24, 1.71. A is clearly the most potent of the four original hypotensive peptides A-D. These data provide insights to which modification of A enhance, retain or abolish hypotensive potencies. Six of the new hypotensive peptides are significantly more potent than A. These are peptides 10, 11, 18, 19, 23 and 24. Peptide 19, a radioiodinatable ligand, is ten times more potent than C or D. The Gln4 modification of C and the N-Me-Arg3, Glu3, D-Arg8 and desGly-NH(9)2 modifications of A abolished hypotensive potency. By contrast, the Eda9, Arg-NH(9)2, [Sar7, Eda9], [Arg7, Eda9<- -Tyr10], [Arg7, desGly9], [Arg7, Lys8, desGly9] modifications of A all led to enhancements of hypotensive potency. This initial structure-activity exploration provides useful clues to the design of (a) more potent vasodepressor peptides and (b) high affinity radioiodinatable ligands for the putative AVP vasodilating receptor. Some of the peptides here may be of value as pharmacological tools for studies on the complex cardiovascular actions of AVP and may lead to the development of a new class of anti-hypertensive agents.

Role of central AT1 and V1 receptors in cardiovascular adaptation to hemorrhage in SD and renin TGR rats

In acute experiments, intracranially applied angiotensin II and vasopressin elicit significant cardiovascular effects. The purpose of the present study was to find out whether chronic intrabrain elevation of these peptides, occurring in the renin transgenic TGR(mRen2)27 (TGR) rats, results in an alteration of the cardiovascular control. Mean arterial blood pressure (MAP) and heart rate responses to hypovolemia were examined in hypertensive TGR and normotensive Sprague-Dawley (SD) rats under control conditions and during blockade of central AT1 or V1 receptors. Both groups received cerebroventricular infusions of either 1) cerebrospinal fluid (series 1), 2) AT1 receptors antagonist (AT1ANT, series 2), or 3) V1 receptors antagonist (V1ANT, series 3). Blockade of AT1 and V1 receptors decreased MAP in TGR but not in SD rats. In SD rats, bleeding elicited a similar decrease of MAP in each series and a transient increase of heart rate in series 3. In TGR, hemorrhage caused bradycardia and decrease of MAP, which was greater than in SD rats. Hemorrhagic hypotension in TGR was abolished by V1ANT and bradycardia by V1ANT or AT1ANT. The results demonstrate remarkable differences in cardiovascular adjustment to hemorrhage in SD and TGR rats and provide evidence for enhanced involvement of central V1 and AT1 receptors in the regulation of blood pressure during hypovolemia in TGR. Central V1 vasopressin receptors play a crucial role in eliciting posthemorrhagic hypotension and bradycardia in this strain.

Increased pressor function of central vasopressinergic system in hypertensive renin transgenic rats

OBJECTIVE

Renin transgenic hypertensive rats [TGR(mRen2)27] have increased contents of angiotensin II and arginine vasopressin (AVP) in the cardiovascular brain regions. The aim of the present study was to evaluate the effects of centrally released AVP on the regulation of baseline blood pressure in TGR(mRen2)27 rats and to determine the interaction between AVP and angiotensin II in the central control of blood pressure in this model of hypertension.

DESIGN

Three basic series of experiments were performed on 20 TGR(mRen2)27 and 20 Hannover Sprague-Dawley conscious rats, chronically instrumented with lateral cerebral ventricle (LCV) cannulae and femoral artery catheters. In series 1, blood pressure and heart rate were recorded during an LCV infusion of artificial cerebrospinal fluid before and after LCV administration of angiotensin II. In series 2, the effects of an LCV administration of angiotensin 11 (100 ng) on mean arterial pressure and the heart rate were determined during LCV infusion of a selective AVP receptor (V1) antagonist [1-(1-mercapto-4-methylcyclohexaneacetic acid)-8-arginine vasopressin (MeCAAVP) and d(CH2)5[Tyr(Me)2,Ala-NH2(9)]AVP] or a selective angiotensin II type 1 (AT1) receptor antagonist (losartan) or both. In series 3, mean arterial pressure and the heart rate were determined after an LCV injection of either AVP (10 ng) or AVP together with angiotensin II.

RESULTS

The LCV infusions of antagonists to V1 and AT1 receptors caused significant comparable decreases in baseline MAP in TGR(mRen2)27 but not in Sprague-Dawley rats. Angiotensin II elicited significant pressor responses, both in TGR(mRen2)27 and in Sprague-Dawley rats. Blockade of V1 receptors significantly reduced the duration and the maximum amplitude of the central pressor response to angiotensin II in TGR(mRen2)27 rats, whereas in Sprague-Dawley rats the maximum pressor effect was not significantly altered. In both strains, the pressor response to angiotensin II was abolished by blockade of AT1 receptors.

CONCLUSIONS

The results indicate that the elevated blood pressure in TGR(mRen2)27 rats is partly caused by increased function of the brain angiotensinergic AT1 and vasopressinergic V1 systems. Centrally released AVP is involved in mediation of the pressor effect exerted by centrally applied angiotensin II in TGR(mRen2)27 rats.

Characterization of oxytocin actions in guinea-pig isolated uterine artery: the effect of pregnancy

While the contractile effect of oxytocin on uterine artery has been reported, little is known about whether pregnancy affects the responsiveness of this artery to oxytocin. If it does, is it a consequence of changed endothelial function, as has been proposed for some other vasoconstrictors. Furthermore, the receptor subtypes involved in oxytocin action on uterine artery has not been yet determined. Therefore the purposes of this study were to (1) determine the receptor subtypes involved in oxytocin action in non-pregnant and pregnant guinea-pig uterine artery and to (2) determine whether possible changes in uterine artery sensitivity to oxytocin during pregnancy are due to altered endothelial function. Therefore, the effect of oxytocin on non-pregnant and pregnant guinea-pig uterine arterial rings with and without endothelium was investigated. In non-pregnant guinea-pig uterine artery oxytocin induced contraction (pEC50 = 7.63) with greater potency than in pregnant guinea-pig uterine artery (pEC50 = 7.17). Removal of the endothelium did not affect oxytocin-induced contractions, regardless of the pregnancy status. The uterine arteries did not respond to [Thr4, Gly7]oxytocin. In the preparations studied, [d(CH2)5Tyr(Me)2]vasopressin and [d(CH2)5, D-Ile2, Ile4]vasopressin antagonized oxytocin action with the following pKB values ([d(CH2)5Tyr(Me)2]vasopressin versus [d(CH2)5, D-Ile2, Ile4]vasopressin): 8.24 versus 7.29 and 8.11 versus 7.17 for non-pregnant guinea-pig uterine artery with and without endothelium, respectively; 8.39 versus 7.25 and 8.35 versus 7.25 for pregnant guinea-pig uterine artery with and without endothelium, respectively. We suggest that, in uterine arteries, oxytocin induces contraction by activation of vasopressin V1A receptors. The potency of oxytocin in uterine artery is decreased during pregnancy and this is not associated with altered endothelial function.

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).

Conformationally readdressed CCK-B/delta-opioid peptide ligands

The sequence of a cholecystokinin (CCK) related peptide was modified to obtain analogues, which interact selectively either with CCK-B, or with delta-opioid receptors. Two kinds of peptides were designed, namely, the cyclic peptides of the H-Tyr-cyclo (D-Pen-Gly-Trp-L/D-3-transmercaptoproline)-Asp-Phe-NH2 sequence (compounds 1a and 1b, respectively), and the linear peptides of the H-Tyr-D-Val-Gly-Trp-L/D-3-trans-methylmercaptoproline-Asp-Phe- NH2 sequence (compounds 2a and 2b, respectively). The only difference between the chemical structures of the linear analogues compared to the cyclic ones is that one covalent bond has been eliminated and a sulfur atom is replaced by a methyl group. Molecular modeling showed that, among low-energy conformers of cyclic compounds 1, there are three-dimensional structures compatible to the model for delta-receptor-bound conformer, suggested earlier [G. V. Nikiforovich, V.J. Hruby, O. Prakash, and C.A. Gehrig (1991) Biopolymers, vol. 31, pp. 941-955]. Results of binding assays fully supported the rationale for the design of compounds 1 and 2. The cyclic analogue 1a has Ki values of 4.5 and > 5000 nM at delta- and mu-opioid receptors, respectively; and IC50 values of 1.6 and > 10,000 nM for CCK-A and CCK-B receptors, respectively. The results of this study demonstrate a possibility to redirect a peptide sequence that interacts with one type of receptors (CCK-B receptors) toward interaction with another type (delta-opioid receptors) belonging to a different physiological system. This redirection could be performed by changing the conformational properties of the peptide with very minimal changes in its chemical structure.

Characterization of arginine vasopressin actions in human uterine artery: lack of role of the vascular endothelium

1. The effect of arginine vasopressin (AVP) on human uterine artery rings, both intact and denuded of endothelium, was investigated. 2. Initially, AVP (63 pM-32 nM) induced concentration-dependent contraction of human uterine artery (pD2 = 8.92 +/- 0.01). Removal of the endothelium did not affect the concentration-response curve for AVP (pD2 = 8.83 +/- 0.03). 3. In contrast, human uterine arteries, both intact and denuded of endothelium, did not respond to the addition of 1-desamino-8-D-arginine vasopressin (dDAVP, 1 nM-1 microM). 4. In both types of preparations, [d(CH2)5Tyr(Me)AVP (1-10 nM) and [d(CH2)5,D-Ile2,Ile4]AVP (300 nM-3 microM) produced parallel rightward shifts of the curves for AVP. The Schild plots constrained to a slope of unity gave the following -log KB values: [d(CH2)5Tyr(Me)]AVP vs. [d(CH2)5,D-Ile2,Ile4]AVP 9.66 vs. 6.69 and 9.61 vs. 6.80 for human uterine artery, intact and denuded of endothelium, respectively. 5. The pKA values for AVP itself also did not differ between preparations: 6.56 and 6.43 for human uterine artery with and without endothelium, respectively. In both types of preparations, the receptor reserve (KA/EC50) was considerably greater than unity (intact vs. denuded: 228 vs. 244). 6. It is concluded that, in human uterine artery, AVP induces contractions that are not modulated by the endothelium. It is likely that AVP acts as a full agonist on human uterine artery, regardless of the endothelial condition. On the basis of differential antagonists affinity and affinity of AVP itself, it is probable that vasopressin receptors involved in AVP-induced contraction in human uterine arteries belong to the V1a or V1a-like subtype.

Effect of pregnancy on vasopressin-mediated responses in guinea-pig uterine arteries with intact and denuded endothelium

The effect of pregnancy on vasopressin-induced contraction of guinea-pig uterine arterial rings was investigated. Initially, vasopressin induced contraction (pD2 = 9.14) in pregnant guinea-pig uterine artery with greater potency than in non-pregnant guinea-pig uterine artery (pD2 = 8.77). Removal of the endothelium did not affect vasopressin-induced contractions, regardless of pregnancy status. In all types of preparations, [d(CH2)5Tyr(Me)2]vasopressin (10-100 nM) and [d(CH2)5,D-Ile2,Ile4]vasopressin (300 nM-3 microM) produced parallel rightward shifts of the curves for vasopressin. The Schild plots constrained to a slope of unity gave the following -log KB values: [d(CH2)5Tyr(Me)2]vasopressin vs. [d(CH2)5,D-Ile2,Ile4]vasopressin 8.74 vs. 6.82 and 8.50 vs. 6.72 for non-pregnant guinea-pig uterine artery with intact and denuded endothelium, respectively; 8.38 vs. 6.49 and 8.36 vs. 6.75 for pregnant guinea-pig uterine artery with intact and denuded endothelium, respectively. The pKA values for vasopressin itself also did not differs between preparations: 6.49 and 6.55 for non-pregnant guinea-pig uterine artery with intact and denuded endothelium, respectively; 6.48 and 6.52 for pregnant guinea-pig uterine artery with intact and denuded endothelium, respectively. The receptor reserve (KA/EC50) was significantly greater in preparations taken from pregnant than from non-pregnant animals. It is concluded that vasopressin-induced contractions of guinea-pig uterine artery are not modulated by the endothelium, regardless of pregnancy status. The receptor reserve for vasopressin in guinea-pig uterine artery is increased during pregnancy, that is not related to the changes of vasopressin receptor affinity for vasopressin. It is probable that vasopressin receptors involved in vasopressin-induced contraction of all types of vessels studied belong to the V1A-like subtype.

Vasopressin stimulation of Ca2+ mobilization, two bivalent cation entry pathways and Ca2+ efflux in A7r5 rat smooth muscle cells

1. Arg8-vasopressin (AVP)-regulated Ca2+ transport were investigated in fura-2-loaded A7r5 cells using both single cell and population measurements. 2. AVP evokes an initial concentration-dependent rise in cytosolic free Ca2+ concentration ([Ca2+ ]i) to a peak which is independent of extracellular Ca2+, and a sustained Ca2+ signal that results from a balance between stimulation of Ca2+ entry and efflux. 3. Depletion of intracellular Ca2+ stores with thapsigargin, ionomycin, or prior treatment with AVP in Ca2(+)-free medium activates 'capacitative' entry of Ca2+, Ba2+ or Mn2+. Capacitative Mn2+ entry is inhibited by refilling stores with Ca2+; neither Sr2+ nor Ba2+ substitute for Ca2+ to give this effect. 4. In cells with empty stores, AVP stimulates further bivalent cation entry, and the effect persists when extracellular Na+ is replaced by N-methyl-D-glucamine or under depolarizing condition (extracellular KCl concentration ([KCl]o), 135 mM). This effect of AVP is not therefore merely a consequence of AVP causing membrane hyperpolarization or stimulation of Na(+)-Ca2+ exchange, but results from opening of a bivalent cation influx pathway. 5. Several lines of evidence indicate that AVP-stimulated bivalent cation entry is not a consequence of more complete emptying of the intracellular stores and consequent further activation of the capacitative pathway. AVP stimulates Ba2+ entry when the intracellular Ca2+ stores have been both emptied by ionomycin and prevented from refilling by thapsigargin. Mn2+ permeates the capacitative pathway, but AVP does not further increase Mn2+ entry, confirming that AVP does not further activate the capacitative pathway and that the two pathways differ in their permeability to Mn2+. When the extracellular [Sr2+] is low, empty stores do not stimulate detectable Sr2+ entry, but addition of AVP causes substantial Sr2+ entry. 6. A decrease in [Ca2+]i occurs when 50 nM AVP is added during a sustained elevation of [Ca2+]i evoked by thapsigargin. Since AVP does not inhibit the capacitative pathway, this result suggests that AVP stimulates Ca2+ extrusion. 7. We conclude that stimulation of Ca2+ mobilization, two modes of bivalent cation entry, and Ca2+ efflux all contribute to the complex concentration-dependent effects of AVP in A7r5 smooth muscle cells.

An exploration of the effects of L- and D-tetrahydroisoquinoline-3-carboxylic acid substitutions at positions 2, 3 and 7 in cyclic and linear antagonists of vasopressin and oxytocin and at position 3 in arginine vasopressin

We have investigated the effects of mono-substitutions with the conformationally restricted amino acid, 1,2,3,4 tetrahydroisoquinoline-3-carboxylic acid (Tic) at position 3 in arginine vasopressin (AVP), at positions 2, 3 and 7 in potent non-selective cyclic AVP V2/V1a antagonists, in potent and selective cyclic and linear AVP V1a antagonists, in a potent and selective oxytocin antagonist and in a new potent linear oxytocin antagonist Phaa-D-Tyr(Me)-Ile-Val-Asn-Orn-Pro-Orn-NH2 (10). We report here the solid-phase synthesis of peptide 10 together with the following Tic-substituted peptides: 1. [Tic3]AVP: 2. dICH2)5[D-TIc2]VAVP: 3, d(CH2)5[D-Tyr(Et)2Tic3]VAVP: 4, d(CH2)5[Tic2Ala-NH2(9)]AVP: 5. d(CH2)5[Tyr]Me)2.Tic3,Ala-NH2(9)]AVP: 6. d(CH2)5 [Tyr(Me)2,Tic7]AVP: 7, Phaa-D-Tyr(Me)-Phe-Gln-Asn-Lys-Tic-Arg-NH2: 8, desGly-NH2,d[CH2]5[Tic2,Thr4]OVT: 9. desGly-NH2d(CH2)5[Tyr(Me)2Thr4, Tic7[OVT; 11, Phaa-D-Tic-Ile-Val-Asn-Orn-Pro-Orn-NH2, using previously described methods. The protected precursors were synthesized by the solid-phase method, cleaved, purified and deblocked with sodium in liquid ammonia to give the free peptides 1-11 which were purified by methods previously described. Peptides 1-11 were examined for agonistic and antagonistic potency in oxytocic (in vitro, without Mg2+) and AVP antidiuretic (V2-receptor) and vasopressor (V1a-receptor) assays. Tic3 substitution in AVP led to drastic losses of V2, V1a and oxytocic agonistic activities in peptide 1, L- and D-Tic2 substitutions led to drastic losses of anti-V2/anti-V1a and anti-oxytocic potencies in peptides 2, 4, 8 and 11 (peptide 2 retained substantial anti-oxytocic potency; pA2 = 7.25 +/- 0.025). Whereas Tic3 substitution in the selective V1a antagonist d(CH2)5[Tyr(Me)2,Ala-NH2(9)]AVP(C) led to a drastic reduction in anti-V1a potency (from anti-V1a pA2 8.75 to 6.37 for peptide 5, remarkably, Tic3 substitution in the V2/V1a antagonist d(CH2)5(D-Tyr(Et)2]VAVP(B) led to full retention of anti-V2 potency and a 95% reduction in anti-V1a potency. With an anti-V2 pA2 = 7.69 +/- 0.05 and anti-V1a pA2 = 6.95 +/- 0.03. d(CH2)5[D-Tyr(Et)2, Tic3]VAVP exhibits a 13-fold gain in anti-V2/anti-V1a selectivity compared to (B). Tic7 substitutions are very well tolerated in peptides 6, 7 and 9 with excellent retention of the characteristic potencies of the parent peptides. The findings on the effects of Tic3 substitutions reported here may provide promising leads to the design of more selective and possibly orally active V2 antagonists for use as pharmacological tools and as therapeutic clinical agents for the treatment of the syndrome of the inappropriate secretion of antidiuretic hormone (SIADH).

Design, synthesis and some uses of receptor-specific agonists and antagonists of vasopressin and oxytocin

Selective agonists and antagonists are powerful tools for studies on AVP and OT receptors and on the physiological and pathophysiological roles of AVP and OT. Here we show how some of these peptides and their radiolabelled derivatives were designed. We also present examples of the currently available cyclic and linear OT and AVP agonists and antagonists from our laboratories.