Differential effects of V2 vasopressin agonist and antagonists on blood pressure regulation in normotensive (WKY) and spontaneously hypertensive (SHR) rats

Body temperature and cardiac changes induced by peripherally administered oxytocin, vasopressin and the non-peptide oxytocin receptor agonist WAY 267,464: a biotelemetry study in rats

BACKGROUND AND PURPOSE

There is current interest in oxytocin (OT) as a possible therapeutic in psychiatric disorders. However, the usefulness of OT may be constrained by peripheral autonomic effects, which may involve an action at both OT and vasopressin V1A receptors. Here, we characterized the cardiovascular and thermoregulatory effects of OT, vasopressin (AVP) and the non-peptide OT receptor agonist WAY 267,464 in rats, and assessed the relative involvement of the OT and V1A receptors in these effects.

EXPERIMENTAL APPROACH

Biotelemetry in freely moving male Wistar rats was used to examine body temperature and heart rate after OT (0.01 - 1 mg kg(-1); i.p.), AVP (0.001 - 0.1 mg kg(-1); i.p.) or WAY 267,464 (10 and 100 mg kg(-1); i.p.). The actions of the OT receptor antagonist Compound 25 (C25, 5 and 10 mg kg(-1)) and V1A receptor antagonist SR49059 (1 and 10 mg kg(-1)) were studied, as well as possible V1A receptor antagonist effects of WAY 267,464.

KEY RESULTS

OT and AVP dose-dependently reduced body temperature and heart rate. WAY 267,464 had similar, but more modest, effects. SR49059, but not C25, prevented the hypothermia and bradycardia induced by OT and AVP. WAY 267,464 (100 mg·kg(-1)) prevented the effects of OT, and to some extent AVP.

CONCLUSIONS AND IMPLICATIONS

Peripherally administered OT and AVP have profound cardiovascular and thermoregulatory effects that appear to principally involve the V1A receptor rather than the OT receptor. Additionally, WAY 267,464 is not a simple OT receptor agonist, as it has functionally relevant V1A antagonist actions.

Vasopressin and angiotensin receptors of the medial septal area in the control of mean arterial pressure induced by vasopressin

Introduction. Brain arginine 8-vasopressin (AVP), through the V1a- and V2-receptors, is essential for the maintenance of mean arterial pressure (MAP). Central AVP interacts with the components of the renin-angiotensin system, which participate in MAP regulation. This study aimed to determine the effects of V1a-, V2- and V1a/V2-AVP selective antagonists and AT1- and AT2-angiotensin II (Ang II) selective antagonists on the MAP induced by AVP injected into the medial septal area (MSA) of the brain.

Materials and methods. Male Holtzman rats with stainless steel cannulae implanted into the MSA were used in experiments. Direct MAP was recorded in conscious rats.

Results. AVP administration into the MSA caused a prompt and potent pressor response in a dose-dependent fashion. Pretreatment with the V1a- and V2-antagonists reduced, whereas prior injection of the V1a/V2-antagonist induced a decrease in the MAP that remained below the baseline. Both AT 1- and AT2-antagonists elicited a decrease, while simultaneous injections of two antagonists were more effective in decreasing the MAP induced by AVP.

Conclusion. These results indicate there is a synergism between the V1a- and V2-AVP and AT1and AT2-Ang II receptors in the MSA in the regulation of MAP.

Differential glomerular response to continuous infusion of vasopressin in spontaneously hypertensive rats and Wistar-Kyoto rats

To determine the difference of glomerular response to exogenous vasopressin (VP) in vivo between normotensive and hypertensive rats, we examined the effects of 14-day continuous infusion of VP (1.0 ng/kg/min) on the physiological and histological aspects in 7-week-old spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) rats. VP infusion did not result in significant changes in systolic blood pressure, heart rate, serum electrolytes, serum creatinine, urinary protein and N-acetyl-beta-glucosaminidase levels in both strains of rats. VP infusion significantly reduced daily urine volume associated with significant concentration of the urine in WKY rats but not SHR. Kidney and heart weights did not differ significantly after VP infusion between both strains. Glomerular mesangial expansion was significantly enhanced in VP infused SHR, but glomerular cellularity was not different between both strains following treatment. Competitive reverse transcription-polymerase chain reaction revealed that the level of glomerular transforming growth factor (TGF)-beta1 mRNA was significantly higher in SHR than WKY rats, and that this difference was significantly augmented after VP infusion in SHR. VP infusion, however, did not change the level of glomerular mRNAs of platelet-derived growth factor (PDGF) B-chain in both strains. Then, exogenous VP infusion contributes to the glomerular mesangial expansion in SHR, which involved overexpression of glomerular TGF-beta1 without any pressor effect. In contrast, the significant changes of glomerular expansion and TGF-beta1 level were not shown in WKY rats. These findings suggest that the glomerular response to the exogenous VP is preferentially enhanced in SHR.

Enhanced renal vasoconstrictor responsiveness to vasopressin after renal denervation

Previous studies from our laboratory and others demonstrated a relative insensitivity of the renal vasculature to the vasoconstrictor influences of arginine vasopressin (AVP). Experiments were therefore conducted to identify the possible roles of renal sympathetic withdrawal and V2-vasopressinergic receptor-mediated vasodilation in this response. Renal hemodynamic responses to AVP or saline vehicle were examined in the presence or absence of either selective V1-vasopressinergic-receptor blockade or nonselective vasopressinergic-receptor antagonism in conscious intact and renal-denervated rats. Our results indicate that renal denervation profoundly augmented AVP-induced renal vasoconstriction, whereas no differences were observed in the responses to V1-receptor blockade versus combined V1, V2-receptor antagonism. These findings are consistent with a role for renal sympathoinhibition in mediating the relative insensitivity of the renal bed to the vasoconstrictor effects of AVP in conscious rats but do not support a modulatory influence of V2-receptor activity in this response.

Desmopressin (DDAVP) acts on platelets to generate platelet microparticles and enhanced procoagulant activity

Desmopressin (DDAVP), an analog of vasopressin (AVP), has wide clinical application as an anti-hemorrhagic (AH) agent. DDAVP in vivo releases vWF from endothelial cells but is reported to have little action on platelets. However, DDAVP is often used to improve hemostasis in platelet dysfunctions. We examined the effect of DDAVP on platelet microparticle (PMP) formation and procoagulant activity in vitro using platelets from normal volunteers and in vivo in six patients receiving DDAVP therapy. In the former, platelets were incubated with DDAVP (0.5 to 25 nM) and PMP released were stained with FITC-labeled MAb alpha-GP IIb/IIIa for flow cytometry. Procoagulant activity was measured in a clot-based assay using Russel's viper venom (RVV) calibrated with cephalin. A mean increase of 2-3 fold was observed in both PMP and procoagulant activity. Parallel to these observations was a dose-dependent rise in organelle-associated Ca2+. The assays were also performed on six patients prior to and at one hour after infusion of DDAVP, and similar but lesser effects were observed. We conclude that DDAVP acts on platelets in vitro, and that these effects may contribute to the hemostatic action of DDAVP in platelet dysfunctions in vivo.

Central cardiovascular effects of AVP and ANP in normotensive and spontaneously hypertensive rats

The purpose of the present study was to compare influence of central arginine vasopressin (AVP) and of atrial natriuretic peptide (ANP) on control of arterial blood pressure (MAP) and heart rate (HR) in normotensive (WKY) and spontaneously hypertensive (SHR) rats. Three series of experiments were performed on 30 WKY and 30 SHR, chronically instrumented with guide tubes in the lateral ventricle (LV) and arterial and venous catheters. MAP and HR were monitored before and after i.v. injections of either vehicle or 1, 10 and 50 ng of AVP and 25, 125 and 500 ng of ANP. Sensitivity of cardiac component of baroreflex (CCB), expressed as a slope of the regression line was determined from relationships between systolic arterial pressure (SAP) and HR period (HRp) during phenylephrine (Phe)-induced hypertension and sodium nitroprusside (SN)-induced hypotension. CCB was measured before and after administration of either vehicle, AVP, ANP, or both peptides together. Increases of MAP occurred after LV administration of 1, 10 and 50 ng of AVP in WKY and of 10 and 50 ng in SHR. ANP did not cause significant changes in MAP in both strains as compared to vehicle, but it abolished AVP-induced MAP increase in WKY and SHR. CCB was reduced in WKY and SHR after LV administration of AVP during SN-induced hypotension. In SHR but not in WKY administration of ANP, AVP and ANP + AVP decreased CCB during Phe-induced MAP elevation. The results indicate that centrally applied AVP and ANP exert differential effects on blood pressure and baroreflex control of heart rate in WKY and SHR and suggest interaction of these two peptides in blood pressure regulation at the level of central nervous system.

Central cardiovascular effects of AVP and AVP analogs with V1, V2 and 'V3' agonistic or antagonistic properties in conscious dog

To determine central cardiovascular effects of arginine vasopressin (AVP) in the dog, and the nature of the receptors involved, blood pressure (BP) and heart rate (HR) responses were monitored in 18 conscious dogs subjected to third ventricle (i.c.v.) infusions of either AVP or AVP analogs with agonistic or antagonistic properties towards V1, V2 and putative 'V3' receptors. Significant blood pressure (BP) increases were elicited by i.c.v. infusions of: (i) AVP at a rate of 0.01, 1 and 100 ng/min; (ii) the selective V1 agonist (F180, Ferring) at a rate of 1 and 100 ng/min, and (iii) the selective V1 antagonist, dEt2 Tyr(Me)DAVP at a rate of 100 ng and 400 ng/min. Pretreatment with another selective V1 antagonist (MeCAAVP) increased baseline BP and prevented AVP induced BP increase. Blood pressure was not altered by i.c.v. infusions of the selective V2 agonist dVDAVP, the selective V2 antagonist d(CH2)5[D-Ile2,Abu4]AVP, the putative 'V3' agonist vasopressin-(4-8) (Akzo Organon), mediating behavioral actions of AVP and by artificial CSF. Heart rate was significantly accelerated by AVP infused at a rate of 100 ng/min. The results reveal high sensitivity of the conscious dog to central pressor action of AVP and indicate that this effect is mediated by V1-like receptors. It is suggested that the pressor effect of the V1 antagonist may result from its partial agonistic properties towards central V1 receptors, however, it can not be excluded that endogenous AVP released under baseline conditions may exert tonic hypotensive effect mediated by a different population of V1 receptors, this effect being abolished by V1 antagonist. The results do not support involvement of V2 or 'V3' receptors in central cardiovascular actions of AVP.