Inhibition of vasopressin action in vascular smooth muscle by the V1 antagonist OPC-21268

Combined Treatment with KV Channel Inhibitor 4-Aminopyridine and either γ-Cystathionine Lyase Inhibitor β-Cyanoalanine or Epinephrine Restores Blood Pressure, and Improves Survival in the Wistar Rat Model of Anaphylactic Shock

Simple Summary

Allergic diseases are presenting a constant increase all over the world and caused by such different substances as food, drugs, and pollens. Anaphylactic shock is the more severe complication of allergy which can induce death if the treatment is not administered immediately. Some patients do not respond to the recommended treatment, intra venous or intramuscular epinephrine. The pathophysiology of anaphylactic shock is still under investigation. The mediators released after the activation of mast cells and basophiles act on endothelial cells and smooth muscle cells, inducing the vasodilation responsible for hypotension and shock. Nitric oxide and hydrogen sulphide are both intracellular mediators that induce vasodilation. The role of potassium voltage dependent channels is suspected. We aimed to demonstrate the ability of a blocker of potassium voltage dependent channels, 4-aminopyridine, alone or in combination with inhibitors of cystathionine γ-lyase to restore blood pressure and improve survival in an ovalbumin rat anaphylactic shock model. The blockade of potassium voltage dependent channels alone or combined with inhibitors of cystathionine γ-lyase, dl-propargylglycine, or β-cyanoalanine restored blood pressure and improved survival. These findings suggest possible investigative treatment pathways for research concerning epinephrine-refractory anaphylactic shock in patients.

Abstract

The mechanism of anaphylactic shock (AS) remains incompletely understood. The potassium channel blocker 4-aminopyridine (4-AP), the inhibitors of cystathionine γ-lyase (ICSE), dl-propargylglycine (DPG) or β-cyanoalanine (BCA), and the nitric oxide (NO) synthase produce vasoconstriction and could be an alternative for the treatment of AS. The aim of this study was to demonstrate the ability of L-NAME, ICSE alone or in combination with 4-AP to restore blood pressure (BP) and improve survival in ovalbumin (OVA) rats AS. Experimental groups included non-sensitized Wistar rats (n = 6); AS (n = 6); AS (n = 10 per group) treated i.v. with 4-AP (AS+4-AP), epinephrine (AS+EPI), AS+DPG, AS+BCA, or with L-NAME (AS+L-NAME); or AS treated with drug combinations 4-AP+DPG, 4-AP+BCA, 4-AP+L-NAME, or 4-AP+EPI. AS was induced by i.v. OVA (1 mg). Treatments were administered i.v. one minute after AS induction. Mean arterial BP (MAP), heart rate (HR), and survival were monitored for 60 min. Plasma levels of histamine, prostaglandin E2 (PGE2) and F2 (PGF2α), leukotriene B4 and C4, angiotensin II, vasopressin, oxidative stress markers, pH, HCO3, PaO2, PaCO2, and K+ were measured. OVA induced severe hypotension and all AS rats died. Moreover, 4-AP, 4-AP+EPI, or 4-AP+BCA normalized both MAP and HR and increased survival. All sensitized rats treated with 4-AP alone or with 4-AP+BCA survived. The time-integrated MAP “area under the curve” was significantly higher after combined 4-AP treatment with ICSE. Metabolic acidosis was not rescued and NO, ICSE, and Kv inhibitors differentially alter oxidative stress and plasma levels of anaphylactic mediators. The AS-induced reduction of serum angiotensin II levels was prevented by 4-AP treatment alone or in combination with other drugs. Further, 4-AP treatment combined with EPI or with BCA also increased serum PGF2α, whereas only the 4-AP+EPI combination increased serum LTB4. Serum vasopressin and angiotensin II levels were increased by 4-AP treatment alone or in combination with other drugs. Moreover, 4-AP alone and in combination with inhibition of cystathionine γ-lyase or EPI normalizes BP, increases serum vasoconstrictor levels, and improves survival in the Wistar rat model of AS. These findings suggest possible investigative treatment pathways for research into epinephrine-refractory anaphylactic shock in patients.

The cloned avian neurohypophysial hormone receptors

Arginine vasotocin (AVT), a neurohypophysial hormone, has many essential functions in birds including the regulation of salt and fluid balance, blood pressure, the stress response and a variety of behaviors. In addition, AVT controls reproductive functions in birds that are served by oxytocin in mammals. In the following review, we examine the functions of AVT in birds with an emphasis on the present state of knowledge concerning the cloned receptors for this important hormone. Receptor and gene structure, signal transduction mechanisms and expression pattern are all discussed. Finally, we explore the phylogenetic relationships between the cloned avian receptors and other vertebrate and invertebrate neurohypophysial hormone receptors.

Vasopressin-induced vasoconstriction is dependent on MAPKerk1/2 phosphorylation

To investigate the involvement of the mitogen-activated protein kinase (MAPK) family of extracellular signal-regulated kinase (ERK) 1 and 2 (MAPKerk1/2) in the vasopressin-mediated vasoconstriction in the rat aorta. Vasopressin-induced vasoconstriction was measured in isolated rat thoracic aortae in the presence or absence of MAPKerk1/2 kinase (MKKmek1/2) inhibitors. Thereafter the MAPKerk1/2 phosphorylation in the rat aorta was quantified using Western blot analysis. Vasopressin (1-300 nm) induced a concentration-dependent vasoconstriction, which could be inhibited concentration dependently by the selective MKKmek1/2 inhibitors, PD 98059 (10 and 100 microm) and U 0126 (10 and 100 microm). Western blot analysis revealed a 2.7 +/- 0.6-fold increase in the MAPKerk1/2 phosphorylation induced by vasopressin (300 nm). This phosphorylation could be dose dependently prevented by both PD 98059 (100 microm) and U 0126 (10 and 100 microm). These results indicate that vasoconstriction induced by vasopressin is partly regulated by the MAPKerk1/2 pathway.

Treatment of brain edema with a nonpeptide arginine vasopressin V1 receptor antagonist OPC-21268 in rats

OBJECTIVE

Recent experimental evidence suggests that centrally released arginine vasopressin plays a significant role in brain capillary water permeability as well as in pathogenesis of vasogenic brain edema. The purpose of this study was to examine the effects of orally administered OPC-21268, a nonpeptide arginine vasopressin V1 receptor antagonist, on cold-induced brain edema in rats.

METHODS

Cold brain injury was induced for 1 minute in 140 rats. Treatment with OPC-21268, at dosages of 100 mg (n = 20), 200 mg (n = 20), and 300 mg/kg (n = 15), or with saline (n = 17) was started 1 hour after the induction of cold injury and was continued every 8 hours for 24 hours. Two percent Evans blue in saline (1 ml/kg) was administered intravenously before cold injury in another group of rats, 15 of which were saline-treated and 55 of which were OPC-21268-treated at the above dosages. After 24 hours, brain tissue water and electrolytes, brain tissue swelling, blood-brain barrier permeability to Evans blue, and plasma electrolytes and osmolality were determined.

RESULTS

Compared with the saline-treated group, OPC-21268 treatment at the dosages of 200 and 300 mg/kg significantly reduced brain water content in both hemispheres (P<0.01). Swelling of the traumatized hemispheres was also significantly reduced at 200 and 300 mg/kg dosages (P<0.05). Brain tissue sodium content was significantly reduced at the dosage of 300 mg/kg (P<0.05). Blood-brain barrier permeability to Evans blue was significantly decreased in a dose-dependent manner compared with the saline-treated group (P<0.01). No significant changes were observed in other parameters.

CONCLUSION

Our results indicate that OPC-21268 predominantly exerts a protective effect in areas where the maximum amount of blood-brain barrier breakdown occurs, and it is effective in the treatment of cold-induced vasogenic brain edema.

Hemolysate-mediated renal vasoconstriction and hypersensitization

The present studies measured vessel diameter, before and after addition of hemolysate, in isolated afferent arterioles (AA) and efferent arterioles (EA) obtained from the rat kidney. Human red blood cells (RBC) were hemolyzed in distilled water and membranes were discarded after centrifugation. Hemolysate added to the bath solution caused vigorous AA and EA contraction and, after washout, hypersensitized the AA and EA to doses of angiotensin II (AII) which would normally only elicit 50% contraction (EC50). Neither the contraction nor the hypersensitization were mimicked by pure human hemoglobin. The vasoconstrictive responses in the AA and EA were accompanied by increased cytosolic-free calcium concentration. Further purification (desalting) of the hemolysate to remove substance of < or = 1000 Da (which include ATP) did not eliminate the vasoconstrictive component from the hemolysate. Finally, cultured rat aortic vascular smooth muscle cells also demonstrated a rapid increase in (Ca2+i) when exposed to hemolysate. This increase in (Ca2+i) was, in part, dependent on Ca2+ influx since it could be attenuated with diltiazem (10(-5) M). In conclusion, hemolysate contains a factor which induces contractions of the isolated rat kidney AA and EA and rapid elevations in (Ca2+i). This factor, from hemolyzed RBC, is not hemoglobin itself.

Arginine vasopressin V1-receptor antagonism in an ovine model of acute myocardial infarction

Arginine vasopressin (AVP) has an uncertain role in the pathogenesis of increased myocardial and other regional vascular resistance after cardiac injury. The extreme increase in plasma AVP levels observed in some individuals after myocardial infarction potentially exacerbates already compromised myocardial perfusion. We assessed whether blockade of AVP, by administration of the specific V1-receptor antagonist OPC-21268, can reduce the severity of myocardial injury after embolization in our ovine model of acute myocardial infarction. Embolizations resulted in a pattern of changes in ECG and cardiac enzymes consistent with moderate to severe acute myocardial infarction, resulting in left ventricular dysfunction [left ventricular ejection fraction (LVEF) reduced from 52-53% to 38%]. However, no statistically significant differences were observed between groups in hemodynamic or neurohumoral indices of myocardial damage. By contrast, the hypothalamus-pituitary-adrenal (HPA) response [including plasma AVP (p < 0.01), adrenocorticotropic hormone (ACTH; p < 0.01), and cortisol (p < 0.01)) to embolizations was significantly increased in the sheep infused with OPC-21268. In conclusion, whereas plasma HPA responses differed between the two groups, the similar responses in cardiac enzymes, LVEF, and hemodynamic and neurohumoral factors in both groups does not support a role of V1 receptor-mediated exacerbation of myocardial injury in this model of myocardial infarction. Assessment of different receptor antagonists or examination of other models of cardiac injury may further clarify the role of the markedly increased AVP levels that can occur in myocardial infarction.

Angiotensin (AT1A) receptor-mediated increases in transcellular sodium transport in proximal tubule cells

Angiotensin II (ANG II), acting through angiotensin type 1A receptors (AT1A), is important in regulating proximal tubule salt and water balance. AT1A are present on apical (AP) and basolateral (BL) surfaces of proximal tubule epithelial cells (PTEC). The molecular mechanism of AT1A function in epithelial tissue is not well understood, because specific binding of ANG II to intact PTEC has not been found and because a number of isoforms of AT receptors are present in vivo. To overcome this problem, we developed a cell line from opossum kidney (OK) proximal tubule cells, which stably express AT1A (Kd = 5.27 nM, Bmax = 6.02 pmol/mg protein). Characterization of nontransfected OK cells revealed no evidence of AT1A mRNA (reverse transcriptase-polymerase chain reaction analysis) or protein (125I-labeled ANG II binding studies) expression. In cells stably expressing AT1A, ANG II binding was saturable, reversible, and regulated by G proteins. Transfected receptors were coupled to increases in intracellular calcium and inhibition of cAMP. To determine the polarity of AT1A expression and function in proximal tubules, transfected cells were grown to confluence on membrane inserts under conditions that allowed selective access to AP or BL surfaces. AT1A were expressed on both AP (Kd = 8.7 nM, Bmax = 3.33 pmol/mg protein) and BL (Kd = 10.1 nM, Bmax = 5.50 pmol/mg protein) surfaces. Both AP and BL AT1A receptors underwent agonist-dependent endocytosis (AP receptor: t1/2 = 7.9 min, Ymax = 78.5%; BL receptor: t1/2 = 2.1 min, Ymax = 86.3%). In cells transfected with AT1A, ANG II caused time- and concentration-dependent increases in transepithelial 22Na transport (2-fold over control at 20 min) by increasing Na/H exchange. In conclusion, we have established a stable proximal tubule cell line that expresses AT1A on both AP and BL surfaces, undergoes agonist-dependent receptor endocytosis, and is functional, as evidenced by inhibition of cAMP and increases in cytosolic calcium mobilization and transepithelial sodium movement. This cell line should prove useful for understanding the molecular and biochemical regulation of AT1A expression and function in PTEC.

Effect of YM087, a potent nonpeptide vasopressin antagonist, on vasopressin-induced protein synthesis in neonatal rat cardiomyocyte

OBJECTIVE

Hypertrophy of cardiomyocytes may play an important role in the pathogenesis of cardiac hypertrophy associated with various cardiovascular diseases such as congestive heart failure. The aim of this study was to investigate whether vasopressin (AVP) induces protein synthesis in cultured neonatal rat cardiomyocytes through its specific receptor and whether YM087, a newly synthesized nonpeptide AVP receptor antagonist, inhibits AVP-induced protein synthesis in vitro.

METHODS

AVP receptors on cardiomyocytes were characterized using the radioligand [3H] AVP. The effects of AVP and YM087 on intracellular free calcium concentration ([Ca2+]i), mitogen-activated protein (MAP) kinase and [3H]-leucine incorporation were investigated in cultured neonatal rat cardiomyocytes.

RESULTS

In cardiomyocytes, Scatchard analysis showed a single population of high-affinity binding sites with the expected AVP V1A receptor subtype profile. YM087 showed high affinity for cardiomyocyte V1A receptors with a Ki value of 0.63 nM. In these same cells, YM087 potently inhibited AVP-induced increases in [CA2+]I and activation of MAP kinase in a concentration-dependent manner. In addition, AVP concentration-dependently stimulated the synthesis of protein without changing the rate of DNA synthesis, and YM087 prevented AVP-induced protein synthesis in a concentration-dependent manner.

CONCLUSIONS

These results suggest that AVP directly causes protein synthesis and YM087 is a potent inhibitor of AVP-induced protein synthesis of cardiomyocytes and thus may have beneficial effects in the development and regression of cardiomyocytic hypertrophy.

Modified low density lipoprotein is a potent stimulus for smooth muscle cell outgrowth from rat aortic explant in vitro

Outgrowth of vascular wall cells from rat aortic tissue explant was studied. In addition to fresh rat serum (3%), the complete culture medium contained either low density lipoprotein (LDL) separated from rat plasma (n-LDL, 100 microg/ml) or rat LDL modified either by activated rat polymorphonuclear leucocytes (pmn-LDL) or by exposure to UV light (uv-LDL). Compared to n-LDL, pmn-LDL significantly increased the start of cell outgrowth and the further rate of growth. High concentration of uv-LDL (500 microg/ml) was cytotoxic. Cells which grew out from aortic tissue in the presence of n-LDL were characterised as endothelial cells by staining with lectin Ulex europaeus, with monoclonal antibody to Factor VIII or with monoclonal antibody to endothelial cells (CD31). However, cells which grew out in the presence either of pmn-LDL or uv-LDL did not stain with any of these endothelial cell markers, instead they showed intense staining with monoclonal anti-alpha-smooth muscle actin, indicating that they were smooth muscle cells. Growth rate of subcultured rat aortic smooth muscles cells was increased (P < 0.05) by the presence of uv-LDL (100 microg/ml). It is concluded that LDL modified either by activated leucocytes or by UV light prevents the normal outgrowth of endothelial cells from aortic explant and at the same time greatly promotes outgrowth of smooth muscle cells. Stimulation of both outgrowth of smooth muscle cells from vascular tissue and their proliferation by modified (oxidised) LDL may have important pathological significance in atherogenesis and restenosis.

Vasopressin stimulates Ca2+ spiking activity in A7r5 vascular smooth muscle cells via activation of phospholipase A2

[Arg8]-vasopressin (AVP) is both a potent vasoconstrictor and a mitogen for vascular smooth muscle cells. AVP binds to a single class of receptors (V1a) in the A7r5 rat aortic smooth muscle cell line (Kd approximately 2 nmol/L). Stimulation of these cells with AVP results in an increase in cytoplasmic free Ca2+ concentration ([Ca2+]i) by releasing intracellular Ca2+ stores and increasing Ca2+ influx; the EC50 for these effects is approximately 5 nmol/L. AVP has recently been reported to stimulate arachidonic acid release in primary cultures of rat aortic smooth muscle over a much lower concentration range (EC50 approximately 0.05 nmol/L). The present study examined the effects of varying concentrations of AVP on spontaneous Ca2+ spiking activity in fura 2-loaded A7r5 cells. Frequency of CA2+ spiking increased with increasing [AVP] in the range of 10 to 500 pmol/L. Higher concentrations of AVP inhibited spiking but elicited the characteristic [Ca2+]i changes ascribed to the release of Ca2+ stores and increased Ca2+ entry. The effects of both low and high concentrations of AVP were inhibited by [1-(beta-mercapto-beta,beta,-pentamethylenepropionic acid),2-0-methyltyrosine]arginine vasopressin, a selective V1a vasopressin antagonist. Nimodipine (50 nmol/L), a blocker of L-type voltage-sensitive Ca2+ channels, abolished the Ca(2+)-spiking activity without inhibiting a maximal [Ca2+]i response to AVP (1 mumol/L). AVP-stimulated Ca2+ spiking, but not release of intracellular Ca2+ stores, was also abolished by ONO-RS-082 (1 mumol/L), an inhibitor of phospholipase A2. These results suggest that occupation of a small fraction of V1a vasopressin receptors by AVP results in stimulation of phospholipase A2 and leads to increased Ca(2+)-spiking activity. This effect may be important for fine tuning of vascular tone, whereas maximal stimulation by AVP (full receptor occupancy) may be required for more vigorous or sustained vasoconstriction or mitogenesis.

Attenuation of genetic hypertension after short-term vasopressin V1A receptor antagonism

Abnormalities of the vasopressin system are found in genetic hypertension. This study compares the delayed effects of a brief period of vasopressin V1A receptor blockade and angiotensin-converting enzyme inhibition in young female and male spontaneously hypertensive rats (SHR) on the development of hypertension in adult life. In a separate study, the role of vasopressin in the maintenance of blood pressure in adult SHR was assessed. Young SHR received either the nonpeptide vasopressin V1A receptor antagonist OPC-21268, the angiotensin-converting enzyme inhibitor ramipril, or vehicle from 6 to 10 weeks of age. During the treatment period, OPC-21268 and ramipril reduced systolic blood pressure compared with control SHR (P < .001). Blood pressure in male SHR 7 weeks after treatment withdrawal was 178 +/- 1 mm Hg in ramipril-treated, 184 +/- 1 mm Hg in OPC-21268-treated, and 200 +/- 2 mm Hg in control SHR (P < .001). Similar results were seen in female SHR, although both OPC-21268 and ramipril were less effective antihypertensive agents in female compared with male SHR. The sustained attenuation in blood pressure was not associated with significant cardiovascular structural changes (left ventricular-to-body weight ratio, renal weight-to-body weight ratio, mesenteric resistance artery media-to-lumen ratio). Results of vasopressin V1A receptor binding kinetics and plasma renin or aldosterone concentrations did not suggest a lasting effect of OPC-21268 on the vasopressin system or of ramipril on the renin-angiotensin system following treatment withdrawal.(ABSTRACT TRUNCATED AT 250 WORDS)