Characterization of SR 121463A, a highly potent and selective, orally active vasopressin V2 receptor antagonist.

SR 121463A, a potent and selective, orally active, nonpeptide vasopressin V2 receptor antagonist, has been characterized in several in vitro and in vivo models. This compound displayed highly competitive and selective affinity for V2 receptors in rat, bovine and human kidney (0.6 < or = Ki [nM] < or = 4.1). In this latter preparation, SR 121463A potently antagonized arginine vasopressin (AVP)-stimulated adenylyl cyclase activity (Ki = 0.26+/-0.04 nM) without any intrinsic agonistic effect. In autoradiographic experiments performed in rat kidney sections, SR 121463A displaced [3H]AVP labeling especially in the medullo-papillary region and confirmed that it is a suitable tool for mapping V2 receptors. In comparison, the nonpeptide V2 antagonist, OPC-31260, showed much lower affinity for animal and human renal V2 receptors and lower efficacy to inhibit vasopressin-stimulated adenylyl cyclase (Ki in the 10 nanomolar range). Moreover, OPC-31260 exhibited a poor V2 selectivity profile and can be considered as a V2/V1a ligand. In normally hydrated conscious rats, SR 121463A induced powerful aquaresis after intravenous (0.003-0.3 mg/kg) or oral (0.03-10 mg/kg) administration. The effect was dose-dependent and lasted about 6 hours at the dose of 3 mg/kg p.o. OPC-31260 had a similar aquaretic profile but with markedly lower oral efficacy. The action of SR 121463A was purely aquaretic with no changes in urine Na+ and K+ excretions unlike that of known diuretic agents such as furosemide or hydrochlorothiazide. In addition, no antidiuretic properties have been detected with SR 121463A in vasopressin-deficient Brattleboro rats. Thus, SR 121463A is the most potent and selective, orally active V2 antagonist yet described and could be a powerful tool for exploring V2 receptors and the therapeutical usefulness of V2 blocker aquaretic agents in water-retaining diseases.

[3H] norepinephrine binding by rat glial cells in culture. Lack of correlation between binding and adenylate cyclase activation.

Subcellular fractions prepared from rat glial cells in culture (clonal line c6) were used in an attempt to characterize the adrenergic receptor involved in adenylate cyclase activation. Both [3H] norepinephrine binding and enzyme activation were measured under identical experimental conditions. Binding sites for norepinephrine could be detected; their main characteristics were: apparant Km: 4 - 10-6 M, macimal capacity: 20 pmol/mg protein. Their stereospecificity towards structually related drugs was found to be different from the stereospecificity of the receptor involved in adenylate cyclase activation. Thus, 3-methoxydopamine (a competitive inhibitor of norepinephrine for adenylate cyclase activation), phenylephrine (a partial adrennergic agonist) and the blocking agent propranolol were unable to compete with [3H] norepinephrine for binding. On the other hand, several molecules like dopa bearing a catechol group and which are unable to interact with the adenylate cyclase as agonists or competitive inhibitors strongly inhibited [3H] norepinephrine binding. As in several other systems so far studied, the presence on the glial cell's membrane of a large number of "catechol-binding sites" makes it difficult to characterize the beta-adrenergic receptor.

Vasopressin induces breakdown of membrane phosphoinositides in adrenal glomerulosa and fasciculata cells.

We have previously shown that vasopressin exerts a marked mitogenic effect on adrenal glomerulosa cells. In the present study, we demonstrate that vasopressin (VP) stimulates the formation of inositol monophosphate (IP), inositol diphosphate (IP2) and inositol triphosphate (IP3) in primary cultures of glomerulosa as well as fasciculata cells 5- to 8-fold over the corresponding basal values. In both cell types, the relative stimulations of IP, IP2, and IP3 formation were similar. Angiotensin II (ATII) also induced glomerulosa cells to produce a dose-dependent (up to 10-fold) increase in IP, IP2, and IP3, but had only a small effect on fasciculata cells. The dose dependencies for ATII-induced IP, IP2, and IP3 formation and aldosterone production were nearly the same. We conclude that VP- and ATII-induced formation of inositol phosphates may represent an early step in the action of these peptides on adrenal cells. However, additional elements must be involved to account for the cell specificity of VP and ATII. In glomerulosa cells, VP stimulates mitotic activity and aldosterone secretion, while ATII is only steroidogenic. On fasciculata cells, VP induces a significant increase in the formation of inositol phosphates in spite of the absence of a known biological function in these cells.

Pharmacological characterization of the angiotensin receptor negatively coupled with adenylate cyclase in rat anterior pituitary gland.

Angiotensin II (AII) inhibited anterior pituitary adenylate cyclase. Whereas GTP was necessary to fully express the AII inhibitory effect, Na+ was not required. The magnitude of inhibition (42 +/- 6%) permitted a pharmacological characterization of the AII receptor involved in adenylate cyclase inhibition. Angiotensin I (AI) was less potent than AII, and deletion of aminoacids in the N-terminal position resulted in a progressive reduction of the Ki (peptide concentration producing half-maximal inhibition). The Ki values were 3 +/- 0.9, 10, and 700 nM for AII, angiotensin III (AIII), and des-Asp, des-Arg-AII, respectively. Sarcosine in position 1 [( Sar, Phe]AII) increased the potency of inhibition (Ki = 0.12 +/- 0.12 nM). Different antagonists of the AII receptors appeared to be partial agonists. There was a very close correlation (r = 0.98) between the respective potencies of a series of AII analogs to inhibit adenylate cyclase and the potencies of these analogs to elicit PRL or ACTH release or to bind to AII-binding sites. Dopamine and AII inhibition of anterior pituitary adenylate cyclase were not additive. This suggests that both receptors are on the same cell and likely on lactotrophs. This hypothesis agrees with the observation that vasoactive intestinal peptide stimulation of adenylate cyclase was inhibited by AII, whereas corticotropin-releasing factor stimulation was unaffected. Although dopamine and AII inhibited the same adenylate cyclase, they had opposing effects on PRL release (inhibition and stimulation, respectively). The possible significance of this observation is related to a model implying that PRL release can be elicited through either a Ca+2 or a cAMP pathway.

Angiotensin II stimulates both inositol phosphate production and human placental lactogen release from human trophoblastic cells.

We studied the functional significance of the binding of angiotensin-II (AII) to human placentas. Human trophoblastic cell suspensions were prepared by trypsin digestion of minced tissue. Cell incubations with increasing doses of [125I](SAR1)AII, ranging from 0.01-2.5 nmol/L, were carried out for 20 min at 37 C. The results indicated the presence of specific low capacity [4300 +/- 1300 (+/- SE) sites/cell], high affinity (Kd = 0.38 +/- 0.06 nmol/L) binding sites for [125I](Sar1)AII. This binding was specific for AII analogs. When placental cells were preloaded with 40 microCi/mL [3H]myoinositol for 2 h at 37 C, AII stimulation resulted in a dose-dependent increase in inositol phosphate (InsP) production [EC50 = 1.4 +/- 0.4 (+/- SE) nmol/L], as measured by ion exchange chromatography. (Sar1)AII also stimulated InsP production, with an EC50 of 0.3 +/- 0.2 nmol/L. AII-stimulated production of InsP was completely blocked by the antagonist (Sar1,Ala8)AII. AII also stimulated human placental lactogen release from trophoblastic cells in a dose-dependent fashion. The EC50 was 18 +/- 9 pmol/L, and the stimulation was blocked by (Sar1,Ala8)AII, as found for AII-stimulated InsP production. These results suggest that stimulation of human placental lactogen release by AII may be mediated by activation of phospholipase-C, which, in turn, produces phosphoinositide breakdown. The results, therefore, provide evidence of a physiological role for the renin-angiotensin system within the human placenta.

Angiotensin II inhibits adenylate cyclase from adrenal cortex glomerulosa zone.

Angiotensin binding and the effects of angiotensin on adenylate cyclase activity were determined on purified membranes from the glomerulosa zone of bovine adrenal cortex. Angiotensin II inhibited adenylate cyclase activity in a dose-dependent manner with an A50-value of 2 nM. The angiotensin effect required the presence of GTP. Angiotensin also inhibited ACTH-stimulated activity. Angiotensin binding was sensitive to the same effectors which influenced angiotensin-induced adenylate cyclase inhibition. In the presence of NaCl 100 mM and magnesium, angiotensin interacted with a single population of binding sites (Kd = 4 nM and maximal binding capacity of 440 fmol/mg protein). These results and published data suggest that the ability to inhibit adenylate cyclase might be a general property of angiotensin receptors.

Muscarinic stimulation of inositol phosphate accumulation and acid secretion in gastric fundic mucosal cells.

The muscarinic agonist, carbachol (CCh), was shown to stimulate the production of inositol phosphates (IP) in isolated cells from rabbit fundic mucosa. This stimulatory effect was time- and dose-dependent: EC50 values for IP1, IP2 and IP3 accumulation were not statistically different. The mean value was 30 +/- 8 microM (n = 6). The corresponding maximal stimulation (% of basal value) observed after 20 min incubation in the presence of 100 microM CCh was 160 +/- 15%. CCh-induced IP accumulation was abolished by atropine (Ki = 0.32 +/- 0.18 nM (n = 3)). The CCh concentrations leading to half-maximal inhibition of N-[3H]methylscopolamine binding and half-maximal IP accumulation were similar. The half-maximal value for CCh-induced aminopyrine accumulation was 8-times lower. These results indicate that IP3-mediated mobilization of intracellular Ca2+ might be involved in CCh-induced acid secretion by parietal cells.

A radioiodinated linear vasopressin antagonist: a ligand with high affinity and specificity for V1a receptors.

A linear vasopressin antagonist, Phaa-D-Tyr(Me)-Phe-Gln-Asn-Arg-Pro-Arg-Tyr-NH2 (Linear AVP Antag) (Phaa = Phenylacetyl), was monoiodinated at the phenyl moiety of the tyrosylamide residue at position 9. This antagonist appeared to be a highly potent anti-vasopressor peptide with a pA2 value in vivo of 8.94. It was demonstrated to bind to rat liver membrane preparations with a very high affinity (Kd = 0.06 nM). The affinity for the rat uterus oxytocin receptor was lower (Ki = 2.1 nM), and affinities for the rat kidney- and adenohypophysis-vasopressin receptors were much lower (Ki = 47 nM and 92 nM, respectively), resulting in a highly specific vasopressin V1a receptor ligand. Autoradiographical studies using rat brain slices showed that this ligand is a good tool for studies on vasopressin receptor localization and characterization.

125I-d(CH2)5[Tyr(Me)2, Tyr(NH2)9]AVP: iodination and binding characteristics of a vasopressin receptor ligand.

A radioiodinated vasopressin antagonist, d(CH2)5[Tyr(NH2)9]AVP has been prepared. Iodination was carried out at the phenyl moiety of the tyrosylamide residue at position 9, followed by HPLC purification. Non-radiolabelled monoiodinated antagonist was used as a reference for identification. 125I-d(CH2)5[Tyr(Me)2, Tyr(NH2)9]AVP binding appeared to take place with a dissociation constant of 0.28 +/- 0.09 nM (Kd +/- SD) to V1 vasopressin receptors on rat liver membranes.

Two aromatic residues regulate the response of the human oxytocin receptor to the partial agonist arginine vasopressin.

We investigated the mechanisms that regulate the efficacy of agonists in the arginine-vasopressin (AVP)/oxytocin (OT) receptor system. In this paper, we present evidence that AVP, a full agonist of the vasopressin receptors, acts as a partial agonist on the oxytocin receptor. We also found that AVP becomes a full agonist when two aromatic residues of the oxytocin receptor are replaced by the residues present at equivalent positions in the vasopressin receptor subtypes. Our results indicate that these two residues modulate the response of the oxytocin receptor to the partial agonist AVP.

Hormone-mediated inositol lipid breakdown in hepatocytes and WRK1 cells: relationship to receptor function.

All hormones and neurotransmitters which provoke their intracellular effects by increasing the cytosolic concentration of Ca2+ in their target cells also stimulate the breakdown of inositol phospholipids. Much evidence suggests that this breakdown is intimately involved in the mechanism which couples cell-surface receptor activation to intracellular Ca2+ mobilization. Recent results indicate that the primary, receptor-mediated event in stimulated cells is a phosphodiesteric hydrolysis of phosphatidylinositol 4,5-bisphosphate to yield inositol trisphosphate and diacylglycerol. It is likely that both products of this reaction fulfill 'second messenger' roles within stimulated cells.

Renal adenylate cyclase activation by amino acylated vasopressin and oxytocin.

Two series of neurohypophysial peptide amino-acylated derivatives were tested for their ability to activate plasma membrane adenylate cyclase prepared from pig or rat kidney. They were firstly [8-lysine]-vasopressin-related derivatives (Na-[Glycyl-Cys]1-[8-Lysine]-vasopressin and Na-[Glycyl-Glycyl-Cys51-[8-Lysine]-vasopressin) and secondly oxytocin-related derivatives (Na-[Glycyl-Cys-a1)-oxytocin, Na-[Leucyl-Glycyl-Glycyl--Cys]-oxytocin, and Na-[Glycyl-Cys]-[2-0methyl tyrosine]-oxtocin). The maximal adenylate cyclase activation induced by these peptides was lower than that induced by their respective parent hormones. After incubation of these analogues with plasma membranes obtained from the renal medulla, no significant release of parent hormones occurred. Good qualitative correlations were observed between relative antidiuretic activities measured in vivo and relative potencies in activating adenylate cyclase. It was concluded that direct action of peptides tested on the kidney is at least partly responsible for their antidiuretic activity in vivo.

Ontogenic development of antidiuretic hormone receptors in rat kidney: comparison of hormonal binding and adenylate cyclase activation.

The development of adenylate cyclase responsiveness to vasopressin and parathyroid hormone was studied using membrane fractions prepared from medullo-papillary and cortical portions of kidneys of 2-46-day-old rats. The development of vasopressin binding capacity was followed on the same preparations, using [3H]vasopressin. The characteristics of medullo-papillary adenylate cyclase response to vasopressin were identical in young and adult control animals as regards apparent Km values for [Lys8]vasopressin (3 X 10(-8) M), specificity towards the natural neurohypophysial peptides and the effects of Mg2+. However, the magnitude of maximal enzyme activation by vasopressin was much lower in very young than adult animals. Accordingly vasopressin responsiveness increased sharply between the 10th and 25th days but the magnitude of the maximal response only reached the adult value between the 30th and 45th days after birth. During both periods basal adenylate cyclase activity was almost independent of age. Specific vasopressin binding sites were detected on kidney medullo-papillary membranes from young animals. Vasopressin binding capacity and adenylate cyclase responsiveness to the hormone followed similar development patterns. However, the appearance of specific binding sites slightly preceded the onset of adenylate cyclase responsiveness. Basal cortical adenylate cyclase activity/mg protein was 12 times higher in 2-day-old rats than in the adult controls. It dropped with age but only fell to the adult value between the 25th and the 35th days after birth. For the youngest animals tested (2 days old), the increase in activity due to parathyroid hormone was about half the increase measured in adults, and gradually rose to about 75% of the adult response between the 2nd and 46th days after birth. Apparent Km values for parathyroid hormone were identical in young and adult animals (3.2 and 3.0 U/ml, respectively).

Activation of rat kidney adenylate cyclase by vasopressin analogues: lack of correlation with antidiuretic activity.

Vasopressin analogues with enhanced antidiuretic activity in vivo (deamino-[D-arg8]-vasopressin, deamino-6-carba-[Orn8]-vasopressin, deamino-6-carba-[Arg8]-vasopressin, and deamino-6-carba-[D-Arg8]-vasopressin) were tested for their ability to activate rat renal medullary adenylate cyclase and compared to the natural antidiuretic hormones [Arg8]- and [Lys8]-vasopressin. The enzyme preparation used did not inactivate the vasopressins or the analogues tested. The analogues activated adenylate cyclase. However, several of them were far less effective than expected on the basis of their very high in vivo antidiuretic activity. It was concluded that the enhanced in vivo activity reflects greater metabolic stability in vivo rather than enhanced affinity for the renal antidiuretic hormone receptor.

Vasopressin and angiotensin induce inositol lipid breakdown in rat adenohypophysial cells in primary culture.

Adenohypophysial cells from female Wistar rats were dispersed and maintained for 4 days in primary culture in the presence of [3H]myoinositol. The effects of several releasing hormones, corticotropin-releasing factor (CRF), arginine vasopressin (AVP), angiotensin II (A II), thyrotropin-releasing hormone (TRH), and luteinizing hormone-releasing hormone (LHRH) on the liberation of labelled inositol phosphate (InsP), inositol-bisphosphate (InsP2), and inositol-trisphosphate (InsP3) from prelabelled inositol lipids were tested alone and in combination. Of the corticotropin (ACTH) secretagogues tested, AVP and A II produced a dose-dependent increase in inositol phosphate accumulation. CRF was inactive. The ED50 values of about 1 nM for both AVP and A II were close to the corresponding dissociation constants for binding to pituitary membranes: and, in the case of A II, close to the ED50 for A II-induced inhibition of pituitary membrane adenylate cyclase. The responses to A II and AVP could be inhibited by [Sar1,Ile8]A II and the AVP antagonist d(Et2)-VAVP, respectively. The magnitude of the maximal effect of AVP on accumulation of inositol phosphates was small (25% increase over basal value) suggesting that this effect was restricted to a minor subpopulation of pituitary cells (probably corticotrophes). CRF did not potentiate AVP-induced inositol phosphates accumulation. Maximal A II-induced increase in inositol phosphates accumulation represented 150% of the basal value and was partially additive with that of TRH suggesting that lactotrophes represent the main A II-sensitive subpopulation.

Vasoactive intestinal polypeptide increases inositol phospholipid breakdown in the rat superior cervical ganglion.

The effects of VIP and of peptides of the VIP family: secretin, glucagon, the porcine histidine isoleucine containing peptide (PHI) and the rat hypothalamic growth hormone-releasing hormone (rhGRF) on the cyclic AMP and inositol phosphate contents of isolated rat superior cervical ganglia were investigated. We demonstrate that VIP is able to provoke a large inositol lipid breakdown by acting directly on ganglionic cells. This observation suggests the presence in rat superior cervical ganglia of a new type of receptors for VIP or for an unidentified peptide structurally related to VIP.

Vasopressin potentiates the noradrenaline-induced accumulation of cyclic AMP in the rat superior cervical ganglion.

Previous experiments gave biochemical and electrophysiological evidence for the presence of functional V1-vasopressin receptors coupled to inositol lipid metabolism, but not to cyclic AMP accumulation in the rat superior cervical ganglion. This work was designed to investigate whether there was an action of vasopressin on the noradrenaline-induced cyclic AMP accumulation through the activation of phospholipase C. Our results clearly demonstrate that arginine-vasopressin potentiates cyclic AMP accumulation induced by noradrenaline or isoproterenol in a concentration-dependent manner. The potentiation was unaffected by phentolamine, but was suppressed by the V1-type vasopressin receptor antagonists. Moreover, the phorbol ester 4 beta-phorbol-12-myristate-13-acetate (TPA) did not affect this potentiation which seemed to be Ca2+-dependent. The results suggest that vasopressin may modulate the activity of autonomous functions in the sympathetic ganglia.

Localization and characterization of vasopressin binding sites in the rat brain using an iodinated linear AVP antagonist.

The binding characteristics and central distribution of 125I-Linear AVP antagonist, a new ligand for vasopressin binding sites, are described in the following studies. Saturation studies performed on rat brain septal membranes demonstrated that 125I-Linear AVP antagonist binds to a single class of sites with high affinity (55 pM) and limited capacity (88 fmol/mg protein). In autoradiographic studies, 125I-Linear AVP antagonist labeled brain areas known to contain vasopressin receptors without binding to neurophysins. 125I-Linear AVP antagonist also labeled sites in cortex, hypothalamus, ventral tegmental area and substantia nigra. In competition studies, 125I-Linear AVP antagonist binding was most readily blocked by AVP and a selective V1a agonist. Oxytocin and a selective V2 ligand were effective only in micromolar concentrations. A selective oxytocin agonist was virtually ineffective in blocking 125I-Linear AVP antagonist binding. In regions that contain a high density of oxytocin binding sites, however, oxytocin-displaceable binding was observed. In agreement with studies on peripheral tissues, the binding profile generated from these studies indicates that 125I-Linear AVP antagonist binds to vasopressin receptors of the V1a subtype. These results suggest that 125I-Linear AVP antagonist is a valuable ligand for the study of central AVP receptors.

Localization and pharmacological characterization of high affinity binding sites for vasopressin and oxytocin in the rat brain by light microscopic autoradiography.

Sites which bind tritiated vasopressin (AVP) with high affinity were detected in the brain of male, adult rats, by light microscopic autoradiography. Their anatomical localization differed markedly from that of high affinity binding sites for tritiated oxytocin (OT) determined in the same animal. Co-labelling was minimized by using low concentrations of [3H]AVP and [3H]OT. Binding of the former occurred predominantly in several structures of the limbic system (septum, amygdala, bed nucleus of the stria terminalis, accumbens nucleus), in two hypothalamic nuclei (suprachiasmatic and dorsal tuber) and in the area of the nucleus of the solitary tract. Binding of OT was evidenced in the olfactory tubercle, the ventromedial hypothalamic nucleus, the central amygdaloid nucleus and the ventral hippocampus. The ligand specificity of the binding sites was assessed in competition experiments. Synthetic structural analogues were used, allowing to discriminate OT receptors (OH[Thr4,Gly7]OT) from V2 receptors (dDAVP and d[Tyr(Me)2]VDAVP), V1 receptors ([Phe2,Orn8]VT) and V1b receptors (desGly9d(CH2)5AVP). Our main conclusions are, firstly, that AVP and OT binding sites can be readily distinguished, and that there is virtually no overlap in their distribution in the rat brain. Second, we showed that the sites which bind AVP with high affinity in the brain are V1 receptors, different both from the renal V2 receptors and from the anterior pituitary V1b receptors. Our results support the conjecture that AVP and OT play a role in interneuronal communication in the brain.

Pharmacological characterization of inositol 1,4,5-trisphosphate binding sites: relation to Ca2+ release.

Two subcellular fractions, one enriched in plasma membranes and the other in endoplasmic reticulum membranes, were obtained from WRK1 cells using a combination of differential centrifugations and Percoll gradient fractionation. Specific inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) binding sites were detected in these two preparations. Endoplasmic reticulum membranes exhibited a binding capacity which was about 5-fold higher than that of plasma membranes. Dose-dependent Ins(1,4,5)P3 binding was determined. Experimental data obtained with endoplasmic reticulum membranes could be adequately fitted with a two-site model (a high-affinity binding site with Kd and Bmax values of 0.7 +/- 0.15 nM and 12.9 +/- 5 fmol/mg protein and a low-affinity binding site with Kd and Bmax values of 44.2 +/- 14.6 nM and 143 +/- 43 fmol/mg protein). Both the high- and low-affinity binding sites were selective for Ins(1,4,5)P3. Besides Ins(1,4,5)P3, Ins(1,3,4,5)P4 also discriminated between the two populations of sites while heparin interacted with the high- and low-affinity binding sites with the same affinity. Ins(1,4,5)P3-induced calcium release from endoplasmic reticulum vesicles was determined by monitoring the calcium concentration in the extravesicular compartment with fura-2. Under experimental conditions where the degradation of Ins(1,4,5)P3 was reduced (incubation at 0 degrees C), a high-affinity Ins(1,4,5)P3-induced calcium release (apparent Kact around 20 nM) could be demonstrated. These results suggest that in WRK1 cells, the endoplasmic reticulum is a major site for Ins(1,4,5)P3 action and that the high-affinity binding sites located on the endoplasmic reticulum membranes may contribute to the physiological regulation of the cytosolic free calcium concentration.