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

Characterization of a functional V1B vasopressin receptor in the male rat kidney: evidence for cross talk between V1B and V2 receptor signaling pathways

Although vasopressin V_1B receptor (V_1BR) mRNA has been detected in the kidney, the precise renal localization as well as pharmacological and physiological properties of this receptor remain unknown. Using the selective V_1B agonist d[Leu⁴, Lys⁸]VP, either fluorescent or radioactive, we showed that V_1BR is mainly present in principal cells of the inner medullary collecting duct (IMCD) in the male rat kidney. Protein and mRNA expression of V_1BR were very low compared with the V₂ receptor (V₂R). On the microdissected IMCD, d[Leu⁴, Lys⁸]VP had no effect on cAMP production but induced a dose-dependent and saturable intracellular Ca²⁺ concentration increase mobilization with an EC₅₀ value in the nanomolar range. This effect involved both intracellular Ca²⁺ mobilization and extracellular Ca²⁺ influx. The selective V_1B antagonist SSR149415 strongly reduced the ability of vasopressin to increase intracellular Ca²⁺ concentration but also cAMP, suggesting a cooperation between V_1BR and V₂R in IMCD cells expressing both receptors. This cooperation arises from a cross talk between second messenger cascade involving PKC rather than receptor heterodimerization, as supported by potentiation of arginine vasopressin-stimulated cAMP production in human embryonic kidney-293 cells coexpressing the two receptor isoforms and negative results obtained by bioluminescence resonance energy transfer experiments. In vivo, only acute administration of high doses of V_1B agonist triggered significant diuretic effects, in contrast with injection of selective V₂ agonist. This study brings new data on the localization and signaling pathways of V_1BR in the kidney, highlights a cross talk between V_1BR and V₂R in the IMCD, and suggests that V_1BR may counterbalance in some pathophysiological conditions the antidiuretic effect triggered by V₂R activation.NEW & NOTEWORTHY Although V_1BR mRNA has been detected in the kidney, the precise renal localization as well as pharmacological and physiological properties of this receptor remain unknown. Using original pharmaceutical tools, this study brings new data on the localization and signaling pathways of V_1BR, highlights a cross talk between V_1BR and V₂ receptor (V₂R) in the inner medullary collecting duct, and suggests that V_1BR may counterbalance in some pathophysiological conditions the antidiuretic effect triggered by V₂R activation.

V1b and CRHR1 receptor heterodimerization mediates synergistic biological actions of vasopressin and CRH

Vasopressin (AVP) and CRH synergistically regulate adrenocorticotropin and insulin release at the level of the pituitary and pancreas, respectively. Here, we first extended these AVP and CRH coregulation processes to the adrenal medulla. We demonstrate that costimulation of chromaffin cells by AVP and CRH simultaneously induces a catecholamine secretion exceeding the one induced by each hormone alone, thus demonstrating a net potentiation. To further elucidate the molecular mechanisms underlying this synergism, we coexpressed human V1b and CRH receptor (CRHR)1 receptor in HEK293 cells. In this heterologous system, AVP also potentiated CRH-stimulated cAMP accumulation in a dose-dependent and saturable manner. This effect was only partially mimicked by phorbol ester or inhibited by a phospholipase C inhibitor respectively. This finding suggests the existence of an new molecular mechanism, independent from second messenger cross talk. Similarly, CRH potentiated the AVP-induced inositol phosphates production. Using bioluminescence resonance energy transfer, coimmunoprecipitation, and receptor rescue experiments, we demonstrate that V1b and CRHR1 receptors assemble as heterodimers. Moreover, new pharmacological properties emerged upon receptors cotransfection. Taken together, these data strongly suggest that direct molecular interactions between V1b and CRHR1 receptors play an important role in mediating the synergistic interactions between these two receptors.

Pharmacological and physiological characterization of d[Leu4, Lys8]vasopressin, the first V1b-selective agonist for rat vasopressin/oxytocin receptors

Recently, we synthesized and characterized the first selective V(1b) vasopressin (VP)/oxytocin receptor agonist, d[Cha(4)]arginine vasopressin. However, this agonist was only selective for the human receptors. We thus decided to design a selective V(1b) agonist for the rodent species. We started from previous observations showing that modifying [deamino(1),Arg(8)]VP in positions 4 and 8 altered the rat VP/oxytocin receptor selectivity. We synthesized a series of 13 [deamino(1),Arg(8)]VP analogs modified in positions 4 and 8. Among them, one seemed very promising, d[Leu(4), Lys(8)]VP. In this paper, we describe its pharmacological and physiological properties. This analog exhibited a nanomolar affinity for the rat, human, and mouse V(1b) VP receptors and a strong V(1b) selectivity for the rat species. On AtT20 cells stably transfected with the rat V(1b) receptor, d[Leu(4), Lys(8)]VP behaved as a full agonist on both phospholipase C and MAPK assays. Additional experiments revealed its ability to induce the internalization of enhanced green fluorescent protein-tagged human and mouse V(1b) receptors as expected for a full agonist. Additional physiological experiments were performed to further confirm the selectivity of this peptide. Its antidiuretic, vasopressor, and in vitro oxytocic activities were weak compared with those of VP. In contrast, used at low doses, its efficiency to stimulate adrenocorticotropin or insulin release from mouse pituitary or perfused rat pancreas, respectively, was similar to that obtained with VP. In conclusion, d[Leu(4), Lys(8)]VP is the first selective agonist available for the rat V(1b) VP receptor. It will allow a better understanding of V(1b) receptor-mediated effects in rodents.

Position 4 analogues of [deamino-Cys1] arginine vasopressin exhibit striking species differences for human and rat V2/V1b receptor selectivity

Arginine vasopressin (AVP) mediates a wide variety of biological actions by acting on three distinct G-protein coupled receptors, termed V(1a) (vascular), V(1b) (pituitary) and V(2) (renal). It also binds to the oxytocin (OT) receptor. As part of a program aimed at the design of selective agonists for the human V(1b) receptor, we recently reported the human V(1b), V(1a), V(2) and OT receptor affinities of the following position 4 substituted analogues of [deamino-Cys(1)] arginine vasopressin (dAVP)-(1) d[Leu(4)]AVP, (2) d[Orn(4)]AVP, (3) d[Lys(4)]AVP, (4) d[Har(4)]AVP, (5) d[Arg(4)]AVP, (6) d[Val(4)]AVP, (7) d[Ala(4)]AVP, (8) d[Abu(4)]AVP, (9) d[Nva(4)]AVP, (10) d[Nle(4)]AVP, (11) d[Ile(4)]AVP, (12) d[Phe(4)]AVP, (13) d[Asn(4)]AVP, (14) d[Thr(4)]AVP: (15) d[Dap(4)]AVP. With the exception of Nos. 7 and 12, all peptides exhibit very high affinities for the human V(1b) receptor. Furthermore, peptides 1-4 exhibit high selectivities for the human V(1b) receptor with respect to the V(1a), V(2) and OT receptors and, with d[Cha(4)]AVP, in functional tests, are the first high affinity selective agonists for the human V(1b) receptor (Cheng LL et al., J. Med. Chem. 47: 2375-2388, 2004). We report here the pharmacological properties of peptides 1-4, 5 (from a resynthesis), 7, 9-13, 15 in rat bioassays (antidiuretic, vasopressor and oxytocic) (in vitro: no Mg(++)) with those previously reported for peptides 5, 6, 8, 14. We also report the rat V(1b), V(1a), V(2) and OT receptor affinities of peptides 1-5 and the rat V(2) receptor affinities for peptides: 7-15.The antidiuretic activities in units/mg of peptides 1-15, are: 1=378; 2=260; 3=35; 4=505; 5=748; 6=1150; 7=841; 8=1020; 9=877; 10=1141; 11=819, 12=110; 13=996; 14=758; 15=1053. Peptides 1-4 exhibit respectively the following rat and human (in brackets) V(2) receptor affinities: 1=3.1 nm (245 nm); 2=3.4 nm (1125 nm); 3=24.6 nm (11,170 nm); 4=0.6 nm (1386 nm). Their rat V(1b) receptor affinities are 1=0.02 nm; 2=0.45 nm; 3=9.8 nm; 4=0.32 nm. Their rat V(1a) receptor affinities are 1=1252 nm; 2=900 nm; 3=1478 nm; 4=32 nm. Their rat oxytocin (OT) receptor affinities are 1=481 nm; 2=997 nm; 3=5042 nm; 4=2996 nm. All four peptides have high affinities and selectivities for the rat V(1b) receptor with respect to the rat V(1a) and OT receptors. However, in contrast to their high selectivity for the human V(1b) receptor with respect to the human V(2) receptor, they are not selective for the V(1b) receptor with respect to the V(2) receptor in the rat. These findings confirm previous observations of profound species differences between the rat and human V(2) receptors. Peptides 1-4 are promising leads to the design of the first high affinity selective agonists for the rat V(1b) receptor.

Angiotensin II receptor subtypes and phosphoinositide hydrolysis in rat adrenal medulla

Angiotensin II (ANG) receptor subtypes were characterized by quantitative autoradiography after incubation with the ANG agonist [124I]Sar1-ANG in rat adrenal medulla. ANG receptors are highly localized in adrenal medulla. Specific binding was displaced by 4% and by 95% with the AT, receptor blocker losartan and the AT2 receptor competitor CGP 42112A, respectively. Analysis of competition curves indicated relative binding potencies for the AT2 population of CGP 42112A>PD 123319> PD 123177. ANG stimulated +-nositol phosphate formation in a dose-dependent manner in rat adrenal medulla. Losartan at concentrations of 10(-9) to 10(-5) M antagonized the effect of ANG, whereas PD 123177 or PD 123319 had no antagonistic action. However, at a higher concentration (10(-5) M) PD 123177 or PD 123319 potentiated the effect of ANG on InsP1-accumulation. In the presence of PD 123319 (10(-5) M) ANG dose-response curve was shifted to the left with no change in the maximal effect. This affect was blocked by the addition of losartan (10(-5) M). On the contrary, the addition of CGP 42112A (10(-6) M) inhibited ANG-induced increase in InsP1-accumulation. On the other hand, ANG and CGP 42112A reduced basal cyclic GMP formation, this effect was partially reverted by sodium orthovanadate, a phosphotyrosine phosphatase inhibitor. Our results further demonstrate the presence of two ANG receptor subtypes in adrenal medulla: ANG binding to AT, receptor stimulates inositol phospholipid metabolism, whereas ANG binding to AT2 receptors decreases both inositol phosphate production and cGMP formation.

Studies of the mechanisms of action of corticotropin-releasing factor (CRF) and arginine vasopressin (AVP) in the ovine anterior pituitary: evidence that CRF and AVP stimulate protein phosphorylation and dephosphorylation

This study was undertaken to determine the roles of corticotropin-releasing factor (CRF) and arginine vasopressin (AVP) in the regulation of adrenocorticotropin (ACTH) secretion in perfused ovine anterior pituitary (AP) cells and their ability to cause protein phosphorylation and dephosphorylation in these cells. Freshly dispersed ovine AP cells were maintained in a miniperifusion chamber and ACTH secretion was monitored every 20 s. When cells were perfused with CRF (1 nM, 5 min) or AVP (1 nM, 5 min), ACTH release was increased 20-fold and 12-fold, respectively. When an ovine AP cell membrane fraction was incubated with either CRF or AVP, CRF stimulated the phosphorylation of at least 11 proteins and the dephosphorylation of at least 5 phosphoproteins, whereas AVP caused the phosphorylation of at least 15 proteins and the dephosphorylation of 5 proteins. A comparison of the proteins phosphorylated by CRF or AVP with those phosphorylated by cAMP or protein kinase C activators suggested that the hormone-stimulated phosphorylation may also involve unidentified protein kinases. Additionally, at least eight proteins appeared to be phosphorylated by both CRF and AVP. Furthermore, in the case of four particular proteins both CRF and AVP stimulated phosphorylation at low concentrations of Ca2+ (0.1-1 microM), but at high concentrations of Ca2+ (10-100 microM) CRF or AVP triggered dephosphorylation of these proteins.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence that the stimulation by arginine vasopressin of the release of adrenocorticotropin from the ovine anterior pituitary involves the activation of protein kinase C

These studies were undertaken to evaluate the role of protein kinase C (PKC) in the regulation by arginine vasopressin (AVP) of adrenocorticotropin (ACTH) secretion from the ovine anterior pituitary. AVP caused the rapid translocation of PKC from the cytosol to the cell membrane in ovine anterior pituitary cells that was maximal at 5 min. This phenomenon, which is a known concomitant of C-kinase activation, was produced to a greater extent by phorbol 12-myristate 13-acetate (PMA) but not by corticotropin-releasing factor (CRF). To determine whether AVP activated corticotrope PKC, we assessed the ability of three different PKC inhibitors (H-7, sphingosine, and retinal) to modify basal, AVP-, PMA-, and CRF-stimulated ACTH release. In addition to inhibiting the in vitro activity of purified PKC, each compound also caused in vitro inhibition of the protein kinase A (PKA) catalytic subunit, indicating that none could be considered to be a specific inhibitor of PKC and the PKA catalytic subunit. As determined by the mean IC50 values required for the in vitro inhibition of PKC and the PKA catalytic subunit, sphingosine was judged to be the most selective and H-7 the least selective PKC inhibitor. A 4 h exposure to each inhibitor caused a dose-dependent increase in basal ACTH release and attenuation of both AVP- and PMA-stimulated ACTH release. H-7 and retinal, in concentrations that caused a 20-50% inhibition of PKA, also attenuated CRF-stimulated ACTH release; however, this effect was not observed with sphingosine in concentrations that caused only a 10-20% inhibition of PKA. We conclude that: (1) AVP causes the direct activation of PKC in the ovine anterior pituitary and that C kinase activation is important in mediating the effect of AVP on ACTH release; (2) the finding that inhibition of PKC elevates ACTH suggests that basal ACTH secretion is also partly regulated by PKC; (3) since CRF does not cause PKC translocation in ovine anterior pituitary cells, it is unlikely that PKC plays a physiological role in the action of CRF on the corticotrope; (4) the finding that H-7 and retinal attenuate CRF-stimulated ACTH secretion suggests that CRF activates PKA in corticotropes.

Inhibition of protein kinase C activity in cultured pituitary cells attenuates both cyclic AMP-independent and -dependent secretion of ACTH

The present study examines the effect of reduction of protein kinase C (PKC) activity, as induced by either phorbol ester (PMA) down-regulation or staurosporine inhibition, on the secretion of ACTH from cultured anterior pituitary (AP) cells. Short-term (3 h) exposure of cells to 5 nM PMA resulted in almost complete desensitization to both PMA and vasopressin (AVP), while there was only a minor incidence on the effect of corticotropin-releasing factor (CRF). In contrast, long-term (12-24 h) exposure of cells to PMA, as well as pretreatment with staurosporine, dramatically reduced the stimulatory influence of CRF. This was shown not to be due to a decline in ACTH cells' stores, nor to the toxicity of phorbol ester or to a negative autofeedback of ACTH. Pretreatment of corticotrophs with PMA failed to dampen the CRF-induced cyclic AMP formation, while it caused a decline in the effects of forskolin and 8-bromoadenosine cyclic AMP. Stimulated ACTH secretion subsequent to either veratridine- or high K(+)-induced cell depolarization was likewise decreased. We conclude that in corticotrophs the stimulatory action of not only AVP, but also of that of CRF on ACTH secretion strongly relies on PKC activity. In the case of CRF, however, this may not be a primary consequence of receptor occupation, as evidence suggests an indirect relationship which may involve PKC regulation of Ca2+ channels and/or the ion's intracellular messenger function.

Activation of protein kinase C differentially regulates corticotropin-releasing factor-stimulated peptide secretion and cyclic AMP formation of intermediate and anterior pituitary cells in culture

The present study was aimed at investigating the effect of protein kinase C (PKC) activation on CRF receptor function of proopiomelanocortin (POMC) cells in culture. Incubation of tissues with the phorbol ester PMA selectively potentiated corticotropin-releasing factor (CRF)-stimulated ACTH secretion and cyclic AMP formation of anterior pituitary (AP) cells, while, in sharp contrast, it failed to similarly affect intermediate pituitary (IP) cells and AtT-20 corticotrophs exposed to CRF. Unexpectedly, however, long-term treatment of cultures with PMA, which depletes cell stores of PKC, resulted in a similar dramatic attenuation of stimulated peptide release from both corticotrophs and melanotrophs, while being without significant effect on cyclic AMP production. Exposure of cells to PMA did not change either basal or CRF-enhanced levels of POMC mRNA. We conclude that activation of PKC fails to synergize with CRF-mediated signalling in IP and AtT-20 cells, although optimal CRF receptor expression requires the presence of a functional kinase C pathway, thus suggesting cross-talks between both messenger systems.

Receptor-effector coupling by G proteins

The primary structure of G proteins as deduced from purified proteins and cloned subunits is presented. When known, their functions are discussed, as are recent data on direct regulation of ionic channels by G proteins. Experiments on expression of alpha subunits, either in bacteria or by in vitro translation of mRNA synthesized from cDNA are presented as tools for definitive assignment of function to a given G protein. The dynamics of G protein-mediated signal transduction are discussed. Key points include the existence of two superimposed regulatory cycles in which upon activation by GTP, G proteins dissociate into alpha and beta gamma and their dissociated alpha subunits hydrolyze GTP. The action of receptors to catalyze rather than regulate by allostery the activation of G proteins by GTP is emphasized, as is the role of subunit dissociation, without which receptors could not act as catalysts. To facilitate the reading of this review, we have presented the various subtopics of this rapidly expanding field in sections 1-1X, each of which is organized as a self-contained sub-chapter that can be read independently of the others.

The role of intracellular messengers in adrenocorticotropin secretion in vitro

Adrenocorticotropin (ACTH), an opiomelanocortin peptide, is secreted from anterior pituitary corticotrophs upon stimulation with corticotropin-releasing hormone (CRH), arginine vasopressin (AVP) and several other neuropeptides. CRH, the most potent secretagogue of ACTH, stimulates ACTH secretion and biosynthesis by increasing the production of cyclic adenosine 3',5'-monophosphate (cAMP) within corticotrophs. AVP, which is a weak secretagogue of ACTH but strongly potentiates CRH-stimulated ACTH secretion, operates through the phosphatidylinositol (PI) transduction pathway. Both CRH and AVP increase cytosolic free [Ca2+] within normal corticotrophs indicating a role for Ca2+ in ACTH secretion. Glucocorticoids inhibit ACTH synthesis by suppressing transcription of the proopiomelanocortin (POMC) gene and attenuate ACTH release by decreasing cAMP accumulation stimulated by CRH. This review focuses on the roles of these intracellular messengers in ACTH secretion from normal anterior pituitary cells in vitro, and discusses the possible interactions between the cAMP, calcium and PI transduction pathways. Future areas of research are suggested such as identification of protein substrates of cAMP-dependent and Ca2(+)-dependent kinases within normal corticotrophs and evaluation of their role in ACTH biosynthesis and secretion.

Roles of G proteins in coupling of receptors to ionic channels and other effector systems

Guanine nucleotide binding (G) proteins are heterotrimers that couple a wide range of receptors to ionic channels. The coupling may be indirect, via cytoplasmic agents, or direct, as has been shown for two K+ channels and two Ca2+ channels. One example of direct G protein gating is the atrial muscarinic K+ channel K+[ACh], an inwardly rectifying K+ channel with a slope conductance of 40 pS in symmetrical isotonic K+ solutions and a mean open lifetime of 1.4 ms at potentials between -40 and -100 mV. Another is the clonal GH3 muscarinic or somatostatin K+ channel, also inwardly rectifying but with a slope conductance of 55 pS. A G protein, Gk, purified from human red blood cells (hRBC) activates K+ [ACh] channels at subpicomolar concentrations; its alpha subunit is equipotent. Except for being irreversible, their effects on gating precisely mimic physiological gating produced by muscarinic agonists. The alpha k effects are general and are similar in atria from adult guinea pig, neonatal rat, and chick embryo. The hydrophilic beta gamma from transducin has no effect while hydrophobic beta gamma from brain, hRBCs, or retina has effects at nanomolar concentrations which in our hands cannot be dissociated from detergent effects. An anti-alpha k monoclonal antibody blocks muscarinic activation, supporting the concept that the physiological mediator is the alpha subunit not the beta gamma dimer. The techniques of molecular biology are now being used to specify G protein gating. A "bacterial" alpha i-3 expressed in Escherichia coli using a pT7 expression system mimics the gating produced by hRBC alpha k.

Indirect relationship between vasopressin-induced secretion of ACTH and cyclic nucleotides in cultured anterior pituitary cells

The present study examines whether a correlation exists between cyclic nucleotides and the mechanism of action of arginine vasopressin (AVP) on adrenocorticotropin hormone (ACTH) secretion from pituitary corticotrophs. Incubation of cultured anterior pituitary cells with 3-isobutyl-1-methylxanthine (IBMX) or Rolipram elevated the basal intracellular content of both adenosine 3':5'-cyclic monophosphate (cAMP) and guanosine 3':5'-cyclic monophosphate (cGMP) or cAMP alone, respectively. Both IBMX and Rolipram enhanced the AVP-stimulated secretion of ACTH in cultured anterior pituitary cells, but not in AtT-20 corticotrophs which lack functional AVP receptors. Rolipram was less potent than IBMX in this regard, which suggests a possible involvement of cGMP. In contrast, both drugs showed similar potency to stimulate CRF-induced ACTH secretion. Incubation of pituitary cells with atrial natriuretic factor elevated tissue cGMP levels and increased the ACTH response to AVP. The results of this study show that, although AVP fails to directly affect the levels of cAMP and cGMP in anterior pituitary cells, the stimulatory effect of AVP on ACTH secretion was modulated by the cellular cAMP content.

Stress and the pituitary-adrenal axis

The hypothalamo-pituitary-adrenal axis is controlled by complex regulatory mechanisms. Numerous factors such as CRF, vasopressin, oxytocin, angiotensin II and conceivably other hormones--all controlled by various substances acting on central locations--stimulate the release of the stress hormone ACTH. On the other hand, glucocorticoids inhibit the secretion of ACTH by acting at the hypothalamic and/or pituitary level. The release of ACTH is therefore the final outcome of the interactions between the hypothalamus, the adrenal gland and possibly other organs. The multimolecular nature of the factors responsible for the control of the pituitary-adrenal axis is an attractive hypothesis because of the great variety of stress stimuli. The various factors could have specific roles in various stress situations. They provide a highly sensitive mechanism regulating very finely the stress hormone in response to a whole variety of endogenous and exogenous stimuli. Depending on the type of stress, they may therefore singly or in combination affect the amount and duration of ACTH and steroid secretion. The released glucocorticoids may then produce their numerous effects on inflammatory and immunological processes, carbohydrate metabolism, shock and water balance. It has been postulated that these effects may be important in order to prevent host responses from over-reacting to stress and threatening homeostasis. However, proof of the necessity of the glucocorticoid hypersecretion in response to stress remains elusive.