Atrial natriuretic factor-induced cGMP accumulation in rat anterior pituitary cells in culture is not coupled to hormonal secretion

Receptor autoradiography as mean to explore the possible functional relevance of neuropeptides: focus on new agonists and antagonists to study natriuretic peptides, neuropeptide Y and calcitonin gene-related peptides

Over the past 20 years, receptor autoradiography has proven most useful to provide clues as to the role of various families of peptides expressed in the brain. Early on, we used this method to investigate the possible roles of various brain peptides. Natriuretic peptide (NP), neuropeptide Y (NPY) and calcitonin (CT) peptide families are widely distributed in the peripheral and central nervous system and induced multiple biological effects by activating plasma membrane receptor proteins. The NP family includes atrial natriuretic peptide (ANP), brain natriuretic peptide (BNP) and C-type natriuretic peptide (CNP). The NPY family is composed of at least three peptides NPY, peptide YY (PYY) and the pancreatic polypeptides (PPs). The CT family includes CT, calcitonin gene-related peptide (CGRP), amylin (AMY), adrenomedullin (AM) and two newly isolated peptides, intermedin and calcitonin receptor-stimulating peptide (CRSP). Using quantitative receptor autoradiography as well as selective agonists and antagonists for each peptide family, in vivo and in vitro assays revealed complex pharmacological responses and radioligand binding profile. The existence of heterogeneous populations of NP, NPY and CT/CGRP receptors has been confirmed by cloning. Three NP receptors have been cloned. One is a single-transmembrane clearance receptor (NPR-C) while the other two known as CG-A (or NPR-A) and CG-B (or NPR-B) are coupled to guanylate cyclase. Five NPY receptors have been cloned designated as Y(1), Y(2), Y(4), Y(5) and y(6). All NPY receptors belong to the seven-transmembrane G-protein coupled receptors family (GPCRs; subfamily type I). CGRP, AMY and AM receptors are complexes which include a GPCR (the CT receptor or CTR and calcitonin receptor-like receptor or CRLR) and a single-transmembrane domain protein known as receptor-activity-modifying-proteins (RAMPs) as well as an intracellular protein named receptor-component-protein (RCP). We review here tools that are currently available in order to target each NP, NPY and CT/CGRP receptor subtype and establish their respective pathophysiological relevance.

Identification of cGMP-dependent protein kinase and its specific substrates in the anterior pituitary

In the anterior pituitary, cGMP is produced in response to a number of stimuli, but intracellular events distal to cGMP production are obscure. Since cGMP-dependent protein kinase (PKG) is a major effector of cGMP actions in other tissues we have determined whether PKG and its specific substrates might be present and responsive to external signals in the ovine anterior pituitary. Photoaffinity labelling with [32P]cGMP revealed a specific 78 kDa protein in ovine anterior pituitary that comigrated with purified bovine lung PKG-I. PKG in protein extracts from anterior pituitary or cultured anterior pituitary cells was enriched by DEAE ion-exchange chromatography and assayed for activity. Both tissue and cultured cells had a relatively high PKG activity by comparison with aortic smooth muscle (known high activity) and brain (known low activity). Subcellular distribution studies showed that in anterior pituitary, aortic and brain, PKG activity was present in both cytosol and triton-extracted membrane fractions, while in platelets the activity was associated with only the membrane fraction. To determine if this PKG might be responsive to extracellular signals an activity ratio assay was used. Incubation of cultured cells with atrial natriuretic peptide (ANP) and sodium nitroprusside, activators of membrane and cytosolic guanylate cyclases respectively, increased the activity of PKG. To determine events distal to PKG activation, a search for potential substrates of PKG was performed. Few substrates were detectable upon addition of purified PKG to tissue lysates due to the high background activity of endogenous protein kinases in the anterior pituitary. However, 19 substrates of PKG were detected in heat-stable and 14 in acid-soluble protein extracts of the anterior pituitary, in which background phosphorylation was almost abolished. After partial purification through Q-Sepharose ion-exchange chromatography some of these proteins were preferentially phosphorylated by addition of PKG-I, while the others were additionally substrates of exogenous cAMP-dependent protein kinase (PKA) or Ca2+ and phospholipid-dependent protein kinase (PKC). A 132-kDa substrate showed an identical phosphopeptide map to a PKG substrate previously described in vascular smooth muscle and platelets. These data demonstrate for the first time the presence of functional PKG activity and multiple PKG substrates in the anterior pituitary where they may play a role in mediating the intracellular actions of cGMP.

Localization of mRNA coding for the three subtypes of atrial natriuretic factor (ANF) receptors in rat anterior pituitary gland cells

Atrial Natriuretic Factor (ANF) action is mediated by highly selective and specific receptors. Three subtypes have been characterized and cloned: ANF receptor-A, -B and -C. These subtypes are all expressed in the anterior pituitary of the rat. In the present study, the mRNA for each subtype was detected by in situ hybridization. The amounts of ANFR-A and -B mRNA were found to be similar, and to be twice that of ANFR-C mRNA. At the ultrastructural level, the three types of ANFR mRNA were expressed in three anterior pituitary cell types, namely lactotrophs, corticotrophs, and gonadotrophs, identified by their hormonal content. No signal was revealed in somatotrophs or thyrotrophs. The different forms of mRNA were similar in terms of subcellular localization: in the cytoplasmic matrix and the nuclear euchromatin. These data indicate that the anterior pituitary is an important target tissue for ANF action.

Atrial natriuretic hormone inhibits corticotropin-releasing hormone-induced prolactin release in man

Atrial natriuretic hormone (ANH) is found in heart myocytes, and also in the CNS. The inhibitory action of ANH on the hypothalamic-pituitary-adrenocortical (HPA) system has been established by in vivo and in vitro experiments, and could be of considerable importance: whereas several synergists to corticotropin-releasing hormone (CRH), the key hormone of the HPA system, are characterized in the past, until now ANH seems to be the only peptide which counterbalances the effects of CRH at the pituitary. As well as at the corticotroph, CRH has a stimulatory influence upon the lactotroph in vivo, and like ACTH and corticosteroids prolactin (PRL) is released in response to physical and cognitive challenges. To test the hypothesis of whether ANH also inhibits the CRH-mediated prolactin release a randomized, placebo-controlled, double-blind study in 12 males aged from 25 to 30 years was conducted. With regard to the diurnal variation of the HPA system activity we compared the prolactin release by 100 micrograms hCRH during a 30 min infusion of placebo, 150 micrograms ANH or 3 IU arginine vasopressin in the morning (08:00 h) and evening (19:00 h). Evaluation of morning and evening effects revealed that administration of hCRH led to a prompt rise of plasma PRL concentration. Infusion of ANH resulted in a significantly reduced maximum increment of PRL compared to placebo (0.83 +/- 0.87 vs 2.85 +/- 1.57 ng/ml, mean +/- SD, n = 12, p < .001). In addition, the AUC values were significantly lower under ANH than in the placebo condition. Infusion of AVP did not significantly change the PRL response to CRH vs placebo.(ABSTRACT TRUNCATED AT 250 WORDS)

Natriuretic peptides stimulate cyclic GMP production in an immortalized LHRH neuronal cell line

The role of cyclic 3',5'-guanosine monophosphate (cGMP) as a second messenger in LHRH neurons is not well understood. Recent studies involving nitric oxide, a direct activator of soluble guanylate cyclase (GC), have implicated cGMP in the regulation of LHRH secretion both in vivo and in vitro. Evidence for the membrane-bound form of GC in LHRH neurons has thus far not been reported. In polymerase chain reaction screening of various cell lines for the natriuretic peptide receptors--which represent GCs--we identified both GC-A and GC-B cDNAs by southern blot hybridization in reverse transcribed and amplified extracts of the GT1-7 cell line, an immortalized LHRH neuronal cell line. Subsequent experiments demonstrated that all of the natriuretic peptides elevated cGMP production with a rank order of potency: CNP > ANP > BNP. Time course studies revealed a rapid intracellular accumulation of cGMP following exposure to CNP with a peak at 2.5 min. CNP was some 200-fold more potent than the NO donor, sodium nitroprusside, in stimulating cGMP accumulation in these cells. These data show for the first time the presence of functional mGCs on LHRH cells, and suggest that the natriuretic peptides may also participate in the regulation of LHRH activity.

Bioactive peptides in anterior pituitary cells

The anterior pituitary (AP) has been shown to contain a wide variety of bioactive peptides: brain-gut peptides, growth factors, hypothalamic releasing factors, posterior lobe peptides, opioids, and various other peptides. The localization of most of these peptides was first established by immunocytochemical methods and some of the peptides were localized in identified cell types. Although intracellular localization of a peptide may be the consequence of internalization from the plasma compartment, there is evidence for local synthesis of most of these peptides in the AP based on the identification of their messenger-RNA (mRNA). In several cases the release of the peptide from the AP cell has been shown and regulation of synthesis, storage and release have also been described. Because the amount of most of the AP peptides is very low (except for POMC peptides and galanin), endocrine functions are not expected. There is more evidence for paracrine, autocrine, or intracrine roles in growth, differentiation, and regeneration, or in the control of hormone release. To demonstrate such functions, in vitro AP experiments have been designed to avoid the interference of hypothalamic or peripheral hormones. The strategy is first to show a direct effect of the peptide after adding it to the in vitro system and, secondly, to explore if the endogenous AP peptide has a similar action by using blockers of peptide receptors or antisera immunoneutralizing the peptide.

Atrial natriuretic peptide effects in AtT-20 pituitary tumour cells

Whether atrial natriuretic peptide (ANP)-evoked inhibition of corticotrophin-releasing factor (CRF)-stimulated ACTH secretion was also manifest in ACTH secreting AtT-20 pituitary tumour cells was investigated. ANP stimulated increases in cGMP accumulation at concentrations of the peptide above 10(-8) M which indicates the presence of the ANP receptors on these cells. CRF stimulated a concentration-dependent increase in ACTH secretion from AtT-20 cells which was unaffected by ANP, 8-bromo-cGMP, or sodium nitroprusside (SNP). Calcium stimulated a concentration-dependent increase in ACTH secretion from electrically permeabilised cells which was unaffected by co-incubation with cGMP but potentiated by cAMP. These results reveal the presence of ANP receptors on AtT-20 cells but suggest that an incomplete expression of the stimulus-secretion coupling mechanisms for ANP, at some point after cGMP production, prevents the effects of natriuretic peptides upon ACTH secretion being manifest in these cells.

An enkephalinergic mechanism for the hypothalamic effect of atrial natriuretic Peptide to inhibit luteinizing hormone secretion

Abstract Previous studies have demonstrated a hypothalamic site of action of atrial natriuretic peptide (ANP) to inhibit luteinizing hormone and an opioid mechanism has been suggested. We have identified the paraventricular nucleus as at least one locus of action of ANP since site-specific injection of 0.1 nmol, bilaterally into this nucleus, but not the medial preoptic nuclei or arcuate nucleus, resulted in a significant inhibition of plasma levels in conscious, ovariectomized rats 90 and 120 min later. This paraventricular site of action suggested an involvement of endogenous enkephalins. The stable enkephalin analog, [D-Pen(2.5)]enkephalin, when injected into the third ventricle in a dose of 1.0 nmol, produced a significant and transient inhibition of luteinizing hormone secretion similar to that seen following injection of 2.0 nmol ANP. Pretreatment of the rats 15 min before peptide injection with the selective 5 opioid antagonist naltrindole (50 mug/2 mul saline) completely prevented the luteinizing hormone-inhibiting effects of both ANP and the enkephalin analog, delta opioid blockade failed to prevent the prolactin-inhibiting effect of ANP but did reverse the prolactin-stimulating effect of [D-Pen(2.5)]enkephalin. Our results suggest a paraventricular nucleus site of action of ANP in addition to probable effects in the median eminence and indicate the possible enkephalinergic mediation of the peptide's ability to inhibit luteinizing hormone secretion.

Atrial natriuretic peptide in peripheral organs other than the heart

The heart atria represent the major site of synthesis of atrial natriuretic peptide (ANP) in mammals including man, and its function as a regulator of water and salt homeostasis has been repeatedly suggested. However, more recently ANP has been located in organs not intimately related to cardiovascular physiology, e.g. the adrenals, lungs, and gut, as well as tissues belonging to the lymphatic, reproductive or endocrine systems. Thus, ANP might serve many more physiological roles than originally thought, but the functional significance of ANP in these non-cardiac tissues is presently poorly understood.

Receptor sites for atrial natriuretic factors in brain and associated structures: an overview

1. Recent data have clearly shown the existence of specific receptor binding sites for atrial natriuretic factors (ANF) or polypeptides in mammalian brain tissues. 2. Ligand selectivity pattern and coupling to cGMP production suggest that brain ANF sites are similar to high-affinity/low-capacity sites found in various peripheral tissues (kidney, adrenal gland, blood vessels). These brain ANF sites possibly are of the B-ANP subtype. 3. High densities of ANF binding sites are found especially in areas of the central nervous system associated with the control of various cardiovascular parameters (such as the subfornical organ and area postrema). However, high densities of sites are also present in other regions such as the hippocampus, cerebellum, and thalamus in the brain of certain mammalian species, suggesting that brain ANF could act as a neuromodulator of noncardiovascular functions. 4. The density of brain ANF binding sites is modified in certain animal models of cardiovascular disorders and during postnatal ontogeny, demonstrating the plasticity of these sites in the central nervous system (CNS). 5. Specific ANF binding sites are also found in various other CNS-associated tissues such as the eye, pituitary gland, and adrenal medulla. In these tissues ANF appears to act as a modulator of fluid production and hormone release. 6. Thus, ANF-like peptides and ANF receptor sites are present in brain and various peripheral tissues, demonstrating the existence of a family of brain/heart peptides.

Atrial natriuretic peptide in the central nervous system of the rat

1. Studies of the presence of atrial natriuretic peptide immunoreactivity and receptor binding sites in the central nervous system have revealed unusual sites of interest. 2. As a result, numerous studies have appeared that indicate that brain atrial natriuretic peptide is implicated in the regulation of blood pressure, fluid and sodium balance, cerebral blood flow, brain microcirculation, blood-brain barrier function, and cerebrospinal fluid production. 3. Alteration of the atrial natriuretic peptide system in the brain could have important implications in hypertensive disease and disorders of water balance in the central nervous system.

Central nervous system actions of atrial natriuretic factor

The recently discovered cardiac peptides, called atrial natriuretic factors (ANF), act peripherally as hormones which control fluid and electrolyte homeostasis. Their renal, adrenal and vascular effects are complemented by central nervous system (CNS) actions to inhibit vasopressin secretion, salt preference, and water intake, and to inhibit the CNS component of the hypothalamo-pituitary-adrenal axis. These central actions of ANF are thought to mirror physiological roles played by endogenous, neuronally derived ANF within the brain. ANF immunoreactivity and binding sites in the anterior pituitary gland and median eminence suggest, as well, neuroendocrine actions of the peptide. We have failed to observe direct pituitary effects of ANF on basal or stimulated pituitary hormone secretion; however, specific hypothalamic actions have been discovered. ANF infusions (IV or cerebroventricular) inhibit luteinizing hormone (LH) secretion via, at least in part, an opioid mechanism since naloxone pretreatment blocks the effect. Additionally ANF inhibits catecholamine stimulation of the release of LH-releasing factor in the median eminence. Direct effects of ANF on tuberoinfundibular dopamine neurons are suggested by the observation that the prolactin-inhibiting action of ANF is prevented by domperidone treatment and is absent following alpha methyl-p-tyrosine inhibition of tyrosine hydroxylase activity. These recent results imply neuromodulatory actions of ANF within the CNS that are expressed via interaction with brain peptide and catecholamine systems.

Synthesis and internalization of atrial natriuretic factor in anterior pituitary cells

Atrial natriuretic factors (ANF) are a family of peptides originally identified in atrial cardiocytes and having natriuretic, diuretic and vasorelaxatory properties. ANF was recently reported to be synthesized in anterior pituitary cells. In the current study, the cell-specific sites of binding and internalization of ANF in the adenohypophysis, a site of ANF action, were investigated. By in vitro autoradiographic techniques, [125I]ANF was found to bind specifically to both anterior and posterior lobes of the pituitary, but not to the intermediate lobe. 5 min following intravenous injection of [125I]ANF, radiolabel was detected by ultrastructural autoradiography on gonadotrophs, corticotrophs and lactotrophs, but not thyrotrophs or somatotrophs. Label was found at both the plasma membrane level and cytoplasmic matrix, but not in the nuclei, of all three cell types. The cellular and subcellular localization of endogenous ANF-like immunoreactivity was also investigated. Specific immunoreactivity was also detected in gonadotrophs, corticotrophs and some lactotrophs. At the subcellular level in all three cell types, immunoreactivity was localized in the cytoplasmic matrix, on secretory granule membranes, and in mitochondria. Less dense staining was observed in nuclear euchromatin; endoplasmic reticulum and Golgi apparatus were not labelled in any of the cells. The data suggest that notwithstanding their reported ability to synthesize ANF, exogenous ANF can also bind and be internalized by adenohypophyseal gonadotrophs; the binding and presence of endogenous ANF-like immunoreactivity in corticotrophs and lactotrophs suggest that these cells may be sites of action for the atrial peptide.

Atrial natriuretic factor stimulates luteal guanylate cyclase

Effect of a synthetic atrial natriuretic peptide, rat atriopeptin II (rAP-II) on the formation of cyclic nucleotides and progesterone production in Percoll-purified rat luteal cells was investigated. Incubation of luteal cells with varying concentrations of rAP-II resulted in a dose-related stimulation of intracellular cyclic GMP content; maximum stimulation being achieved with 10 nM rAP-II. The increase in cyclic GMP formation was extremely rapid and a 12-fold increase in the cyclic GMP content over basal level was attained within 5 min of incubation of the cells with 10 nM rAP-II. In the presence of phosphodiesterase inhibitor, 3-isobutyl-1-methyl-xanthine, both basal and rAP-II-stimulated levels of cyclic GMP were increased approximately 10 times, but the magnitude of stimulation remained similar in the presence or absence of the inhibitor. The atrial peptide at the concentration of 1-100 nM, however, had no effect on either basal or gonadotropin-stimulated progesterone production and cyclic AMP formation by the luteal cells. Furthermore, the increase in the level of cellular cyclic GMP content of rAP-II was demonstrated to result from a selective activation of particulate guanylate cyclase.