Regulation of brain atrial natriuretic peptide and angiotensin receptors: quantitative autoradiographic studies

Natriuretic peptide receptor-C signaling and regulation

The natriuretic peptides (NP) are a family of three polypeptide hormones termed atrial natriuretic peptide (ANP), brain natriuretic peptide (BNP), and C-type natriuretic peptide (CNP). ANP regulates a variety of physiological parameters by interacting with its receptors present on the plasma membrane. These are of three subtypes NPR-A, NPR-B, and NPR-C. NPR-A and NPR-B are guanylyl cyclase receptors, whereas NPR-C is non-guanylyl cyclase receptor and is coupled to adenylyl cyclase inhibition or phospholipase C activation through inhibitory guanine nucleotide regulatory protein (Gi). ANP, BNP, CNP, as well as C-ANP(4-23), a ring deleted peptide that specifically interacts with NPR-C receptor inhibit adenylyl cyclase activity through Gi protein. Unlike other G-protein-coupled receptors, NPR-C receptors have a single transmembrane domain and a short cytoplasmic domain of 37 amino acids, which has a structural specificity like those of other single transmembrane domain receptors. A 37 amino acid cytoplasmic peptide is sufficient to inhibit adenylyl cyclase activity with an apparent Ki similar to that of ANP(99-126) or C-ANP(4-23). In addition, C-ANP(4-23) also stimulates phosphatidyl inositol (PI) turnover in vascular smooth muscle cells (VSMC) which is attenuated by dbcAMP and cAMP-stimulatory agonists, suggesting that NPR-C receptor-mediated inhibition of adenylyl cyclase and resultant decreased levels of cAMP may be responsible for NPR-C-mediated stimulation of PI turnover. Furthermore, the activation of NPR-C receptor by C-ANP(4-23) and CNP inhibits the mitogen-activated protein kinase activity stimulated by endothelin-3, platelet-derived growth factor, phorbol-12 myristate 13-acetate, suggesting that NPR-C receptor might also be coupled to other signal transduction system or that there may be an interaction of the NPR-C receptor and some other signaling pathways. In this review article, NPR-C receptor coupling to different signaling pathways and their regulation will be discussed.

Changes of hypothalamic and plasma vasopressin in rats with deoxycorticosterone-acetate induced salt appetite

Mineralocorticoids play a predominant role in development of salt appetite and hypertension. Since vasoactive peptides could mediate the central effects of mineralocorticoids, we evaluated changes of immunoreactive (IR) arginine vasopressin (AVP) in the paraventricular (PVN) and supraoptic (SON) hypothalamic nucleus during DOCA-induced salt appetite. In one model, rats having free access to water and 3% NaCl during 9 (prehypertensive stage) or 21 days (hypertensive stage) received DOCA (s.c., 10 mg/rat/in alternate days). A decrease in the IR cell area, number of IR cells and staining intensity was obtained in magnocellular PVN of rats treated during 9 days. After 21 days IR cell area and number of cells in the PVN also decreased, but staining intensity of remaining cells was normal. The same parameters were unchanged in the SON. In another model, animals treated with DOCA during 9 days had only access to 3% NaCl or water. The IR cell area in PVN and SON significantly increased in mineralocorticoid-treated and control animals, both drinking 3% NaCl. Staining intensity (PVN and SON) and number of IR cells (PVN) also augmented in DOCA-treated animals drinking salt respect of a group drinking water. Plasma AVP in rats treated with DOCA and offered salt and water, exhibited a 2-2.5 fold increase at the time of salt appetite induction. Plasma AVP was substantially higher in rats drinking salt only, while the highest levels were present in salt-drinking DOCA-treated rats. Thus, peptide depletion in the PVN may be due to increased release, because reduced levels of hypothalamic and posterior pituitary AVP were measured in this model. In rats drinking salt only the substantial increase of IR AVP in the PVN and SON, may be due to dehydration and hyperosmosis. Because DOCA-salt treated rats showed higher AVP levels in the PVN compared to untreated rats drinking salt only, it is possible that DOCA sensitized PVN cells to increase AVP production. The results suggest the vasopressinergic system could mediate some central functions of mineralocorticoids.

Atrial natriuretic peptide-C receptor and membrane signalling in hypertension

Atrial natriuretic peptide (ANP) regulates a variety of physiological parameters, including the blood pressure and intravascular volume, by interacting with its receptors present on the plasma membrane. ANP receptors are of three subtypes: ANP-A, -B and -C receptors. ANP-A and ANP-B receptors are guanylyl cyclase receptors, whereas ANP-C receptors are coupled to adenylyl cyclase inhibition or phospholipase C activation through inhibitory guanine nucleotide-regulating protein. Unlike other G protein-coupled receptors, ANP-C receptors have a single transmembrane domain and a short cytoplasmic domain of 37 amino acids, the cytoplasmic domain has a structural specificity like those of other single-transmembrane-domain receptors and 37 amino-acid cytoplasmic domain peptide is able to exert is inhibitory effect on adenylyl cyclase. The activation of ANP-C receptor by C-ANP(4-23) (a ring-deleted peptide of ANP) and C-type natriuretic peptide inhibits the mitogen-activated protein kinase activity stimulated by endothelin-3, platelet-derived growth factor and phorbol-12 myristate 13-acetate. C-ANP also inhibits mitogen-induced stimulation of DNA synthesis, indicating that the ANP-C receptor plays a role in cell proliferation through an inhibition of mitogen-activated protein kinase and suggesting that the ANP-C receptor might also be coupled to other signal transduction mechanism(s) or that there might be an interaction of the ANP-C receptor with some other signalling pathways. ANP receptor binding is decreased in most organs in hypertensive subjects and hypertensive animals. This decrease is consistent with there being fewer guanylyl cyclase-coupled receptors in the kidney and vasculature and selective inhibition of the ANP-C receptor in the thymus and spleen. Platelet ANP-C receptors are decreased in number in hypertensive patients and spontaneously hypertensive rats. ANP-A, -B and -C receptors are decreased in number in deoxycorticosterone acetate-salt-treated kidneys and vasculature; however, the responsiveness of adenylyl cyclase to ANP is augmented in the vasculature and heart and is attenuated completely in platelets. These alterations in ANP receptor subtypes may be related to the pathophysiology of hypertension. Several hormones such as angiotensin II, ANP and catecholamines, the levels of which are increased in hypertension, downregulate or upregulate ANP-C receptors and ANP-C receptor-mediated inhibition of adenylyl cyclase. It can be suggested that the antihypertensive action of several types of drugs such as angiotensin converting enzyme inhibitors, angiotensin type 1 receptor antagonists and beta2-adrenergic antagonists may partly be attributed to their ability to modulate the expression and function of the ANP-C receptor.

Defective ANF-R2/ANP-C receptor-mediated signalling in hypertension

In the present studies we have shown that atrial natriuretic factor (peptide) receptor of ANF-R2/ANP-C type is coupled to adenylyl cyclase/cAMP signal transduction system through Gi-regulatory protein and is implicated in mediating some of the physiological responses of atrial natriuretic factor or peptide (ANP). ANF-R2/ANP-C receptor-mediated adenylyl cyclase inhibition was altered in hypertension. This alteration was tissue specific. In heart, aorta, brain and adrenal, the extent of inhibition of adenylyl cyclase by ANP was enhanced in SHR as compared to age-matched WKY, whereas in platelets, the ANP-mediated inhibition was completely attenuated. The enhanced inhibition of adenylyl cyclase by ANP was also observed in heart and aorta from DOCA-salt hypertensive rats. In addition, the augmented inhibition of adenylyl cyclase by ANP was observed in 2 weeks and older SHR but not in 3-5 days old SHR. Similarly, in DOCA-salt hypertensive rats, the enhanced inhibition of adenylyl cyclase by ANP was observed after 2 weeks of DOCA-salt treatment when the blood pressure was also enhanced, however one week older SHR but not in 3-5 days old SHR. Similarly, in DOCA-salt hypertensive rats, the enhanced inhibition of adenylyl cyclase by ANP was observed after 2 weeks of DOCA-salt treatment when the blood pressure and augmented ANP-mediated inhibition of adenylyl of DOCA-salt treatment did not result in an augmented blood pressure and augmented ANP-mediated inhibition of adenylyl cyclase, suggesting that blood pressure increase may be responsible for the enhanced responsiveness of ANP to adenylyl cyclase inhibition. However, in genetic model of hypertension, the increased inhibition of adenylyl cyclase by ANP at 2 weeks of age (when the blood pressure is normal) may be implicated in the pathogenesis of hypertension. The augmented inhibition of adenylyl cyclase in cardiovascular tissues from SHR and DOCA-salt hypertensive rats may be due to the upregulation of ANF-R2/ANP-C receptors or due to the amplification of post-receptor signalling mechanisms.

Chronic manipulation of dietary salt modulates renal physiology and kidney dopamine receptor subtypes: functional and autoradiographic studies

1. Compared to rats maintained on the normal NaCl (0.33%) diet, animals maintained on the low NaCl (0%) diet for 4 weeks exhibited increased plasma aldosterone and chloride and decreased urinary sodium excretion. 2. Rats maintained on the high NaCl (8%) diet for 4 weeks showed increased systolic blood pressure, water intake, urine volume, sodium and dopamine excretion and decreased plasma aldosterone and glomerular filtration rate. 3. Administration of SCH 23390 (10 mg/kg, po), but not domperidone to the high salt diet rats attenuated the diuretic effect, indicating the involvement of DA1 rather than DA2 receptors. The dopamine decarboxylase inhibitor, carbidopa (30 mg/kg, i.p.), also reduced the high salt-induced diuresis. 4. Kidney sections from rats fed the low NaCl diet showed a 63-100% decrease (P < 0.001-0.02) in cortical and medullary DA1 and DA2 binding sites, while rats fed the high NaCl diet demonstrated only a 70% decrease (P < 0.01-0.02) in cortical DA1 binding, without affecting DA2 binding. 5. These data indicate that chronic modification of dietary salt profoundly affects the sodium, water and dopamine excretion and leads to selective modulation of renal dopamine receptor subtypes.

Quantitative autoradiography demonstrates selective modulation of rat brain regional dopamine (D1 and D2) receptor subtypes after chronic manipulation of dietary salt

The effects of chronic dietary sodium chloride (NaCl) consumption on renal function and brain dopamine receptors were studied in adult, male normotensive rats. Compared to rats maintained on the normal NaCl (0.33%) diet, animals maintained on the low NaCl (0%) diet for 4 weeks exhibited significant increases in plasma aldosterone, chloride and changes in urinary electrolyte excretion. In contrast, rats maintained on the high NaCl (8%) diet for 4 weeks demonstrated significant increases in urine volume and urinary sodium, chloride and dopamine excretions and water intake. Rats fed the high NaCl diet displayed a 42-59% decrease (p < 0.001-0.05) in D1 binding in the nucleus accumbens (NA), olfactory tubercle (OT) and the striatum (STM), without any effects on D2 binding in these brain regions. Rats maintained on the low NaCl diet also demonstrated decreased D1 binding in the ventral (24%, p < 0.02) and lateral (29%, p < 0.01) STM, but not in the OT, NA, entopeduncular nucleus and substantia nigra. Rats fed low or high NaCl diets exhibited a 35-180% increase (p < 0.01-0.05) in D2 binding in several mid-brain areas (e.g. hypothalamus, thalamus and hippocampus) and hindbrain regions (e.g. superior colliculus and nucleus tractus solitarius) without affecting the D1 binding. These data indicate that chronic modification of dietary salt intake profoundly affects the renal handling of sodium/water excretion and leads to selective up- and/or down-regulation of DA receptor subtypes in different areas of the brain.(ABSTRACT TRUNCATED AT 250 WORDS)

Adrenalectomy reduces atrial natriuretic peptide stimulated guanylate cyclase activity in rat paraventricular nucleus

Atrial natriuretic peptide (ANP) or an Atriopeptin III analog (PL 058) stimulated cGMP formation in the membrane fraction of rat olfactory bulb, median eminence and paraventricular nucleus, in a dose-dependent manner. The effect of the Atriopeptin III analog was 20-40% greater than that of ANP. Bilateral adrenalectomy, with or without mineralo- or glucocorticoid-replacement, on ANP-stimulated cGMP formation was investigated in rat paraventricular nucleus. 11 days after bilateral adrenalectomy a reduced responsiveness to ANP- or PL 058-induced cGMP production was observed. This effect was prevented by deoxycorticosterone, but not by dexamethasone administration. Our results further support the presence of guanylate cyclase-coupled ANP receptors in brain localized target sites; and they provide evidence suggesting that guanylate cyclase-linked ANP binding sites in the PVN are susceptible to regulatory changes after adrenalectomy-induced activation of the hypothalamus-hypophyso-adrenocortical system.

Effects of deoxycorticosterone acetate (DOCA) and aldosterone on Sar1-angiotensin II binding and angiotensin-converting enzyme binding sites in brain

1. It is known that regulation of salt appetite is a complex behavior controlled in the brain by interaction of mineralocorticoids (MC) and angiotensin II (ANGII). To investigate the effects of MC on ANGII receptors and ANGII synthesis, we have studied two models of salt appetite control. 2. In the first model, doses of DOCA sufficient to induce salt appetite of intact rats were given. In the second one, we studied the effects of aldosterone (ALDO) in doses sufficient to suppress salt appetite developed by prior adrenalectomy (ADX). 3. Binding to ANGII receptors was determined in brain sections incubated with 3 nM [125I]Sar1 ANGII, exposed to [3H]Hyperfilm with an optical density of autoradiograms measured by computerized densitometry. Sar1-ANGII binding was increased by DOCA treatment in the median preoptic nucleus (MnPO) and subfornical organ (SFO) but not in the paraventricular nucleus (PVN) in comparison to vehicle-treated rats. ALDO treatment was without effect on the MnPO but increased ANGII binding in the SFO and PVN. Neither hormone affected binding in the median eminence or anterior pituitary (AP). 4. In contrast to effects on Sar1-ANGII binding in selected areas, [125I]351A binding to angiotensin-converting enzyme (ACE) was unchanged by DOCA or ALDO administration in the SFO, caudate putamen, AP, or posterior pituitary. 5. These findings suggest that MC modulation of the renin-angiotensin system is exerted at the central, and not at the pituitary level. ANGII receptors were modulated in a dose- and region-specific manner: while DOCA may promote their actions upon the MnPO and SFO, ALDO actions may occur at the PVN and SFO. This mechanism may not require increased generation of ANGII in the brain or pituitary.

A modified quantitative autoradiographic assay for atrial natriuretic peptide receptors in rat brain

New conditions for quantitative autoradiographic determination of natriuretic peptide (ANP) binding in rat brain were developed in order to inhibit hormone degradation during incubation without the use of proteinase inhibitors. This procedure includes 1-h preincubation of tissue slices in hypotonic buffer and incubation at 4 degrees C in the presence of 4% proteinase-free albumin. ANP binding to choroid plexus and subfornical organ under above conditions reached equilibrium after 90 min. ANP degradation at the end of the incubation did not exceed 2%. Binding data analysis revealed the previously undetected presence of two distinct ANP binding sites in the rat choroid plexus.

Increased dithiothreitol-insensitive, type 2 angiotensin II receptors in selected brain areas of young rats

1. Angiotensin II receptors have been studied by quantitative autoradiography in selected brain areas of young (2-week-old) and adult (8-week-old) rats. 2. In young rats, angiotensin II receptors were present in brain areas which did not express receptors in the adult brain, such as thalamic nuclei, cortical areas, and the cerebellum. 3. Young rats had more angiotensin II receptors in the subfornical organ than adult rats. In the inferior olive, the number of angiotensin receptors in young animals was 10 times higher than that in adult rats. Angiotensin II binding in the inferior olive was insensitive to incubation in the presence of dithiothreitol. 4. Conversely, the number of angiotensin II receptors in the nucleus of the solitary tract was lower in young rats compared to adults. Incubation in the presence of dithiothreitol resulted in a more than 90% inhibition of angiotensin II binding in the nucleus of the solitary tract. 5. Our results indicate the presence of two types of angiotensin II receptor in brain, one sensitive (type 1) and one insensitive (type 2) to the reducing agent dithiothreitol. 6. The expression of type 2 angiotensin II receptors, insensitive to dithiothreitol, is more marked in young rats, indicating a role for this type of angiotensin receptors in brain development.

Quantitative autoradiography reveals different angiotensin II receptor subtypes in selected rat brain nuclei

Heterogeneity of rat brain angiotensin II receptors was revealed by quantitative autoradiography after incubation with 125I-Sar1-angiotensin II and displacement with the angiotensin II antagonists CGP 42112 A and DuP-753 and by receptor sensitivity to dithiothreitol. Receptors in areas involved in cardiovascular and fluid control--the subfornical organ, nucleus of the solitary tract, paraventricular nucleus, and area postrema--are displaced by DuP-753 with an IC50 of 1 x 10(-7) M, are sensitive to 5 mM dithiothreitol, and thus are angiotensin II type-1. Receptors in the inferior olive are displaced by CGP 42112 A (IC50, 1 x 10(-9) M) but not by DuP-753 in concentrations up to 10(-4) M, are insensitive to 5 mM dithiothreitol, and thus are angiotensin II type-2.

Effect of chronic administration of the converting enzyme inhibitor enalapril (MK 421) on brain atrial natriuretic peptide receptors in Wistar-Kyoto and spontaneously hypertensive rats

Spontaneously hypertensive rats (SHR) showed lower brain ANP binding density when compared with normotensive Wistar-Kyoto (WKY) rats. In the WKY, angiotensin converting enzyme inhibitor, enalapril (25 mg/kg, p.o. for 14 days), decreased the number of ANP binding sites selectively in the subfornical organ and area postrema. Conversely, enalapril increased ANP binding density in the SHR, but only in the area postrema. Enalapril has central effects on ANP binding sites, specific to the circumventricular organs.