Hyperosmolality rapidly reduces atrial-natriuretic-peptide-dependent cyclic guanosine monophosphate production in cultured rat inner medullary collecting duct cells

Upregulation of ANP and NPR-C mRNA in the kidney and heart of eNOS knockout mice

OBJECTIVES

The aim of the present studywas to examine the question of whether the atrial natriuretic peptide (ANP) system is altered by endothelial nitric-oxide synthase (eNOS).

METHODS

Male eNOS-deficient mice (eNOS-/-) and wild type control mice (eNOS+/+, C57B1/6J) were used. Blood pressure was measured in anesthetized mice by tail cuff plethysmography and renal function was measured. Expression of ANP, natriuretic peptide receptor (NPR)-A, NPR-C, and tonicity-responsive enhancer binding protein (TonEBP) mRNA was determined by real-time PCR. Localization of (125)I-ANP binding sites was measured using in vitro autoradiography.

RESULTS

In eNOS-/- mice, systolic blood pressure increased and left ventricular hypertrophy was observed. Urine volume and osmolarity did not change. Expression of ANP markedly increased in the heart and kidney of eNOS-/- mice. Expression of NPR-A and NPR-C increased in the heart and tended to increase in the kidney of eNOS-/- mice. In the renal medulla in particular, increased expression of NPR-C was more prominent. Expression of TonEBP mRNA was markedly decreased in the renal medulla, but not in the renal cortex. Maximum binding capacity (B(max)) of ANP and C-ANP increased in the renal medulla in eNOS-/- mice.

CONCLUSION

These results suggest that the eNOS-NO system may be partly involved in regulation of ANP, NPR-A, -C, and TonEBP mRNA expression in the kidney.

Ligand-mediated endocytosis and intracellular sequestration of guanylyl cyclase/natriuretic peptide receptors: role of GDAY motif

The guanylyl cyclase/natriuretic peptide receptor-A (GC-A/NPRA), also referred to as GC-A, is a single polypeptide molecule having a critical function in blood pressure regulation and cardiovascular homeostasis. GC-A/NPRA, which resides in the plasma membrane, consists of an extracellular ligand-binding domain, a single transmembrane domain, and an intracellular cytoplasmic region containing a protein kinase-like homology domain (KHD) and a guanylyl cyclase (GC) catalytic domain. After binding with atrial and brain natriuretic peptides (ANP and BNP), GC-A/NPRA is internalized and sequestered into intracellular compartments. Therefore, GC-A/NPRA is a dynamic cellular macromolecule that traverses different subcellular compartments through its lifetime. This review describes the roles of short-signal sequences in the internalization, trafficking, and intracellular redistribution of GC-A/NPRA from cell surface to cell interior. Evidence indicates that, after internalization, the ligand-receptor complexes dissociate inside the cell and a population of GC-A/NPRA recycles back to the plasma membrane. Subsequently, the disassociated ligands are degraded in the lysosomes. However, a small percentage of the ligand escapes the lysosomal degradative pathway, and is released intact into culture medium. Using pharmacologic and molecular perturbants, emphasis has been placed on the cellular regulation and processing of ligand-bound GC-A/NPRA in terms of receptor trafficking and down-regulation in intact cells. The discussion is concluded by examining the functions of short-signal sequence motifs in the cellular life-cycle of GC-A/NPRA, including endocytosis, trafficking, metabolic processing, inactivation, and/or down-regulation in model cell systems.

Osmoregulation of natriuretic peptide receptors in bromoethylamine-treated rat kidney

Extracellular osmolarity is known as an important factor for the regulation of natriuretic peptide receptors (NPRs). We investigated the intra-renal osmoregulation of NPRs using renal medullectomized rats with bromoethylamine hydrobromide (BEA, 200mg/kg). The administration of BEA caused the decreased food intake and body weight. Water intake was decreased on the first day and then increased from the second day. Urine volume was persistently increased from the first day and free water clearance was also increased from the second day. Urinary excretions of sodium and potassium were decreased on the second day and then recovered to control level. Plasma levels of atrial natriuretic peptide (ANP) and Dendroaspis natriuretic peptide (DNP) in BEA-treated rats were not different from control rats. The inactive renin was increased. The maximum binding capacities of (125)I-ANP as well as (125)I-DNP decreased in glomeruli and medulla of BEA-treated rat kidneys but the binding affinity was not changed. In renal cortex, the gene expressions of ANP, NPR-A, and NPR-B were not changed but that of NPR-C decreased. In renal medulla, the gene expressions of NPR-A, -B, and -C decreased without change in ANP mRNA. Both renal medullary osmolarity and sodium concentration by BEA treatment were lower than those in control kidney. The cGMP concentrations in renal medulla and urine in BEA-treated rats were higher than those in control rats. These results suggest that the increased cGMP production may be partly involved in the decrease in NPRs mRNA expression and their binding capacities by BEA-induced medullectomy.

Regulation of guanylyl cyclase/natriuretic peptide receptor-A gene expression

Natriuretic peptide receptor-A (NPRA) is the biological receptor of the peptide hormones atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP). The level and activity of this receptor determines the biological effects of ANP and BNP in different tissues mainly directed towards the maintenance of salt and water homeostasis. The core transcriptional machinery of the TATA-less Npr1 gene, which encodes NPRA, consists of three SP1 binding sites and the inverted CCAAT box. This promoter region of Npr1 gene has been shown to contain several putative binding sites for the known transcription factors, but the functional significance of most of these regulatory sequences is yet to be elucidated. The present review discusses the current knowledge of the functional significance of the promoter region of Npr1 gene and its transcriptional regulation by a number of factors including different hormones, growth factors, changes in extracellular osmolarity, and certain physiological and patho-physiological conditions.

Calcium-dependent dephosphorylation mediates the hyperosmotic and lysophosphatidic acid-dependent inhibition of natriuretic peptide receptor-B/guanylyl cyclase-B

C-type natriuretic peptide binding to natriuretic peptide receptor-B (NPR-B) stimulates cGMP synthesis, which regulates vasorelaxation, cell proliferation, and bone growth. Here, we investigated the mechanistic basis for hyperosmotic and lysophosphatidic acid-dependent inhibition of NPR-B. Whole cell cGMP measurements and guanylyl cyclase assays indicated that acute hyperosmolarity decreased NPR-B activity in a reversible, concentration- and time-dependent manner, whereas chronic exposure had no effect. Acute hyperosmolarity elevated intracellular calcium in a concentration-dependent fashion that paralleled NPR-B desensitization. A calcium chelator, but not a protein kinase C inhibitor, blocked both calcium elevations and desensitization. Hyperosmotic medium stimulated NPR-B dephosphorylation, and the receptor was rapidly rephosphorylated and resensitized when the hypertonic media was removed. Lysophosphatidic acid also inhibited NPR-B in a calcium- and phosphorylation-dependent process, consistent with calcium being a universal regulator of NPR-B. The absolute requirement of dephosphorylation in this process was demonstrated by showing that a receptor with glutamates substituted at all known NPR-B phosphorylation sites is unresponsive to hyperosmotic stimuli. This is the first study to measure the phosphorylation state of an endogenous guanylyl cyclase and to link intracellular calcium elevations with its dephosphorylation.

Structure, Regulation, and Function of Mammalian Membrane Guanylyl Cyclase Receptors, With a Focus on Guanylyl Cyclase-A

Besides soluble guanylyl cyclase (GC), the receptor for NO, there are at least seven plasma membrane enzymes that synthesize the second-messenger cGMP. All membrane GCs (GC-A through GC-G) share a basic topology, which consists of an extracellular ligand binding domain, a short transmembrane region, and an intracellular domain that contains the catalytic (GC) region. Although the presence of the extracellular domain suggests that all these enzymes function as receptors, specific ligands have been identified for only three of them (GC-A through GC-C). GC-A mediates the endocrine effects of atrial and B-type natriuretic peptides regulating arterial blood pressure and volume homeostasis and also local antihypertrophic actions in the heart. GC-B is a specific receptor for C-type natriuretic peptide, having more of a paracrine function in vascular regeneration and endochondral ossification. GC-C mediates the effects of guanylin and uroguanylin on intestinal electrolyte and water transport and on epithelial cell growth and differentiation. GC-E and GC-F are colocalized within the same photoreceptor cells of the retina and have an important role in phototransduction. Finally, the functions of GC-D (located in the olfactory neuroepithelium) and GC-G (expressed in highest amounts in lung, intestine, and skeletal muscle) are completely unknown. This review discusses the structure and functions of membrane GCs, with special emphasis on the physiological endocrine and cardiac functions of GC-A, the regulation of hormone-dependent GC-A activity, and the relevance of alterations of the atrial natriuretic peptide/GC-A system to cardiovascular diseases.

The renal medullary interstitium: focus on osmotic hypertonicity

1. There has been continued interest in the functional role of the renal medullary interstitium and intense research in this area has furnished new information regarding the extent, dynamics and mechanisms determining fluctuations in medullary osmotic hypertonicity. 2. Any change in the tonicity (interstitial solute concentration) indicates an imbalance of the rate of solute delivery to the interstitium (by tubular transport) and solute removal therefrom (by the microcirculation). It is often difficult to establish whether alteration of the delivery or removal triggered the change in medullary tissue tonicity. 3. Newer in vivo studies have confirmed earlier predictions and indirect evidence indicating that the rate of NaCl transport in the ascending limb of the loop of Henle is the major determinant of medullary ionic hypertonicity. 4. The hypothesis of a 'washout' of medullary solutes during increased medullary blood flow (MBF) has been re-evaluated. A novel experimental approach has provided direct evidence of a modest dissipation of medullary solutes with increasing MBF and a modest accumulation of solutes with decreasing MBF. 5. Increasing evidence is reviewed indicating that medullary tonicity is not only a regulated variable, but also that it may itself modulate the activity of multiple local endocrine and paracrine control systems and thereby affect local microcirculation and the function of medullary interstitial and tubular cells.

Chronic hyperosmolality enhances ANP-dependent cGMP production via stimulation of transcription and protein synthesis in cultured rat IMCD cells

Recently, we found that hyperosmolality acutely inhibited atrial natriuretic peptide (ANP) dependent cGMP production by reducing ANP binding sites in cultured rat inner medullary collecting duct (IMCD) cells. Therefore, the present study was undertaken to evaluate the chronic effect of hyperosmolality on ANP dependent cGMPproduction in IMCD cells. Cell culture was carried out either in an iso-osmotic or hyperosmotic solution consisting of equi-isomolar NaCl and/or urea. Incubations with ANP and/or other agents were performed under the same osmotic conditions. ANP or SNP stimulated cGMP production was enhanced in a chronically hyperosmotic medium. These changes occurred in an osmolality-dependent manner. Hyperosmolality with sodium alone or with sodium and urea, but not with urea alone, was effective for the enhancement of ANP action. There was no significant difference between 125I-ANP specific bindings under iso-osmotic and hyperosmotic conditions. Incubation with cytoskeleton modulators did not affect ANP-dependent cGMP production stimulated by hyperosmolality. On the other hand, both actinomycin D, an inhibitor of transcription, and cycloheximide, an inhibitor of protein synthesis, prevented the stimulatory effects of hyperosmolality. The results suggest that chronic hyperosmolality enhances ANP-dependent cGMP production via stimulation of transcription and protein synthesis in cultured rat IMCD cells.

Osmoregulation of natriuretic peptide receptor signaling in inner medullary collecting duct. A requirement for p38 MAPK

In the inner medullary collecting duct of the terminal nephron, the type A natriuretic peptide receptor (NPR-A) plays a major role in determining urinary sodium content. This nephron segment, by virtue of its medullary location, is subject to very high levels of extracellular tonicity. We have examined the ability of medium tonicity to regulate the activity and expression of this receptor in cultured rat inner medullary collecting duct cells. We found that NaCl (75 mm) and sucrose (150 mm), but not urea (150 mm), increased natriuretic peptide receptor activity, gene expression, and promoter activity. The osmotic stimulus also activated extracellular signal-regulated kinase (ERK), c-Jun NH(2)-terminal kinase (JNK), and p38 mitogen-activated protein kinase (p38 MAPK). In the latter instance the beta isoform was selectively activated. Inhibition of p38 MAPK with SB203580 blocked the osmotic induction of receptor activity and expression, as well as receptor gene promoter activity, whereas inhibition of ERK with PD98059 had no effect. Cotransfection of p38 beta MAPK together with the receptor gene promoter resulted in amplification of the osmotic stimulation of the latter, whereas cotransfection of dominant negative MKK6, but not dominant-negative MEK, completely blocked the osmotic induction of receptor promoter activity. Collectively, the data indicate that extracellular osmolality stimulates receptor activity and receptor gene expression through a specific p38 beta-dependent mechanism, raising the possibility that changes in medullary tonicity could play an important role in the regulation of renal sodium handling in the terminal nephron.

Aquaporin-1: an osmoinducible water channel in cultured mIMCD-3 cells

The expressions of aquaporin-1 (AQP-1) in cultured mIMCD-3 cells were studied. There was no detectable AQP-1 in cells grown in serum-containing growth medium (SM, 297 +/- 2 mOsm/kg. H2O). When SM was supplemented with NaCl (406 +/- 2 mOsm/kg. H2O), cellular AQP-1 was induced. A further increase in medium osmolarity with NaCl (493 +/- 3 mOsm/kg. H2O) had conferred cells an 2.5 to 3-fold increase of AQP-1 expression over those grown in the 406 +/- 2 mOsm/kg. H2O medium. Moreover, AQP-1 was found to be translocated from cytosol to membrane. In addition, exposing the mIMCD-3 cells to vasopressin (AVP, 10(-8) M) and/or NaCl-supplemented serum-free media (496 +/- 3 mOsm/kg. H2O) for 6h did not render them to produce AQP-1. However, AQP-1 was induced after 24h of incubation, with an 1.5-fold additive effect by AVP. Our RT-PCR data had confirmed the NaCl inducibility and AVP synergism in AQP-1 expression at both mRNA and protein levels. This suggests a new role for cellular AQP-1 and AVP in overcoming osmotic stress in an in vitro system.

Functional significance of cell volume regulatory mechanisms

To survive, cells have to avoid excessive alterations of cell volume that jeopardize structural integrity and constancy of intracellular milieu. The function of cellular proteins seems specifically sensitive to dilution and concentration, determining the extent of macromolecular crowding. Even at constant extracellular osmolarity, volume constancy of any mammalian cell is permanently challenged by transport of osmotically active substances across the cell membrane and formation or disappearance of cellular osmolarity by metabolism. Thus cell volume constancy requires the continued operation of cell volume regulatory mechanisms, including ion transport across the cell membrane as well as accumulation or disposal of organic osmolytes and metabolites. The various cell volume regulatory mechanisms are triggered by a multitude of intracellular signaling events including alterations of cell membrane potential and of intracellular ion composition, various second messenger cascades, phosphorylation of diverse target proteins, and altered gene expression. Hormones and mediators have been shown to exploit the volume regulatory machinery to exert their effects. Thus cell volume may be considered a second message in the transmission of hormonal signals. Accordingly, alterations of cell volume and volume regulatory mechanisms participate in a wide variety of cellular functions including epithelial transport, metabolism, excitation, hormone release, migration, cell proliferation, and cell death.

Dipyridamole enhances interleukin-1beta-stimulated nitric oxide production by cultured rat vascular smooth muscle cells

We examined whether dipyridamole affected interleukin-1beta-stimulated nitric oxide (NO) production by cultured rat vascular smooth muscle cells. Interleukin-1beta stimulated the production of nitrite and nitrate, stable metabolites of NO, in a dose- and time-dependent manner in vascular smooth muscle cells. Dipyridamole (1-100 mu M) enhanced interleukin-1beta-induced nitrite production in a dose- and time-dependent manner. The mRNA expression of inducible NO synthase was up-regulated by dipyridamole (0.3-10 mu M) treatment in a dose-dependent manner. Both 8-bromo-guanosine 3',5'-cyclic monophosphate (8-bromo-cGMP) and dibutyryl adenosine 3',5'-cyclic monophosphate (db-cAMP) enhanced the nitrite production in the presence of interleukin-1beta. Dipyridamole up-regulated the effect of both 8-bromo-cGMP and db-cAMP on the interleukin-1beta-induced nitrite production. Dipyridamole increased the intracellular cAMP content in the presence of interleukin-1beta (10 ng/ml), but did not affect the intracellular cGMP content. 8R*,9S*,11S*-(-)-9-hydroxy-9-n-hexyloxy-8-methyl-2,3,9,10- tetrahydro-8,11-epoxy-1H,8H,11H-2,7b,11a-triazadibenzo-(a,g)-cy cloocta ++-(c,d,e)-trinden-1-one (KT 5720), a selective inhibitor of cAMP-dependent protein kinase, abolished the enhancement of interleukin-1beta-induced nitrite production by dipyridamole, whereas 8R*,9S*,11S*-(-)-9-methoxy-carbamyl-8-methyl-2,3,9,10-tetrahydro-8,11-ep oxy-1H,8H,11H-2,7b,11a-trizadibenzo-(a,g)-cyclo-octa-(c,d,e)-tr inden-1-one (KT 5823), an inhibitor of cGMP-dependent protein kinase, did not attenuate the enhancement. Furthermore, Rolipram and 4-(3-butoxy-4-methoxybenzyl)-2-imidazolidinone (Ro-20-1724), cAMP-specific phosphodiesterase type IV inhibitors, augmented the interleukin-1beta-induced nitrite production. We concluded that dipyridamole enhanced the interleukin-1beta-induced NO production via an increase in intracellular cAMP content in cultured rat vascular smooth muscle cells.

Losses, maintenance and recruitment of patients in a peritoneal dialysis home training program

The St. Peter Hospital Kidney Dialysis Center began a peritoneal dialysis home training program in 1988, and demonstrated steady growth until the end of its sixth year at which time the census plummeted. Peritoneal dialysis staff sought to determine a cause for the reduction in census by evaluating the reasons for drop-out. They concluded that drop-out rates were not unexpected or unusual and that maintenance and referrals are influenced by: (a) patient predialysis education and (b) continual conversation and collaboration between the nephrologists and peritoneal dialysis home training staff.