Regulation of atrial natriuretic peptide receptors in glomeruli during chronic salt loading

Mechanisms of Oxidative Stress-Induced Increase in Salt Sensitivity and Development of Hypertension in Sprague-Dawley Rats

High salt intake produces vascular changes that contribute to the development of hypertension in salt-sensitive individuals. Because reactive oxygen species play a role in the pathogenesis of cardiovascular diseases, we investigated whether oxidative stress contributes to salt-sensitive hypertension. Sprague-Dawley rats were divided in different groups and received tap water (vehicle), 30 mmol/L of l -buthionine sulfoximine ([BSO] an oxidant), high salt ([HS] 1% NaCl), and BSO plus HS without and with antioxidant tempol (1 mmol/L) in drinking water for 12 days. Compared with vehicle, BSO treatment caused oxidative stress and mild increase in blood pressure. Thoracic aortic rings from BSO-treated rats exhibited decreased response to endothelium-independent vasorelaxants. In HS-treated rats, the response to vasoactive agents, as well as blood pressure, was unaffected. Concomitant treatment of rats with BSO and HS produced a marked increase in blood pressure and a decreased response to both endothelium-dependent and endothelium-independent vasorelaxants with an increase in EC 50 . Incubation of aortic tissue from BSO-treated rats with sodium nitroprusside showed decreased cGMP accumulation, whereas HS rats had decreased basal NO synthase activity. Tempol decreased oxidative stress, normalized blood pressure, and restored NO signaling and responses to vasoactive compounds in BSO and BSO plus HS rats. We conclude that BSO increases oxidative stress and reduces NO signaling, whereas HS reduces NO levels by decreasing the NO synthase activity. These phenomena collectively result in reduced responsiveness to both endothelium -dependent and endothelium- independent vasorelaxants and may contribute to salt-sensitive hypertension.

Indomethacin decreases particulate guanylyl cyclase activity in rat kidney

1. Effects of non-steroidal anti-inflammatory drugs on the local atrial natriuretic peptide (ANP) and nitric oxide (NO) systems in the kidney were investigated. 2. Male Sprague-Dawley rats were treated with indomethacin (5 mg/kg, every 12 h, i.p.) for 2 days. The expression of ANP and natriuretic peptide receptor-A (NPR-A) mRNA was determined in the kidney, as was that of endothelial NO synthase (NOS) proteins. Particulate and soluble guanylyl cyclase activities were determined separately. 3. Following treatment with indomethacin, urinary sodium excretion decreased significantly. Although the renal expression of ANP was not changed significantly, that of NPR-A decreased significantly. The expression of NOS increased significantly. Particulate guanylyl cyclase activity was decreased, whereas the activity of soluble guanylyl cyclase was increased. The catalytic activity of Na(+)/K(+)-ATPase was increased, with no significant changes in its expression. The expression of the type 3 Na/H exchanger and Na-K-2CL cotransporters increased significantly. 4. The indomethacin-induced decrease in urinary sodium excretion may be attributed, at least in part, to decreased activity of the local ANP/cGMP system. The increased activity of the NO/cGMP system may be a compensatory response to the diminished activity of the prostaglandin system.

Intracerebroventricular infusion of hypertonic NaCl increases urinary CGMP in healthy and cirrhotic rats

Implication of serum atrial natriuretic peptide (ANP) and endothelin-1 (ET1) in the central nervous system (CNS)-induced natriuresis and hypertension respectively, was investigated in healthy and cirrhotic rats. Both healthy and nonascitic CCl(4)-induced cirrhotic rats under pentobarbital anesthesia received either normotonic (140 mmol/L) or hypertonic (320 mmol/L) NaCl artificial cerebrospinal fluid into the CNS lateral ventricle at a rate of 8.3 microl/min for 120 min. A sham operated group, but not centrally infused, served as matched control. Hypertonic NaCl solution significantly increased mean arterial pressure (MAP) similarly in both healthy (n = 5) ((MAP: 16 mm Hg, 13%) and cirrhotic rats (n = 6) ((MAP: 20 mm Hg, 15%) (ANOVA, p <.001) although the latter showed a slower increment. Under hypertonic NaCl infusion, natriuresis was also significantly increased in a similar manner in both healthy (U (Na) V: baseline: 0.38 +/- 0.22 micromol/min x 100 g; experiment: 2.36 +/- 0.90 micromol/min x 100 g; mean +/- SD) and cirrhotic rats (0.69 +/- 0.48 vs. 3.16 +/- 0.87; p <.001). By contrast, central hypertonic NaCl solutions did not show a significant modification of serum ANP in neither healthy (62 +/- 18 fmol/ml vs. 51 +/- 17 fmol/ml) nor cirrhotic rats (126 +/- 61 vs. 115 +/- 30). Likewise, ET-1 was not significantly modified under central hypertonic NaCl infusion in neither healthy (352 +/- 46 pg/ml vs. 344 +/- 39 pg/ml) nor cirrhotic rats (287 +/- 58 vs. 277 +/- 61). Despite no modification in serum ANP, there was a significant increment in urinary excretion of cGMP under central hypertonic NaCl infusions in bo th healthy (6.8 +/- 4.1 pmol/min x 100 g vs. 13.0 +/- 6.5 pmol/min x 100 g; p <.05) and cirrhotic rats (8.6 +/- 1.7 vs. 11.1 +/- 1.3; p <.05). Our data indicate the preservation of the mechanisms of central natriuresis in a model of non-ascitic CCl(4 )-induced cirrhosis in rats. An increment in urinary cGMP could potentially be implicated in the natriuretic response obtained by intracerebroventricular hypertonic NaCl stimulus in both healthy and cirrhotic rats. The lack of modification of serum ANP and ET-1 does not appear to support a systemic implication of these peptides in the natriuretic and hypertensive responses respectively induced by this manoeuvre.

Role of natriuretic peptide receptor type C in Dahl salt-sensitive hypertensive rats

The natriuretic peptide system is suggested to be involved in the pathogenesis of salt-sensitive hypertension; a recent report indicated that disruption of the atrial natriuretic peptide precursor gene caused salt-sensitive hypertension. However, natriuretic peptide receptor (NPR)-A knockout mice did not show enhanced salt sensitivity of blood pressure. The aim of the present study was to investigate the role of NPR-C, the other receptor for atrial natriuretic peptide, in increased salt sensitivity of blood pressure. Dahl salt-sensitive (DS) and salt-resistant (DR) rats were placed on a 0.3% or 8% NaCl diet for 4 weeks. Blood pressure was elevated by salt loading only in DS rats. RNase protection assay demonstrated that NPR-C transcript level in the kidney was reduced by chronic salt loading in both DR and DS rats, whereas expression of NPR-A and NPR-B was not altered. The reduction of NPR-C mRNA in response to salt loading was enhanced in DS compared with DR rats. In situ hybridization indicated that the salt-induced NPR-C change was attributed mainly to suppressed expression of NPR-C in the podocytes. NPR-C gene expression was regulated by salt loading in a tissue-specific manner; the marked decrease in NPR-C mRNA by salt loading was seen only in the kidney. These data suggest that the exaggerated salt-induced reduction of NPR-C in the kidney of DS rats may play an important role in the pathogenesis of salt hypertension in this animal, possibly related to impaired renal sodium excretion.

Ontogeny of ovine fetal renal atrial natriuretic factor receptors

Ovine fetal renal responses to ANF decrease during the last third of gestation, although circulating fetal plasma atrial natriuretic factor (ANF) levels are higher than in the maternal circulation, and do not change with gestation. This study examined whether previously reported maturational changes in fetal renal responses to ANF are due to changes in renal ANF receptor numbers and/or affinity during gestation. ANF receptor numbers (Bmax) and dissociation constants (Kd) were measured in isolated renal glomeruli from early (95 and 110 day; mean 103 +/- 2) and late gestation (131 and 145 day; mean 138 +/- 2) fetal and maternal sheep. Fetal renal ANF receptor Bmax values significantly increased between 103 and 138 days gestation (13 +/- 3 to 29 +/- 4 fmol/mg protein) but were significantly lower than maternal values (60 +/- 13 fmol/mg protein). Fetal ANF receptor Kd values also increased significantly (245 +/- 34 to 370 +/- 36 pM), with early gestation values significantly lower than maternal values (470 +/- 69 pM). Thus, the blunted fetal renal response to ANF in late as compared to early gestation is not due to reduced ANF receptor numbers. Rather, an increased proportion of ANF clearance receptors, reduced post-receptor function and/or altered intrarenal hemodynamics may contribute.

Renal receptors and effects of atrial natriuretic factor in compensatory renal hypertrophy

In the present study we investigated the in vivo and in vitro renal responsiveness to ANF, and the adaptation of ANF receptors in compensatory renal hypertrophy in the rat. One week after left nephrectomy (UNx), plasma levels of immunoreactive ANF, blood pressure (MAP), hematocrit (Hct), and urine flow rate (V) were unaltered compared to control (C) rats. Baseline GFR and potassium excretion (UKV) were significantly higher, and sodium excretion (UNaV) tended to be elevated in UNx rats. Administered ANF led to similar dose-related decreases in MAP and increases in Hct in UNx and C rats. However, at each dose of infused ANF, absolute values and the increase in GFR and UNaV were higher in UNx than in C rats. Hypertrophied (H) kidneys were removed from UNx and perfused in vitro to determine distribution and density of ANF receptors, responsiveness to ANF, and receptor-mediated organ clearance of 125I-ANF1-28. The density of ANF receptors in cortex, outer medulla, and papilla of H kidneys was not significantly different from that in C kidneys. In H isolated kidneys, ANF led to dose-related increases in GFR, V, UNaV, and UKV that were indistinguishable (P greater than 0.05) from those in C kidneys. Receptor-mediated organ clearance of 125I-ANF1-28 in isolated H kidneys was 2.8 +/- .02 ml/min, a value not significantly different (P greater than 0.05) from that in C kidneys.(ABSTRACT TRUNCATED AT 250 WORDS)

Atrial natriuretic factor: its (patho)physiological significance in humans

The first human studies using relatively high-doses of ANF revealed similar effects as observed in the preceding animal reports, including effects on systemic vasculature (blood pressure fall, decrease in intravascular volume), renal vasculature (rise in GFR, fall in renal blood flow), renal electrolyte excretion (rises in many electrolytes), and changes in release of a number of different hormones. Whether all these changes are the result of direct ANF effects or secondary to a (single) primary event of the hormone remains to be determined. Certainly, it has been proven that more physiological doses of ANF fail to induce short-term changes in many of these parameters leaving only a rise in hematocrit, natriuresis and an inhibition of the RAAS as important detectable ANF effects in humans. This leads us to hypothesize that ANF is a "natriuretic" hormone with physiological significance. The primary function in humans is to regulate sodium homeostasis in response to changes in intravascular volume (cardiac atrial stretch). Induction of excess renal sodium excretion and extracellular volume shift appear to be the effector mechanisms. The exact mechanism of the natriuresis in humans still needs to be resolved. It appears however, that possibly a small rise in GFR, a reduction in proximal and distal tubular sodium reabsorption, as well as an ensuing medullary washout, are of importance. The pathophysiological role of ANF in human disease is unclear. One may find elevated plasma irANF levels and/or decreased responses to exogenous ANF in some disease states. Whether these findings are secondary to the disease state rather than the cause of the disease remains to be resolved. Therapeutic applications for ANF, or drugs that intervene in its production or receptor-binding, seem to be multiple. Most important could be the antihypertensive effect, although areas such as congestive heart failure, renal failure, liver cirrhosis and the nephrotic syndrome cannot be excluded. Although the data that have been gathered to date allowed us to draw some careful conclusions as to the (patho)physiological role of ANF, the exact place of ANF in sodium homeostatic control must still be better defined. To achieve this, we will need more carefully designed low-dose ANF infusion, as well as ANF-breakdown inhibitor studies. Even more promising, however, is the potential area of studies open to us when ANF-receptor (ant)agonists become available for human use.

Atrial natriuretic peptide in portal vein-ligated rats: alterations in cardiac production, plasma level and glomerular receptor density and affinity

The atrial natriuretic peptide hormonal system is altered to a variable degree in patients with cirrhosis. Portal pressure and portal-systemic shunting are also varied in cirrhosis. We used a portal vein-ligated rat model with predictable portal hypertension to study the effects of portal hypertension alone on the atrial natriuretic peptide hormonal system. Sham-operated rats were used as controls. Mean portal pressure was significantly increased in portal vein-ligated rats (portal vein-ligated rats, 21.7 +/- 0.74 cm H2O; sham-operated rats, 13.7 +/- 0.47 cm H2O; p less than 0.0001). Plasma atrial natriuretic peptide decreased 50% in the portal vein-ligated rats (p less than 0.0001). Atrial natriuretic peptide messenger RNA level was decreased by 40% to 60% in the left and right atria and in the ventricles of portal vein-ligated rats (p less than 0.05 for each chamber). Only one class of glomerular binding site was identified by competitive binding studies. The atrial natriuretic peptide glomerular receptor density increased in the portal vein-ligated rats (portal vein-ligated rats, 1,660 +/- 393; sham-operated 725 +/- 147 fmol/mg protein, p less than 0.02), whereas affinity decreased (portal vein-ligated, 1.69 +/- 0.49; sham-operated, 0.55 +/- 0.12 nmol/L, p less than 0.02). No difference was seen in the amount of cyclic GMP generated by atrial natriuretic peptide stimulation in isolated glomeruli from portal vein-ligated and sham-operated rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Increased glomerular atrial natriuretic peptide receptor affinity in experimental nephrotic syndrome

Atrial natriuretic peptide (ANP) is a cardiac hormone with natriuretic and diuretic effects. To better define the ANP hormonal system in the nephrotic syndrome, a condition associated with renal sodium retention, we undertook a study of glomerular ANP receptors in rats with puromycin aminonucleoside-induced nephrotic syndrome and in pair-fed controls. Nephrotic rats had significantly decreased serum albumin and total protein and significantly increased serum cholesterol, triglycerides and 24 hour urinary protein excretion. Plasma level of atrial natriuretic peptide was similar in both groups of rats. Competition binding inhibition studies in isolated glomeruli demonstrated one binding site in both groups of rats. The density of ANP binding sites in isolated glomeruli was similar in nephrotic and pair-fed rats while the binding affinity was increased significantly in the nephrotic rats. This is the first study to demonstrate alterations in renal ANP receptors in the nephrotic syndrome. Further studies will be necessary to determine whether alterations in glomerular ANP receptors contribute to renal sodium retention in the nephrotic syndrome.

Effects of atrial natriuretic peptide on systemic and renal hemodynamics and renal excretory function in patients with chronic renal failure

We examined the effects of 60 min alpha-hANP infusion (24 ng/min/kg) on glomerular filtration rate (GFR), renal blood flow (RBF), cardiac index (CI) and blood pressure (BP) in 8 patients with chronic renal failure (CRF) with GFR ranging from 18 to 80 ml/min/1.73 m2 and in 8 control (C) subjects with normal renal function. Basal plasma levels of ANP and cGMP were elevated in CRF (ANP: 60.6 +/- 9.1 vs 13.6 +/- 1.9 pmol/l, p less than 0.05; cGMP: 14.3 +/- 2.9 vs 6.6 +/- 1.1 pmol/ml, p less than 0.05). During ANP infusion, peak levels of cGMP were higher in CRF than in C (27.5 +/- 3.2 vs. 17.3 +/- 1.3 pmol/ml, p less than 0.05). During ANP infusion, GFR increased in CRF by 70.7 +/- 4.2% from 34.5 +/- 6.8 to 57.4 +/- 9.9 ml/min/1.73 m2 (p less than 0.001) as compared to 16.2 +/- 1.4% in C (p less than 0.001 vs CRF). RBF increased in CRF by 43.6 +/- 6.4% and in C by 3.1 +/- 1.2% (p less than 0.01). Basal urinary sodium excretion (UNaV) was slightly lower in CRF than in C but rose to the same level in both groups during ANP infusion. In CRF, as opposed to C, UNaV remained elevated above baseline after the end of the infusion. The effect of ANP on fractional sodium excretion (FENa), however, was more pronounced in C.(ABSTRACT TRUNCATED AT 250 WORDS)