Atrial natriuretic peptide inhibits mineralocorticoid receptor function in rat colonic surface cells.

Atrial natriuretic peptide (ANP) inhibits and aldosterone (ALDO) stimulates Na conductive transport. Therefore, the effects of ANP and its second messenger cGMP on mineralocorticoid receptor (MR) function in rat colon surface and crypt cells were examined. 100 nM 8-Br-cGMP decreased surface [3H]ALDO binding by 42 +/- 4% but increased crypt [3HvALDO binding by 52+/-16%. ANP decreased surface [3H]ALDO binding by approximately 50% after a 2.5-h lag period but had no effect on crypt ALDO binding. ANP and cGMP rapidly (< 15 min) inhibited surface cell ALDO-induced MR nuclear translocation but did not affect crypt MR nuclear translocation. Inhibition of cGMP-dependent protein kinase with KT5823 blocked the inhibitory effects of ANP and 8-Br-cGMP on surface cell ALDO binding and MR nuclear translocation. In crypt, KT5823 increased baseline [3H]ALDO binding but did not inhibit the stimulatory effect of exogenous cGMP. DEAE-cellulose chromatography and gel mobility shift assay showed that ANP did not inhibit surface MR activation. ANP inhibited ALDO stimulated short circuit current in distal colon. These data demonstrate cell-specific regulation of MR function. In surface cells, ANP rapidly inhibits MR nuclear translocation and ALDO-induced short circuit current. ANP inhibition of MR function may be an additional mechanism of ANP antagonism of Na reabsorption.     

Downregulation of natriuretic peptide clearance receptor mRNA in vascular smooth muscle cells by angiotensin II

Angiotensin II can downregulate atrial natriuretic peptide binding to rat vascular smooth muscle cells (VSMCs), but the mechanism is not known. Because protein kinase C (PKC) mimetic phorbol myristate acetate (PMA) can destabilize natriuretic peptide clearance receptor (NPR‐C) mRNA and angiotensin II activates several PKC isoforms in VSMCs, we hypothesized that angiotensin II treatment decreases NPR‐C mRNA stability and exerts this effect through PKC. This study demonstrated that angiotensin II induced time‐ and concentration‐dependent downregulation of NPR‐C, which was completely inhibited by an angiotensin II type I receptor blocker losartan. NPR‐C mRNA disappearance rate over 6 h was nearly doubled by exposure of VSMCs to 100 nm angiotensin II, compared with that observed after inhibition of RNA synthesis alone. However, this response to angiotensin II was undiminished by the PKC inhibitor chelerythrine, or by depletion of PKC by prior exposure of cells to PMA for 48 h. Inhibitors of tyrosine kinases, phospholipase C, or mitogen‐activated protein kinase kinase also failed to reverse the angiotensin II effect. We conclude that at least two distinct proximal signaling pathways, one involved and one independent of phorbol ester‐sensitive protein kinase C, lead to downregulation of NPR‐C gene expression by destabilizing its mRNA.     

Physiologic regulation of atrial natriuretic peptide receptors in rat renal glomeruli.

Isolated rat renal glomeruli and cultured glomerular mesangial and epithelial cells were examined for atrial natriuretic peptide (ANP) receptors, and for ANP-stimulated cyclic guanosine monophosphate (cGMP) generation. In glomeruli from normal rats, human (1-28) 125I-ANP bound to a single population of high affinity receptors with a mean equilibrium dissociation constant of 0.46 nM. Human (1-28) ANP markedly stimulated cGMP generation, but not cAMP generation in normal rat glomeruli. Analogues of ANP that bound to the glomerular ANP receptor with high affinity stimulated cGMP accumulation, whereas the (13-28) ANP fragment, which failed to bind to the receptor, was devoid of functional activity. Cell surface receptors for ANP were expressed on cultured glomerular mesangial but not epithelial cells, and appreciable ANP-stimulated cGMP accumulation was elicited only in mesangial cells. Approximately 12,000 ANP receptor sites were present per mesangial cell, with an average value for the equilibrium dissociation constant of 0.22 nM. Feeding of a low-salt diet to rats for 2 wk resulted in marked up regulation of the glomerular ANP receptor density to a mean of 426 fmol/mg protein, compared with 116 fmol/mg in rats given a high-salt diet. A modest reduction in the affinity of glomerular ANP receptors was also observed in rats fed the low-salt diet. ANP-stimulated cGMP generation in glomeruli did not change with alterations in salt intake. We conclude that high salt feeding in the rat results in reduced glomerular ANP receptor density relative to values in salt restricted rats. Furthermore, the mesangial cell is a principal target for ANP binding in the glomerulus.     

Brain natriuretic peptide as a novel cardiac hormone in humans. Evidence for an exquisite dual natriuretic peptide system, atrial natriuretic peptide and brain natriuretic peptide.

Using a specific radioimmunoassay for human brain natriuretic peptide (hBNP) with a monoclonal antibody, we have investigated its synthesis, secretion, and clearance in comparison with those of atrial natriuretic peptide (ANP) in normal subjects and patients with congestive heart failure (CHF). Mean BNP-like immunoreactivity (-LI) levels in normal atrium and ventricle were 250 and 18 pmol/g, respectively. The plasma BNP-LI level in normal subjects was 0.90 +/- 0.07 fmol/ml, which was 16% of the ANP-LI level. In contrast, the plasma BNP-LI level markedly increased in patients with CHF in proportion to its severity, and surpassed the ANP-LI level in severe cases. There was a significant step-up of the plasma BNP-LI level in the coronary sinus (CS) compared with that in the aortic root (Ao) and the difference between these BNP-LI levels, delta(CS-Ao)BNP, also increased with the severity of CHF. In addition, the step-up of the BNP-LI level in the anterior interventricular vein [delta(AIV-Ao)BNP] was comparable to delta(CS-Ao)BNP, indicating that BNP is secreted mainly from the ventricle. Predominant BNP synthesis in the ventricle was also confirmed by Northern blot analysis. Catheterization and pharmacokinetic studies revealed that hBNP is cleared from the circulation more slowly than alpha-hANP; this was in part attributed to lower (about 7%) binding affinity of hBNP to clearance receptors than that of alpha-hANP. A predominant molecular form of BNP-LI in the heart and plasma was a 3-kD form corresponding to hBNP. These results indicate that BNP is a novel cardiac hormone secreted predominantly from the ventricle, and that the synthesis, secretion and clearance of BNP differ from those of ANP, suggesting discrete physiological and pathophysiological roles of BNP in a dual natriuretic peptide system.     

Pulsatile secretion of atrial natriuretic peptide and brain natriuretic peptide in healthy humans.

Both atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP) are involved in sodium and water homoeostasis in healthy humans. The plasma concentrations of the natriuretic peptides can be used to differentiate between dyspnoea of cardiac and pulmonary origin, and the degree of elevation of the peptide levels in the plasma in heart failure is a measure of the severity of the disease. However, the patterns of secretion of ANP and BNP are not clear either in healthy humans or in patients. The purpose of the present study was to test the hypotheses that both ANP and BNP are secreted in pulses in healthy humans, and that this phenomenon can be revealed by determination of ANP and BNP in peripheral venous blood samples. In 12 healthy subjects, blood samples were drawn every 2 min through an intravenously inserted plastic needle over a period of 2 h. Plasma concentrations of ANP and BNP were determined by RIAs, and the results were analysed for pulsatile behaviour by Fourier transformation. Pulsatile secretion of ANP was seen in 10 out of 12 subjects [nu=0.028 min(-1) (median; range 0.013-0.047 min(-1)), i.e. a pulse of ANP with an interval of 36 min (range 21-77 min)]. Pulsatile secretion of BNP was seen in nine out of 12 patients [nu=0. 021 min(-1) (range 0.013-0.042 min(-1)), i.e. a pulse of BNP with an interval of 48 min (range 24-77 min)]. The main conclusion is that the secretion patterns of both ANP and BNP are pulsatile in most healthy humans. Consequently, it is important to study whether pulsatile secretion also occurs in heart failure in order to obtain the most informative predictive values both in the differential diagnosis of dyspnoea and in the evaluation of the severity of the disease.     

Atrial natriuretic peptide protects against acute ischemic renal failure in the rat.

Because of its ability to increase glomerular filtration, antagonize the actions of vasoconstrictors, and produce vasodilation, alpha human atrial natriuretic peptide (alpha-hANP) was evaluated for its potentially beneficial effects in experimental ischemic renal failure induced by 45-60 min of renal artery occlusion in bilaterally or unilaterally renally intact Sprague-Dawley rats. After ischemia, a 4-h intrarenal infusion of alpha-hANP restored 14C-inulin clearances in bilaterally and unilaterally intact animals from 0.05 +/- 0.006 and 0.05 +/- 0.01 ml/min per 100 g to 0.314 +/- 0.04 and 0.25 +/- 0.01 ml/min per 100 g, respectively (P less than 0.001, n = 8), compared with normal values of 0.49 +/- 0.023 ml/min per 100 g. Histologically, there was a progressive decrease in medullary hyperemia and prevention of intratubular cell shedding and granulocyte margination as a result of the 4-h alpha-hANP infusion such that after 24 and 48 h the histological appearance of the tissue was essentially normal. The results show that a 4-h intrarenal infusion of alpha-hANP after renal ischemia can preserve glomerular filtration rate and reduce renal tissue damage.     

Plasma N-terminal pro-brain natriuretic peptide and brain natriuretic peptide in assessment of acute dyspnea.

We examined the analytical correlation between non-radioimmunometric plasma N-terminal pro-brain natriuretic peptide (NT-proBNP) and B-type natriuretic peptide (BNP) and evaluated whether NT-proBNP or BNP was superior in the emergency diagnosis of heart failure and whether this was influenced by age, gender, body mass index (BMI) and renal function. Data were collected prospectively from patients admitted to the emergency department for acute dyspnea. Plasma BNP (Triage, Biosite) and NT-proBNP (Elecsys, Roche diagnostic(R)) were measured at admission in addition to other standard biological parameters and clinical variables. Reference diagnosis was adjudicated by two independent cardiologists using the European society of cardiology guidelines. We evaluated the influence of creatinine clearance, age, gender and BMI on plasma BNP and NT-proBNP levels. One hundred and sixty consecutive patients were included: 84 females and 76 males, mean age 80.1 + 13.5 (16-98). The analytical correlation between the automated electro-chemiluminescence immunoassay for NT-proBNP and the single use fluorescence immunoassay for BNP was satisfactory using the equation: NT-proBNP = 1.1 BNP + 0.57 and a correlation r = 0.93. This was established over a wide range of concentration (5-6400 pg/ml for BNP). Areas under receiver operating characteristic (ROC) curve for BNP and NT-proBNP as a diagnostic marker for heart failure were 0.82 and 0.84, respectively and a BNP level of 150 pg/ml has similar sensitivity and specificity that NT-proBNP level of 1000 pg/ml. The correlation was not influenced by age, gender and BMI of patients. Renal dysfunction did not affect significantly this correlation (r = 0.93). We conclude that NT-proBNP, as assayed in the present study, correlates closely with BNP. This correlation is only slightly modulated by creatinine clearance values. The NT-proBNP appears as accurate as BNP according to area under ROC curve. Used in conjunction with other clinical information, rapid measurement of BNP or NT-proBNP is useful in establishing or excluding the diagnosis of congestive heart failure in patients with acute dyspnea.     

Pressure-independent enhancement of cardiac hypertrophy in natriuretic peptide receptor A-deficient mice

Mice lacking natriuretic peptide receptor A (NPRA) have marked cardiac hypertrophy and chamber dilatation disproportionate to their increased blood pressure (BP), suggesting, in support of previous in vitro data, that the NPRA system moderates the cardiac response to hypertrophic stimuli. Here, we have followed the changes in cardiac function in response to altered mechanical load on the heart of NPRA-null mice (Npr1-/-). Chronic treatment with either enalapril, furosemide, hydralazine, or losartan were all effective in reducing and maintaining BP at normal levels without affecting heart weight/body weight. In the reverse direction, we used transverse aortic constriction (TAC) to induce pressure overload. In the Npr1-/- mice, TAC resulted in a 15-fold increase in atrial natriuretic peptide (ANP) expression, a 55% increase in left ventricular weight/body weight (LV/BW), dilatation of the LV, and significant decline in cardiac function. In contrast, banded Npr1+/+ mice showed only a threefold increase in ANP expression, an 11% increase in LV/BW, a 0.2 mm decrease in LV end diastolic dimension, and no change in fractional shortening. The activation of mitogen-activated protein kinases that occurs in response to TAC did not differ in the Npr1+/+ and Npr1-/- mice. Taken together, these results suggest that the NPRA system has direct antihypertrophic actions in the heart, independent of its role in BP control.     

Atrial natriuretic peptide transcription, secretion, and glomerular receptor activity during mineralocorticoid escape in the rat

The mechanisms that mediate renal "escape" from the sodium-retaining effects of mineralocorticoids are incompletely understood. This study was undertaken to determine whether atrial natriuretic peptide (ANP) may play a role in the escape phenomenon. Immunoreactive ANP in rat plasma increased 2.5-fold above baseline values at 12 and 24 h after a single depot injection of desoxycorticosterone acetate in oil and returned to baseline thereafter. In addition, specific pre-pro-ANP messenger RNA content in rat atria was significantly elevated as early as 12 h after mineralocorticoid administration and remained elevated at 24, 48, and 72 h, indicating a prompt and sustained increase in ANP biosynthesis. Renal glomerular ANP receptor density was down-regulated appropriately with rising plasma ANP levels, and receptor affinity was unchanged. Thus, mineralocorticoid administration in the rat is a powerful stimulus for ANP release and for atrial myocyte ANP synthesis, which suggests a potential role for this hormone in overriding mineralocorticoid-induced renal sodium retention.     

Clearance of human brain natriuretic peptide in rabbits; effect of the kidney, the natriuretic peptide clearance receptor, and peptidase activity

Although the synthetic version of the cardiac peptide human brain natriuretic peptide (hBNP) has demonstrated beneficial cardiovascular effects in clinical studies, little is known about mechanisms governing its elimination from the blood. This study measured the role of the kidney, the natriuretic peptide clearance (NP-C) receptor, and peptidase digestion on the elimination of synthetic hBNP from the plasma compartment of rabbits. The estimated plasma steady state resulting from a continuous i.v. infusion was achieved within 50 min and was related in a linear manner with the infusion rate of the drug. Complete restriction of kidney blood flow by bilateral suture-ligation of the renal arteries compared with sham-treated animals reduced the clearance of hBNP by approximately half (24 +/- 9 ml/min versus 47 +/- 14 ml/min, respectively, p <. 007). Pharmacological blockade of the NP-C receptor with a clearance receptor-specific analog of atrial natriuretic peptide increased in a statistically significant and dose-related manner the plasma steady-state level of hBNP during continuous i.v. infusion of hBNP (maximum effect of 1.9 +/- 0.3-fold, p <.01). The peptidase inhibitor phosphoramidon increased in a dose-related manner the plasma steady-state level of hBNP 1.7 +/- 0.4-fold during continuous i.v. infusion of hBNP in rabbits. These data suggest that the kidney, the NP-C receptor, and peptidases are all important in the elimination of hBNP from the plasma compartment.