Atrial natriuretic factor receptors in rat kidney, adrenal gland, and brain: autoradiographic localization and fluid balance dependent changes

The ANF-RGC gene motif (669)WTAPELL(675) is vital for blood pressure regulation: biochemical mechanism

ANF-RGC is the prototype membrane guanylate cyclase, both the receptor and the signal transducer of the hormones ANF and BNP. After binding them at the extracellular domain, it, at its intracellular domain, signals activation of the C-terminal catalytic module and accelerates production of the second messenger, cyclic GMP. This, in turn, controls the physiological processes of blood pressure, cardiovascular function, fluid secretion, and others: metabolic syndrome, obesity, and apoptosis. The biochemical mechanism by which this single molecule controls these diverse processes, explicitly blood pressure regulation, is the subject of this study. In line with the concept that the structural modules of ANF-RGC are designed to respond to more than one yet distinctive signals, the study demonstrates the construction of a novel ANF-RGC-In-gene-(669)WTAPELL(675) mouse model. Through this model, the study establishes that (669)WTAPELL(675) is a vital ANF signal transducer motif of the guanylate cyclase. Its striking physiological features linked with their biochemistry are the following. (1) It controls the hormonally dependent cyclic GMP production in the kidney and the adrenal gland. Its deletion causes (2) hypertension and (3) cardiac hypertrophy. (4) These mice show higher levels of the plasma aldosterone. For the first time, a mere seven-amino acid-encoded motif of the mouse gene has been directly linked with the physiological control of blood pressure regulation, a detailed biochemistry of this linkage has been established, and a model for this linkage has been described.

Atrial natriuretic peptides elevate cyclic GMP levels in primary cultures of rat ependymal cells

The aim of this study was to examine the effect of atrial natriuretic peptides on primary cultures of ependymal cells, as measured by changes in intracellular levels of cyclic GMP. Incubation of ependymal cells with rat atrial natriuretic peptide-(1-28) (rANP) elicited a 30-fold increase in ependymal cGMP content within 1 min and more than a 100-fold increase within 10 min to a plateau value of approximately 30 pmol/mg protein. The C-type natriuretic peptide (CNP) elicited a similar increase in cGMP levels; however the maximal effect was observed within 1 min and the levels subsequently dropped by 90% to a low plateau within 10 min. A comparison of the concentration-response curves for rANP, human ANP-(1-28) (hANP) and CNP showed that rANP, hANP and CNP had similar effects, with regards to elevation of cGMP levels at high concentrations, but with differing EC50 values. These results demonstrate the presence of a heterogenous population of functional ANP receptors i n cultured ependymalcells suggesting that ANP may regulate specific ependymal cell activity.

Atrial natriuretic peptide-induced inhibition of aldosterone secretion: a quest for mediator(s)

Atrial natriuretic peptide (ANP) inhibits aldosterone secretion evoked by its physiological secretagogues by a mechanism(s) likely to involve intracellular messengers. When one examines the results of various investigations so far, this premise, although not definitive yet, seems to be supported. Therefore a brief perspective on the cellular messengers of the various secretagogues is provided before the inquiry into the possible mechanism of action of ANP. The receptors of ANP in the adrenal cells have been identified and characterized. ANP inhibits adenylate cyclase in various tissues through an inhibitory G protein, which appears to explain in part the inhibitory effect of ANP on adrenocorticotropin-induced aldosterone secretion. However, there could be other possible effects of ANP as discussed. ANP probably inhibits aldosterone secretion evoked by angiotensin II and potassium by interfering with the appropriate changes in calcium flux and cell calcium concentration, concomitants of stimulation by these secretagogues. The potential modes of these effects are probed. The role of guanosine 3',5'-cyclic monophosphate, which is increased by receptor activation of guanylate cyclase by ANP and is thought to play a major role in the biological effects of ANP in some other tissues, remains controversial in the aldosterone-lowering effect of ANP, and this is also discussed extensively in this review.

Functional implications of decreased renal cortical atrial natriuretic peptide binding in experimental diabetes

Glomerular hyperfiltration in streptozotocin-induced diabetes mellitus in rats may be mediated by atrial natriuretic peptide (ANP). We wanted to evaluate plasma levels of ANP and plasma volume in relation to renal ANP receptor density and affinity in rats 6 weeks after induction of diabetes. Plasma levels of immunoreactive ANP were significantly higher in hyperglycemic diabetic (75.2 +/- 8.3 pg/ml) than in control animals (34.7 +/- 8.1 pg/ml; p less than 0.01). Administration of insulin to keep diabetic rats normoglycemic normalized plasma levels of immunoreactive ANP (30.5 +/- 5.2 pg/ml). In contrast, plasma volume did not show significant differences among the groups (hyperglycemic diabetes, 46.6 +/- 3.8; normoglycemic diabetes, 42.4 +/- 3.2; controls, 43.2 +/- 2.0 ml/kg body wt). No correlation was found between plasma levels of immunoreactive ANP and plasma volume. By autoradiography a significant reduction in the number of renal cortical ANP receptors was observed in hyperglycemic diabetic rats as compared with controls. At variance, ANP receptor affinity did not change either in the cortex or in the medulla in hyperglycemic diabetics in comparison with control animals. The pathophysiological implication of cortical ANP receptor down-regulation was underscored by the blunted response of glomerular filtration rate to ANP infusion in diabetic animals as compared with controls.(ABSTRACT TRUNCATED AT 250 WORDS)

Alteration of atrial natriuretic peptide receptors in the choroid plexus of rats with induced or congenital hydrocephalus

Specific binding sites for atrial natriuretic peptide (ANP) in the choroid plexus of rats with induced or congenital hydrocephalus were examined using in vitro quantitative receptor autoradiographic methods. The number of 125I-ANP binding sites in the choroid plexus of rats with kaolin-induced hydrocephalus was significantly higher as compared to findings in the control rats, whereas no differences in the binding affinity were observed 3 days and 3 weeks after the intracisternal injection of kaolin. Conversely, rats with congenital hydrocephalus (LEW-HYR and HTX rats) had a small number of binding sites for 125I-ANP in the choroid plexus, as compared to findings in the control rats. These alterations may relate to the pathophysiology of hydrocephalus. The possibility that atrial natriuretic peptide may be involved in the regulation of cerebrospinal fluid production in the choroid plexus must be considered.

Atrial natriuretic peptide-LI following subarachnoid haemorrhage in man

Atrial natriuretic peptide-like immunoreactivity (ANP-LI) was measured in plasma from the external jugular vein (EJV) in the postoperative course of 11 patients with aneurysmal subarachnoid haemorrhage. Samples were taken on day, 1, 2, 3, 5, 7 and 9 after operation and ANP-LI levels were determined using radioimmunoassay. Ten healthy volunteers were investigated with one EJV plasma sample. Comparing the whole group of SAH patients with the control group, no significant differences in ANP-LI levels were found. In one patient very high ANP-LI levels were found together with high mean plasma sodium levels and high urine sodium excretion. This suggests that there is no general correlation between plasma ANP-LI and SAH; in occasional patients such a correlation may be secondary to changes in plasma sodium levels.

Protection of atrial natriuretic factor against degradation: diuretic and natriuretic responses after in vivo inhibition of enkephalinase (EC 3.4.24.11) by acetorphan

Atrial natriuretic factor (ANF) might be beneficial in several cardiovascular disorders, but its poor oral absorption and rapid inactivation in vivo have so far prevented its use in therapeutics. We have assessed the role of enkephalinase (membrane metallo-endopeptidase, EC 3.4.24.11) in the in vivo inactivation of ANF in mice and healthy human volunteers by evaluating the effects of acetorphan, a potent inhibitor. In mice, the degradation of 125I-labeled ANF was markedly delayed, as shown by the levels of the intact peptide in the plasma and the kidney, a major target organ. The effect of acetorphan was due to the inhibition of enkephalinase activity, since it occurred at an ED50 very close to this drug's ID50 for the inhibition of the specific binding of radioactive material to the kidney or lung peptidase that was measured after administration of [3H]acetorphan. The effects of acetorphan were also studied in eight healthy human volunteers by using a randomized double-blind, placebo-controlled design. Oral administration of acetorphan elicited a lasting elevation of plasma ANF-like immunoreactivity, with a time course parallel to that of the inhibition of plasma enkephalinase activity. These effects were accompanied by significant increases in urinary volume and sodium excretion, two well-established renal responses to ANF peptides. These results indicate that enkephalinase plays a critical role in ANF degradation in vivo and that its inhibition enhances the levels of circulating endogenous ANF, which, in turn, results in diuresis and natriuresis. Enkephalinase inhibition may constitute another therapeutic approach to the treatment of cardiovascular diseases, such as congestive heart failure or essential hypertension, on which ANF is postulated to have a beneficial effect.

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.

Role of endopeptidase-24.11 in the inactivation of atrial natriuretic peptide

The circulating form of atrial natriuretic factor is a 28-residue peptide containing a 17-residue disulphide-linked ring. It has important actions on the kidney, largely on its haemodynamics, and at other sites including the adrenal cortex and CNS. It has a short half-life in vivo and is rapidly inactivated when incubated with kidney microvillar membranes. Of the battery of peptidases present in that membrane, only one, endopeptidase-24.11, is responsible for initiating the attack, and this commences with hydrolysis of the Cys7-Phe8 bond within the ring. Hydrolysis at this and other points has been shown to inactivate the peptide and this information has pointed the way to the synthesis of resistant analogues.

Decreased atrial natriuretic peptide binding in renal medulla in rats with chronic heart failure

The relations between atrial natriuretic peptide (ANP) binding sites in the renal medulla, plasma ANP concentration, and ventricular dysfunction have been studied in rats 4 weeks after myocardial infarction induced by left coronary artery ligation. Plasma ANP concentration was measured by radioimmunoassay, and quantitation of receptors was performed by computerized in vitro autoradiography with 125I-labeled alpha-rat ANP (1-28) as the radioligand. When compared with controls, rats with myocardial infarction had markedly elevated plasma immunoreactive ANP concentrations (462 +/- 82 versus 124 +/- pg/ml, p less than 0.01) and reduced densities of ANP binding in the inner renal medulla (2.93 +/- 0.19 versus 3.53 +/- 0.22 fmol/mg protein, p less than 0.01). Extensive myocardial infarction was associated with a significant decrease in receptor numbers in the inner medulla (33.6 +/- 5.7 versus 95.6 +/- 9.6 fmol/mg protein, p less than 0.01) without significantly altering the affinity constant (1.76 +/- 0.51 versus 1.03 +/- 0.15 x 10(9) M-1, p greater than 0.05). Right ventricular weight increased in proportion to infarct size (r = 0.71, p less than 0.01), and both were correlated with plasma immunoreactive ANP levels (r = 0.74, p less than 0.01 and r = 0.75, p less than 0.01, respectively). Binding densities in the inner medulla of rats with infarcts were negatively correlated with right ventricular weight, plasma immunoreactive ANP concentrations, and also with infarct size (r = -0.92, p less than 0.001; r = -0.78, p less than 0.001; r = -0.77, p less than 0.01, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)

Clinical application of atrial natriuretic polypeptide in patients with congestive heart failure: beneficial effects on left ventricular function

Synthetic alpha-human atrial natriuretic polypeptide was infused in patients with congestive heart failure (CHF) (New York Heart Association class III or IV) and in those without CHF. The infusion of atrial natriuretic polypeptide (ANP) at a rate of 0.1 microgram/kg/min significantly decreased pulmonary capillary wedge pressure and increased stroke volume index in all of the patients with CHF, whereas it decreased pulmonary capillary wedge pressure but caused no significant change in stroke volume index in the patients without CHF. Concomitant significant reductions in total systemic resistance were observed in both groups of patients. The ANP infusion significantly increased the urine volume, the excretion of sodium, and endogenous creatinine clearance in the patients without CHF. In the patients with CHF, it also showed a tendency to increase all these variables, but the urine volume did not correlate with the reduction in pulmonary capillary wedge pressure. The ANP infusion also decreased plasma aldosterone concentrations in these patients, although no significant difference was observed in the decrement of the plasma aldosterone concentration in the patients with and those without CHF. These findings indicate that the ANP infusion improves left ventricular function in patients with CHF, and suggest that this improvement results mainly from the vasodilating activity of ANP.

Atrial natriuretic peptide effects on cGMP and cAMP contents in microdissected glomeruli and segments of the rat and rabbit nephrons

A microradioimmunoassay has been developed in order to measure the changes in cGMP cell content induced in vitro by atrial natriuretic peptides (ANP) in either glomeruli or defined portions of tubules microdissected from collagenase treated rat and rabbit kidneys. When tested at 0.1 microM or 1 microM, all ANP analogues used produced in rat glomeruli a 20-25 fold increase in cGMP accumulation compared to basal values. Threshold responses were obtained with about 1 nM ANP and apparent Ka values ranged between 5 and 50 nM. Atriopeptin III led to similar results in glomeruli isolated from rabbit. Under the same experimental conditions, no cGMP could be detected in any ANP-treated nephron segment from the rat kidney (namely, from the proximal convoluted tubule up to the outer medullary collecting tubule) nor in cortical collecting tubules isolated from the rabbit kidney. Moreover, ANP did not alter the forskolin-induced increase in cAMP content in glomeruli or collecting tubules, nor the AVP-induced increase in cAMP content in collecting tubules. Our data confirm the marked effect of ANP on cGMP generation by isolated glomeruli from rat and rabbit; however, they are not compatible with a direct action of ANP stimulating cGMP generation in tubules or inhibiting vasopressin-induced cAMP generation in collecting tubules.

Atrial natriuretic peptide receptors along the rat and rabbit nephrons: [125I] alpha-rat atrial natriuretic peptide binding in microdissected glomeruli and tubules

Binding of [125I] alpha-rat atrial natriuretic peptide ([125I] alpha-RANP) was measured in glomeruli and pieces of tubule microdissected from rat and rabbit nephrons. High densities of specific ANP binding sites were found only in the glomeruli (10-30 X 10(-18) mol X glom-1), whereas no specific binding could be detected in the proximal tubule, the thin segments of the Henle's loop, the thick ascending limb, the distal tubule and the cortical and outer medullary collecting tubules. Rising the temperature from 4 degrees C to 35 degrees C resulted in biphasic kinetics of binding, suggesting a temperature-dependent inactivation of labelled hormone by glomeruli. At 4 degrees C, specific binding of [125I] alpha-RANP was time and dose-dependent and Scatchard analysis of data indicated an apparent equilibrium dissociation constant of 0.63 nM. Competition experiments revealed the following sequence of stereospecificity for binding to rat glomeruli: RANP 3-28 greater than [125I] alpha-RANP = [125I] alpha-HANP = alpha-RANP = antriopeptin III greater than antriopeptin II, whereas binding was unaffected by pharmacological doses of unrelated peptide hormones, prostaglandins, adrenergic agonists, dopamine, histamine and carbamylcholine. The results indicate that glomerular binding sites might be the physiological ANP receptors.

The hydrolysis of alpha-human atrial natriuretic peptide by pig kidney microvillar membranes is initiated by endopeptidase-24.11

alpha-Human atrial natriuretic peptide, a 28-amino-acid-residue peptide, was rapidly hydrolysed by pig kidney microvillar membranes in vitro, with a t1/2 of 8 min, comparable with the rate observed with angiotensins II and III. The products of hydrolysis were analysed by h.p.l.c., the pattern obtained with membranes being similar to that with purified endopeptidase-24.11 (EC 3.4.24.11). No hydrolysis by peptidyl dipeptidase A (angiotensin I converting enzyme, EC 3.4.15.1) was observed. The contribution of the various microvillar membrane peptidases was assessed by including specific inhibitors. Phosphoramidon, an inhibitor of endopeptidase-24.11, caused 80-100% suppression of the products. Captopril and amastatin (inhibitors of peptidyl dipeptidase A and aminopeptidases respectively) had no significant effect. Hydrolysis at an undefined site within the disulphide-linked ring occurred rapidly, followed by hydrolysis at other sites, including the Ser25--Phe26 bond.