Cellular mechanism of release of atrial natriuretic factor

Tissue extracts of rat heart atria contain a family of peptides with natriuretic and vasorelaxant properties. We have shown previously that this "atrial natriuretic factor" may be released in vitro from incubated atria by muscarinic cholinergic stimulation. Experiments reported here demonstrate that incubation with adrenalin or arginine vasopressin, but not with deamino-8-D-arginine vasopressin, also results in liberation of atrial natriuretic factor. Since the effective agonists have in common activation of the cellular polyphosphoinositide system with consequent production of inositol triphosphate, we suggest that inositol triphosphate is the second messenger of stimulus-secretion coupling in atrial cells.

Molecular cloning and characterization of DNA sequences encoding rat and human atrial natriuretic factors

A cDNA copy of the message encoding rat atrial natriuretic factor (ANF) has been cloned in Escherichia coli, and its nucleotide sequence was determined. ANF appears to be synthesized as a larger precursor, atrial pronatriodilatin. The cDNA has an open reading frame potentially encoding a protein of 152 amino acids, of which the first 24 amino acids strongly resemble a signal sequence. This is followed by a sequence with 80% homology to a second vasoactive protein, porcine cardiodilatin. The ANF peptide is contained in the COOH-terminal portion of the protein. The DNA sequence corresponding to human ANF is also presented and displays a high degree of homology to its rat counterpart. These data provide further evidence for the expression in cardiac atria of a multifactor system that may contribute to the regulation of blood pressure and extracellular fluid volume.

Specific membrane receptors for atrial natriuretic factor in renal and vascular tissues

Membranes from rabbit aorta and from rabbit and rat kidney cortex possess high-affinity (Kd = 10(-10) M) specific binding sites for atrial natriuretic factor (ANF). Similar high-affinity sites are present in an established cell line from pig kidney, LLC-PK1. Results of fractionation studies indicate that the receptors are localized in the plasma membrane of these tissues. The binding is time-dependent and saturable. An excellent quantitative correlation was found between the affinity of synthetic ANF and analogs of intermediate activity to aorta membranes and the half-maximal concentration needed for relaxation of rabbit aorta rings contracted by addition of serotonin. Furthermore, the binding affinity of the receptor in kidney membranes is consistent with the concentration required for in vivo natriuresis in the rat. Biologically inactive synthetic ANF fragments and other peptide hormones such as angiotensin II and vasopressin do not significantly inhibit binding. These data suggest that the receptors for ANF in vascular and renal tissues are responsible for mediating the physiological actions of this peptide in these target tissues.

Atrial Natriuretic Factor: Sodium Transport in Human Erythrocytes

1. The effect of partially purified rat atrial natriuretic factor on sodium efflux from sodium-loaded human erythrocytes was studied.

2. High molecular weight (10000–30000) and low molecular weight (3000–10000) fractions of atrial extract were prepared by gel filtration; they had natriuretic activity in rats.

3. Neither fraction affected sodium efflux in erythrocytes.

4. The results suggest that atrial natriuretic factor acts in the kidney by mechanisms other than through inhibition of the Na+-K+ exchange pump or the Na+-K+ cotransport system.

Specific receptor-mediated inhibition by synthetic atrial natriuretic factor of hormone-stimulated steroidogenesis in cultured bovine adrenal cells

The effect of synthetic atrial natriuretic factor (ANF) on adrenal steroidogenesis has been studied in primary culture of bovine adrenal cells. ANF-(8-33) produced a potent 40-70% inhibition of angiotensin II-, ACTH-, PGE1-, and forskolin-stimulated secretion of aldosterone production from zona glomerulosa cells with an ED50 of 120 pM. An equipotent inhibitory effect of the natriuretic factor on cortisol production was also observed in cultured zona fasciculata cells. Nicotine-stimulated secretion of catecholamines from medullary cells was only slightly inhibited by the factor at doses above 10 nM. [125I]iodo-ANF-(8-33) binding to glomerulosa membranes displayed an apparent affinity of 100-150 pM for specific receptor sites and was not inhibited by angiotensin II or ACTH. Conversely, the natriuretic factor had no affinity for angiotensin II receptor sites. The results demonstrate that part of the natriuretic effect of this new factor might be due to inhibition of adrenal steroidogenesis by action through a distinct receptor.

Endocrine physiology of electrolyte metabolism

Historically, the sodium ion has been given prominence in relation to cardiovascular disease, perhaps to the exclusion of other ions. Recently, other ions, including chloride, potassium, magnesium and calcium have received increasing attention in relation to hypertension, cardiac arrhythmias, and metabolic derangements. Endocrine factors controlling these ions have also received increasing attention; they include classic hormonal actions as well as neurotransmission and paracrine hormonal actions. Studies indicate that control of the renin-angiotensin-aldosterone system resides in cytosolic calcium ion levels in the juxtaglomerular cell, as well as chloride ion and prostaglandins at the macula densa. Renin release is stimulated by hyperpolarisation of the juxtaglomerular cell induced by beta 1-agonists, parathyroid hormone, glucagon, magnesium and low cytosol calcium. Renin release is inhibited by high calcium, potassium and angiotensin II. Subsequent to renin release, hormonal regulation includes stimulation of converting enzyme activity by cortisol and prostaglandin (PGE2). Other hormonal control includes antidiuretic hormone producing dilution of extracellular electrolytes and augmented peripheral resistance. A recently identified natriuretic factor isolated from cardiac atria appears to be a potent diuretic with actions similar to that of frusemide (furosemide). Other electrolytes have received closer scrutiny. Chloride may play a dominant role in renal sodium reabsorption, responding to prostaglandin levels. Calcium has been recognised as a basic regulator of the secretion of such hormones as noradrenaline, renin, and aldosterone. As well, calcium ion changes are the means by which smooth muscle contraction is effected. Parathyroid hormone and vitamin D regulate the level of this ion in the body. In addition, a high dietary calcium intake appears to play a protective role against hypertension, while calcium channel blockers appear to reduce blood pressure. Endocrine systems play a major role in the protection against acute elevations in serum potassium by means of insulin action and adrenergic modulation of extrarenal potassium disposal. Aldosterone is recognised as the delayed regulator of potassium excretion. Magnesium levels fall in hyperaldosteronism, hyperparathyroidism, and diabetic keto-acidosis, as well as in malnutrition states. A coexisting potassium deficiency may be refractory to therapy until hypomagnesaemia is corrected. The integrated action of these hormones and electrolytes are thus of major importance in regulation of the cardiovascular system.

Effect of native and synthetic atrial natriuretic factor on cyclic GMP

Mammalian atrial cardiocyte granules contain a potent natriuretic and diuretic peptide. Since cGMP appears to be involved in the modulation of cholinergic and toxin-induced sodium transport, we examined the effect of atrial natriuretic factor (ANF) on this nucleotide. Atrial but not ventricular extracts elicited approximately a 28-fold increase of urinary cGMP excretion parallel to the natriuresis and diuresis. The atrial extracts also elevated cGMP levels in kidney slices and primary cultures of renal tubular cells. The effect of ANF on cGMP appeared to be specific since antibodies which were capable of inhibiting the ANF-induced diuresis also suppressed cGMP excretion. Furthermore, during the course of ANF purification, the ANF-induced increase of cGMP production by kidney cells paralleled the heightened specific natriuretic activity of the atrial factor. A synthetic peptide (8-33)-ANF similarly increased urinary plasma and kidney tubular cGMP levels. The exact mechanism of action of ANF on cGMP remains to be elucidated, but indirect inhibition of cGMP phosphodiesterase appears to participate in its effect.

[Influence of lithium carbonate on the effects of the natriuretic factor]

It has been shown in rat experiments that administration of lithium carbonate for 3 days (100 mg/kg) enhances renal excretion of water and electrolytes following an increase in the volume of extracellular fluid. In this case renal excretion of natriuretic factor rises, whereas its concentration in blood plasma does not ascend. Lithium carbonate raises the sensitivity of the kidneys to natriuretic factor and reduces its effect on transport activity in the small intestine.

Effect of atrial natriuretic factor (ANF)-related peptides on aldosterone secretion by adrenal glomerulosa cells: critical role of the intramolecular disulphide bond

We previously demonstrated that synthetic 48-73 atrial natriuretic factor (ANF) (previously called 8-33 ANF) blocked the response of rat adrenal glomerulosa cells to angiotensin II, ACTH and potassium. We have now investigated the effects of natural 43-73 ANF, oxidised synthetic 48-73 ANF and the natural 1-73 ANF on aldosterone output by rat glomerulosa cells. The natural 43-73 ANF and the natural 1-73 ANF were equipotent to 48-73 ANF in inhibiting the stimulation of aldosterone secretion produced by angiotensin II with an IC50 of 2 X 10(-9)M. Similar results were obtained with ACTH and potassium. After oxidation with performic acid, 48-73 ANF was completely devoid of activity on the response of aldosterone to angiotensin II, ACTH and potassium. We conclude that the intramolecular disulphide bond in 48-73 ANF is critical for maintaining the active conformation of ANF.

Effect of a purified atrial natriuretic factor on rat and rabbit vascular strips and vascular beds

Rat atrium cardiocytes contain a powerful natriuretic and diuretic peptide that has been localized in the specific granules. This atrial natriuretic factor (ANF) produced a potent, dose-dependent relaxant effect on rabbit and rat arterial strips previously made to contract by application of either norepinephrine (NE) or angiotensin II. The effect was not seen if KCl was used as contractile agent or under any conditions with rabbit mesenteric strips. After the application of ANF the vascular strips were refractory to subsequent stimulation by either NE or angiotensin II. The infusion of ANF into a high-resistance isolated perfused rat kidney produced a rapid decrease (33 +/- 5 mmHg) in perfusion pressure that lasted for 18 +/- 3 min. This effect was not seen in the isolated rat mesenteric arterial preparation, even when the perfusion pressure was raised by the infusion of NE. These effects of ANF on vascular smooth muscle are not mediated by prostaglandins, by alpha- and beta-adrenergic and muscarinic receptors, or by an impairment of Ca2+ influx, but they are mimicked by sodium nitroprusside. A low- and a high-molecular-weight ANF produced the same effects. The existence of specific receptive sites for these peptides is suggested.

Atrial natriuretic factor inhibits adenylate cyclase activity

The synthetic atrial natriuretic factor (ANF) (8- 33AA ) inhibited adenylate cyclase activity in aorta washed particles, mesenteric artery, and renal artery homogenates in a concentration dependent manner with an apparent Ki between 0.1 to 1nM . The extent of inhibition of adenylate cyclase by ANF varied from tissue to tissue. The adenylate cyclase from mesenteric artery and renal artery was inhibited to a greater extent as compared to that from aorta. ANF was also able to inhibit the stimulatory effects of hormones on adenylate cyclase activity and of agents such as F- and forskolin which activate adenylate cyclase by receptor- independent mechanism. In addition, ANF showed an additive effect with the inhibitory response of angiotensin II on adenylate cyclase from rat aorta. These studies for the first time demonstrate that ANF is an inhibitor of adenylate cyclase of several systems.

Purification, sequencing and synthesis of natriuretic and vasoactive rat atrial peptide

Mammalian atria contain potent natriuretic and diuretic substances which exist in high- and low-molecular-weight forms and which appear to be associated with atrium-specific granules. The natriuretic effect of atrial extract is largely accountable for by its renal haemodynamic effects; atrial extracts also antagonize hormone- and non-hormone-induced contraction of the isolated rabbit aorta and isolated rat kidney vasculature. We have completely purified a low-molecular-weight natriuretic and vasoactive substance from rat atria and characterized it as a 24-amino acid peptide. Synthetic peptide, produced by solid-phase synthesis, mimics biological effects of crude atrial extract and purified peptide; its activity is enhanced by slow oxidation, suggesting a disulphide (Cys 4-Cys 20) configuration for the native peptide. If secreted into blood, this atrial natriuretic peptide (' auriculin B') could be a novel peptide hormone of considerable importance to renal and cardiovascular homeostasis.

Identification in rat atrial tissue of multiple forms of natriuretic polypeptides of about 3,000 daltons

Rat atrial natriuretic peptides of relatively low molecular weight have been isolated from the alpha-component of rectum relaxant activity corresponding to about 3,000 daltons, which was obtained as a side fraction in our previous isolation of beta-rat atrial natriuretic polypeptide (beta- rANP ). In contrast to the same fraction from human atria, the rat atrial alpha-component was found to contain six or more distinct but related peptides, eliciting a potent natriuretic activity. Six of them (B-II, C, D, E, B-I and A), containing 35, 33, 32, 31, 28 and 25 amino acid residues, respectively, have been purified to homogeneity and sequenced. All these peptides were found to correspond to the C-terminal sequence of beta- rANP composed of 48 residues, with varying N-terminal elongations. This indicates that these peptides are derived from beta- rANP . Peptide B-I, composed of 28 residues, is identical to alpha-human atrial polypeptide(alpha- hANP ), with a single replacement of Ile for Met at position 12.

Atrial natriuretic factor and urinary kallikrein in the rat: antagonistic factors?

The rat atrium contains a potent natriuretic factor which appears to inhibit the sodium reabsorption in the collecting tubules of the kidneys. We examined the effects of the injection of partially purified atrial natriuretic factor (ANF) and synthetic ANF (8-33) into rats with simultaneous infusions of dextrose or aprotinin. Aprotinin, an inhibitor of serine proteases, increases the natriuretic and diuretic effects of the atrial factor by 50%. Urinary kallikrein excretion is also slightly increased by ANF but is not affected by aprotinin. As a comparison, aprotinin has no effect on the diuretic or natriuretic responses of furosemide, although it inhibits by 50% the kallikrein excretion induced by furosemide. When ANF is incubated with purified rat urinary kallikrein, the natriuretic and diuretic effects are decreased by more than 50%. We conclude that glandular kallikrein or a similar serine protease may be involved in the catabolism of ANF.

Water immersion and the kidney: implications for volume regulation

Studies from this laboratory have demonstrated that head-out immersion in isothermic water causes a cephalad redistribution of blood volume. The resultant central hypervolemia induces a marked natriuresis and diuresis and suppression of plasma renin activity, plasma aldosterone, and plasma arginine vasopressin. All of these changes are thought to be attributable to stimulation of cardiopulmonary receptors. Immersion also produces an augmentation of prostaglandin E (PGE) excretion, which reflects increased renal PGE synthesis. The ability of immersion in induce a prompt and profound central hypervolemia, without concomitant alterations in plasma composition, indicates that immersion might be a preferred investigative tool for assessing the effects of volume expansion on renal function and hormonal responsiveness in both normal individuals and patients with edematous disorders. In addition, this model constitutes an appropriate tool for simulating weightlessness.

Isolation and sequence determination of peptide components of atrial natriuretic factor

The independent isolation and sequence determination in our laboratories of three closely related Atrial Natriuretic Factor peptides from rat atria confirm the sequences of ANF peptides reported by Seidah et al and synthesized by Nutt et al [Proc. Natl. Acad. Sci., (1984) in press] and contain the sequences reported by Flynn et al [Biochem. Biophys. Res. Commun. (1983) 117: 859-865] and by Currie et al [Science (1984) 223: 67-69]. In addition, we provide proof for a C-terminal tyrosine rather than tyrosine amide in our isolated peptides.

Role of prostaglandins in the production of natriuretic factor by the isolated rat kidney

The renal origin of natriuretic factor (NF) of high molecular weight (MW 45,000) has been previously demonstrated. Its role in the control of sodium excretion by the kidney has been strongly suggested. The aim of the present work was to study the relationship between NF and prostaglandins, which are known to modify the renal regulation of sodium. (1) The activity of NF, obtained by ultrafiltration and gel filtration from pooled plasma of salt-loaded rats, was tested on an isolated rat kidney perfused by a cell-free medium. A significant natriuresis was observed in the kidneys isolated either from normal rats or from indomethacin-pretreated rats after injection of NF. Moreover, a dose-response curve was obtained with increasing amounts of NF. (2) The ability of isolated rat kidney to produce NF was studied in two series of salt-deprived rats, the second series being pretreated with indomethacin. The production of NF by isolated kidneys was induced by addition of saline to the fluid perfusing kidneys isolated from normal or indomethacin-pretreated rats fed a salt-depleted diet. Before addition of saline, no natriuretic activity was detectable. After the addition of 0.9% NaCl, NF was only found in the perfusate from kidneys isolated from normal rats. Perfusate extracts from indomethacin-pretreated kidneys did not induce any significant rise in natriuresis. (3) Kidneys isolated from rats previously fed a free fatty acid-deficient diet and thus prostaglandin-depleted were unable to produce NF even if they were salt-loaded. Intraarterial perfusion of prostaglandin E2 restored their ability to release or secrete NF. It is concluded that the release or the synthesis of NF is only possible in the presence of prostaglandins. Its action is not influenced by prostaglandin inhibition or deficiency.

[Essential arterial hypertension, cellular intolerance to sodium]

Much experimental and clinical evidence points to the fact that increased intracellular Na+ concentration could play an important pathogenic role in hypertension, especially where cells of excitable tissues are concerned. Active extrusion of Na+ (from the cells to the extracellular space) depends mainly on the activity of the system of membrane transport known as the Na+ pump. The extrusion of Na+ occurs in exchange for K+ and the activity of the pump depends on the enzyme Na+-K+-ATPase. The Na+ pump is inhibited by cardiotonic glycosides such as digitalis and ouabain . The results obtained in our laboratory suggest that the activity of the Na+ pump is controlled by an endogenic system acting in a similar manner to ouabain and digitalis. Experimental and clinical studies show: -- that the administration of Na+ increases the inhibitor effect both in urine and plasma; -- that the inhibitor is increased in about 50 p. 100 of patients with essential hypertension; -- that this increase seems to depend on familial or genetic factors. The biochemical identification of this endogenic inhibitor is now under way; it has a small molecular weight (less than 3000), is thermostable and anionic. This factor could have both a regulating role on Na+ turnover and a pathological role in hypertension. Its activity comes into competition cations also capable of inhibiting the Na+ pump, including K+.