The physiology of atrial natriuretic factor

Following the discovery of the natriuretic effect of atrial extract, our laboratory attempted to dissect the possible physiological role of atrial natriuretic factor. Initial micropuncture experiments demonstrated that the reduction of tubular sodium reabsorption was localized in the medullary collecting duct, a nephron site in which sodium transport was known to be inhibited after acute hypervolemia. Partial removal of the endogenous source of atrial natriuretic factor was associated with a reduced renal response to hypervolemia, confirming that the factor is causally involved in acute sodium balance. In vitro incubation of atrial tissue was used to investigate mechanisms of release of atrial natriuretic factor. It was found that agonists known to activate the intracellular polyphosphoinositide system in other tissues were effective in releasing natriuretic activity from the atria into the incubation medium. To determine whether atrial natriuretic factor might play a role in hypertension, atrial natriuretic content was measured in spontaneously hypertensive rats and their normotensive controls. Hypertension was associated with increased content. Since the renal response to exogenous factor was not impaired in these animals, we suggested that the increased content might play a compensatory role. Our early studies thus indicated that atrial natriuretic factor was a previously unrecognized hormone involved in cardiovascular regulation.     

Antisera to atrial natriuretic factor reduces urinary sodium excretion and increases plasma renin activity in rats

Although the presence of atrial natriuretic factor in the blood has been demonstrated by radioimmunoassay, its biological activity and physiological significance has not been elucidated. Using specific antiserum against atrial natriuretic factor, we investigated the effect of passive immunization in rats. A significant reduction of urine output and urinary sodium excretion lasted for about 30 min after intravenous administration of antiserum. The effects were more pronounced in rats pretreated with deoxycorticosterone acetate and saline. Plasma renin activity was increased after the administration of antiserum. No significant effects on the urinary sodium excretion was observed following injection of normal rabbit serum. The results of this study provide evidence indicating that endogenous atrial natriuretic factor plays an important role in the regulation of urinary water and sodium excretion and plasma renin activity.     

Responses of plasma immunoreactive atrial natriuretic factor, aldosterone and urinary dopamine to salt-loading in a patient with severe idiopathic edema

A patient with severe idiopathic edema and long history of diuretic abuse had, in response to salt loading, an inability to increase urinary sodium excretion associated with a paradoxical response (decrease) of urinary dopamine excretion, a non suppressible aldosterone and non stimulable immunoreactive atrial natriuretic factor in plasma. These patterns distinguished this patient from those with a milder form of idiopathic edema who did not abuse diuretics and had, in comparison with controls, marginally decreased urinary sodium and dopamine responses but normal aldosterone suppressibility and ANF stimulability. Since the natriuretic action of ANF appears to be mediated by dopaminergic mechanisms, this severe natriuretic handicap may be due to a chronic diuretic abuse-induced combined ANF and dopamine deficiency.     

Evidence for a dopaminergic mechanism for the diuretic and natriuretic action of centrally administered atrial natriuretic factor

1. Intracerebroventricular (IVT) administration of rat atrial natriuretic factor (ANF) (99-126) to conscious male hydrated rats induces a dose-dependent increase in urine and sodium excretion. The possible involvement of brain dopaminergic system in the IVT-ANF-induced diuresis and natriuresis was evaluated. 2. Central sympathectomy (6-OHDA, 250 micrograms/5 microliters, IVT; 72 and 48 hr before IVT-ANF) inhibited both the diuretic and the natriuretic action of centrally administered ANF, suggesting that in the brain ANF requires the integrity of central noradrenergic and/or dopaminergic systems function for its actions. 3. Intracerebroventricular injection of haloperidol and intragastric administration of domperidone prevent the diuretic and natriuretic response to centrally administered ANF. 4. Our data suggest a neuromodulatory action of ANF within the brain and demonstrate an interaction of the peptide with brain dopaminergic systems.     

A possible role of atrial natriuretic peptide in ethanol-induced acute diuresis.

The acute effects of ethanol on plasma atrial natriuretic peptide levels were investigated in 4 clinically healthy males, aged 24-26 years, consumed either 750 ml of water as a control study, or the same beverage with 1 ml/kg alcohol added, which increased the plasma alcohol concentration to 99.12 +/- 15.10 mg/dl at 60 min. Plasma atrial natriuretic peptide levels were significantly higher in the alcohol study compared to the control study at each time point (10, 20, 30, 60, 120 min after drinking onset), and with a peak at 10 min. Atrial natriuretic peptide levels showed a positive significant correlation with plasma antidiuretic hormone in the control group, while no relationship was found between the two peptides in the alcohol study. Moreover, a significant correlation exists between plasma atrial natriuretic peptide levels and systolic arterial blood pressure, and heart rate, and between the variations in atrial natriuretic peptide values and the variations in plasma sodium, serum ethanol, and plasma osmolality in the alcohol study. Acute ethanol intake causes an increase in urinary volume, and a decrease in urinary potassium excretion and urinary osmolality, and no change in urinary sodium excretion. These data suggest that acute ethanol administration causes a rapid increase in plasma levels of atrial natriuretic peptide, which could be an important factor of ethanol-induced diuresis. The main mechanisms for increased atrial natriuretic peptide release from atria after acute ethanol ingestion seem to be atrial stretch, due to the increase in arterial blood pressure, in heart rate, in sympathetic tone, and in plasma osmolality, and to a direct secretory effect by antidiuretic hormone.     

Effects of atrial natriuretic factor on renal handling of water and electrolytes in rats

The intravenous injection of an extract of atrial myocardium into anesthetized rats during a hypotonic diuresis resulted in an increase in the renal excretion of water, sodium, potassium, calcium, magnesium, and phosphate. There was an increase in urine concentration which was probably a result of the secretion of vasopressin since it did not occur in Brattleboro (di/di) rats. A transient increase in glomerular filtration rate and renal plasma flow occurred during the first five minutes with a more sustained rise in filtration fraction. Injection of atrial extract also caused a partial inhibition of solute-free water formation in Brattleboro rats subjected to water diuresis and a partial inhibition of solute-free water reabsorption in rats subjected to maximal antidiuresis by infusing vasopressin. In neither case was the degree of inhibition as profound as that observed after injecting furosemide in a dose which caused a comparable natriuretic response. A large dose of furosemide blocked the natriuretic response to atrial extracts whereas, when a comparable level of sodium and water output was produced by massive infusions of saline, the natriuretic response to atrial extract was increased. It is suggested that atrial natriuretic factor might inhibit sodium transport in nephron segments beyond the medullary thick ascending limb. Furosemide might also act at the same tubular site or inhibit tubular secretion of the atrial natriuretic factor.     

Effects of atrial appendectomy on circulating atrial natriuretic factor during volume expansion in the rat

This study examined the changes in the circulating level of endogenous atrial natriuretic factor during diuresis and natriuresis produced by acute volume expansion in anesthetized rats with either bilateral atrial appendectomy (n = 9) or sham operation (n = 9). Following control measurements in the sham-operated rats, 1% body weight volume expansion with isotonic saline produced an increment in urinary sodium excretion of over 4 mueq/min (P less than 0.05) while urine volume increased by more than 20 microliter/min (P less than 0.05). These responses were associated with a significant increase in immunoreactive plasma atrial natriuretic factor from a baseline value of 82 +/- 10 pg/ml to a level of 120 +/- 14 pg/ml (P less than 0.05). In contrast, in the group of rats with bilateral atrial appendectomy an identical degree of volume expansion increased urinary sodium excretion and urine volume by only 0.61 mueq/min (P less than 0.05) and 3.07 microliter/min (P less than 0.05), respectively. In this group, immunoreactive plasma atrial natriuretic factor remained statistically unchanged from a control value of 70 +/- 12 pg/ml to a level of 82 +/- 16 pg/ml (P greater than 0.05). Comparison of the two groups indicates that the natriuresis, diuresis, and plasma atrial natriuretic factor levels during volume expansion were significantly reduced in the rats with bilateral atrial appendectomy. No differences in mean arterial pressure and heart rate were observed between the two groups. These data demonstrate that removal of both atrial appendages in the rat attenuated the release of atrial natriuretic factor during volume expansion; and this effect, in turn, was associated with a reduction in the natriuretic and diuretic responses.     

The effect of atrial natriuretic factor on force development in rat cardiac trabeculae

Studies in single cardiac muscle cells have demonstrated that atrial natriuretic factor decreases the L-type calcium current. Recent investigations in human atrial cells have also demonstrated that atrial natriuretic factor causes a voltage-dependent reduction in sodium channel activity and thus may reduce intracellular calcium via decreased activity of the sodium-calcium exchange mechanism. By reducing intracellular calcium, atrial natriuretic factor may have a negative inotropic effect on cardiac muscle. To characterize the effect of atrial natriuretic factor on the development of force, we studied the force-sarcomere length relationship in 11 right ventricular rat trabeculae, both before and after exposure of the muscles to increasing concentrations of atrial natriuretic factor. Sarcomere length was measured by laser diffraction techniques and controlled by a servomotor system. The addition of atrial natriuretic factor to the superfusion solution, at concentrations of 10(-9)-10(-7) M, increased stimulus threshold, reduced peak twitch force in a dose-dependent manner by 38% (maximum), and reduced time to peak twitch force by 15% (maximum). Incubation of muscle preparations with concentrations of atrial natriuretic factor below 10(-9) M had no effect on force generation. The negative inotropic effect of atrial natriuretic factor was associated with a change in the shape of the force-sarcomere length relationship, similar to a reduction of the extracellular calcium concentration. ANF (10(-7) M) had no effect on the rate of decay of force following post extra-systolic potentiation. These observations are consistent with the assumption that the negative inotropic effect of atrial natriuretic factor is mediated by reduction of calcium entry into the cardiac cell.     

Dopaminergic mediation of the diuretic and natriuretic effects of ANF in the rat

Atrial natriuretic factor (ANF) increases sodium (Na+) and water excretion 8-10 fold on repeated administration to anesthetized rats. SCH-23390 (80 micrograms/kg i.v.) and R-sulpiride (80 micrograms/kg i.v.), selective antagonists of dopamine receptors in the renal vasculature, inhibited diuresis and natriuresis induced by AP III and dopamine. These findings suggest that ANF exerts its effects on renal Na+ and water handling via a dopaminergic mechanism; however, changes in intrarenal hemodynamics secondary to dopamine receptor blockade may attenuate the actions of ANF.     

Dopamine receptor antagonists inhibit the natriuretic response to atrial natriuretic factor (ANF)

The diuretic and natriuretic response of anesthetized rats to low doses of semi-purified atrial extracts or synthetic alpha-hANP was completely blocked by intravenous injection of 50 micrograms of haloperidol or chlorpromazine. Sulpiride or metoclopramide at the same doses did not show this effect. We conclude from these results that dopamine receptors, probably of the D1-type, are involved in the natriuretic effect of the atrial peptides.     

Multiple forms of atrial natriuretic factor receptor in human placenta

Multiple forms of atrial natriuretic factor receptor have been identified in human placental membranes. Atrial natriuretic factor binds specifically to placental membranes and the binding activity could be solubilized using non ionic detergent, Triton X-100. Binding to the detergent solubilized preparation was inhibited 80% by the addition of 0.5 M sodium chloride. Affinity cross-linking analysis indicated that this binding was associated with a single protein band of molecular weight 170-kDa. On the other hand, if sodium chloride was added together with a chelator, o-phenanthroline, ANF binding to this preparation was stimulated 300%. Binding under these conditions was not to the 170-kDa protein but was associated with a broad band in the region of 100/110-kDa and a minor band at 200-kDa. These observations clearly indicated that in human placental membranes, atrial natriuretic factor binds to distinctly different molecular species depending on the presence or absence of certain ions and chelators. The two types of binding could be conveniently assayed in the presence of each other by elimination or inclusion of sodium chloride and o-phenanthroline in the assay system.     

Atrial natriuretic factor inhibits the sympathetic nervous activity in one-kidney, one-clip hypertension in the rat

The one-kidney, one-clip model of rat hypertension was found to have an increased natriuresis following chronic infusion of atrial natriuretic factor (ANF). We have now found that this natriuretic effect of ANF is associated with a suppression of the initially elevated urinary excretion of norepinephrine and epinephrine and increase of the excretion of the main dopamine metabolite-dihydroxyphenylacetic acid as well as of the urinary dopamine to norepinephrine ratio. These data are compatible with the hypothesis that ANF suppresses the increased sympathetic activity in this model of hypertension and this action combined with opposite changes of dopamine may contribute to the natriuretic effect of ANF.     

Plasma atrial natriuretic factor concentrations and renal actions in the domestic fowl

Plasma atrial natriuretic factor concentrations in Rhode Island red hens averaged 72.1±6.9 pg·ml^-1, range 33.4–136.0 pg·ml^-1. The intravenous infusion of isotonic saline containing 3% dextran for 2 h produced no significant changes in plasma osmotic or electrolyte concentrations; however, haematocrit changes indicated vascular expansions of 14.4% after 1 h and 21.3% after 2 h and plasma atrial natriuretic factor concentrations were elevated by 190% and 257%, respectively. The intravenous infusion of chicken atrial natriuretic factor at rates of 10, 25, 50 and 100 ng·kg^-1·min^-1 for 20 min produced levels of plasma atrial natriuretic factor that were directly related to the infusion rate and which, in birds undergoing a steady-state diuresis/natriuresis driven by the intravenous infusion of isotonic saline at 1 ml·min^-1, produced dose-dependent increases of 19, 26, 38 and 55% in urine flow rate and of 8, 30, 49 and 77% in sodium excretion. Potassium excretion was significantly increased only at the two highest atrial natriuretic factor infusion rates. The observed correlation between plasma atrial natriuretic factor concentration and vascular volume together with the atrial natriuretic factor-induced modulation of renal salt and water elimination is consistent with the concept that in the chicken this peptide has a physiological role as a regulatory hormone in volume homeostasis.Abbreviations AII angiotensin II - ANF atrial natriuretic factor - AVT arginine vasotocin - BV blood volume - chANF chicken atrial natriuretic factor - CHE chicken heart extract - ECF extracellular fluid - EDTA ethylenediaminetetra-acetate - Hct haematocrit - i.v. intravenous - PCR plasma clearance rate - PRA plasma renin activity - RIA radioimmunoassay     

The effects of hydraulic pressure on atrial natriuretic peptide during rehabilitative head-out water immersion

To study the hydraulic effects of subtotal immersion as a rehabilitative hydrotherapy, we examined the change in serum levels of atrial natriuretic peptide, catecholamine, cortisol and interleukins in 12 healthy volunteers. The subjects soaked in 42 degrees C water of 70 cm depth up to chin level in the upright seated position for 10 min. The serum level of atrial natriuretic peptide increased significantly 15 min after the start of subtotal immersion, though that of brain natriuretic peptide did not change. The serum dopamine level increased significantly 15 min after immersion, though neither the serum epinephrine nor norepinephrine levels did. In addition, 30 min after the start of immersion, the serum levels of atrial natriuretic peptide and dopamine decreased to those before immersion. The serum level of adrenocorticotropic hormone increased 15 min after immersion, though those of cortisol, interleukin-1beta and 6, and tumor necrotic factor-alpha did not change. It is suggested that 10-min head-out water immersion increased atrial natriuretic peptide partly due to increased venous return or right atrial load by hydraulic pressure.     

Interaction among atrial natriuretic factor (ANF), vasopressin, and renal nerves in terms of renal responses in rats.

The relationship between the renal nerves and vasopressin in terms of the natriuretic and diuretic responses to atrial natriuretic factor (ANF--0.25 microgram/kg/min for 15 min), was investigated in unilaterally denervated anesthetized rats before and after the administration of a vasopressin V2 specific antagonist (AVPX)--(40 micrograms/kg bolus followed by 0.4 microgram/kg/min infusion). Administration of the AVPX or ANF did not alter the arterial pressure. Acute renal denervation or AVPX administration independently produced significant increases in sodium and water excretion. ANF infusion by itself produced a greater increase in urine flow and sodium excretion from the denervated kidney compared to the intact kidney before the administration of AVPX. However, after the administration of AVPX renal responses to ANF from the intact kidneys were enhanced such that they were not significantly different from the denervated kidneys. These results suggest that the full physiological response to ANF may be masked by tonic renal nerve activity or antidiuretic actions of vasopressin. Furthermore, since combined renal denervation and AVPX administration does not produce any greater potentiation of the renal responses to ANF than either of these manipulations alone, it is suggested that they may act via a common mechanism, possibly altering activity in the renal nerves.     

Dissociation between right atrial pressure and plasma atrial natriuretic factor following prolonged high salt intake

The influence of prolonged high salt intake on intravascular volume, right atrial pressure, plasma atrial natriuretic factor, and extra-atrial tissue (lung, kidney, and liver) COOH- and NH2-terminal atrial natriuretic factor content was investigated in normotensive rats. Despite prolonged high salt (8% NaCl) intake for 5 weeks, total intravascular volume was not impaired. However, right atrial pressure was increased by 54% (p less than 0.01) after salt loading. Although this increment in right atrial pressure should favor atrial natriuretic factor release after NaCl intake, plasma atrial natriuretic factor (COOH-terminal) concentrations markedly decreased from 97.8 +/- 27 to 38.9 +/- 8 pg/mL. Sodium and circulatory homeostasis was, however, well preserved. The lungs contained the highest levels of COOH- and NH2-terminal atrial natriuretic factor. Salt loading resulted in increased concentrations of low as well as high molecular weight atrial natriuretic factor in the lung but not in the kidney or the liver. Our study indicates a limited role of atrial natriuretic factor in adaptation to prolonged salt consumption in rats. Dissociation between right atrial pressure and plasma atrial natriuretic factor after salt intake implicates other factors regulating circulating peptide levels. Prolonged salt intake increases lung generation of atrial natriuretic factor.     

Human alpha atrial natriuretic peptide inhibits angiotensin stimulated aldosterone biosynthesis in bovine adrenal glands

The behaviour of aldosterone output was evaluated in isolated and superfused bovine adrenal glands during superfusion with human alpha atrial natriuretic peptide on its own or with angiotensin II or a antagonist dopaminergic drug: metoclopramide. H alpha-ANP even in high concentrations did not reduce the basal amount of aldosterone released from bovine adrenal glands, nor did it modify aldosterone response to metoclopramide, but it partially inhibited aldosterone stimulation by angiotensin II. These data suggest that atrial natriuretic factor may affect sodium secretion through the modulation of aldosterone secretion.     

Renal effects of atrial natriuretic factor in the rats of different ages

Renal effects of an atrial natriuretic factor preparation were compared in 15, 28 and 66-day-old rats. This factor, prepared from atrial tissue of adult rats, was more effective in 28 and 66-day-old rats than in 15-day-old rats. There was a 6 fold increase of sodium excretion in 15-day-old rats and a 60 fold increase in 28-day-old rats. There was also a 15 fold increase in renal sodium excretion in 66-day-old rats receiving a higher dose (0.1 ml/animal). As indicated by the sodium potassium ratio, the increase in renal excretion of sodium was distinctly more pronounced than the increase in renal potassium excretion. In 15, 28 and 60-day-old rats, the increase of urine volume was 2 fold, 4 fold and 5 fold, respectively. The increase of fractional sodium excretion (FE) in rats receiving an atrial factor preparation was distinctly more pronounced than the increase of GFR. In all experiments, the preparation from ventricular tissue of the same animals was ineffective in producing natriuresis or diuresis.     

Involvement of the adrenal glands in the action of the atrial natriuretic factor

Adrenalectomized, medullectomized and sham operated rats were treated with either a chronic infusion or a bolus injection of the synthetic atrial natriuretic factor (ANF). ANF did not enhance natriuresis and diuresis in sham operated conscious animals during chronic infusion, but it had a potent action when injected as a bolus into anesthetized rats. The absence of the whole adrenal glands, but not adrenal medulla profoundly modified the renal response to ANF: a) following chronic administration of ANF, the baseline natriuresis paradoxically decreased in adrenalectomized rats, and b) in response to a bolus injection of ANF the natriuretic and diuretic actions of the peptide were attenuated in these animals. The medullectomy-induced decreased natriuresis and dopamine excretion were corrected by ANF infusion. Furthermore, ANF suppressed the compensatory increase of norepinephrine excretion secondary to adrenalectomy. The data suggest that the presence of the adrenal cortex is necessary for the natriuretic and diuretic actions of ANF. The decrease in urinary DA excretion may reflect diminished dopaminergic activity and contribute to the post-medullectomy antinatriuresis, a phenomenon which can be corrected by ANF infusion. ANF may also have a depressing activity on the increased sympathetic tone.     

Effect of chronic treatment with deoxycorticosterone acetate on content of a natriuretic substance in atria of rats

Chronic (72 days) administration of deoxycorticosterone acetate (DOCA), with or without saline as the sole drinking fluid, depleted atria of rats of their diuretic and natriuretic activities. Chronic ingestion of saline as the sole drinking fluid did not affect the diuretic, natriuretic, and kaliuretic activities of atria compared with those of rats receiving water to drink. Since systolic blood pressure of the DOCA-treated group did not differ significantly from that of the untreated control group, the decrease in potency of atrial extract from DOCA-treated rats most likely occurred in response to increases in extracellular and vascular volumes. The ability of DOCA to decrease diuretic and natriuretic activities of atria was dose dependent. The decreased activities of the atria of DOCA-treated rats could reflect an increased production and turnover of atrial natriuretic factor. Additional studies revealed an increased diuretic and natriuretic responsiveness of DOCA-treated recipients to atrial extract from untreated rats. Thus, the results of these studies suggest that chronic treatment with DOCA reduced the natriuretic and diuretic potencies of atrial extract and increased renal responsiveness to it.