Atrial natriuretic peptide during mineralocorticoid escape in the human

A case of primary aldosteronism combined with acquired nephrogenic diabetes insipidus

Aldosterone-producing adrenal adenoma can induce various clinical manifestations as a result of chronic exposure to aldosterone. We report a rare case of a 37-year-old man who complained of general weakness and polyuria. He was diagnosed with aldosterone-producing adrenal adenoma and nephrogenic diabetes insipidus. Aldosterone enhances the secretion of potassium in the collecting duct, which can lead to hypokalemia. By contrast, nephrogenic diabetes insipidus, which manifests as polyuria and polydipsia, can occur in several clinical conditions such as acquired tubular disease and those attributed to toxins and congenital causes. Among them, hypokalemia can also damage tubular structures in response to vasopressin. The patient’s urine output was >3 L/d and was diluted. Owing to the ineffectiveness of vasopressin, we eventually made a diagnosis of nephrogenic diabetes insipidus. Laparoscopic adrenalectomy and intraoperative kidney biopsy were subsequently performed. The pathologic finding of kidney biopsy revealed a decrease in aquaporin-2 on immunohistochemical stain.

A case of primary aldosteronism with end stage renal disease

A 52-year-old woman was referred to our hospital due to chronic renal failure with a 10-year history of hypertension. We found polycystic kidney disease, pulmonary tuberculosis and an aldosterone-producing adrenocortical mass. At this time, her serum potassium level and blood pressure were within the normal range. She refused hemodialysis and then was hospitalized because of uremic encephalopathy. On admission, her serum potassium level was normal without treatment and plasma aldosterone concentration highly elevated. She received hemodialysis, and thereafter hypokalemia developed. We then administered spironolactone, whereupon serum potassium level returned to the normal range. In this case, we thought that normokalemia was balanced hypokalemia of primary aldosteronism with hyperkalemia of chronic renal failure, and that hypokalemia developed after hemodialysis was due to an imbalanced primary aldosteronism with end stage renal disease.

Thiazolidinediones and the renal and hormonal response to water immersion-induced volume expansion in type 2 diabetes mellitus

Thiazolidinediones cause sodium retention and edema by a direct effect on the kidneys. The aim of this study was to use the technique of head-out water immersion to investigate the effects of rosiglitazone on sodium and volume homeostasis in subjects with type 2 diabetes mellitus. The volume expansion response to water immersion was compared with the response on a non-immersion control day in 12 nondiabetic male subjects and 8 diet-controlled male type 2 diabetic subjects with hourly blood and urine sampling over a 4-h period. This was repeated after both groups had taken 4 mg of rosiglitazone daily for 7 days. Immersion produced a natriuresis in both groups (P < 0.001). An impairment of this natriuresis was seen in the diabetic subjects (P = 0.006). However, when rosiglitazone was taken, there was no significant difference in immersion-induced natriuresis compared with nondiabetic controls (P = 0.2). There was an immersion-induced rise in atrial natriuretic peptide (ANP) and urinary cyclic guanosine monophosphate (cGMP), in the healthy subjects (ANP P = 0.001, cGMP P = 0.043), which was not seen in the diabetic subjects (ANP P = 0.51, cGMP P = 0.74). Rosiglitazone restored the immersion-induced increase in cGMP excretion and rise of ANP in the diabetic group (ANP P = 0.048, cGMP P = 0.009). This study confirms that type 2 diabetic subjects have an impaired natriuretic response to acute volume expansion, which appears to be enhanced rather than diminished by rosiglitazone. This may be related to its effects in increasing natriuretic peptides and restoring the impaired cGMP excretion to volume expansion.

Aldosterone biosynthesis, regulation, and classical mechanism of action

Circulating aldosterone is principally made in the glomerulosa zone of the adrenal cortex by a series of enzyme steps leading to the conversion of cholesterol to aldosterone. Uniquely, aldosterone's production is regulated at two critical enzyme steps: (1) early in its biosynthetic pathway (the conversion of cholesterol to pregnenolone cholesterol side chain cleavage enzyme) and (2) late (the conversion of corticosterone to aldosterone by aldosterone synthase). A variety of factors modify aldosterone secretion--the most important are angiotensin II (AngII), the end-product of the renin-angiotensin system (RAS), and potassium. However ACTH, neural mediators and natriuretic factors also contribute at least over the short run. Aldosterone's classical epithelial effect is to increase the transport of sodium across the cell in exchange for potassium and hydrogen ions. Although still controversial, there is an increasing body of data that supports the hypothesis that aldosterone can be synthesized in tissues outside of the adrenal cortex, specifically in the heart and the vasculature. Aldosterone's biosynthesis appears to be regulated in these tissues similar to what occurs in the adrenal cortex. The role of this extra adrenal aldosterone production in health and disease is as of yet undetermined.

Salt-sensitive hypertension in ANP knockout mice: potential role of abnormal plasma renin activity

Atrial natriuretic peptide (ANP), a peptide hormone produced by the heart, exerts a chronic hypotensive effect. Knockout mice with a homozygous disruption of the pro-ANP gene (-/-) are incapable of producing ANP and are hypertensive relative to their wild-type (+/+) siblings. Previous studies showed that arterial blood pressure (ABP) was further increased in conscious -/- mice kept for 2 wk on 2% salt, but not in anesthetized -/- mice after 1 wk on 8% salt. To determine whether inconsistencies in observed effects of salt on ABP of -/- mice are due to duration of increased salt intake and/or the state of consciousness of the animals, we measured ABP from an exteriorized carotid catheter during and after recovery from anesthesia with ketamine-xylazine in adult +/+ and -/- mice kept on low (LS; 0.008% NaCl)- or high (HS; 8% NaCl)-salt diets for 3-4 wk. Conscious ABP +/- SE (mmHg) of +/+ mice did not differ significantly on either diet (HS, 113 +/- 3; LS, 110 +/- 5). However, on HS diet -/- mice had significantly higher ABP (135 +/- 3; P < 0.001) than both -/- (115 +/- 2) and +/+ (110 +/- 5) mice on LS diet. Anesthesia decreased ABP in all groups, but the the genotype- and diet-related differences were preserved. Plasma renin activity (PRA, ng ANG I.ml-1.h-1) in blood collected at termination of experiment was appropriately different on the 2 diets in +/+ mice (HS, 4.9 +/- 1.9; LS, 21 +/- 2.8). However, PRA failed to decrease in -/- mice on HS diet (HS, 18 +/- 2.9; LS, 19 +/- 3.7). Independent of genotype, concentration of endothelin-1 (ET-1, pg/mg protein) and endothelial constitutive NOS (ecNOS, density/100 micrograms protein) was significantly elevated in kidneys of mice fed on HS diet (ET-1 -/-, 31 +/- 4.7 and +/+, 32 +/- 4.1; ecNOS -/-, 160 +/- 19 and +/+, 156 +/- 19) compared with mice fed on LS diet (ET-1 -/-, 19 +/- 1.9 and +/+, 21 +/- 1.8; ecNOS -/-, 109 +/- 13 and +/+, 112 +/- 18). We conclude that, regardless of the state of alertness, -/- mice develop salt-sensitive hypertension after prolonged feeding on HS, in part due to their inability to reduce PRA, whereas the specific renal upregulation of ecNOS and ET-1 in response to HS intake may be an ANP-independent adaptive adjustment aimed at improving kidney function and counteracting the pressor effect of salt.

Hormonal and humeral considerations in hypertensive disease

This article has discussed the classic hormonal causes of hypertension. Pheochromocytoma and hyperaldosteronism have been discussed, and a clinical approach to evaluation of patients for these problems has been presented. Other humeral factors that influence volume pressure homeostasis have been discussed. It is likely that pharmacologic agents affecting these other factors will become available to patients with hypertension in the future.

Reduced renal extraction of atrial natriuretic peptide in primary aldosteronism

We investigated renal and peripheral forearm extraction of atrial natriuretic peptide in patients with primary aldosteronism to determine whether alterations in extraction may contribute to the elevated levels of circulating atrial natriuretic peptide observed in primary aldosteronism. We obtained simultaneous venous blood samples from the left renal vein and a peripheral vein and from the radial artery in 28 patients with primary aldosteronism and 10 patients with essential hypertension. Renal extraction of atrial natriuretic peptide was significantly (P < .001) reduced (40 +/- 2%) in primary aldosteronism compared with essential hypertensive patients (62 +/- 3%). Peripheral forearm extraction was also reduced (P < .01) in primary aldosteronism compared with essential hypertensive patients (24 +/- 3% versus 38 +/- 4%). These findings are consistent with widespread downregulation of atrial natriuretic peptide receptors in primary aldosteronism. Consistent with reports that marked reduction in glomerular filtration rate is required before the renal extraction of atrial natriuretic peptide is reduced, no significant relationship between renal extraction of atrial natriuretic peptide and plasma creatinine was seen in primary aldosteronism or essential hypertension. Although the major regulators of atrial natriuretic peptide secretion in primary aldosteronism are presumably alterations in arterial blood pressure and plasma volume, reduced renal and peripheral extraction of atrial natriuretic peptide in primary aldosteronism may also contribute significantly to the elevated circulating levels observed.

Atrial natriuretic factor significantly contributes to the mineralocorticoid escape phenomenon. Evidence for a guanylate cyclase-mediated pathway

The mechanism underlying the mineralocorticoid escape phenomenon remains unknown. To assess the possible contribution of natriuretic peptides to mineralocorticoid escape, rats were injected with 5 mg deoxycorticosterone acetate for 3 d. Plasma atrial natriuretic factor (ANF) rose to twice basal levels and atrial ANF content decreased significantly by 24 h of treatment. This coincided with renal escape and with a significant increase in urinary cGMP excretion. Plasma ANF remained elevated and atrial ANF content continued to decline by 48 and 72 h while atrial ANF mRNA levels increased significantly only at 72 h. Plasma brain natriuretic peptide did not increase during escape although atrial brain natriuretic peptide mRNA levels increased significantly. Chronically administered HS-142-1 (HS), a specific antagonist of the guanylate cyclase-coupled natriuretic peptide receptors, significantly and dose-dependently impaired the escape phenomenon. The highest dose of HS completely suppressed the increase in urinary cGMP. Despite the continued suppression, partial escape was observed by the end of the observation period. HS alone influenced neither plasma nor tissue or urine parameters. These findings show that despite activation of atrial ANF, blockade of the guanylate cyclase-coupled natriuretic peptide receptors impairs the ability of the kidney to escape the Na+ retaining effect of excess mineralocorticoid in a dose-dependent fashion. Later-acting, unknown mechanisms eventually come into play to mediate the escape phenomenon through a guanylate cyclase-independent pathway. Therefore, ANF of cardiac origin appears to be a major factor initiating mineralocorticoid escape through a guanylate cyclase-dependent pathway.

Potassium supplementation ameliorates mineralocorticoid-induced sodium retention

Potassium depletion induced by dietary potassium restriction causes sodium retention while potassium supplementation augments urinary sodium excretion. The role of external potassium balance in modulating mineralocorticoid-induced sodium retention in humans is unknown. Accordingly, eight healthy subjects were studied at the Clinical Research Center receiving a constant diet providing (per kg body wt) sodium 2.5 mmol, potassium 1.1 mmol daily. After establishing basal sodium and potassium balance over three days, each subject received 9 alpha-fludrocortisone 0.4 mg/day for 10 days. Subjects were studied twice, four to eight weeks apart, in a double blind, randomized crossover design receiving either placebo or additional KCl (80 mmol/day) over the 10 day study period. Serum potassium concentrations were unchanged from basal values on KCl while the values fell (4.1 +/- 0.1 vs. 3.4 +/- 0.1 mmol/liter, P = 0.01) on placebo. Urinary sodium excretion decreased with fludrocortisone administration in both groups, but this decrease reached significance only in the placebo group. Furthermore, during fludrocortisone administration the sodium excretion rates on KCl were significantly higher compared to the values noted on placebo (134 +/- 8 vs. 112 +/- 13 mmol/day, P = 0.01). Body weight recorded after 10 days of fludrocortisone administration was higher on placebo compared to KCl (72.3 +/- 2.8 vs. 71.6 +/- 2.8 kg, P = 0.01). Plasma renin activity, and aldosterone concentrations decreased on fludrocortisone while atrial natriuretic peptide levels increased. These studies suggest that amelioration of hypokalemia attenuates mineralocorticoid-induced sodium retention. Therefore, potassium depletion may contribute to the mineralocorticoid-induced sodium retention.

Mineralocorticoid escape in patients with compensated cirrhosis and portal hypertension

Failure to escape from mineralocorticoids in compensated cirrhosis is considered a major argument supporting the overflow theory of ascites. To assess the frequency and mechanism of mineralocorticoid escape in cirrhosis, 9-alpha-fluorohydrocortisone (0.6 mg/day) was administered to 19 patients with compensated cirrhosis, portal hypertension, and no history of ascites who were able to maintain sodium balance on a 250 mmol Na+ diet. Fifteen patients (78.9%) escaped from mineralocorticoids, while 4 patients (21.1%) did not escape and developed ascites. Patients who did not escape had significantly higher cardiac index (4.97 +/- 0.42 vs 3.46 +/- 0.21 L.min-1.m-2) and lower peripheral vascular resistance (485.9 +/- 37.5 vs. 665.8 +/- 32.9 dyne.s.cm-5/m2) than those who escaped. Hepatic venous pressure gradient was not significantly different. The escape phenomenon was associated with a significant increase in mean arterial pressure, creatinine clearance, and atrial natriuretic factor and suppression of plasma renin activity. All of these parameters showed minimal or no changes in patients who did not escape. These results indicate that failure to escape from mineralocorticoids is uncommon in patients with compensated cirrhosis, is related to an inadequate expansion of effective plasma volume due to the accumulation of ascites, and occurs in patients with marked peripheral arteriolar vasodilation.

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.

Basal and saline-stimulated levels of plasma atrial natriuretic factor in acromegaly

We have studied plasma ANF before and after a 4-h intravenous infusion of normal saline in eight subjects with active acromegaly and in eight age and sex-matched control subjects. Plasma ANF, serum aldosterone and blood pressure were measured basally and after 2 and 4 h and plasma renin activity basally and after 4 h. Basal plasma ANF was similar in each group (4.4 +/- 1.5 pmol/l (mean +/- SEM) in acromegalic subjects and 5.3 +/- 0.7 pmol/l in controls NS). Plasma ANF did not rise significantly after saline in the acromegalic group (2-h value, 5.9 +/- 0.9; 4-h value, 5.1 +/- 0.9 pmol/l) but did rise significantly in the control group (2-h value, 8.9 +/- 1.9; 4-h value 9.5 +/- 1.3 pmol/l, both values P less than 0.05 vs basal level). The 4-h ANF value was significantly higher in the control group than in the acromegalic group (P less than 0.05). Basal and stimulated serum aldosterone values were similar in the two groups. Plasma renin activity suppressed to a lesser extent in the acromegalic group after 4 h. The facts that basal plasma ANF was not raised in acromegalic subjects and did not respond to saline stimulation demonstrate that an abnormality of ANF control may be an important factor in the aetiology of the expanded sodium status of patients with acromegaly and hence may contribute to the hypertension seen in patients with growth hormone excess.

Effect of sodium intake on fasting and postprandial levels of atrial natriuretic factor in humans

The effect of chronic dietary sodium intake on fasting and postprandial plasma atrial natriuretic factor (ANF) levels was examined in 2 studies of normal humans. In Study I, 3 separate groups of normals (n = 8 for each) received diets of either low (L), normal (N) or high (H) daily sodium intake for 7 days. Twenty-four h urines for sodium were obtained on days 6 and 7. Urine sodium excretion for each group was (L) 13.1 +/- 3.7, (N) 150.1 +/- 19.4 and (H) 271.3 +/- 33.6 mEq/day. On the completion of day 7, fasting plasma ANF showed no change with alteration in sodium intake. In contrast, when blood samples were obtained postprandially, significant increases in plasma ANF were observed in the group maintained on high sodium diet. In Study II, a continuous group of normals (n = 8) received the 3 sodium controlled diets for 7 days sequentially (L/N/H). No significant changes in fasting levels of ANF were detected between L/N/H sodium diets. In conclusion, these studies show that the maintenance of sodium balance during chronic changes in sodium intake can occur despite no significant increase in plasma ANF under normal steady state conditions. However, plasma ANF is significantly elevated during chronic high sodium intake, when measured postprandially.

Norethandrolone produces temporary loss of the ability to escape from salt-retaining steroids

Patients with diseases characterized by salt retention manifest a loss of the normal ability of healthy persons to escape from repeat injections of aldosterone or other salt-retaining steroids. This phenomenon may be a clue to the pathophysiological mechanisms of salt retention. Administration of norethandrolone to a subject who had demonstrated the ability to escape from the salt-retaining effect of corticosteroid administration temporarily and reversibly deleted his ability to escape. Thus norethandrolone administration provides the basis for a model system for exploring the mechanisms of escape (and therefore of salt retention).

Enhancement of renal but not haemodynamic effects of atrial natriuretic peptide (1-28) in sheep treated with ACTH

1. The haemodynamic and renal effects of short-term infusion of human atrial natriuretic peptide (ANP) (1-28) were examined in sheep treated with ACTH and compared with the responses previously observed in normotensive sheep. 2. Infusion of ANP at 100 micrograms/h for 60 min in ACTH-treated sheep (5 micrograms/kg per day for 5 days) decreased blood pressure and produced a fall in both cardiac output and stroke volume. No changes were seen in heart rate and total peripheral resistance. 3. ANP produced large increases in urine volume, urinary sodium and chloride excretion, and further decreased plasma potassium concentration in the ACTH-treated sheep. Compared with normal sheep studied previously under the same conditions, the ACTH-treated sheep showed a much greater diuretic and natriuretic response to ANP, although the blood pressure response to ANP was similar in both states. 4. The change in renal responsiveness to ANP in sheep may be related to the increased blood volume of the ACTH-treated animals because volume expansion is known to enhance the renal effects of ANP.

Enhanced natriuretic effect of atrial natriuretic factor during mineralocorticoid escape in humans

We examined the question of whether escape from the sodium-retaining effect of mineralocorticoid involves an increased natriuretic effect of atrial natriuretic factor (ANF). Seven healthy volunteers taking a 170 mmol Na/100 mmol K diet received an intravenous bolus (25 micrograms) followed by a 1-hour infusion (0.02 micrograms/kg/min) of ANF (human ANF-[99-126]) before and after 10 days of 9-fludrocortisone acetate, 0.5 mg b.i.d. Escape was accompanied by an increase in body weight (from 72.2 +/- 12.9 to 74.0 +/- 12.6 kg; p less than 0.05), mean arterial pressure (from 95 +/- 4 to 109 +/- 3 mm Hg; p less than 0.01), plasma ANF (from 9 +/- 2 to 24 +/- 4 pmol/L; p less than 0.01), and inulin clearance (from 124 +/- 9 to 137 +/- 7 ml/min; p less than 0.05). Indexes for renal sodium handling (lithium and free water clearance) were compatible with a decreased "proximal" and an increased "distal" tubular reabsorption fraction. ANF infusion raised inulin clearance comparably before and after escape to 138 +/- 10 and 152 +/- 7 ml/min, respectively, but the natriuretic effect was much larger (p less than 0.05) after escape (from 366 +/- 34 to 1294 +/- 278 mumol/min) than before (from 248 +/- 48 to 630 +/- 124 mumol/min). Indexes for tubular reabsorption were consistent with greater suppression of both "proximal" and "distal" tubular sodium reabsorption by ANF after versus before mineralocorticoid expansion. These results indicate that escape is accompanied not only by a rise in plasma ANF but also by potentiation of the natriuretic effect of ANF.(ABSTRACT TRUNCATED AT 250 WORDS)

Atrial natriuretic peptide: blood levels in human disease and their measurement

The atrial hormonal system consists of 126 amino acid-containing prohormone (proANP) stored in the secretory granules of atrial myocytes and 28 amino acid-containing hormone (ANP) that is secreted into the bloodstream in response to raised atrial pressure. ANP participates in the homeostasis of body fluid volume through its main receptor-mediated effects; natriuresis, inhibition of renin and aldosterone secretion, and vasodilation. It counteracts the renin-angiotensin system with the putative primary role of regulating the circulating blood volume. Although in man, the physiologic volume stimuli lead to relatively modest increases of ANP secretion, its plasma level undergoes striking changes in pathology. Marked elevations in conditions accompanied by fluid retention, most conspicuously in heart failure and renal failure, have been explained as a compensatory reaction to volume overload. The recent data suggest a decreased target organ responsiveness as one of the causes of a relative inefficiency of the high circulating levels of ANP in inducing an appropriate natriuresis in these volume overload conditions. The well established radioimmunoassay and the more recent methods of plasma ANP measurement are reviewed, and the authors' results with a commercial RIA are presented.

Cardio-renal-endocrine dynamics during stepwise infusion of physiologic and pharmacologic concentrations of atrial natriuretic factor in the dog

Infusion of alpha-human atrial natriuretic factor (alpha-h-ANF) into pentobarbital anesthetized dogs (n = 10) at 0.0025, 0.005, 0.01, and 0.3 micrograms/kg/min was performed to differentiate the physiologic actions of atrial natriuretic factor from its pharmacologic actions. The lowest doses of atrial natriuretic factor infusion resulted in circulating levels that were previously produced by 0-10% saline volume expansion. At the lowest infusion rate, circulating ANF increased 31 +/- 3 pg/ml, resulting in a significant increase in absolute sodium excretion, fractional excretion of sodium, and fractional excretion of lithium, and a significant decrease in urine osmolality. A greater change in circulating atrial natriuretic factor (96 +/- 12 pg/ml) was required to significantly decrease right atrial pressure, cardiac output, and plasma renin activity, and to increase systemic vascular resistance and total and fractional excretion of potassium. The highest dose of atrial natriuretic factor infused was required to decrease arterial pressure and renal vascular resistance. The present study demonstrates that atrial natriuretic factor is natriuretic and diuretic at physiologic concentrations; at low concentrations, atrial natriuretic factor appears to decrease the whole kidney proximal tubular reabsorption of sodium and does not affect glomerular filtration rate; a greater (but physiologic) change in circulating atrial natriuretic factor is required to significantly decrease cardiac output, cardiac filling pressure, and plasma renin activity than is required to significantly increase sodium excretion; and a decrease in systemic arterial pressure and vascular resistance does not occur at physiologic concentrations of atrial natriuretic factor.

Sodium excretion and atrial natriuretic peptide levels during mineralocorticoid administration. A mechanism for the escape from hyperaldosteronism

Urinary sodium excretion initially decreases when mineralocorticoid levels are increased, but if high plasma levels of hormone are maintained, sodium excretion rises to again equal sodium intake. To ascertain if atrial natriuretic peptide (ANP) plays a role in reestablishing sodium balance during mineralocorticoid ingestion, 0.3 to 0.5 mg per day of fludrocortisone were administered for 18 days to four healthy male subjects. The average daily intake of sodium was regulated at 180 +/- 2 meq. ANP levels rose from a mean of 91.7 +/- 13.0 pg/ml during the control week to 179.7 +/- 39.2 pg/ml during the final week on fludrocortisone (p less than 0.05). Urinary sodium excretion fell 27% immediately after fludrocortisone administration was initiated but returned to baseline levels in an average of 5 days. Levels of ANP, normalized for each subject to the mean of his control week values, correlated with the amount of sodium excreted in the subsequent 24 hours (p less than 0.05). Simultaneous with the rise in ANP values, levels of plasma renin activity (PRA) and aldosterone decreased. ANP concentrations throughout the study were inversely correlated with PRA and aldosterone levels (p less than 0.001 for both correlations). Values of serum osmolality and plasma arginine vasopressin did not change significantly during the study. The results obtained demonstrate that increased secretion of ANP is associated with escape from the sodium retaining effect of chronically high mineralocorticoid levels in man and suggest that ANP plays a prominent role in the mechanism of this escape.