Atrial natriuretic peptide in Bartter's syndrome

Bartter's syndrome: the unsolved puzzle

Bartter's syndrome is a congenital abnormality characterized by metabolic alkalosis [corrected], hyperreninemic hyperaldosteronism, and hypokalemia. Most patients present early in life with symptoms such as muscle weakness and polyuria, which may be attributed to potassium depletion. Despite the hyperaldosteronism, the patients tend to be normotensive, which is at least partially explained by vascular hyporesponsiveness to pressor hormones. Numerous studies have documented increased renal excretion of prostaglandins. Several different patterns of aberrant renal ion transport have been observed in patients with the syndrome, suggesting that it actually may represent a family of related but distinct tubular disorders. Therapeutic approaches to Bartter's syndrome include potassium supplementation, prostaglandin synthesis inhibitors (nonsteroidal anti-inflammatory agents), aldosterone antagonists, and converting enzyme inhibitors. During the first two decades following its initial description, Bartter's syndrome was the focus of widespread interest, based on the likelihood that its investigation might provide insight into the normal functioning of the renin-angiotensin-aldosterone and prostanoid hormone systems. During the past decade, however, little additional progress has been made in Bartter's syndrome, and its patho-physiology remains poorly understood.

[Bartter syndrome]

Bartter's syndrome, sometimes a familial autosomal recessive condition, is characterized by hypokalaemia with normal kaliuresis, hyperreninaemia with secondary hyperaldosteronism, vascular resistance to angiotensin and overproduction of prostaglandins by the kidneys. This syndrome is rare but sometimes envisaged in patients with unexplained hypokalaemia, the main difficulty being to exclude intoxication with diuretics which is very similar in all respects. Its physiopathology is unknown, and the various hypotheses put forward since it was first described (vascular insensitiveness to angiotensin, defect of sodium or chloride reabsorption, excess of atrial natriuretic factor, general abnormality of membrane permeability) were unable to demonstrate their primary character, each disorder described seeming, subsequently, secondary to another. For this reason, treatment is difficult and disappointing, but although the hypokalaemia is sometimes worrying, Bartter's syndrome is usually a benign condition.

Vasoactive peptides in Bartter's syndrome

Patients with Bartter's syndrome exhibit an increased vascular resistance to the pressor effects of angiotensin II and noradrenaline. Further, an increased production of vasodilating renal prostaglandins, perhaps mediating the vascular unresponsiveness, has been hypothesized in this syndrome based on high urinary prostaglandins. To determine whether different peptides might contribute to blood pressure control in this syndrome, the basal immunoreactive plasma levels of an array of vasoactive peptides and catecholamines were analysed in six patients with Bartter's syndrome. Among the vasoconstrictors analyzed, the mean plasma levels of noradrenaline (NA), adrenaline (A) and neuropeptide Y-like immunoreactivity (NPY-LI) were significantly increased as compared to healthy subjects (P = 0.030, 0.046 and 0.001, respectively). The plasma level of the vasodilator substance P (SP-LI) was also higher in these patients (P = 0.057). These results indicate that in Bartter's syndrome the vasoconstrictive effect of catecholamines and angiotensin II may be enhanced by concomitant NPY release. Whether a release of the vasodilator substance P is an independent mechanism or represents a reflex response to the increased secretion of angiotensin II, catecholamines and/or NPY remains to be established. However, the significance of these biochemical findings for blood pressure maintenance in Bartter's syndrome remains to be settled.

The effect of indomethacin on basal and saline-stimulated plasma atrial natriuretic factor levels in normal man

Surprisingly inappropriately high levels of plasma atrial natriuretic factor (ANF) in subjects with Bartter's syndrome are lowered by indomethacin therapy. Indomethacin in normal man causes sodium retention. One might therefore expect plasma ANF to increase in subjects taking indomethacin as a secondary phenomenon. On the other hand a decrease of plasma ANF in normal man similar to that reported in Bartter's subjects may explain the sodium retention caused by the drug in normals. We have studied plasma ANF before and during a two litre, four hour normal saline infusion in eight healthy male subjects both before and following five days of oral indomethacin. Plasma ANF basally was 4.2 +/- 0.9 pmol/l (mean +/- SEM) on no drug and 5.2 +/- 0.6 pmol/l on indomethacin (NS). It increased in response to saline in both studies (7.8 +/- 1.5 pmol/l after two litres of saline on control day; 10.6 +/- 1.5 pmol/l on the drug at the equivalent time, both p less than 0.05 vs basal value). Overall response to saline as assessed by the area under the curve above the basal value of hourly measurements, was not different in the two studies. Basal serum aldosterone and plasma renin activity were reduced by indomethacin. Urinary sodium excretion was not different between groups during the 12 hours before, four hours during and eight hours after the infusion. We have shown that indomethacin does not alter basal or saline stimulated plasma ANF in normal man, a finding in contrast to that reported in subjects with Bartter's syndrome. The sodium retention caused by indomethacin in normal man is not therefore due to a decrease of plasma ANF.

Does magnesium play a role in the hypokalemia of Bartter's syndrome?

A patient with Bartter's syndrome manifested hypomagnesemia in addition to hypokalemia. Under conditions of maximal free water production, he had a fractional distal solute reabsorption of 0.65, a value consistent with a renal defect in sodium chloride reabsorption in the thick ascending limb of the loop of Henle. This is also the site of 65% to 70% of urinary magnesium reabsorption. With magnesium supplementation and amiloride, the urinary potassium decreased and the serum potassium increased. Atrial natriuretic peptide concentrations in the plasma were low. Evaluation of family members showed five of nine offspring had hypokalemia with no disorder in the parents, an apparent autosomal recessive mode of inheritance. This is a US government work. There are no restrictions on its use.

Raised plasma levels of atrial natriuretic peptides in Addison's disease

Plasma levels of atrial natriuretic peptides (ANP) were significantly higher in 7 patients with treated Addison's disease (15.8 +/- 8.8 pg/ml, mean +/- SD) than in 7 control subjects (6.1 +/- 3.8 pg/ml) matched for sex, age, body weight and blood pressure. All subjects were studied on their usual sodium intake and had similar urinary sodium excretions. These findings indicate inappropriately high levels of plasma ANP in patients with treated Addison's disease and are possibly due to the lack of adrenal control on ANP synthesis and/or secretion in these patients.

Changes of molecular forms of atrial natriuretic peptide after treatment for congestive heart failure

In the present study, an attempt was made to clarify whether ANP molecular forms in the plasma of severe congestive heart failure patients differ from those in healthy persons and whether ANP molecular forms in the plasma of the patients were changed by successful treatment of cardiac disease. Twenty patients with congestive heart failure were treated at Kitasato University Hospital. They were classified as class III or IV by New York Heart Association criteria at the time of admission. Plasma ANP concentrations decreased after treatment from 356 +/- 58.2 to 72.3 +/- 14.8 pg/ml. The gel permeation chromatograms from the plasma of healthy persons showed low, or low and high molecular weight ANP peaks which correspond to the elution positions of authentic alpha-ANP or ribonuclease A (mol. wt., 13.7 kdalton). In patients with severe congestive heart failure at a severe stage, middle molecular weight ANP consisted with the elution position of authentic beta-ANP was particularly noted in addition of low and high molecular weight ANP peaks. This middle molecular weight peak disappears in most of cases by successful treatment. Molecular forms in the plasma obtained from the coronary sinus and the inferior or superior vena cava were essentially the same. These results indicate that the middle molecular weight ANP supposed as beta-ANP may particularly be secreted in severe congestive heart failure patients.

Effects of posture and saline infusion on atrial natriuretic peptide and haemodynamics in patients with Bartter's syndrome and healthy controls

In order to explore the role of atrial natriuretic peptide (ANP) in Bartter's syndrome, five patients and five healthy controls matched for age and sex were studied. The study was designed to stimulate and suppress ANP secretion by manipulation of right atrial pressure with different body positions and mild volume expansion with saline. Other vasoactive hormones were also measured, and heart rate and blood pressure were recorded at 5-min intervals. Plasma ANP levels increased after head-down tilt and returned to baseline in the upright position. Infusion of saline failed to increase plasma ANP both in the control group and in four of the patients. No significant differences were found in plasma atrial natriuretic peptide concentrations between both groups. In view of previously reported elevated plasma ANP levels, Bartter's syndrome may be heterogeneous in this respect. Plasma renin activity was higher in the patients, but plasma aldosterone, adrenaline and noradrenaline were similar in both groups. Mean arterial blood pressure was similar in both groups, but rose significantly in the upright position in the control group only, while changes in heart rate were similar in both groups. We conclude that atrial natriuretic peptide does not seem to play a causal role in our patients with Bartter's syndrome.

Influence of exercise on plasma atrial natriuretic factor levels in patients with myocardial infarction

The influence of dynamic exercise on plasma atrial natriuretic factor (ANF) levels was studied in a group of 10 patients with myocardial infarction (MI) and five patients with atypical chest pain (control group). Exercise protocol consisted of three fixed workloads (25, 50, and 75 watts) every 4 minutes with the use of a supine bicycle ergometer. Plasma ANF levels and hemodynamic indices were measured before, during, and 10 minutes after exercise. In the MI group, plasma ANF levels significantly increased at the 75-watt workload and significantly decreased at 10 minutes after exercise, whereas in the control group, the increase in plasma ANP levels after a 75-watt workload, compared with those at rest, was not significant. Significant correlations of pulmonary artery wedge pressure, right atrial pressure, mean arterial pressure, and heart rate to plasma ANF levels were observed at four points obtained before and during each stage of exercise in the MI group. Furthermore, a significant correlation between maximal creatine kinase levels and plasma ANF levels at a 75-watt workload and a significant inverse correlation between left ventricular ejection fraction and plasma ANF levels at a 75-watt workload were observed. These results suggest that the increase in the circulating ANF level during exercise in MI is associated with elevated atrial pressure resulting from left ventricular dysfunction and that measurement of ANF during exercise may be an indication of the severity of MI and associated left ventricular dysfunction.

Altering angiotensin levels by administration of captopril or indomethacin, or by angiotensin infusion, contributes to an understanding of atrial natriuretic peptide regulation in man

1. Plasma atrial natriuretic peptide (ANP) levels were positively correlated with plasma renin activity (PRA) levels, when blood volume and blood pressure (BP) were not raised in normal subjects (NLS) or patients with postoperative aldosterone-producing adenoma (APA), Bartter's syndrome (BS), Addison's disease, anorexia nervosa, diuretic abuse or salt-losing congenital adrenal hyperplasia. 2. Angiotensin II infusion raised ANP levels in NLS, and patients with BS, pre- and postoperative APA, only when BP rose, suggesting that this effect might be mediated by the rise in BP. 3. Captopril lowered aldosterone and ANP levels in renal artery stenosis, but falling BP levels could mediate this effect. Captopril lowered aldosterone and BP in BS, but did not lower ANP, perhaps because angiotensin remained elevated. 4. Indomethacin lowered ANP when PRA was initially normal or raised (NLS and BS), but not when PRA was suppressed (APA). This effect could not be mediated by BP, which rose, but could be mediated by renin-angiotensin, which fell. 5. Factors other than central blood volume and atrial stretch may modulate ANP levels. Plasma angiotensin II may be such a factor, and may exert an important influence at high levels, especially when blood volume is low.

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.

Red blood cell Li+-Na+ countertransport, Na+-K+ cotransport, and the hemodynamics of hypertension

Red blood cell Li+-Na+ countertransport and Na+-K+ cotransport activities, home blood pressure, invasive systemic hemodynamics, and limb venous compliance were measured in 65 white men (23 normotensive, 22 borderline hypertensive, and 20 mild essential hypertensive subjects). Li+-Na+ countertransport activity was positively and significantly correlated with subject-determined home systolic blood pressure (r = 0.31, p less than 0.02) and with directly measured systolic (r = 0.29, p less than 0.02) and diastolic (r = 0.27, p less than 0.03) blood pressures in the hemodynamic laboratory, independent of potential confounding variables. Analysis of the hemodynamic determinants of blood pressure revealed a significant positive correlation of countertransport with vascular resistance (r = 0.30, p less than 0.02) but not with cardiac output or cardiac index. High red blood cell Na+-K+ cotransport activity was not independently associated with hypertension or with a characteristic hemodynamic pattern but was related to decreased venous compliance. Red blood cell Li+-Na+ countertransport deserves further study as a marker for the genetic substrate of human essential hypertension. Red cell Na+-K+ cotransport may be altered secondarily by factors related to high blood pressure and seems to be a valid marker for abnormalities of the venous system in hypertension.