In vivo measurement of ANP overall turnover and identification of its main metabolic pathways under steady state conditions in humans

Different factors affecting human ANP amyloid aggregation and their implications in congestive heart failure

Aims

Atrial Natriuretic Peptide (ANP)-containing amyloid is frequently found in the elderly heart. No data exist regarding ANP aggregation process and its link to pathologies. Our aims were: i) to experimentally prove the presumptive association of Congestive Heart Failure (CHF) and Isolated Atrial Amyloidosis (IAA); ii) to characterize ANP aggregation, thereby elucidating IAA implication in the CHF pathogenesis.

Methods and Results

A significant prevalence (85%) of IAA was immunohistochemically proven ex vivo in biopsies from CHF patients. We investigated in vitro (using Congo Red, Thioflavin T, SDS-PAGE, transmission electron microscopy, infrared spectroscopy) ANP fibrillogenesis, starting from α-ANP as well as the ability of dimeric β-ANP to promote amyloid formation. Different conditions were adopted, including those reproducing β-ANP prevalence in CHF. Our results defined the uncommon rapidity of α-ANP self-assembly at acidic pH supporting the hypothesis that such aggregates constitute the onset of a fibrillization process subsequently proceeding at physiological pH. Interestingly, CHF-like conditions induced the production of the most stable and time-resistant ANP fibrils suggesting that CHF affected people may be prone to develop IAA.

Conclusions

We established a link between IAA and CHF by ex vivo examination and assessed that β-ANP is, in vitro, the seed of ANP fibrils. Our results indicate that β-ANP plays a crucial role in ANP amyloid deposition under physiopathological CHF conditions. Overall, our findings indicate that early IAA-related ANP deposition may occur in CHF and suggest that these latter patients should be monitored for the development of cardiac amyloidosis.

Measurement of brain natriuretic peptide in plasma samples and cardiac tissue extracts by means of an immunoradiometric assay method

We evaluated the analytical characteristics and clinical usefulness of a commercial immunoradiometric assay (IRMA) kit for brain natriuretic peptide (BNP). Mean (+/-SD) plasma BNP concentrations measured in 129 normal subjects were 2.9+/-2.7 pmol/l (median 2.2 pmol/l; range 0.1-12.4 pmol/l). The mean (+/- SD) value observed in healthy men (2.1 +/- 2.0 pmol/l, n = 49) was significantly (p=0.0009) different to that found in women (3.4 +/- 2.9 pmol/l, n=80). A positive relationship (R=0.214, p=0.0174) was found between BNP values and age. In 65 patients with cardiac diseases, BNP levels increased with the progression of clinical severity of disease; patients with more severe disease [NYHA functional class III-IV, mean (+/- SD) BNP +/- 254 +/- 408 pmol/l, n=22] showed significantly (p<0.0001) increased values compared to patients with mild symptoms of disease (NYHA functional class I-II, mean (+/- SD) BNP=19.6 +/- 17.2 pmol/l, n=43). Furthermore, in 32 patients with chronic renal failure, greatly increased (p<0.0001) BNP values were found both before (mean +/- SD=88. 1+/- 111.1 pmol/l) and after haemodialysis (mean +/- SD=65.6 +/- 76.7 pmol/l), with a significant reduction after haemodialysis (p=0.0004) compared to pre-haemodialysis. The mean (+/- SD) BNP value found in atrial extracts collected during aorto-coronary bypass operations in 15 patients was 14.5 +/- 51.9 pmol/g of cardiac tissue. Moreover, the mean (+/- SD) tissue levels of BNP in 7 heart transplant recipients were 128.4 +/- 117.2 pmol/g of cardiac tissue in atrium, 68.4 +/- 76.7 pmol/g in ventricle, and 10.9 +/- 8.5 pmol/g in interventricular septum. Finally, BNP values found in cardiac tissues of two subjects collected at autopsy were considerably lower (on average 1/1000) than those observed in cardiac tissues of patients with cardiac diseases. The IRMA method for BNP determination evaluated in this study showed a good degree of sensitivity, precision and practicability. Therefore, this method should be a reliable tool for the measurement of plasma BNP levels for both experimental studies and routine assay.

Circulating levels of cardiac natriuretic hormones measured in women during menstrual cycle

Alterations in fluid and electrolyte balance represent a common complaint by women during different stages of the menstrual cycle; however, conflicting results concerning the possible role of plasma Atrial Natriuretic Peptide (ANP) modifications during the menstrual cycle have been reported. This may be due to differences in assay methods or in the clinical protocol adopted. Moreover, possible variations in plasma Brain Natriuretic Peptide (BNP) levels during the menstrual cycle have not been studied. We measured the plasma levels of ANP and BNP by means of two highly sensitive and specific immunoradiometric assay (IRMA) methods in 19 normal women without premenstrual symptoms, in order to evaluate whether significant modifications of these hormones are present during the menstrual cycle. Because it is well-known that circulating levels of cardiac hormones show great variations in normal subjects due to their rapid plasma half lives, blood samples were collected at 2.5-min intervals over a 15-min period on the 5th and 24th days of the cycle. The mean (+/-SD) values of ANP (follicular phase=15.1+/-8.7 pg/ml; luteal phase=14.8+/-9.5 pg/ml) and of BNP (follicular phase=13.0+/-15.0 pg/ml; luteal phase=11.2+/-11.4 pg/ml) did not show significant variations during the menstrual cycle. Moreover, the variability of ANP values (CV=24.8+/-13.2%) was significantly higher (p=0.0318) than that of BNP values (CV=16.5+/-8.9%), and a significant correlation was found between the mean ANP and BNP values of the individual women studied (R=0.407, p=0.0437). The values of estradiol, progesterone, LH, FSH and prolactin did not correlate with the ANP or BNP values. In conclusion, our results indicate that circulating levels of cardiac hormones do not show any significant modifications during the menstrual cycle in healthy women.

Circulating levels of cardiac natriuretic peptides (ANP and BNP) measured by highly sensitive and specific immunoradiometric assays in normal subjects and in patients with different degrees of heart failure

Plasma atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP) levels increase in patients with heart failure with the progression of clinical symptoms and with the deterioration of hemodynamics; consequently, assay methods for these peptides may be useful in the follow-up of cardiac patients. Non-competitive immunoradiometric assay (IRMA) methods for ANP or BNP do not generally require preliminary extraction and/or purification of the plasma sample, and so may be more suitable than competitive immunoradiometric assay (RIA) methods for the routine assay of plasma peptide concentrations. We evaluated the analytical characteristics and clinical usefulness of two IRMAs for plasma ANP and BNP, to verify whether these methods may be considered suitable for the follow-up of patients with heart failure. Both methods are based on the solid-phase sandwich IRMA system, which uses two monoclonal antibodies prepared against two sterically remote epitopes of peptide molecule; the first antibody was coated on the beads solid-phase and the second was radiolabeled with 125I. Blood samples were collected from a brachial vein in ice-chilled disposable polypropylene tubes containing aprotinin and EDTA after the patient had rested for at least 20 min in the recumbent position. Plasma samples were immediately separated by centrifugation and stored at -20 C until assay. The IRMA methods showed a better sensitivity and a wider working range sensitivity (about 2 ng/l) than those of RIA methods. Moreover, the normal range found with these methods (ANP = 16.1 +/- 8.6 ng/l, 5.2 +/- 2.8 pmol/l, BNP = 8.6 +/- 8.2 ng/l, 2.5 +/- 2.4 pmol/l) was similar to that generally reported using the most accurate methods, such as the other IRMAs or RIAs, using a preliminary extraction and purification of plasma samples with chromatographic procedures. Our results obtained in patients with different degrees of heart failure indicate that plasma ANP and BNP increase with the progression of clinical symptoms (NYHA class) (ANOVA p < 0.0001). Indeed, circulating levels of ANP (R = -0.701, no. = 86) and BNP (R = -0.745, no. = 55) were significantly (p < 0.0001) and negatively correlated with the left ventricular ejection fraction values. Furthermore, a close curvilinear regression (R = 0.960, no. = 215) was found between ANP and BNP values, because plasma BNP progressively increases more than plasma ANP in patients with different stages of heart failure. In conclusion, IRMA methods are preferable for the measurement of plasma ANP and BNP for experimental studies and routine assay because they are more practicable, sensitive and accurate than RIA procedures. Finally, BNP assay appears to be better than ANP for discriminating between normal subjects and patients with different degrees of heart failure.

Alterations in metabolic clearance of atrial natriuretic peptides in heart failure: how do they relate to the resistance to atrial natriuretic peptides?

Deficiencies in activity of the atrial natriuretic peptide (ANP) system may be able to explain the disturbed electrolyte and fluid homeostasis occurring in chronic heart failure. Generally, in studies concerning the possible pathophysiologic role of ANP in heart failure, only the circulating levels of the hormone were measured. It has been shown, however, that plasma ANP levels exhibit marked variability attributable to the pulsatory pattern of secretion and to its very short plasma half-life. An evaluation of the main turnover parameters might represent a significant improvement in the assessment of the functioning of the overall ANP system. By using a tracer method, which does not alter the steady-state condition, a disturbed peripheral metabolism of ANP and a resistance to its biologic effects was demonstrated in patients with idiopathic dilated cardiomyopathy, even in those in the asymptomatic phase of the disease, showing ANP circulating levels, atrial pressure and volume, and cardiac index within the normal range. The altered degradation and distribution of ANP in patients with heart failure were demonstrated by a great increase in metabolic clearance (on average, 2.5-fold), production (on average, 6-fold), or both, and by a progressive reduction in the distribution spaces of the hormone when compared with normal subjects at the same sodium intake. The ratio between ANP disposal and the daily excretion of sodium (equal to the sodium intake in subjects at a strictly controlled sodium balance) may give a good index of the biologic activity (natriuresis) of the ANP system. Preliminary studies suggest that an index of biologic activity of the ANP system may be a useful tool for the comparison of the efficacy of different therapeutic strategies in heart failure. Indeed, after a 2-month treatment with an angiotensin-converting enzyme inhibitor, this index was returned to within the normal range, suggesting that the drug was able to "normalize" the peripheral distribution and degradation of the hormone in these patients. Although future studies in a large series of patients will be useful to assess this important issue, the setup of accurate methods able to evaluate the presence and degree of resistance to biologic activity of ANP may be a useful tool in the follow-up evaluation of patients with heart failure, and may pave the way for further progress in the knowledge of more general physiologic and pathophysiologic mechanisms of this important clinical condition.