Atrial natriuretic peptide release at rest and with exercise after cardiac transplantation with bicaval anastomoses

Atrial autonomic innervation remodelling and atrial fibrillation inducibility after epicardial ganglionic plexi ablation

AIMS

The effects of ganglionated plexi (GP) ablation on atrial fibrillation (AF) inducibility and atrial autonomic innervation remodelling have not been elucidated.

METHODS AND RESULTS

Thirteen dogs were randomly divided into sham-operated group and GP ablation group. All animals underwent a right thoracotomy at the fourth intercostal space. Atrial fibrillation inducibility was assessed by burst rapid pacing at right atrium (RA). After anterior right GP and inferior right GP ablation, AF inducibility was assessed in the GP ablation group. The animals were allowed to recover for 8 weeks, after which, AF was measured again. The levels of atrial natriuretic peptide (ANP) in blood and atrial tissues were examined by radioimmunoassay. Immunocytochemical staining of cardiac nerves was performed in tissues from the dogs. Atrial fibrillation was induced easily in the GP ablation group after 8 weeks although AF was not observed in the sham-operated group, and after instant GP ablation. Compared with that in the sham-operated group, the levels of ANP in the blood and RA increased significantly 8 weeks after GP ablation (111.4 +/- 18.2 vs. 175.1 +/- 25.9; 184.9 +/- 36.3 vs. 299.1 +/- 32.5; P < 0.05). In the GP ablation group, the density of growth-associated protein 43-positive, tyrosine hydroxylase-positive, and choline acetyltransferase-positive nerves in the RA was 821 +/- 752, 481 +/- 627, and 629 +/- 644 per mm(2), respectively, which was significantly (P < 0.01) lower than the nerve density in sham-operated tissues (2590 +/- 841, 1752 +/- 605, and 3147 +/- 886 per mm(2), respectively).

CONCLUSION

Atrial autonomic innervations remodelling may be the mechanism of induced AF after GP ablation.

Suppression of the secretion of atrial and brain natriuretic peptide after total cavopulmonary connection

OBJECTIVES

Among the modifications of the Fontan operation currently being used, total cavopulmonary connection offers the greatest potential for optimizing early and long-term postoperative outcomes. Although studies have established that abrupt increases in right atrial pressure elevate the plasma atrial natriuretic peptide level after the Fontan procedure, changes in plasma natriuretic peptide levels after total cavopulmonary connection have not been clarified. Our hypothesis is that secretion of atrial natriuretic peptide may be suppressed in patients undergoing total cavopulmonary connection because the atrium continues to function at low pressure in those patients. In this study, we measured plasma levels of atrial and brain natriuretic peptide before and during the postoperative period in patients undergoing total cavopulmonary connection.

METHODS

We evaluated 60 patients: 30 patients underwent total cavopulmonary connection and 30 patients underwent definitive repair for Fallot's tetralogy. Blood samples for measurement of atrial and brain natriuretic peptide were obtained before the operation and in the postoperative period.

RESULTS

Plasma levels of atrial and brain natriuretic peptide were significantly lower during the early postoperative period in patients undergoing total cavopulmonary connection than in patients undergoing definitive repair for Fallot's tetralogy. No correlations were identified between the atrial natriuretic peptide levels and central venous pressure after total cavopulmonary connection.

CONCLUSION

These results suggest that total cavopulmonary connection attenuates the secretion of atrial and brain natriuretic peptide in the early postoperative period. The suppressed release of atrial and brain natriuretic peptide after total cavopulmonary connection may have clinical importance.

Role of natriuretic peptides in ion transport mechanisms

Natriuretic peptides (NP) act as ligands on the guanylyl cyclase family of receptors. The NP binding site on these receptors is extracellular and the guanylyl cyclase and protein kinase domains are intracellular. The guanylyl cyclase receptor catalyzes the synthesis of the second messenger molecule, cGMP, which activates protein kinase. This in turn is involved in the phosphorylation of various ion transport proteins. Ion transport proteins, which are modulated by NP and are thought to underlie the natriuretic and diuretic actions of NP, include: (a) calcium-activated K+ channels; (b) ATP-sensitive K+ channels; (c) inwardly-rectifying K+ channels; (d) outwardly-rectifying K+ channels; (e) L-type Ca2+ channels; (f) Cl- channels including cystic fibrosis transmembrane conductance regulator Cl- channels; (g) Na+- K+ 2Cl- co-transporter; (h) Na+- K+ ATPase; (i) Na+ channels; (j) stretch-activated channels; and (k) water channels. It appears that NP modulate the kinetics, rather than the conductance, of ion channels. Some of these channels, like the Ca2+, ATP-sensitive K+ and stretch-activated channels, are also involved in NP secretion. In addition, the structural properties of the NP, e.g., ovCNP-22 and ovCNP-39, appear to confer on them the ability to form ion channels. These CNP-formed ion channels can modify the trans-membrane signal transduction and second messenger systems underlying NP-induced pathological effects.

Enhanced brain natriuretic peptide response to peak exercise in heart transplant recipients

We investigated the atrial (ANP) and brain natriuretic peptides (BNP), catecholamines, heart rate, and blood pressure responses to graded upright maximal cycling exercise of eight matched healthy subjects and cardiac-denervated heart transplant recipients (HTR). Baseline heart rate and diastolic blood pressure, together with ANP (15.2 +/- 3.7 vs. 4.4 +/- 0.8 pmol/l; P < 0.01) and BNP (14.3 +/- 2. 6 vs. 7.4 +/- 0.6 pmol/l; P < 0.01), were elevated in HTR, but catecholamine levels were similar in both groups. Peak exercise O2 uptake and heart rate were lower in HTR. Exercise-induced maximal ANP increase was similar in both groups (167 +/- 34 vs. 216 +/- 47%). Enhanced BNP increase was significant only in HTR (37 +/- 8 vs. 16 +/- 8%; P < 0.05). Similar norepinephrine but lower peak epinephrine levels were observed in HTR. ANP and heart rate changes from rest to 75% peak exercise were negatively correlated (r = -0.76, P < 0.05), and BNP increase was correlated with left ventricular mass index (r = 0.83, P < 0.01) after heart transplantation. Although ANP increase was not exaggerated, these data support the idea that the chronotropic limitation secondary to sinus node denervation might stimulate ANP release during early exercise in HTR. Furthermore, the BNP response to maximal exercise, which is related to the left ventricular mass index of HTR, is enhanced after heart transplantation.

Significance of raised natriuretic peptides after bicaval and standard cardiac transplantation

BACKGROUND

High levels of plasma atrial natriuretic peptide (ANP) and ventricular natriuretic peptide (BNP) have been identified after standard orthotopic cardiac transplantation. It has been postulated that the high ANP levels are a result of persistent secretion from the large residual atrial mass after transplantation. This study was undertaken to investigate the significance of raised ANP and BNP levels after standard and bicaval orthotopic heart transplantation.

METHODS

Plasma ANP and BNP levels were measured in 40 ambulatory, randomly selected cardiac transplant patients (group A, n = 20 had bicaval transplantation; group B, n = 20 had standard transplantation) and 10 healthy volunteers (group C). Cardiac transplant patients underwent endomyocardial biopsy and hemodynamic evaluation.

RESULTS

Plasma levels of ANP and BNP were elevated in the transplant recipients in comparison with normal volunteers (p = 0.0001 and p < 0.0001, respectively). There was no significant difference in the ANP levels between group A and group B, whereas BNP levels were higher in group B compared with group A (p = 0.03). Linear regression analysis showed that a faster heart rate, high mean pulmonary artery pressure, high pulmonary capillary wedge pressure, and high transpulmonary gradient were associated with higher levels of BNP (p < 0.05). Lower mean systemic pressure was associated with higher levels of ANP (p < 0.05).

CONCLUSIONS

High levels of ANP and BNP are synthesized and secreted by the transplanted denervated human heart regardless of the surgical technique. The level of BNP correlates with ventricular performance and afterload. The bicaval technique seems to be associated with better left ventricular and right ventricular diastolic performance.