High concentrations of atrial natriuretic peptide and brain natriuretic peptide in rat pericardial fluid and their reduction by reserpine in vivo

We determined concentrations and molecular sizes of natriuretic peptides in rat pericardial fluid and plasma by use of specific radioimmunoassays (RIA) and gel filtration HPLC. Our study shows that pericardial fluid forms a local extracellular storage of immunoreactive (ir) atrial natriuretic peptide (irANP) and brain natriuretic peptide (irBNP) near the heart where these peptides can be found in high concentrations in vivo. The concentrations of irANP, irBNP and NH2-terminal fragment of proANP (irNT-proANP) in pericardial fluid were 9.8 +/- 3.7, 0.49 +/- 0.47 and 28.9 +/- 11.8 nmol/l, respectively. IrBNP had the lowest (20 +/- 11) and irANP the highest (90 +/- 32) concentration ratio between pericardial fluid and plasma. The elution positions of irANP, irBNP and irNT-proANP in pericardial fluid and plasma were similar as examined by gel filtration HPLC. Furthermore, we show that the reduction of noradrenaline content of the heart muscle by reserpine reduces concentration of irANP in pericardial fluid by 39.6% and in plasma by 30.3% when compared to respective control group values. The concentration of irBNP is reduced by 44.1% in pericardial fluid but in plasma its reduction was not statistically significant. Vasoactive peptides released into the interstitial space and from there into pericardial fluid may have a more active role in the regulation of cardiac function than previously considered.