Concentration and molecular forms of brain natriuretic peptide in rat plasma and spinal cord

To characterize the biological functions of rat brain (B-type) natriuretic peptide (BNP), which has been shown to be present mainly in the heart and only faintly in the spinal cord, the concentration and molecular forms of BNP in plasma and spinal cord were determined. The concentration of immunoreactive (ir-) BNP was 2.00 fmol/ml in normal rat and 13.29 fmol/ml in morphine-treated rat, being respectively about 1/20 and 1/80 those of ir-atrial (A-type) natriuretic peptide (ANP). In morphine-treated rats, ir-BNP was shown to circulate mainly as BNP-45, which is identical to a major storage form found in cardiac atrium. In the spinal cord, BNP was also shown to be present as BNP-45, but its concentration was only 0.057 pmol/g, being about 1/60 that of spinal cord ANP. These results confirm that BNP mainly functions as a circulating hormone in the molecular form of BNP-45. Morphine stimulates secretion of ANP and BNP but by different ratios, suggesting different regulation systems for storage and secretion of ANP and BNP.