1. The aim of this study was to determine plasma levels of N-terminal atrial natriuretic peptide and atrial natriuretic peptide in normal subjects and in patients with essential hypertension, cardiac transplant and chronic renal failure, using radioimmunoassays directed towards the-mid-portion pro-atrial natriuretic peptide (31-67) and pro-atrial natriuretic peptide (1-30) of the N-terminal atrial natriuretic peptide and atrial natriuretic peptide (99-126). The circulating form(s) of the immunoreactive N-terminal atrial natriuretic peptide in plasma extracts has been investigated using all three radioimmunoassays by means of gel filtration chromatography to further clarify the major immunoreactive molecular circulating form(s) of N-terminal atrial natriuretic peptide in man. 2. The plasma level (mean +/- SEM) of N-terminal pro-atrial natriuretic peptide (31-67) in the normal subjects was 547.2 +/- 32.7 pg/ml (n = 36) and was significantly elevated in patients with essential hypertension (730.2 +/- 72.3 pg/ml, P < 0.025, n = 39), in cardiac transplant recipients (3214.0 +/- 432.2 pg/ml, P < 0.001, n = 9) and in patients with chronic renal failure (3571.8 +/- 474.1 pg/ml, P < 0.001, n = 11). Plasma levels of N-terminal pro-atrial natriuretic peptide (1-30) and atrial natriuretic peptide were similarly elevated in the same patient groups when compared with the mean plasma values in the normal subjects. 3. There were positive associations between pro-atrial natriuretic peptide (31-67) and atrial natriuretic peptide, pro-atrial natriuretic peptide (31-67) and proatrial natriuretic peptide (1-30) and between proatrial natriuretic peptide (1-30) and atrial natriuretic peptide in the normal subjects, hypertensive patients, cardiac transplant recipients and patients with chronic renal failure. The correlation coefficient for all groups taken together was 0.86 (P < 0.001, n = 95) for pro-atrial natriuretic peptide (31-67) and atrial natriuretic peptide, 0.93 (P < 0.001, n = 95) for pro-atrial natriuretic peptide (31-67) and pro-atrial natriuretic peptide (1-30), and 0.82 (P < 0.001, n = 95) for proatrial natriuretic peptide (1-30) and atrial natriuretic peptide. 4. Gel filtration of extracted plasma from cardiac transplant patients and patients with chronic renal failure indicated a single peak of immunoreactivity for N-terminal atrial natriuretic peptide using both the pro-atrial natriuretic peptide (31-67) and proatrial natriuretic peptide (1-30) radioimmunoassays, suggesting a major single high-molecular-mass circulating immunoreactive N-terminal atrial natriuretic peptide, probably pro-atrial natriuretic peptide (1-98). Atrial natriuretic peptide immunoreactivity, as measured by the radioimmunoassay for atrial natriuretic peptide (99-126), showed a separate and distinct peak from that of the N-terminal atrial natriuretic peptide, which co-eluted with the synthetic human standard atrial natriuretic peptide (99-126). 5. These results show that immunoreactive N-terminal atrial natriuretic peptide and atrial natriuretic peptide are elevated in patients with essential hypertension, in cardiac transplant recipients and in patients with chronic renal failure. The major immunoreactive form of N-terminal atrial natriuretic peptide cross-reacting in both the pro-atrial natriuretic peptide (31-67) and pro-atrial natriuretic peptide (1-30) radioimmunoassays is of a high molecular mass, probably pro-atrial natriuretic peptide (1-98). Since pro-atrial natriuretic peptide (1-98) is unlikely to cross-react identically with antibodies for proatrial natriuretic peptide (31-67) or pro-atrial natriuretic peptide (1-30), this could account for the differences in plasma levels obtained by the assays for pro-atrial natriuretic peptide (31-67) and proatrial natriuretic peptide (1-30).
