Radioimmunoassay and Characterization of Atrial Natriuretic Peptide in Human Plasma*

Abstract

A RIA for a-human atrial natriuretic peptide (βhANP) in plasma was developedand used to study the immunoreactive components secreted by the heart and circulating in peripheral venous plasma. The assay used [¹²⁵I]diiodotyrosylahANP, purified byhigh pressure liquid chromatography (HPLC), and a C-terminal-specific antiserum purchasedfrom Peninsula Laboratories. Serial dilution curves of coronary sinus plasma samples wereparallel with the standard curve, but significant nonparallelism was found in peripheral plasma samples of low immunoreactivity. When plasma was extracted using C-18 Sep-Pak cartridges, serial dilution curves from both coronary sinus and peripheral plasma samples were parallel to the standard curve. Although values for plasma samples assayed before and after extraction agreed closely (r = 0.99; n = 76), immunoreactive ANP in unextracted plasma was consistently greater(70-79 pmol/liter) than in extracts of plasma, suggesting nonspecific interference by a component in plasma when assayed without extraction. Mean plasma immunoreactive ANP in 19 normal subjects consuming a normal salt intake was 14 ±1 (± SE) pmol/liter. In 5 normal men, increasing dietary sodium intake from 10 to 200 mmol sodium/day was associated with a 2-fold increment in ANP levels, and similar changesaccompanied acute sodium loading using iv saline. Elevated values were found in patients with congestive heart failure (mean, 58 pmol/liter; range, 0-200; n = 9), chronic renal failure(mean, 118 pmol/liter; range, 30-290; n =8), and primary aldosteronism (range, 32-90 pmol/liter; n = 3). HPLC and gel cc harnoaml-atographi ysis of the immunoreactive material found in coronary sinus plasma extracts showed that a large amount of the materialelutedin the position of βhANP. A smaller quantity of immunoreactive material with a mol wt of about 1600was also identified. Peripheral venous plasma extracts also contained several immunoreactive components, the largest amount of which corresponded to βhANP. The patternof immunoreactive components in peripheral venous plasma, as identified by both gel chromotography and HPLC, was similar to that in coronary sinus plasma drawn during an active phase of hormone secretion. These findings indicate that the heart secretes ahANP or a closely similar peptide which is also present in peripheral venous plasma. Plasma immunoreactive ANP is responsive to sodium loading in normal man and is elevated in patients with hypervolemic disorders.