A highly specific radioreceptor assay for the active circulating form of atrial natriuretic factor in human plasma

This highly sensitive radioreceptor assay (RRA) for the active circulating form of atrial natriuretic factor (ANF), fragment 99-126, in human plasma requires 125I-labeled ANF, bovine zona-glomerulosa membrane receptors, and amiloride HCl. The amiloride elicits an increase in the binding affinity of ANF to its receptors. ANF is extracted from human plasma with "Sep-Pak" cyano cartridges. After a 90-min incubation at 25 degrees C, bound and free fractions are separated by filtration. The ANF concentration that inhibits receptor binding of the radioligand by 50% is 12.4 fmol of unlabeled ANF per tube. The minimum detectable concentration is 0.2 fmol per tube. Using ANF-supplemented plasma samples, there was a good correlation (r = 0.99) between ANF concentrations found and those expected. Only the active circulating form of ANF fully cross-reacts in this assay, which confirms its high selectivity. Results correlated strongly (r = 0.93) for clinical samples tested by RRA and RIA.