Differential levels of “urotensin-II-like” activity determined by radio-receptor and radioimmuno-assays

Abstract Plasma and urinary levels of “urotensin(U)-II-like” substances determined in healthy human volunteers were 12.4 ± 0.6 ng/ml and 2.2 ± 0.3 ng/ml by RIA, an order of magnitude lower than that seen by RRA, 167.5 ± 9.5 ng/ml and 65.2 ± 4.3 ng/ml. HPLC demonstrated the existence of at least three prominent activity peaks in plasma and urine, the more hydrophobic of which did not co-elute with U-II, degradation products or URP. RRA and RIA recognized these peaks with contrasting efficacy. As such, published levels of “U-II-like” activity should be interpreted with caution until a better understanding is obtained regarding what species specific RIA and RRA assay reagents interact with.