Atrial natriuretic peptide in eel plasma, heart and brain characterized by homologous radioimmunoassay.

A highly specific and sensitive radioimmunoassay has been developed for the measurement of eel atrial natriuretic peptide (ANP). The antiserum, raised against eel ANP-(1-27) did not cross-react with two other eel natriuretic peptides, i.e. eel ventricular natriuretic peptide and C-type natriuretic peptide (CNP), or with any mammalian ANPs, CNPs or brain natriuretic peptides so far identified. The minimal detectable amount was 0.39 fmol (0.90 pg)/tube with more than 99% confidence. Because of its high sensitivity, the radioimmunoassay makes it possible to measure eel ANP directly with only a few microlitres of plasma without extraction. Using the radioimmunoassay we found high levels of ANP in the atrium (11 +/- 2 pmol/mg wet tissue, n = 8), and much lower levels in the ventricle (56 +/- 8 fmol/mg, n = 8) and the brain (22 +/- 1 fmol/mg, n = 8) of eels. Eel plasma contained a large amount of ANP (247 +/- 66 fmol/ml, n = 8) compared with the levels reported in mammals, although atrial levels are similar between eels and mammals. Gel-permeation chromography revealed that a major form of ANP stored in the eel atrium, ventricle and brain has a molecular mass of approximately 14 kDa but low molecular forms of about 3 kDa are predominant in eel plasma. A detailed analysis with reverse-phase high-performance liquid chromatography showed that a major molecular form circulating in eel plasma is ANP-(1-27). ANP-(1-27) was also detected in small amounts in the eel atrium, ventricle and brain.