Eel ventricular natriuretic peptide: isolation of a low molecular size form and characterization of plasma form by homologous radioimmunoassay

Ventricular natriuretic peptide (VNP) with 25 amino acid residues was isolated from the low molecular weight fraction of acid extracts of eel cardiac ventricles. No other short forms of VNP were recovered from the fraction. This peptide was named eel VNP(1–25) because it was a C-terminally truncated form of the previously isolated eel VNP(1–36) As observed before with eel VNP(1–36), eel VNP(1–25) had a much higher (146-fold) vasodepressor activity than human atrial natriuretic peptide (ANP) in eels, but was a third to a half as active in rats with respect to vasodepressor and natriuretic activities. Eel VNP(1–25) was generally less potent than eel VNP(1–36) for vasodepressor and natriuretic effects. A specific radioimmunoassay (RIA) has been developed for the measurement of eel VNP. The antiserum, raised against eel VNP(1–36), was highly specific and did not exhibit significant cross-reactivity with eel ANP and C-type natriuretic peptide, even though their amino acid sequences have more than 60% homology with that of eel VNP. The sensitivity of assay was 0·5 fmol/tube for eel VNP(1–36) with more than 99% confidence. Such high sensitivity permitted direct assaying of VNP with only a few microlitres of plasma. In fresh water eels, the concentration of VNP in the cardiac ventricle was higher than those in the atrium or brain and that of ANP in the ventricle. Thus, VNP seems to be a ventricular hormone. Although ANP is a major circulating hormone in mammals, the plasma concentration of VNP was threefold higher than that of ANP. The RIA coupled with gel-permeation chromatography revealed that a 14 kDa form, probably proVNP, and smaller forms (3–6 kDa) circulate in eel plasma. Reversephase high performance liquid chromatography identified both VNP(1–36) and VNP(1–25) in eel plasma; VNP(1–36) appeared to be a major form. Journal of Endocrinology (1994) 141, 81–89