ventricular natriuretic peptide, eel

Properties

The Mr of mature eel VNP is 3940, and the pI is about 10. It is freely soluble in water, acid, and 67% acetone, but insoluble in 99% acetone. VNP solution in water at >10^-4M is stable for more than a year at -20°C.

Gene, mRNA, and precursor

By linkage mapping, the rainbow trout VNP gene was found to be localized in tandem with the atrial natriuretic peptide (ANP) and B-type natriuretic peptide (BNP) genes on the same chromosome. The VNP gene is on the same scaffold as those of ANP and BNP in the eel genome database. However, the precise chromosomal location of the VNP gene has not been determined yet in the two species. The size of eel VNP mRNA is 1024 bp. Unlike BNP mRNA, there is no repetitive AUUUA motif in the 30 -untranslated region of VNP mRNA. Eel proVNP1–128 is cleaved into VNP-36 (VNP93–128) and N-terminal (NT)-proVNP1–92 by prohormone convertase.

Synthesis and release

In eels, a major stimulus for VNP release is osmotic stimulus, particularly an acute increase in plasma osmolality. The plasma eel VNP level is transiently and rapidly increased after seawater transfer, or injections of hypertonic NaCl or mannitol. Increased blood volume (volume stimulus) also induces VNP release, although it is less potent when compared to mammals. Increased VNP is cleared from the circulation quickly due to the high metabolic clearance rate (2.7±0.2mL/min: metabolic clearance rate of eel ANP is 1.7±0.1mL/min). Similarly to ANP in mammals, the chronic volume load, but not the salt load, is a major stimulus for VNP secretion in the trout. The promoter region and potential transcrip-tion factors have not been identified yet for the VNP gene.

Receptors

Eel VNP binds to the eel A-type NP receptor (NPR-A: Kd=0.1nM), the C-type NP receptor (NPR-C: Kd=0.15nM), and the D-type NP receptor (NPR-D: Kd=1nM) when they are transiently expressed in COS-7 cells. Although the binding affinity to the eel B-type NP receptor (NPR-B) has not been determined yet, eel VNP stimulates the guanylyl cyclase activity of eel NPR-B expressed in COS-7 cells at 10nM. Therefore, it is assumed that VNP is a ligand not only for NPR-A but also for NPR-B when its secretion is enhanced. Eel NPR-A, -B, and -C are widely distributed while eel NPR-D is specifically expressed in the brain and gills. A VNP-specific receptor has not yet been found.

Agonists and Antagonists

C-ANF is a selective agonist for both NPR-C and -D. Osteocrin containing the NP motif selectively binds to the NPR-C, but not to the NPR-A or -B. HS-142-1 blocks the binding of VNP to eel NPR-A, -B, and -D, but not NPR-C.

Biological functions

VNP is as potent as ANP, and more potent than BNP for cardiovascular effects in eels and trout. The systemic injection of eel VNP at doses of 0.1–1 nmol/kg decreases blood pressure and increases hematocrit. The hypotensive action of eel VNP lasts longer than that of eel ANP. In seawater-adapted eels, VNP decreases the plasma Na+ concentration by inhibiting the drinking rate and subsequent intestinal NaCl absorption. Thus, VNP is thought to be an important hormone for seawater acclimation in eels. Eel VNP potentiates the steroidogenic action of ACTH. In rats, the natriuretic and hypotensive effects of eel VNP are observed at 1–10 nmol/kg.

Description

VNP was purified in 1991 from the eel ventricle. Deduced eel proVNP consists of 128 aa residues. Bioactive mature eel VNP (36 aa residues) is located at the C-terminus. Eel VNP has 17 aa residues of an intramolecular ring, from which a long C-terminal tail sequence (14 aa residues) extends. The C-terminally truncated form, VNP1–25, is also present in eel plasma. An NP gene abundantly expressed in the chicken kidney, named renal NP (RNP), may be an ortholog of VNP.

Structure and conformation

Deduced eel proVNP consists of 128 aa residues. Bioactive mature eel VNP (36 aa residues) is located at the C-terminus. Eel VNP has 17 aa residues of an intramolecular ring, from which a long C-terminal tail sequence (14 aa residues) extends. The C-terminally truncated form, VNP1–25, is also present in eel plasma. An NP gene abundantly expressed in the chicken kidney, named renal NP (RNP), may be an ortholog of VNP. The sequence identity is >75% in the mature sequences of the eel, salmon, sturgeon, and bichir VNP. The VNP gene is absent in the genome database of several advanced teleost species (e.g., the medaka and pufferfish) and other advanced classes of vertebrates, except in birds (the chicken).