The
natriuretic peptide system of a euryhaline teleost, the Japanese eel (Anguilla japonica), consists of three types of
hormones [
atrial natriuretic peptide (
ANP), ventricular
natriuretic peptide (VNP), and
C-type natriuretic peptide (CNP)] and four types of receptors [
natriuretic peptide receptors (NPR)-A, -B, -C, and -D]. Although
ANP is recognized as a volume-regulating
hormone that extrudes both Na(+) and water in mammals,
ANP more specifically extrudes Na(+) in eels. Accumulating evidence shows that
ANP is secreted in response to
hypernatremia and acts to inhibit the uptake and to stimulate the excretion of Na(+) but not water, thereby promoting seawater (SW) adaptation. In fact,
ANP is secreted immediately after transfer of eels to SW and ameliorates sudden increases in plasma Na(+) concentration through inhibition of drinking and intestinal absorption of NaCl.
ANP also stimulates the secretion of
cortisol, a long-acting
hormone for SW adaptation, whereas
ANP itself disappears quickly from the circulation. Thus
ANP is a primary
hormone responsible for the initial phase of SW adaptation. By contrast, CNP appears to be a
hormone involved in freshwater (FW) adaptation. Recent data show that the gene expression of CNP and its specific receptor, NPR-B, is much enhanced in FW eels. In fact, CNP infusion increases (22)Na uptake from the environment in FW eels. These results show that
ANP and CNP, despite high sequence identity, have opposite effects on salinity adaptation in eels. This difference apparently originates from the difference in their specific
receptors, ANP for NPR-A and CNP for NPR-B. VNP may compensate the effects of
ANP and CNP for adaptation to respective media, because it has high affinity to both receptors. On the basis of these data, the authors suggest that the
natriuretic peptide system is a key endocrine system that allows this euryhaline fish to adapt to diverse osmotic environments, particularly in the initial phase of adaptation.