title: Involvement of drinking and intestinal sodium absorption in hyponatremic effect of atrial natriuretic peptide in seawater eels.
abstract: Atrial natriuretic peptide (ANP) decreases plasma Na+ concentration and promtes seawater (SW) adaptation in eels. The hyponatremia may most probably be caused by increased branchial extrusion of Na+, but the mechanism has not been determined yet. The present study examined initially the effects of ANP on branchial Na+ efflux in vivo using isotopic 22Na. However, the efflux rate was not altered by infusion of a hyponatremic dose of ANP (5 pmol.kg(-1).min(-1)). Therefore, we sought to examine whether the ANP-mediated hyponatremia is caused by a decrease in the uptake of Na+ from the environment. Since a decrease in drinking was highly correlated with a degree of hyponatremia, conscious SW eels were infused with dilute SW into the stomach at a normal drinking rate to offset the antidipsogenic effect of ANP. Under this regimen, the hyponatremic effect of ANP was abolished. Then, we examined the site of Na+ absorption in the alimentary tract by measuring the changes in ion composition of intraluminal fluid along the tract. Since Na+ was absorbed at the esophagus and anterior/middle intestine, a sac was prepared at each site and the effects of ANP were examined in situ in conscious SW eels. ANP infusion did not alter Na+ absorption at the esophagus, but it profoundly reduced the absorption at the intestine. Together with our previous finding that ANP does not alter renal Na+ excretion, we propose that ANP reduces plasma Na+ concentration in SW eels by inhibiting drinking and subsequent absorption of Na+ by the intestine.
authors.affiliation: Ocean Research Institute, University of Tokyo, Tokyo 164-8639, Japan. tsuka@ori.u-tokyo.ac.jp
authors.forename: Takehiro
authors.initials: T
authors.lastname: Tsukada
authors.forename: J Cliff
authors.initials: JC
authors.lastname: Rankin
authors.forename: Yoshio
authors.initials: Y
authors.lastname: Takei
mesh_terms: D000704:Analysis of Variance; D000811:Anguilla / Q000378:metabolism* / Q000502:physiology; D000818:Animals; D009320:Atrial Natriuretic Factor / Q000378:metabolism*; D001693:Biological Transport, Active; D004326:Drinking / Q000502:physiology*; D007010:Hyponatremia / Q000378:metabolism*; D007413:Intestinal Mucosa / Q000378:metabolism*; D017136:Ion Transport; D007564:Japan; D012623:Seawater; D012979:Sodium Radioisotopes / Q000097:blood / Q000378:metabolism*; D014882:Water-Electrolyte Balance / Q000502:physiology*
chemical_list: D012979:Sodium Radioisotopes; D009320:Atrial Natriuretic Factor