Integrative approach to osmoregulatory action of atrial natriuretic peptide in seawater eels.

Atrial natriuretic peptide (ANP) reduces plasma Na+ concentration and promotes seawater (SW) adaptation in SW eels. However, little is known about the mechanisms for the hyponatremic effect of ANP. In order to evaluate the role of ANP in the whole-body Na+ homeostasis of marine teleost, we reviewed previous in vivo experiments using exogenously administered ANP and present additional experiments to assess the role of endogenous ANP in Na+ homeostasis in conscious SW eels. The Na+ influx and efflux rate across the body surfaces including the gills measured with isotopic 22Na were not altered by the hyponatremic dose (5 pmol kg-1 min-1) of ANP infusion in SW eels. ANP infusion also had no effect on renal Na+ excretion in SW eels. In contrast, ANP strongly inhibited drinking, and the inhibition was quantitatively correlated with the hyponatremic effect of ANP. Further, intestinal absorption of Na+ was inhibited by ANP as examined in situ using intestinal sac in conscious SW eels. The combined inhibitory actions of ANP on drinking and intestinal absorption were sufficient to explain the decrease in plasma Na+ concentration. In addition, removal of endogenous circulating ANP by immunoneutralization increased plasma Na+ concentration with a concomitant increase in drinking rate in SW eels. These results strongly suggest that endogenous ANP is involved in the hyponatremic regulation through actions on drinking, and probably on intestine, in SW eels.