Central angiotensin II stimulates cutaneous water intake behavior via an angiotensin II type-1 receptor pathway in the Japanese tree frog Hyla japonica

Abstract

Angiotensin II (Ang II) stimulates oral water intake by causing thirst in all terrestrial vertebrates except anurans. Anuran amphibians do not drink orally but absorb water osmotically through ventral skin. In this study, we examined the role of Ang II on the regulation of water-absorption behavior in the Japanese tree frog ( Hyla japonica). In fully hydrated frogs, intracerebroventricular (ICV) and intralymphatic sac (ILS) injection of Ang II significantly extended the residence time of water in a dose-dependent manner. Ang II-dependent water uptake was inhibited by ICV pretreatment with an angiotensin II type-1 (AT 1) receptor antagonist but not a type-2 (AT 2) receptor antagonist. These results suggest that Ang II stimulates water-absorption behavior in the tree frog via an AT 1-like but not AT 2-like receptor. We then cloned and characterized cDNA of the tree frog AT 1 receptor from the brain. The tree frog AT 1 receptor cDNA encodes a 361 amino acid residue protein, which is 87% identical to the toad ( Bufo marinus) AT 1 receptor and exhibits the functional characteristics of an Ang II receptor. AT 1 receptor mRNAs were found to be present in a number of tissues including brain (especially in the diencephalon), lung, large intestine, kidney and ventral pelvic skin. When tree frogs were exposed to dehydrating conditions, AT 1 receptor mRNA significantly increased in the diencephalon and the rhombencephalon. These data suggest that central Ang II may control water intake behavior via an AT 1 receptor on the diencephalon and rhombencephalon in anuran amphibians and may have implications for water consumption in vertebrates.