Angiotensin's contribution to dipsogenic additivity in several rodent species

Significant positive correlations were obtained between plasma angiotensin levels and drinking following the administration of polyethylene glycol (PEG) in rats, gerbils, and hamsters and following isoproterenol (ISOP) in rats and gerbils, but not in a South American rodent, Octodon degus (degus). Hamsters revealed elevations in plasma angiotensin following ISOP but no drinking, whereas degus failed to show changes in plasma angiotensin to either treatment, although drinking occurred following PEG. All species drank to 2.5 M NaCl injections with no measurable changes in plasma angiotensin concentrations. A second experiment addressed our previous inability to measure [125I] angiotensin II (AII) specific binding in the brains of gerbils and degus and indicated that these animals possess circumventricular organ (CVO) angiotensin III (AIII) receptors; thus, circulating AIII may be the ligand in these species. A final experiment examined members of these species for dipsogenic additivity following the pairing of extracellular and intracellular thirst challenges. Rats and gerbils revealed additivity when challenged with either PEG or ISOP paired with 2.5 M NaCl. Degus and hamsters indicated additivity only with PEG and 2.5 M NaCl combined. Despite the presence of CVO angiotensin receptors in degus and hamsters, it is concluded that the important component of the hypovolemic dipsogenic stimulus in members of these species may be activation of volume receptors rather than brain angiotensin receptors.