Intra- and extracellular dehydration has no effect on plasma levels of angiotensin II in an amphibian

Previous studies have demonstrated that both dehydration (intra and extrace llular) and treatment with angiotensin II (A-II) induce changes in thirst-r elated behavior in the spadefoot toad, Scaphiopus couchii. One of the steps in determining a causal relationship between a hormone and a behavior is t o determine that there is association between an animal's performance of th e behavior and changes in endogenous hormonal concentrations. The hypothesi s tested that plasma levels of the peptide hormone A-II would change as a r esult of dehydration known to induce water absorption response (WR) behavio r in the spadefoot toad. Plasma samples were taken from toads dehydrated in tracellularly by injection of hypertonic solutions of NaCl or sucrose at le vels known to induce WR behavior. As an osmotic control, a group of animals was injected with urea, which has been demonstrated to not induce WR behav ior. In order to determine the effects of extracellular dehydration on plas ma, A-II levels in toads dehydrated by plasma volume depletion via cardiac puncture were compared to sham-punctured controls. None of the treatments i n any experiment resulted in significant differences in plasma levels of an giotensin II among groups sampled at the time when WR behavior occurs. Thes e results do not support the hypothesis that dehydration-induced thirst is stimulated by changes in plasma A-II concentrations at the onset of WR beha vior. (C) 2000 Wiley-Liss, Inc.