Correlation between aquaporin and water permeability in response to vasotocin, hydrin and {beta}-adrenergic effectors in the ventral pelvic skin of the tree frog Hyla japonica.

The ventral pelvic skin of the tree frog Hyla japonica expresses two kinds of arginine vasotocin (AVT)-stimulated aquaporins (AQP-h2 and AQP-h3), which affect the capacity of the frog's skin to absorb water. As such, it can be used as a model system for analyzing the molecular mechanisms of water permeability. We investigated AQP dynamics and water permeability in the pelvic skin of H. japonica following challenge with AVT, hydrins (intermediate peptides of pro-AVT) and beta-adrenergic effectors. In the in vivo experiment, both AQP-h2 and AQP-h3 proteins were translocated to the apical plasma membrane in the principal cells of the first-reacting cell (FRC) layer in the pelvic skin following challenge with AVT, hydrin 1 and hydrin 2, thereby increasing the water permeability of the pelvic skin. The beta-adrenergic receptor agonist isoproterenol (IP) and its anatagonist propranolol (PP) in combination with AVT or hydrins were used as challenge in the in vitro experiment. IP increased water permeability whereas PP inhibited it, and both events were well correlated with the translocation of the AQPs to the apical membrane. In the PP+AVT-treated skins, labels for AQP-h2 and AQP-h3 were differentially visible among the principal cells; the apical plasma membrane of some cells was labeled while others were not, indicating that the response of PP or AVT is different from cell to cell. These results provide morphological evidence that the principal cells of the FRC layers may have two kinds of receptors: a V2 receptor and beta-adrenergic receptor.