Abstract
The vasopressin/vasotocin-V2-type receptor-aquaporin 2 (VP/VT-V2R-AQP2) axis plays a pivotal role in renal water reabsorption in tetrapods. It is widely thought that this axis evolved with the emergence of the tetrapods, reflecting a requirement of water retention in terrestrial environment. Here we report that lungfish, the closest living relatives of tetrapods, already possess a system similar to the VT-V2R-AQP2 axis in the kidney, but the system is functional only in the terrestrial estivating condition. In the research process, we discovered a novel AQP from the kidney of estivating lungfish. Characterization of the lungfish AQP showed that the AQP is a paralogous gene of AQP0 which is proposed as the ancestral gene of tetrapod AQP2 and AQP5. Furthermore, we identified a functional V2R in medaka and bichir, but the physiological role of fish V2R still remains to be clarified. In the present study, we showed that VT appears to act on NaCl retention via the VT-V2R-NCC axis in FW teleosts. This mechanism probably plays a key role in the maintenance of body fluid homeostasis in a FW environment. The evolution of VT functions from teleosts to tetrapods is of great interest from the perspective of phylogenic evolution and the habitat expansion of tetrapods to terrestrial environments.
