Gas exchange, ionoregulation, acid-base balance and their interactions in fish: The evolution of complex physiological systems.

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The long-term goal of my research program is to understand the interaction between gas-exchange, ion regulation and acid-base balance in fish during environmental acclimation/adaptation and development. In this proposal I will investigate three areas that interact with acid-base regulation as a common theme, with the focus largely upon "primitive" and developing fish. 1) The fish gill is multifunctional, comprising the dominant surface for gas exchange, ion regulation, and acid-base balance, all processes crucial for survival. In general, the vertebrate gill is thought to have evolved primarily to enhance gas exchange and secondarily for other processes, however, based upon our recent findings we hypothesize that the vertebrate gill evolved for acid-base regulation first, and gas exchange secondarily. 2) While the gills are the primary site for blood pH regulation in fish, they become limited in this capacity at high CO2 levels. Fish that can tolerate high CO2 appear to abandon blood pH regulation and instead defend pHi of important tissues, which we have termed preferential pHi (ppHi) regulation. We will investigate the evolution of ppHi regulation, its functional significance and cellular mechanisms and we hypothesize it was associated with the evolution of air-breathing in fish. 3) Teleost fish (half of all vertebrates) possesses a very pH-sensitive haemoglobin (Root effect), where a reduction in pH in specialized structures greatly enhances O2 delivery to the eye and swimbladder which is thought to be associated with the extensive adaptive radiation of teleosts. However, we have discovered a mechanism where a mild acid-base disturbance in the presence of a Root effect can greatly enhance muscle O2 delivery. We will further investigate this mechanism and its' functional significance to determine whether enhanced general O2 delivery may represent the early selection for the Root effect in fish. The proposed experiments are interdisciplinary, spanning the range of molecular, cellular, and whole animal. They are designed to test novel hypotheses and findings will add significantly to our understanding of the evolution of complex physiological systems and have the potential to alter textbook dogma.