CO2 and acid-base sensing

Carbon dioxide (CO2) and its hydration products hydrogen (H+), bicarbonate HCO 3 − and carbonate CO 3 2 − ions collectively contribute to the acid-base status of aqueous solutions, and have major effects on the physiology of organisms. Correspondingly, organisms have developed the ability to sense specific acid-base disturbances that routinely arise from metabolic and environmental sources, and to coordinate a variety of homeostatic responses. A common requirement for all homeostatic mechanisms is the ability to sense specific acid-base disturbances and to coordinate appropriate responses. This chapter synthetizes our knowledge concerning the sensory pathways that allow fish to sense acid-base disturbances of both metabolic and environmental origin and the ensuing downstream physiological responses that promote homeostasis in different organs. We focus largely on the peripheral, and to a lesser extent, the central sites of CO2/H+ detection, emphasizing the cellular sites and molecular mechanisms of acid-base sensing.