Invited review: Development of acid-base regulation in vertebrates

Acid-base regulation at intra- and extracellular levels is of fundamental importance for developing animals, as it is for adults. Although adult animals exhibit well established acid-base regulatory capacities to offset acid-base disturbances, early developmental stages must develop some of the mechanisms to compensate for these disturbances. Thus, in this review we consider the multiple mechanisms that are involved in acid-base regulation in the early developmental stages of vertebrates. We start with a brief general review of the three basics mechanisms that adult animals use for acid-base regulation: 1) rapidly acting bicarbonate, phosphate and protein buffer systems; 2) respiratory elimination of metabolically produced CO₂ with a time-course of minutes; and 3) excretion of HCO₃^- and H⁺ via gills, kidneys or other excretory organs, the slowest of the mechanisms with a time-course of hours or days. We then consider the ability of developing fishes, amphibians, reptiles and birds to regulate acid-base balance using these three mechanisms, as well as the timing of the onset of these various mechanisms during development. Overall, the ability to regulate acid-base balance in the face of environmental stressors becomes apparent early in developing vertebrates, especially when the early stages are free living (e.g. embryos in eggs) and have no parental support via placental structures. However, we still have only a poor understanding of the dynamics and actual partitioning of acid-base regulation between buffer systems, respiratory elimination of CO₂, and urinary excretion of HCO₃^- and H⁺ excretion in developing or adult fishes, amphibians, reptiles and birds. Yet, such studies could help understand key questions ranging from the evolution of endothermy to the relative importance α-stat and pH-stat acid-base regulation.