Did Acidic Stress Resistance in Vertebrates Evolve as Na⁺/H⁺ Exchanger-Mediated Ammonia Excretion in Fish?

How vertebrates evolved different traits for acid excretion to maintain body fluid pH homeostasis is largely unknown. The evolution of Na⁺/H⁺ exchanger (NHE)-mediated NH₄ ⁺ excretion in fishes is reported, and the coevolution with increased ammoniagenesis and accompanying gluconeogenesis is speculated to benefit vertebrates in terms of both internal homeostasis and energy metabolism response to acidic stress. The findings provide new insights into our understanding of the possible adaptation of fishes to progressing global environmental acidification. In human kidney, titratable H⁺ and NH₄ ⁺ comprise the two main components of net acid excretion. V-type H⁺-ATPase-mediated H⁺ excretion may have developed in stenohaline lampreys when they initially invaded freshwater from marine habitats, but this trait is lost in most fishes. Instead, increased reliance on NHE-mediated NH₄ ⁺ excretion is gradually developed and intensified during fish evolution. Further investigations on more species will be needed to support the hypothesis. Also see the video abstract here https://youtu.be/vZuObtfm-34.