Effects of carbonic anhydrase inhibition on the acid base status in lamprey and trout

Inhibition of red cell carbonic anhydrase (CA) activity resulted in the rapid development of a respiratory acidosis (0.25 pH depression within 15 min post-injection) in the blood of trout. In the lamprey, however, the onset of the respiratory acidosis was delayed and its magnitude was less (0.18 pH depression at 6 h post-injection). Erythrocyte pH of both species decreased by about 0.12 units by 1 h after CA inhibition. These data, combined with the lack of rapid anion (Cl-/HCO3-) exchange in the red cells of agnathans but not in other lower vertebrates, support the hypotheses that (1) the majority of total CO2 in lamprey is transported within the erythrocyte, and (2) the limiting step in the evolution of a functioning Jacobs-Stewart cycle, and thus the evolution of the common mechanism of systemic CO2 transport in vertebrate blood, was the incorporation of the band-3 anion exchange protein into the membrane of the red cell.