Plasma-accessible carbonic anhydrase at the tissue of a teleost fish may greatly enhance oxygen delivery:in vitroevidence in rainbow trout,Oncorhynchus mykiss

Abstract

SUMMARYDuring a generalized acidosis in rainbow trout, catecholamines are released into the blood, activating red blood cell (RBC) Na+/H+ exchange (βNHE), thus protecting RBC intracellular pH (pHi) and subsequent O₂ binding at the gill. Because of the presence of a Root effect (a reduction in oxygen carrying capacity of the blood with a reduction in pH), the latter could otherwise be impaired. However, plasma-accessible carbonic anhydrase (CA) at the tissues (and absence at the gills) may result in selective short-circuiting of RBC βNHE pH regulation. This would acidify the RBCs and greatly enhance O₂ delivery by exploitation of the combined Bohr-Root effect, a mechanism not previously proposed. As proof-of-principle, an in vitro closed system was developed to continuously monitor extracellular pH (pHe) and O₂ tension (PO₂) of rainbow trout blood. In this closed system, adding CA to acidified, adrenergically stimulated RBCs short-circuited βNHE pH regulation, resulting in an increase in PO₂ by >30 mmHg, depending on the starting Hb-O₂ saturation and degree of initial acidification. Interestingly, in the absence of adrenergic stimulation, addition of CA still elevated PO₂, albeit to a lesser extent, a response that was absent during general NHE inhibition. If plasma-accessible CA-mediated short-circuiting is operational in vivo, the combined Bohr-Root effect system unique to teleost fishes could markedly enhance tissue O₂ delivery far in excess of that in vertebrates possessing a Bohr effect alone and may lead to insights about the early evolution of the Root effect.