Invited Review Role of tubular secretion and carbonic anhydrase in vertebrate renal sulfate excretion

Pelis, Ryan M., and J. Larry Renfro. Role of tubular secretion and carbonic anhydrase in vertebrate renal sulfate excretion. Am J Physiol Regul Integr Comp Physiol 287: R491–R501, 2004; 10.1152/ajpregu.00084.2004.—The renal proximal tubule of vertebrates performs an essential role in controlling plasma SO4 2 concentration ([SO4 2 ]). Although net tubular SO4 2 reabsorption is the predominate control process in terrestrial vertebrates, a facilitated secretory flux is also present. In contrast, marine teleosts obtain excess SO4 2 from drinking, and increased plasma [SO4 2 ] is prevented predominately through net tubular secretion. Tubular SO4 2 secretion is accomplished by at least two electroneutral anion exchange processes in series. Movement of SO4 2 into the cell across the basolateral membrane is pH dependent, suggesting SO4 2 /OH exchange. Luminal HCO3 and Cl can facilitate SO4 2 movement out of the cell across the brush-border membrane. The molecular identities of the anion exchangers are unknown but are probably homologues of SO4 2 transporters in the mammalian SLC26 gene family. In all species tested, glucocorticoids increase renal SO4 2 excretion. Whereas glucocorticoids downregulate SO4 2 reabsorptive mechanisms in terrestrial vertebrates, they may also stimulate a mediated secretory flux. In the marine teleost, cortisol increases the level of SO4 2 /HCO3 exchange at the brushborder membrane, tubular carbonic anhydrase (CA) activity, CAII protein, and a proportion of tubular SO4 2 secretion that is CA dependent. CA activity is required for about one-half of this net SO4 2 secretion but is also required for about one-half of the net reabsorption in bird proximal epithelium. A CA-SO4 2 /anion exchanger metabolon arrangement is proposed that may speed both the secretory and reabsorptive processes.