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INVITED REVIEW

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

Department of Physiology and Neurobiology, University of Connecticut, Storrs, Connecticut 06269

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 brush-border 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.

renal proximal tubule; transport metabolon

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