Role of tubular secretion and carbonic anhydrase in vertebrate renal sulfate excretion.
The renal proximal tubule of vertebrates performs an essential role in controlling plasma SO(4)(2-) concentration ([SO(4)(2-)]). Although net tubular SO(4)(2-) reabsorption is the predominate control process in terrestrial vertebrates, a facilitated secretory flux is also present. In contrast, marine teleosts obtain excess SO(4)(2-) from drinking, and increased plasma [SO(4)(2-)] is prevented predominately through net tubular secretion. Tubular SO(4)(2-) secretion is accomplished by at least two electroneutral anion exchange processes in series. Movement of SO(4)(2-) into the cell across the basolateral membrane is pH dependent, suggesting SO(4)(2-)/OH(-) exchange. Luminal HCO(3)(-) and Cl(-) can facilitate SO(4)(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 SO(4)(2-) transporters in the mammalian SLC26 gene family. In all species tested, glucocorticoids increase renal SO(4)(2-) excretion. Whereas glucocorticoids downregulate SO(4)(2-) reabsorptive mechanisms in terrestrial vertebrates, they may also stimulate a mediated secretory flux. In the marine teleost, cortisol increases the level of SO(4)(2-)/HCO(3)(-) exchange at the brush-border membrane, tubular carbonic anhydrase (CA) activity, CAII protein, and a proportion of tubular SO(4)(2-) secretion that is CA dependent. CA activity is required for about one-half of this net SO(4)(2-) secretion but is also required for about one-half of the net reabsorption in bird proximal epithelium. A CA-SO(4)(2-)/anion exchanger metabolon arrangement is proposed that may speed both the secretory and reabsorptive processes.
Pelis
Ryan M
RM
Department of Physiology and Neurobiology, U-4156, University of Connecticut, 3107 Horsebarn Hill Rd., Storrs, CT 06269-4156, USA.
Renfro
J Larry
JL
D000042:Absorption; D000818:Animals; D001692:Biological Transport; D002256:Carbonic Anhydrases / Q000378:metabolism*; D007668:Kidney / Q000378:metabolism*; D007684:Kidney Tubules / Q000378:metabolism*; D008954:Models, Biological; D013431:Sulfates / Q000378:metabolism* / Q000652:urine
D013431:Sulfates; D002256:Carbonic Anhydrases