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Dev Dyn. 2005 May;233(1):142-4.

Pronephric regulation of acid-base balance; coexpression of carbonic anhydrase type 2 and sodium-bicarbonate cotransporter-1 in the late distal segment.

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Department of Biological Science, University of Calgary, 2500 University Drive NW, Calgary, Alberta T2N 1N4, Canada.


The kidney plays a critical role in regulating the composition of the blood, including pH. The major buffer regulating pH of the blood is bicarbonate. Bicarbonate is freely filtered through the glomerulus and must be recovered from the primary filtrate to maintain appropriate buffering capacity and blood pH. In this report, the expression of carbonic anhydrase type 2 within the late distal segment of pronephroi is described. This expression precisely overlaps that of the sodium-bicarbonate transporter XNBC1. In mammalian kidneys, these two proteins collaborate in the transepithelial recovery of bicarbonate from the kidney filtrate. Coexpression of the genes encoding these proteins in Xenopus embryos indicates that pronephroi use the same recovery system and that this system is largely localized to the late portion of the pronephric distal segment. Not only do the simple pronephroi regulate water and solute balance, they also appear to regulate blood pH in a manner akin to that of the vastly more complex mammalian metanephric kidneys.

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