spacer gif spacer gif spacer gif spacer gif spacer gif
QUICK SEARCH:   [advanced]


spacer gif
     Home     Help     Feedback     Subscriptions     Archive     Search     Table of Contents    

First published online May 15, 2009
Journal of Experimental Biology 212, 1684-1696 (2009)
Published by The Company of Biologists 2009
doi: 10.1242/jeb.027730
This Article
Right arrow Summary Freely available
Right arrow Full Text
Right arrow Full Text (PDF)
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Email this article to a friend
Right arrow Related articles in JEB
Right arrow Similar articles in this journal
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via HighWire
Google Scholar
Right arrow Articles by Grosell, M.
Right arrow Articles by Taylor, J. R.
PubMed
Right arrow Articles by Grosell, M.
Right arrow Articles by Taylor, J. R.
Social Bookmarking
Add to CiteULike   Add to Complore   Add to Connotea   Add to Del.icio.us   Add to Digg   Add to Reddit   Add to Technorati   Add to Twitter  
What's this?

High rates of HCO3– secretion and Cl– absorption against adverse gradients in the marine teleost intestine: the involvement of an electrogenic anion exchanger and H+-pump metabolon?

M. Grosell*, E. M. Mager, C. Williams and J. R. Taylor

RSMAS, University of Miami, 4600 Rickenbacker Causeway, Miami, FL 33149, USA


Figure 1
View larger version (19K):
[in this window]
[in a new window]

  		Fig. 1. Schematic presentation of accepted and putative transport processes in the intestinal epithelium of marine teleost fish. Transcellular or/and paracellular water transport (broken lines) is driven by active NaCl absorption, providing a hyperosmotic coupling compartment in the lateral interspace (lis). Apical Na+ entry via NKCC2 and NC co-transporters and extrusion across the basolateral membrane via Na+/K+-ATPase accounts for transepithelial Na+ movement. Apical entry of Cl– occurs via both co-transporters and Cl–/HCO3– exchange conducted at least in part by the SLC26a6 anion exchanger whereas basolateral Cl– channels allow for movement of Cl– from the cell across the basolateral membrane. Cellular substrate (HCO3–) for apical anion exchange is provided in part by HCO3– entry across the apical membrane via NBC1 and in part by hydration of endogenous CO2. Cytosolic carbonic anhydrase (CAc) found mainly in the apical region of the enterocytes facilitates the CO2 hydration reaction. Protons arising from the hydration of CO2 are extruded mainly across the basolateral membrane by a Na+-dependent pathway and possibly by vacuolar H+ pumps. Recent findings revealed that some H+ extrusion occurs across the apical membrane via H+-pumps and that this H+ secretion masks some of the apical HCO3– secretion by dehydration yielding molecular CO2. This molecular CO2 may diffuse back into the enterocytes for re-hydration and continued apical anion exchange. Luminal conversion of HCO3– to CO2 is facilitated by membrane-bound carbonic anhydrase (CAIV) and possibly other isoforms, a process that consumes H+ and thereby contributes to luminal alkalinization and CO 2–3 formation. The titration of luminal HCO3– and formation of CO 2–3, which facilitates formation of CaCO3 precipitates both act to reduce luminal osmotic pressure and thus aid water absorption. The electrogenic anion exchanger SLC26a6 exports nHCO3– in exchange for 1Cl– and its activity is therefore enhanced by the hyperpolarizing effect of the H+-pump. The constellation of an apical electrogenic nHCO3–/Cl– exchanger and electrogenic H+-pump constitutes a transport metabolon perhaps accounting for the apparently active secretion of HCO3– and the uphill movement of Cl– across the apical membrane. Note that the value for osmotic pressure and pH in the absorbed fluids are based on measured net movements of H2O and electrolytes, including H+s but that the degree of hypertonicity and acidity in lis is probably much less than indicated due to rapid equilibration with sub-epithelial fluid compartments. See text for further details. NKCC2, Na+:K+:2Cl– co-transporter; NC, Na+:Cl– co-transporter; NBC, Na+:HCO3– co-transporter; TJ, tight junction.

 

Figure 2
View larger version (62K):
[in this window]
[in a new window]

  		Fig. 2. Predicted amino acid sequence and membrane topology of toadfish SLC26a6 (tfSLC26a6) modified from conpred/TMpred predictions. Assumptions of intracellular amino- and carboxy-terminal domains have been employed (Moseley et al., 1999Go; Saier et al., 1999Go) to predict the 12 transmembrane domains that fall within the range of 10–14 transmembrane domains typically predicted for SLC26 proteins (Mount and Romero, 2004Go). The tfSLC26a6 contains the STAS domain, which has been suggested to function in coordinating SLC26a6 and CFTR function for Cl– and fluid secretion in mammalian systems (Ko et al., 2002Go; Ko et al., 2004Go).

 

Figure 3
View larger version (8K):
[in this window]
[in a new window]

  		Fig. 3. Tissue distribution of toadfish SLC26a6 mRNA expression. Expression levels normalized to EF1{alpha} are reported relative to the lowest tissue expression level observed (rectum). Means ± s.e.m., N=8.

 

Figure 4
View larger version (6K):
[in this window]
[in a new window]

  		Fig. 4. Expression of toadfish SLC26a6 in anterior (Ant int), mid (Mid int), posterior (Post int) intestine as well as rectum (Rec) and gill following transfer from seawater to 60 p.p.t. Expression levels normalized to EF1{alpha} and 18S are reported relative to the lowest expression level observed in the segment or tissue. Means ± s.e.m., N=8.

 

Figure 5
View larger version (7K):
[in this window]
[in a new window]

  		Fig. 5. (A) 36Cl uptake 72 h post-injection of H2O (control) or toadfish SLC26a6 mRNA by individual Xenopus oocytes (N=14–16) and (B) membrane potential 48 h post-injections of H2O (control) or toadfish SLC26a6 mRNA in individual Xenopus oocytes (N=6). Means ± s.e.m., *indicates statistically significant difference from control (see text for detail).

 

Figure 6
View larger version (16K):
[in this window]
[in a new window]

  		Fig. 6. (A) HCO3– secretion, (B) transepithelial potential (TEP) and (C) epithelial conductance (G) in anterior intestinal epithelium from the gulf toadfish Opsanus beta under control conditions (0–60 min) and after the addition to 1 µmol l–1 bafilomycin to the luminal saline (60–120 min). Experiments were performed on a total of eight preparations, of which seven showed a response to bafilomycin addition (black bars, black line, black symbols, means ± s.e.m., N=7). A single preparation (gray bars, gray line, gray symbols) did not respond and was not included in the calculation of means or statistical evaluation. Note that the preparation that did not respond to bafilomycin exhibited an unusual absolute TEP value and declining TEP over time of measurement. *Indicates statistically significant difference from control (see text for detail).

 

Figure 7
View larger version (60K):
[in this window]
[in a new window]

  		Fig. 7. Localization of the vacuolar H+-pump in the anterior intestine of the gulf toadfish by immunohistochemistry. Both panels (A; control without primary antibody and B; H+-pump staining) are overlays of two images collected for H+-pump immunoreactivity (green) and nuclei visualization by DAPI (blue). Scale bar, 10 µm.

 

Add to CiteULike CiteULike   Add to Complore Complore   Add to Connotea Connotea   Add to Del.icio.us Del.icio.us   Add to Digg Digg   Add to Reddit Reddit   Add to Technorati Technorati   Add to Twitter Twitter    What's this?



© The Company of Biologists Ltd 2009