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CO2-induced ion and fluid transport in human retinal pigment epithelium.

Adijanto J, Banzon T, Jalickee S, Wang NS, Miller SS - J. Gen. Physiol. (2009)

Bottom Line: Our analysis of native and cultured fetal human RPE shows that the apical membrane is significantly more permeable (approximately 10-fold; n = 7) to CO2 than the basolateral membrane, perhaps due to its larger exposed surface area.The activity of this transporter was increased by elevating apical bath CO2 and was reduced by dorzolamide.This homeostatic process would preserve the close anatomical relationship between photoreceptor outer segments and RPE in the dark and light, thus protecting the health of the photoreceptors.

View Article: PubMed Central - PubMed

Affiliation: Department of Chemical and Biomolecular Engineering, University of Maryland, College Park, MD 20742, USA.

ABSTRACT
In the intact eye, the transition from light to dark alters pH, [Ca2+], and [K] in the subretinal space (SRS) separating the photoreceptor outer segments and the apical membrane of the retinal pigment epithelium (RPE). In addition to these changes, oxygen consumption in the retina increases with a concomitant release of CO2 and H2O into the SRS. The RPE maintains SRS pH and volume homeostasis by transporting these metabolic byproducts to the choroidal blood supply. In vitro, we mimicked the transition from light to dark by increasing apical bath CO2 from 5 to 13%; this maneuver decreased cell pH from 7.37 +/- 0.05 to 7.14 +/- 0.06 (n = 13). Our analysis of native and cultured fetal human RPE shows that the apical membrane is significantly more permeable (approximately 10-fold; n = 7) to CO2 than the basolateral membrane, perhaps due to its larger exposed surface area. The limited CO2 diffusion at the basolateral membrane promotes carbonic anhydrase-mediated HCO3 transport by a basolateral membrane Na/nHCO3 cotransporter. The activity of this transporter was increased by elevating apical bath CO2 and was reduced by dorzolamide. Increasing apical bath CO2 also increased intracellular Na from 15.7 +/- 3.3 to 24.0 +/- 5.3 mM (n = 6; P < 0.05) by increasing apical membrane Na uptake. The CO2-induced acidification also inhibited the basolateral membrane Cl/HCO3 exchanger and increased net steady-state fluid absorption from 2.8 +/- 1.6 to 6.7 +/- 2.3 microl x cm(-2) x hr(-1) (n = 5; P < 0.05). The present experiments show how the RPE can accommodate the increased retinal production of CO2 and H(2)O in the dark, thus preventing acidosis in the SRS. This homeostatic process would preserve the close anatomical relationship between photoreceptor outer segments and RPE in the dark and light, thus protecting the health of the photoreceptors.

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13% apical CO2 increases net solute and fluid absorption across the RPE. The transporters and channels depicted in this model are adopted from earlier studies of frog, bovine, human, and cultured human RPE. Apical membrane proteins: Na/K ATPase, Na/K/2Cl cotransporter (NKCC1), Na/H exchanger (NHE), and Na/2HCO3 cotransporter (NBC1). Basolateral membrane proteins: Ca2+-activated Cl channels, cAMP-sensitive CFTR, Cl/HCO3 exchanger (AE2), and Na/nHCO3 cotransporter (NBC). Increasing apical CO2 increases Na (Cl + HCO3) and fluid absorption from the SRS to the choroid.
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fig1: 13% apical CO2 increases net solute and fluid absorption across the RPE. The transporters and channels depicted in this model are adopted from earlier studies of frog, bovine, human, and cultured human RPE. Apical membrane proteins: Na/K ATPase, Na/K/2Cl cotransporter (NKCC1), Na/H exchanger (NHE), and Na/2HCO3 cotransporter (NBC1). Basolateral membrane proteins: Ca2+-activated Cl channels, cAMP-sensitive CFTR, Cl/HCO3 exchanger (AE2), and Na/nHCO3 cotransporter (NBC). Increasing apical CO2 increases Na (Cl + HCO3) and fluid absorption from the SRS to the choroid.

Mentions: The RPE expresses several different HCO3 transport proteins at the apical and basolateral membranes as illustrated in Fig. 1 (Hughes et al., 1989; Kenyon et al., 1997). As in other epithelia, the activities of these transporters in human RPE can be facilitated by carbonic anhydrases (CAs) expressed in the cytosol (CA II), on the apical membrane (CA IV, CA IX, CA XII, and CA XIV), or on the basolateral membrane (CA IX) (Casey, 2006; Purkerson and Schwartz, 2007; Zhi, C.G., F.E. Wang, T. Banzon, S. Jalickee, R. Fariss, A. Maminishkis, and S.S. Miller. 2007. Membrane-Bound Carbonic Anhydrases in Human Fetal Retinal Pigment Epithelial Cells (hfRPE)). The removal of CO2 from the SRS can be achieved by diffusion across the RPE or by conversion to HCO3 using the catalytic activity of CAs for transmembrane transport via HCO3 transporters, or both. CO2 diffusion into the RPE drives CA-mediated formation of HCO3 and protons that acidify the cell. Transporter-mediated HCO3 entry mitigates this acidification and helps maintain pH homeostasis in the RPE (Kenyon et al., 1997) and in the SRS (Borgula et al., 1989).


CO2-induced ion and fluid transport in human retinal pigment epithelium.

Adijanto J, Banzon T, Jalickee S, Wang NS, Miller SS - J. Gen. Physiol. (2009)

13% apical CO2 increases net solute and fluid absorption across the RPE. The transporters and channels depicted in this model are adopted from earlier studies of frog, bovine, human, and cultured human RPE. Apical membrane proteins: Na/K ATPase, Na/K/2Cl cotransporter (NKCC1), Na/H exchanger (NHE), and Na/2HCO3 cotransporter (NBC1). Basolateral membrane proteins: Ca2+-activated Cl channels, cAMP-sensitive CFTR, Cl/HCO3 exchanger (AE2), and Na/nHCO3 cotransporter (NBC). Increasing apical CO2 increases Na (Cl + HCO3) and fluid absorption from the SRS to the choroid.
© Copyright Policy - openaccess
Related In: Results  -  Collection

License 1 - License 2
Show All Figures
getmorefigures.php?uid=PMC2713148&req=5

fig1: 13% apical CO2 increases net solute and fluid absorption across the RPE. The transporters and channels depicted in this model are adopted from earlier studies of frog, bovine, human, and cultured human RPE. Apical membrane proteins: Na/K ATPase, Na/K/2Cl cotransporter (NKCC1), Na/H exchanger (NHE), and Na/2HCO3 cotransporter (NBC1). Basolateral membrane proteins: Ca2+-activated Cl channels, cAMP-sensitive CFTR, Cl/HCO3 exchanger (AE2), and Na/nHCO3 cotransporter (NBC). Increasing apical CO2 increases Na (Cl + HCO3) and fluid absorption from the SRS to the choroid.
Mentions: The RPE expresses several different HCO3 transport proteins at the apical and basolateral membranes as illustrated in Fig. 1 (Hughes et al., 1989; Kenyon et al., 1997). As in other epithelia, the activities of these transporters in human RPE can be facilitated by carbonic anhydrases (CAs) expressed in the cytosol (CA II), on the apical membrane (CA IV, CA IX, CA XII, and CA XIV), or on the basolateral membrane (CA IX) (Casey, 2006; Purkerson and Schwartz, 2007; Zhi, C.G., F.E. Wang, T. Banzon, S. Jalickee, R. Fariss, A. Maminishkis, and S.S. Miller. 2007. Membrane-Bound Carbonic Anhydrases in Human Fetal Retinal Pigment Epithelial Cells (hfRPE)). The removal of CO2 from the SRS can be achieved by diffusion across the RPE or by conversion to HCO3 using the catalytic activity of CAs for transmembrane transport via HCO3 transporters, or both. CO2 diffusion into the RPE drives CA-mediated formation of HCO3 and protons that acidify the cell. Transporter-mediated HCO3 entry mitigates this acidification and helps maintain pH homeostasis in the RPE (Kenyon et al., 1997) and in the SRS (Borgula et al., 1989).

Bottom Line: Our analysis of native and cultured fetal human RPE shows that the apical membrane is significantly more permeable (approximately 10-fold; n = 7) to CO2 than the basolateral membrane, perhaps due to its larger exposed surface area.The activity of this transporter was increased by elevating apical bath CO2 and was reduced by dorzolamide.This homeostatic process would preserve the close anatomical relationship between photoreceptor outer segments and RPE in the dark and light, thus protecting the health of the photoreceptors.

View Article: PubMed Central - PubMed

Affiliation: Department of Chemical and Biomolecular Engineering, University of Maryland, College Park, MD 20742, USA.

ABSTRACT
In the intact eye, the transition from light to dark alters pH, [Ca2+], and [K] in the subretinal space (SRS) separating the photoreceptor outer segments and the apical membrane of the retinal pigment epithelium (RPE). In addition to these changes, oxygen consumption in the retina increases with a concomitant release of CO2 and H2O into the SRS. The RPE maintains SRS pH and volume homeostasis by transporting these metabolic byproducts to the choroidal blood supply. In vitro, we mimicked the transition from light to dark by increasing apical bath CO2 from 5 to 13%; this maneuver decreased cell pH from 7.37 +/- 0.05 to 7.14 +/- 0.06 (n = 13). Our analysis of native and cultured fetal human RPE shows that the apical membrane is significantly more permeable (approximately 10-fold; n = 7) to CO2 than the basolateral membrane, perhaps due to its larger exposed surface area. The limited CO2 diffusion at the basolateral membrane promotes carbonic anhydrase-mediated HCO3 transport by a basolateral membrane Na/nHCO3 cotransporter. The activity of this transporter was increased by elevating apical bath CO2 and was reduced by dorzolamide. Increasing apical bath CO2 also increased intracellular Na from 15.7 +/- 3.3 to 24.0 +/- 5.3 mM (n = 6; P < 0.05) by increasing apical membrane Na uptake. The CO2-induced acidification also inhibited the basolateral membrane Cl/HCO3 exchanger and increased net steady-state fluid absorption from 2.8 +/- 1.6 to 6.7 +/- 2.3 microl x cm(-2) x hr(-1) (n = 5; P < 0.05). The present experiments show how the RPE can accommodate the increased retinal production of CO2 and H(2)O in the dark, thus preventing acidosis in the SRS. This homeostatic process would preserve the close anatomical relationship between photoreceptor outer segments and RPE in the dark and light, thus protecting the health of the photoreceptors.

Show MeSH
Related in: MedlinePlus