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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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Effect of apical bath CO2 on basolateral membrane Na/nHCO3 cotransporter. Low HCO3 (2.62 mM) Ringer was perfused into the basal bath to record the initial control response, and this maneuver was repeated in (A) 13% or (B) 1% apical bath CO2. After returning to control Ringer, low basal bath [HCO3]-induced control response was obtained. Solid bars above the graphs represent solution changes from control Ringer as described in the legend to Fig. 2.
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fig12: Effect of apical bath CO2 on basolateral membrane Na/nHCO3 cotransporter. Low HCO3 (2.62 mM) Ringer was perfused into the basal bath to record the initial control response, and this maneuver was repeated in (A) 13% or (B) 1% apical bath CO2. After returning to control Ringer, low basal bath [HCO3]-induced control response was obtained. Solid bars above the graphs represent solution changes from control Ringer as described in the legend to Fig. 2.

Mentions: In confluent monolayers of hfRPE, reducing basal bath [HCO3] 10-fold (5% CO2) acidified the cells by 0.20 ± 0.05, with an equivalent H+ flux of 6.2 ± 1.5 mM × min−1 (n = 45), and increased TEP by 1.18 ± 0.60 mV (n = 53; Figs. 8–12 and Tables I and II). This TEP response is consistent with a basolateral membrane depolarization resulting from electrogenic Na/nHCO3 cotransport activity. We evaluated the DIDS sensitivity of this basolateral membrane Na/nHCO3 cotransporter by comparing the basal bath Δ[HCO3]-induced pHi and TEP responses in the presence of 0.5 mM of basal DIDS to that in control (Fig. 8). In five experiments, basal DIDS reduced the basal bath Δ[HCO3]-induced acidification from 0.20 ± 0.04 to 0.09 ± 0.05 (P < 0.05), and reduced the TEP response from 1.41 ± 0.69 to 0.42 ± 0.29 mV (P < 0.05). The inhibitory effect of DIDS on the pHi and TEP responses was irreversible after a 5-min washout with control Ringer (ΔpHi = 0.12 ± 0.03; ΔTEP = 0.56 ± 0.25 mV; P > 0.05).


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)

Effect of apical bath CO2 on basolateral membrane Na/nHCO3 cotransporter. Low HCO3 (2.62 mM) Ringer was perfused into the basal bath to record the initial control response, and this maneuver was repeated in (A) 13% or (B) 1% apical bath CO2. After returning to control Ringer, low basal bath [HCO3]-induced control response was obtained. Solid bars above the graphs represent solution changes from control Ringer as described in the legend to Fig. 2.
© Copyright Policy - openaccess
Related In: Results  -  Collection

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

fig12: Effect of apical bath CO2 on basolateral membrane Na/nHCO3 cotransporter. Low HCO3 (2.62 mM) Ringer was perfused into the basal bath to record the initial control response, and this maneuver was repeated in (A) 13% or (B) 1% apical bath CO2. After returning to control Ringer, low basal bath [HCO3]-induced control response was obtained. Solid bars above the graphs represent solution changes from control Ringer as described in the legend to Fig. 2.
Mentions: In confluent monolayers of hfRPE, reducing basal bath [HCO3] 10-fold (5% CO2) acidified the cells by 0.20 ± 0.05, with an equivalent H+ flux of 6.2 ± 1.5 mM × min−1 (n = 45), and increased TEP by 1.18 ± 0.60 mV (n = 53; Figs. 8–12 and Tables I and II). This TEP response is consistent with a basolateral membrane depolarization resulting from electrogenic Na/nHCO3 cotransport activity. We evaluated the DIDS sensitivity of this basolateral membrane Na/nHCO3 cotransporter by comparing the basal bath Δ[HCO3]-induced pHi and TEP responses in the presence of 0.5 mM of basal DIDS to that in control (Fig. 8). In five experiments, basal DIDS reduced the basal bath Δ[HCO3]-induced acidification from 0.20 ± 0.04 to 0.09 ± 0.05 (P < 0.05), and reduced the TEP response from 1.41 ± 0.69 to 0.42 ± 0.29 mV (P < 0.05). The inhibitory effect of DIDS on the pHi and TEP responses was irreversible after a 5-min washout with control Ringer (ΔpHi = 0.12 ± 0.03; ΔTEP = 0.56 ± 0.25 mV; P > 0.05).

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