Passive ion fluxes across the corneal endothelium

Corneal Endothelial Pump Coupling to Lactic Acid Efflux in the Rabbit and Mouse

Purpose

Confirm that the corneal endothelial pump uses a lactate-coupled water efflux mechanism.

Methods

Corneal thickness, lactate efflux, and stromal [lactate] were measured in de-epithelialized swollen and nonswollen ex vivo-mounted rabbit corneas perfused with bicarbonate-rich and bicarbonate-free Ringers, ouabain, or acetazolamide to determine if the relationships among these parameters were similar to previous data using intact corneas. The role of barrier function was tested by perfusion with calcium-free EGTA. Predictions of [lactate] in endothelial dystrophy were examined in the Slc4a11 knock out mouse.

Results

De-epithelialized corneal swelling, lactate efflux, and stromal [lactate] in response to bicarbonate-free Ringers, ouabain, and acetazolamide perfusion had the same relationship as in intact corneas. The absolute amount of lactate efflux and stromal [lactate] in the de-epithelialized corneas was about half of intact corneas. De-epithelialized, swollen corneas deswelled fully with bicarbonate-rich, partially in the presence of acetazolamide, but continued to swell with bicarbonate-free or ouabain. The relationship among corneal thickness, lactate efflux, and [lactate] was the same as with nonswollen de-epithelialized corneas. In intact corneas swollen by perfusion with calcium-free EGTA, the relationship between swelling and lactate flux was the inverse of control corneas. The relationship between corneal swelling and [lactate] of intact corneas exposed to ouabain, but perfused with 7 mM lactate to simulate aqueous humor, was the same as without lactate. Corneal [lactate] in Slc4a11 knock out was twice that of wild type.

Conclusions

The corneal endothelial pump works via a lactate efflux mechanism that requires an intact osmotic barrier.

Fluid transport by the cornea endothelium is dependent on buffering lactic acid efflux

Maintenance of corneal hydration is dependent on the active transport properties of the corneal endothelium. We tested the hypothesis that lactic acid efflux, facilitated by buffering, is a component of the endothelial fluid pump. Rabbit corneas were perfused with bicarbonate-rich (BR) or bicarbonate-free (BF) Ringer of varying buffering power, while corneal thickness was measured. Perfusate was collected and analyzed for lactate efflux. In BF with no added HEPES, the maximal corneal swelling rate was 30.0 ± 4.1 μm/h compared with 5.2 ± 0.9 μm/h in BR. Corneal swelling decreased directly with [HEPES], such that with 60 mM HEPES corneas swelled at 7.5 ± 1.6 μm/h. Perfusate [lactate] increased directly with [HEPES]. Similarly, reducing the [HCO3 (-)] increased corneal swelling and decreased lactate efflux. Corneal swelling was inversely related to Ringer buffering power (β), whereas lactate efflux was directly related to β. Ouabain (100 μM) produced maximal swelling and reduction in lactate efflux, whereas carbonic anhydrase inhibition and an monocarboxylic acid transporter 1 inhibitor produced intermediate swelling and decreases in lactate efflux. Conversely, 10 μM adenosine reduced the swelling rate to 4.2 ± 0.8 μm/h and increased lactate efflux by 25%. We found a strong inverse relation between corneal swelling and lactate efflux (r = 0.98, P < 0.0001). Introducing lactate in the Ringer transiently increased corneal thickness, reaching a steady state (0 ± 0.6 μm/h) within 90 min. We conclude that corneal endothelial function does not have an absolute requirement for bicarbonate; rather it requires a perfusing solution with high buffering power. This facilitates lactic acid efflux, which is directly linked to water efflux, indicating that lactate flux is a component of the corneal endothelial pump.

Lactate-H⁺ transport is a significant component of the in vivo corneal endothelial pump

PURPOSE

To confirm the expression of monocarboxylate transporters (MCT) 1, 2, and 4 in rabbit CE and to test the hypothesis that cellular buffering contributed by HCO₃⁻, NBCe1, and carbonic anhydrase (CA) activity facilitates lactate-H⁺ efflux thereby controlling corneal hydration in vivo.

METHODS

MCT1-4 expression of rabbit endothelium was examined by Western blotting and immunofluorescence staining. Lactate-induced acidification (LIA) was measured in perfused CE in the presence and absence of HCO₃⁻ and acetazolamide (ACTZ) using tissue treated with siRNA specific to MCT1, 2, and 4. Corneal thickness and lactate concentration were measured in New Zealand White rabbits treated with the topical CA inhibitor Azopt, and from eyes that were injected intracamerally with ouabain, disodium 4,4'-diisothiocyanatostilbene-2,2'-disulfonate (DIDS), and shRNA specific to the 1Na⁺:2HCO₃⁻ cotransporter NBCe1.

RESULTS

MCT1 and MCT4 are localized to the lateral membrane, while MCT2 is apical. Cell pH measurements showed LIA in response to 40 mM lactate in bicarbonate free (BF) Ringer's that was inhibited by niflumic acid and by MCT siRNA knockdown, and significantly reduced in the presence of HCO₃⁻. Lactate-dependent proton flux in vitro was not significantly greater in the presence of HCO₃⁻ or reduced by ACTZ. However, when active transport, NBCe1, or CA activity was disrupted in vivo, corneal edema ensued and was associated with significant corneal lactate accumulation.

CONCLUSIONS

MCT1, 2, and 4 are expressed in rabbit CE on both the apical and basolateral surfaces and function to transport lactate-H⁺. Lactate-H⁺ flux is facilitated by active transport, HCO₃⁻ transport and CA activity, disruption of which causes corneal edema in vivo and indicates that facilitation of lactate efflux is a component of the endothelial pump.

Dependence of cAMP meditated increases in Cl- and HCO(3)- permeability on CFTR in bovine corneal endothelial cells

The cystic fibrosis transmembrane conductance regulator (CFTR) is present on the apical membrane of corneal endothelial cells. Increasing intracellular [cAMP] with forskolin stimulates an NPPB and glibenclamide-inhibitable apical Cl(-) and HCO(3)(-) permeability [Sun, X.C., Bonanno, J.A., 2002. Expression, localization, and functional evaluation of CFTR in bovine corneal endothelial cells. Am. J. Physiol. Cell Physiol. 282, C673-C683]. To definitively determine that the increased permeability is dependent on CFTR, we used an siRNA knockdown approach. Apical Cl(-) and HCO(3)(-) permeability and steady-state HCO(3)(-) flux were measured in the presence or absence of forskolin using cultured bovine corneal endothelial cells that were transfected with CFTR siRNA or a scrambled sequence control. CFTR protein expression was reduced by approximately 80% in CFTR siRNA treated cultures. Forskolin (10 microM) increased apical chloride permeability by 7-fold, which was reduced to control level in siRNA treated cells. CFTR siRNA treatment had no effect on baseline apical chloride permeability. Apical HCO(3)(-) permeability was increased 2-fold by 10 microM forskolin, which was reduced to control level in siRNA treated cultures. Similarly, there was no effect on baseline apical HCO(3)(-) permeability by knocking down CFTR expression. The steady-state apical-basolateral pH gradient (DeltapH) at 4h in control cultures was increased approximately 2.5-fold by forskolin. In CFTR siRNA treated cells, the baseline DeltapH was similar to control, however forskolin did not have a significant effect. We conclude that forskolin induced increases in apical HCO(3)(-) permeability in bovine corneal endothelium requires CFTR. However, CFTR does not have a major role in determining baseline apical chloride or HCO(3)(-) permeability.

An update on corneal hydration control

An account is provided of developments in our understanding of the mechanism of corneal hydration control, particularly as regards the possibility of an active system for its regulation. Emphasis is given to issues that are contentious, such as the role of bicarbonate in the endothelial pump and the significance of water channels in both corneal limiting cell layers.

Predicted permeability of the cornea to topical drugs

PURPOSE

To develop a theoretical model to predict the passive, steady-state permeability of cornea and its component layers (epithelium, stroma, and endothelium) as a function of drug size and distribution coefficient (phi). The parameters of the model should represent physical properties that can be independently estimated and have physically interpretable meaning.

METHODS

A model was developed to predict corneal permeability using 1) a newly developed composite porous-medium approach to model transport through the transcellular and paracellular pathways across the epithelium and endothelium and 2) previous work on modeling corneal stroma using a fiber-matrix approach.

RESULTS

The model, which predicts corneal permeability for molecules having a broad range of size and lipophilicity, was validated by comparison with over 150 different experimental data points and showed agreement with a mean absolute fractional error of 2.43, which is within the confidence interval of the data. In addition to overall corneal permeability, the model permitted independent analysis of transcellular and paracellular pathways in epithelium, stroma and endothelium. This yielded strategies to enhance corneal permeability by targeting epithelial paracellular pathways for hydrophilic compounds (phi < 0.1 - 1), epithelial transcellular pathways for intermediate compounds, and stromal pathways for hydrophobic compounds (phi > 10 - 100). The effects of changing corneal physical properties (e.g., to mimic disease states or animals models) were also examined.

CONCLUSIONS

A model based on physicochemical properties of the cornea and drug molecules can be broadly applied to predict corneal permeability and suggest strategies to enhance that permeability.

An oneiropenic account of an ophthalmological career

The author has done pretty much what he wanted to do throughout his professional life. Little harm resulted. A few findings may survive.

Ocular pharmacokinetics of calcium, chloride and sulphate ions after instillation in the rabbit

The ionic environment of the eye is involved in major biochemical processes which are essential for preserving the integrity of cornea and lens. The purpose of the present study was to determine the intra-ocular penetration and the pharmacokinetic parameters of calcium, chloride and sulphate ions in the cornea, iris-ciliary body (ICB) and lens, after administration by instillation in the rabbit eye. In order to extrapolate our results to the processes occurring in man, we followed a precise instillation protocol using a low volume (5 microliters) of 45Calcium, 36Chloride and 35Sulphate, which is less than the lacrimal volume determined during the palpebral closing and performing manual blinking at a frequency of 2 min-1. The results indicate an immediate trans-corneal permeability and a rapid ocular distribution of these ions. We observe that, relative to dose, an important percentage of calcium (67.20 per cent) was entrapped in the cornea; this parameter was less important for chloride (10.19 per cent) and for sulphate ions (3.25 per cent). These values are in agreement with those predicted theoretically for trans-corneal penetration by such compounds. On the other hand, the total degrees of penetration by chloride and sulphate ions in ICB (1.40 per cent and 0.90 per cent, respectively) and lens (0.35 per cent and 0.41 per cent, respectively) are quite similar. Calcium retention is much higher in these tissues (25.39 per cent in ICB and 16.03 per cent in lens).(ABSTRACT TRUNCATED AT 250 WORDS)