Ionic selectivity of the paracellular shunt path across rabbit corneal endothelium

Epithelial Fluid Transport is Due to Electro-osmosis (80%), Plus Osmosis (20%)

Epithelial fluid transport, an important physiological process shrouded in a long-standing enigma, may finally be moving closer to a solution. We propose that, for the corneal endothelium, relative proportions for the driving forces for fluid transport are 80% of paracellular electro-osmosis, and 20% classical transcellular osmosis. These operate in a cyclical process with a period of 9.2 s, which is dictated by the decrease and exhaustion of cellular Na⁺. Paracellular electro-osmosis is sketched here, and partially discussed as much as the subject still allows; transcellular osmosis is presented at length.

Claudins and the modulation of tight junction permeability

Claudins are tight junction membrane proteins that are expressed in epithelia and endothelia and form paracellular barriers and pores that determine tight junction permeability. This review summarizes our current knowledge of this large protein family and discusses recent advances in our understanding of their structure and physiological functions.

Wavelet analysis of corneal endothelial electrical potential difference reveals cyclic operation of the secretory mechanism

The corneal endothelium is a fluid-transporting epithelium. As other similar tissues, it displays an electrical potential of ~1 mV (aqueous side negative) across the entire layer [transendothelial potential difference (TEPD)]. It appears that this electrical potential is mainly the result of the transport of anions across the cell layer (from stroma to aqueous). There is substantial evidence that the TEPD is related linearly to fluid transport; hence, under proper conditions, its measure could serve as a measure of fluid transport. Furthermore, the TEPD is not steady; instead, it displays a spectrum of frequency components (0-15 Hz) recognized recently using Fourier transforms. Such frequency components appear due to charge-separating (electrogenic) processes mediated by epithelial plasma membrane proteins (both ionic channels and ionic cotransporters). In particular, the endothelial TEPD oscillations of the highest amplitude (1-2 Hz) were linked to the operation of so-called sodium bicarbonate cotransporters. However, no time localization of that activity could be obtained with the Fourier methodology utilized. For that reason we now characterize the TEPD using wavelet analysis with the aim to localize in time the variations in TEPD. We find that the mentioned high-amplitude oscillatory components of the TEPD appear cyclically during the several hours that an endothelial preparation survives in vitro. They have a period of 4.6 ± 0.4 s on average (n=4). The wavelet power value at the peak of such oscillations is 1.5 ± 0.1 mV(2) Hz on average (n = 4), and is remarkably narrow in its distribution.

Fluid Transport Across Leaky Epithelia: Central Role of the Tight Junction and Supporting Role of Aquaporins

The mechanism of epithelial fluid transport remains unsolved, which is partly due to inherent experimental difficulties. However, a preparation with which our laboratory works, the corneal endothelium, is a simple leaky secretory epithelium in which we have made some experimental and theoretical headway. As we have reported, transendothelial fluid movements can be generated by electrical currents as long as there is tight junction integrity. The direction of the fluid movement can be reversed by current reversal or by changing junctional electrical charges by polylysine. Residual endothelial fluid transport persists even when no anions (hence no salt) are being transported by the tissue and is only eliminated when all local recirculating electrical currents are. Aquaporin (AQP) 1 is the only AQP present in these cells, and its deletion in AQP1 null mice significantly affects cell osmotic permeability (by ∼40%) but fluid transport much less (∼20%), which militates against the presence of sizable water movements across the cell. In contrast, AQP1 null mice cells have reduced regulatory volume decrease (only 60% of control), which suggests a possible involvement of AQP1 in either the function or the expression of volume-sensitive membrane channels/transporters. A mathematical model of corneal endothelium we have developed correctly predicts experimental results only when paracellular electro-osmosis is assumed rather than transcellular local osmosis. Our evidence therefore suggests that the fluid is transported across this layer via the paracellular route by a mechanism that we attribute to electro-osmotic coupling at the junctions. From our findings we have developed a novel paradigm for this preparation that includes 1) paracellular fluid flow; 2) a crucial role for the junctions; 3) hypotonicity of the primary secretion; and 4) an AQP role in regulation rather than as a significant water pathway. These elements are remarkably similar to those proposed by the laboratory of Adrian Hill for fluid transport across other leaky epithelia.

Corneal endothelium transports fluid in the absence of net solute transport

The corneal endothelium transports fluid from the corneal stroma to the aqueous humor, thus maintaining stromal transparency by keeping it relatively dehydrated. This fluid transport mechanism is thought to be driven by the transcellular transports of HCO(3)(-) and Cl(-) in the same direction, from stroma to aqueous. In parallel to these anion movements, for electroneutrality, there are paracellular Na(+) and transcellular K(+) transports in the same direction. The resulting net flow of solute might generate local osmotic gradients that drive fluid transport. However, there are reports that some 50% residual fluid transport remains in nominally HCO(3)(-) free solutions. We have examined the driving force for this residual fluid transport. We confirm that in nominally HCO(3)(-) free solutions, 48% of control fluid transport remains. When in addition Cl(-) channels are inhibited, 30% of control fluid movement still remains. Addition of a carbonic anhydrase inhibitor has no further effect. These manipulations combined inhibit the transcellular transport of all anions, without which there cannot be any net transport of solute and consequently no local osmotic gradients, yet there is residual fluid movement. Only the further addition of benzamil, an inhibitor of epithelial Na(+) channels, abolishes fluid transport completely. Our data are inconsistent with transcellular local osmosis and instead support the paradigm of paracellular fluid transport driven by electro-osmotic coupling.

Modulation of tight junction properties relevant to fluid transport across rabbit corneal endothelium

Paracellular junctions could play an important role in corneal endothelial fluid transport. In this study we explored the effects of different reagents on the tight junctional barrier by assessing the translayer specific electrical resistance (TER) across rabbit corneal endothelial preparations and cultured rabbit corneal endothelial cells' (CRCEC) monolayers, the paracellular permeability (Papp) for fluorescein isothiocyanate (FITC) dextrans across CRCEC, and fluid transport across de-epithelialized rabbit corneal endothelial preparations. Palmitoyl carnitine (PC), poly-L-lysine (PLL), adenosine triphosphate (ATP), and dibutyryl adenosine 3',5'-cyclic monophosphate (dB-cAMP) were used to modulate corneal endothelial fluid transport and tight junctions (TJs). After seeding, the TER across CRCEC reached maximal values (29.2+/-1.0 Omega cm2) only after the 10th day. PC (0.1 mM) caused decreases both in TER (by 40%) and fluid transport (swelling rate: 18.5+/-0.3 microm/h), and an increase in Papp. PLL resulted in increased TER rose and Papp but decreased fluid transport (swelling rate: 10+/-0.3 microm/h). dB-cAMP (0.1 mM) and ATP (0.1 mM) decreased TER by 16% and 6%, increased Papp slightly, and stimulated fluid transport; the rates of de-swelling (in microm/h) were -5.4+/-0.3 and -12.1+/-0.4, respectively. PC might cause the junctions to open up unspecifically and thus increase passive leak. PLL is a known junctional charge modifier that may be adding steric hindrance to the tight junctions. The results with dB-cAMP and ATP are consistent with fluid transport via the paracellular route.

The Role of the Tight Junction in Paracellular Fluid Transport across Corneal Endothelium. Electro-osmosis as a Driving Force

The mechanism of epithelial fluid transport is controversial and remains unsolved. Experimental difficulties pose obstacles for work on a complex phenomenon in delicate tissues. However, the corneal endothelium is a relatively simple system to which powerful experimental tools can be applied. In recent years our laboratory has developed experimental evidence and theoretical insights that illuminate the mechanism of fluid transport across this leaky epithelium. Our evidence points to fluid being transported via the paracellular route by a mechanism requiring junctional integrity, which we attribute to electro-osmotic coupling at the junctions. Fluid movements can be produced by electrical currents. The direction of the movement can be reversed by current reversal or by changing junctional electrical charges by polylysine. Aquaporin 1 (AQP1) is the only AQP present in these cells, and its deletion in AQP1 null mice significantly affects cell osmotic permeability but not fluid transport, which militates against the presence of sizable water movements across the cell. By contrast, AQP1 null mice cells have reduced regulatory volume decrease (only 60% of control), which suggests a possible involvement of AQP1 in either the function or the expression of volume-sensitive membrane channels/transporters. A mathematical model of corneal endothelium predicts experimental results only when based on paracellular electro-osmosis, and not when transcellular local osmosis is assumed instead. Our experimental findings in corneal endothelium have allowed us to develop a novel paradigm for this preparation that includes: (1) paracellular fluid flow; (2) a crucial role for the junctions; (3) hypotonicity of the primary secretion; (4) an AQP role in regulation and not as a significant water pathway. These elements are remarkably similar to those proposed by the Hill laboratory for leaky epithelia.

On the mechanism of fluid transport across corneal endothelium and epithelia in general

The mechanism by which fluid is transported across epithelial layers is still unclear. The prevalent idea is that fluid traverses these layers transcellularly, driven by local osmotic gradients secondary to electrolyte transport and utilizing the high osmotic permeability of aquaporins. However, recent findings that some aquaporin knockout mice epithelia transport fluid sow doubts on local osmosis. This review discusses recent evidence in corneal endothelium that points instead to electro-osmosis as the mechanism underlying fluid transport. In this concept, a local recirculating electrical current would result in electro-osmotic coupling at the level of the intercellular junctions, dragging fluid via the paracellular route. The text also mentions possible mechanisms for apical bicarbonate exit from endothelial cells, and discusses whether electro-osmosis could be a general mechanism.

Characterization of corneal endothelium cell cultured on microporous membrane filters

In these experiments we characterize rabbit and bovine corneal endothelia cell cultured on microporous membrane filters (0.6cm2). Cell cultured bovine or rabbit corneal endothelial cells (subcultures 1-3) were seeded onto Millicell-HA filter inserts. Electrical resistance measured across the cultured monolayers increased steadily through 14 days of culture, reaching 34.2 +/- 0.8 ohm-cm2 (mean +/- SE) for rabbit cells and 33.1 +/- 1.1 ohm-cm2 for bovine cells. Alizarin red staining of the monolayers showed a polygonal morphology comparable to that observed in situ. Transmission electron microscopy showed well developed apical junctional complexes and flaps. Exposure of the monolayers to calcium-free medium resulted in the disruption of intercellular junctions, rounding-up of the cells and a decrease in electrical resistance (to near 0). Transmonolayer fluxes of inulin and dextran correlated well to the resistance measurements. Results of this study demonstrate that corneal endothelium, both bovine and rabbit, grown on filter inserts is comparable in morphology and ultrastructure to corneal endothelium in situ. The cells cultured in this system form functional apical junctional complexes that effect a barrier function comparable to that of the endothelium in situ.

Physiological state of the rabbit cornea following 4 degrees C moist chamber storage

Eyes from female albino rabbits (1.9-2.3 kg) were enucleated immediately following euthanasia along with the lids and conjunctiva. The globes were moist chamber-stored (in a 4 degrees C refrigerator with the lids closed and the cornea facing downwards) for 12-500 hr. Central corneal thickness (by ultrasound) increased from 350 to approximately 650 microns within 72 hr but changed little thereafter. In the latter period, the relative fluid pressure of the globe (pneumatonography) dropped to less than 10 mmHg, aqueous bicarbonate (tCO2) levels fell to less than 5 mM and anterior chamber fluid tonicity decreased progressively (especially after 24 hr storage) to reach values of around 200 mosmol l-1 by 7 days. Storma-endothelium preparations of the corneas, after 2 hr of in vitro equilibration with a bicarbonate-Ringer solution (supplemented with glucose, adenosine and glutathione) were evaluated for their ability to undergo deturgescence under silicone oil or to pump fluid against a hydrostatic pressure of 20 cmH2O. Corneal preparations from up to 7 days storage showed rapid (60 to 135 microns hr-1) and complete deturgescence (net change in thickness of 140-180 microns) that was maintained. Thereafter, the ability to show deturgescence declined to zero by 10 days. In marked contrast, endothelial fluid pump activity (of approximately 5 microliters hr-1) was manifest for only 36 hr after which time this function rapidly declined. Most corneas stored for periods longer than 48 hr exhibited a continuous leak (of -1 to -5 microliters hr-1). The results indicate that corneal deturgescence and endothelial fluid pump function are not necessarily coupled in vitro.

The movement of sodium across short-circuited rabbit corneal endothelium

Unidirectional sodium and bicarbonate ion fluxes were measured under short-circuit conditions across rabbit corneal endothelium. A net flux of bicarbonate was measured, direction tears to lens, however no net flux of sodium was measured. From these experiments it seems that the inequality between electrically measured short-circuit current and net trans-endothelial bicarbonate ion flux cannot be bridged by any supposed net sodium flux.

The mechanism of fluid and electrolyte transport across corneal endothelium: critical revision and update of a model

A model for endothelial transport is updated to include recent evidence. We discuss electrolyte movements based on a Na+-K+ ATPase, a Na+-H+ exchanger, a Na+-HCO3 coupler, a Cl- -HCO-3 exchanger, a K+-Cl-coupler, and K+ and anion channels. We discuss near-isotonic transport of fluid on the basis of recent findings of high endothelial osmotic permeability.

Amiloride inhibition of Na+-entry into corneal endothelium

Amiloride in 10(-3) M concentration inhibits incompletely the short circuit current and active potential difference across the bovine corneal endothelium in vitro. The drug effect is reversible and unilateral, e.g. the drug is effective only from the aqueous side. The amiloride effect is compared to the effect of ouabain, nystatin and vasopressin on the same electrical parameters. The effect of these drugs support a model for active Na+ transport across the corneal endothelium with two separate pathways for Na+ transport - one for extrusion and one for reentry.

Streaming potentials and diffusion potentials across rabbit proximal convoluted tubule

The streaming potential, defined as the transepithelial potential appearing in the presence of an osmotic water flow, was measured in rabbit kidney proximal convoluted tubules perfused in vitro. The S2 segments studied were dissected from mid-cortical and juxtamedullary portions of the kidney and the streaming potential induced by the addition of raffinose in bath was compared for each tubule with the diffusion potential corresponding to an imposed NaCl gradient in the absence of osmotic gradient. The amplitude of the measured streaming potential was found to vary from positive to negative values (+0.9 to -1.8 mV) according to the location of the dissected tubule: the more juxtamedullary the nephron, the more lumen negative was the streaming potential. This correlated well with the diffusion potentials recorded on the same tubules and the amplitude of the streaming potentials was a close function of the PNa/PCl ratios calculated from these diffusion potentials. This is in agreement with the hypothesis of solute polarization in an unstirred layer as the origin of the streaming potential; a calculation of hydraulic permeability (Pf) of the proximal tubule, taking the role of such an unstirred layer into consideration is proposed.

Evidence for coupled transport of bicarbonate and sodium in cultured bovine corneal endothelial cells

Using intracellular microelectrode technique, the response of the voltage V across the plasma membrane of cultured bovine corneal endothelial cells to changes in sodium and bicarbonate concentrations was investigated. (1) The electrical response to changes in [HCO3-]o (depolarization upon lowering and hyperpolarization upon raising [HCO3-]o) was dependent on sodium. Lithium could fairly well be substituted for sodium, whereas potassium or choline were much less effective. (2) Removal of external sodium caused a depolarization, while a readdition led to a hyperpolarization, which increased with time of preincubation in the sodium-depleted medium. (3) The response to changes in [Na+]o was dependent on bicarbonate. In a nominally bicarbonate-free medium, its amplitude was decreased or even reversed in sign. (4) Application of SITS or DIDS (10(-3) M) had a similar effect on the response to sodium as bicarbonate-depleted medium. (5) At [Na+]o = 151 mM and [HCO3-]o = 46 mM, the transients of V depended, with 39.0 +/- 9.0 (SD) mV/decade, on bicarbonate and, with 15.3 +/- 5.8 (SD) mV/decade, on sodium. (6) After the preincubation of cells with lithium, replacement of Li by choline led to similar effects as the replacement of sodium by choline, though the response of V was smaller with Li. This response could be reduced or reversed by the removal of bicarbonate or by the application of SITS. (7) Amiloride (10(-3) M) caused a reversible hyperpolarization of the steady-state potential by 8.5 +/- 2.6 mV (SD). It did not affect the immediate response to changes in [Na+]o or [HCO3-]o, but reduced the speed of regaining the steady-state potential after a change in [HCO3-]o. (8) Ouabain (10(-4) M) caused a fast depolarization of -6.8 +/- 1.1 (SD) mV, which was followed by a continuing slower depolarization. The effect was almost identical at 10(-5) M. (9) It is suggested, that corneal endothelial cells possess a cotransport for sodium and bicarbonate, which transports net negative charge with these ions. It is inhibitable by stilbenes, but not directly affected by amiloride or ouabain. Lithium is a good substitute for sodium with respect to bicarbonate transport and is transported itself. In addition, the effect of amiloride provides indirect evidence for the existence of a Na+/H+-antiport. A model for the transepithelial transport of bicarbonate across the corneal endothelium is proposed.