The lens is an asymmetric structure bounded on the anterior surface by a single layer of epithelial cells and on the posterior surface by fiber cells (Fig. 1). This apparent asymmetry initiated a number of double-chamber experiments that appeared to show a translens (posterior to anterior) flux of sodium ions correlated with a translens short circuit current.^11,12 Studies such as these have given rise to the “pump-leak model” of lens transport. However, more recent studies performed using a vibrating probe to measure radiating currents^13,14 have shown asymmetries that are more marked between the equator and the two poles than there are between the two poles themselves, and this has given rise to the “internal flow model” for the lens.⁵

There is little doubt that the membranes of the anterior epithelial cells have a very different array of channels and pumps than the fiber cell membranes. This fact, allied to an asymmetric distribution of internal gap junctions that seem to favor an internal flow of current toward the equator rather than from posterior to anterior, makes the two models difficult to reconcile.⁵ However, if an amalgam of the two asymmetry models is considered, then it is possible to merge the two separate ideas into one working hypothesis. The model is presented here and the evidence scrutinized later.

It is suggested that the lens consists of two symmetric and superimposed structures that are only intimately connected at the equator. In this model (see Fig. 1), the anterior epithelium and dynamic bow cells represent one system and the mature fiber cells another. The cells within each system are known to be in good electrical communication with other cells within the same system,^15,16 but it is likely that good communication between the systems exists only at the bow region.¹⁷ If we also assume that the resting potential of the epithelial cells, on average, is higher than the mature fiber cell membrane potential (again on average), then current flows in at the fiber cell membranes and out through the epithelial cells. If the anterior epithelial layer is assumed to act as a leaky epithelium with little potential difference between the apical and basal surfaces, then the only net current that is observed is inward at the poles and outward at the equator. Furthermore, when the anterior and posterior surfaces are isolated, then the anterior epithelium lies in series with the rest of the lens and a small voltage difference (the net transepithelial voltage) wouldbe expected to be observed. In fact, using an oil to isolate the two surfaces of the bovine lens in a nondisruptive manner, Duncan and colleagues¹⁸ observed a maximum potential difference of + 6.5 mV (anterior positive). It should be noted that if the anterior epithelial cells were well-connected to the whole mass of underlying fibers, then a much greater asymmetry in polar currents would be expected than that actually observed as the high voltage of the anterior face membranes would give a strong outward current across the whole of the anterior surface.

It should be noted that there are two types of intercellular communication possible: electrical and molecular (or metabolic). It has been shown in several systems, including the lens, that metabolic communication can be severed while ionic communication remains intact.¹⁶ This important point should be borne in mind when picking one’s way through the mine field of lens junction literature. It also should be remembered that there are possible species differences in the extent of communication between different regions of the lens and, furthermore, the pattern of communication can change as the lens develops.

The molecules (connexins) that form junctional complexes have been reviewed in detail recently,^5,19 and only a brief summary is given here. Connexin 43 is the major junctional molecule in epithelial cells, whereas there is a much greater heterogeneity of connexins found in fiber cells, including connexins 46 and 50, although the latter is not found in mature fibers. Interestingly, connexin mutations can give rise to congenital cataracts,⁵ showing the importance of junction communication to the normal function of the lens. There is general agreement among lens physiologists that the fiber cells arecoupled electrically and that all epithelial cells are extremely well coupled, both electrically and metabolically.^5,16,17,20 There is not the same degree of uniformity of opinion concerning the metabolic coupling between epithelial and fiber cells.^17,20,21

The permeability properties of the lens can be measured by a number of different methods including radiotracer techniques to assess directly the passage of ions and molecules and electrophysiologic techniques both to assess the overall electrical conductance of the lens but also to investigate the specific channel mechanisms involved. In the latter case, it is emerging from patch-clamp experiments that the lens possesses a remarkable variety of potassium, sodium, chloride, and calcium channels. The different methods used to identify the various channels have been reviewed recently in a number of publications.^5,39,40 This review deals rather with the overall changes in lens conductance that occur with age and on exposure to oxidation, but it also deals with the changes that occur after the activation of certain receptors. It also should be remembered that ions may traverse membranes by a number of electrically silent systems, and they include the Na⁺ /H⁺,Na⁺ /Ca²⁺ HCO3-/Cl- exchange mechanisms as well as active transporters such as Na-K-ATPase. The movement of certain ions and even water itself also is restricted by interactions with other species within the lens, and this has to be taken into account in the overall picture of movement across the outer barriers and through the internal matrix of the lens. In the human lens, for example, although almost 100% of K⁺ ions are in free solution, ion-sensitive microelectrode measurements have shown that approximately 50% of internal sodium is complexed, whereas more than 99% of the total calcium is so tightly bound or sequestered that it does not contribute to the measured activity.^33,41 Interestingly, a large fraction of the calcium that enters during cortical cataract also finally resides in the lens in a complexed form.^42,43

It is now generally accepted that the main ionic permeability barriers lie at the outer surface of the lens, whereas the inner fiber membranes present less of a barrier to the flow of ions because adjacentfibers are coupled through gap junctions or become fused or “degenerate.”^5,15,31 The relative permeability of the different ionic species can be computed by fitting the Goldman equation⁴⁴ to the membrane potential and (E_m) ionic data:

where α = P_Na/P_K and β = P_Cl/P_K. In most lensesso far studied, P_Na <m25> P_K, while P_Cl ≈ P_K. In the amphibian lens, the relative permeabilities obtained in this way are in good agreement with the individual permeabilities calculated from radioisotope flux data obtained for the individual ions.⁴⁵ The ⁴²K (or ⁸⁶Rb) and ³⁶Cl efflux data are easiest to interpret as the major component follows single exponential kinetics.^31,45 The ²²Na and ⁴⁵Ca²⁺ data kinetics are multiexponential in form, presumably because significant proportions of both of these ions are in a bound or complexed form.^45,46

Sodium is distributed far from equilibrium in the normal lens and energy must be expended continuously to maintain the normal sodium, potassium, and water content of the lens.³¹ When the pump is inhibited by ouabain, then the sodium and water content increases and there is some loss of transparency due to localized refractive index changes. Several studies have shown that ouabain is only effective when applied to the anterior surface^11,12,18 and, indeed, the largest fraction of Na-K-ATPase activity resides in the anterior epithelium. It should be noted, however, that ATPase activity is found associated with the membranes of all superficial lens cells.⁴⁷ Interestingly, Western blot studies show that pump molecules are present on all lens membranes, even although activity is only found in those near the surface.⁴⁷

The Na-K-ATPase pump consists of an α-catalytic subunit with an associated β subunit and is, in fact, a member of a multigene family of proteins.⁴⁸ Three α subunits (α₁, α₂, α₃) and three β subunits (β₁, β₂, β₃) have been identified so far, and the expression of the three isoforms (the α form in particular) is quite tissue specific. Most kidney cell membranes express α₁ and α₃. Interestingly, the lens appears to express all three polypeptides in anterior epithelial cells but only the α₁ form in fiber cells.⁴⁹ It further appears that all three isoforms are not expressed homogeneously across the anterior epithelium, with α₃ being located at the anterior pole and α₁ predominating in the equatorial region.⁵⁰

Sodium pump activity is under dynamic control itself and more α subunits are found in lenses stressed by exposure to amphotericin, for example. Interestingly, expression of the α₂ subunit appears to be preferentially upregulated.^51,52 More recently, it has been shown that Na-K-ATPase activity can be modulated by tyrosine kinase (TK) activation, which raises the very interesting possibility thatthis critical pump mechanism is under second-messenger control in the lens.⁵³

The membrane potential of the human lens depolarizes with age (see Fig. 2) and the decline becomes most significant after the age of 50 years.⁵⁴ Theion concentration and electrical data indicate thatP_Na/P_K progressively increases and as the measured electrical conductance and free Ca²⁺ activity also increases, Duncan and coworkers⁵⁴ have suggested that all of these age-related changes could be explained by the progressive activation of a nonselective cation channel. Several studies have shown the presence of such a channel in a number of lens species.^5,55,56 Interestingly, in the amphibian lens at least, this channel can be activated by pressure.⁵⁶ After the age of 50, the lens becomes increasingly unable to accommodate. This is not because the ciliary muscles cease to function, but rather the lens properties became less elastic.⁵⁷ The stretching and relaxation energy of the muscles would then be dissipated in the lens, leading to an increase in pressure on the lens membranes after the age of 50. The lens nonselective cation conductance also appears to be activated by oxidation⁵⁸ and a progressive decline in glutathione content of the lens with age⁵⁹ would also, therefore, tend to activate such a channel. It should be noted that there is a catastrophic decline in membrane potential in human cortical cataract with a concomitant increase in membrane permeability^7,42,60 (see Fig. 2).

Although tetrodotoxin-sensitive sodium-specific conductance channels have been identified in tissue-cultured cells,⁵ there is as yet no evidence to suggest that neuronal-like sodium channels exist in native human lens cells.

Potassium ions are distributed near, or above, equilibrium levels in most lenses so far studied, and there is a huge diversity in the range of channelsthrough which K⁺ ions can diffuse. Patch-clamp and molecular cloning methods currently are in use to identify the channels that are responsible in thelens for K⁺ movement.^5,39,40 The three types of channels identified by Mathias and his colleagues⁵ include inward rectifiers, large conductance calcium-activated channels (BK), and delayed rectifiers. The amphibian lens has been most extensively studied so far, and the overall conductance has beenmapped out by current-clamp techniques.⁶¹ The I-Vcurves show outward-rectifying properties and the I-V relationship in the normal lens is so steep, it cannot be predicted by Goldman theory. The overall conductance-voltage relationship is best explained by a combination of nonvoltage-sensitive and outwardly rectifying conductances. Removal of external calcium increases the former, largely by increasing Na⁺ conductance, while the latter is largely inactivated so that the conductance under Ca-free conditions is actually lower than the control conductance at voltages more depolarized than-70 mV. There are significant differences in the behavior of the overall potassium conductances of frog and rat lenses. In the former, the voltage-sensitive conductance is largely inactivated at the resting potential and the conductance is insensitive to blockade by TEA or quinine.^62,63 However, on depolarizing by voltage or by adding external K⁺, the voltage-sensitive conductance is activated and can be blocked by TEA. Interestingly, the voltage-sensitive conductance in the frog can be activated by low concentrations of the nonpermeant SH complexing agent PCMPS. Under these circumstances, the lens voltage hyperpolarizes to near -100 mV, and the conductance can now be blocked by TEA.⁶³ In the rat lens, conversely, the resting conductance is very sensitive to blockage by TEA or quinine.⁶⁴ Under these circumstances, the K⁺ conductance also can be blocked by the addition of external Ca²⁺ and the voltage depolarizes. Hence, although the K⁺ conductance in the rat lens can be modulated by Ca²⁺, it is not strictly a Ca-activated conductance but rather an inactivated conductance. Significantly, ⁸⁶Rb⁺ efflux from the human lens is reduced significantly by increasing external (so presumably internal) Ca²⁺ .⁶⁰ At present, the only potassium channel identified by molecular cloning techniques from native human lens epithelial tissue is the inwardly rectifying channel 1RK1.⁶⁵ This channel also has been identified in a number of animal lenses.^5,39,40

Chloride ions are distributed near equilibrium across the lens membranes of a variety of species,^31,45,66 and in the amphibian lens, the measured electrical conductance decreases when chloride ions are removed from the external solution.⁶⁷ The isotopic chloride efflux rates from both amphibian and mammalian lenses are high. In addition, it should be noted that part of this efflux arises from exchange mechanisms and these would not be expectedto contribute to the electrical conductance.^66,67 Examples of such processes included the DIDS-sensitive HCO3-/Cl- and the Na⁺ /HCO3-/H⁺ /Cl- exchangers^66,68 as well as the Na⁺ Cl-cotransporter.⁶⁹ Undoubtedly, other exchange systems are likely to exist. Chloride channels have been identified by patch-clamp techniques in both epithelial and fiber membranes⁷⁰ and are likely to play important roles in both volume⁷⁰ and pH⁶⁶ regulation.

The cytoplasm of the lens, in common with most animal cells, is isosmotic with the surrounding media.⁷¹ However, this is not an equilibrium situation, because of the large concentration of fixed negative charges with the membranes of the lens, and the lens has to expend energy continuously to maintain osmotic equilibrium.⁷² When the sodium pump is inhibited or the sodium permeability is increased, then the lens swells.³¹ The water permeability of lens membranes can either be determined by creating an osmotic gradient across the lens membranes and observing the rate of change of volume of the lens, or it can be determined by measuring the efflux of radioactive water. The first measurement determines the hydraulic permeability (P_os) and the second the diffusional permeability (P_d). Both permeabilities have been determined for the amphibian lens and the values are 100 μm-¹ and 0.4 μm-¹, respectively. These values are remarkably close to the two permeabilities measured in a wide range of cell types,⁷¹ and the large difference between the two indicates that much of the water movement can take place through water-filled pores. Water channels are known as aquaporins and lens epithelial cells contain an abundance of these.⁷³ Interestingly, MIP26 is believed to be a very old (and not very efficient) member of the aquaporin family and is termed aquaporin O. When MIP26 is expressed in oocytes, it does, indeed, give rise to a small increase in water permeability and, significantly, in agreement with some other members of the same family, it also acts as a glycerol transporter.⁷⁴ MIP26 could be termed the Swiss army knife of lens membrane proteins as it appears to have had a very large number of functions ascribed to it.⁷⁴ The one role it seems to have escaped so far is of a “membrane chaperone” and its huge abundance in the lens might, indeed, indicate such a function!

The lens must continually and actively extrude protons for two main reasons. The first reason, in common with most cells, is that the lens has a relatively high negative resting potential and thus there is an inwardly directed electrical gradient for mobile positive charge. For example, with a transmembrane potential of -60 mV and an external pH of the humors of approximately 7.3,²⁹ the internal pH (pH_i) at equilibrium would be predicted to be 6.2 from the Nernst equation. The measured pH_i from a range of mammalian lens cells (including human) is close to ^7,28,29,41 and so protons are not in equilibrium. The second important reason arises from the fact that because the lens is largely an anaerobic tissue, there is a continuous production of H⁺ from lactic acid.