Contribution from a pH- and tonicity-sensitive K+ conductance to toad translens short-circuit current

Fluid circulation determined in the isolated bovine lens

PURPOSE

In 1997, a theoretical model was developed that predicted the existence of an internal, Na(+)-driven fluid circulation from the poles to the equator of the lens. In the present work, we demonstrate with a novel system that fluid movement can be measured across the polar and equatorial surface areas of isolated cow lenses. We have also determined the effects of ouabain and reduced bath [Na(+)].

METHODS

Lenses were isolated in a chamber with three compartments separated by two thin O-rings. Each compartment, anterior (A), equatorial (E), and posterior (P), was connected to a vertical capillary graduated in 0.25 μL. Capillary levels were read every 15 minutes. The protocols consisted of 2 hours in either open circuit or short circuit. The effects of ouabain and low-Na(+) solutions were determined under open circuit.

RESULTS

In 21 experiments, the E capillary increased at a mean rate of 0.060 μL/min while the A and P levels decreased at rates of 0.044 and 0.037 μL/min, respectively, closely accounting for the increase in E. The first-hour flows under short circuit were approximately 40% larger than those in open-circuit conditions. The first-hour flows were always larger than those during the second hour. Preincubation of lenses with either ouabain or low-[Na(+)] solutions resulted in reduced rates of fluid transport. When KCl was used to replace NaCl, a transitory stimulation of fluid transport occurred.

CONCLUSIONS

These experiments support that a fluid circulation consistent with the 1997 model is physiologically active.

Changes in rabbit and cow lens shape and volume upon imposition of anisotonic conditions

In vivo, mammalian lenses have the capacity to effect fully reversible changes in shape, and possibly volume, during the accommodation process. Isolated lenses also change shape by readily swelling or shrinking when placed in anisotonic media. However, the manner by which the lens changes its shape when its volume is changed osmotically is not firmly established. Putatively, the lens could swell or shrink evenly in all directions, or manifest distinctive swelling and/or shrinking patterns when exposed to anisotonic media. The present study measured physical changes in lenses consistent with the latter alternative using methods we developed for determining rapid changes in lens shape and volume. It was found in isolated rabbit and cow lenses that the length of the axis between the anterior and posterior poles (A-P length) primarily increases under hypotonic conditions (-40 to -100 mOsM), with smaller, or no changes, in equatorial diameter (ED). Hypertonic conditions (+50 to +100 mOsM) on rabbit lenses elicited a predominant reduction in ED, while the A-P length was only marginally reduced. Hypertonic solutions of +150 mOsM were required to obtain similar changes in cow lens shape. The ratio of the A-P length to the ED was taken as a measure of "circularity". This ratio increased gradually in rabbit and cow lenses bathed in hypotonic solutions because of the increase in the A-P length. The calculated lens volume increased in tandem with the increase in "circularity". Lens circularity also increased under hypertonic conditions due to the decrease in ED, but this increase in circularity during shrinkage was not as pronounced as that which occurred during swelling. As such, the lens has a tendency upon swelling to change its shape by approaching the structure of a globular spheroid (as occurs during accommodation for near focusing), but lens shrinkage does not result in a flatter lens with a reduced A-P length as occurs during dis-accommodation for distance focusing. Moreover, osmotically evoked shape changes appear irreversible, in contrast to the mechanically elicited shape changes of accommodation.

Fluid transport phenomena in ocular epithelia

This article discusses three largely unrecognized aspects related to fluid movement in ocular tissues; namely, (a) the dynamic changes in water permeability observed in corneal and conjunctival epithelia under anisotonic conditions, (b) the indications that the fluid transport rate exhibited by the ciliary epithelium is insufficient to explain aqueous humor production, and (c) the evidence for fluid movement into and out of the lens during accommodation. We have studied each of these subjects in recent years and present an evaluation of our data within the context of the results of others who have also worked on electrolyte and fluid transport in ocular tissues. We propose that (1) the corneal and conjunctival epithelia, with apical aspects naturally exposed to variable tonicities, are capable of regulating their water permeabilities as part of the cell-volume regulatory process, (2) fluid may directly enter the anterior chamber of the eye across the anterior surface of the iris, thereby representing an additional entry pathway for aqueous humor production, and (3) changes in lens volume occur during accommodation, and such changes are best explained by a net influx and efflux of fluid.

Electrolyte and fluid transport across corneal, conjunctival and lens epithelia

All ocular epithelia examined to date transport fluid as a consequence of a sufficiently high water permeability bestowed by endogenous water channels (aquaporins) and transepithelial solute movement due to active transport mechanisms. This article provides a synopsis of the current understanding of electrolyte and fluid transport across corneal, conjunctival and lens epithelia.

Beta-adrenergic stimulation of Na(+)-K(+)-2Cl(-) cotransport activity in the rabbit lens

Experimental maneuvers known to increase cellular cAMP levels evoked a stimulation in the K(+) influx across the anterior surfaces of isolated rabbit lenses, as measured by 86Rb(+) uptake. For this, the lenses were mounted in a modified Ussing-type chamber and exposed to the radiolabel under short-circuit conditions. The enhanced, cAMP-elicited flux was attributed to the basolateral Na(+)-K(+)-2Cl(-) cotransporter given its preclusion by bumetanide, a highly selective inhibitor of this symport, and the ineffectiveness of ouabain in mitigating the stimulation. The ouabain- plus bumetanide-insensitive K(+) uptake, which is about 10% of the total influx and represents passive entry of the radiolabel, was not affected by cAMP-elevating conditions. Forskolin, an activator of adenylyl cyclase; epinephrine, a non-selective adrenergic agonist; and the beta-selective agents, isoproterenol and terbutaline, were among the drugs used to elicit the increase in bumetanide-sensitive K(+) inflow. In experiments with isoproterenol, the stimulated influx evoked by the agonist was inhibited in lenses simultaneously exposed to propranolol. Other observations included that the stimulation of bumetanide-sensitive K(+) influx with forskolin was eliminated in lenses pretreated with the protein kinase inhibitors, staurosporine or H-89. However, these drugs were ineffective in preventing the increased influx produced by calyculin A, a phosphatase inhibitor, suggesting modulation of the cotransporter by at least two independent pathways. The cAMP-generating stimuli also produced an inhibition of the short-circuit current across the lens and an increase in translens resistance. These latter effects suggest that cAMP elevation also evokes an inhibition in an epithelial conductance(s) simultaneously to the stimulation of the cotransporter. As such, this study provides the first indication for the regulation of lens transport by adrenoceptors, presumably of the beta-2 subtype.

Distribution of acetylcholine-sensitive currents around the rabbit crystalline lens

The relative distribution of acetylcholine (ACh) receptors on the surface of the isolated ocular lens of the rabbit was determined from induced changes in translens short-circuit current (I(SC)) and the translenticular resistance (R(t)) at seven delineated, parallel zones from the anterior to the posterior pole. For this, one O-ring (from among several having different diameters) was used to separate two zones in a vertically arranged Ussing-type chamber. Different O-rings separated different zone pairs. Earlier experiments from this laboratory used a conventional divided chamber, which occluded the equatorial surface, to demonstrate that anterior applications of ACh transiently decreased the I(SC) due to an intracellular Ca(2+) release and inhibition of anteriorly located K(+) channels. Measurements obtained with the newly designed zonal arrangement determined that the entire epithelial surface from its anterior-most aspect to the equatorial region responds electrically to ACh exposure, while the posterior-most region does not. Furthermore, lens-mounting positions that resulted in separation of the epithelium so that portions of its surface were present in each hemichamber resulted in inverse current changes upon bilateral ACh addition to the bathing solutions. Reductions in outward cationic current across the anterior surface into the anterior bath upon ACh treatment were accompanied by an increase in translens resistance consistent with a closure of basolateral K(+) channels. Overall, these results suggest that the posterior fiber cells may lack ACh receptors, which are clearly present in the lens epithelium that covers about two-thirds of the rabbit lens surface area, and indicate that an ACh-evoked Ca(2+) signal does not spread throughout the epithelial layer. A functional role for lens acetylcholine receptors remains to be determined.

Regional distribution of the Na(+) and K(+) currents around the crystalline lens of rabbit

Early studies described asymmetrical electrical properties across the ocular lens in the anterior-to-posterior direction. More recent results obtained with a vibrating probe indicated that currents around the lens surface are not uniform by showing an outwardly directed K(+) efflux at the lens equator and Na(+) influx at the poles. The latter studies have been used to support theoretical models for fluid recirculation within the avascular lens. However, the existence of a nonuniform current distribution in the lens epithelium from the anterior pole to the equator has never been confirmed. The present work developed a modified short-circuiting technique to examine the net flows of Na(+) and K(+) across arbitrarily defined lens surface regions. Results indicate that passive inflows of Na(+) occur at both the anterior polar region and posterior lens surface, consistent with suggestions derived from the vibrating probe data, whereas K(+) efflux plus the Na(+)-K(+) pump-generated current comprise the currents at the equatorial surface and an area anterior to it. Furthermore, Na(+)-K(+) pump activity was absent at the posterior surface and its polar region in all lenses examined, as well as from the anterior polar region in most lenses. The latter unexpected observation suggests that the monolayered epithelium, which is confined to the anterior surface of the lens, does not express an active Na(+)-K(+) pump at its anterior-most aspect. Nevertheless, this report represents the first independent confirmation that positive currents leave the lens around the equator and reenter across the polar and posterior surfaces.

Localization of a Na(+)-K(+)-2Cl(-) cotransporter in the rabbit lens

Earlier work from this laboratory demonstrated a bumetanide-inhibitable K(+) uptake activity in cultured bovine lens epithelial cells, but not at the anterior surfaces of intact bovine lenses isolated in an Ussing-type chamber. Presently the distribution of the bumetanide-sensitive Na(+)-K(+)-2Cl(-) cotransporter within the lens was re-examined. To complement previous results, (86)Rb(+) uptake experiments were done in a chamber design that limited exposure of the radiolabel to specific surfaces of rabbit lenses under short-circuit conditions. In addition, the cotransporter protein (NKCC1, but not NKCC2) was immune-detected in Western blots. For the latter, membrane preparations were obtained from capsule-plus-epithelial specimens, and from three cortical fractions, i.e. the anterior, equatorial, and posterior regions, as well as a fifth, nuclear fraction. K(+) influxes across the anterior-polar, equatorial, and posterior-polar surfaces were 0.375, 0.348 and 0.056 microEq (hr cm(2))(-1) respectively, rates that were not significantly reduced by the presence of 0.1 mM bumetanide (P > 0.15, as unpaired data). In contrast, bumetanide-sensitive K(+) influx rates were measured across the anterior and equatorial surfaces under hypertonic, but not under hypo-osmotic conditions. In culture, bumetanide and ouabain were equipotent in reducing by approximately half the K(+) uptake of quiescent, rabbit lens epithelial cells under control, iso-osmotic conditions, indicating a cell-culture induced up-regulation of the cotransport activity by an undetermined mechanism. The immunoblotting of lens membrane proteins elicited approximately 170-180 kDa bands accordant with the identity of the NKCC1 isoform in the epithelial and cortical equatorial fractions. Thus, NKCC1 was readily demonstrated using membrane specimens taken from within the lens. Its activity in the intact organ may be activated by conditions fostering cell shrinkage, and perhaps, agents stimulating epithelial cell elongation, given its distribution within the lens.

Potassium current oscillations across the rabbit lens epithelium

Rabbit lenses expressing spontaneous oscillations in translens short-circuit current (Isc) are obtained somewhat frequently, with this phenomenon observed in approximately 30% of isolated lenses as described earlier (Exp. Eye Res. 61, 129-140, 1995). Since pharmacological protocols to consistently elicit Isc oscillations were not found, characterizations of the underlying transport processes have been limited to the application of various inhibitors on the spontaneous phenomenon. The present report extends the initial observations by confirming that oscillations are immediately inhibited upon the anterior addition of the Ca2+ channel blocker nifedipine (10 microM), and by demonstrating that other treatments which should affect epithelial Ca2+ homeostasis are also inhibitory (e.g., Bay K 8644 (10 microM), diltiazem (10 microM), EGTA (2 mm), and Ca2+-free media). Furthermore, Isc oscillations are immediately inhibited by the K+ channel blocker, Ba2+, but not by the Na+-K+ pump inhibitor, ouabain. The intracellular Ca2+ mobilizing agents thapsigargin (0.1 microM) or acetylcholine (1 microM) modified but did not permanently inhibit the oscillations, confirming earlier observations. At 50 microM, however, acetylcholine addition was inhibitory, but reversible, for oscillations restarted upon its subsequent removal. In addition, lens oscillations were also characterized under open-circuit conditions with microelectrodes inserted in the superficial cells near the equator of lenses isolated in a divided chamber. The potential difference (PD) across each lens face was recorded, as was the translens PD (PDt), which equals the difference between the PDs across each lens surface. Oscillations in PDt were obtained in 7 of 26 lenses. The oscillations arose only from an oscillation in the PD across the anterior face (PDa). While PDa and PDt oscillated with the same amplitude (approximately 12 mV) and period (approximately 70 sec), the PD across the posterior surface remained stable. During these oscillations the conductance of the anterior surface was maximal at the most positive voltage of the anterior bath with respect to the lens interior (46 mV), whereas, minimal conductance occurred at the least positive PDa (34 mV). Overall, these observations are consistent with the likely presence of voltage-operated Ca2+ channels in parallel with various Ca2+-sensitive K+ channels in the epithelial basolateral membrane. A model to explain the oscillatory pattern across the anterior face while the PD across the posterior face remains unaltered is presented.

A mechanism for regulatory volume decrease in cultured lens epithelial cells

PURPOSE

To identify mechanisms contributing to regulatory volume decrease in lens epithelial cells.

METHODS

Cells of the lens epithelial cell line alpha TN4 were cultured in four-well culture dishes in Dulbecco's Modified Eagle Medium containing 10% fetal bovine serum. After confluence cell water space was determined by measuring the equilibrium distribution of 3-O-methylglucose. Potassium influx and efflux in isotonic and hypotonic solutions were measured using 86rubidium (86Rb) as tracer. Total cell potassium and sodium content were determined with atomic absorption spectroscopy. Protein content per well was assayed with a modified Lowry assay and flux data and ion concentrations were normalized per mg of protein.

RESULTS

Lens epithelial cells responded to hypotonic solutions with rapid swelling followed by regulatory volume decrease (RVD). During swelling and subsequent volume decrease the unidirectional Rb efflux was increased proportionaly to the osmotic challenge. Rubidium efflux was highly sensitive to changes in extracellular osmolarity and responded with a measurable activation to changes of 12.5 mOsm. No changes in 86Rb influx were observed with small changes (< 20%) in osmolarity and only relatively small changes occurred with larger changes in osmolarity. The resulting net loss of 86Rb and potassium (K+) was demonstrated by measuring the change of intracellular [K+] in hypotonic solutions using atomic absorption spectroscopy. The K(+)-channel blockers quinine-HCl and BaCl2 and the Cl(-)-channel blockers diphenyl-2-carboxylate (DPC) and 5-nitro-2-(3-phenyl propylamino) benzoic acid (NPPB) did not significantly affect the 86Rb efflux induced by hypotonic solutions. However, [(dihydroindenyl)oxy]alkanoic acid (DIOA), reported to be a specific inhibitor of the K-Cl cotransporter, inhibited the activation of 86Rb efflux. 86Rb efflux could be activated in isosmotic solutions by the addition of 1 mM N-ethylmaleimide (NEM). This activation of Rb efflux could be prevented by the addition of 1 mM dithiothreitol and could be 90% blocked by DIOA. The activation of rubidium efflux by NEM led to a significant decrease of the intracellular water content. The volume regulatory changes in NEM and in hypotonic solutions could be inhibited in DIOA.

CONCLUSIONS

The observations are consistent with the presence in lens epithelial cells of a K-Cl cotransporter serving as a mechanism for regulatory volume decrease.

Effects of Ca2+ on rabbit translens short-circuit current: evidence for a Ca2+ inhibitable K+ conductance

PURPOSE

To characterize the effects of medium Ca2+ levels on rabbit lens electrical properties. Early studies with wholly submerged lenses had shown that Ca2+ removal from the bath resulted in an increased Rb+ efflux, a consequence of an increased Na+ Permeability and lens depolarization.

METHODS

Lenses were bathed with Ussing-type chambers under short-circuited conditions, an arrangement in which the translens short-circuit current (Isc) is carried out across the posterior lens surface mainly by an influx of Na+, and across the anterior face largely by a K+ efflux.

RESULTS

Under the present conditions in which the effects of Ca2+ were characterized unilaterally, the above established effects could only be ascribed to the posterior surface. When Ca2+ removal was limited to the anterior face, the Isc increased from 11.87 +/- 1.17 to 17.04 +/- 1.52 microA/cm2 (means +/- SE's, n = 18; an accompanying translens resistance (Rt) decrease of 0.23 +/- 0.049 K omega.cm2 was also recorded). Conversely, increasing the control, anterior-bath [Ca2+] from 1.8 to 3.6 mM reduced the K+ efflux-dependent Isc from 10.54 +/- 1.09 to 8.93 +/- 1.02 (n = 10, with an Rt increase of 0.11 +/- 0.013). These changes were reversible Na(+)-independent, and fully inhibited by the presence of K+ channel blockers (quinidine or Ba2+). Inhibitions of the Ca2+ effects were also obtained with strontium, a Ca2+ surrogate. The Isc was less responsive to changes in the Ca2+ content of the posterior bath. Removal of the cation caused a gradual 1.65 +/- 0.72 microA/cm2 increase (n = 9, with an Rt decrease of 0.090 +/- 0.021 K omega.cm2). In the absence of posterior Na+, Ca2+ withdrawal resulted in highly variable responses, with some specimens exhibiting salient current increases, suggesting that an outwardly directed, posterior efflux of an anion could also have been affected. During the course of this study it was consistently observed that the removal of Na+ from the anterior bath led to an Isc decrease of 2.62 +/- 0.22 microA/cm2 (n = 32, with an Rt increase of 0.35 +/- 0.029 k omega.cm2). This change occurred in both the presence of ouabain and the absence of Ca2+, suggesting that it did not result from an inhibition of the Na(+)-K+ pump current nor from a reversal in putative Na+/Ca2+ exchange activity. Small Isc increases upon anterior Na+ withdrawal (1.68 +/- 0.17, n = 7), consistent with Na+ efflux from the lens, could only be observed with K+ channels inhibited with Ba2+. Also congruent with the observations of a relatively limited anterior Na+ permeability, was the finding that the induction of nonspecific cation channels with amphotericin B reduced the Isc by following Na+ from the anterior bath to enter the lens. Thus, changes in lens Isc can differentiate changes in K+ permeability across the native anterior epithelium from changes in Na+ permeability.

CONCLUSIONS

Overall, these results suggest that lens Ca2(+)-mobilizing agents (e.g. acetylcholine) could trigger the inhibition of epithelial K+ conductance(s) by the direct action of Ca2+ on K+ channels.

Altered lens short-circuit current in adult cataract-prone Dahl hypertensive rats

We assessed components of lenticular short-circuit current in adult hypertensive Dahl salt-sensitive rats (DS) during chronic control (0.4% sodium) versus high (3% sodium) dietary NaCl intake begun at the age of 4 weeks until rats were studied. We also evaluated the influence of barium, a potassium channel blocker, and ouabain, a specific inhibitor of Na+, K(+)-ATPase activity, by adding them to the anterior lens surface, thus measuring barium-sensitive, ouabain-sensitive, and barium- and ouabain-in-sensitive short-circuit currents. During control NaCl intake, short-circuit current in DS and their control group, Dahl salt-resistant rats (DR), did not differ significantly. DS were subclassified into cataract-prone rats and rats unlikely to develop cataracts on the basis of their initial pressor response to the change from a normal to high NaCl diet during the first weeks of age. Although only transparent lenses were studied, total lens short-circuit current was already markedly decreased in the cataract-prone subgroup compared with DS unlikely to develop cataracts and control DR. This was in sharp contrast to the increase in short-circuit current previously reported in Sprague-Dawley rats and now observed in control DR in response to high dietary NaCl. The decrease in lens short-circuit current in cataract-prone rats was associated with lower absolute values of barium- and ouabain-sensitive short-circuit currents as well as with low barium- and ouabain-insensitive short-circuit current. Although the barium- and ouabain-sensitive components of the short-circuit current were similar in DS unlikely to develop cataracts and DR, the barium- and ouabain-insensitive component of the short-circuit current was lower in DS unlikely to develop cataracts than values in DR. Interestingly, this component of lens short-circuit current also increased in DR during chronic high NaCl, whereas the opposite change occurred in cataract-prone DS and DS unlikely to develop cataracts. Thus, the barium- and ouabain-insensitive short-circuit current may be a mechanism that protects the normal lens from developing cataracts. Possible candidates for this short-circuit current component are voltage-dependent potassium channels, calcium-activated potassium channels, or both. Our studies show altered lens short-circuit current in response to high NaCl intake in cataract-prone DS and suggest the possibility of altered lens potassium transport during sustained hypertension but before loss of lens transparency.

Active sodium and chloride transport across the isolated rabbit conjunctiva

The bulbar-palpebral conjunctiva from albino rabbits was dissected as a cylinder and cut longitudinally to convert it to a flat epithelium that was mounted as a partition between Using-type chambers, exposing 0.38 cm2 of cross-sectional area. The tissue was bathed with a modified Tyrode's solution at 37 degrees C, pH 7.5. The tear-facing side (apical) was 14.6 +/- 1.5 mV negative relative to the basolateral side. Transepithelial resistance was 1.23 +/- 0.01 K omega.cm2 and the short-circuit current (Isc) was 14.4 +/- 1.3, microA/cm2. Sixty percent of the Isc could be accounted for by a Na(+)-dependent, bumetanide-inhibitable Cl- transport directed towards the apical side. The remainder of the Isc reflected a Na+ absorptive process at the apical surface that was amiloride resistant. Evidence was obtained that a likely contributor to this activity is an electrogenic Na(+)-glucose co-carrier. The Cl-dependent Isc was stimulated by forskolin and epinephrine. Permeabilization of the apical membrane with amphotericin B evinced a current carried by a basolateral Na+:K+ pump. An effect by heptanol suggested that part of the Isc traverse the epithelium via gap junctions. Our results imply that transport processes at the conjunctiva could influence the composition of the tear film.

Acetylcholine modulation of the short-circuit current across the rabbit lens

Rabbit lenses were bathed within a bicameral Ussing-type chamber under short-circuit conditions. In this situation the short-circuit current (Isc) reflects, across the anterior aspect, the presence of anteriorly facing K+ conductance(s) plus the Na(+)-K+ pump current. Across the posterior surface the Isc is primarily carried by the movement of Na+ from the posterior bathing solution to the lens. Addition of acetylcholine (ACh) to the posterior hemichamber did not affect the translens electrical parameters; but, its introduction to the anterior bath at 1 microM immediately reduced the Isc from 8.91 +/- 1.47 to 5.84 +/- 1.28 microA cm-2 and increased the translens resistance from 1.50 +/- 0.08 to 1.59 +/- 0.09 K omega cm2 (+/- S.E.S; P < 0.05 as paired values, n = 25 lenses). The suppressed Isc gradually recovered and reached 75% of the control value 5 min after the introduction of the neurotransmitter. In six cases the recovery was nearly complete (> or = 95% of control) within this time. The preaddition of 0.1 microM atropine prevented an effect by 1 microM ACh. When atropine was added within 1 min of ACh, the suppressed Isc immediately recovered. The ACh-elicited Isc suppression was averted in lenses pre-exposed to either K+ channel blockers (quinidine or barium) or to the endoplasmic reticular Ca(2+)-ATPase inhibitor thapsigargin (Tg: 0.1 microM), which in itself produced Isc inhibitions similar to those seen with ACh under control conditions. Similarly comparable were the ACh-evoked Isc inhibitions garnered upon introduction of the agonist to lenses bathed in the absence of extracellular Ca2+. In these cases, however, the Isc recovered fully within 2-3 min. This condition also revealed that the anterior removal of medium Ca2+ increased the Isc by about 50%, a completely reversible phenomenon; Ca2+ restoration in the presence of the Ca2+ channel blocker, nifedipine (10 microM), blunted markedly the reversal to the control Isc. Overall, these results suggest that ACh receptor activation induces the release of intracellularly stored Ca2+, which in turn leads to the temporary deactivation of a K+ conductance(s); in addition, secondary Ca2+ inflow may further extend the observed inhibition. During this study, the Isc of about 30% of the lenses used spontaneously oscillated (common duration of 30 min, with a mean peak frequency of 0.76 +/- 0.32 cycle min-1 and mean amplitude of 4.07 +/- 2.65 microA cm-2; +/- S.D.S, n = 24). Experiments attempted to determine the sensitivity of the oscillatory activity to ACh. Tg, nifedipine, and the phorbol ester PMA.(ABSTRACT TRUNCATED AT 400 WORDS)

Potassium currents from isolated frog lens epithelial cells

Using the perforated patch version of whole-cell recording, we have measured currents from isolated frog lens epithelial cells. Three types of currents were seen. A time-independent outwardly rectifying potassium current was identified that sets the resting voltage. This potassium current differs significantly from any of the potassium currents recorded with the whole-cell technique in mammalian lens epithelial cells. In addition to the potassium current, the two other currents present were both outwardly rectifying: one was time-independent while the other showed distinct activation.