Effect of intracellular potassium upon the electrogenic pump of frog retinal pigment epithelium

Prph2 disease mutations lead to structural and functional defects in the RPE

Prph2 is a photoreceptor-specific tetraspanin with an essential role in the structure and function of photoreceptor outer segments. PRPH2 mutations cause a multitude of retinal diseases characterized by the degeneration of photoreceptors as well as defects in neighboring tissues such as the RPE. While extensive research has analyzed photoreceptors, less attention has been paid to these secondary defects. Here, we use different Prph2 disease models to evaluate the damage of the RPE arising from photoreceptor defects. In Prph2 disease models, the RPE exhibits structural abnormalities and cell loss. Furthermore, RPE functional defects are observed, including impaired clearance of phagocytosed outer segment material and increased microglia activation. The severity of RPE damage is different between models, suggesting that the different abnormal outer segment structures caused by Prph2 disease mutations lead to varying degrees of RPE stress and thus influence the clinical phenotype observed in patients.

Electrical signaling in control of ocular cell behaviors

Epithelia of the cornea, lens and retina contain a vast array of ion channels and pumps. Together they produce a polarized flow of ions in and out of cells, as well as across the epithelia. These naturally occurring ion fluxes are essential to the hydration and metabolism of the ocular tissues, especially for the avascular cornea and lens. The directional transport of ions generates electric fields and currents in those tissues. Applied electric fields affect migration, division and proliferation of ocular cells which are important in homeostasis and healing of the ocular tissues. Abnormalities in any of those aspects may underlie many ocular diseases, for example chronic corneal ulcers, posterior capsule opacity after cataract surgery, and retinopathies. Electric field-inducing cellular responses, termed electrical signaling here, therefore may be an unexpected yet powerful mechanism in regulating ocular cell behavior. Both endogenous electric fields and applied electric fields could be exploited to regulate ocular cells. We aim to briefly describe the physiology of the naturally occurring electrical activities in the corneal, lens, and retinal epithelia, to provide experimental evidence of the effects of electric fields on ocular cell behaviors, and to suggest possible clinical implications.

The retinal pigment epithelium in visual function

Located between vessels of the choriocapillaris and light-sensitive outer segments of the photoreceptors, the retinal pigment epithelium (RPE) closely interacts with photoreceptors in the maintenance of visual function. Increasing knowledge of the multiple functions performed by the RPE improved the understanding of many diseases leading to blindness. This review summarizes the current knowledge of RPE functions and describes how failure of these functions causes loss of visual function. Mutations in genes that are expressed in the RPE can lead to photoreceptor degeneration. On the other hand, mutations in genes expressed in photoreceptors can lead to degenerations of the RPE. Thus both tissues can be regarded as a functional unit where both interacting partners depend on each other.

Na,K-ATPase inhibition alters tight junction structure and permeability in human retinal pigment epithelial cells

Na,K-ATPase regulates a variety of transport functions in epithelial cells. In cultures of human retinal pigment epithelial (RPE) cells, inhibition of Na,K-ATPase by ouabain and K(+) depletion decreased transepithelial electrical resistance (TER) and increased permeability of tight junctions to mannitol and inulin. Electrophysiological studies demonstrated that the decrease in TER was due to an increase in paracellular shunt conductance. At the light microscopy level, this increased permeability was not accompanied by changes in the localization of the tight junction proteins ZO-1, occludin, and claudin-3. At the ultrastructural level, increased tight junction permeability correlated with a decrease in tight junction membrane contact points. Decreased tight junction membrane contact points and increased tight junction permeability were reversible in K(+)-repletion experiments. Confocal microscopy revealed that in control cells, Na,K-ATPase was localized at both apical and basolateral plasma membranes. K(+) depletion resulted in a large reduction of apical Na,K-ATPase, and after K(+) repletion the apical Na,K-ATPase recovered to control levels. These results suggest a functional link exists between Na,K-ATPase and tight junction function in human RPE cells.

Cotransport of H+, lactate and H2O by membrane proteins in retinal pigment epithelium of bullfrog

1. The interaction between H+, lactate and H2O fluxes in the retinal membrane of the pigment epithelium from bullfrog Rana catesbiana was studied by means of ion-selective micro-electrodes. 2. Changes in intracellular pH and cell volume were recorded in response to abrupt changes in retinal solution concentration and/or osmolarity. 3. Two parallel pathways for water transport were identified across the retinal membrane, an osmotic one with a hydraulic water permeability of 3.2 x 10(-4) cm s-1 (osmol l-1)-1 and one which depended on the presence of lactate. 4. Addition of sodium lactate to the retinal solution caused cell shrinkages that were small compared with those produced by mannitol. The reflection coefficient for sodium lactate was 0.25. 5. Isosmotic replacement of Cl- with lactate caused an influx of water. Simultaneous acidification of the retinal solution from pH 7.4 to 6.4 enhanced the effect. The influx of water could proceed against osmotic gradients elicited by mannitol. 6. The interdependence of the fluxes of H+, lactate and H2O can be described as cotransport: the fluxes had a fixed ratio of about 109 mmol of lactic acid per litre of water, the flux of one species was able to energize the flux of the other two, and the fluxes exhibited saturation for increasing driving forces. 7. The Gibbs equation gives an accurate quantitative description of these coupled fluxes.

Effects of Ba2+ and Cs+ on apical membrane K+ conductance in toad retinal pigment epithelium

Intracellular microelectrode techniques were employed to characterize the blocker sensitivity of the K+ conductance (gK) at the apical membrane of the toad retinal pigment epithelium (RPE). Increasing the K+ concentration in the apical bath ([K+]o) from 2 to 5 mM produced a rapid depolarization of the apical membrane potential (VA). The addition of 0.5 mM Ba2+ or 5 mM Cs+ to the apical bath rapidly depolarized VA and increased the transepithelial resistance and ratio of apical-to-basolateral membrane resistance. In the presence of apical Ba2+ or Cs+, the response of VA to delta [K+]o was markedly reduced, indicating that these ions are effective blockers of apical gK. The Ba(2+)- and Cs(+)-induced decreases in the apparent apical-to-basolateral membrane conductance ratio were concentration dependent, with apparent dissociation constants of 17 microM and 0.5 mM, respectively. The apparent blocker sensitivity of apical gK is similar to that previously demonstrated for the inwardly rectifying K+ conductance in isolated toad RPE cells, suggesting that the inwardly rectifying K+ conductance comprises much of apical gK.

The delayed basolateral membrane hyperpolarization of the bovine retinal pigment epithelium: mechanism of generation

1. Conventional and ion-selective double-barrelled microelectrodes were used in an in vitro preparation of bovine retinal pigment epithelium (RPE)-choroid to measure the changes in membrane voltage, resistance and intracellular Cl- activity (aCli) produced by small, physiological changes in extracellular potassium concentration ([K+]o). These apical [K+]o changes approximate those produced in the extracellular (subretinal) space between the photoreceptors and the RPE following transitions between light and dark. 2. Changing apical [K+]o from 5 to 2 mM in vitro elicited membrane voltage responses with three distinct phases. The first phase was generated by an apical membrane hyperpolarization, followed by a (delayed) basolateral membrane hyperpolarization (DBMH); the third phase was an apical membrane depolarization. The present experiments focus on the membrane and cellular mechanisms that generate phase 2 of the response, the DBMH. 3. The DBMH was abolished in the presence of apical bumetanide (100 microM); this response was completely restored after bumetanide removal. 4. Reducing apical [K+]o, adding apical bumetanide (500 mM), or removing apical Cl- decreased aCli by 25 +/- 6 (n = 8), 28 +/- 1 (n = 2) and 26 +/- 5 mM (n = 3), respectively; adding 100 microM apical bumetanide decreased aCli by 12 +/- 2 mM (n = 3). Adding apical bumetanide or removing apical bath Cl- hyperpolarized the basolateral membrane and decreased the apparent basolateral membrane conductance (GB). 5. DIDS (4,4'-diisothiocyanostilbene-2,2'-disulphonic acid) blocked the RPE basolateral membrane Cl- conductance and inhibited the DBMH and the basolateral membrane hyperpolarization produced by apical bumetanide addition or by removal of apical Cl-o. The present results show that the DBMH is caused by delta[K]o-induced inhibition of the apical membrane Na(+)-K(+)-2Cl- cotransporter; the subsequent decrease in aCli generated a hyperpolarization at the basolateral membrane Cl- channel.

Regulatory volume decrease in frog retinal pigment epithelium

To measure changes in cell water during cell volume regulation, retinal pigment epithelial cells were loaded with tetramethylammonium (TMA). Regulatory volume decrease (RVD) in TMA-loaded retinal pigment epithelial (RPE) cells was measured using double-barreled K(+)-specific microelectrodes. Hyposmotic removal of 12.5 mM NaCl from the apical bath caused bullfrog RPE cells to rapidly swell by approximately 10% and to recover to control level within 10-15 min. Hyposmotic RVD was inhibited by 5 mM basal but not apical BaCl2. Raising K+ in the basal bath from 2 to 12 mM also inhibited RVD. Hyposmotic swelling was accompanied by an increase in the ratio of apical to basolateral membrane resistance (Ra/Rb). The swelling-induced increase in Ra/Rb was inhibited by 5 mM BaCl2. Together, the above findings suggest that hyposmotic swelling enhances basolateral K+ conductance such that K+ and presumably anion efflux mediate net solute and water loss during RVD. RPE cells can also regulate their volume when swollen in isosmotic Ringer solution under certain conditions. When urea or apical HCO3- was used to induce cell swelling, RPE cells underwent an RVD. In contrast, isosmotic elevation of apical K+ from 2 to 5 mM resulted in an increase in RPE cell volume with no subsequent RVD. Thus the method used to swell RPE cells is an important determinant of RVD. Because changes in RPE cell volume in vivo may alter the volume and composition of the extracellular (subretinal) space surrounding the photoreceptors, isosmotic volume regulation may play an important physiological role in maintaining the integrity and health of the neural retina under normal and pathophysiological conditions.

Whole-cell K+ currents in fresh and cultured cells of the human and monkey retinal pigment epithelium

1. Whole-cell potassium currents of freshly isolated human (adult and fetal) and monkey (adult) retinal pigment epithelial (RPE) cells, as well as cultured human and monkey RPE cells were studied using the patch-clamp technique. 2. In freshly isolated adult cells of both species, two currents were observed in the voltage range from -150 to +50 mV: an outwardly rectifying current and an inwardly rectifying current. These currents were also found in cultured cells of both species. 3. The outwardly rectifying current in freshly isolated adult human and monkey cells and some cultured cells was evoked by depolarizing voltage pulses more positive that -30 mV. The current activated with a sigmoidal time course after a brief delay, and was virtually non-inactivating. The conductance associated with the current was half-maximal at -16.4 mV for fresh human cells and -13.5 mV for fresh monkey cells, but was shifted 16.0 and 17.7 mV in the positive direction in cultured human and monkey cells, respectively. The reversal potential of the current in both human and monkey cells matched the potassium equilibrium potential (EK) over a wide range of external potassium concentrations. This current was blocked by 20 mM tetraethylammonium. 4. A membrane current that exhibited inward rectification was observed with hyperpolarizing voltage pulses. The zero-current potential of this current was close to EK. This current was blocked by 2 mM Ba2+ and 2 mM Cs+. In cultured human and monkey cells, but not in fresh cells, this current exhibited an inactivation when voltage pulses were more negative than -120 mV. External Na+ was responsible for the inactivation, as the inactivation was removed in a Na(+)-free solution. 5. Membrane currents in freshly isolated fetal human RPE cells were remarkably different from those in adult cells. A transient outward current resembling the A-type potassium current was observed as the dominant membrane current in freshly isolated fetal human cells. This current activated when voltage pulses were more positive than -30 mV. It inactivated rapidly after reaching a maximal level. Application of 5 mM 4-aminopyridine (4-AP) completely blocked this current. Although this current was never observed in fresh adult cells, it was found in 33% of the cultured adult cells with similar kinetics, ion selectivity, and pharmacological properties. 6. In about 26% of the freshly isolated fetal human cells, a more slowly activating outward current, which resembled the delayed rectifier, was found to co-exist with the transient outward current.(ABSTRACT TRUNCATED AT 400 WORDS)

pHi regulation in frog retinal pigment epithelium: two apical membrane mechanisms

This study demonstrates that the apical membrane of frog retinal pigment epithelium (RPE) contains two intracellular pH (pHi) regulatory mechanisms, an electrogenic Na-HCO3 cotransporter blocked by DIDS and an amiloride-inhibitable Na-H antiporter. pHi was studied using the pH-sensitive dye 2',7'-bis(2-carboxyethyl)-5,6-carboxyfluorescein (BCECF). In these cells resting pHi equals 7.26 +/- 0.09 (n = 58). After an acid load (NH4Cl prepulse), pHi recovery required apical extracellular Na concentration ([Na]o) in HCO3 or HCO3-free Ringer. In HCO3 Ringer recovery was completely blocked by 1 mM apical DIDS (n = 5) but was not affected by absence of Cl. In HCO3-free Ringer, recovery was completely blocked by 1 mM apical amiloride (n = 3). At resting pHi, the intrinsic pH-buffering capacity of the cell is approximately 7.1 mM/pH and rises monotonically as pHi decreases. In HCO3 Ringer, the initial rate of acidification caused by apical Na removal, 0.39 +/- 0.03 pH/min (n = 26), was 80-90% inhibited by apical DIDS (n = 5) and 16% inhibited by 1 mM apical amiloride (n = 7), but not affected by absence of Cl. In HCO3 Ringer, initial rates of acidification induced by apical DIDS or amiloride were 0.11 +/- 0.06 (n = 5) and 0.03 +/- 0.02 pH/min (n = 7), respectively. These results indicate that the Na-HCO3 cotransporter accounts for 80-90% of the acid extrusion from frog RPE cells. Increasing apical [K]o from 2 to 5 mM approximates the in vivo apical [K]o changes during a light-dark transition and alkalinizes the cells. [K]o-induced alkalinization had an initial rate of 0.11 +/- 0.02 pH/min (n = 16), which was approximately 75% inhibited by apical DIDS (to 0.04 +/- 0.01 pH/min, n = 7) and completely blocked by HCO3/CO2 removal from both bathing solutions. [K]o-induced pHi changes alter RPE transport mechanisms and may affect RPE-photoreceptor interactions.

Apical and basal membrane ion transport mechanisms in bovine retinal pigment epithelium

1. Intracellular voltage recordings using conventional and double-barrelled chloride-selective microelectrodes have been used to identify several transport mechanisms at the apical and basolateral membranes of the isolated bovine retinal pigment epithelium (RPE)-choroid preparation. Intracellular recordings were obtained from two cell populations, melanotic (pigmented) and amelanotic (non-pigmented). The electrical properties of these two populations are practically identical. For melanotic cells the average apical resting membrane potential (VA) is -61 +/- 2 mV (mean +/- S.E.M., n = 49 cells, thirty-three eyes). For these cells the ratio of apical to basolateral membrane resistance (a) was 0.22 +/- 0.02. The mean transepithelial voltage and resistance were 6 +/- 1 mV and 138 +/- 7 omega cm2, respectively. 2. The apical membrane, which faces the distal retina, contains a Ba(2+)-inhibitable K+ conductance and a ouabain-inhibitable, electrogenic Na(+)-K+ pump. In addition it contains a bumetanide-sensitive mechanism, the putative Na(+)-K(+)-Cl- cotransporter. The basolateral membrane contains a DIDS (4,4'-diisothiocyanostilbene-2,2'-disulphonic acid)-inhibitable chloride channel. The relative conductances of the apical and basolateral membranes to K+ and Cl- are TK approximately 0.9 and TCl approximately 0.7, respectively. 3. The ouabain-induced fast phase of apical membrane depolarization (0-30 s) was used to calculate the equivalent resistances of the apical (RA) and basolateral (RB) cell membranes, as well as the paracellular or shunt resistance (RS). They are: 3190 +/- 400, 17920 +/- 2730 and 2550 +/- 200 omega (mean +/- S.E.M., n = 9 tissues), respectively. From these data the equivalent electromotive forces (EMF) at the apical (EA) and basolateral (EB) membranes were also calculated. They are: -69 +/- 5.0 and -24 +/- 5.0 mV, respectively. 4. Intracellular Cl- activity (aiCl) was measured using double-barreled ion-selective microelectrodes. In the steady state aiCl = 61 +/- 4.0 mM and the Nernst potential ECl = -13.5 +/- 1.5 mV (mean +/- S.E.M., n = 4). 5. In the intact eye or in retina, RPE-choroid preparations it has been shown that the transition between light and dark alters the K+ concentration in the extracellular (or subretinal) space between the photoreceptors and the apical membrane of the RPE. These light-induced changes in subretinal [K+]o were qualitatively simulated in vitro by altering apical K+ between 5 and 2 mM. This produced a sequence of voltage changes at the apical and basolateral membranes that had three operationally distinct phases. Phase 1 is generated by the combination of an apical membrane K+ diffusion potential and inhibition of the electrogenic Na(+)-K+ pump.(ABSTRACT TRUNCATED AT 400 WORDS)

Voltage-dependent currents in isolated cells of the frog retinal pigment epithelium

1. Retinal pigment epithelial (RPE) cells were isolated enzymatically from bullfrog retinae. The patch-clamp technique was employed to investigate whole-cell currents under voltage-clamp conditions. 2. Isolated RPE cells were columnar or cuboidal in form, often with long processes protruding from the apical surface. Distinct apical and basal membrane domains were maintained for several hours following isolation. 3. The mean membrane capacitance was 62 pF. The resting potential averaged -30 mV, but it was as high as -75 mV in some cells. 4. Three voltage-dependent currents were observed: a time-independent and inwardly rectifying current and two time-dependent outwardly rectifying currents that had distinct kinetic properties. 5. Voltage pulses from a holding potential of -70 mV to potentials ranging from -30 to -120 mV produced membrane currents that were essentially time independent. The I-V relationship in this voltage range depended on the resting potential. It was usually inwardly rectifying in cells with resting potentials negative to about -50 mV, but tended to be linear in cells with more positive potentials. Three observations strongly suggested that the inwardly rectifying current is carried by K+. First, increasing the extracellular K+ concentration [( K+]) from 2 to 112 mM shifted the zero-current potential of the I-V relationship in the positive direction from an average value of -60 mV to 0 mV. Second, the addition of the K+ channel blockers Ba2+ (2 mM) or Cs+ (5 mM) to the extracellular solution inhibited a major component of the inwardly rectifying current. Finally, the reversal potential (Vr) of the Ba2(+)-sensitive current averaged -90 mV, near the K+ equilibrium potential (EK). 6. In approximately 50% of the cells, depolarizing voltage pulses to potentials more negative than -30 mV evoked an outward current that resembled the delayed rectifier present in other non-excitable cells. It activated with sigmoidal kinetics in less than 100 ms following a brief delay and then declined exponentially with a time constant of approximately 1 s. The peak chord conductance associated with this current was half-maximal at +14 mV. Several observations indicated that this outwardly rectifying current is carried primarily by K+: its Vr closely matched EK over a wide range of extracellular [K+]; it was inhibited 80% by exposure to the K+ channel blockers 4-aminopyridine (1 mM) and tetraethylammonium (20 mM); and it was abolished by intracellular dialysis with a K(+)-free solution.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of taurine on the isolated retinal pigment epithelium of the frog: electrophysiologic evidence for stimulation of an apical, electrogenic Na+-K+ pump

The apical surface of the retinal pigment epithelium (RPE) faces the neural retina whereas its basal surface faces the choroid. Taurine, which is necessary for normal vision, is released from the retina following light exposure and is actively transported from retina to choroid by the RPE. In these experiments, we have studied the effects of taurine on the electrical properties of the isolated RPE of the bullfrog, with a particular focus on the effects of taurine on the apical Na+-K+ pump. Acute exposure of the apical, but not basal, membrane of the RPE to taurine decreased the normally apical positive transepithelial potential (TEP). This TEP decrease was generated by a depolarization of the RPE apical membrane and did not occur when the apical bath contained sodium-free medium. With continued taurine exposure, the initial TEP decrease was sometimes followed by a recovery of the TEP toward baseline. This recovery was abolished by strophanthidin or ouabain, indicating involvement of the apical Na+-K+ pump. To further explore the effects of taurine on the Na+-K+ pump, barium was used to block apical K+ conductance and unmask a stimulation of the pump that is produced by increasing apical [K+]o. Under these conditions, increasing [K+]o hyperpolarized the apical membrane and increased TEP. Taurine reversibly doubled these responses, but did not change total epithelial resistance or the ratio of apical-to-basal membrane resistance, and ouabain abolished these responses. Collectively, these findings indicate the presence of an electrogenic Na+/taurine cotransport mechanism in the apical membrane of the bullfrog RPE. They also provide direct evidence that taurine produces a sodium-dependent increase in electrogenic pumping by the apical Na+-K+ pump.

Single-channel recordings from cultured human retinal pigment epithelial cells

We have applied patch-clamp techniques to on-cell and excised-membrane patches from human retinal pigment epithelial cells in tissue culture. Single-channel currents from at least four ion channel types were observed: three or more potassium-selective channels with single-channel slope conductances near 100, 45, and 25 pS as measured in on-cell patches with physiological saline in the pipette, and a relatively nonselective channel with subconductance states, which has a main-state conductance of approximately 300 pS at physiological ion concentrations. The permeability ratios, PK/PNa, measured in excised patches were 21 for the 100-pS channels, 3 for the 25-pS channels, and 0.8 for the 300-pS nonselective channel. The 45-pS channels appeared to be of at least two types, with PK/PNa's of approximately 41 for one type and 3 for the other. The potassium-selective channels were spontaneously active at all potentials examined. The average open time for these channels ranged from a few milliseconds to many tens of milliseconds. No consistent trend relating potassium-selective channel kinetics to membrane potential was apparent, which suggests that channel activity was not regulated by the membrane potential. In contrast to the potassium-selective channels, the activity of the nonselective channel was voltage dependent: the open probability of this channel declined to low values at large positive or negative membrane potentials and was maximal near zero. Single-channel conductances observed at several symmetrical KCl concentrations have been fitted with Michaelis-Menten curves in order to estimate maximum channel conductances and ion-binding constants for the different channel types. The channels we have recorded are probably responsible for the previously observed potassium permeability of the retinal pigment epithelium apical membrane.

cAMP stimulates the Na+-K+ pump in frog retinal pigment epithelium

Adenosine 3', 5'-cyclic monophosphate (cAMP) induced increases in active Na+ secretion and K+ absorption that were blocked by apical ouabain (10(-4) M), suggesting stimulation of the Na+-K+ pump. cAMP also produced rapid membrane voltage and resistance changes that could be divided chronologically into three phases. In phase 1, the basolateral membrane depolarized at a faster rate than the apical membrane, probably as a result of an increase in basolateral membrane conductance. In phase 2, the apical membrane repolarized toward control faster than the basal membrane, whereas in phase 3 the basolateral membrane repolarized faster than the apical membrane. Apical ouabain completely inhibited the cAMP-induced repolarization of the apical membrane during phase 2. Thus the stimulation of the Na+-K+ pump occurs within minutes of cAMP elevation. Na+ removal from the basal side did not block the cAMP-induced voltage changes, indicating that the initial conductance increase is not due to Na+. In contrast, Na+ removal from the apical bath inhibited all phases of the cAMP response. This suggests that apical membrane Na+-dependent transport mechanisms mediate the stimulation of the Na+-K+ pump. cAMP also caused a significant drop in intracellular K+ activity (approximately 5 mM) that preceded phase 2. This drop could stimulate the Na+-K+ pump, as suggested by previous experiments.

Potassium transport of the frog retinal pigment epithelium: autoregulation of potassium activity in the subretinal space

The K+ transport of the isolated retinal pigment epithelium from the bull-frog was studied using micropuncture with double-barrelled ion-selective micro-electrodes. Transient changes of intracellular values of electrical potential and K+ activity were monitored in response to abrupt changes in the K+ concentration on the retinal side of the tissue. The data were interpreted in terms of a simple three-compartment model of the epithelium in which the retinal (or apical) and choroidal (or basal) membranes separate the cellular compartment from the retinal and choroidal compartments. K+ transport across the retinal membrane was described by an active ouabain-sensitive K+ influx in parallel with a passive electrodiffusive K+ efflux. In steady state under control conditions, the active K+ influx (pump rate) averaged 0.18 X 10(-9) mol cm-2 s-1. The electrodiffusive K+ efflux was described by a K+ permeability, which in steady state under control conditions averaged 1.7 X 10(-5) cm s-1. K+ transport across the choroidal membrane was described as purely electrodiffusive. In steady state under control conditions, the K+ permeability of the choroidal membrane averaged 0.6 X 10(-5) cm s-1. When the K+ concentration on the retinal side of the tissue was increased from its control value, the K+ permeability of the retinal membrane decreased and the K+ permeability of the choroidal membrane increased. This caused the epithelium to attain a new steady state in which the cells transported K+ away from the retinal compartment at a high rate. When the K+ concentration on the retinal side of the tissue was decreased from its control value, the K+ permeability of the retinal membrane increased and the pump rate decreased. This caused the epithelial cells to transport K+ from the cellular compartment into the retinal compartment. In effect, the K+ transport of the retinal pigment epithelium depends on the K+ concentration in the retinal compartment in such a way as to keep variations in this concentration at a minimum.

Interactions of pH and K+ conductance in cultured bovine retinal pigment epithelial cells

Passive ion transport properties were studied in confluent monolayers of cultured bovine retinal pigment epithelial cells using intracellular microelectrode technique. The mean stable intracellular (designated by subscript i) potential was -59.1 +/- 0.8 (SE) mV. Extracellular (designated by subscript o) acidification induced a depolarization, whereas alkalinization induced a hyperpolarization. These effects were observed both in bicarbonate-free as well as in HCO3- Ringer (pHo changed by varying [HCO3-]o at constant pCO2). Acidification of pHi (changed by addition and removal of butyrate, CO2 or NH3) also caused a depolarization. Complete removal of HCO3-/CO2 at constant pHo caused a hyperpolarization. K+ transference, checked by applying high K+o, increased with K+o. It decreased with both extra and intracellular acidification and increased with alkalinization. In the presence of Ba2+, voltage reactions to changes in either pHo or pHi were greatly reduced. Depolarization by 40 mM K+ caused a similar reduction. It is suggested that K+ conductance of bovine retinal pigment epithelial cells is reduced by either intra- or extracellular acidification at normal [K+]o. Depolarization by high K+ induces an increase in K+ transference and reduces pH sensitivity.

Interactions between the retinal pigment epithelium and the neural retina

The retinal pigment epithelium (RPE) interacts with the photoreceptors, which it faces across the subretinal space. In these interactions the RPE acts as three types of cell - epithelium, macrophage, and glia. This review briefly describes selected interactions between the RPE and photoreceptors in ion and water transport, Vitamin A transport, phagocytosis of shed portions of outer segments, ensheathment of photoreceptors outer segments, and electrical responses. The electrical interactions can be recorded at the cornea in the c-wave, fast oscillation, and light peak of the DC electroretinogram (DC-ERG) and electrooculogram (EOG). Each response reflects photoreceptor-RPE interactions in a distinct way. The three responses taken together provide perhaps the best opportunity to learn how pathophysiological conditions alter the interactions between the RPE and photoreceptors.

Cyclic AMP modulation of ion transport across frog retinal pigment epithelium. Measurements in the short-circuit state

In the frog retinal pigment epithelium (RPE), the cellular levels of cyclic AMP (cAMP) were measured in control conditions and after treatment with substances that are known to inhibit phosphodiesterase (PDE) activity (isobutyl-1-methylxanthine, SQ65442) or stimulate adenylate cyclase activity (forskolin). The cAMP levels were elevated by a factor of 5-7 compared with the controls in PDE-treated tissues and by a factor of 18 in forskolin-treated tissues. The exogenous application of cAMP (1 mM), PDE inhibitors (0.5 mM), or forskolin (0.1 mM) all produced similar changes in epithelial electrical parameters, such as transepithelial potential (TEP) and resistance (Rt), as well as changes in active ion transport. Adding 1 mM cAMP to the solution bathing the apical membrane transiently increased the short-circuit current (SCC) and the TEP (apical side positive) and decreased Rt. Microelectrode experiments showed that the elevation in TEP is due mainly to a depolarization of the basal membrane followed by, and perhaps also accompanied by, a smaller hyperpolarization of the apical membrane. The ratio of the apical to the basolateral membrane resistance increased in the presence of cAMP, and this increase, coupled with the decrease in Rt and the basolateral membrane depolarization, is consistent with a conductance increase at the basolateral membrane. Radioactive tracer experiments showed that cAMP increased the active secretion of Na (choroid to retina) and the active absorption of K (retina to choroid). Cyclic AMP also abolished the active absorption of Cl across the RPE. In sum, elevated cellular levels of cAMP affect active and passive transport mechanisms at the apical and basolateral membranes of the bullfrog RPE.

Effects of cyclic AMP on fluid absorption and ion transport across frog retinal pigment epithelium. Measurements in the open-circuit state

A modified version of a capacitance probe technique has been used to measure fluid transport across the isolated retinal pigment epithelium (RPE)-choroid of the bullfrog. The accuracy of this measurement is 0.5-1.0 nl/min. Experiments carried out in the absence of external osmotic or hydrostatic gradients show that the RPE-choroid transports fluid from the retinal to the choroid side of the tissue at a rate of approximately 10 nl/min (4-6 microliters/cm2 X h). Net fluid absorption (Jv) was abolished within 10 min by the mitochondrial uncoupler 2,4-dinitrophenol. It was also inhibited (70%) by the removal of bicarbonate from the bulk solutions bathing the tissue. Ouabain caused a slow decrease in Jv (no effect at 10 min, 70% at 3 h), which indicates that RPE fluid transport is not directly coupled to the activity of the Na-K pump located at the apical membrane of this epithelium. In contrast to ouabain, cyclic AMP (cAMP) produced a quick decrease in Jv (84% within 5 min). Radioisotope experiments in the open circuit show that cAMP stimulated secretory fluxes of Na and Cl, which accounted for the observed cAMP-induced decrease in Jv. The direction of net fluid absorption, the magnitudes of the net ionic fluxes in the open circuit, and the dependence of Jv on external bicarbonate concentration strongly suggest that fluid absorption is generated primarily by the active absorption of bicarbonate.

Effects of maintained illumination upon [K+]0 in the subretinal space of the isolated retina of the toad

Illumination of the vertebrate retina evokes a transient decrease in extracellular potassium concentration, [K+]0, in the subretinal space. During maintained illumination, [K+]0 recovers toward its dark-adapted level. The mechanisms most likely to contribute to this recovery process were examined by using K+-selective microelectrodes to measure [K+]0 in the isolated retina of the toad. Bufo marinus. Although both diffusion of K+ and changes in the rod membrane voltage contribute to the recovery of [K+]0 during maintained illumination, other factors are likely to be involved as well. It is suggested that this recovery process could be due in part to inhibition of the Na+/K+ pump in the rod photo-receptors during maintained illumination.

Potassium transport across the frog retinal pigment epithelium

Previous experiments indicate that the apical membrane of the frog retinal pigment epithelium contains electrogenic Na:K pumps. In the present experiments net potassium and rubidium transport across the epithelium was measured as a function of extracellular potassium (rubidium) concentration, [K]0 ( [Rb]0). The net rate of retina-to-choroid 42K(86Rb) transport increased monotonically as [K]0 ( [Rb]0) increased from approximately 0.2 to 5 mM on both sides of the tissue or on the apical (neural retinal) side of the tissue. No further increase was observed when [K]0 ( [Rb]0) was elevated to 10 mM. Net sodium transport was also stimulated by elevating [K]0. The net K transport was completely inhibited by 10-4 M ouabain in the solution bathing the apical membrane. Ouabain inhibited the unidirectional K flux in the direction of net flux but had no effect on the back-flux in the choroid-to-retina direction. The magnitude of the ouabain-inhibitable 42K(86Rb) flux increased with [K]0 ( [Rb]0). These results show that the apical membrane Na:K pumps play an important role in the net active transport of potassium (rubidium) across the epithelium. The [K]0 changes that modulate potassium transport coincide with the light-induced [K]0 changes that occur in the extracellular space separating the photoreceptors and the apical membrane of the pigment epithelium.

Origin and sensitivity of the light peak in the intact cat eye

1. The light peak is a large light-induced change in the DC potential across the eye (standing potential) that reaches its maximum in 5-13 min in mammals. The light peak of the intact cat eye was studied in order to define its cellular origin and stimulus-response characteristics. Direct-coupled recordings were made with a vitreal electrode and also with intraretinal and intracellular micro-electrodes. Light peaks were generally evoked with 300 sec periods of diffuse white illumination.2. Micro-electrode recordings made in the subretinal space just outside the apical membrane of the retinal pigment epithelium (r.p.e.) showed that the light peak was a change in trans-epithelial potential. No component was generated in the neural retina.3. Intracellular recordings from r.p.e. cells showed that the change in trans-epithelial potential resulted from a depolarization of the basal membrane (facing the choroid). This depolarization came after the hyperpolarization of the apical membrane that gave rise to the r.p.e. component of the c-wave of the e.r.g.4. The light peak amplitude at a constant retinal illumination was nearly linear with stimulus duration over the range 15-180 sec, and saturated at about 300 sec. The time-to-peak remained nearly constant at about 300 sec over this range. Large light peaks could be evoked with flashes as short as 10 sec if the retinal illumination was several log units above rod saturation.5. When stimulus duration was held constant at 300 sec, light peak amplitude was graded with illumination over a wide range, from 3 log units below to 2 log units above rod saturation. The threshold of the light peak was below that of the e.r.g. and only about 1.5-2.5 log units above the absolute threshold of the most sensitive ganglion cells. The increase of light peak amplitude above rod saturation was not due primarily to cones.6. The trans-epithelial light peak had an unusual dependence on stimulus area, being at least twice as large in response to diffuse light as it was in response to a large spot (10 deg diameter) of the same retinal illumination.7. These findings indicate that the light peak represents a normal physiological interaction between the retina and the r.p.e. They also suggest that the interaction involves a change in the concentration of a diffusible substance in the retina, which then either enters the r.p.e. itself, or triggers an internal messenger to cause the basal depolarization.

Origin of the light peak: in vitro study of Gekko gekko

1. The light peak is a large, light-evoked increase in standing potential recorded in mammals, birds and reptiles. We have studied the cellular origin of the light peak in an in vitro preparation of neural retina-pigment epithelium (r.p.e)-choroid from the lizard, Gekko gekko. The tissue was mounted between two separate bathing solutions; the trans-tissue potential was recorded retinal-side positive; micro-electrodes were introduced to measure the trans-epithelial potential (t.e.p.) and to record intracellularly from the r.p.e.2. A 10 min stimulus of diffuse white light evoked an increase in trans-tissue potential that reached maximum amplitude, the light peak, about 15 min after stimulus onset. Since the light peak is present in vitro, it must originate in either the neural retina or the r.p.e.3. A micro-electrode was positioned in the subretinal space and the trans-retinal potential and t.e.p. were measured simultaneously. A 10 min stimulus produced an increase in t.e.p. equal in magnitude and time course to the trans-tissue light peak; no potential was present across the retina. The light peak is therefore generated solely across the r.p.e.4. Intracellular r.p.e. recordings were made to determine whether the light peak was generated at the apical or basal membrane or across the paracellular shunt. A 10 min stimulus first caused a hyperpolarization of both membranes with a time course similar to the r.p.e. c-wave followed by a depolarization of both membranes with the time course of the light peak. We conclude that whereas the r.p.e. c-wave results from a hyperpolarization of the apical membrane, the light peak is generated by a depolarization of the basal membrane of the r.p.e.5. Changes in tissue resistance, R(t), and the ratio of apical to basal membrane resistances, a, were monitored during the light peak by passing current across the tissue and measuring the appropriate current-induced voltages. R(t) decreased and a increased with the time course of the light peak. Assuming that the paracellular shunt resistance is constant, we conclude that the light peak is accompanied by an increase in basal membrane conductance.6. This and the following paper present the first direct demonstration of an interaction between the neural retina and the basal membrane of the r.p.e. The light peak, initiated by absorption of light by photoreceptors, results in a depolarization and conductance increase of the basal membrane.

Ion diffusion modified by tortuosity and volume fraction in the extracellular microenvironment of the rat cerebellum

1. The validity of the macroscopic laws of ion diffusion was critically examined within the microenvironment of the extracellular space in the rat cerebellum using ion-selective micropipettes and ionophoretic point sources. 2. The concepts of volume averaging, volume fraction (alpha) and tortuosity (lambda) were defined and shown to be theoretically appropriate for quantifying diffusion in a complex medium such as the brain. 3. Diffusion studies were made with the cations tetramethylammonium and tetraethylammonium and the anions alpha-naphthalene sulphonate and hexafluoro-arsenate, all of which remained essentially extracellular during the measurements. Diffusion parameters were measured for a period of 50s and over distances of the order of 0.1 mm. 4. Measurements of the diffusion coefficients of the ions in agar gel gave values that were very close to those derivable from the literature, thus confirming the validity of the method. 5. Measurements in the cerebellum did not reveal any systematic influences of ionophoretic current strength, electrode separation, anisotropy, inhomogeneity, charge discrimination or uptake, within the limits tested. 6. The pooled data from measurements with all the ions gave alpha = 0.21 +/- 0.02 (mean +/- S.E. of mean) and lambda = 1.55 +/- 0.05 (mean +/- S.E. of mean). 7. These results show that the extracellular space occupies about 20% of the rat cerebellum and that the diffusion coefficient for small monovalent extracellular ions is reduced by a factor of 2.4 (i.e. lambda 2) without regard to charge sign. The over-all effect of this is to increase the apparent strength of any ionic source in the cerebellum by a factor of lambda 2/alpha, about 12-fold in the present case, and to modify the time course of diffusion. 8. These conclusions confirm that the laws of macroscopic diffusion are closely obeyed in the cerebellum for small ions in the extracellular space, provided that volume fraction and tortuosity are explicitly taken into account. It is likely that these conclusions are generally applicable to other brain regions and other diffusing substances.

Early effects of sodium iodate on the directly recorded standing potential of the eye and on the c-wave of the DC registered electroretinogram in albino rabbits

The early effects of intravenously administered sodium iodate (NaIO3) on the directly recorded standing potential (SP) of the eye and on the c- and b-waves of the DC registered ERG were studied in 8 anaesthetized albino rabbits. In 5 of 6 animals obtaining 40 mg NaIO3/kg bwt. the SP decreased immediately following the injection, and had attained a level 3.5 - 4 mV below the original one after 1 h. The c-wave declined rapidly and 6 min after the injection it was replaced by a large cornea-negative potential. The b-wave was relatively unchanged except in one animal. In 2 rabbits treated with 30 mg NaIO3/kg btw. and in the 6th animal obtaining 40 mg NaIO3/kg bwt. an SP increase instead of a decrease was seen, and the c-wave was more slowly (about 22 min after the injection) replaced by the cornea-negative potential. The b-wave was somewhat increases. These results demonstrate the dose-related and inter-individual variability in the SP reaction to NaIO3 and are in good agreement with the well-known ultrastructural pigment epithelial injury and c-wave changes produced by this substance.

Transmembrane components of taurine flux across frog retinal pigment epithelium

In previous work from this laboratory, a net transepithelial flux of the amino acid taurine was measured across the in vitro frog retinal pigment epithelium. This flux was from retina to choroid and could be modulated by small (0.5 mM) changes in K+ concentration, by changes in taurine concentration, and by ouabain. In the present experiments we measured the unidirectional transmembrane fluxes across each of the two cell membranes, the apical membrane (facing the neural retina) and the basal membrane (facing the choroid) of the retinal pigment epithelium. In modified Ringer's solution containing 2mM taurine + 2mM K+, we found that the apparent outward permeability of the basal membrane, corrected for its actual area, was 26 times that of the apical membrane. As expected from the direction of net flux, the inward apical and outward basal fluxes dominated the transmembrane fluxes. When the apical Na+:K+ pump was inhibited, the ratio of the apparent permeability of the basal membrane relative to the apical decreased from 26 to 4.4. The data are consistent with the previous suggestion of Na+:taurine co-transport into the retinal pigment epithelium across the apical membrane. The basal membrane response to ouabain and reduced K+ concentration suggests that a K+-dependent mechanism is responsible, at least in part, for the inward basal taurine flux.

The electrogenic sodium pump of the frog retinal pigment epithelium

It was previously shown that ouabain decreases the potential difference across an in vitro preparation of bullfrog retinal pigment epithelium (RPE) when applied to the apical, but not the basal, membrane and that the net basal-to-apical Na+ transport is also inhibited by apical ouabain. The suggested the presence of a Na+ - K+ pump on the apical membrane of the RPE. In the present experiments, intracellular recordings from RPE cells show that this pump is electrogenic and contributes approximately - 10 mV to the apical membrane potential (VAP). Apical ouabain depolarized VAP in two phases. The initial, fast phase was due to the removal of the direct, electrogenic component. In the first one minute of th response to ouabain, VAP depolarized at an average rate of 4.4 +/- 0.42 mV/min (n = 10, mean +/- SEM) and VAP depolarized an average of 9.6 +/- 0.5 mV during the entire fast phase. A slow phase of membrane depolarization, due to ionic gradients running down across both membranes, continued for hours at a much slower rate, 0.4 mV/min. Using a simple diffusion model and K+-specific microelectrodes, it was possible to infer that the onset of the ouabain-induced depolarization coincided with the arrival of ouabain molecules at the apical membrane. This result must occur if ouabain affects an electrogenic pump. Other metabolic inhibitors, such as DNP and cold, also produced a fast depolarization of the apical membrane. For a decrease in temperature of congruent to 10 degrees C, the average depolarization of the apical membrane was 7.1 +/- 3.4 mV (n = 5) and the average decrease in transepithelial potential was 3.9 +/- 0.3 mV (n = 10). These changes in potential were much larger than could be explained by the effect of temperature on an RT/F electrodiffusion factor. Cooling the tissue inhibited the same mechanism as ouabain, since prior exposure to ouabain greatly reduced the magnitude of the cold effect. Bathing the tissue in 0 mM [K+] solution for 2 hr inhibited the electrogenic pump, and subsequent re-introduction of 2 mM [K+] solution produced a rapid membrane hyperpolarization. We conclude that the electrogenic nature of this pump is important to retinal function, since its contribution to the apical membrane potential is likely to affect the transport of ions, metabolites, and fluid across the RPE.