Ion selectivity of the cation transport system of isolated intact cattle rod outer segments: evidence for a direct communication between the rod plasma membrane and the rod disk membranes

Na(+)-Ca(2+)-K(+) currents measured in insect cells transfected with the retinal cone or rod Na(+)-Ca(2+)-K(+) exchanger cDNA

The recently cloned retinal cone Na(+)-Ca(2+)-K(+) exchanger (NCKX) was expressed in cultured insect cells, and whole-cell patch clamp was used to measure transmembrane currents generated by this transcript and compare them with currents generated by retinal rod NCKX or by a deletion mutant rod NCKX from which the two large hydrophilic loops were removed. We have characterized the ionic currents generated by both the forward (Ca(2+) extrusion) and reverse (Ca(2+) influx) modes of all three NCKX proteins. Reverse NCKX exchange generated outward current that required the simultaneous presence of both external Ca(2+) and external K(+). Forward NCKX exchange carried inward current with Na(+), but not with Li(+) in the bath solution. The cation dependencies of the three NCKX tested (external K(+), external Na(+), internal Ca(2+)) were very similar to each other and to those reported previously for the in situ rod NCKX. These findings provide the first electrophysiological characterization of cone NCKX and the first electrophysiological characterization of potassium-dependent Na(+)-Ca(+) exchangers in heterologous systems. Our results demonstrate the feasibility of combining heterologous expression and biophysical measurements for detailed NCKX structure/function studies.

Calcium homeostasis in vertebrate retinal rod outer segments

The outer segments of vertebrate retinal rod photoreceptors (ROS) exhibit dynamic Ca2+ fluxes. In darkness, Ca2+ continuously enters via the light-sensitive, cGMP-gated channels and this requires the presence of a powerful Ca2+ extrusion mechanism in the ROS plasma membrane. Our laboratory has characterized a Na/Ca+K exchanger in the ROS plasma membrane, which utilizes both inward Na+ gradient and outward K+ gradient to extrude Ca2+. Here, I review our work on the functional properties of the Na/Ca+K exchanger including the stoichiometry, ion binding sites and regulation of Ca2+ transport via Na/Ca+K exchange. Inactivation of the Ca2+ extrusion mode of the Na/Ca+K exchanger will be discussed as a mechanism to prevent lowering of cytosolic free Ca2+ to undesirably low values of < 1 nM that are expected from the coupling stoichiometry of the Na/Ca+K exchanger and that are expected to occur when Ca2+ influx via the cGMP-gated channels is interrupted during saturation of rod photoreceptors in bright light. This review also reexamines the contribution of internal Ca2+ stores (i.e. disks) to Ca2+ homeostasis in ROS.

How does the retinal rod Na-Ca+K exchanger regulate cytosolic free Ca2+?

The roles of 1) inactivation of Na-Ca+K exchange and 2) Ca2+ release from discs in regulation of cytosolic free Ca2+ were examined in intact rod outer segments (ROS) purified from bovine retinas. Measurements of cytosolic free Ca2+ (with fluo-3) were combined with Ca2+ flux measurements (45Ca) in ROS that contained about 600 microM total Ca2+. Na(+)-induced Ca2+ extrusion was measured in a Ca(2+)-free medium and did not lower cytosolic free Ca2+ to below 1 nM as expected from a coupling stoichiometry of 4Na+:(1Ca(2+) + 1K+). Instead, cytosolic free Ca2+ was rapidly (20 s) lowered from about 1300 nM to 100-150 nM, while at the same time about 35% of total ROS Ca2+ was removed. During the next 40 min cytosolic free Ca2+ remained virtually steady, but total ROS Ca2+ was reduced by a further 50% at a 100-fold lower rate than that observed for the initial fast phase. The steady cytosolic Ca2+ concentration resulted from Ca2+ release from discs and subsequent removal across the plasma membrane by Na-Ca+K exchange operating at a greatly reduced rate. Addition of the alkali cation channel ionophore gramicidin led to a persistent increase in cytosolic free Ca2+ concentration to about 400 nM, presumably caused by an increase in intracellular Na+. It is suggested that cytosolic free Ca2+ is not determined by the Na+:Ca2+ coupling ratio of the exchanger, but rather by a sensor on its cytoplasmic domain that controls inactivation of the Ca2+ extrusion mode and is sensitive to intracellular Ca2+, Na+, and K+.

A Na+/Ca2+ exchange mechanism in apical membrane vesicles of the retinal pigment epithelium

The retinal pigment epithelium (RPE) lying between the neural retina and the choroid, performs as a transport organ for solutes and water between the choriocapillaries and the subretinal space. It also has the function to maintain the microenvironment of photoreceptors including the regulation of calcium ions during light or dark adaptation. In order to further elucidate the transport functions of the RPE, apical membranes were isolated from RPE by differential precipitation with divalent ions. In this work bovine tissues were used as well as elasmobranch tissues. For the latter, we have already purified and characterized membrane vesicles in a previous paper. Na(+)-K(+)-ATPase, alkaline phosphatase, and 5'-nucleotidase, which are marker enzymes of the apical membrane, were highly enriched in the final membrane fraction. The majority of the fraction consists of right side out vesicles. The fluorescent indicator for sodium, SBFI, or the calcium specific indicator, Fura-2, were pre-loaded into the apical membrane vesicles of RPE of either dogfish eyes or bovine eyes. When an outwardly-directed Ca2+ gradient was formed across the vesicular membranes, the Ca2+ influx was also enhanced by 136% for dogfish RPE and 167% for bovine RPE. This Na+ gradient dependent Ca2+ influx was blocked by bepridil, an antiarrhythmic agent which is a Na+/Ca2+ exchanger inhibitor. These results indicate that a Na+/Ca2+ exchanger is present in the apical membrane of bovine and dogfish RPE.

The stoichiometry of Na-Ca+K exchange in rod outer segments isolated from bovine retinas

Ca2+ extrusion in the outer segments of retinal rods (ROS) is mediated by a protein that couples both the inward Na+ gradient and the outward K+ gradient to Ca2+ extrusion. Na(+)-stimulated Ca2+ release from ROS requires internal K+ and is accompanied by release of internal K+, whereas a slow component of Na(+)-stimulated Ca2+ release does not require K+. In this paper we discuss our observations on the K+ transport via Na-Ca+ K exchange in bovine ROS, on the electrogenicity and stoichiometry of the ROS Na-Ca+ K exchanger, and on the mechanism on coupling Ca2+ to K+ via this protein. Finally, we discuss briefly the physiological implications of Na-Ca+ K exchange.

Sodium-calcium exchange in renal epithelial cells: dependence on cell sodium and competitive inhibition by magnesium

Kinetic properties of Na(+)-Ca2+ exchange in a renal epithelial cell line (LLC-MK2) were assessed by measuring cytosolic free Ca2+ with fura-2 and 45Ca2+ influx. Replacing external Na+ with K+ produced relatively small increases in free Ca2+ and 45Ca2+ uptake unless the cells were incubated with ouabain. Ouabain markedly increased cell Na+ and strongly potentiated the effect of replacing external Na+ with K+ on free Ca2+ and 45Ca2+ uptake. 45Ca2+ influx in 140 mM K+ or N-methyl-D-glucamine minus influx in 140 mM Na+ was used to quantify Na(+)-Ca2+ exchange activity of Na(+)-loaded cells. The dependence of exchange on cell Na+ was sigmoidal; the K0.5 was 26 +/- 3 mmol/liter cell water space, and the Hill coefficient was 3.1 +/- 0.2. The kinetic features of the dependence of exchange on cell Na+ partly account for the small increase in Ca2+ influx when all external Na+ is replaced by K+. Besides raising cell Na+ ouabain appears to activate the exchanger. Magnesium competitively inhibited exchange activity. The potency of Mg2+ was 8.2-fold lower with potassium instead of N-methyl-D-glucamine or choline as the replacement for external Na+. Potassium also increased the Vmax of exchange by 86% and had no effect on the Km for Ca2+. The exchanger does not cause detectable 22Na(+)-Mg2+ exchange and does not appear to require K+ or transport 86Rb+. Although exchange activity was plentiful in the epithelial cells from monkey kidney, others from amphibian, canine, opossum, and porcine kidney had no detectable exchange activity. All of the measured kinetic properties of Na(+)-Ca2+ exchange in the renal epithelial cells are very similar to those of the exchanger in rat aortic myocytes.

Glycolysis and glucose uptake in intact outer segments isolated from bovine retinal rods

Glucose transport across the plasma membrane of isolated bovine rod outer segments (ROS) was measured by uptake of 14C-labeled 3-O-methylglucose and 2-deoxyglucose and was inferred from deenergization of ROS with 2-deoxyglucose. Glucose transport was mediated by a facilitated diffusion glucose transporter that equilibrated external and internal free hexose concentrations. Glucose transport in ROS displayed two components as judged from kinetic analysis of hexose equilibration and as judged from inhibition by cytochalasin B and phloretin. Transport under exchange conditions was considerably faster as compared with net hexose uptake, similar to that observed for the erythrocyte glucose transporter. Sensitivity to cytochalasin B and affinity to 3-O-methylglucose were similar to those observed for the hepatocyte glucose transporter. The cytochalasin-insensitive component appears unique to ROS and did not reflect leakage transport as judged from a comparison with L-glucose uptake. Glucose transport feeds glycolysis localized to ROS. We suggest that a major role for glycolysis in ROS is phosphorylation of GDP to GTP via pyruvate kinase and PEP, while phosphorylation of ADP to ATP can use the creatine kinase/phosphocreatine pathway as well.

Na+-Ca2+ exchange in bovine rod outer segments requires and transports K+

Intact outer segments isolated from bovine retinas (bovine ROS) display a high activity of Na+-Ca2+ exchange, and Na+-Ca2+ exchange appears to be the only functional ion transporter present. Here we demonstrate for the first time that Na+-Ca2+ exchange requires and transports K+ from the following observations. 1) Na+-Ca2+ exchange in bovine ROS required the simultaneous presence of K+ and Ca2+ on one side of the membrane and the presence of Na+ on the other side. 2) Na+-stimulated Ca2+ release from bovine ROS was accompanied by an equally large release of K+. We used the electrogenic protonophore carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP) as an added electrical shunt; in the intact rod cell, electrogenic Na+-Ca2+ exchange is shunted by K+ channels present in the rod inner segment. In the presence of FCCP, an inward Na+-Ca2+ exchange current was accompanied by an outward current of protons with a stoichiometry of 1 H+/Ca2+; in the absence of FCCP, no Na+-induced proton current was observed. Addition of FCCP did not uncouple Na+-induced K+ release from Na+-induced Ca2+ release. We conclude that Na+-Ca2+ exchange in bovine rod photoreceptors operates at an electrogenic stoichiometry of 4 Na+:(1 Ca2+ + 1 K+). In isolated ROS and in the absence of an external electrical shunt, Na+-Ca2+ exchange operated at an electroneutral stoichiometry of 3 Na+:(1 Ca2+ + 1 K+).

Silver ions induce a rapid Ca2+ release from isolated intact bovine rod outer segments by a cooperative mechanism

Micromolar concentrations of silver ion activate large Ca2+ fluxes across the plasma membrane of intact rod outer segments isolated from bovine retinas (intact ROS). The rate of Ag+-induced Ca2+ efflux from intact ROS depended on the Ag+ concentration in a sigmoidal manner suggesting a cooperative mechanism with a Hill coefficient between 2 and 3. At a concentration of 50 microM Ag+ the rate of Ca2+ efflux was 7 x 10(6) Ca2+/outer segment/sec; this represents a change in total intracellular Ca2+ by 0.7 mM/outer segment/sec. Addition of the nonselective ionophore gramicidin in the absence of external alkali cations greatly reduced the Ag+-induced Ca2+ efflux from intact ROS, apparently by enabling internal alkali cations to leak out. Adding back alkali cations to the external medium restored Ag+-induced Ca2+ efflux when gramicidin was present. In the presence of gramicidin, Ag+-induced Ca2+ efflux from intact ROS was blocked by 50 microM tetracaine or L-cis diltiazem, whereas without gramicidin both blockers were ineffective. Both L-cis diltiazem and tetracaine are blockers of one kinetic component of cGMP-induced Ca2+ flux across ROS disk membranes. The ion selectivity of the Ag+-induced pathway proved to be broad with little discrimination between the alkali cations Li+, Na+, K+, and Cs+ or between Ca2+ and Mg2+. The properties of the Ag+-induced pathway(s) suggest that it may reflect the cGMP-dependent conductance opened in the absence of cGMP by silver ions.

Calcium in dark-adapted toad rods: evidence for pooling and cyclic-guanosine-3'-5'-monophosphate-dependent release

1. We have used laser micromass analysis (l.a.m.m.a.) to investigate Ca uptake and release in intact 'red' rod photoreceptors in the dark-adapted retina of the toad, Bufo marinus. 2. With l.a.m.m.a. it is possible to measure separately the concentrations of each of the Ca isotopes. Rods normally containing almost exclusively 40Ca can be incubated in Ringer solution containing the stable isotopes 42Ca or 44Ca. In this way, the movements of Ca into and out of the rod can be separately determined. 3. When rods are incubated in darkness in high 44Ca (up to 20 mM), large amounts of 44Ca accumulate in the outer segment at a rate which increases with increasing external 44Ca concentration. However, this 44Ca appears not to exchange with the 40Ca originally present within the rod. This result suggests that the 40Ca may be sequestered within a pool which normally exchanges slowly with external Ca. 4. We explored Ca exchange in high-Ca solutions in more detail with double-isotope labelling. In these experiments, rods were first pre-loaded with Ca of one isotope (42Ca) and then incubated in Ringer solution containing another (44Ca). We could then measure separately the rate of exchange of the pre-loaded 42Ca with the 44Ca in the Ringer solution and with the 40Ca originally present within the rod in the sequestered pool. 5. These experiments show that the pre-loaded-Ca exchanges rapidly with Ca in the Ringer solution, at least in part by Ca-Ca exchange, but much more slowly with the Ca originally present within the rod. Thus Ca in the outer segments can exist in (at least) two pools: one which exchanges rapidly across the plasma membrane and is probably Ca free or loosely bound within the cytosol, and another which exchanges slowly and is probably Ca within the disks. 6. Although Ca sequestered within the outer segment normally exchanges quite slowly, it can be rapidly released if the extracellular free Ca is buffered to low levels with EGTA. The rate-limiting step for Ca release under these conditions appears not to be Na-Ca exchange, since the rate of Ca efflux is unchanged if the Na in the Ringer solution is substituted with choline. 7. Ca can also be released from the sequestered pool if rods are incubated in Ringer solution containing 100 or 500 microM-IBMX (3-isobutyl-1-methylxanthine).(ABSTRACT TRUNCATED AT 400 WORDS)

Identification of frog photoreceptor plasma and disk membrane proteins by radioiodination

Several functions have been identified for the plasma membrane of the rod outer segment, including control of light-dependent changes in sodium conductance and a sodium-calcium exchange mechanism. However, little is known about its constituent proteins. Intact rod outer segments substantially free of contaminants were prepared in the dark and purified on a density gradient of Percoll. Surface proteins were then labeled by lactoperoxidase-catalyzed radioiodination, and intact rod outer segments were reisolated. Membrane proteins were identified by polyacrylamide gel electrophoresis and autoradiography. The surface proteins labeled included rhodopsin, the major membrane protein, and 12 other proteins. Several control experiments indicated that the labeled proteins are integral membrane proteins and that label is limited to the plasma membrane. To compare the protein composition of plasma membrane with that of the internal disk membrane, purified rod outer segments were lysed by hypotonic disruption or freeze-thawing, and plasma plus disk membranes were radioiodinated. In these membrane preparations, rhodopsin was the major iodinated constituent, with 12 other proteins also labeled. Autoradiographic evidence indicated some differences in protein composition between disk and plasma membranes. A quantitative comparison of the two samples showed that labeling of two proteins, 24 kilodaltons (kDa) and 13 kDa, was enriched in the plasma membrane, while labeling of a 220-kDa protein was enriched in the disk membrane. These plasma membrane proteins may be associated with important functions such as the light-sensitive conductance and the sodium-calcium exchanger.

Na+- and cGMP-induced Ca2+ fluxes in frog rod photoreceptors

We have examined the Ca2+ content and pathways of Ca2+ transport in frog rod outer segments using the Ca2+-indicating dye arsenazo III. The experiments employed suspensions of outer segments of truncated, but physiologically functional, frog rods (OS-IS), intact isolated outer segments (intact OS), and leaky outer segments (leaky OS with a plasma membrane leaky to small solutes, but with sealed disk membranes). We observed the following. Intact OS or OS-IS isolated and purified in Percoll-Ringer's solution contained an average of 2.2 mM total Ca2+, while leaky OS contained 2.0 mM total Ca2+. This suggests that most of the Ca2+ in OS-IS is contained inside OS disks. Phosphodiesterase inhibitors increased the Ca2+ content to approximately 4.2 mM in intact OS or OS-IS, whereas the Ca2+ content of leaky OS was not altered. Na-Ca exchange was the dominant pathway for Ca2+ efflux in both intact and leaky OS/OS-IS. The rate of Na-Ca exchange in intact OS/OS-IS was half-maximal between 30 and 50 mM Na+; at 50 mM Na+, this amounted to 5.8 X 10(7) Ca2+/OS X s or 0.05 mM total Ca2+/s. This is much larger than the Ca2+ component of the dark current. Other alkali cations could not replace Na+ in Na-Ca exchange in either OS-IS or leaky OS. They inhibited the rate of Na-Ca exchange (K greater than or equal to Rb greater than Cs greater than or equal to Li greater than TMA) and, as the inhibition became greater, a delay developed in the onset of Na-Ca exchange. The inhibition of Na-Ca exchange by alkali cations correlates with the prolonged duration of the photoresponse induced by these cations (Hodgkin, A. L., P. A. McNaughton, and B. J. Nunn. 1985. Journal of Physiology. 358:447-468). In addition to Na-Ca exchange, disk membranes in leaky OS showed a second pathway of Ca2+ transport activated by cyclic GMP (cGMP). The cGMP-activated pathway required the presence of alkali cations and had a maximal rate of 9.7 X 10(6) Ca2+/OS X s. cGMP caused the release of only 30% of the total Ca2+ from leaky OS. The rate of Na-Ca exchange in leaky OS amounted to 1.9 X 10(7) Ca2+/OS X s.(ABSTRACT TRUNCATED AT 400 WORDS)

Transduction persists in rod photoreceptors after depletion of intracellular calcium

We have examined the role of Ca++ in phototransduction by manipulating the intracellular Ca++ concentration in physiologically active suspensions of isolated and purified rod photoreceptors (OS-IS). The results are summarized by the following. Measurement of Ca++ content using arsenazo III spectroscopy demonstrates that incubation of OS-IS in 10 nM Ca++-Ringer's solution containing the Ca++ ionophore A23187 reduces their Ca++ content by 93%, from 1.3 to 0.1 mol Ca++/mol rhodopsin. Virtually the same reduction can be accomplished in 10 nM Ca++-Ringer's without ionophore, presumably via the plasma membrane Na/Ca exchange mechanism. Hundreds of photoresponses can be obtained from the Ca++-depleted OS-IS for at least 1 h in 10 nM Ca++-Ringer's with ionophore. The kinetics and light sensitivity of the photoresponse are essentially the same in the presence or absence of the ionophore in 10 nM Ca++. The addition of A23187 in 1 mM Ca++-Ringer's results in a Ca++ influx that rapidly suppresses the dark current and the photoresponse. This indicates that there is an intracellular site at which Ca++ can modulate the light-regulated conductance. Both the current and photoresponse can be restored if intracellular Ca++ is reduced by lowering the external Ca++ to 10 nM. During the transition from high to low Ca++, the response duration becomes shorter, which suggests that it can be regulated by a Ca++-dependent mechanism. If the dark current and the photoresponse are suppressed by adding A23187 in 1 mM Ca++-Ringer's, the subsequent addition of the cyclic GMP phosphodiesterase inhibitor isobutylmethylxanthine can restore the current and photoresponse. This implies that under conditions where the rod can no longer control its intracellular Ca++, the elevation of cyclic GMP levels can restore light regulation of the channels. The persistence of normal flash responses under conditions where intracellular Ca++ levels are reduced and perturbed suggests that changes in the intracellular Ca++ concentration do not cause the closure of the light-regulated channel.

Sodium-calcium exchange in the outer segments of bovine rod photoreceptors

Intact rod outer segments (r.o.s.) isolated from bovine retinas were used to measure net Ca2+ fluxes using the optical Ca2+ indicator Arsenazo III. Ca2+ fluxes were observed, which could change the internal Ca2+ content of isolated r.o.s. by as much as 0.5 mM s-1. The Ca2+ content of isolated intact r.o.s. was strongly dependent on the Na/Ca ratio in the isolation medium, and could be made less than 0.1 mol Ca2+ mol-1 rhodopsin (zero Ca2+ in isolation medium) or up to 7 mol Ca2+ mol-1 rhodopsin (zero Na+ in isolation medium). Ca2+ efflux from r.o.s. rich in Ca2+ was observed only when Na+ was added to the external medium (as opposed to any other alkali cation); in Ca2+-depleted r.o.s. Ca2+ uptake required the presence of internal Na+ and was inhibited selectively by external Na+. These results suggest that Na-Ca exchange across the plasma membrane operated freely in both directions and controlled the internal Ca2+ concentration in r.o.s. Na+-stimulated Ca2+ efflux depended on the external Na+ concentration in a sigmoidal way. This suggests that the simultaneous binding of two Na ions is rate limiting for transport. In Ca2+-depleted r.o.s. and in the absence of external Na+, 1 mol Ca2+ mol-1 rhodopsin (or 3 mM-total Ca2+) could be taken up within 1 min by intact r.o.s. at a free external Ca2+ concentration of about 1 microM. Only part of the internal Ca2+ was available for Na-Ca exchange. The external Na+ and K+ concentration as well as the temperature were factors controlling the accessibility of internal Ca2+ to participate in Na-Ca exchange. Ca2+ fluxes in r.o.s. with a permeabilized plasma membrane but intact disk membranes were very similar to those observed in intact r.o.s.; Na-Ca exchange could operate in both directions across the disk membrane. In addition to Na-Ca exchange, leaky r.o.s. also showed a guanosine 3', 5'-cyclic monophosphate (cyclic GMP)-induced Ca2+ release that was about 1/20 of the rate of Na-Ca exchange. Na-Ca exchange could release 1.5 mol Ca2+ mol-1 rhodopsin from disks as compared with a cyclic-GMP-induced release of 0.15 mol Ca2+ mol-1 rhodopsin.

Light induced sodium dependent accumulation of calcium and potassium in the extracellular space of bee retina

Intense illumination of long duration induced a large transient increase in extracellular calcium (delta[Ca2+]o) and potassium (delta[K+]o) during and after light in bee retina when measured with ion-selective microelectrodes. Whenever a large delta[Ca2+]o appeared, it was accompanied by a transient afterdepolarization (TA). Both the increase in [Ca2+]o, [K+]o and the TA were reduced or abolished when sodium was replaced by arginine, choline or lithium (Li+) ions. At 0-Na conditions a Na independent decrease in [Ca2+]o was observed during illumination only. A pronounced transient depolarization of the photoreceptor in the dark due to transient anoxia did not result in a significant change in [Ca2+]o. In some retinae the elevated level of [K+]o after light was absent, however a small Na-dependent TA was still observed. The above findings suggest that intense long illumination induces a large Ca2+ influx into the photoreceptors which is followed by Na-dependent Ca2+ efflux due to Na-Ca exchange. The light-induced afterdepolarization arises mainly from K+ accumulation in the extracellular space but partially from the electrogenicity of Na-Ca exchange.

Calcium content and calcium exchange in dark-adapted toad rods

We have used laser-activated micro mass analysis (l.a.m.m.a.) and energy-dispersive X-ray analysis (e.d.x.) to measure Ca content and Ca movements in 'red' rod photoreceptors in the dark-adapted retina of the toad, Bufo marinus. Measurements with both l.a.m.m.a. and e.d.x. show that intact rod outer segments contain 4-5 mmol total Ca/l wet tissue volume, or 1-2 Ca per rhodopsin. We could detect no significant variation in the total Ca as a function of distance across or up and down the outer segment. In the inner segment, Ca could be detected only within the mitochondria-rich ellipsoid body, where the total Ca concentration was of the order of 100-400 mumol/l wet tissue volume. To measure the exchange of Ca in outer segments from intact photoreceptors, we exposed the dark-adapted retina to Ringer containing the stable isotope 44Ca. Since l.a.m.m.a. can measure separately the concentrations of each of the isotopes of the elements, and since native rods contain almost exclusively 40Ca, the increase in 44Ca and decrease in 40Ca could be used as a measure of Ca influx and efflux. Ca exchange in intact rod outer segments in darkness is very slow. The rate of accumulation of 44Ca was only 10(5) Ca/rod.s, or about 10% of the total outer segment Ca/h. This slow rate of exchange is apparently not the result of restricted movement of Ca across the plasma membrane. Ca exchange was also measured in outer segments which were either partially or entirely detached from the rest of the photoreceptor. In broken-off outer segments, Ca exchange is faster than in the intact organelles, and in 1 h, half of the 44Ca exchanges for 40Ca. When the retina was incubated in Ringer for which all of the Na was substituted with Li or choline, there was an increase in the rate of 44Ca accumulation in intact outer segments, probably due to an inhibition of Na-Ca counter transport across the plasma membrane. Our measurements indicate that the great majority of the Ca in the rod appears to be inaccessible to exchange under physiological conditions, probably because it is sequestered within the disks which in intact rods appear to be nearly impermeable to Ca in darkness.

Ca2+ buffer sites in intact bovine rod outer segments: introduction to a novel optical probe to measure ionic permeabilities in suspensions of small particles

The nature of the Ca2+ buffer sites in intact rod outer segments isolated from bovine retinas (ROS) was investigated. The predominant Ca2+ buffer in intact ROS was found to be negatively charged groups confined to the surface of the disk membranes. Accordingly, Ca2+ buffering in ROS was strongly influenced by the electrostatic surface potential. The concentration of Ca2+ buffer sites was about 30 mM, 80% of which were located at the membrane surface in the intradiskal space. A comparison with observations in model systems suggests that phosphatidylserine is the major Ca2+ buffer site in ROS. Protons and alkali cations could replace Ca2+ as mobile counterions for the fixed negatively charged groups. At physiological ionic strength, the total number of these diffusible, but osmotically inactive, counterions was as large as the number of osmotically active cations in ROS. The surface potential is dependent on the concentration of cations in ROS and can be measured with the optical dye neutral red. Addition of cations to the external solution led to the release of the internally bound dye as the cations crossed the outer membrane. The chemical and spectral properties of the dye enable its use as a real-time indicator of cation transport across the outer envelope of small particles in suspension. In this study, the dye method is illustrated by the use of well-defined ionophores in intact ROS and in liposomes. In the companion paper this method is used to describe the cation permeabilities native to ROS.

Ionic permeabilities of the plasma membrane of isolated intact bovine rod outer segments as studied with a novel optical probe

The permeability properties of the plasma membrane of intact rod outer segments purified from bovine retinas (ROS) were studied with the aid of the optical probe neutral red as described in the companion paper. The following observations were made: Electrical shunting of ROS membranes greatly stimulated Na+ and K+ transport, suggesting that this transport reflects Na+ and K+ currents, respectively. The dissipation of a Na+ gradient across the plasma membrane occurred with a half-time of 30 sec at 25 degrees C. The Na+ permeability was progressively inhibited when the external Ca2+ concentration was raised from 1 microM to 20 mM. A similar Ca2+ dependence was observed for H+ and Li+ transport. The Na+ permeability was not affected when the total internal Ca2+ content of ROS was varied between 0.1 mol Ca2+/mol rhodopsin and 7 mol Ca2+/mol rhodopsin, or when the free internal Ca2+ concentration was varied between 0.1 and 50 microM. The K+ permeability was progressively stimulated when the external Ca2+ concentration was raised from 0.001 to 1 microM, whereas a further increase to 20 mM was without effect. A similar Ca2+ dependence was observed for Rb+ and Cs+ transport. At an external Ca2+ concentration in the micromolar range the rate of transport decreased in the order: Na+ greater than K+ = H+ greater than Cs+ greater than Li+. Na+ fluxes depended in a sigmoidal way on the external Na+ concentration, suggesting that sodium ions move in pairs. The concentration dependence of uniport Na+ transport and that of Na+-stimulated Ca2+ efflux (exchange or antiport transport) were very similar.

Effect of cGMP and cations on the permeability of cattle retinal disks

Guanosine 3',5'-monophosphate and Na or Ca ions affect the transmembrane movements of the same pool of intradiskal ions. Extradiskal Na ions activate the efflux of intradiskal Na ions. Extradiskal Ca ions activate the efflux of intradiskal Rb ions. Na and Ca ions activate Na/Ca or Ca/Ca exchange, as previously described. cGMP activates a membrane permeability for all the cations tested, as previously described. The reciprocal relations between cGMP and the other pathways for ion movements through disk membranes are systematically examined. Some analogies between the cGMP-activated permeability of the disk membranes and the light-sensitive conductance of the rod plasma outer membrane are discussed.

Light changes the membrane potential and ion balances of retinal rod disks

Light stimulation of rod cells in vertebrate eyes may cause Ca2+ release from the intracellular disks. Radiolabelled tracers show that light causes a small hyperpolarization of intact disk stacks and redistribution of the ions Ca2+ and Cl-.

Effect of light on Na-Ca exchange in rod outer segments in frog

The effect of illumination on the calcium (Ca) translocation mechanisms in isolated frog rod outer segments (ROS) was studied. The ATP-dependent Ca uptake and the Ca-Ca exchange mechanisms were unaffected by light. In contrast, we report a light-evoked Ca efflux which is mediated by the Na-Ca exchange system. The ratio of released Ca to rhodopsin bleaching was measured and the stoichiometry obtained was 5 Ca molecules released per mole of rhodopsin bleached. Concomitant to the Ca release, light induced Ca uptake, which increase the total Ca content of ROS. Physiological relevance of results to the phototransduction process is discussed.

Frog rod outer segments with attached inner segment ellipsoids as an in vitro model for photoreceptors on the retina

Purified suspensions of frog rod outer segments still attached to the mitochondria-rich inner segment portion of the receptor cell (OS-IS) can be obtained in quantities (0.1 mg/retina) sufficient for chemical analysis. In oxygenated glucose-bicarbonate Ringer's medium with added Percoll, they display normal dark currents, light sensitivity, and photocurrent kinetics for several hours. Two millimolar cytoplasmic levels of ATP and GTP are maintained, fivefold higher than in isolated OS. The levels are not altered by abolition of the dark current with ouabain. Nucleoside triphosphates are more effectively buffered than in isolated OS, and their levels remain constant during changes in external calcium levels. 32Pi is incorporated into endogenous ATP and GTP pools twice as efficiently as in isolated OS, and is used in the phosphorylation of rhodopsin. OS-IS take up and release 45Ca++ by Na+-, Ca++-, and IBMX-sensitive mechanisms. Illumination causes release of 45Ca++, which confirms retinal studies by other groups using Ca++-sensitive electrodes. Thus, OS-IS suspensions model the behavior of photoreceptors still attached to the living retina. Their availability permits the simultaneous assay and correlation of electrophysiological and chemical changes occurring during excitation and adaptation.

Light-dependent calcium release from photoreceptors measured by laser micro-mass analysis

Yoshikami and Hagins first suggested that calcium is sequestered within membranous disks in the outer segments of vertebrate rods and that the bleaching of visual pigment molecules by light causes the release of Ca from the disks. Once released, the Ca was postulated to bind to Na+ channels or carrier molecules in the plasma membrane to produce the electrical response. This theory, termed the 'calcium hypothesis', is supported by much evidence but remains controversial, largely because of the difficulty in measuring calcium in rods and of demonstrating light-induced release. Here we describe direct measurements of total rod Ca using a new microprobe method, called laser micro-mass analysis, or LAMMA . Using this technique, we show that rods contain large amounts of Ca concentrated in their outer segments. Physiological levels of illumination produce a graded efflux of rod Ca content, amounting to about 10(4) ions per rhodopsin molecule bleached in dim light. As light does not change the rate of Ca influx, the total Ca content of the rod decreases. In bright light, as much as half the total Ca leaves the rod during only 1 min of illumination.

Effect of ions on retinal rods from Bufo marinus

The effect of ions on the light-sensitive current of isolated retinal rods from the toad Bufo marinus was studied by sucking the inner segment into a tightly fitting pipette. The outer segment projected into flowing solution whose composition could be changed rapidly. Reducing the external Na concentration, [Na]o, round the outer segment caused rapid and reversible reductions in the light-sensitive current. With the outer segment in the pipette, reductions of [Na]o round the inner segment had little effect on the light-sensitive current. The current about 15 s after a change in [Na]o was approximately proportional to [Na]2o. The current decreased in elevated external Ca concentration, [Ca]o, and increased in reduced [Ca]o. Between 10 and 0.5 mM-external Ca the current 15 s after a change was approximately inversely proportional to [Ca]o. Reducing [Ca]o from 1 mM to 1 microM or less transiently increased the current by about 15-fold. After a change in [Na]o or [Ca]o the current did not approach its final value monotonically but with a characteristic overshoot or underswing, followed by a slow relaxation of current which may reflect the time course of change in internal Na. Reducing [Na]o from 110 to 70 mM or less prolonged the response to a flash; very long responses were observed in solutions containing Li rather than Na and also in rods that had been returned to Ringer solution after exposure to low Ca. All these effects might be explained if Ca extrusion in exchange for Na determines the reactivation of current after a flash. The rod current was not changed if the ratio [Na]No/[Ca]o was held constant, N being about 2.5. Between 5 mM and 10 microM-Ca the change in peak current produced by absorption of a single quantum was roughly proportional to the dark current. Responses in the absence of external Na were not normally seen if the solution contained 0.1 mM-Ca or more. Responses of normal polarity were regularly observed in 0 Na, 0 Ca EGTA solutions containing 1.6 mM-Mg. Removal of Mg from such solutions gave inverted responses. Other conditions which promote responses of normal and inverted polarity in Na-free solutions are described briefly. We conclude that Li, Ca, Mg and perhaps K can pass through the light-sensitive channel. The above results suggest that external Na has two distinct effects: (1) it provides ions to carry inward current, and (2) it keeps the light-sensitive conductance open by maintaining the internal Ca concentration, [Ca]i, at a low level.

Membrane potential affects photocurrent kinetics in salamander rods and cones

In both rod and cone photoreceptors, polarization of the cell to positive potentials produces a slowing of the photocurrent. This could occur because, at positive potentials, the rate of removal of internal transmitter is slowed, or because the time for which the channels stay closed after interaction with internal transmitter is prolonged. Ion concentration changes within the cell may also contribute to the time course of the photocurrent.

Activation of electrogenic Na-Ca exchange by light in fly photoreceptors

Illumination of white-eyed Musca photoreceptors following hypoxia or the application of ruthenium red (RR, a known blocker of Ca2+ uptake into intracellular organelles) induced a transient after depolarization (TA). The TA was enhanced when external [Ca2+] was reduced; it was abolished when external [Na+] was reduced to a level that affected the receptor potential to a small degree. The TA was enhanced or depressed when the activity of Na/K pump, which controls the Na+ gradient, was enhanced or depressed respectively. This effect was observed even when the receptor potential was not affected. All of the above observations are consistent with the hypothesis that the TA is triggered by a light-induced increase in the concentration of intracellular free Ca2+ which appear to be very high, following treatments with hypoxia or RR. The high sensitivity of the TA to Na+ and Ca2+ gradients across the photoreceptors membrane strongly suggests that the TA is due to a transient activation of an electrogenic Na-Ca exchange mechanism which depolarizes the cell.

Protons block the dark current of isolated retinal rods

Membrane currents of isolated frog rods were recorded with the suction pipette technique and tested by perfusion techniques for their sensitivity to H+. The following facts have been established. (i) Increased [H+] suppresses the Na+ conductance of the outer segment rapidly and reversibly. (ii) H+ acts in the rod interior. (iii) The [H+] necessary to cause a 50% decrement in Na+ conductance is inversely related to the [Ca2+] over 5 orders of magnitude. (iv) The sensitivity to H+ and the sensitivity to light, as a function of [Ca2+], have the same slope. Thus, H+ act like light in effecting membrane current suppression but behave as if their effect is mediated through Ca2+. Based on these results and properties of rod disk membrane phosphodiesterase, we propose that protons produced in the light-activated hydrolysis of cGMP liberate Ca2+ from the disks by ion exchange.

Adapting lights and lowered extracellular free calcium desensitize toad photoreceptors by differing mechanisms

Extracellular recordings were made across the outer segment layer of isolated, superfused toad retinas. Under these recording conditions, the photovoltage reflects primarily the current flowing through the outer-segment membrane of red rods. In normal toad Ringer solution, a dim conditioning flash desensitized a test flash response. The desensitization reached a peak 1.8-2.0 s after the conditioning flash and then declined approximately as an exponential with time constant 6 s. Lowered extracellular calcium, [Ca2+]o, desensitized the photoresponse. It required approximately ten times more light to reach a half-maximal response for each ten-fold change in [Ca2+]o from 10(-6) to 10(-9) M. When [Ca2+]o was less than 10(-7) M, substitution of Li+ for Na+ as the predominant monovalent cation in the superfusate permitted responses to continue and a resensitization of up to approximately 1 log unit was observed. The effects of lowered [Ca2+]o on response kinetics were markedly different from the effects of background lights producing a comparable desensitization. Low [Ca2+]o increased absolute latency and time-to-peak of the flash response. Background lights decreased time-to-peak, leaving latency unchanged. The effects of background lights and lowered [Ca2+]o are not additive. Moderate backgrounds had little effect on the intensity/response function in low [Ca2+]o. Conditioning flashes facilitated the test flash response in 10(-7) M-[Ca2+]o superfusate. These results can be understood in terms of the Ca2+ hypothesis of transduction (Hagins & Yoshikami, 1974) if it is assumed that lowered [Ca2+]o exposes an endogenous Ca2+ buffer. The data also provide evidence for a role of Na+/Ca2+ exchange in regulating intracellular Ca2+ concentration in the toad photoreceptor. A quantitative model based on these assumptions is derived and compared with the experimental data.

On the relation between rapid light-induced Ca2+ release and proton uptake in rod outer segment disk membranes

In this paper we review our experiments on the light-induced Ca2+ release and proton uptake at the rod outer segment (ROS) disk membrane using flash-spectrophotometry and the indicating dyes arsenazo III and bromcresol purple. We used three different ROS preparations in order to locate the intracellular site of Ca2+ release. The ionophore A23187 was required to communicate the Ca2+ release to the indicator located in the external medium in both ROS with an intact and with a leaky plasma membrane. A23187 was also required to observe the Ca2+ released in the interior of vesicles prepared by sonication of ROS. From this we conclude that the site of Ca2+ release is located at the luminal side of the disk membrane, whereas this Ca2+ was not transported across the disk membrane under our experimental conditions and on the time scale of our experiments (20 s). Light-induced Ca2+ release was inhibited by electrolysis in the suspension medium provided that the electrolytes gained access to the compartment where Ca2+ was released. The effectivity to inhibit Ca2+ release markedly increased from monovalent to divalent to trivalent cations. The results strongly suggest that electrolytes (cations) act by screening the electrostatic potential at the disk membrane surface due to the presence of a net fixed negative surface charge. The surface potential controls the free Ca2+ concentration at the membrane surface and, therefore, controls the amount of Ca2+ bound to the disk membrane. The kinetics of light-induced Ca2+ release and proton uptake showed a similar dependence on the structural status of the ROS. In sonicated ROS almost linear Arrhenius plots were observed for metarhodopsin II formation, Ca2+ release and proton uptake (energy of activation 150 kJ/mol). In intact ROS both Ca2+ release and proton uptake showed a nonlinear Arrhenius plot with rate constants up to 30-fold slower than metarhodopsin II formation. At temperatures above 10 degrees C a process other than metarhodopsin II formation rate limited both ligh-induced proton uptake and Ca2+ release (energy of activation 42 kJ/mol). A model is discussed in which metarhodopsin II formation triggers the uptake of proton(s) into the disk membrane lowering the surface potential. A reduction potential of the surface in turn decreases the free Ca2+ concentration at the surface thereby causing the release of part of the bound Ca2+.

Light-mediated cyclic GMP hydrolysis controls important aspects of kinetics of retinal rod voltage response

Pulsatile injections of cyclic GMP into rod outer segments of the isolated toad retina cause transient depolarizations that are reduced in amplitude in proportion with the receptor potential by low Na+ Ringer's. This reduction in the amplitude of the cyclic GMP depolarization may be due to the direct effect of external Na+ concentration on dark current and an indirect effect resulting from the inactivation of a sodium-calcium exchange mechanism raising the intracellular Ca2+ concentration. By comparison the reduction in cyclic GMP response amplitude effected by illumination is accompanied by faster kinetics. This difference suggests that the reduced amplitude and speedier response reflect a light induced increase in phosphodiesterase (PDE) activity rather than the effects of Ca2+. Large doses of cyclic GMP can distort the kinetics of both the light response and the recovery from a depolarization caused by a pulse of cyclic GMP by similarly slowing both types of responses. This similarity in the kinetics of the cyclic GMP response and the initial hyperpolarizing phase of the receptor potential suggests that the kinetics of the initial phase of the receptor potential are controlled by light-mediated cyclic GMP hydrolysis.

The effects of low calcium and background light on the sensitivity of toad rods

1. We have examined the effects of decreases in extracellular Ca(2+) concentration on the intracellularly recorded light responses of rods from the toad, Bufo marinus. In agreement with previous results (Brown & Pinto, 1974; Lipton, Ostroy & Dowling, 1977), Ca(2+) concentrations below 10(-6) M produced a depolarization of rod resting membrane potential of approximately 30-40 mV and a corresponding increase in the maximum amplitude of the rod's light responses, so that saturating flashes in normal and low Ca(2+) Ringer produced hyperpolarizations to approximately the same membrane potential.2. The rod's sensitivity was reduced in low Ca(2+) Ringer by an amount dependent upon the extracellular Ca(2+) concentration. At 10(-6) M-Ca(2+), sensitivity was approximately 0.6 log units below normal. Thereafter, it dropped nearly linearly with [Ca(2+)](o) to a value approximately 4.0 log units below normal at 10(-9) M-Ca(2+). Most of the decline occurred within 1-2 min after the solution change as the membrane potential depolarized, but sensitivity continued to fall slowly with time at the lowest Ca(2+) concentrations. Exposure to low Ca(2+) solutions altered the kinetics of the receptor response to brief flashes, delaying response onset and time-to-peak but affecting the time course of decay very little.3. The sensitivity of the rod to maintained steps of light was also reduced in low Ca(2+). Furthermore, the changes in sensitivity produced by background illumination were very much smaller in low Ca(2+) than in normal Ringer. In some cases backgrounds actually increased sensitivity.4. In 10(-8) M-Ca(2+), backgrounds which themselves produced no response in the rod and no changes in rod sensitivity produced large decreases in response latency for responses of all amplitudes, and pronounced changes in time-to-peak and time-to-decay for moderate and large amplitude responses.5. Since the effects of background light and low Ca(2+) on the wave form of the rod are distinct and in some cases antagonistic, and since the changes in receptor sensitivity produced by backgrounds and low Ca(2+) are not additive, the decreases in sensitivity produced by exposure to low Ca(2+) appear to be caused by a mechanism distinct from normal light adaptation. We suggest that they are caused by an increase in the buffering capacity of the receptor cytosol for Ca(2+) and that Ca(2+) is the excitatory messenger or ;internal transmitter', as originally suggested by Yoshikami & Hagins (1971).

X-ray diffraction and electron microscope study of phase separation in rod outer segment photoreceptor membrane multilayers

Phase separation in artificially stacked multilayers of isolated bovine retinal rod outer segment (ROS) membranes has been examined via x-ray diffraction and electron microscopy. Specimens were prepared by isopotential spin drying followed with partial hydration by equilibration against moist gas streams. Upon dehydration, the multilamellar membrane phase assumes a binary phase composition consisting of concentrated protein-containing lamellae interspersed with microdomains of hexagonally packed tubes of lipid in a HII configuration. The HII lattice is geometrically coupled to the lamellar phase with one set of hexagonal crystal planes co-planar to the local membrane lamellae. The hexagonal microdomains bear a striking resemblance to the "paracrystalline inclusions" observed in fast-frozen, intact frog ROS (Corless and Costello. 1981. Exp. Eye Res. 32:217). The lamellar lattice is characterized by an unusually small degree of disorder. Sharp lamellar diffraction with a 120 A unit cell is observed (at near total dehydration) to a resolution of 6 A. A model consistent with the data is that a multilamellar array of ROS disks is stable as long as the external disk surfaces are kept sufficiently far apart. If the distance between the membranes is allowed to shrink below a certain critical value, the disk lipids spontaneously convert to a nonbilayer phase. This suggests that the structure of the ROS is stabilized by an internal framework that acts to keep the disks apart from one another and from the plasmalemma. Thus, the necessity of avoiding phase separations may provide a rationale for the peculiar morphology of the ROS.

The effects of sodium replacement on the responses of toad rods

1. We have investigated the effects of Na(+) substitution on the membrane potential and light responses of rods in the superfused retina of the toad, Bufo marinus.2. When all of the Na(+) in the Ringer was replaced with Li(+), the effects on the rods depended upon the external free Ca(2+) concentration ([Ca(2+)](o)). At [Ca(2+)](o) >/= 10(-6) M, the membrane potential (E(m)) hyperpolarized and light responses were greatly diminished or abolished. At [Ca(2+)](o) </= 10(-7) M, Li(+) replacement had little effect on E(m) or response amplitude.3. We interpret these results as revealing a Na(+)-Ca(+) counter-transport in rods. At high [Ca(2+)](o), replacing Na(+) with Li(+) would have produced an increase in the rod cytosol free Ca(2+) concentration ([Ca(2+)](i)) and the blockage of the light-dependent conductance, leading directly to the suppression of light responses. At [Ca(2+)](o) </= 10(-7) M, this presumably would not have occurred.4. Since at these low Ca(2+) concentrations we observed light responses of nearly normal amplitude in Li(+), our results suggest that the light-dependent conductance is permeable to Li(+).5. Substitution of Na(+) with K(+) in low Ca(2+) produced a complete suppression of the responses. However, it was still possible to measure large light-induced changes in rod input resistance.6. Substitution of Na(+) with tetramethylammonium, tetraethylammonium, Tris, or choline in low Ca(2+) produced a large hyperpolarization of the membrane potential and a diminution of response amplitude. However, we were unable to observe a complete suppression of the responses for these cations.7. Substitution of Na(+) with tetrapropylammonium or with an uncharged substance (glucose or urea) in low Ca(2+) produced a large hyperpolarization of membrane potential and a considerable decrease in the light responses. In about half our attempts, the responses were observed to decline reversibly to less than 20% of their peak amplitude in Na(+).8. Results with tetrapropylammonium were indistinguishable from those of glucose or urea, indicating that the light-dependent conductance probably is not permeable to TPA. The resistance changes measured with K(+) substitution and the responses observed in the presence of the organic ions TMA, TEA, Tris and choline suggest that these species may be permeable, but we are unable to discount alternative explanations.

Calcium metabolism in vertebrate photoreceptors

So far all attempts to demonstrate a rapid, light-stimulated release of calcium from disks into the cytosol at a sufficiently high stoichiometry have failed. Either the release stoichiometry was too small or the velocity too slow to account for the amplification in visual transduction. The multitude of failures demonstrate that regulation of intracellular calcium is a very delicate process and the idea of a robust calcium channel in the disk membrane that is opened by rhodopsin itself is certainly an oversimplification. The strongest evidence in favour of the "calcium transmitter hypothesis" is the large calcium efflux from rods in a retina. However as long as the source of the calcium efflux inside the rod cells is unknown conclusions about the role of this calcium efflux are premature. Unfortunately, measurements of intracellular calcium, such as those by Brown and coworkers (93,94) in their pioneering work on photoreceptors in the ventral eye of Limulus, have not yet been feasible in vertebrates.