Extrusion of calcium from rod outer segments is driven by both sodium and potassium gradients

Actions of Ca2+ on an early stage in phototransduction revealed by the dynamic fall in Ca2+ concentration during the bright flash response

To study the actions of Ca2+ on "early" stages of the transduction cascade, changes in cytoplasmic calcium concentration (Ca2+i) were opposed by manipulating Ca2+ fluxes across the rod outer segment membrane immediately following a bright flash. If the outer segment was exposed to 0 Ca2+/0 Na+ solution for a brief period immediately after the flash, then the period of response saturation was prolonged in comparison with that in Ringer solution. But if the exposure to 0 Ca2+/0Na+ solution instead came before or was delayed until 1 s after the flash then it had little effect. The degree of response prolongation increased with the duration of the exposure to 0 Ca2+/0 Na+ solution, revealing a time constant of 0.49 +/- 0.03 s. By the time the response begins to recover from saturation, Ca2+i seems likely to have fallen to a similar level in each case. Therefore the prolongation of the response when Ca2+i was prevented from changing immediately after the flash seems likely to reflect the abolition of actions of the usual dynamic fall in Ca2+i on an early stage in the transduction cascade at a site which is available for only a brief period after the flash. One possibility is that the observed time constant corresponds to the phosphorylation of photoisomerized rhodopsin.

Characterization of sodium-calcium exchange in rabbit renal arterioles

Experiments were performed to test the hypothesis that renal arterioles exhibit Na-Ca exchange capability and that this process is regulated by protein kinase C (PKC). Glomeruli with attached arterioles were dissected from rabbit kidney and loaded with fura-2 for measurement of intracellular calcium concentration ([Ca2+]i) using microscope-based photometry. In tissue bathed in Ringer's solution containing 150 mM Na+ and 1.5 mM Ca2+, afferent and efferent arteriolar [Ca2+]i averaged 136 +/- 6 and 154 +/- 7 nM, respectively. Removal of extracellular Na+ increased afferent arteriolar [Ca2+]i by 70 +/- 7 mM, while efferent arteriolar [Ca2+]i only increased by 39 +/- 5 nM (P < 0.01 vs. afferent arteriole). These responses were inhibited by 6 nM Ni2+ and required extracellular Ca2+, but were unaffected by 10 microM diltiazem. After incubation in 500 microM ouabain, 5 microM monensin, and 5 microM nigericin, [Ca2+]i responses to removal of extracellular Na+ were exaggerated significantly, averaging 174 +/- 50 nM in afferent arterioles and 222 +/- 82 nM in efferent arterioles (NS vs. afferent arterioles). Moreover, responses to removal of extracellular Na+ were enhanced by 100 nM phorbol 12-myristate 13-acetate, an affect which was blocked by PKC inhibition (25 nM K252b). These data indicate that both afferent and efferent arterioles express the Na-Ca exchanger, and that PKC activity impacts on exchange capacity in these vessels.

The action of cytoplasmic calcium on the cGMP-activated channel in salamander rod photoreceptors

1. Truncated salamander rod photoreceptors were internally perfused to investigate the action of cytoplasmic Ca2+ on cGMP-activated channels in the outer segment. 2. Switching from 1 microM Ca2+ to 0 Ca2+ increased the cGMP-activated current by a factor of 7.1 +/- 0.5 when measured in the first 60 s after the outer segment was opened to the bath, but only 2-fold after 5 min or more. This was attributed to the loss from the outer segment of a soluble factor required for Ca2+ to inhibit the cGMP-activated channel. 3. Short exposures to 0 Ca2+ caused an irreversible increase in the cGMP-activated current measured in 1 microM Ca2+, indicating that lowering [Ca2+] accelerated the loss of the channel inhibitor from the outer segment. 4. Channel activation occurred with a half-time of 6.7 s on switching to 0 Ca2+. Replacing 1 microM Ca2+ inhibited the current again with a half-time of 11.0 s. 5. The inhibition of the cGMP-activated current by Ca2+ could be described by a Hill curve with half-maximal suppression at 55 +/- 13 nM Ca2+ and a Hill coefficient of 1.4 +/- 0.4. 6. Addition of calmodulin (1 microM), or the calmodulin inhibitors mastoparan and calmidazolium (5 microM), did not alter the action of Ca2+ on the cGMP-activated current. 7. The increased affinity of the cGMP-activated channels in response to a fall in [Ca2+] has the magnitude, speed and Ca2+ dependence to suggest that it will promote recovery of the cGMP-activated current in response to the light-induced fall in [Ca2+] that normally occurs inside the outer segment.

Evidence for a Na+-Ca2+ exchanger in rat pancreatic ducts

Only recently has it been recognized that intracellular Ca2+ is an important cellular mediator in pancreatic ducts. The aim of the present study was to characterize the Ca2+ efflux pathway in ducts freshly prepared from rat pancreas. Lowering of extracellular Na+ concentration resulted in a significant increase in intracellular Ca2+. This effect was fast, reversible, dependent on the extracellular Na+ concentration and did not correlate with intracellular pH changes. It was abolished in Ca2+-free solutions, indicating that the outwardly directed Na+ gradient was directly coupled to a flufenamate insensitive Ca2+ influx. Removal and reintroduction of extracellular Na+ induced transient hyperpolarization and depolarization of Vm, respectively. Taken together, our data indicate that pancreatic ducts possess an electrogenic Na+-Ca2+ exchanger, which under control conditions is responsible for transporting Ca2+ out of resting duct cells.

Molecular origin of continuous dark noise in rod photoreceptors

Noise in the rod photoreceptors limits the ability of the dark-adapted visual system to detect dim lights. We investigated the molecular mechanism of the continuous component of the electrical dark noise in toad rods. Membrane current was recorded from intact, isolated rods or truncated, internally dialyzed rod outer segments. The continuous noise was separated from noise due to thermal activation of rhodopsin and to transitions in the cGMP-activated channels. Selectively disabling different elements of the phototransduction cascade allowed examination of their contributions to the continuous noise. These experiments indicate that the noise is generated by spontaneous activation of cGMP phosphodiesterase (PDE) through a process that does not involve transducin. The addition of recombinant gamma, the inhibitory subunit of PDE, did not suppress the noise, indicating that endogenous gamma does not completely dissociate from the catalytic subunit of PDE during spontaneous activation. Quantitative analysis of the noise provided estimates of the rate constants for spontaneous PDE activation and deactivation and the catalytic activity of a single PDE molecule in situ.

Dark adaptation in vertebrate photoreceptors

Exposure of the eye to bright light bleaches a significant fraction of the photopigment in rods and cones and produces a prolonged decrease in the sensitivity of vision, which recovers slowly as the photopigment is regenerated. This sensitivity decrease is larger than would be expected merely from the decrease in the concentration of the pigment. Recent experiments have shown that the decrease in sensitivity is produced largely by an excitation of the phototransduction cascade by bleached pigment; even in darkness, it produces an equivalent background similar to that produced by real steady background illumination. Thus, excitation produced by a form of rhodopsin thought previously to be inactive has a profound effect on the physiology of the photoreceptor. This raises the possibility that forms of other G protein-coupled receptors thought to be inactive might also play an important role in signal transduction and disease.

Kinetics of oxygen consumption and light-induced changes of nucleotides in solitary rod photoreceptors

We made simultaneous measurements of light-induced changes in the rate of oxygen consumption (QO2) and transmembrane current of single salamander rod photoreceptors. Since the change of PO2 was suppressed by 2 mM Amytal, an inhibitor of mitochondrial respiration, we conclude that it is mitochondrial in origin. To identify the cause of the change of QO2, we measured, in batches of rods, the concentrations of ATP and phosphocreatine (PCr). After 3 min of illumination, when the QO2 had decreased approximately 25%, ATP levels did not change significantly; in contrast, the amount of PCr had decreased approximately 40%. We conclude that either the light-induced decrease of QO2 is not caused by an increase in [ATP] or [PCr], or that the light-induced change of [PCr] is highly heterogeneous in the rod cell.

Functional expression of Na-Ca exchanger clones measured with the fluorescent Ca(2+)-indicating dye fluo-3

The process of Ca2+ homeostasis is of prime importance to all cells because of the ubiquitous role of cytoplasmic Ca2+ as an intracellular messenger and the cytotoxicity of sustained elevated cytosolic Ca2+ concentrations. Two classes of plasma membrane proteins are responsible for maintaining cytosolic free Ca2+ in the submicromolar range against a very large electrochemical Ca2+ gradient across the plasma membrane, the ATP-driven Ca2+ pump and Na-Ca exchangers. Two types of Na-Ca exchangers are known, the 3Na:1Ca exchangers found in heart, brain, kidney, and most other tissues and the 4Na:1Ca+ 1K exchanger found in retinal rod and cone photoreceptors. Functional expression of Na-Ca(/K) exchangers is most often measured as 45Ca uptake in Na(+)-loaded cells or as Na-Ca exchange currents with the giant excised patch technique. In this study, two functional assays used to detect expression of the bovine heart Na-Ca exchanger in CHO cells are described. Both assays are based on measurements of cytosolic free Ca2+ with the fluorescent Ca(2+)-indicating dye fluo-3 and should be equally applicable in the study of functional expression of both Na-Ca and Na-Ca/K exchanger clones.

Modeling and analysis of spatio-temporal change in [Ca2+]i in a retinal rod outer segment

The change in [Ca2+]i, in a retinal rod outer segment to flash and step stimuli was simulated. The present model included inward and outward calcium fluxes through cation channels and an Na-Ca-K exchanger, respectively, calcium buffers, and diffusion through the interdiskal space of cytoplasm and incisures. Under control conditions (with the diffusion constant for calcium (DCa) of 10(-10) m2/s and the total concentration ([B]t) of 240 microM) the decrease in [Ca2+]i to flash stimuli was found to be localized around the edge of a disk irrespective of the presence or absence of incisures. Homogeneous but limited degree of decrease in [Ca2+]i was seen under a larger DCa of 10(-9) m2/s with no incisure. For the step illumination by which all the cation channels on a plasma membrane were closed, the decrease in [Ca2+]i around the edge of a disk was quick, while that at the center of an interdiskal space was slow (50% of the resting level at 5 s after the onset of the step illumination). These results indicate that the calcium feedback on guanylate cyclase and possibly on S-modulin in response to flash stimuli proceeds only around the edge of disk membranes or on the plasma membrane. This implicates localized mechanisms for the signal transduction and early phase of adaptation within rod outer segments.

Ca2+ fluxes and channel regulation in rods of the albino rat

By use of microelectrodes, changes in the receptor current and the Ca2+ concentration were measured in the rod layer of the rat retina after stimulation by flashes or steady light. Thereby light induced Ca2+ sources, and sinks along a rod were determined in dependence of time. Thus, the Ca2+ fluxes across the plasma membrane of a mammalian rod could be studied in detail. By light stimulation, Ca2+ sources are evoked along the outer segment only. Immediately after a saturating flash, a maximum of Ca2+ efflux is observed which decays exponentially with tau = 0.3 s at 37 degrees C (4.2 s at 23 degrees C). During regeneration of the dark current, the outer segment acts as a Ca2+ sink, indicating a restoration of the Ca(2+)-depleted outer segment. These findings agree with earlier reports on amphibian rods. Further experiments showed that the peak Ca2+ efflux and tau are temperature dependent. The peak amplitude also depends on the external Ca2+ concentration. In contrast to the reports on amphibian rods, only a part of the Ca2+ ions extruded from the outer segment is directly restored. Surprisingly, during steady light the Ca2+ efflux approaches a permanent residual value. Therefore, in course of a photoresponse, Ca2+ must be liberated irreversibly from internal Ca2+ stores. There is certain evidence that the inner segment acts as a Ca2+ store. Our results show that the Ca2+ fraction of the ions carrying the dark current is proportional to the extracellular Ca2+ concentration. This indicates that the Ca2+ permeability of the plasma membrane of the rod outer segment is independent of the Ca2+ concentration.

Regulation of sensitivity in vertebrate rod photoreceptors by calcium

Over the past decade and a half, there have been great advances in our understanding of how light is transduced into electrical signals by the retinal rod and cone photoreceptors in vertebrates. One essential feature of these sensory neurons is their ability to adapt to background illumination, which allows them to function over a broad range of light intensities. This adaptation appears to arise mostly from negative feedback on phototransduction that is mediated by calcium ions. Recent work has suggested that this feedback is fairly complex, and involves several pathways directed at different components of phototransduction. From direct measurements of these feedback pathways in rods, it is possible to evaluate their relative contributions to the overall sensitivity of the cell. At the same time, these feedback mechanisms, as currently known, appear to be sufficient for explaining the change in sensitivity of rods during adaptation to light.

How photons start vision

Recent studies have elucidated how the absorption of a photon in a rod or cone cell leads to the generation of the amplified neural signal that is transmitted to higher-order visual neurons. Photoexcited visual pigment activates the GTP-binding protein transducin, which in turn stimulates cGMP phosphodiesterase. This enzyme hydrolyzes cGMP, allowing cGMP-gated cationic channels in the surface membrane to close, hyperpolarize the cell, and modulate transmitter release at the synaptic terminal. The kinetics of reactions in the cGMP cascade limit the temporal resolution of the visual system as a whole, while statistical fluctuations in the reactions limit the reliability of detection of dim light. Much interest now focuses on the processes that terminate the light response and dynamically regulate amplification in the cascade, causing the single photon response to be reproducible and allowing the cell to adapt in background light. A light-induced fall in the internal free Ca2+ concentration coordinates negative feedback control of amplification. The fall in Ca2+ stimulates resynthesis of cGMP, antagonizes rhodopsin's catalytic activity, and increases the affinity of the light-regulated cationic channel for cGMP. We are using physiological methods to study the molecular mechanisms that terminate the flash response and mediate adaptation. One approach is to observe transduction in truncated, dialyzed photoreceptor cells whose internal Ca2+ and nucleotide concentrations are under experimental control and to which exogenous proteins can be added. Another approach is to observe transduction in transgenic mouse rods in which specific proteins within the cascade are altered or deleted.

Inhibition and acceleration of Na+/Ca2+/K+ exchange fluxes by Ag+ in bovine retinal rod outer segments

The effect of Ag+ on Ca2+ fluxes mediated by the retinal rod Na+/Ca2+/K+ exchanger was investigated in intact bovine rod outer segments (ROS). Intracellular Na+ concentration ([Na+]in)-dependent Ca2+ influx and extracellular Na+ concentration ([Na+]out)-dependent Ca2+ efflux were monitored by changes in cytosolic free Ca2+ measured with the fluorescent Ca(2+)-indicating dye fluo 3. Ag+ was the most effective inhibitor of Na+/Ca2+/K+ exchange fluxes described to date, with half-maximal inhibition observed at 2-8 microM Ag+. Inhibition by Ag+ could be reversed by addition of beta-mercaptoethanol but not by addition of cysteine. Reversal by beta-mercaptoethanol resulted in a marked acceleration of [Na+]out-dependent lowering of cytosolic free Ca2+ but not of [Na+]in-dependent Ca2+ influx. We suggest that Ag+ inhibits and accelerates Na+/Ca2+/K+ exchange fluxes by binding to cysteine residues on the cytosolic surface of the exchanger protein.

Mechanisms of injury-induced calcium entry into peripheral nerve myelinated axons: in vitro anoxia and ouabain exposure

In the present investigation, electron probe X-ray microanalysis was used to characterize the effects of in vitro ouabain (2 mM) or anoxia on elemental composition (e.g. Na, K, Ca) and water content of rat peripheral (tibial) nerve myelinated axons and Schwann cells. Results showed that independent of axon size, both ouabain and anoxia markedly increased axoplasmic Na and decreased K concentrations. However, only anoxia was associated with significant elevation of axonal Ca content. Mitochondrial areas from ouabain- or anoxia-exposed fibers exhibited changes in element and water contents that were similar to axoplasmic alterations. Schwann cells and myelin displayed small increases in Na and substantial losses of K in response to ouabain exposure. In contrast, these glial compartments were relatively resistant to anoxia as indicated by the modest and delayed nature of the elemental changes. Nonetheless, neither treatment significantly affected glial Ca concentrations. Our results suggest that Ca2+ accumulation in peripheral nerve axons is complex and involves not only deregulation of Na+ and K+ but other fundamental pathogenic changes as well. In addition to providing baseline information, we have identified an in vitro model (anoxia) which features Ca2+ build-up in PNS myelinated axons. Thus, the present study offers a foundation for investigation into mechanisms of Ca2+ entry following peripheral nerve injury.

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.

Rods, cones and calcium

A transduction cascade in the outer segments of vertebrate photoreceptors amplifies the visual signal, resulting in the metabolism of cGMP and the closure of ionic channels. The intracellular calcium concentration declines after a light response, and this decline is the key regulator responsible for controlling the gain of the transduction cascade. Calcium turnover in the outer segment is determined by three processes: influx through light-sensitive channels; buffering within the outer segment; and extrusion by a Na/Ca,K exchange mechanism.

Control of photoreceptor proteins by Ca2+

A decrease of cytoplasmic Ca(2+)-concentration in vertebrate photoreceptor cells after illumination is necessary for light adaptation. Although the mechanisms of adaptation is not completely understood, several Ca(2+)-dependent cellular processes have been discovered. Some involve calcium-binding proteins like recoverin, guanylyl cyclase-activating protein and calmodulin, and their target proteins rhodopsin kinase, guanylyl cyclase, the cGMP-gated channel, and NO synthase. The activity of several enzymes or channels is directly controlled by Ca2+ and does not involve calcium-binding proteins. These proteins are pyrophosphatase, protein kinase C and the cGMP-gated channel.

Calcium homeostasis in guinea pig type-I vestibular hair cell: possible involvement of an Na(+)-Ca2+ exchanger

In type-I vestibular hair cells (VHCs), the mechanisms involved in intracellular calcium homeostasis have not yet been established. In order to investigate the involvement of an Na(+)-dependent ionic exchanger in the regulation of cytosolic free calcium concentration, we analyzed the effect of the removal of external sodium on the cytosolic concentration of calcium ions ([Ca2+]i), sodium ions ([Na+]i), and protons (pHi). These concentrations were measured in type-I VHCs isolated from guinea pig labyrinth, using Fura-2, sodium benzofuran isophtalate (SBFI), and 1,4 diacetoxy-2,3 dicyanobenzol (ADB) respectively. Complete replacement of Na+ in the superfusion solution with N-methyl-D-glucamine (NMDG+), reversibly increased [Ca2+]i by 276 +/- 89% (n = 46) and decreased [Na+]i by 23 +/- 6% (n = 14). Both responses were prevented by removing external Ca2+ or chelating internal Ca2+. This suggests the presence of coupled Ca2+ and Na+ transport. The [Ca2+]i increase evoked by Na(+)-free solution was reduced by about 55% with the application of amiloride derivatives and was totally abolished in the presence of high [Mg2+]o. No pHi variation was detected during [Na+]o reduction. In the absence of external K+, the Na(+)-free solution failed to induce [Ca2+]i increase; the readmission of external K+ restored the [Ca2+]i response. These results are consistent with a Na(+)-Ca2+ exchanger operating in reverse mode. An K+ dependence of this exchange is also suggested.

Effect of blocking the Na+/K+ ATPase on Ca2+ extrusion and light adaptation in mammalian retinal rods

Membrane current and light response were recorded from rods of monkey and guinea pig by means of suction electrodes. The correlation between adaptation and the Na+/K+ pump was investigated by measuring light-dependent changes in sensitivity with and without inhibition of Na+/K+ ATPase by strophanthidin. Strophanthidin was found to reduce the dark current, to slow the time course of the photoresponse, and to increase light sensitivity. At concentrations between 20 and 500 nM, the pump inhibitor suppressed in a reversible way the current re-activation occurring during prolonged illumination and modified the light-dependent decrease in sensitivity, which in control conditions approximates to a Weber-Fechner function. The effects of the pump inhibitor on the adaptive properties of rods are associated with an increased time constant of the membrane current attributed to the operation of the Na+:Ca2+,K+ exchanger. The effects of rapid application of the pump inhibitor on the current re-activation are consistent with the idea that significant changes in the internal sodium occur in rods of mammals during background illumination and that they play an important role in the process of light adaptation.

Transport of K+ by Na(+)-Ca2+, K+ exchanger in isolated rods of lizard retina

Transport of K+ by the photoreceptor Na(+)-Ca2+, K+ exchanger was investigated in isolated rod outer segments (OS) by recording membrane current under whole-cell voltage-clamp conditions. Known amounts of K+ were imported in the OS through the Ca(2+)-activated K+ channels while perfusing with high extracellular concentration of K+, [K+]o. These channels were detected in the recordings from the OS, which probably retained a small portion of the rest of the cell. The activation of forward exchange (Na+ imported per Ca2+ and K+ extruded) by intracellular K+, Ki+, was described by first-order kinetics with a Michaelis constant, Kapp(Ki+), of about 2 mM and a maximal current, Imax, of about -60 pA. [Na+]i larger than 100 mM had little effect on Kapp(Ki+) and Imax, indicating that Nai+ did not compete with Ki+ for exchange sites under physiological conditions, and that Na+ release at the exchanger intracellular side was not a rate-limiting step for the exchange process. Exchanger stoichiometry resulted in one K+ ion extruded per one positive charge imported. Exchange current was detected only if Ca2+ and K+ were present on the same membrane side, and Na+ was simultaneously present on the opposite side. Nonelectrogenic modes of ion exchange were tested taking advantage of the hindered diffusion found for Cai2+ and Ki+. Experiments were carried out so that the occurrence of a putative nonelectrogenic ion exchange, supposedly induced by the preapplication of certain extracellular ion(s), would have resulted in the transient presence of both Cai2+ and Ki+. The lack of electrogenic forward exchange in a subsequent switch to high Nao+, excluded the presence of previous nonelectrogenic transport.

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+.

The Na(+)-Ca2+ exchange system in rat glial cells in culture: activation by external monovalent cations

Cultured rat glial cells display a Na(+)-Ca2+ exchange system located at the plasma membrane levels. This was evidenced by the Na+ (i)-dependency of a Na+ (o)-inhibitable influx of Ca2+, or reversal exchange mode. This antiporter has an external site where monovalent cations (K+, Li+, and Na+ were investigated) stimulate the exchange by a chemical action. The monovalent cation is not transported during the exchange cycle. The mechanism of that stimulation agrees with an increase in the apparent affinity of the carrier for Ca2+(o) without effect on the maximal translocation rate. Two models can equally well account for the data: i) the formation of ECa(o) is essential for the binding of the monovalent cation, or ii) the activating cation can bind even when the carrier is free of Ca2+(o). The cations K+ and Li+ produced only stimulation, although that of K+ seem to require actions other than the chemical effect. The response to Na+ was biphasic; this can be fully explained considering that at low concentrations, Na+(o) binds preferentially to the activating monovalent site while at high concentrations it displaces Ca2+ from its external transporting site. Pure type I astrocytes displayed the same Na(+)-Ca2+ exchange mechanism.

Kinetic properties of the sodium-calcium exchanger in rat brain synaptosomes

1. The kinetic properties of the internal Na+ (Na+i)- dependent 45Ca2+ influx and external Na+ (Na+o)-dependent 45Ca2+ efflux were determined in isolated rat brain nerve terminals (synaptosomes) under conditions which the concentrations of internal Na+ ([Na+]i), external Na+ ([Na+]o), external Ca2+ (Ca2+]o), and external K+ ([K+]o) were varied. Both fluxes are manifestations of Na(+)-Ca2+ exchange. 2. Ca2+ uptake was augmented by raising [Na+]i and / or lowering [Na+]o. The increase in Ca2+ uptake induced by removing external Na+ was, in most instances, quantitatively equal to the Na+i-dependent Ca2+ uptake. 3. The Na+i-dependent Ca2+ uptake (measured at 1 s) was activated with an apparent half-maximal [Ca2+]o (KCa(o)) of about 0.23 mM. External Na+ inhibited the uptake in a non- competitive manner: increasing [Na+]o from 4.7 to 96 mM reduced the maximal Na+(i)-dependent Ca2+ uptake but did not affect KCa(o). 4. The inhibition of Ca2+ uptake by Na+o was proportional to ([Na+]o)2, and had a Hill coefficient (nH) of approximately 2.0. The mean apparent half-maximal [Na+]o for inhibition (KI(Na)) was about 60mM, and was independent of [Ca2+]o between 0.1 and 1.2mM; this, too, is indicative of non-competitive inhibition. 5. Low concentrations of alkali metal ions (M+) in the medium, including Na+, stimulated the Na+i-dependent uptake. The external Na+ and K+ concentrations required for apparent half-maximal activation (KM(Na) and KM(K), respectively) were 0.12 and 0.10mM. Thus, the relationship between Ca2+ uptake and [Na+]o was biphasic: uptake was stimulated by [Na+]o < or = 10 mM, and inhibited by higher [Na+]o. 6. The calculated maximal Na+i-dependent Ca2+ uptake (Jmax) was about 1530 pmol (mg protein) -1s-1 at 30 degrees C saturating [Ca2+]o and external M+ concentration ([M+]o), and with negligible inhibition by external Na+. 7. Internal Na+ activated the Ca2+ uptake with an apparent half-maximal concentration (KNa(i)) of about 20 mM and a Hill coefficient, nH, of approximately 3.0. 8. The Jmax for the Na+o-dependent efflux of Ca2+ from 45Ca(2+)-loaded synaptosomes treated with carbonyl cyanide p-trifluormethoxy-phenylhydrazone (FCCP) and caffeine (to release stored Ca2+ and raise the internal Ca2+ concentration ([Ca2+]i) was about 1800-2000 pmol (mg protein -1s-1 at 37 degrees C. 9. When the membrane potential (Vm) was reduced (depolarized) by increasing [K+]o, the Na+i-dependent Ca2+ influx increased, and the Na+o-dependent Ca2+ efflux declined. Both fluxes changed about 2-fold per 60 mV change in Vm. This voltage sensitivity corresponds to the movement of one elementary charge through about 60% of the membrane electric field. The symmetry suggests that the voltage-sensitive step is reversible. 10. The Jmax values for both Ca2P influx and efflux correspond to a Na+-Ca2+ exchange-mediated flux of about 425-575 jumol Ca2P (1 cell water)-' s-' or a turnover of about one quarter of the total synaptosome Ca2P in 1 s. We conclude that the Na+-Ca2P exchanger may contribute to Ca2P entry during nerve terminal depolarization; it is likely to be a major mechanism mediating Ca2P extrusion during subsequent repolarization and recovery.

Intracellular calcium and its sodium-independent regulation in voltage-clamped snail neurones

1. We have used both Ca(2+)-sensitive microelectrodes and fura-2 to measure the intracellular free calcium ion concentration ([Ca2+]i or its negative log, pCai) of snail neurones voltage clamped to -50 or -60 mV. Using Ca(2+)-sensitive microelectrodes, [Ca2+]i was found to be approximately 174 nM and pCai, 6.76 +/- 0.09 (mean +/- S.E.M.; n = 11); using fura-2, [Ca2+]i was approximately 40 nM and pCai, 7.44 +/- 0.06 (mean +/- S.E.M., n = 10). 2. Depolarizations (1-20 s) caused an increase in [Ca2+]i which was abolished by removal of extracellular Ca2+, indicating that the rise in [Ca2+]i was due to Ca2+ influx through voltage-activated Ca2+ channels. 3. Caffeine (10-20 mM) caused an increase in [Ca2+]i in the presence or absence of extracellular Ca2+. The effects of caffeine on [Ca2+]i could be prevented by ryanodine. 4. Thapsigargin, an inhibitor of the endoplasmic reticulum Ca(2+)-ATPase, caused a small increase in resting [Ca2+]i and slowed the rate of recovery from Ca2+ loads following 20 s depolarizations. 5. Neither replacement of extracellular sodium with N-methyl-D-glucamine (NMDG), nor loading the cells with intracellular sodium, had any effect on resting [Ca2+]i or the rate of recovery of [Ca2+]i following depolarizations. 6. The mitochondrial uncoupling agent carbonyl cyanide m-chlorophenylhydrazone (CCmP) caused a small gradual rise in resting [Ca2+]i. Removal of extracellular sodium during exposure to CCmP had no further effect on [Ca2+]i. 7. Intracellular orthovanadate caused an increase in resting [Ca2+]i and prevented the full recovery of [Ca2+]i following small Ca2+ loads, but removal of extracellular sodium did not cause a rise in [Ca2+]i. We conclude that there is no Na(+)-Ca2+ exchanger present in the cell body of these neurones and that [Ca2+]i is maintained by an ATP-dependent Ca2+ pump.

Ca2+ modulation of the cGMP-gated channel of bullfrog retinal rod photoreceptors

1. The outer segment of an isolated rod photoreceptor from the bullfrog retina was drawn into a pipette containing choline solution for recording membrane current. The rest of the cell was sheared off with a glass probe to allow internal dialysis of the outer segment with a bath potassium solution ('truncated rod outer segment' preparation). The potential between the inside and the outside of the pipette was held at 0 mV. 2. Application of bath cGMP, in the presence of 3-isobutyl-1-methylxanthine (IBMX), gave rise to an outward membrane current. At saturating cGMP concentrations, this current was insensitive to intracellular Ca2+ at concentrations between 0 and 10 microM. At subsaturating cGMP concentrations, however, this current was inhibited by intracellular Ca2+. This sensitivity to Ca2+ declined after dialysis with a low-Ca2+ solution, suggesting the involvement of a soluble factor. 3. At low (nominally 0) Ca2+, the half-maximal activation constant and Hill coefficient for the activation of the cGMP-gated current by cGMP were 27 microM and 2.0, respectively. At high (ca 10 microM) Ca2+, the corresponding values were 40 microM cGMP and 2.4. 4. The inhibition of the current by Ca2+ was characterized at 20 microM cGMP. Ca2+ inhibited the current by up to 60%, with half-maximal inhibition at 48 nM Ca2+ and a Hill coefficient of 1.6.

Ca2+ regulation in the presynaptic terminals of goldfish retinal bipolar cells

1. To investigate regulation of the intracellular free Ca2+ concentration ([Ca2+]i) in presynaptic terminals, the Ca2+ current (ICa) and [Ca2+]i in axon terminals were simultaneously monitored in acutely dissociated retinal bipolar cells under whole-cell voltage clamp. 2. The recovery phase of the Ca2+ transient, which was evoked by activation of ICa, became slower when the Na(+)-Ca2+ exchanger was suppressed by removing extracellular Na+. 3. Inhibition of the plasma membrane Ca2+ pump produced by raising extracellular pH to 8.4 increased the basal [Ca2+]i and caused incomplete recovery from the Ca2+ transient. These effects were not observed in orthovanadate-loaded bipolar cells. 4. The Ca2+ transient was not significantly affected by ryanodine, caffeine, thapsigargin, Ruthenium Red or FCCP. Internal Ca2+ stores may not participate in shaping the Ca2+ transient. 5. The ratio of the peak amplitude of the Ca2+ transient to the total amount of Ca2+ influx became smaller as the size of the Ca2+ influx increased. This action was not affected by blockage of Ca2+ transporters in the plasma membrane, or by reduction of the rate of Ca2+ influx. The peak amplitude of the Ca2+ transient seemed to be determined by Ca2+ buffering substances with a positive co-operativity.

Cyclic GMP diffusion coefficient in rod photoreceptor outer segments

Cyclic GMP (cGMP) is the intracellular messenger that mediates phototransduction in retinal rods. As photoisomerizations of rhodopsin molecules are local events, the longitudinal diffusion of cGMP in the rod outer segment should be a contributing factor to the response of the cell to light. We have employed the truncated rod outer segment preparation from bullfrog (Rana catesbeiana) and tiger salamander (Ambystoma tigrinum) to measure the cGMP diffusion coefficient. In this preparation, the distal portion of a rod outer segment was drawn into a suction pipette for measuring membrane current, and the rest of the cell was then sheared off with a glass probe, allowing bath cGMP to diffuse into the outer segment and activate the cGMP-gated channels on the surface membrane. Addition and removal of bath cGMP were fast enough to produce effectively step changes in cGMP concentration at the open end of the outer segment. When cGMP hydrolysis is inhibited by isobutylmethylxanthine (IBMX), the equation for the diffusion of cGMP inside the truncated rod outer segment has a simple analytical solution, which we have used to analyze the rise and decay kinetics of the cGMP-elicited currents. From these measurements we have obtained a cGMP diffusion coefficient of approximately 70 x 10(-8) cm2 s-1 for bullfrog rods and approximately 60 x 10(-8) cm2 s-1 for tiger salamander rods. These values are six to seven times lower than the expected value in aqueous solution. The estimated diffusion coefficient is the same at high (20-1000 microM) and low (5-10 microM) concentrations of cGMP, suggesting no significant effect from buffering over these concentration ranges.

Cultured rat astrocytes possess Na(+)-Ca2+ exchanger

Na(+)-Ca2+ exchange activity in its reverse mode was demonstrated in cultured rat astrocytes. Combination of ouabain (1 mM) and monensin (20 microM) caused a marked increase in 45Ca2+ uptake in astrocytes. 45Ca2+ uptake was also stimulated by lowering the external Na+ concentration. Ouabain plus monensin-stimulated 45Ca2+ uptake was blocked by 3,4-dichlorobenzamil (IC50, 16 microM), an inhibitor of Na(+)-Ca2+ exchanger, but not by nifedipine (0.1 microM). The stimulated-45Ca2+ uptake was observed even in K(+)-free medium, and external K+ at 5-10 mM caused a 2.2-fold increase in the uptake. Microspectrofluorimetry using the Ca(2+)-sensitive dye fura-2 showed that ouabain plus monensin increased intracellular Ca2+ concentration in single astrocytes. The Ca2+ signal was dependent on external Ca2+ (EC50, 1.4 mM), and blocked by 20 microM 3,4-dichlorobenzamil, but not by Ca2+ channel blockers (Cd2+, 20 microM; Ni2+, 100 microM). Antiserum of cardiac Na(+)-Ca2+ exchanger recognized 160 and 120-135 kDa proteins on SDS-polyacrylamide gel electrophoresis of astrocyte homogenate. Northern blot analysis revealed the presence of mRNA for the exchanger protein in astrocytes. These findings indicate that Na(+)-Ca2+ exchanger which is modulated by K+ is present in cultured rat astrocytes.

Free calcium concentrations in bullfrog rods determined in the presence of multiple forms of Fura-2

We employed the fluorescent calcium indicator Fura-2, loaded into intact retinas of the bullfrog Rana catesbeiana, to measure free calcium concentrations in the rod outer-segment cytosol. We determined that traditional methods of calculation yielded erroneous values of calcium. This error results from the presence of at least two distinct pools of Fura-2 in rod outer segments. Application of manganese quenches each pool, but quenching occurs at different rates. Using this fact, we show that the pools can be isolated by brief exposure to manganese and examined separately. One of these pools has the same fluorescent properties as the free salt of Fura-2 we use in our in vitro calibrations. The other source of fluorescence has more unusual properties. Although insensitive to calcium concentrations in the physiological range, it contributes significant anomalous fluorescence when cytosolic free calcium concentrations are elevated by application of IBMX. Nevertheless, the experimentally isolated, classic pool of Fura-2 is well behaved and allows us to calculate calcium concentrations relative to the Kd of Fura-2 by the usual ratio method. We show that when rods are exposed to saturating light, the free calcium concentration in their outer segments falls to a level not significantly different from zero within 20-30 s.

Photoreceptor light-adaptation mediated by S-modulin, a member of a possible regulatory protein family of protein phosphorylation in signal transduction

In vertebrate retinal photoreceptors, cytoplasmic [Ca2+] decreases upon exposure to light. A Ca(2+)-binding protein, S-modulin, detects this [Ca2+] decrease and reduces the light-sensitivity of the cell to induce light-adaptation. The reduction of the sensitivity is attained by disinhibition or facilitation of rhodopsin phosphorylation, a quenching mechanism of light-activated rhodopsin. S-modulin-like proteins are found in the brain as well. Several of these proteins show similar S-modulin effects, suggesting that these proteins also participate in the regulation of protein phosphorylation in the signal transduction in their host cells.

Voltage dependence of Na-Ca exchanger conformational currents

Properties of a transient current (Icont) believed to reflect a conformational change of the Na-Ca exchanger molecules after Ca2+ binding were investigated. Intracellular Ca2+ concentration jumps in isolated cardiac myocytes were generated with flash photolysis of caged Ca2+ dimethoxynitrophenamine, and membrane currents were simultaneously measured using the whole-cell variant of the patch-clamp technique. A previously unresolved shallow voltage dependence of Icont was revealed after developing an experimental protocol designed to compensate for the photoconsumption of the caged compound. This voltage dependence can be interpreted to reflect the distribution of Na-Ca exchanger conformational states with the Ca2+ binding site exposed to the inside of the cell immediately before the flash. Analysis performed by fitting a Boltzmann distribution to the observed data suggests that under control conditions most exchanger molecules reside in states with the Ca2+ binding site facing the outside of the cell. Dialysis of the cytosol with 3',4'-dichlorobenzamil, an organic inhibitor of the Na-Ca exchange, increased the magnitude of Icont and changed the voltage dependence, consistent with a parallel shift of the charge/voltage curve. This shift may result from intracellular DCB interfering with an Na(+)-binding or Na(+)-translocating step. These observations are consistent with Icont arising from a charge movement mediated by the Na-Ca exchanger molecules after binding of Ca2+.

Calcium pump in the disk membranes isolated from bovine retinal rod outer segments

The existence of a Ca2+ pump in rod outer segment disks of bovine retina is strongly suggested by the isolation on sodium dodecyl sulfate polyacrylamide gel electrophoresis of a hydroxylamine-sensitive phosphorylated intermediate (E-P) of molecular mass of about 100 kDa as well as by measurements of active calcium transport and adenosine 5'-triphosphate (ATP) hydrolysis. Active Ca2+ uptake by disks was dependent on the presence of Mg(2+)-ATP, was inhibited by vanadate or lanthanum and appeared poorly sensitive to calmodulin. ATP hydrolysis by disk membranes was a function of free Ca2+ concentration in the absence of exogenous Mg2+. The presence of a Ca2+ pump on disk membranes is discussed in terms of its possible role in Ca2+ ion buffering during photoreceptor cell functioning.

Purification and physiological evaluation of a guanylate cyclase activating protein from retinal rods

In retinal rods light triggers a cascade of enzymatic reactions that increases cGMP hydrolysis and generates an electrical signal by causing closure of cGMP-gated ion channels in the photoreceptor outer segment. This leads to a decrease in internal Ca, which activates guanylate cyclase and promotes photoresponse recovery by stimulating the resynthesis of cGMP. We report here that the activation of guanylate cyclase by low Ca is mediated by an approximately 20-kDa protein purified from bovine rod outer segments by using DEAE-Sepharose, hydroxylapatite, and reverse-phase chromatographies. In a reconstituted system, this protein restores the Ca-sensitive regulation of guanylate cyclase and when dialyzed into functionally intact lizard rod outer segment decreases the sensitivity, time to peak, and recovery time of the flash response.

The role of cytoplasmic calcium in photoreceptor light adaptation

The process of light adaptation in vertebrate rod and cone photoreceptors is believed to involve a diffusible cytoplasmic messenger. Two lines of evidence indicate that photoreceptor light adaptation is mediated by a light-induced fall in cytoplasmic calcium concentration (Ca2+i). First, if changes in calcium concentration are slowed by the incorporation of calcium chelators into the photoreceptor cytoplasm then light adaptation is slowed also. Second, if the normal control of Ca2+i is prevented by simultaneously minimising calcium influx and efflux across the outer segment membrane by means of external solution changes, then all of the manifestations of light adaptation are abolished. Furthermore, recent results show that changes in Ca2+i imposed in the absence of light are sufficient to cause at least some of the manifestations of light adaptation. Together these results indicate that calcium acts as the messenger of light adaptation in the photoreceptors of both lower and higher vertebrates.

Critical processes which characterize the photocurrent of retinal rod outer segments to flash stimuli

The chemical reactions of retinal rod outer segments (ROS) were modeled aimed at finding the critical process for the reconstruction of the photocurrent to flash stimuli. The differential equations, which were derived from the chemical reactions, were numerically integrated. According to the present model, it was found that the most critical process for the recovery of the photocurrent was the synthesis of cGMP by guanylate cyclase in the [Ca2+]i-dependent manner. The other recovery processes, such as rhodopsin phosphorylation, transducin and phosphodiesterase inactivation seemed not to be involved in the recovery of the photocurrent to flash stimuli. Finally, a recently proposed scheme in which transducin remained bound to phosphodiesterase after its activation was examined. The simulation for this scheme showed that the ROS sensitivity was greatly reduced because of the limited amplification in the transduction cascades.

Steady state feedback in mammalian phototransduction illustrated by a nomogram

Published data characterizing the four reactions responsible for the Ca(2+)-mediated negative feedback in mammalian rod phototransduction were used to generate graphs which are combined in a circular fashion so that the y-axis of one serves as the x-axis of the next. The nomogram thus created makes it possible to determine by inspection the steady state situation in darkness, and the quasi-steady state situations that pertain shortly after exposure to light of different intensities. The results predicted by the nomogram suggest that Ca(2+)-mediated negative feedback is responsible for the Weber-Fechner relationship between stimulus and response.

After transduction: response shaping and control of transmission by ion channels of the photoreceptor inner segments

Photoreceptors convert the elements of the visual image into the elements of a neural image. This process involves well-studied molecular events occurring at the outer segment, but also employs important molecular events in the proximal regions of the photoreceptor, including the synaptic terminal, encompassed here as the inner segment. Integral to neural processing at this level in the visual system, the inner segment mechanisms modify the visual signal before transmission to second order cells at the photoreceptor output synapse. This commentary, emphasizing the author's own work, discusses biophysical properties of the ensemble of ion channels in the photoreceptor inner segment that shape the light response and enable its transmission. Examples that illustrate ion channels whose biophysical properties seem well suited for their roles in photoreceptor function include: h channels, cation-selective channels activated by hyperpolarization, which carry current that counteracts the strong hyperpolarizing influence of cGMP-gated channel closure accompanying bright light; Kx channels, carrying potassium current which shares the kinetic properties of the M-current found in many other cell types, which shape responses to dim light and set the dark resting potential; and Ca channels that regulate calcium influx to control Ca-gated channel activity and synaptic output, "re-transducing" the neural signal now into a chemical one. The role of chloride current, carried in Ca-activated Cl channels dependent on the unknown chloride equilibrium potential in photoreceptors, is also discussed.

Sodium/calcium exchange regulates cytoplasmic calcium in smooth muscle

The sodium/calcium (Na+/Ca2+) exchanger is often considered to be a key regulator of the cytoplasmic calcium concentration ([Ca2+]) in smooth muscle but neither its precise role in Ca2+ homeostasis nor even its existence in smooth muscle are generally agreed upon. Here we directly assessed the role Na+/Ca2+ exchange plays in regulating [Ca2+] in single voltage-clamped smooth muscle cells. Following an elevation of [Ca2+], its decline was found to have both voltage-dependent and voltage-independent components. The voltage-dependent component was abolished when Na+ was removed from the external bathing solution. During the fall of [Ca2+] a small and declining Na(+)-dependent inward current was observed of a magnitude predicted by 3:1 Na+/Ca2+ exchange stoichiometry. At [Ca2+] above 400 nM the principal efflux of Ca2+ above rest was attributed to this Na(+)-dependent removal mechanism. These results establish that a Na+/Ca2+ exchanger exists in smooth muscle and argue that it can regulate [Ca2+] at physiological Ca2+ concentrations.

Calcium controls light-triggered formation of catalytically active rhodopsin

Background light reduces the gain of phototransduction in retinal rods so that the ability to register changes in light intensity is not prevented by saturation of the cell's response. The gain is reduced by a light-induced fall in the intracellular calcium concentration which results from blockage of Ca2+ entry through the channels closed by light and continued Ca2+ extrusion by the Na:Ca,K exchanger. Calcium seems to exert several coordinated effects on the cyclic GMP cascade: a fall in [Ca2+] stimulates cGMP synthesis, increases the affinity of the cGMP-gated channel for cGMP and accelerates rhodopsin deactivation by phosphorylation. We now report that lowering intracellular [Ca2+] reduces the catalytic rhodopsin activity produced by light. The effect is operationally equivalent to a fourfold reduction in the number of rhodopsin molecules available for activation. The reduction in gain is cooperative and half-maximal at about 35 nM Ca2+, suggesting that it is mediated by a specific Ca(2+)-binding protein. Reduced rhodopsin activity in low Ca2+ should contribute to adaptation in background light.

Na(+)-Ca2+,K+ exchange in bovine retinal rod outer segments: quantitative characterization of normal and reversed mode

Ca2+ homeostasis of bovine retinal rod outer segments is maintained through Na(+)-Ca2+,K+ exchangers and cGMP-gated channels in the plasma membrane. It has recently been demonstrated that both proteins are associated. This novel finding allowed us to investigate quantitatively normal and reversed mode Na(+)-Ca2+,K+ exchange in rod outer segment membrane vesicles and reconstituted proteoliposomes both containing exchangers in rightside-out and inside-out orientations. Addition of Na+ activated both normal and reversed mode exchange; if, however, initially Ca2+ from vesicles containing inside-out oriented exchangers has been released by activation of the associated channels, only normal mode exchange was observed upon addition of Na+. Using this approach, the fractions of vesicles containing rightside-out and inside-out oriented exchangers were about similar in these vesicle preparations. Normal and reversed mode exchange had similar Na+ concentrations of about 70 mM for half maximal activation (in the presence of 115 mM K+) and cooperativity parameters, nHill, of about 2.0. Furthermore, both modes were electrogenic, and showed only little Na(+)-Ca2+,K+ exchange in the absence of K+. The two modes of exchange differed, however, in the maximal exchange rate, the normal mode being about twice as fast as the reversed mode.

Na(+)-Ca2+ exchanger in arteries: identification by immunoblotting and immunofluorescence microscopy

Antibodies raised against dog cardiac Na(+)-Ca2+ exchanger were employed to determine the presence and distribution of the exchanger in arterial smooth muscle (ASM) cells. The antiserum cross-reacted with protein bands of approximately 70, 120, and 150-160 kDa from the membranes of ASM cells, as well as heart sarcolemma. A cardiac Na(+)-Ca2+ exchanger cDNA probe hybridized to 7-kilobase (kb) mRNA from myocytes of the mesenteric artery. Thus ASM cells possess a "cardiac type" Na(+)-Ca2+ exchanger. The relative amounts of 7-kb mRNA and antigen detected on Northern and Western blots, respectively, however, indicate that vascular myocytes contain much less of this transporter than do cardiac myocytes. Immunofluorescence studies on cultured arterial myocytes suggest that the exchanger molecules are organized in reticular patterns over the cell surfaces. A similar pattern is observed when cells are stained for sarcoplasmic reticulum (SR) Ca(2+)-ATPase. This raises the possibility that the exchanger in the plasmalemma of arterial myocytes may be associated, perhaps functionally as well as structurally, with the underlying SR. The antiserum also cross-reacted with endothelial cell membranes, but labeling was lighter and more diffuse than in the myocytes.

Is there a Na+/Ca2+ exchanger in macrophages and in lymphocytes?

In two blood cell types, peritoneal murine macrophages and Jurkat cells (a human T cell line), we have examined whether a Na+/Ca2+ exchange was present and what could be its functional importance. In non-stimulated macrophages, the intracellular Ca2+ concentration, [Ca2+]i, was unchanged when Li+ was substituted for external Na+. However, after stimulation by platelet-activating factor (PAF), the Ca2+ response was larger when the extracellular solution contained Li+ rather than Na+ ions. In stimulated macrophages, the rate of Ca2+ extrusion was smaller in a Li(+)- than in a Na(+)-containing medium. The net electrochemical gradient for ionic movements through the Na+/Ca2+ exchanger, during the course of the response of macrophages to PAF, was determined by combining the measurements of membrane potential (in patch-clamp), of [Ca2+]i (with fura-2), and of the intracellular Na+ concentration (with sodium-binding benzofuran isophthalate). These results show that macrophages possess a Na+/Ca2+ exchange that only functions as a Ca2+ extruder, and this only when [Ca2+]i has been increased, for instance following PAF stimulation. In T lymphocytes, before or after stimulation by an anti-CD3 antibody, no Na+/Ca2+ activity could be detected by measuring either [Ca2+]i, or the rate of Ca2+ extrusion. Even if a Na+/Ca2+ exchanger was present in these cells, its equilibrium potential would be such that it would not allow Ca2+ influx but only Ca2+ extrusion.

Activation of Na-Ca exchange current by photolysis of "caged calcium"

Intracellular photorelease of Ca2+ from "caged calcium" (DM-nitrophen) was used to investigate the Ca(2+)-activated currents in ventricular myocytes isolated from guinea pig hearts. The patch-clamp technique was applied in the whole-cell configuration to measure membrane current and to dialyze the cytosol with a pipette solution containing the caged compound. In the presence of inhibitors for Ca2+, K+, and Na+ channels, concentration jumps of [Ca2+]i induced a rapidly activating inward Na-Ca exchange current which then decayed slowly (tau approximately 500 ms). The initial peak of the inward current and the time-course of current decay were voltage-dependent, and no reversal of the current direction was found between -100 and +100 mV. The observed shallow voltage dependence can be described in terms of the movement of an apparently fractional elementary charge (+0.44e-) across an energy barrier located symmetrically in the electrical field of the membrane. The currents were dependent on extracellular Na+ with a half-maximal activation at 73 mM and a Hill coefficient of 2.8. No change of membrane conductance was activated by the Ca2+ concentration jump when extracellular Na+ was completely replaced by Li+ or N-methyl-D-glucamine (NMG) or when the Na-Ca exchange was inhibited by extracellular Ni2+, La3+, or dichlorobenzamil (DCB). The velocity of relengthening after a twitch induced by photorelease of Ca2+ was only reduced drastically when both the sarcoplasmic reticulum and the Na-Ca exchange were inhibited suggesting that all other Ca2+ removing mechanisms have a low transport capacity under these conditions. In conclusion, we have used a novel approach to study Na-Ca exchange activity with photolysis of "caged" calcium. We found that in guinea pig heart muscle cells the Na-Ca exchange is a potent mechanism for Ca2+ extrusion, is weakly voltage-dependent (118 mV for e-fold change) and can be studied without contamination with other Ca(2+)-activated currents.

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.