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

Contrasting dynamics of water and mineral nutrients in stems shown by stable isotope tracers and cryo-SIMS

Lateral exchange of water and nutrients between xylem and surrounding tissues helps to de-couple uptake from utilization in all parts of a plant. We studied the dynamics of these exchanges, using stable isotope tracers for water (H(2)(18)O), magnesium ((26)Mg), potassium ((41)K) and calcium ((44)Ca) delivered via a cut stem for various periods to the transpiration stream of bean shoots (Phaseolus vulgaris cv. Fardenlosa Shiny). Tracers were subsequently mapped in stem cross-sections with cryo-secondary ion mass spectrometry. The water tracer equilibrated within minutes across the entire cross-section. In contrast, the nutrient tracers showed a very heterogeneous exchange between xylem vessels and the different stem tissues, even after 4 h. Dynamics of nutrients in the tissues revealed a fast and extensive exchange of nutrients in the xylem parenchyma, with, for example, calcium being completely replaced by tracer in less than 5 min. Dilution of potassium tracer during its 30 s transit in xylem sap through the stem showed that potassium concentration was up-regulated over many hours, to the extent that some of it was probably supplied by phloem recirculation from the shoot.

Imaging nutrient distributions in plant tissue using time-of-flight secondary ion mass spectrometry and scanning electron microscopy

A new approach to trace the transport routes of macronutrients in plants at the level of cells and tissues and to measure their elemental distributions was developed for investigating the dynamics and structure-function relationships of transport processes. Stem samples from Phaseolus vulgaris were used as a test system. Shock freezing and cryo-preparation were combined in a cryogenic chain with cryo-time-of-flight secondary ion mass spectrometry (cryo-ToF-SIMS) for element and isotope-specific imaging. Cryo-scanning electron microscopy (cryo-SEM) was integrated into the cryogenic workflow to assess the quality of structural preservation. We evaluated the capability of these techniques to monitor transport pathways and processes in xylem and associated tissues using supplementary sodium (Na) and tracers for potassium (K), rubidium (Rb), and (41)K added to the transpiration stream. Cryo-ToF-SIMS imaging produced detailed mappings of water, K, calcium, magnesium, the K tracers, and Na without quantification. Lateral resolutions ranged from 10 microm in survey mappings and at high mass resolution to approximately 1 microm in high lateral resolution imaging in reduced areas and at lower mass resolution. The tracers Rb and (41)K, as well as Na, were imaged with high sensitivity in xylem vessels and surrounding tissues. The isotope signature of the stable isotope tracer was utilized for relative quantification of the (41)K tracer as a fraction of total K at the single pixel level. Cryo-SEM confirmed that tissue structures had been preserved with subcellular detail throughout all procedures. Overlays of cryo-ToF-SIMS images onto the corresponding SEM images allowed detailed correlation of nutrient images with subcellular structures.

Retina expresses a novel variant of the ryanodine receptor

Calcium released from intracellular stores via the ryanodine receptor (RyR) mediates a variety of signalling processes. We previously showed that retina expresses the three known types of RyR, but retinal membrane preparations exhibit unique characteristics such as Ca2+-independent [3H]ryanodine-binding and inhibition by caffeine. We have heretofore suggested that the major retinal RyR isoform is novel. The present study aimed to identify this receptor isoform and to localize RyR in mammalian retina. Immunoblotting with specific and pan-antibodies showed that the major retinal RyR has a mobility similar to that of RyR2 or RyR3. Real-time PCR revealed that the major type is RyR2, and RT-PCR followed by sequencing showed a transcript that encodes a protein with approximately 99% identity to RyR2, yet lacking two regions of seven and 12 amino acids and including an additional insertion of eight amino acids. An antibody against RyR2 localized this type to somas and primary dendrites of most retinal neurons. An antibody against RyR1 localized RyR to most somas but also revealed staining in photoreceptor outer segments, concentrated on the disk membranes at their rim. The ryanodine-binding properties and the electrophoretic mobility of RyR from the outer segments were similar to those of the whole retinal preparation. The results thus identify a novel variant of RyR2 which can contribute to regulating photoreceptor Ca2+ concentrations. The restricted localization of the outer segment RyR to the disk rim suggests that its activation mechanism involves a coupling between retinal RyR and the cGMP-gated channel.

Time course and magnitude of the calcium release induced by bright light in salamander rods

Changes in fluorescence were measured with an argon ion laser from the outer segments of isolated salamander rods containing the Ca(2+)-sensitive fluorescent dye fluo-5F. When the outer segments were exposed to a 0Ca(2+)/0Na(+) solution designed to minimise surface membrane Ca(2+) fluxes, exposure to intense light from the laser evoked a slow increase in fluorescence, reflecting a light-induced rise in outer segment [Ca(2+)](i). The time course of this slow fluorescence rise could be fitted with the sum of two asymptotic exponential functions of approximately equal amplitude, having time constants of approximately 200 ms and 5.7 s. When rods were exposed to saturating background light to reduce outer segment [Ca(2+)](i) before laser illumination, the relative amplitude of the two exponentials was altered so as to reduce the contribution from the one with the shorter time constant. Examination of the initial time course of fluorescence when recording at high temporal resolution revealed a further rapid rise with a time constant of 1-2 ms, which could be observed even from rods in Ringer solution. This initial rapid rise could be abolished by pre-exposing the rod to bleaching illumination, whether the bleach was given in Ringer solution or in 0Ca(2+)/0Na(+) solution. It would therefore appear that the rapid rise in fluorescence is generated in some way by the bleaching of the photopigment. Unlike the slower components of fluorescence increase, the rapid initial rise was virtually unaffected in waveform or amplitude when rods were pre-exposed in Ringer solution to light which was bright enough to suppress completely the circulating current but which bleached a negligible fraction of the photopigment. Furthermore, pre-incubation with the AM ester of the Ca(2+) chelator BAPTA, although completely abolishing the slower components of fluorescence increase, had virtually no effect on the rapid rise. These results indicate that the rapid component, though triggered by rhodopsin bleaching, does not reflect an increase in outer segment [Ca(2+)](i). Neither the rapid nor the slower components of fluorescence increase were affected by exposure of the outer segment to 10 microM of the membrane-permeant compound N,N,N',N'-tetrakis(2-pyridyl-methyl)ethylenediamine (TPEN), which chelates heavy metals such as Zn(2+), or 100 microM 2-aminoethoxydiphenylborate (2-APB), a membrane-permeant blocker of IP(3) receptors. These results appear to exclude a role for changes in heavy metal concentration or Ca(2+) release via IP(3) receptors in the light-induced increases in dye fluorescence. Estimates of absolute Ca(2+) concentration and of rod buffering capacity suggest that the slower components of fluorescence increase represent the release of around 10-50 micromoles Ca(2+) per litre cytoplasmic volume from bound or sequestered stores after bleaching.

A light-dependent increase in free Ca2+ concentration in the salamander rod outer segment

1. The Ca(2+) indicator dye fluo-5F was excited by an argon ion laser to measure changes in free Ca(2+) concentration ([Ca2+]i) in the outer segments of isolated salamander rods rapidly exposed to a 0 Ca(2+), 0 Na(+) solution designed to minimise surface membrane Ca(2+) fluxes. Over 30-60 s of laser illumination, the fluorescence first increased rapidly and then declined at a rate that was much slower than in Ringer solution and consistent with previous physiological evidence that 0 Ca(2+), 0 Na(+) solution greatly retards light-induced changes in [Ca(2+)]i. 2. The initial increase in fluorescence was investigated with a sequence of 100 ms laser flashes presented at 5 s intervals. The fluorescence evoked by the second laser flash was on average 30 % larger than the first, and subsequent responses exhibited a slow decline like that measured with continuous laser exposures. The initial increase in fluorescence did not depend upon the timing of exposure to 0 Ca(2+), 0 Na(+) solution but appeared to be evoked by exposure to the laser light. 3. Both the increase and subsequent decline in fluorescence measured with brief laser flashes could be reduced by incorporation of the Ca(2+) chelator BAPTA. This and other results indicate that the fluorescence increase was unlikely to have been caused by a change in the affinity of fluo-5F for Ca(2+) or an increase in the quantity of incorporated dye available to bind Ca(2+) but reflects an actual release of intracellular Ca(2+) within the outer segment. 4. The pool of Ca(2+) available to be released could be decreased if, before the first laser flash, the rod was exposed to light bright enough to bleach a substantial fraction of the photopigment. The releasable pool could also be depleted by exposure to saturating light of much lower intensity if delivered in Ringer solution but not if delivered in 0 Ca(2+), 0 Na(+) solution. We conclude that Ca(2+) can be released within the outer segment both by the bleaching of rhodopsin and by the reduction in [Ca(2+)]i which normally accompanies illumination in Ringer solution. 5. The activation of rhodopsin appears somehow to induce the release of Ca(2+) from a binding site or store within the outer segment. Substantial release, however, required stimulating light of an intensity sufficient to bleach a considerable fraction of the visual pigment. It therefore seems unlikely that such release contributes to the normal Ca(2+)-mediated modulation of transduction during light adaptation. The mechanism and physiological function of light-induced Ca(2+) release are unknown.

Adaptation in vertebrate photoreceptors

When light is absorbed within the outer segment of a vertebrate photoreceptor, the conformation of the photopigment rhodopsin is altered to produce an activated photoproduct called metarhodopsin II or Rh(*). Rh(*) initiates a transduction cascade similar to that for metabotropic synaptic receptors and many hormones; the Rh(*) activates a heterotrimeric G protein, which in turn stimulates an effector enzyme, a cyclic nucleotide phosphodiesterase. The phosphodiesterase then hydrolyzes cGMP, and the decrease in the concentration of free cGMP reduces the probability of opening of channels in the outer segment plasma membrane, producing the electrical response of the cell. Photoreceptor transduction can be modulated by changes in the mean light level. This process, called light adaptation (or background adaptation), maintains the working range of the transduction cascade within a physiologically useful region of light intensities. There is increasing evidence that the second messenger responsible for the modulation of the transduction cascade during background adaptation is primarily, if not exclusively, Ca(2+), whose intracellular free concentration is decreased by illumination. The change in free Ca(2+) is believed to have a variety of effects on the transduction mechanism, including modulation of the rate of the guanylyl cyclase and rhodopsin kinase, alteration of the gain of the transduction cascade, and regulation of the affinity of the outer segment channels for cGMP. The sensitivity of the photoreceptor is also reduced by previous exposure to light bright enough to bleach a substantial fraction of the photopigment in the outer segment. This form of desensitization, called bleaching adaptation (the recovery from which is known as dark adaptation), seems largely to be due to an activation of the transduction cascade by some form of bleached pigment. The bleached pigment appears to activate the G protein transducin directly, although with a gain less than Rh(*). The resulting decrease in intracellular Ca(2+) then modulates the transduction cascade, by a mechanism very similar to the one responsible for altering sensitivity during background adaptation.

Light-dependent changes in outer segment free-Ca2+ concentration in salamander cone photoreceptors

Simultaneous measurements of photocurrent and outer segment Ca2+ were made from isolated salamander cone photoreceptors. While recording the photocurrent from the inner segment, which was drawn into a suction pipette, a laser spot confocal technique was employed to evoke fluorescence from the outer segment of a cone loaded with the Ca2+ indicator fluo-3. When a dark-adapted cone was exposed to the intense illumination of the laser, the circulating current was completely suppressed and fluo-3 fluorescence rapidly declined. In the more numerous red-sensitive cones this light-induced decay in fluo-3 fluorescence was best fitted as the sum of two decaying exponentials with time constants of 43 +/- 2.4 and 640 +/- 55 ms (mean +/- SEM, n = 25) and unequal amplitudes: the faster component was 1.7-fold larger than the slower. In blue-sensitive cones, the decay in fluorescence was slower, with time constants of 140 +/- 30 and 1,400 +/- 300 ms, and nearly equal amplitudes. Calibration of fluo-3 fluorescence in situ from red-sensitive cones allowed the calculation of the free-Ca2+ concentration, yielding values of 410 +/- 37 nM in the dark-adapted outer segment and 5.5 +/- 2.4 nM after saturating illumination (mean +/- SEM, n = 8). Photopigment bleaching by the laser resulted in a considerable reduction in light sensitivity and a maintained decrease in outer segment Ca2+ concentration. When the photopigment was regenerated by applying exogenous 11-cis-retinal, both the light sensitivity and fluo-3 fluorescence recovered rapidly to near dark-adapted levels. Regeneration of the photopigment allowed repeated measurements of fluo-3 fluorescence to be made from a single red-sensitive cone during adaptation to steady light over a range of intensities. These measurements demonstrated that the outer segment Ca2+ concentration declines in a graded manner during adaptation to background light, varying linearly with the magnitude of the circulating current.

A depolarizing chloride current contributes to chemoelectrical transduction in olfactory sensory neurons in situ

Recent biophysical investigations of vertebrate olfactory signal transduction have revealed that Ca2+-gated Cl- channels are activated during odorant detection in the chemosensory membrane of olfactory sensory neurons (OSNs). To understand the role of these channels in chemoelectrical signal transduction, it is necessary to know the Cl--equilibrium potential that determines direction and size of Cl- fluxes across the chemosensory membrane. We have measured Cl-, Na+, and K+ concentrations in ultrathin cryosections of rat olfactory epithelium, as well as relative element contents in isolated microsamples of olfactory mucus, using energy-dispersive x-ray microanalysis. Determination of the Cl- concentrations in dendritic knobs and olfactory mucus yielded an estimate of the Cl--equilibrium potential ECl in situ. With Cl- concentrations of 69 mM in dendritic knobs and 55 mM in olfactory mucus, we obtained an ECl value of +6 +/- 12 mV. This indicates that Ca2+-gated Cl- channels in olfactory cilia conduct inward currents in vivo carried by Cl- efflux into the mucus. Our results show that rat OSNs are among the few known types of neurons that maintain an elevated level of cytosolic Cl-. In these cells, activation of Cl- channels leads to depolarization of the membrane voltage and can induce electrical excitation. The depolarizing Cl- current in mammalian OSNs appears to contribute a major fraction to the receptor current and may sustain olfactory function in sweet-water animals.

Effect of different illumination conditions and ionic environment on the guanylate cyclase activity in retina, optic nerve and optic chiasm of the rat

Guanylate cyclase is sensitive to changes of light and dark periods in incubated extracts obtained from soluble fractions of the retina, optic nerve and optic chiasm. The changes in guanylate cyclase activity found, about 100-fold between dark and light periods in those tissues, indicate a key role for this enzyme. The results showed that light inhibits strongly the retinal guanylate cyclase activity, while it increases the activity of this enzyme in the optic nerve. A generalized photo-inhibited response of guanylate cyclase was observed in all studied tissues in animals adapted to the dark. This suggests that light could act as a double stimulus gating the central circuit which promotes the hydrolysis of cGMP via cGMP phosphodiesterase-rhodopsin-transducin cascade, and by direct inhibition of the retinal guanylate cyclase activity. Finally, different responses have been observed in the guanylate cyclase activity in relation with the ion exposure depending on the studied tissue. In summary, all indicate an important role for the soluble guanylate cyclase activity in retina, and other tissues involved in the visual process such as optic nerve and optic chiasm, which have not been examined until now.

betagamma-Transducin stimulates hydrolysis and synthesis of phosphatidylinositol 4,5-bisphosphate in bovine rod outer segment membranes

T betagamma was shown to stimulate the hydrolysis and synthesis of PtdInsP2 in dark-adapted bovine retinal rod outer segments. In contrast, T alphaGDP blocked the effect of betagamma-transducin. It was also demonstrated that T betagamma was a stimulator of 32P incorporation into PtdInsP2 in ROS. These findings explain the modulating actions of GTP and light on PtdInsP2 hydrolysis and synthesis in ROS. The possible existence of cross-talk between the cGMP and phosphoinositide cascades in retinal rods was discussed.

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.

Accumulation of calcium in degenerating photoreceptors of several Drosophila mutants

The hypothesis that a large, possibly toxic, increase in cellular calcium accompanies photoreceptor cell degeneration in several different Drosophila mutants was tested. The calcium content of wild type and mutant photoreceptors of Drosophila was measured using rapid freezing of the eyes and energy-dispersive x-ray analysis (e.d.x.) of cryosections and semithin sections of cryosubstituted material. Light- and dark-raised mutants of the following strains were studied: retinal degeneration B (rdgB); retinal degeneration C (rdgC); neither inactivation nor afterpotential C (ninaC), and no receptor potential A (norpA). These are light-dependent retinal degeneration mutants in which the affected gene products had been previously shown as myosin-kinase (ninaC), calcium-dependent phosphoprotein phosphatase (rdgC), phosphoinositide transfer protein (rdgB), and phospholipase C (norpA). In light-raised mutants, ommatidia of variable degrees of degeneration were observed. Mass-dense globular bodies of 200-500 nm diameter in relatively large quantities were found in the degenerating photoreceptor of all the mutants tested. These subcellular globules were found to have a very high calcium content, which was not found in wild type or in nondegenerating photoreceptors of the mutants. Nondegenerating photoreceptors were found not only in dark-raised mutants, but in smaller quantities also in light-raised mutants. Usually these globular structures contained high levels of phosphorus, indicating that at least part of the calcium in the mutant photoreceptors is precipitated as calcium phosphate. The results indicate that a large increase in cellular calcium accompanies light-induced photoreceptor degeneration in degenerating Drosophila mutants even when induced by very different mutations, suggesting that the calcium accumulation is a secondary rather than a primary effect in the degeneration process.

Calcium homeostasis in the outer segments of retinal rods from the tiger salamander

1. The processes regulating intracellular calcium in the outer segments of salamander rods have been investigated. The main preparation used was the isolated rod loaded with the Ca(2+)-sensitive photoprotein aequorin, from which outer segment membrane current and free [Ca2+]i could be recorded simultaneously. Two other preparations were also used: outer segment membrane current was recorded from intact, isolated rods using a suction pipette, and from detached outer segments using a whole-cell pipette. 2. Measurements of free intracellular [Ca2+] in Ringer solution were obtained from two aequorin-loaded rods. Mean [Ca2+]i in darkness was 0.41 microM, and after a bright flash [Ca2+]i fell to below detectable levels ( < 0.3 microM). No release of intracellular Ca2+ by a bright flash of light could be detected ( < 0.2 microM). 3. Application of the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX) caused an increase in the size of the light-sensitive current and a rise in [Ca2+]i, but application of IBMX either when the light-sensitive channels had been closed by a bright light or in the absence of external Ca2+ caused no detectable rise in [Ca2+]i. It is concluded that IBMX increases [Ca2+]i by opening light-sensitive channels, and does not release Ca2+ from stores within the outer segment. 4. Removal of external Na+ caused a rise in [Ca2+]i to around 2 microM and completely suppressed the light-sensitive current. 5. The Na(+)-Ca2+, K+ exchange current in aequorin-loaded rods was activated in first-order manner by internal free calcium, with a mean Michaelis constant, KCa, of 1.6 microM. 6. The KCa of the Na(+)-Ca2+, K+ exchange was increased by elevating internal [Na+]. 7. The Michaelis relation between [Ca2+]i and the activity of the Na(+)-Ca2+, K+ exchange was used to calculate the change in [Ca2+]i occurring during the response to a bright light. In aequorin-loaded rods in Ringer solution the mean change in free [Ca2+]i after a bright flash was 0.34 microM. In these rods 10% of the dark current was carried by Ca2+. 8. Most of the calcium entering the outer segment was taken up rapidly and reversibly by buffer systems. The time constant of equilibration between free and rapidly bound Ca2+ was less than 20 ms. No slow component of calcium uptake was detected. 9. Two components of calcium buffering could be distinguished in the outer segments of aequorin-loaded rods.(ABSTRACT TRUNCATED AT 400 WORDS)

Phototransduction: different mechanisms in vertebrates and invertebrates

The photoreceptor cells of invertebrate animals differ from those of vertebrates in morphology and physiology. Our present knowledge of the different structures and transduction mechanisms of the two animal groups is described. In invertebrates, rhodopsin is converted by light into a meta-rhodopsin which is thermally stable and is usually re-isomerized by light. In contrast, photoisomerization in vertebrates leads to dissociation of the chromophore from opsin, and a metabolic process is necessary to regenerate rhodopsin. The electrical signals of visual excitation have opposite character in vertebrates and invertebrates: the vertebrate photoreceptor cell is hyperpolarized because of a decrease in conductance and invertebrate photoreceptors are depolarized owing to an increase in conductance. Single-photon-evoked excitatory events, which are believed to be a result of concerted action (the opening in invertebrates and the closing in vertebrates) of many light-modulated cation channels, are very different in terms of size and time course of photoreceptors for invertebrates and vertebrates. In invertebrates, the single-photon events (bumps) produced under identical conditions vary greatly in delay (latency), time course and size. The multiphoton response to brighter stimuli is several times as long as a response evoked by a single photon. The single-photon response of vertebrates has a standard size, a standard latency and a standard time course, all three parameters showing relatively small variations. Responses to flashes containing several photons have a shape and time scale that are similar to the single-photon-evoked events, varying only by an amplitude scaling factor, but not in latency and time course. In both vertebrate and invertebrate photoreceptors the single-photon-evoked events become smaller (in size) and faster owing to light adaptation. Calcium is mainly involved in these adaptation phenomena. All light adaptation in vertebrates is primarily, or perhaps exclusively, attributable to calcium feedback. In invertebrates, cyclic AMP (cAMP) is apparently another controller of sensitivity in dark adaptation. The interaction of photoexcited rhodopsin with a G-protein is similar in both vertebrate and invertebrate photoreceptors. However, these G-proteins activate different photoreceptor enzymes (phosphodiesterases): phospholipase C in invertebrates and cGMP phosphodiesterase in vertebrates. In the photoreceptors of vertebrates light leads to a rapid hydrolysis of cGMP which results in closing of cation channels. At present, the identity of the internal terminal messenger in invertebrate photoreceptors is still unsolved.(ABSTRACT TRUNCATED AT 400 WORDS)

Identification of the sodium-calcium exchanger as the major ricin-binding glycoprotein of bovine rod outer segments and its localization to the plasma membrane

After neuraminidase treatment the Na+/Ca2+ exchanger of bovine rod outer segments was found to specifically bind Ricinus communis agglutinin. SDS gel electrophoresis and Western blotting of ricin-binding proteins purified from rod outer segment membranes by lectin affinity chromatography revealed the existence of two major polypeptides of Mr 215K and 103K, the former of which was found to specifically react with PMe 1B3, a monoclonal antibody specific for the 230-kDa non-neuraminidase-treated Na+/Ca2+ exchanger. Reconstitution of the ricin affinity-purified exchanger into calcium-containing liposomes revealed that neuraminidase treatment had no significant effect on the kinetics of Na+/Ca2+ exchange activation by sodium. We further investigated the density of the Na+/Ca2+ exchanger in disk and plasma membrane preparations using Western blotting, radioimmunoassays, immunoelectron microscopy, and reconstitution procedures. The results indicate that the Na+/Ca2+ exchanger is localized in the rod photoreceptor plasma membrane and is absent or present in extremely low concentrations in disk membranes, as we have previously shown to be the case for the cGMP-gated cation channel. Previous reports describing the existence of Na+/Ca2+ exchange activity in rod outer segment disk membrane preparations may be due to the fusion of plasma membrane components and/or the presence of contaminating plasma membrane vesicles.

Transbilayer coupling mechanism for the formation of lipid asymmetry in biological membranes. Application to the photoreceptor disc membrane

An equilibrium transmembrane asymmetry in charged lipids is shown to arise as a result of oriented, bipolar proteins in the membrane. The basic interaction giving rise to the asymmetry is between a lipid molecule and a transbilayer potential generated by the asymmetric charge distribution in the protein. Thus, a protein can generate a lipid asymmetry without a direct binding interaction between lipid and protein. The generation of an asymmetry in charged lipid by this mechanism can also lead to a concomitant asymmetry in neutral lipids if deviations from ideality in the lipid mixture are taken into account. It is shown that regular solution theory applied to the lipid phase predicts an asymmetry in all components of a ternary mixture as long as one component is electrostatically oriented according to the mechanism mentioned above. The resulting asymmetry is not strongly salt dependent. The mechanism quantitatively accounts for the experimentally determined phospholipid asymmetry in the rod outer segment disc membrane of the vertebrate photoreceptor.

A model for open-close control of cation channels in the plasma membrane of retinal rod outer segments

A model for open-close control of cation channels in the plasma membrane of retinal rod outer segments is presented. A channel is assumed to open when 3 cGMP molecules bind to it and close as soon as one of the 3 cGMP molecules is released from it. The calcium ion (divalent cation) is a modulator of the channel conductance. The channel conductance is low when Ca2+ binds to it, while it is high when it is free from Ca2+. From the above assumptions, the reaction scheme of channels with cGMP and Ca2+ is created and the fraction of channels in the open and closed states was calculated using equations for this scheme. The kinetic constants used in the model are estimated from the experimental results of many studies and from the theories. From this estimation, it was found that at the physiological concentrations of intracellular and extracellular Ca2+, almost all channels are bound with Ca2+ and are in the low conductance state. The present model accounts for the reported dose(cGMP)-response(membrane current or conductance) relationship, where the Hill coefficient decreases as the cGMP concentration increases. The dark-level cGMP concentration of 8.13 microM is estimated from the model. This is in good agreement with the reported values. Moreover, the model predicts the invariance of current noise at relatively low Ca2+ concentrations when the cGMP concentration is raised from the dark level to a saturation level. The dynamic properties (opening and closing actions) of the channels in the present model are also in good agreement with the reported observations. The burst mode opening and closing of a channel is predicted by the present model, and it was found that the number of openings in a burst is controlled by the forward and backward rate constants between a channel protein and cGMP molecules. The simulated waveform of a single channel is similar to the reported observations.

Electron microscopic cytochemical localization of Ca-ATPase in the rod outer segments of the toad Bufo marinus

Transmission electron microscopy and ultracytochemistry were employed in an attempt to localize the enzyme calcium adenosine triphosphatase (Ca-ATPase) in the rod outer segments (ROS) of the toad retina. Utilizing a one-step incubation procedure, Ca-ATPase was identified as an electron-dense precipitate in the intradiskal spaces of the rod disks (vesicles) of the ROS. Analytical microscopy identified the reaction product as lead phosphate. The formation of the reaction product was dependent on the presence of ATP (the substrate) and calcium ions. However, calcium ions could be substituted for by magnesium ions. In addition, the reaction was vanadate sensitive. The latter is known to inhibit Ca-ATPase activity. Such data appear to indicate the presence of a Ca-Mg-ATPase in association with the rod disks. Since cyclic guanosine monophosphate (cyclic GMP), rather than calcium ions, is currently believed to be the primary intracellular messenger associated with phototransduction, the presence of an ROS Ca-ATPase may indicate other functions for this cation in the physiology and biochemistry of the visual process. Ca-ATPase might play a role in directional calcium fluxes between intracellular compartments.

Evidence for two functionally different membrane fractions in bovine retinal rod outer segments

1. Cyclic GMP-induced and Na+-induced Ca2+ releases from Ca2+-containing photoreceptor membrane vesicles were investigated using the Ca2+-sensitive dye Arsenazo III. Bovine photoreceptor membrane vesicles were prepared by osmotic lysis and hypotonic wash of purified rod outer segments. 2. Calcium was entrapped into these vesicles either by utilizing their passive membrane permeability ('passive' Ca2+ loading), or by activating cyclic GMP-dependent channels (cyclic GMP-stimulated Ca2+ loading), or by Na+-Ca2+ exchange (Na+-stimulated Ca2+ loading). 3. After passive Ca2+ loading, cyclic GMP released at most about 23% of the total Ca2+ which could be released with the Ca2+ ionophore A23187 (Km = 6.5 +/- 0.92 microM (6); Hill coefficient 1.68 +/- 0.19 (6]. 4. Millimolar concentrations of Na+ also induce Ca2+ releases from passively Ca2+-loaded outer segment membrane vesicles of at most 26% of the total releasable Ca2+. 5. For passively Ca2+-loaded outer segment membrane vesicles, the sum of a partial cyclic GMP-induced Ca2+ release and a consecutive saturating Na+-induced Ca2+ release, or vice versa, the sum of a partial Na+-induced Ca2+ release and a consecutive saturating cyclic GMP-induced Ca2+ release, was about 25% of the total releasable Ca2+. 6. Both cyclic GMP-induced and Na+-induced Ca2+ release was greater than 25%, up to 50% of the total releasable Ca2+ if either cyclic GMP-stimulated or Na+-stimulated Ca2+ loading of outer segment membrane vesicles was employed. 7. When the outer segment membrane vesicles were mildly sonicated in a water bath, the maximal percentages of Ca2+ releasable by both cyclic GMP and Na+ dropped, reaching final values, after a 60 s sonication period, of 2.3% for the cyclic GMP-induced Ca2+ release and 7% for the Na+-induced Ca2+ release. 8. It is concluded from these results that outer segment membrane vesicles comprise two populations of vesicles, one which contains cyclic GMP-dependent channels and Na+-Ca2+ exchanging proteins, and another one which contains neither of these proteins. 9. The sonication experiments are interpreted as suggesting that these two vesicle populations correspond to plasma and disc membranes of the rod outer segments, respectively.

Calcium and magnesium fluxes across the plasma membrane of the toad rod outer segment

1. Membrane current was recorded from an isolated, dark-adapted toad rod by sucking either its inner segment or outer segment into a tight-fitting glass pipette containing Ringer solution. The remainder of the cell was exposed to bath solution which could be changed rapidly. 2. In normal Ringer solution the current response of a cell to a saturating flash or step of light showed a small secondary rise at its initial peak. The profile of this secondary rise (i.e. amplitude and time course) was independent of both the intensity and the duration of illumination once the light response had reached a plateau level. 3. This secondary rise disappeared when external Na+ around the outer segment was replaced by Li+ or guanidinium, suggesting that it represented an electrogenic Na+-dependent Ca2+ efflux which was declining after the onset of light. 4. This Na+-Ca2+ exchange activity showed a roughly exponential decline, with a time constant of about 0.5 s. Exponential extrapolation of the exchange current to the time at half-height of the light response gave an initial amplitude of about 2 pA. Using La3+ as a blocker, we did not detect any steady exchange current after the initial exponential decline. 5. An intense flash superposed on a just-saturating steady background light failed to produce any incremental exchange current transient. 6. Our interpretation of the above results is that in darkness there are counterbalancing levels of Ca2+ influx (through the light-sensitive conductance) and efflux (through the Na+-Ca2+ exchange) across the plasma membrane of the rod outer segment. The exchange current transient at the onset of light merely represents the unidirectional Ca2+ efflux which becomes revealed as a result of the stoppage of the Ca2+ influx, rather than a de novo Ca2+ efflux triggered by light. 7. Consistent with this interpretation, a test light delivered soon after a saturating, conditioning light elicited little exchange current, which then gradually recovered to control value with a time course parallel to the restoration of the dark current. Conversely, when the dark current was increased above its physiological level by IBMX (isobutylmethylxanthine) the exchange current transient became larger than control.(ABSTRACT TRUNCATED AT 400 WORDS)

Mitochondrial hydroxyapatite deposits in spinocerebellar degeneration

We report the presence of crystalline deposits of calcium hydroxyapatite in the mitochondria of 2 children with sporadic spinocerebellar degeneration. The deposits, identified by electron microscopy, were found in the mitochondria of neurons and smooth muscle cells in one patient and in only smooth muscle cells in the second child, but not in other cell types. The calcific nature of the deposits was confirmed by laser microprobe mass analysis. The calcium overload may interfere with mitochondrial function, as has been shown in the cardiomyopathic strain of the Syrian hamster, a model of the cardiomyopathy of Friedreich's ataxia.

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)

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.

The opsin family of proteins

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Interplay between calcium and activated cGMP phosphodiesterase from retinal rod outer segments

Hypotonic extraction of bovine retinal rod outer segments after bleaching in isotonic buffer yielded an extract exhibiting activated cGMP phosphodiesterase properties. Since this extract was virtually devoid of other proteins involved in the rod outer segment cGMP enzymatic cascade, it was used to study phosphodiesterase catalytic activity. The hypotonic extract required Mg2+ in the range 0.1-1.0 mM for optimal cGMP hydrolysis. At these Mg2+ concentrations hydrolysis could be effectively inhibited by Ca2+ at concentrations which might be attainable in rod outer segments. Since higher Ca2+ concentrations were required to give a chosen degree of inhibition at higher Mg2+ concentrations, this inhibition was probably due to competition by Ca2+ for Mg2+ binding site(s) on the phosphodiesterase catalytic unit. Other divalent cations were also able to inhibit cGMP hydrolysis, many of them (especially those with ionic radii close to that of magnesium) more effectively than calcium. It is suggested that Ca2+ may play a role in phototransduction by participating in the control of photoreceptor sensitivity, and that this is achieved by modulating rod outer segment cGMP hydrolysis.

Coupling between transmembrane calcium transport and membrane potential in retinal rod discs

Changes in the transmembrane potential of bovine rod discs were studied by use of the potential-sensitive fluorescence probes diS-C3-(5) and diBA-C4-(5). The disc membrane was shown to be impermeable to potassium ions. Their concentration in the disc is as high as 2.1 +/- 0.3 mM. The permeability of the disc membrane to Ca2+ was shown to be selective. The accumulation and release of Ca2+ were found to be accompanied by the generation of inside positive and inside negative transmembrane potentials, respectively. The uptake of Ca2+ in the discs may operate against the concentration gradient of the ion. The value of the potential developed is directly proportional to the logarithm of free Ca2+ concentration in the medium (delta phi m = 11.2 +/- 1.6 mV at 4.85 microM Ca2+fr). The accumulation of Ca2+ is decreased by sodium ions and totally inhibited by monensin. This indicates that a Na-Ca exchange process participates in Ca2+ uptake of photoreceptor discs.

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.

Incorporation of calcium buffers into salamander retinal rods: a rejection of the calcium hypothesis of phototransduction

The suction-electrode technique was used to monitor the photocurrent of isolated retinal rods from the tiger salamander, by drawing in the light-sensitive outer segment, or sometimes the inner segment. Calcium buffers or other agents were then introduced into the rod cytoplasm by the 'whole-cell patch-clamp' technique. A patch pipette was sealed against the region of the rod protruding from the suction pipette (usually the inner segment), and the membrane patch was ruptured to obtain a whole-cell recording. Several lines of evidence indicated that the pipette contents diffused into the outer segment, and showed that the cell could be adequately voltage clamped. With only trace quantities of chelator in the patch pipette (to bind stray calcium), a gradual decline of the dark current and slowing of responses was usually observed over a period of 10-20 min after rupture of the patch. When the patch pipette contained no added calcium and 10 mM of the calcium chelator BAPTA (1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid) (free Ca2+ ca. 10(-9) M) rupture of the patch led, over a period of a few minutes, to an increase in mean dark current, an increased duration of responses, a substantial increase in flash sensitivity, and a pronounced overshoot in the recovery phase, but with virtually no change in the rising phase of the response to bright flashes. Similar results were obtained when EGTA was used in place of BAPTA, and also in the few cases when successful rupture of the outer segment membrane was obtained. With the free calcium concentration in the patch pipette buffered to the higher level of 1 microM (with 10 mM-Ca2+/11 mM-BAPTA) the results were qualitatively similar to those obtained with BAPTA alone, except that the mean dark current did not increase. This is consistent with a resting free calcium concentration in darkness in the region of 1 microM. In the presence of bright steady illumination with BAPTA in the cell the suppression of outer segment current could be maintained for at least 15 min. Upon extinction of the light a very large current transient developed (similar to the overshoot with flashes) which was light suppressible. With backgrounds of moderate intensity the incorporation of buffer led to a gradual reduction of the residual current.(ABSTRACT TRUNCATED AT 400 WORDS)

Applications of laser microprobe mass analysis in medicine

An instrument for laser microprobe mass analysis (LAMMA) has been available for applied research for several years. The value of this sensitive microanalytical technique has been demonstrated in various fields of science, including medicine and biology. The LAMMA instrument comprises two laser systems. The first is a weak continuous pilot laser that can be aimed at a region of interest of the sample, e.g., a histological microtome section, using a light microscope. The second is a powerful pulsed laser that can evaporate the selected area. The generated ions are then analyzed in a time-of-flight mass spectrometer. The advantages of LAMMA present new perspectives in biomedical research. Elements can be located and analyzed down to the parts per million-level with a 1 micron spatial resolution. Speciation and/or fingerprinting of (in)organic molecules may be obtained. However, the technique is destructive, the quality of the light-microscopic view is poor, and quantification is still under investigation. In this review, the analytical characteristics of LAMMA are evaluated, including instrumental features, detection sensitivity, ionization parameters, molecular speciation, and quantification. Applications of LAMMA in medicine are reviewed, covering the microanalysis of a wide range of physiological and toxic constituents (alkali metals, calcium, aluminum, titanium, fluorine and iodine, heavy metals, transition elements, and molecular compounds).

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.

Laser microprobe mass analysis: a review of applications in the life sciences

The characteristics and analytical utility of laser microprobe mass analysis (LAMMA) are described and evaluated, and a short history of this recent microanalytical technique is presented. A review of the areas of application of LAMMA and related laser microprobes is presented with special emphasis on applications in the life sciences.

The Drosophila ninaE gene encodes an opsin

The Drosophila ninaE gene was isolated by a multistep protocol on the basis of its homology to bovine opsin cDNA. The gene encodes the major visual pigment protein (opsin) contained in Drosophila photoreceptor cells R1-R6. The coding sequence is interrupted by four short introns. The positions of three introns are conserved with respect to positions in mammalian opsin genes. The nucleotide sequence has intermittent regions of homology to bovine opsin coding sequences. The deduced amino acid sequence reveals significant homology to vertebrate opsins; there is strong conservation of the retinal binding site and two other regions. The predicted protein secondary structure strikingly resembles that of mammalian opsins. We conclude the Drosophila and vertebrate opsin genes are derived from a common ancestor.

Light-induced reduction of cytoplasmic free calcium in retinal rod outer segment

The response of retinal rod photoreceptors to light consists of a membrane hyperpolarization resulting from the decrease of a light-sensitive conductance in the outer segment. According to the calcium hypothesis, this conductance is blocked by a rise in intracellular free Ca triggered by light, a notion supported by the findings that an induced rise in internal Ca leads to blockage of the light-sensitive conductance and that light triggers a net Ca efflux from the outer segment via a Na-Ca exchanger, suggesting a rise in internal free Ca in the light. We have now measured both Ca influx and efflux through the outer segment plasma membrane and find that, contrary to the calcium hypothesis, light seems to decrease rather than increase the free Ca concentration in the rod outer segment. This result implies that Ca does not mediate visual excitation but it probably has a role in light adaptation.

ATP-dependent calcium uptake activity associated with a disk membrane fraction isolated from bovine retinal rod outer segments

Ca2+ sequestration and release from disks of rod outer segments may play a critical role in visual transduction. An ATP-dependent Ca2+ uptake activity has been identified in association with purified disks of bovine rod outer segments. A crude preparation of osmotically active disks was obtained from rod outer segments by hypoosmotic shock and subsequent flotation on a 5% Ficoll 400 solution. These "crude" disks were further purified by separation into two distinct components by centrifugation in a linear Ficoll gradient. Disks comprised the major component; at least 60% of the protein was rhodopsin. This fraction also contained a Ca2+ uptake activity stimulated approximately 4-fold by ATP. The initial rate was approximately 0.21 nmol of Ca2+ (mg of protein)-1 min-1. Most of the ATP-dependent accumulation of 45Ca2+ was released by the calcium ionophore A23187. The uptake activity was sensitive to vanadate (Ki approximately 20 microM) and insensitive to the mitochondrial Ca2+ uptake inhibitor ruthenium red (10 microM). The ATP-dependent Ca2+ uptake exhibited Michaelis-Menten activation kinetics with respect to [Ca2+] (Km approximately 6 microM). The osmotic properties of the sealed disk membranes were exploited to determine whether the association of Ca2+ transport activity with the disks was merely coincidental. The sedimentation properties of these disks, upon centrifugation on a second Ficoll linear density gradient, varied with the osmolarity of the gradient solution. In several separate gradient solutions of differing osmotic and ionic strengths, the Ca2+ uptake activity always comigrated with the disks. These results indicate that the ATP-dependent Ca2+ uptake activity was physically associated with sealed native disk membranes. The characteristics of the Ca2+ uptake activity suggest that it may play a major role in the regulation of cytosolic Ca2+ levels in rod cells in vivo.