Calcium metabolism in vertebrate photoreceptors

Cytoplasmic free calcium concentration in dark-adapted retinal rod outer segments

We measured the cytoplasmic free Ca concentration in the outer segment of intact, dark-adapted rods of the toad retina. The Ca indicator dye, Quin2, was loaded at concentrations of 0.273 +/- 0.06 mM into the rod cytoplasm by incubation of isolated retinas in the hydrophobic ester, Quin2AM. Quin2 did not alter the cytoplasmic Ca concentration in the dark, but it buffered light-dependent concentration transients and, hence, modified the rod photoresponse. In the presence of 1 mM external Ca, the cytoplasmic Ca concentration in the dark was 273 +/- 129 nM.

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.

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.

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.

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.

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.

Elemental distribution in Rana pipiens retinal rods: quantitative electron probe analysis

The composition of dark-adapted and illuminated retinal rod outer and inner segments and mitochondria was determined with electron probe X-ray micro-analysis of cryosections. The concentration of Ca in the outer segment was 0.4 mmol/kg dry wt. (0.1 Ca/rhodopsin) and did not measurably change upon illumination with saturating light for 5 min. The non-mitochondrial regions of the inner segment contained the highest concentrations (up to 13 mmol/kg dry wt.) of Ca in rods; these regions probably represent the endoplasmic reticulum. The equilibrium potentials estimated from the measured elemental concentrations and the known water content of dark-adapted outer segments were (mV): ENa = +17, EK = -83, ECl = -27. The respective values in the inner segment were: ENa = +20, EK = -89, ECl = -26. The above values were obtained in frog rods bathed in 0.18 mM-Ca Ringer solution. In the outer segment of toad rods bathed in 1.8 mM-Ca Ringer, ENa = +33 mV. The Mg content of the rods was high. The (computed) concentration in the dark-adapted retinae was 11 mM in the outer segment and 24 mM in the inner segment. Illumination caused a reduction in Mg to 9 mM (outer segment) and 16 mM (inner segment). Illumination caused a highly significant reduction in Na and Cl concentrations, and an increase in K concentration in both outer and inner segments. Exposure to Na-free (choline Ringer) solution resulted in reduction in Na to just-detectable levels (3 +/- 1 mmol/kg dry wt.) in the outer segment and to 5 +/- 1 mM in the inner segment. This was associated with a significant loss of Cl and decrease in ECl to -50 mV. The low Na content of the outer segment in the Na-depleted rods is not compatible with an extracellular concentration (105 mM) of inexchangeable Na in the intradiskal space. Mitochondrial Na and Mg paralleled the changes in the cytoplasmic concentrations: both mitochondrial Na and Mg were significantly decreased in illuminated, compared to dark-adapted rods. There was no detectable Ca (0 +/- 0.2 mmol/kg dry wt.) in mitochondria of dark-adapted rods containing high concentrations of Na; mitochondrial Ca was slightly higher (0.5 +/- 0.2 mmol/kg dry wt.) in the mitochondria that contained low Na following illumination.

Signal mechanisms of phototransduction in retinal rod

The levels of intracellular molecules are modulated by illumination of rod photoreceptor cells in the vertebrate retina. Among these are Ca ions, cyclic nucleotides (cGMP in particular), and phosphate nucleotides (ATP and GTP). It is presumed now that at least two of these molecules, Ca and cGMP, may function as chemical linkers between the absorption of light by rhodopsin and the ionic channels of the plasma membrane of the rod outer segment that close when the rod is illuminated. The manuscript will review the physiology of the rod cell, the evidence in support of light-dependent changes in the intracellular levels of various small molecules, and the role of these changes in coupling rhodopsin excitation to the control of the light-sensitive membrane channels in the rod.

Phase behavior of isolated photoreceptor membrane lipids is modulated by bivalent cations

The phase behavior of isolated photoreceptor membrane lipids is further investigated by 31P-NMR, in view of earlier discrepant results [(1979) Biochim. Biophys. Acta 558, 330-337; (1982) FEBS Lett. 124, 93-99]. We present evidence that the discrepancy is due to bivalent cations. When resuspended in aqueous media at neutral pH in the absence of bivalent cations, the isolated photoreceptor membrane lipids largely adopt the bilayer configuration. However, upon addition of such cations (Ca2+ greater than Mg2+) or when resuspended in their presence, the formation of other phases (hexagonal H11, lipidic particles) results. The rate of this transition depends on cation concentration and temperature. The transition is not easily reversed by addition of EDTA. Implications with regard to photoreceptor membrane structure and function need further study.

Cyclic GMP releases calcium from leaky rod outer segments

We show that cyclic GMP promotes the release of calcium from disks in whole rod outer segments by two different techniques. Most likely cyclic GMP causes a change in the permeability of the disk membrane.

Effects of temperature changes on toad rod photocurrents

Rod current responses were measured over the range 5-30 degrees C. Following a rapid decrease in temperature the amplitude of the dark current decreased without detectable delay (less than 3 s). Over a period of several minutes the amplitude of the dark current sometimes relaxed slightly towards its previous value. The rapid change cannot be accounted for simply by altered activity of the sodium pump and instead indicates that the conductance of the outer segment in darkness changes with temperature. Over the range 10-30 degrees C the amplitude of the dark current increased approximately linearly with temperature, and the straight line of best fit extrapolated to zero current at about 5 degrees C. The few points available below 10 degrees C indicated that the relationship flattened out, but this could not be investigated properly. The kinetics of responses to dim flashes accelerated with a Q10 of about 2.2, and were well described by an Arrhenius plot with an activation energy of 13.8 kcal mol-1 (HEPES Ringer solution). The time course of recovery of dark current following a saturating flash showed a similar temperature dependence to that of the dim flash kinetics. A simple explanation of the previous two findings is that the delays determining the time course of responses to both dim and bright flashes are largely determined by the fluidity of the disk membrane. The sensitivity to dim flashes had a broad peak at about 22 degrees C, decreasing at both lower and higher temperatures. The relative sensitivity to long wave-length light increased slightly with temperature. The sensitivity at 700 nm relative to that at 500 nm increased by 0.225 log10 units (1.68 times) upon a temperature increase from 11.5 to 29.3 degrees C (from approximately -5.0 log10 units to approximately -4.8 log10 units). This change appears to be approximately what would be expected theoretically.

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

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