Intracellular calcium changes in Balanus photoreceptor. A study with calcium ion-selective electrodes and Arsenazo III

Ionic mechanisms of phototransduction in photoreceptor cells from the epistellar body of the octopus eledone cirrhosa

Intracellular recordings were made from extraocular photoreceptor cells within isolated epistellar bodies of the lesser or northern octopus Eledone cirrhosa. The cells had resting potentials around −41+/−5 mV (mean +/− s.d., N=60) and showed light-flash-induced membrane depolarisation. The evoked response to a brief light flash consisted of a transient peak depolarisation, followed by a plateau component. The magnitude of the light-induced peak depolarisation response was decreased by bathing the epistellar body in artificial sea water (ASW) low in Na+, where choline+ replaced Na+, or by passing steady depolarising current. Replacement of external Na+ by Li+ had no effect on the light-stimulated response. The external application of the Na+ channel blocker tetrodotoxin (3 micromol l-1) increased the light-evoked response, but this was accompanied by a loss of action potential activity. The amplitude and duration of the response to a light flash was increased by bathing the epistellar body in ASW low in Ca2+, or in ASW containing 10 mmol l-1 Co2+, and after intracellular microinjection of the Ca2+ buffer EGTA. Intracellular microinjection of Ca2+ or inositol 1,4,5-trisphosphate, or external application of the phospholipase C inhibitor U-73122, had no apparent effect on the light-evoked response. These results are consistent with the interpretation that (1) the majority of the light-induced inward current is carried by Na+, probably via a non-selective cation channel, and (2) an increase in the intracellular free Ca2+ concentration, mediated by the phototransduction process, is involved in regulating the light-induced inward photocurrent and thus, in effect, determines the amplitude, time course and sensitivity of the receptor potential.

The Limulus ventral photoreceptor: light response and the role of calcium in a classic preparation

The ventral nerve photoreceptor of the horseshoe crab Limulus polyphemus has been used for many years to investigate basic mechanisms of invertebrate phototransduction. The activation of rhodopsin leads in visual cells of invertebrates to an enzyme cascade at the end of which ion channels in the plasma membrane are transiently opened. This allows an influx of cations resulting in a depolarization of the photoreceptor cell. The receptor current of the Limulus ventral photoreceptor consists of three components which differ in several aspects, such as the time course of activation, the time course of recovery from light adaptation, and the reversal potential. Each component is influenced in a different, characteristic way by various pharmacological manipulations. In addition, at least two types of single photon-evoked events (bumps) and three elementary channel conductances are observed in this photoreceptor cell. These findings suggest that the receptor current components are controlled by three different light-activated enzymatic pathways using three different ligands to increase membrane conductance. Probably one of these ligands is cyclic GMP, another one is activated via the IP3-cascade and calcium, the third one might be cyclic AMP. Calcium ions are very important for the excitation and adaptation of visual cells in invertebrates. The extracellular and intracellular calcium concentrations determine the functional state of the visual cell. A rise in the cytosolic calcium concentration appears to be an essential step in the excitatory transduction cascade. Cytosolic calcium is the major intracellular mediator of adaptation. If the cytosolic calcium level exceeds a certain threshold value after exposure to light it causes the desensitization of the visual cell. On the other hand, from a slight rise in cytosolic calcium facilitation results, i.e. increased sensitivity of the photoreceptor.

Current issues in invertebrate phototransduction. Second messengers and ion conductances

Investigation of phototransduction in invertebrate photoreceptors has revealed many physiological and biochemical features of fundamental biological importance. Nonetheless, no complete picture of phototransduction has yet emerged. In most known cases, invertebrate phototransduction involves polyphosphoinositide and cyclic GMP (cGMP) intracellular biochemical signaling pathways leading to opening of plasma membrane ion channels. Excitation is Ca(2+)-dependent, as are adaptive feedback processes that regulate sensitivity to light. Transduction takes place in specialized subcellular regions, rich in microvilli and closely apposed to submicrovillar membrane systems. Thus, excitation is a highly localized process. This article focuses on the intracellular biochemical signaling pathways and the ion channels involved in invertebrate phototransduction. The coupling of signaling cascades with channel activation is not understood for any invertebrate species. Although photoreceptors have features that are common to most or all known invertebrate species, each species exhibits unique characteristics. Comparative electrophysiological, biochemical, morphological, and molecular biological approaches to studying phototransduction in these species lead to fundamental insights into cellular signaling. Several current controversies and proposed phototransduction models are evaluated.

Activation of light-dependent K+ channels in ciliary invertebrate photoreceptors involves cGMP but not the IP3/Ca2+ cascade

The activation of light-dependent K+ channels in ciliary photoreceptors from Pecten was investigated using intracellular dialysis of putative messengers and modulators. Neither elevated [Ca2+] nor BAPTA changed the membrane current in the dark or the light response. IP3 and the antagonists heparin and decavanadate were similarly ineffective, indicating that in these cells the IP3/Ca2+ signaling pathway is not crucial for phototransduction. By contrast, 8-Br-cGMP and cGMP induced an outward current accompanied by an increase in membrane conductance; 8-Br-cAMP was ineffective. The identity between the cGMP-induced and the light-induced currents is suggested by the following: both are carried by K+ and blocked by 4-AP, and both show outward rectification. In addition, guanine cyclic nucleotides depressed the photoresponse and induced single-channel currents in excised patches of light-sensitive membrane. These light-dependent channels therefore appear to represent a link between the families of cyclic nucleotide-gated channels and voltage-dependent K+ channels.

Comparative methodological investigations on the cytochemical localization of calcium in brain and inner ear of cichlid fish

Four different methods for calcium precipitation are compared in the optic tectum and the inner ear of the cichid fish, Oreochromis mossambicus. Several parameters are investigated concerning their influences on the reaction product. Three procedures (bichromate, fluoride, and oxalate-pyroantimonate) produce fine-grained deposits, often flocculent in the latter method. The fourth method (potassium-pyroantimonate) generates predominantly coarse-grained reaction product. The calcium content of the deposits is always proven with energy-filtering transmission electron microscopy (EFTEM). In both tissues fine-grained reaction product is found in endoplasmic reticulum and synaptic vesicles, and in addition in some mitochondria and at the cytoskeleton. The coarse-grained deposits of the potassium-pyroantimonate method have a more unspecific distribution. This is the only method which produces extracellular deposits in the inner ear, whereas in the optic tectum extracellular precipitates are always present except with the oxalate-pyroantimonate procedure. Two factors have an influence on the reaction product: the duration of fixation and the type of resin. The prolongation of the fixation time up to 24 hours leads to an increase of the reaction product, which also becomes coarse-grained. These observations are corroborated by quantification with image analysis. Furthermore the use of an epoxy resin compared to acrylic resins decreases the amount of reaction product produced. We show that the application of several methods is meaningful in order to understand the calcium properties of the investigated tissue, but it is necessary to optimize a certain method for a given tissue.

Measurement of cytosolic Ca2+ concentration in Limulus ventral photoreceptors using fluorescent dyes

Several Ca-sensitive fluorescent dyes (fura-2, mag-fura-2 and Calcium Green-5N) were used to measure intracellular calcium ion concentration, Cai, accompanying light-induced excitation of Limulus ventral nerve photoreceptors. A ratiometric procedure was developed for quantification of Calcium Green-5N fluorescence. A mixture of Calcium Green-5N and a Ca-insensitive dye, ANTS, was injected in the cell and the fluorescence intensities of both dyes were used to calculate the spatial average of Cai within the light-sensitive R lobe of the photoreceptor. In dark-adapted photoreceptors, the initial Cai was 0.40 +/- 0.22 microM (SD, n = 7) as measured with fura-2. Cai peaked in the light-sensitive R lobe at 700-900 ms after the onset of an intense measuring light step, when the spatial average of Cai within the R lobe reached 68 +/- 14 and 62 +/- 37 microM (SD, n = 5) as measured with mag-fura-2 and Calcium Green-5N, respectively. The rate of Cai rise was calculated to be approximately 350 microM/s under the measuring conditions. The resting level of Mg2+ was estimated to be 1.9 +/- 0.9 mM, calculated from mag-fura-2 measurements. To investigate the effect of adapting light on the initial Cai level in the R lobe, a 1-min step of 420 nm background light was applied before each measurement. The first significant (P < 0.05) change in the initial level of Cai occurred even at the lowest adapting light intensity, which delivered approximately 3 x 10(3) effective photons/s. The relative sensitivity of the light-adapted photoreceptors was linearly related to the relative Cai on a double log plot with slope between -4.3 and -5.3. We were unable to detect a Cai rise preceding the light-activated receptor potential. The Cai rise, measured with Calcium Green-5N, lagged 14 +/- 5 ms (SD, n = 32) behind the onset of the receptor potential at room temperature in normal ASW. In the absence of extracellular Ca2+ and at 10 degrees C, this lag increased to 44 +/- 12 ms (SD, n = 17).

A light emitting diode microspectrophotometer: intracellular Ca2+ measurements in isolated stretch receptor

A microspectrophotometer was designed to measure absorbance changes in single cells. The device utilizes sequentially activated light emitting diodes (LED) to provide different wave lengths of light. The instrument has the advantage of relative simplicity and less cost compared to other devices. The spectrophotometer was tested by measuring absorbance changes of the metallochromic Ca2+ indicator Arsenazo III (AIII) injected into the crayfish (Astacus astacus) stretch receptor. Under the conditions described the detection limit of the concentration of AIII was 0.05 mM and absorbance changes of 0.0005 can be reliably determined which correspond to a detection limit of 10-20 nM for free Ca2+ changes assuming a light path length of 0.003 cm and an apparent dissociation constant (KD) of 2 microM for the Ca(2+)-AIII complex. The upper frequency limit of the device is 3000 Hz. The absorbance measurements of AIII injected into the crayfish stretch receptor neurons revealed a Ca(i) of 375 +/- 177 nM (mean +/- SD: 14 cells). LiCl substituted for NaCl increased Ca(i) 45-100 nM in different cells, suggesting that a Na+ gradient is necessary for Ca2+ homeostasis in this receptor.

The trp gene is essential for a light-activated Ca2+ channel in Drosophila photoreceptors

Invertebrate phototransduction is an important model system for studying the ubiquitous inositol-lipid signaling system. In the transient receptor potential (trp) mutant, one of the most intensively studied transduction mutants of Drosophila, the light response quickly declines to baseline during prolonged intense light. Using whole-cell recordings from Drosophila photoreceptors, we show that the wild-type response is mediated by at least two functionally distinct classes of light-sensitive channels and that both the trp mutation and a Ca2+ channel blocker (La3+) selectively abolish one class of channel with high Ca2+ permeability. Evidence is also presented that Ca2+ is necessary for excitation and that Ca2+ depletion mimics the trp phenotype. We conclude that the recently sequenced trp protein represents a class of light-sensitive channel required for inositide-mediated Ca2+ entry and suggest that this process is necessary for maintained excitation during intense illumination in fly photoreceptors.

Intracellular free calcium concentration in the blowfly retina studied by Fura-2

The retinal tissue of blowflies was loaded with the fluorescent Ca2+ indicator Fura-2 by incubating cut heads in saline solutions which contained the membrane permeable acetoxymethylester of Fura-2 (Fura-2/AM). The spectral analysis of the tissue fluorescence showed that Fura-2/AM was intracellularly hydrolysed to Fura-2. In order to monitor the intracellular free Ca2+ concentration ([Ca2+]i) the Fura-2 fluorescence was excited by short light flashes. The fluorescence was calibrated by incubating the tissue in Ca2+ buffers of high buffering capacity and subsequent disruption of the cell membranes by freeze/thawing, which gave a dissociation constant for the Ca(2+)-Fura-2 complex of 100 nM. When the extracellular Ca2+ concentration ([Ca2+]o) was altered [Ca2+]i reversibly changed. The changes were most pronounced when [Ca2+]o was varied in the millimolar range, e.g. [Ca2+]i increased from 0.07 microM at [Ca2+]o = 0.1 mM to 1 microM at [Ca2+]o = 10 mM. When extracellular Na+ was replaced by Li+ or other monovalent ions, [Ca2+]i rapidly increased which supports the view that electrogenic Na+/Ca2+ exchange contributes to the control of [Ca2+]i. However, [Ca2+]i decreased again when the tissue was superfused with Na(+)-free media for longer periods, which points to a Ca(2+)-transporting system different from Na+/Ca2+ exchange. Light adaptation had only a small effect on [Ca2+]i. Even after intense stimulation [Ca2+]i increased by a factor of 1.5 only, which is in line with results obtained in the photoreceptors of Balanus and Apis.

Biophysical processes in invertebrate photoreceptors: recent progress and a critical overview based on Limulus photoreceptors

Limulus ventral nerve photoreceptor, a classical preparation for the study the phototransduction in invertebrate eyes, seems to have a very complex mechanism to transform light energy into a physiological signal. Although the main function of the photoreceptor is to change the membrane conductance according to the illumination, the cell has voltage-activated conductances as well. The voltage-gated conductances are matched to the light-activated ones in the sense that they make the function of the cell more efficient. The complex mechanism of phototransduction and the presence of four different voltage-gated conductance in Limulus ventral nerve photoreceptors indicate that these cells are far less differentiated than the photoreceptor cells of vertebrates. Indications accumulated in recent years support the view that the ventral photoreceptor of Limulus has different light-activated macroscopic current components, ion channels and terminal transmitters. After conclusions from macroscopic current measurements (Payne, 1986; Payne et al. 1986 a, b), direct evidence was presented by single-channel (Nagy & Stieve, 1990 a, b; Nagy, 1990 a, b) and macroscopic current measurements (Deckert et al. 1991 a, b) for three different light-activated conductances. It has been shown that two of these conductances are stimulated by two different excitation mechanisms. The two mechanisms, having different kinetics, release probably two different transmitters. One of them might be the cGMP (Johnson et al. 1986), the other one the calcium ion (Payne et al. 1986 a, b). However, the biochemical processes which link the rhodopsin molecules and the ion channels are not known. The unknown chemical details of the phototransduction result in a delay for the mathematical description of the biophysical mechanisms. More biochemical details are known about the adaptation mechanism. It was found that inositol 1,4,5-trisphosphate is a messenger for the release of calcium ions from the intracellular stores and that calcium ions are the messengers for adaptation (Payne et al. 1986 b; Payne & Fein, 1987). Concerning the mechanism of calcium release, it was revealed that a negative feedback acts on the enzyme cascade to regulate the internal calcium level and to protect the stores against complete emptying (Payne et al. 1988, 1990). Calcium ions also play an important role in the excitation mechanism. (a) In [Ca2+]i-depleted cells the light-induced current was increased after intracellular Ca2+ injection, suggesting that calcium is necessary for the transduction mechanism (Bolsover & Brown, 1985).(ABSTRACT TRUNCATED AT 400 WORDS)

Microvillar components of light adaptation in blowflies

The process of light adaptation in blowfly photoreceptors was analyzed using intracellular recording techniques and double and triple flash stimuli. Adapting flashes of increasing intensity caused a progressive reduction in the excitability of the photoreceptors, which became temporarily suppressed when 3 x 10(6) quanta were absorbed by the cell. This suppression was confirmed by subsequently applying an intense test flash that photoactivated a considerable fraction of the 10(8) visual pigment molecules in the cell. The period of temporary desensitization is referred to as the refractory period. The stimulus intensity to render the receptor cell refractory was found to be independent of the extracellular calcium concentration over a range of 10(-4) and 10(-2) M. During the refractory period (30-40 ms after the adapting flash) the cell appears to be "protected" against further light adaptation since light absorption during this period did not affect the recovery of the cell's excitability. Calculations showed that the number of quantum absorptions necessary to induce receptor refractoriness is just sufficient to photoactivate every microvillus of the rhabdomere. This coincidence led to the hypothesis that the refractoriness of the receptor cells is due to the refractoriness of the individual microvilli. The sensitivity of the receptor cells after relatively weak adapting flashes was reduced considerably more than could be accounted for by the microvilli becoming refractory. A quantitative analysis of these results suggests that a photoactivated microvillus induces a local adaptation over a relatively small area of the rhabdomere around it, which includes several tens of microvilli. After light adaptation with an intense flash, photoactivation of every microvillus by the absorption of a few quanta produced only a small receptor response whereas photoactivation of every rhodopsin molecule in every microvillus produced the maximum response. The excitatory efficiency of the microvilli therefore increases with the number of quanta that are absorbed simultaneously.

Dimethyl sulfoxide elevates intracellular Ca2+ and mimics effects of increased light intensity in a photoreceptor

A 1% (v/v) solution of dimethyl sulfoxide (DMSO) added to the saline bath of isolated Balanus eburneus photoreceptors increased receptor potential amplitude by 40-50% and shortened time to peak amplitude and latency by 20-25%. The light-sensitive membrane current of voltage-clamped cells was increased systematically as DMSO concentration was increased from 1% to 10%. The null potential of the light sensitive current was unaffected by DMSO with short pulses of light, indicating that DMSO has no direct effect on ion selectivity of the light-sensitive channel. Absorbance changes of cell injected with the calcium indicator arsenazo III show that DMSO elevates intracellular Ca2+ (Cai). Current-voltage relations in darkness reveal that DMSO induces a small sustained inward current (approximately 5 nA) which has a null potential similar to the light-induced current. DMSO may activate the light-sensitive conductance via the increase in Cai. However, the altered kinetics and increased amplitude of the receptor current are opposite to the desensitizing effects normally observed with increased Cai.