Phosphatidylinositol-4,5-bisphosphate phospholipase C in bovine rod outer segments

Distinct properties of the two isoforms of CDP-diacylglycerol synthase

CDP-diacylglycerol synthases (CDS) are critical enzymes that catalyze the formation of CDP-diacylglycerol (CDP-DAG) from phosphatidic acid (PA). Here we show in vitro that the two isoforms of human CDS, CDS1 and CDS2, show different acyl chain specificities for its lipid substrate. CDS2 is selective for the acyl chains at the sn-1 and sn-2 positions, the most preferred species being 1-stearoyl-2-arachidonoyl-sn-phosphatidic acid. CDS1, conversely, shows no particular substrate specificity, displaying similar activities for almost all substrates tested. Additionally, we show that inhibition of CDS2 by phosphatidylinositol is also acyl chain-dependent, with the strongest inhibition seen with the 1-stearoyl-2-arachidonoyl species. CDS1 shows no acyl chain-dependent inhibition. Both CDS1 and CDS2 are inhibited by their anionic phospholipid end products, with phosphatidylinositol-(4,5)-bisphosphate showing the strongest inhibition. Our results indicate that CDS1 and CDS2 could create different CDP-DAG pools that may serve to enrich different phospholipid species with specific acyl chains.

Calcium stores in vertebrate photoreceptors

This review lays out the emerging evidence for the fundamental role of Ca(2+) stores and store-operated channels in the Ca(2+) homeostasis of rods and cones. Calcium-induced calcium release (CICR) is a major contributor to steady-state and light-evoked photoreceptor Ca(2+) homeostasis in the darkness whereas store-operated Ca(2+) channels play a more significant role under sustained illumination conditions. The homeostatic response includes dynamic interactions between the plasma membrane, endoplasmic reticulum (ER), mitochondria and/or outer segment disk organelles which dynamically sequester, accumulate and release Ca(2+). Coordinated activation of SERCA transporters, ryanodine receptors (RyR), inositol triphosphate receptors (IP3Rs) and TRPC channels amplifies cytosolic voltage-operated signals but also provides a memory trace of previous exposures to light. Store-operated channels, activated by the STIM1 sensor, prevent pathological decrease in [Ca(2+)]i mediated by excessive activation of PMCA transporters in saturating light. CICR and SOCE may also modulate the transmission of afferent and efferent signals in the outer retina. Thus, Ca(2+) stores provide additional complexity, adaptability, tuneability and speed to photoreceptor signaling.

Haploinsufficiency of RanBP2 is neuroprotective against light-elicited and age-dependent degeneration of photoreceptor neurons

Prolonged light exposure is a determinant factor in inducing neurodegeneration of photoreceptors by apoptosis. Yet, the molecular bases of the pathways and components triggering this cell death event are elusive. Here, we reveal a prominent age-dependent increase in the susceptibility of photoreceptor neurons to undergo apoptosis under light in a mouse model. This is accompanied by light-induced subcellular changes of photoreceptors, such as dilation of the disks at the tip of the outer segments, prominent vesiculation of nascent disks, and autophagy of mitochondria into large multilamellar bodies. Notably, haploinsufficiency of Ran-binding protein-2 (RanBP2) suppresses apoptosis and most facets of membrane dysgenesis observed with age upon light-elicited stress. RanBP2 haploinsufficiency promotes decreased levels of free fatty acids in the retina independent of light exposure and turns the mice refractory to weight gain on a high-fat diet, whereas light promotes an increase in hydrogen peroxide regardless of the genotype. These studies demonstrate the presence of age-dependent and RanBP2-mediated pathways modulating membrane biogenesis of the outer segments and light-elicited neurodegeneration of photoreceptors. Furthermore, the findings support a mechanism whereby the RanBP2-dependent production of free fatty acids, metabolites thereof or the modulation of a cofactor dependent on any of these, promote apoptosis of photoreceptors in concert with the light-stimulated production of reactive oxygen species.

Protein kinase C activity and light sensitivity of single amphibian rods

Biochemical experiments by others have indicated that protein kinase C activity is present in the rod outer segment, with potential or demonstrated targets including rhodopsin, transducin, cGMP-phosphodiesterase (PDE), guanylate cyclase, and arrestin, all of which are components of the phototransduction cascade. In particular, PKC phosphorylations of rhodopsin and the inhibitory subunit of PDE (PDE ) have been studied in some detail, and suggested to have roles in downregulating the sensitivity of rod photoreceptors to light during illumination. We have examined this question under physiological conditions by recording from a single, dissociated salamander rod with a suction pipette while exposing its outer segment to the PKC activators phorbol-12-myristate,13-acetate (PMA) or phorbol-12,13-dibutyrate (PDBu), or to the PKC-inhibitor GF109203X. No significant effect of any of these agents on rod sensitivity was detected, whether in the absence or presence of a background light, or after a low bleach. These results suggest that PKC probably does not produce any acute downregulation of rod sensitivity as a mechanism of light adaptation, at least for isolated amphibian rods.

Molecular cloning of a diacylglycerol kinase isozyme predominantly expressed in rat retina

We have cloned and characterized a new diacylglycerol kinase (DGK) isozyme which is expressed in the retina and the brain of rat. The cDNA contains an open reading frame of 567 amino acid residues with a predicted protein of 64 kDa and shows very high homology to human DGK epsilon. The new DGK isozyme contains two distinctive zinc-finger structures and a putative catalytic domain. This DGK expressed predominantly in the inner and outer nuclear layers of retina. This expression pattern is different from those of the previously cloned DGKs including the human DGK epsilon, suggesting that this DGK isozyme has potential importance in visual functions as was the case in Drosophila retinal cells.

Gene cloning and characterization of CDP-diacylglycerol synthase from rat brain

A cDNA encoded a 462-amino acid protein, which showed CDP-diacylglycerol synthase (CDS) activity was cloned for the first time as the vertebrate enzyme molecule from rat brain cDNA library. The deduced molecular mass of this rat CDS was 53 kDa, and putative primary structure included several possible membrane- spanning regions. At the amino acid sequence level, rat CDS shared 55.5%, 31. 7%, and 20.9% identity with already known Drosophila, Saccharomyces cerevisiae, and Escherichia coli CDS, respectively. This rat CDS preferred 1-stearoyl-2-arachidonoyl phosphatidic acid as a substrate, and its activity was strongly inhibited by phosphatidylglycerol 4, 5-bisphosphate. By immunoblotting analysis of COS cells overexpressed with the epitope-tagged for rat CDS, a 60-kDa band was detected. By epitope-tag immunocytochemistry, the CDS protein was mainly localized in close association with the membrane of the endoplasmic reticulum of the transfected cells. The intense mRNA expression of CDS was localized in the cerebellar Purkinje cells, the pineal body, and the inner segment of photoreceptor cells. Additionally, very intense expression was detected in postmitotic spermatocytes and spermatids.

Identification of components of a phosphoinositide signaling pathway in retinal rod outer segments

Phototransduction in retinal rods involves a G protein-coupled signaling cascade that leads to cGMP hydrolysis and the closure of cGMP-gated cation channels that are open in darkness, producing a membrane hyperpolarization as the light response. For many years there have also been reports of the presence of a phosphoinositide pathway in the rod outer segment, though its functions and the molecular identities of its components are still unclear. Using immunocytochemistry with antibodies against various phosphoinositide-specific phospholipase C (PLC) isozymes (beta1-4, gamma1-2, and delta1-2), we have found PLCbeta4-like immunoreactivity in rod outer segments. Similar experiments with antibodies against the alpha-subunits of the G(q) family of G proteins, which are known to activate PLCbeta4, have also demonstrated G(alpha11)-like immunoreactivity in this location. Immunoblots of total proteins from whole retina or partially purified rod outer segments with anti-PLCbeta4 and anti-G(alpha11) antibodies gave, respectively, a single protein band of the expected molecular mass, suggesting specific labelings. The retinal locations of the two proteins were also supported by in situ hybridization experiments on mouse retina with probes specific for the corresponding mouse genes. These two proteins, or immunologically identical isoforms, therefore likely mediate the phosphoinositide signaling pathway in the rod outer segment. At present, G(alpha11) or a G(alpha11)-like protein represents the only G protein besides transducin (which mediates phototransduction) identified so far in the rod outer segment. Although absent in the outer segment layer, other PLC isoforms as well as G(alpha q) (another G(q) family member), are present elsewhere in the retina.

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.

Light adaptation of bovine retinas in situ stimulates phosphatidylinositol synthesis in rod outer segments in vitro

Light-stimulated phosphoinositide turnover has been reported in both vertebrate retina and isolated rod outer segments (ROS). In the current investigation, we examined the incorporation of [3H]-inositol in vitro in bovine ROS isolated from dark adapted (DROS) or bleached (BROS) retinas. Incorporation of [3H]-inositol into phosphoinositides in BROS was 3-5 fold higher than in DROS. The majority (approximately 90%) of [3H]-inositol was found in phosphatidylinositol (PI), whereas phosphatidylinositol 4-phosphate (PIP) and phosphatidylinositol 4,5-bisphosphate (PIP2) accounted for 7-8% of the label. The enhanced labelling of PI was only observed when bovine retinas were light-adapted prior to ROS preparation, suggesting the requirement for an intact photoreceptor for the observed effect. Our data strongly suggest that bleaching of bovine retina in situ stimulates PI synthesis in isolated ROS in vitro.

Diacylglycerol analogs inhibit the rod cGMP-gated channel by a phosphorylation-independent mechanism

The electrical response to light in retinal rods is mediated by cyclic nucleotide-gated, nonselective cation channels in the outer segment plasma membrane. Although cGMP appears to be the primary light-regulated second messenger, cellular levels of other substances, including Ca2+ and phosphatidylinositol-4,5-bisphosphate, are also sensitive to the level of illumination. We now show that diacylglycerol (DAG) analogs reversibly suppress the cGMP-activated conductance in excised patches from frog rod outer segments. This suppression did not require nucleoside triphosphates, indicating that a phosphorylation reaction was not involved. DAG was more effective at low than at high [cGMP]: with 50 microM 8-Br-cGMP, the DAG analog 1,2-dioctanoyl-sn-glycerol (1,2-DiC8) reduced the current with an IC50 of approximately 22 microM (Hill coefficient, 0.8), whereas with 1.2 microM 8-Br-cGMP, only approximately 1 microM 1,2-DiC8 was required to halve the current. DAG reduced the apparent affinity of the channels for cGMP: 4 microM 1,2-DiC8 produced a threefold increase in the K1/2 for channel activation by 8-Br-cGMP, as well as a threefold reduction in the maximum current, without changing the apparent stoichiometry or cooperativity of cGMP binding. Inhibition by 1,2-DiC8 was not relieved by supersaturating concentrations of 8-Br-cGMP, suggesting that DAG did not act by competitive inhibition of cGMP binding. Furthermore, DAG did not seem to significantly reduce single-channel conductance. A DAG analog similar to 1,2-DiC8--1,3-dioctanoyl-sn-glycerol (1,3-DiC8)--suppressed the current with the same potency as 1,2-DiC8, whereas an ethylene glycol of identical chain length (DiC8-EG) was much less effective. Our results suggest that DAG allosterically interferes with channel opening, and raise the question of whether DAG is involved in visual transduction.

Kinetics and localization of the phosphorylation of rhodopsin by protein kinase C

Protein kinase C isolated from retina catalyzes the stoichiometric phosphorylation of bovine rhodopsin. Enzymological studies using receptor in rod outer segment membranes stripped of peripheral proteins reveal that the phosphorylation is independent of receptor conformation or liganded state; the half-time for phosphorylation of unbleached (dark-adapted) rhodopsin, bleached (light-activated) rhodopsin, and opsin (chromophore removed) is the same. The phosphorylation by protein kinase C is Ca2+ and lipid regulated; the Km for Ca2+ decreases with increasing concentrations of membrane, consistent with known properties of Ca(2+)-regulated protein kinase Cs. The Km for ATP is 27 microM, with an optimal concentration for MgCl2 of approximately 1 mM. The phosphorylation of rhodopsin by protein kinase C is inhibited by the protein kinase C-selective inhibitor sangivamycin. Proteolysis by Asp-N reveals that all the protein kinase C phosphorylation sites are on the carboxyl terminus of the receptor. Cleavage with trypsin indicates that Ser338, the primary phosphorylation site of rhodopsin kinase, is not phosphorylated significantly; rather, the primary phosphorylation site of protein kinase C is on the membrane proximal half of the carboxyl terminus. The protein kinase C-catalyzed phosphorylation of rhodopsin is analogous to the ligand-independent phosphorylation of other G protein-coupled receptors that is catalyzed by second messenger-regulated kinases.

Inositol-1,4,5-trisphosphate receptors in the vertebrate retina

Evidence has shown an activation of phosphatidylinositol 4,5-bisphosphate (PIP2) specific phospholipase C (PtdIns-PLC) by light in the vertebrate retina and rod outer segments (ROS), suggesting important roles for its two metabolites, 1,2-diacylglycerol (DG) and inositol-1,4,5-trisphosphate [Ins(1,4,5)P3]. DG activates protein kinase C (PKC) and Ins(1,4,5)P3 releases bound intracellular calcium. Since Ca2+ plays an important role in light adaptation, the presence of Ins(1,4,5)P3 receptors in ROS may indicate a regulatory role of Ins(1,4,5)P3 to the free Ca2+ content. In the present study, we investigated the Ins(1,4,5)P3 receptors in whole retinal membranes and several subcellular fractions prepared from bovine retinas. Scatchard analyses of binding data for retinal membrane preparations showed a single, high-affinity binding site with equilibrium dissociation constant (Kd) of 24 +/- 2 nM and maximal binding capacity (Bmax) of 353 +/- 15 fmol/mg protein at pH 7.4. Specific binding was found in both small and large synaptosomal preparations representing inner and outer plexiform layers, respectively. A detectable, but low abundance of Ins(1,4,5)P3-specific binding in ROS was observed at both pH 7.4 and 8.3, but no specific binding of Ins(1,4,5)P3 was found in isolated outer segment discs. The binding of Ins(1,4,5)P3 in ROS was reduced by addition of ATP, suggesting a regulatory role for this nucleotide. Addition of calcium, sodium, and potassium ions also reduced specific binding of Ins(1,4,5)P3. Immunocytochemical studies indicate intense staining in the inner segment and extending to the ROS. Inner and outer plexiform layers were also stained. These findings show that the Ins(1,4,5)P3 receptor is present in photoreceptor cells and inner and outer plexiform layers in the vertebrate retina.

Phosphorylation of bovine rod photoreceptor cyclic GMP phosphodiesterase

The cyclic GMP phosphodiesterase (PDE) of retinal rods plays a key role in phototransduction and consists of two catalytic subunits (PDE alpha and PDE beta) and two identical inhibitory subunits (PDE gamma). Here we report that PDE alpha and PDE gamma are phosphorylated by protein kinase(s) C (PKC) from brain and rod outer segments (ROS). These same two types of PKC also phosphorylate PDE alpha in trypsin-activated PDE (without PDE gamma). In contrast, cyclic-AMP-dependent protein kinase catalytic subunit phosphorylates both PDE alpha and PDE beta, but not PDE gamma. This kinase does not phosphorylate trypsin-activated PDE. The synthetic peptides AKVISNLLGPREAAV (PDE alpha 30-44) and KQRQTRQFKSKPPKK (PDE gamma 31-45) inhibited phosphorylation of PDE by PKC from ROS. These data suggest that sites (at least one for each subunit) for phosphorylation of PDE by PKC are localized in these corresponding regions of PDE alpha and PDE gamma. Isoenzyme-specific PKC antibodies against peptides unique to the alpha, beta, gamma, delta, epsilon and zeta isoforms of protein kinase C were used to show that a major form of PKC in ROS is PKC alpha. However, other minor forms were also present.

Light evoked inositol trisphosphate release in the rat retina in vitro

Light exposure not only elicits a photic response but may also alter the metabolism and functional properties of the retina. This may be evoked by the stimulation of phospholipid derived second messenger systems. In this study, we investigated the light-evoked release of inositol 1,4,5-triphosphate in the isolated rat retina in vitro by means of high performance liquid chromatography (HPLC) detection. After prelabelling of isolated retinae with tritiated myo-inositol in darkness, they were exposed to no light or to white fluorescent light of 10,000 lux illuminance for 3,5 and 10 sec, respectively. We observed a 200% increase in the release of inositol 1,4,5-trisphosphate compared to basal values in darkness after 3 sec of light exposure with a decline after 5 sec and a return to basal values after 10 sec indicating a rapid breakdown of inositol 1,4,5-trisphosphate. Our data confirm previous studies in the amphibian retina and photoreceptors and demonstrate for the first time a light evoked inositol 1,4,5-trisphosphate release in the mammalian retina.

Distinctive subtypes of bovine phospholipase C that have preferential expression in the retina and high homology to the norpA gene product of Drosophila

The Drosophila norpA gene encodes a phospholipase C involved in phototransduction. However, phospholipase C apparently is not directly involved in phototransduction in vertebrate photoreceptors, although light-activated phospholipase C activity has been reported in vertebrate rod outer segments. Conserved regions of norpA cDNA were used to isolate bovine cDNAs that would encode four alternative forms of phospholipase C of the beta class that are highly homologous to the norpA protein and expressed preferentially in the retina. Two of the variants are highly unusual in that they lack much of the N-terminal region present in all other known phospholipases C. The sequence conservation between these proteins and the norpA protein is higher than that between any other known phospholipases C. GTPase sequence motifs found in proteins of the GTPase superfamily are found conserved in all four variants of the bovine retinal protein as well as the norpA protein but not in other phospholipases C. Results suggest that these proteins together with the norpA protein constitute a distinctive subfamily of phospholipases C that are closely related in structure, function, and tissue distribution. Mutations in the norpA gene, in addition to blocking phototransduction, cause light-dependent degeneration of photoreceptors. In view of the strong similarity in structure and tissue distribution, a defect in these proteins may have similar consequences in the mammalian retina.

Purification and characterization of phosphoinositide-specific phospholipase C from bovine iris sphincter smooth muscle

Two forms (I and II) of phosphoinositide-specific phospholipase C (PLC) were purified from the cytosol of bovine iris sphincter by sequential chromatography on DEAE-Sepharose, EAH-Sepharose, heparin-Sepharose, Sephacryl S-200 gel filtration and Mono Q HR columns. The final step resulted in specific activities of PLC-I and PLC-II of 4.3 and 5.9 mumol of phosphatidylinositol (PI) cleaved/min per mg of protein, which represented up to 295-fold purification compared with that of the starting supernatant. The purified enzymes were further investigated for the presence of isoenzymes and characterized for molecular mass, substrate specificity, pH, Ca2+ requirements and kinetic parameters. Using monoclonal antibodies, PLC-I was identified as PLC-delta 1. The apparent molecular mass of PLC-I as determined by SDS/PAGE and gel filtration was 85 kDa. PLC-II contained an apparently invisible protein band that reacted with the antibody against PLC-gamma 1, and a major 109 kDa protein band that was not recognized by any of the PLC monoclonal antibodies. Further purification of PLC-II by size-exclusion h.p.l.c. resulted in elution of the enzyme activity as a single peak which corresponded to 109 kDa position. Again, this PLC activity was not recognized by any of the PLC monoclonal antibodies. However, the 109 kDa protein activity was recognized by a polyclonal antibody raised against a rat PLC-gamma 1 fragment (amino acids 1272-1287), thus suggesting that this protein is a proteolytic product of PLC-gamma 1. PLC-delta 1 and PLC-gamma 1 were identified in the supernatant fraction and PLC-beta 1 in the membrane fraction of the iris sphincter. Although immunologically different, the catalytic properties of PLC-I and PLC-II were quite similar. The Vmax and Km values for phosphatidylinositol 4,5-bisphosphate (PIP2) hydrolysis were three to five times greater than those for PI hydrolysis. Both forms preferred PIP and PIP2 over PI and both were inactive against phosphatidylcholine. With PIP2 as substrate, the optimal pH values for PLC-I and PLC-II were 6.5 and 7.5 respectively. Unlike PIP2, PI hydrolysis by both forms was dependent on the presence of free Ca2+. The maximal hydrolysis of PI and PIP2 by both forms occurred at 200 and 5 microM Ca2+ respectively. Incubation of the purified enzymes with the catalytic subunit of protein kinase A (PKA) and [gamma-32P]ATP resulted in increased phosphorylation of PLC-I and PLC-II, but it had no inhibitory effect on their enzyme activities.(ABSTRACT TRUNCATED AT 400 WORDS)

Phosphatidic acid and polyphosphoinositide metabolism in rod outer segments. Differential role of soluble and peripheral proteins

The phosphorylation of endogenous diacylglycerol (DAG) and phosphoinositides by [tau-32P]ATP was studied in bovine rod outer segments (ROS) selectively depleted of soluble or peripheral and soluble proteins by treatment with moderate (100 mM) or low (5 mM) ionic strength medium, respectively. DAG kinase activity was similar in bleached and non-bleached ROS extracted with 100 mM medium, and amounted to 70% of that observed in the corresponding non-extracted ROS. Phosphatidic acid (PtdH) labelling in ROS extracted in the dark with low ionic strength medium was markedly lower than in those extracted in light. Thus, even when a major proportion of DAG kinase was associated to the membrane, a soluble form also occurred. Most of the membrane-bound fraction behaved as a peripherally associated protein, its binding to the membrane being modified by light. Ir ROS extracted at moderate ionic strength the labelling of inositides was similar to that in non-extracted ROS. A marked enhancement in polyphosphoinositide labelling was observed in ROS extracted in the dark with low ionic strength. Alkaline treatment of ROS also produced inhibition of polyphosphoinositide phosphorylation. A peripheral form of a type C phospholipase, or a peripheral protein-mediated activation of a particulate form thereof, is suggested. Labelled polyphosphoinositides were more actively hydrolyzed in the light and in the dark plus GTP tau S than in the dark-incubated membranes. The results of phosphorylation experiments in membranes where differential extraction of the alpha subunit of transducin was carried out suggest that alpha and beta tau subunits may play opposite modulating roles in PtdH and polyphosphoinositide metabolism.

Binding of inositol phosphates to arrestin

Arrestin binds to phosphorylated rhodopsin in its light-activated form (metarhodopsin II), blocking thereby its interaction with the G-protein, transducin. In this study, we show that highly phosphorylated forms of inositol compete against the arrestin-rhodopsin interaction. Competition curves and direct binding assays with free arrestin consistently yield affinities in the micromolar range; for example, inositol 1,3,4,5-tetrakisphosphate (InP4) and inositol hexakisphosphate (InP6 bind to arrestin with dissociation constants of 12 microM and 5 microM, respectively. Only a small control amount of inositol phosphates is bound, when arrestin interacts with phosphorylated rhodopsin. This argues for a release of bound inositol phosphates by interaction with rhodopsin. Transducin, rhodopsin kinase, or cyclic GMP phosphodiesterase are not affected by inositol phosphates. These observations open a new way to purify arrestin and to inhibit its interaction with rhodopsin. Their physiological significance deserves further investigation.

Identification and immunolocalization of phospholipase C in bovine rod outer segments

Bovine rod outer segments (ROS) contain a phospholipase C (PLC) that hydrolyzes phosphatidylinositol 4,5-bisphosphate. Approximately 60-70% of PLC activity is recovered in soluble extracts of ROS. Moreover, the specific activity of this soluble PLC is approximately 10-fold higher than that of resealed ROS enzyme activity. Peptide-specific antiserum (Ab 1109) directed against a highly conserved sequence of the Y-region found in several PLC isozymes was used to detect any PLC belonging to this family. This antibody specifically recognized a protein of apparent molecular mass of approximately 140 kDa present in immunoblots of soluble extracts of both ROS and whole retina. The elution profile of this 140-kDa antigen from a Sephadex G-150 column coincided with the peak of PLC activity, suggesting PLC activity is associated with the 140-kDa protein. Immunocytochemical studies of bovine retina using Ab 1109 showed pronounced immunoreactive labeling in the photoreceptor layer. In resealed ROS and washed ROS membranes, Ab 1109 recognized an additional protein of apparent molecular mass of 70 kDa not usually detectable in soluble extracts of ROS, suggesting the presence of at least two isozymes of PLC in ROS.

Calcium and light adaptation in retinal photoreceptors

In the past several years there has been great progress in the understanding of the phototransduction process in retinal photoreceptors. Recently, this knowledge has expanded and we now understand the mechanism of background light adaptation in these cells and the role that calcium has to play.