Control of light-sensitive current in salamander rods

Light adaptation and the rising phase of the flash photocurrent of salamander retinal rods

1. Both theory and analysis of photocurrents in retinal rods show that phosphodiesterase activity after a flash rises initially as a delayed ramp. 2. The effect of light adaptation on the flash-induced rise in phosphodiesterase activity deduced from photocurrent responses was investigated. 3. Background adaptation reduces the deduced rate of rise of phosphodiesterase activity. The effect is most prominent for bright backgrounds and moderate flashes. There is little reduction for bright flashes, even in bright backgrounds. There is no effect for weak backgrounds. 4. Light adaptation after bleaching visual pigment produces a reduction in the deduced rise of phosphodiesterase activity for all flashes. For bright flashes, the reduction is explained by the reduction in quantum catch. For moderate flashes, there is an extra reduction, similar to the reduction produced by the equivalent background. 5. The results provide support for the idea that a reduction in the amplification step of phototransduction functions as part of the mechanism of light adaptation in rods. The dependence on flash intensity of the background-induced reduction in phosphodiesterase activation could imply a feedback mechanism on the activation steps of phototransduction.

Genetic and functional complexity of inherited retinal degeneration

Recent findings emphasize the complexity, both genetic and functional, of the manifold genes and mutations causing inherited retinal degeneration in humans. Knowledge of the genetic bases of these diseases can contribute to design of rational therapy, as well as elucidating the function of each gene product in normal visual processes.

Channel structure and divalent cation regulation of phototransduction

The identification of additional subunits of the cGMP-gated cation channel suggests exciting questions about their regulatory roles and about structure/functional relationships. How do the different subunits interact? How is the complex assembled into the plasma membrane? Divalent cations have been implicated in the regulation of adaptation. One often overlooked cation is magnesium. Could this ion play a role in phototransduction?

Structure of the cGMP-gated channel

The subunit structure of the cGMP-gated cation channel of rod photoreceptors is rapidly being defined, and in the process the mode of regulation by Ca2+-calmodulin unraveled. Intriguingly, early results suggest that additional subunits of unknown function are associated with the channel and remain to be identified.

Linking genotypes with phenotypes in human retinal degenerations: Implications for future research and treatment

Although undoubtedly it will be incomplete by the time it is published, the target article by Daiger et al. organizes mutations in genes that produce retinal degenerations in humans into categories of clinically relevant phenotypes. Such classifications should help us understand the link between altered photoreceptor cell proteins and subsequent cell death, and they may yield insight into methods for preventing consequent blindness.

Genetic and clinical heterogeneity in tapetal retinal dystrophies

Large scale DNA-mutation screening in patients with hereditary retinal diseases greatly enhances our knowledge about retinal function and diseases. Scientists, clinicians, patients, and families involved with retinal disorders may directly benefit from these developments. However, certain aspects of this expanding knowledge, such as the correlation between genotype and phenotype, may be much more complicated than we expect at present.

The determination of rhodopsin structure may require alternative approaches

The structure of rhodopsin is a subject of intense interest. Solving the structure by traditional methods has proved exceedingly challenging. It may therefore be useful to confront the problem by a combination of alternate techniques. These include FTIR (Fourier transform infrared spectroscopy) and AFM (atomic force microscopy) on the intact protein. Furthermore, additional insights may be gained through structural investigations of discrete rhodopsin domains.

Na-Ca + K exchanger and Ca2+ homeostasis in retinal rod outer segments: Inactivation of the Ca2+ efflux mode and possible involvement of intracellular Ca2+ stores in Ca2+ homeostasis

Inactivation of the Ca2+ extrusion mode of the retinal rod Na- Ca + K exchanger is suggested to be the mechanism that prevents lowering of cytosolic free Ca2+ to < 1 nM when rod cells are saturated for a prolonged time under bright light conditions. Under these conditions, Ca2+ fluxes across disk membranes can contribute significantly to Ca2+ homeostasis in rods.

Nuclear magnetic resonance studies on the structure and function of rhodopsin

Magic angle spinning (MAS) NMR methods provide a means of obtaining high resolution structural data on rhodopsin and its photoin termediates. Current work has focused on the structure of the retinal chromophore and its interactions with surrounding protein charges. The recent development of MAS NMR methods for measuring internuclear distances with a resolution of ∼0.2 will complement diffraction methods for addressing key mechanistic questions.

Glutamate accumulation in the photoreceptor-presumed final common path of photoreceptor cell death

Genetic abnormalities of three factors related to the photoreceptor mechanism have been reported in both animal models and humans. Apoptotic mechanism has also been suggested as a final common pathway of photoreceptor cell death. Our findings of increased level of glutamate in photoreceptor cells in rds mice suggest that amino acid might mediate between these two pathological mechanisms.

Unique lipids and unique properties of retinal proteins

Amino-terminal heteroacylation has been identified in retinal proteins including recoverin and α subunit of G-protein, transducin. The tissue-specific modification seems to mediate not only a proteinmembrane interaction but also a specific protein-protein interaction. The mechanism generating the heterogeneity and its physiological role are still unclear, but an interesting idea for the latter postulates a fine regulation of the signal transduction pathway by distinct N-acyl groups.

Further insight into the structural and regulatory properties of the cGMP-gated channel

Recent studies from several different laboratories have provided further insight into structure-function relationships of cyclic nucleotide-gated channel and in particular the cCMPgated channel of rod photoreceptors. Site-directed mutagenesis and rod-olfactory chimeria constructs have defined important amino acids and peptide segments of the channel that are important in ion blockage, ligand specificity, and gating properties. Molecular cloning studies have indicated that cyclic nucleotide-gated channels consist of two subunits that are required to reproduce the properties of the native channels. Biochemical analysis of the cGMP-gated channel of rodcells have indicated that the 240 kDa protein that co-purifies with the 63 kDa channel subunit contains both the previously cloned second subunit of the channel and a glutamic acid-rich protein. The regulatory properties of the cGMP-gated channel from rod cells has also been studied in more detail. Studies indicate that the beta subunit of the cGMP-gated channel of rod cells contains the binding site for calmodulin. Interaction of calmodulin with the channel alters the apparent affinity of the channel for cGMP in all in vitro systems that have been studied. The significance of these recent studies are discussed in relation to the commentaries on the target article.

Unsolved issues in S-modulin/recoverin study

S-Modulin is a frog homolog of recoverin. The function and the underlying mechanism of the action of these proteins are now understood in general. However, there remain some unsolved issues including; two distinct effects of S-modulin; Ca2+-dependent binding of S-modulin to membranes and a possible target protein; S-modulin-like proteins in other neurons. These issues are considered in this commentary.

Mechanisms of photoreceptor degenerations

The candidate gene approach has identified many causes of photoreceptor rod cell death in retinitis pigmentosa. Some mutations lead to increased cyclicGMP concentrations in rods. Rod photoreceptors are also particularly susceptible to some mutations in housekeeping genes. Although many more cases of macular degeneration than retinitis pigmentosa occur each year, there is much less known about both genetic and sporadic forms of this disease.

Reduced cytoplasmic calcium concentration may be both necessary and sufficient for photoreceptor light adaptation

Light adaptation is modulated almost exclusively by changes in intracellular Ca2+ concentration, and other Ca2+-independent mechanisms are likely to play only a minor role. Changes in Ca2+i may be not only necessary for light adaptation to take place but sufficient to cause it.

The genetic kaleidoscope of vision

Site-specific phenotypic effects of the 73 known alleles in the rhodopsin gene that cause retinal degeneration are difficult to interpret because most alleles are documented in only one case or one family, which means variation in effects could actually arise from interactions with other loci. However, sample sizes necessary to detect epistatic interaction may place an answer to this question beyond our grasp.

Evidence that the type I adenylyl cyclase may be important for neuroplasticity: Mutant mice deficient in the gene for type I adenylyl cyclase show altered behavior and LTP

The regulatory properties of the neurospecific, type I adenylyl cyclase and its distribution within brain have suggested that this enzyme may be important for neuroplasticity. To address this issue, the murine, Ca2+ -stimulated adenylyl cyclase (type I), was inactivated by targeted mutagenesis. Ca2+ -stimulated adenylyl cyclase activity was reduced 40% to 60% in the hippocampus, neocortex, and cerebellum. Long term potentiation in the CA1 region of the hippocampus from mutants was perturbed relative to controls. Both the initial slope and maxim um extent of changes in synaptic response were reduced. Although mutant mice learned to find a hidden platform normally in the Morris water task, they did not display a preference for the region where the platform had been when it was removed. The behavioral phenotype of these mice is very similar to that exhibited by mice which have been surgically lesioned in the hippocampus. These results indicate that disruption of the gene for the type I adenylyl cyclase produces changes in spatial memory and indicate that the cAMP signal transduction pathway may play an important role for synaptic plasticity.

Calcium/calmodulin-sensitive adenylyl cyclase as an example of a molecular associative integrator

Evidence suggests that the Ca2+/calmodulin-sensitive adenylyl cyclase may play a key role in neural plasticity and learning in Aplysia, Drosophila, and mammals. This dually-regulated enzyme has been proposed as a possible site of stimulus convergence during associative learning. This commentary discusses the evidence that is required to demonstrate that a protein in a second messenger cascade actually functions as a molecular site of associative integration. It also addresses the issue of how a dually-regulated protein could contribute to the temporal pairing requirements of classical conditioning: that relationship between stimuli display both temporal contiguity and predictability.

The key to rhodopsin function lies in the structure of its interface with transducin

Light activated rhodopsin functions by catalyzing the exchange of GTP for GDP on numerous copies of transducin. Peptide mapping has shown that at least six regions, three on rhodopsin and three on the transducin alpha subunit, are involved in the active interface between the two proteins. The most informative structural studies of rhodopsin should include focus on the transducin interaction.

Effects of lowered cytoplasmic calcium concentration and light on the responses of salamander rod photoreceptors

1. In order to study the interactions between cytoplasmic calcium concentration ([Ca2+]i) and light in modulating the responses of rod photoreceptors, [Ca2+]i was held at different levels by manipulating Ca2+ fluxes across the outer segment membrane. 2. If [Ca2+]i was reduced by the removal of external Ca2+ in the continued presence of Na+, and then held near this reduced level by exposure to 0 Ca(2+)-0 Na+ solution, the onset of the recovery phase of the response to a bright flash delivered just before the return to Ringer solution was accelerated, much as is the case during light adaptation, provided that precautions were taken to minimize Na+ influx. 3. If the rod was first allowed to adapt to steady light, [Ca2+]i held near the appropriate light-adapted level by superfusion with 0 Ca(2+)-0 Mg(2+)-0 Na+ solution and the light extinguished, the onset of the recovery phase of the bright flash response varied with the original background intensity in the same way as in the continued presence of steady light. These results indicate that reduction of [Ca2+]i is sufficient to induce this manifestation of light adaptation in darkness. 4. When [Ca2+]i was held at a reduced level in darkness, not only was the sensitivity to dim flashes reduced, but the response rising phase was also delayed and its amplitude increased supralinearly with flash intensity, neither of which changes is seen during light adaptation. However, similar changes in response kinetics resulted when [Ca2+]i was held near its normal dark level and the phosphodiesterase was partially inhibited by 3-isobuty-1-methylxanthine (IBMX), suggesting that they arose indirectly from an elevated cyclic GMP concentration rather than from a direct effect of Ca2+. 5. If [Ca2+]i was held near the normal dark level and bright steady light presented, the circulating current was completely suppressed. Partial inhibition of the phosphodiesterase by superfusion with 0 Ca(2+)-0 Na+ solution including IBMX resulted in restoration of the circulating current. Dim flash responses recorded under these conditions exhibited kinetics similar to those recorded in 0 Ca(2+)-0 Na+ solution in darkness, in contrast to the response acceleration seen when [Ca2+]i was held near the appropriate light-adapted level. These results indicate that the kinetics of the flash response depend on [Ca2+]i rather than on the steady light intensity.

Bleached pigment activates transduction in isolated rods of the salamander retina

1. We have used suction electrode recording together with rapid steps into Li+ solution and 0.5 mM IBMX solution to estimate the rates of the guanylyl phosphodiesterase (PDE) and guanylyl cyclase in isolated rods of the salamander, Ambystoma tigrinum. 2. We show that both the PDE and cyclase velocities are accelerated by steady background light. The steady velocities of both enzymes appear to be saturating functions of background intensity. 3. Bleaching also accelerates both the PDE and cyclase. This effect is maintained long after the bleaching stimulus is removed (up to 2 h) and is reversed only if the photopigment is regenerated with exogenous chromophore. 4. The estimated steady-state PDE and cyclase velocities appear to be linear functions of the amount of pigment bleached, as if each bleached pigment molecule activated the transduction cascade with the same probability and gain. 5. The effectiveness of bleached pigment in activating transduction is only 10(-6) to 10(-7) times that of activated rhodopsin (Rh*), but this is sufficient after large bleaches to produce an 'equivalent background' excitation of the rod, which is probably responsible, at least in part, for bleaching desensitization.

Stochastic simulation of activation in the G-protein cascade of phototransduction

Activation of the G-protein cascade underlying phototransduction has been modeled by simulating the two-dimensional diffusional interactions that occur at the rod disc membrane between the three reacting protein species, which are the activated rhodopsin (R*), the G-protein (G), and the effector protein (E, the phosphodiesterase, PDE). The stochastic simulations confirm the main predictions of a simplified analytical model (Lamb, T. D., and E. N. Pugh, 1992, Journal of Physiology 449:719-758), and extend that treatment to more complicated cases, where there is a finite probability of reaction or a finite time for reaction. The simulations also provide quantitative estimates of the efficiency of coupling from activated G-protein (G*) to activated effector (E*) in terms of the concentrations, lateral diffusion coefficients, and binding rate constants of the participating molecules; the efficiency of coupling from G* to E* is found to be not as high as in the previous simplified analytical theory. The findings can be extended to other G-protein cascades, provided that the physical parameters of those cascades are specified.

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

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

Changes in the light-sensitive current of salamander rods upon manipulation of putative pH-regulating mechanisms in the inner and outer segment

The light-sensitive current of dark-adapted rods isolated from the Ambystoma retina was recorded while either the inner or the outer segment (IS or OS) protruding from the suction pipette was exposed to treatments intended to reveal the physiological roles of pH-regulating transport mechanisms. Applied to the IS, both amiloride (presumed to block Na+/H+ exchange, 2 mM) and 4-4'-diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS) (presumed to block bicarbonate transport, 0.1 mM) generally abolished light sensitivity completely but reversibly, consistent with acidification of the IS. Yet, the circulating ("dark") current often persisted, implying that the OS was not acidified. Applied to the OS, amiloride depressed but DIDS increased the dark current and photoresponses. Given the fact that the current increases with rising OS-pHi, this suggests alkalinization, which could be due to DIDS inhibiting bicarbonate extrusion by HCO3-/Cl- exchangers in the OS. Consistent with this idea, replacing external Cl- by other anions increased the current as would be expected if HCO3-/Cl- exchange is reversed. We propose that the IS and OS manage their acid balances independently and with different sets of transport mechanisms. Acidosis in either compartment suppresses the photosensitivity of the rod, but by differing mechanisms.

The role of cytoplasmic calcium in photoreceptor light adaptation

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

Modulation of transduction gain in light adaptation of retinal rods

The effect of light adaptation on the period of photocurrent saturation induced by a bright stimulating flash was examined in rod photoreceptors of the larval-stage tiger salamander (Ambystoma tigrinum). Using suction electrodes, photocurrent responses to brief flashes were recorded from single, isolated rods in the presence and absence of steady background illumination. Background light decreased the saturation period (T) measured at fixed flash intensity (fixed If) and in this respect light-adapted the saturating response. Effects of the background on responses to weak (i.e. subsaturating) and bright flashes were compared with changes in a parameter, phi = e-delta T/TR*, where delta T is the decrease in saturation period, and where TR* is the slope of the line that relates T and ln If in a given state of adaptation. Dark- and light-adapted responses to flash intensities IDf and ILf, respectively, exhibited similar absolute peak photocurrent and falling-phase kinetics when IDf and ILf satisfied the relation, IDf = phi (ILf + IbTR*), where Ib is the background intensity. It is argued that phi approximates the relative PDE*/R* gain of transduction, i.e. the relative peak level of activated cGMP phosphodiesterase (PDE*) produced by a given, small amount of photoactivated visual pigment (R*). Interpreted on this view, the results imply that light adaptation derives largely from a decrease in PDE*/R gain, rather than from the stimulation of guanylate cyclase activity. The data are consistent with the possibility that modulation of the lifetime of PDE* underlies the background dependence of phi.

Regulation of intracellular cyclic GMP concentration by light and calcium in electropermeabilized rod photoreceptors

This study examines the regulation of cGMP by illumination and by calcium during signal transduction in vertebrate retinal photoreceptor cells. We employed an electropermeabilized rod outer segment (EP-ROS) preparation which permits perfusion of low molecular weight compounds into the cytosol while retaining many of the features of physiologically competent, intact rod outer segments (ROS). When nucleotide-depleted EP-ROS were incubated with MgGTP, time- and dose-dependent increases in intracellular cGMP levels were observed. The steady state cGMP concentration in EP-ROS (0.007 mol cGMP per mol rhodopsin) approached the cGMP concentration in intact ROS. Flash illumination of EP-ROS in a 250-nM free calcium medium resulted in a transient decrease in cGMP levels; this occurred in the absence of changes in calcium concentration. The kinetics of the cGMP response to flash illumination of EP-ROS were similar to that of intact ROS. To further examine the effects of calcium on cGMP metabolism, dark-adapted EP-ROS were incubated with MgGTP containing various concentrations of calcium. We observed a twofold increase in cGMP steady state levels as the free calcium was lowered from 1 microM to 20 nM; this increase was comparable to the behavior of intact ROS. Measurements of guanylate cyclase activity in EP-ROS showed a 3.5-fold increase in activity over this range of calcium concentrations, indicating a retention of calcium regulation of guanylate cyclase in EP-ROS preparations. Flash illumination of EP-ROS in either a 50- or 250-nM free calcium medium revealed a slowing of the recovery time course at the lower calcium concentration. This observation conflicts with any hypothesis whereby a reduction in free calcium concentration hastens the recovery of cytoplasmic cGMP levels, either by stimulating guanylate cyclase activity or by inhibiting phosphodiesterase activity. We conclude that changes in the intracellular calcium concentration during visual transduction may have more complex effects on the recovery of the photoresponse than can be accounted for solely by guanylate cyclase activation.

R2D5 antigen: a calcium-binding phosphoprotein predominantly expressed in olfactory receptor neurons

R2D5 is a mouse monoclonal antibody that labels rabbit olfactory receptor neurons. Immunoblot analysis showed that mAb R2D5 recognizes a 22-kD protein with apparent pI of 4.8, which is abundantly contained in the olfactory epithelium and the olfactory bulb. We isolated cDNA for R2D5 antigen and confirmed by Northern analysis and neuronal depletion technique that R2D5 antigen is expressed predominantly, but not exclusively, in olfactory receptor neurons. Analysis of the deduced primary structure revealed that R2D5 antigen consists of 189 amino acids with calculated M(r) of 20,864 and pI of 4.74, has three calcium-binding EF hands, and has possible phosphorylation sites for Ca2+/calmodulin-dependent protein kinase II (CaM kinase II) and cAMP-dependent protein kinase (A kinase). Using the bacterially expressed protein, we directly examined the biochemical properties of R2D5 antigen. R2D5 antigen binds Ca2+ and undergoes a conformational change in a manner similar to calmodulin. R2D5 antigen is phosphorylated in vitro by CaM kinase II and A kinase at different sites, and 1.81 and 0.80 mol of Pi were maximally incorporated per mol of R2D5 antigen by CaM kinase II and A kinase, respectively. Detailed immunohistochemical study showed that R2D5 antigen is also expressed in a variety of ependymal cells in the rabbit central nervous system. Aside from ubiquitous calmodulin, R2D5 antigen is the first identified calcium-binding protein in olfactory receptor neurons that may modulate olfactory signal transduction. Furthermore our results indicate that olfactory receptor neurons and ependymal cells have certain signal transduction components in common, suggesting a novel physiological process in ependymal cells.

A GTPase-accelerating factor for transducin, distinct from its effector cGMP phosphodiesterase, in rod outer segment membranes

Hydrolysis of GTP by the photoreceptor G protein transducin (Gt alpha) was found to occur with kinetics identical to the inactivation of its effector cGMP phosphodiesterase (PDE), but was too slow (tens of seconds) in dilute rod outer segment (ROS) suspensions to account for subsecond recovery of the light response. Raising the concentration of ROS membranes increased the rates of GTP hydrolysis and PDE inactivation in parallel as much as 6-fold. Holo-PDE and its gamma subunit had weak effects on GTPase kinetics (< 1.6-fold and < 1.3-fold, respectively). ROS membranes stripped of PDE retained approximately 90% of a GTPase accelerating activity that was protease sensitive, indicating that they contain a GTPase-accelerating factor distinct from PDE.

In retinal cones, membrane depolarization in darkness activates the cGMP-dependent conductance. A model of Ca homeostasis and the regulation of guanylate cyclase

We measured outer segment currents under voltage clamp in solitary, single cone photoreceptors isolated from the retina of striped bass. In darkness, changes in membrane voltage to values more positive than 10 mV activate a time- and voltage-dependent outward current in the outer segment. This dark, voltage-activated current (DVAC) increases in amplitude with a sigmoidal time course up to a steady-state value, reached in 0.75-1.5 s. DVAC is entirely suppressed by light, and its current-voltage characteristics and reversal potential are the same as those of the light-sensitive currents. DVAC, therefore, arises from the activation by voltage in the dark of the light-sensitive, cGMP-gated channels of the cone outer segment. Since these channels are not directly gated by voltage, we explain DVAC as arising from a voltage-dependent decrease in cytoplasmic Ca concentration that, in turn, activates only guanylate cyclase and results in net synthesis of cGMP. This explanation is supported by the finding that the Ca buffer BAPTA, loaded into the cytoplasm of the cone outer segment, blocks DVAC. To link a decrease in cytoplasmic Ca concentration to the synthesis of cGMP and the characteristics of DVAC, we develop a quantitative model that assumes cytoplasmic Ca concentration can be continuously calculated from the balance between passive Ca influx via the cGMP-gated channel and its active efflux via a Na/Ca,K exchanger, and that further assumes that guanylate cyclase is activated by decreasing cytoplasmic Ca concentration with characteristics identical to those described for the enzyme in rods. The model successfully simulates experimental data by adjusting the Ca conductance of the cGMP-gated channels as a function of voltage and the Ca buffering power of the cytoplasm. This success suggests that the activity of guanylate cyclase in cone outer segments is indistinguishable from that in rods.

Amplification and kinetics of the activation steps in phototransduction

We can summarize our investigation of amplification in the activation steps of vertebrate phototransduction as follows. (1) A theoretical analysis of the activation steps of the cGMP cascade shows that after a brief flash of phi photoisomerizations the number of activated PDE molecules should rise as a delayed ramp with slope proportional to phi, and that, as a consequence, the cGMP-activated current should decay as a delayed Gaussian function of time (Eqn. 20). (i) Early in the response to a flash, the normalized response R(t) can be approximated as rising as 1/2 phi At2 (after a short delay), where A is the amplification constant characteristic of the individual photoreceptor. (ii) The delayed ramp behavior of PDE activation and the consequent decline of current in the form of the delayed Gaussian are confirmed by experiments in a variety of photoreceptors; the analysis thus yields estimates of the amplification constant from these diverse photoreceptors. (iii) Eqn. 20 further predicts that the response-intensity relation at any fixed time should saturate exponentially, as has been found experimentally. (2) The amplification constant A can be expressed as the product of amplification factors contributed by the individual activation steps of phototransduction, i.e., A = nu RG cGP beta sub n (Eqns. 9 and 21), where (i) nu RG is the rate of G* production per Rh*; (ii) cGP is the efficiency of the coupling between G* production and PDE* production; (iii) beta sub is the increment in hydrolytic rate constant produced by one PDE*, i.e., a single activated catalytic subunit of PDE; and (iv) n is the Hill coefficient of opening of the cGMP-activated channels. (3) The amplification factor beta sub includes the ratio kcat/Km, which characterizes the hydrolytic activity of the PDE in vivo where cG << Km. Two different analyses based upon photocurrents were developed which provide lower bounds for kcat/Km in vivo; these analyses establish that kcat/Km probably exceeds 10(7) M-1 s-1 (and is likely to be higher) in both amphibian and mammalian rods. Few biochemical studies (other than those using trypsin activation) have yielded such high values. A likely explanation of many of the relatively low biochemical estimates of kcat/Km is that Km may have been overestimated by a factor of about 4 in preparations in which stacks of disks are left intact, due to diffusion with hydrolysis in the stacks.(ABSTRACT TRUNCATED AT 400 WORDS)

The effect of recoverin-like calcium-binding proteins on the photoresponse of retinal rods

The rod photoresponse is triggered by an enzyme cascade that stimulates cGMP hydrolysis. The resulting fall in cGMP leads to a decrease in Ca2+, which promotes photoresponse recovery by activating guanylate cyclase, causing cGMP resynthesis. In vitro biochemical studies suggest that Ca2+ activation of guanylate cyclase is medicated by recoverin, a 26 kd Ca(2+)-binding protein. To evaluate this, exogenous bovine recoverin and two other homologous Ca(2+)-binding proteins from chicken and Gecko retina were dialyzed into functionally intact Gecko rods using whole-cell recording. All three proteins prolonged the rising phase of the photoresponse without affecting the kinetics of response recovery. These results suggest that recoverin-like proteins affect termination of the transduction cascade, rather than mediate Ca(2+)-sensitive activation of guanylate cyclase.

Interactions between divalent cations and the gating machinery of cyclic GMP-activated channels in salamander retinal rods

The effects of divalent cations on the gating of the cGMP-activated channel, and the effects of gating on the movement of divalent cations in and out of the channel's pore were studied by recording macroscopic currents in excised membrane patches from salamander retinal rods. The fractional block of cGMP-activated Na+ currents by internal and external Mg2+ as well as internal Ca2+ was nearly independent of cGMP concentration. This indicates that Mg2+ and Ca2+ bind with similar affinity to open and closed states of the channel. In contrast, the efficiency of block by internal Cd2+ or Zn2+ increased in proportion to the fraction of open channels, indicating that these ions preferentially occupy open channels. The kinetics of block by internal Ni2+, which competes with Mg2+ but blocks more slowly, were found to be unaffected by the fraction of channels open. External Ni2+, however, blocked and unblocked much more rapidly when channels were mostly open. This suggests that within the pore a gate is located between the binding site(s) for ions and the extracellular mouth of the channel. Micromolar concentrations of the transition metal divalent cations Ni2+, Cd2+, Zn2+, and Mn2+ applied to the cytoplasmic surface of a patch potentiated the response to subsaturating concentrations of cGMP without affecting the maximum current induced by saturating cGMP. The concentration of cGMP that opened half the channels was often lowered by a factor of three or more. Potentiation persisted after the experimental chamber was washed with divalent-free solution and fresh cGMP was applied, indicating that it does not result from an interaction between divalent cations and cGMP in solution; 1 mM EDTA or isotonic MgCl2 reversed potentiation. Voltage-jump experiments suggest that potentiation results from an increase in the rate of cGMP binding. Lowering the ionic strength of the bathing solution enhanced potentiation, suggesting that it involves electrostatic interactions. The strong electrostatic effect on cGMP binding and absence of effect on ion permeation through open channels implies that the cGMP binding sites on the channel are well separated from the permeation pathway.

Effect of lithium on rod photoreceptor rhodopsin-coupled G-protein (transducin)

1. Lithium was known to inhibit both adrenergic and cholinergic agonist-induced activation of G-proteins in cerebral cortex. 2. Doly et al (1989) observed that lithium reduced the b-wave of the electroretinogram and suggested that the effect was due to inhibition of G-protein in photoreceptor cells. 3. This study was undertaken to test this hypothesis directly on photoreceptor cell membranes. Rod disk membranes containing the visual transduction machinery were isolated in the dark and the effect of lithium was tested on (a) activation by bright light of G-protein-mediated cyclic GMP hydrolysis, (b) light sensitivity of the activation, (c) the lifetime of the light-activated receptor, and (d) light activation of GTP-gamma-S binding to the membranes. 4. None of these processes were affected by lithium. It is therefore concluded that the effects of lithium on the b-wave of the electroretinogram should be due to influences on G-proteins in other parts of the retina.

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)

G-protein cascades: gain and kinetics

G-protein cascades provide amplification in a wide variety of biological signal transducers--from hormonal and synaptic systems to the receptor cells of vision and olfaction. Through recent understanding of the molecular mechanisms involved, it is possible to construct a quantitative description of the amplification and speed of response of the cascade. The gain and kinetics can now be described in terms of physical parameters, such as enzyme activities and the densities and lateral diffusion coefficients of the proteins involved.

A quantitative account of the activation steps involved in phototransduction in amphibian photoreceptors

1. We have undertaken a theoretical analysis of the steps contributing to the phototransduction cascade in vertebrate photoreceptors. We have explicitly considered only the activation steps, i.e. we have not dealt with the inactivation reactions. 2. From the theoretical analysis we conclude that a single photoisomerization leads to activation of the phosphodiesterase (PDE) with a time course which approximates a delayed ramp; the delay is contributed by several short first-order delay stages. 3. We derive a method for extracting the time course of PDE activation from the measured electrical response, and we apply this method to recordings of the photoresponse from salamander rods. The results confirm the prediction that the time course of PDE activation is a delayed ramp, with slope proportional to light intensity; the initial delay is about 10-20 ms. 4. We derive approximate analytical solutions for the electrical response of the photoreceptor to light, both for bright flashes (isotropic conditions) and for single photons (involving longitudinal diffusion of cyclic GMP in the outer segment). The response to a brief flash is predicted to follow a delayed Gaussian function of time, i.e. after an initial short delay the response should begin rising in proportion to t2. Further, the response-intensity relation is predicted to obey an exponential saturation. 5. These predictions are compared with experiment, and it is shown that the rising phase of the flash response is accurately described over a very wide range of intensities. We conclude that the model provides a comprehensive description of the activation steps of phototransduction at a molecular level.

Light-dependent delay in the falling phase of the retinal rod photoresponse

Using suction electrodes, photocurrent responses to 100-ms saturating flashes were recorded from isolated retinal rods of the larval-stage tiger salamander (Ambystoma tigrinum). The delay period (Tc) that preceded recovery of the dark current by a criterion amount (3 pA) was analyzed in relation to the flash intensity (If), and to the corresponding fractional bleach (R*0/Rtot) of the visual pigment; R*0/Rtot was compared with R*s/Rtot, the fractional bleach at which the peak level of activated transducin approaches saturation. Over an approximately 8 ln unit range of I(f) that included the predicted value of R*s/Rtot, Tc increased linearly with ln I(f). Within the linear range, the slope of the function yielded an apparent exponential time constant (tau c) of 1.7 +/- 0.2 s (mean +/- S.D.). Background light reduced the value of Tc measured at a given flash intensity but preserved a range over which Tc increased linearly with ln I(f); the linear-range slope was similar to that measured in the absence of background light. The intensity dependence of Tc resembles that of a delay (Td) seen in light-scattering experiments on bovine retinas, which describes the period of essentially complete activation of transducin following a bright flash; the slope of the function relating Td and ln flash intensity is thought to reflect the lifetime of photoactivated visual pigment (R*) (Pepperberg et al., 1988; Kahlert et al., 1990). The present data suggest that the electrophysiological delay has a similar basis in the deactivation kinetics of R*, and that tau c represents TR*, the lifetime of R* in the phototransduction process. The results furthermore suggest a preservation of the "dark-adapted" value of TR* within the investigated range of background intensity.

P26-calcium binding protein from bovine retinal photoreceptor cells

The primary structure of bovine retinal calcium binding protein P26 has been determined by parallel analysis of protein and corresponding cDNA. This protein is identical to recovering and shares 59% homology with visinin, a cone specific calcium binding protein from chicken retina. Preliminary data are presented on expression of P26 as a fusion protein in E. coli.

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.

Light adaptation in turtle cones. Testing and analysis of a model for phototransduction

Light adaptation in cones was characterized by measuring the changes in temporal frequency responses to sinusoidal modulation of light around various mean levels spanning a range of four log units. We have shown previously that some aspects of cone adaptation behavior can be accounted for by a biochemical kinetic model for phototransduction in which adaptation is mediated largely by a sigmoidal dependence of guanylate cyclase activity on the concentration of free cytoplasmic Ca2+, ([Ca2+]i) (Sneyd and Tranchina, 1989). Here we extend the model by incorporating electrogenic Na+/K+ exchange, and the model is put to further tests by simulating experiments in the literature. It accounts for (a) speeding up of the impulse response, transition from monophasic to biphasic waveform, and improvement in contrast sensitivity with increasing background light level, I0; (b) linearity of the response to moderate modulations around I0; (c) shift of the intensity-response function (linear vs. log coordinates) with change in I0 (Normann and Perlman, 1979); the dark-adapted curve adheres closely to the Naka-Rushton equation; (d) steepening of the sensitivity vs. I0 function with [Ca2+]i fixed at its dark level, [Ca2+]i dark; (Matthews et al., 1988, 1990); (e) steepening of the steady-state intensity-response function when [Ca2+]i is held fixed at its dark level (Matthews et al., 1988; 1990); (f) shifting of a steep template saturation curve for normalized photocurrent vs. light-step intensity when the response is measured at fixed times and [Ca2+]i is held fixed at [Ca2+]i dark (Nakatani and Yau, 1988). Furthermore, the predicted dependence of guanylate cyclase activity on [Ca2+] closely matches a cooperative inhibition equation suggested by the experimental results of Koch and Stryer (1988) on cyclase activity in bovine rods. Finally, the model predicts that some changes in response kinetics with background light will still be present, even when [Ca2+]i is held fixed at [Ca2]i dark.

Incorporation of chelator into guinea-pig rods shows that calcium mediates mammalian photoreceptor light adaptation

1. The effects of steady light on the sensitivity and kinetics of the photocurrent response were studied in the rod photoreceptors of the guinea-pig, using suction pipette recordings of circulating current. 2. The sensitivity of the flash response decreased with increasing background intensity according to Weber's law. Ultimately for the brightest backgrounds saturation ensued. The recovery phase of the flash response was accelerated by steady light, while the early rising phase was little affected. 3. These results indicate that guinea-pig rods adapt to light in much the same way as do the rods and cones of lower vertebrates. 4. The role of cytoplasmic calcium concentration in this adaptation was studied by incorporation of the calcium chelator bis(o-aminophenoxy)ethane- N,N,N',N'-tetraacetic acid (BAPTA) into the rod cytoplasm. Superfusion with a solution containing the membrane-permeant acetoxymethyl ester resulted in progressive changes in the response to light. 5. BAPTA incorporation retarded the falling phase of the flash response, thereby increasing receptor sensitivity, but did not affect the early rising phase of the response. BAPTA also slowed the adaptation of the response to steady illumination. 6. These results indicate that cytoplasmic calcium concentration plays a similar role in the light adaptation of guinea-pig rods to that in the adaptation of the rods and cones of lower vertebrates. Calcium therefore appears to act as the messenger of light adaptation in mammalian rods.