Role of calcium in regulating the cyclic GMP cascade of phototransduction in retinal rods

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.

Diffusion coefficient of cyclic GMP in salamander rod outer segments estimated with two fluorescent probes

Experiments have demonstrated that single photoisomerizations in amphibian and primate rods can cause the suppression of 3-5% of the dark circulating current at the response peak (Baylor, D. A., T. D. Lamb, and K. W. Yau. 1979. J. Physiol. (Lond.). 288:613-634; Baylor, D. A., B. J. Nunn, and J. L. Schnapf. 1984. J. Physiol. (Lond.). 357:575-607). These results indicate that the change in [cGMP] effected by a single isomerization must spread longitudinally over at least the corresponding fractional length of the outer segment. The effective longitudinal diffusion coefficient, Dx, of cGMP is thus an important determinant of rod sensitivity. We report here measurements of the effective longitudinal diffusion coefficients, Dx, of two fluorescently labeled molecules: 5/6-carboxyfluorescein and 8-(fluoresceinyl)thioguanosine 3',5'-cyclic monophosphate, introduced into detached outer segments via whole-cell patch electrodes. For these compounds, the average time for equilibration of the entire outer segment with the patch pipette was approximately 6 min. Fluorescence images of rods were analyzed with a one-dimensional diffusion model that included limitations on transfer between the electrode and outer segment and the effects of intracellular binding of the dyes. The analyses yielded estimates of Dx of 1.9 and 1.0 microns 2.s-1 for the two dyes. It is shown that these results place an upper limit on Dx for cGMP of 11 microns2.s-1. The actual value of Dx for cGMP in the rod will depend on the degree of intracellular binding of cGMP. Estimates of the effective buffering power for cGMP in the rod at rest range from two to six (Lamb and Pugh, 1992; Cote and Brunnock, 1993). When combined with these estimates, our results predict that for cGMP itself, Dx falls within the range of 1.4-5.5 microns 2.s-1.

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.

Rhodopsin phosphorylation as a mechanism of cyclic GMP phosphodiesterase regulation by S-modulin

During light-adaptation by the vertebrate eye, the rods are desensitized and the light response is accelerated. When light is absorbed by the rods, a phosphodiesterase is activated that hydrolyses cyclic GMP. A light-induced decrease in cytoplasmic Ca2+ concentration is part of this light-adaptation process. The protein S-modulin (M(r) 26,000) is known to increase the fraction of light-activated cyclic GMP-phosphodiesterase (PDE) at high Ca2+ concentrations in frog rod photoreceptors. Here I present evidence that S-modulin lengthens the lifetime of active PDE (PDE*) at high Ca2+ concentrations. These S-modulin effects are observed in the physiological range of Ca2+ concentration (30 nM to 1 microM; half-maximum effects at 200-400 nM). At the high Ca2+ concentrations at which S-modulin prolongs the lifetime of PDE*, S-modulin inhibits rhodopsin phosphorylation (half-maximum effect at approximately 100 nM Ca2+). ATP is necessary for the S-modulin effects on PDE activation. I therefore conclude that the Ca(2+)-dependent regulation of PDE by S-modulin is mediated by rhodopsin phosphorylation. This regulation seems to be the principal mechanism of light adaptation in vertebrate photoreceptors.

Calcium-calmodulin dependence of actin accretion and lethality in cultured HEp-2 cells infected with enteropathogenic Escherichia coli

Infection of cultured HEp-2 cells with enteropathogenic Escherichia coli causes substantial actin accretion at points of bacterial contact and cell death. Loss of viability was delayed by chelating intracellular free calcium. Actin accretion was partially inhibited by preventing elevation of free cytosolic calcium and prevented by treatment with a calmodulin inhibitor.

Modulation of the cGMP-gated channel of rod photoreceptor cells by calmodulin

Photobleaching of rhodopsin in rod photoreceptors activates the visual cascade system leading to a decrease in cyclic GMP and the closure of cGMP-gated channels in the rod outer segment plasma membrane. Calcium plays an important role in the recovery of the rod outer segment to its dark state by regulating the resynthesis of cGMP by guanylate cyclase. Here we report that calmodulin, a Ca(2+)-binding protein present in the rod outer segment, increases the apparent Michaelis constant of the channel for cGMP. This results in a decrease in the rate of cation influx into the rod outer segment by two- to sixfold at low cGMP concentrations and has the effect of increasing the sensitivity of the channel to small changes in cGMP levels during phototransduction. Biochemical studies indicate that calcium-calmodulin binds to a protein of M(r) 240K which is tightly associated with the channel. On the basis of these studies, Ca2+ is suggested to play a central role in photorecovery and light adaptation, not only by regulating guanylate cyclase, possibly through recoverin, but also by modulating the cGMP-gated channel through calmodulin interaction with the 240K protein.

The electrical responses of the trout pineal photoreceptors to brief and prolonged illumination

Intracellular recordings from 103 photoreceptors in the excised pineal body of adult trouts were obtained by using single electrode current- and voltage-clamp techniques. The photoresponses to brief flashes showed the same polarity but a slower time course than those previously recorded from retinal photoreceptors of lower vertebrates. Pineal photoreceptors showed spectral sensitivity peaks at about 495 and 521 nm and absolute sensitivity comparable to retinal cone cells of the same species. The photoreceptor membrane conductance, measured under voltage clamp during moderate illumination was about 10% lower than in the dark, and the extrapolated reversal potential of the response was at 60 mV above the dark membrane potential. The addition of 3-isobutyl-1-methylxanthine (IBMX) to the perfusate was followed by a receptor depolarization in the dark and by a slow-down of the response kinetic. Pineal receptor cells produce constant amplitude responses during steady illumination, without displaying the delayed slow depolarization typically associated with light adaptation of retinal photoreceptors. Photoresponses to brief flashes superimposed on a steady illumination are decreased in amplitude by an amount directly related to the background intensity. Increase of the background intensity leads to threshold increments without significant changes of the saturation intensity, resulting in a gradual compression of the cell dynamic range. These results were discussed relative to light adaptation in retinal photoreceptors. The conclusion can be drawn that the response properties of pineal photoreceptors during steady illumination are part of an unknown, self-regulating mechanism to lock the rate of metabolism and secretion of indolamines to the absolute level of diurnal light.

Modulation of synaptic gain by light

Although synaptic transmission in the retina has been assumed to be static, it appears that the voltage gains of the synapses between photoreceptors and second-order cells can be enhanced by light. Voltage gains of the synapses between rods and bipolar (or horizontal) cells are about 10 times higher in the presence of dim background light than in darkness. This increase in synaptic gain may compensate for the loss of rod light responsiveness caused by weak background light so that the animal can maintain good rod sensitivity under moonlight or starlight, the natural lighting condition for mating and food catching.

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.

Association of cyclic GMP-gated channels and Na(+)-Ca(2+)-K+ exchangers in bovine retinal rod outer segment plasma membranes

1. Cyclic GMP-gated channels and Na(+)-Ca(2+)-K+ exchangers from bovine photoreceptors were examined by investigation of the Ca2+ fluxes from vesicles of rod outer segment (ROS) membranes and from proteoliposomes obtained by solubilization of the ROS membrane proteins and reconstitution in soy bean L-alpha-phosphatidylcholine (PC). 2. Whereas vesicles obtained by mild sonication of ROS membranes in a Ca(2+)-containing buffer yielded a maximal cyclic GMP-induced Ca2+ release of about 2.5% and a maximal Na(+)-induced Ca2+ release of about 7%, freezing and thawing of ROS membranes prior to sonication elevated these maximal Ca2+ releases to about 17% for cyclic GMP, and to about 34% for Na+. These observations are in agreement with the view that cyclic GMP-gated channels and Na(+)-Ca(2+)-K+ exchangers are localized only in the plasma membrane of the photoreceptors (which in bovine ROS makes up about 6% of the total membrane), whereas freezing and thawing results in fusion of disc and plasma membranes, thus leading to a distribution of these proteins over a much larger membrane area. 3. For fused ROS membranes, the cyclic GMP-releasable fraction of Ca2+ of 17% is an upper bound; assuming that the cyclic GMP-gated channels are randomly distributed we estimate that about 37% of the vesicles contain at least one cyclic GMP-gated channel. The mean diameter of the vesicles prepared by sonication was determined to be 0.12 +/- 0.04 micron, and therefore the fused ROS membranes contain about sixteen cyclic GMP-gated channels/microns 2. If all cyclic GMP-gated channels originated from the plasma membrane, we estimate that the plasma membrane contains about 270 cyclic GMP-gated channels/microns 2. 4. In vesicles prepared from fused ROS membranes, Na(+)-Ca2+ exchange after activation of the cyclic GMP-gated channels. On the other hand, after an exhaustive Na(+)-Ca2+ exchange, only little, if any, Ca2+ was released upon addition of cyclic GMP, demonstrating that cyclic GMP-gated channels and Na(+)-Ca(2+)-K+ exchangers occur on the same vesicle fraction. This observation suggests that Na(+)-Ca(2+)-K+ exchangers do not distribute independently of the cyclic GMP-gated channels upon membrane fusion but are apparently associated with the cyclic GMP-gated channels.(ABSTRACT TRUNCATED AT 400 WORDS)

A molecular basis for Weber's law

A mathematical model is presented that obeys a strong form of Weber's law--over a range of adapting and stimulus intensities, equal contrast stimuli evoke identical responses. To account for the strong Weber's law, the adaptive stage in the proposed model employs a "delayed" reverse reaction along with a power-law input. It is suggested that this Weber's law mechanism is responsible for a slow, voltage-uncorrelated component of adaptation in the vertebrate photoreceptor. A plausible biochemical mechanism is the G-protein cycle with phosphorylation of photoactivated photopigment (and binding of arrestin to the phosphorylated photopigment) as the adaptive process. In an Appendix, features of the general model and implications of a specific biochemical model are examined by computer simulation.

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.

Light adaptation in retinal rods of the rabbit and two other nonprimate mammals

The responses of rabbit rods to light were studied by drawing a single rod outer segment projecting from a small piece of retina into a glass pipette to record membrane current. The bath solution around the cells was maintained at near 40 degrees C. Light flashes evoked transient outward currents that saturated at up to approximately 20 pA. One absorbed photon produced a response of approximately 0.8 pA at peak. At the rising phase of the flash response, the relation between response amplitude and flash intensity (IF) had the exponential form 1-e-kappa FIF (where kappa F is a constant denoting sensitivity) expected from the absence of light adaptation. At the response peak, however, the amplitude-intensity relation fell slightly below the exponential form. At times after the response peak, the deviation was progressively more substantial. Light steps evoked responses that rose to a transient peak and rapidly relaxed to a lower plateau level. The response-intensity relation again indicated that light adaptation was insignificant at the early rising phase of the response, but became progressively more prominent at the transient peak and the steady plateau of the response. Incremental flashes superposed on a steady light of increasing intensity evoked responses that had a progressively shorter time-to-peak and faster relaxation, another sign of light adaptation. The flash sensitivity changed according to the Weber-Fechner relation (i.e., inversely) with background light intensity. We conclude that rabbit rods adapt to light in a manner similar to rods in cold-blooded vertebrates. Similar observations were made on cattle and rat rods.

Calcium-dependent regulation of cyclic GMP phosphodiesterase by a protein from frog retinal rods

In vertebrate photoreceptors, light reduces cyclic GMP concentration and closes cGMP-activated channels to induce a hyperpolarizing response. As Ca2+ can permeate the channels and the Na(+)-Ca2+ exchanger continuously extrudes Ca2+, closure of the channel results in a reduction of the inter-rod Ca2+ concentration. This is believed to be one of the mechanisms of light-adaptation produced by activation of guanylate cyclase. Effects of Ca2+ on the cGMP phosphodiesterase (PDE) have been reported, but their physiological significance has remained unclear. We have perfused the inside-out preparation of a frog rod outer segment (I/O ROS, originally termed truncated ROS, and find that Ca2+ in a physiological range regulates the light-activation of PDE. Therefore, PDE regulation by Ca2+ must be involved in light-adaptation in rods. The effect is mediated by a newly found protein which binds to disk membranes at high Ca2+ concentrations and prolongs PDE activation.

Phototransduction: different mechanisms in vertebrates and invertebrates

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

Calcium and the mechanism of light adaptation in vertebrate photoreceptors

Vertebrate photoreceptors transduce the absorption of light into a hyperpolarizing change in membrane potential. The mechanism of transduction is becoming fairly well understood and has been shown to occur via a G protein-coupled decrease in cyclic GMP. Attention is now turning to the way the enzymatic machinery in the outer segment of the photoreceptor cell is modulated during light adaptation. Recent studies show that light adaptation cannot occur if changes in the concentration of cytoplasmic free calcium in the outer segment are prevented, suggesting that calcium functions as a second messenger in sensitivity regulation.

Recovery from bleaching in photoreceptors promoted by biotin, pyruvate, and glucose

In the rods of Bufo marinus and other species, bleaching of the rhodopsin in isolated cells leads to a loss of sensitivity and response amplitude and to a shortened response duration. These changes are permanent for cells bathed in Ringer's solution. They are due to as yet unknown modulations in the transduction biochemistry. In this paper, we report that these changes can be partly or completely reversed by supplying biotin, pyruvate, and elevated glucose to the rod. The time course of this reversal and the substances which promote it imply that these are metabolically mediated effects. Based on the reported action of biotin and pyruvate on the one hand and on the changes of the response waveforms on the other hand, we believe that the phenomena we observe involve the later steps of the transduction cycle.

Light adaptation in cone photoreceptors of the salamander: a role for cytoplasmic calcium

1. Light adaptation has been studied in isolated red-sensitive cone photoreceptors of the salamander, using suction pipette recordings of circulating current. 2. In the presence of background illumination, the response to incremental dim flashes became desensitized according to the Weber-Fechner law. The recovery phase of the flash response was accelerated significantly, although the time-to-peak was reduced only slightly, and for dim backgrounds the rising phase was unaltered. 3. The role of cytoplasmic calcium concentration, Cai2+, in mediating cone adaptation was investigated by minimizing light-induced changes in Cai2+, either by incorporating calcium buffer into the cytoplasm or by exposing the outer segment to low-Ca2+, 0-Na+ solution. Both treatments appeared to slow dramatically or even to eliminate the onset of light adaptation in the cone. 4. When the low-Ca2+, 0-Na+ solution was presented in darkness, responses to subsequent illumination were affected in a characteristic manner: (i) the response-intensity relation was steepened and shifted to lower intensities, (ii) the response to a step of light could be predicted by integration and compression of the flash response, and (iii) the flash sensitivity declined steeply as a function of background intensity. 5. After extended exposure of the cone to bright backgrounds, the sensitivity in darkness failed to return to its original level. The flash response kinetics were faster and more biphasic than for dark-adapted responses or for responses desensitized to a comparable degree by exposure to steady background illumination. 6. The results indicate that, in cones isolated from the pigment epithelium, the primary factor influencing the adaptational state of the cell is the cytoplasmic concentration of free calcium, but that at high intensities the effects of pigment bleaching are likely to be significant.

Kinetics of phototransduction in retinal rods of the newt Triturus cristatus

1. The kinetics of photoresponses to flashes and steps of light of rods, from the retina of the newt Triturus cristatus, were analysed by recording the membrane current with a suction electrode. 2. In dark-adapted conditions the relation between the normalized amplitude of the photoresponse at a fixed time 1 s after the onset of light and the light intensity could be fitted by an exponential or a polynomial relation. In the presence of a steady bright light the same relation could be fitted by a Michaelis-Menten relation. 3. The kinetics of photoresponses to light stimuli was reconstructed using a model in which: (i) three molecules of guanosine 3'.5'-cyclic monophosphate (cyclic GMP) open a light-sensitive channel; (ii) light activates the enzyme phosphodiesterase, which hydrolyses cyclic GMP, thus closing light-sensitive channels: (iii) Ca2+ ions permeate through light-sensitive channels: and (iv) intracellular Ca2+ inhibits, in a co-operative way, the enzyme cyclase, which synthesizes cyclic GMP. 4. The model reproduces the shortening of the time to peak of brief flash photoresponses from about 1080 ms to about 690 ms with brighter lights. The model also explains the shortening of the time to peak to 350 ms observed in the presence of a steady light and the lack of a further acceleration with brighter flashes of lights. 5. The presence in the model of an intracellular calcium buffer accounts for the partial reactivation of the photocurrent following a step of light, lasting several seconds. The time course of this reactivation is not accelerated by a steady bright light both experimentally and in the model. 6. After the extinction to a long step of light the photocurrent showed a rapid partial reactivation, which was followed by a slow component of the photoresponse which extinguished with a rate constant of about 0.05 s-1. The model explains the origin of this slow component by assuming that the inactivation of excited rhodopsin is partially reversible. 7. The model is also able to explain the particular changes of kinetics when different amounts of exogenous calcium buffers are incorporated into rods (Torre, Matthews & Lamb, 1986).

Activation and adaptation of transducer currents in turtle hair cells

1. Transducer currents were recorded in turtle cochlear hair cells during mechanical stimulation of the hair bundle. The currents were measured under whole-cell voltage clamp in isolated cells that were firmly stuck to the floor of the recording chamber. 2. Stimuli were calibrated by projecting the image of the hair bundle onto a rapidly scanned 128 photodiode array. This technique showed that, while the cell body was immobilized, the tip of the bundle would follow faithfully the motion of an attached glass probe up to frequencies of more than 1 kHz. 3. The relationship between inward transducer current and bundle displacement was sigmoidal. Maximum currents of 200-400 pA were observed for deflections of the tip of the bundle of 0.5 microns, equivalent to rotating the bundle by about 5 deg. 4. In response to a step deflection of the bundle, the current developed with a time constant (about 0.4 ms for small stimuli) that decreased with the size of displacement. This suggests that the onset of the current was limited by the gating kinetics of the transduction channel. The onset time course was slowed about fourfold for a 20 degrees C drop in temperature. 5. For small maintained displacements, the current relaxed to about a quarter of the peak level with a time constant of 3-5 ms. This adaptation was associated with a shift of the current-displacement relationship in the direction of the stimulus. The rate and extent of adaptation were decreased by lowering external Ca2+. 6. Adaptation was strongly voltage sensitive, and was abolished at holding potentials positive to the reversal potential of the transducer current of about 0 mV. It was also diminished by loading cells with 10 mM of the Ca2+ chelator BAPTA. These observations suggest that adaptation may be partly controlled by influx of Ca2+ through the transducer channels. 7. Removal of adaptation produced asymmetric responses, with fast onsets but slow decays following return of the bundle to its resting position; the offset time course depended on both the magnitude and duration of the prior displacement. 8. In some experiments, hair bundles were deflected with a flexible glass fibre whose motion was monitored using a dual photodiode arrangement. Positive holding potentials abolished adaptation of the transducer currents, but had no influence on the time course of motion of the fibre. We have no evidence therefore that adaptation is caused by a mechanical reorganization within the bundle.

Cytoplasmic calcium as the messenger for light adaptation in salamander rods

1. In order to study the role of cytoplasmic calcium concentration (Ca2+i) in rod photoreceptor light adaptation, we have attempted to prevent light-induced changes in Ca2+i by minimizing calcium fluxes across the outer segment plasma membrane. This was achieved by exposing the outer segment to a low-Ca2+, 0-Na+ solution, in which sodium was replaced with either guanidinium or lithium and the external calcium concentration (Ca2+o) was reduced to micromolar levels. 2. With guanidinium and 1-3 microM-Ca2+o, the circulating current in darkness was maintained for a period of at least 15 s, consistent with approximate stability of Ca2+i. With Li+ rather than guanidinium most of the initial current was suppressed, but the residual current was again relatively stable. 3. During prolonged exposures (greater than 30 s) to low-Ca2+, 0-Na+ solution followed by dim illumination, the circulating current did not remain constant but slowly increased. Incorporation of calcium buffer into the cytoplasm greatly reduced the rate of change of current, consistent with the idea that the increase arose from a gradual decrease in Ca2+i. 4. Light responses of rods exposed to low-Ca2+, 0-Na+ solution in darkness were altered in a characteristic manner. Although the initial rising phase of the light response was little changed, the peak amplitude of the response was larger and occurred later, and the response decayed more slowly than in control. The response-intensity relation was steepened and was shifted towards lower intensities both for flashes and for steps of light. The normal sag in the response to steps disappeared, and the waveform of the step response could be predicted to a close approximation from the integral of the dim flash response. 5. Presentation of background illumination in Ringer solution produced a marked acceleration of the response to a subsequent bright flash. No such acceleration was observed if the background was given in low-Ca2+, 0-Na+ solution. 6. The results described in paragraphs 4 and 5 indicate that, under conditions expected to minimize changes in Ca2+i, all manifestations of light adaptation disappear, and the rod simply sums the effects of incident photons with an invariant integration time. 7. Exposure of a light-adapted rod to low-Ca2+, 0-Na+ solution altered the responses to superimposed test flashes in much the same way as for rods in darkness. The initial rising phases in low-Ca2+, 0-Na+ solution were unchanged, but the responses were larger, reached peak later and decayed more slowly.(ABSTRACT TRUNCATED AT 400 WORDS)

Calcium dependence of the activation and inactivation kinetics of the light-activated phosphodiesterase of retinal rods

The Ca2+ dependence of the kinetics and light sensitivity of light-activated phosphodiesterase was studied with a pH assay in toad and bovine rod disk membranes (RDM), and in a reconstituted system containing GTP-binding protein, phosphodiesterase and rhodopsin kinase. Three statistics, peak hydrolytic velocity, turnoff time, and time to peak velocity, were measured. ATP decreased phosphodiesterase light sensitivity nearly 10-fold and accelerated the dim-flash kinetics of cGMP hydrolysis when compared to those with GTP alone. CA2+ reversed all of the effects of ATP, Ca2+ increased peak velocity, turnoff time, and time to peak velocity, to the values obtained with GTP alone. The Ca2+ dependence of peak velocity and turnoff time can be characterized as hyperbolic saturation functions with a K0.5 for Ca2+ of 1.0-1.5 mM in toad RDM. In bovine RDM the Ca2+ dependence of peak velocity and turnoff time has a K0.5 of 0.1 mM Ca2+. The Ca2+ dependence in the reconstituted system is similar to that in bovine RDM for peak velocity (K0.5 = 0.1 mM Ca2+) but differs for turnoff time (K0.5 = 2.5 mM Ca2+). We tested the hypothesis that a soluble modulator, normally required to confer submicromolar Ca2+ sensitivity, was too dilute in our assay by comparing data obtained at one RDM concentration with those obtained at 10-fold higher RDM, and therefore a constituent protein, concentration. We observe no difference and present a formal analysis of these data that excludes the hypothesis that the soluble modulator binds its target protein with Kd less than 5 microM. The lack of submicromolar Ca2+ dependence of any of the steps in the cGMP cascade that underlie cGMP phosphodiesterase activation and inactivation in vitro argues against Ca2+ regulation of these steps having a significant role in the light adaptation of the intact rod.

Direct activation of cGMP-dependent channels of retinal rods by the cGMP phosphodiesterase

The cationic conductances of purified bovine retinal rod membranes were studied by incorporation of vesicles into planar lipid bilayers. When the membranes were stripped of all peripheral proteins [guanine nucleotide-binding protein (G protein) and cGMP phosphodiesterase (3',5'-cyclic-GMP 5'-nucleotidohydrolase), EC 3.1.4.35], sodium and calcium fluxes were almost only observed in the presence of cGMP. Reconstitution experiments in which purified cGMP phosphodiesterase alone or with G protein were reassociated to the vesicles in proportions similar to those found in the native rod provide evidence for a direct interaction between the cGMP-dependent channel protein and the phosphodiesterase. (i) In its inhibited state, phosphodiesterase markedly stimulates the activity of the channels in the presence of cGMP (situation in the dark-adapted rod) but is not capable of activating the channels in the absence of cGMP. (ii) In the absence of cGMP, activation of the phosphodiesterase by G protein with GTP bound (equivalent to photoexcitation) induces the opening of cation channels that have the same conductance for sodium ions as cGMP-activated channels (20-22 pS, with two sublevels of about 7 pS and 13 pS).

Sodium-dependent calcium extrusion and sensitivity regulation in retinal cones of the salamander

1. Membrane current was recorded from an isolated, dark-adapted salamander cone by sucking its inner segment into a tight-fitting glass pipette containing Ringer solution. The outer segment of the cell was exposed to a bath solution that could be changed rapidly. 2. After removing Na+ from the bath Ringer solution for a short period of time in darkness (the 'loading period'), a transient inward current was observed upon restoring it in bright light. A similar but longer-lasting current was observed when Na+ was restored in the light after a large Ca2+ influx was induced through the light-sensitive conductance in darkness. 3. The above transient current was not observed if Li+ or guanidinium was substituted for Na+ in the light, or if Ba2+ was substituted for Ca2+ during the dark loading period. However, a current was observed if Sr2+ was the substituting ion for Ca2+ during loading. These observations suggested that the current was associated with an electrogenic Na+-dependent Ca2+ efflux at the cone outer segment. 4. The saturated amplitude of the exchange current was 12-25 pA with a mean around 16 pA. This is very comparable to that measured in the outer segment of a salamander rod under similar conditions. 5. By comparing a known Ca2+ load in a cone outer segment to the subsequent charge transfer through the exchange, we estimated that the stoichiometry of the exchange was near 3Na+:1Ca2+. 6. With a small Ca2+ load, or in the presence of Cs+ around the inner segment, the final temporal decline of the Na+-Ca2+ exchange current was roughly exponential, with a mean time constant of about 100 ms. This decline is about four times faster than that measured in rods. We interpret the shorter time constant in cones to reflect a faster rate of decline of intracellular free Ca2+ in their outer segments resulting from the exchange activity. 7. In the absence of external Na+, and hence any Na+-dependent Ca2+ efflux, the absolute sensitivity of a cone to a dim flash was several times higher than in normal Ringer solution. 8. A roughly similar increase in light sensitivity was observed for a rod under the same conditions. 9. We conclude that the Na+-dependent Ca2+ efflux, through lowering intracellular free Ca2+ in the light, has a role in regulating the absolute light sensitivity in cones as it does in rods.(ABSTRACT TRUNCATED AT 400 WORDS)

Incorporation of analogues of GTP and GDP into rod photoreceptors isolated from the tiger salamander

1. Analogues of GTP and GDP were introduced into isolated rod photoreceptors using the whole-cell patch clamp technique, while simultaneously recording the photocurrent with a suction pipette. After several minutes of whole-cell recording the patch pipette was disengaged, thus trapping the analogue inside the cell. 2. During the introduction of the hydrolysis-resistant GTP analogues guanosine-5'-O-(3-thio-triphosphate) (GTP-gamma-S) and guanylyl-imidodiphosphate (GMP-PNP) the dark current progressively declined, and the duration of responses to flashes of light which had previously been just-saturating increased slightly. The form of the rising phases of the responses to dim or bright flashes was little affected. 3. Following the incorporation of these GTP analogues the response to an intense flash was prolonged by a factor of up to 300, and the circulating current remained suppressed for up to 1 h. Ultimately the circulating current recovered and the duration of the flash response returned to near its control value. 4. Superfusion of the outer segment with the phosphodiesterase inhibitor 3-isobutyl-1-methyl-xanthine (IBMX) during the extended period of saturation resulted in a rapid increase in the circulating current, suggesting that the analogues had their major effect on the duration of phosphodiesterase activation by light. 5. Introduction of the phosphorylation-resistant GDP analogue guanosine-5'-O-(2-thio-diphosphate) (GDP-beta-S) resulted in a decrease in light sensitivity and a reduction in the slope of the rising phase of the flash response. 6. The response to an intense flash was also prolonged in cells containing GDP-beta-S, recovery becoming progressively slower on successive presentations of the flash following the withdrawal of the patch pipette. This observation suggests that GDP-beta-S may be slowly converted within the cell to form a hydrolysis-resistant product. 7. These results indicate that the presence of a hydrolysis-resistant analogue of GTP within the cell causes light activation of the transduction mechanism for an extended period. Our interpretation of this finding is that hydrolysis of the bound guanosine nucleotide is necessary for the quenching of activated GTP-binding protein.

Control of light-sensitive current in salamander rods

1. The exponential decline of light-sensitive current seen after switch from Na+ to Li+ in the presence of Ca2+ probably depends on the activity of the phosphodiesterase (PDE) which hydrolyses cyclic GMP. 2. This probability is supported by experiments with suction electrodes which show that in toad and salamander rods the rate constant, b, of the exponential decline of current was increased at least 10-fold by moderate light intensities and decreased about 10-fold by 3-isobutyl-1-methylxanthine (IBMX), an inhibitor of PDE. 3. The rate constant b is about 3 times more sensitive to weak lights or to IBMX than the membrane current. This may be explained by a feed-back involving calcium ions which tends to hold current constant, perhaps by calcium inhibition of guanylate cyclase. 4. The time course of b, which probably represents the changes in PDE activity, was measured by switching from Na+ to Li+ at various times after a flash. The results suggest that a moderate flash (140 Rh) increased b about 7 times in 0.5 s and that b then declined with a time constant of 1.5-2 s. 5. Extrapolated values of the parameter b suggest that strong flashes (5000-10,000 Rh) increased b from 1 s-1 in the dark to perhaps 60 s-1 and that b continued to increase with flash strength for several log units after the current had reached saturation. 6. The observations in 4 and 5 fit well with the idea that b is related to PDE activity and that changes in the latter are sufficient to account for the rising phase of the flash response. 7. After a flash the light-sensitive current recovers much more rapidly than the time constant b-1, a discrepancy which is explained if a light flash causes a delayed increase in guanylate cyclase activity. 8. The apparent delayed increase in cyclase activation is consistent with an inhibitory effect of [Ca2+]i which is reduced when calcium is pumped out during the plateau of the response. 9. Experiments in which pulses of IBMX were applied at different times during a flash response support the idea that a flash causes a delayed increase in the rate of supply of cyclic GMP. Quantitative analysis of these and other tests with IBMX gave rate constants similar to those obtained by the Na+----Li+ method.

External and internal actions in the response of salamander retinal rods to altered external calcium concentration

1. The membrane current was recorded from retinal rods isolated from Ambystoma tigrinum using the suction pipette and whole-cell patch pipette techniques, while the concentration of calcium bathing the outer segment was rapidly reduced. 2. The increase in outer segment current induced by lowered external calcium in darkness could be resolved into two components, one as rapid as the time course of the solution change (as judged by the junction current) and the other somewhat slower. 3. Introduction of the calcium buffer BAPTA (1,2-bis(o-aminophenoxy)ethane-N ,N ,N' ,N'-tetraacetic acid) into the cell from a patch pipette led to a progressive slowing of the second component of current increase. 4. When several minutes had elapsed following rupture of the patch, to allow a substantial amount of BAPTA into the cell (ca. 10 mM in the patch pipette), the second component was slowed by a factor of about 20-fold, while the first component continued to have the same rapid time course as the solution change. 5. The rapid component is attributed to a direct effect of external calcium, Ca2+o, and the delayed component to an indirect effect mediated by a reduction in internal calcium, Ca2+i. 6. These results confirm that, in previous experiments in which BAPTA was introduced into photoreceptors, the internal calcium concentration was very significantly buffered. 7. When Ca2+o drops from 1 mM to less than 10(-8) M, the rapid external component corresponds to an increase in circulating current of 3- to 4-fold, and the internal component corresponds to an increase of at least 4- to 5-fold. However, the total current at late times is limited by electrical factors, so that the size of the internal effect is bound to be considerably greater.

The ionic selectivity of the light-sensitive current in isolated rods of the tiger salamander

1. Using the method of Hodgkin, McNaughton & Nunn (1985) for rapidly changing the extracellular medium, we analysed the effect of divalent cations on the photocurrent of isolated retinal rods of the tiger salamander. 2. When the extracellular NaCl was replaced by equiosmolar amounts of BaCl2, SrCl2, CaCl2, MgCl2 and MnCl2 the efficacy in carrying the photocurrent at early times was Ba2+ greater than Sr2+ greater than Ca2+ greater than Mg2+ greater than Mn2+. At early times Ba2+ could carry a photocurrent similar to or larger than that carried by Na+. 3. The photocurrent carried by Ba2+ increased by about 50% when [Ca2+]o was reduced from 1 to 0.1 mM. In the presence of 0.1 mM-Ca2+ in the extracellular medium the photocurrent carried by Ba2+ saturated when [Ba2+]o was close to 50 mM and was half-activated at 15 mM [Ba2+]o. 4. The photocurrent which can be carried by Sr2+ is not larger than that carried by Ba2+ and does not saturate for [Sr2+]o up to 70 mM. 5. When extracellular Na+ is replaced by the impermeant organic ion choline it is possible to observe a transient photocurrent which is carried by Ca2+. This current has a maximal value of about 11 pA and has a half-activation constant of about 50 microM. 6. Movements of Mg2+ across the light-sensitive channel can be seen only when extracellular Ca2+ is reduced below 10 microM. Under these conditions the maximal photocurrent which can be carried by Mg2+ at early times is about 8 pA and has a half-activation of about 2 mM. Under normal conditions Mn2+ is hardly permeable through the light-sensitive channel. 7. It is concluded that the selectivity of the light-sensitive channel in the low ionic concentration range is Ca2+ greater than Sr2+ greater than Ba2+ greater than Mg2+ greater than Na+.

Photoreceptor light adaptation is mediated by cytoplasmic calcium concentration

The vertebrate visual system can operate over a large range of light intensities. This is possible in part because the sensitivity of photoreceptors decreases approximately in inverse proportion to the background light intensity. This process, called photoreceptor light adaptation, is known to be mediated by a diffusible intracellular messenger, but the identity of the messenger is still unclear. There has been considerable speculation that decreased cytoplasmic Ca2+ concentration (Cai2+) may play a role in light adaptation, and recent experiments in which Ca2+ buffer was incorporated into rod-cells have supported this notion. The extent of the contribution of calcium, however, remains unresolved. We now show that light-dependent changes in sensitivity in amphibian photoreceptors can be abolished by preventing movements of Ca2+ across the outer-segment plasma membrane. These experiments demonstrate that light adaptation in photoreceptors is mediated in cones primarily, and in rods perhaps exclusively, by changes in Cai2+.

Calcium and light adaptation in retinal rods and cones

Retinal rods and cones respond to light with a membrane hyperpolarization. This hyperpolarization is mediated by an ionic conductance (the light-regulated conductance) that is kept open in darkness by cyclic GMP acting as a ligand, and which closes in the light as a result of an increase in cGMP hydrolysis triggered by illumination. Calcium ions appear to have a role in this phototransduction process: they provide negative feedback between the conductance, which is permeable to Ca2+ (refs 4, 5), and the concentration of cGMP, which is sensitive to Ca2+ (refs 6-8). This feedback down-regulates the sensitivity to light of a photoreceptor and probably contributes to the important phenomenon of light adaptation in vision. It is still not clear, however, how much of the light adaptation is actually attributable to this Ca2+ feedback. We have examined the responses of amphibian rods and cones to light with the Ca2+ feedback removed. Normally, the response of a cell to a step of light rises transiently to a peak, but rapidly relaxes to a lower level, indicative of light adaptation. When the feedback is removed, however, the relaxation of the response is completely absent; furthermore, the steady response levels at different light-step intensities are well predicted by a statistical superposition of invariant single-photon responses. We therefore conclude that the Ca2+ feedback underlies essentially all light adaptation in rods and cones.

Ion transport by the Na-Ca exchange in isolated rod outer segments

The inward membrane current generated by the coupled exchange of external sodium for internal calcium has been investigated in isolated rod outer segments. The exchange rate is sensitive to voltage, with a reduction by a factor of e occurring for a 70-mV depolarization in normal Ringer's solution. The voltage sensitivity is not a constant property of the exchange, as it is reduced by an increase in external Na+ or by the removal of external Ca2+, Mg2+, or K+. Changes in membrane potential do not appear to affect the affinity of the exchange mechanism for internal Ca2+, but hyperpolarization increases the affinity for external Na+. When the external Na+ concentration is raised sufficiently to saturate the exchange mechanism, the voltage sensitivity is no longer apparent. We propose that the voltage dependence of the exchange is due to the external Na+-binding site being sensitive to membrane potential, perhaps because it is located within the membrane electric field.

Rod light adaptation may be mediated by acceleration of the phosphodiesterase-guanylate cyclase cycle

We compare the retinal rod photocurrent before and after introduction of an hydrolysis-resistant analog of GTP into the outer segment by the whole-cell patch technique. Others have shown that GTP bound to transducin leads to the hydrolysis of cyclic GMP, causing the response to light--a decrease in dark current. The hydrolysis-resistant GTP analog prolongs the response to a bright flash, which leads us to suggest that prolonged transducin activation by bright light desensitizes the rod by a prolonged decrease in dark current. Recovery from the response to a bright flash does occur after introduction of the analog; that recovery requires acceleration of cyclase activity rather than inhibition of phosphodiesterase. The analog mimics light adaptation by desensitizing the rod and speeding the recovery from a dim flash. The analog plus light or light adaptation prolongs the activities of transducin and phosphodiesterase (oligonucleate 5'-nucleotidohydrolase, EC 3.1.4.1) to mediate desensitization by reducing the dark current. Hence, this faster recovery from a dim flash would be by increased activity of guanylate cyclase [GTP pyrophosphate-lyase (cyclizing), EC 4.6.1.2] rather than by inhibited phosphodiesterase. Accelerated activity of guanylate cyclase may speed recovery by response truncation. We conclude that transducin, activated by photolyzed rhodopsin, may lead to increased activity of both phosphodiesterase and guanylate cyclase to mediate the desensitization and the faster recovery of the light-adapted response.

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

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

Guanosine 3',5'-cyclic monophosphate-activated conductance studied in a truncated rod outer segment of the toad

1. In darkness, a single rod outer segment isolated from the toad retina was sucked partially, tip first, into a tight-fitting, Ringer solution-filled glass pipette for recording membrane current. The basal end of the outer segment outside the pipette was sheared off with a probe to allow internal dialysis. The potential between the inside and the outside of the pipette was held at 0 mV. 2. With cyclic GMP and IBMX (isobutylmethylxanthine) in the dialysis solution, a large inward current appeared across the plasma membrane of the outer segment; this current saturated at around 1 mM-cyclic GMP. IBMX by itself was ineffective. 3. The saturated cyclic GMP-induced current recorded varied in size with the length of outer segment (L) within the suction pipette. For L less than 25 micron, the relation was linear, with a current density of 4-20 pA micron-1. 4. At short L (less than 25 micron), the dose-response relation between current magnitude and cyclic GMP concentration was sigmoidal, with a Hill coefficient (n) of 1.8-3.1 and a half-saturating cyclic GMP concentration (K1/2) of 30-85 microM. 5. In the presence of IBMX and the absence of GTP, the dose-response relation was the same in continuous bleaching light as in darkness. This indicates that both the characteristics of cyclic GMP binding and the intrinsic conduction properties of the open conductance are not affected by light. 6. Removing IBMX from the dialysing solution had little effect on the saturated current, but substantially reduced the current induced at low concentrations of cyclic GMP. When the analogue 8-bromo cyclic GMP was used instead, however, the presence of IBMX was relatively unimportant even at low agonist concentrations. These observations indicated that significant phosphodiesterase activity was present within the truncated outer segment. 7. In the absence of IBMX and the presence of GTP, the cyclic GMP-induced current could be suppressed by light. When ATP was also present in the dialysing solution, the effect of light was significantly reduced and the suppression also became more transient. 8. We conclude from the above results that the cyclic GMP-gated conductance is indeed present in the plasma membrane of the rod outer segment, and that this conductance and the light-sensitive conductance are one and the same entity. 9. From the results, we estimate that only about 1% of the conductance is normally open in darkness. This fraction of open conductance corresponds to a free cyclic GMP concentration of a few micromolar.

Intracellular biochemical manipulation of phototransduction in detached rod outer segments

Recent progress in understanding phototransduction has come primarily from studies on cell-free systems. To investigate the transduction process under physiological conditions, a fully functional preparation of retinal rod outer segments without attached inner segments was developed that allows electrical recording of light-sensitive current during intracellular dialysis with defined solutions. No light-sensitive current is recorded from detached outer segments dialyzed with nucleotide-free solutions, whereas cells detached from the retina into Ringer's solution containing 3-isobutyl-1-methyl-xanthine (a phosphodiesterase inhibitor) develop a light-sensitive inward dark current. This indicates that there is a basal level of cGMP-specific phosphodiesterase activity in the dark. Detached outer segments dialyzed with greater than or equal to 20 microM cGMP rapidly develop a light-suppressible current. A current of similar magnitude is generated more slowly during dialysis with a 50-fold greater concentration of GTP. Apparently, cGMP can be synthesized from GTP by guanylate cyclase in the outer segment. Cells dialyzed with cGMP alone show a reduced light sensitivity that is restored to normal by addition of 20 microM GTP. This action of GTP is antagonized by guanosine 5'-[beta-thio]diphosphate. These findings are in good agreement with biochemical evidence indicating that a GTP-binding protein (transducin) plays a pivotal role in the generation of responses to light. The recovery of photocurrent following a brief flash is delayed or abolished by dialysis with solutions that lack ATP or contain guanosine 5'-[gamma-thio]triphosphate, a nonhydrolyzable GTP analog. These results support the view that both GTP hydrolysis by activated transducin and ATP-dependent phosphorylation of a rhodopsin photoproduct are necessary for termination of the transduction process.

cGMP-dependent cation channel of retinal rod outer segments

Light-modulated cytoplasmic cGMP simultaneously controls plasma membrane Na+ conductance in visual excitation and Ca2+ entry into rods by direct interaction with the cation channel. Cytoplasmic Ca2+ in turn may set operating points and contribute to the dynamics of several enzymes that regulate cGMP levels in the dark, recovery from excitation and receptor adaptation or down regulation. Similar channels may couple electrical activity to internal nucleotide metabolism in other tissues. We here report the identification, partial purification and behaviour after reconstitution of a protein of relative molecular mass 39,000 (Mr 39K) present in both disk and plasma membranes from bovine rod outer segments that mediates these cGMP-dependent cation fluxes. Its cGMP agonist specificity, kinetic cooperativity, ionic selectivity, membrane density and other features closely match the properties of the visual cGMP-dependent conductance inferred from electrophysiological measurements.