Single-channel recordings demonstrate that cGMP opens the light-sensitive ion channel of the rod photoreceptor

Evaluating letter recognition, flicker fusion, and the Talbot-Plateau law using microsecond-duration flashes

Four experiments examined the ability of respondents to identify letters that were displayed on an LED array with flashes lasting little more than a microsecond. The first experiment displayed each letter with a single, simultaneous flash of all the dots forming the letter and established the relation of flash intensity to the probability of letter identification. The second experiment displayed the letters with multiple flashes at different frequencies to determine the probability that the sequence of flashes would be perceived as fused. The third experiment displayed the letters at a frequency that was above the flicker-fusion frequency, varying flash intensity to establish the amount needed to elicit a given probability of letter identification. The fourth experiment displayed each letter twice, once at a frequency where no flicker was perceived and also with steady light emission. The intensity of each flash was fixed and the steady intensity was varied; respondents were asked to judge whether the fused-flicker display and the steady display appeared to be the same brightness. Steady intensity was about double the average flash intensity where the two conditions were perceived as being equal in brightness. This is at odds with Talbot-Plateau law, which predicts that these two values should be equal. The law was formulated relative to a flash lasting half of each period, so it is surprising that it comes this close to being correct where the flash occupies only a millionth of the total period.

Speed, sensitivity, and stability of the light response in rod and cone photoreceptors: facts and models

The light responses of rod and cone photoreceptors in the vertebrate retina are quantitatively different, yet extremely stable and reproducible because of the extraordinary regulation of the cascade of enzymatic reactions that link photon absorption and visual pigment excitation to the gating of cGMP-gated ion channels in the outer segment plasma membrane. While the molecular scheme of the phototransduction pathway is essentially the same in rods and cones, the enzymes and protein regulators that constitute the pathway are distinct. These enzymes and regulators can differ in the quantitative features of their functions or in concentration if their functions are similar or both can be true. The molecular identity and distinct function of the molecules of the transduction cascade in rods and cones are summarized. The functional significance of these molecular differences is examined with a mathematical model of the signal-transducing enzymatic cascade. Constrained by available electrophysiological, biochemical and biophysical data, the model simulates photocurrents that match well the electrical photoresponses measured in both rods and cones. Using simulation computed with the mathematical model, the time course of light-dependent changes in enzymatic activities and second messenger concentrations in non-mammalian rods and cones are compared side by side.

Localized activation of proteins in a free intracellular space: dependence of cellular morphologies and reaction schemes

Localized activation of proteins in a cell is crucial for the segregation of cellular functions leading, for example, to the development of polarized cells and chemotaxis. If there is a physical diffusion barrier, localized activation of proteins will emerge. In case of no physical barrier, however, it is not clear to what extent the protein activation is localized within a three dimensional intracellular space. In the previous report we showed a simulation result of localized activation of Ca(2+)/calmodulin-dependent kinase II (CaMKII) within a dendritic spine of a neuron, and this localization was enhanced by the diffusion of calmodulin. However, a dendritic spine will act as a physical diffusion barrier. Here, we report that the localization of activated proteins is seen in more simplified morphology with no diffusion barrier. Furthermore, this localization was seen with a simple reaction scheme. In case that a Ca(2+) source was located at the center of the spherical cell with diameter of 20μm, which is the extreme case without any physical diffusion barrier, the simulation results showed localized activation of a protein around the Ca(2+) source. This localized activation was also enhanced by the diffusion of calmodulin. These localizations were not blurred with time within the tested time range. The reason for the increase in the localization by the diffusion of calmodulin was the replenishment of free calmodulin from surrounding regions. These simulation results indicate that the localized activation of proteins emerges in biological cells without any physical diffusion barrier, and the replenishment of proteins by diffusion can act as an enhancer of localized activation of downstream proteins.

Electrophysiological recordings in solitary photoreceptors from the retina of squid, Loligo pealei

A protocol was developed to isolate enzymatically photoreceptors from the retina of the squid, Loligo pealei. The procedure routinely results in a high yield of intact cells. Examination of solitary photoreceptors under Nomarski optics revealed that the fine morphological features described in anatomical studies of retinal sections are retained. The distal segment is up to 250 μm long, 4–7 μm wide, covered in part by short microvilli; the inner segment and the cell body, with the initial portion of the axon, are also clearly discernible in solitary cells. Suction electrode measurements performed from the cell body confirmed that responsiveness to light survived cell isolation. Macroscopic membrane currents were measured using the whole-cell tight-seal technique, and the perforated-patch method. Step depolarizations of membrane voltage administered in the dark elicited a slowly activating, sustained outward current. Light stimulation evoked an inward current graded with stimulus intensity; the peak current could amply exceed 1000 pA. Intense photostimulation gave rise to a prolonged inward aftercurrent that lasted for tens of seconds. On-cell patch recording along the intermediate segment and most of the smooth areas of the distal segment showed a large incidence of silent patches, with the occasional presence of voltage-dependent channels. On the other hand, channel activity could be recorded more frequently from electrode placements near the apical tip of the cell, where the presence of microvilli could be confirmed visually. Some patches were unresponsive to voltage Stimulation applied in the dark but produced distinct bursts of channel openings after illumination. The feasibility of single-cell electrophysiology in isolated photoreceptors, together with the growing body of biochemical information on cephalopod preparations, makes squid an attractive model system to investigate the visual process in invertebrates using multiple experimental approaches.

Cilium-attached and excised patch-clamp recordings of odourant-activated Ca-dependent K channels from chemosensory cilia of olfactory receptor neurons

It has previously been proposed that a Ca2+-dependent K+ conductance is implicated in the inhibitory odourant response in rat and toad olfactory receptor neurons. Previous whole-cell and single-channel measurements on inside-out excised patches, in addition to immunochemical evidence, indicated the presence of Ca2+-dependent K+ channels in olfactory cilia, the transducing structures of these sensory cells. Ca2+-dependent K+ channels opened in 'on-cilium' membrane patches from C. caudiverbera upon odourant stimulation. Furthermore, after excision in the inside-out configuration, the channel could be opened by micromolar Ca2+, in a Ca2+-dependent fashion, but it was unresponsive to cyclic AMP. We estimated that the Ca2+ concentration in the proximity of a Ca2+-dependent K+ channel within the cilia reaches at least 100 microM during the odour response. The K+ channel displayed a higher selectivity for K+ than for Na+. Our results support a role for this Ca2+-dependent K+ channel in chemotransduction.

Optimal detection of flash intensity differences using rod photocurrent observations

The rod photocurrent contains two noise components that may limit the detectability of flash intensity increments. The limits imposed by the low- and high-frequency noise components were assessed by computing the performance of an optimal detector of increments in flash intensity. The limits imposed by these noise components depend on the interval of observation of the photocurrent signal. When the entire photocurrent signal, lasting 3 or more seconds, is observed, the low-frequency component of the photocurrent noise (attributed to the quantal noise of the incoming light, as well as random isomerizations of enzymes within the phototransduction cascade) is the most significant limitation on detectability. When only the first 380 ms or less is observed, the high-frequency component of the noise (due to the thermal isomerizations of the cGMP-gated channel) presents a significant limit on the detectability of flashes.

Phosphorylation of the alpha-subunits of the Na+/K+-ATPase from mammalian kidneys and Xenopus oocytes by cGMP-dependent protein kinase results in stimulation of ATPase activity

Phosphorylation of Na+/K+-ATPase by cGMP-dependent protein kinase (PKG) has been studied in enzymes purified from pig, dog, sheep and rat kidneys, and in Xenopus oocytes. PKG phosphorylates the alpha-subunits of all animal species investigated. Phosphorylation of the beta-subunit was not observed. The stoichiometry of phosphorylation estimated for pig, sheep and dog renal Na+/K+-ATPase is 3.5, 2.2 and 2.1 mol Pi per mol alpha-subunit, respectively. Proteolytic fingerprinting of the pig alpha1-subunits phosphorylated by PKG using specific antibodies raised against N-terminus or C-terminus reveals that phosphorylation sites are located within the intracellular loop of the alpha-subunit between the 35 kDa N-terminal and 27 kDa C-terminal fragments. Phosphorylation sites within the alpha1-subunit of the purified Na+/K+-ATPase do not appear to be easily accessible for PKG since incorporation of Pi requires 0.2% of Triton X-100. Administration of cGMP and PKG in the presence of 5 mm ATP, which prevents inactivation of the Na+/K+-ATPase by detergent, leads to stimulation of hydrolytic activity by 61%. Administration of 50 microm of cGMP or dbcGMP in yolk-free homogenates of Xenopus oocytes leads to stimulation of ouabain-dependent ATPase activity by 130-198% and to incorporation of 33P into the alpha-subunit without the detergent. Hence, PKG plays regulatory role in active transmembraneous transport of Na+ and K+ via phosphorylation of the catalytic subunit of the Na+/K+-ATPase.

Neurochemical organization of the macaque retina: effect of TTX on levels and gene expression of cytochrome oxidase and nitric oxide synthase and on the immunoreactivity of Na+ K+ ATPase and NMDA receptor subunit I

The present study examined the relationship between an important energy-generating enzyme (cytochrome oxidase; CO), a key energy-consuming enzyme (Na+ K+ ATPase) and neurochemicals associated with excitatory glutamatergic synapses (NMDAR1 and neuronal nitric oxide synthase, nNOS) in the adult macaque retina. Polyclonal antibodies against neuronal nitric oxide synthase and N-methyl-D-aspartate receptor subunit I were generated for immunohistochemical examination and labeled sites not previously reported were found. We have also isolated cDNAs for cytochrome oxidase subunits III (mitochondrial-encoded) and IV (nuclear-encoded), as well as for a fragment of neuronal nitric oxide synthase, from a human cDNA library. The distributions of mRNAs of these genes were analyzed by in situ hybridization. We found that three or more of the markers examined coexisted in a number of sites: (a) In the inner segments of photoreceptors, high energy demand for maintaining the dark current was placed by Na+ K+ ATPase. This was partially met by ATP-generating enzymes such as CO. Neuronal NOS was also present there for the synthesis of NO and the cascading event leading to the generation of cGMP and the gating of channels for visual transduction. (b) Both the outer and inner plexiform layers had detectable amounts of all four markers, although the levels varied among them. This was most likely due to the presence of depolarizing glutamatergic synapses arising from photoreceptors and bipolar cells and such synaptic events were energy-demanding. The involvement of NMDA receptors and nNOS in these synaptic layers is strongly implicated in the present study. (c) All four markers were present in the majority of retinal ganglion cells, with some inherent heterogeneity related to intensity and size. Retinal ganglion cells are known to receive excitatory synapses from glutamatergic bipolar cells and are themselves highly active. The presence of both NMDAR1 and nNOS in these cells were verified in the present study and the energy demands related to these synaptic activities were necessarily high. Thus, active ion transporting functions related to synaptic or non-synaptically induced repolarization from the basis for an interrelationship between the neurochemicals/enzymes studied. Finally, (d) all four markers and the gene expression of CO and nNOS in the macaque retina were regulated by neuronal activity.

Electrogenic hyperpolarization-elicited chloride transporter current in blue cones of zebrafish retinal slices

Voltage-activated currents in blue cones of the retinal slice of zebrafish were characterized using whole cell recording techniques. Depolarizing-elicited currents were recorded: an outward tetraethylammonium (TEA)-sensitive K+ current (IKx), an outward Ca(2+)-activated Cl- current (ICl(Ca)), from which we inferred an inward Ca2+ current (ICa) as well as a hyperpolarizing-elicited nonselective inward cation current (Ih). In addition, hyperpolarizing steps elicited an outward current (Iout-h) in about one-third of the blue cones. Iout-h seems to be carried by inward transported Cl- because it was abolished by equimolar substitution of bath Cl- with acetate; equimolar substitution of Na+ with choline or TEA had no effect; it was not affected by Cl- channel blockers, anthracene-9-carboxylic acid, 4,4'-diisothiocyanostilbene-2.2'-disulfonic acid, N-phenylanthranilic acid (DPC), niflumic acid, and 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid but was suppressed by Cl- transporter blockers acetalzolamide, bumetanide, N-ethylmaleimide, furosemide, and vanadate, and no reversal potential was found. In addition, this current was suppressed by ouabains but unrelated to their Na(+)-K(+)-ATPase inhibitory effect, was not suppressed by Co2+ or nifedipine, was not affected by the gap junction decoupler, 2-octanol, was increased by bath application of Cs+, presumably due to suppression of Ih, which was masked by Iout-h, and was suppressed by intensive light. Similar current also was found in the short cones and double cones. As Iout-h operates over the same voltage range, and with similar magnitude and time course as Ih, we suggest that Iout-h contributes to the modulation of the photoresponse of cones.

Conductance and kinetics of single cGMP-activated channels in salamander rod outer segments

1. The conductance and kinetics of single 3',5'-cyclic guanosine monophosphate (cGMP)-activated channels of retinal rod outer segments were studied in inside-out membrane patches. The size of the single channel currents was increased by using low concentrations of divalent cations. 2. At saturating cGMP concentration, the current flickered at high frequency. Occasionally, the current was interrupted by closures lasting tens or hundreds of milliseconds. At +50 mV the maximum current during an opening was slightly more than 1 pA, but the open channel level was poorly resolved due to the speed of the gating transitions. 3. Amplitude histograms confirmed the presence of a sublevel of current, roughly a quarter the size of the peak current, at low cGMP concentrations. The fraction of time in the sublevel decreased with increasing cGMP concentration, suggesting that the sublevel may be due to opening by the partially liganded channel. 4. Consistent with previous macroscopic current recordings, single channel activation by cGMP had an apparent dissociation constant of 8.6 microM, and a Hill coefficient of 2.8. 5. At saturating cGMP concentrations, the channel was modelled as a two-state system with the following parameters. The open channel conductance was 25 pS. The opening rate constant, beta, was 1.5 x 10(4) s-1 at 0 mV, and had a voltage sensitivity equivalent to the movement of 0.23 electronic charges outward through the membrane electric field. The closing rate constant, alpha, was 2.1 x 10(4) s-1 and was voltage insensitive. Assuming that the open-state chord conductance was voltage independent, the inferred voltage dependence of beta largely accounted for the outward rectification in the steady-state macroscopic current-voltage relation of multichannel patches, at saturating cGMP concentration.

Gating, selectivity and blockage of single channels activated by cyclic GMP in retinal rods of the tiger salamander

1. Patches in the inside-out configuration were excised from the membrane of outer and inner segments of the larval tiger salamander, Ambystoma tigrinum. The current flowing through single channels opened by cyclic GMP was studied with the voltage clamp technique. 2. Amplitude histograms of current recordings from patches containing only one flickering channel, excised from the inner segment and in the presence of 100 microM cyclic GMP, could be fitted by a theoretical scheme in which the single channel conductance was at least 55 pS at +40 mV and at least 45 pS at -40 mV. The mean open time was no longer than the time constant of our recording system, about 35 microseconds. Similar results were obtained by analysis of the amplitude histograms of patches from the outer segment containing many channels, and in the presence of 1-5 microM cyclic GMP. 3. In membrane patches excised from the outer segment, reducing the temperature from 24 to 8 degrees C did not reduce the flickering, but changed the amplitude histograms of current fluctuations activated by 1 microM cyclic GMP in a way consistent with a decrease of 50% in the single channel conductance and a decrease of 50% in the open probability. 4. In the presence of 1 microM cyclic GMP at +60 mV, when Na+ was replaced by NH4+ or K+, brief outward current transients flowing through single channels were observed. When Na+ was replaced with Li+, Rb+ or Cs+, current transients were very small. 5. The shape of the power spectrum of current fluctuations induced by 1 microM cyclic GMP at +60 mV did not change when the permeating ion was Na+, K+ or NH4+. Analysis of the amplitude histogram did not show any effect of the tested monovalent cations on the open probability or on channel gating. At +60 mV, the estimated single channel currents were at least 4, 2.8 and 2 pA for NH4+, Na+ and K+ respectively. 6. The addition of 0.5 or 1 mM Ca2+ to the medium bathing the cytoplasmic side of the membrane greatly reduced the frequency of openings, but single channel activity could still be observed. The blocking effect of 1 mM Ca2+ on the channel activity induced by 2 microM cyclic GMP could be counterbalanced by increasing the cyclic GMP concentration. The addition of 0.5 or 1 mM Ca2+ did not change the shape of power spectra obtained at membrane voltages between -100 and +100 mV.(ABSTRACT TRUNCATED AT 400 WORDS)

Single-channel properties of cloned cGMP-activated channels from retinal rods

Single-channel properties of a cloned channel activated by cyclic GMP have been analysed. The mRNA encoding for the channel was injected into oocytes of Xenopus laevis and the current flowing through a single ionic channel activated by cGMP was studied in excised patches under voltage-clamp conditions. The ionic channel activated by cGMP had a single-channel conductance of 32 +/- 2 pS at +120 mV and 25 +/- 4 pS at -120 mV, and its conductance was not significantly affected by increasing the cGMP concentration from 20 microM to 200 microM. The single-channel currents in the presence of NH+4, Na+, K+, Li+ and Rb+ in the medium bathing the cytoplasmic side of the membrane at +140 mV were 5.3, 4.7, 3.8, 1.3 and 0.8 pA, respectively. The single-channel current in the presence of Cs+ was less than 0.5 pA. Ca2+ and Mg2+ (both 0.5 mM) in the presence of 100 microM cGMP did not appreciably affect the channel activity at membrane potentials more negative than -80 mV, whereas at +100 mV they reduced the single-channel conductance by about threefold. The ionic selectivity and the blockage by divalent cations of the native channel found in amphibian rods and in the cloned channel from bovine rods are quite similar. However, the cloned channel has well-resolved openings, especially at positive membrane voltages, whereas the native channel is characterized by a continuous flickering between the open and closed state.

Gating of retinal rod cation channel by different nucleotides: comparative study of unitary currents

Single channels are observed after incorporation of native vesicles from bovine rod outer segment membranes into planar lipid bilayers. The activity of a single channel in the presence of cGMP is compared to that induced by the analog 8-bromo-cGMP and by cAMP. At +80 mV, K0.5 is about 3 microM for 8Br-cGMP, 18 microM for cGMP and 740 microM for cAMP. In cAMP, the amplitude of the current is smaller than in cGMP or 8Br-cGMP and depends on the filter cut-off frequency. The open/closed transition rates of the channel are slightly slower with 8Br-cGMP than with cGMP while they are 5 to 10 times faster with cAMP. Addition of Ni2+ ions to either cGMP or cAMP increases the open probability: the open/closed transition rates and amplitude of the current in cAMP are then comparable to those in cGMP. A dual effect of the addition of cAMP on the cGMP- or 8Br-cGMP-dependent activity previously reported (Furman, R.E., Tanaka, J.C. 1989. Biochemistry 28:2785-2788) is observed with a single channel: addition of subthreshold cAMP concentrations to cGMP (or to 8Br-cGMP) markedly increases Po; addition of cAMP concentrations higher than about 70 microM progressively accelerates the kinetics and reduces the amplitude to values observed in cAMP alone. The results are discussed in relation with the model previously proposed to account for the existence of four current levels (Ildefonse, M., Bennett, N. 1991. J. Membrane Biol. 123:133-147).

Light-activated ion channels in solitary photoreceptors of the scallop Pecten irradians

Retinas from the scallop Pecten irradians were enzymatically dispersed, yielding a large number of isolated photoreceptors suitable for tight-seal recording. Whole-cell voltage clamp measurements demonstrated that the phototransducing machinery remained intact: quantum bumps could be elicited by dim illumination, while brighter flashes produced larger, smooth photocurrents. Single-channel currents specifically activated by light were recorded in cell-attached patches, and were almost exclusively confined to the rhabdomeric region. Their density is sufficiently high to account for the macroscopic photoresponse. Channel activation is graded with stimulus intensity in a range comparable to that of the whole-cell response, and can be recorded with illumination sufficiently dim to evoke only quantum bumps. Light-dependent channel openings are very brief, on average 1 ms or less at 20-22 degrees C, apparently not because of blockage by extracellular divalent cations. The mean open time does not change substantially with stimulus intensity. In particular, since dwell times are in the millisecond range even with the dimmest lights, the channel closing rate does not appear to be the rate-limiting step for the decay kinetics of discrete waves. The latency of the first opening after light onset is inversely related to light intensity, and the envelope of channel activity resembles the time course of the whole-cell photocurrent. Unitary currents are inward at resting potential, and have a reversal voltage similar to that of the macroscopic light response. Voltage modulates the activity of light-sensitive channels by increasing the opening rate and also by lengthening the mean open times as the patch is depolarized. The unitary conductance of the predominant class of events is approximately 48 pS, but at least one additional category of smaller-amplitude openings was observed. The relative incidence of large and small events does not appear to be related in a simple way to the state of adaptation of the cell.

Cation interactions within the cyclic GMP-activated channel of retinal rods from the tiger salamander

1. The ionic dependence of current through the 3',5'-cyclic guanosine monophosphate (cyclic GMP)-activated channels of salamander rods was studied in excised inside-out membrane patches from isolated outer segments. Voltage-clamp experiments on transducing rods were performed so that the channels in intact cells could be compared with those in excised patches. 2. The reversal potential of the cyclic GMP-induced patch current was close to the Na+ equilibrium potential when the concentration of NaCl on the cytoplasmic surface of a patch was varied at constant external NaCl concentration. Fitting the Goldman-Hodgkin-Katz equation indicated that the apparent ratio of permeabilities for Na+ and Cl- was at least 50. This confirms a previous report that the channel's Na+ permeability is much larger than its Cl- permeability. 3. Na+ currents through the channel did not obey the independence principle. The outward patch current at large positive potential began to saturate with increasing concentrations of internal Na+, as if permeation required Na+ to bind to a site with an apparent dissociation constant around 180 mM. 4. In symmetrical NaCl solutions containing very low concentrations of divalent cations the current-voltage relation measured from excised patches 50 microseconds after switching the voltage showed mild outward rectification. By 1 ms the rectification was more pronounced. The rectification at 50 microseconds is attributed to voltage dependence of Na+ permeation. The additional rectification at later times is attributed to voltage dependence of the channel's probability of being open, depolarization favouring the open state. 5. In symmetrical Mg2+ solutions the cyclic GMP-induced patch currents were smaller and the outward rectification was more pronounced. 6. Addition of Mg2+ or Ca2+ to an internal Na+ solution blocked the cyclic GMP-induced Na+ current through the channels, as if by occupying a single binding site with an affinity in the 0.1-2 mM range. Block by Mg2+ was voltage dependent, suggesting that the binding site was within the channel's transmembrane electric field. Raising the Mg2+ concentration on the external surface of the patch increased the apparent dissociation constant of block by internal Mg2+, as expected if external and internal Mg2+ compete for the same binding site. 7. Block by internal Ca2+ had an opposite and weaker voltage dependence than block by internal Mg2+. 8. In symmetrical solutions containing both Na+ and Mg2+ the outward rectification was more pronounced than in solutions containing Na+ alone. In solutions thought to be close to physiological the outward patch current increased e-fold for a depolarization of 24-30 mV.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Light-dependent channels from excised patches of Limulus ventral photoreceptors are opened by cGMP

The identity of the second messenger that directly activates the light-dependent conductance in invertebrate photoreceptors remains unclear; the available evidence provides some support for cGMP and Ca2+. To resolve this issue we have applied these second messengers to membrane patches excised from the light-sensitive lobe of Limulus ventral photoreceptors. Our results show that these patches contain channels that can be opened by cGMP, but not by Ca2+. These cGMP-activated channels closely resemble the channels activated by light in cell-attached patches. This evidence suggests that cGMP is the messenger that opens the light-dependent channel in invertebrate photoreceptors.

Four cases of direct ion channel gating by cyclic nucleotides

Four different nucleotide-gated ion channels are discussed in terms of their biophysical properties and their importance in cell physiology. Channels activated directly by cGMP are present in vertebrate and invertebrate photoreceptors. In both cases cGMP increases the fraction of time the channel remains in the open state. At least three cGMP molecules are involved in channel opening in vertebrate photoreceptors and the concentration of the cyclic nucleotide to obtain the half maximal effect is about 15 microM. The light-dependent channel of both vertebrates and invertebrates is poorly cation selective. The vertebrate channel allows divalent cations to pass through 10-15-fold more easily than monovalent ions. In agreement with their preference for divalent cations, this channel is blocked by l-cis Dialtazem, a molecule that blocks certain types of calcium channels. In olfactory neurons a channel activated by both cAMP and cGMP is found and, as in the light-dependent channel, several molecules of the nucleotide are needed to open the channel with a half maximal effect obtained in the range of 1-40 microM. The channel is poorly cationic selective. A K+ channel directly and specifically activated by cAMP is found in Drosophila larval muscle. At least three cAMP molecules are involved in the opening reaction. Half-maximal effect is obtained at about 50 microM. This channel is blocked by micromolar amount of tetraethylammonium applied internally. Interestingly, this channel has a probability of opening 10-20-fold larger in the mutant dunce, a mutant that possesses abnormally elevated intracellular cAMP level, than in the wild type.

Single-channel study of the cGMP-dependent conductance of retinal rods from incorporation of native vesicles into planar lipid bilayers

Unitary currents through cGMP-dependent channels of retinal rods are observed following incorporation into planar lipid bilayers of native vesicles from purified rod outer segment membranes washed free of soluble and peripheral proteins. The influence of the concentration of cGMP, inhibitors (cis-diltiazem, tetracaine and Ag+) and divalent cations (Ca2+, Mg2+, and Co2+) on the conductance and open probability of the channel is described, as well as the voltage dependence of these effects. The cGMP dependence suggests the existence of four binding sites for cGMP and reveals that sequential binding of four cGMP molecules corresponds to the opening of four discrete conductance levels. Finally, we provide conclusive evidence that activated G-protein does not directly inactivate the cGMP-dependent channels of bovine retinal rods.

Similar properties of cGMP-activated channels between cones and rods in the carp retina

Using patch-clamp techniques, properties of cGMP-activated channel were studied at a single-channel level in order to examine (1) whether any differences are recognized between the cGMP-activated channels of rods and cones in the same animal species, and (2) whether the channel properties of the same photoreceptor class differ in different animal species. Experiments were performed on inside-out membrane patches excised from outer segments of rods and morphological subtypes of cones in the carp retina. Single-channel activities could be recorded when the patches were perfused with low concentrations of cGMP (less than 10 microM). Throughout five morphological subtypes of cones and rod, single-channel currents showed no significant rectification at membrane hyperpolarization in a low divalent cation solution, and single-channel conductances were almost the same: 13.8 +/- 0.2 pS (mean +/- S.E.M., n = 23) in cones and 12.7 +/- 0.8 pS (n = 3) in rods. These values were significantly smaller than that reported in catfish cones (about 50 pS), and that in rods of the toad and the tiger salamander (about 25 pS). In rods and all subtypes of cones of the carp, open durations of cGMP-activated channels were brief. In addition, kinetic parameters of channel openings and closings showed no differences throughout all subtypes of cones and rod.

Electrophysiological properties of isolated photoreceptors from the eye of Lima scabra

Photoreceptor cells were enzymatically dissociated from the eye of the file clam, Lima scabra. Micrographs of solitary cells reveal a villous rhabdomeric lobe, a smooth soma, and a heavily pigmented intermediate region. Membrane voltage recordings using patch electrodes show resting potentials around -60 mV. Input resistance ranges from 300 M omega to greater than 1 G omega, while membrane capacitance is of the order of 50-70 pF. In darkness, quantum bumps occur spontaneously and their frequency can be increased by dim continuous illumination in a fashion graded with light intensity. Stimulation with flashes of light produces a depolarizing photoresponse which is usually followed by a transient hyperpolarization if the stimulus is sufficiently intense. Changing the membrane potential with current-clamp causes the early phase to invert around +10 mV, while the hyperpolarizing dip disappears around -80 mV. With bright light, the biphasic response is followed by an additional depolarizing wave, often accompanied by a burst of action potentials. Both Na and Ca ions are required in the extracellular solution for normal photoexcitation: the response to flashes of moderate intensity is greatly degraded either when Na is replaced with Tris, or when Ca is substituted with Mg. By contrast, quantum bumps elicited by dim, sustained light are not affected by Ca removal, but they are markedly suppressed in a reversible way in 0 Na sea water. It was concluded that the generation of the receptor potential is primarily dependent on Na ions, whereas Ca is probably involved in a voltage-dependent process that shapes the photoresponse. Light adaptation by repetitive flashes leads to a decrease of the depolarizing phase and a concomitant enhancement of the hyperpolarizing dip, eventually resulting in a purely hyperpolarizing photoresponse. Dark adaptation restores the original biphasic shape of the photoresponse.

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)

The magnitude, time course and spatial distribution of current induced in salamander rods by cyclic guanine nucleotides

1. Cyclic GMP was introduced into isolated salamander rods through a tight-seal electrode attached to the inner segment while the outer segment was held in a suction electrode; nucleotide-induced membrane current was recorded by both electrodes. After 3-15s of nucleotide exposure the cells were stimulated with intense, brief flashes, which suppressed 90-95% of the induced membrane current. 2. The magnitude of the induced light-sensitive current depended little on the pipette cyclic GMP concentration in the range 10-20 nM: the mean whole-cell current magnitude was 1256 +/- 160 pA (mean +/- 2 S.E.M., n = 41). 3. Experiments and analyses addressed hypotheses about the nature of the magnitude limitation on the induced current. It was shown that the spatial distribution of nucleotide, the residual series resistance of the whole-cell electrode, and the diminution of the ion gradients driving the induced current did not limit the current magnitude by more than 20%. 4. In contrast, the hypothesis that outer segment internal longitudinal resistance severely limits the magnitude of the cyclic GMP-induced current was supported by experiments in which various lengths of the outer segment were drawn into the suction electrode. These showed that the ratio of nucleotide-induced light-sensitive current collected by the suction electrode to that collected by a whole-cell electrode decreased steeply as a function of outer segment length excluded from the suction electrode, having an apparent space constant of 5-7 microns. 5. A cable model of the rod was developed and used to analyse the magnitude of the nucleotide-induced currents. The data are consistent with an outer segment longitudinal resistance of 1-4 M omega microns-1, and a maximum (space-clamped) light-sensitive current density of 313 pA microns-1, equivalent to a total induced current of 7200 pA (23 microns outer segment). 6. A diffusion model was developed and combined with the non-linear cable model to provide an account for the time course of the induced membrane currents. The results are consistent with an effective longitudinal diffusion coefficient of cyclic GMP in the outer segment of 3-10 microns2 s-1, and Hill coefficient of 2-3 for the cyclic GMP gating of the light-sensitive conductance. 7. 8-Bromo-cyclic GMP also caused the light-sensitive membrane current to increase to about the same magnitude as did cyclic GMP.(ABSTRACT TRUNCATED AT 400 WORDS)

Suppression by glutamate of cGMP-activated conductance in retinal bipolar cells

Depolarizing bipolar cells (DBCs) of the retina are the only neurons in the vertebrate central nervous system known to be hyperpolarized by the neurotransmitter glutamate. Both glutamate and its analogue L-2-amino-4-phosphonobutyrate (APB) hyperpolarize DBCs by decreasing membrane conductance. Furthermore, glutamate responses in DBCs slowly decrease during whole-cell recording, suggesting that the response involves a second messenger system. Here we report that intracellular cyclic GMP or GTP activates a membrane conductance that is suppressed by APB, resulting in an enhanced APB response. In the presence of GTP-gamma-S, APB causes an irreversible suppression of the conductance. Inhibitors of G-protein activation or phosphodiesterase activity decrease the APB response. Thus, the DBC glutamate receptor seems to close ion channels by increasing the rate of cGMP hydrolysis by a G protein-mediated process that is strikingly similar to light transduction in photoreceptors.

Cyclic GMP-activated channels of rod photoreceptors show neither fast nor slow desensitization

Desensitization of cGMP-activated channels was examined in excised, inside-out patches obtained from rod photoreceptors. Cyclic GMP was applied using a rapid-flow system in which concentration jumps are complete within 10-50 ms. In outer-segment patches containing many channels, the cGMP-dependent conductance reached a steady plateau that was maintained for tens of seconds in the presence of cGMP; thus, there was no indication of slow desensitization. However, rapid desensitization on the scale of milliseconds could not be ruled out because of limited speed of access of cGMP to the inner face of the patch membrane. To test for rapid desensitization, inner-segment patches containing only a single cGMP-activated channel were used. In these one-channel patches, there was no change in activity of the channel with time from its earliest onset after application of cGMP, indicating that rapid desensitization also did not occur.

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).

Silver ions induce a rapid Ca2+ release from isolated intact bovine rod outer segments by a cooperative mechanism

Micromolar concentrations of silver ion activate large Ca2+ fluxes across the plasma membrane of intact rod outer segments isolated from bovine retinas (intact ROS). The rate of Ag+-induced Ca2+ efflux from intact ROS depended on the Ag+ concentration in a sigmoidal manner suggesting a cooperative mechanism with a Hill coefficient between 2 and 3. At a concentration of 50 microM Ag+ the rate of Ca2+ efflux was 7 x 10(6) Ca2+/outer segment/sec; this represents a change in total intracellular Ca2+ by 0.7 mM/outer segment/sec. Addition of the nonselective ionophore gramicidin in the absence of external alkali cations greatly reduced the Ag+-induced Ca2+ efflux from intact ROS, apparently by enabling internal alkali cations to leak out. Adding back alkali cations to the external medium restored Ag+-induced Ca2+ efflux when gramicidin was present. In the presence of gramicidin, Ag+-induced Ca2+ efflux from intact ROS was blocked by 50 microM tetracaine or L-cis diltiazem, whereas without gramicidin both blockers were ineffective. Both L-cis diltiazem and tetracaine are blockers of one kinetic component of cGMP-induced Ca2+ flux across ROS disk membranes. The ion selectivity of the Ag+-induced pathway proved to be broad with little discrimination between the alkali cations Li+, Na+, K+, and Cs+ or between Ca2+ and Mg2+. The properties of the Ag+-induced pathway(s) suggest that it may reflect the cGMP-dependent conductance opened in the absence of cGMP by silver ions.

Opsin exhibits cGMP-activated single-channel activity

Low concentrations of cGMP evoked reversible single-channel currents in patches excised from liposomes that contained purified bovine opsin. Two elementary conductances, of 32 and 17 pS, were observed in the presence of 10-200 microM cGMP. Both individual channel openings (mean open times, approximately 1.6 ms for the 32-pS conductance and approximately 1.0 ms for the 17-pS conductance) and bursts of openings (mean burst duration, approximately 2-3 ms for the large events) were observed. The cGMP-activated channel activity could be observed in the presence or absence of Ca2+. These results raise the possibility that opsin or rhodopsin, an opsin/rhodopsin isoform, or an opsin/isoform multimer serves as a cGMP-modulated pore in the rod outer segment.

Light-induced losses and dark recovery rates of guanosine 3',5'-cyclic monophosphate in rod outer segments of intact amphibian photoreceptors

We used an apparatus in which pieces of dark-adapted amphibian retinas (Rana pipiens, Bufo marinus) obtained under infrared illumination were exposed to precise intervals of 500-nm illuminations, and then frozen by contact of their outer segment surface with a liquid helium-cooled copper mirror. Sections of the frozen outer segment layer were obtained in a cryostat and then assayed for total extractable cyclic 3',5'-guanosine monophosphate (cGMP). Significant losses of cGMP with respect to the dark level were evident as early as 60 ms after light onset. With dim subsecond illuminations these losses were surprisingly large, which suggests a previously underestimated magnification in the cGMP cascade, or a transient substantial inhibition of guanylate cyclase activity in combination with increased cyclic GMP phosphodiesterase activity. Within the subsecond period, significant losses that were proportional to light intensity (2-log-unit range) and duration (60-550 ms) were generally not evident. However, losses significantly proportional to these factors became evident with durations of 1 s or longer. When pieces of retina were first illuminated (10 or 60 ms), then held in darkness for increasing periods before freezing, we observed a continuous loss of cGMP during the early postillumination dark period, followed by a recovery of the total cGMP level. The times for recovery to the preillumination level appear to be significantly longer than times reported for the recovery of the photoreceptor membrane potential after similar light exposures.

Activation of single ion channels from toad retinal rod inner segments by cyclic GMP: concentration dependence

1. Patch-clamp recordings of single cyclic GMP-activated channels from toad rod photoreceptors were made in inside-out membrane patches containing only one such channel. Patches were obtained from the inner segment, where the density of cyclic GMP-activated channels is lower than in the outer segment, making one-channel patches possible. The dependence of channel gating on cyclic GMP concentration ([cyclic GMP]) was studied. At low [cyclic GMP] (5-10 microM), channel openings were infrequent and occurred as bursts of rapid opening and closing. As [cyclic GMP] was increased, bursts became more frequent, until at 1 mM the activity fused into long bouts of rapid flicker between open and closed states. 2. The duration of brief openings and closings (flicker) within bursts was not affected by [cyclic GMP]. This suggests that the rapid flicker within bursts results from an intrinsic channel property not associated with agonist-induced receptor activation. 3. At 10 microM-cyclic GMP, the distribution of closed times was fitted by a sum of three exponential components. The briefest, with time constant averaging 0.29 ms, corresponded to the brief closings within bursts, while the two longer components, with time constants averaging 3.5 and 32 ms, corresponded to much longer closings between bursts. At 0.5 or 1 mM-cyclic GMP (saturation), the longer components disappeared, and the distribution of closed times was fitted by a single-exponential equation with the same time constant as the briefest component observed at lower concentrations. 4. Because the channel continued to flicker even at high [cyclic GMP], the maximal probability of being in the open state (Po) did not approach 1.0, averaging 0.30 +/- 0.05 (N = 8). The relation between Po and [cyclic GMP] was fitted by the Hill equation with an exponent of 3, suggesting that binding of cyclic GMP to multiple sites is required to open the channel.

Evidence for two functionally different membrane fractions in bovine retinal rod outer segments

1. Cyclic GMP-induced and Na+-induced Ca2+ releases from Ca2+-containing photoreceptor membrane vesicles were investigated using the Ca2+-sensitive dye Arsenazo III. Bovine photoreceptor membrane vesicles were prepared by osmotic lysis and hypotonic wash of purified rod outer segments. 2. Calcium was entrapped into these vesicles either by utilizing their passive membrane permeability ('passive' Ca2+ loading), or by activating cyclic GMP-dependent channels (cyclic GMP-stimulated Ca2+ loading), or by Na+-Ca2+ exchange (Na+-stimulated Ca2+ loading). 3. After passive Ca2+ loading, cyclic GMP released at most about 23% of the total Ca2+ which could be released with the Ca2+ ionophore A23187 (Km = 6.5 +/- 0.92 microM (6); Hill coefficient 1.68 +/- 0.19 (6]. 4. Millimolar concentrations of Na+ also induce Ca2+ releases from passively Ca2+-loaded outer segment membrane vesicles of at most 26% of the total releasable Ca2+. 5. For passively Ca2+-loaded outer segment membrane vesicles, the sum of a partial cyclic GMP-induced Ca2+ release and a consecutive saturating Na+-induced Ca2+ release, or vice versa, the sum of a partial Na+-induced Ca2+ release and a consecutive saturating cyclic GMP-induced Ca2+ release, was about 25% of the total releasable Ca2+. 6. Both cyclic GMP-induced and Na+-induced Ca2+ release was greater than 25%, up to 50% of the total releasable Ca2+ if either cyclic GMP-stimulated or Na+-stimulated Ca2+ loading of outer segment membrane vesicles was employed. 7. When the outer segment membrane vesicles were mildly sonicated in a water bath, the maximal percentages of Ca2+ releasable by both cyclic GMP and Na+ dropped, reaching final values, after a 60 s sonication period, of 2.3% for the cyclic GMP-induced Ca2+ release and 7% for the Na+-induced Ca2+ release. 8. It is concluded from these results that outer segment membrane vesicles comprise two populations of vesicles, one which contains cyclic GMP-dependent channels and Na+-Ca2+ exchanging proteins, and another one which contains neither of these proteins. 9. The sonication experiments are interpreted as suggesting that these two vesicle populations correspond to plasma and disc membranes of the rod outer segments, respectively.

Kinetics of the hydrolysis of 8-bromo-cyclic GMP by the light-activated phosphodiesterase of toad rods

The hypothesis that cyclic GMP is the internal transmitter of retinal rod phototransduction, when combined with the observations that 8-bromo-cyclic GMP opens the cyclic GMP-dependent outer segment conductance and that rods into which 8-bromo-cyclic GMP has been injected still respond to light, predicts that the light-activated phosphodiesterase (EC 3.1.4.17) must catalyze the hydrolysis of 8-bromo-cyclic GMP. This hypothesis was tested by measuring light-activated toad rod disk membrane phosphodiesterase with a pH assay technique. Phosphodiesterase-catalyzed hydrolysis of 8-bromo-cyclic GMP was confirmed: at pH 8.0, total proton production after flash activation was identical to total amount of 8-bromo-cyclic GMP added as substrate. Photoactivated phosphodiesterase was remarkably less efficient in catalyzing the hydrolysis of 8-bromo-cyclic GMP than of cyclic GMP: Vmax for 8-bromo-cyclic GMP was 0.063 M/M rhodopsin/s, whereas that for cyclic GMP was 11 M/M rhodopsin/s--170 times greater. The Km for 8-bromo-cyclic GMP was 160 microM, and for cyclic GMP, 590 microM. 8-bromo-cyclic GMP competitively inhibited phosphodiesterase-catalyzed hydrolysis of cyclic GMP with a Ki of 1.2 mM. Complete reaction progress curves were analyzed for obedience to Michaelis-Menten kinetics: cyclic GMP hydrolysis, 8-bromo-cyclic GMP hydrolysis, and cyclic GMP hydrolysis in the presence of 8-bromo-cyclic GMP as competitive inhibitor were found to follow the integrated form of the Michaelis-Menten equation over the time course of the reactions, assuming phosphodiesterase was activated as a step. The kinetic parameters extracted from reaction progress curves were consistent with those derived from analysis of the initial velocity.(ABSTRACT TRUNCATED AT 250 WORDS)

Gating kinetics of the cyclic-GMP-activated channel of retinal rods: flash photolysis and voltage-jump studies

The gating kinetics of the cGMP-activated cation channel of salamander retinal rods have been studied in excised membrane patches. Relaxations in patch current were observed after two kinds of perturbation: (i) fast jumps of cGMP concentration, generated by laser flash photolysis of a cGMP ester ("caged" cGMP), and (ii) membrane voltage jumps, which perturb activation of the channel by cGMP. In both methods the speed of activation increased with the final cGMP concentration. The results are explained by a simple kinetic model in which activation involves three sequential cGMP binding steps with bimolecular rate constants close to the diffusion-controlled limit; fully liganded channels undergo rapid open-closed transitions. Voltage perturbs activation by changing the rate constant for channel closing, which increases with hyperpolarization. Intramolecular transitions of the fully liganded channel limit the kinetics of activation at high cGMP concentrations (greater than 50 microM), whereas at physiological cGMP concentrations (less than 5 microM), the kinetics of activation are limited by the third cGMP binding step. The channel appears to be optimized for rapid responses to changes in cytoplasmic cGMP concentration.

cGMP-dependent channel protein from photoreceptor membranes: single-channel activity of the purified and reconstituted protein

The cGMP-dependent channel protein has been purified from bovine rod photoreceptor membranes and incorporated into planar lipid membranes. At low divalent cation concentrations, cGMP stimulated single-channel current fluctuations. The probability Po of the channel being open strongly depended on the cGMP concentration (EC50 = 31 microM; Hill coefficient, n = 2.3); whereas the single-channel conductance (lambda = 26 pS) was independent of the agonist concentration. The agonist-stimulated increase in the probability of an open channel was largely due to shorter closed times and, to a lesser extent, due to the channel staying open for a longer time. The current-voltage relationship of the single open channel deviated from ohmic behavior, and the open probability decreased at more negative membrane potentials. The rectification of the macroscopic cGMP-induced current in artificial bilayers that contained many channel copies can be accounted for by the voltage dependence of channel gating together with the nonlinearity of the current-voltage curve of an open channel. Current fluctuations exhibited a variety of sublevels, indicating that the channel may exist in more than one conductive state.

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.

Kinetics and components of the flash photocurrent of isolated retinal rods of the larval salamander, Ambystoma tigrinum

1. Membrane currents initiated by intense, 20 microseconds flashes (photocurrents) were recorded from isolated salamander rods by combined extracellular suction electrodes and intracellular tight-seal electrodes either in current or voltage clamp mode. The magnitudes (mean +/- 2 S.E.M.) of the maximal photoresponses recorded by the suction and by the intracellular electrode respectively were 40 +/- 5 pA (n = 18) and 35 +/- 7 mV (n = 8) for current clamp at zero current; 43 +/- 9 pA and 66 +/- 13 (n = 11) pA for voltage clamp at the zero-current holding potential, -24 +/- 3 mV. 2. Photocurrents initiated by flashes isomerizing 0.1% or more of the outer segment's rhodopsin achieved a saturated velocity and were 95% complete in less than 50 ms. The effect of incrementing flash intensity above 0.1% isomerization can be described as a translation of the photocurrent along the time axis towards the origin. Within the interval 0-50 ms the latter two-thirds of the velocity-saturated photocurrent is well described as a single-exponential decay. The decay was much faster in voltage clamp (2.8 +/- 1.2 ms, n = 11) than in current clamp mode (17 +/- 5 ms, n = 17). 3. The initial third of the velocity-saturated photocurrent, occurring over the interval from the flash to the onset of exponential decay, followed about the same time course in current and voltage clamp. The time interval occupied by this initial 'latent' phase decreased with increasing flash intensity and attained an apparent minimum of about 7 ms in response to flashes isomerizing 10% or more of the rhodopsin at ca. 22 degrees C. 4. The hypothesis that the decay of outer segment light-sensitive membrane current is the same in current and voltage clamp was supported by an analysis of the difference between outer segment currents measured successively in the two recording modes. First, the tail of the difference current decayed exponentially with a time constant approximately equal to R x C, where R and C are independently estimated slope resistance and capacitance of the rod. Secondly, the integral of the difference current, when divided by outer segment capacitance, closely approximated the hyperpolarizing light response measured under current clamp. Thus, displacement current accounted for the difference between photocurrents measured in current and voltage clamp.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

A derivative of amiloride blocks both the light-regulated and cyclic GMP-regulated conductances in rod photoreceptors

Vertebrate rod photoreceptors in the dark maintain an inward current across the outer segment membrane. The photoresponse results from a light-induced suppression of this dark current. The light-regulated current is not sensitive to either tetrodotoxin or amiloride, potent blockers of Na+ channels. Here, we report that a derivative of amiloride, 3',4'-dichlorobenzamil (DCPA), completely suppresses the dark current and light response recorded from rod photoreceptors. DCPA also blocks a cyclic GMP-activated current in excised patches of rod plasma membrane and a cGMP-induced Ca++ flux from rod disk membranes. These results are consistent with the notion that the Ca++ flux mechanism in the disk membrane and the light-regulated conductance in the plasma membrane are identical. DCPA also inhibits the Na/Ca exchange mechanism in intact rods, but at a 5-10-fold-higher concentration than is required to block the cGMP-activated flux and current. The blocking action of DCPA in 10 nM Ca++ is different from that in 1 mM Ca++, which suggests either that the conductance state of the light-regulated channel may be modified in high and low concentrations of Ca++, or that there may be two ionic channels in the rod outer segment membrane.

Properties of ion channels closed by light and opened by guanosine 3',5'-cyclic monophosphate in toad retinal rods

1. In patch-clamp recordings from outer segments of dark-adapted rod photoreceptors, single-channel recordings were obtained from the light-sensitive conductance when divalent cations were omitted from the pipette solution bathing the extracellular face of the recorded patch of membrane. 2. Activity of the light-sensitive channel was suppressed by light within the normal response range of the dark-adapted rod. During dim, steady illumination, the rate of opening of the channel fluctuated dramatically, as expected qualitatively from statistical fluctuations in the number of photoisomerizations occurring within the effective collecting area of the recorded patch. 3. The light-sensitive channel flickered rapidly in the open state, so that individual events appeared as a burst of openings and closings. The average duration of a burst was 0.78 +/- 0.03 ms (mean +/- S.E.). The average duration of an individual opening was 0.18 +/- 0.008 ms. The average closed duration within a burst was 0.37 +/- 0.02 ms. 4. Hyperpolarization of the recorded patch had no effect on average burst or open duration, although opening frequency increased slightly (+18.6 +/- 4.9%; n = 13; mean +/- S.E.). Average single-channel current increased linearly with hyperpolarization, giving an estimated single-channel conductance of 20.5 +/- 1.1 pS. By extrapolation of the relation between channel current and hyperpolarization, the dark driving force was estimated to be about 48 mV. 5. In addition to reducing the rate of channel events, dim non-saturating light also reduced the average duration of a burst of openings and the average duration of openings within a burst. 6. About 50% of cell-attached patches showed no channel activity in darkness. Light-suppressable channel activity could be induced in these silent patches by perfusing the outer segment with low-Ca2+ Ringer solution. Similarly, activity could be increased dramatically by low-Ca2+ Ringer solution in patches that did show channel activity in the dark. From the maximal channel activity observed during low-Ca2+ perfusion, the lower limit for the number of channels per patch was 20-70, corresponding to an estimated channel density of 100-350 channels micron-2. 7. After recording light-sensitive channel activity in the intact rod, the patch of membrane was excised, exposing the intracellular membrane face. Application of guanosine 3',5'-cyclic monophosphate (cyclic GMP) to the intracellular face activated channels (Haynes, Kay & Yau, 1986; Zimmerman & Baylor, 1986; Matthews, 1986d, 1987) whose properties could then be compared directly with the light-sensitive channels recorded earlier in the same patch of membrane.(ABSTRACT TRUNCATED AT 400 WORDS)