Ionic movements through light-sensitive channels of toad rods

The logic of ionic homeostasis: Cations are for voltage, but not for volume

Neuronal activity is associated with transmembrane ionic redistribution, which can lead to an osmotic imbalance. Accordingly, activity-dependent changes of the membrane potential are sometimes accompanied by changes in intracellular and/or extracellular volume. Experimental data that include distributions of ions and volume during neuronal activity are rare and rather inconsistent partly due to the technical difficulty of performing such measurements. However, progress in understanding the interrelations among ions, voltage and volume has been achieved recently by computational modelling, particularly “charge-difference” modelling. In this work a charge-difference computational model was used for further understanding of the specific roles for cations and anions. Our simulations show that without anion conductances the transmembrane movements of cations are always osmotically balanced, regardless of the stoichiometry of the pump or the ratio of Na⁺ and K⁺ conductances. Yet any changes in cation conductance or pump activity are associated with changes of the membrane potential, even when a hypothetically electroneutral pump is used in calculations and K⁺ and Na⁺ conductances are equal. On the other hand, when a Cl^- conductance is present, the only way to keep the Cl^-equilibrium potential in accordance with the changed membrane potential is to adjust cell volume. Importantly, this voltage-evoked Cl^--dependent volume change does not affect intracellular cation concentrations or the amount of energy that is necessary to support the system. Taking other factors into consideration (i.e. the presence of internal impermeant poly-anions, the activity of cation-Cl^- cotransporters, and the buildup of intra- and extracellular osmolytes, both charged and electroneutral) adds complexity, but does not change the main principles.

We have developed software that calculates membrane potential and cell volume that result from redistribution of principal ions (K⁺, Na⁺, and Cl^-) during normal cellular activity and experimental manipulations. Calculations in the model are done by an iterative charge-difference method that makes few assumptions about governing equations. Most of the features that were considered to be important for volume and voltage regulation were incorporated in the model, including the unique capability to perform calculations with different values of transmembrane water permeability. We have used the program to reexamine interactions between ionic fluxes, membrane potential, and cell volume and found that there was a previously unappreciated difference in the way that the distribution of cations and anions affect the cell. Na⁺ and K⁺, which are distributed unevenly across the membrane by the Na⁺/K⁺-ATPase, are primarily responsible for the membrane potential, but, contrary to popular belief, do not directly participate in volume regulation. On the other hand, the Cl^- conductance determines the extent of volume changes, because Cl^- has to follow the changes of membrane potential, which inevitably leads to changes in cell volume. The software is available to download and use for other investigations.

Effect of the ILE86TER mutation in the γ subunit of cGMP phosphodiesterase (PDE6) on rod photoreceptor signaling

The light-dependent decrease in cyclic guanosine monophosphate (cGMP) in the rod outer segment is produced by a phosphodiesterase (PDE6), consisting of catalytic α and β subunits and two inhibitory γ subunits. The molecular mechanism of PDE6γ regulation of the catalytic subunits is uncertain. To study this mechanism in vivo, we introduced a modified Pde6g gene for PDE6γ into a line of Pde6g(tm1)/Pde6g(tm1) mice that do not express PDE6γ. The resulting ILE86TER mice have a PDE6γ that lacks the two final carboxyl-terminal Ile(86) and Ile(87) residues, a mutation previously shown in vitro to reduce inhibition by PDE6γ. ILE86TER rods showed a decreased sensitivity and rate of activation, probably the result of a decreased level of expression of PDE6 in ILE86TER rods. More importantly, they showed a decreased rate of decay of the photoresponse, consistent with decreased inhibition of PDE6 α and β by PDE6γ. Furthermore, ILE86TER rods had a higher rate of spontaneous activation of PDE6 than WT rods. Circulating current in ILE86TER rods that also lacked both guanylyl cyclase activating proteins (GCAPs) could be increased several fold by perfusion with 100μM of the PDE6 inhibitor 3-isobutyl-1-methylxanthine (IBMX), consistent with a higher rate of dark PDE6 activity in the mutant photoreceptors. In contrast, IBMX had little effect on the circulating current of WT rods, unlike previous results from amphibians. Our results show for the first time that the Ile(86) and Ile(87) residues are necessary for normal inhibition of PDE6 catalytic activity in vivo, and that increased basal activity of PDE can be partially compensated by GCAP-dependent regulation of guanylyl cyclase.

State-dependent block of CNG channels by dequalinium

Cyclic nucleotide–gated (CNG) ion channels are nonselective cation channels with a high permeability for Ca²⁺. Not surprisingly, they are blocked by a number of Ca²⁺ channel blockers including tetracaine, pimozide, and diltiazem. We studied the effects of dequalinium, an extracellular blocker of the small conductance Ca²⁺-activated K⁺ channel. We previously noted that dequalinium is a high-affinity blocker of CNGA1 channels from the intracellular side, with little or no state dependence at 0 mV. Here we examined block by dequalinium at a broad range of voltages in both CNGA1 and CNGA2 channels. We found that dequalinium block was mildly state dependent for both channels, with the affinity for closed channels 3–5 times higher than that for open channels. Mutations in the S4-S5 linker did not alter the affinity of open channels for dequalinium, but increased the affinity of closed channels by 10–20-fold. The state-specific effect of these mutations raises the question of whether/how the S4-S5 linker alters the binding of a blocker within the ion permeation pathway.

Dequalinium: a novel, high-affinity blocker of CNGA1 channels

Cyclic nucleotide-gated (CNG) channels have been shown to be blocked by diltiazem, tetracaine, polyamines, toxins, divalent cations, and other compounds. Dequalinium is an organic divalent cation which suppresses the rat small conductance Ca(2+)-activated K(+) channel 2 (rSK2) and the activity of protein kinase C. In this study, we have tested the ability of dequalinium to block CNGA1 channels and heteromeric CNGA1+CNGB1 channels. When applied to the intracellular side of inside-out excised patches from Xenopus oocytes, dequalinium blocks CNGA1 channels with a K(1/2) approximately 190 nM and CNGA1+CNGB1 channels with a K(1/2) approximately 385 nM, at 0 mV. This block occurs in a state-independent fashion, and is voltage dependent with a zdelta approximately 1. Our data also demonstrate that dequalinium interacts with the permeant ion probably because it occupies a binding site in the ion conducting pathway. Dequalinium applied to the extracellular surface also produced block, but with a voltage dependence that suggests it crosses the membrane to block from the inside. We also show that at the single-channel level, dequalinium is a slow blocker that does not change the unitary conductance of CNGA1 channels. Thus, dequalinium should be a useful tool for studying permeation and gating properties of CNG channels.

Pore topology of the hyperpolarization-activated cyclic nucleotide-gated channel from sea urchin sperm

The current flow through hyperpolarization-activated cyclic nucleotide-gated (HCN) channels, referred to as I(h), plays a major role in several fundamental biological processes. The sequence of the presumed pore region of HCN channels is reminiscent of that of most known K(+)-selective channels. In the present work, the pore topology of an HCN channel from sea urchin sperm, called SpHCN, was investigated by means of the substituted-cysteine accessibility method (SCAM). The I(h) current in the wild-type (w.t.) SpHCN channel was irreversibly blocked by intracellular Cd(2+). This blockage was not observed in mutant C428S. Extracellular Cd(2+) did not cause any inhibition of the I(h) current in the w.t. SpHCN channel, but blocked the current in mutant channels K433C and F434C. Large extracellular anions blocked the current both in the w.t. and K433Q mutant channel. These results suggest that 1) cysteine in position 428 faces the intracellular medium; 2) lysine and phenylalanine in position 433 and 434, respectively, face the extracellular side of the membrane; and 3) lysine 433 does not mediate the anion blockade. Additionally, our study confirms that the K(+) channel signature sequence GYG also forms the inner pore in HCN channels.

Cyclic nucleotide-gated ion channels

Cyclic nucleotide-gated (CNG) channels are nonselective cation channels first identified in retinal photoreceptors and olfactory sensory neurons (OSNs). They are opened by the direct binding of cyclic nucleotides, cAMP and cGMP. Although their activity shows very little voltage dependence, CNG channels belong to the superfamily of voltage-gated ion channels. Like their cousins the voltage-gated K+ channels, CNG channels form heterotetrameric complexes consisting of two or three different types of subunits. Six different genes encoding CNG channels, four A subunits (A1 to A4) and two B subunits (B1 and B3), give rise to three different channels in rod and cone photoreceptors and in OSNs. Important functional features of these channels, i.e., ligand sensitivity and selectivity, ion permeation, and gating, are determined by the subunit composition of the respective channel complex. The function of CNG channels has been firmly established in retinal photoreceptors and in OSNs. Studies on their presence in other sensory and nonsensory cells have produced mixed results, and their purported roles in neuronal pathfinding or synaptic plasticity are not as well understood as their role in sensory neurons. Similarly, the function of invertebrate homologs found in Caenorhabditis elegans, Drosophila, and Limulus is largely unknown, except for two subunits of C. elegans that play a role in chemosensation. CNG channels are nonselective cation channels that do not discriminate well between alkali ions and even pass divalent cations, in particular Ca2+. Ca2+ entry through CNG channels is important for both excitation and adaptation of sensory cells. CNG channel activity is modulated by Ca2+/calmodulin and by phosphorylation. Other factors may also be involved in channel regulation. Mutations in CNG channel genes give rise to retinal degeneration and color blindness. In particular, mutations in the A and B subunits of the CNG channel expressed in human cones cause various forms of complete and incomplete achromatopsia.

Electrophysiological characteristics of rat gustatory cyclic nucleotide--gated channel expressed in Xenopus oocytes

The complementary DNA encoding gustatory cyclic nucleotide--gated ion channel (or gustCNG channel) cloned from rat tongue epithelial tissue was expressed in Xenopus oocytes, and its electrophysiological characteristics were investigated using tight-seal patch-clamp recordings of single and macroscopic channel currents. Both cGMP and cAMP directly activated gustCNG channels but with markedly different affinities. No desensitization or inactivation of gustCNG channel currents was observed even in the prolonged application of the cyclic nucleotides. Single-channel conductance of gustCNG channel was estimated as 28 pS in 130 mM of symmetric Na(+). Single-channel current recordings revealed fast open-close transitions and longer lasting closure states. The distribution of both open and closed events could be well fitted with two exponential components and intracellular cGMP increased the open probability (P(o)) of gustCNG channels mainly by increasing the slower opening rate. Under bi-ionic conditions, the selectivity order of gustCNG channel among divalent cations was determined as Na(+) approximately K(+) > Rb(+) > Li(+) > Cs(+) with the permeability ratio of 1:0.95:0.74:0.63:0.49. Magnesium ion blocked Na(+) currents through gustCNG channels from both intracellular and extracellular sides in voltage-dependent manners. The inhibition constants (K(i)s) of intracellular Mg(2+) were determined as 360 +/- 40 microM at 70 mV and 8.2 +/- 1.5 mM at -70 mV with z delta value of 1.04, while K(i)s of extracellular Mg(2+) were as 1.1 +/- 0.3 mM at 70 mV and 20.0 +/- 0.1 microM at -70 mV with z delta of 0.94. Although 100 microM l-cis-diltiazem blocked significant portions of outward Na(+) currents through both bovine rod and rat olfactory CNG channels, the gustCNG channel currents were minimally affected by the same concentration of the drug.

Effects of mutation at a conserved N-glycosylation site in the bovine retinal cyclic nucleotide-gated ion channel

Bovine retinal cyclic nucleotide-gated (CNG) ion channel contains an evolutionary conserved N-glycosylation site in the external loop between the fifth transmembrane segment and the pore-forming region. The effect of tunicamycin treatment and the site-specific mutation suggested that the channel is glycosylated when expressed in Xenopus oocytes. To test the role of glycosylation in this channel, N-glycosylation was abolished by mutation, and the detailed permeation and the gating characteristics of the mutant channel were investigated. The charge contribution turned out to be detectable, although the mutation of the N-glycosylation site did not affect expression and functionality of the CNG channel in oocytes.

Mechanism of cGMP-gated channel block by intracellular polyamines

Polyamines block the retinal cyclic nucleotide-gated channel from both the intracellular and extracellular sides. The voltage-dependent mechanism by which intracellular polyamines inhibit the channel current is complex: as membrane voltage is increased in the presence of polyamines, current inhibition is not monotonic, but exhibits a pronounced damped undulation. To understand the blocking mechanism of intracellular polyamines, we systematically studied the endogenous polyamines as well as a series of derivatives. The complex channel-blocking behavior of polyamines can be accounted for by a minimal model whereby a given polyamine species (e.g., spermine) causes multiple blocked channel states. Each blocked state represents a channel occupied by a polyamine molecule with characteristic affinity and probability of traversing the pore, and exhibits a characteristic dependence on membrane voltage and cGMP concentration.

Divalent cation interactions with light-dependent K channels. Kinetics of voltage-dependent block and requirement for an open pore

The light-dependent K conductance of hyperpolarizing Pecten photoreceptors exhibits a pronounced outward rectification that is eliminated by removal of extracellular divalent cations. The voltage-dependent block by Ca(2+) and Mg(2+) that underlies such nonlinearity was investigated. Both divalents reduce the photocurrent amplitude, the potency being significantly higher for Ca(2+) than Mg(2+) (K(1/2) approximately 16 and 61 mM, respectively, at V(m) = -30 mV). Neither cation is measurably permeant. Manipulating the concentration of permeant K ions affects the blockade, suggesting that the mechanism entails occlusion of the permeation pathway. The voltage dependency of Ca(2+) block is consistent with a single binding site located at an electrical distance of delta approximately 0.6 from the outside. Resolution of light-dependent single-channel currents under physiological conditions indicates that blockade must be slow, which prompted the use of perturbation/relaxation methods to analyze its kinetics. Voltage steps during illumination produce a distinct relaxation in the photocurrent (tau = 5-20 ms) that disappears on removal of Ca(2+) and Mg(2+) and thus reflects enhancement or relief of blockade, depending on the polarity of the stimulus. The equilibration kinetics are significantly faster with Ca(2+) than with Mg(2+), suggesting that the process is dominated by the "on" rate, perhaps because of a step requiring dehydration of the blocking ion to access the binding site. Complementary strategies were adopted to investigate the interaction between blockade and channel gating: the photocurrent decay accelerates with hyperpolarization, but the effect requires extracellular divalents. Moreover, conditioning voltage steps terminated immediately before light stimulation failed to affect the photocurrent. These observations suggest that equilibration of block at different voltages requires an open pore. Inducing channels to close during a conditioning hyperpolarization resulted in a slight delay in the rising phase of a subsequent light response; this effect can be interpreted as closure of the channel with a divalent ion trapped inside.

Blockade of a retinal cGMP-gated channel by polyamines

The cyclic nucleotide-gated (CNG) channel in retinal rods converts the light-regulated intracellular cGMP concentration to various levels of membrane potential. Blockade of the channel by cations such as Ca2+ and Mg2+ lowers its effective conductance. Consequently, the membrane potential has very low noise, which enables rods to detect light with extremely high sensitivity. Here, we report that three polyamines (putrescine, spermidine, and spermine), which exist in both the intracellular and extracellular media, also effectively block the CNG channel from both sides of the membrane. Among them, spermine has the greatest potency. Extracellular spermine blocks the channel as a permeant blocker, whereas intracellular spermine appears to block the channel in two conformations-one permeant, and the other non- (or much less) permeant. The membrane potential in rods is typically depolarized to approximately -40 mV in the dark. At this voltage, K1/2 of the CNG channel for extracellular spermine is 3 microM, which is 100-1,000-fold higher affinity than that of the NMDA receptor-channel for extracellular spermine. Blockade of the CNG channel by polyamines may play an important role in suppressing noise in the signal transduction system in rods.

Calmodulin controls the rod photoreceptor CNG channel through an unconventional binding site in the N-terminus of the beta-subunit

Calmodulin (CaM) controls the activity of the rod cGMP-gated ion channel by decreasing the apparent cGMP affinity. We have examined the mechanism of this modulation using electrophysiological and biochemical techniques. Heteromeric channels, consisting of alpha- and beta-subunits, display a high CaM sensitivity (EC50 </=5 nM) similar to the native channel. Using surface plasmon resonance spectroscopy, we identified two unconventional CaM-binding sites (CaM1 and CaM2), one in each of the N- and the C-terminal regions of the beta-subunit. Ca2+ co-operatively stimulates binding of CaM to these sites exactly within the range of [Ca2+] occurring during a light response. Deletion of the N-terminal CaM1 site results in channels that are no longer CaM-sensitive, whereas deletion of CaM2 has only minor effects. We discuss different models to explain the high-affinity binding of CaM.

The determination of total cGMP levels in rod outer segments from intact toad photoreceptors in response to light superimposed on background and to consecutive flashes: a second light flash accelerates the dark recovery rate of cGMP levels in control media, but not in Na(+)-free, low Ca2+ medium

In previous experiments we established that a light flash reduced cGMP levels of toad rod outer segments within the transduction time interval, but that recovery of the dark level of cGMP occurred more slowly than reported electrophysiological recovery of membrane potential. We now report that a second light flash accelerates the recovery rate of total cGMP following an initial flash, but that this acceleration is blocked in a medium which is both sodium and calcium deficient. We also noted that calcium deficiency only elevated cGMP levels when sodium was present. For other experiments, we recorded ERG or aspartate isolated PIII responses from eyecups or retinas mounted on our quick-freeze apparatus, the light stimuli originating from the double light-bench of the latter. Whereas background illumination depressed cGMP, no detectable further cGMP loss accompanied the electrical response to a flash superimposed on the background.

The permeability of the cGMP-activated channel to organic cations in retinal rods of the tiger salamander

1. The permeability of the channel activated by guanosine 3',5'-cyclic monophosphate (cGMP) to many organic monovalent cations was determined by recording macroscopic currents in excised inside-out patches of plasma membrane from isolated retinal rod outer segments of the tiger salamander. 2. Current-voltage relations were measured when the NaCl of the bathing medium was replaced by salts of organic cations. Permeability ratios relative to Na+ ions were calculated with the Goldman-Hodgkin-Katz potential equation from the measured changes of reversal potentials. 3. Hydroxylammonium+, hydrazinium+ and methylammonium+, which are molecules of very similar shape and size, permeate the channel with very different permeability ratios: 5.92, 1.99 and 0.60 respectively. 4. Methylated and ethylated ammonium+ compounds were investigated. It was found that, not only methylammonium+, but also dimethylammonium+ and ethylammonium+ were permeant with permeability ratios of 0.6, 0.14 and 0.16 respectively. Trimethylammonium+, tetramethylammonium+, diethylammonium+, triethylammonium+, and tetraethylammonium+ were not permeant. 5. Guanidinium+ and its derivatives formamidinium+, aminoguanidinium+, acetamidinium+ and methylguanidinium+ were all permeant with permeability ratios 1.12, 1.00, 0.63, 0.36 and 0.33 respectively. 6. The cGMP-activated channel was found to be permeable to at least thirteen organic cations. Molecular models of the permeant cations indicate that the cross-section of the narrowest part of the pore must be at least as large as a rectangle of 0.38 x 0.5 nm dimensions.

On the relation between ERG waves and retinal function: inverted rod photoresponses from the frog retina

In rod mass receptor photoresponses recorded across the isolated frog retina, a paradoxical cornea-positive wave may precede the response of normal polarity. We present a model which shows that the light-induced decrease in rod current can give rise to inverted or biphasic ERG signals if the distal part (tip) of the rod outer segment responds more slowly and/or less sensitively than the proximal part (base). The condition is that current entering at the tip is represented with greater weight in the ERG. The model reproduces recorded ERG waveforms well. It further predicts that if there is a light-insensitive conductance in the tip membrane, ERG photoresponses may be non-recordable although current photoresponses are only slightly reduced. The model reveals a type of complexity in the relation between mass potentials and underlying physiological processes which has not previously received attention.

Diltiazem at high concentration increases the ionic permeability of biological membranes

The effects of diltiazem, a drug which inhibits the calcium channels in cardiac muscle as well as the light-sensitive channels in photoreceptor cells, were studied on ionic fluxes in both membrane and intact cell preparations. Diltiazem nonselectively increased the ionic permeability to both anions and cations in photoreceptor rod outer segment and synaptic membrane vesicles as well as in intact erythrocytes. Under our conditions, the estimated threshold for the diltiazem effect varied between 12.5 and 200 microM. In each case the concentration dependence exhibited the sigmoidal shape characteristic of positive cooperativity. The effect of diltiazem on ionic fluxes from phospholipid vesicles were strongly influenced by phospholipid composition and membrane charge. By contrast, diltiazem inhibited the efflux of 86Rb from photoreceptor cells of intact aspartate-isolated retina, an effect opposite to that of diltiazem on ionic permeabilities in photoreceptor membrane vesicle preparations. These data raise serious doubts on the specificity of diltiazem as a calcium channel blocker or as a cGMP channel blocker when used at concentrations higher than 10 microM.

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.

Partial characterization of a high affinity [Ca2+ + Mg2+]-dependent adenosinetriphosphatase from bovine retina

Examination of retinal tissue homogenates indicated the presence of a [Ca2+ + Mg2+]-dependent adenosinetriphosphatase activity that exhibited high affinity for Ca2+ (K0.5 = 0.17 microM) and moderately high affinity for Mg2+ and ATP (K0.5 = 12.5 microM and Km = 22.8 microM, respectively). Maximum ATP hydrolysis occurred at pH 7.4. Under conditions of optimal substrate, cation and hydrogen ion concentrations, specific activity ranged from 15 to 18 nmol phosphate released min-1 mg-1 protein. Although the retinal [Ca2+ + Mg2+] adenosinetriphosphatase hydrolyzes both ATP and dATP, other nucleotides (CTP, GTP, ITP and UTP) were not hydrolyzed to any great extent. The monovalent cations, Li+, K+ and Na+, had no effect upon hydrolysis of ATP; whereas Cs+ and NH4+ ions were moderately (approximately 30%) inhibitory. All divalent cations tested were stimulatory. With the exception of rotenone which inhibited ATP hydrolysis approximately 25%; retinal adenosinetriphosphatase activity was insensitive to mitochondrial inhibitors (NaN3, KCN, ruthenium red and oligomycin). Adenosinetriphosphatase activity was observed to be very sensitive to low concentrations (I50 approximately 2 microM) of vanadate; whereas, lanthanum administration resulted in no inhibition. Removal of calmodulin (80%) resulted in reducing adenosinetriphosphatase activity 60% but addition of exogenous calmodulin back to calmodulin deficient membranes did not restore activity to starting levels. Calmodulin antagonists trifluoperazine and calmidazolium reduced significantly Ca2+ stimulated, Mg2+ dependent ATP hydrolysis. We conclude that the [Ca2+ + Mg2+]-dependent adenosinetriphosphatase of bovine retina is a non-mitochondrial protein exhibiting very high affinity for Ca2+ and appears to require calmodulin for maximum activity. Because of its high affinity for Ca2+, this protein may play an important role in reducing intracellular Ca2+ to nanomolar levels.

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)

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.

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

1. By using the method of Hodgkin, McNaughton & Nunn (1985) for rapidly changing the extracellular medium, we analysed the effect of the organic compound IBMX (3-isobutyl-1-methylxanthine) on the movement of divalent cations through the light-sensitive channels of isolated retinal rods of the tiger salamander. 2. When the rod is treated with 0.5 mM-IBMX it is possible to observe photocurrents larger than 50 pA carried by Ba2+, Sr2+, Ca2+, Mg2+ and Mn2+. Under these conditions Ca2+, Mg2+ and Mn2+ carry photocurrents of similar amplitude, while Ba2+ and Sr2+ usually carry larger photocurrents. 3. The movement of Mn2+ through the light-sensitive channel, which is hardly detected under normal conditions, can also be observed after treating the rod for a few seconds with a solution containing 35 mM[Na+]o and 10(-7) M[Ca2+]o. Under these conditions the photocurrent carried by Mn2+ is fully saturated in the presence of 1 mM-extracellular Mn2+. 4. When the rod is pre-treated with an extracellular solution containing 0.5 mM-IBMX the maximal photocurrent which can be carried by 10 mM [Ca2+]o increases from about 10 pA to approximately 200 pA. In these conditions the half-activation of the Ca2+ current is between 1 and 10 mM, that is 20-50 times higher than in normal conditions (Menini, Rispoli & Torre, 1988). 5. When the rod is pre-treated with an extracellular solution containing 0.5 mM-IBMX the half-activation of the photocurrent which can be carried by Mg2+, Ba2+ and Sr2+ is equivalent to or greater than 10 mM. In the absence of pre-treatment with IBMX the half-activation of the photocurrent carried by Mg2+, Ba2+ and Sr2+ is less than 5 mM. 6. We conclude that the light-sensitive channel can exist in at least two distinct open states. The selectivity of the channel in the first open state is as described in a previous paper (Menini et al. 1988). Mn2+, which is hardly permeable through the light-sensitive channel in the first open state, can move through the light-sensitive channel in the second open state. Ca2+, Mg2+, Ba2+ and Sr2+ permeate more freely through the light-sensitive channel in the second open state, probably because the electrostatic interactions between these ions and the channel are less strong.

Regulation of cyclic GMP binding to retinal rod membranes by calcium

The apparently cooperative binding of 8-(5-thioacetamidofluorescein)-cGMP (SAF-cGMP) to cGMP-binding sites of the rod outer segments is regulated by Ca2+ in the 0.1-1 microM activity range. High Ca2+ reduces, and low Ca2+ increases the affinity of SAF-cGMP binding. This regulation involves only intrinsic membrane components. It is proposed that an allosteric regulation of cGMP binding by Ca2+ can contribute to photoreceptor potential adaptation.

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+.

The blocking effect of l-cis-diltiazem on the light-sensitive current of isolated rods of the tiger salamander

The effect of the organic compound l-cis-diltiazem on the light-sensitive current of isolated rods of the tiger salamander was analysed by rapidly changing the extracellular medium using the method of Hodgkin et al. (1985). Addition to the extracellular medium of small amounts of l-cis-diltiazem rapidly inhibits the photocurrent. Complete suppression of the current was observed with 1 mM l-cis-diltiazem. Half blockage of the photocurrent occurred with about 150 microM l-cis-diltiazem. The blocking effect of l-cis-diltiazem was enhanced by light and by a reduction of extracellular Na. A concentration of l-cis-diltiazem of 140 microM, which suppresses one third of the photocurrent, was able to completely suppress the photocurrent carried by Ba. It is suggested that l-cis-diltiazem blocks the light-sensitive channel, possibly competing with cyclic guanosine-3'-5'-monophosphate (cGMP) for an internal regulatory site.

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)

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.

Measurement of sodium-calcium exchange in salamander rods

1. Methods employing suction electrodes to measure the small inward currents associated with the exchange of internal Ca2+ for external Na+ in salamander rod outer segments are described. 2. The ratio of the integral of the exchange current to the integral of the Ca2+ current during the loading period averaged 0.37, which is consistent with 1 Ca2+ ion exchanging with 2.7 Na+ ions, in approximate agreement with Yau & Nakatani (1984b). 3. The transient pumping current observed when external Na+ was restored after a few seconds in isotonic Ca2+ with IBMX (3-isobutyl-1-methylxanthine) consisted of a phase with current at a constant saturated level followed by a phase in which current declined along a characteristic S-shaped curve that was much steeper than expected from the Michaelis equation. 4. The relation between Ca2+ load and pumping current was also steeper than a Michaelis relation. 5. In Ringer solution at 20 degrees C the saturated exchange current was about 20 pA and the value of charge at which the current was half-saturated was 1-5 pC corresponding to 6-30 X 10(6) Ca2+ ions per rod outer segment. 6. The Ca2+ exchange current after small loads declined along the same curve as that determined with medium loads but fell more slowly after large loads. 7. The exchange current at the beginning of the plateau of a strong flash response usually declined along the curve determined with small or medium Ca2+ loads. 8. There was evidence that the exchange current at the tip of the outer segment remained saturated for longer than at the base. 9. The time to pump out Ca2+ through the Na+-Ca2+ exchange system is largely responsible for the delay in the recovery of the light-sensitive current after a Ca2+ load. 10. A theoretical analysis of some of the observations in this and the succeeding paper is based on assumptions about the binding of Ca2+ by exchange sites and by cytoplasmic Ca2+ buffers.

Divalent cations directly affect the conductance of excised patches of rod photoreceptor membrane

Phototransduction in rod cells is likely to involve an intracellular messenger system that links the absorption of light by rhodopsin to a change in membrane conductance. The direct effect of guanosine 3',5'-monophosphate (cGMP) on excised patches of rod outer segment membrane strongly supports a role for cGMP as an intracellular messenger in phototransduction. It is reported here that magnesium and calcium directly affect the conductance of excised patches of rod membrane in the absence of cGMP and that magnesium, applied to intact rod cells, blocks a component of the cellular light response. The divalent cation-suppressed conductance in excised patches showed outward rectification and cation-selective permeability resembling those of the light-suppressed conductance measured from the intact rod cell. The divalent cation-suppressed conductance was partly blocked by a concentration of the pharmacological agent L-cis-diltiazem that blocked all of the cGMP-activated conductance. Divalent cations may act, together with cGMP, as an intracellular messenger system that mediates the light response of the rod photoreceptor cell.

Cyclic GMP-sensitive conductance of retinal rods consists of aqueous pores

The surface membrane of retinal rod and cone outer segments contains a cation-selective conductance which is activated by 3',5'-cyclic guanosine monophosphate (cGMP). Reduction of this conductance by a light-induced decrease in the cytoplasmic concentration of cGMP appears to generate the electrical response to light, but little is known about the molecular nature of the conductance. The estimated unitary conductance is so small that ion transport might occur via either a carrier or a pore mechanism. Here we report recordings of cGMP-activated single-channel currents from excised rod outer segment patches bathed in solutions low in divalent cations. Two elementary conductances, of approximately 24 and 8 pS, were observed. These conductances are too large to be accounted for by carrier transport, indicating that the cGMP-activated conductance consists of aqueous pores. The dependence of the channel activation on the concentration of cGMP suggests that opening of the pore is triggered by cooperative binding of at least three cGMP molecules.

Incorporation of calcium buffers into salamander retinal rods: a rejection of the calcium hypothesis of phototransduction

The suction-electrode technique was used to monitor the photocurrent of isolated retinal rods from the tiger salamander, by drawing in the light-sensitive outer segment, or sometimes the inner segment. Calcium buffers or other agents were then introduced into the rod cytoplasm by the 'whole-cell patch-clamp' technique. A patch pipette was sealed against the region of the rod protruding from the suction pipette (usually the inner segment), and the membrane patch was ruptured to obtain a whole-cell recording. Several lines of evidence indicated that the pipette contents diffused into the outer segment, and showed that the cell could be adequately voltage clamped. With only trace quantities of chelator in the patch pipette (to bind stray calcium), a gradual decline of the dark current and slowing of responses was usually observed over a period of 10-20 min after rupture of the patch. When the patch pipette contained no added calcium and 10 mM of the calcium chelator BAPTA (1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid) (free Ca2+ ca. 10(-9) M) rupture of the patch led, over a period of a few minutes, to an increase in mean dark current, an increased duration of responses, a substantial increase in flash sensitivity, and a pronounced overshoot in the recovery phase, but with virtually no change in the rising phase of the response to bright flashes. Similar results were obtained when EGTA was used in place of BAPTA, and also in the few cases when successful rupture of the outer segment membrane was obtained. With the free calcium concentration in the patch pipette buffered to the higher level of 1 microM (with 10 mM-Ca2+/11 mM-BAPTA) the results were qualitatively similar to those obtained with BAPTA alone, except that the mean dark current did not increase. This is consistent with a resting free calcium concentration in darkness in the region of 1 microM. In the presence of bright steady illumination with BAPTA in the cell the suppression of outer segment current could be maintained for at least 15 min. Upon extinction of the light a very large current transient developed (similar to the overshoot with flashes) which was light suppressible. With backgrounds of moderate intensity the incorporation of buffer led to a gradual reduction of the residual current.(ABSTRACT TRUNCATED AT 400 WORDS)

Electrical properties of the light-sensitive conductance of rods of the salamander Ambystoma tigrinum

The light-sensitive conductance of isolated rods from the retina of the tiger salamander was studied using a voltage-clamp method. The membrane current of the outer segment was collected with a suction electrode while the internal voltage was measured and controlled with a pair of intracellular electrodes. Saturating light blocked the outer segment current at all potentials, the residual conductance usually becoming less than 20 pS. This suggests that light-sensitive channels comprise the main ionic conductance in the surface membrane of the outer segment. Current-voltage relations determined 10-40 ms after changing the voltage showed outward-going rectification, the outward current increasing e-fold for a depolarization of 11-14 mV. The reversal potential of the light-sensitive current was estimated as 5 +/- 4 mV. This is consistent with other evidence indicating that the channel is not exclusively permeable to Na. Applying steady light, lowering external Ca, or changing the intracellular voltage to a new steady level scaled the light-sensitive current without altering the reversal potential or the form of the rectification. This suggests that all three manipulations change the number of channels in the conducting state without changing the ionic concentration gradients or the mechanism of permeation through an 'open' channel. Hyperpolarizing voltage steps slowly increased the light-sensitive current and depolarizing steps reduced it. A gating variable Y expressing the fractional activation of the light-sensitive conductance in the steady state was derived from the ratio of the instantaneous and steady-state currents. Y declined at voltages positive to -100 mV and usually reached a minimum near 0 mV, with a secondary rise positive to 0 mV. Around the dark voltage Y changed e-fold in roughly 25 mV. The voltage-dependent gating in (6). appeared to involve two delays similar in magnitude to those of the four principal delays in the rod's response to a dim flash. Steady background light shortened the time-scale of gating and flash responses to a similar degree. Clamping the voltage at the dark level had little effect on the photocurrent evoked by a flash. The small, delayed effect actually observed is explained by the slow voltage-dependent gating of the light-sensitive conductance. Hyperpolarization had little effect on the kinetics of the response to a flash, but depolarization slowed the response, causing it to reach a larger, later peak. Depolarization also prolonged the blockage of the light-sensitive current after a saturating flash.(ABSTRACT TRUNCATED AT 400 WORDS)

The effects of phosphodiesterase inhibitors and lanthanum ions on the light-sensitive current of toad retinal rods

The light-sensitive current of isolated toad rods was recorded using the method of Yau, McNaughton & Hodgkin (1981) and the effects of the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX) and of La ions were examined. IBMX caused an increase in the light-sensitive current and a prolongation of the time course of the response. A small inward current which may reflect the operation of an Na-Ca exchange pump was also increased in IBMX. With low doses of IBMX the time course of the dim flash response could be mapped onto that in Ringer solution by a linear transformation of the time scale. Light adaptation had opposite effects to those of IBMX on the time course of the dim flash response, and a steady background light could exactly neutralize the effects of IBMX on the time course. Light adaptation had the additional effect of strongly reducing the amplitude of the dim flash response. La ions caused a rapid inhibition (t1/2 less than 1 s) followed by slow inhibition (t1/2 approximately equal to 30 s) of the light-sensitive current. In low [Ca] the rapid inhibition became more prominent, perhaps because of a competition between La and Ca for a blocking site near the light-sensitive channel. The time constants of the falling phases of responses to both bright and dim flashes were slowed by La. The dim flash response could be fitted by a model in which a single time constant in the chain underlying the flash response is slowed by La. La reduced the rate of activation of light-sensitive current in response to a reduction of external [Ca]. A concentration of La sufficient to block the activation of current in low [Ca] did not prevent the activation of current in IBMX. Light-sensitive currents carried by Mn, Ca or Mg in the absence of Na and in the presence of IBMX were inhibited by La. An outward current observed in the absence of permeable ions was inhibited by La. The effects of La on the time course of the response and on the rate of activation of current when [Ca] is reduced are consistent with an inhibition of the Ca pump. La ions also have a high affinity for the light-sensitive channel and can block current carried by another ion.

Ionic permeabilities of the plasma membrane of isolated intact bovine rod outer segments as studied with a novel optical probe

The permeability properties of the plasma membrane of intact rod outer segments purified from bovine retinas (ROS) were studied with the aid of the optical probe neutral red as described in the companion paper. The following observations were made: Electrical shunting of ROS membranes greatly stimulated Na+ and K+ transport, suggesting that this transport reflects Na+ and K+ currents, respectively. The dissipation of a Na+ gradient across the plasma membrane occurred with a half-time of 30 sec at 25 degrees C. The Na+ permeability was progressively inhibited when the external Ca2+ concentration was raised from 1 microM to 20 mM. A similar Ca2+ dependence was observed for H+ and Li+ transport. The Na+ permeability was not affected when the total internal Ca2+ content of ROS was varied between 0.1 mol Ca2+/mol rhodopsin and 7 mol Ca2+/mol rhodopsin, or when the free internal Ca2+ concentration was varied between 0.1 and 50 microM. The K+ permeability was progressively stimulated when the external Ca2+ concentration was raised from 0.001 to 1 microM, whereas a further increase to 20 mM was without effect. A similar Ca2+ dependence was observed for Rb+ and Cs+ transport. At an external Ca2+ concentration in the micromolar range the rate of transport decreased in the order: Na+ greater than K+ = H+ greater than Cs+ greater than Li+. Na+ fluxes depended in a sigmoidal way on the external Na+ concentration, suggesting that sodium ions move in pairs. The concentration dependence of uniport Na+ transport and that of Na+-stimulated Ca2+ efflux (exchange or antiport transport) were very similar.

Effect of cGMP and cations on the permeability of cattle retinal disks

Guanosine 3',5'-monophosphate and Na or Ca ions affect the transmembrane movements of the same pool of intradiskal ions. Extradiskal Na ions activate the efflux of intradiskal Na ions. Extradiskal Ca ions activate the efflux of intradiskal Rb ions. Na and Ca ions activate Na/Ca or Ca/Ca exchange, as previously described. cGMP activates a membrane permeability for all the cations tested, as previously described. The reciprocal relations between cGMP and the other pathways for ion movements through disk membranes are systematically examined. Some analogies between the cGMP-activated permeability of the disk membranes and the light-sensitive conductance of the rod plasma outer membrane are discussed.

Induction by cyclic GMP of cationic conductance in plasma membrane of retinal rod outer segment

Vertebrate rod photoreceptors hyperpolarize when illuminated, due to the closing of cation-selective channels in the plasma membrane. The mechanism controlling the opening and closing of these channels is still unclear, however. Both 3',5'-cyclic GMP and Ca2+ ions have been proposed as intracellular messengers for coupling the light activation of the photopigment rhodopsin to channel activity and thus modulating light-sensitive conductance. We have now studied the effects of possible conductance modulators on excised 'inside-out' patches from the plasma membrane of the rod outer segment (ROS), and have found that cyclic GMP acting from the inner side of the membrane markedly increases the cationic conductance of such patches (EC50 30 microM cyclic GMP) in a reversible manner, while Ca2+ is ineffective. The cyclic GMP-induced conductance increase occurs in the absence of nucleoside triphosphates and, hence, is not mediated by protein phosphorylation, but seems rather to result from a direct action of cyclic GMP on the membrane. The effect of cyclic GMP is highly specific; cyclic AMP and 2',3'-cyclic GMP are completely ineffective when applied in millimolar concentrations. We were unable to recognize discrete current steps that might represent single-channel openings and closings modulated by cyclic GMP. Analysis of membrane current noise shows the elementary event to be 3 fA with 110 mM Na+ on both sides of the membrane at a membrane potential of -30 mV. If the initial event is assumed to be the closure of a single cyclic GMP-sensitive channel, this value corresponds to a single-channel conductance of 100 fS. It seems probable that the cyclic GMP-sensitive conductance is responsible for the generation of the rod photoresponse in vivo.

The ionic selectivity and calcium dependence of the light-sensitive pathway in toad rods

A new method is described for determining the effects of rapid changes in ionic concentration on the light-sensitive currents of rod outer segments. Replacing Na with another monovalent cation caused a rapid change in current followed by an exponential decline of time constant 0.5-2 s. From the magnitude of the initial rapid change in current we conclude that Li, Na, and K and Rb ions pass readily through the light-sensitive channel in the presence of 1 mM-Ca, whereas Cs crosses with difficulty and choline, tetramethylammonium and tetraethylammonium not at all. The effect of reducing Ca in the external medium indicates that the residual inward current recorded for a few seconds when Na is replaced by an impermeant ion is carried largely by Ca ions. With 1 microM-Ca in the external medium the relative ability of monovalent cations to carry light-sensitive current is Li:Na:K:Rb:Cs = 1.4:1:0.8:0.6:0.15. The same order applied in the physiological region but the values are less certain. Large transient inward currents are seen if external Ca is raised form 1 microM to 5 mM or more; these currents which are maximal in an isotonic Ca solution are presumably carried by Ca. The effect of monovalent cations on the number of open light-sensitive channels was tested by adding the cation to a solution containing 55 mM-Na. Na ions open light-sensitive channels with a delay, probably by promoting Na-Ca exchange; K and Rb close channels by inhibiting exchange; Li and Cs seem inert in the exchange mechanism. The rate at which inward current declines in low [Na]o or high [Ca]o is accelerated by weak background lights and slowed by 3-isobutyl-1-methylxanthine (IBMX), which inhibits the hydrolysis of cGMP. On returning to Ringer solution after a period in low [Na]o the current recovers with a delay of about 1 s which decreases as the Ca concentration of the low [Na]o medium is reduced. We conclude that intracellular Ca has a strong effect on the number of open light-sensitive channels. None the less, several observations are inconsistent with channel closure being dependent simply on combination with internal Ca.

Signal mechanisms of phototransduction in retinal rod

The levels of intracellular molecules are modulated by illumination of rod photoreceptor cells in the vertebrate retina. Among these are Ca ions, cyclic nucleotides (cGMP in particular), and phosphate nucleotides (ATP and GTP). It is presumed now that at least two of these molecules, Ca and cGMP, may function as chemical linkers between the absorption of light by rhodopsin and the ionic channels of the plasma membrane of the rod outer segment that close when the rod is illuminated. The manuscript will review the physiology of the rod cell, the evidence in support of light-dependent changes in the intracellular levels of various small molecules, and the role of these changes in coupling rhodopsin excitation to the control of the light-sensitive membrane channels in the rod.

Cation selectivity of light-sensitive conductance in retinal rods

Vertebrate photoreceptors respond to light by a membrane hyperpolarization which is thought to result from the decrease of a Na-selective conductance in the outer segment. One hypothesis is that light increases intracellular free Ca which reversibly blocks the Na conductance; at least part of this Ca is then extruded to the cell exterior by a Na-Ca exchanger at the plasma membrane. We describe experiments here which show that the light-sensitive conductance in rods is also highly permeable to K. While external Na acts to keep the conductance open, external K tends to keep it closed, both actions probably involving the Na-Ca exchanger. The conductance is also permeable to the monovalent cations Li, Rb and Cs and the divalent cations Ca, Sr and Ba. The ability of both Na and K to go through the light-sensitive conductance explains the long-standing puzzle as to why the reversal potential for the light response is at 0 to + 10 mV.

Effect of ions on retinal rods from Bufo marinus

The effect of ions on the light-sensitive current of isolated retinal rods from the toad Bufo marinus was studied by sucking the inner segment into a tightly fitting pipette. The outer segment projected into flowing solution whose composition could be changed rapidly. Reducing the external Na concentration, [Na]o, round the outer segment caused rapid and reversible reductions in the light-sensitive current. With the outer segment in the pipette, reductions of [Na]o round the inner segment had little effect on the light-sensitive current. The current about 15 s after a change in [Na]o was approximately proportional to [Na]2o. The current decreased in elevated external Ca concentration, [Ca]o, and increased in reduced [Ca]o. Between 10 and 0.5 mM-external Ca the current 15 s after a change was approximately inversely proportional to [Ca]o. Reducing [Ca]o from 1 mM to 1 microM or less transiently increased the current by about 15-fold. After a change in [Na]o or [Ca]o the current did not approach its final value monotonically but with a characteristic overshoot or underswing, followed by a slow relaxation of current which may reflect the time course of change in internal Na. Reducing [Na]o from 110 to 70 mM or less prolonged the response to a flash; very long responses were observed in solutions containing Li rather than Na and also in rods that had been returned to Ringer solution after exposure to low Ca. All these effects might be explained if Ca extrusion in exchange for Na determines the reactivation of current after a flash. The rod current was not changed if the ratio [Na]No/[Ca]o was held constant, N being about 2.5. Between 5 mM and 10 microM-Ca the change in peak current produced by absorption of a single quantum was roughly proportional to the dark current. Responses in the absence of external Na were not normally seen if the solution contained 0.1 mM-Ca or more. Responses of normal polarity were regularly observed in 0 Na, 0 Ca EGTA solutions containing 1.6 mM-Mg. Removal of Mg from such solutions gave inverted responses. Other conditions which promote responses of normal and inverted polarity in Na-free solutions are described briefly. We conclude that Li, Ca, Mg and perhaps K can pass through the light-sensitive channel. The above results suggest that external Na has two distinct effects: (1) it provides ions to carry inward current, and (2) it keeps the light-sensitive conductance open by maintaining the internal Ca concentration, [Ca]i, at a low level.