The effect of ions on sodium-calcium exchange in salamander rods

Bicarbonate boosts flash response amplitude to augment absolute sensitivity and extend dynamic range in murine retinal rods

Rod photoreceptors in the retina adjust their responsiveness and sensitivity so that they can continue to provide meaningful information over a wide range of light intensities. By stimulating membrane guanylate cyclases in the outer segment to synthesize cGMP at a faster rate in a Ca²⁺-dependent fashion, bicarbonate increases the circulating "dark" current and accelerates flash response kinetics in amphibian rods. Compared to amphibian rods, mammalian rods are smaller in size, operate at a higher temperature, and express visual cascade proteins with somewhat different biochemical properties. Here, we evaluated the role of bicarbonate in rods of cpfl3 mice. These mice are deficient in their expression of functional cone transducin, Gnat2, making cones very insensitive to light, so the rod response to light could be observed in isolation in electroretinogram recordings. Bicarbonate increased the dark current and absolute sensitivity and quickened flash response recovery in mouse rods to a greater extent than in amphibian rods. In addition, bicarbonate enabled mouse rods to respond over a range that extended to dimmer flashes. Larger flash responses may have resulted in part from a bicarbonate-induced elevation in intracellular pH. However, high pH alone had little effect on flash response recovery kinetics and even suppressed the accelerating effect of bicarbonate, consistent with a direct, modulatory action of bicarbonate on Ca²⁺- dependent, membrane guanylate cyclase activity.

Effects of cell size and bicarbonate on single photon response variability in retinal rods

Accurate photon counting requires that rods generate highly amplified, reproducible single photon responses (SPRs). The SPR is generated within the rod outer segment (ROS), a multilayered structure built from membranous disks that house rhodopsin. Photoisomerization of rhodopsin at the disk rim causes a local depletion of cGMP that closes ion channels in the plasmalemma located nearby with relative rapidity. In contrast, a photoisomerization at the disk center, distant from the plasmalemma, has a delayed impact on the ion channels due to the time required for cGMP redistribution. Radial differences should be greatest in large diameter rods. By affecting membrane guanylate cyclase activity, bicarbonate could impact spatial inhomogeneity in cGMP content. It was previously known that in the absence of bicarbonate, SPRs are larger and faster at the base of a toad ROS (where the ROS attaches to the rest of the cell) than at the distal tip. Given that bicarbonate enters the ROS at the base and diffuses to the tip and that it expedites flash response recovery, there should be an axial concentration gradient for bicarbonate that would accentuate the base-to-tip SPR differences. Seeking to understand how ROS geometry and bicarbonate affect SPR variability, we used mathematical modeling and made electrophysiological recordings of single rods. Modeling predicted and our experiments confirmed minor radial SPR variability in large diameter, salamander rods that was essentially unchanged by bicarbonate. SPRs elicited at the base and tip of salamander rods were similar in the absence of bicarbonate, but when treated with 30 mM bicarbonate, SPRs at the base became slightly faster than those at the tip, verifying the existence of an axial gradient for bicarbonate. The differences were small and unlikely to undermine visual signaling. However, in toad rods with longer ROSs, bicarbonate somehow suppressed the substantial, axial SPR variability that is naturally present in the absence of bicarbonate. Modeling suggested that the axial gradient of bicarbonate might dampen the primary phototransduction cascade at the base of the ROS. This novel effect of bicarbonate solves a mystery as to how toad vision is able to function effectively in extremely dim light.

The effect of external sodium concentration on sodium-calcium exchange in frog olfactory receptor cells

During the response of vertebrate olfactory receptor cells to stimulation, Ca(2+) enters the cilia via cyclic nucleotide-gated channels and is extruded by Na(+)-Ca(2+) exchange. The rise in Ca(2+) concentration opens a Ca(2+)-activated Cl(-) conductance which carries most of the inward receptor current. The dependence of Ca(2+) extrusion upon external Na(+) concentration was studied by using the falling phase of the Ca(2+)-activated Cl(-) current following a brief exposure to the phosphodiesterase inhibitor IBMX to monitor indirectly the decay in intraciliary Ca(2+) concentration. External Na(+) concentration was reduced by partial substitution with guanidinium, an ion which permeates the cyclic nucleotide-gated channel but does not support Na(+)-Ca(2+) exchange. The time constant describing the decay in current following IBMX stimulation was surprisingly little affected by substitution of external Na(+), being substantially retarded only when its concentration was reduced to a third or less of its normal value in Ringer solution. When the cilia were returned to Ringer solution after a period in reduced-Na(+) solution, the time constant for the final decay of current was similar to that seen when returning immediately to IBMX-free Ringer solution. This observation suggests that Ca(2+) extrusion via Na(+)-Ca(2+) exchange dominates the falling phase of the response to IBMX, which can therefore be used to assess exchanger activity. Rate constants derived from the time constants for current decay at different external Na(+) concentrations could be fitted by the Hill equation with a K(d) of 54 +/- 4 mm and Hill coefficient of 3.7 +/- 0.4. The cooperativity of the dependence upon external Na(+) concentration indicates that at least three Na(+) ions enter for each exchanger cycle, while the high affinity for external Na(+) contrasts with the photoreceptor and cardiac exchangers. The functional importance of this observation is that the relative insensitivity of the Na(+)-Ca(2+) exchanger to external Na(+) concentration allows normal response termination even following partial dilution or concentration of the olfactory mucus.

Simultaneous measurement of current and calcium in the ultraviolet-sensitive cones of zebrafish

In rods and visible cone photoreceptors, multiple measurements cannot be made of intracellular Ca2+ concentration from the same cell using fluorescent dyes, because a single exposure of the measuring light bleaches too large a fraction of the rod or cone photopigment. We have therefore identified and characterized UV-sensitive cones of the zebrafish, whose wavelength of maximum sensitivity is at 360 nm which is far enough from the wavelength of our measuring light (514.5 nm) so that it has been possible to make multiple determinations of photocurrent and Ca2+ concentration from the same cells. We show that for a limited number of measurements, for which the bleaching of the cone photopigment is too small to affect flash kinetics, the outer segment Ca2+ concentration closely follows the wave form of the flash response convolved with the dominant time constant for Ca2+ removal by Na+-Ca2+-K+ exchange. For a larger number of measurements, significant acceleration of the response kinetics by pigment bleaching inevitably occurs, but the Ca2+ concentration nevertheless rises and falls in approximate agreement with the flash wave form. During exposure to steady background light, the Ca2+ concentration falls in proportion to the steady-state current for dim backgrounds at all times and for bright backgrounds at steady state. At early times following the onset of bright backgrounds, however, the Ca2+ concentration is markedly higher than expected from the current of the cone. We show this to be the result of light-dependent Ca2+ release by bright background light, which can be abolished by pre-exposure of the cone to the membrane-permeant acetoxymethyl ester of the Ca2+ chelator BAPTA. Our results therefore demonstrate that the cone outer segment Ca2+ concentration is predominantly a function of the rate of influx and efflux of Ca2+ across the plasma membrane, but that a release of Ca2+ in bright light most probably from buffer sites within the cell can transiently elevate the Ca2+ concentration above the level expected from the open probability of the light-dependent channels.

Generation of time delays: simplified models of intracellular signalling in cerebellar Purkinje cells

In many neuronal systems, information is encoded in temporal spike patterns. The recognition and storage of temporal patterns requires the generation and modulation of time delays between inputs and outputs. In cerebellar Purkinje cells, stimulation of metabotropic glutamate receptors (mGluRs) results in a delayed calcium and voltage response that has been implicated in classical conditioning and temporal pattern recognition. Here, we analyse and simplify a complex model of the intracellular signalling network that has been proposed as a substrate for this delayed response. We systematically simplify the original model, present a minimal model of time delay generation, and show that a delayed response can be produced by the combination of negative feedback and autocatalysis, without any intervening signalling steps that would contribute additive delays. The minimal model is analysed using phase plane methods, and classified as an excitable system. We discuss the implication of excitability for computations performed by intracellular signalling networks in general.

A biophysical model of synaptic delay learning and temporal pattern recognition in a cerebellar Purkinje cell

It has been suggested that information in the brain is encoded in temporal spike patterns which are decoded by a combination of time delays and coincidence detection. Here, we show how a multi-compartmental model of a cerebellar Purkinje cell can learn to recognise temporal parallel fibre activity patterns by adapting latencies of calcium responses after activation of metabotropic glutamate receptors (mGluRs). In each compartment of our model, the mGluR signalling cascade is represented by a set of differential equations that reflect the underlying biochemistry. Phosphorylation of the mGluRs changes the concentration of receptors which are available for activation by glutamate and thereby adjusts the time delay between mGluR stimulation and voltage response. The adaptation of a synaptic delay as opposed to a weight represents a novel non-Hebbian learning mechanism that can also implement the adaptive timing of the classically conditioned eye-blink response.

Light-dependent changes in outer segment free-Ca2+ concentration in salamander cone photoreceptors

Simultaneous measurements of photocurrent and outer segment Ca2+ were made from isolated salamander cone photoreceptors. While recording the photocurrent from the inner segment, which was drawn into a suction pipette, a laser spot confocal technique was employed to evoke fluorescence from the outer segment of a cone loaded with the Ca2+ indicator fluo-3. When a dark-adapted cone was exposed to the intense illumination of the laser, the circulating current was completely suppressed and fluo-3 fluorescence rapidly declined. In the more numerous red-sensitive cones this light-induced decay in fluo-3 fluorescence was best fitted as the sum of two decaying exponentials with time constants of 43 +/- 2.4 and 640 +/- 55 ms (mean +/- SEM, n = 25) and unequal amplitudes: the faster component was 1.7-fold larger than the slower. In blue-sensitive cones, the decay in fluorescence was slower, with time constants of 140 +/- 30 and 1,400 +/- 300 ms, and nearly equal amplitudes. Calibration of fluo-3 fluorescence in situ from red-sensitive cones allowed the calculation of the free-Ca2+ concentration, yielding values of 410 +/- 37 nM in the dark-adapted outer segment and 5.5 +/- 2.4 nM after saturating illumination (mean +/- SEM, n = 8). Photopigment bleaching by the laser resulted in a considerable reduction in light sensitivity and a maintained decrease in outer segment Ca2+ concentration. When the photopigment was regenerated by applying exogenous 11-cis-retinal, both the light sensitivity and fluo-3 fluorescence recovered rapidly to near dark-adapted levels. Regeneration of the photopigment allowed repeated measurements of fluo-3 fluorescence to be made from a single red-sensitive cone during adaptation to steady light over a range of intensities. These measurements demonstrated that the outer segment Ca2+ concentration declines in a graded manner during adaptation to background light, varying linearly with the magnitude of the circulating current.

Regulation of intracellular pH in salamander retinal rods

1. We measured intracellular pH (pHi) in rods isolated from the retina of the axolotl salamander, Ambystoma mexicanum, using the fluorescent indicator 2',7'-bis(carboxyethyl)-5(and -6)-carboxyfluorescein (BCECF). 2. The light exposures associated with data acquisition had no marked effect on pHi. There was no sharp change between the value obtained from the first exposure of dark-adapted rods and subsequent readings. Increasing the acquisition frequency from 1 to 10 min-1 either had no effect, or brought about a slow acidification, which was stopped or reversed when the low frequency was restored. 3. In nominally HCO3(-)-free solution at pH 7.5, the rods had a steady-state pHi of 7.09 +/- 0.02 (n = 46) and a buffering power (beta i) of 24 +/- 1 mM (pH unit)-1 (n = 48). The buffering power was virtually constant in the pH range 6.6-8.0. In the same range, pHi dependent linearly on perfusion pH (pHo) with regression coefficients of 0.4-0.5. 4. There were no significant differences between the inner and outer segment of intact rods as regards steady-state pHi or responses to experimental treatments. 5. Recovery from an intracellular acid load imposed by sodium propionate or an NH4Cl prepulse in nominally bicarbonate-free perfusate was completely blocked by decreasing the extracellular Na+ concentration to 7 mM, and slowed by 86% by applying 1 mM amiloride. 6. Introduction of 2% CO2-13 mM HCO3- caused an alkalinization that was often preceded by a transient acidification. Steady-state pHi was on average 0.1 pH units higher than in nominally bicarbonate-free solution. The mean acid extrusion rate, calculated on the assumption that CO2-HCO3- behaves as an open system, was 19% higher (31 +/- 2 mM h-1) than in a solution buffered only by Hepes (26 +/- 2 mM h-1). 7. In the presence of CO2-HCO3-, 100 microM 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS) decreased the acid extrusion rate by 20% on average. Lowering the extracellular Cl-concentration to 7 mM raised pHi, but did not significantly affect the acid extrusion rate. 8. We conclude that retinal rods regulate pHi by both Na(+)-H+ exchange and mechanism(s) involving HCO3(-)-Cl- exchange. In the present conditions, the Na(+)-H+ exchanger appears as the dominant mechanism for acid extrusion.

Metabotropic glutamate receptor activation in cerebellar Purkinje cells as substrate for adaptive timing of the classically conditioned eye-blink response

To understand how the cerebellum adaptively times the classically conditioned nictitating membrane response (NMR), a model of the metabotropic glutamate receptor (mGluR) second messenger system in cerebellar Purkinje cells is constructed. In the model, slow responses, generated postsynaptically by mGluR-mediated phosphoinositide hydrolysis and calcium release from intracellular stores, bridge the interstimulus interval (ISI) between the onset of parallel fiber activity associated with the conditioned stimulus (CS) and climbing fiber activity associated with unconditioned stimulus (US) onset. Temporal correlation of metabotropic responses and climbing fiber signals produces persistent phosphorylation of both AMPA receptors and Ca(2+)-dependent K+ channels. This is responsible for long-term depression (LTD) of AMPA receptors. The phosphorylation of Ca(2+)-dependent K+ channels leads to a reduction in baseline membrane potential and a reduction of Purkinje cell population firing during the CS-US interval. The Purkinje cell firing decrease disinhibits cerebellar nuclear cells, which then produce an excitatory response corresponding to the learned movement. Purkinje cell learning times the response, whereas nuclear cell learning can calibrate it. The model reproduces key features of the conditioned rabbit NMR: Purkinje cell population response is timed properly; delay conditioning occurs for ISIs of up to 4 sec, whereas trace conditioning occurs only at shorter ISIs; mixed training at two different ISIs produces a double-peaked response; and ISIs of 200-400 msec produce maximal responding. Biochemical similarities between timed cerebellar learning and photoreceptor transduction, and circuit similarities between the timed cerebellar circuit and a timed dentate-CA3 hippocampal circuit, are noted.

Metabotropic glutamate receptor activation in cerebellar Purkinje cells as substrate for adaptive timing of the classically conditioned eye-blink response

To understand how the cerebellum adaptively times the classically conditioned nictitating membrane response (NMR), a model of the metabotropic glutamate receptor (mGluR) second messenger system in cerebellar Purkinje cells is constructed. In the model, slow responses, generated postsynaptically by mGluR-mediated phosphoinositide hydrolysis and calcium release from intracellular stores, bridge the interstimulus interval (ISI) between the onset of parallel fiber activity associated with the conditioned stimulus (CS) and climbing fiber activity associated with unconditioned stimulus (US) onset. Temporal correlation of metabotropic responses and climbing fiber signals produces persistent phosphorylation of both AMPA receptors and Ca(2+)-dependent K+ channels. This is responsible for long-term depression (LTD) of AMPA receptors. The phosphorylation of Ca(2+)-dependent K+ channels leads to a reduction in baseline membrane potential and a reduction of Purkinje cell population firing during the CS-US interval. The Purkinje cell firing decrease disinhibits cerebellar nuclear cells, which then produce an excitatory response corresponding to the learned movement. Purkinje cell learning times the response, whereas nuclear cell learning can calibrate it. The model reproduces key features of the conditioned rabbit NMR: Purkinje cell population response is timed properly; delay conditioning occurs for ISIs of up to 4 sec, whereas trace conditioning occurs only at shorter ISIs; mixed training at two different ISIs produces a double-peaked response; and ISIs of 200-400 msec produce maximal responding. Biochemical similarities between timed cerebellar learning and photoreceptor transduction, and circuit similarities between the timed cerebellar circuit and a timed dentate-CA3 hippocampal circuit, are noted.

Ca2+ fluxes and channel regulation in rods of the albino rat

By use of microelectrodes, changes in the receptor current and the Ca2+ concentration were measured in the rod layer of the rat retina after stimulation by flashes or steady light. Thereby light induced Ca2+ sources, and sinks along a rod were determined in dependence of time. Thus, the Ca2+ fluxes across the plasma membrane of a mammalian rod could be studied in detail. By light stimulation, Ca2+ sources are evoked along the outer segment only. Immediately after a saturating flash, a maximum of Ca2+ efflux is observed which decays exponentially with tau = 0.3 s at 37 degrees C (4.2 s at 23 degrees C). During regeneration of the dark current, the outer segment acts as a Ca2+ sink, indicating a restoration of the Ca(2+)-depleted outer segment. These findings agree with earlier reports on amphibian rods. Further experiments showed that the peak Ca2+ efflux and tau are temperature dependent. The peak amplitude also depends on the external Ca2+ concentration. In contrast to the reports on amphibian rods, only a part of the Ca2+ ions extruded from the outer segment is directly restored. Surprisingly, during steady light the Ca2+ efflux approaches a permanent residual value. Therefore, in course of a photoresponse, Ca2+ must be liberated irreversibly from internal Ca2+ stores. There is certain evidence that the inner segment acts as a Ca2+ store. Our results show that the Ca2+ fraction of the ions carrying the dark current is proportional to the extracellular Ca2+ concentration. This indicates that the Ca2+ permeability of the plasma membrane of the rod outer segment is independent of the Ca2+ concentration.

Effect of blocking the Na+/K+ ATPase on Ca2+ extrusion and light adaptation in mammalian retinal rods

Membrane current and light response were recorded from rods of monkey and guinea pig by means of suction electrodes. The correlation between adaptation and the Na+/K+ pump was investigated by measuring light-dependent changes in sensitivity with and without inhibition of Na+/K+ ATPase by strophanthidin. Strophanthidin was found to reduce the dark current, to slow the time course of the photoresponse, and to increase light sensitivity. At concentrations between 20 and 500 nM, the pump inhibitor suppressed in a reversible way the current re-activation occurring during prolonged illumination and modified the light-dependent decrease in sensitivity, which in control conditions approximates to a Weber-Fechner function. The effects of the pump inhibitor on the adaptive properties of rods are associated with an increased time constant of the membrane current attributed to the operation of the Na+:Ca2+,K+ exchanger. The effects of rapid application of the pump inhibitor on the current re-activation are consistent with the idea that significant changes in the internal sodium occur in rods of mammals during background illumination and that they play an important role in the process of light adaptation.

Transport of K+ by Na(+)-Ca2+, K+ exchanger in isolated rods of lizard retina

Transport of K+ by the photoreceptor Na(+)-Ca2+, K+ exchanger was investigated in isolated rod outer segments (OS) by recording membrane current under whole-cell voltage-clamp conditions. Known amounts of K+ were imported in the OS through the Ca(2+)-activated K+ channels while perfusing with high extracellular concentration of K+, [K+]o. These channels were detected in the recordings from the OS, which probably retained a small portion of the rest of the cell. The activation of forward exchange (Na+ imported per Ca2+ and K+ extruded) by intracellular K+, Ki+, was described by first-order kinetics with a Michaelis constant, Kapp(Ki+), of about 2 mM and a maximal current, Imax, of about -60 pA. [Na+]i larger than 100 mM had little effect on Kapp(Ki+) and Imax, indicating that Nai+ did not compete with Ki+ for exchange sites under physiological conditions, and that Na+ release at the exchanger intracellular side was not a rate-limiting step for the exchange process. Exchanger stoichiometry resulted in one K+ ion extruded per one positive charge imported. Exchange current was detected only if Ca2+ and K+ were present on the same membrane side, and Na+ was simultaneously present on the opposite side. Nonelectrogenic modes of ion exchange were tested taking advantage of the hindered diffusion found for Cai2+ and Ki+. Experiments were carried out so that the occurrence of a putative nonelectrogenic ion exchange, supposedly induced by the preapplication of certain extracellular ion(s), would have resulted in the transient presence of both Cai2+ and Ki+. The lack of electrogenic forward exchange in a subsequent switch to high Nao+, excluded the presence of previous nonelectrogenic transport.

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

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

Ca2+ regulation in the presynaptic terminals of goldfish retinal bipolar cells

1. To investigate regulation of the intracellular free Ca2+ concentration ([Ca2+]i) in presynaptic terminals, the Ca2+ current (ICa) and [Ca2+]i in axon terminals were simultaneously monitored in acutely dissociated retinal bipolar cells under whole-cell voltage clamp. 2. The recovery phase of the Ca2+ transient, which was evoked by activation of ICa, became slower when the Na(+)-Ca2+ exchanger was suppressed by removing extracellular Na+. 3. Inhibition of the plasma membrane Ca2+ pump produced by raising extracellular pH to 8.4 increased the basal [Ca2+]i and caused incomplete recovery from the Ca2+ transient. These effects were not observed in orthovanadate-loaded bipolar cells. 4. The Ca2+ transient was not significantly affected by ryanodine, caffeine, thapsigargin, Ruthenium Red or FCCP. Internal Ca2+ stores may not participate in shaping the Ca2+ transient. 5. The ratio of the peak amplitude of the Ca2+ transient to the total amount of Ca2+ influx became smaller as the size of the Ca2+ influx increased. This action was not affected by blockage of Ca2+ transporters in the plasma membrane, or by reduction of the rate of Ca2+ influx. The peak amplitude of the Ca2+ transient seemed to be determined by Ca2+ buffering substances with a positive co-operativity.

Calcium gradients and light-evoked calcium changes outside rods in the intact cat retina

We have studied light-evoked changes in extracellular Ca2+ concentration ([Ca2+]o) in the intact cat eye using ion-sensitive double-barreled microelectrodes. Two prominent changes in Ca2+ concentration were observed that differed in retinal location. There was a light-evoked increase in [Ca2+]o, accompanied by brief ON and OFF transients, which was maximal in the inner plexiform layer and was not further studied. There was an unexpected sustained light-evoked decrease in [Ca2+]o, of relatively rapid onset and offset, which was maximal in the distalmost region of the subretinal space (SRS). [Ca2+]o in the SRS was 1.0 mM higher than in the vitreous humor during dark adaptation and this transretinal gradient disappeared during rod-saturating illumination. After correcting for the light-evoked increase in the volume of the SRS, an increase in the total Ca2+ content of the SRS during illumination was revealed, which presumably represents the Ca2+ released by rods. To explain the light-evoked [Ca2+]o changes, we used the diffusion model described in the accompanying paper (Li et al., 1994b), with the addition of light-dependent sources of Ca2+ at the retina/retinal pigment epithelium (RPE) border and rod outer segments. We conclude that a drop in [Ca2+]o around photoreceptors, which persists during illumination and reduces a transretinal Ca2+ gradient, is the combined effect of the light-evoked SRS volume increase, Ca2+ release from photoreceptors, and an unidentified mechanism(s), which is presumably Ca2+ transport by the RPE. The relatively rapid onset and offset of the [Ca2+]o decrease remains unexplained. These steady-state shifts in [Ca2+]o should have significant effects on photoreceptor function, especially adaptation.

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

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

Modulation of transmission gain by protons at the photoreceptor output synapse

Synaptic transmission of the light response from photoreceptors to second-order cells of the retina was studied with the whole-cell patch-clamp technique in tiger salamander (Ambystoma tigrinum) retinal slices. Synaptic strength is modulated by extracellular pH in a striking manner: Light-sensitive postsynaptic currents in horizontal and bipolar cells were found to be exponential functions of pH, exhibiting an e-fold increase per 0.23 pH unit over the pH range from 7 to 8. Calcium channel currents in isolated photoreceptors were measured and also exhibited proton sensitivity. External alkalinization from pH 7 to 8 shifted the voltage dependence of channel activation negative by 12 mV. A model of the synaptic transfer function suggested that presynaptic Ca channels could be the primary sites of proton action. Increased Ca influx and transmitter release brought about by alkalinization give rise to larger postsynaptic currents. These results suggest that activity-dependent interstitial pH changes known to occur in the retina, while not alleviating signal clipping at this synapse, may provide an adaptative mechanism controlling gain at the photoreceptor output synapse.

The mechanism of ion transport by the Na(+)-Ca2+,K+ exchange in rods isolated from the salamander retina

1. Membrane currents caused by the operation of electrogenic Na(+)-Ca2+,K+ exchange were recorded from isolated rod outer segments under voltage-clamp using a whole-cell electrode. 2. Reversed mode exchange currents (Na+i-Ca2+o,K+o) were recorded with a high internal [Na+] and when both Ca2+ and K+ were present in the external solution. Omission of either Ca2+ or K+ completely suppressed both the reversed exchange current and the entry of Ca2+. 3. The charge transferred by the exchange per Ca2+ ion transported was identical in both forward and reversed modes. 4. The reversed exchange current declined as Ca2+ accumulated inside the outer segment, and the form of this decline was consistent with a first-order inhibition by internal Ca2+. 5. The reversed exchange current was increased e-fold by a 230 mV depolarization over the range -51 to +29 mV. 6. The activation of reversed exchange by external Ca2+ was well described by first-order kinetics with a Michaelis constant, KappCao, of 34 microM in the presence of 20 mM external K+. KappCao was reduced by lowering external [K+], was increased by adding external Na+ and was unaffected by membrane potential. 7. External K+ also activated the exchange in a first-order manner with a Michaelis constant, KappKo, of 151 microM in the presence of 0.5 mM external Ca2+. KappKo was reduced by lowering external [Ca2+], increased by adding external Na+ and was unaffected by membrane potential. 8. When the level of internal Ca2+ was increased via reversed exchange, KappCao diminished in proportion to the reduction in the maximum current, but KappKo remained approximately constant. 9. These observations cannot be reconciled with simple models of the exchange in which ions bind simultaneously at opposite faces of the membrane before transport occurs. The results are broadly consistent with a consecutive model of the exchange in which unbinding of Na+ at either the external or the internal membrane surface is followed by binding of Ca2+ and then K+, and are fully reproduced by a model in which Ca2+ binds before all of the Na+ has dissociated from the exchange molecule.

Visual transduction in dialysed detached rod outer segments from lizard retina

1. Properties of a new preparation for studying the physiology and biochemistry of phototransduction in retinal rods are described. Whole-cell voltage clamp was used to record the generation, maintenance and light-sensitivity of dark current in rod outer segments that had been isolated from the rest of the receptor cell by detachment at the connecting cilium. 2. Detached outer segments dialysed with standard internal solution supplemented with physiological amounts of ATP (5 mM) and GTP (1 mM) developed a standing inward dark current that was the sum of three components: approximately 91% light-sensitive current, approximately 6% Na(+)-Ca2+,K+ exchange current and approximately 3% leakage current. Light-sensitive dark current (mean amplitude approximately -63 pA) was suppressed transiently by brief flashes in an intensity-dependent manner. Light responses had the same kinetics, sensitivity and intensity-response relationship as those recorded from intact rods. 3. Dialysed outer segments differed from intact rods in that intense flashes evoked saturating responses that recovered incompletely to a plateau of reduced dark current caused by incomplete inactivation of the transduction cascade. Light sensitivity was reduced for a short time following an intense flash and then recovered despite persistent reduction of dark current. This suggests that there is no fixed relationship between dark current amplitude and light sensitivity. 4. Light-sensitive dark current faded rapidly when outer segments were not supplied with nucleotides. Outer segments dialysed with solution that contained cyclic GMP, but no ATP or GTP, supported dark current at a level that increased with [cyclic GMP]. When basal phosphodiesterase (PDE) activity is inhibited, 8 microM cyclic GMP supports a dark current of approximately 70 pA. 5. Light sensitivity decreased during recordings made with solution that contained only cyclic GMP, consistent with the inhibition of G protein activation by loss of GTP. After thorough nucleoside triphosphate depletion, however, intense illumination evoked a transient increase rather than a decrease in dark current, i.e. an inverted light response. This result suggests that isomerized rhodopsin may generate a signal that causes either inhibition of basal PDE activity or release of bound cyclic GMP. 6. Sustained Na(+)-Ca2+,K+ exchange current was recorded during steady illumination when Ca2+, but not when Mg2+, was added to the dialysis solution. Exchange current increased with the amount of added Ca2+ and saturated at approximately 18 pA when the dialysis solution contained > or = 10 mM Ca2+.(ABSTRACT TRUNCATED AT 400 WORDS)

Sodium-calcium exchange: derivation of a state diagram and rate constants from experimental data

A mechanism is developed for Na(+)-Ca2+ exchange using a new approach made possible by the availability of computer software that allows the systematic search of a large parameter space for optimum sets of parameters to fit multiple sets of experimental data. The approach was to make the experimental data dictate the form of the mechanism: the qualitative features of the data dictating the number and nature of the states of the exchanger and their interrelationship, and the quantitative aspects of the data dictating the values of the rate constants that govern the amount of each state relative to the total amount of exchanger. A single set of experimental data served this initial purpose, namely, observations of equilibrium Ca(2+)-Ca2+ exchange in cardiac sarcolemmal vesicles (Slaughter et al., 1983, J. biol. Chem. 258, 3183-3190). From this data a minimum mechanism was induced having 56 states (SYM56), which gave satisfactory quantitative fits to the experimental data. With this set of parameters additional experimental data were fitted, from the same preparation, the single cardiac cell and the squid giant axon, with some changes in parameters, but none dramatic. In spite of the symmetric nature of the mechanism, i.e. binding constants for Na+ and Ca2+ do not depend on the orientation of the binding sites, the mechanism exhibits marked asymmetric behavior similar to that observed experimentally. Finally, in accounting for Ca(2+)-Ca2+ exchange in the absence of monovalent cations, Ca2+ influx becomes dependent on intracellular Ca(2+)--an unexpected outcome--exactly in keeping with the "essential activator" role of intracellular Ca2+ observed by DiPolo & Beaugé (1987, J. gen. Physiol. 90, 505-525). Observations of Na(+)-Ca2+ exchange in the retinal rod outer segment are well fitted with a simplified version of SYM56 comprising 25 states (namely, SYM25), supporting the notion that the exchanger in the retinal rod outer segment differs from that in cardiac sarcolemma and squid axon. Maximum turnover rate of 840 sec-1 for SYM56 and 20 sec-1 for SYM25 are comparable to those reported for the exchanger in cardiac muscle and retinal rod outer segment, respectively.

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

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

Kinetic models of Na-Ca exchange in ferret red blood cells. Interaction of intracellular Na, extracellular Ca, Cd, and Mn

The kinetic equation that best describes the intracellular Na dependence of Ca influx into ferret red cells is sequential; whether this implies that there is a conformation of the protein that has both Na and Ca ions bound remains to be determined. Cd and Mn substitute very well for Ca on the exchanger in ferret red cells; this suggests that the Ca-binding site does not contain an important thiol and that the one of the Na steps may be rate limiting.

Translocation mechanism of cardiac Na-Ca exchange

In single cardiac ventricular cells of guinea pig, we have studied the ionic translocation mechanism of the electrogenic Na-Ca exchange, that is, whether Na and Ca ions countercross the membrane simultaneously or consecutively. The dose-response relations between the external Ca ([Ca]o) and the outward Na-Ca exchange current were measured at three different internal Na concentrations ([Na]i) in the absence of external Na. Hyperbolic regression curves and Hanes-Woolf linear plots of the dose-response relation revealed that apparent Km values for external Ca (K'mCao) decrease progressively as [Na]i decreases. The ratio of K'mCao to apparent Imax value (I'max) showed a slight increasing tendency as [Na]i decreased. We previously interpreted the data as consistent with the simultaneous mechanism but without statistical analysis. Here we performed careful statistical analysis, which indicated that the K'max/I'max values were not significantly different among the different [Na]i at most of the potentials. This result suggests that Na-Ca exchange is likely to be a consecutive mechanism.

Na-Ca exchange: evidence against a ping-pong mechanism and against a Ca pool in ferret red blood cells

To determine the mechanism of Na-Ca exchange, we estimated the ratio of maximum velocity to Michaelis constant for extra-cellular Ca by measuring the rate of Ca uptake at very low extracellular Ca. In a Ping-Pong mechanism, one set of sites alternatively transports Ca and Na. In a sequential mechanism, Ca and Na sites are both filled during part of the transport cycle. In each set of experiments, two intracellular Na concentrations were studied. The Ca uptake rate (at low Ca) increased as Na increased; this is consistent with a sequential model, as has been found in other cells. We also examined the alternative hypothesis that the exchanger followed Ping-Pong kinetics and that the red blood cells had a submembrane pool for Ca that limited mixing with the cytosol. In these experiments Ca pump activity was monitored by measuring ATP hydrolysis. This model was disproven by experiments that indicated that greater than 80% of the Ca that entered the cell became bound to EGTA and less than 20% resulted in Ca efflux by the Ca pump.

Molecular operations of the sodium-calcium exchanger revealed by conformation currents

The sodium-calcium exchanger is critical in the normal functioning of many cells. In heart muscle, it is the principal way by which the cells keep the concentration of intracellular calcium low, pumping out the Ca2+ that enters the cytosol through L-type Ca2+ channels. The exchanger may also contribute to the triggering of Ca2+ release during voltage-activated excitation-contraction coupling in heart. Time resolved examination of the conformational changes of macromolecules in living cells has so far been largely restricted to ion-channel proteins whose gating is voltage-dependent. We have now directly measured electrical currents arising from the molecular rearrangements of the sarcolemmal Na-Ca exchanger. Changes in the conformation of the exchanger protein were activated by a rapid increase in the intracellular calcium concentration produced by flash photolysis of caged calcium in voltage-clamped heart cells. Two components of membrane current were produced, reflecting a calcium-dependent conformational change of the transporter proteins and net transport of ions by the exchanger. The properties of these components provide evidence that the Na-Ca exchanger protein undergoes two consecutive membrane-crossing molecular transitions that each move charge, and that there are at least 250 exchangers per micron 2 turning over up to 2,500 times per second.

The effects of quinidine on sodium-dependent calcium efflux in isolated rod photoreceptors of the salamander retina

The effect of quinidine on the membrane current generated by the Na:Ca, K exchange has been investigated in the outer segment of isolated rod photoreceptors from the retina of the larval tiger salamander. The inward exchange current associated with the efflux of Ca2+ was selectively recorded by introducing a Ca2+ load through the light-sensitive channels, and then shutting these channels with a bright light. Quinidine (20-1000 microM) reduced the magnitude of the exchange current and slowed its decay during the removal of a Ca2+ load. Quinidine did not alter the form of the relation between the exchange current and the total concentration of exchangeable calcium remaining within the outer segment. [Ca]T, showing that it does not change the affinity of the exchange mechanism for internal Ca2+. The relation between exchange current inhibition and the quinidine concentration could be described by a simple Michaelis relation with a Ki of 287 microM and a maximum inhibition of 50%. The incomplete block of the Na:Ca, K exchange current by quinidine shows that it does not act by simple competition with external Na+, and suggests that the inhibition of the exchange by quinidine may be non-specific.

Evidence for Na+/H+ exchange in vertebrate rod photoreceptors

In this paper, we have documented our investigation of pH regulation in the rod photoreceptor of the toad, Bufo marinus. Unlike other neural tissues, the retina depends upon aerobic glycolysis to meet its energy requirements. A consequence of its reliance on glycolysis is a large metabolic production of protons (H+) which must be extruded by pH regulating mechanisms. Based on the work of previous authors, we propose that rods share these same characteristics with whole retina. Our results in rods are consistent with the hypothesis that under nominally bicarbonate-free conditions, an amiloride-sensitive Na+/H+ exchanger contributes to pH regulation in this cell.

Measurement of reversal potential of Na+-Ca2+ exchange current in single guinea-pig ventricular cells

1. To identify the Na+- or Ca2+-induced current as Na+-Ca2+ exchange current and to determine the stoichiometry of the Na+-Ca2+ exchange, the reversal potential was measured in a wide range of external Na+ [( Na+]o) or Ca2+ [( Ca2+]o) concentrations. The Na+- or Ca2+-induced current was recorded in single ventricular cells enzymatically dispersed from guinea-pig hearts, using the technique of whole-cell voltage clamp combined with internal perfusion. 2. In the presence of 10-40 mM-Na+ and 55-803 nM-Ca2+ in the internal solution, an increase of [Ca2+]o from 0.1 to 0.5-20 mM or an increase of [Na+]o from 30 to 50-140 mM induced an extra current associated with an increase in membrane conductance. The reversal potential of these extra currents was determined from an intersection of the current-voltage (I-V) relations obtained in the absence and presence of a Na+-Ca2+ exchange blocker, Ni2+ (2 mM). 3. Ba2+ in the external solution failed to induce the extra current, but inhibited the background conductance having a reversal potential at around 0 mV. Thus, 1 mM-Ba2+ was added to all external solutions, so that a change in the background current was minimized during application of Ca2+ or Ni2+. 4. The relation between [Ca2+]o and amplitude of the Ca2+-induced current was examined in the presence and absence of Ni2+. Lineweaver-Burk analysis revealed that the action of Ni2+ on the extra current might be a mixed type of competitive and non-competitive inhibition. 5. During the application of Ca2+, the Ca2+-induced outward current decayed in a time-dependent manner, resulting in a shift of the I-V relations towards positive potentials. This current decay was inhibited by increasing the capacity of the internal Ca2+-buffer, using BAPTA (1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid) or higher concentrations of EGTA. The result indicates that [Ca2+]i, at least under the cell membrane, changes due to ion fluxes through the Na+-Ca2+ exchange and that control of the ion concentrations within the cell is prerequisite for measuring the reversal potential of the Na+-Ca2+ exchange. 6. The shift of both the holding current and the I-V relations during stimulation of the exchange was suppressed, when the membrane potential was clamped at the equilibrium potential of 3Na+:1Ca2+ exchange.(ABSTRACT TRUNCATED AT 400 WORDS)

Extrusion of calcium from rod outer segments is driven by both sodium and potassium gradients

Calcium is transported across the surface membrane of both nerve and muscle by a Na+-dependent mechanism, usually termed the Na:Ca exchange. It is well established from experiments on rod outer segments that one net positive charge enters the cell for every Ca2+ ion extruded by the exchange, which is generally interpreted to imply an exchange stoichiometry of 3 Na+:1 Ca2+. We have measured the currents associated with the operation of the exchange in both forward and reversed modes in isolated rod outer segments and we find that the reversed mode, in which Ca2+ enters the cell in exchange for Na+, depends strongly on the presence of external K+. The ability of changes in external K+ concentration ([K+]o) to perturb the equilibrium level of [Ca2+]i indicates that K+ is co-transported with calcium. From an examination of the relative changes of [Ca2+]o, [Na+]o, [K+]o and membrane potential required to maintain the exchange at equilibrium, we conclude that the exchange stoichiometry is 4 Na+:1 Ca2+, 1 K+ and we propose that the exchange should be renamed the Na:Ca, K exchange. Harnessing the outward K+ gradient should allow the exchange to maintain a Ca2+ efflux down to levels of internal [Ca2+] that are considerably lower than would be possible with a 3 Na+:1 Ca2+ exchange.

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

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

Control of light-sensitive current in salamander rods

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

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

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

On the mechanism of a pH-induced rise in membrane potassium conductance in hamster eggs

1. The effect of external pH (pHo) on the membrane potential and resistance of unfertilized zona-free hamster eggs was investigated by intracellular recording techniques. 2. A hyperpolarization of the hamster egg membrane was induced by raising the extracellular pH above 8.0. This hyperpolarization was accompanied by a rise in membrane conductance and was reversible by washing the egg. 3. The estimated value of the reversal potential of the hyperpolarizing response to a solution with pHo 9.5 was about -85 mV. The membrane potential changed linearly with log [K+]o with a slope of 43 +/- 2 mV (mean +/- S.D.; n = 4) for a 10-fold change in [K+]o, while it was unaltered by the removal of Cl- from the solution. 4. The amplitude of the pHo-induced hyperpolarization decreased substantially as [Ca2+]o was lowered from 20 to 1 mM. Sr2+ could substitute for Ca2+ in sustaining the response to high pHo, whereas Ba2+ or Mg2+ could not. 5. Injection of the Ca2+ chelator EGTA into the egg prevented the pHo-induced hyperpolarization suggesting that a rise in [Ca2+]i is required. 6. The rate of rise of Ca2+ action potentials was reversibly enhanced by raising pHo. However, influx through the voltage-gated Ca2+ channels is not involved in initiation and maintenance of the pHo-induced response, as responses were not affected by the Ca2+ channel blocker La3+. 7. The duration of the hyperpolarization evoked by intracellular Ca2+ injection in eggs bathed in normal solution or Na+-free solution was greatly prolonged by raising pHo. 8. It is suggested that a rise in external pH produces an increase in [Ca2+]i, activating a Ca2+-mediated K+ conductance which hyperpolarizes the egg membrane. 9. It is concluded that both a Na+-Ca2+ exchange system and a Ca2+ pump are responsible for Ca2+ extrusion and that inhibition of the Ca2+ pump by high pHo is the chief mechanism underlying the pH-induced hyperpolarization in hamster eggs. Although the Na+-Ca2+ exchange system is facilitated at high pHo, the effect of this facilitation of efflux is outweighed by the inhibition of the Ca2+ pump.

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+.

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.

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

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