The initiation of excitation and light adaptation in Limulus ventral photoreceptors

Role of calsequestrin evaluated from changes in free and total calcium concentrations in the sarcoplasmic reticulum of frog cut skeletal muscle fibres

Calsequestrin is a large-capacity Ca-binding protein located in the terminal cisternae of sarcoplasmic reticulum (SR) suggesting a role as a buffer of the concentration of free Ca in the SR ([Ca2+](SR)) serving to maintain the driving force for SR Ca2+ release. Essentially all of the functional studies on calsequestrin to date have been carried out on purified calsequestrin or on disrupted muscle preparations such as terminal cisternae vesicles. To obtain information about calsequestrin's properties during physiological SR Ca2+ release, experiments were carried out on frog cut skeletal muscle fibres using two optical methods. One - the EGTA-phenol red method - monitored the content of total Ca in the SR ([Ca(T)](SR)) and the other used the low affinity Ca indicator tetramethylmurexide (TMX) to monitor the concentration of free Ca in the SR. Both methods relied on a large concentration of the Ca buffer EGTA (20 mM), in the latter case to greatly reduce the increase in myoplasmic [Ca2+] caused by SR Ca2+ release thereby almost eliminating the myoplasmic component of the TMX signal. By releasing almost all of the SR Ca, these optical signals provided information about [Ca(T)](SR) versus [Ca2+](SR) as [Ca2+](SR) varied from its resting level ([Ca2+](SR,R)) to near zero. Since almost all of the Ca in the SR is bound to calsequestrin, this information closely resembles the binding curve of the Ca-calsequestrin reaction. Calcium binding to calsequestrin was found to be cooperative (estimated Hill coefficient = 2.95) and to have a very high capacity (at the start of Ca2+ release, 23 times more Ca was estimated to initiate from calsequestrin as opposed to the pool of free Ca in the SR). The latter result contrasts with an earlier report that only approximately 25% of released Ca2+ comes from calsequestrin and approximately 75% comes from the free pool. The value of [Ca2+](SR,R) was close to the K(D) for calsequestrin, which has a value near 1 mm in in vitro studies. Other evidence indicates that [Ca2+](SR,R) is near 1 mM in cut fibres. These results along with the known rapid kinetics of the Ca-calsequestrin binding reaction indicate that calsequestrin's properties are optimized to buffer [Ca2+](SR) during rapid, physiological SR Ca2+ release. Although the results do not entirely rule out a more active role in the excitation-contraction coupling process, they do indicate that passive buffering of [Ca2+](SR) is a very important function of calsequestrin.

Protein kinase C-induced disorganization and endocytosis of photosensitive membrane in Limulus ventral photoreceptors

Protein kinase C (PKC) desensitizes the light response in photoreceptors from the ventral optic nerve of the horseshoe crab Limulus. Photoisomerization of Limulus rhodopsin leads to phosphoinositide hydrolysis, resulting in the production of inositol trisphosphate and diacylglycerol (DAG). Inositol trisphosphate mobilizes intracellular stores of Ca(2+), resulting in photoreceptor excitation in Limulus, while DAG may activate PKC. We investigated whether PKC-mediated desensitization of the photoresponse is accompanied by ultrastructural changes in the rhodopsin-bearing photosensitive membrane (rhabdom) in Limulus ventral photoreceptors. PKC activation by (-)-indolactam V in darkness induces disorganization and swelling of the rhodopsin-containing microvilli and endocytosis of rhabdomeral membrane. The effects of (-)-indolactam V on dark-adapted photoreceptor ultrastructure are reversible, are stereospecific, are blocked by coapplication of PKC inhibitors, and closely match those induced by continuous, bright light. Rhabdom disorganization and endocytosis via PKC activation may, therefore, contribute to desensitization of the light-adapted photoreceptor.

Effect of sarcoplasmic reticulum (SR) calcium content on SR calcium release elicited by small voltage-clamp depolarizations in frog cut skeletal muscle fibers equilibrated with 20 mM EGTA

Cut muscle fibers from Rana temporaria (sarcomere length, 3.5-3.9 micro(m); 14-16 degreesC) were mounted in a double Vaseline-gap chamber and equilibrated with an external solution that contained tetraethyl ammonium- gluconate and an internal solution that contained Cs as the principal cation, 20 mM EGTA, and 0 Ca. Fibers were stimulated with a voltage-clamp pulse protocol that consisted of pulses to -70, -65, -60, -45, and -20 mV, each separated by 400-ms periods at -90 mV. The change in total Ca that entered into the myoplasm (Delta[CaT]) and the Ca content of the SR ([CaSR]) were estimated with the EGTA/phenol red method (Pape, P.C., D.-S. Jong, and W.K. Chandler. 1995. J. Gen. Physiol. 106:259-336). Fibers were stimulated with the pulse protocol, usually every 5 min, so that the resting value of [CaSR] decreased from its initial value of 1,700-2, 300 microM to values near or below 100 microM after 18-30 stimulations. Three main findings for the voltage pulses to -70, -65, and -60 mV are: (a) the depletion-corrected rate of Ca release (release permeability) showed little change when [CaSR] decreased from its highest level (>1,700 microM) to approximately 1,000 microM; (b) as [CaSR] decreased below 1,000 microM, the release permeability increased to a maximum level when [CaSR] was near 300 microM that was on average about sevenfold larger than the values observed for [CaSR] > 1,000 microM; and (c) as [CaSR] decreased from approximately 300 microM to <100 microM, the release permeability decreased, reaching half its maximum value when [CaSR] was approximately 110 microM on average. It was concluded that finding b was likely due to a decrease in Ca inactivation, while finding c was likely due to a decrease in Ca-induced Ca release.

On the molecular origins of thermal noise in vertebrate and invertebrate photoreceptors

Retinal photoreceptors generate discrete electrical events in the dark indistinguishable from those evoked by light and the resulting dark signals limit visual sensitivity at low levels of illumination. The random spontaneous events are strongly temperature dependent and in both vertebrate and invertebrate photoreceptors require activation energies usually in the range of 23 to 28 kcal mol-1. Recent molecular orbital studies and pH experiments on horseshoe crabs (Limulus) suggest that the thermal isomerization of a relatively unstable form of rhodopsin, one in which the Schiff-base linkage between the chromophore and protein is unprotonated, is responsible for thermal noise. This mechanism is examined in detail and compared to other literature models for photoreceptor noise. We conclude that this two-step process is likely to be the principal source of noise in all vertebrate and invertebrate photoreceptors. This model predicts that the rate of photoreceptor noise will scale in proportion to 10- xi, where xi is the pKa of the Schiff base proton on the retinyl chromophore. Nature minimizes photoreceptor noise by selecting a binding site geometry which shifts the pKa of the Schiff base proton to > 16, a value significantly larger than the pKa of the chromophore in bacteriorhodopsin (pKa approximately 13) or model protonated Schiff bases in solution (pKa approximately 7).

On the molecular origin of photoreceptor noise

Retinal photoreceptors are noisy. They generate discrete electrical events in the dark indistinguishable from those evoked by light and thereby limit visual sensitivity at low levels of illumination. The random spontaneous events are strongly temperature-dependent and have been attributed to thermal isomerizations of the vitamin A chromophore of rhodopsin, the light-sensitive molecule in photoreceptors. But thermal generation of dark events in both vertebrate and invertebrate photoreceptors requires activation energies in the range of 23 to 27 kcal mol-1, which are significantly less than the energy barrier of 45 kcal mol-1, for photoisomerization of the chromophore of native rhodopsin. We propose that photoreceptor noise results from the thermal isomerization of a relatively unstable form of rhodopsin, one in which the Schiff-base linkage between the chromophore and protein is unprotonated. This molecular mechanism is supported by both theoretical calculations of the properties of rhodopsin and experimental measurements of the properties of photoreceptor noise.

Biophysical processes in invertebrate photoreceptors: recent progress and a critical overview based on Limulus photoreceptors

Limulus ventral nerve photoreceptor, a classical preparation for the study the phototransduction in invertebrate eyes, seems to have a very complex mechanism to transform light energy into a physiological signal. Although the main function of the photoreceptor is to change the membrane conductance according to the illumination, the cell has voltage-activated conductances as well. The voltage-gated conductances are matched to the light-activated ones in the sense that they make the function of the cell more efficient. The complex mechanism of phototransduction and the presence of four different voltage-gated conductance in Limulus ventral nerve photoreceptors indicate that these cells are far less differentiated than the photoreceptor cells of vertebrates. Indications accumulated in recent years support the view that the ventral photoreceptor of Limulus has different light-activated macroscopic current components, ion channels and terminal transmitters. After conclusions from macroscopic current measurements (Payne, 1986; Payne et al. 1986 a, b), direct evidence was presented by single-channel (Nagy & Stieve, 1990 a, b; Nagy, 1990 a, b) and macroscopic current measurements (Deckert et al. 1991 a, b) for three different light-activated conductances. It has been shown that two of these conductances are stimulated by two different excitation mechanisms. The two mechanisms, having different kinetics, release probably two different transmitters. One of them might be the cGMP (Johnson et al. 1986), the other one the calcium ion (Payne et al. 1986 a, b). However, the biochemical processes which link the rhodopsin molecules and the ion channels are not known. The unknown chemical details of the phototransduction result in a delay for the mathematical description of the biophysical mechanisms. More biochemical details are known about the adaptation mechanism. It was found that inositol 1,4,5-trisphosphate is a messenger for the release of calcium ions from the intracellular stores and that calcium ions are the messengers for adaptation (Payne et al. 1986 b; Payne & Fein, 1987). Concerning the mechanism of calcium release, it was revealed that a negative feedback acts on the enzyme cascade to regulate the internal calcium level and to protect the stores against complete emptying (Payne et al. 1988, 1990). Calcium ions also play an important role in the excitation mechanism. (a) In [Ca2+]i-depleted cells the light-induced current was increased after intracellular Ca2+ injection, suggesting that calcium is necessary for the transduction mechanism (Bolsover & Brown, 1985).(ABSTRACT TRUNCATED AT 400 WORDS)

Calcium/calmodulin-stimulated phosphorylation of photoreceptor proteins in Limulus

Calcium (Ca2+) is thought to play a major role in the photoresponse of both vertebrates and invertebrates, but the mechanisms through which Ca2+ exerts its effects are unclear. In many systems, some effects of Ca2+ on cellular processes are thought to be mediated via activation of calcium/calmodulin protein kinase (CaCAM-PK) and the phosphorylation of specific proteins. Thus, protein substrates for CaCAM-PK in photoreceptor cells may be important in mediating the effects of Ca2+ on the photoresponse. In this study, we identify eight substrates for CaCAM-PK found in both the ventral and lateral eyes of Limulus. We focus on a characterization of one of these, a 46-kD substrate. We show that its subcellular distribution in ventral photoreceptors and its isoelectric forms are identical to the 46-kD light-stimulated phosphoprotein (46A) described by Edwards et al. (1989). Furthermore, we present evidence that 46A is unique to photoreceptor cells, and that it is present throughout the cell. Based on the results of this study, and the previous study by Edwards et al. (1989), we propose that 46A is involved in mediating the effects of Ca2+ on Limulus photoreceptor cell function, and that it may be involved in dark adaptation.

Simultaneous monitoring of changes in magnesium and calcium concentrations in frog cut twitch fibers containing antipyrylazo III

Antipyrylazo III was introduced into frog cut twitch fibers (17-19 degrees C) by diffusion. After action potential stimulation, the change in indicator absorbance could be resolved into two components that had different time courses and wavelength dependences. The first component was early and transient and due to an increase in myoplasmic free [Ca] (Maylie, J., M. Irving, N.L. Sizto, and W.K. Chandler, 1987, Journal of General Physiology, 89:83-143). The second component, usually measured at 590 nm (near the isosbestic wavelength for Ca), developed later than the Ca transient and returned towards baseline about 100 times more slowly. Although the wavelength dependence of this component is consistent with an increase in either free [Mg] or pH, its time course is clearly different from that of the signals obtained with the pH indicators phenol red and 4',5'-dimethyl-5-(and -6-) carboxyfluorescein, suggesting that it is mainly due to an increase in free [Mg]. After a single action potential in freshly prepared cut fibers that contained 0.3 mM antipyrylazo III, the mean peak amplitude of delta A (590) would correspond to an increase in free [Mg] of 47 microM if all the signal were due to a change in [Mg] and all the intracellular indicator reacted with Mg as in cuvette calibrations. With either repetitive action potential stimulation or voltage-clamp depolarization, the delta A (590) signal continued to develop throughout the period when free [Ca] was elevated and then recovered to within 40-90% of the prestimulus baseline with an average rate constant between 0.5 and 1.0 s-1. With prolonged voltage-clamp depolarization, both the amplitude and rate of development of the delta A(590) signal increased with the amplitude of the depolarization and appeared to saturate at levels corresponding to an increase in free [Mg] of 0.8-1.4 mM and a maximum rate constant of 3-4 s-1, respectively. These results are consistent with the idea that the delta A(590) signal is primarily due to changes in myoplasmic free [Mg] produced by a change in the Mg occupancy of the Ca,Mg sites on parvalbumin that results from the Ca transient.

Light-controlled adaptation kinetics in Phycomyces: evidence for a novel yellow-light absorbing pigment

When sporangiophores of the fungus Phycomyces blakesleeanus adapt from high to low fluence rate, dark adaptation (sensitivity recovery) can be accelerated by dim subliminal light [Galland et al. (1989) Photochem. Photobiol. 49, 485-491]. We measured fluence rate-response curves for this acceleration under the following conditions. After sporangiophores were initially adapted symmetrically to a fluence rate of 1 W m-2 (447 nm), they were exposed to unilateral subliminal light (subthreshold for phototropism) of variable wavelength and fluence rate, and then to unilateral test light (447 nm) of fluence rate either 10(-3) or 10(-5) W m-2. The duration of the subliminal light was chosen so that phototropism would not occur during this period. Phototropic latencies could be shortened by subliminal light that was less intense than the test light by several orders of magnitude. In experiments with the final unilateral light of fluence rate 10(-3) W m-2, the 447 nm subliminal light had a threshold (for the acceleration effect) of about 10(-11) W m-2. Yellow light of wavelength 575 nm, which itself is extremely ineffective for phototropism was extremely effective in shortening phototropic latencies in response in response to the test light. At 575 nm, the threshold was about 2 x 10(-12) W m-2. Conversely, near-UV light of wavelength 347 nm, which is highly effective for phototropism, was relatively ineffective (threshold approximately 7 x 10(-8) W m-2) in shortening the phototropic latency. Our results suggest the presence of a novel yellow-light absorbing pigment in Phycomyces that specifically regulates dark adaptation.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence for electrogenic Na+/Ca2+ exchange in Limulus ventral photoreceptors

The Ca2+ indicator photoprotein, aequorin, was used to estimate and monitor intracellular Ca2+ levels in Limulus ventral photoreceptors during procedures designed to affect Na+/Ca2+ exchange. Dark levels of [Ca2+]i were estimated at 0.66 +/- 0.09 microM. Removal of extracellular Na+ caused [Ca2+]i to rise transiently from an estimated 0.5-0.6 microM in a typical cell to approximately 21 microM; [Ca2+]i approached a plateau level in 0-Na+ saline of approximately 5.5 microM; restoration of normal [Na+]o lowered [Ca2+]i to baseline with a time course of 1 log10 unit per 9 s. The apparent rate of Nao+-dependent [Ca2+]i decline decreased with decreasing [Ca2+]i. Reintroduction of Ca2+ to 0-Na+, 0-Ca2+ saline in a typical cell caused a transient rise in [Ca2+]i from an estimated 0.36 microM (or lower) to approximately 16.5 microM. This was followed by a decline in [Ca2+]i approaching a plateau of approximately 5 microM; subsequent removal of Cao2+ caused [Ca2+]i to decline slowly (1 log unit in approximately 110 s). Intracellular injection of Na+ in the absence of extracellular Na+ caused a transient rise in [Ca2+]i in the presence of normal [Ca2+]o; in 0-Ca2+ saline, however, no such rise in [Ca2+]i was detected. Under constant voltage clamp (-80 mV) inward currents were measured after the addition of Nao+ to 0-Na+ 0-Ca2+ saline and outward currents were measured after the addition of Cao2+ to 0-Na+ 0-Ca2+ saline. The results suggest the presence of an electrogenic Na+/Ca2+ exchange process in the plasma membrane of Limulus ventral photoreceptors that can operate in forward (Nao+-dependent Ca2+ extrusion) or reverse (Nai+-dependent Ca2+ influx) directions.

Accessibility of colloidal gold and horseradish peroxidase to cytosolic spaces in Limulus ventral photoreceptors

Physiological studies of intracellular messengers frequently employ intracellular injections of large molecules that either monitor or modulate the metabolism of the messenger cascade. Injected molecules have unknown mobility in the cytosol and unknown accessibility to various cytosolic compartments, including those postuiated to be traversed by intracellular messenger molecules. In order to determine whether injected molecules have access to the confined spaces through which messenger molecules must diffuse, we injected 5-nm colloidal gold or horseradish peroxidase, or both, into Limulus ventral photoreceptors. Injections were made by applying pressure pulses to the back of an intracellular micropipette that also monitored membrane voltage. The tissue was fixed at varying times after injection and processed for electron microscopy by conventional techniques. Cells fixed 1-3 min after injection contained HRP reaction product only in the cell body. HRP reaction product was found at varying distances down axons in direct relation to the interval between injection and fixation. Colloidal gold particles were found throughout the cell body but not in axons of tissue fixed 1-3 min after injection. Both HRP reaction product and 5-nm colloidal gold particles were observed within the microvillar projections of internal and external rhabdomere, as well as within the extracisternal spaces of endoplasmic reticulum. We conclude that large molecules injected from an intracellular micropipette into an arbitary locus of ventral photoreceptor cells have access to all of the presumed sites of the phototranduction cascade.

The quantal source of area supralinearity of flash responses in Limulus photoreceptors

The time integrals of the responses of dark-adapted Limulus ventral photoreceptors to flashes exhibit a supralinear dependence on intensity at intermediate intensities. By decomposing the responses into their elementary single-photon components ("bumps"), we are able to calculate the overall quantum efficiency and to display the time courses of the bump amplitude and rate of appearance. Since the time course of the flash response is not slow compared with that of the bump, it was necessary, in order to carry out the decomposition, to develop a new technique for noise analysis of dynamic signals. This new technique should have wide applications. Our main finding is that the supralinearity of the flash responses corresponds to an increase in bump amplitude, with little change in bump duration or quantum efficiency. The time courses of the bump rate and of the change in bump amplitude are peaked and have widths similar to that of the response itself. The peaks of the time courses of the bump rate and amplitude displayed against the starting times of the bumps do not coincide and occur approximately 80 and approximately 40 ms, respectively, before the peak of the response. The time from the start of a bump to its centroid is approximately 70 ms, which means that the time at which the bump centroid reaches its maximum follows the response peak by 30 ms. These results impose constraints on possible mechanisms for the amplitude enhancement.

Biophysical evidence that light adaptation in Limulus photoreceptors is due to a negative feedback

The steady-state stimulus-response curve of the Limulus ventral photoreceptor comprises a linear portion followed by a less-than-unity power law dependence, which is maintained over at least 4 decades of intensity. This progressive desensitization corresponds to light adaptation. For flash stimulation of dark-adapted cells, the stimulus-response curve again has an initial linear portion, but this is followed by a region of supralinearity before the curve saturates. In a previous article, we showed that the distribution of time integrals of the single-photon responses is consistent with a model of a single chain of first-order reactions. Starting with such a model, we have looked at relevant elementary nonlinear biochemical mechanisms to determine which of them can modulate the enzymatic amplifications of the chain in such a way as to lead to these behaviors. We assume that each of the two phenomena, adaptation and supralinearity, derives from a single mechanism that acts on a single enzymatic stage. We then conclude that the adaptation must be a cooperative negative feedback, in which an accessory material activated by a late stage of the transduction chain acts cooperatively to inhibit an earlier enzymatic amplification. In Limulus, the number of molecules that cooperate is between 3 and 5. We were not able to discard any of the mechanisms tested for the supralinearity, except to say that they must act at a stage of the chain later than that on which the adaptive material acts. If we assume the conclusions of a previous work which shows that the supralinearity mechanism is active during the steady state, we can also conclude that the supralinearity stage must precede the stage that is the source of the adaptive material.

Responses of crayfish photoreceptor cells following intense light adaptation

After intense orange adapting exposures that convert 80% of the rhodopsin in the eye to metarhodopsin, rhabdoms become covered with accessory pigment and appear to lose some microvillar order. Only after a delay of hours or even days is the metarhodopsin replaced by rhodopsin (Cronin and Goldsmith 1984). After 24 h of dark adaptation, when there has been little recovery of visual pigment, the photoreceptor cells have normal resting potentials and input resistances, and the reversal potential of the light response is 10-15 mV (inside positive), unchanged from controls. The log V vs log I curve is shifted about 0.6 log units to the right on the energy axis, quantitatively consistent with the decrease in the probability of quantum catch expected from the lowered concentration of rhodopsin in the rhabdoms. Furthermore, at 24 h the photoreceptors exhibit a broader spectral sensitivity than controls, which is also expected from accumulations of metarhodopsin in the rhabdoms. In three other respects, however, the transduction process appears to be light adapted: The voltage responses are more phasic than those of control photoreceptors. The relatively larger effect (compared to controls) of low extracellular Ca++ (1 mmol/l EGTA) in potentiating the photoresponses suggests that the photoreceptors may have elevated levels of free cytoplasmic Ca++. The saturating depolarization is only about 30% as large as the maximal receptor potentials of contralateral, dark controls, and by that measure the log V-log I curve is shifted downward by 0.54 log units.(ABSTRACT TRUNCATED AT 250 WORDS)

Relationship between light sensitivity and intracellular free Ca concentration in Limulus ventral photoreceptors. A quantitative study using Ca-selective microelectrodes

The possible role of Ca ions in mediating the drop in sensitivity associated with light adaptation in Limulus ventral photoreceptors was assessed by simultaneously measuring the sensitivity to light and the intracellular free Ca concentration (Cai); the latter was measured by using Ca-selective microelectrodes. In dark-adapted photoreceptors, the mean resting Cai was 3.5 +/- 2.5 microM SD (n = 31). No correlation was found between resting Cai and absolute sensitivity from cell to cell. Typically, photoreceptors are not uniformly sensitive to light; the Cai rise evoked by uniform illumination was 20-40 times larger and faster in the most sensitive region of the cell (the rhabdomeral lobe) than it was away from it. In response to a brief flash, the Cai rise was barely detectable when 10(2) photons were absorbed, and it was saturated when approximately 10(5) photons were absorbed. During maintained illumination, starting near the threshold of light adaptation, steady Cai increases were associated with steady desensitizations over several log units of light intensity: a 100-fold desensitization was associated with a 2.5-fold increase in Cai. After a bright flash, sensitivity and Cai recovered with different time courses: the cell was still desensitized by approximately 0.5 log units when Cai had already recovered to the prestimulus level, which suggests that under those conditions Cai is not the rate-limiting step of dark adaptation. Ionophoretic injection of EGTA markedly decreased the light-induced Cai rise and increased the time to peak of the light response, but did not alter the resting Cai, which suggests that the time to peak is affected by a change in the capacity to bind Ca2+ and not by resting Cai. Lowering the extracellular Ca2+ concentration (Cao) first decreased Cai and increased sensitivity. Longer exposure to low Cao resulted in a further decrease of Cai but decreased rather than increased sensitivity, which suggests that under certain conditions it is possible to uncouple Cai and sensitivity.

3',5'-cyclic adenosine monophosphate and adenylate cyclase in phototransduction by limulus ventral photoreceptors

Biochemical and electrophysiological measurements were made on photoreceptor cells from Limulus ventral eyes to investigate the possible role of cyclic AMP and adenylate cyclase in the visual transduction mechanism. Cyclic AMP content in a photoreceptor-enriched fraction (the end organs) of Limulus ventral eyes was approximately 15 pmol/mg protein. The cyclic AMP content was increased by bathing eyes in 1-methyl-3-isobutyl xanthine or forskolin and was increased almost 100-fold when bathed in both. Illumination did not change cyclic AMP content significantly in any of these conditions. Discrete events that can be recorded electrophysiologically occur spontaneously in darkness. An increase in the frequency of discrete events is evoked by dim illumination. The discrete events are a sign of excitation of Limulus photoreceptor cells. Drug-induced changes in the rate of occurrence of discrete events recorded electrophysiologically in darkness were not correlated with changes in cyclic AMP content. Adenylate cyclase activity measured from a small number of pooled photoreceptor clusters was stimulated by fluoride and vanadate ions, hydrolysis-resistant analogues of GTP, cholera toxin and forskolin. The Limulus enzyme is similar pharmacologically to mammalian and avian adenylate cyclases. Activation of adenylate cyclase by drugs was not correlated with changes in the rate of occurrence of discrete events recorded electrophysiologically in darkness. A heat-treated Lubrol extract of membranes from Limulus ventral eyes reconstituted the adenylate cyclase activity of membranes from S49 mouse lymphoma cyc- mutant cells which lack a functional regulatory protein. These findings suggest that Limulus ventral eye photoreceptors contain a regulatory protein that mediates the activation of adenylate cyclase by guanine nucleotides, fluoride or cholera toxin. This regulatory protein is homologous with that found in mammalian and avian adenylate cyclases. Our findings suggest that neither cyclic AMP nor adenylate cyclase activation is a necessary or obligatory component of the excitation mechanism in Limulus ventral photoreceptors.

Effects of extracellular calcium and of light adaptation on the response to dim light in honey bee drone photoreceptors

Light responses in honey bee drone photoreceptors were recorded with intracellular micro-electrodes in superfused slices of retina. The effects of changes in extracellular calcium on the size and the shape of the response to dim light were studied and compared with the effects of light adaptation. Dim light stimuli were used so that the amplitude of the response was linearly related to the number of the photons absorbed, the effects of voltage-dependent mechanisms were negligible and no detectable light adaptation was produced by the stimulus. Lowering the extracellular calcium concentration increased the amplitude and the duration of the response. Raising the extracellular calcium concentration produced the opposite effects. Changing the extracellular calcium concentration modified the response without altering either the linearity of the intensity--response relation or the resting membrane potential in the dark. Light adaptation decreased the amplitude and the duration of the response in a manner that could be quantitatively simulated, in the same photoreceptors, by an increase in the extracellular calcium concentration. Changing the extracellular calcium concentration, or light-adapting the preparation, modified the response without altering its early depolarizing phase. Lowering external calcium either did not affect, or slightly increased, the maximum rate of the light-induced depolarization; raising external calcium, or light-adapting the preparation, either did not affect, or slightly decreased, the maximum rate of the light-induced depolarization. The experimental data can be quantitatively described by a mathematical model with the basic assumption that calcium acts in the process of light adaptation by decreasing the mean open time of the light-activated channels.

Colour dependence of the early receptor potential and late receptor potential in scallop distal photoreceptor

1. Intracellular voltage and current responses to short (blue) and long (red) wave-length lights were measured in the distal hyperpolarizing photoreceptor (;off receptor') of the isolated and perfused scallop (Pecten irradians) retina.2. The early receptor potential (e.r.p.) was isolated by holding membrane potential at the reversal potential for the late receptor potential (l.r.p.) or by working at temperatures (< 5.0 degrees C) that abolished the l.r.p.3. The e.r.p., measured using intense flashes of white light, consisted of a positive phase followed by a negative phase, but was converted to a monophasic, negative-going wave following pre-adaptation with red light and to a monophasic, positive-going wave following pre-adaptation with blue light.4. The spectral sensitivity curve for the negative e.r.p. was maximum at 500 nm, whereas the spectral sensitivity curve for the positive e.r.p. was maximum at 575 nm.5. The positive or negative e.r.p.s approached their maximum amplitude exponentially when tested with red or blue flashes of increasing intensity. The results suggest that the positive (or negative) e.r.p. is proportional to the number of photopigment molecules photo-isomerized.6. The photosensitivity maximum of rhodopsin calculated at 500 nm, using the exponential constant and the spectral sensitivity data, was estimated to be 2.1 x 10(-16) cm(2) photon(-1), whereas the photosensitivity maximum of metarhodopsin calculated at 575 nm was estimated to be 2.6 x 10(-16) cm(2) photon(-1).7. In cells pre-adapted with white light, stimulation with blue light caused a hyperpolarizing l.r.p. which was followed by a prolonged hyperpolarizing after-potential (p.h.a.). Stimulation with red light under similar conditions caused an initial hyperpolarization which was followed by a small depolarization during the stimulus, but no after-potential.8. The duration of the p.h.a. was increased by pre-adaptation with a red light, which caused the maximum net transfer of metarhodopsin to rhodopsin; however, its decay was always complete in 5 min or less.9. The photo-isomerization of metarhodopsin by red light suppressed the p.h.a. and caused an after-depolarizing response that decayed in less than 1 min.10. The spectral sensitivity curve for the induction of the p.h.a. was maximum at 500 nm and corresponded to the spectral sensitivity for the negative e.r.p. and for the l.r.p. studied in the dark-adapted retina, whereas the spectral sensitivity curve for the suppression of the p.h.a. and for the induction of the after-depolarization was maximum at 575 nm and corresponded to the spectral sensitivity for the positive e.r.p.11. In photoreceptors clamped to the resting potential in normal ASW, the photo-isomerization of rhodopsin, in the absence of light absorption by metarhodopsin, activated a persistent outward current that had the same time course of decay as the p.h.a. The photo-isomerization of metarhodopsin suppressed the persistent outward current and activated an inward current whose decay took longer than the decay of the after-depolarizing response.12. In the absence of external Ca(2+) and Na(+) ions, the persistent outward current produced by light absorption by rhodopsin, and the inward current produced by light absorption by metarhodopsin, both reversed at the K(+) equilibrium potential. The results show that the induction of the prolonged hyperpolarizing after-potential and the after-depolarizing response involve only the movement of K(+) ions through the same light-dependent K(+) channels that determine the hyperpolarizing l.r.p. of the distal cells.

Optical measurements of intracellular pH and magnesium in frog skeletal muscle fibres

1. Single twitch fibres were isolated from frog muscle, then mounted in a chamber which was positioned on an optical bench. The fibres were immobilized by high stretch (sarcomere spacing 3.9-4.3 mum) and by placement on a pedestal. Their optical properties were determined by illuminating a 35-65 mum diameter spot with quasimonochromatic light of intensity I(0) and measuring the intensity I of the transmitted light. Since the main purpose of the experiments was to draw inferences from the absorbance spectra of different indicator dyes injected into the fibres, all results were expressed in terms of absorbance A calculated from the equation [Formula: see text]. Changes in absorbance DeltaA were calculated from the differential form of the equation [Formula: see text].2. The absorbance of a normal, non-injected fibre was, on average, equal to 0.03 at 570 nm and varied approximately inversely with wavelength between 450 and 750 nm.3. The earliest change in absorbance following an action potential was a small, transient increase which was followed by a larger decrease. The decrease in fibre absorbance varied from 0.5 x 10(-4) to 3 x 10(-4) units.4. Resting myoplasmic pH was determined by comparing the absorbance spectrum from fibres injected with Phenol Red with that obtained from calibrating solutions in cuvettes. The muscle measurements were corrected for the intrinsic absorbance of fibre without dye. The average value of pH in two fibres was 6.9. The change in absorbance following an action potential in these highly stretched fibres was small. In one experiment the change, if due to pH alone, corresponded to an increase in pH of 0.004 peak and 0.002 maintained (relative to a resting level of 6.9). The maintained signal can be satisfactorily explained by the known amount of phosphocreatine hydrolysis.5. Estimates of myoplasmic free [Mg(2+)] were made using three metallochromic indicator dyes. A different estimate was obtained with each dye as indicated below. Since these dyes are sensitive to pH, as well as [Mg(2+)], the estimate depends on the assumed value of intracellular pH. [List: see text] This variability probably means that at least two, and possibly all three dyes behave differently inside muscle fibres than they do in calibrating solutions. The most likely explanation is that dye, once injected, can bind to cellular contents and that this alters its properties.6. Changes in absorbance of fibres injected with Arsenazo I, a dye three times more sensitive to Mg(2+) than to Ca(2+), were used to determine whether changes in free [Mg(2+)] occur following an action potential. The observed changes were small and could be due to a small increase in pH, of the magnitude measured with Phenol Red, and/or free [Mg(2+)]. In terms of a change in free [Mg(2+)], the results set an upper limit of 2%.7. The conclusion from the action potential experiments is that neither intracellular pH nor free [Mg(2+)] changes appreciably in highly stretched fibres. Changes in these two quantities can therefore be neglected in analysing the relatively large 650-660 nm Ca(2+) signal in fibres injected with the Ca(2+) (but also pH and Mg(2+)) sensitive indicator dye Arsenazo III.

Ca2+-sequestering smooth endoplasmic reticulum in an invertebrate photoreceptor. I. Intracellular topography as revealed by OsFeCN staining and in situ Ca accumulation

Two ultrastructural approaches were used in photoreceptor cells of the leech, Hirudo medicinalis, to (a) investigate the intracellular topography of the smooth endoplasmic reticulum (SER) and (b) identify among the various subregions of the SER those which might function as Ca-sequestering sites. When the cells are prefixed with CaCl2-containing glutaraldehyde and postfixed with osmium tetroxide-ferricyanide (OsFeCN), only a part of the total SER is specifically stained. The stained SER cisternae include the submicrovillar cisternae (SMC), subsurface cisternae (SSC), the nuclear envelope, Golgi-associated SER, paracrystalline SER, and SER associated with glycogen areas. An extensive tubular SER cisternal system always remains unstained. When the cells are permeabilized by saponin and subsequently incubated with Ca2+, MgATP, and oxalate, the SMC (Walz, 1979, Eur. J. Cell Biol. 20:83-91), the SSC and the nuclear envelope contain electron-opaque Ca-oxalate precipitates indicating their ability to function as an effective Ca2+ sink. The results show that the very elaborate SER in this photoreceptor cell includes many functionally heterogeneous subregions. Of special physiological significance are those components (SMC and SSC) which are effective in Ca2+-buffering in the immediate vicinity of the plasma membrane.

Functional significance of voltage-dependent conductances in Limulus ventral photoreceptors

The influence of voltage-dependent conductances on the receptor potential of Limulus ventral photoreceptors was investigated. During prolonged, bright illumination, the receptor potential consists of an initial transient phase followed by a smaller plateau phase. Generally, a spike appears on the rising edge of the transient phase, and often a dip occurs between the transient and plateau. Block of the rapidly inactivating outward current, iA, by 4-aminopyridine eliminates the dip under some conditions. Block of maintained outward current by internal tetraethylammonium increases the height of the plateau phase, but does not eliminate the dip. Block of the voltage-dependent Na+ and Ca2+ current by external Ni2+ eliminates the spike. The voltage-dependent Ca2+ conductance also influences the sensitivity of the photoreceptor to light as indicated by the following evidence: depolarizing voltage-clamp pulses reduce sensitivity to light. This reduction is blocked by removal of external Ca2+ or by block of inward Ca2+ current with Ni2+. The reduction of sensitivity depends on the amplitude of the pulse, reaching a maximum at or approximately +15 mV. The voltage dependence is consistent with the hypothesis that the desensitization results from passive Ca2+ entry through a voltage-dependent conductance.

The contribution of pigment transitions to sensitivity changes in the barnacle photoreceptor and the correlation with the prolonged depolarizing afterpotential

A conditioning light can cause a decrease (adaptation) or an increase (facilitation) in the sensitivity of barnacle photoreceptors, as measured by the amplitude of the late receptor potential (LRP). We show that a net transfer of visual pigment from the rhodopsin (R) to the metarhodopsin (M) state induces a large facilitation whereas the reverse transfer results in a much smaller facilitation or even an adaptation. These effects were not due to the response to the conditioning light but to the pigment reactions. When the conditioning light did not alter the pigment population (i.e., M leads to M, R leads to R) it was followed by an intermediate degree of facilitation. These conclusions are correct for cells which have relatively low sensitivity. In sensitive cells, all pigment transitions produce adaptation. LRP facilitation and the prolonged depolarizing afterpotential (PDA) show several common characteristics with respect to pigment transitions: 1. Their magnitude increases with the amount of pigment transferred from R to M. 2. Both are depressed by the M leads to R transition. 3. Their production is impeded by the M leads to R transition. 4. The PDA itself is facilitated by the R leads to M transition and this facilitation decays with a time course comparable to that of LRP facilitation. These results suggest that there may be an underlying process common to LRP facilitation and PDA.

Properties of the pH-sensitive site that controls the lambda max of Limulus metarhodopsin

A pH-sensitive site controls the lambda max of Limulus metarhodopsin. The properties of this site were examined using intracellular recordings of the early receptor potential (ERP) as a pigment assay. ERPs recorded over a range of extracellular pHs indicate that the apparent pK of the site is in the range of 8.3-8.6. Several lines of evidence indicate that the site responds directly to changes in extracellular pH (pHo) rather than to changes in intracellular pH(pHi) that follow as a secondary result of changing pHo : (a) the effect of changing pHo was rapid (less than 60 s); (b) when pHo was raised, the simultaneous rise in pHi, as measured with phenol red, was relatively small; (c) raising pHi by intracellular injection of pH 10 glycine buffer did not affect the site; and (d) the effect of changing pH0 could not be blocked by increasing the intracellular pH buffering capacity. It is concluded that the pH-sensitive site on metarhodopsin is on the extracellular surface of the plasma membrane.