Spatially nonuniform changes in intracellular calcium ion concentrations

The influence of the chloride currents on action potential firing and volume regulation of excitable cells studied by a kinetic model

In excitable cells, the generation of an action potential (AP) is associated with transient changes of the intra- and extracellular concentrations of small ions such as Na(+), K(+) and Cl(-). If these changes cannot be fully reversed between successive APs cumulative changes of trans-membrane ion gradients will occur, impinging on the cell volume and the duration, amplitude and frequency of APs. Previous computational studies focused on effects associated with excitation-induced changes of potassium and sodium. Here we present a model based study on the influence of chloride on the fidelity of AP firing and cellular volume regulation during excitation. Our simulations show that depending on the magnitude of the basal chloride permeability two complementary types of responsiveness and volume variability exist: (i) At high chloride permeability (typical for muscle cells), large excitatory stimuli are required to elicit APs; repetitive stimuli of equal strength result in almost identical spike train patterns (Markovian behavior), however, long excitation may lead to after discharges due to an outward directed current of intracellular chloride ions which accumulate during excitation; cell volume changes are large. (ii) At low chloride permeability (e.g., neurons), small excitatory stimuli are sufficient to elicit APs, repetitive stimuli of equal strength produce spike trains with progressively changing amplitude, frequency and duration (short-term memory effects or non-Markovian behavior); cell volume changes are small. We hypothesize that variation of the basal chloride permeability could be an important mechanism of neuronal cells to adapt their responsiveness to external stimuli during learning and memory processes.

Immunohistochemical localization of nitric oxide synthase and soluble guanylyl cyclase in the ventral cochlear nucleus of the rat

The diffusible messenger nitric oxide (NO) is implicated in auditory processing. It acts in the brain largely through activation of soluble guanylyl cyclase (sGC), a heterodimer comprised of alpha and beta subunits. The authors used immunohistochemistry to study the NO/guanosine 3',5'-cyclic monophosphate (cGMP) pathway in the cochlear nucleus of Sprague-Dawley rats. Central fibers of the cochlear nerve were stained for neuronal nitric oxide synthase (NOS-I) but not for sGCbeta. Within the ventral cochlear nucleus, a large fraction of principal cells were immunopositive for both NOS-I and sGCbeta; these cells could be seen at times receiving contacts from NOS-I-positive fibers. sGC staining of somatic cytoplasm extended into the distal dendritic tree. At variance with this pattern, NOS-I was concentrated mainly in somata. Double-labeling experiments showed that most of the principal neurons expressed both antigens. By contrast, in the granule cell domain, small cells that were immunopositive for NOS-I rarely corresponded to those that were immunopositive for sGC. To assess whether NOS-I and sGC immunoreactivities colocalize with their respective catalytic activities, the authors performed multiple labeling with L-citrulline (a by-product of the formation of NO from L-arginine) and cGMP, respectively. L-citrulline was restricted to NOS-I-positive elements, and the large majority of NOS-expressing neurons were positive for citrulline. Multiple labeling revealed that almost all sGC-positive neurons also accumulated cGMP both in the ventral cochlear nucleus and in the granule cell domain. These data suggest that NO is a signaling molecule in the cochlear nucleus, perhaps functioning in both a paracrine manner and an autocrine manner.

Light activated calcium release in Limulus ventral photoreceptors as revealed by laser confocal microscopy

Using confocal imaging and fluorescent calcium indicators, light-induced elevation of intracellular Ca2+ concentration ([Ca2+]i) in Limulus ventral photoreceptors was shown to be initiated within 4 microns of the light-sensitive plasma membrane. Within 500 ms, elevation of [Ca2+]i spread throughout the light-sensitive rhabdomeral lobe of the photoreceptor, but barely penetrated the arhabdomeral lobe. During saturating illumination of measurement spots near the plasma membrane, [Ca2+]i rose at rates of 1-2 mM/s after a latent period of 14-40 ms, reaching peak concentrations of approximately 150 microM. Rapid elevation of [Ca2+]i persisted in the absence of extracellular Ca2+ and was therefore ascribed to release from intracellular stores. The elevation of [Ca2+]i was always detectable within 5 ms of the electrical response of the photoreceptor to light. In 14 out of 54 measurements, detection of elevated calcium preceded the electrical response. Cyclopiazonic acid, an inhibitor of endoplasmic reticulum Ca-pumps, greatly reduced the elevation of [Ca2+]i during bright flashes and the sensitivity of the electrical response to dim flashes. However, the maximal response to bright flashes was not diminished. Therefore, although the calcium release that we detect may be fast enough to contribute to the electrical response we are unable to demonstrate that it is absolutely required.

Intracellular injection of heparin and polyamines. Effects on phototransduction in limulus ventral photoreceptors

Heparin is thought to inhibit InsP3 binding to receptors involved in the intracellular release of Ca2+. Injection of heparin into Limulus ventral photoreceptors to high intracellular concentrations reduces the amplitude and slows the rate of rise of voltage-clamp currents induced by brief flashes, tends to make the responses to long flashes more "square," and tends to block the light-induced rise in [Ca2+]i detected by arsenazo III. In these ways, intracellular heparin mimics the effects of high concentrations of intracellular BAPTA or EGTA. In addition, the effects of heparin are attenuated by prior injection of BAPTA to high intracellular concentrations. Neomycin and spermine are thought to inhibit phospholipase C activity. Injections of spermine or neomycin to low intracellular concentrations largely mimic the effects of intracellular heparin. These findings suggest that the predominant effect of polyamines is to inhibit light-induced production of InsP3 by phospholipase C activity and thereby reduce the light-induced increase in [Ca2+]i. Our findings suggest that excitation can proceed in the absence of InsP3-induced increases in [Ca2+]i, but (a) the gain and speed of transduction are reduced and (b) adaptation is largely blocked.

Electrical-ionic control of gene expression

1. Changes in turgor, in cell volume, in membrane potential, in intracellular ionic activities and, more recently, in spontaneous electrical activity have been reported to be causally linked to the expression of specific genes. 2. As a result, it has become clear that changes in membrane properties and/or in the intracellular "ionic environment" can play an important role in generating cell type specific physiological responses which indirectly--or maybe directly--affect gene expression. 3. Possible targets of the ionic "environment" are: the selective transport across biological membranes; the activity of certain (regulatory) enzymes; the conformation of some (regulatory) proteins; of chromatin; of the cytoskeleton; of the nuclear matrix; the association of the cytoskeleton with plasmamembrane proteins or RNA; the association chromatin-nuclear matrix; protein-DNA and protein-protein interactions etc. All these sites may be instrumental to "fine or coarse" tuning of gene expression. 4. The exact mechanisms by which changes in intracellular ionic environment are transduced, directly or indirectly, into alterations of the activity of trans-acting factors have not yet been fully uncovered. Changes in the degree of phosphorylation of regulatory proteins and/or of trans-acting factors may provoke fine tuning effects on cell type specific gene expression activity. 5. The intranuclear ionic environment is difficult to measure in an exact way. It can be influenced in a number of ways. The location of a gene, as determined by the position of the nucleus in the cytoplasm and by the association of chromatin to the nuclear matrix may be especially important in cells which can generate some type of intracellular gradient or in excitable cells. 6. In some somatic cell types--germinal vesicles may behave differently--the intranuclear inorganic ionic "environment" has been reported to be distinct from the cytoplasmic one. This challenges the widespread assumption that the nuclear envelope is always freely permeable to small molecules and inorganic ions. 7. It can be expected that the fast progress in the cloning of "electrically" controlled genes, in the identification of trans-acting factors, in their mode of interaction with genes and in the precise localization of genes within the nucleus may soon lead to substantial progress in this domain.

Intracellular free calcium concentration in the blowfly retina studied by Fura-2

The retinal tissue of blowflies was loaded with the fluorescent Ca2+ indicator Fura-2 by incubating cut heads in saline solutions which contained the membrane permeable acetoxymethylester of Fura-2 (Fura-2/AM). The spectral analysis of the tissue fluorescence showed that Fura-2/AM was intracellularly hydrolysed to Fura-2. In order to monitor the intracellular free Ca2+ concentration ([Ca2+]i) the Fura-2 fluorescence was excited by short light flashes. The fluorescence was calibrated by incubating the tissue in Ca2+ buffers of high buffering capacity and subsequent disruption of the cell membranes by freeze/thawing, which gave a dissociation constant for the Ca(2+)-Fura-2 complex of 100 nM. When the extracellular Ca2+ concentration ([Ca2+]o) was altered [Ca2+]i reversibly changed. The changes were most pronounced when [Ca2+]o was varied in the millimolar range, e.g. [Ca2+]i increased from 0.07 microM at [Ca2+]o = 0.1 mM to 1 microM at [Ca2+]o = 10 mM. When extracellular Na+ was replaced by Li+ or other monovalent ions, [Ca2+]i rapidly increased which supports the view that electrogenic Na+/Ca2+ exchange contributes to the control of [Ca2+]i. However, [Ca2+]i decreased again when the tissue was superfused with Na(+)-free media for longer periods, which points to a Ca(2+)-transporting system different from Na+/Ca2+ exchange. Light adaptation had only a small effect on [Ca2+]i. Even after intense stimulation [Ca2+]i increased by a factor of 1.5 only, which is in line with results obtained in the photoreceptors of Balanus and Apis.

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.

The localization of calcium release by inositol trisphosphate in Limulus photoreceptors and its control by negative feedback

Microvillar photoreceptors of invertebrates exhibit a light-induced rise in the intracellular concentration of free calcium (Cai) that results in part from release of calcium from an intracellular compartment. This light-induced release of calcium appears to result from a cascade of reactions that involve rhodopsin, a GTP-binding protein and a phospholipase-C which releases inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) from the plasma membrane; the Ins(1,4,5)P3 acts to release calcium from smooth endoplasmic reticulum. In the ventral photoreceptor of the horseshoe crab Limulus polyphemus not all of the endoplasmic reticulum is subject to calcium release by Ins(1,4,5)P3. Only endoplasmic reticulum in the light-sensitive region of the cell is competent to release calcium in response to Ins(1,4,5)P3. The release of calcium by Ins(1,4,5)P3 in ventral photoreceptors appears to be subject to feedback inhibition through elevated Cai. We suggest that this feedback inhibition contributes to sensory adaptation in the photoreceptor and may account for oscillatory membrane responses sometimes observed with large injections of Ins(1,4,5)P3.

A spatial-temporal model of cell activation

A spatial-temporal model of calcium messenger function is proposed to account for sustained cellular responses to sustained stimuli, as well as for the persistent enhancement of cell responsiveness after removal of a stimulus, that is, cellular memory. According to this model, spatial separation of calcium function contributes to temporal separation of distinct phases of the cellular response. At different cellular sites, within successive temporal domains, the calcium messenger is generated by different mechanisms and has distinct molecular targets. In particular, prolonged cell activation is brought about by the interaction of calcium with another spatially confined messenger, diacylglycerol, to cause the association of protein kinase C with the plasma membrane. Activity of the membrane-associated protein kinase C is controlled by the rate of calcium cycling across the plasma membrane. In some instances, a single stimulus induces both protein kinase C activation and calcium cycling and thus causes prolonged activation; but in others, a close temporal association of distinct stimuli brings about cell activation via interaction of these intracellular messengers. Persistent enhancement of cell responsiveness after removal of stimuli is suggested to be due to the continued association, or anchoring, of protein kinase C to the membrane.

Activation of mitochondrial oxidative metabolism by calcium ions in Limulus ventral photoreceptor

Cells regulate their metabolic energy production to meet the requirements of their energy consuming activities. For most animal cells the prime site of energy production, in the form of ATP, is the mitochondrion. Extensive in vitro studies of isolated mitochondria have provided detailed information about the specific biochemical reactions involved in energy production. At present there is a debate about whether respiration in excitable cells is controlled by the availability of ADP to the mitochondrion and/or by calcium ions. Using the large ventral photoreceptor of the horseshoe crab (Limulus polyphemus) we describe a method for measuring the transient increase in the mitochondrial O2 consumption (delta QO2) following a flash of light of a single photoreceptor. We then show that this delta QO2 results in part from a rise in the intracellular concentration of calcium (Cai).

Inositol 1,4,5 trisphosphate releases calcium from specialized sites within Limulus photoreceptors

We have investigated the subcellular distribution and identity of inositol trisphosphate (InsP3)-sensitive calcium stores in living Limulus ventral photoreceptor cells, where light and InsP3 are known to raise intracellular calcium. We injected ventral photoreceptor cells with the photoprotein aequorin and viewed its luminescence with an image intensifier. InsP3 only elicited detectable aequorin luminescence when injected into the light-sensitive rhabdomeral (R)-lobe where aequorin luminescence induced by light was also confined. Calcium stores released by light and InsP3 are therefore localized to the R-lobe. Within the R-lobe, InsP3-induced aequorin luminescence was further confined around the injection site, due to rapid dilution and/or degradation of injected InsP3. Prominent cisternae of smooth endoplasmic reticulum are uniquely localized within the cell beneath the microvillar surface of the R-lobe (Calman, B., and S. Chamberlain, 1982, J. Gen. Physiol., 80:839-862). These cisternae are the probable site of InsP3 action.

Chelation of intracellular calcium blocks insulin action in the adipocyte

The hypothesis that intracellular Ca2+ is an essential component of the intracellular mechanism of insulin action in the adipocyte was evaluated. Cells were loaded with the Ca2+ chelator quin-2, by preincubating them with quin-2 AM, the tetrakis(acetoxymethyl) ester of quin-2. Quin-2 loading inhibited insulin-stimulated glucose transport (IC50, 26 microM quin-2 AM) without affecting basal activity. The ability of insulin to stimulate glucose uptake in quin-2-loaded cells could be partially restored by preincubating cells with buffer supplemented with 1.2 mM CaCl2 and the Ca2+ ionophore A23187. These conditions had no effect on basal activity and omission of CaCl2 from the buffer prevented the restoration of insulin-stimulated glucose uptake by A23187. Quin-2 loading also inhibited insulin-stimulated glucose oxidation (IC50, 11 microM quin-2 AM) and the ability of insulin to inhibit cAMP-stimulated lipolysis (IC50, 78 microM quin-2 AM), without affecting their basal activities. Incubation of cells with 100 microM quin-2 or quin-2 AM had no effect on intracellular ATP concentration or the specific binding of 125I-labeled insulin to adipocytes. These findings suggest that intracellular Ca2+ is an essential component in the coupling of the insulin-activated receptor complex to cellular physiological/metabolic machinery. Furthermore, differing quin-2 AM dose-response profiles suggest the presence of dual Ca2+-dependent pathways in the adipocyte. One involves insulin stimulation of glucose transport and oxidation, whereas the other involves the antilipolytic action of insulin.

Probability of quantal transmitter release from nerve terminals: theoretical considerations in the determination of spatial variation

The release of transmitter occurs in discrete quantal units, such that the number released (m) is equal to the number available (n) times the average probability of release (p). Although a common method of estimating these parameters is to use simple binomial statistics, results may be biased if there is spatial or temporal variation in n and p (vars p, vart n, vart p). The problem arises in the simultaneous analysis of five variables, which is impractical due to the complexity and margin of error involved. The proposed solution is to eliminate two variables (vart n, vart p) by assuming stationarity and to obtain the required information from the first three moments of m. The resulting quadratic equation gives two solutions, p1 and p2. Computer simulation of quantal output as a function of vars p indicates that p1 is the better estimator of p when vars p is small, but that p2 is better when vars p is large. This changeover or "inflection" occurs at points which correspond to the maximum vars p obtainable by unimodal distributions of p (larger vars p being obtained by bimodal distributions). Comparison of the simulated histogram of m with those predicted by p1 and p2 shows that p1 provides the better fit, whether vars p is large or small. This discrepancy indicates that histogram analysis is unable to distinguish the appropriate estimate. The major limitations in the procedure can be met by assuming (1) stationarity (which can be attained and tested experimentally), and (2) normal distribution of p (since vars p is then less than "inflection" point, p1 will always be the correct estimate). The overall findings demonstrate that vars p and unbiased estimates of n and p may be calculated, provided reasonable assumptions are made. This in turn should allow the continued use of quantal parameters for describing transmitter release.

Involvement of mitochondria in the age-dependent decrease in calcium uptake of rat brain synaptosomes

Calcium uptake in rat brain synaptosomes decreases during ageing. The possible involvement of mitochondria in altered calcium homeostasis has been investigated. Mitochondria isolated from old rat brain showed decreased calcium uptake rates. Since neither the mitochondrial membrane potential nor the delta pCa decreases with age, it was concluded that variations in the driving force for calcium uptake were not the cause for impaired calcium transport in mitochondria from aged rat brain. The steady state calcium distribution in isolated aged rat brain mitochondria was achieved at higher extramitochondrial calcium concentrations than that of adults. Studying the effects of the selective release of calcium from the mitochondrial pool by the addition of an uncoupler to 45Ca loaded synaptosomes incubated in high-potassium media, it was found that the intrasynaptic mitochondrial pool and the intra/extramitochondrial 45Ca distribution also decreased considerably in 24-month-old rats. Steady state fluorescence anisotropy (rs) of diphenylhexatriene-labelled mitoplasts from 'free' brain mitochondria increased with ageing. However, since no changes in rs from synaptosomal mitochondria were found in 24-month-old rats, it is suggested that alterations in lipid dynamics are not involved in the impaired calcium uptake observed in brain mitochondria from aged rats. The implications of these findings in the calcium homeostasis of brain endings are discussed.

Pressure injection of calcium both excites and adapts Limulus ventral photoreceptors

Single pressure injections of 1-2 mM calcium aspartate into the light-sensitive region of Limulus ventral photoreceptors resulted in a rapid, 20-40-mV depolarization lasting approximately 2 s. The depolarization closely followed the rise in intracellular free calcium caused by the injection, as indicated by aequorin luminescence. The depolarization was followed by reversible desensitization (adaptation) of responses to both light and inositol 1,4,5 trisphosphate. Similar single injections of calcium into the light-insensitive region of the receptor were essentially without effect, even though aequorin luminescence indicated a large, rapid rise in intracellular free calcium. The depolarization caused by injection of calcium arose from the activation of an inward current with rectification characteristics and a reversal potential between +10 and +20 mV that were similar to those of the light-activated conductance, which suggests that the same channels were activated by light and by calcium. The reversal potentials of the light- and calcium-activated currents shifted similarly when three-fourths of the extracellular sodium was replaced by sucrose, but were not affected by a similar replacement of sodium by lithium. The current activated by calcium was abolished by prior injection of a calcium buffer solution containing EGTA. The responses of the same cells to brief light flashes were slowed and diminished in amplitude, but were not abolished after the injection of calcium buffer. Light adaptation and prior injection of calcium diminished the calcium-activated current much less than they diminished the light-activated current.

Effect of common agonists on cytoplasmic ionized calcium concentration in platelets. Measurement with 2-methyl-6-methoxy 8-nitroquinoline (quin2) and aequorin

Because of controversy regarding the relationship of cytoplasmic ionized calcium concentration ([Cai2+]) to platelet activation, we studied the correlation of platelet aggregation and ATP secretion with [Cai2+] as determined by 2-methyl-6-methoxy 8-nitroquinoline (quin2) and aequorin in response to ADP, epinephrine, collagen, the Ca2+ ionophore A23187, and thrombin. Both indicators showed a concentration-dependent increase in [Cai2+] in response to all agonists except epinephrine when gel-filtered platelets were suspended in media containing 1 mM Ca2+. With epinephrine, a rise in [Cai2+] was indicated by aequorin, but not by quin2; [Cai2+] signals, aggregation, and secretion were suppressed by EGTA. ADP [0.5 microM] produced a rise in [Cai2+] that was registered by both aequorin and quin2 in platelets in Ca2+-containing media; addition of EGTA to the medium raised the threshold concentration of ADP to 5.0 microM for both indicators. Collagen produced progressive concentration-related increases in [Cai2+] and aggregation in aspirin-treated aequorin-loaded platelets. Quin2 failed to indicate a rise in [Cai2+]at lower collagen concentrations with EGTA or aspirin. [Cai2+] response to A23187 and thrombin was reduced by addition of EGTA to platelets loaded with either aequorin or quin2. With all five agonists in all conditions tested, aequorin [Cai2+] signals occurred at the same agonist concentration as that or lower than that which produced platelet shape change, aggregation, or secretion. Platelet activation was better correlated with changes in [Cai2+] indicated by aequorin than with the response of quin2, possibly because aequorin is more sensitive to local zones of [Cai2+] elevation.

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)

Calcium-binding proteins in whole brain and synaptic subfractions

At least 19 calcium-binding proteins were detected in avian brain subfraction using 45Ca2+ binding to proteins immobilized in polyacrylamide gels. Half of the 45Ca2+ binding proteins were observed in presynaptic cytoplasm. Two-dimensional gel electrophoresis of this material revealed at least 14 45Ca2+ binding polypeptides besides calmodulin. These proteins may be important in brain and nerve terminal function.

Regional distribution of calcium influx into bursting neurons detected with arsenazo III

Absorbance changes of the metallochromic indicator arsenazo III were used in conjunction with an array of 100 photodiodes to measure changes in intracellular calcium concentration at many positions simultaneously in identified neurons of the crab stomatogastric ganglion. When stimulated with intrasomatically injected current, several of these neurons showed calcium changes all over the cell, indicating that calcium channels were distributed widely in the neuropil and on the soma. When the membrane potential was allowed to oscillate without stimulation, absorbance oscillations were detected all over the neuropil but not in the soma. A comparison between the membrane potential recorded in the soma and the calcium signal in the neuropil shows that calcium entry followed the slow voltage oscillation with the peak calcium signal detected 50-150 msec after the end of the voltage plateau.

Calcium ion, an intracellular messenger of light adaptation, also participates in excitation of Limulus photoreceptors

Photoreceptor cells of Limulus ventral eyes were bathed in artificial sea water (ASW) that contained 10 mM-EGTA and no added Ca2+ (EGTA-ASW). Test flashes elicited responses that increased to a maximum size within 10 min in EGTA-ASW but did not change further when dark-adapted cells were bathed for an additional 35 min in this solution. Light responses progressively declined from this maximum size if the cells were repetitively illuminated in EGTA-ASW. In this state of reduced responsiveness, response amplitudes were further reduced by intracellular ionophoretic injection of EGTA; response amplitudes were increased by intracellular ionophoretic injection of Ca2+. Both of these findings are opposite to what is normally observed for cells bathed in ASW. Also, after repetitive illumination in EGTA-ASW, both the slope of the response versus intensity relationship became steeper and light responses often had a delayed increase in amplitude. The light responses and the response versus intensity relation returned to normal when the bathing medium was changed back to ASW containing 10 mM-Ca2+. The light-induced rise in luminescence recorded from photoreceptors injected with the photoprotein aequorin (the 'aequorin response') declined by at most 50% after dark-adapted photoreceptors were bathed in EGTA-ASW for 45 min. However, the aequorin response progressively declined by 98% if cells were repetitively illuminated while bathed in EGTA-ASW. The total intracellular Ca content of whole end-organs was measured by atomic absorption spectroscopy. Total intracellular Ca content did not change significantly while photoreceptors were bathed in EGTA-ASW even after repetitive illumination. We suggest that cytosolic Ca2+ is required by one or more steps in the mechanisms that link rhodopsin isomerization to both (i) an increase in the conductance of the cell membrane to Na+ and (ii) a release of Ca2+ from a light-labile store.

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.

Reduction of the metallochromic indicators arsenazo III and antipyrylazo III to their free radical metabolites by cytoplasmic enzymes

At a concentration much lower than that usually employed for measuring cytosolic ionized Ca2+ concentrations, arsenazo III underwent a one-electron reduction by rat liver cytosolic fraction or a hypoxanthine-xanthine oxidase system to produce an azo anion radical metabolite. NADH, NADPH, N1-methylnicotinamide, hypoxanthine, and xanthine, in that order, could serve as a source of reducing equivalents for the production of this free radical by the cytosolic fraction. The steady-state concentration of the azo anion radical and the arsenazo III-stimulated O2 consumption were enhanced by calcium and magnesium. Antipyrylazo III was ineffective in increasing O2 consumption by rat liver cytosolic fraction and gave a much weaker ESR signal of an azo anion radical with both the liver cytosolic fraction, in the presence of NADH, and the hypoxanthine-xanthine oxidase system.

myo-Inositol polyphosphate may be a messenger for visual excitation in Limulus photoreceptors

Photoreceptor excitation begins with the absorption of a photon by rhodopsin and proceeds through an unknown sequence of steps that leads to changes in specific ionic conductances. These conductance changes produce the receptor potential. It has been proposed that hydrolysis of phosphoinositides is involved in the control of a variety of physiological processes. Recent studies have implicated inositol 1,4,5-trisphosphate as an intracellular messenger in the cascade mediating hormone-stimulated secretion. We propose that one of the steps in the excitatory cascade in Limulus ventral photoreceptors may be an increase in intracellular concentration of myo-inositol polyphosphates, derived from hydrolysis of the membrane component phosphatidylinositol bisphosphate by a phospholipase. Here we present biochemical and electrophysiological evidence that an inositol polyphosphate may be an intracellular messenger in the cascade mediating excitation, based on the following criteria: the cells possess the synthetic and degradative metabolism for the messenger; the natural stimulus leads to a change in the concentration of the messenger within the cells; and intracellular injection of exogenous messenger mimics naturally occurring electrophysiological events.