Intracellular calcium and its sodium-independent regulation in voltage-clamped snail neurones

The Ca(2+): H(+) coupling ratio of the plasma membrane calcium ATPase in neurones is little sensitive to changes in external or internal pH

To explore the effects of both external and internal pH (pH_o and pH_i) on the coupling between Ca²⁺ extrusion and H⁺ uptake by the PMCA activity in snail neurones H⁺ uptake was assessed by measuring surface pH changes (ΔpH_s) with pH-sensitive microelectrodes while Ba²⁺ or Ca²⁺ loads were extruded. Ru360 or ruthenium red injection showed that injected Ca²⁺ was partly taken up by mitochondria, but Ca²⁺ entering through channels was not. External pH was changed using a mixture of three buffers to minimise changes in buffering power. With depolarisation-induced Ca²⁺ or Ba²⁺ loads the ΔpH_s were not changed significantly over the pH range 6.5–8.5. With Ca²⁺ injections into cells with mitochondrial uptake blocked the ΔpH_s were significantly smaller at pH 8.5 than at 7.5, but this could be explained in part by the slower rate of activity of the PMCA. Low intracellular pH also changed the ΔpH_s responses to Ca²⁺ injection, but not significantly. Again this may have been due to reduced pump activity at low pH_i. I conclude that in snail neurones the PMCA coupling ratio is either insensitive or much less sensitive to pH than in red blood cells or barnacle muscle.

Genetically encoded optical sensors for monitoring of intracellular chloride and chloride-selective channel activity

This review briefly discusses the main approaches for monitoring chloride (Cl⁻), the most abundant physiological anion. Noninvasive monitoring of intracellular Cl⁻ ([Cl⁻]i) is a challenging task owing to two main difficulties: (i) the low transmembrane ratio for Cl⁻, approximately 10:1; and (ii) the small driving force for Cl⁻, as the Cl⁻ reversal potential (E_Cl) is usually close to the resting potential of the cells. Thus, for reliable monitoring of intracellular Cl⁻, one has to use highly sensitive probes. From several methods for intracellular Cl⁻ analysis, genetically encoded chloride indicators represent the most promising tools. Recent achievements in the development of genetically encoded chloride probes are based on the fact that yellow fluorescent protein (YFP) exhibits Cl⁻-sensitivity. YFP-based probes have been successfully used for quantitative analysis of Cl⁻ transport in different cells and for high-throughput screening of modulators of Cl⁻-selective channels. Development of a ratiometric genetically encoded probe, Clomeleon, has provided a tool for noninvasive estimation of intracellular Cl⁻ concentrations. While the sensitivity of this protein to Cl⁻ is low (EC₅₀ about 160 mM), it has been successfully used for monitoring intracellular Cl⁻ in different cell types. Recently a CFP–YFP-based probe with a relatively high sensitivity to Cl⁻ (EC₅₀ about 30 mM) has been developed. This construct, termed Cl-Sensor, allows ratiometric monitoring using the fluorescence excitation ratio. Of particular interest are genetically encoded probes for monitoring of ion channel distribution and activity. A new molecular probe has been constructed by introducing into the cytoplasmic domain of the Cl⁻-selective glycine receptor (GlyR) channel the CFP–YFP-based Cl-Sensor. This construct, termed BioSensor-GlyR, has been successfully expressed in cell lines. The new genetically encoded chloride probes offer means of screening pharmacological agents, analysis of Cl⁻ homeostasis and functions of Cl⁻-selective channels under different physiological and pathological conditions.

The plasma membrane calcium ATPase (PMCA) of neurones is electroneutral and exchanges 2 H+ for each Ca2+ or Ba2+ ion extruded

The coupling between Ca2+ extrusion and H+ uptake by the ubiquitous plasma membrane calcium ATPase (PMCA) has not been measured in any neurone. I have investigated this with Ca2+- and pH-sensitive microelectrodes in large voltage-clamped snail neurones, which have no Na+-Ca2+ exchangers. The recovery of [Ca2+]i and surface pH after a brief depolarization or Ca2+ injection was not slowed by hyperpolarization to -90 mV from a holding potential of -50 mV, consistent with a 1 Ca2+ : 2 H+ coupling ratio. Since Ca2+ injections proved difficult to quantify, and Ca2+ currents through channels were obscured by K+ currents, Ba2+ was used as a substitute. When the cell was bathed in Ca2+-free Ba2+ Ringer solution, the K+ currents were blocked and large inward currents were revealed on depolarization. The Ca2+-sensitive microelectrodes were sensitive to intracellular Ba2+ as well as Ca2+. With equal depolarizations Ba2+ entry appeared larger than Ca2+ entry and generated similar but slower pH changes. Ba2+ extrusion was insensitive to hyperpolarization, blocked by eosin or high pH, and about 5 times slower than Ca2+ extrusion. The ratio of the pH change caused by the extrusion of unit charge of Ba2+ influx to that caused by unit charge of H+ injection was 0.85 +/- 0.08 (s.e.m., n = 8), corresponding to a Ba2+ : H+ ratio of 1 : 1.7. Both this ratio and the electroneutrality of the PMCA suggest that the Ca2+ : H+ ratio is 1 : 2, ensuring that after a Ca2+ influx [Ca2+]i recovery is not influenced by the membrane potential and maximizes the conversion of Ca2+ influxes into possible pH signals.

Dynamic and static calcium gradients inside large snail (Helix aspersa) neurones detected with calcium-sensitive microelectrodes

We have used quartz Ca²⁺-sensitive microelectrodes (CASMs) in large voltage-clamped snail neurones to investigate the inward spread of Ca²⁺ after a brief depolarisation. Both steady state and [Ca²⁺]_i transients changed with depth of penetration. When the CASM tip was within 20 μm of the far side of the cell the [Ca²⁺]_i transient time to peak was 4.4 ± 0.5 s, rising to 14.7 ± 0.7 s at a distance of 80 μm. We estimate that the Ca²⁺ transients travelled centripetally at an average speed of 6 μm² s⁻¹ and decreased in size by half over a distance of about 45 μm. Cyclopiazonic acid had little effect on the size and time to peak of Ca²⁺ transients but slowed their recovery significantly. This suggests that the endoplasmic reticulum curtails rather than reinforces the transients. Injecting the calcium buffer BAPTA made the Ca²⁺ transients more uniform in size and increased their times to peak and rates of recovery near the membrane. We have developed a computational model for the transients, which includes diffusion, uptake and Ca²⁺ extrusion. Good fits were obtained with a rather large apparent diffusion coefficient of about 90 ± 20 μm² s⁻¹.This may assist fast recovery by extrusion.

Electrical coupling induces bistability of rhythms in networks of inhibitory spiking neurons

Information processing in higher brain structures is thought to rely on the synchronization of spiking neurons. Increasing evidence indicates that, within these structures, inhibitory neurons are linked by both chemical and electrical synapses. However, how synchronized states may emerge from such circuits is not fully understood. Using snail neurons interconnected through a dynamic-clamp system, we show that networks of spiking neurons linked by both reciprocal inhibition and electrical coupling can express two coexisting coordination patterns of different rhythms. One of these patterns consists of antiphase firing of the network partners whereas, in the other, neurons fire synchronously. Switching between patterns may be evoked immediately by transient stimuli, demonstrating bistability of the network. Thus electrical coupling can provide a potent way for instantaneous reconfiguration of activity patterns in inhibitory spiking networks without alteration of intrinsic network properties by modulatory processes.

Ca2+ clearance at growth cones produced by crayfish motor axons in an explant culture

Intracellular free Ca2+ concentration ([Ca2+]i) plays an important role in the regulation of growth cone (GC) motility; however, the mechanisms responsible for clearing Ca2+ from GCs have not been examined. We studied the Ca2+-clearance mechanisms in GCs produced by crayfish tonic and phasic motor axons by measuring the decay of [Ca2+]i after a high [K+] depolarizing pulse using fura-2AM. Tonic motor axons regenerating in explant cultures develop GCs with more rapid Ca2+ clearance than GCs from phasic axons. When Na/Ca exchange was blocked by replacing external Na+ with N-methyl-d-glucamine (NMG), [Ca2+]i decay was delayed in both tonic and phasic GCs. Tonic GCs appear to have higher Na/Ca exchange activity than phasic ones since reversal of Na/Ca exchange by lowering external Na+ caused a greater increase in [Ca2+]i for tonic than phasic GCs. Application of the mitochondrial inhibitors, Antimycin A1 (1 microM) and CCCP (10 microM), demonstrated that mitochondrial Ca2+ uptake/release was more prominent in phasic than tonic GCs. When both Na/Ca exchange and mitochondria were inhibited, the plasma membrane Ca2+ ATPase was effective in extruding Ca2+ from tonic, but not phasic GCs. We conclude that Na/Ca exchange plays a prominent role in extruding large Ca2+ loads from both tonic and phasic GCs. High Na/Ca exchange activity in tonic GCs contributes to the rapid decay of [Ca2+]i in these GCs; low rates of Ca2+ extrusion plus the release of Ca2+ from mitochondria prolongs the decay of [Ca2+]i in the phasic GCs.

Ca2+ Clearance at Growth Cones Produced by Crayfish Motor Axons in an Explant Culture

Intracellular free Ca2+ concentration ([Ca2+]i) plays an important role in the regulation of growth cone (GC) motility; however, the mechanisms responsible for clearing Ca2+ from GCs have not been examined. We studied the Ca2+-clearance mechanisms in GCs produced by crayfish tonic and phasic motor axons by measuring the decay of [Ca2+]i after a high [K+] depolarizing pulse using fura-2AM. Tonic motor axons regenerating in explant cultures develop GCs with more rapid Ca2+ clearance than GCs from phasic axons. When Na/Ca exchange was blocked by replacing external Na+ with N-methyl-d-glucamine (NMG), [Ca2+]i decay was delayed in both tonic and phasic GCs. Tonic GCs appear to have higher Na/Ca exchange activity than phasic ones since reversal of Na/Ca exchange by lowering external Na+ caused a greater increase in [Ca2+]i for tonic than phasic GCs. Application of the mitochondrial inhibitors, Antimycin A1 (1 μM) and CCCP (10 μM), demonstrated that mitochondrial Ca2+ uptake/release was more prominent in phasic than tonic GCs. When both Na/Ca exchange and mitochondria were inhibited, the plasma membrane Ca2+ ATPase was effective in extruding Ca2+ from tonic, but not phasic GCs. We conclude that Na/Ca exchange plays a prominent role in extruding large Ca2+ loads from both tonic and phasic GCs. High Na/Ca exchange activity in tonic GCs contributes to the rapid decay of [Ca2+]i in these GCs; low rates of Ca2+ extrusion plus the release of Ca2+ from mitochondria prolongs the decay of [Ca2+]i in the phasic GCs.

The effects of HCl and CaCl(2) injections on intracellular calcium and pH in voltage-clamped snail (Helix aspersa) neurons

To investigate the mechanisms by which low intracellular pH influences calcium signaling, I have injected HCl, and in some experiments CaCl(2), into snail neurons while recording intracellular pH (pH(i)) and calcium concentration ([Ca(2+)](i)) with ion-sensitive microelectrodes. Unlike fluorescent indicators, these do not increase buffering. Slow injections of HCl (changing pH(i) by 0.1-0.2 pH units min(-1)) first decreased [Ca(2+)](i) while pH(i) was still close to normal, but then increased [Ca(2+)](i) when pH(i) fell below 6.8-7. As pH(i) recovered after such an injection, [Ca(2+)](i) started to fall but then increased transiently before returning to its preinjection level. Both the acid-induced decrease and the recovery-induced increase in [Ca(2+)](i) were abolished by cyclopiazonic acid, which empties calcium stores. Caffeine with or without ryanodine lowered [Ca(2+)](i) and converted the acid-induced fall in [Ca(2+)](i) to an increase. Injection of ortho-vanadate increased steady-state [Ca(2+)](i) and its response to acidification, which was again blocked by CPA. The normal initial response to 10 mM caffeine, a transient increase in [Ca(2+)](i), did not occur with pH(i) below 7.1. When HCl was injected during a series of short CaCl(2) injections, the [Ca(2+)](i) transients (recorded as changes in the potential (V(Ca)) of the Ca(2+)-sensitive microelectrode), were reduced by only 20% for a 1 pH unit acidification, as was the rate of recovery after each injection. Calcium transients induced by brief depolarizations, however, were reduced by 60% by a similar acidification. These results suggest that low pH(i) has little effect on the plasma membrane calcium pump (PMCA) but important effects on the calcium stores, including blocking their response to caffeine. Acidosis inhibits spontaneous calcium release via the RYR, and leads to increased store content which is unloaded when pH(i) returns to normal. Spontaneous release is enhanced by the rise in [Ca(2+)](i) caused by inhibiting the PMCA.

The effect of calcium pump inhibitors on the response of intracellular calcium to caffeine in snail neurones

We have measured intracellular free calcium ([Ca(2+)]i) using Fura-2 or Ca(2+)-sensitive microelectrodes in voltage-clamped neurones of the snail, Helix aspersa. Caffeine-induced transient increases in [Ca(2+)]i were normally followed by a brief fall of [Ca(2+)]i below its pre-caffeine level. We investigated the cause of this undershoot by raising [Ca(2+)]i; and by inhibiting the plasma membrane or endoplasmic reticulum Ca ATPases (PMCA or SERCA respectively). When the cell membrane potential was decreased from -60 to -25mV, steady-state [Ca(2+)]i increased. The caffeine-induced transients were smaller while the undershoots were larger than in control conditions. When the PMCA was inhibited by high pH the steady-state [Ca(2+)]i increased by 100-400nM. The caffeine-induced [Ca(2+)]i increase and the subsequent undershoot both became larger. Injection of orthovanadate, which inhibits the PMCA and increases [Ca(2+)]i, did not block either effect of caffeine. But when the SERCA was inhibited by cyclopiazonic acid the undershoot disappeared. The phosphodiesterase inhibitor IBMX did not influence the undershoot. These results suggest that the undershoot is generated by the Ca(2+)] ATPase of the stores rather than that of the plasma membrane. Since the undershoot increased as [Ca(2+)]i increased, we conclude that at higher levels of [Ca(2+)]i the stores refill more rapidly.

The ouabain-induced [Ca2+]i increase in leech Retzius neurones is mediated by voltage-dependent Ca2+ channels

In leech Retzius neurones the inhibition of the Na+/K+ pump by ouabain causes an increase in the cytosolic free calcium concentration ([Ca2+]i). To elucidate the mechanism of this increase we investigated the changes in [Ca2+]i (measured by Fura-2) and in membrane potential that were induced by inhibiting the Na+/K+ pump in bathing solutions of different ionic composition. The results show that Na+/K+ pump inhibition induced a [Ca2+]i increase only if the cells depolarized sufficiently in the presence of extracellular Ca2+. Specifically, the relationship between [Ca2+]i and the membrane potential upon Na+/K+ pump inhibition closely matched the corresponding relationship upon activation of the voltage-dependent Ca2+ channels by raising the extracellular K+ concentration. It is concluded that the [Ca2+]i increase caused by inhibiting the Na+/K+ pump in leech Retzius neurones is exclusively due to Ca2+ influx through voltage-dependent Ca2+ channels.

The effects of intracellular pH changes on resting cytosolic calcium in voltage-clamped snail neurones

We have investigated the effects of changing intracellular pH on intracellular free calcium concentration ([Ca2+]i) in voltage-clamped neurones of the snail Helix aspersa. Intracellular pH (pHi) was measured using the fluorescent dye 8-hydroxypyrene-1,3,6-trisulphonic acid (HPTS) and changed using weak acids and weak bases. Changes in [Ca2+]i were recorded using either fura-2 or calcium-sensitive microelectrodes. Acidification of the neurones with 5 mM or 20 mM propionate (approximately 0.2 or 0.3 pH units acidification, respectively) caused a small reduction in resting [Ca2+]i of 5 +/- 2 nM (n = 4) and 7 +/- 16 nM (n = 4), respectively. The removal of the 20 mM propionate after approximately 40 min superfusion resulted in an alkalinization of approximately 0.35 pH units and an accompanying rise in resting [Ca2+]i of 31 +/- 9 nM (n = 4, P < 0.05). The removal of 5 mM propionate did not significantly affect [Ca2+]i. Alkalinizations of approximately 0.2-0.4 pH units of Helix neurones induced by superfusion with 3 mM concentrations of the weak bases trimethylamine (TMA), ammonium chloride (NH4Cl) and procaine were accompanied by significant (P < 0.05) increases in resting [Ca2+]i of 42 +/- 4 nM (n = 26), 30 +/- 7 nM (n = 5) and 36 +/- 4 nM (n = 3), respectively. The effect of TMA (0.5-6 mM) on [Ca2+]i was dose dependent with an increase in [Ca2+]i during pHi increases of less than 0.1 pH units (0.5 mM TMA). Superfusion of neurones with zero calcium (1 mM EGTA) Ringer solution inhibited depolarization-induced calcium increases but not the calcium increase produced by the first exposure to TMA (3 mM). In the prolonged absence of extracellular calcium (approximately 50 min) TMA-induced calcium rises were decreased by 64 +/- 10% compared to those seen in the presence of external calcium (P < 0.05). The calcium rise induced by TMA (3 mM) was reduced by 60 +/- 5% following a 10 min period of superfusion with caffeine (10 mM) to deplete the endoplasmic reticulum (ER) stores of calcium (P < 0.05). Cyclopiazonic acid (10-30 microM CPA), an inhibitor of the ER calcium pump, inhibited the calcium rise produced by TMA (3 mM) and NH4Cl (3 mM) by 61 +/- 4% compared to controls (P < 0.05). These data are consistent with physiological intracellular alkaline shifts stimulating release of calcium, or inhibiting re-uptake of calcium by an intracellular store. The calcium increase was much reduced following application of caffeine, treatment with CPA or prolonged removal of external calcium. Hence the ER was likely to be the source of mobilized calcium.

Role played by sodium in activity-dependent secretion of neurotrophins - revisited

In previous experiments, a causal relationship between sodium influx and secretion of nerve growth factor (NGF) was deduced from the observation that the sodium substitute N-methyl-D-glucamine (NMDG) abolished any activity-mediated NGF secretion that depends on intact internal calcium stores. However, all available experimental evidence speaks against sodium-mediated calcium mobilization from these stores under physiological conditions. We now report that rapid sodium influx initiated by monensin or ouabain did not induce brain-derived neurotrophic factor (BDNF) secretion from either native hippocampal slices or BDNF-transduced hippocampal neuronal cultures. Additionally, we found marked differences between the replacement of sodium by NMDG and sucrose on the one hand, and choline and lithium on the other. Replacement of 100% (and as little as 10%) sodium by NMDG or sucrose not only blocked the activity-mediated neurotrophin secretion, but itself led to a rapid and substantial increase of neurotrophin secretion. In contrast, the replacement of sodium (10% and 100%) by lithium and choline did not result in a release of neurotrophins, and only 100% replacement blocked the activity-mediated neurotrophin secretion. We conclude that the blocking effects of NMDG and sucrose on neurotrophin secretion do not reflect the sodium replacement, but instead represent an independent blocking effect. These differences were also reflected in part by electrophysiological investigations in individually patched hippocampal neurons. The importance of the present observations lies not only in the reevaluation of the involvement of sodium in activity-dependent neurotrophin secretion, but also in the demonstration that sodium replacement may initiate 'side effects' that are unrelated to sodium replacement.

Ratiometric confocal Ca2+ measurements with visible wavelength indicators in d-amphetamine-treated central snail neuron

1. The bursting firing of action potentials and cytosolic calcium content in d-amphetamine-treated RP4 neurons of the African snail, Achatina fulica Ferussac, were studied elelctrophysiologically and optopharmacologically. 2. d-Amphetamine elicited bursting of firing of action potentials in central RP4 neuron. The bursting firing of action potentials was blocked following extracellular application of ruthenium red, the calcium uptake and release inhibitor from mitochondria. 3. At the same neuron, image analysis using ratiometric measurement on calcium green 1 and Texas red dextran (70,000 MW) with laser-scanning confocal microscopy revealed that cytosolic calcium content was increased after d-amphetamine treatment. 4. Our results support the view that the bursting firing of action potentials elicited by d-amphetamine is associated with cytosolic calcium content.

Trifluoperazine enhancement of Ca2+-dependent inactivation of L-type Ca2+ currents in Helix aspersa neurons

The effects of trifluoperazine hydrochloride (TFP), a calmodulin antagonist, on L-type Ca2+ currents (L-type ICa2+) and their Ca(2+)-dependent inactivation, were studied in identified Helix aspersa neurons, using two microelectrode voltage clamp. Changes in [Ca2+]i were measured in unclamped fura-2 loaded neurons. Bath applied TFP produced a reversible and dose-dependent reduction in amplitude of L-type ICa2+ (IC50 = 28 microM). Using a double-pulse protocol, we found that TFP enhances the efficacy of Ca(2+)-dependent inactivation of L-type ICa2+. Trifluoperazine sulfoxide (50 microM), a TFP derivative with low calmodulin-antagonist activity, did not have any effects on either amplitude or inactivation of L-type ICa2+. TFP (20 microM) increased basal [Ca2+]i from 147 +/- 37 nM to 650 +/- 40 nM (N = 7). The increase in [Ca2+]i was prevented by removal of external Ca2+ and curtailed by depletion of caffeine-sensitive intracellular Ca2+ stores. Since TFP may also block protein kinase C (PKC), we tested the effect of a PKC activator (12-C-tetradecanoyl-phorbol-13-acetate) on L-type Ca2+ currents. This compound produced an increase in L-type ICa2+ without enhancing Ca(2+)-dependent inactivation. The results show that 1) TFP reduces L-type ICa2+ while enhancing the efficacy of Ca(2+)-dependent inactivation. 2) TFP produces an increase in basal [Ca2+]i which may contribute to the enhancement of Ca(2+)-dependent inactivation. 3) PKC up-regulates L-type ICa2+ without altering the efficacy of Ca(2+)-dependent inactivation. 4) The TFP effects cannot be attributed to its action as PKC blocker.

Intracellular Ca2+ regulation by the leech giant glial cell

1. We have measured the intracellular Ca2+ concentration, [Ca2+]i, and the intracellular Na+ concentration, [Na+]i, with the fluorescent dyes fura-2 (for Ca2+) and SBFI (for Na+) in situ in giant glial cells of the central nervous system of the leech Hirudo medicinalis. 2. The basal [Ca2+]i was 79 +/- 35 nM (n = 27) in cells voltage clamped at -70 to -80 mV, and 75 +/- 29 nM (mean +/- S.D., n = 82) in unclamped cells at a mean membrane potential of -67 +/- 6 mV. 3. Removal of external Na+ evoked a small reversible [Ca2+]i increase of 29 +/- 21 nM (n = 27) in cells voltage clamped at -70 to -80 mV, and of 35 +/- 18 nM (n = 37) in unclamped cells. This [Ca2+]i increase, and the time constant of the subsequent [Ca2+]i recovery after Na+ re-addition, did not change significantly with the holding potential between -110 and -60 mV. 4. The basal [Na+]i was 5.6 +/- 1.3 mM (n = 18). Increasing [Na+]i by inhibiting the Na+-K+ pump with 100 microM ouabain had no effect on the [Ca2+]i rise upon removal of external Na+. 5. The time course of recovery from a [Ca2+]i load mediated by voltage-dependent Ca2+ influx during depolarization in high K+ was unaffected by the removal of external Na+. 6. Cyclopiazonic acid (10 muM), an inhibitor of the endoplasmic reticulum Ca2+-ATPase, caused a transient increase in [Ca2+]i of 28 +/- 11 nM (n = 5), and significantly slowed the recovery from imposed [Ca2+]i loads. 7. Iontophoretic injection of orthovanadate, an inhibitor of P-type ATPases including the plasma membrane Ca2+-ATPase, caused a persistent increase in the basal [Ca2+]i of 163 +/- 101 nM (n = 5) in standard saline, and of 427 +/- 338 nM in Na+-free saline (n = 5). Vanadate injection significantly slowed the recovery from [Ca2+]i loads. Removal of external Na+ during vanadate injection induced an additional, reversible [Ca2+]i increase of 254 +/- 64 nM (n = 3). 8. The results suggest that the low basal [Ca2+]i in these glial cells is predominantly maintained by a Ca2+-ATPase in the plasma membrane. This ATPase is also the main Ca2+ extruder after an intracellular Ca2+ load, while intracellular stores appear to contribute little to this recovery. A Na+-Ca2+ exchanger seems to play a minor role in the maintenance of basal [Ca2+]i in these cells, but becomes prominent when the plasma membrane Ca2+-ATPase is blocked.

Calcium homeostatic mechanisms operating in cultured postnatal rat hippocampal neurones following flash photolysis of nitrophenyl-EGTA

1. We examined Ca2+ homeostatic mechanisms in cultured postnatal rat hippocampal neurones by monitoring the recovery of background-subtracted fluo-3 fluorescence levels at 20-22 degrees C immediately following a rapid increase in Ca2+ levels induced by flash photolysis of the caged Ca2+ compound nitrophenyl-EGTA (NP-EGTA). 2. A variety of methods or drugs were used in attempt to block specifically efflux of Ca2+ by the plasmalemmal Na(+)-Ca2+ exchanger or uptake of Ca2+ into mitochondria. 3. Many of the experimental manipulations produced a decrease in intracellular pH (pHi) measured in sister cultures using the pH-sensitive dye 2',7'-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein (BCECF). Accordingly, in each case, we determined the appropriate amount of the weak base trimethylamine (TMA) required to restore baseline pHi prior to flash photolysis. 4. Blockade of the plasmalemmal Na(+)-Ca2+ exchanger by replacement of external Na+ with either Li+ or N-methyl-D-glucamine (NMDG) markedly reduced pHi but did not affect the rate of recovery of fluo-3 fluorescence intensities once pHi was restored. 5. Inhibition of mitochondrial Ca2+ uptake, using the protonophore carbonyl cyanide m-chloro-phenylhydrazone (CCCP), resulted in a reduction in pHi, which could be restored by the addition of 2 mM TMA. Under these conditions the rate of recovery of Ca2+ levels was significantly slower than in the controls. Similar results were found using the respiratory chain inhibitor rotenone. 6. We conclude that, when the potential effects of changes in pHi are taken into account, mitochondria appear to sequester significant amounts of Ca2+ in the neuronal preparations used.

Ca2+ influx and activation of a cation current are coupled to intracellular Ca2+ release in peptidergic neurons of Aplysia californica

1. Stimulation of inputs to bag cell neurons in the abdominal ganglion of Aplysia californica causes an increase in their intracellular Ca2+ concentration ([Ca2+]i). We have used thapsigargin, a specific inhibitor of the endoplasmic reticulum Ca2+ pump, to analyse the effects of Ca2+ released from intracellular stores on the electrophysiological responses of bag cell neurons. 2. Using digital imaging of fura-2-loaded isolated bag cell neurons we found that thapsigargin rapidly evoked an increase in [Ca2+]i in somata, with smaller increases in neurites. Thapsigargin-induced elevation of [Ca2+]i peaked at about 1 microM within 5-10 min and then decayed to basal levels by 30 min. 3. Placement of an extracellular vibrating Ca(2+)-selective microelectrode to within 1 micron of somata revealed a relatively large steady-state Ca2+ efflux. Thapsigargin produced a rapid increase in Ca2+ influx. Changes in Ca2+ flux were not detected at neurites. 4. Thapsigargin produced a small depolarization in isolated bag cell neurons in artificial sea water (ASW). Sometimes enhanced depolarizations were observed when extracellular Na+ was replaced by TEA or Tris, but not N-methyl-D-glucamine (NMDG). The depolarization was not blocked by 100 microM tetrodotoxin (TTX), removal of extracellular Ca2+ (0.5 mM EGTA) or addition of 10 mM Co2+ to the bath solution. 5. In voltage-clamp experiments, thapsigargin induced an inward current (ITg) that was recorded in Ca(2+)-free media containing TEA or Tris substituted for Na+. The apparent reversal potential of ITg was -16.8 +/- 1.2 mV in TEA-ASW. Induction of ITg was inhibited in neurons that were microinjected with the Ca2+ chelator BAPTA-Dextran70 or treated with the membrane-permeant analogue BAPTA AM. Activation of ITg was not observed when Na+ was replaced with NMDG. Manipulation of [Na+]o and [K+]o produced shifts in the reversal potential of ITg consistent with the underlying channels being permeable to both Na+ and K+. 6. Thapsigargin did not alter the amplitude or kinetics of voltage-activated Ba2+ currents, but in some experiments it did increase the amplitude of a component of outward K+ current. 7. Thapsigargin neither induced bag cell neurons within the intact ganglion to depolarize and fire spontaneously, nor did it alter the frequency or duration of firing of an electrically stimulated bag cell after-discharge. 8. We conclude that thapsigargin-sensitive Ca2+ pools are present predominantly in the somata of bag cell neurons. Ca2+ that is released from thapsigargin-sensitive Ca2+ stores activates a non-selective cation current that may help sustain depolarization of the somata, but does not by itself trigger an after-discharge.

Effects of injecting calcium-buffer solution on [Ca2+]i in voltage-clamped snail neurons

We have investigated why fura-2 and Ca(2+)-sensitive microelectrodes report different values for the intracellular free calcium ion concentration ([Ca(2+)]i or its negative log, pCa(i)) of snail neurons voltage-clamped to -50 or -60 mV. Both techniques were initially calibrated in vitro, using calcium calibration solutions that had ionic concentrations similar to those of snail neuron cytoplasm. Pressure injections of the same solutions at resting and elevated [Ca(2+)]i were used to calibrate both methods in vivo. In fura-2-loaded cells these pressure injections generated changes in [Ca(2+)]i that agreed well with those expected from the in vitro calibration. Thus, using fura-2 calibrated in vitro, the average resting [Ca(2+)]i was found to be 38 nM (pCa(i) 7.42 +/- 0.05). With Ca(2+)-sensitive microelectrodes, the first injection of calibration solutions always caused a negative shift in the recorded microelectrode potential, as if the injection lowered [Ca2+]i. No such effects were seen on the fura-2 ratio. When calibrated in vivo the Ca(2+)-sensitive microelectrode gave an average resting [Ca2+]i of approximately 25 nM (pCa(i) 7.6 +/- 0.1), much lower than when calibrated in vitro. We conclude that [Ca(2+)]i in snail neurons is approximately 40 nM and that Ca(2+)-sensitive microelectrodes usually cause a leak at the point of insertion. The effects of the leak were minimized by injection of a mobile calcium buffer.

Relationship between intracellular calcium and its muffling measured by calcium iontophoresis in snail neurones

1. We have measured intracellular free calcium ion concentration ([Ca2+]i) with fura-2, and intracellular chloride with chloride-sensitive microelectrodes, in voltage-clamped snail neurones. By making iontophoretic injections of CaCl2 we have investigated calcium muffling, the sum of the processes which minimize the calcium transient, at different values of [Ca2+]i. 2. By injection of calcium into cell-sized droplets of buffer we measured the calcium transport index. It was stable over the range pCa 6-7.4 (0.48 +/- 0.06 measured at pCa 6.70 +/- 0.12, n = 5). 3. Measurement of intracellular chloride activity during a series of fura-2-KCl pressure injections revealed a nearly linear relationship between fura-2 Ca(2+)-insensitive fluorescence and the sum of the increments in intracellular chloride. This allowed us to calculate the intracellular fura-2 concentration ([fura-2]i). 4. The rate of recovery of [Ca2+]i following a depolarization-induced load was increased by low [fura-2]i (10-20 microM) but decreased by higher [fura-2]i (40-80 microM). These effects are consistent with the addition of a mobile buffer to the cytoplasm. 5. Iontophoresis of Ca2+ at various membrane potentials allowed us to calculate the intracellular calcium muffling power (the amount of calcium required to cause a transient tenfold increase in [Ca2+]i per unit volume) and calcium muffling ratio (number of Ca2+ ions injected divided by the maximum increase in [Ca2+]i per unit volume) at different values of [Ca2+]i. 6. Calcium muffling power at resting [Ca2+]i was approximately 40 microM Ca2+ (pCa unit)-1, (about 250 times less than for hydrogen ions). It increased linearly about fivefold with [Ca2+]i over the range 20-120 nM (10 cells, 153 measurements) and therefore exponentially with decreasing pCa. 7. The calcium muffling ratio appeared to be constant (361 +/- 14, n = 10 cells, 130 measurements) over the range 20-120 nM Ca2+. 8. In three experiments we modelled the additional calcium buffering power produced by multiple pressure injections of fura-2 into voltage-clamped snail neurones. Back-extrapolation of the increases in calcium buffering power allowed us to calculate the calcium muffling power of the neurones. 9. Small increases in [fura-2]i (approximately 10 microM) significantly increased intracellular calcium muffling power in individual experiments. However, the variability among neurones in intracellular calcium muffling power was large enough to obscure the additional buffering produced by fura-2 in pooled experiments.

Effects of low doses of caffeine on [Ca2+]i in voltage-clamped snail (Helix aspersa) neurones

1. We have measured cytosolic free Ca2+ concentrations ([Ca2+]i) in voltage-clamped snail neurones using fura-2. Transient increases in [Ca2+]i were induced by depolarizing voltage steps of 20-60 mV for 0.1-10 s from a holding potential of -50 or -60 mV. 2. Low doses of caffeine, 0.2-1 mM, increased the size of the [Ca2+]i transients by both increasing the peak and producing an undershoot. 3. Ryanodine, an inhibitor of Ca2+ release from the intracellular Ca2+ stores, and cyclopiazonic acid (CPA), an inhibitor of the Ca(2+)-ATPase of the intracellular Ca2+ stores, both reduced the size of the [Ca2+]i transients and blocked the effects of caffeine on the transients. 4. The effects of caffeine and CPA were greater on transients produced by long, small, rather than short, large depolarizations. This suggests that calcium-induced calcium release (CICR) played a greater role in the [Ca2+]i increase resulting from longer, smaller depolarizations. 5. Increasing the extracellular pH from 7.5 to over 9, which inhibits the plasmalemmal Ca(2+)-H(+)-ATPase, increased the resting [Ca2+]i level. Depolarization-induced [Ca2+]i transients became much larger while the two effects of caffeine remained. CPA was ineffective at high pH. 6. In some experiments the increase in basal [Ca2+]i caused by alkaline pH was reduced by 0.2 or 0.5 mM caffeine. The increase in basal [Ca2+]i caused by maintained depolarization was reduced, after a transient increase, by 0.5 mM caffeine. Both reduction and increase were blocked by CPA. 7. We conclude that low doses of caffeine can increase uptake by intracellular Ca2+ stores. Caffeine could also release Ca2+ from ryanodine-insensitive Ca(2+)-ATPase-dependent stores as well as facilitating normal ryanodine-sensitive CICR.

Localization and functional significance of the Na+/Ca2+ exchanger in presynaptic boutons of hippocampal cells in culture

Immunocytochemical evidence for localized distribution of the Na+/Ca2+ exchange protein in nerve terminals of cultured hippocampal cells is presented together with results on the functional relevance of the exchanger in the control of [Ca2+]i and of synaptic vesicle recycling. The monoclonal antibody R3F1, directed against an epitope on the intracellular loop of the protein, revealed higher densities of expression in synaptic regions than in other parts of the neurons. Removal of extracellular Na+ produced enhanced and prolonged elevation of [Ca2+]i in nerve terminals during and after electrical stimulation of the cells. Correspondingly, initial rates of exocytosis, measured by fluorescence changes of FM 1-43 during stimulation, were faster in LiCl-containing solution than in NaCl-containing solution. By contrast, endocytosis at 20 s was the same in both solutions.