Basal Ca2+ and the oscillation of Ca2+ in caffeine-treated bullfrog sympathetic neurones

Caffeine-induced Ca2+ oscillations in type I horizontal cells of the carp retina and the contribution of the store-operated Ca2+ entry pathway

The mechanisms of release, depletion, and refilling of endoplasmic reticulum (ER) Ca²⁺ were investigated in type I horizontal cells of the carp retina using a fluo-3-based Ca²⁺ imaging technique. Exogenous application of caffeine, a ryanodine receptor agonist, induced oscillatory intracellular free Ca²⁺ concentration ([Ca²⁺]_i) responses in a duration- and concentration-dependent manner. In Ca²⁺-free Ringer’s solution, [Ca²⁺]_i transients could also be induced by a brief caffeine application, whereas subsequent caffeine application induced no [Ca²⁺]_i increase, which implied that extracellular Ca²⁺ was required for ER refilling, confirming the necessity of a Ca²⁺ influx pathway for ER refilling. Depletion of ER Ca²⁺ by thapsigargin triggered a Ca²⁺ influx which could be blocked by the store-operated channel inhibitor 2-APB, which proved the existence of the store-operated Ca²⁺ entry pathway. Taken together, these results suggested that after being depleted by caffeine, the ER was replenished by Ca²⁺ influx via store-operated channels. These results reveal the fine modulation of ER Ca²⁺ signaling, and the activation of the store-operated Ca²⁺ entry pathway guarantees the replenishment of the ER so that the cell can be ready for response to the subsequent stimulus.

Store-operated Ca2+ entry in sensory neurons: functional role and the effect of painful nerve injury

Painful nerve injury disrupts levels of cytoplasmic and stored Ca(2+) in sensory neurons. Since influx of Ca(2+) may occur through store-operated Ca(2+) entry (SOCE) as well as voltage- and ligand-activated pathways, we sought confirmation of SOCE in sensory neurons from adult rats and examined whether dysfunction of SOCE is a possible pathogenic mechanism. Dorsal root ganglion neurons displayed a fall in resting cytoplasmic Ca(2+) concentration when bath Ca(2+) was withdrawn, and a subsequent elevation of cytoplasmic Ca(2+) concentration (40 ± 5 nm) when Ca(2+) was reintroduced, which was amplified by store depletion with thapsigargin (1 μm), and was significantly reduced by blockers of SOCE, but was unaffected by antagonists of voltage-gated membrane Ca(2+) channels. We identified the underlying inwardly rectifying Ca(2+)-dependent I(CRAC) (Ca(2+) release activated current), as well as a large thapsigargin-sensitive inward current activated by withdrawal of bath divalent cations, representing SOCE. Molecular components of SOCE, specifically STIM1 and Orai1, were confirmed in sensory neurons at both the transcript and protein levels. Axonal injury by spinal nerve ligation (SNL) elevated SOCE and I(CRAC). However, SOCE was comparable in injured and control neurons when stores were maximally depleted by thapsigargin, and STIM1 and Orai1 levels were not altered by SNL, showing that upregulation of SOCE after SNL is driven by store depletion. Blockade of SOCE increased neuronal excitability in control and injured neurons, whereas injured neurons showed particular dependence on SOCE for maintaining levels of cytoplasmic and stored Ca(2+), which indicates a compensatory role for SOCE after injury.

Axotomy depletes intracellular calcium stores in primary sensory neurons

BACKGROUND

The cellular mechanisms of neuropathic pain are inadequately understood. Previous investigations have revealed disrupted Ca signaling in primary sensory neurons after injury. The authors examined the effect of injury on intracellular Ca stores of the endoplasmic reticulum, which critically regulate the Ca signal and neuronal function.

METHODS

Intracellular Ca levels were measured with Fura-2 or mag-Fura-2 microfluorometry in axotomized fifth lumbar (L5) dorsal root ganglion neurons and adjacent L4 neurons isolated from hyperalgesic rats after L5 spinal nerve ligation, compared to neurons from control animals.

RESULTS

Endoplasmic reticulum Ca stores released by the ryanodine-receptor agonist caffeine decreased by 46% in axotomized small neurons. This effect persisted in Ca-free bath solution, which removes the contribution of store-operated membrane Ca channels, and after blockade of the mitochondrial, sarco-endoplasmic Ca-ATPase and the plasma membrane Ca ATPase pathways. Ca released by the sarco-endoplasmic Ca-ATPase blocker thapsigargin and by the Ca-ionophore ionomycin was also diminished by 25% and 41%, respectively. In contrast to control neurons, Ca stores in axotomized neurons were not expanded by neuronal activation by K depolarization, and the proportionate rate of refilling by sarco-endoplasmic Ca-ATPase was normal. Luminal Ca concentration was also reduced by 38% in axotomized neurons in permeabilized neurons. The adjacent neurons of the L4 dorsal root ganglia showed modest and inconsistent changes after L5 spinal nerve ligation.

CONCLUSIONS

Painful nerve injury leads to diminished releasable endoplasmic reticulum Ca stores and a reduced luminal Ca concentration. Depletion of Ca stores may contribute to the pathogenesis of neuropathic pain.

Depletion of calcium stores regulates calcium influx and signal transmission in rod photoreceptors

Tonic synapses are specialized for sustained calcium entry and transmitter release, allowing them to operate in a graded fashion over a wide dynamic range. We identified a novel plasma membrane calcium entry mechanism that extends the range of rod photoreceptor signalling into light-adapted conditions. The mechanism, which shares molecular and physiological characteristics with store-operated calcium entry (SOCE), is required to maintain baseline [Ca(2+)](i) in rod inner segments and synaptic terminals. Sustained Ca(2+) entry into rod cytosol is augmented by store depletion, blocked by La(3+) and Gd(3+) and suppressed by organic antagonists MRS-1845 and SKF-96365. Store depletion and the subsequent Ca(2+) influx directly stimulated exocytosis in terminals of light-adapted rods loaded with the activity-dependent dye FM1-43. Moreover, SOCE blockers suppressed rod-mediated synaptic inputs to horizontal cells without affecting presynaptic voltage-operated Ca(2+) entry. Silencing of TRPC1 expression with small interference RNA disrupted SOCE in rods, but had no effect on cone Ca(2+) signalling. Rods were immunopositive for TRPC1 whereas cone inner segments immunostained with TRPC6 channel antibodies. Thus, SOCE modulates Ca(2+) homeostasis and light-evoked neurotransmission at the rod photoreceptor synapse mediated by TRPC1.

Physiology and Pathophysiology of the Calcium Store in the Endoplasmic Reticulum of Neurons

The endoplasmic reticulum (ER) is the largest single intracellular organelle, which is present in all types of nerve cells. The ER is an interconnected, internally continuous system of tubules and cisterns, which extends from the nuclear envelope to axons and presynaptic terminals, as well as to dendrites and dendritic spines. Ca2+release channels and Ca2+pumps residing in the ER membrane provide for its excitability. Regulated ER Ca2+release controls many neuronal functions, from plasmalemmal excitability to synaptic plasticity. Enzymatic cascades dependent on the Ca2+concentration in the ER lumen integrate rapid Ca2+signaling with long-lasting adaptive responses through modifications in protein synthesis and processing. Disruptions of ER Ca2+homeostasis are critically involved in various forms of neuropathology.

Repetitive application of caffeine sensitizes caffeine-induced Ca2+ release in bullfrog sympathetic ganglion neurons

Cytosolic free calcium concentration ([Ca(2+)](i)) was recorded from cultured bullfrog sympathetic ganglion cells loaded with the Ca(2+)-indicator Fura-2 or Fura-6F. Repetitive application of caffeine at a low concentration, which either failed to produce any [Ca(2+)](i) elevation or induced a small gradual increase in [Ca(2+)](i) at first challenge, produced a drastic increase in the amplitude of Ca(2+) release (caffeine response). The caffeine response eventually reached peak amplitude and then remained constant even if caffeine application were continued. This augmentation was maintained for up to 2 h, and was achieved not only by repetitive application but also by a long exposure of caffeine. However, this augmentation was neither achieved by repetitive administration of high K(+)-solution, nor caused by inhibition of phosphodiesterase by caffeine. The repetitive or sustained application of caffeine is suggested to increase the caffeine sensitivity of the calcium release channel to calcium, thus causing the potentiation of the caffeine response.

Differential regulation of ER Ca2+ uptake and release rates accounts for multiple modes of Ca2+-induced Ca2+ release

The ER is a central element in Ca(2+) signaling, both as a modulator of cytoplasmic Ca(2+) concentration ([Ca(2+)](i)) and as a locus of Ca(2+)-regulated events. During surface membrane depolarization in excitable cells, the ER may either accumulate or release net Ca(2+), but the conditions of stimulation that determine which form of net Ca(2+) transport occurs are not well understood. The direction of net ER Ca(2+) transport depends on the relative rates of Ca(2+) uptake and release via distinct pathways that are differentially regulated by Ca(2+), so we investigated these rates and their sensitivity to Ca(2+) using sympathetic neurons as model cells. The rate of Ca(2+) uptake by SERCAs (J(SERCA)), measured as the t-BuBHQ-sensitive component of the total cytoplasmic Ca(2+) flux, increased monotonically with [Ca(2+)](i). Measurement of the rate of Ca(2+) release (J(Release)) during t-BuBHQ-induced [Ca(2+)](i) transients made it possible to characterize the Ca(2+) permeability of the ER ((~)P(ER)), describing the activity of all Ca(2+)-permeable channels that contribute to passive ER Ca(2+) release, including ryanodine-sensitive Ca(2+) release channels (RyRs) that are responsible for CICR. Simulations based on experimentally determined descriptions of J(SERCA), and of Ca(2+) extrusion across the plasma membrane (J(pm)) accounted for our previous finding that during weak depolarization, the ER accumulates Ca(2+), but at a rate that is attenuated by activation of a CICR pathway operating in parallel with SERCAs to regulate net ER Ca(2+) transport. Caffeine greatly increased the [Ca(2+)] sensitivity of ((~)P(ER)), accounting for the effects of caffeine on depolarization-evoked [Ca(2+)](i) elevations and caffeine-induced [Ca(2+)](i) oscillations. Extending the rate descriptions of J(SERCA), ((~)P(ER)), and J(pm) to higher [Ca(2+)](i) levels shows how the interplay between Ca(2+) transport systems with different Ca(2+) sensitivities accounts for the different modes of CICR over different ranges of [Ca(2+)](i) during stimulation.

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.

Modifications of intracellular Ca2+ signalling during nerve growth factor-induced neuronal differentiation of rat adrenal chromaffin cells

Postnatal sympathetic neurons (SNs) and chromaffin cells (CCs) derive from neural crest precursors. CCs can differentiate in vitro into SN-like cells after nerve growth factor (NGF) exposure. This study examines changes of intracellular Ca2+ homeostasis and dynamics of CCs under conditions that promote a neuronal phenotype. Spontaneous Ca2+ fluctuations, a frequent observation in early cultures of CCs, diminished after > 10 days in vitro in control cells and ceased in NGF-treated ones. At the same time, Ca2+ rises resulting from entry upon membrane depolarization, gradually increased both their size and peak d[Ca2+]i/dt, resembling those recorded in SNs. Concomitantly, caffeine-induced Ca2+ rises, resulting from Ca2+ release from intracellular stores, increased their size and their peak d[Ca2+]i/dt by > 1000%, and developed transient and sustained release components, similar to those of SNs. The transient component, linked to regenerative Ca2+ release, appeared after > 10 days of NGF treatment, suggesting a delayed steep enhancement of Ca2+-induced Ca2+ release (CICR). Immunostaining showed that proteins coded by the three known isoforms of ryanodine receptors (RyRs) are present in CCs, but that only RyR2 increased significantly after NGF treatment. Since the transient release component increased more steeply than RyR2 immunostaining, we suggest that the development of robust CICR requires both an increased expression of RyRs and more efficient functional coupling among them. NGF-induced transdifferentiation of chromaffin cells involves the enhancement of both voltage-gated Ca2+ influx and Ca2+ release from intracellular stores. These modifications are likely to complement the extensive morphological and functional reorganization required for the replacement of the endocrine phenotype with the neuronal one.

Expression and function of ryanodine receptors in human melanocytes

We investigated the expression of ryanodine receptors (RyRs) in cultured human melanocytes with immunocytochemistry and reverse transcriptase-polymerase chain reaction. With the use of a monoclonal antibody, RyR immunoreactivity was detected in the cytoplasm of melanocytes, and was further confirmed by RT-PCR assay. The PCR products were cut with restriction enzymes specific for each RyR isoform. Using the RyR1-specific restriction enzyme SacI yielded fragments of 300, 100, and 130 base pairs, consistent with the expression of RyR1 isoforms. The function of RyR in Ca(2+) signaling was investigated using single-cell fura-2 imaging. Ryanodine (1 to approximately 100 microM) induced significant elevation of cytoplasmic Ca(2+) in single human melanocytes in a dose-dependent manner. The ryanodine-induced [Ca(2+)](i) increase was inhibited by neomycin. Furthermore, ryanodine inhibited proliferation and stimulated pigmentation of human melanocytes. This study demonstrates that the RyR1 isoform is expressed in cultured human melanocytes, and suggests that the RyR may be involved in regulating the intracellular Ca(2+) responses involved in proliferation and pigmentation of cultured human melanocytes.

Investigation of the role of intracellular Ca(2+) stores in generation of the muscarinic agonist-induced slow afterdepolarization (sADP) in guinea-pig olfactory cortical neurones in vitro

1. Intracellular recordings were made from guinea-pig olfactory cortical brain slice neurones to assess the possible role of intracellular Ca(2+) stores in the generation of the slow post-stimulus afterdepolarization (sADP) and its underlying tail current (I(ADP)), induced by muscarinic receptor activation. 2. Caffeine or theophylline (0.5 - 3 mM) reduced the amplitude of the I(ADP) (measured under 'hybrid' voltage clamp) induced in the presence of the muscarinic agonist oxotremorine-M (OXO-M, 10 microM) by up to 96%, without affecting membrane properties or muscarinic depolarization of these neurones. 3. The L-type Ca(2+) channel blocker nifedipine (1, 10 microM) also inhibited I(ADP) (by up to 46%), while ryanodine (10 microM) (a blocker of Ca(2+) release from internal stores) produced a small ( approximately 10%) reduction in I(ADP) amplitude; however, neither 10 microM dantrolene (another internal Ca(2+) release blocker) nor the intracellular Ca(2+) store re-uptake inhibitors thapsigargin (3 microM) or cyclopiazonic acid (CPA, 15 microM) affected I(ADP) amplitude. 4. IBMX (100 microM), a phosphodiesterase inhibitor, also had no effect on I(ADP). Furthermore, inhibition of I(ADP) by caffeine was not reversed by co-application of 100 microM adenosine. 5. Caffeine (3 mM) or nifedipine (10 microM) reduced the duration of presumed Ca(2+) spikes revealed by intracellular Cs(+) loading. When applied in combination, nifedipine and caffeine effects were occlusive, rather than additive, suggesting a common site of action on L-type calcium channels. 6. We conclude that Ca(2+)-induced Ca(2+) release (CICR) from internal stores does not contribute significantly to muscarinic I(ADP) generation in olfactory cortical neurones. However caffeine and theophylline, which enhance CICR in other systems, blocked I(ADP) induction. We suggest that this action might involve a combination of L-type voltage-gated Ca(2+) channel blockade, and a direct inhibitory action on the putative I(ADP) K(+) conductance.

Ca2+ influx in resting rat sensory neurones that regulates and is regulated by ryanodine-sensitive Ca2+ stores

1. Store-operated, voltage-independent Ca2+ channels are activated by depletion of intracellular Ca2+ stores and mediate Ca2+ influx into non-excitable cells at resting membrane potential. We used microfluorimetry, patch-clamp and Mn2+-quench techniques to explore the possibility that a similar mechanism exists in rat dorsal root ganglion (DRG) neurones in primary culture. 2. Following caffeine-induced depletion, ryanodine-sensitive Ca2+ stores refilled with Ca2+ at resting membrane potential. The refilling process required extracellular Ca2+, was blocked by 2 mM Ni2+, and was facilitated by membrane hyperpolarization from -55 to -80 mV, indicating a key role for Ca2+ influx. This influx of Ca2+ was not affected by the voltage-operated Ca2+ channel (VOCC) antagonists nicardipine (10 microM), nimodipine (10 microM) or omega-grammotoxin SIA (1 microM). 3. When ryanodine-sensitive Ca2+ stores were depleted in Ca2+-free media, a return to 2 mM external Ca2+ resulted in a pronounced [Ca2+]i overshoot, indicating an increased permeability to Ca2+. Depletion of Ca2+ stores also produced a 2-fold increase in the rate of Mn2+ influx. The [Ca2+]i overshoot and Mn2+ entry were both inhibited by Ni2+, but not by VOCC antagonists. 4. Caffeine induced periodic Ca2+ release from, and reuptake into, ryanodine-sensitive stores. The [Ca2+]i oscillations were arrested by removal of extracellular Ca2+ or by addition of Ni2+, but they were not affected by VOCC antagonists. Hyperpolarization increased the frequency of this rhythmic activity. 5. These data suggest the presence of a Ca2+ entry pathway in mammalian sensory neurones that is distinct from VOCCs and is regulated by ryanodine-sensitive Ca2+ stores. This pathway participates in refilling intracellular Ca2+ stores and maintaining [Ca2+]i oscillations and thus controls the balance between intra- and extracellular Ca2+ reservoirs in resting DRG neurones.

Long-term use-dependent enhancement of impulse-induced exocytosis by adrenaline at frog motor nerve terminals

Adrenaline (5-20 microM) use-dependently increased end-plate potentials (EPPs) in normal Ringer solution (containing d-tubocurarine to partially block acetylcholine receptors) and a low Ca2+, high Mg2+ solution for more than several hours and decreased the coefficient of variation of EPP amplitude in the latter solution in frog neuromuscular junctions. The amplitude and frequency of miniature EPPs and impulse-induced increases in intraterminal Ca2+ concentration were unaffected. Adrenaline thus causes sustained enhancement of impulse-induced exocytosis by acting at a mechanism of exocytosis downstream to Ca2+ entry.

Caffeine-induced [Ca2+] oscillations in neurones of frog sympathetic ganglia

1. Single cell fluorimetry was used to monitor caffeine-induced oscillations of cytosolic [Ca2+] in frog sympathetic ganglion neurones in 2.0 mM K+ Ringer solution. 2. [Ca2+] oscillations decreased in frequency and exhibited three different amplitude patterns after the first large peak of [Ca2+]: (a) a series of big oscillations (BOs) of constant large amplitude (300-400 nM), (b) a series of much smaller oscillations (SOs) (40-60 nM), or (c) a series of decaying oscillations (DOs) of rapidly decreasing amplitude. 3. A model in which the oscillation amplitude was determined by the Ca2+ content of the endoplasmic reticulum (ER) whereas the oscillation frequency was controlled by how rapidly the cytosolic [Ca2+] reached the threshold for Ca2+-induced Ca2+ release (CICR) was able to simulate each observed pattern by varying the level of activity of the ER Ca2+ pump (SERCA), CICR and release-activated Ca2+ transport (RACT). A cumulative, cytosolic Ca2+-dependent inactivation of the plasma membrane (PM) Ca2+ influx or of the Ca2+-sensitive leak coefficient of the ryanodine receptors caused the oscillation frequency to decrease in the model. 4. Transitions between BOs and SOs and changes in [Ca2+] oscillations caused by ryanodine, thapsigargin, lanthanum and FCCP could also be simulated. 5. We conclude that RACT, SERCA, CICR and Ca2+-dependent PM Ca2+ influx are major mechanisms underlying [Ca2+] oscillations in these neurones.

Calcium transients evoked by action potentials in the somata of chick ciliary neurons

The effect of action potentials on the calcium concentration in the somata of chick ciliary neurons ([Ca2+]s) was determined by loading these with the calcium indicator calcium green-1. Following trains of 1-10 impulses (30 Hz) to the postganglionic nerve, the [Ca2+]s increased rapidly and then declined along a single exponential with a time constant of 0.70 +/- 0.04 s (fast phase). After trains of 20 or 50 impulses, the elevated [Ca2+]s declined as the sum of two exponentials, with time constants of 0.78 +/- 0.12 s (fast phase) and 4.0 +/- 0.4 s (moderate phase). After a 600-impulse postganglionic train of impulses, the elevated [Ca2+]s declined quickly over about 1 s, and then as the sum of two exponentials: that of the moderate phase and a slower component with a time constant of 109 +/- 16 s (slow phase). Similar time courses were observed following stimuli to the preganglionic nerve. Caffeine (3 mM) and ryanodine (20 microM) both sped the fast phase and slowed the moderate phase of [Ca2+]s decline. Carbonyl cyanide m-chlorophenyl hydrazone (CCCP, 2 microM) slowed the slow phase, without affecting the other phases of decline. These results are discussed in relation to identifying the mechanisms responsible for these different phases of Ca2+ removal.

Nerve growth factor maintains regulation of intracellular calcium in neonatal sympathetic neurons but not in mature or aged neurons

We examined the effects of nerve growth factor on the regulation of intracellular calcium levels of superior cervical ganglion neurons in terms of postnatal maturation and ageing. Rat superior cervical ganglion neurons from three age groups (neonatal: 0 to one-day-old, young adult: three to six-month-old, and aged: more than 24-month-old) were dissociated and cultured in the presence or absence of 100 ng/ml of nerve growth factor. Intracellular free calcium levels ([Ca2+]i) were measured using the fura-2 microfluorometry. Nerve growth factor treatment increased the resting [Ca2+]i of neonatal neurons, although it had no effect on those of mature and aged neurons. We further examined the effects of nerve growth factor on the transient increase of [Ca2+]i induced by methacholine (0.1 mM), caffeine (20 mM) or high-potassium medium (40 mM K+). Nerve growth factor pre-treatment significantly increased the population of neonatal superior cervical ganglion neurons which responded to methacholine, whereas almost all young adult and aged neurons responded to methacholine regardless of pre-treatment of nerve growth factor. Caffeine induced a cyclic alteration of [Ca2+]i (oscillation) in 45% of the neonatal superior cervical ganglion neurons when they were maintained without nerve growth factor, but nerve growth factor treatment suppressed the oscillation to 10% of neurons. In contrast to neonatal neurons, all of the young adult and aged neurons showed only a transient increase of [Ca2+]i in response to caffeine independent of nerve growth factor treatment. There was no significant effect of nerve growth factor on K+ depolarization-induced [Ca2+]i elevations at any of the ages studied. Nerve growth factor did not substantially alter the pattern of the transients induced by these three agents. Our results indicate that exogenous nerve growth factor is necessary to maintain normal acetylcholine receptor-mediated [Ca2+]i responses as well as Ca(2+)-induced Ca2+ release from intracellular calcium storage in neonatal superior cervical ganglion neurons. In mature superior cervical ganglion neurons, Ca2+ homeostasis becomes independent of exogenous nerve growth factor, and Ca2+ homeostasis and its independency are well preserved in aged neurons.

Evidence for the calcium-dependent potentiation of M-current obtained by the ratiometric measurement of the fura-2 fluorescence in bullfrog sympathetic neurons

Intracellular Ca2+ concentration ([Ca]i) was measured following the activation of an inward Ca2+ current and subsequent potentiation of an M-type K+ current (IM) in bullfrog sympathetic neurons. Fura-2 was used as an indicator for [Ca]i. The fluorescence ratio at 340 and 380 nm (F340/F380) was elevated from 0.36 to 1.22 when IM was potentiated by 68% following the Ca2+ current. Based on the in vivo calibration curve obtained from cells permeabilized with digitonin (20 microM), the F340/F380 value of 1.22 was equivalent to a [Ca]i of 0.97 microM. We therefore propose that a rise in [Ca]i into the micromolar range can lead to the potentiation of IM in amphibian autonomic neurons.

Caffeine mediates cation influx and intracellular Ca2+ release in leech P neurones

We investigated the effect of caffeine on the intracellular free Ca2+ concentration ([Ca2+]i) of leech P neurones by using the fluorescent indicator Fura-2. Caffeine induced a [Ca2+]i increase that was strongly reduced, but not abolished, in Ca(2+)-free solution. The effect of caffeine on [Ca2+]i was dose-dependent: while 5 mM caffeine evoked a persistent [Ca2+]i increase that could be elicited repetitively, 10 mM caffeine or more induced a transient [Ca2+]i increase that was strongly reduced upon subsequent applications at the same concentration. Surprisingly, the cells remained fully responsive to a moderately increased caffeine concentration. The caffeine-induced [Ca2+]i increase was not blocked by millimolar concentrations of La3+, Mg2+, Cd2+, Zn2+, Co2+, Ni2+, or Mn2+. While La3+ and Mg2+ had no effect on the caffeine response, the other cations caused irreversible changes in the Fura-2 fluorescence. The inhibitors of intracellular Ca2+ pumps-thapsigargin, cyclopiazonic acid (CPA), and 2,5-di-(t-butyl)-1,4-hydroquinone (BHQ)--had no effect on the caffeine-induced [Ca2+]i increase at normal extracellular Ca2+ concentration, but they reduced it in Ca(2+)-free solution. Ryanodine had no effect on the caffeine-induced [Ca2+]i increase at normal extracellular Ca2+ concentration, and also in Ca(2+)-free solution it seemed to be largely ineffective. Caffeine evoked complete fluctuations of the membrane potential. The effect in Ca2+ free and in Na(+)-free solution suggests that the depolarizing response components were mainly due to Na+ influx, while Ca2+ reduced the Na+ influx and/or activated mechanisms which re- or hyperpolarize the cells. It is concluded that leech P neurones possess caffeine-sensitive intracellular Ca2+ stores, as well as caffeine-sensitive ion channels, in the plasma membrane that are activated by a voltage-independent mechanism. The plasma membrane channels are permeable to various divalent cations including Ca2+, and possibly also to Na+.

Release-activated Ca2+ transport in neurons of frog sympathetic ganglia

Frog sympathetic ganglion neurons exhibit a novel Ca2+ uptake mechanism, release-activated calcium transport or RACT, which is manifest in both cytosolic and store [Ca2+] signals as greatly accelerated Ca2+ uptake after Ca2+ release from internal stores. RACT is activated by Ca2+ release but not by Ca2+ entry and serves to selectively refill Ca2+ stores after release. RACT lowers cytosolic [Ca2+] with a rate constant about 1.6 times that of the SERCA pump with empty ER. RACT is thapsigargin-insensitive, was eliminated by ryanodine, but was not affected by blocking mitochondrial or plasma membrane Ca2+ transport. A Ca2+ flux model with RACT in the ER membrane reproduced the cytosolic and store [Ca2+] responses to all stimuli.

Low concentrations of caffeine raise intracellular calcium concentration only in the presence of extracellular calcium in cultured molluscan neurons

1. The effects of low concentrations of caffeine (100 and 300 microM) on the intracellular calcium concentration [Ca2+]i in four cultured, identified neurons of the pond snail Lymnaea stagnalis (L) were investigated. 2. Intracellular CA2+ levels in these neurons were measured with the cell-permeable Ca2+ indicator Fura-2/AM, both in the presence and absence of extracellular Ca2 (o-Ca2+/EGTA). 3. In the presence of Ca2+ in the external medium, caffeine was found to induce a substantial elevation in the free [Ca2+]i in all cell types. 4. In some cases, the rise in [Ca2+]i was found to be both time- and concentration-dependent. 5. Low doses of caffeine did not produce any appreciable rise in [Ca2+]i in the absence of Ca2+ in the external medium, but calcium was still available from stores, as clinical concentrations of halothane rose [Ca2+]i in the absence of extracellular calcium. 6. These results indicate that the actions of caffeine, when applied at low concentrations, are dependent on extracellular calcium.

Calcium-dependent potentiation of M-current in bullfrog sympathetic neurons

Whole-cell voltage-clamp recordings were made from cultured bullfrog sympathetic neurons to measure the steady-state activation curve of M-type potassium current. When measured with a calcium-deficient (10 nM) pipette solution M-conductance was 4.8 nS at -35 mV having the 50%-activation voltage at-20 mV. Respective values were 17.2 nS at -35 mV with the 50%-activation voltage at -42 mV when measured with a calcium-rich (1 microM) solution, indicating the hyperpolarizing displacement of the activation curve with high internal calcium. It is suggested that intracellular calcium ions can modulate kinetics of M-current which thereby regulate the number of M-channels being open at given membrane potentials.

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.

Calcium-induced calcium release in rat sensory neurons

1. In isolated dorsal root ganglion cells (DRG neurons), changes in the concentration of global cytosolic Ca2+ (delta [Ca2+]c) were measured by the fluorescence of K5-indo-1. Depolarizations from -60 to 0 mV (500 ms) and Ca2+ influx through Ca2+ channels (ICa) increased [Ca2+]c by 480 +/- 113 nM, the peak occurring 542 +/- 76 ms (mean +/- S.E.M.) after repolarization. 2. Ryanodine (10 microM) reduced depolarization-induced delta [Ca2+]c by up to 80% and blocked delta [Ca2+]c induced by 20 mM caffeine. 3. Peak delta [Ca2+]c and peak ICa followed a similar bell-shaped voltage dependence. Removal of extracellular Ca2+ abolished depolarization-induced delta [Ca2+]c; its elevation from 2 to 8 mM increased peak ICa by 30% and delta [Ca2+]c by 108%. 4. Ca2+ influx at 0 mV was graded by pulse durations between 20 and 500 ms. Up to 200 ms, delta [Ca2+]c increased linearly with Ca2+ influx. Depolarizations longer than 200 ms induced a supralinear increase in delta [Ca2+]c that was abolished by caffeine (20 mM). 5. The supralinear increase in delta [Ca2+]c and the caffeine-induced delta [Ca2+]c were measured only in thirteen of nineteen DRG neurons; in the other six of nineteen cells both properties were absent. The results suggest that Ca(2+)-induced Ca2+ release (CICR) is expressed differently in different populations of DRG neurons. 6. A single action potential did not significantly increase [Ca2+]c. Trains of stimuli (20 Hz) induced delta [Ca2+]c that linearly increased with the number of action potentials. Delta [Ca2+]c due to 100 action potentials had a significant ryanodine-sensitive component. 7. It is discussed that CICR can contribute to the depolarization-induced [Ca2+]c, provided the Ca2+ influx lasts for a certain minimum period of time.

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.

Ryanodine receptor-mediated intracellular calcium release in rat cerebellar Purkinje neurones

1. Ryanodine receptor-mediated Ca2+ release was investigated in Purkinje neurones of rat cerebellar slices by using whole-cell patch-clamp recordings combined with fluorometric digital imaging of cytoplasmic Ca2+ concentration ([Ca2+]i). 2. Caffeine caused a transient increase in [Ca2+]i in the somata and dendrites of Purkinje neurones. Caffeine-induced Ca2+ transients were not associated with a membrane inward current and persisted in Ca(2+)-free external solutions, indicating that they are caused by Ca2+ released from intracellular stores. The amplitudes of the caffeine-mediated elevations in [Ca2+]i were strongly dependent on the baseline level of [Ca2+]i. 3. Intracellular application of Ruthenium Red through the patch pipette blocked caffeine-induced Ca2+ transients in Purkinje neurones. Ryanodine when applied either intra- or extracellularly caused a use-dependent block of caffeine-induced Ca2+ release. 4. Depolarization-induced Ca2+ transients were strongly prolonged by caffeine. Several lines of evidence suggest that these prolongations reflect Ca(2+)-induced Ca2+ release. 5. Despite the presence of skeletal muscle type ryanodine receptors in Purkinje neurones, depolarizing pulses failed to induce any changes in [Ca2+]i when the influx of Ca2+ through voltage-gated channels was prevented by using Ca(2+)-free solution, or when applying blockers of voltage-gated Ca2+ channels. 6. Dendritic Ca2+ transients produced by stimulation of the excitatory climbing fibre synaptic input were also prolonged by caffeine, indicating that ryanodine receptor-mediated release of Ca2+ may be involved in synaptic signalling in cerebellar Purkinje neurones. 7. Ryanodine receptor-mediated release of Ca2+ in cerebellar Purkinje neurones can be explained by a model in which release of Ca2+ is strongly facilitated by the co-operative action of Ca2+, caffeine and/or ryanodine. Our results suggest that Ca2+ release in these central neurones becomes prominent only during episodes of intensive electrical activity associated with increased Ca2+ entry.

Properties of the ryanodine-sensitive release channels that underlie caffeine-induced Ca2+ mobilization from intracellular stores in mammalian sympathetic neurons

The most compelling evidence for a functional role of caffeine-sensitive intracellular Ca2+ reservoirs in nerve cells derives from experiments on peripheral neurons. However, the properties of their ryanodine receptor calcium release channels have not been studied. This work combines single-cell fura-2 microfluorometry, [3H]ryanodine binding and recording of Ca2+ release channels to examine calcium release from these intracellular stores in rat sympathetic neurons from the superior cervical ganglion. Intracellular Ca2+ measurements showed that these cells possess caffeine-sensitive intracellular Ca2+ stores capable of releasing the equivalent of 40% of the calcium that enters through voltage-gated calcium channels. The efficiency of caffeine in releasing Ca2+ showed a complex dependence on [Ca2+]i. Transient elevations of [Ca2+]i by 50-500 nM were facilitatory, but they became less facilitatory or depressing when [Ca2+]i reached higher levels. The caffeine-induced Ca2+ release and its dependence on [Ca2+]i was further examined by [3H]ryanodine binding to ganglionic microsomal membranes. These membranes showed a high-affinity binding site for ryanodine with a dissociation constant (KD = 10 nM) similar to that previously reported for brain microsomes. However, the density of [3H]ryanodine binding sites (Bmax = 2.06 pmol/mg protein) was at least three-fold larger than the highest reported for brain tissue. [3H]Ryanodine binding showed a sigmoidal dependence on [Ca2+] in the range 0.1-10 microM that was further increased by caffeine. Caffeine-dependent enhancement of [3H]ryanodine binding increased and then decreased as [Ca2+] rose, with an optimum at [Ca2+] between 100 and 500 nM and a 50% decrease between 1 and 10 microM. At 100 microM [Ca2+], caffeine and ATP enhanced [3H]ryanodine binding by 35 and 170% respectively, while binding was reduced by > 90% with ruthenium red and MgCl2. High-conductance (240 pS) Ca2+ release channels present in ganglionic microsomal membranes were incorporated into planar phospholipid bilayers. These channels were activated by caffeine and by micromolar concentrations of Ca2+ from the cytosolic side, and were blocked by Mg2+ and ruthenium red. Ryanodine (2 microM) slowed channel gating and elicited a long-lasting subconductance state while 10 mM ryanodine closed the channel with infrequent opening to the subconductance level. These results show that the properties of the ryanodine receptor/Ca2+ release channels present in mammalian peripheral neurons can account for the properties of caffeine-induced Ca2+ release. Our data also suggest that the release of Ca2+ by caffeine has a bell-shaped dependence on Ca2+ in the physiological range of cytoplasmic [Ca2+].

Ca(2+)-induced Ca2+ release and its activation in response to a single action potential in rabbit otic ganglion cells

1. Ryanodine-sensitive intracellular Ca2+ release activated by Ca2+ entry was studied with fura-2 fluorescence and intracellular voltage recording techniques in rabbit otic ganglion cells. 2. The removal of extracellular Ca2+ reduced sustained, transient or oscillatory rises in intracellular Ca2+ ([Ca2+]i) induced at high extracellular K+ and abolished the [Ca2+]i oscillation in cultured neurones. 3. Ryanodine (10 microM) transiently increased [Ca2+]i and reduced the amplitude and rate of rise of the high-K(+)-induced rise in [Ca2+]i, while caffeine (5 mM) produced a few transient rises in [Ca2+]i in most cultured cells and [Ca2+]i oscillation only in one cell. 4. The two components of the slow after-hyperpolarization (AHP) of an action potential in neurones of freshly isolated ganglia were dependent on extracellular Ca2+ and abolished by Ca2+ channel blockers, Cd2+ or Co2+. 5. The late component of AHP (LAHP), but not the initial component, in 'fresh' neurones increased in area with an increase in the preceding interval, was abolished by ryanodine (10 microM) and intracellularly injected EGTA, and mimicked by intracellular injection of Ca2+. 6. A ryanodine-sensitive Ca(2+)-induced Ca2+ release thus exists, operates in response to an action potential-induced Ca2+ entry and underlies LAHP in rabbit otic ganglion cells.

Characterization of spontaneous calcium transients in nerve growth cones and their effect on growth cone migration

This study examines the mechanisms of spontaneous and induced [Ca2+]i spiking in nerve growth cones and the effect of spikes on growth cone migration. Over a 10-20 min observation period, 29% of DRG growth cones undergo spontaneous and transient elevations in physiological extracellular Ca2+ ((Ca2+)o; 2 mM), whereas 67% of growth cones exposed to 20 mM (Ca2+)o exhibit similar [Ca2+]i spikes. Spontaneous [Ca2+]i spiking was not observed in neuronal cell bodies or nonneuronal cells. Ca2+ influx through non-voltage-gated Ca2+ channels was required for spontaneous [Ca2+]i spikes in growth cones, since removal of (Ca2+)o, or addition of the general Ca2+ channel blockers La3+ or Ni2+, reversibly blocked [Ca2+]i spiking, while blockers of the voltage-gated Ca2+ channels did not. Experiments using agents that influence intracellular Ca2+ stores suggest that Ca2+ stores may buffer and release Ca2+ during growth cone [Ca2+]i spikes. Growth cone migration was immediately and transiently inhibited by [Ca2+]i spikes, but eventually returned to prespike rates.

Cyclic ADP-ribose modulates Ca2+ release channels for activation by physiological Ca2+ entry in bullfrog sympathetic neurons

Although Ca(2+)-induced Ca2+ release (CICR) via ryanodine receptors has been found to occur in intact neurons, little is known about the physiological processes that regulate it. We studied the effects of cyclic ADP-ribose (cADPR) on CICR in cultured bullfrog sympathetic neurons by fura-2 fluorescence recording and patch-clamp techniques. cADPR applied through a patch pipette augmented action potential- or depolarizing pulse-induced rises in intracellular Ca2+ without a change in Ca2+ entry initiating the responses, but not in the presence of ryanodine. Likewise, cADPR enhanced a single or oscillatory rise(s) in intracellular Ca2+ induced by caffeine. These results strongly suggest that cADPR can be an endogenous modulator of ryanodine receptors in neurons.

Caffeine-induced calcium release from internal stores in cultured rat sensory neurons

Free intracellular calcium concentration ([Ca2+]in) was recorded at 22 degrees C by means of Indo-1 or Fura-2 single-cell microfluorometry in cultured dorsal root ganglion neurons obtained from neonatal rats. The resting [Ca2+]in in dorsal root ganglion neurons was 73 +/- 21 nM (mean +/- S.D., n = 94). Fast application of 20 mM caffeine evoked [Ca2+]in transient which reached a peak of 269 +/- 64 nM within 5.9 +/- 1.1 s. After reaching the peak the [Ca2+]in level started to decline in the presence of caffeine and for 87.2 +/- 10.6 s cytoplasmic calcium returned to an initial resting value. In 40% of neurons tested [Ca2+]in decreased to subresting levels following the washout of caffeine (the so-called post-caffeine undershoot). On average, the undershoot level was 19 +/- 2.5 nM below the resting [Ca2+]in value. Prolonged exposure of caffeine depleted the caffeine-sensitive stores of releasable Ca2+; the degree of this depletion depended on caffeine concentration. The depletion of the caffeine-sensitive internal stores to some extent was linked to calcium extrusion via La(3+)-sensitive plasmalemmal Ca(2+)-ATPases. The stores could be partially refilled by the uptake of cytoplasmic Ca2+, but the complete recovery of releasable Ca2+ content of the caffeine-sensitive pools required the additional calcium entry via voltage-operated calcium channels. Caffeine-evoked [Ca2+]in transients were effectively blocked by 10 microM ryanodine, 5 mM procaine, 10 microM dantrolene or 0.5 mM Ba2+, thus sharing the basic properties of the Ca(2+)-induced-Ca2+ release from endoplasmic reticulum. Pharmacological manipulation with caffeine-sensitive stores interfered with the depolarization-induced [Ca2+]in transients. In the presence of low caffeine concentration (0.5-1 mM) in the extracellular solution the rate of rise of the depolarization-triggered [Ca2+]in transients significantly increased (by a factor 2.15 +/- 0.29) suggesting the occurrence of Ca(2+)-induced Ca2+ release. When the caffeine-sensitive stores were emptied by prolonged application of caffeine, the amplitude and the rate of rise of the depolarization-induced [Ca2+]in transients were decreased. These facts suggest the involvement of internal caffeine-sensitive calcium stores in the generation of calcium signal in sensory neurons.

Caffeine- and muscarinic receptor agonist-sensitive Ca2+ stores in chick ciliary ganglion cells

To investigate the presence and the role of intracellular Ca2+ stores in chick ciliary ganglion cells, the concentration of cytosolic free Ca2+ ([Ca]in) was measured in acutely isolated neurons, using fura-2 microfluorometry. Caffeine caused a substantial increase in [Ca]in following or during high K+ depolarization; this response was inhibited by treatment of the cells with thapsigargin or with caffeine plus ryanodine. The peak value and the rate of the depolarization-induced [Ca]in increase were not much altered by either of these treatments, which deplete caffeine-sensitive Ca2+ stores. The muscarinic receptor agonists muscarine, oxotremorine M, and methacholine, caused substantial increases in [Ca]in, in a manner that was partially dependent on Ca2+. These agonists also caused a rise in [Ca]in during K+ depolarization, which rise was inhibited by treatment with thapsigargin or with caffeine plus ryanodine. The response to oxotremorine M during depolarization was strongly inhibited by 10 nM 4-DAMP, but was not inhibited by 1 microM pirenzepine or by 1 microM AF-DX 116. These results indicate that chick ciliary ganglion cells possess Ca2+ stores that are activated by both caffeine and a second messenger generated by the activation of the M3 muscarinic receptor subtype.

Characteristics of Ca2+ release induced by Ca2+ influx in cultured bullfrog sympathetic neurones

1. A rise in intracellular Ca2+ ([Ca2+]i) and a Ca2+ current (ICa) induced by a depolarizing pulse were simultaneously recorded by fura-2 or indo-1 fluorescence and whole-cell patch clamp techniques in cultured bullfrog sympathetic ganglion cells. 2. [Ca2+]i (calculated from the ratio of fura-2 fluorescences excited at 380 and 340 nm and recorded with a photomultiplier at > 492 nm) rose regeneratively (in most cells) during a command pulse (from -60 to 0 mV, 100 ms), continued to rise thereafter, peaked at 666 ms (on average) and decayed slowly with a half-decay time of 22.8 s. 3. Scanning a single horizontal line across the cytoplasm with an ultraviolet argon ion laser (351 nm) and recording indo-1 fluorescences at two wavelengths (peaked at 410 and 475 nm) with a confocal microscope demonstrated that [Ca2+]i beneath the cell membrane rose much faster than that in the deeper cytoplasm. The time course of the spatial integral of [Ca2+]i, however, corresponded well with that recorded with fura-2 fluorescence using a photomultiplier. 4. [Ca2+]i measured by fura-2 fluorescence ratio using a photomultiplier did not increase during a strong depolarizing pulse (-60 to +80 mV), but sometimes rose after the pulse. A depolarization-induced rise in [Ca2+]i ([Ca2+]i transient) was blocked in a Ca(2+)-free, EGTA solution, reduced by lowering the extracellular Ca2+ concentration ([Ca2+]o) to 0.45 or 0.9 mM and enhanced by raising [Ca2+]o to 7.2 or 14.4 nM. 5. The extracellular Ca2+ dependence was non-linear when long depolarizing pulses (up to 500 ms) were applied; the amplitude of [Ca2+]i transient/Ca2+ entry (unit [Ca2+]i transient) increased with an increase in Ca2+ entry. 6. Increasing the duration of depolarization (-50 or -60 to 0 mV) from 20 to 500 ms enhanced asymptotically the integral of ICa (due to inactivation), and progressively the magnitude of [Ca2+]i transients, leading to the apparent non-linear dependence of unit [Ca2+]i transient on Ca2+ entry as well as on the duration of membrane depolarization. The peak time of [Ca2+]i transient was unchanged for pulse durations up to 300 ms, but prolonged with an increase in pulse duration to 500 ms. 7. Inhibitors of Ca2+ release from intracellular Ca2+ reservoirs, dantrolene (10 microM) and ryanodine (50 microM), blocked the [Ca2+]i transient to 56 and 30%, respectively, of the control. 8. The higher the basal [Ca2+]i level, the greater was the magnitude of the [Ca2+]i transients.(ABSTRACT TRUNCATED AT 400 WORDS)

Intracellular calcium dynamics in response to action potentials in bullfrog sympathetic ganglion cells

1. Dynamic changes in the intracellular free Ca2+ concentration ([Ca2+]i) following electrical membrane activity, were recorded from the neurone soma of the excised bullfrog sympathetic ganglion, using Fura-2 fluorescence and compared with the accompanying Ca(2+)-dependent electrical membrane responses. 2. The resting [Ca2+]i was about 100 nM, a value little changed by penetration with an intracellular electrode. 3. A net rise in fluorescence at a wavelength of 340 nm (Ca2+ transient) induced by a single action potential in Ringer solution rose almost in parallel with the initial decay phase of a slow Ca(2+)-dependent after-hyperpolarization; decayed in parallel with the late phase; and increased in amplitude and duration in the presence of tetraethylammonium (20 mM). 4. A Ca2+ transient induced by repetitive action potentials was increased asymptotically in amplitude and progressively in duration by increasing the number of spikes, and was slower in time course than the associated Ca(2+)-dependent K+ current. 5. Scanning a single horizontal line across the cytoplasm with an ultraviolet argon ion laser (351 nm) and recording Indo-1 fluorescence with a confocal microscope demonstrated an inward spread of a rise in [Ca2+]i following a tetanus. 6. Both single spike- and tetanus-induced Ca2+ transients were abolished in a Ca(2+)-free solution, while single or repetitive transient rises in [Ca2+]i induced by caffeine (5-10 mM) were generated under the same conditions. 7. Ryanodine (10-50 microM) did not affect tetanus-induced Ca2+ transients, whereas it blocked completely the caffeine-induced oscillation of [Ca2+]i. 8. Ca2+ transients induced by a tetanus in Ringer solution were independent of the interval from the preceding tetanus. The amplitude of Ca2+ transients induced by a tetanus in the presence of caffeine (5 mM) was equal to, or greater than, that generated in Ringer solution in any of the phases of [Ca2+]i oscillation. 9. It is suggested that under the physiological conditions here, the induction of action potentials does not cause the release of Ca2+ in the cells of the freshly excised bullfrog sympathetic ganglion, and that Ca(2+)-buffering systems contribute not only to lowering a transient rise in [Ca2+]i but also to sustaining an increased [Ca2+]i after a large Ca2+ load into the cell.