Calcium domains associated with individual channels can account for anomalous voltage relations of CA-dependent responses

Effects of rapid buffers on Ca2+ diffusion and Ca2+ oscillations

Based on realistic mechanisms of Ca2+ buffering that include both stationary and mobile buffers, we derive and investigate models of Ca2+ diffusion in the presence of rapid buffers. We obtain a single transport equation for Ca2+ that contains the effects caused by both stationary and mobile buffers. For stationary buffers alone, we obtain an expression for the effective diffusion constant of Ca2+ that depends on local Ca2+ concentrations. Mobile buffers, such as fura-2, BAPTA, or small endogenous proteins, give rise to a transport equation that is no longer strictly diffusive. Calculations are presented to show that these effects can modify greatly the manner and rate at which Ca2+ diffuses in cells, and we compare these results with recent measurements by Allbritton et al. (1992). As a prelude to work on Ca2+ waves, we use a simplified version of our model of the activation and inhibition of the IP3 receptor Ca2+ channel in the ER membrane to illustrate the way in which Ca2+ buffering can affect both the amplitude and existence of Ca2+ oscillations.

Ca(2+)-dependent inactivation of a cloned cardiac Ca2+ channel alpha 1 subunit (alpha 1C) expressed in Xenopus oocytes

The alpha 1 subunit of cardiac Ca2+ channel, expressed alone or coexpressed with the corresponding beta subunit in Xenopus laevis oocytes, elicits rapidly inactivating Ca2+ currents. The inactivation has the following properties: 1) It is practically absent in external Ba2+; 2) it increases with Ca2+ current amplitudes; 3) it is faster at more negative potentials for comparable Ca2+ current amplitudes; 4) it is independent of channel density; and 5) it does not require the beta subunit. These findings indicate that the Ca2+ binding site responsible for inactivation is encoded in the alpha 1 subunit and suggest that it is located near the inner channel mouth but outside the membrane electric field.

Ca(2+)-dependent inactivation of P-type calcium channels in nerve terminals

Rapid Ca2+ signals evoked by K+ depolarization of rat cerebral cortical synaptosomes were measured by dual-channel Ca2+ spectrofluorometry coupled to a stopped-flow device. Kinetic analysis of the signal rise phase at various extracellular Ca2+ concentrations revealed that the responsible voltage-dependent Ca2+ channels, previously identified as P-type Ca2+ channels, inactivate owing to the rise in intracellular Ca2+ levels. At millimolar extracellular Ca2+ concentrations the channels were inactivated very rapidly and the rate was dependent on the high influx rate of Ca2+, thus limiting the Ca2+ signal amplitudes to 500-600 nM. A slower, probably voltage-dependent regulation appears to be effective at lower Ca2+ influx rates, leading to submaximal Ca2+ signal amplitudes. The functional feedback regulation of calcium channels via a sensor for intracellular Ca2+ levels appears to be responsible for the different inhibition characteristics of Cd2+ versus omega-agatoxin IVa.

A caffeine- and ryanodine-sensitive intracellular Ca2+ store can act as a Ca2+ source and a Ca2+ sink in PC12 cells

We have investigated the modulation of stimulus-induced changes in intracellular Ca2+ concentration ([Ca2+]i) by a caffeine-and ryanodine-sensitive Ca2+ store in PC12 cells. In populations of fura-2-loaded cells, caffeine cause a concentration-dependent increase in [Ca2+]i that was saturable, reversible and inhibited in a use-dependent fashion by ryanodine. Maximal Ca2+ release occurred with 40 mM caffeine, with an EC50 of 13 mM caffeine and a Hill coefficient (h) of 2.7, indicating that the release mechanism was co-operative. Pretreatment of intact cell populations with increasing concentrations of caffeine in nominally Ca(2+)-free medium inhibited the subsequent Ca2+ response to a maximal concentration of ATP, in a dose-dependent manner. In permeabilized cells, a maximal concentration (40 microM) of InsP3 still released Ca2+ in the presence of a supramaximal concentration (50 mM) of caffeine, whereas caffeine was unable to release Ca2+ after the InsP3-sensitive store had been completely emptied. These data suggest that PC12 cells contain a uniquely InsP3-sensitive Ca2+ store, and a store that is sensitive to both InsP3 and caffeine. Depletion of the caffeine-sensitive Ca2+ store by caffeine and ryanodine pretreatment in intact cells attenuated the Ca2+ response to ATP, but not to 55 mM K+, suggesting that the caffeine-sensitive Ca2+ store acts as a Ca2+ source after ATP stimulation, but not after depolarization with 55 mM K+. Pretreatment of intact cells with ATP and ryanodine resulted in a use-dependent block of both caffeine- and ATP-mediated Ca2+ release, confirming that ATP stimulation of PC12 cells brings about activation of ryanodine receptors. The rate of recovery, but not the magnitude or rate of onset, of the depolarization-induced [Ca2+]i transient was modulated by the state of filling of the caffeine-sensitive Ca2+ store such that recovery was prolonged if the store was either full, or empty and unable to refill. We conclude that the caffeine- and ryanodine-sensitive Ca2+ store can act as a Ca2+ source and a Ca2+ sink in PC12 cells, and that its role may in part be governed by the nature of the stimulating agent.

Cytoplasmic Ca2+ activity regulation as measured by a calcium-activated current

Calcium-activated non-selective cation (CAN) currents were activated by quantitative injections of Ca2+ into voltage clamped bursting neurons of the snails Helix aspersa or Helix pomatia. Membrane potential was held at the potassium equilibrium potential and CAN currents were fit with a rising and falling exponential function. Ca2+ transporters and pumps of the cell membrane, endoplasmic reticulum, and mitochondria were selectively blocked with pharmacological agents. Bath solutions containing 0 Na Ringers, chlorpromazine, Na3VO4, or thapsigargin did not significantly change the CAN current decay constants from those measured in Ringers. External 2,4-dinitrophenol or internal ruthenium red, however, significantly lengthened the CAN current decay constant. It is concluded that mitochondria are the most important sink for sub-membrane Ca2+ activity in the range necessary to effectively activate CAN currents.

Inactivation of single Ca2+ channels in rat sensory neurons by extracellular Ca2+

1. Single Ca2+ channels conducting 20 mM Ba2+ from adult rat dorsal root ganglion cells were characterized using the two-electrode patch-clamp technique configuration. 2. Channels demonstrating specific characteristics of conductance, voltage dependence and dihydropyridine sensitivity were classified as high-threshold or L-type Ca2+ channels. 3. Mean single-channel current in 20 mM Ba2+ did not show inactivation, but inactivation occurred when using Ca2+ as a permeating ion. 4. Stimulus protocols were delivered alternately in the cell-attached and whole-cell electrode, while recording single-channel activity and total Ca2+ current simultaneously. 5. A mean single-channel Ba2+ current from a stimulated patch did not show inactivation. However, stimulation of a physiological whole-cell Ca2+ current induced a marked inactivation of mean single-channel Ba2+ current. 6. Complete Ca2+ current block by the addition of 200 microM Cd2+ in the external solution removed single-channel inactivation in patches stimulated through a whole-cell electrode.

Functional dependence of Ca(2+)-activated K+ current on L- and N-type Ca2+ channels: differences between chicken sympathetic and parasympathetic neurons suggest different regulatory mechanisms

The influx of Ca2+ ions controls many important processes in excitable cells, including the regulation of the gating of Ca(2+)-activated K+ channels (the current IK[Ca]). Various IK[Ca] channels contribute to the regulation of the action-potential waveform, the repetitive discharge of spikes, and the secretion of neurotransmitters. It is thought that large-conductance IK[Ca] channels must be closely colocalized with Ca2+ channels (ICa) to be gated by Ca2+ influx. We now report that IK[Ca] channels can be preferentially colocalized with pharmacologically distinct subtypes of voltage-activated Ca2+ channel and that this occurs differently in embryonic chicken sympathetic and parasympathetic neurons. The effects of various dihydropyridines and omega-conotoxin on voltage-activated Ca2+ currents (ICa) and Ca(2+)-activated K+ currents (IK[Ca]) were examined by using perforated-patch whole-cell recordings from embryonic chicken ciliary and sympathetic ganglion neurons. Application of nifedipine or omega-conotoxin each caused a 40-60% reduction in ICa, whereas application of S-(-)-BAY K 8644 potentiated ICa in ciliary ganglion neurons. But application of omega-conotoxin had little or no effect on IK[Ca], whereas nifedipine and S-(-)-BAY K 8644 inhibited and potentiated IK[Ca], respectively. These results indicate that IK[Ca] channels are preferentially coupled to L-type, but not to N-type, Ca2+ channels on chicken ciliary ganglion neurons. Chicken sympathetic neurons also express dihydropyridine-sensitive and omega-conotoxin-sensitive components of ICa. However, in those cells, application of omega-conotoxin caused a 40-60% reduction in IK[Ca], whereas nifedipine reduced IK[Ca] but only in a subpopulation of cells. Therefore, IK[Ca] in sympathetic neurons is either coupled to N-type Ca2+ channels or is not selectively coupled to a single Ca(2+)-channel subtype. The preferential coupling of IK[Ca] channels with distinct ICa subtypes may be part of a mechanism to allow for selective modulation of neurotransmitter release. Preferential coupling may also be important for the differentiation and development of vertebrate neurons.

Presynaptic calcium concentration microdomains and transmitter release

n-Aequorin J, a luminescent protein which responds to calcium concentration changes in the order of several hundred micromoles, was injected into the preterminal fiber in the squid giant synapse. The activation of the presynaptic terminal leading to release of transmitter was accompanied by light emission at well-defined sites at the active zone in the presynaptic terminal. Location of these light emission sites was very much the same from one stimulus to the next, indicating that light emission was triggered by the inward calcium current occurring at specific and invariant locations. The distribution, size and number of these QEDs (quantum emission domains) coincides well with the location and number of active zones in the presynaptic terminal. The results imply that transmitter release is triggered by very well-localized calcium concentration changes that may be as high as several hundred micromoles.

"Strong" and "weak" synaptic differentiation in the crayfish opener muscle: structural correlates

The single excitor motoneuron to the limb opener muscle in the crayfish Procambarus clarkii provides multiterminal innervation to individual muscle fibers. At low impulse frequencies, these neuromuscular synapses generate a threefold larger junctional potential in fibers of the proximal region of the muscle compared to those in the central region. Focal extracellular recording from synapse-bearing "boutons" showed more quantal release at low frequencies in the proximal region. Structural correlates for the physiological differences were sought. Fluorescence microscopy of surface innervation stained with a vital fluorescent dye, 4-Di-2-Asp, showed that density of innervation was not greater in the proximal region and thus could not account for the overall differences in synaptic strength. Freeze fracture studies showed that the intramembrane organization of excitatory synapses and their active zones was qualitatively similar in proximal and central sites. Serial section electron microscopy of several innervation sites in proximal and central regions showed homogeneity in number and size of synapses. However, presynaptic dense bars (at release sites, or active zones) were longer and occurred at a higher density in proximal than in central synapses. The differences in number and length of presynaptic dense bars correlate positively with the differences in synaptic strength represented by junctional potential amplitudes and quantal contents of individual surface recording sites. Since many individual proximal synapses have multiple dense bars, co-operativity among these may serve to enhance transmitter output. It is concluded that occurrence of dense bars is a significant presynaptic correlate of synaptic strength in this neuron.

Exocytosis elicited by action potentials and voltage-clamp calcium currents in individual mouse pancreatic B-cells

1. Measurements of membrane capacitance, as an indicator of exocytosis, and intracellular Ca2+ concentration ([Ca2+]i) were used to determine the Ca2+ dependence of secretion in single pancreatic B-cells. 2. Exocytosis was dependent on a rise in [Ca2+]i and could be evoked by activation of voltage-dependent Ca2+ currents. The threshold for depolarization-induced release was 0.5 microM [Ca2+]i. Once the [Ca2+]i threshold was exceeded, exocytosis was rapidly (< 50 ms) initiated. When individual pulses were applied, exocytosis stopped immediately upon repolarization and the Ca2+ channels closed, although [Ca2+]i remained elevated for several seconds. 3. During repetitive stimulation (1 Hz), when [Ca2+]i attained micromolar levels, exocytosis also took place during the interpulse intervals albeit at a slower rate than during the depolarizations. 4. Exocytosis could be initiated by simulated action potentials. Whereas a single action potential only produced a small capacitance increase, and in some cells even failed to stimulate release, larger and more consistent responses were obtained with > or = four action potentials. 5. Comparison of the rates of exocytosis measured in response to depolarization, mobilization of Ca2+ from intracellular stores or infusion of Ca2+ through the patch pipette suggests that [Ca2+]i at the secretory sites attains a concentration of several micromolar. This is much higher than the average [Ca2+]i detected by microfluorimetry suggesting the existence of steep spatial gradients of [Ca2+]i within the B-cell. 6. Inclusion of inhibitors of Ca2+/calmodulin-dependent protein kinase II in the intracellular solution reduced the depolarization-induced exocytotic responses suggesting this enzyme may be involved in the coupling between elevation of [Ca2+]i to stimulation of the secretory machinery. 7. The size of the unitary exocytotic event was 2 fF, corresponding to a secretory granule diameter of 250 nm. 8. Over short periods, exocytosis may be extremely fast (1 pF/s or 500 granules/s), which is much higher than the rate of endocytosis (18 fF/s or 9 granules/s). Since the latter is in better agreement with the maximum rate of insulin secretion from islets (approximately 2 granules/s), we suggest that membrane retrieval may set an upper limit on the rate of exocytosis during extended periods of secretion.

Single calcium channels and acetylcholine release at a presynaptic nerve terminal

The relationship between calcium influx and the gating of transmitter release was examined at the release face of a cholinergic presynaptic nerve terminal using a technique that allows the simultaneous recording of both calcium channels at the single-channel level and quantal acetylcholine secretion. Acetylcholine release occurred during large inward calcium currents through many simultaneously open channels but was also gated by very small calcium transients, admitting less than 200 ions, when only one channel was open at a time. These findings provide functional support for a highly structured model of the transmitter release face in which the synaptic vesicle release mechanism is closely tethered to one or more presynaptic calcium channels and the opening of only one of these may be sufficient to trigger quantal secretion.

Calcium buffering in bursting Helix pacemaker neurons

Bursting pacemaker neurons of the snail Helix pomatia were voltage-clamped and Ca currents in response to depolarizing steps were recorded. Simultaneously, changes in intracellular Ca concentrations were measured using the fluorescent dye fura-2 and a highly sensitive digital camera. Ca influx through voltage-gated channels induced a spatially non-uniform increase in intracellular Ca. The Ca signals decayed with a time constant of about 5 s. By increasing the concentration of the indicator dye, its Ca-buffering capacity was enhanced and Ca transients in response to depolarization were diminished. Thereby, the endogenous Ca buffer capacity could be determined and was calculated to be about 480 buffered ions for every free Ca ion. The buffer capacity did not vary significantly with the amount of Ca influx within the range tested, suggesting that the buffer is not saturated at Ca concentrations of up to 1 microM.

Stable co-expression of calcium channel alpha 1, beta and alpha 2/delta subunits in a somatic cell line

1. The high-voltage-activated L-type calcium channel is a multi-protein complex of alpha 1, alpha 2/delta, beta and gamma subunits. The alpha 1 subunit contains the voltage-dependent calcium-conducting pore. Chinese hamster ovary (CHO) cells were stably transfected with the complementary DNA of the alpha 1, beta and alpha 2/delta subunits. These subunits were not detected in wild-type CHO cells. 2. The alpha 1 (CaCh2b) subunit itself directed the expression of functional calcium channels which bound calcium channel blockers and showed voltage-dependent activation and inactivation. 3. The co-expression of the alpha 1 subunit with the beta subunit (CaB1 gene) enhanced the density of the dihydropyridine binding sites 2- to 3-fold and increased dihydropyridine-sensitive barium inward currents (IBa) up to 3.5-fold from -13.3 microA/cm2 (alpha 1 subunit) to -46.7 microA/cm2 (alpha 1 and beta subunits). 4. Co-expression of the beta subunit did not change the sensitivity of IBa towards dihydropyridines, but accelerated current activation and inactivation and shifted the half-maximal steady-state activation and inactivation to slightly more hyperpolarizing potentials. 5. The co-expression of the alpha 2/delta subunit together with alpha 1 and beta subunits accelerated the inactivation kinetics of the channel without a major effect on the other parameters. 6. These results indicate that the beta and alpha 2/delta subunit interact with the alpha 1 subunit and modulate thereby the properties of the alpha 1 subunit-dependent inward current.

Microdomains with high Ca2+ close to IP3-sensitive channels that are sensed by neighboring mitochondria

Microdomains of high intracellular calcium ion concentration, [Ca2+]i, have been hypothesized to occur in living cells exposed to stimuli that generate inositol 1,4,5-trisphosphate (IP3). Mitochondrially targeted recombinant aequorin was used to show that IP3-induced Ca2+ mobilization from intracellular stores caused increases of mitochondrial Ca2+ concentration, [Ca2+]m, the speed and amplitude of which are not accounted for by the relatively small increases in mean [Ca2+]i. A similar response was obtained by the addition of IP3 to permeabilized cells but not by perfusion of cells with Ca2+ at concentrations similar to those measured in intact cells. It is concluded that in vivo, domains of high [Ca2+]i are transiently generated close to IP3-gated channels and sensed by nearby mitochondria; this may provide an efficient mechanism for optimizing mitochondrial activity upon cell stimulation.

Ca channel kinetics during the spontaneous heart beat in embryonic chick ventricle cells

The ability of Ca ions to inhibit Ca channels presents one of the most intriguing problems in membrane biophysics. Because of this negative feedback, Ca channels can regulate the current that flows through them. The kinetics of the channels depend on voltage, and, because the voltage controls the current, a strong interaction exists between voltage dependence and Ca dependence. In addition to this interaction, the proximity of pores and the local concentration of ions also determine how effectively the Ca ions influence channel kinetics. The present article proposes a model that incorporates voltage-dependent kinetics, current-dependent kinetics, and channel clustering. We have based the model on previous voltage-clamp data and on Ca and Ba action currents measured during the action potential in beating heart cells. In general we observe that great variability exists in channel kinetics from patch to patch: Ba or Ca currents have low or high amplitudes and slow or fast kinetics during essentially the same voltage regime, either applied step-protocols or spontaneous cell action potentials. To explain this variability, we have postulated that Ca channels interact through shared ions. The model we propose expands on our previous model for Ba currents. We use the same voltage-dependent rate constants for the Ca currents that we did for the Ba currents. However, we vary the current-dependent rate constants according to the species of the conducting ion. The model reproduces the main features of our data, and we use it to predict Ca channel kinetics under physiological conditions. Preliminary reports of this work have appeared (DeFelice et al., 1991, Biophys. J. 59:551a; Risso et al., 1992, Biophys. J. 61:248a).

Ca(2+)-dependent inactivation of Ca2+ current in Aplysia neurons: kinetic studies using photolabile Ca2+ chelators

1. The kinetics and sensitivity of the Ca(2+)-dependent inactivation of calcium current (ICa) were examined in intact cell bodies from the abdominal ganglion of Aplysia californica under two-electrode voltage clamp. 2. Rapid changes in the level of intracellular free calcium ([Ca2+]i) were generated at the cell surface by photolytic release of Ca2+ (nitr-5 and dimethoxy nitrophen) or Ca2+ buffer (diazo-4). 3. Diazo-4 increased ICa by 10-15% and slowed the rate of ICa decay when photolysed before a test pulse or between a prepulse and a test pulse. The predominant effect of further light flashes was to increase the amount of non-inactivating current (I infinity) remaining at the end of long (> 1 s) depolarizing pulses. 4. A rapid increase in [Ca2+]i buffering during ICa inactivation did not cause a rapid recovery of current but merely reduced the rate and extent of subsequent inactivation. This effect was not seen when Ba2+ was the charge carrier. 5. Photolytic release of Ca2+ from nitr-5 produced estimated Ca2+ jumps of 3-4 microM at the front surface of the cell but failed to augment inactivation either before or during ICa. In contrast, photolysis of DM-nitrophen 10-90 ms before the test pulse decreased peak ICa by about 30%. A flash given during ICa rapidly blocked 41 +/- 3% of peak current with a time constant of 3-4 ms at 17 degrees C. Similar results were seen with the barium current (IBa). 6. Microinjection of the potent phosphatase inhibitor microcystin-LR (5 microM) had variable effects on ICa inactivation and augmented the cyclic AMP-induced depression of the delayed rectifier (IK(V) by forskolin (100 microM) and 3-isobutyl-1-methylxanthine (IBMX; 200 microM). 7. Full recovery from inactivation measured in two-pulse experiments took at least 20 s. This slow recovery process was unaffected by increases in intracellular cyclic AMP elicited by direct injection or by bath application of forskolin and IBMX. It was also unaffected by decreases in cyclic AMP induced by injecting 2',5'-dideoxyadenosine (1 mM) or bath application of the Rp isomer of cyclic adenosine 3',5'-monophosphothioate (Rp-cAMPS; 200 microM). 8. A 'shell' model relating submembrane Ca2+ to inactivation was inconsistent with the experimental results since it greatly overestimated the effects of diazo-4 and predicted significant inactivation by nitr-5 photolysis. 9. A model linearly relating [Ca2+]i in a single Ca2+ channel 'domain' to inactivation more closely matched the experimental results with diazo-4 and DM-(ABSTRACT TRUNCATED AT 400 WORDS)

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)

Annexin I interactions with human neutrophil specific granules: fusogenicity and coaggregation with plasma membrane vesicles

The interactions of annexin I with specific granules isolated from human neutrophils were investigated. Unfractionated cytosol induced Ca(2+)-dependent granule self-aggregation and fusion of granules with model phospholipid vesicles. High Ca2+ concentrations were required for these processes (500-600 microM for the half-maximal rate of granule self-aggregation; 100-200 microM for the half-maximal rate of fusion with phospholipid vesicles). These activities were inhibited by a monoclonal antibody specific for annexin I and immunodepletion of cytosol by this antibody greatly reduced activity, implicating annexin I as the major mediator of these processes in neutrophil cytosol. The fact that the Ca2+ concentration dependences differed for different membranes suggests that specificity may be controlled by the type of intracellular membrane involved and the local Ca2+ concentration. Trypsin treatment of granules enhanced the rate of fusion of phospholipid vesicles with granules, suggesting that access to phospholipids in the granule membrane may be modulated by granule proteins or that a fusogenic protein factor in the granule membrane is activated by trypsin treatment. Coaggregation of specific granules with plasma membrane vesicles mediated by Ca2+ and annexin I was suggested by the fact that granules preincubated with Ca2+, cytosol and plasma membrane vesicles blocked the fusion of subsequently added phospholipid vesicles with the plasma membrane vesicles. These data suggest a role for annexin I as part of a multiprotein system involved in membrane-membrane contact necessary for exocytosis of specific granules in human neutrophils.

The relation between transmitter release and Ca2+ entry at the mouse motor nerve terminal: role of stochastic factors causing heterogeneity

The relation between quantal transmitter release and presynaptic Ca2+/Ba2+ entry at the mouse neuromuscular junction was studied, making use of the finding that in the presence of Ba2+ trains of nerve stimuli or brief nerve terminal depolarizations elicit "tails" of raised miniature end-plate potential frequency (fm) that reflect entry of Ba2+ per pulse, and hence effectiveness of pulses in opening Ca2+/Ba2+ channels; at the same time these pulses elicit end-plate potentials. With nerve stimulation in the presence of Ba2+ and Ca2+ and modulation of release by raised Mg2+ or bekanamycin, slopes of log quantal content (m) vs log apparent Ba2+ entry per pulse were close to 4, which is the same as the Hill coefficient for Ba2+ cooperativity derived from other data. With depolarizing pulses of varied intensity, however, similar plots gave slopes close to 2, with Ba2+ alone or in a mixture of Ca2+ and Ba2+. Thus, the relation between transmitter release and Ca2+ (or Ba2+) entry apparently depends upon how entry is varied; varying the numbers of channels opened is not the same as varying ion entry per channel. A mathematical model was developed to examine the consequences of heterogeneity of local Ca2+ (or Ba2+) between release sites, arising because of stochastic variation of number and time course of Ca2+ channels opened per site; the experimental results were consistent with this model. It was therefore concluded that release is normally governed by intracellular Ca2+ close to points of Ca2+ entry through channels; stochastic factors give rise to more release than if Ca2+ were homogeneously distributed. If Ca2+ channels are uniformly close to release sites the average number of channels opened per site per action potential may be as low as 4.

Time courses of calcium and calcium-bound buffers following calcium influx in a model cell

Fixed and diffusible calcium (Ca) buffers shape the spatial and temporal distribution of free Ca following Ca entry through voltage-gated ion channels. This modeling study explores intracellular Ca levels achieved near the membrane and in deeper locations following typical Ca currents obtained with patch clamp experiments. Ca ion diffusion sets an upper limit on the maximal average Ca concentration achieved near the membrane. Fixed buffers restrict Ca elevation spatially to the outermost areas of the cell and slow Ca equilibration. Fixed buffer bound with Ca near the membrane can act as Ca source after termination of Ca influx. The relative contribution of fixed versus diffusible buffers to shaping the Ca transient is determined to a large extent by the binding rate of each buffer, with diffusible buffer dominating at equal binding rates. In the presence of fixed buffers, diffusible buffers speed Ca equilibration throughout the cell. The concentration profile of Ca-bound diffusible buffer differs from the concentration profile of free Ca, reflecting theoretical limits on the temporal resolution which can be achieved with commonly used diffusible Ca indicators. A Ca indicator which is fixed to an intracellular component might more accurately report local Ca concentrations.

Cytosolic Ca2+ gradients, Ca2+ binding proteins and synaptic plasticity

Trains of spikes encoded by presynaptic neurons are decoded into rises in cytosolic Ca2+ concentration both in presynaptic terminals and in postsynaptic dendrites. Intracellular [Ca2+] rises trigger neurotransmitter release and also induce short- and long-term modifications of synaptic efficacy. These modifications can be potentiation or depression depending on the intensity of stimuli. A dynamic mechanism, "dynamic decoding", is proposed to understand the multiplicity of the functions of Ca2+, based on recent knowledge of Ca2+ binding proteins and of the dynamics of Ca2+ signaling. The dynamic model is in many ways superior to static models, and may be applied to various neuronal functions including the induction of long-term plasticity in cerebral cortex.

Macroscopic and unitary properties of physiological ion flux through T-type Ca2+ channels in guinea-pig heart cells

1. We sought to distinguish two types of Ca2+ channel in guinea-pig ventricular cells (T-type and L-type) and to characterize their respective gating and permeation properties when Ca2+ (1-10 mM) is the charge carrier, as is the case physiologically. 2. Na+ was removed from both the external and internal solutions to eliminate currents through Na+ channels and Na(+)-Ca2+ exchange. Major differences in the voltage dependence of steady-state inactivation were exploited to separate the two Ca2+ current components. 3. From a holding potential of -50 mV, only L-type channels were available to open with depolarization. When holding at -90 mV, T-type channels contributed an additional rapidly inactivating component superimposed upon the L-type current. Only the L-type channels thus identified were sensitive to the dihydropyridine Ca2+ channel blocker nitrendipine. 4. T-type currents, measured by taking the difference between the currents elicited from a holding potential of -90 mV and those elicited from -50 mV, peaked within 10 ms and decayed completely within 50-100 ms. 5. Macroscopic T-type currents were largest during depolarizing pulses between -40 and -30 mV (peak current density of 0.62 +/- 0.21 nA nF-1) and decreased at more positive potentials, becoming unmeasurably small above 0 mV. 6. Unitary currents recorded with similar ionic conditions and voltage protocols exhibited a single-channel conductance of 4-5 pS in 10 mM Ca2+. Ensemble average currents through a single channel reproduced accurately the time course of whole-cell T-type current. Permeation properties could not explain the absence of macroscopic T-type currents at positive test potentials, which must therefore be attributable to gating. 7. Convolution analysis was employed to clarify the single-channel basis of the rapidly decaying current waveform of T-type channels. The latencies to first opening and reopening, which reflect activation and deactivation, influenced the waveform most strikingly. Open times were sufficiently brief that they contributed little to shaping the average current. Thus, macroscopic inactivation largely reflects rate-limiting activation events. 8. The unitary current amplitudes and peak open probabilities measured for single T-type channels, when compared to the average macroscopic T-type current density, predict 10.6 functional channels per picofarad, or approximately 1700 T-type channels per typical ventricular myocyte.

Evaluation of cellular mechanisms for modulation of calcium transients using a mathematical model of fura-2 Ca2+ imaging in Aplysia sensory neurons

A theoretical model of [Ca++]i diffusion, buffering, and extrusion was developed for Aplysia sensory neurons, and integrated with the measured optical transfer function of our fura-2 microscopic recording system, in order to fully simulate fura-2 video or photomultiplier tube measurements of [Ca++]i. This allowed an analysis of the spatial and temporal distortions introduced during each step of fura-2 measurements of [Ca++]i in cells. In addition, the model was used to evaluate the plausibility of several possible mechanisms for modulating [Ca++]i transients evoked by action potentials. The results of the model support prior experimental work (Blumenfeld, Spira, Kandel, and Siegelbaum, 1990. Neuron. 5: 487-499), suggesting that 5-HT and FMRFamide modulate action potential-induced [Ca++]i transients in Aplysia sensory neurons through changes in Ca++ influx, and not through changes in [Ca++]i homeostasis or release from internal stores.

Time course of transmitter release calculated from simulations of a calcium diffusion model

A three-dimensional presynaptic calcium diffusion model developed to account for characteristics of transmitter release was modified to provide for binding of calcium to a receptor and subsequent triggering of exocytosis. When low affinity (20 microM) and rapid kinetics were assumed for the calcium receptor triggering exocytosis, and stimulus parameters were selected to match those of experiments, the simulations predicted a virtual invariance of the time course of transmitter release to paired stimulation, stimulation with pulses of different amplitude, and stimulation in different calcium solutions. The large temperature sensitivity of experimental release time course was explained by a temperature sensitivity of the model's final rate limiting exocytotic process. Inclusion of calcium tail currents and a saturable buffer with finite binding kinetics resulted in high peak calcium transients near release sites, exceeding 100 microM. Models with a single class of calcium binding site to the secretory trigger molecule failed to produce sufficient synaptic facilitation under this condition. When at least one calcium ion binds to a different site having higher affinity and slow kinetics, facilitation again reaches levels similar to those seen experimentally. It is possible that the neurosecretory trigger molecule reacts with calcium at more than one class of binding site.

Propagation of a calcium pulse between osteoblastic cells

Using rat calvaria cells in primary culture monolayers and bone-like nodules, and isolated rat osteosarcoma cells, we show via laser scanning confocal microscopy and fluorescent indicator fluo-3/AM, that mechanical perturbation of a cell results in a transient increase (pulse) of measured intracellular calcium concentration that propagates from cell to cell, even between cells connected only by a thin process. The calcium pulse does not occur in the mechanically perturbed cell in calcium-free bathing medium, nor is there pulse propagation under this condition. Halothane, which blocks gap junctions, inhibits propagation. Propagation velocity does not decrease with successive cell to cell steps. These observations suggest the existence of a self-regenerating calcium signaling mechanism that may be based on a form of calcium-induced calcium release.

Submicroscopic Ca2+ diffusion mediates inhibitory coupling between individual Ca2+ channels

Dihydropyridine-sensitive Ca2+ channels in heart demonstrate an important negative feedback property: they close, or inactivate, in response to prior Ca2+ entry. We now find that Ca2+ influx through one channel can selectively contribute to the inactivation of another adjacent channel, without a generalized elevation of bulk intracellular Ca2+ concentration. Intracellular application of the Ca2+ chelator BAPTA greatly diminishes such negative interactions within Ca2+ channel pairs. These findings demonstrate that Ca2+ currents are controlled not only by intrinsic channel properties, but also by local diffusive interactions among neighboring channels. Such inhibitory coupling among channels provides a concrete example of localized Ca2+ signaling, long proposed to exist on the basis of theoretical calculations.

Conditional probability analysis for a domain model of Ca(2+)-inactivation of Ca2+ channels

The domain model of Ca2+ inactivation of Ca2+ channels, which has been used to explain rapid inactivation of whole cell Ca2+ currents in pancreatic beta cells, is applied to single-time and conditional open probability measurements on guinea pig ventricular myocyte Ca2+ channels. These two measurements greatly constrain the choice of kinetic constants in the model. Calculations with the model provide a simple quantitative explanation of recent experimental results, including a slow increase in the inactivation rate.

Calcium-activated currents in cultured neurones from rat dorsal root ganglia

1. Voltage-activated Ca2+ currents and caffeine (1 to 10 mM) were used to increase intracellular Ca2+ in rat cultured dorsal root ganglia (DRG) neurones. Elevation of intracellular Ca2+ resulted in activation of inward currents which were attenuated by increasing the Ca2+ buffering capacity of cells by raising the concentration of EGTA in the patch solution to 10 mM. Low and high voltage-activated Ca2+ currents gave rise to Cl- tail currents in cells loaded with CsCl patch solution. Outward Ca2+ channel currents activated at very depolarized potentials (Vc + 60 mV to + 100 mV) also activated Cl- tail currents, which were enhanced when extracellular Ca2+ was elevated from 2 mM to 4 mM. 2. The Ca(2+)-activated Cl- tail currents were identified by estimation of tail current reversal potential by use of a double pulse protocol and by sensitivity to the Cl- channel blocker 5-nitro 2-(3-phenyl-propylamino) benzoic acid (NPPB) applied at a concentration of 10 microM. 3. Cells loaded with Cs acetate patch solution and bathed in medium containing 4 mM Ca2+ also had prolonged Ca(2+)-dependent tail currents, however these smaller tail currents were insensitive to NPPB. Release of Ca2+ from intracellular stores by caffeine gave rise to sustained inward currents in cells loaded with Cs acetate. Both Ca(2+)-activated tail currents and caffeine-induced inward currents recorded from cells loaded with Cs acetate were attenuated by Tris based recording media, and had reversal potentials positive to 0 mV suggesting activity of Ca(2+)-activated cation channels.4. Our data may reflect (a) different degrees of association between Ca2+-activated channels with voltage-gated Ca2+ channels, (b) distinct relationships between channels and intracellular Ca2" stores and Ca2+ homeostatic mechanisms, (c) regulation of Ca2+-activated channels by second messengers, and (d) varying channel sensitivity to Ca2 , in the cell body of DRG neurones.

Neuronal Ca2+ signalling takes the local route

The past year has seen several sets of experimental results demonstrate that fast, large and highly localized rises in intracellular Ca2+ concentration can occur in neurons. These results confirm previous theoretical predictions of acute spatial compartmentalization of Ca2+ signalling, and document a form of signalling that may occur whenever rapid and local signal processing is the goal. The dimensions involved present severe challenges for attempts to directly measure these signalling events.

Calcium gradients and buffers in bovine chromaffin cells

1. Digital imaging and photometry were used in conjunction with the fluorescent Ca2+ indicator, Fura-2, to examine intracellular Ca2+ signals produced by depolarization of single adrenal chromaffin cells. 2. Depolarization with a patch pipette produced radial gradients of Ca2+ within the cell, with Ca2+ concentration highest in the vicinity of the plasma membrane. These gradients dissipated within a few hundred milliseconds when the voltage-gated Ca2+ channels were closed. 3. Dialysis of Fura-2 into the chromaffin cell caused concentration-dependent changes in the depolarization-induced Ca2+ signal, decreasing its magnitude and slowing its recovery time course. These changes were used to estimate the properties of the endogenous cytoplasmic Ca2+ buffer with which Fura-2 competes for Ca2+. 4. The spatially averaged Fura-2 signal was well described by a model assuming fast competition between Fura-2 and an endogenous buffer on a millisecond time scale. Retrieval of calcium by pumps and slow buffers occurs on a seconds-long time scale. No temporal changes indicative of buffers with intermediate kinetics could be detected. 5. Two independent estimates of the capacity of the fast endogenous Ca2+ buffer suggest that 98-99% of the Ca2+ entering the cell normally is taken up by this buffer. This buffer appears to be immobile, because it does not wash out of the cell during dialysis. It has a low affinity for Ca2+ ions, because it does not saturate with 1 microM-Ca2+ inside the cell. 6. The low capacity, affinity and mobility of the endogenous Ca2+ buffer makes it possible for relatively small amounts of exogenous Ca2+ buffers, such as Fura-2, to exert a significant influence on the characteristics of the Ca2+ concentration signal as measured by fluorescence ratios. On the other hand, even at moderate Fura-2 concentrations (0.4 mM) Fura-2 will dominate over the endogenous buffers. Under these conditions radiometric Ca2+ concentration signals are largely attenuated, but absolute fluorescence changes (at 390 nm) accurately reflect calcium fluxes.

Calcium requirements for secretion in bovine chromaffin cells

1. Measurements of membrane capacitance and intracellular Ca2+ concentration, [Ca2+]i, were used to examine the Ca2+ dependence of secretion in single adrenal chromaffin cells. 2. Intracellular dialysis of Ca2+, through a patch pipette, promoted secretion; the rate of secretion increased monotonically as [Ca2+]i was elevated, while the total amount of secretion reached a maximum at 1.5 microM-Ca2+ and declined at high [Ca2+]i. 3. Release of Ca2+ from internal stores, using bradykinin or ionomycin, transiently elevated [Ca2+]i and the rate of secretion. 4. Considering responses to both Ca2+ dialysis and release from internal stores, it appears that the rate of secretion increases over a range of [Ca2+]i levels above 0.2 microM and saturates at concentrations greater than 10 microM, if at all. Secretion appears to have a Hill coefficient for Ca2+ of about 2. At [Ca2+]i greater than 1-2 microM, prolonged elevation of [Ca2+]i, via dialysis, produced lower rates of secretion than transient elevation of [Ca2+]i caused by release from internal stores. This may have been caused by a depletion of readily releasable chromaffin granules during prolonged elevation of [Ca2+]i. 5. Brief depolarizing pulses produced transient rises in both [Ca2+]i and the rate of secretion. The ability of these pulses to evoke secretion 'washed out' during prolonged intracellular dialysis, due to both reduced Ca2+ influx and a diminished ability of the cell to secrete in response to a given Ca2+ load. 6. The kinetics of the secretory response depended upon the size of the depolarization-induced Ca2+ load; small rises in [Ca2+]i increased membrane capacitance only during the depolarization, while larger rises in [Ca2+]i produced increases both during and following the depolarization. The secretory responses that outlasted the depolarization appeared to be due to persistent elevation of [Ca2+]i. Secretory responses were sometimes followed by a slower decline in membrane capacitance, probably due to endocytosis of membrane. 7. Comparison of the rates of secretion measured during depolarization to those produced by Ca2+ dialysis or release from internal stores suggests that [Ca2+]i at secretory sites can exceed 10 microM during depolarization. The spatially averaged measurements of [Ca2+]i indicate much smaller levels of [Ca2+]i; thus, there must be pronounced spatial gradients of [Ca2+]i during depolarization.

Microdomains of high calcium concentration in a presynaptic terminal

Increases in intracellular calcium concentration are required for the release of neurotransmitter from presynaptic terminals in all neurons. However, the mechanism by which calcium exerts its effect is not known. A low-sensitivity calcium-dependent photoprotein (n-aequorin-J) was injected into the presynaptic terminal of the giant squid synapse to selectively detect high calcium concentration microdomains. During transmitter release, light emission occurred at specific points or quantum emission domains that remained in the same place during protracted stimulation. Intracellular calcium concentration microdomains on the order of 200 to 300 micromolar occur against the cytoplasmic surface of the plasmalemma during transmitter secretion, supporting the view that the synaptic vesicular fusion responsible for transmitter release is triggered by the activation of a low-affinity calcium-binding site at the active zone.

Kinetics of diffusion in a spherical cell. I. No solute buffering

Realistic neuron models that involve effects of concentration changes of second messengers on ion channels must include processes such as diffusion and solute buffering. These processes, which span a wide range of spatial and temporal scales, may impose a severe computational burden. In this paper and its companion, we examine the kinetics of diffusion and present methods for stimulating it accurately and efficiency. The problem of calcium diffusion in a spherical cell is used as a device to demonstrate the practical application of our analysis. However, the scope of these papers is not limited to this problem. The same analysis that we apply and concerns that we raise are germane to the spread of any second messenger, and can be adapted to other geometries. The focus of this paper is the simplest case: diffusion in the absence of solute buffering. This analysis also applies whenever buffering is so fast that it is instantaneous compared to diffusion, or so slow that concentration gradients have dissipated before substantial buffering takes place. The second paper investigates the more difficult situation where diffusion and buffering occur at comparable rates. In the absence of buffering, concentration changes produced by diffusion can be fit by an infinite series of exponential terms. We show how to design a model with N + 1 compartments that fits the N slowest terms of this series exactly in a shell just inside the cell membrane.

Dihydropyridine-sensitive and omega-conotoxin-sensitive calcium channels in a mammalian neuroblastoma-glioma cell line

1. Pharmacological and kinetic properties of high-voltage-activated (HVA) Ca2+ channel currents were studied using the whole-cell and perforated patch-clamp methods in a mouse neuroblastoma and rat glioma hybrid cell line, NG108-15, differentiated by dibutyryl cyclic AMP or by prostaglandin E1 and theophylline. 2. The HVA currents were separated into two components by use of two organic Ca2+ channel antagonists, omega-conotoxin GVIA (omega CgTX) and a dihydropyridine (DHP) compound, nifedipine. One current component, IDHP, was blocked by nifedipine (Kd = 8.2 nM) and was resistant to omega CgTX. Conversely, the other component, I omega CgTX, was irreversibly blocked by omega CgTX and was resistant to DHPs. Thus, IDHP could be studied in isolation by a short application of omega CgTX, while I omega CgTX could be studied in the presence of nifedipine. 3. The voltage for half-activation of IDHP was smaller than that of I omega CgTX by 13 mV. IDHP was activated at potentials that were subthreshold for voltage-dependent K+ currents of the cell, whereas I omega CgTX was not. 4. Time courses of activation and deactivation of IDHP were faster than those of I omega CgTX. 5. Voltage-dependent inactivation was small for both IDHP and I omega CgTX at any potential. 6. Ca(2+)-dependent inactivation of IDHP was faster and more prominent than that of I omega CgTX. The time course of the Ca(2+)-dependent inactivation of IDHP, but not I omega CgTX, was slowed as the membrane potential was made more positive between -20 and 30 mV, although amplitude of the current was increased. 7. Alkaline earth metal ions carried the two components of IHVA in the same order: Ba2+ greater than Sr2+ greater than Ca2+. 8. Metal ions blocked the two components of IHVA in the same order of potency: Gd3+ greater than La3+ greater than Cd2+ greater than Cu2+ greater than Mn2+ greater than Ni2+. 9. An alkylating agent, N-ethylmaleimide (NEM, 0.1 mM), selectively augmented IDHP by 30%. 10. During the course of cellular differentiation induced by dibutyryl cyclic AMP, IDHP appeared earlier than I omega CgTX. 11. These results indicate that two classes of Ca2+ channels contribute to the HVA currents of this cell line. The DHP-sensitive channel is more apt to generate Ca2+ spikes and Ca2+ plateau potentials than the omega CgTX-sensitive channel.

Role of residual calcium in synaptic depression and posttetanic potentiation: fast and slow calcium signaling in nerve terminals

Trains of action potentials evoked rises in presynaptic Ca2+ concentration ([Ca2+]i) at the squid giant synapse. These increases in [Ca2+]i were spatially nonuniform during the trains, but rapidly equilibrated after the trains and slowly declined over hundreds of seconds. The trains also elicited synaptic depression and augmentation, both of which developed during stimulation and declined within a few seconds afterward. Microinjection of the Ca2+ buffer EGTA into presynaptic terminals had no effect on transmitter release or synaptic depression. However, EGTA injection effectively blocked both the persistent Ca2+ signals and augmentation. These results suggest that transmitter release is triggered by a large, brief, and sharply localized rise in [Ca2+]i, while augmentation is produced by a smaller, slower, and more diffuse rise in [Ca2+]i.

Ca(2+)- and voltage-dependent inactivation of Ca2+ currents in rat intermediate pituitary

We used single electrode voltage-clamp methods to investigate the inactivation of Ca2+ currents in melanotrophs of the intermediate lobe of the pituitary. The low threshold transient current was inactivated by brief prepulses to potentials above -30 mV and inhibition remained complete as prepulse potential was increased from 0 to +70 mV. Both the high threshold transient and sustained currents, however, were inhibited to the greatest extent (60%) by prepulses to 0 mV. Prepulses to more positive potentials close to the Ca2+ reversal potential produced much less (15%) inactivation. Buffering intracellular Ca2+ by including BAPTA in the recording electrode or replacing extracellular Ca2+ with Ba2+ reduced the effect of prepulses. Slowing Ca2+ extrusion by reducing the Na+ gradient across the cell increased the duration of the effect of prepulses. We conclude that the low threshold, transient current is inactivated primarily by membrane voltage while both the high threshold currents are inhibited by elevation of intracellular Ca2+ although the two currents display different sensitivities to Ca2+ concentration. Inhibition of the high threshold transient current by the neurotransmitter dopamine, however, acts by a different mechanism not mediated by Ca(2+)-dependent inactivation.

Gating of L-type Ca2+ channels in embryonic chick ventricle cells: dependence on voltage, current and channel density

1. L-type calcium channels in embryonic chick heart ventricle have voltage-dependent, time-variant kinetics when they conduct inward currents carried by 20 mM-Ba2+. Depolarizing the membrane from -20 to 20 mV increases mean open time from 1.4 to 4.2 ms. Mean open time increases monotonically with voltage. The single-channel conductance, 18 +/- 2 pS, is approximately linear over this voltage range, and the extrapolated reversal potential is 38 +/- 5 mV. 2. In cell-attached patches with five or more L-type Ca2+ channels in the patch, the currents elicited by 500 ms depolarizing steps, from a -80 mV holding potential, inactivate rapidly and have large tail currents. In the same patch, currents from a -40 mV holding potential are smaller, inactivate more slowly, and have practically no tail currents. 3. In cell-attached patches containing one of two L-type Ca2+ channels, currents from -80 or -40 mV are virtually identical, and they are similar to the currents from multichannel patches held at -40 mV. 4. The voltage-dependent, time-variant kinetics of individual L-type Ca2+ channels are unaltered if the patch is removed from the cell and forms an inside-out configuration. In these experiments the internal membrane was bathed with an artificial, intracellular-like solution containing no phosphorylating enzymes or substrates. 5. Cells bathed in 20 mM-Ba2+ solutions and held at -80 mV have currents with an early phase that inactivates in tens of milliseconds, a late phase that inactivates in hundreds of milliseconds, and a large, slow tail current. Currents from -40 mV have only the late phase and practically no tails. However, if the maximum current is less than 0.1 pA pF-1, records from either -80 or -40 mV are virtually identical, and they are similar to currents from cells with higher channel density held at -40 mV. Furthermore, if cells are stimulated before full recovery from inactivation, the reduced current is accompanied by slower inactivation. 6. Whole-cell currents in 1.5 mM-Ca2+ solutions are entirely abolished by addition of 20 microM-nifedipine, and they are enhanced 2-3 times by addition of 30 microM-cyclic AMP and 3 mM-ATP to the whole-cell recording electrode. The whole-cell currents in 20 mM-Ba2+ solutions are also completely blocked by 20 microM-nifedipine, regardless of kinetics or holding potential. Thus, by definition, the cells we are studying contain only L-type channels.(ABSTRACT TRUNCATED AT 400 WORDS)

Inactivation kinetics of calcium current of acutely dissociated CA1 pyramidal cells of the mature guinea-pig hippocampus

1. The process of inactivation of the Ca2+ current of acutely dissociated pyramidal cells from the CA1 subfield of mature guinea-pig hippocampus was characterized. The decline of the current after rapid activation could be approximated well by the sum of two exponentials (time constants approximately 200 ms and 2 s) and a constant offset. 2. The time constants of inactivation exhibited a voltage dependence consistent with a voltage-dependent mechanism. However, under conditions which normally counteract Ca(2+)-dependent inactivation (viz. intracellular bis(O-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid (BAPTA) and external Ba2+) all three showed a U-shaped inactivation curve, characteristic of Ca(2+)-dependent inactivation. 3. The rate of inactivation was found to increase with current at a given voltage; however, increasing external divalent ion concentrations did not accelerate inactivation. 4. Calcium imaging experiments, using the Ca(2+)-sensitive probe, Fura-2, were performed to estimate the accumulation of Ca2+ in the presence of 10 mM-intracellular BAPTA. Under these conditions voltage steps which induced maximal Ca2+ currents lead to free Ca2+ concentrations of less than 500 nM in the bulk of the cytoplasm. 5. Elevation of the intracellular free Ca2+ concentration to above 1 microM suppressed all the components of the Ca2+ current. However, even at a concentration of 3 microM-Ca2+ the U-shaped inactivation curve persisted. 6. Substitution of Ca2+ for Ba2+ led to an acceleration of inactivation through an increase in the proportion of the fast process of inactivation and an acceleration of both the fast and slow rates of inactivation. 7. During the slow decline of Ca2+ current ('run-down') the proportion of all three components remained approximately constant and there was little change in the rate of inactivation. 8. On the basis of the results I suggest that inactivation results fro a dual process of voltage- and Ca(2+)-dependent inactivation. Ca(2+)-dependent inactivation seems to result from the accumulation of Ca2+ close to the channel mouth. 9. The macroscopic properties of the Ca2+ channel are consistent with the existence of one channel type in the CA1 pyramidal cells.

Bursting response to current-evoked depolarization in rat CA1 pyramidal neurons is correlated with lucifer yellow dye coupling but not with the presence of calbindin-D28k

Calbindin-D28k (CaBP) immunohistochemistry has been combined with electrophysiological recording and Lucifer Yellow (LY) cell identification in the CA1 region of the rat hippocampal formation. CaBP is shown to be contained within a distinct sub-population of CA1 pyramidal cells which is equivalent to the superficial layer described by Lorente de Nó (1934). The neurogenesis of these CaBP-positive neurons occurs 1-2 days later than the CaBP-negative neurons in the deep pyramidal cell layer, as shown by 3H-thymidine autoradiography. No correlation could be found between the presence or absence of CaBP and the type of electrophysiological response to current-evoked depolarizing pulses. The latter could be separated into bursting or non-bursting types, and the bursting-type response was nearly always found to be associated with the presence of LY dye coupling. Furthermore, when dye coupling involved three neurons, a characteristic pattern was observed which may represent the coupling of phenotypically identical neurons into distinct functional units within the CA1 pyramidal cell layer. In this particular case the three neurons were all likely to be CaBP-positive.

A model for intracellular calcium signaling and the coordinate regulation of hormone biosynthesis, receptors and secretion

A two-state model for the stimulus-induced nongraded response of a single cell is formulated. Individual metestrus gonadotropes stimulated with LHRH operate as a simple switch: either on or off. At a given concentration of stimulus some gonadotropes switch on, while others do not switch on, secretion. The probability of a gonadotrope being in the secretory state is enhanced with each increment of LHRH concentration. Individual gonadotropes in a secretory state are envisioned to decrease their number of LHRH receptors and to switch off LH biosynthesis. On the other hand, individual gonadotropes that are not in a secretory state are thought to increase their number of LHRH receptors and to switch on LH biosynthesis. The group of individuals in the population that have thresholds falling in the range of a given stimulus initiate secretion. And, the group of individuals in the population that have thresholds that fall above the range of a given stimulus do not initiate secretion. More remarkable is evidence that the cells that are protected from hormone secretion nevertheless respond with a set of intracellular signals and this provides a new perspective of how they switch on hormone biosynthesis and up-regulate the LHRH receptors. These changes are envisioned to reduce the threshold of an individual cell and accordingly to enhance the probability that the cell responds in the secretory state with the next stimulus. This scheme would appear to lead to automatic cycles of secretion and biosynthesis since an individual cell can occupy only one of two states at any time and occupancy of either state promotes change to the other. This may provide a solution to the problem of how an endocrine gland might reconcile differences in the time-course of hormone secretion which occurs rapidly and hormone biosynthesis that requires a longer period of time. Parenthetically, the model may also be adapted to the case where the vast majority of individuals in the population are generally subthreshold in relation to the physiological stimulus: such an adaption leads to interesting ways of viewing the mammalian reproductive cycle and the regulation of the preovulatory LH surge. A two-state model of the internal Ca2+ store is outlined here to stimulate thought on how the intracellular signals of each binary state may switch a variety of cellular responses either on or off. The model provides a new perspective on the coordinate regulation of hormone biosynthesis, receptors, and secretion that may be useful in the final reconciliation of population studies with insights about individual cells.

L-type Ca2+ currents in ascidian eggs

We have studied Ca2+ currents in ascidian eggs using the whole-cell clamp technique. T and L components, as observed in somatic cells, are present and the L-type current predominates. Since the IV relationship for these inward currents overlap at -30 mV, separation of the two components using different voltage regimes is not feasible. Increasing external Ca2+ results in larger currents. The L-type current decreases in a dose-dependent fashion in the presence of Mn2+ and Nifedipine, while the T-type current is inhibited in Ni2+. When Ba2+ was used as the carrier ion, channel kinetics and conductance were completely altered. Considering the density and kinetics of L-type channels in unfertilized eggs it is probable they play an important role in regulating cytosolic Ca2+ during early developmental processes.

Whole-cell clamp of dissociated photoreceptors from the eye of Lima scabra

Voltage-dependent membrane currents were investigated in enzymatically dissociated photoreceptors of Lima scabra using the whole-cell clamp technique. Depolarizing steps to voltages more positive than -10 mV elicit a transient inward current followed by a delayed, sustained outward current. The outward current is insensitive to replacement of a large fraction of extracellular Cl- with the impermeant anion glucuronate. Superfusion with tetraethylammonium and 4-aminopyridine reversibly abolishes the outward current, and internal perfusion with cesium also suppresses it, indicating that it is mediated by potassium channels. Isolation of the inward current reveals a fast activation kinetics, the peak amplitude occurring as early as 4-5 ms after stimulus onset, and a relatively rapid, though incomplete inactivation. Within the range of voltages examined, spanning up to +90 mV, reversal was not observed. The inward current is not sensitive to tetrodotoxin at concentrations up to 10 microM, and survives replacement of extracellular Na with tetramethylammonium. On the other hand, it is completely eliminated by calcium removal from the perfusing solution, and it is partially blocked by submillimolar concentrations of cadmium, suggesting that it is entirely due to voltage-dependent calcium channels. Analysis of the kinetics and voltage dependence of the isolated calcium current indicates the presence of two components, possibly reflecting the existence of separate populations of channels. Barium and strontium can pass through these channels, though less easily than calcium. Both the activation and the inactivation become significantly more sluggish when these ions serve as the charge carrier. A large fraction of the outward current is activated by preceding calcium influx. Suppression of this calcium-dependent potassium current shows a small residual component resembling the delayed rectifier. In addition, a transient outward current sensitive to 4-aminopyridine (Ia) could also be identified. The relevance of such conductance mechanisms in the generation of the light response in Lima photoreceptors is discussed.