Fast kinetics of calcium liberation induced in Xenopus oocytes by photoreleased inositol trisphosphate

Hindered cytoplasmic diffusion of inositol trisphosphate restricts its cellular range of action

The range of action of intracellular messengers is determined by their rates of diffusion and degradation. Previous measurements in oocyte cytoplasmic extracts indicated that the Ca²⁺-liberating second messenger inositol trisphosphate (IP₃) diffuses with a coefficient (~280 μm² s^-1) similar to that in water, corresponding to a range of action of ~25 μm. Consequently, IP₃ is generally considered a "global" cellular messenger. We reexamined this issue by measuring local IP₃-evoked Ca²⁺ puffs to monitor IP₃ diffusing from spot photorelease in neuroblastoma cells. Fitting these data by numerical simulations yielded a diffusion coefficient (≤10 μm² s^-1) about 30-fold slower than that previously reported. We propose that diffusion of IP₃ in mammalian cells is hindered by binding to immobile, functionally inactive receptors that were diluted in oocyte extracts. The predicted range of action of IP₃ (<5 μm) is thus smaller than the size of typical mammalian cells, indicating that IP₃ should better be considered as a local rather than a global cellular messenger.

Imaging local Ca2+ signals in cultured mammalian cells

Cytosolic Ca2+ ions regulate numerous aspects of cellular activity in almost all cell types, controlling processes as wide-ranging as gene transcription, electrical excitability and cell proliferation. The diversity and specificity of Ca2+ signaling derives from mechanisms by which Ca2+ signals are generated to act over different time and spatial scales, ranging from cell-wide oscillations and waves occurring over the periods of minutes to local transient Ca2+ microdomains (Ca2+ puffs) lasting milliseconds. Recent advances in electron multiplied CCD (EMCCD) cameras now allow for imaging of local Ca2+ signals with a 128 x 128 pixel spatial resolution at rates of >500 frames sec(-1) (fps). This approach is highly parallel and enables the simultaneous monitoring of hundreds of channels or puff sites in a single experiment. However, the vast amounts of data generated (ca. 1 Gb per min) render visual identification and analysis of local Ca2+ events impracticable. Here we describe and demonstrate the procedures for the acquisition, detection, and analysis of local IP3-mediated Ca2+ signals in intact mammalian cells loaded with Ca2+ indicators using both wide-field epi-fluorescence (WF) and total internal reflection fluorescence (TIRF) microscopy. Furthermore, we describe an algorithm developed within the open-source software environment Python that automates the identification and analysis of these local Ca2+ signals. The algorithm localizes sites of Ca2+ release with sub-pixel resolution; allows user review of data; and outputs time sequences of fluorescence ratio signals together with amplitude and kinetic data in an Excel-compatible table.

Inositol trisphosphate receptor and ion channel models based on single-channel data

The inositol trisphosphate receptor (IPR) plays an important role in controlling the dynamics of intracellular Ca(2+). Single-channel patch-clamp recordings are a typical way to study these receptors as well as other ion channels. Methods for analyzing and using this type of data have been developed to fit Markov models of the receptor. The usual method of parameter fitting is based on maximum-likelihood techniques. However, Bayesian inference and Markov chain Monte Carlo techniques are becoming more popular. We describe the application of the Bayesian methods to real experimental single-channel data in three ion channels: the ryanodine receptor, the K(+) channel, and the IPR. One of the main aims of all three studies was that of model selection with different approaches taken. We also discuss the modeling implications for single-channel data that display different levels of channel activity within one recording.

Targeting and clustering of IP3 receptors: key determinants of spatially organized Ca2+ signals

Inositol 1,4,5-trisphosphate receptors (IP3R) are intracellular Ca2+ channels that are almost ubiquitously expressed in animal cells. The spatiotemporal complexity of the Ca2+ signals evoked by IP3R underlies their versatility in cellular signaling. Here we review the mechanisms that contribute to the subcellular targeting of IP3R and the dynamic interplay between IP3R that underpin their ability to generate complex intracellular Ca2+ signals.

Calcium-dependent inactivation and the dynamics of calcium puffs and sparks

Localized intracellular Ca(2+) elevations known as puffs and sparks arise from the cooperative activity of inositol 1,4,5-trisphosphate receptor Ca(2+) channels (IP(3)Rs) and ryanodine receptor Ca(2+) channels (RyRs) clustered at Ca(2+) release sites on the surface of the endoplasmic reticulum or sarcoplasmic reticulum. When Markov chain models of these intracellular Ca(2+)-regulated Ca(2+) channels are coupled via a mathematical representation of a Ca(2+) microdomain, simulated Ca(2+) release sites may exhibit the phenomenon of "stochastic Ca(2+) excitability" reminiscent of Ca(2+) puffs and sparks where channels open and close in a concerted fashion. To clarify the role of Ca(2+) inactivation of IP(3)Rs and RyRs in the dynamics of puffs and sparks, we formulate and analyze Markov chain models of Ca(2+) release sites composed of 10-40 three-state intracellular Ca(2+) channels that are inactivated as well as activated by Ca(2+). We study how the statistics of simulated puffs and sparks depend on the kinetics and dissociation constant of Ca(2+) inactivation and find that puffs and sparks are often less sensitive to variations in the number of channels at release sites and strength of coupling via local [Ca(2+)] when the average fraction of inactivated channels is significant. Interestingly, we observe that the single channel kinetics of Ca(2+) inactivation influences the thermodynamic entropy production rate of Markov chain models of puffs and sparks. While excessively fast Ca(2+) inactivation can preclude puffs and sparks, moderately fast Ca(2+) inactivation often leads to time-irreversible puffs and sparks whose termination is facilitated by the recruitment of inactivated channels throughout the duration of the puff/spark event. On the other hand, Ca(2+) inactivation may be an important negative feedback mechanism even when its time constant is much greater than the duration of puffs and sparks. In fact, slow Ca(2+) inactivation can lead to release sites with a substantial fraction of inactivated channels that exhibit puffs and sparks that are nearly time-reversible and terminate without additional recruitment of inactivated channels.

Rapid ligand-regulated gating kinetics of single inositol 1,4,5-trisphosphate receptor Ca2+ release channels

The ubiquitous inositol 1,4,5-trisphosphate receptor (InsP(3)R) intracellular Ca(2+) release channel is engaged by thousands of plasma membrane receptors to generate Ca(2+) signals in all cells. Understanding how complex Ca(2+) signals are generated has been hindered by a lack of information on the kinetic responses of the channel to its primary ligands, InsP(3) and Ca(2+), which activate and inhibit channel gating. Here, we describe the kinetic responses of single InsP(3)R channels in native endoplasmic reticulum membrane to rapid ligand concentration changes with millisecond resolution, using a new patch-clamp configuration. The kinetics of channel activation and deactivation showed novel Ca(2+) regulation and unexpected ligand cooperativity. The kinetics of Ca(2+)-mediated channel inhibition showed the single-channel bases for fundamental Ca(2+) release events and Ca(2+) release refractory periods. These results provide new insights into the channel regulatory mechanisms that contribute to complex spatial and temporal features of intracellular Ca(2+) signals.

MGluR-mediated calcium waves that invade the soma regulate firing in layer V medial prefrontal cortical pyramidal neurons

Factors that influence the activity of prefrontal cortex (PFC) pyramidal neurons are likely to play an important role in working memory function. One such factor may be the release of Ca2+ from intracellular stores. Here we investigate the hypothesis that metabotropic glutamate receptors (mGluRs)-mediated waves of internally released Ca2+ can regulate the intrinsic excitability and firing patterns of PFC pyramidal neurons. Synaptic or focal pharmacological activation of mGluRs triggered Ca2+ waves in the dendrites and somata of layer V medial PFC pyramidal neurons. These Ca2+ waves often evoked a transient SK-mediated hyperpolarization followed by a prolonged depolarization that respectively decreased and increased neuronal excitability. Generation of the hyperpolarization depended on whether the Ca2+ wave invaded or came near to the soma. The depolarization also depended on the extent of Ca2+ wave propagation. We tested factors that influence the propagation of Ca2+ waves into the soma. Stimulating more synapses, increasing inositol trisphosphate concentration near the soma, and priming with physiological trains of action potentials all enhanced the amplitude and likelihood of evoking somatic Ca2+ waves. These results suggest that mGluR-mediated Ca2+ waves may regulate firing patterns of PFC pyramidal neurons engaged by working memory, particularly under conditions that favor the propagation of Ca2+ waves into the soma.

Synapse specificity of calcium release probed by chemical two-photon uncaging of inositol 1,4,5-trisphosphate

Biological messengers can be "caged" by adding a single photosensitive group that can be photolyzed by a light flash to achieve spatially and temporally precise biochemical control. Here we report that photolysis of a double-caged form of the second messenger inositol 1,4,5-trisphosphate (IP3) triggers focal calcium release in Purkinje cell somata, dendrites, and spines as measured by two-photon microscopy. In calbindin knock-out Purkinje cells, peak calcium increased with flash energy with higher cooperativity for double-caged IP3 than for conventional single-caged IP3, consistent with a chemical two-photon effect. Spine photolysis of double-caged IP3 led to local calcium release. Uncaging of glycerophosphoryl-myo-inositol 4,5-bisphosphate (gPIP2), a poorly metabolizable IP3 analog, led to less well localized release. Thus, IP3 breakdown is necessary for spine-specificity. IP3- and gPIP2-evoked signals declined from peak with similar, slow time courses, indicating that release lasts hundreds of milliseconds and is terminated not by IP3 degradation but by intrinsic receptor dynamics. Based on measurements of spine-dendrite coupling, IP3-evoked calcium signals are expected to be at least 2.4-fold larger in their spine of origin than in nearby spines, allowing IP3 to act as a synapse-specific second messenger. Unexpectedly, single-caged IP3 led to less release in somata and was ineffective in dendrites and spines. Calcium release using caged gPIP2 was inhibited by the addition of single-caged IP3, suggesting that single-caged IP3 is an antagonist of calcium release. Caging at multiple sites may be an effective general approach to reducing residual receptor interaction.

A kinetic model of single and clustered IP3 receptors in the absence of Ca2+ feedback

Ca2+ liberation through inositol 1,4,5-trisphosphate receptor (IP3R) channels generates complex patterns of spatiotemporal cellular Ca2+ signals owing to the biphasic modulation of channel gating by Ca2+ itself. These processes have been extensively studied in Xenopus oocytes, where imaging studies have revealed local Ca2+ signals ("puffs") arising from clusters of IP3R, and patch-clamp studies on isolated oocyte nuclei have yielded extensive data on IP3R gating kinetics. To bridge these two levels of experimental data, we developed an IP3R model and applied stochastic simulation and transition matrix theory to predict the behavior of individual and clustered IP3R channels. The channel model consists of four identical, independent subunits, each of which has an IP3-binding site together with one activating and one inactivating Ca2+-binding site. The channel opens when at least three subunits undergo a conformational change to an "active" state after binding IP3 and Ca2+. The model successfully reproduces patch-clamp data; including the dependence of open probability, mean open duration, and mean closed duration on [IP3] and [Ca2+]. Notably, the biexponential distribution of open-time duration and the dependence of mean open time on [Ca2+] are explained by populations of openings involving either three or four active subunits. As a first step toward applying the single IP3R model to describe cellular responses, we then simulated measurements of puff latency after step increases of [IP3]. Assuming that stochastic opening of a single IP3R at basal cytosolic [Ca2+] and any given [IP3] has a high probability of rapidly triggering neighboring channels by calcium-induced calcium release to evoke a puff, optimal correspondence with experimental data of puff latencies after photorelease of IP3 was obtained when the cluster contained a total of 40-70 IP3Rs.

Models of the inositol trisphosphate receptor

The inositol (1,4,5)-trisphosphate receptor (IPR) plays a crucial role in calcium dynamics in a wide range of cell types, and is often a central feature in quantitative models of calcium oscillations and waves. We review deterministic and stochastic mathematical models of the IPR, from the earliest ones of the 1970s and 1980s, to the most recent. The effects of IPR stochasticity on Ca2+ dynamics are briefly discussed.

Polycystin 2 interacts with type I inositol 1,4,5-trisphosphate receptor to modulate intracellular Ca2+ signaling

Autosomal dominant polycystic kidney disease, a common cause of renal failure, arises from mutations in either the PKD1 or the PKD2 gene. The precise function of both PKD gene products polycystins (PCs) 1 and 2 remain controversial. PC2 has been localized to numerous cellular compartments, including the endoplasmic reticulum, plasma membrane, and cilia. It is unclear what pools are the most relevant to its physiological function as a putative Ca2+ channel. We employed a Xenopus oocyte Ca2+ imaging system to directly investigate the role of PC2 in inositol 1,4,5-trisphosphate (IP3)-dependent Ca2+ signaling. Cytosolic Ca2+ signals were recorded following UV photolysis of caged IP3 in the absence of extracellular Ca2+. We demonstrated that overexpression of PC2, as well as type I IP3 receptor (IP3R), significantly prolonged the half-decay time (t1/2) of IP3-induced Ca2+ transients. However, overexpressing the disease-associated PC2 mutants, the point mutation D511V, and the C-terminally truncated mutation R742X did not alter the t1/2. In addition, we found that D511V overexpression significantly reduced the amplitude of IP3-induced Ca2+ transients. Interestingly, overexpression of the C terminus of PC2 not only significantly reduced the amplitude but also prolonged the t1/2. Co-immunoprecipitation assays indicated that PC2 physically interacts with IP3R through its C terminus. Taken together, our data suggest that PC2 and IP3R functionally interact and modulate intracellular Ca2+ signaling. Therefore, mutations in either PC1 or PC2 could result in the misregulation of intracellular Ca2+ signaling, which in turn could contribute to the pathology of autosomal dominant polycystic kidney disease.

Modeling stochastic Ca2+ release from a cluster of IP3-sensitive receptors

We focused our attention on Ca(2+) release from the endoplasmic reticulum through a cluster of inositol(1,4,5)-trisphosphate (IP(3)) receptor channels. The random opening and closing of these receptors introduce stochastic effects that have been observed experimentally. Here, we present a stochastic version of Othmer-Tang model (OTM) for IP(3) receptor clusters. We address the average behavior of the channels in response to IP(3) stimuli. In our stochastic simulation we found that the fraction of open channels versus [IP(3)] follows a Hill curve, whose associate Hill coefficient increases when intracellular Ca(2+) level increase. This finding suggests that feedback from cytosolic Ca(2+) plays a key role in the channel response to IP(3). We also study several aspects of the stochastic properties of Ca(2+) release and we compare with experimental observations.

Gating mechanisms of the type-1 inositol trisphosphate receptor

A large amount of data and observations on inositol 1,4,5-trisphosphate (IP(3)) binding to the IP(3) receptor/Ca(2+) channel, the steady-state activity of the channel, and its inactivation by IP(3) can be explained by assuming one activation and one inhibition module, both allosterically operated by Ca(2+), IP(3), and ATP, and one adaptation element, driven by IP(3), Ca(2+), and the interconversion between two possible conformations of the receptor. The adaptation module becomes completely insensitive to a second IP(3) pulse within 80 s. Observed kinetic responses are well reproduced if, in addition, two module open states are rendered inactive by the current charge carrier Mn(2+). The inactivation time constants are 59 s in the activation, and 0.75 s in the adaptation module. The in vivo open probability of the channel is predicted to be almost in coincidence with the behavior in lipid bilayers for IP(3) levels of 0.2 and 2 microM and one-order-higher at 0.02 microM IP(3), whereas at 180 microM IP(3) the maximal in vivo activity may be 2.5-orders higher than in bilayers and restricted to a narrower Ca(2+) domain (approximately 10 microM-wide versus approximately 100 microM-wide). IP(3) is likely to inhibit channel activity at < or =120 nM Ca(2+) in vivo.

Release currents of IP(3) receptor channel clusters and concentration profiles

We simulate currents and concentration profiles generated by Ca(2+) release from the endoplasmic reticulum (ER) to the cytosol through IP(3) receptor channel clusters. Clusters are described as conducting pores in the lumenal membrane with a diameter from 6 nm to 36 nm. The endoplasmic reticulum is modeled as a disc with a radius of 1-12 microm and an inner height of 28 nm. We adapt the dependence of the currents on the trans Ca(2+) concentration (intralumenal) measured in lipid bilayer experiments to the cellular geometry. Simulated currents are compared with signal mass measurements in Xenopus oocytes. We find that release currents depend linearly on the concentration of free Ca(2+) in the lumen. The release current is approximately proportional to the square root of the number of open channels in a cluster. Cytosolic concentrations at the location of the cluster range from 25 microM to 170 microM. Concentration increase due to puffs in a distance of a few micrometers from the puff site is found to be in the nanomolar range. Release currents decay biexponentially with timescales of <1 s and a few seconds. Concentration profiles decay with timescales of 0.125-0.250 s upon termination of release.

A comparison of three models of the inositol trisphosphate receptor

The inositol (1,4,5)-trisphosphate receptor (IPR) plays a crucial role in calcium dynamics in a wide range of cell types, and is often a central feature in quantitative models of calcium oscillations and waves. We compare three mathematical models of the IPR, fitting each of them to the same data set to determine ranges for the parameter values. Each of the fits indicates that fast activation of the receptor, followed by slow inactivation, is an important feature of the model, and also that the speed of inositol trisphosphate IP3 binding cannot necessarily be assumed to be faster than Ca2+ activation. In addition, the model which assumed saturating binding rates of Ca2+ to the IPR demonstrated the best fit. However, lack of convergence in the fitting procedure indicates that responses to step increases of Ca2+ and IP3 provide insufficient data to determine the parameters unambiguously in any of the models.

Spatiotemporal patterning of IP3-mediated Ca2+ signals in Xenopus oocytes by Ca2+-binding proteins

Ca(2+)-binding proteins (CaBPs) are expressed in a highly specific manner across many different cell types, yet the physiological basis underlying their selective distribution patterns remains unclear. We used confocal line-scan microscopy together with photo-release of IP(3) in Xenopus oocytes to investigate the actions of mobile cytosolic CaBPs on the spatiotemporal properties of IP(3)-evoked Ca(2+) signals. Parvalbumin (PV), a CaBP with slow Ca(2+)-binding kinetics, shortened the duration of IP(3)-evoked Ca(2+) signals and 'balkanized' global responses into discrete localized events (puffs). In contrast, calretinin (CR), a presumed fast buffer, prolonged Ca(2+) responses and promoted 'globalization' of spatially uniform Ca(2+) signals at high [IP(3)]. Oocytes loaded with CR or PV showed Ca(2+) puffs following photolysis flashes that were subthreshold in controls, and the spatiotemporal properties of these localized events were differentially modulated by PV and CR. In comparison to results we previously obtained with exogenous Ca(2+) buffers, PV closely mimicked the actions of the slow buffer EGTA, whereas CR showed important differences from the fast buffer BAPTA. Most notably, puffs were never observed after loading BAPTA, and this exogenous buffer did not show the marked sensitization of IP(3) action evident with CR. The ability of Ca(2+) buffers and CaBPs with differing kinetics to fine-tune both global and local intracellular Ca(2+) signals is likely to have significant physiological implications.

Buffer kinetics shape the spatiotemporal patterns of IP3-evoked Ca2+ signals

Ca2+ liberation through inositol 1,4,5-trisphosphate receptors (IP3Rs) plays a universal role in cell regulation, and specificity of cell signalling is achieved through the spatiotemporal patterning of Ca2+ signals. IP3Rs display Ca2+-induced Ca2+ release (CICR), but are grouped in clusters so that regenerative Ca2+ signals may remain localized to individual clusters, or propagate globally between clusters by successive cycles of Ca2+ diffusion and CICR. We used confocal microscopy and photoreleased IP3 in Xenopus oocytes to study how these properties are modulated by mobile cytosolic Ca2+ buffers. EGTA (a buffer with slow 'on-rate') speeded Ca2+ signals and 'balkanized' Ca2+ waves by dissociating them into local signals. In contrast, BAPTA (a fast buffer with similar affinity) slowed Ca2+ responses and promoted 'globalization' of spatially uniform Ca2+ signals. These actions are likely to arise through differential effects on Ca2+ feedback within and between IP3R clusters, because Ca2+ signals evoked by influx through voltage-gated channels were little affected. We propose that cell-specific expression of Ca2+-binding proteins with distinct kinetics may shape the time course and spatial distribution of IP3-evoked Ca2+ signals for specific physiological roles.

Kinetic model of the inositol trisphosphate receptor that shows both steady-state and quantal patterns of Ca2+ release from intracellular stores

The release of Ca(2+) from intracellular stores via InsP(3) receptors shows anomalous kinetics. Successive additions of low concentrations of InsP(3) cause successive rapid transients of Ca(2+) release. These quantal responses have been ascribed to all-or-none release from stores with differing sensitivities to InsP(3) or, alternatively, to a steady-state mechanism where complex kinetic properties of the InsP(3) receptor allow partial emptying of all the stores. We present here an adaptive model of the InsP(3) receptor that can show either pattern, depending on the imposed experimental conditions. The model proposes two interconvertible conformational states of the receptor: one state binds InsP(3) rapidly, but with low affinity, whereas the other state binds slowly, but with high affinity. The model shows repetitive increments of Ca(2+) release in the absence of a Ca(2+) gradient, but more pronounced incremental behaviour when released Ca(2+) builds up at the mouth of the channel. The sensitivity to Ins P (3) is critically dependent on the density of InsP(3) receptors, so that different stores can respond to different concentration ranges of Ins P (3). Since the model generates very high Hill coefficients (h approximately 7), it allows all-or-none release of Ca(2+) from stores of differing receptor density, but questions the validity of the use of h values as a guide to the number of InsP(3) molecules needed to open the channel. The model presents a mechanism for terminating Ca(2+) release in the presence of positive feedback from released Ca(2+), thereby providing an explanation of why elementary Ca(2+) signals ('blips' and 'puffs') do not inevitably turn into regenerative waves.

IP(3)-mediated Ca(2+) signals in human neuroblastoma SH-SY5Y cells with exogenous overexpression of type 3 IP(3) receptor

Human neuroblastoma SH-SY5Y cells, predominantly expressing type 1 inositol 1,4,5-trisphosphate (IP(3)) receptor (IP(3)R), were stably transfected with IP(3)R type 3 (IP(3)R3) cDNA. Immunocytochemistry experiments showed a homogeneous cytoplasmic distribution of type 3 IP(3)Rs in transfected and selected high expression cloned cells. Using confocal Ca(2+) imaging, carbachol (CCh)-induced Ca(2+) release signals were studied. Low CCh concentrations (< or = 750 nM) evoked baseline Ca(2+) oscillations. Transfected cells displayed a higher CCh responsiveness than control or cloned cells. Ca(2+) responses varied between fast, large Ca(2+) spikes and slow, small Ca(2+) humps, while in the clone only Ca(2+) humps were observed. Ca(2+) humps in the transfected cells were associated with a high expression level of IP(3)R3. At high CCh concentrations (10 microM) Ca(2+) transients in transfected and cloned cells were similar to those in control cells. In the clone exogenous IP(3)R3 lacked the C-terminal channel domain but IP(3)-binding capacity was preserved. Transfected cells mainly expressed intact type 3 IP(3)Rs but some protein degradation was also observed. We conclude that in transfected cells expression of functional type 3 IP(3)Rs causes an apparent higher affinity for IP(3). In the clone, the presence of degraded receptors leads to an efficient cellular IP(3) buffer and attenuated IP(3)-evoked Ca(2+) release.

Fast biphasic regulation of type 3 inositol trisphosphate receptors by cytosolic calcium

In cytosol-like medium (CLM) with a free [Ca(2+)] of 200 nm, a supramaximal concentration of inositol 1,4,5-trisphosphate (IP(3)) (30 microm) evoked (45)Ca(2+) release from type 3 IP(3) receptors only after a latency of 48 +/- 6 ms; this latency could not be reduced by increasing the IP(3) concentration. In CLM containing a low free [Ca(2+)] ( approximately 4 nm), 300 microm IP(3) evoked (45)Ca(2+) release after a latency of 66 +/- 11 ms; this was reduced to 14 +/- 3 ms when the [Ca(2+)] was 1 mm. Preincubation with CLM containing 100 microm Ca(2+) caused a rapid (half-time = 33 +/- 9 ms), complete, and fully reversible inhibition that could not be overcome by a high concentration of IP(3) (300 microm). Hepatic (type 2) IP(3) receptors were not inhibited by Ca(2+) once they had bound IP(3), but 100 microm Ca(2+) rapidly inhibited type 3 IP(3) receptors whether it was delivered before addition of IP(3) or at any stage during a response to IP(3). Ca(2+) increases the affinity of IP(3) for hepatic receptors by slowing IP(3) dissociation, but Ca(2+) had no effect on IP(3) binding to type 3 receptors. The rate of inhibition of type 3 IP(3) receptors by Ca(2+) was faster than the rate of IP(3) dissociation, and occurred at similar rates whether receptors had bound a high (adenophostin) or low affinity (3-deoxy-3-fluoro-IP(3)) agonist. Dissociation of agonist is not therefore required for Ca(2+) to inhibit type 3 IP(3) receptors. We conclude that type 2 and 3 IP(3) receptors are each biphasically regulated by Ca(2+), but by different mechanisms. For both, IP(3) binding causes a stimulatory Ca(2+)-binding site to be exposed allowing Ca(2+) to bind and open the channel. IP(3) binding protects type 2 receptors from Ca(2+) inhibition, but type 3 receptors are inhibited by Ca(2+) whether or not they have IP(3) bound. Increases in cytosolic [Ca(2+)] will immediately inhibit type 3 receptors, but inhibit type 2 receptors only after IP(3) has dissociated.

A dynamic model of the type-2 inositol trisphosphate receptor

The dynamic properties of the inositol (1,4,5)-trisphosphate (IP(3)) receptor are crucial for the control of intracellular Ca(2+), including the generation of Ca(2+) oscillations and waves. However, many models of this receptor do not agree with recent experimental data on the dynamic responses of the receptor. We construct a model of the IP(3) receptor and fit the model to dynamic and steady-state experimental data from type-2 IP(3) receptors. Our results indicate that, (i) Ca(2+) binds to the receptor using saturating, not mass-action, kinetics; (ii) Ca(2+) decreases the rate of IP(3) binding while simultaneously increasing the steady-state sensitivity of the receptor to IP(3); (iii) the rate of Ca(2+)-induced receptor activation increases with Ca(2+) and is faster than Ca(2+)-induced receptor inactivation; and (iv) IP(3) receptors are sequentially activated and inactivated by Ca(2+) even when IP(3) is bound. Our results emphasize that measurement of steady-state properties alone is insufficient to characterize the functional properties of the receptor.

Calcium signalling: IP3 rises again...and again

Recent results indicate that 'regulators of G-protein signalling' may contribute to the generation of receptor-specific patterns of cytosolic Ca2+ oscillations by associating with specific receptors, accelerating G-protein inactivation and responding to changes in cytosolic Ca2+.

Synthesis of potent agonists of the D-myo-inositol 1,4, 5-trisphosphate receptor based on clustered disaccharide polyphosphate analogues of adenophostin A

Clustered disaccharide analogues of adenophostin A (2), i.e. mono-, di-, and tetravalent derivatives 6-8, respectively, were synthesized and evaluated as novel ligands for the tetrameric D-myo-inositol 1,4, 5-trisphosphate receptor (IP(3)R). The synthesis was accomplished via Sonogashira coupling of propargyl 2-O-acetyl-5-O-benzyl-3-O-(3, 4-di-O-acetyl-2, 6-di-O-benzyl-alpha-D-glucopyranosyl)-beta-D-ribofuranoside (16) with iodobenzene 18, 22, or 25, followed by deacetylation, phosphorylation, and deprotection. The abilities of the target compounds 6-8, as well as ribophostin 4, propylphostin 5, and IP(3) (1), to evoke Ca(2+) release from permeabilized hepatocytes or displacement of [(3)H]IP(3) from its receptor in hepatic membranes were compared. Although the binding affinities of 4-8 were similar, there were modest though significant differences in their potencies in Ca(2+) release assays: tetraphostin 8 > IP(3) approximately diphostin 7 > phenylphostin 6 > ribophostin 4 approximately propylphostin 5.

Control of InsP3-induced Ca2+ oscillations in permeabilized blowfly salivary gland cells: contribution of mitochondria

Many agonists linked to the generation of inositol 1,4, 5-trisphosphate (InsP3) and release of Ca2+ from intracellular stores induce repetitive transients in cytosolic Ca2+ whose frequency increases over a certain range of agonist concentrations. In order to investigate the mechanisms underlying this frequency modulation, the fluorescent Ca2+ sensor mag-fura-2 was loaded into intracellular calcium stores and used to monitor InsP3-induced dynamics of the intraluminal calcium concentration ([Ca2+]L) in secretory cells of permeabilized blowfly Calliphora vicina salivary glands. In this preparation, increasing concentrations of InsP3 induced graded decreases in [Ca2+]L that were often superimposed with repetitive [Ca2+]L transients produced by sequential Ca2+ release and re-uptake. These [Ca2+]L oscillations developed at frequencies of 3-11 min-1 unrelated to the concentration of InsP3 present. In contrast, incremental concentrations of InsP3 applied in the presence of the oxidizable mitochondrial substrates citrate, succinate, or pyruvate-malate induced repetitive [Ca2+]L transients whose frequency increased with the concentration of InsP3. This InsP3 concentration-dependent modulation of oscillation frequency was abolished after dissipating the mitochondrial membrane potential (Delta psi m) by combined treatment with carbonyl cyanide p-trifluoromethoxyphenyl hydrazone + oligomycin or after application of Ruthenium Red, an inhibitor of mitochondrial Ca2+ uptake. Taken together, the data indicate that energized mitochondria exert negative control over the frequency of InsP3-induced Ca2+ oscillations. It is concluded that mitochondria play a crucial role in determining the duration of the interspike period and, therefore, for the encoding of amplitude-modulated, InsP3-liberating stimuli into the frequency of cytosolic Ca2+ oscillations.

Ca(2+)-dependent activation of Cl(-) currents in Xenopus oocytes is modulated by voltage

Ca(2+)-activated Cl(-) currents (I(Cl,Ca)) were examined using fluorescence confocal microscopy to monitor intracellular Ca(2+) liberation evoked by flash photolysis of caged inositol 1,4, 5-trisphosphate (InsP(3)) in voltage-clamped Xenopus oocytes. Currents at +40 mV exhibited a steep dependence on InsP(3) concentration ([InsP(3)]), whereas currents at -140 mV exhibited a higher threshold and more graded relationship with [InsP(3)]. Ca(2+) levels required to half-maximally activate I(Cl,Ca) were about 50% larger at -140 mV than at +40 mV, and currents evoked by small Ca(2+) elevations were reduced >25-fold. The half-decay time of Ca(2+) signals shortened at increasingly positive potentials, whereas the decay of I(Cl,Ca) lengthened. The steady-state current-voltage (I-V) relationship for I(Cl,Ca) exhibited outward rectification with weak photolysis flashes but became more linear with stronger stimuli. Instantaneous I-V relationships were linear with both strong and weak stimuli. Current relaxations following voltage steps during activation of I(Cl,Ca) decayed with half-times that shortened from about 100 ms at +10 mV to 20 ms at -160 mV. We conclude that InsP(3)-mediated Ca(2+) liberation activates a single population of Cl(-) channels, which exhibit voltage-dependent Ca(2+) activation and voltage-independent instantaneous conductance.

Mapping of IP(3)-mediated Ca(2+) signals in single human neuroblastoma SH-SY5Y cells: cell volume shaping the Ca(2+) signal

Fast confocal laser-scanning microscopy was used to study spatiotemporal properties of IP(3)-mediated Ca(2+) release signals in human SH-SY5Y neuroblastoma cells. [Ca(2+)](i) increases were not affected by ryanodine (30 microgM) or caffeine (10 mM) and largely insensitive to removal of external Ca(2+), indicating predominance of IP(3)-induced Ca(2+) release. Ca(2+) signals evoked by high concentration (10 microM) of the muscarinic agonist carbachol appeared as self-propagating waves initiating in cell processes. At low carbachol concentrations (500 nM) Ca(2+) changes in most cells displayed striking spatiotemporal heterogeneity. The Ca(2+) response in the cell body was delayed and had a smaller amplitude and a slower rise time than that in processes. Ca(2+) changes in processes either occurred in a homogeneous manner throughout the whole process or were sometimes confined to hot spots. Regional differences in surface-to-volume ratio appear to be critical clues that determine the spatiotemporal pattern of intracellular Ca(2+) release signals.

Lateral inhibition of inositol 1,4,5-trisphosphate receptors by cytosolic Ca(2+)

Ryanodine and inositol 1,4,5-trisphosphate (IP(3)) receptors - two related families of Ca(2+) channels responsible for release of Ca(2+) from intracellular stores [1] - are biphasically regulated by cytosolic Ca(2+) [2] [3] [4]. It is thought that the resulting positive feedback allows localised Ca(2+)-release events to propagate regeneratively, and that the negative feedback limits the amplitude of individual events [5] [6]. Stimulation of IP(3) receptors by Ca(2+) occurs through a Ca(2+)-binding site that becomes exposed only after IP(3) has bound to its receptor [7] [8]. Here, we report that rapid inhibition of IP(3) receptors by Ca(2+) occurs only if the receptor has not bound IP(3). The IP(3) therefore switches its receptor from a state in which only an inhibitory Ca(2+)-binding site is accessible to one in which only a stimulatory site is available. This regulation ensures that Ca(2+) released by an active IP(3) receptor may rapidly inhibit its unliganded neighbours, but it cannot terminate the activity of a receptor with IP(3) bound. Such lateral inhibition, which is a universal feature of sensory systems where it improves contrast and dynamic range, may fulfil similar roles in intracellular Ca(2+) signalling by providing increased sensitivity to IP(3) and allowing rapid graded recruitment of IP(3) receptors.

Regulatory and spatial aspects of inositol trisphosphate-mediated calcium signals

Hormones that act to release Ca2+ from intracellular stores initiate a signaling cascade that culminates in the production of inositol 1,4,5-trisphosphate (InsP3). The Ca2+ response mediated by InsP3 is not a sustained increase in the cytosolic Ca2+ concentration, but rather a series of periodic spikes that manifest as waves in larger cells. In vitro studies have determined that the key positive feedback parameter driving spikes and waves is a highly localized direct Ca(2+)-activation of InsP3-gated Ca2+ channels. Advances in fluorescent Ca2+ imaging have facilitated the resolution of individual positive feedback units. These studies have revealed that there are several modes of channel coupling underlying global Ca2+ signals; single channel openings or Ca2+ "blips," synchronized clusters of channels or Ca2+ "puffs," and cell wide calcium waves. It appears that the channel clusters that produce Ca2+ puffs are synchronized by the highly localized positive feedback that was predicted by the in vitro studies of channel regulation. Localization of InsP3-induced Ca2+ signals has been shown to be important for activation of several cellular processes including uni-directional salt flow and mitochondrial activation.

Radial localization of inositol 1,4,5-trisphosphate-sensitive Ca2+ release sites in Xenopus oocytes resolved by axial confocal linescan imaging

The radial localization and properties of elementary calcium release events ("puffs") were studied in Xenopus oocytes using a confocal microscope equipped with a piezoelectric focussing unit to allow rapid (>100 Hz) imaging of calcium signals along a radial line into the cell with a spatial resolution of <0.7 micrometer. Weak photorelease of caged inositol 1,4,5-trisphosphate (InsP3) evoked puffs arising predominantly within a 6-micrometer thick band located within a few micrometers of the cell surface. Approximately 25% of puffs had a restricted radial spread, consistent with calcium release from a single site. Most puffs, however, exhibited a greater radial spread (3.25 micrometer), likely involving recruitment of radially neighboring release sites. Calcium waves evoked by just suprathreshold stimuli exhibited radial calcium distributions consistent with inward diffusion of calcium liberated at puff sites, whereas stronger flashes evoked strong, short-latency signals at depths inward from puff sites, indicating deep InsP3-sensitive stores activated at higher concentrations of InsP3. Immunolocalization of InsP3 receptors showed punctate staining throughout a region corresponding to the localization of puffs and subplasmalemmal endoplasmic reticulum. The radial organization of puff sites a few micrometers inward from the plasma membrane may have important consequences for activation of calcium-dependent ion channels and "capacitative" calcium influx. However, on the macroscopic (hundreds of micrometers) scale of global calcium waves, release can be considered to occur primarily within a thin, essentially two-dimensional subplasmalemmal shell.

Local calcium signalling by inositol-1,4,5-trisphosphate in Purkinje cell dendrites

The second messenger inositol-1,4,5-trisphosphate (InsP3) releases Ca2+ from intracellular Ca2+ stores by activating specific receptors on the membranes of these stores. In many cells, InsP3 is a global signalling molecule that liberates Ca2+ throughout the cytoplasm. However, in neurons the situation might be different, because synaptic activity may produce InsP3 at discrete locations. Here we characterize InsP3 signalling in postsynaptic cerebellar Purkinje neurons, which have a high level of InsP3 receptors. We find that repetitive activation of the synapse between parallel fibres and Purkinje cells causes InsP3-mediated Ca2+ release in the Purkinje cells. This Ca2+ release is restricted to individual postsynaptic spines, where both metabotropic glutamate receptors and InsP3 receptors are located, or to multiple spines and adjacent dendritic shafts. Focal photolysis of caged InsP3 in Purkinje cell dendrites also produces Ca2+ signals that spread only a few micrometres from the site of InsP3 production. Uncaged InsP3 produces a long-lasting depression of parallel-fibre synaptic transmission that is limited to synapses where the Ca2+ concentration is raised. Thus, in Purkinje cells InP3 acts within a restricted spatial range that allows it to regulate the function of local groups of parallel-fibre synapses.

Restricted photorelease of biologically active molecules near the plasma membrane

An evanescent wave of ultraviolet light was successfully used to release biologically active molecules from caged compounds in living cells. The evanescent wave was generated by the total internal reflection in a limited region near the plasma membrane attached to the illuminated interface. At first, the photolysis efficiency of the evanescent wave of ultraviolet laser light was studied using caged glutamic acid in vitro. Then, caged Ca2+ introduced in the living cultured cell was similarly photolyzed by the evanescent wave and the resulting elevations of the concentration of intracellular Ca2+ in the proximity of the plasma membrane and in the cytosol were observed with a simultaneously introduced fluorescent calcium indicator. Inositol trisphosphate can also be photoreleased near the plasma membrane, which enables study of the temporal and spatial pathways of signal transduction. The method developed here provides a useful tool for studying signal transduction near the plasma membrane in a living cell.

Inositol trisphosphate receptors: Ca2+-modulated intracellular Ca2+ channels

The three subtypes of inositol trisphosphate (InsP3) receptor expressed in mammalian cells are each capable of forming intracellular Ca2+ channels that are regulated by both InsP3 and cytosolic Ca2+. The InsP3 receptors of many, though perhaps not all, tissues are biphasically regulated by cytosolic Ca2+: a rapid stimulation of the receptors by modest increases in Ca2+ concentration is followed by a slower inhibition at higher Ca2+ concentrations. Despite the widespread occurrence of this form of regulation and the belief that it is an important element of the mechanisms responsible for the complex Ca2+ signals evoked by physiological stimuli, the underlying mechanisms are not understood. Both accessory proteins and Ca2+-binding sites on InsP3 receptors themselves have been proposed to mediate the effects of cytosolic Ca2+ on InsP3 receptor function, but the evidence is equivocal. The effects of cytosolic Ca2+ on InsP3 binding and channel opening, and the possible means whereby the effects are mediated are discussed in this review.

Hemispheric asymmetry of macroscopic and elementary calcium signals mediated by InsP3 in Xenopus oocytes

1. The mechanisms underlying hemispheric asymmetry of the inositol 1, 4,5-trisphosphate (InsP3)-calcium signalling pathway in Xenopus oocytes were examined by fluorescence imaging of calcium signals and recording calcium-activated Cl- currents (ICl,Ca) evoked by intracellular calcium injections and photorelease of InsP3. 2. The maximal ICl,Ca evoked by strong photorelease of InsP3 was 8 times greater in the animal than the vegetal hemisphere, but the average threshold amounts of InsP3 required to evoke detectable currents were similar in each hemisphere. 3. Currents evoked by injections of calcium were about 2.5 times greater near the animal pole than near the vegetal pole, whereas fluorescence signals evoked by injections were similar in each hemisphere. 4. Calcium waves were evoked by photolysis flashes of similar strengths in both hemispheres of albino oocytes, but peak calcium levels evoked by supramaximal stimuli were 70 % greater in the animal hemisphere. 5. Elementary calcium release events (puffs) in the animal hemisphere had amplitudes about double that in the vegetal hemisphere, and more often involved coupled release from adjacent sites. Calcium release sites were more closely packed in the animal hemisphere, with a mean spacing of about 1.5 micro m compared with 2.25 micro m in the vegetal hemisphere. 6. The larger amplitude of currents mediated by InsP3 in the animal hemisphere, therefore, involves an increased flux of calcium at individual release units, a more dense packing of release units and a higher density of Cl- channels.

Activation and co-ordination of InsP3-mediated elementary Ca2+ events during global Ca2+ signals in Xenopus oocytes

1. The activation of elementary calcium release events ('puffs') and their co-ordination to generate calcium waves was studied in Xenopus oocytes by confocal linescan imaging together with photorelease of inositol 1,4,5-trisphosphate (InsP3) from a caged precursor. 2. Weak photolysis flashes evoked no responses or isolated calcium puffs, whereas flashes of increasing strength evoked more frequent puffs, often occurring in flurries as abortive waves, and then a near-simultaneous calcium liberation originating at multiple sites. The numbers of sites activated increased initially as about the fourth power of photoreleased [InsP3]. 3. Following repeated, identical photolysis flashes, puffs arose after stochastically varying latencies of a few hundred milliseconds to several seconds. The cumulative number of events initially increased as about the third power of time. No rise in free [Ca2+] was detected preceding the puffs, suggesting that this co-operativity arises through binding of multiple InsP3 molecules, rather than through calcium feedback. 4. The mean latency to onset of calcium liberation shortened as about the square of the flash strength, and the dispersion in latencies between events reduced correspondingly. 5. Weak stimuli often evoked coupled puffs involving adjacent sites, and stronger flashes evoked saltatory calcium waves, propagating with non-constant velocity. During waves, [Ca2+] rose slowly between puff sites, but more abruptly at active sites following an initial diffusive rise in calcium. 6. Initial rates of rise of local [Ca2+] at release sites were similar during puffs and release induced by much (> 10-fold) greater [InsP3]. In contrast, macroscopic calcium measurements averaged over the scan line showed a graded dependence of rate of calcium liberation upon [InsP3], due to recruitment of additional sites and decreasing dispersion in activation latencies. 7. We conclude that the initiation of calcium liberation depends co-operatively upon [InsP3] whereas the subsequent regenerative increase in calcium flux depends upon local calcium feedback and is largely independent of [InsP3]. Wave propagation is consistent with the diffusive spread of calcium evoking regenerative liberation at heterogeneous discrete sites, the sensitivity of which is primed by InsP3.

Calcium

Ca2+ is involved in an intriguing variety of different biological events. The rapid development of techniques such as region- or organelle-directed fluorescent probes and laser scanning confocal microscopy for studying cellular biological events at a molecular level provides us with a rich daily intake of new results. While detailed three-dimensional structures of many intracellular and extracellular Ca2+-binding proteins have become available, structural information on key membrane proteins is still lacking. An integrated picture of the molecular events behind the multifunctional roles of Ca2+ in biological systems is still pending.

Effects of sympathetic nerve stimulation on membrane potential, [Ca2+]i and force in the arrested sinus venosus of the toad, Bufo marinus

1. The effects of sympathetic nerve stimulation on membrane potential and on the intracellular concentration of calcium ions, [Ca2+]i, were recorded concurrently from the sinus venosus of the toad, Bufo marinus, in preparations where beating had been abolished by adding an organic calcium antagonist to the physiological saline. In a separate set of experiments the effects of sympathetic nerve stimulation on force production were examined. 2. Stimulation of the sympathetic nerves caused a membrane depolarization and a simultaneous increase in [Ca2+]i. Both responses were reduced by dihydroergotamine (20 microM). 3. The membrane depolarization and increase in [Ca2+]i evoked by sympathetic nerve stimulation were abolished by ryanodine (10 microM), or caffeine (3 mM). The effects of caffeine, but not those of ryanodine, were fully reversible. 4. Although the Ca(2+)-ATPase inhibitor thapsigargin (30 microM) itself had little effect on the responses to sympathetic nerve stimulation, in its presence caffeine (3 mM) irreversibly abolished the responses. 5. In the presence of nifedipine (10 microM), sympathetic nerve stimulation caused contractions of the sinus venosus. These responses were abolished by either ryanodine (10 microM) or caffeine (3 mM). 6. The results suggest that neuronally released transmitter activates a complex biochemical pathway which triggers the release of Ca2+ from internal stores.

Kinetics of Ca2+ release by InsP3 in pig single aortic endothelial cells: evidence for an inhibitory role of cytosolic Ca2+ in regulating hormonally evoked Ca2+ spikes

1. The role of the InsP3 receptor and its interaction with Ca2+ in shaping endothelial Ca2+ spikes was investigated by comparing InsP3-evoked intracellular Ca2+ release with hormonally evoked Ca2+ spikes in single endothelial cells. 2. InsP3 was generated by flash photolysis of intracellular caged InsP3. InsP3 at 0.2 microM or higher released Ca2+ from stores with a time course comprising a well-defined delay, a fast rise of free [Ca2+] to a peak where net flux into the cystosol is zero, and a slow decline to preflash levels. InsP3-evoked Ca2+ flux into unit cytosolic volume was measured as the rate of change of free [Ca2+]i during the fast rise, d[Ca2+]i/dt (mol s-1 l-1). 3. The mean delay decreased from 433 ms at 0.2 microM to 30 ms at 5 microM. At very high InsP3 concentrations, 78 microM, the delay was shorter, < 10 ms. At low InsP3 concentration the delay was reduced by approximately 30% by prior elevation of free [Ca2+]i, supporting a co-operative action of free [Ca2+] and InsP3 in activation. 4. Both Ca2+ flux and peak free [Ca2+]i increased with InsP3 concentration within each cell. Maximal activation was at > 5 microM, 50% maximum Ca2+ flux was at 1.6 microM InsP3 and the Hill coefficient was between 3.6 and 4.3. A large variation of Ca2+ flux and peak [Ca2+]i was found from cell to cell at the same InsP3 concentration. 5. Strong inhibition of InsP3-evoked flux was produced by an immediately preceding response, with complete inhibition at peak free [Ca2+]i due to the first pulse. InsP3 sensitivity returned over 1-2 min, with 50% recovery at approximately 25 s. The recovery of InsP3 sensitivity may determine the minimum interval between hormonally evoked spikes. 6. Ca2+ flux due to a pulse of InsP3 terminated rapidly, in the continued presence of InsP3, producing a well-defined peak [Ca2+]. A reciprocal relation was found between the duration and the rate of Ca2+ flux, such that high Ca2+ flux was of brief duration. The rate of termination of flux measured as the reciprocal of the 10-90% rise time of free [Ca2+]i showed a linear correlation with Ca2+ flux over a large range in all cells. A systematic deviation from linearity at low InsP3 concentration showed a greater rate of termination at low InsP3 concentration than at high for the same flux. 7. Elevating cytosolic free [Ca2+] by 0.1-2.5 microM strongly inhibited Ca2+ release by InsP3, and buffering free [Ca2+] to low levels greatly prolonged Ca2+ release. Both results support the idea that Ca2+ flux quickly produces locally high free [Ca2+] which inhibits the receptor and terminates Ca2+ release. 8. Hormonally evoked Ca2+ spikes showed a similar reciprocal relation between rise time and Ca2+ flux, seen in the initial Ca2+ spike evoked by extracellular ATP in porcine aortic endothelial cells and by acetylcholine in rat aortic endothelial cells in situ, supporting the idea that the same mechanism of cytosolic Ca2+ inhibition determines the duration of hormonally and InsP3-evoked Ca2+ spikes.

Calcium signalling: how do IP3 receptors work?

The intracellular receptor for inositol 1,4,5-trisphosphate (IP3) is responsible for generation and control of very complex Ca2+ signals. New experimental approaches to studying the kinetics of the IP3 receptor are now beginning to give some insight into the mechanisms behind its rather bizarre properties.

Cooperative activation of IP3 receptors by sequential binding of IP3 and Ca2+ safeguards against spontaneous activity

BACKGROUND

Ca2+ waves allow effective delivery of intracellular Ca2+ signals to cytosolic targets. Propagation of these regenerative Ca2+ signals probably results from the activation of intracellular Ca2+ channels by the increase in cytosolic [Ca2+] that follows the opening of these channels. Such positive feedback is potentially explosive. Mechanisms that limit the spontaneous opening of intracellular Ca2+ channels are therefore likely to have evolved in parallel with the mechanism of Ca2+-induced Ca2+ release.

RESULTS

Maximal rates of 45Ca2+ efflux from permeabilised hepatocytes superfused with medium in which the [Ca2+] was clamped were cooperatively stimulated by inositol 1,4,5-trisphosphate (IP3). A minimal interval of approximately 400 msec between IP3 addition and the peak rate of Ca2+ mobilisation indicate that channel opening does not immediately follow binding of IP3. Although the absolute latency of Ca2+ release was unaffected by further increasing the IP3 concentration, it was reduced by increased [Ca2+].

CONCLUSIONS

We propose that the closed conformation of the IP3 receptor is very stable and therefore minimally susceptible to spontaneous activation; at least three (probably four) IP3 molecules may be required to provide enough binding energy to drive the receptor into a stable open conformation. We suggest that a further defence from noise is provided by an extreme form of coincidence detection. Binding of IP3 to each of its four receptor subunits unmasks a site to which Ca2+ must bind before the channel can open. As IP3 binding may also initiate receptor inactivation, there may be only a narrow temporal window during which each receptor subunit must bind both of its agonists if the channel is to open rather than inactivate.

Analytical calculation of intracellular calcium wave characteristics

We present a theoretical analysis of intracellular calcium waves propagated by calcium feedback at the inositol 1,4,5-trisphosphate (IP3) receptor. The model includes essential features of calcium excitability, but is still analytically tractable. Formulas are derived for the wave speed, amplitude, and width. The calculations take into account cytoplasmic Ca buffering, the punctate nature of the Ca release channels, channel inactivation, and Ca pumping. For relatively fast buffers, the wave speed is well approximated by V(infinity) = (J(eff)D(eff)/C0)1/2, where J(eff) is an effective, buffered source strength; D(eff) is the effective, buffered diffusion constant of Ca; and C(0) is the Ca threshold for channel activation. It is found that the saturability and finite on-rate of buffers must be taken into account to accurately derive the wave speed and front width. The time scale governing Ca wave propagation is T(r), the time for Ca release to reach threshold to activate further release. Because IP3 receptor inactivation is slow on this time scale, channel inactivation does not affect the wave speed. However, inactivation competes with Ca removal to limit wave height and front length, and for biological parameter ranges, it is inactivation that determines these parameters. Channel discreteness introduces only small corrections to wave speed relative to a model in which Ca is released uniformly from the surface of the stores. These calculations successfully predict experimental results from basic channel and cell parameters and explain the slowing of waves by exogenous buffers.

Regulation of Ca2+ release by InsP3 in single guinea pig hepatocytes and rat Purkinje neurons

The repetitive spiking of free cytosolic [Ca2+] ([Ca2+]i) during hormonal activation of hepatocytes depends on the activation and subsequent inactivation of InsP3-evoked Ca2+ release. The kinetics of both processes were studied with flash photolytic release of InsP3 and time resolved measurements of [Ca2+]i in single cells. InsP3 evoked Ca2+ flux into the cytosol was measured as d[Ca2+]i/dt, and the kinetics of Ca2+ release compared between hepatocytes and cerebellar Purkinje neurons. In hepatocytes release occurs at InsP3 concentrations greater than 0.1-0.2 microM. A comparison with photolytic release of metabolically stable 5-thio-InsP3 suggests that metabolism of InsP3 is important in determining the minimal concentration needed to produce Ca2+ release. A distinct latency or delay of several hundred milliseconds after release of low InsP3 concentrations decreased to a minimum of 20-30 ms at high concentrations and is reduced to zero by prior increase of [Ca2+]i, suggesting a cooperative action of Ca2+ in InsP3 receptor activation. InsP3-evoked flux and peak [Ca2+]i increased with InsP3 concentration up to 5-10 microM, with large variation from cell to cell at each InsP3 concentration. The duration of InsP3-evoked flux, measured as 10-90% risetime, showed a good reciprocal correlation with d[Ca2+]i/dt and much less cell to cell variation than the dependence of flux on InsP3 concentration, suggesting that the rate of termination of the Ca2+ flux depends on the free Ca2+ flux itself. Comparing this data between hepatocytes and Purkinje neurons shows a similar reciprocal correlation for both, in hepatocytes in the range of low Ca2+ flux, up to 50 microM. s-1 and in Purkinje neurons at high flux up to 1,400 microM. s-1. Experiments in which [Ca2+]i was controlled at resting or elevated levels support a mechanism in which InsP3-evoked Ca2+ flux is inhibited by Ca2+ inactivation of closed receptor/channels due to Ca2+ accumulation local to the release sites. Hepatocytes have a much smaller, more prolonged InsP3-evoked Ca2+ flux than Purkinje neurons. Evidence suggests that these differences in kinetics can be explained by the much lower InsP3 receptor density in hepatocytes than Purkinje neurons, rather than differences in receptor isoform, and, more generally, that high InsP3 receptor density promotes fast rising, rapidly inactivating InsP3-evoked [Ca2+]i transients.

Single-channel kinetics, inactivation, and spatial distribution of inositol trisphosphate (IP3) receptors in Xenopus oocyte nucleus

Single-channel properties of the Xenopus inositol trisphosphate receptor (IP3R) ion channel were examined by patch clamp electrophysiology of the outer nuclear membrane of isolated oocyte nuclei. With 140 mM K+ as the charge carrier (cytoplasmic [IP3] = 10 microM, free [Ca2+] = 200 nM), the IP3R exhibited four and possibly five conductance states. The conductance of the most-frequently observed state M was 113 pS around 0 mV and approximately 300 pS at 60 mV. The channel was frequently observed with high open probability (mean P(o) = 0.4 at 20 mV). Dwell time distribution analysis revealed at least two kinetic states of M with time constants tau < 5 ms and approximately 20 ms; and at least three closed states with tau approximately 1 ms, approximately 10 ms, and >1 s. Higher cytoplasmic potential increased the relative frequency and tau of the longest closed state. A novel "flicker" kinetic mode was observed, in which the channel alternated rapidly between two new conductance states: F1 and F2. The relative occupation probability of the flicker states exhibited voltage dependence described by a Boltzmann distribution corresponding to 1.33 electron charges moving across the entire electric field during F1 to F2 transitions. Channel run-down or inactivation (tau approximately 30 s) was consistently observed in the continuous presence of IP3 and the absence of change in [Ca2+]. Some (approximately 10%) channel disappearances could be reversed by an increase in voltage before irreversible inactivation. A model for voltage-dependent channel gating is proposed in which one mechanism controls channel opening in both the normal and flicker modes, whereas a separate independent mechanism generates flicker activity and voltage-reversible inactivation. Mapping of functional channels indicates that the IP3R tends to aggregate into microscopic (<1 microm) as well as macroscopic (approximately 10 microm) clusters. Ca2+-independent inactivation of IP3R and channel clustering may contribute to complex [Ca2+] signals in cells.

Modulation of the kinetics of inositol 1,4,5-trisphosphate-induced [Ca2+]i oscillations by calcium entry in pituitary gonadotrophs

Inositol 1,4,5-trisphosphate (InsP3) binds to its receptor channels and causes liberation of Ca2+ from intracellular stores, frequently in an oscillatory manner. In addition to InsP3, the activation and inactivation properties of these intracellular channels are controlled by Ca2+. We studied the influence of Ca2+ entry on the kinetics of InsP3-triggered oscillations in cytosolic calcium ([Ca2+]i) in gonadotrophs stimulated with gonadotropin-releasing hormone, an agonist that activates InsP3 production. The natural expression of voltage-gated Ca2+ channels (VGCC) in these cells was employed to manipulate Ca2+ entry by voltage clamping the cells at different membrane potentials (Vm). Under physiological conditions, the frequency of the GnRH-induced oscillations increased with time, while the amplitude decreased, until both reached stable values. However, in cells with Vm held at -50 mV or lower, both parameters progressively decreased until the signal was abolished. These effects were reverted by a depolarization of the membrane positive to -45 mV in both agonist- and InsP3-stimulated gonadotrophs. Depolarization also led to an increase in the fraction of time during which the [Ca2+]i remained elevated; this effect originated from both an increase in the mean duration of spikes and a decrease in the interval between spikes. The frequency and amplitude of spiking depended on the activity of VGCC, but displayed different temporal courses and voltage relationships. The depolarization-driven recovery of the frequency was instantaneous, whereas the recovery of the amplitude of spiking was more gradual. The midpoints of the Vm sensitivity curve for amplitude and duration of spiking (-15 mV) were close to the value observed for L-type Ca2+ current and for depolarization-induced increase in [Ca2+]i, whereas this parameter was much lower (-35 mV) for interval between spikes and frequency of oscillations. These observations are compatible with at least two distinct effects of Ca2+ entry on the sustained [Ca2+]i oscillations. Calcium influx facilitates its liberation from intracellular stores by a direct and instantaneous action on the release mechanism. It also magnifies the Ca2+ signal and decreases the frequency because of its gradual effect on the reloading of intracellular stores.

Elementary events of InsP3-induced Ca2+ liberation in Xenopus oocytes: hot spots, puffs and blips

Liberation of sequestered Ca2+ ions in Xenopus oocytes by the second messenger inositol 1,4,5-trisphosphate (InP3) occurs from functionally discrete sites, which are spaced at intervals of several microns and probably represent clusterings of InsP3 receptor/channels (InsP3R) in the endoplasmic reticulum. As well as requiring InsP3, opening of release channels is regulated by dual positive and negative feedback by cytosolic Ca2+, leading to regenerative Ca2+ transients. Because the sensitivity of this process is determined by [InsP3], the ability of Ca2+ ions diffusing from one location to activate increasingly distant InsP3R is enhanced by increasing [InsP3]. Together with the spatial distribution of receptors, this results in generation of a hierarchy of Ca2+ release events, which may involve individual InsP3R (Ca2+ 'blips'), concerted activation of several receptors within a single release site (Ca2+ 'puffs'), and recruitment of successive sites by Ca2+ diffusing over micron distances to produce propagating Ca2+ waves. Thus, Ca2+ signalling in the oocyte is organized as at least two sizes of elemental 'building blocks'; highly localized Ca2+ transients that arise autonomously and stochastically from discrete sites at low [InsP3], but which become coordinated at higher [InsP3] to produce global Ca2+ responses.

Reversible desensitization of inositol trisphosphate-induced calcium release provides a mechanism for repetitive calcium spikes

Repetitive transient increases in cytosolic calcium concentration (calcium spikes or calcium oscillations) are a common mode of signal transduction in receptor-mediated cell activation. Repetitive calcium spikes are initiated by phospholipase C-mediated production of inositol 1,4,5-trisphosphate (InsP3) and are thought to be generated by a positive feedback mechanism in which calcium potentiates its own release, a negative feedback mechanism by which calcium release is terminated, and a slow recovery process that defines the time interval between calcium spikes. The molecular mechanisms that terminate each calcium spike and define the spike frequency are not yet known. Here we show, in intact rat basophilic leukemia cells, that calcium responses induced by InsP3 are diminished for a period of 30-60 s following an InsP3-induced calcium spike. The sensitivity of calcium release for InsP3 was probed by UV laser-mediated photorelease of InsP3, and calcium responses were monitored by fluorescence calcium imaging. A maximal loss in sensitivity (desensitization) was observed for InsP3 increases that resulted in a near maximal calcium spike and was expressed as an 80-100% reduction in the calcium response to an equal amount of InsP3, released 10 s after the first UV pulse. When the amount of released InsP3 in the second pulse was increased 2-3-fold, desensitization was overcome and a second calcium response of equal amplitude to the first was produced. A power dependence of 3.2 was measured between the amount of released InsP3 and the amplitude of the triggered calcium response, explaining how a small decrease in InsP3 sensitivity can lead to a nearly complete reduction in the calcium response. Desensitization was abolished by the addition of the calcium buffers BAPTA and EGTA and could be induced by microinjection of calcium, suggesting that it is a calcium-dependent process. Half-maximal desensitization was observed at a free calcium concentration of 290 nM and increased with a power of 3.7 with peak calcium concentration. These studies suggest that reversible desensitization of InsP3-induced calcium release serves as a "saw-tooth" parameter that controls the termination of each spike and the frequency of calcium spikes.