Calcium signalling and cancer cell growth.

Cancer is caused by defects in the mechanisms underlying cell proliferation and cell death. Calcium ions are central to both phenomena, serving as major signalling agents with spatial localization, magnitude and temporal characteristics of calcium signals ultimately determining cell's fate. There are four primary compartments: extracellular space, cytoplasm, endoplasmic reticulum and mitochondria that participate in the cellular Ca2+ circulation. They are separated by own membranes incorporating divers Ca2(+)-handling proteins whose concerted action provides for Ca2+ signals with the spatial and temporal characteristics necessary to account for specific cellular response. The transformation of a normal cell into a cancer cell is associated with a major re-arrangement of Ca2+ pumps, Na/Ca exchangers and Ca2+ channels, which leads to the enhanced proliferation and impaired ability to die. In the present chapter we examine what changes in Ca+ signalling and the mechanisms that support it underlie the passage from normal to pathological cell growth and death control. Understanding this changes and identifying molecular players involved provides new prospects for cancers treatment.

Hepatitis B virus-related insertional mutagenesis implicates SERCA1 gene in the control of apoptosis.

We have used the Hepatitis B Virus DNA genome as a probe to identify genes clonally mutated in vivo, in human liver cancers. In a tumor, HBV-DNA was found to be integrated into the gene encoding Sarco/Endoplasmic Reticulum Calcium ATPase (SERCA), which pumps calcium, an important intracellular messenger for cell viability and growth, from the cytosol to the endoplasmic reticulum. The HBV X gene promoter cis-activates chimeric HBV X/SERCA1 transcripts, with splicing of SERCA1 exon 11, encoding C-terminally truncated SERCA1 proteins. Two chimeric HBV X/SERCA1 proteins accumulate in the tumor and form dimers. In vitro analyses have demonstrated that these proteins localize to the ER, determine its calcium depletion and induce cell death. We have also shown that these biological effects are related to expression of the SERCA, rather than of the viral moiety. This report involves for the first time the expression of mutated SERCA proteins in vivo in a tumor cell proliferation and in vitro in the control of cell viability. Oncogene (2000).

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.

Internal perfusion of guinea-pig hepatocytes with buffered Ca2+ or inositol 1,4,5-trisphosphate mimics noradrenaline activation of K+ and Cl- conductances.

External application of noradrenaline to voltage-clamped guinea-pig isolated hepatocytes evoked membrane conductance increases to K+ and Cl-. This effect was reproduced by internal perfusion of the cells with 2 microM buffered Ca2+ and with 20 microM inositol 1,4,5-trisphosphate (IP3). The kinetic properties of the K+ conductance and its selective block by the toxin apamin were the same in each case. Cyclical fluctuations of conductance observed with noradrenaline were reproduced by internal IP3 but not by Ca2+ perfusion, indicating that oscillations of intracellular free Ca2+ may arise from properties of the Ca2+ sequestration mechanism at constant IP3 concentration.

Effects of noradrenaline, vasopressin and angiotensin on the Na-K pump in rat isolated liver cells.

The effects of noradenaline (via alpha 1-adrenoceptors) and of the peptidic hormones vasopressin and angiotensin on the Na-K pump have been studied in rat isolated liver cells. The three hormones increased the cytosolic Ca concentration, stimulated the Na-K pump and decreased the internal Na concentration of the cells. The effects were dose-dependent and were blocked by the corresponding antagonists. The simultaneous addition of maximal doses of noradrenaline and angiotensin or vasopressin were not additive suggesting that the hormones use a common mechanism to stimulate the carrier. Incubating the cells in Ca-free medium for long periods (Ca-depletion) increased the Na-K pump activity and reduced the stimulatory action of vasopressin, angiotensin and noradrenaline. The effect of the Ca indicator quin2, used as an intracellular Ca chelator, was also studied. The cells were loaded with a maximal concentration of [3H]-quin2 acetoxymethyl ester in the presence of external Ca for 6 min. The final cell content was 3.1 nmol quin2 mg-1 cell dry wt. In these cells the cytosolic Ca, as monitored from the fluorescence emission of the indicator, was about 200 nM and Na-K pump activity was normal and the cells remained responsive to the three hormones. Loading the cells with quin2 in the absence of external Ca reduced the [Ca]i from 200 nM to about 40 nM and increased the Na-K pump activity but not as a result of a rise in internal Na concentration. In addition, the rat hepatocytes were no longer sensitive to the hormones. It is proposed that Ca inhibits the Na-K pump by binding the internal sites and that vasopressin, angiotensin and noradrenaline stimulate the carrier by interfering with the inhibitory Ca sites.

Sustained calcium signalling and caspase-3 activation involve NMDA receptors in thymocytes in contact with dendritic cells.

L-glutamate, the major excitatory neurotransmitter, also has a role in non-neuronal tissues and modulates immune responses. Whether NMDA receptor (NMDAR) signalling is involved in T-cell development is unknown. In this study, we show that mouse thymocytes expressed an array of glutamate receptors, including NMDARs subunits. Sustained calcium (Ca(2+)) signals and caspase-3 activation in thymocytes were induced by interaction with antigen-pulsed dendritic cells (DCs) and were inhibited by NMDAR antagonists MK801 and memantine. NMDARs were transiently activated, triggered the sustained Ca(2+) signal and were corecruited with the PDZ-domain adaptor postsynaptic density (PSD)-95 to thymocyte-DC contact zones. Although T-cell receptor (TCR) activation was sufficient for relocalization of NMDAR and PSD-95 at the contact zone, NMDAR could be activated only in a synaptic context. In these T-DC contacts, thymocyte activation occurred in the absence of exogenous glutamate, indicating that DCs could be a physiological source of glutamate. DCs expressed glutamate, glutamate-specific vesicular glutamate transporters and were capable of fast glutamate release through a Ca(2+)-dependent mechanism. We suggest that glutamate released by DCs could elicit focal responses through NMDAR-signalling in T cells undergoing apoptosis. Thus, synapses between T and DCs could provide a functional platform for coupling TCR activation and NMDAR signalling, which might reflect on T-cell development and modulation of the immune response.

ATP-activated cation currents in single guinea-pig hepatocytes.

1. Responses of single guinea-pig liver cells to the application of external ATP were studied using the whole-cell voltage clamp technique. 2. When the cells were loaded with 5 mM EGTA in the absence of K+ and Cl- in both internal and external solutions, application of ATP (0.03-100 microM) elicited a large cation-selective inward current at negative holding potentials. The current densities at the peak of the response to 100 microM ATP were 4.5 +/- 0.5 pA pF-1 (mean +/- s.e.m., n = 18) in the presence of Na+ and Ca2+ in the external medium and 3.3 +/- 0.7 pA pF-1 (n = 6) with Ca2+ as the major permeant ion. 3. Divalent cations, when added during the response to ATP in the presence of Na+ and Ca2+, exerted different effects: CdSO4 (2 mM) totally and NiSO4 (2 mM) partially blocked the inward current whereas MnSO4 (2 mM) did not block it. The ATP-activated conductance was permeable to all the divalent cations tested in this study, i.e. Ca2+, Cd2+, Ni2+, Mn2+ and Mg2+. No response to ATP was observed in the absence of external cations. 4. The activation of the inward current was not maintained in the continuous presence of ATP. The effect of Ca2+ ions on the desensitization of the response was studied in different external solutions. The decline in the amplitude of the inward current after the peak was fitted with a single exponential with a time constant of about 2 s for pure Ca2+, Cd2+ or Ni2+ currents, 3 s for Mg2+ or Mn2+ and 4 s in the presence of both Na+ and Ca2+. 5. Under more physiological conditions, the entry of Ca2+ evoked after the stimulation of P2X purinoceptors was associated with an increase in fluo-3 fluorescence and a marked reduction in the delay before the mobilization of internal Ca2+ stores following the activation of P2Y purinoceptors.

Properties of membrane ion conductances evoked by hormonal stimulation of guinea-pig and rabbit isolated hepatocytes.

Membrane conductance changes evoked in isolated guinea-pig or rabbit hepatocytes by hormonal stimulation were studied with the whole-cell patch clamp technique. In Cl-containing solutions, noradrenaline (NA), ATP or angiotensin II (AII) evoked an increase of conductance to both K (GK) and Cl (GCl) ions. Activation of GK occurred after a delay of several seconds and was sustained in the presence of hormone. Activation of GCl was transient, lasting several seconds, and arose either at the same time or shortly after the increase in GK. Conductances showed an initial rapid rise and slow oscillatory changes during maintained hormone application. The NA-induced current reversed at -19 mV in Cl solutions, between the equilibrium potentials for chloride (ECl = 0 mV) and potassium ions (EK = -85 mV), and at -75 mV, near EK, in Cl-free solution. In both conditions whole-cell current-voltage curves were linear in the range -100 mV to +40 mV. The conductance increase produced by NA to Cl- ions was about 50 nS, that to K+ ions was 6 nS. The potassium conductance increase was abolished by the polypeptide toxin apamin (50 nM). An increase in membrane current noise was associated with NA-evoked outward K+ current and blocked by apamin. Spectral analysis gave estimates of the elementary K channel conductance of 1.7 pS. Power spectra were fitted by two Lorentzian components, with average half-power frequencies of 2 and 190 Hz. These results are discussed in relation to the single-channel properties and indicate that the open probability of K channels during the NA response is high. In Cl solutions, with apamin to block the K conductance, no increase in current noise was detected during the large Cl conductance evoked by NA. This suggests either that Cl channels are of very low unitary conductance (less than 1 pS) or that Cl transport is due to a membrane carrier. The complex time-course of hormonally evoked conductances is not due to the properties of ion conductances per se but probably to underlying changes of intracellular second-messenger concentration.

The properties of calcium-activated potassium ion channels in guinea-pig isolated hepatocytes.

1. Unitary currents due to calcium-activated potassium ion channels were studied in inside-out or outside-out excised membrane patches from guinea-pig hepatocytes. 2. Potassium ion channels were identified which were activated by internal calcium ions and blocked by external apamin (50 nM) or (+)-tubocurarine (10 microM). These properties are characteristic of the whole-cell potassium conductance increase evoked in guinea-pig hepatocytes by hormonal stimulation. 3. The single-channel conductance was 20 pS in inside-out or outside-out patches with external and internal K+ ion concentrations of 150 and 135 mM respectively and gluconate anion. Reducing external K+ concentration to 5 mM reduced the unitary conductance for outward current to 6 pS. 4. The calcium sensitivity was investigated with buffered internal Ca2+ ion concentrations in the range 0.3-2.2 microM. Tubocurarine-sensitive channels had an open probability of less than 0.05 at 0.3 microM-internal Ca2+. This increased steeply to a maximum of 0.85 at concentrations of 1.1 microM-Ca2+ or higher. 5. In patches with a single channel active, analysis of open and closed intervals showed that openings occurred in bursts. The increase of open probability at high internal Ca2+ concentration was associated with prolonged bursts of channel opening. 6. Comparison of these results with data from whole-cell conductance changes and with published levels of intracellular Ca2+ ion concentration (Woods, Cuthbertson & Cobbold, 1987) suggests that a large proportion, more than 40%, of potassium ion channels in guinea-pig hepatocytes are activated by hormonal stimulation.

Ca2+ depletion and inositol 1,4,5-trisphosphate-evoked activation of Ca2+ entry in single guinea pig hepatocytes.

Store-operated Ca(2+) entry was investigated by monitoring the Ca(2+)-dependent K(+) permeability in voltage-clamped guinea pig hepatocytes. In physiological conditions, intracellular Ca(2+) stores are discharged following agonist stimulation, but depletion of this stores can be achieved using Ca(2+)-Mg(2+)-ATPase inhibitors such as 2,5-di(tert-butyl)-1,4-benzohydroquinone and thapsigargin. The effect of internal Ca(2+) store depletion on Ca(2+) influx was tested in single cells using inositol 1,4,5-trisphosphate (InsP(3)) release from caged InsP(3) after treatment of the cells with 2, 5-di(tert-butyl)-1,4-benzohydroquinone or thapsigargin in Ca(2+)-free solutions. We show that the photolytic release of 1-d-myo-inositol 1,4-bisphosphate 5-phosphorothioate, a stable analog of InsP(3), and Ca(2+) store depletion have additive effects to activate a high level of Ca(2+) entry in single guinea pig hepatocytes. These results suggest that there is a direct functional interaction between InsP(3) receptors and Ca(2+) channels in the plasma membrane, although the nature of these Ca(2+) channels in hepatocytes is unclear.

3':5'-cyclic guanosine monophosphate (cGMP) potentiates the inositol 1,4,5-trisphosphate-evoked Ca2+ release in guinea-pig hepatocytes.

The effect of cGMP on noradrenaline-induced intracellular Ca2+ mobilization was investigated in whole-cell voltage-clamped guinea-pig hepatocytes. Treatment of the cells with 8-Br-cGMP (1-500 microM) resulted in an increase in the sensitivity of the cells to noradrenaline and to inositol 1,4,5-trisphosphate (InsP3) photo-released from caged InsP3. The positive effect of 8-Br-cGMP on the Ca2+ release evoked by Ca(2+)-mobilizing agonists or InsP3 was blocked by a protein kinase G (PKG; cGMP-dependent protein kinase) inhibitor, the RP-8-(4-chlorophenylthio)guanosine 3':5'-monophosphorothioate. 8-Br-cGMP affected neither the basal InsP3 concentration nor the noradrenaline-induced production of InsP3. In permeabilized hepatocytes, the dose-response curve for InsP3-induced Ca2+ release was shifted to the left in the presence of 8-Br-cGMP. Furthermore, the treatment with 8-Br-cGMP did not affect the Ca2+ content of the InsP3-sensitive Ca2+ stores. These results indicate that intracellular cGMP potentiates the noradrenaline-induced Ca2+ response by enhancing Ca2+ release from the intracellular Ca2+ stores. We suggest that cGMP increases the apparent affinity of InsP3 receptors for InsP3 in guinea-pig hepatocytes probably by phosphorylation via the activation of PKG.

Effect of the alpha-agonist noradrenaline on total and 45Ca2+ movements in mitochondria of rat liver cells.

Ca(2+) movements triggered by noradrenaline were determined in isolated cells and mitochondria from rat livers. It has been shown that these depend on experimental conditions. In cells incubated in 1.8mm-Ca(2+), results suggest that noradrenaline mobilizes Ca(2+) from reticulum before releasing Ca(2+) from mitochondria.

Cyclic AMP-evoked oscillations of intracellular [Ca2+] in guinea-pig hepatocytes.

The effects of the beta-adrenoceptor agonist isoprenaline and cyclic AMP (cAMP) on cytosolic free Ca2+ ([Ca2+]i) were studied in the single guinea-pig hepatocyte. In common with InsP3-dependent agonists such as noradrenaline or angiotensin II, isoprenaline (0.5-10 microM) and cAMP (50-100 mM, perfused into the cell via the patch-pipette), were able to generate fast and slow fluctuations of [Ca2+]i. Responses to isoprenaline and cAMP also were observed in the absence of external Ca2+. Isoprenaline-evoked [Ca2+]i rises were not blocked by the intracellular perfusion of heparin, suggesting that these fluctuations are independent of the binding of InsP3 to its receptor.

Evidence for bile acid-evoked oscillations of Ca2(+)-dependent K+ permeability unrelated to a D-myo-inositol 1,4,5-trisphosphate effect in isolated guinea pig liver cells.

In single liver cells, the D-myo-inositol 1,4,5-triphosphate (InsP3)-dependent agonists such as noradrenaline and angiotensin II evoke oscillations in intracellular calcium [Ca2+]i resulting mostly from the periodic release and reuptake of calcium from intracellular stores. In the present work, we have reexamined the effects of these agonists and investigated whether the natural bile acid taurolithocholic acid 3-sulfate (TLC-S), which permeabilizes the endoplasmic reticulum, could initiate oscillations of [Ca2+]i. Oscillations of [Ca2+]i were monitored with the Ca2(+)-dependent K+ permeability in whole-cell voltage-clamped guinea pig liver cells. Our results confirm the presence of two types of oscillations induced by hormones. They could be distinguished by their frequency periods. The fast (type I) had periods ranging from 5 to 12 s and the slow (type II) from 60 to 240 s. They have been respectively attributed to second messenger- and receptor-controlled oscillations, respectively. Our results also show that TLC-S, as noradrenaline and angiotensin II, induced the activation of this Ca(+)-dependent K+ current and was able to reproduce both types of oscillations. The bile acid effect was not blocked by intracellular perfusion of heparin known to inhibit both InsP3 binding and InsP3-evoked Ca2+ release in several tissues. In these conditions, TLC-S only evoked type I oscillations, suggesting that these fluctuations could originate from a mechanism that is independent of InsP3 and is an intrinsic property of internal Ca2+ stores.

Oscillations of cytosolic free calcium concentration in the presence of intracellular antibodies to phosphatidylinositol 4,5-bisphosphate in voltage-clamped guinea-pig hepatocytes.

In liver cells, the stimulation of alpha 1-adrenoceptors by noradrenaline induces the production of Ins(1,4,5)P3 through the degradation of membrane polyphosphoinositides [PtdIns(4,5)P2]. InsP3 evokes in turn the release of Ca2+ from internal stores. Our results show that the internal perfusion of single guinea-pig hepatocytes with monoclonal anti-PtdInsP2 antibody blocks the rise in cytosolic free Ca2+ concn. ([Ca2+]i) evoked by noradrenaline, an InsP3-dependent agonist, but not by the monohydroxylated bile acid taurolithocholate 3-sulphate, which is known to permeabilize the endoplasmic reticulum. In these conditions, the bile acid elicited either fast or slow fluctuations of [Ca2+]i independently of any InsP3 production. The responses to the bile acid were also observed in the absence of external Ca2+. The presence of intracellular anti-PtdInsP2 antibody does not affect the response to a photolytic release of InsP3 (1.5 microM final concn.) from a caged precursor.

A rhoptry-protein-associated mechanism of clonal phenotypic variation in rodent malaria.

The recognition and invasion of host cells are mediated by components of the apical complex of the ookinete, sporozoite and merozoite stages of Plasmodium parasites. The paired rhoptries (organelles involved in host-cell recognition) in the apical complex contain many proteins of as-yet unknown function. In the rodent malaria agent P. yoelii yoelii, a multigene family codes for merozoite rhoptry proteins of relative molecular mass 235,000 (p235 proteins); these proteins are thought to determine the subset of erythrocytes that the parasites invade. Further support for this idea came from the identification of a region in p235 with weak but significant homology to reticulocyte-binding protein-2 of P. vivax and the demonstration that at least one p235 member binds to the erythrocyte surface membrane. Here, using single, micromanipulated P.y.yoelii parasites, we describe a new mechanism of gene expression by which the merozoites originating from a single schizont each express a distinct member of this multigene family. We propose that this new type of clonal phenotypic variation provides the parasite with a survival strategy in the mammalian host; this strategy contributes to the observed chronicity of malarial infections. This phenomenon is genetically and functionally distinct from classical antigenic variation, which is mediated by the var multigene family of P. falciparum.

Kinetics of the conductance evoked by noradrenaline, inositol trisphosphate or Ca2+ in guinea-pig isolated hepatocytes.

1. Guinea-pig hepatocytes respond to noradrenaline (NA, 5-10 microM) with a large membrane conductance increase to K+ and Cl-. The response has a long initial delay (range 2-30 s). Following the delay, the K+ conductance (studied in Cl(-)-free solutions) rises quickly to a peak in 1-2 s and is maintained in the continued presence of NA, though often with superimposed oscillations of conductance. The roles of intracellular Ca2+ and D-myo-inositol 1,4,5-trisphosphate (InsP3) in this complex response have been investigated by rapid photolytic release of intracellular Ca2+ (from Nitr5-Ca2+ buffers) or InsP3 from 'caged' InsP3. 2. A rapid increase of intracellular [Ca2+] produced an immediate membrane conductance increase which rose approximately exponentially to a new steady level, consistent with a direct activation of Ca2(+)-dependent ion channels. 3. Following a pulse of InsP3, conductance rose after a brief delay (range 70-1500 ms) which was shortest at high [InsP3] or if the initial cytosolic [Ca2+] had been raised above normal levels. The maximum conductance produced by InsP3 was similar in each cell to the peak recorded with NA and could be evoked by InsP3 concentrations of 0.5-1 microM. 4. The rates of rise of conductance increased with InsP3 concentration in the range 0.25-12.5 microM (range 10-90%, rise times 90-1000 ms), indicating that InsP3-evoked Ca2(+)-efflux from stores increases with InsP3 concentration in this range. 5. Photochemically released InsP3 and Ca2+ activate at physiological concentrations the same membrane conductances as NA. The results indicate that the long initial delay in NA action occurs prior to or during generation of InsP3. The mechanism of the delay and the subsequent apparently all-or-none conductance increase during NA action are discussed in terms of the high co-operativity in InsP3 and Ca2+ actions and an additional positive feedback step. 6. Evidence was found of a negative interaction between [Ca2+] and InsP3-evoked Ca2+ release. The time course of the recovery of InsP3-evoked Ca2+ release following a rise of cytosolic [Ca2+] suggests that this interaction may be important in regulating oscillatory responses of [Ca2+] during hormonal stimulation of guinea-pig hepatocytes.

Mechanisms of intracellular calcium release during hormone and neurotransmitter action investigated with flash photolysis.

To understand the complex time course of cytosolic Ca2+ signalling evoked by hormones and neurotransmitters, it is necessary to know the kinetics of steps in the second-messenger cascade, particularly cooperative and inhibitory interactions between components that might give rise to periodic fluctuations. In the case of inositol trisphosphate (InsP3)-evoked Ca2+ release, fast perfusion studies with subcellular fractions or permeabilised cells can be made if sufficient homogeneous tissue is available. Single-cell studies can be made by combining whole-cell patch-clamp techniques and microspectrofluorimetry with flash photolytic release of InsP3 to give quantitative, time-resolved data of Ca2+ release from stores. A technical description is given here of flash photolysis of caged InsP3, and the results of fast perfusion and flash photolytic experiments are reviewed. Studies of kinetics of Ca2+ release have shown that the InsP3 receptor/channel is regulated first by positive and then by negative feedback by free cytosolic Ca2+ concentration, producing a pulse of Ca2+ release having properties that may be important in the spatial propagation of Ca2+ signals within and between cells. The properties of InsP3-evoked Ca2+ release in single cells differ between peripheral tissues, such as the liver, and Purkinje neurones of the cerebellum. Purkinje neurones need 20-50 times higher InsP3 concentrations and release Ca2+ to change the free cytosolic concentration 30 times faster and to higher peak concentrations than in liver. The InsP3 receptors in the two cell types appear to differ in apparent affinity, and the greater Ca2+ efflux from stores in Purkinje cells is probably due to a high receptor density.