Capacitative Calcium Entry

Role of IP3 Receptors in Shaping the Carotid Chemoreceptor Response to Hypoxia But Not to Hypercapnia in the Rat Carotid Body: An Evidence Review

This article addresses the disparity in the transduction pathways for hypoxic and hypercapnic stimuli in carotid body glomus cells. We investigated and reviewed the experimental evidence showing that the response to hypoxia, but not to hypercapnia, is mediated by 1,4,5-inositol triphosphate receptors (IP₃R/s) regulating the intracellular calcium content [Ca²⁺]_c in glomus cells. The rationale was based on the past observations that inhibition of oxidative phosphorylation leads to the explicit inhibition of the hypoxic chemoreflex. [Ca²⁺]_c changes were measured using cellular Ca²⁺-sensitive fluorescent probes, and carotid sinus nerve (CSN) sensory discharge was recorded with bipolar electrodes in in vitro perfused-superfused rat carotid body preparations. The cell-permeant, 2-amino-ethoxy-diphenyl-borate (2-APB; 100 μM) and curcumin (50 μM) were used as the inhibitors of IP₃R/s. These agents suppressed the [Ca²⁺]_c, and CSN discharge increases in hypoxia but not in hypercapnia, leading to the conclusion that only the hypoxic effects were mediated via modulation of IP₃R/s. The ATP-induced Ca²⁺ release from intracellular stores in a Ca²⁺-free medium was blocked with 2-APB, supporting this conclusion.

Regulation of nuclear factor of activated T cells (NFAT) in vascular endothelial cells

Proteins of the NFAT family (nuclear factor of activated T cells) are Ca(2+)-sensitive transcription factors, which are involved in hypertrophic cardiovascular remodeling. Activation and nuclear translocation is mediated by dephosphorylation by the Ca(2+)-sensitive phosphatase calcineurin (CaN). We identified Ca(2+) signals that induced nuclear translocation of NFAT in cultured calf pulmonary artery endothelial (CPAE) cells using confocal fluorescence microscopy to measure simultaneously [Ca(2+)](i) and subcellular localization of NFAT-GFP (isoforms NFATc1 and NFATc3). The vasoactive agonists ATP (5 microM) or bradykinin (20 microM) in the presence of 2 mM extracellular Ca(2+) induced Ca(2+) release from the endoplasmic reticulum (ER) and activated capacitative Ca(2+) entry (CCE), which caused robust translocation of NFAT to the nucleus. This effect was sensitive to the CaN-inhibitor cyclosporin A (1 microM). Influx of extracellular Ca(2+) via CCE, but not ER Ca(2+) release was identified as the activating Ca(2+) source. NFAT was also activated by Ca(2+) influx induced by cell swelling, reverse mode Na/Ca exchange or ionomycin treatment. NFAT regulation was isoform-specific. Whereas activation of NFATc1-GFP by ATP resulted in persistent nuclear localization, NFATc3-GFP was only transiently imported into the nucleus, followed by rapid export back to the cytoplasm. Inhibition of nuclear kinases, which mediate export of NFAT via phosphorylation, or direct block of nuclear export (Leptomycin B) resulted in stable nuclear localization of NFATc3. These data demonstrate that extracellular Ca(2+) entry mediates NFAT activation. Furthermore, the regulation of nuclear localization of NFAT is isoform-specific and dependent on nuclear export processes.

Microtubule remodeling mediates the inhibition of store-operated calcium entry (SOCE) during mitosis in COS-7 cells

Regulation of the intracellular calcium ion concentration ([Ca(2+)](i)) is critical, because calcium signaling controls diverse and vital cellular processes such as secretion, proliferation, division, gene transcription, and apoptosis. Store-operated calcium entry (SOCE) is the main mechanism through which non-excitable cells replenish and thus maintain this delicate balance. There is limited evidence which indicates that SOCE may be inhibited during mitosis, and the mechanisms leading to the presumed inhibition has not been elucidated. In the present study, we examined and compared the [Ca(2+)](i) dynamics of COS-7 cells in mitotic and non-mitotic phases with special reference paid to SOCE. Laser scanning confocal microscopy to monitor [Ca(2+)](i) dynamics revealed that SOCE was progressively inhibited in mitosis and became virtually absent during the metaphase. We used various cytoskeletal modifying drugs and immunofluorescence to assess the contribution of microtubule and actin filaments in SOCE signaling. Nocodazole treatment caused microtubule reorganization and retraction from the cell periphery that mimicked the natural mitotic microtubule remodeling that was also accompanied by SOCE inhibition. Short exposure to paclitaxel, a microtubule-stabilizing drug, bolstered SOCE, whereas long exposure resulted in microtubule disruption and SOCE inhibition. Actin-modifying drugs did not affect SOCE. These findings indicate that mitotic microtubule remodeling plays a significant role in the inhibition of SOCE during mitosis.

Ca2+ store determines gating of store operated calcium entry in mammalian skeletal muscle

This work describes the gating of the store operated calcium entry (SOCE) in adult mammalian skeletal muscle. Flexor digitorum brevis fibers (FDB) were isolated from adult mice and exposed to conditions to deplete the sarcoplasmic reticulum (SR). A transient SR depletion caused either by repetitive depolarizations, chlorocresol (CMC) or, cyclopiazonic acid (CPA) induced a bell shaped calcium entry that raised the [Ca(2+)](i) to a maximum of 27.09 +/- 4.35 nM from the resting value. The activation time to reach 10-90% of the maximum amplitude was 112 +/- 10 s (n = 22). On the other hand, any mechanism that caused a permanent SR depletion (like thapsigargin, continuous CPA, or continuous CMC) triggered a calcium entry pathway that lasted 325 +/- 23 s and raised the [Ca(2+)](i )to 129.50 +/- 13.05 nM from the resting level (n = 28). Then, a prolonged depletion triggered an increase in [Ca(2+)](i) to higher values and for a longer time than when the SR is transiently depleted (p < 0.001). Our results, in skeletal muscle, showed that calcium store depletion was the signal for SOCE activation and how the SR got depleted was not relevant. Also, we found that SOCE deactivation was not caused by [Ca(2+)](i) but by the SR content. Our results suggest that the SR calcium content plays an important role in SOCE gating in mammalian skeletal muscle and a calcium sensor is located inside the SR.

Role of glycolytically generated ATP for CaMKII-mediated regulation of intracellular Ca2+ signaling in bovine vascular endothelial cells

The role of glycolytically generated ATP in Ca(2+)/calmodulin-dependent kinase II (CaMKII)-mediated regulation of intracellular Ca(2+) signaling was examined in cultured calf pulmonary artery endothelial (CPAE) cells. Exposure of cells (extracellular Ca(2+) concentration = 2 mM) to glycolytic inhibitors 2-deoxy-D-glucose (2-DG), pyruvate (pyr) + beta-hydroxybutyrate (beta-HB), or iodoacetic acid (IAA) caused an increase of intracellular Ca(2+) concentration ([Ca(2+)](i)). CaMKII inhibitors (KN-93, W-7) triggered a similar increase of [Ca(2+)](i). The rise of [Ca(2+)](i) was characterized by a transient spike followed by a small sustained plateau of elevated [Ca(2+)](i). In the absence of extracellular Ca(2+) 2-DG caused an increase in [Ca(2+)](i), suggesting that inhibition of glycolysis directly triggered release of Ca(2+) from intracellular endoplasmic reticulum (ER) Ca(2+) stores. The inositol-1,4,5-trisphosphate receptor (IP(3)R) inhibitor 2-aminoethoxydiphenyl borate abolished the KN-93- and 2-DG-induced Ca(2+) response. Ca(2+) release was initiated in peripheral cytoplasmic processes from which activation propagated as a [Ca(2+)](i) wave toward the central region of the cell. Focal application of 2-DG resulted in spatially confined elevations of [Ca(2+)](i). Propagating [Ca(2+)](i) waves were preceded by [Ca(2+)](i) oscillations and small, highly localized elevations of [Ca(2+)](i) (Ca(2+) puffs). Inhibition of glycolysis with 2-DG reduced the KN-93-induced Ca(2+) response, and vice versa during inhibition of CaMKII 2-DG-induced Ca(2+) release was attenuated. Similar results were obtained with pyr + beta-HB and W-7. Furthermore, 2-DG and IAA caused a rapid increase of intracellular Mg(2+) concentration, indicating a concomitant drop of cellular ATP levels. In conclusion, CaMKII exerts a profound inhibition of ER Ca(2+) release in CPAE cells, which is mediated by glycolytically generated ATP, possibly through ATP-dependent phosphorylation of the IP(3)R.

Ca2+ signalling in urethral interstitial cells of Cajal

Interstitial cells of Cajal (ICC) in the urethra have been proposed as specialized pacemakers that are involved in the generation of urethral tone and therefore the maintenance of urinary continence. Recent studies on freshly dispersed ICC from the urethra of rabbits have demonstrated that pacemaker activity in urethra ICC is characterized by spontaneous transient depolarizations (STDs) under current clamp and spontaneous transient inward currents (STICs) under voltage clamp. When these events were simultaneously recorded with changes in intracellular Ca(2+) (using a Nipkow spinning disk confocal microscope) they were found to be associated with global Ca(2+) oscillations. In this short review we will consider some of these recent findings regarding the contribution of intracellular Ca(2+) stores and Ca(2+) influx to the generation of pacemaker activity in urethral ICC with particular emphasis on the contribution of reverse Na(+)/Ca(2+) exchange (NCX).

The cellular mechanisms by which adenosine evokes release of nitric oxide from rat aortic endothelium

Adenosine and nitric oxide (NO) are important local mediators of vasodilatation. The aim of this study was to elucidate the mechanisms underlying adenosine receptor-mediated NO release from the endothelium. In studies on freshly excised rat aorta, second-messenger systems were pharmacologically modulated by appropriate antagonists while a NO-sensitive electrode was used to measure adenosine-evoked NO release from the endothelium. We showed that A1-mediated NO release requires extracellular Ca2+, phospholipase A2 (PLA2) and ATP-sensitive K+ (KATP) channel activation whereas A2A-mediated NO release requires extracellular Ca2+ and Ca2+-activated K+ (KCa) channels. Since our previous study showed that A1- and A2A-receptor-mediated NO release requires activation of adenylate cyclase (AC), we propose the following novel pathways. The K+ efflux resulting from A1-receptor-coupled KATP-channel activation facilitates Ca2+ influx which may cause some stimulation of endothelial NO synthase (eNOS). However, the increase in [Ca2+]i also stimulates PLA2 to liberate arachidonic acid and stimulate cyclooxygenase to generate prostacyclin (PGI2). PGI2 acts on its endothelial receptors to increase cAMP, so activating protein kinase A (PKA) to phosphorylate and activate eNOS resulting in NO release. By contrast, the K+ efflux resulting from A2A-coupled KCa channels facilitates Ca2+ influx, thereby activating eNOS and NO release. This process may be facilitated by phosphorylation of eNOS by PKA via the action of A2A-receptor-mediated stimulation of AC increasing cAMP. These pathways may be important in mediating vasodilatation during exercise and systemic hypoxia when adenosine acting in an endothelium- and NO-dependent manner has been shown to be important.

Pacemaker activity in urethral interstitial cells is not dependent on capacitative calcium entry

The aim of the present study was to investigate the properties and role of capacitative Ca(2+) entry (CCE) in interstitial cells (IC) isolated from the rabbit urethra. Ca(2+) entry in IC was larger in cells with depleted intracellular Ca(2+) stores compared with controls, consistent with influx via a CCE pathway. The nonselective Ca(2+) entry blockers Gd(3+) (10 microM), La(3+) (10 microM), and Ni(2+) (100 microM) reduced CCE by 67% (n = 14), 65% (n = 11), and 55% (n = 9), respectively. These agents did not inhibit Ca(2+) entry when stores were not depleted. Conversely, CCE in IC was resistant to SKF-96365 (10 microM), wortmannin (10 microM), and nifedipine (1 microM). Spontaneous transient inward currents were recorded from IC voltage-clamped at -60 mV. These events were not significantly affected by Gd(3+) (10 microM) or La(3+) (10 microM) and were only slightly decreased in amplitude by 100 microM Ni(2+). The results from this study demonstrate that freshly dispersed IC from the rabbit urethra possess a CCE pathway. However, influx via this pathway does not appear to contribute to spontaneous activity in these cells.

Modulation of intracellular Ca2+ release and capacitative Ca2+ entry by CaMKII inhibitors in bovine vascular endothelial cells

The effects of inhibitors of CaMKII on intracellular Ca2+ signaling were examined in single calf pulmonary artery endothelial (CPAE) cells using indo-1 microfluorometry to measure cytoplasmic Ca2+ concentration ([Ca2+]i). The three CaMKII inhibitors, KN-93, KN-62, and autocamtide-2-related inhibitory peptide (AIP), all reduced the plateau phase of the [Ca2+]i transient evoked by stimulation with extracellular ATP. Exposure to KN-93 or AIP alone in the presence of 2 mM extracellular Ca2+ resulted in a dose-dependent increase of [Ca2+]i consisting of a rapid and transient Ca2+ spike followed by a small sustained plateau phase of elevated [Ca2+]i. Exposure to KN-93 in the absence of extracellular Ca2+ caused a transient rise of [Ca2+]i, suggesting that exposure to CaMKII inhibitors directly triggered release of Ca2+ from intracellular endoplasmic reticulum (ER) Ca2+ stores. Repetitive stimulation with KN-93 and ATP, respectively, revealed that both components released Ca2+ largely from the same store. Pretreatment of CPAE cells with the membrane-permeable inositol 1,4,5-trisphosphate (IP3) receptor blocker 2-aminoethoxydiphenyl borate caused a significant inhibition of the KN-93-induced Ca2+ response, suggesting that exposure to KN-93 affects Ca2+ release from an IP3-sensitive store. Depletion of Ca2+ stores by exposure to ATP or to the ER Ca2+ pump inhibitor thapsigargin triggered robust capacitative Ca2+ entry (CCE) signals in CPAE cells that could be blocked effectively with KN-93. The data suggest that in CPAE cells, CaMKII modulates Ca2+ handling at different levels. The use of CaMKII inhibitors revealed that in CPAE cells, the most profound effects of CaMKII are inhibition of release of Ca2+ from intracellular stores and activation of CCE.

Nitric oxide inhibits capacitative Ca2+ entry and enhances endoplasmic reticulum Ca2+ uptake in bovine vascular endothelial cells

In vascular endothelial cells, elevation of cytosolic free calcium concentration ([Ca2+]i) causes activation of nitric oxide synthase (NOS) and release of nitric oxide (NO). The goal of the study was to characterize the interplay between [Ca2+]i and NO production in this cell type. Simultaneous measurements of [Ca2+]i and intracellular NO concentration ([NO]i) in cultured bovine vascular endothelial cells (CPAE cell line) with the fluorescent indicators fura-2 and DAF-2, respectively, revealed that Ca2+ influx following agonist-induced intracellular Ca2+ store depletion (capacitative Ca2+ entry, CCE) represents the preferential Ca2+ source for the activation of the Ca2+-calmodulin-dependent endothelial NOS (eNOS). Exposure to the NO donor sodium nitroprusside (SNP) showed that high NO levels suppressed CCE and had an inhibitory effect on Ca2+ extrusion by the plasmalemmal Ca2+-ATPase. This inhibitory effect on CCE was mimicked by the membrane-permeant cGMP analogue 8-bromo-cGMP, but was reversed by the NO scavenger haemoglobin and prevented by the inhibitor of the NO-sensitive guanylate cyclase ODQ. Brief exposure to SNP reduced the peak of ATP-induced Ca2+ release from the endoplasmic reticulum (ER) and accelerated Ca2+ reuptake into the ER. Prolonged incubation with SNP resulted in enhanced Ca2+ loading of the ER, as revealed by direct measurements of store content with the ER-entrapped low-affinity Ca2+ indicator mag-fura-2. The results suggest that in vascular endothelial cells, NO synthesis is under autoregulatory control that involves NO-dependent [Ca2+]i regulation. Via cGMP-dependent inhibition of CCE and acceleration of Ca2+ sequestration into the ER, NO can lower [Ca2+]i and therefore exert an autoregulatory negative feedback on its own Ca2+-dependent synthesis.

Ca(2+) signalling and PKCalpha activate increased endothelial permeability by disassembly of VE-cadherin junctions

1. The role of intracellular Ca(2+) mobilization in the mechanism of increased endothelial permeability was studied. Human umbilical vein endothelial cells (HUVECs) were exposed to thapsigargin or thrombin at concentrations that resulted in similar increases in intracellular Ca(2+) concentration ([Ca(2+)](i)). The rise in [Ca(2+)](i) in both cases was due to release of Ca(2+) from intracellular stores and influx of extracellular Ca(2+). 2. Both agents decreased endothelial cell monolayer electrical resistance (a measure of endothelial cell shape change) and increased transendothelial (125)I-albumin permeability. Thapsigargin induced activation of PKCalpha and discontinuities in VE-cadherin junctions without formation of actin stress fibres. Thrombin also induced PKCalpha activation and similar alterations in VE-cadherin junctions, but in association with actin stress fibre formation. 3. Thapsigargin failed to promote phosphorylation of the 20 kDa myosin light chain (MLC(20)), whereas thrombin induced MLC(20) phosphorylation consistent with formation of actin stress fibres. 4. Calphostin C pretreatment prevented the disruption of VE-cadherin junctions and the decrease in transendothelial electrical resistance caused by both agents. Thus, the increased [Ca(2+)](i) elicited by thapsigargin and thrombin may activate a calphostin C-sensitive PKC pathway that signals VE-cadherin junctional disassembly and increased endothelial permeability. 5. Results suggest a critical role for Ca(2+) signalling and activation of PKCalpha in mediating the disruption of VE-cadherin junctions, and thereby in the mechanism of increased endothelial permeability.

Inactivation and tachyphylaxis of heat-evoked inward currents in nociceptive primary sensory neurones of rats

Membrane currents evoked by repeated noxious heat stimuli (43-47 degrees C) of 3 s duration were investigated in acutely dissociated dorsal root ganglion (DRG) neurones of adult rats. The heat stimuli generated by a fast solution exchanger had a rise time of 114 +/- 6 ms and a fall time of 146 +/- 13 ms. When heat stimuli were applied to heat-sensitive small (< or = 32.5 microm) DRG neurones, an inward membrane current (I(heat)) with a mean peak of 2430 +/- 550 pA was observed (n = 19). This current started to activate and deactivate with no significant latency with respect to the heat stimulus. The peak of I(heat) was reached with a rise time of 625 +/- 115 ms. When the heat stimulus was switched off I(heat) deactivated with a fall time of 263 +/- 17 ms. During constant heat stimulation I(heat) decreased with time constants of 4-5 s (inactivation). At the end of a 3 s heat stimulus the peak current was reduced by 44 +/- 5 % (n = 19). Current-voltage curves revealed outward rectifying properties of I(heat) and a reversal potential of -6.3 +/- 2.2 mV (n = 6). Inactivation was observed at all membrane potentials investigated (-80 to 60 mV); however, inactivation was more pronounced for inward currents (37 +/- 5 %) than for outward currents (23 +/- 6 %, P < 0.05). When neurones were investigated with repeated heat stimuli (3 to 5 times) of the same temperature, the peak current relative to the first I(heat) declined by 48 +/- 6 % at the 3rd stimulus (n = 19) and by 54 +/- 18 % at the 5th stimulus (n = 4; tachyphylaxis). In the absence of extracellular Ca2+ (buffered with 10 mM EGTA) inactivation (by 53 +/- 6 %) and tachyphylaxis (by 42 +/- 7 % across three stimuli) were still observed (n = 8). The same was true when intracellular Ca2+ was buffered by 10 mM BAPTA (inactivation by 49 +/- 4 %, tachyphylaxis by 52 +/- 7 % across three stimuli; n = 13). Thus, inactivation and tachyphylaxis were mainly independent of intra- and extracellular Ca2+. These results indicate that inactivation and tachyphylaxis of heat-evoked inward currents can be observed in vitro, similar to adaptation and suppression of action potential discharges elicited by comparably fast heat stimuli in vivo. Whereas the voltage dependence of I(heat) resembles that of capsaicin-induced membrane currents (I(Caps)), the independence of inactivation and tachyphylaxis of I(heat) from calcium is in clear contrast to I(Caps). A similar difference in calcium dependence of inactivation has been reported between heat-evoked and capsaicin-induced currents through the cloned capsaicin receptor channel VR1. Thus, the properties of I(heat) and of VR1 largely account for the adaptation and suppression of heat-evoked nociceptor discharges.

Histamine-induced calcium entry in rat cerebellar astrocytes: evidence for capacitative and non-capacitative mechanisms

We have investigated the effects of histamine on the intracellular calcium concentration ([Ca2+]i) of cultured rat cerebellar astrocytes using fura-2-based Ca2+ imaging microscopy. Most of the cells responded to the application of histamine with an increase in [Ca2+]i which was antagonized by the H1 receptor blocker mepyramine. When histamine was applied for several minutes, the majority of the cells displayed a biphasic Ca2+ response consisting of an initial transient peak and a sustained component. In contrast to the initial transient [Ca2+]i response, the sustained, receptor-activated increase in [Ca2+]i was rapidly abolished by chelation of extracellular Ca2+ or addition of Ni2+, Mn2+, Co2+ and Zn2+, but was unaffected by nifedipine, an antagonist of L-type voltage-activated Ca2+ channels. These data indicate that the sustained increase in [Ca2+]i was dependent on Ca2+ influx. When intracellular Ca2+ stores were emptied by prolonged application of histamine in Ca2+-free conditions, Ca2+ re-addition after removal of the agonist did not lead to an 'overshoot' of [Ca2+]i indicative of store-operated Ca2+ influx. However, Ca2+ stores were refilled despite the absence of any substantial change in the fura-2 signal. Depletion of intracellular Ca2+ stores using cyclopiazonic acid in Ca2+-free saline and subsequent re-addition of Ca2+ to the saline resulted in an increase in [Ca2+]i that was significantly enhanced in the presence of histamine. The results suggest that besides capacitative mechanisms, a non-capacitative, voltage-independent pathway is involved in histamine-induced Ca2+ entry into cultured rat cerebellar astrocytes.

Capacitative Ca2+ entry is graded with degree of intracellular Ca2+ store depletion in bovine vascular endothelial cells

1. In endothelial cells, release of Ca2+ from endoplasmic reticulum (ER) Ca2+ stores activates Ca2+ influx via the capacitative Ca2+ entry (CCE) pathway. In cultured bovine pulmonary artery endothelial cells, we investigated the relationship between intracellular Ca2+ store load and CCE activity, as well as the kinetics of CCE activation and deactivation, by simultaneously measuring changes in [Ca2+]i and unidirectional manganese (Mn2+) entry through the CCE pathway. 2. Submaximal concentrations of ATP caused quantal release of Ca2+ from the ER, resulting in a dose-dependent depletion of Ca2+ stores and acceleration of Mn2+ entry. Mn2+ entry rate, as a measure of CCE activity, was graded with the amount of released Ca2+. Maximal activation of CCE did not require complete store depletion. 3. Slow depletion of the ER by exposure to the ER Ca2+ pump inhibitor cyclopiazonic acid resulted in a delayed activation of CCE, revealing a temporal dissociation between release of Ca2+ from intracellular stores and activation of CCE. 4. During [Ca2+]i oscillations, at frequencies higher than 0.5 spikes min-1, each Ca2+ spike resulted in a progressive acceleration of CCE without leading to oscillations of Ca2+ entry. In contrast, low frequency [Ca2+]i oscillations were paralleled by transient CCE that was activated and deactivated with each Ca2+ spike, resulting in an oscillatory pattern of Ca2+ entry. 5. It is concluded that CCE is a rapidly activating process which is graded with store depletion and becomes fully activated before complete depletion. The duration of CCE activation correlates with the degree of store depletion and the time that is required to refill depleted stores. Overall, a mechanism of graded CCE prevents exhaustion of intracellular Ca2+ reserves and provides an efficient way to respond to variable degrees of intracellular store depletion.

Focal agonist stimulation results in spatially restricted Ca2+ release and capacitative Ca2+ entry in bovine vascular endothelial cells

1. Intracellular Ca2+ ([Ca2+]i) signals were studied with spatial resolution in bovine vascular endothelial cells using the fluorescent Ca2+ indicator fluo-3 and confocal laser scanning microscopy. Single cells were stimulated with the purinergic receptor agonist ATP resulting in an increase of [Ca2+]i due to intracellular Ca2+ release from inositol 1,4,5-trisphosphate (IP3)-sensitive stores. ATP-induced Ca2+ release was quantal, i.e. submaximal concentrations mobilized only a fraction of the intracellularly stored Ca2+. 2. Focal receptor stimulation in Ca2+-free solution by pressure application of agonist-containing solution through a fine glass micropipette resulted in a spatially restricted increase in [Ca2+]i. Ca2+ release was initiated at the site of stimulation and frequently propagated some tens of micrometres into non-stimulated regions. 3. Local Ca2+ release caused activation of capacitative Ca2+ entry (CCE). CCE was initially colocalized with Ca2+ release. Following repetitive focal stimulation, however, CCE became detectable at remote sites where no Ca2+ release had been observed. In addition, the rate of Ca2+ store depletion with repetitive local activation of release in Ca2+-free solution was markedly slower than that elicited by ATP stimulation of the entire cell. 4. From these experiments it is concluded that both intracellular IP3-dependent Ca2+ release and activation of CCE are controlled locally at the subcellular level. Moreover, redistribution of intracellular Ca2+ stored within the endoplasmic reticulum efficiently counteracts local store depletion and accounts for the spatial spread of CCE activation.