Functional homogeneity of the non-mitochondrial Ca2+ pool in intact mouse lacrimal acinar cells

Calcium signaling in lacrimal glands

Lacrimal glands provide the important function of lubricating and protecting the ocular surface. Failure of proper lacrimal gland function results in a number of debilitating dry eye diseases. Lacrimal glands secrete lipids, mucins, proteins, salts and water and these secretions are at least partially regulated by neurotransmitter-mediated cell signaling. The predominant signaling mechanism for lacrimal secretion involves activation of phospholipase C, generation of the Ca(2+)-mobilizing messenger, IP3, and release of Ca(2+) stored in the endoplasmic reticulum. The loss of Ca(2+) from the endoplasmic reticulum then triggers a process known as store-operated Ca(2+) entry, involving a Ca(2+) sensor in the endoplasmic reticulum, STIM1, which activates plasma membrane store-operated channels comprised of Orai subunits. Recent studies with deletions of the channel subunit, Orai1, confirm the important role of SOCE in both fluid and protein secretion in lacrimal glands, both in vivo and in vitro.

Role of Orai1 and store-operated calcium entry in mouse lacrimal gland signalling and function

Lacrimal glands function to produce an aqueous layer, or tear film, that helps to nourish and protect the ocular surface. Lacrimal glands secrete proteins, electrolytes and water, and loss of gland function can result in tear film disorders such as dry eye syndrome, a widely encountered and debilitating disease in ageing populations. To combat these disorders, understanding the underlying molecular signalling processes that control lacrimal gland function will give insight into corrective therapeutic approaches. Previously, in single lacrimal cells isolated from lacrimal glands, we demonstrated that muscarinic receptor activation stimulates a phospholipase C-coupled signalling cascade involving the inositol trisphosphate-dependent mobilization of intracellular calcium and the subsequent activation of store-operated calcium entry (SOCE). Since intracellular calcium stores are finite and readily exhausted, the SOCE pathway is a critical process for sustaining and maintaining receptor-activated signalling. Recent studies have identified the Orai family proteins as critical components of the SOCE channel activity in a wide variety of cell types. In this study we characterize the role of Orai1 in the function of lacrimal glands using a mouse model in which the gene for the calcium entry channel protein, Orai1, has been deleted. Our data demonstrate that lacrimal acinar cells lacking Orai1 do not exhibit SOCE following activation of the muscarinic receptor. In comparison with wild-type and heterozygous littermates, Orai1 knockout mice showed a significant reduction in the stimulated tear production following injection of pilocarpine, a muscarinic receptor agonist. In addition, calcium-dependent, but not calcium-independent exocytotic secretion of peroxidase was eliminated in glands from knockout mice. These studies indicate a critical role for Orai1-mediated SOCE in lacrimal gland signalling and function.

Role of the microtubule cytoskeleton in the function of the store-operated Ca2+ channel activator STIM1

We examined the role of the microtubule cytoskeleton in the localization and store-operated Ca(2+) entry (SOCE) function of the endoplasmic reticulum (ER) Ca(2+) sensor stromal interaction molecule 1 (STIM1) in HEK 293 cells. STIM1 tagged with an enhanced yellow fluorescent protein (EYFP-STIM1) exhibited a fibrillar localization that colocalized with endogenous alpha-tubulin. Depolymerization of microtubules with nocodazole caused a change from a fibrillar EYFP-STIM1 localization to one that was similar to that of the ER. Treatment of HEK 293 cells with nocodazole had a detrimental impact on SOCE and the associated Ca(2+) release-activated Ca(2+) current (I(CRAC)). This inhibition was significantly reversed in cells overexpressing EYFP-STIM1, implying that the primary inhibitory effect of nocodazole is related to STIM1 function. Surprisingly, nocodazole treatment alone induced significant SOCE and I(CRAC) in cells expressing EYFP-STIM1, and this was accompanied by an increase in EYFP-STIM1 fluorescence near the plasma membrane. We conclude that microtubules play a facilitative role in the SOCE signaling pathway by optimizing the localization of STIM1.

Expression of polycystin-1 enhances endoplasmic reticulum calcium uptake and decreases capacitative calcium entry in ATP-stimulated MDCK cells

Autosomal dominant polycystic kidney disease (ADPKD) types 1 and 2 arise as a consequence of mutations in the PKD1 or PKD2 genes, encoding polycystins-1 and -2. Because loss of function of either of the polycystins leads to a very similar phenotype and the two proteins are known to interact, polycystins-1 and -2 are probably active in the same pathway. The way in which loss of either polycystin leads to the development of ADPKD remains to be established, but disturbances of cell calcium regulation are likely to play an important role. Here, we demonstrate that polycystin-1, heterologously expressed in Madin-Darby canine kidney cells, had a pronounced effect on intracellular calcium homeostasis. ATP-induced calcium responses in transfection control cells exhibited a double peak and relatively gradual return to baseline. By contrast, cells expressing heterologous polycystin-1 showed a brief, uniphasic peak and an accelerated rate of decay. Heterologously expressed polycystin-1 accelerated endoplasmic reticulum (ER) calcium reuptake and inhibited capacitative calcium entry; we found no effect of the protein on mitochondrial calcium buffering or plasma membrane calcium extrusion. We therefore propose that polycystin-1 accelerated the decay of the cell calcium response to ATP by upregulation of ER calcium reuptake and consequent minimization of the stimulus for capacitative calcium entry. It is possible that cellular dedifferentiation, fluid secretion, and proliferation might therefore arise in ADPKD as a consequence of disturbances in cytoplasmic and ER calcium homeostasis and aberrant capacitative calcium entry.

Differences in stimulus induced calcium increases in lacrimal gland acinar cells from normal and NZB/NZW F1 female mice

PURPOSE

To compare the changes in intracellular free calcium in response to both cholinergic and adrenergic agonists in cells isolated from exorbital lacrimal glands of NZB/NZW F1 female mice (NZB/W) and normal Swiss Webster mice (SW).

METHODS

We have loaded cells with Fura-2 and measured total intracellular calcium using ratiomicrofluorometric methods. Isolated cells were also patch-clamped using perforated patch.

RESULTS

In all cells, both carbachol (CCh) and phenylephrine (PE) increased intracellular calcium. The calcium increase to both CCh and PE was less in cells from NZB/W animals than in cells from SW animals. In cells from young animals, the baseline calcium levels were the same, but in cells from older (6 months) NZB/W animals, the baseline levels of calcium were 50% higher than those seen in SW animals.

CONCLUSION

These data suggest that cells from the NZB/W mice have different calcium dynamics, which could contribute to their compromised fluid secretion.

The mechanism of the decrease in cytosolic Ca2+ concentrations induced by angiotensin II in the high K(+)-depolarized rabbit femoral artery

1. Using front-surface fluorometry of fura-2-loaded strips, and measuring the transmembrane 45Ca2+ fluxes of ring preparations of the rabbit femoral artery, the mechanism underlying a sustained decrease in the cytosolic Ca2+ concentration ([Ca2+]i) induced by angiotensin II (AT-II) was investigated. 2. The application of AT-II during steady-state 118 mM K(+)-induced contractions caused a sustained decrease in [Ca2+]i following a rapid and transient increase in [Ca2+]i, while the tension was transiently enhanced. 3. When the intracellular Ca2+ stores were depleted by thapsigargin, the initial rapid and transient increase in [Ca2+]i was abolished, however, neither the sustained decrease in [Ca2+]i nor the enhancement of tension were affected. 4. Depolarization with 118 mM K+ physiological salt solution containing 1.25 mM Ba2+ induced a sustained increase in both the cytosolic Ba2+ concentration ([Ba2+]i) level and tension. However, the application of 10(-6) M AT-II during sustained Ba(2+)-contractions was found to have no effect on [Ba2+]i, but it did enhance tension. 5. After thapsigargin treatment, AT-II neither decreased nor increased the enhanced Ca2+ efflux rate induced by 118 mM K(+)-depolarization, whereas AT-II did increase the enhanced 45Ca2+ influx and the 45Ca2+ net uptake induced by 118 mM K(+)-depolarization. 6. Pretreatment with calphostin-C, partially, but significantly inhibited the decrease in [Ca2+]i induced by AT-II. 7. These findings therefore suggest that AT-II stimulates Ca2+ sequestration into the thapsigargin-insensitive Ca2+ stores, and thus induces a decrease in [Ca2+]i in the high external K(+)-stimulated rabbit femoral artery.

Hypocalcemia modifies the intracellular calcium response to the alpha 1-adrenergic agent phenylephrine in rat hepatocytes

In vivo, extracellular calcium ([Ca2+]e) homeostasis is maintained within a very narrow range by the calcium regulating hormones. At the cellular level, the response to many agents is transduced by changes in cytosolic Ca2+ ([Ca2+]i) which involves both mobilization of cellular pools and entry of [Ca2+]e through plasma membrane channels. To investigate the cellular effects of chronic hypocalcemia (Ca-) on [Ca2+]i homeostasis, hepatocytes, a cell type well characterized for its [Ca2+]i response, were used. Data indicate that Ca- leads to a significant shift to the left in the basal resting cytosolic Ca2+ concentration distribution curve with half-maximum cumulative frequency of 119 versus 149 nM in Ca- and normal rats (N) respectively (P < 0.0001). The response to the alpha 1-adrenergic agonist phenylephrine (Phe) was also influenced by Ca- with a dampening of the dose-response curve, a significant decrease in the frequency of sustained responses (P < 0.001), and significant changes in the oscillation pattern. Indeed, hepatocytes obtained from Ca- exhibited a higher frequency of large amplitude, low frequency oscillations than N most particularly at the 2 and 5 microM Phe dose while N predominantly exhibited low amplitude, high frequency oscillations on sustained plateaus (P < 0.001). IP3 receptor (IP3R) binding studies and Ca2+ mobilization from IP3-sensitive pools showed that IP3R was highly sensitive to the prevailing Ca2+ with, in the range of resting [Ca2+]i, R affinity significantly lower in Ca- than in N. Upon exposure of permeabilized cells to 25 microM IP3, Ca2+ mobilization from the IP3-sensitive intracellular pool was significantly reduced by Ca- (P < 0.05) suggesting a decrease in the IP3-mobilizable Ca2+ pool in Ca-. Our results indicate that hypocalcemia significantly alters [Ca2+]i signalling by perturbing the initial response to agonist and the [Ca2+]i response pattern. In addition, the decrease in Ca2+ mobilization from IP3-sensitive pools suggests that hypocalcemia may also lead to a decrease in the Ca2+ content of intracellular pools.

Intracellular calcium stores are not required for Bcl-2-mediated protection from apoptosis

The ability of Bcl-2 to inhibit cell death is well documented but its mechanism of action remains elusive. Recent reports have suggested that Bcl-2 prevents apoptosis by inhibiting the release of Ca2+ from the thapsigargin-sensitive Ca2+ store. The mobilization of Ca2+ from this store has been implicated as a signal regulating apoptotic cell death induced by glucocorticoids and by interleukin-3 withdrawal. The present study was designed to determine if Bcl-2 would still inhibit apoptosis after depletion of intracellular Ca2+ stores. We compared the response of two Chinese hamster ovary cell lines (5AHSmyc and 5A300bcl-2.2) following incubation with the calcium ionophore ionomycin to deplete intracellular Ca2+ stores. Continued incubation of 5AHSmyc cells in calcium-free media induced substantial apoptotic DNA fragmentation within 4 h and >95% loss of viability within 48 h. However, 5A300bcl-2.2 cells showed no evidence of DNA fragmentation or loss of viability over the same time period. Intracellular Ca2+ was analyzed with the Ca2+-sensitive fluorescent dye INDO-1 and confirmed that ionomycin was capable of releasing Ca2+ from intracellular stores in both cell lines. These results show that depletion of intracellular Ca2+ stores induces apoptosis and that these Ca2+ stores are not required for the protection afforded by Bcl-2.

Regulation of angiotensin II-stimulated Ca2+ oscillations by Ca2+ influx mechanisms in adrenal glomerulosa cells

In adrenal glomerulosa cells, angiotensin II (Ang II) evokes repetitive [Ca2+]i transients and increases Ca2+ influx through voltage-sensitive calcium channels (VSCCs) as well as the capacitative Ca2+ entry pathway. This study analyzed the relationships between these Ca2+ influx pathways and intracellular Ca2+ signaling in bovine glomerulosa cells, in which Ca2+ oscillation frequency was regulated by Ang II concentration over the range of 50-300 p. In the absence of external Ca2+, such oscillations were maintained for prolonged periods of time, but their frequency was significantly reduced (0.23 min-1 versus 0.38 min-1). Restoration of [Ca2+]o to 0.6 mM increased the frequency of Ca2+ oscillations in cells that showed narrow spikes of constant amplitude and caused a plateau response in cells with broad spikes of rapidly decreasing amplitude. In the presence of Ca2+, nifedipine reduced the frequency of the oscillatory Ca2+ response to 100 pM Ang II by 49%, and BAY K 8644 increased oscillation frequency by 86%, or caused plateau-type responses typical of higher Ang II concentrations. In contrast to their prominent actions on Ca2+ spiking frequency, dihydropyridines caused only minor changes in Ang II (100 pM)-induced inositol phosphate production. Dihydropyridines also had minimal effects on the nonoscillatory Ca2+ signals evoked by high Ang II concentrations (10 nM). These findings indicate that Ca2+ influx through VSCCs modulates the frequency of Ca2+ oscillations induced by low agonist concentrations by a mechanism that does not involve major changes in inositol trisphosphate formation. However, VSCCs make relatively little contribution to the nonoscillatory Ca2+ signals generated by high agonist concentrations, when Ca2+ influx occurs predominantly through the capacitative Ca2+ entry pathway.

Functional co-localization of transfected Ca(2+)-stimulable adenylyl cyclases with capacitative Ca2+ entry sites

Three adenylyl cyclases (ACI, ACIII, and ACVIII) have been described, which are putatively Ca(2+)-stimulable, based on in vitro assays. However, it is not clear that these enzymes can be regulated by physiological rises in [Ca2+]i when expressed in intact cells. Furthermore, it is not known whether transfected adenylyl cyclases might display the strict requirement for capacitative Ca2+ entry that is shown by the Ca(2+)-inhibitable ACVI, which is indigenous to C6-2B glioma cells (Chiono, M., Mahey, R., Tate, G., and Cooper, D. M. F. (1995) J. Biol. Chem. 270, 1149-1155). In the present study, ACI, ACIII, and ACVIII were heterologously expressed in HEK 293 cells, and conditions were devised that distinguished capacitative Ca2+ entry from both internal release and nonspecific elevation in [Ca2+]i around the plasma membrane. Remarkably, not only were ACI and ACVIII largely insensitive to Ca2+ release from stores, but they were robustly stimulated only by capacitative Ca2+ entry and not al all by a substantial increase in [Ca2+]i at the plasma membrane elicited by ionophore. (ACIII, reflecting its feeble in vitro sensitivity to Ca2+, was unaffected by any [Ca2+]i rise.) These results suggest a quite unsuspected, essential association of Ca(2+)-sensitive adenylyl cyclases with capacitative Ca2+ entry sites, even when expressed heterologously.

Evidence that quantal Ca2+ release in HSG cells is not due to 'all-or-none' release from discrete Ca2+ stores with differing sensitivities to IP3

We demonstrate that the application of the muscarinic agonist carbachol to the human salivary epithelial cell line HSG elicits the now well-known phenomenon of 'quantal' Ca2+ release; namely, that the application of a submaximal concentration of agonist results in the release of only a portion of the agonist-sensitive intracellular Ca2+ pool. One explanation that has been proposed to account for this effect is that there are multiple intracellular Ca2+ stores, each with a different agonist sensitivity, which release Ca2+ in an 'all-or-none' fashion. We test this hypothesis in intact HSG cells with an experimental protocol designed to preferentially load less-agonist-sensitive stores with 40Ca2+ and more-agonist-sensitive stores with 45Ca2+. However, contrary to the expectations of the above explanation, these cells do not preferentially release 45Ca2+ in response to low concentrations of agonist. Thus our data suggest that quantal Ca2+ release must arise from some other property of the stores or their Ca2+ release channels.

Effect of inositol 1,3,4,5-tetrakisphosphate on inositol trisphosphate-activated Ca2+ signaling in mouse lacrimal acinar cells

In mouse lacrimal acinar cells, microinjection of the metabolically stable analog of inositol 1,4,5-trisphosphate, inositol 2,4,5-trisphosphate ((2,4,5)IP3), stimulated both intracellular Ca2+ mobilization and Ca2+ entry. Microinjection of inositol 1,3,4,5-tetrakisphosphate ((1,3,4,5)IP4), the inositol 1,4,5-trisphosphate-3-kinase product, was ineffective at mobilizing intracellular Ca2+ or activating Ca2+ entry. In lacrimal cells previously microinjected with submaximal levels of (2,4,5)IP3, the subsequent microinjection of low to moderate concentrations of (1,3,4,5)IP4 did not result in additional release of intracellular Ca2+, nor did it potentiate the Ca2+ entry phase attributable to (2,4,5)IP3. However, as previously demonstrated (Bird, G. S. J., Rossier, M. F., Hughes, A. R., Shears, S. B., Armstrong, D. L., and Putney, J. W., Jr. (1991) Nature 352, 162-165), additional injections of (2,4,5)IP3 induced further mobilization of intracellular Ca2+ and increased the elevated and sustained Ca2+ entry phase. Introduction of high concentrations of (1,3,4,5)IP4 appeared to inhibit or block the (2,4,5)IP3-induced Ca2+ entry phase. These results were consistent with the observed effect of (1,3,4,5)IP4 in permeabilized lacrimal cells, where (1,3,4,5)IP4 did not release cellular 45Ca2+ but at high concentrations inhibited the ability of submaximal concentrations of (2,4,5)IP3 to release 45Ca2+. Likewise, injection of a high concentration of (1,3,4,5)IP4 prior to injection of (2,4,5)IP3 blocked both release and influx of Ca2+. The inhibitory action of (1,3,4,5)IP4 on Ca2+ signaling observed in intact cells occurred at concentrations that might be obtained in agonist-stimulated cells. However, in permeabilized cells, (1,3,4,5)IP4 inhibited Ca2+ mobilization at concentrations exceeding those likely to occur in agonist-stimulated cells. These results suggest that physiologically relevant levels of (1,3,4,5)IP4 in the cell cytoplasm do not release Ca2+, nor do they potentiate inositol trisphosphate-induced Ca2+ entry across the plasma membrane. Rather, the possibility is raised that (1,3,4,5)IP4 or one of its metabolites could function as a negative feedback on Ca2+ mobilization by inhibiting inositol 1,4,5-trisphosphate-induced Ca2+ release.

Synchronization of calcium waves by mitochondrial substrates in Xenopus laevis oocytes

In Xenopus oocytes, as well as other cells, inositol-1,4,5-trisphosphate (Ins(1,4,5)P3)-induced Ca2+ release is an excitable process that generates propagating Ca2+ waves that annihilate upon collision. The fundamental property responsible for excitability appears to be the Ca2+ dependency of the Ins(1,4,5)P3 receptor. Here we report that Ins(1,4,5)P3-induced Ca2+ wave activity is strengthened by oxidizable substrates that energize mitochondria, increasing Ca2+ wave amplitude, velocity and interwave period. The effects of pyruvate/malate are blocked by ruthenium red at the Ca2+ uniporter, by rotenone at complex I, and by antimycin A at complex III, and are subsequently rescued at complex IV by ascorbate tetramethylphenylenediamine (TMPD). Our data reveal that potential-driven mitochondrial Ca2+ uptake is a major factor in the regulation of Ins(1,4,5)P3-induced Ca2+ release and clearly demonstrate a physiological role of mitochondria in intracellular Ca2+ signalling.

Slow kinetics of InsP3-induced Ca2+ release: differences between uni- and bi-directional 45Ca2+ fluxes

The effects of a long-lasting stimulation with inositol 1,4,5-trisphosphate (InsP3) have been studied in monolayers of permeabilized A7r5 cells. When measured under unidirectional 45Ca2+ efflux conditions, i.e. in the presence of 2 microM thapsigargin, an initial fast release was observed which then progressively slowed down into a slow phase which persisted for up to 20 min. When measured under bidirectional 45Ca2+ flux conditions with functional Ca2+ pumps, a transient phase of re-uptake occurred between the initial fast and the subsequent slow release phase. These kinetics are compatible with intrinsic inactivation of the InsP3 receptor. However, this inactivation did not prevent the slow release component. The slow component was not due to the accumulation of an InsP3 metabolite nor to a GTP-dependent translocation of Ca2+ between stores. The slow release phase was more pronounced when the Ca2+ pumps were active than when they were inhibited. This observation is compatible with other findings indicating that the InsP3 receptor is controlled by luminal Ca2+. The decreasing effectiveness of a 20 min lasting InsP3 challenge in mobilizing Ca2+ from less filled stores is most likely due to a progressive depletion of the store and cannot be considered as an experimental artifact caused by a preferential emptying of InsP3-sensitive Ca2+ stores. We conclude that the InsP3 receptor can intrinsically inactivate but that this inactivation is unable to prevent the slow release, which is especially pronounced when Ca2+ pumps are active.

Inhibitors of ER Ca(2+)-ATPase activity deplete the ATP- and thrombin-sensitive Ca2+ pool in UMR 106-01 osteosarcoma cells

While calcium release from intracellular stores is a signaling mechanism used universally by cells responding to hormones and growth factors, the compartmentalization and regulated release of calcium is cell type-specific. We employed thapsigargin and 2,5,-di-(tert-butyl)-1,4-benzohydroquinone (tBuHQ), two inhibitors of endoplasmic reticulum (ER) Ca(2+)-ATPase activity which block the transport of Ca2+ into intracellular stores, to characterize free Ca2+ compartmentalization in UMR 106-01 osteoblastic osteosarcoma cells. Each drug elicited transient increases in cytosolic free Ca2+ ([Ca2+]i), followed by a stable plateau phase which was elevated above the control [Ca2+]i. The release of Ca2+ from intracellular stores was coupled to an increased plasma membrane Ca2+ permeability which was not due to L-type Ca2+ channels. Thapsigargin and tBuHQ emptied the intracellular calcium pool which was released in response to either ATP or thrombin, identifying it as the inositol 1,4,5-trisphosphate-sensitive calcium store. The results of sequential and simultaneous additions of thapsigargin and tBuHQ indicate that both drugs depleted the same Ca2+ store and inhibited the same Ca(2+)-ATPase activity.

Cyclic ADP-ribose and inositol 1,4,5-triphosphate mobilizes Ca2+ from distinct intracellular pools in permeabilized lacrimal acinar cells

In permeabilized lacrimal acinar cells, cyclic ADP-ribose (cADP-ribose) and inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) release Ca2+ in a dose dependent manner from distinct thapsigargin-sensitive Ca2+ pools. Ryanodine specifically blocks the Ca2+ response to cADP-ribose, whereas heparin strongly reduces the response to Ins(1,4,5)P3 application. GTP causes a rapid Ca2+ release by a ryanodine- and heparin-insensitive mechanism and potentiates Ins(1,4,5)P3 but not cADP-ribose evoked Ca2+ release. It is estimated that cADP-ribose can release 16 mumol Ca2+/l cells, whereas Ins(1,4,5)P3 can mobilize 55 mumol Ca2+/l cells. The results suggest that cADP-ribose and Ins(1,4,5)P3 release Ca2+ from distinct internal stores and that a third Ca2+ pool exists which can selectively interact with the Ins(1,4,5)P3-sensitive Ca2+ store by a GTP-mediated process.

Intracellular pH modulates the activity of chloride channels in isolated lacrimal gland acinar cells

The effects of intracellular pH (pHi) on Ca(2+)-activated Cl- currents in rat lacrimal gland acinar cells were examined. Cl- currents were recorded by conventional whole cell patch-clamp methods using K(+)-free and Na(+)-free solutions. pHi was varied by using electrode solutions with pH at 6.8, 7.3, or 7.8, and Ca2+ activity was buffered at 100 nM with ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid. Increasing pH from 6.8 to 7.8 was found to increase whole cell currents. The currents observed exhibited time-dependent activation at depolarizing potentials and time-dependent inactivation at hyperpolarizing potentials (pH 7.8). This behavior is characteristic of Ca(2+)-activated Cl- channels in lacrimal gland cells. The selectivity of the current was examined at pH 7.8 by removing Cl- from the bath solution. This maneuver caused a positive shift in the reversal potential, as expected for a Cl(-)-selective current. Thus increasing pHi appears to activate Ca(2+)-activated Cl- channels. The possibility that an increase in pHi may help sustain Cl- channel activity during secretory activity is discussed.

Multiple intracellular Ca2+ pools exist in human foreskin fibroblast cells: the effect of BK on release and filling of the non-cytosolic Ca2+ pools

Previously, we have used the classical approach to examine intracellular calcium stores in human foreskin fibroblasts (HSWP) cells. In this classical protocol cells are first permeabilized and then allowed to fill their Ca2+ reservoirs with 45Ca2+ in the presence of ATP. In this paper we present an alternative method to examine intracellular calcium pools. In this alternate protocol, whole cells are loaded to isotopic steady-state with 45Ca2+ and then permeabilized using digitonin. Comparison of the Ca2+ content of cells treated with these two methodologies reveals that cells treated with the alternate protocol have a Ca2+ content 3 orders of magnitude higher than those treated with the classical protocol. Using this alternative technique we demonstrate that there are 3 intracellular calcium pools in HSWP cells. These pools are: (i) an IP3-sensitive, thapsigargin-sensitive Ca2+ pool; (ii) an IP3-insensitive, thapsigargin-sensitive Ca2+ pool; and (iii) an ionomycin sensitive Ca2+ pool. The relationship between the Ca2+ pool mobilized by BK treatment and by IP3 treatment is also explored. Microinjection data shown here suggest that IP3 can mobilize all of the intracellular Ca2+ mobilized by BK. However, in the permeabilized system BK pretreatment followed by IP3 treatment can release more Ca2+ than can be release by IP3 treatment alone. We suggest one plausible explanation for this observation: when cells are treated using the alternative permeabilization protocol, communication occurs between an IP3-sensitive and an IP3-insensitive calcium pool. Thus BK pretreatment would empty the IP3-sensitive Ca2+ pool. This pool would subsequently be refilled with Ca2+ from a previously untapped, IP3-insensitive, Ca2+ reservoir and more Ca2+ would be available for subsequent release by IP3 treatment.

Intracellular free Ca2+ in dissociated cells of the chick pineal gland: regulation by membrane depolarization, second messengers and neuromodulators, and evidence for release of intracellular Ca2+ stores

The regulation of intracellular free Ca2+ concentration was examined in single dissociated chick pineal cells using the fura-2 technique. Approximately 10% of cells examined exhibited spontaneous Ca2+ oscillations while the rest were quiescent. Application of salines containing 80 mM KCl evoked large increases in intracellular free Ca2+ that were dependent upon external Ca2+ ions. These responses were inhibited by 10 microM nifedipine indicating involvement of L-type Ca2+ channels. Application of the tumor promoter thapsigargin (2 microM) evoked increases in intracellular free Ca2+. These responses could be observed in the absence of external Ca2+ indicating mobilization of internal stores. In the absence of external Ca2+, the responses to thapsigargin gradually decayed due to depletion of internal Ca2+ pools. A subsequent exposure to saline containing 5.8 mM CaCl2 caused a rapid increase in intracellular Ca2+ that was consistently larger than the peak response to thapsigargin. Application of 100 nM vasoactive intestinal peptide (VIP), a neurohormone that stimulates melatonin secretion from pineal cells, induced a sustained increase in intracellular free Ca2+ in a subpopulation of cells. In a small number of cells, VIP evoked Ca2+ oscillations. Approximately half of the cells examined showed no response to VIP. Application of 200 microM norepinephrine, which inhibits melatonin secretion from the chick pineal, had no effect on intracellular free Ca2+ in any quiescent or spontaneously oscillating cells. Application of 5 mM 8-Br-cAMP evoked sustained increases in intracellular Ca2+. Similar effects were obtained with the phosphodiesterase inhibitors papaverine (50 microM) or isobutylmethylxanthine (100 microM). Application of 200 nM forskolin, an activator of adenylate cyclase, evoked increases in intracellular free Ca2+ that could be detected in the presence of 10 microM nifedipine. The responses to forskolin gradually decayed in Ca(2+)-free external salines due to depletion of intracellular Ca2+ stores. Subsequent exposure to external Ca2+ caused a rapidly developing increase in intracellular Ca2+ that was larger than the peak response to forskolin. These results indicate that the regulation of intracellular free Ca2+ in chick pineal cells is complex. These cells exhibit Ca2+ oscillations and can mobilize both external and internal Ca2+ pools. Agents that increase intracellular cAMP cause mobilization of internal Ca2+ stores, possibly secondary to effects on other second messenger systems. Chick pineal cells, like many other cell types, possess mechanisms to allow for refilling of depleted internal Ca2+ stores. These results suggest new mechanisms for the regulation of melatonin synthesis and secretion and possible sites of action for the intrinsic circadian oscillator.

Intracellular Ca2+ stores of T lymphocytes: changes induced by in vitro and in vivo activation

Intracellular Ca2+ stores were investigated in resting and activated splenic T lymphocytes from Lewis rats. Activation was obtained either in vitro (spleen cells isolated from "naive" rats exposed to concanavalin A for 24 h) or in vivo (spleen cells from rats with fully developed symptoms of experimental allergic encephalomyelitis). In both experimental conditions several changes of Ca2+ homeostasis were observed with respect to resting lymphocytes: (1) a threefold increase of the total intracellular calcium (from 1.15 to 3.5 mmol/l); (2) a moderate increase of the pool sensitive to inositol 1,4,5-trisphosphate (IP3), investigated both in intact T lymphocytes (fura-2 and 45Ca(2+)-release techniques in cells challenged with phytohemagglutinin) and in T lymphocytes permeabilized with beta-escin (45Ca2+ release induced by saturating concentrations of IP3); and (3) the appearance of a pool released by the endoplasmic reticulum (ER) Ca2+ ATPase inhibitor thapsigargin (Tg), but insensitive to IP3, which, therefore, appears to be localized in areas of the ER devoid of the cognate receptor. The latter two findings were paralleled in activated lymphocytes by an increase of expression of ER markers, involved (calreticulin; Ca2+ ATPase) or not (protein disulfide isomerase) in the regulation of Ca2+ homeostasis. In contrast, calnexin (another ER marker) and the receptor for IP3 were increased to only a moderate extent. Finally, an enlargement of non-ER Ca2+ pools was observed in the cells pretreated with Tg in which 45Ca2+ release was induced by the Ca2+ ionophore ionomycin. Our results document structural and functional changes of intracellular Ca2+ stores which might play an important regulatory role in activated T lymphocytes.

The inositol phosphate-calcium signalling system in lacrimal gland cells

From the above discussion, it is clear that the regulation of Ca2+ signalling in exocrine cells is a complex process involving activation of both intracellular Ca2+ release as well as the entry of Ca2+ across the plasma membrane. A poorly understood mechanism links these two phases of Ca2+ signalling thereby providing both rapid as well as sustained signals for the initiation and maintenance of appropriate exocrine responses. Further work is needed to better understand the mechanisms controlling this important and ubiquitous signalling system.