Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “Ionomycin”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 19 recordsLinked to original sources
PubMed1992

Mechanism of release of Ca2+ from intracellular stores in response to ionomycin in oocytes of the frog Xenopus laevis.

1. The mechanism of Ca2+ release from intracellular stores was studied in defolliculated Xenopus laevis oocytes by measuring whole-cell currents using the two-electrode voltage-clamp method. 2. The extracellular application of ionomycin, a selective Ca2+ ionophore, evoked an inward current consisting of a spike-like fast component followed by a long-lasting slow component with few superimposed current oscillations (fluctuations). The ionomycin response occurred in a dose-dependent manner and was dependent on Cl-. 3. No apparent refractory period was observed for repetitively evoked small ionomycin responses when the concentration of ionomycin was low (0.1 microM). In contrast, a larger ionomycin response (1 microM), consisting of fast and slow components, was followed by refractory period. Washing for 50-90 min was necessary for full recovery of the ionomycin response. 4. The response to ionomycin was suppressed by the extracellular application of acetoxymethyl ester of bis-(O-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid (BAPTA AM, 1-10 microM), a membrane-permeable intracellular Ca2+ chelator. 5. The ionomycin response was not affected by pertussis toxin (PTX, 0.3-2.0 microgram/ml), a blocker of guanine nucleotide-binding regulatory proteins (G proteins). In contrast, the response to acetylcholine (ACh), which is known to occur via a G protein, was suppressed by PTX. 6. The fast component was not affected by removing Ca2+ from the bathing medium or by replacing extracellular Ca2+ with Ba2+ or Mn2+ (all of these solutions were supplemented with 2 mM EGTA), whereas the slow component was suppressed. 7. Injection of inositol 1,4,5-trisphosphate (IP3) following a response to extra-cellularly applied ionomycin did not evoke an appreciable membrane current. In contrast, ionomycin evoked a small inward current when it was applied after an inward-current response evoked by IP3 injection, whereas a second injection of IP3 did not evoke any appreciable current. 8. The results indicate that (a) ionomycin releases Ca2+ from its intracellular stores without the involvement of G proteins, resulting in activation of Ca(2+)-activated Cl- channels, (b) ionomycin mainly acts on the same intracellular Ca2+ stores as IP3, and (c) entry of Ca2+ from outside the cell considerably contributes to the slow component of the ionomycin response, whereas its fast component is predominantly dependent on the release of Ca2+ from the intracellular stores.

Acetylcholine↗
PubMed1999

Calcium ionophore, ionomycin inhibits growth of human bladder cancer cells both in vitro and in vivo with alteration of Bcl-2 and Bax expression levels.

PURPOSE: The objective of this study was to characterize the antiproliferative effects of the calcium ionophore, ionomycin on the human bladder cancer cell line HT1376 both in vitro and in vivo. MATERIALS AND METHODS: The in vitro growth rate of HT1376 after treatment with ionomycin was measured by using the MTT assay. The apoptotic features in HT1376 after ionomycin treatment were evaluated by flow cytometric analysis and DNA degradation assay. Bcl-2 and Bax expression levels in HT1376 after ionomycin treatment were examined by Northern and Western blot analyses. The effects of intratumoral injection of ionomycin on the growth of subcutaneous HT1376 tumors established in athymic nude mice were then tested. The efficacy of combined treatment with ionomycin and cisplatin against HT1376 growth was also examined. RESULTS: The in vitro growth rate of HT1376 was suppressed by ionomycin in a dose- and time-dependent manner, and characteristic apoptotic DNA degradation was observed in HT1376. Ionomycin treatment caused a marked decrease in the ratios of Bcl-2 to Bax mRNA and protein in HT1376 cells. Intratumoral injection of ionomycin into subcutaneous HT1376 tumors reduced the tumorigenicity in nude mice. Furthermore, these in vivo growth-inhibitory effects of ionomycin were significantly enhanced by pretreatment with cisplatin. CONCLUSIONS: These findings suggest that ionomycin-based therapy could be used as a novel therapeutic strategy for advanced bladder cancer through the effective induction of apoptosis by decreasing the ratio of Bcl-2 to Bax.

Animals↗
PubMed1989

Correlation of intracellular and extracellular calcium ion concentrations with synergy between 1,2-dioctanoyl-sn-glycerol and ionomycin in platelet arachidonic acid mobilization.

The potentiation by 1,2-dioctanoyl-sn-glycerol (DiC8) of ionomycin-induced platelet production of 12-hydroxy-5,8,10-heptadecatrienoic acid (HHT) and 12-hydroxy-5,8,10,14-eicosatetraenoic acid (12-HETE) was investigated in correlation with extracellular Ca2+ concentrations and increases in [Ca2+]i, as detected with aequorin and fura-2. Extracellular Ca2+ concentrations greatly influenced the production of arachidonic acid metabolites induced by DiC8 and ionomycin, while that induced by ionomycin alone was minimally affected by variation of the extracellular Ca2+ concentration. In the synergy between ionomycin and 20 microM DiC8, the optimal concentrations of ionomycin shifted from high to low with increasing concentrations of extracellular Ca2+, suggesting that there might be a range of optimal [Ca2+]i for the production of the arachidonic acid metabolites. This hypothesis was confirmed by simultaneous measurements of [Ca2+]i increases, and the production of the arachidonic acid metabolites. With the aequorin method, the optimal concentrations of [Ca2+]i fell to between 10 microM and 20 microM, and with the fura-2 method, it fell to between 800 nM and 1800 nM. Direct measurements of [14C]arachidonic acid release suggested that the DiC8-potentiated production of arachidonic acid metabolites induced by ionomycin was attributable to increased arachidonic acid release. Since ionomycin and DiC8 induced relatively low levels of phosphatidic acid production, an indicator of phospholipase C activation, it was suggested that the increased arachidonic acid release was largely dependent upon phospholipase A2. Synergy between DiC8 and ionomycin was also observed with aggregation and serotonin release. Aggregation was induced by lower concentrations of ionomycin, and appeared to be more dependent upon extracellular Ca2+, while serotonin release required higher concentrations of ionomycin, and variations in extracellular Ca2+ affected the response minimally. These findings suggest that the mechanisms underlying the synergy between protein kinase C activation and Ca2+ mobilization differ among the three functions evaluated in this study.

12-Hydroxy-5,8,10,14-eicosatetraenoic Acid↗
PubMed1993

Mechanism of ionomycin-induced intracellular alkalinization of rat hepatocytes.

Calcium ionophores such as ionomycin and A23187 are often used to determine the role of intracellular Ca++ in cellular processes. Ionomycin but not Ca+(+)-mobilizing agonists increases basal intracellular pH in hepatocytes. To explain this difference in effects of agents that increase intracellular Ca++ concentration, the mechanism of ionomycin-induced increases in basal intracellular pH in isolated rat hepatocytes was studied. Changes in intracellular pH and intracellular Ca++ concentration were measured with the fluorescent probes BCECF (2',7'-bis-2-[carboxyethyl ester]-5[6]carboxyfluorescein) and quin-2, respectively. Ionomycin produced dose-dependent increases in intracellular pH and intracellular Ca++ concentration, with the increase in intracellular Ca++ concentration preceded by the increase in intracellular pH. Ionomycin-induced increases in intracellular pH were not affected by 1 mmol/L amiloride, 100 mumol/L diisothiocyanostilbene disulfonate or removal of extracellular Na+, indicating that the effect is not mediated by Na+/H+ exchange, Cl-/HCO3- exchange or Na+/HCO3- cotransport. Ionomycin failed to increase intracellular pH or intracellular Ca++ concentration in the absence of extracellular Ca++, and both intracellular pH and intracellular Ca++ concentration increased promptly when extracellular Ca++ was reintroduced. Ionomycin-induced increases in intracellular Ca++ concentration but not intracellular pH were smaller in hepatocytes loaded with the Ca++ buffering agent MAPTA. Thapsigargin increased intracellular Ca++ concentration but failed to increase intracellular pH. Thus the effect of ionomycin is independent of the effect of ionomycin on intracellular Ca++ concentration and dependent on extracellular intracellular Ca++ concentration. Experimental conditions that produce cell depolarization did not increase basal intracellular pH but lowered ionomycin-induced increases in intracellular pH by 25% without affecting increases in intracellular Ca++ concentration.(ABSTRACT TRUNCATED AT 250 WORDS)

Alkalies↗
PubMed1995

Surface binding and uptake of nickel(II) in human epithelial kidney cells: modulation by ionomycin, nicardipine and metals.

The carcinogenic potency of nickel compounds depends on the ability of nickel ions to enter target cells. The presumptive preventive potential of several metals against nickel-induced cancer may depend on their capacity to inhibit nickel uptake. Surface binding and uptake of 63Ni2+ in immortalized human kidney epithelial (IHKE) cells suspended in a salts/glucose minimal medium was studied at 0.085 mM nickel. Nickel uptake, after a rapid phase of about 2 h, continued at a slower rate for several hours. Nicardipine (50 microM) decreased uptake to about 25% of control values. Ionomycin (3 microM) increased uptake 4- to 5-fold. Nickel uptake into IHKE cells was decreased by metal cations. In the absence of ionomycin, this effect followed the order Zn > Cd > Co > Mn > Mg >> Ca. In the presence of ionomycin the order was Zn > Cd > Co > Mn >> Ca > Mg. These metals inhibited uptake more strongly than binding to the cell surface. However, Co2+ reduced surface binding strongly in ionomycin-potentiated uptake. With no ionomycin present, 0.15 mM Zn2+, 1 mM Mn2+, 4 mM Mg2+ or 70 mM Ca2+ caused 80-85% inhibition of nickel uptake. In the absence of ionomycin, calcium was a weaker inhibitor of nickel uptake than magnesium. In the presence of ionomycin, calcium was a stronger inhibitor than magnesium. The results indicate that nickel can be taken up through calcium channels in IHKE cells. It is suggested that ionomycin-potentiated transport mechanisms, under the conditions applied, are different from normal mechanisms of transport functioning in the absence of ionomycin. Different metals, essential and non-essential, seem to be inhibitors or competitors for the transport mechanisms.

Biological Transport↗
PubMed1986

Ionomycin acts as an ionophore to release TRH-regulated Ca2+ stores from GH4C1 cells.

In the GH4C1 strain of rat pituitary cells, ionomycin, a divalent cation ionophore, induces a rapid and transient spike in cytosolic free Ca2+ concentrations [( Ca2+]i) similar to that induced by the Ca2+-mobilizing hormone thyrotropin-releasing hormone (TRH). To test directly the hypothesis that ionomycin causes the spike in [Ca2+]i by altering cellular Ca2+ stores, we have measured ionomycin-induced changes in 45Ca2+ fluxes and have compared these to previously characterized changes induced by TRH. Ionomycin (half-maximal concentration = 30 nM) rapidly (within 1 min) induced a release into the medium of 50-60% of cell-associated 45Ca2+, paralleling the spike in [Ca2+]i. The ionomycin-induced 45Ca2+ efflux was greater than with TRH, and TRH did not induce further 45Ca2+ efflux in the presence of ionomycin. Ionomycin pretreatment blocked induction of the spike in [Ca2+]i elicited by TRH but did not alter basal or TRH-induced enhancement of inositol phosphate levels. These results provide evidence that the spike in [Ca2+]i induced by ionomycin or TRH is produced largely by release of Ca2+ into the cytosol from the same intracellular pool, followed by rapid extrusion of the released Ca2+ into the extracellular space. However, unlike TRH, ionomycin appears to release cellular Ca2+ directly, acting as an ionophore, without the generation of known second messengers.

Animals↗
PubMed1982

Actions of ionomycin in rat parotid gland.

The effects of ionomycin (SQ 23,377), a carboxylic acid Ca-ionophore, on the rat parotid acinar cell were investigated. Ionomycin stimulated 86Rb efflux from parotid slices and was substantially more potent and efficacious than the Ca-ionophore, A-23187. The release of 86Rb was dependent on the concentration of ionomycin and of Ca. Ionomycin also stimulated 22Na uptake and 3H-protein secretion, but did not stimulate the incorporation of 32PO4 into phosphatidylinositol. These observations are consistent with an action of ionomycin in increasing cytosolic Ca by acting as an ionophore and not involving endogenous receptors. Pretreatment with ionomycin inhibited the transient, Ca-independent responses to carbachol or physalaemin. When ionomycin was added to parotid cells pre-equilibrated with 45Ca, a net loss of radiocalcium was observed. These observations suggest that ionomycin can release the receptor-regulated cellular Ca pool. Morphological studies did not reveal any nonspecific deleterious effects in the cells after incubation with 2.67 microM ionomycin.

Animals↗
PubMed1991

Differential inhibition of interleukin 2- and interleukin 4-mediated human B cell proliferation by ionomycin: a possible regulatory role for apoptosis.

Surface immunoglobulin (Ig) cross-linking by anti-IgM (mu) antibodies leads to B cell activation resulting in numerous early biochemical events including an increase in intracellular [Ca2+]. Furthermore, anti-mu-activated B cells become able to proliferate in response to interleukin (IL)2 and IL4. These studies examined the effect of the calcium ionophore ionomycin, an enhancer of cytoplasmic [Ca2+] levels, on IL2 and IL4-mediated proliferation of anti-mu-stimulated normal human B cells. Ionomycin inhibited the proliferative response of anti-mu-activated B cells to IL4. In contrast, IL2 and phorbol 12,13 dibutyrate (PBu2)-mediated B cell proliferation was refractory to the growth inhibitory effects of ionomycin. In an attempt to delineate a possible mechanism(s) for this differential growth effect of ionomycin, we first studied direct effects of ionomycin on activated B cells. Our data suggested that ionomycin induced DNA fragmentation in anti-mu-costimulated B cells. Interestingly, in contrast to PBu2, IL4 did not prevent ionomycin-dependent DNA fragmentation. Importantly, H7, an inhibitor of protein kinase C activation, down-regulated only the IL2 and PBu2-driven B cell proliferation but not B cell proliferative response to IL4. These results suggest that putative protein kinase C activation, either by direct treatment with phorbol ester or during IL2 signaling, counteracts the inhibitory effects of ionomycin. In contrast, IL4 signaling does not exhibit the same protective properties.

1-(5-Isoquinolinesulfonyl)-2-Methylpiperazine↗
PubMed1996

Excitation of Drosophila photoreceptors by BAPTA and ionomycin: evidence for capacitative Ca2+ entry?

It has been suggested that excitation in Drosophila photoreceptors may be mediated by the depletion of intracellular Ca2+ stores (capacitative Ca2+ entry). To investigate this hypothesis, simultaneous whole-cell recordings and Indo-1 Ca2+ measurements were made from dissociated Drosophila photoreceptors, whilst testing the effects of Ca2+ releasing agents. In Ca2+ free Ringer's solution, thapsigargin raised cytosolic Ca2+ by approximately 80 nM; subsequent application of ionomycin released further Ca2+ (approximately 100 nM). A possible third compartment was indicated by the ability of monensin to mobilize further Ca2+ after saturating doses of ionomycin. Under most conditions, none of these agents activated an inward conductance, and their effects on the light response were consistent with their effects on cytosolic Ca2+. However, in the absence of both external Ca2+ and Mg2+ (to relieve a Mg2+ block of the light-sensitive channels), and after loading cells with BAPTA buffering cytosolic free Ca2+ at approximately 10 nM, ionomycin (but not thapsigargin) activated inward currents of approximately 800 pA. The response to ionomycin was enhanced (10 nA) by buffering cytosolic Ca2+ at 250 nM. A similar current also developed after approximately 3 min in cells loaded with Ca-BAPTA without any ionomycin application. The current-voltage relationships of currents activated by Ca-BAPTA or ionomycin were indistinguishable from that of the light-activated conductance and were similarly affected by a null mutation of the transient receptor potential (trp) gene which is believed to encode a subunit of the light-sensitive channels. These experiments provide some evidence for the suggestion that the light-activated and trp-dependent conductance in Drosophila photoreceptors can be activated by depletion of internal stores. However, activation by Ca-BAPTA and ionomycin had an absolute requirement for cytosolic Ca2+ as no currents could be activated by ionomycin in cells loaded with BAPTA and no Ca2+.

Animals↗
PubMed1994

Ionomycin enhances Ca2+ influx by stimulating store-regulated cation entry and not by a direct action at the plasma membrane.

In fura-2-loaded ECV304 cells ionomycin elicited a saturable biphasic change in intracellular Ca2+ concentration ([Ca2+]i), where the initial phase represented mobilization of intracellular stores and the sustained component represented Ca2+ influx. To examine whether ionomycin could stimulate influx via a store-dependent mechanism. Mn2+ entry was monitored by the quenching of fura-2 fluorescence: influx was enhanced even after ionomycin wash-out, provided that internal stores were not refilled with Ca2+. Moreover, the maximal rate of histamine-stimulated Mn2+ entry was unaffected by ionomycin, suggesting a common route of entry. The Ca(2+)-entry blocker SK&F 96365 inhibited both the ionomycin-induced Mn2+ entry and the sustained [Ca2+]i response to the ionophore (leaving the initial peak [Ca2+]i response unaffected). In other experiments, although addition of ionomycin further increased the plateau phase induced by 100 microM histamine, the increase was completely abolished by pretreatment with the store Ca(2+)-ATPase inhibitor cyclopiazonic acid (CPA). Furthermore, in store-depleted cells, re-addition of 1 mM extracellular Ca2+ (in the presence of CPA plus histamine) led to a rapid rise in [Ca2+]i, dependent on Ca2+ influx, with kinetics that were not enhanced by ionomycin. These data suggest that ionomycin acts primarily at the level of the internal Ca2+ stores, so that, at the concentrations used here (< or = 1 microM), it increases Ca2+ (and Mn2+) influx via activation of endogenous entry pathways and not by plasmalemmal translocation.

Biological Transport↗
PubMed1991

Proliferation and activation of human mononuclear cells induced by ionomycin in a serum-free medium.

Ionomycin was capable of inducing the proliferation and activation of human peripheral blood mononuclear cells (MNC) in serum-free culture medium. Optimal responses, evaluated by tritiated thymidine ([3H]Thy) incorporation and by HLA-DR antigen expression, were obtained with 0.5 micrograms/ml ionomycin in 72-hour cultures. The two ionomycin-induced effects required the presence of monocytes from the start of the culture. It was observed that no cell adhered to the support after three days of culture in the presence of ionomycin, when compared with an ionomycin-free control. The cells stimulated with ionomycin had a large cytoplasm, were metabolically highly active and had a morphology comparable to that of PHA-stimulated cells. At the end of culture, 23 +/- 4% of cells were DR+; this percentage was only partially found (15 +/- 3%) in all the cell populations studied (T and B lymphocytes, NK cells, monocytes). This led us to believe that this expression occurred in other, probably non-differentiated cells. These later were not stimulated to mature by ionomycin. Thus, ionomycin constitutes an essential early signal, but is insufficient to trigger the complete maturation of cells.

Cell Division↗
PubMed1987

Priming of neutrophils and macrophages for enhanced release of superoxide anion by the calcium ionophore ionomycin. Implications for regulation of the respiratory burst.

Phagocytic cells can be primed for enhanced stimulated release of superoxide anion (O2-) by exposure to a variety of biologic agents, including gamma-interferon and lipopolysaccharide. We examined the role of calcium ion in this priming, using the calcium ionophore ionomycin. Preincubation with ionomycin, 1 to 10 nM, primed human neutrophils to release up to 7-fold more O2- during stimulation with 1 microM formyl-methionyl-leucyl-phenylalanine (f-Met-Leu-Phe). With 160 nM phorbol myristate acetate as stimulus, ionomycin caused a doubling of O2- production in mouse peritoneal macrophages. Incubation of phagocytes with ionomycin at priming concentrations did not directly stimulate O2- release. Priming of neutrophils occurred in 1-2 min and was associated with a marked reduction in the lag time for O2- release after f-Met-Leu-Phe stimulation and with an increase in the rate of O2- production. Kinetic analysis of NADPH-dependent O2(-)-producing activity in sonicates of resting human neutrophils incubated with sodium dodecyl sulfate suggested that modification of the enzyme responsible for the respiratory burst was not responsible for priming. Priming of neutrophils with ionomycin had no apparent effect on either the activity or subcellular distribution of protein kinase C. The effect of ionomycin on the cytosolic free calcium concentration ([Ca2+]c) was assessed in neutrophils using the calcium-sensitive fluorescent dye fura-2. Ionomycin at priming concentrations caused an approximate doubling of the base-line [Ca2+]c. When neutrophils were exposed to various concentrations of ionomycin, a parallel rise in [Ca2+]c and priming was observed. A rise in [Ca2+]c of approximately 0.8 microM caused half-maximal priming. These results suggest that an increase in [Ca2+]c is not sufficient to initiate release of O2-, but they support the concept that Ca2+ can serve as a second messenger in this event.

Animals↗
PubMed1995

Differential activation of a calcium-dependent endonuclease in human B lymphocytes. Role in ionomycin-induced apoptosis.

The state of B cell maturation profoundly influences the outcome, i.e., activation, growth arrest, or programmed cell death, of a variety of stimuli, including the calcium ionophore, ionomycin. Initial studies confirmed the observation that cell lines representative of immature B cells, i.e., Burkitt lymphoma cell lines, were induced to undergo apoptosis in response to ionomycin, whereas more mature B cell lines did not, and instead underwent cell cycle arrest in the G1 interval. To understand this differential outcome, we have focused on comparing the expression and activation of an endonuclease(s) in cells induced by ionomycin to undergo programmed cell death (Ramos) with cells resistant to ionomycin-induced programmed cell death (Ly1). Our results demonstrated that a low m.w. fraction of an endogenous Ca2+/Mg(2+)-dependent endonuclease was activated in Ramos cells, but not in activated Ly1 cells, following the addition of ionomycin. Of interest, however, low m.w. endogenous endonuclease(s) activity was induced when isolated Ly1 cell nuclei were treated with exogenous calcium instead. Use of field inversion gel electrophoresis further indicated that cleavage of DNA into large m.w. (> 50 kbp) DNA fragments does not precede ionomycin-induced internucleosomal cleavage in Ramos cells or in ionomycin-resistant Ly1 cells. In summary, these data support the conclusion that ionomycin-induced apoptosis involves the activation of a latent, low m.w., calcium-responsive endonuclease and suggest that control of endonuclease depression may contribute to cell-specific regulation of calcium ionophore-induced apoptosis.

Anti-Bacterial Agents↗
PubMed1993

Estradiol modulation of PMA--and ionomycin-stimulated LH secretion from pituitaries of castrated rats.

Quartered pituitaries from castrated (72 h) +/- estradiol (E2)-treated (24 h) rats were used in a perifusion system to investigate estradiol modulation of ionomycin and ionomycin + PMA stimulated LH secretion. Estradiol enhanced the LH responses to GnRH (1 nM) and ionomycin (10 microM), and was necessary for the manifestation of PMA-stimulated (1 microM) LH secretion. Cycloheximide (5 microM) inhibited the E2-enhanced responses to GnRH, ionomycin and PMA. The protein synthesis inhibitor also partially suppressed the GnRH response from pituitaries of castrates, but was totally ineffective against the ionomycin-induced LH secretion. Protein synthesis-dependent, synergistic interactions between PMA and ionomycin were evident from pituitaries of castrates (even though PMA alone was an ineffective secretagogue). Synergistic interactions were not apparent from pituitaries of castrated + E2-treated rats. These results indicate that: (i) estradiol enhances the responsiveness of male gonadotropes to ionomycin and PMA by protein synthesis-dependent mechanisms which appear to mask their synergistic interactions; and (ii) increases in cytoplasmic Ca2+ might be a prerequisite for an expression of the involvement of PKC as a mediator of LH secretion in the absence of high concentrations of estradiol.

Animals↗
PubMed1993

Ionomycin activates electrogenic Ca2+ influx in rat thymic lymphocytes.

Increases in intracellular Ca2+ concentration ([Ca2+]i) after addition of the carboxylic Ca(2+)-transporting ionophore ionomycin have been reported to be inhibited by depolarization in mouse T- and B-lymphocytes, a result attributed to effects on the plasma-membrane Ca2+ pump. The present experiments were undertaken to test the hypothesis that activation of electrogenic Ca2+ uptake, due to release of Ca2+i from intracellular pools, underlies the membrane-potential (Em)-sensitivity of ionomycin-mediated Ca2+ uptake in T-lymphocytes. To address this proposal we have compared the characteristics of Ca2+ influx induced by ionomycin with those of Ca2+ influx activated by release of Ca2+ from intracellular stores (store-regulated Ca2+ uptake, SRCU). The endosomal Ca(2+)-ATPase inhibitor thapsigargin was used to bring about loss of Ca2+ from intracellular pools. In support of this hypothesis, we report that: (1) ionomycin releases Ca2+ from a thapsigargin-sensitive endosomal pool, a pool previously demonstrated to regulate electrogenic SRCU in rat thymic lymphocytes, (2) Ca2+ and Mn2+ uptake mediated by low doses of ionomycin are inhibited by compounds previously reported to inhibit SRCU, and (3) in the absence of SRCU, ionomycin-mediated Ca2+ uptake displays no sensitivity to Em. We conclude that activation of electrogenic SRCU can adequately account for the Em-sensitivity of Ca2+ flux previously attributed to alterations in Ca(2+)-pump activity. Such a mechanism of action may underlie previous reports of electrogenic Ca2+ transport mediated by ionomycin in other tissues.

Animals↗
PubMed1995

Ionomycin-stimulated phasic myometrial contractions.

Ionomycin, a calcium ionophore, facilitates the sustained entry of extracellular calcium; however, in myometrial tissue it stimulates phasic contractions. This study sought to define further this unanticipated effect of ionomycin and to begin to explore the possible mechanism(s) involved. Utilizing rat uterine strips, in vitro isometric contraction studies were performed to determine the effects of ionomycin with and without membrane-permeant inhibitors of cytosolic calcium oscillations. To determine the effects of ionomycin on phospholipase C, qualitative inositol phosphate production studies were performed. The in vitro contraction studies confirmed that ionomycin-stimulated phasic myometrial contractions were potentially dependent on stimulation of phospholipase C, calcium-induced calcium release, and additional calcium influx through dihydropyridine-sensitive membrane calcium channels. The inositol phosphate production studies confirmed that ionomycin stimulated phospholipase C in a dose-related fashion to levels comparable to oxytocin. In summary, these observations have confirmed the ability of ionomycin to generate dose-related phasic myometrial contractions through mechanisms potentially involving the phosphatidylinositol-signaling pathway.

Animals↗
PubMed1995

Irsogladine inhibits ionomycin-induced decrease in intercellular communication in cultured rabbit gastric epithelial cells.

Effects of irsogladine on ionomycin-induced decrease in intercellular communication and increase in intracellular concentration of Ca2+ ([Ca2+]i) were investigated in cultured rabbit gastric epithelial cells. Ionomycin (10(-7)-10(-6) M) transiently and concentration-dependently inhibited intercellular communication concomitantly with the elevation of [Ca2+]i in the presence and absence of extracellular Ca2+. Irsogladine (10(-5) M), which has been shown to facilitate intercellular communication, suppressed the ionomycin-induced elevation of [Ca2+]i and decrease in intercellular communication. The suppression of the ionomycin effects by irsogladine was independent of extracellular Ca2+. TMB-8 [8-(diethylamino)octyl-3,4,5-trimethoxy-benzoate hydrochloride] (10(-6) M) also suppressed the ionomycin-induced elevation of [Ca2+]i and decrease in intercellular communication. These results indicate that the ionomycin-induced decrease in intercellular communication may be due to Ca2+ mobilization from intracellular stores. Inhibitory effects of irsogladine and TMB-8 on the ionomycin-induced decrease in intercellular communication may be produced by suppressing Ca2+ mobilization.

Animals↗
PubMed1992

Regulation of cell division of mature B cells by ionomycin and phorbol ester.

The growth of a human B lymphoma cell line B104, an experimental model for mature B cells, was inhibited by ionomycin but not 12-O-tetradecanoylphorbol-13-acetate (TPA). Ionomycin inhibited B104 cells from entering into the M phase of the cell cycle without affecting DNA synthesis. The inhibition of cell division of B104 cells by ionomycin occurred within 24 h after stimulation. Because such a mode of action resembles that of anti-IgM antibodies, signals transduced by Ca2+ may be responsible for the inhibition of cell division of B104 cells by anti-IgM antibodies. Indeed, EGTA suppressed the inhibition of cell division of B104 cells caused not only by ionomycin, but also by anti-IgM antibody. Although TPA itself did not have any ability to promote the growth of B104 cells, it could cancel the inhibition of cell division of B104 cells by ionomycin and increase the proportion of B104 cells entering into the M phase of the cell cycle. Staphylococcus aureus Cowan I causes the greatest proliferation of normal human peripheral blood B cells during the period from 48 to 72 h after stimulation. When ionomycin was added to S. aureus Cowan I-stimulated peripheral blood B cells at 48 h of culture, it inhibited cell division during this period without affecting DNA synthesis. In the presence of TPA, this activity of ionomycin was suppressed, and the proportion of M-phase cells increased. These results suggest that cell division of mature B cells is regulated by the signals mediated by Ca2+ and protein kinase C in a mode quite different from that of regulation of DNA synthesis.

B-Lymphocytes↗