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PubMed1991

Calcium dependency of adrenergic and muscarinic cholinergic stimulation of mucin release from dog submandibular gland cells.

Stimulation of muscarinic cholinergic, alpha-adrenergic and beta-adrenergic receptors elicited mucin release from dispersed dog submandibular cells. The secretory response to acetylcholine was much more pronounced than to adrenergic agonists, and largely dependent on the presence of extracellular Ca2+, but the dependency on extracellular Na+ was slight. Ionomycin also stimulated mucin release. In rat submandibular cells, neither muscarinic cholinergic agonists nor ionomycin were as effective mucosecretagogues as beta-adrenergic agonists. alpha-Adrenoceptor-mediated release was decreased by chelating extracellular Ca2+ with EGTA. The beta-adrenoceptor-mediated response was diminished by extensive exposure of cells to EGTA, due at least in part to the requirement of Ca2+ for beta-adrenoceptor stimulation of cAMP formation. 8-br-cAMP stimulated 45Ca2+ release from cells preloaded with 45Ca2+. The 8-br-cAMP-induced mucin release was eliminated in ionomycin-pretreated cells, but not inhibited by chelating extracellular Ca2+ and by the treatment of the cells with TMB-8 or in the cells loaded with BAPTA. These results suggest that not only the adrenergic system but also the muscarinic cholinergic system may participate in the regulation of mucin release in dog submandibular gland, and also provide the possibility that, in addition to a cAMP-mediated mechanism, Ca(2+)-dependent mechanisms may be involved in mucosecretion in dog submandibular acini.

8-Bromo Cyclic Adenosine Monophosphate↗
PubMed1987

Elevated intracellular Ca2+ affects Lii-Nao countertransport in human red blood cells.

Changes in cytoplasmic Ca2+ concentration and in Lii-Nao countertransport activity have been shown to be associated with essential hypertension. Elevated intracellular free [Ca2+], as well as abnormalities of Ca2+ binding and transport have been reported in cells from different tissues of hypertensive laboratory animals and essential hypertensive patients. Similarly, enhanced rates of Lii-Nao countertransport and the modified pattern of the temperature dependence of this activity in red blood cells from essential hypertensive patients have been previously demonstrated. The aim of the present study was to investigate possible interaction between changes in intracellular free [Ca2+] and the Lii-Nao exchange in human red blood cells. The ionophore ionomycin was used to allow Ca2+ incorporation into the cells in a dose-dependent manner. The elevation of intracellular [Ca2+], in turn, resulted in enhanced Li+ efflux from the cells. At 3 microM, ionomycin selectively and significantly enhanced the Lii-Nao countertransport but not Li+ leakage from the cells. EGTA totally abolished the effect of ionomycin, indicating that the effect is directly related to Ca2+. As low as 0.4 microM Ca2+ caused a statistically significant effect. The maximal effect of Ca2+ on the Lii-Nao countertransport was achieved around the external pH range of 6.8-7.5. In contrast, the leakage of Li+ was significantly enhanced by Ca2+ at a pH of 7.4 and above. Ca2+ did not affect the Km of the Lii-Nao countertransport for Li+. Amiloride, which inhibits Na+/H+ exchange, inhibited only 10% of the Ca2+-enhanced countertransport. It is concluded that Ca2+ may play a role in the regulation of Lii-Nao countertransport in erythrocytes.

Antiporters↗
PubMed1988

A role for Ca2+ in mediating hormone-induced biphasic pepsinogen secretion from the chief cell determined by luminescent and fluorescent probes and X-ray microprobe.

In isolated chief cells from the guinea pig, cholecystokinin (10 nM) and a high concentration of ionomycin each caused a biphasic pattern of pepsinogen secretion. The initial fast response to cholecystokinin was not dependent on medium Ca2+ ans was mimicked by low concentration of ionomycin (100 nM). Inositol 1,4,5-trisphosphate caused a similar fast release from permeabilized cells. The slow component of release was dependent on medium Ca2+, however, and was mimicked by the phorbol ester 12-O-tetradecanoylphorbol 13-acetate (TPA) (100 nM) or the diacylglycerol analogue 1-oleoyl-2-acetylglycerol (OAG) (100 microM). Ionomycin (100 nM) and TPA (and/or OAG), when applied together, reproduced the biphasic pattern of pepsinogen secretion, suggesting that the signalling pathways utilized by both types of agonist contribute to the response evoked by cholecystokinin-hormone stimulation. Both fura-2 and aequorin were used to monitor changes of intracellular Ca2+. Three pathways were found to contribute to the Ca2+ transient. A rapid release of Ca2+ from intracellular store(s), a rapid Ca2+ entry from the extracellular space, and a more sustained Ca2+ entry from the extracellular space. Cholecystokinin induced a rapid increase in cytoplasmic Ca2+ ([Ca2+]i) as estimated with fura-2 and aequorin. This rise was reduced but not abolished upon removal of extracellular Ca2+, suggesting that both Ca2+ entry from the extracellular space and Ca2+ mobilization from the intracellular store(s) contribute to the initial, fast component of the Ca2+ transient. A second, more sustained component of the Ca2+ transient induced by cholecystokinin was abolished by lanthanum. TPA and OAG induced a biphasic Ca2+ transient that could be detected only with aequorin. The late, sustained component of this response was again abolished by lanthanum as well as by removal of extracellular Ca2+. It appears that the late component of the Ca2+ transient is dependent on Ca2+ influx from the extracellular space and is too localized to be detected by fura-2. Prestimulation of cells with TPA or OAG prevented the aequorin transient caused by cholecystokinin and vice versa, suggesting that TPA, OAG and cholecystokinin activate the same pathways of Ca2+ entry into the cytosol from the intracellular store(s) or the extracellular space. The stimulation-sensitive Ca2+ pool was examined with electron probe X-ray microanalysis. It appears to be restricted to an area enriched in secretory granules or peripheral endoplasmic reticulum just beneath the apical plasma membrane and in close association with the microtubular-microfilamentous system.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1993

Signal-transduction mechanisms of ATP-stimulated phosphatidylcholine secretion in rat type II pneumocytes: interactions between ATP and other surfactant secretagogues.

ATP stimulates phosphatidylcholine secretion in type II cells, an effect that is mediated by both adenosine A2 receptors coupled to adenylate cyclase and P2 receptors coupled to phosphoinositide-specific phospholipase C. Activation of these effector enzymes leads to formation of cAMP, diacylglycerols and inositol trisphosphate (IP3). cAMP in turn activates cAMP-dependent protein kinase, diacylglycerols activate protein kinase C and IP3 promotes Ca2+ mobilization. To further investigate the signal-transduction mechanisms mediating the ATP effect, we examined its action in combination with that of other surfactant secretagogues: 5'(N-ethylcarboxyamido)adenosine (NECA), a A2 agonist that activates adenylate cyclase; TPA (12-O-tetradecanoylphorbol-13-acetate), a direct activator of protein kinase C; and ionomycin, an ionophore that increases intracellular Ca2+. The effects of NECA, TPA and ionomycin were additive and thus consistent with independent signaling mechanisms. However, the effects of all combinations of three or four secretagogues that contained ATP were 10-20% less than additive. This suggested that ATP and other secretagogues act via common mechanisms. Calmodulin antagonists decreased the effects of ionomycin and ATP by approx. 60% and 30%, respectively, but did not decrease the effects of NECA, terbutaline or TPA. Complete inhibition of the effect of ATP was achieved with a combination of a calmodulin antagonist, an A2 antagonist and a protein kinase C inhibitor. These and previous data suggest that the stimulatory effect of ATP on phosphatidylcholine secretion in type II cells is mediated by three signal-transduction mechanisms: activation of cAMP-dependent protein kinase; activation of protein kinase C; and a calmodulin-dependent mechanism.

1-(5-Isoquinolinesulfonyl)-2-Methylpiperazine↗
PubMed1994

Chlorpromazine protection against Ca(2+)-dependent and oxidative cell injury. Limitations due to depressed mitochondrial function.

Chlorpromazine (CPZ), a phenothiazine, demonstrated both cytoprotective and toxic effects on cardiomyocytes. CPZ markedly reduced cytotoxicity caused by two toxic challenges, each with a distinct cytotoxic mechanism. Lethal cell injury was induced in cultured neonatal cardiomyocytes by either: (1) ionomycin, a Ca2+ ionophore that caused Ca(2+)-dependent cell injury; or (2) ethacrynic acid (EA), a glutathione (GSH) depletor that killed cells primarily via peroxidative damage. Pretreatment with 50 microM CPZ reduced the extent of ionomycin-induced cell death, as measured by lactate dehydrogenase (LDH) leakage, but enhanced the loss of intracellular ATP and collapsed the mitochondrial transmembrane potential (delta psi). In EA-treated cultures, 50 microM CPZ also lowered LDH leakage and diminished the peroxidative damage responsible for the cytotoxicity, but again enhanced the loss of intracellular ATP and collapsed the delta psi. CPZ protection was incomplete and limited to a narrow concentration range that was essentially identical for both toxic challenges. Maximum protection was observed with 50 microM CPZ, yet the amount of residual damage was similar to the degree of injury caused by a mitochondrial uncoupler, carbonylcyanide-m-chlorophenylhydrazone alone. In the absence of either challenge, 50 microM CPZ did not affect cellular energy status or kill the cells, but a higher concentration of CPZ (150 microM) did deenergize unchallenged cardiomyocytes. These data demonstrate that CPZ can reduce cytotoxicity caused by either Ca(2+)-dependent events or oxidative stress. However, even at an optimally protective level, CPZ in combination with either ionomycin or EA deenergized the cells, although neither toxic challenge nor 50 microM CPZ alone seriously affected delta psi. It would appear that intracellular perturbations induced by either challenge promote a depression of mitochondrial function by CPZ, which limits the protective action of the drug. Since both of the challenges used contain toxicologic features exhibited by a wide variety of toxic insults, results of this study have both mechanistic and clinical implications.

Adenosine Triphosphate↗
PubMed1986

Interleukin 1 and protein kinase C activator are dissimilar in their effects on Il-2 receptor expression and Il-2 secretion by T lymphocytes.

T lymphocytes respond to mitogenic stimulation by expressing the receptor for interleukin 2 (Il-2) and secreting Il-2; once the receptor is expressed, Il-2 induces these cells to proliferation. In the present report using mouse T lymphocytes, thymocytes, and the lymphoma cell line EL4, we studied receptor expression and Il-2 secretion as early parameters for T-lymphocyte activation in response to ionomycin, concanavalin A (Con A), 12-O-tetradecanoyl-phorbol 13-acetate (TPA), and interleukin 1 (Il-1). Il-1 is required for mitogenic response of lymphocyte preparations that are rigorously depleted of macrophages. On its own, Il-1 had very little effect on Il-2 secretion and Il-2 receptor expression by T lymphocytes. TPA strongly synergized with ionomycin both for Il-2 secretion and for Il-2 receptor expression whereas Il-1 did not. Il-1 required the simultaneous presence of ionomycin and TPA to have any demonstrable effect on T lymphocytes from spleen and on thymocytes. However, on EL4 cells which were also partially responsive to TPA alone, Il-1 showed strong synergy with TPA to induce Il-2 secretion and Il-2 receptor expression. The effect of Il-1 on EL4 cells was dose dependent where increasingly higher concentrations of Il-1 in the presence of a fixed concentration of TPA caused higher percentage of EL4 cells to become Il-2 receptor positive. The present results suggest that Il-1 does not cause its effect on T lymphocytes via the same mechanism of protein kinase C activation that has been proposed for TPA.

Animals↗
PubMed1986

Stimulation of murine B cells by the combination of calcium ionophore plus phorbol ester.

The ability of calcium ionophore and phorbol ester to stimulate entry into S phase was assessed during short-term cultures of murine B cells extensively depleted of T cells. Neither ionomycin alone nor PMA alone stimulated DNA synthesis. However, the combination of ionomycin plus PMA acted in synergy to induce initiation of DNA synthesis in murine B cells. A role for protein kinase C was implied by the inability of 4 alpha-phorbol didecanoate to reproduce the effects of PMA. The combination of ionomycin plus PMA also stimulated DNA synthesis in xid-defective B cells.

Animals↗
PubMed1988

Proliferative effects of 12-O-tetradecanoylphorbol-13-acetate (TPA) and calcium ionophores on human large granular lymphocytes (LGL).

The proliferative responses of natural killer (NK) cells to 12-O-tetradecanoylphorbol-13-acetate (TPA), which directly activates protein kinase c(PKC), and to the Ca2+ ionophores A23817 and ionomycin, known to enhance the intracellular calcium, have been investigated. Highly purified large granular lymphocytes (LGL) were cultured for 12-30 hr in the presence of TPA, ionomycin, or A23817. TPA alone (1-20 ng/ml) triggered rapid LGL proliferation, whereas the calcium ionophores were ineffective. The addition of either calcium ionophore to suboptimal doses or TPA (0.1-0.5 ng/ml) resulted in a synergistic effect on LGL proliferation. Under these conditions high levels of IL-2 activity were released by the LGL. Phenotypic analysis revealed the rapid loss of the Fc gamma receptors (CD16) on LGL and the induction of the expression of IL-2 (CD25) and transferrin receptors and of HLA-DR, but not of CD3. Removal of extracellular Ca2+ by addition of EGTA at the beginning of the culture greatly depressed LGL proliferation and IL-2 production, and blocked phenotypic changes, such as the expression of Tac antigen. Finally, progression to the proliferative phase of LGL, activated by TPA alone or with ionomycin, was completely abrogated by a hyperimmune anti-IL-2 antiserum.

Antigens, Surface↗
PubMed1990

Adoptive transfer of experimental allergic encephalomyelitis: conditions influencing memory and effector cell development.

The cellular transfer of clinical experimental allergic encephalomyelitis (EAE) with immune spleen cells is only accomplished following lymphoid cell stimulation during an intervening in vitro culture activation period. Recipients of these cells recover from the ensuing adoptively transferred paralytic episode and subsequently respond to active challenge with myelin basic protein (BP)-CFA in an accelerated time frame consistent with the presence of memory cells in the initial cell transfer inoculum. We have found that the addition of anti-CD4 antibody or dexamethasone during the activation period inhibits the development of the transfer active EAE effector cell subpopulation, but does not alter the in vitro development and subsequent expression of the BP-specific memory cell subpopulation. Additional experiments also suggest the development of memory cells in the absence of effector cell activity. PMA + ionomycin when used as a stimulus during the culture activation period leads to effector and memory cell development. The stimulation response is dose dependent, in that a reduced concentration of PMA + ionomycin does not lead to EAE effector cell development; however, at these reduced levels of PMA + ionomycin, memory cell development still occurred. Additional evidence which supports the concept of independent development of memory cells and effector cells was obtained with a BP-specific cell line. Following recovery from cell line-mediated clinical EAE, as well as following adoptive transfer of the cell lines in the precursor stage, cell recipients did not develop an early onset of active EAE when subsequently immunized with BP-CFA. Thus the BP-specific T-cell line appears to contain the precursors of the effector cell subpopulation but does not appear to contain the BP memory cell subpopulation. Collectively these observations suggest the existence of distinct T-cell subsets or pathways of development that are followed during the response to BP as measured by the development of clinical EAE.

Animals↗
PubMed1990

Macrophage-dependent stimulation of T cell-depleted spleen cells by Clostridium difficile toxin A and calcium ionophore.

Clostridium difficile toxin A causes severe intestinal inflammation and fluid secretion in rabbit ileum and is chemotactic for neutrophils in vitro. The mechanism of intestinal injury produced by toxin A appears to involve direct epithelial cell damage as well as recruitment of an inflammatory cell response. The current study was undertaken to determine if toxin A can directly stimulate a proliferative response in lymphocytes. Highly purified toxin A, in the presence of the calcium ionophore, ionomycin, stimulated substantial [3H]thymidine incorporation by murine splenic lymphocytes, which was maximal at 10(-9) M toxin A and 800 ng/ml ionomycin. Removal of T cells with anti-Thy-1.2 antibody plus complement had no effect on the proliferative response induced by toxin A. However, [3H]thymidine incorporation in response to toxin A was significantly inhibited (P less than 0.001) by the removal of macrophages from splenocyte suspensions and was restored by the addition of peritoneal macrophages or cell-free supernatant from toxin A-treated macrophage cultures. Analysis of the toxin A-treated macrophage supernatants showed high levels of IL-1, but not IL-2 or IL-4. The combination of recombinant IL-1 plus ionomycin was found to stimulate [3H]thymidine incorporation by T cell-depleted splenic lymphocytes. These results suggest that toxin A stimulates the release of IL-1, and possibly other factors, from macrophages which can costimulate murine B lymphocytes.

Animals↗
PubMed1991

Actin polymerization in murine B lymphocytes is stimulated by cytochalasin D but not by anti-immunoglobulin.

One might predict that cytochalasin D, which slows polymerization of actin in solution and which inhibits actin-containing microfilament function in live B lymphocytes, would also prevent actin polymerization in these cells. However, we have used the NBD-Phallacidin flow cytometric assay for F-actin and the DNase I inhibition assay for G-actin to demonstrate that cytochalasin D (at 20 micrograms/ml and higher) stimulates actin polymerization in murine B lymphocytes within the first 30 sec of exposure. A similar response was seen in human neutrophils. Actin polymerization induced in neutrophils by chemotactic peptides has been linked to activation of the polyphosphoinositide-calcium increase-protein kinase C signal transduction pathway. As B lymphocytes also transduce signals using this pathway, we investigated whether cytochalasin D induced actin polymerization by activating this pathway. Cytochalasin D and ionomycin both stimulated a rapid increase in internal calcium (by 1 min) in the B cell which was inhibitable by EGTA, implicating calcium influx. Ionomycin also induced actin polymerization, detectable later, by 10 min. EGTA blocked the ionomycin-induced actin polymerization, but not that induced by cytochalasin D. Cytochalasin D-induced actin polymerization was not associated with detectable hydrolysis of polyphosphoinositides, nor was it inhibited by H7 (a protein kinase C inhibitor) or by HA1004 (an inhibitor of cyclic nucleotide-dependent kinases). Furthermore, anti-immunoglobulin antibodies, which stimulate B lymphocytes through the polyphosphoinositide hydrolysis-calcium increase-protein kinase C pathway, failed to induce actin polymerization in these cells. These antibodies did, however, stimulate the cells to perform activities that involve actin-containing microfilaments. Other primary activators of B lymphocytes (dextran sulfate, PMA, and LPS) and a panel of lymphokines previously shown to enhance B lymphocyte activation (IL-1, IL-2, IL-4, IL-5) were also screened in the F-actin assay and no evidence for actin polymerization was found. We conclude that the actin polymerization response to cytochalasin D in the B cell does not involve the polyphosphoinositide hydrolysis-calcium increase-protein kinase C pathway, nor does it depend on cyclic nucleotide-dependent kinases. Furthermore, our studies failed to provide any evidence that early actin polymerization occurs in murine B lymphocyte activation.

Actins↗
PubMed1989

Depolarization of human neuroblastoma cells as a result of muscarinic receptor-induced rise in cytosolic Ca2+.

The role of intracellular free Ca2+ in muscarinic-receptor linked depolarization of SH-SY5Y neuroblastoma cells has been determined by using the bisoxonol membrane potential probe DiBaC4-(3) and intracellular Ca2+ indicator fura-2 respectively. Carbachol and the Ca2+ ionophore, ionomycin, at concentrations which caused similar rises in intracellular Ca2+ increased the bisoxonol fluorescence (depolarization) to the same extent. The membrane potential responses, but not the changes in intracellular Ca2+, were dependent on extracellular Na+. Ionomycin depletion of intracellular Ca2+ with EGTA and ionomycin or loading the cells with a Ca2+ buffer, BAPTA, reduced the carbachol-induced depolarization. The results suggest that a rise in intracellular Ca2+ may cause depolarization through an increase in the Na+ permeability.

Calcium↗
PubMed1990

Use of a pituitary cell dispersion method and primary culture system for the studies of gonadotropin-releasing hormone action in the goldfish, Carassius auratus. II. Extracellular calcium dependence and dopaminergic inhibition of gonadotropin responses.

Primary static cultures of dispersed goldfish pituitary cells obtained by controlled trypsinization released gonadotropin (GTH) in response to 2-hr stimulations of 0.1 nM to 1 microM [Trp7,Leu8]-gonadotropin-releasing hormone (sGnRH), [D-Arg6,Pro9-N-ethylamide]-sGnRH (sGnRHa), and [His5,Trp7,Tyr8]-GnRH (cGnRH-II) in a dose-dependent manner. Coincubation with 10 to 1000 nM of a dopamine agonist, apomorphine, dose dependently reduced the GTH response to increasing concentrations of sGnRH. Apomorphine at 1 microM completely abolished the dose-dependent GTH response to sGnRHa and cGnRH-II, but only partially inhibited the GTH-releasing action of high concentrations of sGnRH. Addition of calcium ionophores, 1 to 100 microM A23187 and 10 to 100 microM ionomycin, significantly increased GTH release. The ED50S of the GTH response to A23187 and ionomycin were 0.88 +/- 0.15 and 13.67 +/- 2.76 microM, respectively. Incubation with Ca2(+)-deficient media (media prepared without the addition of Ca2+ salts) did not significantly affect basal GTH release, but severely decreased the hormone response to increasing concentrations of sGnRH, A23187, and ionomycin. These results confirm the direct inhibitory dopaminergic influence on GTH release in goldfish and further suggest that extracellular Ca2+ plays a role in mediating GnRH action on gonadotropes in fish.

Animals↗
PubMed1988

Ionic signalling in mast cells; antigen and ionophore induced changes in cytosolic pH.

Stimulation of cells of the rat basophilic leukemia line RBL-2H3, which are used as a model in biochemical studies of mast cells, by antigen or by the calcium ionophore ionomycin, are known to cause secretion of mediators of inflammation. These stimuli have now been found to cause a decrease in the cells' cytosolic pH. This acidification process was monitored by the fluorescent indicator 2',7'-bis (carboxyethyl)-5(6)-carboxyfluorescein (BCECF) introduced into these cells. The antigen induced acidification was the result of specific aggregation of membrane residing IgE, reached values up to 0.03 pH units and required the presence of sodium and calcium ions in the incubation medium. It was amiloride resistant but was blocked by the metabolic inhibitor deoxyglucose. Ionomycin caused a dose dependent decrease in cytosolic pH which was also sensitive to the pH of the extracellular medium. The acidification reached more than 0.1 pH units at optimal, non-cytotoxic, doses of ionomycin (1 microM) and decreased markedly as the medium pH increased from 7.0 to 8.0. The antigen and ionophore induced cytosolic acidification processes are interpreted as being the result of the increased concns of free cytosolic calcium ions rather than the effect of direct activation of a sodium-proton exchanger. Further investigation of this process is in progress.

Animals↗
PubMed1990

Degranulation in human neutrophils primes the cells for subsequent responsiveness to the chemoattractant N-formylmethionylleucylphenylalanine but does not increase the sensitivity of the NADPH-oxidase to an intracellular calcium rise.

Both the chemotactic peptide formylmethionylleucylphenylalanine (FMLP) and the calcium-specific ionophore ionomycin can activate the NADPH-oxidase in human neutrophils. However, since ionomycin and FMLP activity differ in their requirement for azide, a potent inhibitor of the hydrogen peroxide consuming enzymes catalase and myeloperoxidase, we propose that the two stimuli can activate different pools of the oxidase. Degranulation, induced in vitro by sn-1,2-dedecaoylglycerol or in vivo by an exudation process, resulted in a priming of the cells using FMLP as stimulating agent as well as in a reduced capacity to generate H2O2 in response to ionomycin. The sensitivity of the plasma membrane-bound NADPH-oxidase to an intracellular [Ca2+] rise, induced by the ionophore was, however, not changed by the degranulation. From these results we propose that FMLP activates the plasma membrane-bound oxidase, whereas the ionophore is capable of activating a granule-bound pool of the oxidase.

Adult↗
PubMed1991

Arachidonic acid release by basophilic leukemia cells and macrophages stimulated by Ca2+ ionophores, antigen and diacylglycerol: essential role for protein kinase C and prevention by glucocorticosteroids.

The role of protein kinase C in phospholipase A2 (PLA2) activation in rat basophilic leukemia cells (RBL-2H3) and macrophages was investigated. 12-O-Tetradecanoyl phorbol 13-acetate (TPA) doubled ionomycin-induced PLA2 activity, assessed by [3H]arachidonate release. Protein kinase C inhibitors, staurosporine and K252a (100 nM) or H-7 (15 micrograms/ml) inhibited ionomycin-stimulation of PLA2 activity by 62, 75 and 80%, respectively. Down-regulation of protein kinase C by prolonged treatment with TPA inhibited Ca2(+)-ionophore A23187 or antigen-stimulation of [3H]arachidonate release by 80%. We examined whether the inhibitory effect of dexamethasone (DEX) on PLA2 activity is related to modulation of protein kinase C activity. The 50% inhibition by DEX of ionomycin elevation of [3H]arachidonate release was almost overcome by addition of TPA. The Ca2+ ionophore and antigen-induced increase in [3H]TPA binding to intact RBL cells was not impaired by DEX. However, DEX markedly reduced phosphorylation of several proteins. 1-Oleoyl-2-acetyl-glycerol (OAG) had a sustained stimulatory effect on PLA2 activity in isolated plasma membranes derived from treated bone-marrow intact mouse macrophages, while both DEX and staurosporine reduced elevated PLA2 activity by 68 and 84%, respectively. The results support an essential role for protein kinase C in regulation of PLA2 activity.

Alkaloids↗
PubMed1989

Calmodulin and protein kinase C activation duplicates the biphasic secretion of luteinizing hormone.

Ionomycin, which activates the Ca2+-calmodulin system, and phorbol 12-myristate 13-acetate (PMA), which activates protein kinase C (PKC), were used to investigate potential roles of these systems as mediators of the biphasic secretion of luteinizing hormone. Quartered pituitaries from diestrous II female rats were perifused at 37 degrees C, and sequential effluent fractions collected every 10 min. Gonadotropin-releasing hormone administration resulted in a biphasic response: an initial, protein synthesis-independent secretion, followed 60 min later by a secondary, augmented, protein synthesis-dependent component. Ionomycin-stimulated gonadotropin secretion was immediate and partially independent of protein synthesis, whereas the PMA-induced secretion was delayed (approximately 70 min), and was completely dependent on protein synthesis. Simultaneous infusions of ionomycin and PMA resulted in an initial, protein synthesis-independent response followed by the secondary, augmented, protein synthesis-dependent component, which exhibited synergistic interactions between calmodulin and PKC. These results suggest that calmodulin mediates the initial, protein synthesis-independent secretion, PKC mediates part of the secondary, augmented response, while calmodulin and PKC synergize to mediate the remaining component of the secondary response.

Animals↗
PubMed1992

Regulatory effect of 1,25-dihydroxycholecalciferol on calcium fluxes in thyroid FRTL-5 cells.

The aim of the present study was to investigate the effect of 1,25-dihydroxycholecalciferol (1,25(OH)2-D3) on the regulation of calcium fluxes in rat thyroid FRTL-5 cells. The ATP-induced uptake of 45Ca2+ was decreased in cells pretreated with 1,25(OH)2D3 for 48 h. No effect was seen on basal uptake of 45Ca2+. At least a 24 h incubation period was required for the effect of 1,25(OH)2D3 to be expressed. Pretreatment with 1,25(OH)2D3 for 48 h did not change resting intracellular Ca2+ ([Ca2+]i) in fura-2-loaded FRTL-5 cells. However, the ATP-induced increase in [Ca2+]i was significantly enhanced in cells preincubated with 1,25(OH)2D3. The effect of 1,25(OH)2D3 was abolished in Ca(2+)-free buffer. No difference in the ionomycin-induced increase in [Ca2+]i was observed between control cells and cells pretreated with 1,25(OH)2D3. However, in Ca(2+)-free buffer the ionomycin response was decreased in cells incubated with 1,25(OH)2D3. The ATP-induced change in [Ca2+]i was decreased when ATP was added after ionomycin to cells treated with 1,25(OH)2D3. The results suggest that 1,25(OH)2D3 has a regulatory effect on Ca2+ fluxes in FRTL-5 cells, possibly by acting on Ca2+ sequestration.

Adenosine Triphosphate↗