Characterization of a single soybean cDNA encoding cytosolic and glyoxysomal isozymes of aspartate aminotransferase

A soybean cDNA clone, pSAT1, which encodes both the cytosolic and glyoxysomal isozymes of aspartate aminotransferase (AAT; EC 2.6.1.1) was isolated. Genomic Southern blots and analysis of genomic clones indicated pSAT1 was encoded by a single copy gene. pSAT1 contained an open reading frame with ca. 90% amino acid identity to alfalfa and lupin cytosolic AAT and two in-frame start codons, designated ATG1 and ATG2. Alignment of this protein with other plant cytosolic AAT isozymes revealed a 37 amino acid N-terminal extension with characteristics of a peroxisomal targeting signal, designated PTS2, including the modified consensus sequence RL-X₅-HF. The second start codon ATG2 aligned with previously reported start codons for plant cytosolic AAT cDNAs. Plasmids constructed to express the open reading frame initiated by each of the putative start codons produced proteins with AAT activity in Escherichia coli. Immune serum raised against the pSAT1-encoded protein reacted with three soybean AAT isozymes, AAT1 (glyoxysomal), AAT2 (cytosolic), and AAT3 (subcellular location unknown). We propose the glyoxysomal isozyme AAT1 is produced by translational initiation from ATG1 and the cytosolic isozyme AAT2 is produced by translational initiation from ATG2. N-terminal sequencing of purified AAT1 revealed complete identity with the pSAT1-encoded protein and was consistent with the processing of the PTS2. Analysis of cytosolic AAT genomic sequences from several other plant species revealed conservation of the two in-frame start codons and the PTS2 sequence, suggesting that these other species may utilize a single gene to generate both cytosolic and glyoxysomal or peroxisomal forms of AAT.     

Dual effects of glucose on the cytosolic Ca2+ activity of mouse pancreatic beta-cells

The cytosolic Ca2+ activity of mouse pancreatic beta-cells was studied with the intracellular fluorescent indicator quin2 . When the extracellular Ca2+ concentration was 1.20 mM, the basal cytosolic Ca2+ activity was 162 +/- 9 nM. Stimulation with 20 mM glucose increased this Ca2+ activity by 40%. In the presence of only 0.20 mM Ca2+ or after the addition of the voltage-dependent Ca2+ -channel blocker D-600, glucose had an opposite and more prompt effect in reducing cytosolic Ca2+ by about 15%. It is concluded that an early result of glucose exposure is a lowering of the cytosolic Ca2+ activity and that this effect tends to be masked by a subsequent increase of the Ca2+ activity due to influx of Ca2+ through the voltage-dependent Ca2+ channels.     

Alpha 1- and beta-adrenergic regulation of intracellular Ca2+ levels in brown adipocytes

In order to monitor changes in cytosolic Ca2+ levels, brown-fat cells were incubated with the fluorescent Ca2+-indicator fura-2 and the fluorescence intensity ratio followed. The addition of norepinephrine led to a rapid and persistent increase in the cytosolic Ca2+ level, which was dose-dependent with a maximal effect at about 1 microM. The response was diminished in the absence of extracellular Ca2+ and was inhibited more efficiently by phentolamine and prazosin than by propranolol or yohimbine, indicating alpha 1-adrenergic mediation. Accordingly, selective alpha 1-adrenergic stimulation also increased the cytosolic Ca2+ level. However, selective beta-adrenergic stimulation, as well as the adenylate cyclase activator forskolin, were also able to increase the cytosolic Ca2+ level in these cells to a certain extent. It was concluded that the major part of the increase in cytosolic Ca2+ was mediated, as in other cell types, via alpha 1-adrenergic receptors, but that Ca2+ levels were also positively modulated by a cAMP-mediated process. These observations are discussed in relation to known alpha 1/beta synergisms in brown adipose tissue.     

Relationship between cellular calcium and prostaglandin synthesis in cultured vascular smooth muscle cells

We have investigated the effects of extracellular and intracellular Ca deficits and of pharmacologic agents thought to inhibit Ca influx or intracellular Ca mobilization on vasopressin-evoked changes of cytosolic Ca2+ levels and PG synthesis in cultured rat mesenteric arterial vascular smooth muscle cells. Vasopressin rapidly increased cytosolic Ca2+ as well as PG synthesis. The increase of cytosolic Ca2+ and the rate of PG synthesis were both maximal within the first minute of incubation. An extracellular Ca deficit of short duration partially inhibited both vasopressin-evoked PG synthesis and the increase of cytosolic Ca2+ by 40 to 60%. Two procedures which deplete cells of some of their intracellular Ca, namely a 30 min incubation in EGTA-supplemented, Ca-lacking media, or a 1 min incubation with ionophore A23187 in Ca-deficient media, decreased PG synthesis by 65% to 100%. The addition of extracellular Ca to Ca-depleted cells restored the ability of vasopressin to stimulate PG synthesis. Two Ca channel antagonists, nifedipine or cinnarizine, had no effect on either vasopressin-evoked PG synthesis or increased cytosolic Ca2+, whereas TMB-8 (10 microM), a putative inhibitor of intracellular Ca mobilization, decreased PG synthesis by 75% by inhibiting acylhydrolase as well as cyclo-oxygenase activities, but had no effect on basal or vasopressin-evoked increase of cytosolic Ca2+, documenting that its inhibitory effect was not a consequence of decreased cytosolic Ca2+. These results demonstrate that decreased cellular Ca levels are associated with decreased cytosolic Ca2+ levels and PG synthesis, and support the hypothesis of a link between, on the one hand, cellular Ca and/or cytosolic Ca2+ and on the other hand, PG synthesis.     

Luminal Ca2+ controls activation of the cardiac ryanodine receptor by ATP

The synergic effect of luminal Ca(2+), cytosolic Ca(2+), and cytosolic adenosine triphosphate (ATP) on activation of cardiac ryanodine receptor (RYR2) channels was examined in planar lipid bilayers. The dose-response of RYR2 gating activity to ATP was characterized at a diastolic cytosolic Ca(2+) concentration of 100 nM over a range of luminal Ca(2+) concentrations and, vice versa, at a diastolic luminal Ca(2+) concentration of 1 mM over a range of cytosolic Ca(2+) concentrations. Low level of luminal Ca(2+) (1 mM) significantly increased the affinity of the RYR2 channel for ATP but without substantial activation of the channel. Higher levels of luminal Ca(2+) (8-53 mM) markedly amplified the effects of ATP on the RYR2 activity by selectively increasing the maximal RYR2 activation by ATP, without affecting the affinity of the channel to ATP. Near-diastolic cytosolic Ca(2+) levels (<500 nM) greatly amplified the effects of luminal Ca(2+). Fractional inhibition by cytosolic Mg(2+) was not affected by luminal Ca(2+). In models, the effects of luminal and cytosolic Ca(2+) could be explained by modulation of the allosteric effect of ATP on the RYR2 channel. Our results suggest that luminal Ca(2+) ions potentiate the RYR2 gating activity in the presence of ATP predominantly by binding to a luminal site with an apparent affinity in the millimolar range, over which local luminal Ca(2+) likely varies in cardiac myocytes.     

Histamine activates H1-receptors to induce cytosolic free calcium transients in cultured vascular smooth muscle cells from rat aorta

Using an intracellularly trapped dye, quin 2, the effects of histamine on cytosolic free calcium concentrations in rat aortic vascular smooth muscle cells in primary culture were recorded, microfluorometrically. When the cells were exposed to histamine, both in the presence and the absence of extracellular Ca2+, there was a rapid, transient and dose-dependent elevation of cytosolic Ca2+ concentrations, with a similar time course. This elevation of cytosolic Ca2+ was dose-dependently inhibited by mepyramine, but not by cimetidine. Thus, histamine activates H1- but not H2- receptors to mediate a release of Ca2+ from the store sites, and there is a rapid and transient elevation of cytosolic Ca2+.     

Cytosolic Ca2+ homeostasis is a constitutive function of the V-ATPase in Saccharomyces cerevisiae

The vacuole is the major site of intracellular Ca(2+) storage in yeast and functions to maintain cytosolic Ca(2+) levels within a narrow physiological range via a Ca(2+) pump (Pmc1p) and a H(+)/Ca(2+) antiporter (Vcx1p) driven by the vacuolar H(+)-ATPase (V-ATPase). We examined the function of the V-ATPase in cytosolic Ca(2+) homeostasis by comparing responses to a brief Ca(2+) challenge of a V-ATPase mutant (vma2Delta) and wild-type cells treated with the V-ATPase inhibitor concanamycin A. The kinetics of the Ca(2+) response were determined using transgenic aequorin as an in vivo cytosolic Ca(2+) reporter system. In wild-type cells, the V-ATPase-driven Vcx1p was chiefly responsible for restoring cytosolic Ca(2+) concentrations after a brief pulse. In cells lacking V-ATPase activity, brief exposure to elevated Ca(2+) compromised viability, even when there was little change in the final cytosolic Ca(2+) concentration. vma2Delta cells were more efficient at restoring cytosolic [Ca(2+)] after a pulse than concanamycin-treated wild-type cells, suggesting long term loss of V-ATPase triggers compensatory mechanisms. This compensation was dependent on calcineurin, and was mediated primarily by Pmc1p.     

Osmotically induced cytosolic free Ca(2+) changes in human neutrophils

Cytosolic free Ca(2+) concentration in neutrophils was measured by ratiometric fluorometry of intracellular fura2. Increasing the extracellular osmolarity, by either NaCl (300-600 mM) or sucrose (600-1200 mM), caused a rise in cytosolic free Ca(2+) (Delta(max) approximately equal to 600 nM). This was not due to cell lysis as the cytosolic free Ca(2+) concentration was reversed by restoration of isotonicity and a second rise in cytosolic free Ca(2+) could be provoked by repeating the change in extracellular osmolarity. Furthermore, the rise in cytosolic free Ca(2+) concentration occurred in the absence of extracellular Ca(2+), demonstrating that release of intracellular fura2 into the external medium did not occur. The osmotically-induced rise in cytosolic free Ca(2+) was not inhibited by either the phospholipase C-inhibitor U73122, or the microfilament inhibitor cytochalasin B, suggesting that neither signalling via inositol tris-phosphate or the cytoskeletal system were involved. However, the rise in cytosolic free Ca(2+) may have resulted from a reduction in neutrophil water volume in hyperosmotic conditions. As these rises in cytosolic Ca(2+) (Delta(max) approximately equal to 600 nM) were large enough to provoke changes in neutrophil activity, we propose that conditions which removes cell water may similarly elevate cytosolic free Ca(2+) to physiologically important levels.     

Ca2+-independent contraction of uterine smooth muscle

Rat uterine smooth muscle shows sustained contraction to oxytocin in Ca2+-free medium with EGTA, that is called "Ca-free contraction"(1). Participation of the rise in cytosolic free Ca2+ in this Ca-free contraction was tested. In Ca-free contraction, the cytosolic free Ca2+ level was not changed at all as measured with fura-2. Further, the chelation of cytosolic free Ca2+ with quin-2 did not at all affect Ca-free contraction. These results strongly suggest that Ca-free contraction is not triggered by Ca2+.     

GTP and GDP will stimulate platelet cytosolic phospholipase C independently of Ca2+

The hydrolysis of [3H]phosphatidylinositol 4,5-bisphosphate (PIP2) by cytosolic phospholipase C from human platelets was determined. Cytosolic fractions were prepared from platelets that had or had not been preactivated with thrombin. Thrombin pretreatment did not affect cytosolic phospholipase C activity. In both cytosolic fractions, phospholipase C was activated by GTP and GTP gamma S. This action is observed in the presence of 2 mM EGTA. GDP was as effective as GTP in stimulating cytosolic phospholipase C in the presence of Ca2+ or EGTA. Partially purified phospholipase C obtained from platelet cytosol is activated by GTP, but not by GTP gamma S, in the presence of 2 mM EGTA. However, in the presence of 6 microM Ca2+, both GTP and GTP gamma S stimulated the partially purified phospholipase C. Our present information indicates that GTP and GDP have a direct effect on the cytosolic phospholipase C.     

Magnesium Sensitizes Slow Vacuolar Channels to Physiological Cytosolic Calcium and Inhibits Fast Vacuolar Channels in Fava Bean Guard Cell Vacuoles

Vacuolar ion channels in guard cells play important roles during stomatal movement and are regulated by many factors including Ca(2+), calmodulin, protein kinases, and phosphatases. We report that physiological cytosolic and luminal Mg(2+) levels strongly regulate vacuolar ion channels in fava bean (Vicia faba) guard cells. Luminal Mg(2+) inhibited fast vacuolar (FV) currents with a K(i) of approximately 0.23 mM in a voltage-dependent manner at positive potentials on the cytoplasmic side. Cytosolic Mg(2+) at 1 mM also inhibited FV currents. Furthermore, in the absence of cytosolic Mg(2+), cytosolic Ca(2+) at less than 10 μM did not activate slow vacuolar (SV) currents. However, when cytosolic Mg(2+) was present, submicromolar concentrations of cytosolic Ca(2+) activated SV currents with a K(d) of approximately 227 nM, suggesting a synergistic Mg(2+)-Ca(2+) effect. The activation potential of SV currents was shifted toward physiological potentials in the presence of cytosolic Mg(2+) concentrations. The direction of SV currents could also be changed from outward to both outward and inward currents. Our data predict a model for SV channel regulation, including a cytosolic binding site for Ca(2+) with an affinity in the submicromolar range and a cytosolic low-affinity Mg(2+)-Ca(2+) binding site. SV channels are predicted to contain a third binding site on the vacuolar luminal side, which binds Ca(2+) and is inhibitory. In conclusion, cytosolic Mg(2+) sensitizes SV channels to physiological cytosolic Ca(2+) elevations. Furthermore, we propose that cytosolic and vacuolar Mg(2+) concentrations ensure that FV channels do not function as a continuous vacuolar K(+) leak, which would prohibit stomatal opening.     

Mechanism of neutrophil activation by an unopsonized inflammatory particulate. Monosodium urate crystals induce pertussis toxin-insensitive hydrolysis of phosphatidylinositol 4,5-bisphosphate.

Monosodium urate crystals are believed to trigger acute inflammation via the direct stimulation of leukocytes. Unopsonized urate crystals activate neutrophil (PMN) membrane G proteins in a pertussis toxin (PT)-sensitive manner, but induce PT-insensitive cytosolic [Ca2+]i elevation. Thus, we have further defined the mechanism of PMN responsiveness to urate crystals in this study. Though urate crystals can increase membrane permeability by lytic effects, we observed elevation of PMN cytosolic [Ca2+]i in the absence of extracellular [Ca2+]i. In addition, the early, crystal-induced cytosolic [Ca2+]i transient was buffered in cells loaded with a [Ca2+]i-chelator. This suggested mobilization of internal [Ca2+]i stores, which was supported by demonstrating rapid phosphatidylinositol bisphosphate (PIP2) hydrolysis, and the formation of inositol (1,4,5) trisphosphate (as well as phosphatidic acid) in a PT-insensitive manner. Importantly, PMN activation by urate crystals was discriminatory, as evidenced by the absence of phosphatidylinositol trisphosphate formation, a PT-sensitive event triggered by chemotactic factors. Urate crystal-induced PIP2 hydrolysis was not a nonspecific consequence of the early cytosolic [Ca2+]i transient itself, and it did not require phagocytosis. However, crystal-induced O2- release was markedly inhibited by buffering of the early cytosolic [Ca2+]i transient under conditions where crystal phagocytosis and PMA-induced O2- release were unaffected. We conclude that urate crystals activate PT-insensitive PIP2 hydrolysis, resulting in IP3 generation, and early urate crystal-induced mobilization of cytosolic [Ca2+]i. This pathway appears to modulate crystal-induced O2- release.     

Mechanisms for activation and subsequent removal of cytosolic Ca2+ in bradykinin-stimulated neuronal and glial cell lines

Mechanisms for activation and for removal of cytosolic Ca2+ after stimulation with bradykinin were investigated in two neural cell lines by measuring cytosolic Ca2+ activity and 45Ca2+ fluxes. In the neuronal (neuroblastoma x glioma hybrid) and in the glial (rat glioma) cell lines, the transient, bradykinin-induced rise in cytosolic Ca2+ activity (determined by fura-2 or indo-1 fluorescence) was blocked by a bradykinin B2 receptor antagonist. Ca2+ ionophores (ionomycin and 4-Br-A23187) caused a comparable transient rise in cytosolic Ca2+ activity. After addition of ionophores, the Ca2+ response to bradykinin was reduced or completely blocked in both cell lines. At the concentrations used, the ionophores primarily depleted intracellular Ca2+ stores and prevented refilling of the stores. Thus, the bradykinin-induced rise of cytosolic Ca2+ activity seems to be mostly due to Ca2+ release from internal stores. In the neuronal but not in the glial cell line, a brief stimulation by bradykinin of 45Ca2+ uptake was followed by a long-lasting inhibition below control values. Thus, in the neuronal cells bradykinin presumably blocks Ca2+ channels by a readily reversible, pertussis toxin-insensitive mechanism. Excess cytosolic Ca2+ of the bradykinin-stimulated cells is mostly not resequestered into the internal Ca2+ pool accessible to bradykinin, but is mainly extruded through the plasma membrane, as indicated by (i) stimulation of 45Ca2+ release by bradykinin, (ii) quick reduction by bradykinin of cellular 45Ca2+ content of cells preequilibrated with 45Ca2+, and (iii) diminution of the ionophore-inducible Ca2+ response after the addition of bradykinin.     

Calcium mobilization and catecholamine secretion in adrenal chromaffin cells. A Quin-2 fluorescence study

To better understand the relation between cell calcium and exocytotic secretion, a quantitative dependence of adrenal catecholamine secretion on cytosolic free calcium has been determined for isolated, intact, bovine chromaffin cells, using the fluorescent probe Quin-2. The cells required a threshold of 250-300 nM cytosolic calcium to be reached before detectable secretion occurred and half-maximal secretion occurred near 2 microM cytosolic calcium. Nicotinic receptors mediated an increase of cytosolic calcium from resting levels near 100 nM to levels in the 1-10 microM range within seconds followed by a decay back to resting levels over several minutes. Muscarinic receptors mediated a smaller rise in cytosolic free calcium from 100 to about 200 nM, within seconds. The nicotinic response required extracellular calcium, while the muscarinic response was largely independent of extracellular calcium, suggesting the latter mobilizes intracellular calcium. The acetylcholine-evoked rise in cytosolic calcium decayed by at least two kinetically distinct processes with half-time constants: t1 = 0.6 min and t2 = 3.2 min. Extracellular Na+ deprivation caused a more prolonged elevation of the acetylcholine-evoked calcium transient, suggesting a possible role of Na+/Ca2+ exchange and/or other Na+ -dependent processes in lowering cytosolic calcium following stimulation. The possible perturbing effects of Quin-2 on resting and stimulated cytosolic calcium levels and on secretion were examined and a novel use of Quin-2 to measure membrane calcium flux was demonstrated.     

Phosphatidylinositol-specific phospholipase C of murine lymphocytes

Phosphatidylinositol-specific phospholipase C (PI-phospholipase C) was found primarily in the cytosolic fraction of murine splenic lymphocytes. However, small but significant amounts of the activity of the enzyme were detected in the microsome and plasma membrane fractions. Both the cytosolic and membrane-bound phospholipases C specifically hydrolyzed inositol phospholipids, phosphatidylinositol, phosphatidylinositol 4-phosphate, and phosphatidylinositol 4,5-bisphosphate. PI-Phospholipase C activity was detected in the cytosolic and microsome fractions from both T-cell-enriched and B-cell-enriched spleen cells. The membrane-bound enzyme was distinguishable from the cytosolic enzyme in the following properties. The cytosolic PI-phospholipase C showed optimal activity at pH 6.0 while the membrane-bound enzyme had two pH optima between pH 5.0 and 7.0. The activity of the cytosolic enzyme was first detected at 1 microM Ca2+, and maximum activity was observed at 100 microM Ca2+, while the membrane-bound PI-phospholipase C required higher Ca2+ concentrations, of millimolar order. The membrane-bound enzyme could hardly be extracted with 1 M NaCl but was extracted with 0.4% cholate.A portion of the membrane-bound PI-phospholipase C activity in the cholate extract was absorbed by concanavalin A-Sepharose and specifically eluted with an alpha-methylmannoside solution. The cytosolic enzyme, which was water soluble, did not bind to concanavalin A-Sepharose. Trypsinization of lymphocytes before subcellular fractionation caused a significant decrease in the PI-phospholipase C activity in the microsome fraction but almost no loss at all of the cytosolic enzyme activity.     

Cytosolic phospholipase C activity: I. Evidence for coupling with cytosolic guanine nucleotide-binding protein,Giα

In a previous report we shwed that glucocorticoed inhibition of cytosolic PLC activity correlated with a reduction in cytosolic Giα levels, suggesting that there may be a functional relationship between cytosolic PLC and cytosolic Giα. In order to establish the nature of the coupliing between cytosolic Giα and cytosolic PLC we examined the effects of Protein activators, and inhibitors on cytosolic PLC activity from rat spenocytes and the rat lymphoma cell line Nb 2, with [³H] PI and [³H]PIP₂ as substrates. (1) Neither GTP nor its nonhydrolyzable analogue, GTPγS, at 100 μm had any effect on the calcium stimulated as well as the basal PLC activity. (2) Howevr, affinity purified antibodies to Giα1 and Giα2 inhibited soluble PLC activity, by 85% and 55%, respectively, with PI as substrate; with PIP₂ as substrate, soluble PLC activity was inhibited 50–70% by antibodies to Gi1, whereas antibodies to Gi2 had little effect. (3)Administration of Giα1 antisense oligonucleotides to splenocytes for 48 h produced 25–40% decrease in cytosolic Giα1 levels compared to control. The soluble PLC activity with both PI and PIP₂ as substrates was also reduced by 25–50% compared to control conditions. This suggest that cytosolic Giα is associated with the activation of splenocyte soluble PLC. (4) Pertussis toxin administered in vivo sugnificantly reduced cytosolic Giα immunoreactivity and soluble PLC activiry when PI was used as substrate, providing additional evidence that cytosolic Giα is associated with the activation of splencyte soluble PLC. (5) Another agent that has beeen used extensively to define G-protein coupled processes is NaF/AlCl₃. NaF(4mM; with or without AlCl₃ inhibited soluble PLC activity with PIP₂ as substrate, in contrast ot the stimulatory effect that has been reported in the activation of membrane PLC. 6) because NaF can act as a protein phosphatase inhibitor, we also tested the effects of trifluoperzine (50 μm, TFP), an inhibitor of protein phosphatase 2B; TFP (50 μm) signigicantly inhibited soluble PLC activity PI was used as substrate. These results suggest a direct involvement of cytosolic Giα in the activation of soluble PLC form splenocytes. Other questions pertaining to the functional significance, the nature, and possible substrate preference of the splenocyte Giα coupled PLC is addressed in the second paper.     

Transient increase in Zn2+ in hippocampal CA1 pyramidal neurons causes reversible memory deficit

The translocation of synaptic Zn(2+) to the cytosolic compartment has been studied to understand Zn(2+) neurotoxicity in neurological diseases. However, it is unknown whether the moderate increase in Zn(2+) in the cytosolic compartment affects memory processing in the hippocampus. In the present study, the moderate increase in cytosolic Zn(2+) in the hippocampus was induced with clioquinol (CQ), a zinc ionophore. Zn(2+) delivery by Zn-CQ transiently attenuated CA1 long-term potentiation (LTP) in hippocampal slices prepared 2 h after i.p. injection of Zn-CQ into rats, when intracellular Zn(2+) levels was transiently increased in the CA1 pyramidal cell layer, followed by object recognition memory deficit. Object recognition memory was transiently impaired 30 min after injection of ZnCl(2) into the CA1, but not after injection into the dentate gyrus that did not significantly increase intracellular Zn(2+) in the granule cell layer of the dentate gyrus. Object recognition memory deficit may be linked to the preferential increase in Zn(2+) and/or the preferential vulnerability to Zn(2+) in CA1 pyramidal neurons. In the case of the cytosolic increase in endogenous Zn(2+) in the CA1 induced by 100 mM KCl, furthermore, object recognition memory was also transiently impaired, while ameliorated by co-injection of CaEDTA to block the increase in cytosolic Zn(2+). The present study indicates that the transient increase in cytosolic Zn(2+) in CA1 pyramidal neurons reversibly impairs object recognition memory.     

Regulation of cytosolic phospholipase A2 in rat endometrial stromal cells: the role of epidermal growth factor

The effect of epidermal growth factor on the levels of cytosolic phospholipase A2 mRNA and protein in cultured rat endometrial stromal cells isolated from uteri sensitized for the decidual cell reaction was examined. Treatment with epidermal growth factor increased the steady-state cytosolic phospholipase A2 mRNA and protein levels as demonstrated by Northern and Western blot analyses, respectively. Immunocytochemical analysis demonstrated an increase of cytosolic phospholipase A2 protein in most cells, as opposed to a small subpopulation of cells in culture. These results show that epidermal growth factor causes an increase in steady-state cytosolic phospholipase A2 mRNA and protein levels in rat endometrial stromal cells from uteri sensitized for the decidual cell reaction. Epidermal growth factor receptor ligands may regulate cytosolic phospholipase A2 and thus prostaglandin production in the endometrial stromal cells during implantation.     

Partial purification and characterization of membrane-bound and cytosolic phosphatidylinositol-specific phospholipases C from murine thymocytes

Membrane-bound and cytosolic phosphatidylinositol (PI)-specific phospholipases C in murine thymocytes have been partially purified and characterized. The membrane-bound enzyme was extracted from microsomes with sodium cholate and purified by sequential column chromatographies on Sephadex G-100, heparin-Sepharose CL-6B, and Sephadex G-100. The cytosolic enzyme was purified from the cytosol by sequential column chromatographies on Sephadex G-100 and FPLC-Mono S. Specific activities of the membrane-bound enzyme and the cytosolic enzyme increased more than 1,800- and 1,400-fold, respectively, compared with those of microsomes and the cytosol. The molecular weights of the both enzymes were estimated to be about 70,000 by gel filtration. These purified enzymes also hydrolyzed phosphatidylinositol 4,5-bisphosphate (PIP2). At neutral pH and low Ca2+ concentrations, the membrane-bound enzyme hydrolyzed PIP2 in preference to PI and showed higher activity than the cytosolic enzyme. These activities were also affected differently by various lipids. For PIP2 hydrolysis, all lipids investigated except lysophosphatidylcholine enhanced the activity of the membrane-bound enzyme, while phosphatidylcholine (PC) and phosphatidylserine (PS) did not significantly affect the activity of the cytosolic enzyme. PC, PE, and PS inhibited the activities of the membrane-bound and cytosolic enzymes for PI hydrolysis. The physiological implications of these results are discussed.     

Swelling of mitochondria in cultured rat hippocampal astrocytes is induced by high cytosolic Ca(2+) load,but not by mitochondrial depolarization

The influence of cytosolic Ca(2+) load and of mitochondrial membrane potential change on mitochondrial morphology was investigated in cultured rat hippocampal astrocytes. The uncoupler FCCP, applied together with oligomycin, depolarized mitochondria rapidly but did not change their morphology. Depolarization was associated with a moderate cytosolic [Ca(2+)](i) rise of up to 0.3 microM. Only high cytosolic Ca(2+) load (above a threshold of 50 microM), which was evoked by application of the ionophore 4-Br-A23187 in Ca(2+)-containing medium, caused drastic change of mitochondrial morphology. The shape change from the typical rod-like to a spherical shape, indicating mitochondrial swelling, was associated with depolarization. Cyclosporin A sensitivity suggests involvement of permeability transition. Thus, a dramatic cytosolic [Ca(2+)](i) rise is required to induce mitochondrial swelling and depolarization. A large but still moderate [Ca(2+)](i) rise evoked by physiological stimulation, however, has no comparable effect.