Subcellular distribution of the calcium-storing inositol 1,4,5-trisphosphate-sensitive organelle in rat liver. Possible linkage to the plasma membrane through the actin microfilaments

The inositol triphosphate receptor family

Specific receptors on intracellular membranes mediate the Ca2+ mobilization induced by the second messenger molecule D-myo-inositol 1,4,5-triphosphate (IP3). Most cell types appear to contain multiple receptor isoforms. The review summarizes recent progress on IP3 receptor biology with a particular emphasis on distinctive structural and regulatory features of the individual isoforms.

Capacitative calcium entry

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Mast cell activation involves plasma membrane potential- and thapsigargin-sensitive intracellular calcium pools

The regulation and role of the intracellular Ca2+ pools were studied in rat peritoneal mast cells. Cytosolic free calcium concentration ([Ca2+]i) was monitored in fura-2 loaded mast cells. In the presence of Ca2+ and K+, compound 48/80 induced a biphasic increase in [Ca2+]i composed of a fast transient phase and an apparent sustained phase. The sustained phase was partially inhibited by the addition of Mn2+. DTPA, a cell-impermeant chelator of Mn2+, reversed this inhibition, suggesting that a quenching of fura-2 fluorescence occurs in the extracellular medium. In the absence of extracellular Ca2+, the transient phase, but not the sustained one, could be preserved, provided that mast cells were depolarized. The transient phase was completely abolished by thapsigargin, a microsomal Ca(2+)-ATPase inhibitor. Maximum histamine release induced by either compound 48/80 or antigen was obtained in the absence of added Ca2+ only when mast cells were depolarized. These histamine releases were inhibited by low doses (< 30 nM) of thapsigargin. Thapsigargin at higher doses induced histamine release which was unaffected by changing the plasma membrane potential, but was completely dependent on extracellular Ca2+, showing that a Ca2+ influx is required for thapsigargin-induced exocytosis. Together, these results suggest that the mobilization of Ca2+ from thapsigargin sensitive-intracellular pools induced by compound 48/80 or antigen is sufficient to trigger histamine release. The modulation of these pools by the plasma membrane potential suggest their localization is close to the plasma membrane.

Evidence that the pertussis toxin-sensitive trimeric GTP-binding protein Gi2 is required for agonist- and store-activated Ca2+ inflow in hepatocytes

The role of a trimeric GTP-binding protein (G-protein) in the mechanism of vasopressin-dependent Ca2+ inflow in hepatocytes was investigated using both antibodies against the carboxyl termini of trimeric G-protein alpha subunits, and carboxyl-terminal alpha-subunit synthetic peptides. An anti-Gi1-2 alpha antibody and a Gi2 alpha peptide (Gi2 alpha) Ile345-Phe355), but not a Gi3 alpha peptide (Gi3 alpha Ile344-Phe354), inhibited vasopressin- and thapsigargin-stimulated Ca2+ inflow, had no effect on vasopressin-stimulated release of Ca2+ from intracellular stores, and caused partial inhibition of thapsigargin-stimulated release of Ca2+. An anti-Gq alpha antibody also inhibited vasopressin-stimulated Ca2+ inflow and partially inhibited vasopressin-induced release of Ca2+ from intracellular stores. Immunofluorescence measurements showed that Gi2 alpha is distributed throughout much of the interior of the hepatocyte as well as at the periphery of the cell. By contrast, Gq/11 alpha was found principally at the cell periphery. It is concluded that the trimeric G-protein, Gi2, is required for store-activated Ca2+ inflow in hepatocytes and acts between the release of Ca2+ from the endoplasmic reticulum (presumably adjacent to the plasma membrane) and the receptor-activated Ca2+ channel protein(s) in the plasma membrane.

Role of cyclic GMP in the control of capacitative Ca2+ entry in rat pancreatic acinar cells

We have investigated the possible roles of cyclic GMP (cGMP) in initiating or regulating capacitiative Ca2+ entry in rat pancreatic acinar cells. In medium containing 1.8 mM external Ca2+, thapsigargin activated Ca2+ entry and slightly but significantly increased intracellular cGMP concentration. This rise in cGMP levels was prevented by pretreating the cells with the guanylate cyclase inhibitor, LY-83583, or by omitting Ca2+ during stimulation by thapsigargin or methacholine. LY-83583 and NG-nitro-L-arginine (L-NA, an inhibitor of NO synthase) both had a small inhibitory effect on Ca2+ entry when they were added after thapsigargin in Ca2(+)-containing medium, and they reduced by 32 and 48% respectively the thapsigargin-induced capacitative Ca2+ entry when added to the cells during a 20 min preincubation period. However, neither dibutyryl cGMP (Bt2cGMP) nor sodium nitroprusside, an NO mimic, affected either basal intracellular Ca2+ concentration [Ca2+]i or thapsigargin-induced capacitative Ca2+ entry. Further, the inhibitory effects observed after preincubation with LY-83583 or L-NA could not be prevented by preincubation with Bt2cGMP, nor could they be reversed by adding Bt2cGMP, 8-bromo-cGMP or sodium nitroprusside acutely after activation of capacitative Ca2+ entry by thapsigargin. Finally, pretreatment of cells with LY-83583 or L-NA did not affect Ca2+ signalling in response to 1 microM methacholine, including the pattern of [Ca2+]i oscillations. In conclusion, in pancreatic acinar cells, the rise in cellular cGMP levels appears to depend on, rather than cause, the increase in [Ca2+]i with agonist stimulation.

Accumulation of phosphatidylalcohol in cultured cells: use of subcellular fractionation to investigate phospholipase D activity during signal transduction

Phosphatidylalcohol accumulates as a product of a phospholipase D (PLD)-catalysed transphosphatidylation reaction in cells incubated in the presence of a primary alcohol. In the presence of ethanol the phorbol ester, phorbol 12-myristate 13-acetate (PMA), stimulated the accumulation of [3H]phosphatidylethanol (PEth) in HeLa cells prelabelled with [3H]palmitic acid. Radioactivity associated with PEth increased linearly during a 30 min incubation, indicating that a sustained activation of PLD is caused by PMA in these cells. This was accompanied by the membrane association of protein kinase C-alpha (PKC-alpha), the PKC isoform that recent studies indicate is involved in the activation of PLD. In similar experiments, the neuropeptide bradykinin stimulated an accumulation of PEth in 3T3 Li cells. The radioactivity associated with PEth increased to a maximal level at 30 s and plateaued after this time, suggesting that bradykinin induces only a transient activation of PLD in these cells. This is consistent with the effects of bradykinin on PKC-alpha, which underwent a rapid and transient association with cell membranes. The subcellular localization of PEth was examined using the technique of subcellular fractionation on Percoll density gradients to isolate organelle-enriched fractions from HeLa and 3T3 Li cells. An accumulation of [3H]PEth was measured in the plasma-membrane (PM)-enriched fractions of both HeLa and 3T3 Li cells after incubation with PMA and bradykinin respectively. This was accompanied by a time-dependent accumulation of [3H]PEth in the combined mitochondrial and endoplasmic reticulum (MER)-enriched fractions of both cell lines. PMA was also found to cause translocation of PKC-alpha to both the PM- and MER-enriched fractions in HeLa cells. However, bradykinin stimulated the translocation of PKC-alpha to the PM-enriched fractions only of 3T3 Li cells. The results show that PLD activation leads to the accumulation of PEth in both the PM and MER fractions. We therefore propose that either bradykinin activates a PM-associated PLD and the PLD reaction product is rapidly translocated to other membrane systems or it activates an MER-associated PLD by a mechanism that does not involve PKC-alpha.

Inositol phosphates in the heart: controversy and consensus

Numerous studies have addressed various aspects of inositol phosphate release and metabolism in myocardial preparations, and many different viewpoints have been expressed. The various results and interpretations presented often appear confusing and extracting a consensus view can be difficult. The differences often derive from the differing cardiac preparations used, especially isolated cells versus intact tissue. Despite these problems there are aspects where consensus prevails. Both the metabolism and the functional activity of inositol phosphates in heart appear to differ from those previously described in non-excitable cells. Inositol phosphates do not appear to be of major importance in the control of cardiac function under physiological conditions but may well have greater influence under pathological conditions such as myocardial ischaemia and reperfusion. Hopefully, the near future will see remaining controversies resolved.

Inositol 1,4,5-trisphosphate receptors, secretory granules and secretion in endocrine and neuroendocrine cells

Recent studies have revealed the presence of inositol 1,4,5-trisphosphate receptors in the secretory granules of endocrine and neuroendocrine cells. This distribution suggests that inositol 1,4,5-trisphosphate-regulated release of granule stores of Ca2+ might facilitate the secretory process. In addition, inositol 1,4,5-trisphosphate receptors might participate directly in the biogenesis of secretory granules. The presence of inositol 1,4,5-trisphosphate receptors in synaptic nerve terminals raises the possibility that they might also be involved in the control of neurotransmitter release.

Identification of the ankyrin-binding domain of the mouse T-lymphoma cell inositol 1,4,5-trisphosphate (IP3) receptor and its role in the regulation of IP3-mediated internal Ca2+ release

In this study we have used several complementary techniques to explore the interaction between the membrane linker molecule, ankyrin, and the inositol 1,4,5-trisphosphate (IP3) receptor in mouse T-lymphoma cells. Using double immunolabeling and laser confocal microscopy, we have found that both cytoplasmic IP3 receptor and ankyrin are preferentially accumulated within ligand-induced lymphocyte receptor-capped structures. The binding between ankyrin and IP3 receptor appears to be very specific. Further analyses indicate that the amino acid sequence GGVGDVLRKPS in the IP3 receptor shares a great deal of structural homology with the ankyrin-binding domain located in certain well characterized ankyrin-binding proteins such as the cell adhesion molecule, CD44. Biochemical studies using competition binding assays and a synthetic peptide identical to GGVGDVLRKPS (a sequence detected in rat brain IP3 receptor (amino acids 2548-2558) and mouse brain IP3 receptor (amino acids 2546-2556)) indicate that this 11-amino acid peptide binds specifically to ankyrin (but not fodrin or spectrin). Furthermore, this peptide competes effectively for ankyrin binding to IP3 receptor-containing vesicles and/or purified IP3 receptor, and it blocks ankyrin-induced inhibitory effects on IP3 binding and IP3-mediated internal Ca2+ release in mouse T-lymphoma cells. These findings suggest that this amino acid sequence, GGVGDVLRKPS, which is located close to the C terminus of the IP3 receptor, resides on the cytoplasmic side (not the luminal side) of IP3 receptor-containing vesicles. This unique region appears to be an important part of the IP3 receptor ankyrin-binding domain and may play an important role in the regulation of IP3 receptor-mediated internal Ca2+ release during lymphocyte activation.

Differences in uptake, storage and release properties between inositol trisphosphate-sensitive and -insensitive Ca2+ stores in permeabilized pancreatic acinar cells

Rabbit pancreatic acinar cells, permeabilized by saponin treatment, were used to study the kinetics of ATP-dependent Ca2+ uptake and release in inositol 1,4,5-trisphosphate (Ins-1,4,5-P3)-sensitive and -insensitive stores. Permeabilized acinar cells rapidly accumulated Ca2+ to steady-state. At steady state, approximately 60% of actively stored Ca2+ resided in the Ins-1,4,5-P3-sensitive store. Kinetic analysis of the Ca2+ uptake process revealed that the initial Ca2+ uptake rate was 1.7 times higher in the Ins-1,4,5-P3-insensitive store as compared to the Ins-1,4,5-P3-sensitive store. On the other hand, the Ca2+ uptake capacity was 1.6 times higher in the Ins-1,4,5-P3-sensitive store as compared to the Ins-1,4,5-P3-insensitive store. The Ca2+ uptake rate in the Ins-1,4,5-P3-sensitive store remained virtually constant for at least 4 min, whereas in the Ins-1,4,5-P3-insensitive Ca2+ store this rate progressively declined with time. These observations are compatible with: (i) an Ins-1,4,5-P3-sensitive store containing relatively few Ca2+ pumps but possessing a relatively high Ca2+ uptake capacity, which may reflect the presence of a substantial amount of Ca2+ binding protein; and (ii) an Ins-1,4,5-P3-insensitive Ca2+ store containing relatively many Ca2+ pumps but possessing a relatively low Ca2+ uptake capacity, which may reflect the presence of little if any Ca2+ binding protein. The data presented are consistent with the idea of a heterogeneous distribution of Ca2+ pumps, Ca2+ binding proteins and Ca2+ release channels between intracellular Ca2+ storage organelles.

Inositol 1,4,5-trisphosphate receptor-like protein in plasmalemmal caveolae is linked to actin filaments

We reported that a plasmalemmal inositol 1,4,5-trisphosphate receptor-like protein (PM InsP3R-L) is localized in caveolae of various non-neuronal cells in vivo (Fujimoto et al. (1992) J. Cell Biol. 119, 1507–1513). In the present study, we investigated the distribution of PM InsP3R-L in cultured cells. In mouse epidermal keratinocytes (Pam 212) cultured in standard Ca2+ (1.8 mM), PM InsP3R-L was distributed densely in the vicinity of cell-to-cell contacts. In contrast, when Pam cells were cultured in low Ca2+ (0.06 mM) without making cell-to-cell contacts, PM InsP3R-L was observed randomly; by restoring the Ca2+ concentration, the circumferential actin filaments became obvious and the density of PM InsP3R-L increased in the contact region. Treatment of Pam cells with cytochalasin D caused aggregation of caveolae where PM InsP3R-L as well as F-actin and fodrin were localized. In bovine aortic endothelial cells, PM InsP3R-L was aligned along actin filaments crossing the cytoplasm in various directions. PM InsP3R-L of Pam cells was hardly extracted by treatment with 0.5% Triton X-100 or 60 mM octyl-glucoside in a cytoskeleton-stabilizing buffer for 15 minutes at 4 degrees C. The results show that the distribution of caveolae bearing PM InsP3R-L changes when the actin cytoskeleton is modified. They also indicate that the association of PM InsP3R-L with actin filaments may mediate the redistribution of caveolae. Since caveolae are thought to be related to signal transduction, their location defined by the actin cytoskeleton may affect the site where cellular reaction is to occur in response to various stimuli.

The role of actin filaments in the organization of the endoplasmic reticulum in honeybee photoreceptor cells

Close to the bases of the photoreceptive microvilli, arthropod photoreceptors contain a dense network of endoplasmic reticulum that is involved in the regulation of the intracellular calcium concentration, and in the biogenesis of the photoreceptive membrane. Here, we examine the role of the cytoskeleton in organizing this submicrovillar endoplasmic reticulum in honeybee photoreceptors. Immunofluorescence microscopy of taxol-stabilized specimens, and electron-microscopic examination of high-pressure frozen, freeze-substituted retinae demonstrate that the submicrovillar cytoplasm lacks microtubules. The submicrovillar region contains a conspicuous F-actin system that codistributes with the submicrovillar endoplasmic reticulum. Incubation of retinal tissue with cytochalasin B leads to depolymerization of the submicrovillar F-actin system, and to disorganization and disintegration of the submicrovillar endoplasmic reticulum, indicating that an intact F-actin cytoskeleton is required to maintain the architecture of this domain of the endoplasmic reticulum. We have also developed a permeabilized cell model in order to study the physiological requirements for the interaction of the endoplasmic reticulum with actin filaments. The association of submicrovillar endoplasmic reticulum with actin filaments appears to be independent of ATP, Ca2+ and Mg2+, suggesting a tight static anchorage.

Cytochalisin D exerts stimulatory and inhibitory effects on insulin-induced glucokinase mRNA expression in hepatocytes

The microfilament cytoskeleton is postulated to have a role in the localization, transport and anchorage of certain specific mRNAs. We investigated the effects of cytochalasin D, a fungal metabolite that binds to actin and disrupts the microfilament structure, on insulin-induced expression of glucokinase mRNA in rat hepatocyte cultures. Cytochalasin-D significantly potentiates insulin-induced glucokinase mRNA expression at 100 nM concentration but counteracts glucokinase expression at 2-20 microM. The latter effect is at least in part due to an increase in glucokinase mRNA degradation. This effect of cytochalasin D cannot be accounted for by an increase in cAMP and is also not due to a non-specific effect on mRNA degradation since albumin mRNA levels were not affected by cytochalasin-D and actin mRNA and tubulin mRNA levels were increased. Measurement of glucokinase mRNA release from digitonin-permeabilized hepatocytes suggests that cytochalasin D does not cause acute dissociation of glucokinase mRNA from its binding site. The increased degradation of glucokinase mRNA suggests involvement of the cytoskeleton in glucokinase mRNA stability. However, an additional effect of cytochalasin D on the insulin signalling mechanism cannot be excluded.

Calcium mobilization by inositol phosphates and other intracellular messengers

Inositol 1,4,5-trisphosphate [Ins(1,4,5)P(3)] is now widely recognized as a messenger controlling the release of calcium from intracellular stores. In oocytes, and also probably in excitable cells, another potential calcium-mobilizing messenger is cyclic ADP ribose, although there is as yet little evidence that its levels are regulated by hormones or other extracellular mediators. In addition to signaling intracellular calcium release, [Ins(1,4,5)P(3)] also regulates calcium entry across the plasma membrane, but not in a direct manner. Rather, the depletion of intracellular stores by the calcium-mobilizing action of [Ins (1,4,5)P(3)] initiates a process of retrograde signaling whereby the depleted stores generate or release a diffusible messenger that is believed to act on the plasma membrane. A phosphorylated metabolite of [Ins(1,4,5)P(3)], inositol 1,3,4,5-tetrakisphosphate [Ins(1,3,4,5)P(4)], has been proposed to modulate this process, but the literature is not consistent on this point. A recently proposed candidate for the retrograde messenger is an activity extracted from Jurkat cells termed CIF (calcium influx factor), which has many properties consistent with such a messenger. There is also evidence that a GTP-dependent process, possibly involving a small G protein, is involved in signaling calcium entry and may be involved in either the formation or action of the diffusible messenger for calcium entry.

A role for a pertussis toxin-sensitive trimeric G-protein in store-operated Ca2+ inflow in hepatocytes

The mechanism of store-operated Ca2+ inflow in hepatocytes was investigated using fluo-3 and fura-2 to monitor changes in the concentration of intracellular free Ca2+ in single cells, and 1-(alpha-glycerophosphoryl)-myo-inositol 4,5-diphosphate, P4(5)-1-(2-nitrophenyl)ethyl ester ('caged' GPIP2) and 'caged' guanosine 5'-[gamma thio]triphosphate (GTP gamma S) (introduced into the cytoplasmic space by microinjection), thapsigargin and 2,5-di-tert- butylhydroquinone (DBHQ) to stimulate Ca2+ inflow. Photolysis of 'caged' GPIP2 or 'caged' GTP gamma S stimulated Ca2+ inflow. The abilities of GPIP2, thapsigargin and DBHQ to stimulate Ca2+ inflow were inhibited by the pre-treatment of hepatocytes with pertussis toxin in vivo for 36 h. Thapsigargin-stimulated Ca2+ inflow was also inhibited by guanosine 5'-[beta-thio]diphosphate (GDP beta S) (introduced by microinjection). It is concluded that, in hepatocytes, store-operated Ca2+ inflow induced by the actions of either inositol 1,4,5-trisphosphate, thapsigargin or DBHQ requires a pertussis toxin-sensitive trimeric G-protein.

Plasmalemmal dihydropyridine receptors modify the function of subplasmalemmal inositol 1,4,5-trisphosphate receptors: a hypothesis

Experimental observations on rat glomerulosa cells inspired a model which postulates that plasmalemmal dihydropyridine receptors are in juxtaposition and interaction with inositol 1,4,5-trisphosphate receptors in subplasmalemmal calciosomes. Activation of dihydropyridine receptors promotes the Ca2+ releasing effect of inositol 1,4,5-trisphosphate. The most important observations compatible with the model are the following: (1) angiotensin II does not influence Ca2+ influx during the peak phase of Ca2+ signal; (2) dihydropyridine drugs modify the initial peak of the Ca2+ signal induced by angiotensin II; (3) inhibitors of the dihydropyridine receptor reduce the initial Ca2+ signal also in the presence of 5 mM Ni2+, an inhibitor of voltage dependent Ca2+ influx; and (4) changes in extracellular K+ concentration within the physiological range also modify the cytoplasmic Ca2+ response to angiotensin II.

A slowly ADP-ribosylated pertussis-toxin-sensitive GTP-binding regulatory protein is required for vasopressin-stimulated Ca2+ inflow in hepatocytes

The roles of heterotrimeric GTP-binding regulatory proteins (G-proteins) and inositol polyphosphates in the mechanism by which vasopressin stimulates Ca2+ inflow in hepatocytes were investigated by using single cells loaded with fura2 by microinjection. Vasopressin-stimulated Ca2+ inflow was mimicked by microinjection of guanosine 5'-[gamma-thio]triphosphate (GTP[S]) or guanosine 5'-[beta gamma-imido]triphosphate to the cells, but not adenosine 5'-[gamma-thio]triphosphate (ATP[S]) or guanosine 5'-[beta-thio]diphosphate (GDP[S]). Extracellular Gd3+ (5 microM) inhibited both vasopressin- and GTP[S]-stimulated Ca2+ inflow. GDP[S], but not GMP, administered to hepatocytes by microinjection, completely inhibited vasopressin-stimulated Ca2+ inflow and partially inhibited vasopressin-induced release of Ca2+ from intracellular stores. The microinjection of pertussis toxin had no effect either on the release of Ca2+ from intracellular stores or on Ca2+ inflow induced by vasopressin, but completely inhibited changes in these processes induced by epidermal growth factor (EGF). Hepatocytes isolated from rats treated with pertussis toxin for 24 h exhibited no vasopressin- or GTP[S]-stimulated Ca2+ inflow, whereas the vasopressin-stimulated release of Ca2+ from intracellular stores was similar to that observed for control cells. Heparin or ATP[S] inhibited, or delayed the onset of, both vasopressin-induced release of Ca2+ from intracellular stores and vasopressin-stimulated Ca2+ inflow. Vasopressin-induced oscillations in intracellular [Ca2+] were observed in some heparin-treated cells. It is concluded that the stimulation by vasopressin of Ca2+ inflow to hepatocytes requires inositol 1,4,5-trisphosphate (InsP3) and, by implication, the pertussis-toxin-insensitive G-protein required for the activation of phospholipase C beta [Taylor, Chae, Rhee and Exton (1991) Nature (London) 350, 516-518], and another G-protein which is slowly ADP-ribosylated by pertussis toxin and acts between InsP3 and the putative plasma-membrane Ca2+ channel. EGF-stimulated Ca2+ inflow involves at least one G-protein which is rapidly ADP-ribosylated and is most likely required for InsP3 formation.

Inhibition of glycogen synthesis by epidermal growth factor in hepatocytes. The role of cell density and pertussis toxin-sensitive GTP-binding proteins

Epidermal growth factor (EGF) counteracts the stimulation of glycogen synthesis by insulin in hepatocytes, but it is not known whether this is due to inhibition of glycogen synthesis or to inhibition of the insulin-signalling mechanism. This study investigates the mechanisms by which EGF affects the basal rate and the insulin stimulation of glycogen synthesis. The basal rate of glycogen synthesis is higher at low than at high cell density. EGF inhibits the basal rate of glycogen synthesis at low cell density but not in confluent cultures and abolishes the difference due to density. However, EGF inhibits the stimulation of glycogen synthesis by insulin irrespective of cell density. Increasing glycogen synthesis by increasing the [glucose] does not abolish the difference in rates of glycogen synthesis due to cell density, neither does it induce responsiveness to EGF at high cell density, establishing that responsiveness to EGF is a function of cell density and not of the basal rate and that inhibition of the insulin stimulation also cannot be accounted for by the higher rate of glycogen synthesis. Cytochalasin D and phalloidin, which alter cell morphology through interactions with the microfilament cytoskeleton, mimic the cell-density-dependent inhibition of glycogen synthesis by EGF. The inhibition of glycogen synthesis by EGF and cytochalasin D is additive and cytochalasin D potentiates the inhibition of glycogen synthesis by EGF, suggesting involvement of a cytoskeletal mechanism. Exogenous phospholipase C inhibits glycogen synthesis at both low and high cell density and the inhibition at low cell density is not additive with that caused by either EGF or cytochalasin D, suggesting that these agonists inhibit glycogen synthesis through changes in Ca2+ and/or diacylglycerol. The inhibition of glycogen synthesis by EGF in the absence of insulin stimulation is blocked by neomycin, which inhibits Ca2+ release from intracellular stores but not by antagonists of protein kinase C. It was also inhibited by pertussis toxin (50%), suggesting that it may involve GTP-binding-protein-mediated release of Ca2+ from intracellular stores. The inhibition of the stimulation of glycogen synthesis by insulin was not affected by neomycin and was only marginally inhibited by pertussis toxin or guanosine 5'-O-[3-thio]triphosphate (GTP[S]). We infer from these findings that the inhibition by EGF of the basal rate of glycogen synthesis and of the insulin stimulation are mediated by different mechanisms. The latter is pertussis toxin insensitive and independent of cell density, whereas the former is expressed only at low cell density, it is potentiated by cytochalasin D and inhibited by pertussis toxin.

Identification of an IP3 receptor in endothelial cells

In this study we have used saponin to permeabilize bovine endothelial cell membranes in order to directly test the involvement of IP3 in regulating internal Ca2+ release. Our results indicate that the release of internal Ca2+ occurs as early as 1-3 seconds after IP3 addition. This IP3-induced internal Ca2+ release can be inhibited by heparin (an IP3 receptor antagonist). Further binding of [3H]IP3 to saponin-permeabilized bovine endothelial cells reveals the presence of a single, high affinity class of IP3 receptor with a dissociation constant (Kd) of approximately 0.50 (+/- 0.03) nM. Using a panel of monoclonal and polyclonal antibodies against IP3 receptor, we have established that the bovine endothelial cell IP3 receptor (approximately 260 kDa) displays immunological cross-reactivity with the rat brain IP3 receptor. Immunofluorescence data indicates that the IP3 receptor is preferentially located at the perinuclear region of the cells. In addition, PCR analysis of first-strand cDNAs from both bovine endothelial cells and rat brain tissues reveals that the IP3 receptor transcript in bovine endothelial cells belongs to the short non-neuronal form and not the long neuronal form detected in rat brain tissue. These findings suggest that the IP3 receptor in endothelial cells is both structurally and functionally analogous to that reported in non-neuronal cell systems and probably plays an important role in agonist-induced endothelial cell activation.

Comparative localization of inositol 1,4,5-trisphosphate and ryanodine receptors in intestinal smooth muscle: an analytical subfractionation study

[3H]Ins(1,4,5)P3- and [3H]ryanodine-binding sites were characterized in membrane fractions from guinea-pig intestinal smooth muscle (longitudinal layer) and their subcellular localization was investigated by analytical cell-fractionation techniques. Fractions collected at low centrifugal fields (N and M fractions) contained predominantly low-affinity [3H]Ins(1,4,5)P3-binding sites (KD 80 nM), whereas microsomal (P) fractions contained only high-affinity binding sites (KD 5 nM). Total sedimentable high-affinity binding sites of [3H]Ins(1,4,5)P3 were 9-10-fold more numerous than those of [3H]ryanodine. Both high-affinity binding sites were purified in microsomal fractions, and their sub-microsomal distribution patterns after isopycnic density-gradient centrifugation were similar to those of presumed endoplasmic reticulum (ER) constituents, indicating that Ins(1,4,5)P3 and ryanodine receptors were localized primarily in ER and probably associated with rough as well as smooth ER. However, the stoichiometric ratio of Ins(1,4,5)P3 to ryanodine receptors was distinctly higher in high-density RNA-rich subfractions than in low-density RNA-poor subfractions, suggesting that Ins(1,4,5)P3 receptors were somewhat concentrated in the ribosome-coated portions of ER. The low overall stoichiometric ratio of ryanodine to Ins(1,4,5)P3 receptors in intestinal smooth muscle (1:9-10) might explain, at least partly, the existence of a Ca(2+)-storage compartment devoid of ryanodine-sensitive Ca2+ channels, but equipped with Ins(1,4,5)P3-sensitive channels, in saponin-permeabilized smooth-muscle cells [Iino, Kobayashi and Endo (1988) Biochem. Biophys. Res. Commun. 152, 417-422].

The inositol 1,4,5-trisphosphate receptor: kinetic properties and regulation

Inositol 1,4,5-triphosphate (InsP3) is a second messenger responsible for the mobilization of intracellular Ca2+ after receptor-mediated hydrolysis of phosphatidylinositol 4,5-bisphosphate. InsP3 binds to a specific receptor located on the membrane of an intracellular compartment and opens a Ca2+ channel causing the cytosolic Ca2+ concentration to increase. Measurement of radiolabelled InsP3 binding and InsP3-induced Ca2+ release in parallel experiments indicated that the liver InsP3 receptor exists in two main states: an active state (A) and an inactive one (I). The "I" form of the receptor is found in the presence of high Ca2+ concentrations (above 1 microM). The binding properties of the "A" and the "I" states of the receptor have been characterized by analysing a membrane fraction enriched in InsP3 receptors. The inactive "I" state displays a high affinity (Kd = 2 nM) and slow rates of association and dissociation. The active state "A" of the receptor displays complex kinetic properties. The rate of association and the rate of dissociation of labelled InsP3 are rapid phenomena probably involving several components. The apparent Kd for the InsP3 binding is about 40 nM in a low Ca2+ medium. The affinity of the "A" state of the receptor is increased by Ca2+ (at concentrations lower than 0.5 microM) and by thiol reagents. The increase of the affinity of the receptor is due to a decrease of the dissociation rate constants. This lowers the threshold such that Ca2+ is released at lower concentrations of InsP3. These data indicate that the binding of InsP3 to its receptor is a complex phenomenon involving the transition among several states.(ABSTRACT TRUNCATED AT 250 WORDS)

Calcium signalling in platelets and other nonexcitable cells

By virtue of their biological simplicity and widespread availability, platelets frequently have been used as a model system to study signal transduction. Such studies have revealed that changes in intracellular free calcium concentration are central to platelet functioning. The following article reviews current concepts of platelet structure and function, with particular emphasis on the mechanisms involved in platelet Ca2+ signalling.

Association of the hepatic IP3 receptor with the plasma membrane: relevance to mode of action

Studies were carried out to characterize the interaction between inositol 1,4,5-trisphosphate (IP3) receptors and the plasma membrane fraction. Extraction of the membranes with the nonionic detergents Nonidet P-40 and Triton X-100, followed by centrifugation at 100,000 g, resulted in the doubling of the IP3 receptor in the pellets, whereas no detectable binding was found in the supernatants. These data indicate that the detergents did not solubilize the receptor, that it remained associated with membrane particles, and that it is likely to be associated with the cytoskeleton. The cytoskeleton proteins actin, ankyrin, and spectrin were identified in the plasma membrane fraction. However, comparison of the amount of these proteins in different fractions of the detergent, or otherwise treated plasma membrane fractions, showed no direct correlation between the presence of any of these proteins in the plasma membrane fraction and their ability to bind [3H]IP3. This is in contrast to the brain and T-lymphoma cells in which the IP3 receptor is attached to ankyrin (L. Y. W. Bourguigon, H. Jin, N. Iida, N. R. Brandt, and S. H. Zhang. J. Biol. Chem. 268: 6477-6486, 1993; and S. K. Joseph and S. Samanta. J. Biol. Chem 268: 6477-6486, 1993). Thus the hepatic IP3 receptor, which is different from the brain receptor, might attach to the cytoskeleton by anchoring to a different protein. Because cytochalasin D treatment of livers diminishes the ability of IP3 to raise cytosolic free Ca2+ levels, the attachment of the IP3 receptor to the cytoskeleton seems to involve an association with microfilaments.

Ca2+ release from platelet intracellular stores by thapsigargin and 2,5-di-(t-butyl)-1,4-benzohydroquinone: relationship to Ca2+ pools and relevance in platelet activation

The effects of the Ca(2+)-ATPase inhibitors thapsigargin (Tg) and 2,5-di-(t-butyl)-1,4-benzohydroquinone (tBuBHQ) were examined by using Ca(2+)-regulatory systems of platelet mixed membranes, saponin-permeabilized and intact platelets. Both agents inhibit Ca(2+)-ATPase activities of platelet mixed membranes, without any effect on the basal Mg(2+)-ATPase activity. Tg is more effective (EC50 = 35 nM) than tBuBHQ (EC50 = 580 nM). The effect of the two inhibitors on 45Ca2+ release from saponin-permeabilized platelets has also been characterized. 45Ca2+ uptake into non-mitochondrial intracellular stores occurs via an ATP-dependent mechanism, and if added at equilibrium the second messenger Ins(1,4,5)P3 releases 50% of the accumulated 45Ca2+. Maximally effective concentrations of Tg (1 microM) and tBuBHQ (50 microM) release 77% and 68% of the accumulated 45Ca2+. Addition of Ins(1,4,5)P3 together with either Tg or tBuBHQ resulted in a non-additive release which was the same as with either Tg or tBuBHQ alone, indicating that the Ins(1,4,5)P3-sensitive Ca2+ pool was a subset of the pool that is sensitive to the Ca(2+)-ATPase inhibitors. Release of 45Ca2+ by either Tg or tBuBHQ was not affected by heparin, which totally blocked Ins(1,4,5)P3-induced Ca2+ release, and Tg was found not to affect [32P]Ins(1,4,5)P3 binding to its receptor on mixed membranes. Thus both Tg and tBuBHQ release Ca2+ from a pool that totally overlaps the Ins(1,4,5)P3-sensitive pool without affecting Ins(1,4,5)P3 function. In intact indomethacin-treated Fura 2-loaded platelets, Tg and tBuBHQ cause Ca2+ elevation, arising from release from intracellular stores and influx from the outside. Both Tg and tBuBHQ elevated Ca2+ to similar levels, which were less and slower than those observed with thrombin. Addition of thrombin to cells already treated with Tg or tBuBHQ produced further elevation of Ca2+, indicating agonist utilization of a Ca(2+)-ATPase inhibitor-insensitive pool. In aggregation experiments Tg and tBuBHQ showed different functional effects. In indomethacin-treated cells Tg induces slow aggregation and secretion responses, whereas tBuBHQ only induces shape change. Both agents show synergistic secretory responses with the protein kinase C activator dioctanoylglycerol (DiC8). Tg also showed greater ability than tBuBHQ to release [3H]arachidonic acid (AA) from [3H]AA-labelled platelets. Additionally, in [32P]Pi-labelled platelets both Tg and tBuBHQ induced phosphorylation of myosin light chain, a 27 kDa protein and the 45 kDa protein pleckstrin, but Tg showed a greater ability than tBuBHQ to cause phosphorylation of pleckstrin. These studies indicate that Tg and tBuBHQ are effective in releasing the Ins(1,4,5)P3-sensitive Ca2+ pool in platelets.(ABSTRACT TRUNCATED AT 400 WORDS)

Inositol 1,4,5-trisphosphate and inositol 1,3,4,5-tetrakisphosphate binding sites in smooth muscle

1. We have previously demonstrated that activation of M3 muscarinic receptors increases inositol 1,4,5-trisphosphate (InsP3) and inositol 1,3,4,5-tetrakisphosphate (InsP4) accumulation in colonic smooth muscle. 2. In the present study, we demonstrate the existence of InsP3 and InsP4 binding sites in colonic circular smooth muscle by use of radioligand binding methods. Both [3H]-InsP3 and [3H]-InsP4 bound rapidly and reversibly to a single class of saturable sites in detergent-solubilized colonic membranes with affinities of 5.04 +/- 1.03 nM and 3.41 +/- 0.78 nM, respectively. The density of [3H]-InsP3 binding sites was 335.3 +/- 19.3 fmol mg-1 protein which was approximately 2.5 fold greater than that of [3H]-InsP4 sites (127.3 +/- 9.1 fmol mg-1 protein). 3. The two high affinity inositol phosphate binding sites exhibited markedly different pH optima for binding of each radioligand. At pH 9.0, specific [3H]-InsP3 binding was maximal, whereas [3H]-InsP4 binding was only 10% that of [3H]-InsP3. Conversely, at pH 5.0, [3H]-InsP4 binding was maximal, while [3H]-InsP3 binding was reduced to 15% of its binding at pH 9.0. 4. InsP3 was about 20 fold less potent (KI = 50.7 +/- 8.3 nM) than InsP4 in competing for [3H]-InsP4 binding sites and could compete for only 60% of [3H]-InsP4 specific binding. InsP4 was also capable of high affinity competition with [3H]-InsP3 binding (KI = 103.5 +/- 1.5 nM), and could compete for 100% of [3H]-InsP3 specific binding. 5. [3H]-InsP3 binding in subcellular fractions separated by discontinuous sucrose density gradients followed NADPH-cytochrome c reductase activity, suggesting an intracellular localization for the majority of InsP3 receptors in this tissue, whereas [3H]-InsP4 binding appeared to be equally distributed between plasma membrane and intracellular membrane populations.6. These results suggest the existence of distinct and specific InsP3 and InsP4 binding sites which may represent the physiological receptors for these second messengers; differences in the subcellular distribution of these receptors may contribute to differences in their putative physiological roles.

Mobilization of Ca2+ by thapsigargin and 2,5-di-(t-butyl)-1,4-benzohydroquinone in permeabilized insulin-secreting RINm5F cells: evidence for separate uptake and release compartments in inositol 1,4,5-trisphosphate-sensitive Ca2+ pool

We characterized and directly compared the Ca(2+)-releasing actions of two inhibitors of endoplasmic-reticulum (ER) Ca(2+)-ATPase, thapsigargin and 2,5-di-(t-butyl)-1,4-benzohydroquinone (tBuBHQ), in electropermeabilized insulin-secreting RINm5F cells. Ambient free calcium concentration ([Ca2+]) was monitored by Ca(2+)-selective mini-electrodes. After ATP-dependent Ca2+ uptake, thapsigargin and tBuBHQ released Ca2+ with and EC50 of approximately 37 nM and approximately 2 microM respectively. Both agents mobilized Ca2+ predominantly from the Ins(1,4,5)P3-sensitive Ca2+ pool, and in this respect thapsigargin was more specific than tBuBHQ. The total increase in [Ca2+] obtained with thapsigargin and Ins(1,4,5)P3 was, on the average, only 7% greater than that with Ins(1,4,5)P3 alone. In contrast, the total increase in [Ca2+] obtained with tBuBHQ and Ins(1,4,5)P3 was 33% greater than that obtained with only InsP3 (P < 0.05). Although Ca2+ was rapidly mobilized by thapsigargin and tBuBHQ, complete depletion of the Ins(1,4,5)P3-sensitive Ca2+ pool was difficult to achieve. After the release by thapsigargin or tBuBHQ, Ins(1,4,5)P3 induced additional Ca2+ release. The additional Ins(1,4,5)P3-induced Ca2+ release was not altered by supramaximal concentrations of thapsigargin and tBuBHQ, or by Bafilomycin A1, an inhibitor of V-type ATPases, but was decreased by prolonged treatment with the ER Ca(2+)-ATPase inhibitors. These results suggest the existence of distinct uptake and release compartments within the Ins(1,4,5)P3-sensitive Ca2+ pool. When treated with the inhibitors, the two compartments became distinguishable on the basis of their Ca2+ permeability. Apparently, thapsigargin and tBuBHQ readily mobilized Ca2+ from the uptake compartment, whereas Ca2+ from the release compartment could be mobilized only very slowly, in the absence of Ins(1,4,5)P3.

Calcium release-activated calcium current in rat mast cells

1. Whole-cell patch clamp recordings of membrane currents and fura-2 measurements of free intracellular calcium concentration ([Ca2+]i) were used to study the biophysical properties of a calcium current activated by depletion of intracellular calcium stores in rat peritoneal mast cells. 2. Calcium influx through an inward calcium release-activated calcium current (ICRAC) was induced by three independent mechanisms that result in store depletion: intracellular infusion of inositol 1,4,5-trisphosphate (InsP3) or extracellular application of ionomycin (active depletion), and intracellular infusion of calcium chelators (ethylene glycol bis-N,N,N',N'-tetraacetic acid (EGTA) or 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA)) to prevent reuptake of leaked-out calcium into the stores (passive depletion). 3. The activation of ICRAC induced by active store depletion has a short delay (4-14 s) following intracellular infusion of InsP3 or extracellular application of ionomycin. It has a monoexponential time course with a time constant of 20-30 s and, depending on the complementary Ca2+ buffer, a mean normalized amplitude (at 0 mV) of 0.6 pA pF-1 (with EGTA) and 1.1 pA pF-1 (with BAPTA). 4. After full activation of ICRAC by InsP3 in the presence of EGTA (10 mM), hyperpolarizing pulses to -100 mV induced an instantaneous inward current that decayed by 64% within 50 ms. This inactivation is probably mediated by [Ca2+]i, since the decrease of inward current in the presence of the fast Ca2+ buffer BAPTA (10 mM) was only 30%. 5. The amplitude of ICRAC was dependent on the extracellular Ca2+ concentration with an apparent dissociation constant (KD) of 3.3 mM. Inward currents were nonsaturating up to -200 mV. 6. The selectivity of ICRAC for Ca2+ was assessed by using fura-2 as the dominant intracellular buffer (at a concentration of 2 mM) and relating the absolute changes in the calcium-sensitive fluorescence (390 nm excitation) with the calcium current integral. This relationship was almost identical to the one determined for Ca2+ influx through voltage-activated calcium currents in chromaffin cells, suggesting a similar selectivity. Replacing Na+ and K+ by N-methyl-D-glucamine (with Ca2+ ions as exclusive charge carriers) reduced the amplitude of ICRAC by only 9% further suggesting a high specificity for Ca2+ ions. 7. The current amplitude was not greatly affected by variations of external Mg2+ in the range of 0-12 mM. Even at 12 mM Mg2+ the current amplitude was reduced by only 23%. 8. ICRAC was dose-dependently inhibited by Cd2+.(ABSTRACT TRUNCATED AT 250 WORDS)

Inositol 1,4,5-trisphosphate binding sites copurify with the putative Ca-storage protein calreticulin in rat liver

Rat liver was homogenized and subjected to differential centrifugation. When the low speed nuclear pellet was processed on a Percoll gradient, plasma membrane markers and Ins(1,4,5)P3 binding activity purified together. The high speed (microsomal) fraction was subfractionated by sucrose density gradient centrifugation, resulting in 10-fold enrichment of [32P]-Ins(1,4,5)P3 binding. In the sucrose density gradient fractions there was an inverse relationship between the enrichment of plasma membrane markers and Ins(1,4,5)P3 binding sites. Endoplasmic reticulum markers showed a moderate enrichment in the fractions displaying high Ins(1,4,5)P3 binding activity. Calcium binding proteins in the homogenate and in the microsomal subfractions were separated by SDS/PAGE. A 60 kD protein, stained metachromatically with Stains-All was identified as calreticulin with immunoblotting. Its enrichment pattern was similar to that of Ins(1,4,5)P3 binding sites, indicating the co-existence of these two elements of Ca(2+)-metabolism in the same intracellular compartment in the liver.

Intracellular Ca2+ pools in Jurkat T-lymphocytes

Jurkat T-lymphocytes comprise at least four intracellular Ca2+ pools. Pool I was agonist-sensitive and contained 23 +/- 8% (n = 18) of the total Ca(2+)-storage capacity, as shown in intact cells in the presence of EGTA. The time courses of the agonist-induced formation of Ins(1,4,5)P3 and of the Ca2+ release from pool I were nearly superimposable, indicating that the agonist-sensitive pool I is emptied by Ins(1,4,5)P3. Likewise, in permeabilized cells, the size of the Ins(1,4,5)P3-sensitive Ca2+ pool I was 27 +/- 11% (n = 14). Pool II contained 26 +/- 5% (n = 9) of intracellularly stored Ca2+ and was liberated by thapsigargin, an inhibitor of the endoplasmic-reticulum (ER) Ca(2+)-ATPase. Addition of thapsigargin before addition of agonist abolished the agonist-induced Ca2+ release in both intact and permeabilized cells, indicating that pool I is a subcompartment of the ER Ca2+ pool. The content of this ER Ca2+ pool (pools I and II) amounted to 51 +/- 15% (n = 9) in intact cells and 49 +/- 16% (n = 16) in permeabilized cells. Caffeine released Ca2+ even when the ER pool (pools I and II) was emptied by previous addition of thapsigargin, indicating the presence of a third pool independent of pools I and II. Pool III contained 23 +/- 6% (n = 8) in intact cells, but 41 +/- 8% (n = 5) in permeabilized cells. The remaining intracellularly stored Ca2+ was released by addition of the Ca2+ ionophore ionomycin. This fourth pool contained 27 +/- 8% (n = 9) in intact cells, but less than 10% in permeabilized cells. The size of pool III was increased when pools I and II were emptied before addition of caffeine, whereas the size of pool IV was decreased under such conditions. In conclusion, this first comprehensive description of intracellular Ca2+ pools in Jurkat T-lymphocytes demonstrates the presence of four different Ca2+ pools, provides estimates of their sizes and describes relationships between each other. Release of Ca2+ from pool I [Ins(1,4,5)P3-sensitive] has previously been shown to play a major role in T-cell activation, whereas the physiological role of pools II-IV remains to be established.

The IP3-sensitive calcium store of HIT cells is located in a surface-derived vesicle fraction

Electron microscopic and biochemical techniques were used to study the cellular localization of the ATP-dependent, IP3-sensitive, Ca2+ store in the glucose- and phosphatidylinositol(PI) agonist-sensitive hamster insulinoma cell line HIT-T15. Scanning electron microscopy revealed conspicuous shape changes of the microvilli following stimulation of these cells with bombesin or thapsigargin. These changes closely resemble those previously shown to accompany stimulation of hexose transport in adipocytes with insulin [J. Cell. Physiol. 142 (1990) 1-14]. Using a hydrodynamic shearing technique for the isolation of microvilli, two cell surface-derived vesicle fractions were prepared containing 80% of the total cellular Ca(2+)-storing activity. In contrast, subcellular fractionation using normal homogenization with a glass/teflon homogenizer yielded the well-known distribution of the Ca(2+)-storing activity which is then predominantly recovered within the microsomal fraction. The surface-derived vesicle fraction was clearly distinguished from the microsomal fraction by its high content of Na+/K(+)-ATPase and an immunoreactive fragment of the GluT-1 glucose transporter isoform which both are not detectable in the microsomal fraction isolated from homogenates from sheared cells. The Ca2+ uptake properties of the cell surface-derived vesicle fractions including the vanadate, A23187, and thapsigargin sensitivity were found to be identical with those described for the microsomal Ca2+ stores of various cell types. Inositol 1,4,5-trisphosphate (IP3) at 1 microM induced a maximal release of 35-40% of the stored Ca2+ from these vesicles.

Intracellular Ca2+ stores of rat cerebellum: heterogeneity within and distinction from endoplasmic reticulum

Rat cerebellum microsomes were subfractionated on isopycnic linear sucrose (20-42%)-density gradients. The distribution of endoplasmic reticulum (ER) markers (RNA, signal-sequence receptor alpha, calnexin, calreticulin, the immunoglobulin-binding protein Bip) and markers of intracellular rapidly exchanging Ca2+ stores [Ca2+ channels sensitive to either Ins(1,4,5)P3 or ryanodine) was investigated biochemically and immunologically. The comparison indicates that: (a) vesicles bearing the InsP3 receptor were separated from those bearing the ryanodine receptor; (b) ER markers, i.e. Bip, calnexin, signal-sequence receptor alpha, RNA, did not sediment as either InsP3 or ryanodine receptors did; (c) calreticulin, an intralumenal low-affinity high-capacity Ca(2+)-binding protein, had a widespread distribution, similar to that of Bip and calnexin, and was present in Purkinje, granule, Golgi and stellate neurons, as indicated by immunofluorescent labelling of cerebellum cortex cryosections. The present results show that the ER is not a homogeneous entity, and that Ca2+ stores are heterogeneous insofar as InsP3 receptors and ryanodine receptors are segregated, either to discrete intracellular organelles or to specialized ER subcompartments.

Inositol (1,4,5)-trisphosphate receptor: characterization of neuron-specific alternative splicing in rat brain and peripheral tissues

One source of diversity in the inositol (1,4,5)-trisphosphate receptors (IP3Rs) is generated at the level of alternative splicing. Our previous studies of splice isoforms of the receptor in various tissues suggested that some tissues, specifically those containing neurons, selectively express a 40 amino acid insert located between 2 sites for phosphorylation by cyclic AMP-dependent protein kinase (PKA), and that the presence of this insert changes the preferred site of phosphorylation of the receptor. Studies of the mouse receptor have also suggested the existence of intermediately spliced forms containing partial versions of the splice and exhibiting different brain distributions. In this study, we have investigated the alternative splicing of the rat receptor in greater detail using RNase protection and PCR analysis. We find little evidence for the existence of intermediately spliced forms in rat, raising the possibility that the degree of alternative splicing at this site differs in the brains of two very similar species. Our screen of tissue distribution supports the selectively neuronal expression of the long spliced form, and suggests that regulation of this receptor in neurons may be different than in other tissues.

Use of intracellular Ca2+ stores from rat basophilic leukemia cells to study the molecular mechanism leading to quantal Ca2+ release by inositol 1,4,5-trisphosphate

Quantal Ca2+ release is a novel motif for the mediation of signal transduction in which the amplitude of a biological response following multiple stepwise increases in agonist concentration is retained. The release of Ca2+ from permeabilized cells in response to the second messenger inositol 1,4,5-trisphosphate (InsP3) proceeds in this fashion. The mechanisms leading to quantal Ca2+ release are unknown. InsP3 releases 50-90% of the Ca2+ sequestered within the intracellular stores of mammalian cells permeabilized with saponin. However, preparation of microsomes results in the loss of this sensitivity. In this report, functionally intact intracellular Ca2+ stores were isolated from rat basophilic leukemia (RBL) cells by osmotic lysis followed by differential and sucrose density gradient centrifugation. From this preparation, 64% of the stored Ca2+ is released by InsP3. We demonstrate that quantal Ca2+ release is retained by isolated Ca2+ stores and is identical to that observed in permeabilized cells. Addition of a subsaturating (28 nM) concentration of InsP3 to permeabilized cells at 37 degrees C results in the release of only a small fraction of the sequestered Ca2+. When the cells are cooled to 11 degrees C, the remaining Ca2+ is rapidly released. Hence, the mechanism leading to the quantal nature of Ca2+ release is reversible and is thus not likely to be the result of a covalent modification of the channel protein or of the Ca2+ store.(ABSTRACT TRUNCATED AT 250 WORDS)

Reconstitution of a T cell receptor-stimulated plasma membrane calcium transporter: lack of dependence on inositol phosphates

The activation of T lymphocytes, like many cells, requires a rapid rise in intracellular Ca2+ secondary to both an influx and a release from intracellular stores. The latter is activated by inositol-1,4,5-trisphosphate [Ins(1,4,5)P3]. It is controversial if inositol phosphates can also stimulate a plasma membrane Ca2+ channel in T cells. We have studied the human T cell line HPB-ALL which, upon stimulation of its antigen receptor, does not generate detectable levels of Ins(1,4,5)P3 or internal Ca2+ release, but does have a Ca2+ influx. We have reconstituted a receptor-activated Ca2+ transporter from plasma membranes from these cells which has properties similar to the transporter observed in vivo and does not require inositol phosphates for activation. These data show that mitogens may activate more than one type of ligand-gated Ca2+ transport mechanism in T lymphocytes.

Subcellular gradients of intracellular free calcium concentration in isolated lacrimal acinar cells

The spatial distribution of intracellular free calcium concentration ([Ca2+]i) was measured in small clusters of isolated rat lacrimal acinar cells by imaging the fluorescence of the Ca(2+)-sensitive dye fura-2. In the absence of extracellular Ca2+, stimulation with acetylcholine (ACh) caused an increase in [Ca2+]i, due to release of intracellular Ca2+ stores, which was maximal at the luminal pole of the cell. In contrast, the organellar Ca(2+)-ATPase inhibitor 2,5-di(tert-butyl)-hydroquinone caused an increase in [Ca2+]i, which was most marked in the basolateral region of the cell. When the cells were stimulated with ACh in a medium containing Ca2+, the gradients of [Ca2+]i (with [Ca2+]i most elevated at the luminal pole) were maintained for the duration of agonist stimulation. The possible implications of these results concerning the location and identity of intracellular Ca2+ stores, and the location of the sites that underlie agonist-stimulated Ca2+ influx, are considered. In particular, it seems likely that intracellular inositol-1,4,5-trisphosphate (InsP3) binding sites may be concentrated in the luminal region of the cell. It is not clear, however, whether this implies that there is a distinct luminally located InsP3-sensitive organelle.

Calcium ion homeostasis in smooth muscle

Ca2+ plays an important role in the regulation of smooth-muscle contraction. In this review, we will focus on the various Ca(2+)-transport processes that contribute to the cytosolic Ca2+ concentration. Mainly the functional aspects will be covered. The smooth-muscle inositol 1,4,5-trisphosphate receptor and ryanodine receptor will be extensively discussed. Smooth-muscle contraction also depends on extracellular Ca2+ and both voltage- and Ca(2+)-release-activated plasma-membrane Ca2+ channels will be reviewed. We will finally discuss some functional properties of the Ca2+ pumps that remove Ca2+ from the cytoplasm and of the Ca2+ regulation of the nucleus.

Inositol lipid hydrolysis contributes to the Ca2+ wave in the activating egg of Xenopus laevis

We have used fluorescence ratio-imaging of fura-2 in the activating egg of Xenopus laevis to study the wave of increased intracellular free Ca2+ concentration ([Ca2+]i) while monitoring that of cortical granule exocytosis. Naturally matured eggs were dejellied, injected with fura-2, and activated by the iontophoresis of 1-30 nCoul of inositol-1,4,5-trisphosphate which triggers an immediate increase in free [Ca2+]i at the injection site. The Ca2+ rise spreads throughout the egg, reaching the opposite side in 5-8 min, and is followed by elevation of the fertilization envelope about 20-30 sec behind the [Ca2+]i wave. [Ca2+]i returns to preactivation levels within about 20 min after activation. We further studied the role of phosphatidylinositol-4,5-bisphosphate (PIP2) hydrolysis by microinjecting antibodies to PIP2 into the egg. PIP2 antibodies did not alter the propagation velocity of the wave but greatly reduced the amount of Ca2+ released in the egg cortex. These data suggest that PIP2 hydrolysis plays a role in the release of [Ca2+]i in the outer regions of the egg following activation.

IP3 receptor: localization to plasma membrane of T cells and cocapping with the T cell receptor

Immune responses in lymphocytes require cellular accumulation of large amounts of calcium (Ca2+) from extracellular sources. In the T cell tumor line Jurkat, receptors for the Ca(2+)-releasing messenger inositol 1,4,5-trisphosphate (IP3) were localized to the plasma membrane (PM). Capping of the T cell receptor-CD3 complex, which is associated with signal transduction, was accompanied by capping of IP3 receptors. The IP3 receptor on T cells appears to be responsible for the entry of Ca2+ that initiates proliferative responses.