GTP- and inositol 1,4,5-trisphosphate-induced release of 45Ca2+ from a membrane store co-localized with pancreatic-islet-cell plasma membrane

Mobilization of Ca2+ stores in individual pancreatic beta-cells permeabilized or not with digitonin or alpha-toxin

The concentration of free Ca2+ in the cytoplasm and organelles of individual mouse pancreatic beta-cells was estimated with dual wavelength microfluorometry and the indicators Fura-2 and furaptra. Measuring the increase of cytoplasmic Ca2+ resulting from intracellular mobilization of the ion in ob/ob mouse beta-cells, most organelle calcium (92%) was found in acidic compartments released when combining the Ca2+ ionophore Br-A23187 with a protonophore. Only 3-4% of organelle calcium was recovered from a pool sensitive to the Ca(2+)-ATPase inhibitor thapsigargin. Organelle Ca2+ was also measured directly in furaptra-loaded beta-cells after controlled plasma membrane permeabilization. The permeabilizing agent alpha-toxin was superior to digitonin in preserving the integrity of intracellular membranes, but digitonin provided more reproducible access to intracellular sites. After permeabilization, the thapsigargin-sensitive fraction of Ca2+ detected by furaptra was as high as 90%, suggesting that the indicator essentially measures Ca2+ in endoplasmic reticulum (ER). Both alpha-toxin- and digitonin-permeabilized cells exhibited ATP-dependent uptake of Ca2+ into thapsigargin-sensitive stores with half-maximal and maximal filling at 6-11 microM and 1 mM ATP respectively. Most of the thapsigargin-sensitive Ca2+ was mobilized by inositol 1,4,5-trisphosphate (IP3), whereas caffeine, ryanodine, cyclic ADP ribose and nicotinic acid adenine dinucleotide phosphate lacked effects both in beta-cells from ob/ob mice and normal NMRI mice. Mobilization of organelle Ca2+ by 4-chloro-3-methylphenol was attributed to interference with the integrity of the ER rather than to activation of ryanodine receptors. The observations emphasize the importance of IP3 for Ca2+ mobilization in pancreatic beta-cells, but question a role for ryanodine receptor agonists.

Microvillar Ca++ signaling: a new view of an old problem

Proceeding from the recent finding that the main components of the Ca++ signal pathway are located in small membrane protrusions on the surface of differentiated cells, called microvilli, a novel concept of cellular Ca++ signaling was developed. The main features of this concept can be summarized as follows: Microvilli are formed on the cell surface of differentiating or resting cells from exocytic membrane domains, growing out from the cell surface by elongation of an internal bundle of actin filaments. The microvillar tip membranes contain all functional important proteins synthesized such as ion channels and transporters for energy-providing substrates and structural components, which are, in rapidly growing undifferentiated cells, distributed over the whole cell surface by lateral diffusion. The microvillar shaft structure, a bundle of actin filaments, forms a dense cytoskeletal matrix tightly covered by the microvillar lipid membrane and represents an effective diffusion barrier separating the microvillar tip compartment (entrance compartment) from the cytoplasm. This diffusion barrier prevents the passage of low molecular components such as Ca++ glucose and other relevant substrates from the entrance compartment into the cytoplasm. The effectiveness of the actin-based diffusion barrier is modulated by various signal pathways and effectors, most importantly, by the actin-depolymerizing/reorganizing activity of the phospholipase C (PLC)-coupled Ca++ signaling. Moreover, the microvillar bundle of actin filaments plays a dual role in Ca++ signaling. It combines the function of a diffusion barrier, preventing Ca++ influx into the resting cell, with that of a high-affinity, ATP-dependent, and IP3-sensitive Ca++ store. Activation of Ca++ signaling via PLC-coupled receptors simultaneously empties Ca++ stores and activates the influx of external Ca++. The presented concept of Ca++ signaling is compatible with all established data on Ca++ signaling. Properties of Ca++ signaling, that could not be reconciled with the basic principles of the current hypothesis, are intrinsic properties of the new concept. Quantal Ca++ release, Ca(++)-induced Ca++ release (CICR), the coupling phenomen between the filling state of the Ca++ store and the activity of the Ca++ influx pathway, as well as the various yet unexplained complex kinetics of Ca++ uptake and release can be explained on a common mechanistic basis.

Activation of calcium signaling in isolated rat hepatocytes is accompanied by shape changes of microvilli

Preceding studies using the hamster insulinoma cell line, HIT, and isolated rat hepatocytes have shown that two essential components of the Ca2+ signaling pathway, the ATP-dependent Ca2+ store and the store-coupled Ca2+ influx pathway, are both located in microvilli covering the surface of these cells. Microvilli-derived vesicles from both cell types exhibited anion and cation pathways which could be inhibited by anion and cation channel-specific inhibitors. These findings suggested that the microvillar tip compartment forms a space which is freely accessible for external Ca2+, ATP, and IP3. The entry of Ca2+ into the cytoplasm, however, is largely restricted by the microvillar core structure, the dense bundle of actin microfilaments acting as a diffusion barrier between the microvillar tip compartment and the cell body. Moreover, evidence has been presented that F-actin may function as ATP-dependent and IP3-sensitive Ca2+ store that can be emptied by profilin-induced depolymerization or reorganization [K. Lange and U. Brandt (1996) FEBS Lett. 395, 137-142]. Here we demonstrate the tight connection between microvillar shape changes and the activation of the Ca2+ signaling system in isolated rat hepatocytes. Using a combination of scanning electron microscopy (SEM) and fura-2 fluorescence technique, we confirmed a consequence of the "diffusion barrier" concept of Ca2+ signaling: Irrespective of the type of the applied stimulus, activation of the Ca2+ influx pathway is accompanied by changes in the structural organization of microvilli indicative of the loss of their diffusion barrier function. We further show that the cell surfaces of unstimulated hepatocytes isolated by either the collagenase or the EDTA perfusion technique are densely covered with microvilli predominantly of a short and slender type. Beside this rather uniformly shaped type of microvilli, a number of dilated surface protrusions were observed. Under these conditions the cells displayed the well known rather high basal [Ca2+]i of 200-250 nM as repeatedly demonstrated for freshly isolated hepatocytes. However, addition of the serine protease inhibitor, phenylmethanesulfonyl fluoride (PMSF), to the cell suspension immediately after its preparation reduced the basal cytoplasmic Ca2+ level to about 100 nM. Concomitantly, dilated surface protrusions disappeared, and cell surfaces exclusively displayed short, slender microvilli. Activation of the Ca2+ signaling pathway by vasopressin, as well as by the IP3-independent acting Ca2+ store inhibitor, thapsigargin, was accompanied by a conspicuous shortening and dilation of microvilli following the same time courses as the respective increases of [Ca2+]i induced by the effectors. Furthermore, the abundance of the large form of surface protrusions on isolated hepatocytes positively correlated with the size of a cellular Ca2+/Fura-2 compartment which is rapidly depleted from Ca2+ by extracellular EGTA. These findings support the postulated localization of the store-coupled Ca2+ influx pathway in microvilli of HIT cells also for hepatocytes and are in accord with the notion of a cytoskeletal diffusion barrier regulating the flux of external Ca2+ via the microvillar tip region in the cytoplasm.

Inhibition of calcium-induced insulin secretion from intact HIT-T15 or INS-1 beta cells by GTP depletion

Using intact rat islets, we previously observed that GTP depletion (achieved through the use of mycophenolic acid or other synthesis inhibitors) impedes nutrient- but not K+-induced insulin secretion. It was concluded that a proximal nutrient-dependent step in stimulus-secretion coupling (but not the process of Ca2+-induced exocytosis itself) is modulated by ambient GTP levels. To examine Ca2+-dependent steps further in intact beta cells, INS-1 cells (which synthesize GTP and ATP similarly to rat islets) and HIT-T15 cells (whose synthesis of purine nucleotides is different) were studied following cell culture for 1-18 hr in various concentrations of mycophenolic acid (MPA) or mizoribine (MZ). Both agents profoundly reduced GTP content (mean: -78%) and lowered the GTP/GDP ratio by an average of -73%; concomitantly, MPA or MZ reduced insulin secretion induced by 10 mM glucose, 30 or 40 mM KCl, or 100 microM tolbutamide, independent of any changes in cell viability, insulin content, ATP content, the ATP/ADP ratio, or cytosolic free Ca2+ concentrations. In INS-1 cells (which appear to have normal nucleobase transport and "salvage" pathway activities), guanine (but not adenine) restored GTP content, the GTP/GDP ratio, and Ca2+-induced secretion. In HIT cells, the phosphoribosylation of exogenous guanine or hypoxanthine is defective; however, provision of 500 microM guanosine (but not adenosine) reversed the effects of MPA. We conclude that, at least in certain situations, a requisite role for GTP in the distal step(s) of exocytosis can be demonstrated.

Local Ca2+ release from internal stores controls exocytosis in pituitary gonadotrophs

Exocytosis and the cell-averaged cytosolic [Ca2+], [Ca2+]i, were tracked in single gonadotrophs. Cells released 100 granules/s at 1 microM = [Ca2+]i when gonadotropin-releasing hormone (GnRH) activated IP3-mediated Ca2+ release from internal stores, but only 1 granule/s when [Ca2+]i was raised uniformly to 1 microM by other means. Strong exocytosis was then seen only at higher [Ca2+]i (half-maximal at 16 microM). Parallel second messengers did not contribute to GnRH-induced exocytosis, because IP3 alone was as effective as GnRH, and because even GnRH failed to trigger rapid exocytosis when the [Ca2+]i rise was blunted by EGTA. When [Ca2+]i was released from stores, exocytosis depended on [Ca2+]i rising rapidly, as if governed by Ca2+ flux into the cytosol. We suggest that IP3 releases Ca2+ selectively from subsurface cisternae, raising [Ca2+] near exocytic sites 5-fold above the cell average.

Dopamine (D2) receptor regulation of intracellular calcium and membrane capacitance changes in rat melanotrophs

1. Indo-1 microfluorimetry and patch clamp techniques were used to study the decrease in cytosolic [Ca2+] ([Ca2+]i) caused by dopamine (D2) receptor activation and the calcium dependence of membrane capacitance changes in single rat melanotrophs. 2. [Ca2+]i decreased when extracellular calcium was removed or when the calcium channel blockers nickel (2 mM) or cadmium (100 microM) were applied by bath perfusion. 3. Quinpirole, a dopamine (D2) receptor agonist, reduced [Ca2+]i by 55 +/- 9 nM and hyperpolarized membrane potential by 29 +/- 9 mV simultaneously. 4. Quinpirole-induced [Ca2+]i decrease required deactivation of voltage-dependent calcium channels. Voltage clamping the membrane potential at -25 mV prevented the quinpirole-induced [Ca2+]i decrease. Nickel (2 mM) reduced [Ca2+]i without hyperpolarization and precluded additional [Ca2+]i decrease by quinpirole. 5. Membrane capacitance measurement of secretion rates in cells dialysed with buffered calcium solutions showed that secretion began at approximately 400 nM Cai2+. 6. Melanotrophs have IP3-sensitive calcium stores, but no caffeine-sensitive calcium stores. Calcium released from IP3-sensitive calcium stores also stimulated secretion. 7. Secretion in melanotrophs is modulated by protein kinase activators. cAMP (200 microM) enhanced secretion at [Ca2+]i > 1000 nM. Phorbol myristate acetate (PMA; 200 nM) enhanced secretion at [Ca2+]i < 400 nM, but not in the absence of calcium. 8. Dopamine receptor activation can reduce secretion by reducing the calcium influx through calcium channels with hyperpolarization of the membrane potential. However downregulation of either cAMP or protein kinase C activity may also contribute to the decrease in secretion.

Characterization of prenylcysteine methyltransferase in insulin-secreting cells

Prenylcysteine carboxymethyltransferase, an enzyme involved in the post-translational modification of many signalling proteins, was characterized in insulin-secreting INS-1 cells and normal rat pancreatic islets. The activity of this enzyme was monitored by the methylation of an artificial substrate (a prenylated cysteine analogue) with S-adenosy1[methyl-3H]methionine as methyl donor. More than 95% of the methyltransferase activity was associated with the membranes, and high-salt treatment only partially extracted the enzyme from the membranes. The highest specific activity was in the insulin-granule-enriched 25000 g pellet obtained by differential centrifugation. However, a highly purified insulin-enriched fraction obtained by density centrifugation in Percoll did not exhibit methyltransferase activity. The analyses of marker enzymes for cellular organelles revealed that the methyltransferase was co-localized, with the plasma membrane and probably the endoplasmic reticulum, but not with the mitochondria or lysosomes. Guanosine 5'-[gamma-thio]-triphosphate failed to increase methyltransferase activity directly, although it promotes the methylation of GTP-binding proteins. Mastoparan, Ca2+, cAMP and the protein kinase C activator phorbol 12-myristate 13-acetate did not alter enzyme activity. In addition, methyltransferase activity was not stably modified by stimulation of intact cells using glucose or other agents. However, the carboxymethylation of certain low-molecular-mass G-proteins is increased by glucose stimulation; conversely, treatment of cells with N-acetyl-S-trans,trans-farnesyl-L-cysteine inhibited glucose- and forskolin-induced insulin secretion. These results suggest that the membrane-associated prenylcysteine carboxymethyltransferase may be constitutively active and that the methylation of target proteins in vivo is regulated by the access of these proteins to the methyltransferase, as well as by their active (GTP-liganded) configuration.

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.

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.

Vasopressin and oxytocin but not glucose stimulate hydrolysis of phosphatidylinositol and phosphatidylcholine in a hamster insulinoma

HIT-T15 cells prelabeled with [3H]-arachidonate were incubated for 15 minutes at 37 degrees C in Krebs Ringer buffer (pH 7.1) in the presence and absence of various agonists. Radioactivity remaining in major phospholipids was measured at the end of incubation period. Oxytocin (1 microM), vasopressin (1 microM), and A23187 (5 microM) stimulated loss of radioactivity from phosphatidylinositol and phosphatidylcholine. No loss of radioactivity from either of the phospholipids, however, was detected in the presence of 10 mM D-glucose, an insulin secretagogue in HIT-T15 cells. The lack of phosphatidylinositol response to glucose was also evident when the cells were prelabeled with myo-[3H] inositol. The formation of inositol phosphates at 15 minutes was readily observed upon the treatment of myo-[3H] inositol-labeled cells with oxytocin or vasopressin but not glucose or A23187. Inability of glucose to stimulate phosphatidylinositol and phosphatidylcholine hydrolysis in beta cell-derived HIT-T15 cells contrasts sharply with results from studies with pancreatic islets, where hydrolysis of these two phospholipids is readily observed and thought to contribute to the signaling mechanism responsible for stimulation of insulin secretion.

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.

Localisation of the [32P]IP3 binding site on human platelet intracellular membranes isolated by high-voltage free-flow electrophoresis

This study reports the localisation of the [32P]IP3 binding site on highly purified membrane fractions prepared using high-voltage free-flow electrophoresis. Binding studies on mixed membranes, carried out at 4 degrees C, revealed a binding site with a Kd = 86 nM and beta max = 5.3 pmol/mg protein. The binding was potently inhibited by heparin. High-voltage free-flow electrophoresis was used to further purify surface and intracellular membranes. The intracellular membranes showed a 5-fold enrichment of binding sites with respect to the parent mixed membranes with the same Kd (80 nM), but the surface membranes showed an absence of binding activity. The results indicate the localisation of the IP3 receptor on highly purified intracellular membranes.

Purification of an inositol 1,4,5-trisphosphate-binding calreticulin-containing intracellular compartment of HL-60 cells

To investigate the identity of Ins(1,4,5)P3-sensitive intracellular Ca2+ stores in myeloid cells, we have developed a method that yields subcellular fractions highly enriched in Ins(1,4,5)P3 binding. HL-60 cells were disrupted by nitrogen cavitation, and subcellular fractions were obtained by differential centrifugation, followed by Percoll- and sucrose-density-gradient separations. A subcellular fraction enriched 26-fold in Ins(1,4,5)P3-binding sites was obtained. This fraction showed no enrichment in plasma-membrane markers and only a comparatively moderate enrichment (7-fold) in endoplasmic-reticulum markers. The ratio between specific enrichment of Ins(1,4,5)P3 binding and endoplasmic-reticulum markers in the different fractions varied over 50-fold, from less than 0.1 to greater than 5. The purified Ins(1,4,5)P3-binding fraction was enriched to a similar extent (27-fold) in the putative intravesicular Ca(2+)-storage protein calreticulin. Our results favour the concept of a distinct Ins(1,4,5)P3-binding, calreticulin-containing compartment (i.e. the calciosome) in HL-60 cells.

Desensitization of the insulin-secreting beta cell

In human diabetes, inherent impaired insulin secretion can be exacerbated by desensitization of the beta cell by chronic hyperglycemia. Interest in this phenomenon has generated extensive studies in genetic or experimentally induced diabetes in animals and in fully in vitro systems, with often conflicting results. In general, although chronic glucose causes decreased beta-cell response to this carbohydrate, basal response and response to alternate stimulating agents are enhanced. Glucose-stimulated insulin synthesis can be increased or decreased depending on the system studied. Using a two-compartment beta-cell model of phasic insulin secretion, a unifying hypothesis is described which can explain some of the apparent conflicting data. This hypothesis suggests that glucose-desensitization is caused by an impairment in stimulation of a hypothetical potentiator singularly responsible for: 1) some of the characteristic phases of insulin secretion; 2) basal release; 3) potentiation of non-glucose stimulators; and 4) apparent "recovery" from desensitization. Review of some of the pathways that regulate insulin secretion suggest that phosphoinositol metabolism and protein kinase-C production are regulated similarly to the theoretical potentiator and their impairment is a major contributor to glucose desensitization in the beta cell.

The identity of the calcium-storing, inositol 1,4,5-trisphosphate-sensitive organelle in non-muscle cells: calciosome, endoplasmic reticulum ... or both?

Although the initial phase of receptor-mediated Ca2+ signaling, involving Ca2+ release from intracellular stores by inositol 1,4,5-trisphosphate, is relatively well characterized, the nature of the organelle releasing Ca2+ is a controversial subject. At issue is the question of whether Ca2+ is released from the endoplasmic reticulum, or from a more specialized organelle called the 'calciosome'. In this review, we attempt to analyse the arguments for and against these two views, and attempt to reconcile some of the apparently conflicting findings by proposing a hypothetical model of the inositol 1,4,5-trisphosphate-sensitive Ca2+ pool.

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 role of Ins(1,4,5)P3 in the mobilization of Ca2+ from intracellular stores of non-muscle cells has been extensively demonstrated; however, the nature of the organelle releasing the Ca2+ is still poorly understood. The distributions of the Ins(1,4,5)P3-binding sites and of the Ins(1,4,5)P3-sensitive Ca2+ pool were investigated in subcellular fractions obtained from rat liver and compared with those of other markers. The Ins(1,4,5)P3-binding vesicles appeared to be completely distinct from the endoplasmic-reticulum-derived microsomes and were enriched in the same fractions which were enriched in alkaline phosphodiesterase I activity. This co-purification of the plasma-membrane marker with the Ins(1,4,5)P3-binding sites was dramatically altered after freezing or after treatment of the homogenate with the microfilament-disruptive drug cytochalasin B, suggesting that the Ins(1,4,5)P3-sensitive organelle may be linked to the plasma membrane through the actin microfilaments. No correlation was observed between the Ins(1,4,5)P3-binding capacity and the portion of the Ca2+ pool that was released by Ins(1,4,5)P3. This may result from the disruption of the native organelle during homogenization, leading to the formation of vesicles containing the Ins(1,4,5)P3 receptor, but lacking the Ca2+ pump. These results are consistent with the idea of a specialized Ins(1,4,5)P3-regulated organelle distinct from the endoplasmic reticulum, and we propose a model of the structural organization of this organelle, in which the anchorage to the cytoskeleton as well as the spatial separation of the Ca2+ pump from the Ins(1,4,5)P3 receptor have important functional significance.

Second messengers derived from inositol lipids

Many hormones, growth factors, and neurotransmitters stimulate their target cells by promoting the hydrolysis of plasma-membrane phosphoinositides to form the two second messengers, diacylglycerol and inositol 1,4,5-trisphosphate [Ins(1,4,5)P3]. In such cells, ligand-receptor interaction stimulates specific phospholipases that are activated by guanyl nucleotide regulatory G proteins or tyrosine phosphorylation. In many cells, the initial rise in cytoplasmic calcium due to Ins(1,4,5)P3-induced mobilization of calcium from agonist-sensitive stores is followed by a sustained phase of cytoplasmic calcium elevation that maintains the target-cell response, and is dependent on influx of extracellular calcium. Numerous inositol phosphates are formed during metabolism of the calcium-mobilizing messenger, inositol 1,4,5-trisphosphate [Ins(1,4,5)P3], to lower and higher phosphorylated derivatives. The cloning of several phospholipase-C isozymes, as well as the Ins(1,4,5)P3-5 kinase and the Ins(1,4,5)P3 receptor, have clarified several aspects of the diversity and complexity of the phosphoinositide-calcium signaling system. In addition to their well-established roles in hormonal activation of cellular responses such as secretion and contraction, phospholipids and their hydrolysis products have been increasingly implicated in the actions of growth factors and oncogenes on cellular growth and proliferation.

Two Ca2+ transport systems are distinguished on the basis of their Mg2+ dependency in a post-nuclear particulate fraction of bovine adrenal cortex

Inositol 1,4,5-trisphosphate (InsP3) is a second messenger responsible for Ca2+ release from an internal store whose nature and location remains undefined. To get more information on this intracellular Ca2+ store, a post-nuclear particulate fraction was prepared from bovine adrenal cortex and its Ca2+ uptake and release activities were monitored with the fluorescent indicator Fura-2. In the presence of Mg2+ (2 mM), the particulate preparation showed high ATP-dependent Ca2+ sequestering activity and decreased the ambient Ca2+ concentration to about 150 nM. In the absence of Mg2+, Ca2+ was still sequestered but less efficiently, reaching a level around 170 nM. In the presence of Mg2+, the Ca2+ released by a maximal dose of InsP3 (2 microM) was completely resequestered whereas in the absence of Mg2+, no resequestration occurred even after complete degradation of InsP3. The use of selective agents such as oligomycin, saponin, ionomycin and biliary salts indicated that Ca2+ was stored in three different pools which are distinct from the mitochondria and from inside-out membrane vesicles. Our data also indicate that InsP3 releases Ca2+ from a pool which is filled up by a Mg2(+) -dependent Ca2+ ATPase.

Structure and function of inositol trisphosphate receptors

Inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) is a soluble intracellular messenger formed rapidly after activation of a variety of cell-surface receptors that stimulate phosphoinositidase C activity. The initial response to Ins(1,4,5)P3 is a rapid Ca2+ efflux from nonmitochondrial intracellular stores which are probably specialized subcompartments of the endoplasmic reticulum, although their exact identities remain unknown. This initial response is followed by more complex Ca2+ signals: regenerative Ca2+ waves propagate across the cell, repetitive Ca2+ spikes occur, and stimulated Ca2+ entry across the plasma membrane contributes to the sustained Ca2+ signal. The mechanisms underlying these complex Ca2+ signals are unknown, although Ins(1,4,5)P3 is clearly involved. The intracellular receptor that mediates Ins(1,4,5)P3-stimulated Ca2+ mobilization has been purified and functionally reconstituted, and its amino acid sequence deduced from its cDNA sequence. These studies demonstrate that the Ins(1,4,5)P3 receptor has an integral Ca2+ channel separated from the Ins(1,4,5)P3 binding site by a long stretch of residues some of which form binding sites for allosteric regulators, and some of which are substrates for phosphorylation. In this review, we discuss the ligand recognition characteristics of Ins(1,4,5)P3 receptors, and their functional properties in their native environment and after purification, and we relate these properties to what is known of the structure of the receptor. In addition to regulation by Ins(1,4,5)P3, the Ins(1,4,5)P3 receptor is subject to many additional regulatory influences which include Ca2+, adenine nucleotides, pH and phosphorylation by protein kinases. Many of the functional and structural characteristics of the Ins(1,4,5)P3 receptor show striking similarities to another intracellular Ca2+ channel, the ryanodine receptor. These properties of the Ins(1,4,5)P3 are discussed, and their possible roles in contributing to the complex Ca2+ signals evoked by extracellular stimuli are considered.

Insulin-dependent contractility of glomerular mesangial cells in response to angiotensin II, platelet-activating factor and endothelin is attenuated by prostaglandin E2

Culture of glomerular mesangial cells in the absence of insulin decreased the degree of contraction of individual cells in response to vasoconstrictive agonists, angiotensin II, platelet-activating factor and endothelin 1, as compared with cells cultured in the presence of insulin (0.7 nM). This change was associated with a decreased sensitivity of the intracellular Ca2+ response to vasoactive agents in fura-2-loaded cells and with an increase in the basal level of prostanoid [prostaglandins (PG) E1 and E2] production estimated by radioimmunoassay. Addition of exogenous PGE2 to insulin-exposed cells decreased the contractile response to that observed in insulin-deficient cells. Inclusion of 8-bromo cyclic AMP had a similar effect. In 45Ca2(+)-release studies it was shown that, in saponin-permeabilized insulin-exposed cells, preincubation with exogenous PGE2 or 8-bromo cyclic AMP decreased the sensitivity of 45Ca2+ release in response to Ins(1,4,5)P3, as demonstrated by an increase in the EC50 (concn. giving half-maximal effect) to 0.182 +/- 0.024 microM and 0.457 +/- 0.031 microM respectively, as compared with untreated permeabilized cells (EC50 0.091 +/- 0.021 microM). A similar decrease in Ins(1,4,5)P3-sensitive 45Ca2+ release was seen in permeabilized cells from insulin-free conditions of culture (EC50 0.20 +/- 0.061 microM). As altered glomerular haemodynamics are found in insulinopaenic diabetic conditions, it is proposed that a decrease in intracellular Ca2+ availability in response to vasoactive agonists and consequent decrease in mesangial-cell contractility contributes to the hyperfiltration seen in this condition.

Interaction of arginine vasopressin and angiotensin II on Ca2+ in vascular smooth muscle cells

The non-osmotic release of arginine vasopressin (AVP) is associated with the concomitant activation of the renin-angiotensin and sympathetic nervous systems. In vivo studies suggest that a positive interaction may occur between AVP and angiotensin II (Ang II), and other Ca2+ mobilizing hormones. In the present study, the cellular mechanisms of this interaction between AVP and Ang II in vascular smooth muscle cell (VSMC) were examined. These results support the existence of a positive interaction between AVP and Ang II on Ca2+ mobilization in VSMC. In fact, the challenge of VSMC with combined AVP and Ang II, in a range from 5 x 10(-11) to 10(-8) M, enhanced cytosolic free Ca2+ ([Ca2+]i) and 45Ca2+ efflux in a more than additive manner. This potentiation, which was not dependent of the presence of extracellular calcium, correlated with an increased VSMC shape change. Moreover, the combination of subthreshold doses of AVP and Ang II (5 x 10(-11) M), which do not release Ca2+ alone, evoked a Ca2+ mobilizing response. A subthreshold dose of Ang II also shifted to the left the concentration-response curve of the AVP-mediated 45Ca2+ efflux. Since there were no changes in receptor binding of either hormone by the other hormone and the interaction of the two hormones on the production of inositol phosphatides was additive, the AVP and AII positive interaction on Ca2+ mobilization on VSMC may occur at the level of the intracellular Ca2(+)-releasing mechanism itself. Such an interaction can occur at hormone concentrations below the Ca2+ release threshold and may explain an increased functional response to the combination of pressor hormones compared to that of each hormone alone.

Diaphragmatic fatigue is associated with decreased inositol 1,4,5-trisphosphate formation

The ability of the rat diaphragm to produce inositol 1,4,5-trisphosphate (InsP3) in response to a maximal contractile stimulus was determine in vitro in both fatigued and nonfatigued diaphragms. InsP3 was produced during a maximal contraction of the diaphragm. After inducing fatigue, there was a significant reduction in the production of InsP3 compared with that in nonfatigued muscle. The maximal force generated by the diaphragm was also decreased after fatigue. A significant positive linear correlation was found between the force developed by the diaphragm and the amount of InsP3 liberated.

Receptor specific for certain nucleotides stimulates inositol phosphate metabolism and Ca2+ fluxes in A431 cells

We have recently reported that extracellular ATP induces a transient rise in cytosolic free Ca2+ [( Ca2+]i) in individual human epidermoid carcinoma A431 cells (Gonzalez et al: Journal of Cellular Physiology 135:269-276, 1988). We have now studied nucleotide specificity and desensitization for several early responses. Extracellular ATP (5-100 microM) caused the rapid formation of inositol trisphosphate and later its metabolites, inositol bisphosphate and inositol monophosphate. ATP also induced the efflux of 45Ca2+ from pre-loaded cells. In addition, an increase in the rate of influx of 45Ca2+ stimulated by extracellular ATP was detected. Based on measurements of 45Ca2+ efflux and influx, desensitization studies, and chlortetracycline fluorimetry, we conclude that ATP mobilizes Ca2+ from internal stores and also stimulates entry across the plasma membrane. These effects were also displayed by UTP and to a lesser extent by ITP, while other nucleoside triphosphates as well as ADP, AMP, and adenosine, were inactive. Furthermore, desensitization of the response to ATP and UTP was seen after prolonged exposure to either nucleotide. This was specific for the nucleotide receptor since a response to bradykinin was not affected by the ATP pretreatment, although pretreatment with phorbol ester inhibited responses to both the nucleotides and bradykinin. Quantitative data on rate of recovery from the desensitized state and the response of desensitized cells to greatly elevated levels of ATP are presented. Extracellular ATP stimulated another early change previously reported for epidermal growth factor, namely, the phosphorylation of an 81-kDa cytoskeletal protein. The stimulation of these events involves an ATP receptor whose properties differ from other ATP receptors that have been described.

Evidence for an alteration in the microsomal Ca2+ release mechanism in senescent rat parotid acinar cells

Previously, we have shown that Ca2+ mobilization following an alpha 1-adrenergic receptor stimulus is reduced in parotid acinar cells from senescent rats as a result of an altered ability of inositol 1,4,5-trisphosphate (IP3) to induce Ca2+ release from a non-mitochondrial, intracellular Ca2+ store (Ishikawa, Y., et al. Biochim. Biophys. Acta 968, 203-210). We have used this model to examine the IP3-induced Ca2+ release mechanism in these cells. 45Ca2+ efflux, after exposure to (-) epinephrine, from cells of young adult (3-6 months) rats was approx. 2-fold that observed from cells from older animals (approx. 24 months) either in the presence or absence of extracellular Ca2+. Similarly, cytosolic Ca2+ levels were greater in cells of young adult rats under these same incubation conditions. However, microsomal membrane preparations, from both age groups displayed similar IP3 binding sites (Kd approximately 90 nM, Bmax approximately 850 fmol/mg protein) and ATP-dependent Ca2+ transport ability (approx. 8 nmol/mg protein.min -1). These data suggest that there is an alteration in the IP3-induced Ca2+ release mechanism in microsomal membranes of parotid glands from senescent rats which may account for the decreased Ca2+ release seen after agonist stimulation of this tissue.

Calcium signaling and gonadotropin secretion

Agonist activation of pituitary gonadotrophs by gonadotropin-releasing hormone (GnRH) stimulates rapid InsP(3)-dependent peaks of calcium mobilization and luteinizing hormone (LH) release, followed by sustained increases in calcium-influx and hormone secretion. Receptor-mediated calcium entry through L-type and dihydropyridine-insensitive calcium channels accounts for the sustained elevation of cytosolic calcium during GnRH action, and for most of the gonadotropin secretory response. Protein kinase C contributes to the phase of sustained LH release from GnRH-stimulated gonadotrophs, and also to gonadotropin synthesis. Calcium-dependent inactivation of L channels occurs during GnRH action, and appears to be a primary factor in the onset of desensitization of gonadotropin secretion.

Effects of GTP on Ca2+ movements across endoplasmic reticulum membranes

Our initial observation that GTP could, under some experimental conditions, have profound effects on Ca2+ movements across endoplasmic reticulum membranes arose from attempts to increase the sensitivity of rat liver microsomes to inositol 1,4,5 trisphosphate (IP3). Most preparations of microsomal fractions from rat liver release only a very small percentage of accumulated Ca2+ on addition of IP3. We found, rather empirically, that the addition of microM concentrations of GTP greatly enhanced the amount of Ca2+ releasable by IP3. The initial, very appealing, hypothesis was to postulate a direct effect of GTP on the IP3-sensitive Ca2+ channel. This idea is no longer tenable, as will be described below. The more likely explanation, that GTP has its effect by either fusing small microsomal vesicles together or by allowing some form of communication between adjacent membranes is considerably more complex mechanistically and also possibly has far reaching implications for the mechanisms by which cells organise and maintain their reticular structures.