Inositol 1,4,5-trisphosphate binds to a specific receptor and releases microsomal calcium in the anterior pituitary gland

Gonadotrope plasticity at cellular, population and structural levels: A comparison between fishes and mammals

Often referred to as "the master gland", the pituitary is a key organ controlling growth, maturation, and homeostasis in vertebrates. The anterior pituitary, which contains several hormone-producing cell types, is highly plastic and thereby able to adjust the production of the hormones governing these key physiological processes according to the changing needs over the life of the animal. Hypothalamic neuroendocrine control and feedback from peripheral tissues modulate pituitary cell activity, adjusting levels of hormone production and release according to different functional or environmental requirements. However, in some physiological processes (e.g. growth, puberty, or metamorphosis), changes in cell activity may be not sufficient to meet the needs and a general reorganization of cell composition and pituitary structure may occur. Focusing on gonadotropes, this review examines plasticity at the cellular level, which allows precise and rapid control of hormone production and secretion, as well as plasticity at the population and structural levels, which allows more substantial changes in hormone production. Further, we compare current knowledge of the anterior pituitary plasticity in fishes and mammals in order to assess what has been conserved or not throughout evolution, and highlight important remaining questions.

Inositol 1,4,5-trisphosphate and its receptors

Activation of cells by many extracellular agonists leads to the production of inositol 1,4,5-trisphosphate (IP₃). IP₃ is a global messenger that easily diffuses in the cytosol. Its receptor (IP₃R) is a Ca(2+)-release channel located on intracellular membranes, especially the endoplasmic reticulum (ER). The IP₃R has an affinity for IP(3) in the low nanomolar range. A prime regulator of the IP₃R is the Ca(2+) ion itself. Cytosolic Ca(2+) is considered as a co-agonist of the IP₃R, as it strongly increases IP(3)R activity at concentrations up to about 300 nM. In contrast, at higher concentrations, cytosolic Ca(2+) inhibits the IP₃R. Also the luminal Ca(2+) sensitizes the IP₃R. In higher organisms three genes encode for an IP₃R and additional diversity exists as a result of alternative splicing mechanisms and the formation of homo- and heterotetramers. The various IP₃R isoforms have a similar structure and a similar function, but due to differences in their affinity for IP₃, their variable sensitivity to regulatory parameters, their differential interaction with associated proteins, and the variation in their subcellular localization, they participate differently in the formation of intracellular Ca(2+) signals and this affects therefore the physiological consequences of these signals.

Ion channels and signaling in the pituitary gland

Endocrine pituitary cells are neuronlike; they express numerous voltage-gated sodium, calcium, potassium, and chloride channels and fire action potentials spontaneously, accompanied by a rise in intracellular calcium. In some cells, spontaneous electrical activity is sufficient to drive the intracellular calcium concentration above the threshold for stimulus-secretion and stimulus-transcription coupling. In others, the function of these action potentials is to maintain the cells in a responsive state with cytosolic calcium near, but below, the threshold level. Some pituitary cells also express gap junction channels, which could be used for intercellular Ca(2+) signaling in these cells. Endocrine cells also express extracellular ligand-gated ion channels, and their activation by hypothalamic and intrapituitary hormones leads to amplification of the pacemaking activity and facilitation of calcium influx and hormone release. These cells also express numerous G protein-coupled receptors, which can stimulate or silence electrical activity and action potential-dependent calcium influx and hormone release. Other members of this receptor family can activate calcium channels in the endoplasmic reticulum, leading to a cell type-specific modulation of electrical activity. This review summarizes recent findings in this field and our current understanding of the complex relationship between voltage-gated ion channels, ligand-gated ion channels, gap junction channels, and G protein-coupled receptors in pituitary cells.

A human endothelial cell feeder system that efficiently supports the undifferentiated growth of mouse embryonic stem cells

Feeder cells are commonly used to culture embryonic stem cells to maintain their undifferentiated and pluripotent status. Conventionally, mouse embryonic fibroblasts (MEFs), supplemented with leukemia inhibitory factor (LIF), are used as feeder cells to support the growth of mouse embryonic stem cells (mESCs) in culture. To prepare for fresh MEF feeder or for MEF-conditioned medium, sacrifice of mouse fetuses repeatedly is unavoidable in these tedious culture systems. Here we report the discovery of a human endothelial cell line (ECV-304 cell line) that efficiently supports growth of mESCs LIF-free conditions. mESCs that were successfully cultured for eight to 20 passages on ECV-304 feeders showed morphological characteristics similar to cells cultured in traditional feeder cell systems. These cells expressed the stem cell markers Oct3/4, Nanog, Sox2, and SSEA-1. Furthermore, cells cultured on the ECV-304 cell line were able to differentiate into three germ layers and were able to generate chimeric mice. Compared with traditional culture systems, there is no requirement for mouse fetuses and exogenous LIF does not need to be added to the culture system. As a stable cell line, the ECV-304 cell line efficiently replaces MEFs as an effective feeder system and allows the efficient expansion of mESCs.

An immobilized metal ion affinity adsorption and scintillation proximity assay for receptor-stimulated phosphoinositide hydrolysis

A novel approach to measuring receptor-stimulated phosphoinositide hydrolysis was developed based on the principles of immobilized metal ion affinity chromatography (IMAC) and scintillation proximity assay (SPA). Hard Lewis metal ions, such as Zr(4+), Ga(3+), Al(3+), Fe(3+), Lu(3+), and Sc(3+), were immobilized on SPA beads via metal chelate and utilized as affinity ligands to entrap inositol phosphates. [3H]Inositol phosphates bound to IMAC-SPA beads through the strong interaction of their phosphate group with the immobilized metal ions. The binding brought [3H]inositol phosphates in close proximity to the scintillant embedded in the SPA beads, thereby allowing the radioactivity to be quantified. Quantification of [3H]inositol phosphate production in cells preincubated with [3H]inositol provided a highly sensitive measurement of phosphoinositide hydrolysis. The utility of this approach was demonstrated in measuring the response mediated by the G-protein-coupled neurokinin NK1 receptor and the tyrosine kinase-linked platelet-derived growth factor (PDGF) receptor. Substance P stimulated phosphoinositide hydrolysis concentration-dependently in CHO cells expressing NK1 receptors with a maximal 12-fold increase in inositol phosphate production. Similarly, PDGF-BB stimulated a 5-fold increase in phosphoinositide hydrolysis in quiescent Swiss 3T3 cells. This new approach is highly sensitive, fast, simple, easily performed on 96-well plates, and amenable for high-throughput screening.

Regulation of the type III InsP(3) receptor by InsP(3) and ATP

Many hormones and neurotransmitters raise intracellular calcium (Ca(2+)) by generating InsP(3) and activating the inositol 1,4, 5-trisphosphate receptor (InsP(3)R). Multiple isoforms with distinct InsP(3) binding properties () have been identified (). The type III InsP(3)R lacks Ca(2+)-dependent inhibition, a property that makes it ideal for signal initiation (). Regulation of the type III InsP(3)R by InsP(3) and ATP was explored in detail using planar lipid bilayers. In comparison to the type I InsP(3)R, the type III InsP(3)R required a higher concentration of InsP(3) to reach maximal channel activity (EC(50) of 3.2 microM versus 0.5 microM for the types III and I InsP(3)R, respectively). However, the type III InsP(3)R did reach a 2.5-fold higher level of activity. Although activation by InsP(3) was isoform-specific, regulation by ATP was similar for both isoforms. In the presence of 2 microM InsP(3), low ATP concentrations (<6 mM) increased the open probability and mean open time. High ATP concentrations (>6 mM) decreased channel activity. These results illustrate the complex nature of type III InsP(3)R regulation. Enhanced channel activity in the presence of high InsP(3) may be important during periods of prolonged stimulation, whereas allosteric modulation by ATP may help to modulate intracellular Ca(2+) signaling.

Threshold for inositol 1,4,5-trisphosphate action

We developed a unidirectional 45Ca2+ efflux technique in which 60 cumulative doses of inositol 1,4,5-trisphosphate (InsP3), each lasting 6 s, were subsequently added to permeabilized A7r5 cells. This technique allowed an accurate determination of the threshold for InsP3 action, which was around 32 nM InsP3 under control conditions. The InsP3-induced Ca2+ release was characterized by an initial rapid phase, after which the normalized rate progressively decreased. The slowing of the release was associated with a shift of the threshold to higher InsP3 concentrations. Stimulatory concentrations of thimerosal (10 microM) shifted the threshold to 4.5 nM InsP3 and increased both the cooperativity and the maximal normalized rate of Ca2+ release. This low threshold was maintained when the thimerosal concentration was increased to inhibitory levels (100 microM) but then the effects on the cooperativity and on the normalized rate of Ca2+ release disappeared. Oxidized glutathione (5 mM) was much less effective in stimulating the release and did not have an effect on the threshold or on the cooperativity. ATP (5 mM) stimulated the release despite a shift in threshold toward higher InsP3 concentrations. Luminal Ca2+ did not affect the threshold for InsP3 action but stimulated the normalized release at each InsP3 concentration. The inhibitory effect of 10 microM free cytosolic Ca2+ was associated with a shift in threshold to higher InsP3 concentrations and a decreased cooperativity of the release process. We conclude that this novel technique of accurately measuring the threshold for InsP3 action under various experimental conditions has allowed us to refine the analysis of the kinetic parameters involved in the regulation of the InsP3 receptor.

Calcium release activity and metabolism of inositol 1,4,5-trisphosphate in T cells. Modulation by novel inositol 1,4,5-trisphosphate 5-phosphatase inhibitors

Stimulation of the T cell antigen receptor/CD3 complex is followed by phospholipase C activation, phosphoinositol lipid metabolism and ultimately by a rapid rise in both myo-inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] and myo-inositol 1,3,4,5-tetrakisphosphate [Ins(1,3,4,5)P4] as well as cytosolic free calcium concentration. A 5-phosphatase plays a pivotal role in the subsequent metabolism of Ins(1,4,5)P3 and Ins(1,3,4,5)P4. Synthetic routes have been developed which have enabled the synthesis of both natural and unnatural inositol phosphates and this approach has yielded several compounds which have been shown to act as inhibitors of Ins(1,4,5)P3 5-phosphatase. These compounds offer considerable potential for investigation of the complex metabolism and function of Ins(1,4,5)P3 and Ins(1,3,4,5)P4 in T cell activation and proliferation. We now report the time course and temperature sensitivity of Ins(1,4,5)P3-induced 45Ca2+ release in the permeabilised leukaemic T cell line Jurkat. Furthermore, we demonstrate that the metabolism of Ins(1,4,5)P3 in the presence of two novel 5-phosphatase inhibitors, namely L-myo-inositol 1,4,5-trisphosphorothioate [L-Ins(1,4,5)PS3] and myo-inositol 1,3,5-trisphosphorothioate [Ins(1,3,5)PS3], can be inhibited with concomitant elevation of the heparin-sensitive Ins(1,4,5)P3-induced release of 45Ca2+. These novel 5-phosphatase inhibitors provide a starting point for development of cell-permeable analogues which may be able to modulate cell function in intact cells and may be used as manipulative tools with which to elucidate the function of Ins(1,4,5)P3 and Ins(1,3,4,5)P4 with respect to T cell activation.

Distribution of the inositol 1,4,5-trisphosphate receptor, P400, in adult rat brain

The distribution of the inositol 1,4,5-trisphosphate receptor protein, P400, was investigated in adult rat brain by immunocytochemistry with the monoclonal antibody 4C11 raised against mouse cerebellar inositol 1,4,5-trisphosphate receptor protein. Immunoreactive neuronal cell bodies were detected in the cerebral cortex, the claustrum, the endopiriform nucleus, the corpus callosum, the anterior olfactory nuclei, the olfactory tubercle, the nucleus accumbens, the lateral septum, the bed nucleus of the stria terminalis, the hippocampal formation, the dentate gyrus, the caudate-putamen, the fundus striatum, the amygdaloid complex, the thalamus, the caudolateral part of the hypothalamus, the supramammillary nuclei, the substantia nigra, the pedunculopontine tegmental nucleus, the ventrotegmental area, the Purkinje cells in the cerebellum, the dorsal cochlear nucleus, the subnucleus oralis and caudalis of trigeminal nerve, and the dorsal horn of the spinal cord. Immunoreactive fibres were found in the medial forebrain bundle, the globus pallidus, the stria terminalis, the pyramidal tract, the spinal tract of trigeminal nerve, and the ventral horn of spinal cord. Nerve fibres forming a dense plexus ending in terminal-like boutons were detected in relation to nonimmunoreactive neurons of the dentate, interpositus, and fastigial nuclei of the cerebellum and around neurons of the vestibular nuclei. This receptor protein binds a specific second messenger, inositol 1,4,5-trisphosphate, which produces a mobilization of intracellular Ca2+ and a modulation of transmitter release.

Identification and Characterization of High-Affinity Binding Sites for Inositol Trisphosphate in Red Beet

Inositol 1,4,5-trisphosphate (InsP3) is thought to play a primary role in intracellular Ca2+ mobilization during signal transduction in plant cells. Although InsP3-elicited Ca2+ release across the vacuolar membrane has been demonstrated in a variety of species, little is known of the properties of the putative InsP3 receptor. Using a 3H-InsP3 ligand-displacement assay with detergent-solubilized microsomes from the storage root of red beet, we determined that InsP3 binds specifically to a single class of high-affinity binding sites (dissociation constant [Kd] = 121 [plus or minus] 10 nM) with an estimated receptor density of 0.84 pmol/mg. Binding of InsP3 is selective, because other inositol phosphates exhibited only supramicromolar affinities for the binding site. Low molecular weight heparin was a potent competitive inhibitor of InsP3 binding (Kd = 301 [plus or minus] 72 nM). High concentrations of ATP also displaced 3H-InsP3 (Kd = 0.66 mM). Preincubation of microsomes with sulfhydryl reagents reduced InsP3-specific binding in an InsP3-protectable manner. Density gradient centrifugation of microsomes led to copurification of InsP3-specific binding with a fraction enriched in vacuolar membrane. Despite a probable difference in cellular location, the putative InsP3 receptor of red beet has characteristics that are very similar to those of animal InsP3 receptors. These studies provide direct evidence of InsP3-specific binding in plant tissue and strengthen the argument that InsP3-induced Ca2+ release is a component in plant cell signal transduction.

Comparison and characterization of retinal pericytes and retinal pigment epithelial cells on subcellular IP3-sensitive Ca2+ pools

A comparative study of inositol 1,4,5-trisphosphate (IP3)-induced Ca2+ mobilization in bovine retinal capillary pericytes (BRCP) and bovine retinal pigment epithelial cells (BRPE) was carried out. Both cells were permeabilized with saponin. The two cell types had similar basal levels of [Ca2+]i (130 nM for BRCP, 132 nM for BRPE) and responded to IP3 in a dose-dependent manner. However, when stimulated by various concentrations of IP3 (1-10 microM), the increase in [Ca2+]i of BRCP was always two- to threefold higher than that in BRPE. Subcellular-fractionation studies showed that a single population of IP3 binding site with a high affinity and high specificity of IP3 mainly localized to plasma membrane in these two cell types. Although the dissociation constant of specific [32P]-IP3 binding sites (Kd 1.9-2.8 nM) was similar, the profile of maximal binding capacity (Bmax) of each fraction was markedly different. In comparison, plasma membrane fractions of BRCP were with Bmax of 165 fmol/mg protein versus 90 fmol/mg protein for BRPE membranes. The ATP-dependent Ca2+ uptake and IP3-dependent Ca2+ release were observed in the both plasma membrane fractions. With quantitative correlation, the membrane fraction (2 mg) of BRCP released 0.2 nmol Ca2+ whereas BRPE only released 0.07 nmol Ca2+ with the same dose of IP3 (5 microM). The selectively higher density of IP3 binding sites in coupling to the larger Ca(2+)-release in the membrane of BRCP suggests that the quantity of Ca2+ mobilized is determined by the spatially preferential distribution of membrane-associated IP3 binding sites. These findings may provide an explanation for the differences observed between BRCP and BRPE in IP3-induced DNA replication.

Gonadotropin releasing hormone activates the lipoxygenase pathway in cultured pituitary cells: role in gonadotropin secretion and evidence for a novel autocrine/paracrine loop

The formation and role of arachidonic acid (AA) and its metabolites during gonadotropin releasing hormone- (GnRH-) induced gonadotropin secretion were investigated in primary cultures of rat pituitary cells. Prelabeled cells ([3H]AA) responded to GnRH challenge with increased formation (about 2-fold) of the leukotrienes LTC4, LTD4, and LTE4 as well as 5- and 15-eicosatetraenoic acids (5- and 15-HETE) as identified by HPLC. Formation of leukotrienes and 15-HETE was further verified by specific radioimmunoassays. No significant increase in the formation of 12-HETE or of the cyclooxygenase products prostaglandin E (PGE) and thromboxane A2 by GnRH was noticed. Addition of physiological concentrations of LTC4 enhanced basal LH release, while subphysiological concentrations of LTC4 (10(-15)-10(-12) M) inhibited GnRH-induced LH release by about 35% (p less than 0.02). Using specific lipoxygenase inhibitors L-656,224 and MK 886, we found inhibition of GnRH-induced LH release by about 40% at concentrations known to specifically inhibit the 5-lipoxygenase pathway. The peptidoleukotriene receptor antagonist ICI 198,615 inhibited LTC4- and LTE4-induced LH release and surprisingly also the effect of GnRH on LH release by 40%. The data strongly suggest a role for AA and its lipoxygenase metabolites in the on/off reactions of GnRH upon LH release. The data also present a novel amplification cycle in which newly formed leukotrienes become first messengers and establish an autocrine/paracrine loop.

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.

Characterization of a novel inositol 1,4,5-trisphosphate receptor in isolated olfactory cilia

Inositol 1,4,5-trisphosphate (InsP3), a product of G-protein-mediated receptor activation of phosphoinositide turnover, plays the role of a second messenger when olfactory neurons are stimulated with certain olfactory stimuli. In this paper we examine the specific binding of [3H]InsP3 to isolated olfactory cilia, microsomes and brain membranes from the channel catfish (Ictalurus punctatus) and, by photoaffinity labelling with an InsP3 analogue (125I-labelled 1-[3-(4-azidosalicyloxy)-aminopropyl]inositol 1,4,5-trisphosphate (125I-ASA-InsP3)], we tentatively identify the major InsP3-binding protein in catfish olfactory cilia. InsP3 binding to ciliary membranes is specific and saturable, with a Kd of 1.10 +/- 0.31 microM and a maximum number of binding sites (Bmax) of 17.6 +/- 5.8 pmol/mg. The rank order for potency of inhibition of [3H]InsP3 binding is Ins(1,4)P2 less than Ins(1,3,4)P3 less than Ins(1,3,4,5)P4 = Ins(1,4,5)P3 less than Ins(2,4,5)P3. Exposure of cilia membranes to u.v. light in the presence of 125I-ASA-InsP3 results in the labelling of a protein with apparent Mr 107,000. Labelling is specifically prevented by Ins(1,4,5)P3, Ins(2,4,5)P3 and Ins(1,3,4,5)P4, but not by Ins(1,4)P2 or Ins(1,3,4)P3. Both specific [3H]InsP3 binding and photoaffinity labelling of the Mr-107,000 protein were displaced by heparin. The Kd and the inhibition of [3H]InsP3 binding and of photoaffinity labelling by inositol phosphates and heparin are consistent with the ability of micromolar concentrations of Ins(1,4,5)P3 [but not Ins(1,3,4)P3] to activate the InsP3-gated currents in patch-clamp experiments with olfactory neurons. These results suggest that InsP3 binding to a Mr-107,000 cilia membrane protein may represent binding to the olfactory InsP3-gated cation channel.

Purification and characterization of the inositol 1,4,5- trisphosphate receptor protein from rat vas deferens

Among rat peripheral tissues examined, Ins(1,4,5)P(3) receptor binding is highest in the vas deferens, with levels about 25% of those of the cerebellum. We have purified the InsP(3) receptor binding protein from rat vas deferens membranes 600-fold. The purified protein displays a single 260 kDa band on SDS/PAGE, and the native protein has an apparent molecular mass of 1000 kDa, the same as in cerebellum. The inositol phosphate specificity, pH-dependence and influence of various reagents are the same for purified vas deferens and cerebellar receptors. Whereas particulate InsP(3) binding in cerebellum is potently inhibited by Ca(2+), particulate and purified vas deferens receptor binding of InsP(3) is not influenced by Ca(2+). Vas deferens appears to lack calmedin activity, but the InsP(3) receptor is sensitive to Ca(2+) inhibition conferred by brain calmedin. The vas deferens may prove to be a valuable tissue for characterizing functional aspects of InsP(3) receptors.

Calcium release mechanisms in smooth muscle

Properties of the intracellular Ca store were studied using saponin-skinned fiber bundles of guinea pig smooth muscles using a fluorescent Ca indicator method. There exist two Ca release mechanisms in the Ca store: Ca-induced Ca release (CICR) and inositol 1,4,5-trisphosphate (IP3)-induced Ca release (IICR) mechanisms. The smooth muscle Ca store consists of two compartments: one (S alpha) has both CICR and IICR, and the other (S beta) has only IICR. The smooth muscle CICR is activated by greater than 1 microM Ca2+, has essentially the same properties with that in striated muscles, and is open-locked by ryanodine. After ryanodine treatment, therefore, the Ca uptake capacity of S alpha is selectively lost ('functional removal') with no effect on S beta. The IICR is Ca2(+)-dependent: Ca2+ enhances the IICR below 300 nM, but has also an inhibitory effect above this concentration. Therefore, Ca2+ acts on the IICR in a positive feedback manner when muscle tension is about to rise, making IP3 more effective, but this feedback is cut off as the tension approaches the maximum. ATP enhances the IICR as is the case in the CICR. 'Functional removal' of S alpha in intact bundles by ryanodine was used to estimate the role of Ca release in agonist-induced contractions. Ca release from the S alpha is important at least in the initial phase of contractions; and in the pulmonary artery, most of the activator Ca2+ originates from S alpha.

Keratinization of rat vaginal epithelium--V. Modulation of intracellular calcium by estradiol

Changes in the calcium levels under the influence of estradiol were investigated in rat vaginal epithelial cells (VEC). After single estradiol injection, the immature rats showed 1.5-fold increase in Ca2+ levels within 15 min when compared to control animals. Progesterone priming brought calcium levels well below control values throughout the experimental period (up to 12 h). Ca2+ levels in serum did not show any appreciable change. Localization of calcium in VEC with electron microscopy showed aggregates of calcium oxalate on the inner nuclear membrane, nucleolus, mitochondria and keratohyaline granules. After 15 min of estradiol priming, maximum electron density was seen on all these cell organelles mentioned above, however, by 30 min the electron density was reduced considerably and did not increase during the experimental period (up to 12 h).

Ultrastructural localization of calcium and Ca(2+)-ATPase activity in gonadotropes and stellate cells of the catfish pituitary

In the pituitary of the African catfish, Clarias gariepinus, calcium precipitates were ultrastructurally visualized with the oxalate-pyroantimonate procedure (OPP). The presence of calcium in these precipitates was validated with several methods, including "Electron Energy Loss Spectrometry" (EELS). In the OPP-treated tissue calcium precipitates were seen in a) non-secretory stellate cells and b) gonadotropic (GTH-) cells. In the latter the amount of precipitate is generally low, but stimulation of the gonadotropin release, either in vivo or in vitro, resulted in a considerable increase. This increase is discussed in relation to the role of calcium as second messenger in the GTH-cells. Ca(2+)-ATPase was exclusively represented in stellate cells and GTH-cells, its strongest activity associated with the plasma membrane and with the membranes of the endoplasmic reticulum. The localization of this enzyme is discussed in relation to its role in the regulation of the intracellular calcium concentration in the GTH-cells. The stellate cells are considered to be involved in the regulation of extracellular calcium concentrations in the pituitary.

The inositol 1,4,5,-trisphosphate receptor in cerebellar Purkinje cells: quantitative immunogold labeling reveals concentration in an ER subcompartment

The Ca2+ mobilization effect of inositol 1,4,5-trisphosphate, the second messenger generated via receptor-stimulated hydrolysis of phosphatidylinositol 4,5-bisphosphate, is mediated by binding to intracellular receptors, which are expressed in high concentration in cerebellar Purkinje cells. Partially conflicting previous reports localized the receptor to various subcellular structures: elements of ER, both rough and smooth-surfaced, the nuclear envelope, and even the plasma membrane. We have now reinvestigated the problem quantitatively by using cryosections of rat cerebellar tissue immunolabeled with polyclonal monospecific antibodies against the inositol 1,4,5-trisphosphate receptor. By immunofluorescence the receptor was detected only in Purkinje cells, whereas the other cells of the cerebellar cortex remained negative. In immunogold-decorated ultrathin cryosections of the Purkinje cell body, the receptor was concentrated in cisternal stacks (piles of up to 12 parallel cisternae separated by regularly spaced bridges, located both in the deep cytoplasm and beneath the plasma membrane; average density, greater than 5 particles/micron of membrane profile); in cisternal singlets and doublets adjacent to the plasma membrane (average density, approximately 2.5 particles/micron); and in other apparently smooth-surfaced vesicular and tubular profiles. Additional smooth-surfaced elements were unlabeled. Perinuclear and rough-surfaced ER cisternae were labeled much less by themselves (approximately 0.5 particles/micron, two- to threefold the background), but were often in direct membrane continuity with heavily labeled, smooth-surfaced tubules and cisternal stacks. Finally, mitochondria, Golgi cisternae, multivesicular bodies, and the plasma membrane were unlabeled. In dendrites, approximately half of the nonmitochondrial, membrane-bound structures (cisternae, tubules, and vesicles), as well as small cisternal stacks, were labeled. Dendritic spines always contained immunolabeled cisternae and vesicles. The dendritic plasma membrane, of both shaft and spines, was consistently unlabeled. These results identify a large, smooth-surfaced ER subcompartment that appears equipped to play a key role in the control of Ca2+ homeostasis: in particular, in the generation of [Ca2+]i transients triggered by activation of specific receptors, such as the quisqualate-preferring trans(+/-)-1-amino-1,3-cyclopentamedicarboxylic acid glutamatergic receptors, which are largely expressed by Purkinje cells.

Thyrotropin-releasing hormone activates a [Ca2+]i-dependent K+ current in GH3 pituitary cells via Ins(1,4,5)P3-sensitive and Ins(1,4,5)P3-insensitive mechanisms

The role of Ins(1,4,5)P3 in receptor-induced Ca2+ mobilization in pituitary cells was studied at the single-cell level. Experimental strategies were developed which allowed a comparative analysis of the effects of Ins(1,4,5)P3 with those of receptor activation under identical conditions. These include microfluorimetry as well as a novel technique which permits the controlled and rapid application of intracellular messenger molecules to individual cells. This latter approach is based on the tight-seal whole-cell recording (WCR) technique, and utilizes two patch-clamp micropipettes, one for electrical recording and the second for the controlled pressure injection. Ins(1,4,5)P3, when applied with this dual-WCR (DWCR) technique, leads rapidly to a marked rise in cytosolic free Ca2+ [( Ca2+]i) and a concomitant stimulation of Ca2(+)-activated K+ current; Ins(1,4,5)P3 can thus mimic the effects of thyrotropin-releasing hormone (TRH) in the same cells under identical conditions. In cells dialysed intracellularly with heparin, a potent antagonist of Ins(1,4,5)P3 action, the rapid response to extracellular stimulation with TRH was abolished, as were the effects of intracellular application of Ins(1,4,5)P3. Heparin, which abolished Ins(1,4,5)P3 action completely, blocked responses to TRH in some cells only partially, revealing that Ca2+ mobilization response to TRH is in part slower in onset than the response to Ins(1,4,5)P3. It is concluded (1) that Ins(1,4,5)P3 is an essential element for the action of TRH, providing a rapid mechanism for Ca2+ mobilization induced by the releasing hormone and (2) that TRH action in mobilizing intracellular Ca2+ is sustained by a slower mechanism which is independent of Ins(1,4,5)P3.

The effect of external calcium and pH on inositol trisphosphate-mediated calcium release from cerebellum microsomal fractions

Binding of D-myo-inositol 1,4,5-trisphosphate (InsP3) to rat cerebellum membranes has previously been shown to be stimulated by alkaline pH and inhibited by low concentrations of Ca2+ [Worley, Baraban, Suppatopone, Wilson & Snyder (1987) J. Biol. Chem. 262, 12132-12136]. In the present study, Scatchard analysis of InsP3 binding to cerebellum microsomes indicates that the effects of Ca2+ and pH are exerted through changes in the apparent affinity of the receptor without effects on maximal binding. The influence of extravesicular Ca2+ and pH on InsP3-mediated 45Ca2+ release was investigated. Extravesicular Ca2+ inhibited InsP3-mediated Ca2+ release. The inhibitory effect of Ca2+ was most marked when a sub-optimal concentration of InsP3 was used. An increase in extravesicular pH produced a decrease in the concentration of InsP3 that yielded half-maximal Ca2+ release. Regulation of the affinity of the InsP3 receptor by Ca2+ and pH can qualitatively account for the observed effects of these factors on InsP3-mediated Ca2+ release. Feedback inhibition of InsP3 binding by Ca2+ could provide a mechanism to generate Ca2+ oscillations, particularly under hormonal conditions that produce sub-optimal elevations of InsP3 concentration.

The Croonian lecture, 1988. Inositol lipids and calcium signalling

The response of cells to many external stimuli requires a decoding process at the membrane to transduce information into intracellular messengers. A major decoding mechanism employed by a variety of hormones, neurotransmitters and growth factors depends on the hydrolysis of a unique inositol lipid to generate two key second messengers, diacylglycerol and inositol 1,4,5-trisphosphate (Ins(1,4,5)P3). Here I examine the second messenger function of Ins(1,4,5)P3 in controlling the mobilization of calcium. We know most about how this messenger releases calcium from internal reservoirs but less is known concerning the entry of external calcium. One interesting possibility is that Ins(1,4,5)P3 might function in conjunction with its metabolic product Ins(1,3,4,5)P4 to control calcium entry through a mechanism employing a region of the endoplasmic reticulum as a halfway house during the transfer of calcium from outside the cell into the cytoplasm. The endoplasmic reticulum interposed between the plasma membrane and the cytosol may function as a capacitor to insure against the cell being flooded with external calcium. When stimulated, cells often display remarkably uniform oscillations in intracellular calcium. At least two oscillatory patterns have been recognized suggesting the existence of separate mechanisms both of which may depend upon Ins(1,4,5)P3. In one mechanism, oscillations may be driven by periodic pulses of Ins(1,4,5)P3 produced by receptors under negative feedback control of protein kinase C. The other oscillatory mechanism may depend upon Ins(1,4,5)P3 unmasking a process of calcium-induced calcium release from the endoplasmic reticulum. The function of these calcium oscillations is still unknown. This Ins(1,4,5)P3/calcium signalling system is put to many uses during the life history of a cell.(ABSTRACT TRUNCATED AT 250 WORDS)

Highly cooperative opening of calcium channels by inositol 1,4,5-trisphosphate

The kinetics of calcium release by inositol 1,4,5-trisphosphate (IP3) in permeabilized rat basophilic leukemia cells were studied to obtain insight into the molecular mechanism of action of this intracellular messenger of the phosphoinositide cascade. Calcium release from intracellular storage sites was monitored with fura-2, a fluorescent indicator. The dependence of the rate of calcium release on the concentration of added IP3 in the 4 to 40 nM range showed that channel opening requires the binding of at least three molecules of IP3. Channel opening occurred in the absence of added adenosine triphosphate, indicating that IP3 acts directly on the channel or on a protein that gates it. The channels were opened by IP3 in less than 4 seconds. The highly cooperative opening of calcium channels by nanomolar concentrations of IP3 enables cells to detect and amplify very small changes in the concentration of this messenger in response to hormonal, sensory, and growth control stimuli.