Inositol 1,4,5-trisphosphate-triggered Ca2+ release from bovine adrenal medullary secretory vesicles

Inositol 1,4,5-trisphosphate- and caffeine-sensitive Ca(2+)-storing organelle in bovine adrenal chromaffin cells

The uptake and release properties of Ca2+ by several subcellular fractions of the bovine adrenal medulla were investigated. Investigation by the 45Ca2+ tracer method showed that permeabilized cells and the fractions of mitochondria (MT) and microsomes (MC) caused ATP-dependent Ca2+ uptake in a Ca2+ concentration-dependent manner (pCa 8-4), whereas permeabilized cells and the fractions of secretory granules (SG) were able to accumulate a significant amount of Ca2+ even in the absence of ATP, which was completed by the addition of hexokinase and glucose. In these organelle fractions, Ca2+ uptake in the presence of ATP at pCa 7 and pCa 5.8 was well-correlated with the activity of the NADPH cytochrome c reductase (marker enzyme for the endoplasmic reticulum) and cytochrome c oxidase (marker enzyme for mitochondria), respectively. As detected by Fura-2 ratiometry, both inositol 1,4,5-trisphosphate (IP3) and caffeine caused concentration-dependent Ca2+ releases from permeabilized cells and MC, but not from MT and SG. In an ATP-depleted condition, homogenates still took up a significant amount of Ca2+ but was not able to respond to IP3 and caffeine. These results suggest that the endoplasmic reticulum is a major Ca(2+)-storing organelle, which releases Ca2+ in response to IP3 and caffeine in bovine adrenal chromaffin cells.

Changes in element concentrations induced by agonist in pig pancreatic acinar cells

Changes in electrolytes of pig pancreatic acinar cells following application of gastrin-cholecystokinin (CCK) were investigated using the technique of X-ray microanalysis of hydrated and dehydrated sections of freshly frozen pancreas. After stimulation by CCK (10(-9) M), Na and Cl increased significantly in the cytoplasm [Na, from 10 mmol/kg wet wt. (48 mmol/kg dry wt.) to 19 mmol/kg (95 mmol/kg); Cl, from 22 mmol/kg (105 mmol/kg) to 49 mmol/kg (245 mmol/kg)] as well as in the luminal interspace [Na, from 53 mmol/kg (189 mmol/kg) to 65 mmol/kg (283 mmol/kg); Cl, from 65 mmol/kg (232 mmol/kg) to 102 mmol/kg (443 mmol/kg)]. In the secretory granules Cl increased significantly from 30 mmol/kg (86 mmol/kg) to 67 mmol/kg (203 mmol/kg). K decreased significantly from 120 mmol/kg (571 mmol/kg) to 81 mmol/kg (405 mmol/kg) in the cytoplasm, while both increased from 38 mmol/kg (109 mmol/kg) to 58 mmol/kg (176 mmol/kg) in the granules and from 46 mmol/kg (164 mmol/kg) to 48 mmol/kg (209 mmol/kg) in the luminal interspace. Ca increased significantly in the cytoplasm as well as in the luminal interspace, and decreased significantly in the secretory granules. CCK evoked Ca release from secretory granules in the secretory pole of acinar cells. The values were measured from dehydrated sections, and agreed well with those from hydrated sections. The effect of furosemide, an inhibitor of the Na+-K+-2Cl- co-transporter, on the ion transport of acinar cell was studied. When furosemide (10(-5) M) was added to the external solution, the cytoplasmic Cl and Ca concentrations decreased significantly, while there was a little decrease in Na and K concentrations under the secretory condition. These results indicate that Na+-K+-2Cl- co-transport, and Na+, Cl- and K+ exits into the lumen are involved in the mechanism of ion secretion in pig pancreatic acinar cells.

Evidence for vacuolar-type proton pumps in nonmitochondrial and inositol 1,4,5-trisphosphate-sensitive calcium stores of insulin-secreting cells

This study examines whether acidic, vacuolar-type, proton-pump-carrying organelles of insulin-secreting cells (clonal endocrine pancreatic cell line INS-1) function as rapidly exchanging, inositol 1,4,5-trisphosphate-sensitive calcium stores. Calcium uptake into calcium stores will be modulated by the proton concentration within the stores, since calcium pumps in general appear to mediate a countertransport of calcium with protons. We therefore tested for sensitivity of calcium sequestration by nonmitochondrial stores (inhibition of mitochondrial calcium uptake by 2 microM ruthenium red) in saponin-permeabilized cells to proton-conducting ionophores and proton pump inhibition, using this as a marker for involvement of acidic organelles. Calcium sequestration was partially inhibited by the protonophores nigericin (10-50 microM) and carbonylcyanide m-chlorophenylhydrazone (CCCP; 20-50 microM), as well as by inclusion of 30 mM NH4Cl. Bafilomycin A1, a potent and selective inhibitor of vacuolar-type proton pumps, alone (1 - 500 nM) had no effect on calcium sequestration. however, it induced an inhibitory effect in the presence of nigericin or CCCP, even at low concentrations (5 microM) of these ionophores, lacking itself an inhibitory action on calcium sequestration. Bafilomycin A1 then was already maximally active at a concentration as low as 10 nM. Corres ponding to inhibition of total nonmitochondrial calcium sequestration, filling of inositol 1,4,5-trisphosphate-sensitive stores was decreased or even abolished by the protonophores alone or the protonophores combined with bafilomycin A1. We conclude that vacuolar-type proton pumps are present in at least a part of nonmitochondrial and inositol 1,4,5-trisphosphate-sensitive calcium stores in INS-1 cells. This assigns these stores to organelles such as secretory granules, the trans Golgi network, or endosomes. Luminal acidity of these stores will stimulate calcium sequestration by providing more protons for countertransport of calcium by calcium pumps. High concentrations of protonophores may be required for inhibitory effects because otherwise the proton pumps may be able to compensate sufficiently for ionophore-mediated proton loss. The lack of effect of bafilomycin A1 without protonophores may be due to a sufficient luminal buffering capacity or to preceding inhibition of the pump by an inside-positive transmembrane potential.

Agonist-releasable intracellular calcium stores and the phenomenon of store-dependent calcium entry. A novel hypothesis based on calcium stores in organelles of the endo- and exocytotic apparatus

Store-dependent calcium entry represents a little characterized calcium permeation pathway that is present in a variety of cell types. It is activated in an unknown way by depletion of intracellular calcium stores, for example in the course of phospholipase C stimulation. Current hypotheses propose that depleted calcium stores signal their filling state to this permeation pathway either by direct, protein-mediated interaction or by release of a small, diffusible messenger. The further characterization of store-dependent calcium entry will benefit from progress in the identification of the intracellular calcium storing compartments. Recent findings reviewed here suggest that these compartments include parts of the organelle system that is involved in endo- and exocytosis. This commentary describes a novel model of store-dependent calcium entry based on calcium stores belonging to the endo- and exocytotic organelle system. Such calcium stores could establish a tubule-like connection with the extracellular space, in analogy to the cellular compartments that contain the insulin-sensitive glucose transporter or the gastric proton pump. This connection will provide a pathway for store-dependent calcium entry. Under store depletion, extracellular calcium will permeate through the tubule-like connection into the store lumen and from there into the cytosol. The consequences of this model for the development of drugs modulating store-dependent calcium entry are discussed.

Localization of Ca2+ extrusion sites in pancreatic acinar cells

We have investigated the localization of Ca2+ extrusion sites in mouse pancreatic acinar cells. Employing a new technique, in which high resolution localization of cellular Ca2+ exit is achieved by confocal microscopy and a Ca2+-sensitive fluorescent probe coupled to heavy dextran to slow down diffusion of extracellular Ca2+, it is shown directly that the secretory pole (secretory granule area) is the major site for Ca2+ extrusion following agonist stimulation. This Ca2+ extrusion appears not to be a consequence of exocytosis, as assessment of secretion under our experimental conditions (low external Ca2+ concentration, room temperature) using the technique of monitoring quinacrine fluorescence shows little loss of secretory granules in spite of sustained Ca2+ exit. We conclude that Ca2+ is primarily extruded by Ca2+ pumps from the secretory pole and propose that this process is useful for maintaining a high Ca2+ concentration in the acinar lumen, which is necessary for promotion of endocytosis.

Inositol trisphosphate and cyclic ADP-ribose-mediated release of Ca2+ from single isolated pancreatic zymogen granules

In pancreatic acinar cells low (physiological) agonist concentrations evoke cytosolic Ca2+ spikes specifically in the apical secretory pole that contains a high density of secretory (zymogen) granules (ZGs). Inositol 1,4,5-trisphosphate (IP3) is believed to release Ca2+ from the endoplasmic reticulum, but we have now tested whether the Ca(2+)-releasing messengers IP3 and cyclic ADP-ribose (cADPr) can liberate Ca2+ from AGs. In experiments on single isolated ZGs, we show using confocal microscopy that IP3 and cADPr evoke a marked decrease in the free intragranular Ca2+ concentration. Using a novel high resolution method, we have measured changes in the Ca2+ concentration in the vicinity of an isolated AG and show that IP3 and cADPr cause rapid Ca2+ release from the granule, explaining the agonist-evoked cytosolic Ca2+ rise in the secretory pole.

pH- and Ca(2+)-induced conformational change and aggregation of chromogranin B. Comparison with chromogranin A and implication in secretory vesicle biogenesis

Chromogranins A and B have been known to undergo pH- and Ca(2+)-dependent aggregation, and this property is considered essential for the proper sorting of the vesicular matrix proteins. In the present study, purified native chromogranin B (CGB) from bovine adrenal medulla was used to study the pH- and Ca(2+)-dependent conformational changes and aggregation property. Similar to chromogranin A (CGA), which had been shown to undergo pH- and Ca(2+)-dependent conformational changes and to be composed of 60-65% random coil with 25-40% alpha-helicity, chromogranin B was also shown to consist of 65-70% random coil, 15-25% alpha-helix, and 10-15% beta-sheet structures. The high percentage of random coil suggests that CGB behaves hydrodynamically as an asymmetric molecule, thus explaining its anomalous migration on SDS-polyacrylamide gels. Further, CGB eluted from a gel filtration column in the volume indicative of a globular protein with molecular weight of approximately 200,000 at both the intravesicular pH of 5.5 and a near physiological pH of 7.5. Considering that dimeric CGA eluted from a gel filtration column in the position suggestive of a 300-kDa protein, this result indicated that CGB exists in a monomeric state at both pH levels. Like CGA, which exhibited greater aggregation at pH 5.5 than at pH 7.5 upon Ca2+ binding, CGB also aggregated much more readily at pH 5.5 than at pH 7.5. However, there was a marked difference in the aggregation properties of CGA and CGB with regard to their sensitivity to Ca2+: CGB was at least 2 orders of magnitude more sensitive to Ca2+ than CGA. This suggested that, in spite of the low concentration of CGB (approximately one-tenth that of CGA) in bovine adrenal chromaffin cells, CGB would start to aggregate well ahead of CGA in the trans-Golgi network. In view of the proposed importance of the pH- and Ca(2+)-induced chromogranin aggregation in vesicle biogenesis, the extreme sensitivity of CGB aggregation to Ca2+ appears to underline the potential importance of CGB aggregation in the early stages of vesicle biogenesis.

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.

Thermodynamic study of the pH-dependent interaction of chromogranin A with an intraluminal loop peptide of the inositol 1,4,5-trisphosphate receptor

The secretory vesicles of adrenal chromaffin cells have previously been identified as a major inositol 1,4,5-trisphosphate (IP3)-sensitive Ca2+ store, and their Ca2+ store role has been attributed to the presence of chromogranin A, a high capacity, low affinity Ca2+ binding protein. Chromogranin A has since been shown to exist primarily in a dimeric state at pH 7.5 and primarily in a tetrameric state at the intravesicular pH of 5.5 and has also been shown to interact with the membrane proteins of secretory vesicles at pH 5.5, including a 260-kDa protein reactive to IP3 receptor antibody [Yoo, S. H. (1994) J. Biol. Chem. 269, 12001-12006]. In a recent study, chromogranin A was shown to interact with one of the intraluminal loop regions of the IP3 receptor at pH 5.5 but not at pH 7.5 [Yoo, S. H., & Lewis, M. S. (1994) FEBS Lett. 341, 28-32]. To gain further insight, we have studied the temperature dependence of the pH-dependent interaction of chromogranin A with the intraluminal peptide of the the IP3 receptor by analytical ultracentrifugation, using multiwavelength scan analysis, and found that four molecules of the intraluminal domain peptide of the IP3 receptor bound to each chromogranin A tetramer with delta Go values ranging from -23.6 to -27.6 kcal mol-1 in the absence and presence of 35 mN Ca2+.(ABSTRACT TRUNCATED AT 250 WORDS)

Inositol trisphosphate and cyclic ADP ribose as long range messengers generating local subcellular calcium signals

The process of messenger-mediated release of Ca2+ from intracellular stores, which is of great importance in virtually all cell types including neurons, can best be studied in cells lacking voltage-gated Ca2+ channels in the plasma membrane. In pancreatic acinar cells agonist-evoked repetitive cytosolic Ca2+ spikes are due to release of Ca2+ via inositoltrisphosphate (IP3) and ryanodine receptors and reuptake into the stores via thapsigargin-sensitive Ca2+ pumps. At low acetylcholine (ACh) or cholecystokinin concentrations the cytosolic Ca2+ spikes are mostly confined to the secretory granule area of the polarized pancreatic acinar cells. Similar results can be obtained by intracellular infusion of IP3 (or one of its non-metabolizable analogues) or cyclic ADP ribose. This suggests that high affinity IP3 and ryanodine receptors are concentrated in the secretory granule area. We have generated an 'artificial synapse' on isolated acinar cells by having a cell-attached patch pipette filled with ACh on the basal membrane. Initially, ACh is prevented from making contact with the receptors by the negative potential applied to the pipette. When the pipette polarity is switched to positive ACh can bind to its receptors. Using digital Ca2+ imaging it could be seen that the first cytosolic rise often occurred in the secretory granule area, a considerable distance away from the site of the agonist-receptor interaction. This shows the long-range action of the messenger(s) IP3 and or cyclic ADP ribose generated by the ACh-receptor interaction. The local Ca2+ spikes in the secretory granule area are sufficient for exocytotic secretory responses as seen in capacitance measurements.(ABSTRACT TRUNCATED AT 250 WORDS)

pH-dependent interaction of an intraluminal loop of inositol 1,4,5-trisphosphate receptor with chromogranin A

The inositol 1,4,5-trisphosphate (IP3)-sensitive Ca2+ store role of the secretory vesicles of adrenal medullary chromaffin cells is attributed to the presence of high capacity, low affinity Ca2+ binding protein chromogranin A. Chromogranin A has recently been shown to interact with the protein component(s) on the intraluminal side of the secretory vesicle membrane at the intravesicular pH of 5.5 but to dissociate from them at the near physiological pH of 7.5. Further, one of the chromogranin A-interacting membrane proteins was tentatively identified as the IP3 receptor. Therefore, the pH-dependent potential interaction of the intraluminal loop domains of the IP3 receptor with chromogranin A was studied by analytical ultracentrifugation utilizing synthetic intraluminal loop peptides of the IP3 receptor labeled with 5-hydroxy-tryptophan at the N-terminus as a chromophore. One of the intraluminal loop domains was found to interact with chromogranin A at pH 5.5 but not at pH 7.5, suggesting the importance of the intraluminal loop domain in transmitting Ca2+ mobilization signals to chromogranin A.

Percoll purification of chromaffin granules inhibits their ability to take up and maintain calcium

Secretory granules of the adrenal medulla have recently been shown to be able to sequester and release Ca2+, in addition to their previously established role as carriers of secretory products. In order to study the ability of these or any other secretory granules to participate in intracellular calcium homeostasis, it is imperative that they should be free of other contaminating Ca2+ sequestering organelles, and that the Ca2+ uptake and release mechanisms of those granules should remain intact throughout any chosen purification procedure. We report here that chromaffin granules which were purified by the isopycnic gradient medium Percoll, or even incubated with it, showed an attenuated ability to sequester Ca2+.

Nature of the pH-induced conformational changes and exposure of the C-terminal region of chromogranin A

Chromogranin A is known to undergo pH induced conformational changes, and the difference in conformation is supposed to be responsible for the difference in Ca2+ binding property. To gain insight regarding the overall structure and the nature of pH-induced conformational changes of chromogranin A, limited trypsin digestions were carried out at pH 5.5 and pH 7.5. The resulting fragments were analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and the amino acid sequences of the tryptic fragments were determined. From these analyses it was shown that the chromogranin A structure consists of an N-terminal compact core region and a rather loosely organized C-terminal region and that the change of pH from 7.5 to 5.5 loosened the overall structure of chromogranin A, exposing the C-terminal region. Since the conserved C-terminal region (residues 407-431) was shown to exist in monomer-dimer and monomer-tetramer equilibria at pH 7.5 and 5.5, respectively, the conformational changes of the region at pH 7.5 and 5.5 were studied by circular dichroism spectroscopy using a synthetic peptide representing the conserved C-terminal region. When the pH was changed from 7.5 to 5.5, the coil structure of the C-terminal peptide decreased with an accompanying increase of alpha-helicity.

Dimerization and tetramerization properties of the C-terminal region of chromogranin A: a thermodynamic analysis

Chromogranin A, which is a high-capacity, low-affinity Ca2+ binding protein, has recently been shown to exist in monomer-dimer and in monomer-tetramer equilibria at pH 7.5 and 5.5, respectively [Yoo, S. H., & Lewis, M. S. (1992) J. Biol. Chem. 267, 11236-11241]. The pH appeared to be a necessary and sufficient factor determining the types of oligomer formed. In the present study, using 14 synthetic peptides representing various portions of chromogranin A, we have identified a region in chromogranin A which exhibited dimerization and tetramerization properties at pH 7.5 and 5.5, respectively. Of the 14 peptides, only the conserved C-terminal region (residues 407-431), represented by peptide 14, showed the oligomerization property, existing in a dimeric state at pH 7.5 and in a tetrameric state at pH 5.5. The delta G degrees values of tetramerization were approximately -18.0 kcal/mol, and the delta G degrees value of dimerization was -4.6 kcal/mol. Although peptide 14 represented only 6% of the entire sequence, the delta G degrees value of -18.0 kcal/mol accounted for 80-83% of the delta G degrees values (-21.6 to -22.7 kcal/mol) of tetramerization of intact chromogranin A. Unlike the tetramerization mechanisms of intact chromogranin A where the presence of 35 mM Ca2+ changed the tetramerization mechanism from an enthalpically driven to an entropically driven reaction, the tetramerization mechanism of peptide 14 remained entropically driven regardless of the presence of Ca2+. Likewise, dimerization of the peptide was also entropically driven.(ABSTRACT TRUNCATED AT 250 WORDS)

pH-dependent association of chromogranin A with secretory vesicle membrane and a putative membrane binding region of chromogranin A

Chromogranin A is a low-affinity, high-capacity Ca2+ binding protein, postulated to be responsible for the Ca2+ buffering role of secretory vesicles, and has been found only in the soluble portions of the vesicular proteins. Contrary to the generally accepted notion of chromogranin A existing as a soluble matrix protein, chromogranin A bound to the secretory vesicle membrane at the intravesicular pH of 5.5 and freed from the membrane when the pH was raised to a more physiological pH of 7.5. Trypsin digestion studies of the vesicle membrane suggested that chromogranin A interacts with the protein component(s) on the intravesicular side of the membrane. Furthermore, in a study using 14 synthetic chromogranin A peptides which represent various portions of chromogranin A, a segment in the N-terminal region (residues 18-37) was shown to bind to the vesicle membrane in a pH-dependent manner. The pH-dependent vesicle membrane binding property of chromogranin A appears to be of fundamental physiological importance with regard to the potential roles of chromogranin A in secretory vesicle biogenesis, particularly in segregating secretory vesicle membranes from others in the trans-Golgi network, and also in transmitting extravesicular signals such as inositol 1,4,5-trisphosphate or inositol 1,3,4,5-tetrakisphosphate for Ca2+ release or uptake to the inside of vesicles.

Ubiquitous presence of chromogranin A in the inner ear of guinea pig

Chromogranin A (CGA), which is supposed to be responsible for the calcium storage of secretory vesicles and is also considered to be a marker protein of neurons and endocrine cells, has been found in a variety of organs and tissues. In the present study, soluble proteins from the organ of Corti, saccule, crista, utricle, tectorial membrane, stria vascularis, and the spiral ligament from the inner ear of guinea pig were extracted, and probed with both polyclonal and monoclonal CGA antibodies to determine the presence of CGA. A 75 kDa protein reactive to both antibodies was found in the organ of Corti, saccule, crista, utricle, stria vascularis, and the spiral ligament, suggesting the widespread presence of CGA in the inner ear.

Frequency modulation of transmitter release

In 1952 Fatt and Katz recorded at a frog neuromuscular junction while stimulating the nerve and found "... that successive endplate potential responses varied in a step-like manner, corresponding to units of miniature endplate potentials" (J Physiol 117, 109-128). This led them to propose that fast neuromuscular transmission is 'quantal'. Quantal release is now commonly ascribed to a vesicular form of neurosecretion since vesicles have routinely been visualized in presynaptic terminals. The vesicular hypothesis (Del Castillo and Katz, 1955) assumes that quanta, or 'transmitter packets of standard size', are assembled and stored in the numerous vesicles routinely identified in micrographs of virtually all central and peripheral presynaptic nerve terminals. Simply stated, this model predicts that each one of the miniature synaptic signals (MSSs) follows from the exocytosis of one vesicle's contents. However, the time required for membrane fusion preceding exocytosis (Almers and Tse, 1990) and the variability in MSS amplitude and time course (Vautrin et al, 1992a,b) cannot readily be reconciled by a simple, exocytotic model of quantal release from preloaded vesicles. These difficulties with the original model have led us to re-evaluate MSSs generated at the classical peripheral synapse, the cholinergic neuromuscular junction of the mouse diaphragm, as well as at central synapses between embryonic hippocampal neurons mediated by gamma-aminobutyric acid (GABA). At these synapses, the release of GABA is also assumed to have classical quantal properties like peripheral acetylcholine release (Edwards et al, 1990). Our results show that at both synapses, progressive alterations in elementary signal properties can be induced in a remarkably rapid manner. The original report of preferred amplitudes and intervals in the spontaneous miniature signals (Fatt and Katz, 1952) has repeatedly been confirmed and is here incorporated into a dynamic model of fast synaptic transmission. Although MSSs exhibit variable rise-times and peak amplitudes, they can both be described in terms of synchronization of transmitter release. We have reviewed many experimental findings, which together strongly suggest that the original interpretation of Fatt and Katz (1952) regarding MSSs as reflecting the non-propagated 'neurogenic' activity of 'terminal spots' may be a useful concept to pursue since it may help to explain part of the underlying molecular basis of quantal release.(ABSTRACT TRUNCATED AT 400 WORDS)

Calcium signalling and the triggering of secretion in adrenal chromaffin cells

The pivotal intracellular message for triggering catecholamine release from bovine adrenal chromaffin cells is an elevation in the concentration of cytosolic free Ca2+ ([Ca2+]i). Studies using video-imaging techniques have shown that a rise in [Ca2+]i at the cell periphery, that is due to Ca2+ entry, is the major activating signal for exocytosis. The cytoskeleton has been identified as a major regulatory site of exocytosis, with Ca(2+)-induced disruption of the cortical actin network being required in order that previously restrained granules may have access to their exocytotic sites. The Ca(2+)- and phospholipid-dependent annexin protein, calpactin, has been strongly implicated in a late stage of interaction between granules and the plasma membrane by both ultrastructural and biochemical studies.

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.