Ryanodine and inositol trisphosphate receptors coexist in avian cerebellar Purkinje neurons

Calcium oscillations in pituitary gonadotrophs: comparison of experiment and theory

We have developed a mathematical model that describes several aspects of agonist-induced Ca2+ signaling in single pituitary gonadotrophs. Our model is based on fast activation of the inositol 1,4,5-trisphosphate (InsP3) receptor Ca2+ channels at low free cytosolic Ca2+ concentration ([Ca2+]i) and slow inactivation at high [Ca2+]i. Previous work has shown that these gating properties, when combined with a Ca(2+)-ATPase, are sufficient to generate simulated Ca2+ oscillations. The Hodgkin-Huxley-like description we formulate here incorporates these different gating properties explicitly and renders their effects transparent and easy to modulate. We introduce regulatory mechanisms of channel opening which enable the model, both in the absence and in the presence of Ca2+ entry, to give responses to a wide range of agonist doses that are in good agreement with experimental findings, including subthreshold responses, superthreshold oscillations with frequency determined by [InsP3], and nonoscillatory "biphasic" responses followed occasionally by small-amplitude oscillations. A particular added feature of our model, enhanced channel opening by reduced concentration of Ca2+ in the lumen of the endoplasmic reticulum, allows oscillations to continue during pool depletion. The model predicts that ionomycin and thapsigargin can induce oscillations with basal [InsP3] and zero Ca2+ entry, while Ca2+ injection cannot. Responses to specific pairings of sub- or superthreshold stimuli of agonist, ionomycin, and thapsigargin are also correctly predicted. Since this model encompasses a wide range of observed dynamic behaviors within a single framework, based on well-established mechanisms, its relevance should not be restricted to gonadotrophs.

Role of ryanodine receptors

Recent findings on the ryanodine receptor of vertebrates, a Ca-release channel protein for the caffeine- and ryanodine-sensitive Ca pools, are reviewed in this article. Three distinct genes, i.e., ryr1, ryr2, and ryr3, express different isoforms in specific locations: Ryr1 in skeletal muscle and Purkinje cells of cerebellum; Ryr2 in cardiac muscle and brain, especially cerebellum; Ryr3 in skeletal muscle of nonmammalian vertebrates, the corpus striatum, and limbic cortex of brain, smooth muscles, and the other cells in vertebrates. While only one isoform (Ryr1) is expressed in mammalian skeletal muscles, two isoforms (alpha- and beta-isoforms expressed by ryr1 and ryr3, respectively) are found in nonmammalian vertebrate skeletal muscles. Although the coexistence of two isoforms may merely be related to differentiation and specialization, the biological significance remains to be clarified. Ryanodine receptors in vertebrate skeletal muscles are believed to mediate two different modes of Ca release: Ca(2+)-induced Ca release and action potential-induced Ca release. All results obtained so far with any isoform of ryanodine receptor are related to Ca(2+)-induced Ca release and show very similar characteristics. Ca(2+)-induced Ca release, however, cannot be the underlying mechanism of Ca release on skeletal muscle activation. Susceptibility of the ryanodine receptor's ryanodine-binding activity to modification by physical factors, such as osmolality of the medium, might be related to action potential-induced Ca release. A hypothesis of molecular interaction in view of the plunger model of action potential-induced Ca release is discussed, suggesting that the model could be compatible with Ryr1 and Ryr3, but incompatible with Ryr2. The functional relevance of ryanodine receptor isoforms, especially Ryr3, in brain also remains to be clarified. Among ryr1 gene-related diseases, malignant hyperthermia was the first to be identified; however, there is still the possibility of involvement of the other genes. Central core disease has been added to the list recently. A molecular approach for the diagnosis and treatment of diseases is now in progress.

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.

Receptor-operated Ca2+ signaling and crosstalk in stimulus secretion coupling

In the cells of higher eukaryotic organisms, there are several messenger pathways of intracellular signal transduction, such as the inositol 1,4,5-trisphosphate/Ca2+ signal, voltage-dependent and -independent Ca2+ channels, adenylate cyclase/cyclic adenosine 3',5'-monophosphate, guanylate cyclase/cyclic guanosine 3',5'-monophosphate, diacylglycerol/protein kinase C, and growth factors/tyrosine kinase/tyrosine phosphatase. These pathways are present in different cell types and impinge on each other for the modulation of the cell function. Ca2+ is one of the most ubiquitous intracellular messengers mediating transcellular communication in a wide variety of cell types. Over the last decades it has become clear that the activation of many types of cells is accompanied by an increase in cytosolic free Ca2+ concentration ([Ca2+]i) that is thought to play an important part in the sequence of events occurring during cell activation. The Ca2+ signal can be divided into two categories: receptor- and voltage-operated Ca2+ signal. This review describes and integrates some recent views of receptor-operated Ca2+ signaling and crosstalk in the context of stimulus-secretion coupling.

Properties of the ryanodine receptor present in the sarcoplasmic reticulum from lobster skeletal muscle

Microsomal sarcoplasmic reticulum (SR) fractions from lobster skeletal muscle were found to bind [3H]-ryanodine. [3H]-ryanodine binding was enhanced by AMP, Ca2+ and caffeine, and significantly diminished by ATP, Ba2+ and Sr2+. Furthermore, dantrolene and ruthenium red, two classical inhibitors of Ca2+ release from the SR, blocked [3H]-ryanodine binding. Similarly, tetracaine, known to block the charge movement associated with excitation-contraction coupling in vertebrate muscle, inhibited the binding of the alkaloid. Our lobster SR preparation exhibited a single high-affinity ryanodine binding site (Kd = 6.6 nM, Bmax = 10 pmol/mg protein). Since SDS-PAGE of the SR proteins revealed a major band c. 565 kDa which comigrated with the putative ryanodine receptor from both rat and chicken skeletal muscle, we concluded that lobster skeletal muscle is equipped with the 565 kDa ryanodine receptor. Finally, incorporation of the SR microsomal fraction from lobster into planar bilayer membranes revealed the presence of a ryanodine-sensitive Ca2+ channel activity (160 pS in symmetrical 200 mM CsCl solutions). We concluded that both the crustacean and vertebrate skeletal muscle ryanodine receptor share the relevant properties such as molecular weight and affinity for ryanodine and inositol 1,4,5 triphosphate. However, there are important differences between the two receptors including differential effects of the alkaloid on the Ca2+ release channel and modulation of the receptor by nucleotides.

Subcellular distribution of Ca2+ release channels underlying Ca2+ waves and oscillations in exocrine pancreas

Agonists trigger Ca2+ waves and oscillations in exocrine gland cells. Our confocal Ca2+ imaging revealed three distinct phases during the Ca2+ waves in the rat pancreatic acinar cell. Rises in Ca2+ concentration were initiated at a small trigger zone, or T zone, in the granular area; then, Ca2+ waves rapidly spread within the area and, at high agonist concentrations, propagated slowly toward the basal pole. Injection of inositol 1,4,5-trisphosphate (IP3) or Ca2+ from patch pipettes demonstrated the presence of high sensitivity IP3 receptors at the T zone, Ca(2+)-induced Ca2+ release channels in the granular area, and low sensitivity IP3 receptors in the basal area. The IP3 receptors at the T zone appeared to generate autonomous Ca2+ spikes and to initiate patterned Ca2+ oscillations. Thus, heterogeneous cytosolic localization of Ca2+ release channels plays a key role in Ca2+ waves and oscillations.

Distribution of ryanodine receptors in the chicken central nervous system

The ryanodine receptor (RR), an intracellular calcium release channel, has been identified in the nervous system but its contributions to neuronal function are unknown. We have utilized immunohistochemical techniques to establish the distribution of RRs in the central nervous system (CNS) of the chick as a step toward elucidating the function of RRs in this system. RR immunoreactivity is observed throughout the brain, most prominently in large neurons. The strongest immunoreactivity is found in cerebellar Purkinje neurons, but nuclei in the motor, visual and vestibular systems are also intensely labeled, and immunoreactive neurons are observed the olfactory bulb and the hippocampus. In these neurons, labeling is prominent in cell bodies, dendrites and axons, but is not observed in the dendritic spines or in plasma membranes. The neuronal RRs bind [3H]ryanodine with high affinity and this activity is regulated by calcium, caffeine, MgCl2/ATP and ionic strength. Multiple forms of the RRs are found in the chicken CNS. Immunoprecipitation and localization studies using RR isoform specific monoclonal antibodies reveal major differences in their distribution. The predominant species in the cerebellum is similar to the skeletal muscle isoform while there is a lower level of expression of either the cardiac or beta skeletal isoforms. In the remainder of the brain, the predominant isoform is similar to the cardiac or beta skeletal muscle isoforms. The broad distribution of RRs in the CNS suggests that calcium release events mediated by these proteins may have a functional role in a diverse array of neurons. Moreover within the populations of neurons expressing RR's, the presence of specific RR isoforms may correlate with specialization in the calcium release events mediated by these proteins.

Redundant mechanisms of calcium-induced calcium release underlying calcium waves during fertilization of sea urchin eggs

Propagating Ca2+ waves are a characteristic feature of Ca(2+)-linked signal transduction pathways. Intracellular Ca2+ waves are formed by regenerative stimulation of Ca2+ release from intracellular stores by Ca2+ itself. Mechanisms that rely on either inositol trisphosphate or ryanodine receptor channels have been proposed to account for Ca2+ waves in various cell types. Both channel types contributed to the Ca2+ wave during fertilization of sea urchin eggs. Alternative mechanisms of Ca2+ release imply redundancy but may also allow for modulation and diversity in the generation of Ca2+ waves.

Failure to make normal alpha ryanodine receptor is an early event associated with the crooked neck dwarf (cn) mutation in chicken

We have investigated the molecular basis of the Crooked Neck Dwarf (cn) mutation in embryonic chickens. Using biochemical and pharmacological techniques we are unable to detect normal alpha ryanodine receptor (RyR) protein in intact cn/cn skeletal muscle. Extremely low levels of alpha RyR immunoreactivity can be observed in mutant muscles, but the distribution of this staining differs from that in normal muscle and colocalizes with the rough endoplasmic reticulum immunoglobulin binding protein, BiP. This suggests the existence of an abnormal alpha RyR protein in mutant muscle. In day E12 cn/cn muscle the levels of RyR mRNA are reduced by approximately 80%, while the levels of other muscle proteins, including the alpha 1 subunit of the dihydropyridine receptor, the SR Ca(2+)-ATPase, calsequestrin, and glyceraldehyde-3-phosphate dehydrogenase, and their associated mRNAs are essentially normal in cn/cn muscle. There is also a failure to express alpha RyR in cn/cn cerebellar Purkinje neurons. Expression of the beta RyR, a second RyR isoform, is not initiated in normal skeletal muscle until day E18. In cn/cn skeletal muscle significant muscle degeneration has occurred by this time and the beta RyR is found at low levels in only a subset of fibers suggesting the reduced levels of this isoform are a secondary consequence of the mutation. The cardiac RyR isoform is found in cn/cn cardiac muscle, which contracts in a vigorous manner. In summary, a failure to make normal alpha RyR receptor appears to be an event closely associated with the cn mutation and one which may be largely responsible for development of the cn/cn phenotype in embryonic skeletal muscle.

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.

Reconstitution of a voltage-activated calcium conducting cation channel from brain microsomes

Many aspects of nerve cell function are controlled by cytosolic Ca2+. Intracellular organelles can sequester the cation and release it in a regulated fashion through specific ion channels including ryanodine-sensitive and inositol trisphosphate (InsP3)-activated intraneuronal Ca2+ channels. We have now used the planar bilayer technique to characterize a distinct high-conductance Ca2+ channel from brain microsomal membranes, which was not found in synaptic plasma membranes. Channel conductance in 50 mM CaCl2 is approximately 100 pS. The channel is permeable to Ca2+, Ba2+, K+ and Cs+, but it is not ideally cation-selective (PCs+:PCl- = 4:1). It opens in bursts in a steeply voltage-dependent manner, with maximal activation around zero mV. Channel activity is unaffected by caffeine or ryanodine (both of which modify the gating of ryanodine-sensitive Ca2+ channels), or by InsP3 or heparin (which act on InsP3-sensitive Ca2+ channels). omega-conotoxin GVIA, ruthenium red, amiloride and procaine all block the channel, the latter two by interacting with one or more negatively-charged binding sites in the voltage gradient within the channel pore. We suggest the channel may have a role in intracellular Ca(2+)-signalling, possibly linked to the operation of intracellular Ca2+ stores.

Multiple/heterogeneous Ca2+ stores in cerebellum Purkinje neurons

1. The rapid and transient redistribution of Ca2+ from intracellular membrane-bound compartments (stores) is a key event of cell activation. 2. The cytological nature and molecular composition of such Ca2+ stores have been the object of intense investigation in recent years. 3. Here we review: (a) the current knowledge on intracellular Ca2+ stores of Purkinje neurons at the functional, biochemical, molecular, morphological and ultrastructural level; and discuss: (b) the relationship between Ca2+ stores and the endoplasmic reticulum, and (c) the occurrence of multiple/heterogeneous Ca2+ stores.

Distribution of ryanodine receptor-like immunoreactivity in mammalian central nervous system is consistent with its role in calcium-induced calcium release

The distributions of ryanodine receptor-like immunoreactivity and Ca-ATPase-like immunoreactivity were identified in the guinea-pig and rat central nervous system using antibodies raised against the rabbit skeletal muscle ryanodine receptor and Ca-ATPase. In both guinea-pig and rat cerebellum, the ryanodine receptor-like immunoreactivity was restricted to the soma and dendrites of Purkinje cells. In the medulla, neuron somata in the hypoglossal nucleus were stained in both species, but in the dorsal motor nucleus of the vagus somata were stained in guinea-pigs but not in rats. This species difference in ryanodine receptor-like immunoreactivity is consistent with the species difference in expression of a ryanodine sensitive, calcium activated potassium conductance in neurons of the dorsal motor nucleus of the vagus. Immunoreactivity to Ca-ATPase was present in vagal motoneurons in both species with denser staining in the guinea-pig. The data further support the idea that, in neurons of the dorsal motor nucleus of the vagus, release of intracellular calcium stores via a ryanodine receptor activates a specific class of potassium channels, thereby modulating cell excitability.

Characterisation and distribution of inositol polyphosphate and Ryanodine receptors in the rat brain

The regional distribution of inositol 1,4,5-trisphosphate (InsP3), inositol 1,3,4,5-tetrakisphosphate (InsP4), and ryanodine binding sites has been characterised and compared in the rat brain using radioligand binding assays. Cortical [3H]InsP3 binding indicated similar positional and stereospecificity as observed in other tissues, with 100-fold selectivity for InsP3 over InsP4. Similarly, high-affinity [32P]InsP4 binding also showed a high degree of positional specificity, with a 1,000-fold selectivity for InsP4 over InsP3. Initial characterisation of [3H]ryanodine binding to cortical membranes demonstrated that specific binding was highly dependent on high salt and micromolar Ca2+ concentrations and inhibited by Ca2+ levels of > 1 mM. [3H]-Ryanodine binding was also enhanced by beta, gamma-methylene-adenosine 5'-trisphosphate and caffeine and inhibited by magnesium and ruthenium red (Ki = 0.81 microM). However, dantrolene (300 microM) was ineffective on the binding. Therefore, although the results indicate a greater similarity to the binding properties of the Ca(2+)-induced Ca2+ release channel isoform present in skeletal, rather than cardiac, muscle, it does not appear to be identical. Detailed binding analysis of ryanodine and polyphosphate sites, with the exception of ruthenium red, indicated no interaction between binding sites. Ruthenium red markedly enhanced the binding of both [3H]InsP3 and [32P]InsP4, an effect most probably due to nonspecific complex formation. Regional binding of InP3, InsP4, and ryanodine in the rat brain was of similar affinity for each ligand in each area, but the density profile for each ligand was clearly different. The highest density of InsP3 sites was in the cerebellum, whereas the highest density of ryanodine sites was in the hippocampus.(ABSTRACT TRUNCATED AT 250 WORDS)

Characteristics of Ca2+ release induced by Ca2+ influx in cultured bullfrog sympathetic neurones

1. A rise in intracellular Ca2+ ([Ca2+]i) and a Ca2+ current (ICa) induced by a depolarizing pulse were simultaneously recorded by fura-2 or indo-1 fluorescence and whole-cell patch clamp techniques in cultured bullfrog sympathetic ganglion cells. 2. [Ca2+]i (calculated from the ratio of fura-2 fluorescences excited at 380 and 340 nm and recorded with a photomultiplier at > 492 nm) rose regeneratively (in most cells) during a command pulse (from -60 to 0 mV, 100 ms), continued to rise thereafter, peaked at 666 ms (on average) and decayed slowly with a half-decay time of 22.8 s. 3. Scanning a single horizontal line across the cytoplasm with an ultraviolet argon ion laser (351 nm) and recording indo-1 fluorescences at two wavelengths (peaked at 410 and 475 nm) with a confocal microscope demonstrated that [Ca2+]i beneath the cell membrane rose much faster than that in the deeper cytoplasm. The time course of the spatial integral of [Ca2+]i, however, corresponded well with that recorded with fura-2 fluorescence using a photomultiplier. 4. [Ca2+]i measured by fura-2 fluorescence ratio using a photomultiplier did not increase during a strong depolarizing pulse (-60 to +80 mV), but sometimes rose after the pulse. A depolarization-induced rise in [Ca2+]i ([Ca2+]i transient) was blocked in a Ca(2+)-free, EGTA solution, reduced by lowering the extracellular Ca2+ concentration ([Ca2+]o) to 0.45 or 0.9 mM and enhanced by raising [Ca2+]o to 7.2 or 14.4 nM. 5. The extracellular Ca2+ dependence was non-linear when long depolarizing pulses (up to 500 ms) were applied; the amplitude of [Ca2+]i transient/Ca2+ entry (unit [Ca2+]i transient) increased with an increase in Ca2+ entry. 6. Increasing the duration of depolarization (-50 or -60 to 0 mV) from 20 to 500 ms enhanced asymptotically the integral of ICa (due to inactivation), and progressively the magnitude of [Ca2+]i transients, leading to the apparent non-linear dependence of unit [Ca2+]i transient on Ca2+ entry as well as on the duration of membrane depolarization. The peak time of [Ca2+]i transient was unchanged for pulse durations up to 300 ms, but prolonged with an increase in pulse duration to 500 ms. 7. Inhibitors of Ca2+ release from intracellular Ca2+ reservoirs, dantrolene (10 microM) and ryanodine (50 microM), blocked the [Ca2+]i transient to 56 and 30%, respectively, of the control. 8. The higher the basal [Ca2+]i level, the greater was the magnitude of the [Ca2+]i transients.(ABSTRACT TRUNCATED AT 400 WORDS)

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

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

One-pool model for Ca2+ oscillations involving Ca2+ and inositol 1,4,5-trisphosphate as co-agonists for Ca2+ release

Experimental observations indicate that Ca(2+)-induced Ca2+ release (CICR) may underlie Ca2+ oscillations in a variety of cells. In its original version, a theoretical model for signal-induced Ca2+ oscillations based on CICR assumed the existence of two types of pools, one sensitive to inositol 1,4,5-trisphosphate (IP3) and the other one sensitive to Ca2+. Recent experiments indicate that Ca2+ channels may sometimes be sensitive to both IP3 and Ca2+. Such a regulation may be viewed as Ca(2+)-sensitized IP3-induced Ca2+ release or, alternatively, as a form of IP3-sensitized CICR. We show that sustained oscillations can still occur in a one-pool model, provided that the same Ca2+ channels are sensitive to both Ca2+ and IP3 behaving as co-agonists. This model and the two-pool model based on CICR both account for a number of experimental observations but differ in some respects. Thus, while in the two-pool model the latency and period of Ca2+ oscillations are of the same order of magnitude and correlate in a roughly linear manner, latency in the one-pool model is always brief and remains much shorter than the period of oscillations. Moreover, the first Ca2+ spike is much larger than the following ones in the one-pool model. These distinctive properties might provide an explanation for the differences in Ca2+ oscillations observed in various cell types.

Differential distribution of the alternative forms of the sarcoplasmic/endoplasmic reticulum Ca(2+)-ATPase, SERCA2b and SERCA2a, in the avian brain

Cellular distribution of the two forms of SERCA2 was examined in adult chicken brain. Four regions of the brain were analyzed with three immunological reagents: a monoclonal antibody that recognizes both forms of SERCA2, and two antisera which are specific for the two alternative forms, SERCA2b or SERCA2a. Cerebellar Purkinje cells express predominantly SERCA2b but also low levels of SERCA2a, as has been reported for mammals. The nucleus isthmo-opticus, nucleus magnocellularis cochlearis, and nucleus laminaris all express high levels of SERCA2 but with different ratios of SERCA2b and SERCA2a. These immunohistochemical results were supported by in situ hybridization analysis. Therefore, it appears that regions within the brain have specific requirements for the two forms of SERCA2. This suggests functional significance for the alternative forms SERCA2b and SERCA2a, and possible functions are discussed.

Intracellular calcium mobilization by inositol 1,4,5-trisphosphate: intracellular movements and compartmentalization

Intracellular calcium ion (Ca2+) changes in NIH-3T3 fibroblasts responding to inositol 1,4,5-trisphosphate (IP3) injections have been monitored using high resolution digital imaging of the calcium indicator Fura-2. Ester loaded and microinjected indicator report radically different patterns of Ca2+ change during the IP3 response. These differences arise from intracellular compartmentalization of the ester loaded indicator which can seriously distort reported Ca2+ levels. Prominent among these aberrant responses is a signal in which Ca2+ levels in the cell nucleus appear to exceed those in the rest of the cell, and an apparent slowing of the Ca2+ recovery time-course throughout the cell when temperature is increased. Similar behavior is observed in other cell types. Judicious use of both loading techniques can provide information on Ca2+ movements into organelles that might otherwise escape detection. The Ca2+ rise normally measured in bulk or integrated single cell measurements is a complex mix of cytosol/nucleus and organellar changes. Much, if not all, of the observable organellar change is an accumulation, not release, of Ca2+ following the IP3 injection. The Golgi apparatus is a conspicuous early site for this accumulation, and mitochondria show a large, temperature sensitive uptake that is capable of limiting the maximal Ca2+ change during the response.

Ryanodine receptors: how many, where and why?

Ryanodine receptors are intracellular Ca2+ channels that have been known for more than a decade to have a role in releasing Ca2+ from the sarcoplasmic reticulum to regulate contraction in skeletal and cardiac muscle fibres. Vincenzo Sorrentino and Pompeo Volpe review some recent developments: the ryanodine receptor channels have now been found to be expressed in the central nervous system, and the cloning of a third ryanodine receptor gene (RYR3) has revealed that this new isoform is widely expressed in several tissues and cells. In consequence, the view of ryanodine receptors as Ca2+ channels of muscle cells is rapidly changing, and these channels seem set to take a more central position on the stage of intracellular Ca2+ signalling.

Inositol trisphosphate and calcium signalling

Inositol trisphosphate is a second messenger that controls many cellular processes by generating internal calcium signals. It operates through receptors whose molecular and physiological properties closely resemble the calcium-mobilizing ryanodine receptors of muscle. This family of intracellular calcium channels displays the regenerative process of calcium-induced calcium release responsible for the complex spatiotemporal patterns of calcium waves and oscillations. Such a dynamic signalling pathway controls many cellular processes, including fertilization, cell growth, transformation, secretion, smooth muscle contraction, sensory perception and neuronal signalling.

Two types of ryanodine receptors in mouse brain: skeletal muscle type exclusively in Purkinje cells and cardiac muscle type in various neurons

Two types of ryanodine receptors, channels for Ca2+ release from intracellular stores, are known. We detected the skeletal muscle type only in cerebellum by immunoblot analysis of microsomes and partially purified proteins. The cardiac muscle type was found in all parts of the mouse brain. Immunohistochemical study showed that the cardiac muscle type was localized mainly at the somata of most neurons. Analysis of mutant cerebella suggested that the skeletal muscle type was present exclusively in Purkinje cells. These results suggest that Ca(2+)-induced Ca2+ release, probably mediated by the cardiac muscle receptor, functions generally in various neurons, whereas depolarization-induced Ca2+ release, probably mediated by the skeletal muscle receptor, functions specifically in Purkinje cells.

Activation of Ca(2+)-dependent currents in cultured rat dorsal root ganglion neurones by a sperm factor and cyclic ADP-ribose

The effects of intracellular application of two novel Ca2+ releasing agents have been studied in cultured rat dorsal root ganglion (DRG) neurones by monitoring Ca(2+)-dependent currents as a physiological index of raised free cytosolic Ca2+ ([Ca2+]i). A protein based sperm factor (SF) extracted from mammalian sperm, has been found to trigger Ca2+ oscillations and to sensitize unfertilized mammalian eggs to calcium induced calcium release (CICR). In this study intracellular application of SF activated Ca(2+)-dependent currents in approximately two-thirds of DRG neurones. The SF induced activity was abolished by heat treatment, attenuated by increasing the intracellular Ca2+ buffering capacity of the cells and persisted when extracellular Ca2+ was replaced by Ba2+. In addition, activity could be triggered or potentiated by loading the cells with Ca2+ by activating a series of voltage-gated Ca2+ currents. Ca(2+)-activated inward current activity was also generated by intracellular application of cyclic ADP-ribose (cADPR), a metabolite of NAD+, which causes Ca2+ release in sea urchin eggs. This activity could also be enhanced by loading the cells with Ca2+. The cADPR induced activity, but not the SF induced activity, was abolished by depleting the caffeine sensitive Ca2+ store. Ruthenium red markedly attenuated SF induced activity but had little action on cADPR induced activity or caffeine induced activity. Our results indicate that both SF and cADPR release intracellular Ca2+ pools in DRG neurones and that they appear to act on subtly distinct stores or distinct intracellular Ca2+ release mechanisms, possibly by modulating CICR.

Expression of a cardiac Ca(2+)-release channel isoform in mammalian brain

Mammalian brain possesses ryanodine-sensitive Ca2+ channels, which in muscle cells mediate rapid Ca2+ release from intracellular stores during excitation-contraction coupling. Analysis of bovine brain ryanodine receptor (RyR) channels suggests specific expression of the cardiac-muscle RyR isoform in mammalian brain. Localization using cardiac-muscle RyR-specific antibodies and antisense RNA revealed that brain RyRs were present in dendrites, cell bodies and terminals of rat forebrain, and highly enriched in the hippocampus. Activity of skeletal-muscle RyR channels is coupled to sarcolemmal voltage sensors, in contrast with cardiac-muscle RyR channels, which are known to be Ca(2+)-induced Ca(2+)-release channels. Thus Ca(2+)-induced Ca2+ release from intracellular stores mediated by brain RyR channels may be a major Ca(2+)-signalling pathway in specific regions of mammalian brain, and hence may play a fundamental role in neuronal Ca2+ homoeostasis.

Biochemical properties and subcellular distribution of an N-type calcium channel alpha 1 subunit

A site-directed anti-peptide antibody, CNB-1, that recognizes the alpha 1 subunit of rat brain class B calcium channels (rbB) immunoprecipitated 43% of the N-type calcium channels labeled by [125I]omega-conotoxin. CNB-1 recognized proteins of 240 and 210 kd, suggesting the presence of two size forms of this alpha 1 subunit. Calcium channels recognized by CNB-1 were localized predominantly in dendrites; both dendritic shafts and punctate synaptic structures upon the dendrites were labeled. The large terminals of the mossy fibers of the dentate gyrus granule neurons were heavily labeled, suggesting that the punctate labeling pattern represents calcium channels in nerve terminals. The pattern of immunostaining was cell specific. The cell bodies of some pyramidal cells in layers II, III, and V of the dorsal cortex, Purkinje cells, and scattered cell bodies elsewhere in the brain were also labeled at a low level. The results define complementary distributions of N- and L-type calcium channels in dendrites, nerve terminals, and cell bodies of most central neurons and support distinct functional roles in calcium-dependent electrical activity, intracellular calcium regulation, and neurotransmitter release for these two channel types.

Immunohistochemical localization of ryanodine binding proteins in the central nervous system of gymnotiform fish

The ryanodine receptor, an integral membrane protein of the sarcoplasmic reticulum in muscle, embodies a high conductance channel permeable to calcium ions. Recent studies have identified ryanodine-binding proteins in avian and mammalian central nervous systems. These neuronal ryanodine receptors appear to function as Ca2+ channels which may gate the release of Ca2+ from caffeine-sensitive intracellular pools in neurons. In the present investigation, we employed monoclonal antibodies against ryanodine-binding proteins of avian muscle cells to the brain of weakly electric gymnotiform fish. Immunoprecipitation and Western blot analysis revealed two isoforms in the fish brain, with molecular weights comparable to those of avian and fish muscle ryanodine-binding proteins. By employing immunohistochemical techniques, we mapped these proteins in fish brain. Ryanodine receptor-like immunoreactivity was found in nerve cell bodies as well as dendrites and axonal processes. The ryanodine-binding protein is distributed throughout the neuraxis in specific cell types of the gymnotiform brain. In the telencephalon, immunoreactive cells were found in the glomerular layer of the olfactory bulb, in the supracommissural subdivision of the ventral telencephalon, and in the intermediate rostral subdivision of the ventral telencephalon. In the diencephalon, immunoreactive cells or fibers were observed in the nucleus prethalamicus and the habenula, within the nucleus at the base of the optic tract and the adjacent dorsal tegmental nucleus, the pretectal nuclei A and B, and the nucleus electrosensorius. In addition, immunopositive cells were seen in several nuclei of the hypothalamus, with the inferior and lateral subdivision of the nucleus recessus lateralis displaying the highest concentration of neurons. In the mesencephalon, the optic tectum contained the greatest number of immunopositive cells. In the rhombencephalon, labelling was seen in the nucleus of the lateral valvula, central gray, lateral tegmental nucleus, in boundary cells of the nucleus praeminentialis, efferent octavolateral nucleus, an area adjacent to the medial edge of the lateral reticular nucleus, nucleus medialis, and electrosensory lateral line lobe. As in avian brain, cerebellar Purkinje cells were positive for ryanodine-binding protein, although only subsets of Purkinje cells were labelled.

Immunohistochemical study on the distribution of sarcoplasmic reticulum calcium ATPase in various human tissues using novel monoclonal antibodies

Novel monoclonal antibodies were raised against sarcoplasmic reticulum calcium (Ca2+)-ATPase of human skeletal muscle. Immunohistochemical analysis demonstrated that these antibodies, designated 6F5 and 7F10, bind Ca(2+)-ATPase of non-muscle tissue of the adult including parathyroid, islets of Langerhans, anterior lobe of the pituitary gland and photoreceptor cells of the retina as well as skeletal muscle. A positive reaction was also found for fetal tissues including skeletal muscle, heart, chondrocytes and peripheral nerves. Our results for distribution suggest that Ca(2+)-ATPase is strongly expressed in the tissues and cells in which signal transduction is actively carried out by Ca2+ release from the cytoplasmic Ca2+ pool.

Identification of calreticulin isoforms in the central nervous system

In the present paper we report the cloning and sequencing of the cDNA encoding two calreticulin isoforms from Xenopus laevis central nervous system. The two isoforms display 93% identity at the amino acid level. The predicted amino acid sequences of the amphibian calreticulins are very similar (76%) to those of mammalian liver and skeletal muscle. Xenopus laevis calreticulins are characterized by a very acidic c-terminal domain endowed with the endoplasmic-reticulum retention signal KDEL. The cDNAs of both clones encode an N-glycosylation consensus sequence. A third clone of calreticulin was also identified. The restriction map of this clone was clearly distinct from that of the two sequenced clones. These results indicate the existence of multiple calreticulin isoforms in the central nervous system and open questions about their functional role in different cells and/or subcellular compartments.

Immunohistochemical localization of ryanodine receptors in mouse central nervous system

The distribution of ryanodine receptor-like immunoreactivity in the mouse central nervous system was studied using two antibodies raised against synthetic peptides. These peptides represented a region conserved between the cardiac and skeletal muscle forms and a region specific to the cardiac form. Western blotting analysis and [3H]ryanodine binding analysis showed ryanodine receptors are expressed in all the brain regions. The activity was prominent in hippocampus and cerebral cortex. Immunohistochemical study demonstrated that the ryanodine receptors were localized unevenly in somata. Some apical and proximal dendrites in some cells were also labeled. In hippocampus pyramidal neurons in CA2-3 region were more labeled than CA1 region. Immunohistochemical distribution revealed by two antibodies was essentially the same but the fibers were more immunoreactive with the antibody raised against the cardiac muscle ryanodine form. The localization of ryanodine receptors was quite different from that of inositol 1,4,5-trisphosphate receptors.

Subcompartments of the endoplasmic reticulum

The endoplasmic reticulum (ER) is the largest continuous endomembrane structure in the cytoplasm. It may be viewed as a series of unique subcompartments. In this review, we examine the rough ER, nuclear envelope and several smooth ER subcompartments. Consideration is given to the characteristic properties and functions of the ER and its domains, and to the formation and maintenance of subcompartments. Associations within the ER membrane bilayer, and with constituents of the cytoplasm and the ER lumen, contribute to the formation of domains and lead to the establishment of subcompartments that reflect specialized functions and vary according to the physiologic state and phenotype of the individual cell. Although the structural complexity of some ER subcompartments (such as the sarcoplasmic reticulum) is highly elaborate, the ER remains a dynamic organelle, subject to assembly and disassembly, capable of extensive remodelling and active in exchange with other organelles through mechanisms of membrane transport.

Interactions between Ca2+ mobilizing mechanisms in cultured rat cerebellar granule cells

1. The interactions between IP3 receptor-mediated and Ca(2+)-induced Ca2+ release were investigated in cerebellar granule cell bodies, using the techniques of microfluorimetry and image analysis. 2. The IP3-sensitive Ca2+ release mechanism was activated using acetylcholine (ACh) and the selective metabotropic glutamate receptor agonist 1-aminocyclopentane-1S,3R-dicarboxylic acid (ACPD). Caffeine was used to activate, and ryanodine to inhibit, the Ca(2+)-induced Ca2+ release process. Thapsigargin was used to deplete intracellular Ca2+ stores. 3. Transient applications of caffeine (5-50 mM), ACPD (50-500 microM) and ACh (0.05-1 microM) mobilized intracellular Ca2+ ([Ca2+]i). Ca2+ mobilizing responses to 50 mM caffeine and 1 microM ACh increased with time in culture until day 4. However, beyond this period the responsiveness of cells to caffeine, but not to ACh, declined markedly. 4. Responses induced by ACPD and ACh were inhibited in the presence of caffeine at concentrations below those which mobilized Ca2+ (1-5 mM). This effect was not due to Ca2+ pool depletion, elevation of cAMP or inhibition of phosphodiesterases. 5. Prior challenge with ACh or ACPD inhibited Ca2+ mobilization induced by caffeine (50 mM). Transient exposure to caffeine inhibited subsequent responses to ACh through a mechanism which involved store depletion. 6. Thapsigargin (0.1-1 microM) inhibited, to a similar extent, Ca2+ mobilization induced by caffeine, ACPD and ACh. 7. Ryanodine (10 microM) antagonized Ca2+ mobilization induced by caffeine, ACh and ACPD. However, the ability of ryanodine to block inositol 1,4,5-trisphosphate-linked agonist responses varied considerably between cells. The sensitivity of ACh-induced responses to ryanodine correlated with the sensitivity of the cells to caffeine. 8. The possible explanations for the pronounced interactions between IP3 receptor-mediated and Ca(2+)-induced Ca2+ release processes in cerebellar granule cells are discussed.

Synergistic release of calcium in sea urchin eggs by caffeine and ryanodine

A transient rise in intracellular Ca2+ during fertilization is necessary for activation of the quiescent sea urchin egg. Several mechanisms contribute to the rise in Ca2+ including influx across the egg plasma membrane and release from intracellular stores. The egg contains both IP3-sensitive and -insensitive Ca2+ release mechanisms and in this study we have used single-cell spectrofluorimetry to examine the effects of caffeine and ryanodine on Ca2+ release in eggs preloaded with fura 2. Caffeine induced a small Ca2+ release that was insensitive to heparin or ruthenium red. Ca2+ liberation by caffeine could be augmented by prior treatment with thapsigargin, an inhibitor of endoplasmic reticulum Ca2+ ATPase. Variable Ca2+ releases were observed in response to microinjection of ryanodine. The action of ryanodine appeared to be enhanced by prior injection of heparin and partially inhibited by ruthenium red. The release of Ca2+ by caffeine or ryanodine was generally insufficient to trigger cortical granule exocytosis, thus these eggs could be fertilized and a second Ca2+ release during fertilization was measured. Unlike the caffeine- and ryanodine-sensitive Ca(2+)-induced Ca2+ release mechanism in somatic cells, the graded responses in eggs suggested this caffeine- and ryanodine-sensitive release mechanism is not sensitive to sudden changes in Ca2+. Thus we could examine the combined actions of caffeine and ryanodine on Ca2+ release, which were synergistic. Caffeine treatment of ryanodine-injected eggs or ryanodine injection of caffeine-treated eggs stimulated a Ca2+ release significantly larger than the release by either drug independently. The experiments presented here suggest that sea urchin eggs liberate Ca2+ in response to caffeine and ryanodine; however, the regulation of this release differs from that described for caffeine- and ryanodine-sensitive Ca(2+)-induced Ca2+ release of somatic cells.

Calcium signaling at fertilization

Calcium is well established as a second messenger in a diverse array of cell activities. Changes in intracellular Ca2+ activities range from localized releases to complex oscillations, which may encode specific cellular signals. The full variety of calcium responses is observed during the fertilization of different animal oocytes and eggs. Current research has focused on the cellular mechanisms that generate these Ca(2+)-activity changes.

Programs for visualization in three-dimensional microscopy

Three-dimensional data representing biological structures can be derived using several methods, including serial section reconstruction, optical sectioning, and tomography. The investigation, comprehension, and communication of structural relationships to others is greatly facilitated by computer-based visualization procedures. We describe SYNU, a suite of programs developed for interactive investigation of three-dimensional structure and for the production of high-quality three-dimensional images and animations. We illustrate the capabilities of SYNU in applications to biological data obtained by confocal light microscopy, serial section, and high-resolution electron microscopy from investigations at the cellular, subcellular, and molecular levels.

Abnormal Ca2+ homeostasis before cell death revealed by whole cell recording of ischemic CA1 hippocampal neurons

Slices were made from the hippocampus of gerbils following transient ischemia achieved by clamping the carotid arteries for 5 min, and changes in the electrophysiology of CA1 pyramidal neurons were studied by whole cell patch-clamp recording as well as conventional intracellular recording. The great majority of CA1 neurons in slices made 2.5-3 days after ischemia showed reduced resting potentials and were easily depolarized by prolonged low-frequency stimulation or by tetanic stimulation of the Schaffer collateral/commissural input. This stimulus-induced depolarization was accelerated by intracellular injection of D-myo-inositol 1,4,5-triphosphate, which depolarized membrane potentials towards 0 mV without synaptic input stimulation. Intracellular application of BAPTA, a Ca2+ chelator, effectively blocked the stimulus-induced depolarization. When recording from ischemic neurons with patch pipettes containing both D-myo-inositol 1,4,5-triphosphate and BAPTA, excitatory postsynaptic currents were transiently potentiated by stimulation, but the membrane potential did not show stimulus-induced depolarization and remained steady for long periods. These results lend support to the view that the intracellular Ca2+ regulation system is severely disturbed following ischemia, and that input fiber stimulation leads to abnormal Ca2+ accumulation in ischemic neurons resulted in neuronal death. The reduction of free Ca2+ inside the ischemic neuron by BAPTA apparently saves neurons which are otherwise destined to delayed neuronal death.

Localization of gene expression of calreticulin in the brain of adult mouse

The localization of gene expression of calreticulin, a calcium-binding protein in the endoplasmic reticulum, was examined throughout the entire brain of adult mice by in situ hybridization. Calreticulin mRNA is expressed widely and heterogeneously in discrete neurons throughout the brain, but the white matters expressed it weekly or faintly. In the olfactory bulb, the mRNA is expresses moderately in the mitral cells, but weakly in the periglomerular cells and internal granule cells. In the cerebrum, the gene is expressed intensely in the piriform cortex, but weakly in neocortex, the entorhinal cortex and the amygdaloid nuclei. In the hippocampal formation, calreticulin mRNA is expressed intensely in the CA1-CA3 regions but less intensely in the granule cells of the dentate gyrus. The caudate-putamen, thalamic and hypothalamic nuclei, and mammillary nuclei express the mRNA weakly or faintly. In the mesencephalon, pons and medulla, moderate expression of the mRNA is detected in the pontine nuclei and the locus ceruleus. Weak expression of the mRNA is detected in several discrete nuclei and zones such as the substantia nigra, the superior colliculus and the central gray. Expression signals of calreticulin mRNA are faint in the inferior olive. In the cerebellum, calreticulin mRNA is expressed moderately in the Purkinje cells whereas no significant expression is detected in the granule cells. The plexus choroideus of the lateral, third and fourth ventriculi express calreticulin mRNA intensely although no distinct expression of the mRNA is discerned in the ependyma.

High affinity [3H]ryanodine binding sites in postmortem human brain: regional distribution and effects of calcium, magnesium and caffeine

The pharmacological properties, regional distribution and autoradiographic localization of [3H]ryanodine binding sites were examined in postmortem human brain. Analyses of binding data from labeled ryanodine titration experiments conducted in frontal cortex revealed a single class of high affinity binding sites with a Kd value of 3.6 nM and a Bmax value of 99 fmol/mg protein. In unlabeled ryanodine titration experiments, Kd and Bmax values were 6.5 nM and 132 fmol/mg protein, respectively. Binding was found to be dependent on free Ca2+ (ED50 value, 89 microM) and was decreased by 35% in the presence of 5 mM Mg2+. This Mg2+ inhibition was abolished by the addition of 10 mM caffeine. Analysis of the regional distribution of [3H]ryanodine binding in membrane preparations revealed high levels of sites in putamen and caudate nucleus, intermediate levels in hippocampus and cortex, and low levels in cerebellum. Autoradiographically, the hippocampus displayed a high density of binding sites in the CA3 region and the dentate gyrus. Ryanodine binding sites in human brain exhibit similar, but not identical binding and pharmacological characteristics to ryanodine receptors previously identified in muscle and more recently in rat and rabbit brain and accordingly may be involved in the regulation of intracellular calcium.

Expression of a ryanodine receptor-Ca2+ channel that is regulated by TGF-beta

Ryanodine receptors (RyRs) are intracellular channels that release calcium ions from the sarcoplasmic reticulum (SR) in response to either plasma membrane depolarization (in skeletal muscle) or increases in the concentration of intracellular free Ca2+ (in the heart). A gene (beta 4) encoding a ryanodine receptor (similar to, but distinct from, the muscle RyRs) was identified. The beta 4 gene was expressed in all tissues investigated, with the exception of heart. Treatment of mink lung epithelial cells (Mv1Lu) with transforming growth factor beta (TGF-beta) induced expression of the beta 4 gene together with the release of Ca2+ in response to ryanodine (but not in response to caffeine, the other drug active on muscle RyRs). This ryanodine receptor may be important in the regulation of intracellular Ca2+ homeostasis.

Widespread tissue distribution of rabbit calreticulin, a non-muscle functional analogue of calsequestrin

Calreticulin was identified in a variety of rabbit tissues by Western blot analysis. Indirect immunofluorescence studies on cultured cells or frozen sections from the corresponding tissues revealed that the protein was distributed to the endoplasmic reticulum or sarcoplasmic reticulum. Calreticulin was found to be an abundant calcium-binding protein in non-muscle and smooth muscle cells and a constituent calcium-binding protein in cardiac and skeletal muscle. From the immunoblot data, calreticulin may exist as an isoform in rabbit neural retina. The present study establishes the ubiquity of calreticulin in intracellular calcium binding.

The endoplasmic reticulum of Purkinje neuron body and dendrites: molecular identity and specializations for Ca2+ transport

Immunofluorescence and immunogold labeling, together with sucrose gradient separation and Western blot analysis of microsomal subfractions, were employed in parallel to probe the endoplasmic reticulum in the cell body and dendrites of rat cerebellar Purkinje neurons. Two markers, previously investigated in non-nerve cells, the membrane protein p91 (calnexin) and the lumenal protein BiP, were found to be highly expressed and widely distributed to the various endoplasmic reticulum sections of Purkinje neurons, from the cell body to dendrites and dendritic spines. An antibody (denominated anti-rough-surfaced endoplasmic reticulum), which recognized two membrane proteins, p14 and p40, revealed a similar immunogold labeling pattern. However, centrifugation results consistent with a widespread distribution were obtained for p14 only, while p40 was concentrated in the rough microsome-enriched subfractions. The areas enriched in the inositol 1,4,5-triphosphate receptor and thus presumably specialized in Ca2+ transport (stacks of multiple smooth-surfaced cisternae; the dendritic spine apparatus) also exhibited labeling for BiP and p91, and were positive for the anti-rough-surfaced endoplasmic reticulum antibody (presumably via the p14 antigen). Additional antibodies, that yielded inadequate immunocytochemical signals, were employed only by Western blotting of the microsomal subfractions, while the ryanodine receptor was studied by specific binding. The latter receptor and the Ca2+ ATPase, known in other species to be concentrated in Purkinje neurons, exhibited bimodal distributions with a peak in the light and another in the heavy subfractions. A similar distribution was also observed with another lumenal protein, protein disulfide isomerase. Taken as a whole, the results that we have obtained suggest the existence in the endoplasmic reticulum of Purkinje neurons of two levels of organization; the first identified by widespread, probably general markers (BiP, p91, possibly p14 and others), the second by specialization markers, such as the inositol 1,4,5-triphosphate receptor and, possibly, p40, which appear restricted to areas where specific functions appear to be localized.

Morphological response of endoplasmic reticulum in cerebellar Purkinje cells to calcium deprivation

Mobilisable intracellular Ca2+ stores are highly enriched in the cerebellum, particularly in Purkinje cells. We have detected, by light and electron microscopy, striking morphological changes in the presumed Ca2+ stores of Purkinje cells when slices of eight-day-old rat cerebellum were incubated in Ca(2+)-deficient media. After 30 min under these conditions, the endoplasmic reticulum became thinned and elongated. By 2 h, it was transformed into multilamellar, whorl-like inclusions with electron-dense cores. These changes were reversed on reintroduction of Ca2+. Analogous changes in other neurons were not observed. The results suggest that Ca2+ storage sites within Purkinje cells are capable of dramatic morphological change depending on the availability of Ca2+. The transformations may reflect, initially, depletion of Ca2+ from the stores and then homeostatic alterations in their capacity.

Calcium pools mobilized by calcium or inositol 1,4,5-trisphosphate are differentially localized in rat heart and brain

Calcium-induced calcium release (CICR) pools have been demonstrated in brain and heart microsomes biochemically and autoradiographically by the sensitivity of 45Ca2+ accumulation to Mg2+, ATP, ruthenium red, caffeine, and tetracaine. The CICR pool colocalizes with [3H]ryanodine binding sites, supporting the notion that [3H]ryanodine labels CICR pools. Sites of CICR pools in the brain contrast with those of inositol 1,4,5-trisphosphate (IP3)-sensitive Ca2+ pools with reciprocal localizations between the two Ca2+ pools in several structures. Thus, in the hippocampus CA-1 is enriched in IP3-sensitive Ca2+ pools, whereas CICR pools are highest in CA-3 and the dentate gyrus. The corpus striatum and cerebellum are enriched in IP3 pools, whereas the medial septum and olfactory bulb have high CICR densities. In cardiac tissue, CICR is localized to atrial and ventricular muscle, whereas IP3 pools are concentrated in coronary vessels and cardiac conduction fibers. The reciprocal enrichment of IP3 and CICR Ca2+ pools implies differential regulation of Ca2+ hemostasis in these tissues.