A cADP-ribose antagonist does not inhibit secretagogue-, caffeine- and nitric oxide-induced Ca2+ responses in rat pancreatic beta-cells

Roles of cADPR and NAADP in pancreatic cells

Cyclic ADP-ribose (cADPR) and nicotinic acid adenine dinucleotide phosphate (NAADP) are Ca(2+)-mobilizing nucleotides that were discovered in the late 1980s. Two decades of investigations have built up a considerable understanding about these two molecules that are related because both are derived from pyridine nucleotides and known to be generated by CD38/ADP-ribosyl cyclases. cADPR has been shown to target the ryanodine receptors in the endoplasmic reticulum whereas NAADP stimulates the two-pore channels in the endo-lysosomes. Accumulating results indicate that cADPR and NAADP are second messenger molecules mediating Ca(2+) signaling activated by a wide range of agonists. This article reviews what is known about these two molecules, especially regarding their signaling roles in the pancreatic cells.

The complex contribution of NOS interneurons in the physiology of cerebrovascular regulation

Following the discovery of the vasorelaxant properties of nitric oxide (NO) by Furchgott and Ignarro, the finding by Bredt and coll. of a constitutively expressed NO synthase in neurons (nNOS) led to the presumption that neuronal NO may control cerebrovascular functions. Consequently, numerous studies have sought to determine whether neuraly-derived NO is involved in the regulation of cerebral blood flow (CBF). Anatomically, axons, dendrites, or somata of NO neurons have been found to contact the basement membrane of blood vessels or perivascular astrocytes in all segments of the cortical microcirculation. Functionally, various experimental approaches support a role of neuronal NO in the maintenance of resting CBF as well as in the vascular response to neuronal activity. Since decades, it has been assumed that neuronal NO simply diffuses to the local blood vessels and produce vasodilation through a cGMP-PKG dependent mechanism. However, NO is not the sole mediator of vasodilation in the cerebral microcirculation and is known to interact with a myriad of signaling pathways also involved in vascular control. In addition, cerebrovascular regulation is the result of a complex orchestration between all components of the neurovascular unit (i.e., neuronal, glial, and vascular cells) also known to produce NO. In this review article, the role of NO interneuron in the regulation of cortical microcirculation will be discussed in the context of the neurovascular unit.

An emerging role for NAADP-mediated Ca2+ signaling in the pancreatic β-cell

Several recent reports, including one in this journal, have reignited the debate about whether the calcium-mobilizing messenger, nicotinic adenine nucleotide diphosphate (NAADP) plays a central role in the regulation of calcium signalling in pancreatic β-cell. These studies have highlighted a role for NAADP-induced Ca(2+) mobilization not only in mediating the effects of the incretin, GLP-1 and the autocrine proliferative effects of insulin, but also possibly a fundamental role in glucose-mediated insulin secretion in the pancreatic β-cell.

Vascular physiology of a Ca2+ mobilizing second messenger - cyclic ADP-ribose

Cyclic ADP-ribose (cADPR) is a novel Ca(2+) mobilizing second messenger, which is capable of inducing Ca(2+) release from the sarcoplasmic reticulum (SR) via activation of ryanodine receptors (RyR) in vascular cells. This signaling nucleotide has also been reported to participate in generation or modulation of intracellular Ca(2+) sparks, Ca(2+) waves or oscillations, Ca(2+)- induced Ca(2+) release (CICR) and spontaneous transient outward currents (STOCs) in vascular smooth muscle cells (VSMCs). With respect to the role of cADPR-mediated signaling in mediation of vascular responses to different stimuli, there is accumulating evidence showing that cADPR is importantly involved in the Ca(2+) response of vascular endothelial cells (ECs) and VSMCs to various chemical factors such as vasoactive agonists acetylcholine, oxotremorine, endothelin, and physical stimuli such as stretch, electrical depolarization and sheer stress. This cADPR-RyR-mediated Ca(2+) signaling is now recognized as a fundamental mechanism regulating vascular function. Here we reviewed the literature regarding this cADPR signaling pathway in vascular cells with a major focus on the production of cADPR and its physiological roles in the control of vascular tone and vasomotor response. We also summarized some publish results that unveil the underlying mechanisms mediating the actions of cADPR in vascular cells. Given the importance of Ca(2+) in the regulation of vascular function, the results summarized in this brief review will provide new insights into vascular physiology and circulatory regulation.

Regulation of intracellular Ca2+ stores by multiple Ca2+-releasing messengers

Although glucose-elicited insulin secretion depends on Ca(2+) entry through voltage-gated Ca(2+) channels in the surface cell membrane of the pancreatic beta-cell, there is also ample evidence for an important role of intracellular Ca(2+) stores, particularly in relation to hormone- or neurotransmitter-induced insulin secretion. There is now direct evidence for Ca(2+) entry-induced release of Ca(2+) from the endoplasmic reticulum in neurons, but with regard to glucose stimulation of beta-cells, there is conflicting evidence about the operation of such a process. This finding suggests that the sensitivity of the Ca(2+) release channels in the endoplasmic reticulum membrane varies under different conditions and therefore is regulated. Recent evidence from studies of pancreatic acinar cells has revealed combinatorial roles of multiple messengers in setting the sensitivity of the endoplasmic reticulum for Ca(2+) release. Here we focus on the possible combinatorial roles of inositol 1,4,5-trisphosphate, cyclic ADP-ribose, and nicotinic acid adenine dinucleotide phosphate in beta-cell function.

The ryanodine receptor calcium channel of beta-cells: molecular regulation and physiological significance

The list of Ca(2+) channels involved in stimulus-secretion coupling in beta-cells is increasing. In this respect the roles of the voltage-gated Ca(2+) channels and IP(3) receptors are well accepted. There is a lack of consensus about the significance of a third group of Ca(2+) channels called ryanodine (RY) receptors. These are large conduits located on Ca(2+) storage organelle. Ca(2+) gates these channels in a concentration- and time-dependent manner. Activation of these channels by Ca(2+) leads to fast release of Ca(2+) from the stores, a process called Ca(2+)-induced Ca(2+) release (CICR). A substantial body of evidence confirms that beta-cells have RY receptors. CICR by RY receptors amplifies Ca(2+) signals. Some properties of RY receptors ensure that this amplification process is engaged in a context-dependent manner. Several endogenous molecules and processes that modulate RY receptors determine the appropriate context. Among these are several glycolytic intermediates, long-chain acyl CoA, ATP, cAMP, cADPR, NO, and high luminal Ca(2+) concentration, and all of these have been shown to sensitize RY receptors to the trigger action of Ca(2+). RY receptors, thus, detect co-incident signals and integrate them. These Ca(2+) channels are targets for the action of cAMP-linked incretin hormones that stimulate glucose-dependent insulin secretion. In beta-cells some RY receptors are located on the secretory vesicles. Thus, despite their low abundance, RY receptors are emerging as distinct players in beta-cell function by virtue of their large conductance, strategic locations, and their ability to amplify Ca(2+) signals in a context-dependent manner.

cAMP-regulated guanine nucleotide exchange factor II (Epac2) mediates Ca2+-induced Ca2+ release in INS-1 pancreatic beta-cells

1. The signal transduction pathway responsible for cAMP-dependent Ca2+-induced Ca2+ release (CICR) from endoplasmic reticulum Ca2+ stores was assessed in the insulin-secreting cell line INS-1. 2. CICR was triggered by the GLP-1 receptor agonist exendin-4, an effect mimicked by caffeine, Sp-cAMPS or forskolin. CICR required influx of Ca2+ through L-type voltage-dependent Ca2+ channels, and was blocked by treatment with nimodipine, thapsigargin, or ryanodine, but not by the IP3 receptor antagonist xestospongin C. 3. Treatment with the cAMP antagonist 8-Br-Rp-cAMPS blocked CICR in response to exendin-4, whereas the PKA inhibitor H-89 was ineffective when tested at a concentration demonstrated to inhibit PKA-dependent gene expression. 4. RT-PCR of INS-1 cells demonstrated expression of mRNA coding for the type-II isoform of cAMP-regulated guanine nucleotide exchange factor (cAMP-GEF-II, Epac2). 5. CICR in response to forskolin was blocked by transient transfection and expression of a dominant negative mutant isoform of cAMP-GEF-II in which inactivating mutations were introduced into the exchange factor's two cAMP-binding domains. 6. It is concluded that CICR in INS-1 cells results from GLP-1 receptor-mediated sensitization of the intracellular Ca2+ release mechanism, a signal transduction pathway independent of PKA, but which requires cAMP-GEF-II.

Mechanisms and physiological significance of the cholinergic control of pancreatic beta-cell function

Acetylcholine (ACh), the major parasympathetic neurotransmitter, is released by intrapancreatic nerve endings during the preabsorptive and absorptive phases of feeding. In beta-cells, ACh binds to muscarinic M(3) receptors and exerts complex effects, which culminate in an increase of glucose (nutrient)-induced insulin secretion. Activation of PLC generates diacylglycerol. Activation of PLA(2) produces arachidonic acid and lysophosphatidylcholine. These phospholipid-derived messengers, particularly diacylglycerol, activate PKC, thereby increasing the efficiency of free cytosolic Ca(2+) concentration ([Ca(2+)](c)) on exocytosis of insulin granules. IP3, also produced by PLC, causes a rapid elevation of [Ca(2+)](c) by mobilizing Ca(2+) from the endoplasmic reticulum; the resulting fall in Ca(2+) in the organelle produces a small capacitative Ca(2+) entry. ACh also depolarizes the plasma membrane of beta-cells by a Na(+)- dependent mechanism. When the plasma membrane is already depolarized by secretagogues such as glucose, this additional depolarization induces a sustained increase in [Ca(2+)](c). Surprisingly, ACh can also inhibit voltage-dependent Ca(2+) channels and stimulate Ca(2+) efflux when [Ca(2+)](c) is elevated. However, under physiological conditions, the net effect of ACh on [Ca(2+)](c) is always positive. The insulinotropic effect of ACh results from two mechanisms: one involves a rise in [Ca(2+)](c) and the other involves a marked, PKC-mediated increase in the efficiency of Ca(2+) on exocytosis. The paper also discusses the mechanisms explaining the glucose dependence of the effects of ACh on insulin release.

Human anti-CD38 autoantibodies raise intracellular calcium and stimulate insulin release in human pancreatic islets

CD38 is involved in transmembrane signaling in many cell types; anti-CD38 autoantibodies have been described in diabetic patients. We tested whether human anti-CD38 antibodies possess signaling properties by measuring their ability to raise intracellular calcium ([Ca2+]i) using the fluo-3-acetoxymethyl ester method in a human-derived T-cell line (Jurkat T-cells, expressing high levels of surface CD38) and in dispersed human islet cells from normal donors. In Jurkat T-cells, 11 of 19 anti-CD38-positive sera raised [Ca2+]i (by > or =20% of baseline), whereas no [Ca2+]i-mobilizing activity was found in 27 anti-CD38-negative sera (chi2 = 20.5, P < 0.0001). In dispersed human islet cells, 5 of 11 anti-CD38-positive sera (and none of three anti-CD38-negative sera) raised [Ca2+]i significantly. When preincubated with Staphylococcus aureus protein A to remove IgG, anti-CD38-positive sera showed a 70 +/- 5% reduction in [Ca2+]i-mobilizing activity. Preincubation with CD38-transfected NIH-3T3 fibroblasts, but not with mock-transfected NIH-3T3 cells, abolished [Ca2+]i mobilization. In blocking experiments, preincubation with nonagonistic anti-CD38 monoclonal antibodies also prevented [Ca2+]i mobilization. In cultured human islets, anti-CD38-positive sera exhibiting [Ca2+]i-mobilizing activity in Jurkat T-cells (n = 6) significantly stimulated insulin release at 3.3 mmol/l glucose (median [interquartile range] 738 microU/ml [234], P = 0.0001 vs. 320 [52] microU/ml of control), whereas 6 anti-CD38-positive sera without [Ca2+]i-mobilizing activity and 10 anti-CD38-negative did not. In further incubations, the five anti-CD38-positive sera displaying [Ca2+]i-mobilizing activity in dispersed islet cells significantly stimulated insulin release at both 3.3 mmol/l glucose (2.2 +/- 0.3% of insulin islet content, P < 0.002 vs. 1.2 +/- 0.1% of control) and 16.7 mmol/l glucose (3.7 +/- 0.3 vs. 2.3 +/- 0.3%, P < 0.002). We conclude that human anti-CD38 autoantibodies with agonistic properties on the CD38 effector system occur in nature; in human islets, their [Ca2+]i-mobilizing activity is coupled with the ability to stimulate insulin release.

Localized calcium influx in pancreatic beta-cells: its significance for Ca2+-dependent insulin secretion from the islets of Langerhans

Ca2+ influx through voltage-dependent Ca2+ channels plays a crucial role in stimulus-secretion coupling in pancreatic islet beta-cells. Molecular and physiologic studies have identified multiple Ca2+ channel subtypes in rodent islets and insulin-secreting cell lines. The differential targeting of Ca2+ channel subtypes to the vicinity of the insulin secretory apparatus is likely to account for their selective coupling to glucose-dependent insulin secretion. In this article, I review these studies. In addition, I discuss temporal and spatial aspects of Ca2+ signaling in beta-cells, the former involving the oscillatory activation of Ca2+ channels during glucose-induced electrical bursting, and the latter involving [Ca2+]i elevation in restricted microscopic "domains," as well as direct interactions between Ca2+ channels and secretory SNARE proteins. Finally, I review the evidence supporting a possible role for Ca2+ release from the endoplasmic reticulum in glucose-dependent insulin secretion, and evidence to support the existence of novel Ca2+ entry pathways. I also show that the beta-cell has an elaborate and complex set of [Ca2+]i signaling mechanisms that are capable of generating diverse and extremely precise [Ca2+]i patterns. These signals, in turn, are exquisitely coupled in space and time to the beta-cell secretory machinery to produce the precise minute-to-minute control of insulin secretion necessary for body energy homeostasis.

A fundamental role for the nitric oxide-G-kinase signaling pathway in mediating intercellular Ca(2+) waves in glia

In this study, we highlight a role for the nitric oxide-cGMP-dependent protein kinase (NO-G-kinase) signaling pathway in glial intercellular Ca(2+) wave initiation and propagation. Addition of the NO donor molsidomine (100-500 microM) or puffing aqueous NO onto primary glial cell cultures evoked an increase in [Ca(2+)](i) in individual cells and also local intercellular Ca(2+) waves, which persisted after removal of extracellular Ca(2+). High concentrations of ryanodine (100-200 microM) and antagonists of the NO-G-kinase signaling pathway essentially abrogated the NO-induced increase in [Ca(2+)](i), indicating that NO mobilizes Ca(2+) from a ryanodine receptor-linked store, via the NO-G-kinase signaling pathway. Addition of 10 microM nicardipine to cells resulted in a slowing of the molsidomine-induced rise in [Ca(2+)](i), and inhibition of Mn(2+) quench of cytosolic fura-2 fluorescence mediated by a bolus application of 2 microM aqueous NO to cells, indicating that NO also induces Ca(2+) influx in glia. Mechanical stress of individual glial cells resulted in an increase in intracellular NO in target and neighboring cells and intercellular Ca(2+) waves, which were NO, cGMP, and G-kinase dependent, because incubating cells with nitric oxide synthase, guanylate cyclase, and G-kinase inhibitors, or NO scavengers, reduced Delta[Ca(2+)](i) and the rate of Ca(2+) wave propagation in these cultures. Results from this study suggest that NO-G-kinase signaling is coupled to Ca(2+) mobilization and influx in glial cells and that this pathway plays a fundamental role in the generation and propagation of intercellular Ca(2+) waves in glia.

Delta-opioid-induced liberation of Gbetagamma mobilizes Ca2+ stores in NG108-15 cells

Activation of delta-opioid receptors in NG108-15 cells releases Ca2+ from an intracellular store through activation of a pertussis toxin-sensitive G protein. We tested the hypothesis that activation of delta-opioid receptors mobilizes inositol 1,4,5-trisphosphate (IP(3))-sensitive Ca2+ stores via liberation of Gbetagamma. Fura-2-based digital imaging was used to study the mechanism of opioid-induced increases in [Ca2+](i) in NG108-15 cells. Exposure to D-Ala(2)-D-Leu(5) enkephalin (100 nM) for 90 s induced increases in [Ca2+](i) that were blocked by microinjection of the IP(3) receptor antagonist heparin (pipette concentration = 100 mg/ml) but not by sham injection. Microinjection of a peptide that binds Gbetagamma (QEHA, 1 mM) decreased the D-Ala(2)-D-Leu(5) enkephalin-evoked response. Microinjection of an inactive peptide (SKEE, 1 mM) that does not bind to Gbetagamma failed to inhibit the opioid-induced increase in [Ca2+](i). Microinjection of a peptide (QLKK, 15 mM) that binds to free Galpha(q) blocked the increase evoked by 3 nM bradykinin, but microinjection of an inactive peptide (ADRK, 15 mM) did not. Microinjection of QLKK did not significantly affect the opioid-induced increase in [Ca2+](i). Collectively, these data demonstrate that activation of delta-opioid receptors induces the release of Ca2+ from IP(3)-sensitive stores in NG108-15 cells through activation of the betagamma subunits of inhibitory G proteins.

cAMP-dependent mobilization of intracellular Ca2+ stores by activation of ryanodine receptors in pancreatic beta-cells. A Ca2+ signaling system stimulated by the insulinotropic hormone glucagon-like peptide-1-(7-37)

Glucagon-like peptide-1 (GLP-1) is an intestinally derived insulinotropic hormone currently under investigation for use as a novel therapeutic agent in the treatment of type 2 diabetes mellitus. In vitro studies of pancreatic islets of Langerhans demonstrated that GLP-1 interacts with specific beta-cell G protein-coupled receptors, thereby facilitating insulin exocytosis by raising intracellular levels of cAMP and Ca2+. Here we report that the stimulatory influence of GLP-1 on Ca2+ signaling results, in part, from cAMP-dependent mobilization of ryanodine-sensitive Ca2+ stores. Studies of human, rat, and mouse beta-cells demonstrate that the binding of a fluorescent derivative of ryanodine (BODIPY FL-X ryanodine) to its receptors is specific, reversible, and of high affinity. Rat islets and BTC3 insulinoma cells are shown by reverse transcriptase polymerase chain reaction analyses to express mRNA corresponding to the type 2 isoform of ryanodine receptor-intracellular Ca2+ release channel (RYR2). Single-cell measurements of [Ca2+]i using primary cultures of rat and human beta-cells indicate that GLP-1 facilitates Ca2+-induced Ca2+ release (CICR), whereby mobilization of Ca2+ stores is triggered by influx of Ca2+ through L-type Ca2+ channels. In these cells, GLP-1 is shown to interact with metabolism of D-glucose to produce a fast transient increase of [Ca2+]i. This effect is reproduced by 8-Br-cAMP, but is blocked by a GLP-1 receptor antagonist (exendin-(9-39)), a cAMP antagonist ((Rp)-cAMPS), an L-type Ca2+ channel antagonist (nimodipine), an antagonist of the sarco(endo)plasmic reticulum Ca2+ ATPase (thapsigargin), or by ryanodine. Characterization of the CICR mechanism by voltage clamp analysis also demonstrates a stimulation of Ca2+ release by caffeine. These findings provide new support for a model of beta-cell signal transduction whereby GLP-1 promotes CICR by sensitizing intracellular Ca2+ release channels to the stimulatory influence of cytosolic Ca2+.

Cyclic ADP-ribose: a novel Ca2+-mobilising second messenger

Cyclic ADP-ribose (cADPR) was discovered as a potent Ca2+-mobilising natural compound in sea urchin eggs. Recently, cADPR was reported to stimulate Ca2+ signalling in several higher eukaryotic cell systems (e.g., smooth and cardiac muscle cells, neuronal cells, adrenal chromaffin cells, macrophages, pancreatic acinar cells and T-lymphocytes). The following aspects of the role of cADPR as a Ca2+-mobilising second messenger are reviewed: coupling of metabolism of cADPR to stimulation of receptors in the plasma membrane, properties and pharmacology of Ca2+ release by cADPR and the involvement of cADPR in Ca2+ entry.

In situ activation of the type 2 ryanodine receptor in pancreatic beta cells requires cAMP-dependent phosphorylation

Molecular mechanisms that regulate in situ activation of ryanodine receptors (RY) in different cells are poorly understood. Here we demonstrate that caffeine (10 mM) released Ca2+ from the endoplasmic reticulum (ER) in the form of small spikes in only 14% of cultured fura-2 loaded beta cells from ob/ob mice. Surprisingly, when forskolin, an activator of adenylyl cyclase was present, caffeine induced larger Ca2+ spikes in as many as 60% of the cells. Forskolin or the phosphodiesterase-resistant PKA activator Sp-cAMPS alone did not release Ca2+ from ER. 4-Chloro-3-ethylphenol (4-CEP), an agent that activates RYs in other cell systems, released Ca2+ from ER, giving rise to a slow and small increase in [Ca2+]i in beta cells. Prior exposure of cells to forskolin or caffeine (5 mM) qualitatively altered Ca2+ release by 4-CEP, giving rise to Ca2+ spikes. In glucose-stimulated beta cells forskolin induced Ca2+ spikes that were enhanced by 3,9-dimethylxanthine, an activator of RYs. Analysis of RNA from islets and insulin-secreting betaTC-3-cells by RNase protection assay, using type-specific RY probes, revealed low-level expression of mRNA for the type 2 isoform of the receptor (RY2). We conclude that in situ activation of RY2 in beta cells requires cAMP-dependent phosphorylation, a process that recruits the receptor in a functionally operative form.

7-Deaza-8-bromo-cyclic ADP-ribose, the first membrane-permeant, hydrolysis-resistant cyclic ADP-ribose antagonist

Cyclic ADP-ribose (cADPR) is a putative second messenger that has been demonstrated to mobilize Ca2+ in many cell types. Its postulated role as the endogenous regulator of ryanodine-sensitive Ca2+ release channels has been greatly supported by the advent and use of specific cADPR receptor antagonists such as 8-NH2-cADPR (Walseth, T. F., and Lee, H. C. (1993) Biochim. Biophys. Acta 1178, 235-242). However, investigations of the role of cADPR in physiological responses, such as fertilization, stimulus-secretion coupling, and excitation-contraction coupling, have been hindered by the susceptibility of cADPR receptor antagonists to hydrolysis and the need to introduce these molecules into cells by microinjection or patch clamp techniques. We have recently reported on the discovery of a poorly hydrolyzable analogue of cADPR, 7-deaza-cADPR (Bailey, V. C., Sethi, J. K., Fortt, S. M., Galione, A., and Potter, B. V. L. (1997) Chem. Biol. 4, 41-51) but this, like cADPR, is an agonist of ryanodine-sensitive Ca2+ release channels. We therefore explored the possibility of combining antagonistic activity with that of hydrolytic resistance and now report on the biological properties of the first hydrolysis-resistant cADPR receptor antagonist, 7-deaza-8-bromo-cADPR. In addition this compound has the advantage of being membrane-permeable. Together these properties make this hybrid molecule the most powerful tool to date for studying cADPR-mediated Ca2+ signaling in intact cells.

Cyclic ADP-ribose measurements in rat pancreatic islets

Cyclic ADP-ribose was measured by a radioimmunological procedure in rat pancreatic islets. The cyclic ADP-ribose content of the islets was much lower immediately after isolation (< or = 2.3 +/- 0.2 fmol/islet) than after a further incubation of 90 min in a salt-balanced medium (26.4 +/- 1.3 fmol/islet). Under the latter condition, the islet cyclic ADP-ribose content was not affected by the concentration of D-glucose and/or D-fructose in the incubation medium and, when expressed per micrograms protein, was 3- to 30-fold higher than that found in other biological samples such as liver, whole pancrease or tumoral islet cells. Although these findings might suggest a physiological role for cyclic ADP-ribose in islet cells, they argue against the view that the cyclic nucleotide acts as a coupling factor in the process of hexosestimulated insulin release.

Adenine nucleotide diphosphates: emerging second messengers acting via intracellular Ca2+ release

Release of Ca2+ from intracellular stores is a widespread mechanism in regulation of cell function. Two hitherto unknown adenine diphosphonucleotides were recently identified, which trigger Ca2+ release from intracellular stores via channels that are distinct from the well-known receptor/channel controlled by inositol 1,4,5,-trisphosphate (IP3): cyclic ADP-ribose (cADPR) and nicotinic acid adenine dinucleotide phosphate (NAADP). Here we review synthesis of cADPR from beta-NAD, its hydrolysis to adenosine diphosphoribose (noncyclic) by cADPR glycohydrolase, as well as our knowledge about the metabolism of NAADP. The Ca2+ release triggered by cADPR, NAADP, or IP3 can be distinguished by the action of inhibitors and by desensitization studies. Evidence now emerges that cADPR synthesis from beta-NAD can be stimulated, at least in some cell types by all-trans-retinoic acid as a first messenger. We then review the properties of cADPR and NAADP as potential second messengers in the intracrine regulation of cell functions. Although their exact role in signaling sequences is not yet known, cADPR and NAADP are likely to play important intracellular regulatory functions, as extensively documented for the process of egg fertilization.

The type 2 ryanodine receptor of neurosecretory PC12 cells is activated by cyclic ADP-ribose. Role of the nitric oxide/cGMP pathway

Of two neurosecretory PC12 cell clones that respond to NO donors and 8-bromo-cGMP with similar increases in cADP-ribose and that possess molecularly similar Ca2+ stores, only one (clone 16A) expresses the type 2 ryanodine receptor, whereas the other (clone 27) is devoid of ryanodine receptors. In PC12-16A cells, activation of the NO/cGMP pathway induced slow [Ca2+]i responses, sustained by release from Ca2+ stores. In contrast, PC12-27 cells were insensitive to NO donors. Likewise, in PC12-16A cells preincubated with NO donors, Ca2+ stores were partially depleted, as revealed by a test with thapsigargin, whereas those in clone 27 were unchanged. The NO-induced Ca2+ release was increased synergistically by caffeine, and the corresponding store depletion was magnified by ryanodine. The specificity for the NO/cGMP pathway was confirmed by the effects of two blockers of cGMP-dependent protein kinase I, while the role of cADP-ribose was demonstrated by the effects of its antagonist, 8-amino-cADP-ribose, administered to permeabilized cells. These results demonstrate in neurosecretory cells a ryanodine receptor activation pathway similar to that known in sea urchin oocytes. The signaling events described here could be of great physiological importance, especially in the nervous system.