Effects of caffeine on cytoplasmic free Ca2+ concentration in pancreatic beta-cells are mediated by interaction with ATP-sensitive K+ channels and L-type voltage-gated Ca2+ channels but not the ryanodine receptor

Chronic fatigue syndrome: Abnormally fast muscle fiber conduction in the membranes of motor units at low static force load

OBJECTIVE

Chronic fatigue syndrome (CFS) and fibromyalgia (FM) are disorders of unknown etiology and unclear pathophysiology, with overlapping symptoms of - especially muscular -fatigue and pain. Studies have shown increased muscle fiber conduction velocity (CV) in the non-painful muscles of FM patients. We investigated whether CFS patients also show CV abnormalities.

METHODS

Females with CFS (n = 25), with FM (n = 22), and healthy controls (n = 21) underwent surface electromyography of the biceps brachii, loaded up to 20% of maximum strength, during short static contractions. The mean CV and motor unit potential (MUP) velocities with their statistical distribution were measured.

RESULTS

The CV changes with force differed between CFS-group and both FM-group and controls (P = 0.01). The CV of the CFS-group increased excessively with force (P < 0.001), whereas that of the controls increased only slightly and non-significantly, and that of the FM-group did not increase at all. In the CFS-group, the number of MUPs conveying very high conduction velocities increased abundantly with force and the MUPs narrowed.

CONCLUSION

Our results suggest disturbed muscle membrane function in CFS patients, in their motor units involved in low force generation. Central neural deregulation may contribute to this disturbance.

SIGNIFICANCE

These findings help to detangle the underlying mechanisms of CFS.

Pleiotropic Effects of Caffeine Leading to Chromosome Instability and Cytotoxicity in Eukaryotic Microorganisms

Caffeine, a methylxanthine analog of purine bases, is a compound that is largely consumed in beverages and medications for psychoactive and diuretic effects and plays many beneficial roles in neuronal stimulation and enhancement of anti-tumor immune responses by blocking adenosine receptors in higher organisms. In single-cell eukaryotes, however, caffeine somehow impairs cellular fitness by compromising cell wall integrity, inhibiting target of rapamycin (TOR) signaling and growth, and overriding cell cycle arrest caused by DNA damage. Among its multiple inhibitory targets, caffeine specifically interacts with phosphatidylinositol 3-kinase (PI3K)-related kinases causing radiosensitization and cytotoxicity via specialized intermediate molecules. Caffeine potentiates the lethality of cells in conjunction with several other stressors such as oxidants, irradiation, and various toxic compounds through largely unknown mechanisms. In this review, recent findings on caffeine effects and cellular detoxification schemes are highlighted and discussed with an emphasis on the inhibitory interactions between caffeine and its multiple targets in eukaryotic microorganisms such as budding and fission yeasts.

4-chloro-orto-cresol activates ryanodine receptor more selectively and potently than 4-chloro-meta-cresol

In this study we performed the comprehensive pharmacological analysis of two stereoisomers of 4-chloro-meta-cresol (4CMC), a popular ryanodine receptor (RyR) agonist used in muscle research. Experiments investigating the Ca²⁺-releasing action of the isomers demonstrated that the most potent isomer was 4-chloro-orto-cresol (4COC) (EC₅₀ = 55 ± 14 μM), although 3-chloro-para-cresol (3CPC) was more effective, as it was able to induce higher magnitude of Ca²⁺ flux from isolated terminal cisterna vesicles. Nevertheless, 3CPC stimulated the hydrolytic activity of the sarcoplasmic reticulum ATP-ase (SERCA) with an EC₅₀ of 91 ± 17 μM, while 4COC affected SERCA only in the millimolar range (IC₅₀ = 1370 ± 88 μM). IC₅₀ of 4CMC for SERCA pump was 167 ± 8 μM, indicating that 4CMC is not a specific RyR agonist either, as it activated RyR in a similar concentration (EC₅₀ = 121 ± 20 μM). Our data suggest that the use of 4COC might be more beneficial than 4CMC in experiments, when Ca²⁺ release should be triggered through RyRs without influencing SERCA activity.

Ionic mechanisms and Ca2+ dynamics underlying the glucose response of pancreatic β cells: a simulation study

To clarify the mechanisms underlying the pancreatic β-cell response to varying glucose concentrations ([G]), electrophysiological findings were integrated into a mathematical cell model. The Ca²⁺ dynamics of the endoplasmic reticulum (ER) were also improved. The model was validated by demonstrating quiescent potential, burst–interburst electrical events accompanied by Ca²⁺ transients, and continuous firing of action potentials over [G] ranges of 0–6, 7–18, and >19 mM, respectively. These responses to glucose were completely reversible. The action potential, input impedance, and Ca²⁺ transients were in good agreement with experimental measurements. The ionic mechanisms underlying the burst–interburst rhythm were investigated by lead potential analysis, which quantified the contributions of individual current components. This analysis demonstrated that slow potential changes during the interburst period were attributable to modifications of ion channels or transporters by intracellular ions and/or metabolites to different degrees depending on [G]. The predominant role of adenosine triphosphate–sensitive K⁺ current in switching on and off the repetitive firing of action potentials at 8 mM [G] was taken over at a higher [G] by Ca²⁺- or Na⁺-dependent currents, which were generated by the plasma membrane Ca²⁺ pump, Na⁺/K⁺ pump, Na⁺/Ca²⁺ exchanger, and TRPM channel. Accumulation and release of Ca²⁺ by the ER also had a strong influence on the slow electrical rhythm. We conclude that the present mathematical model is useful for quantifying the role of individual functional components in the whole cell responses based on experimental findings.

Ca-induced Ca Release from Internal Stores in INS-1 Rat Insulinoma Cells

The secretion of insulin from pancreatic β-cells is triggered by the influx of Ca(2+) through voltage-dependent Ca(2+) channels. The resulting elevation of intracellular calcium ([Ca(2+)](i)) triggers additional Ca(2+) release from internal stores. Less well understood are the mechanisms involved in Ca(2+) mobilization from internal stores after activation of Ca(2+) influx. The mobilization process is known as calcium-induced calcium release (CICR). In this study, our goal was to investigate the existence of and the role of caffeine-sensitive ryanodine receptors (RyRs) in a rat pancreatic β-cell line, INS-1 cells. To measure cytosolic and stored Ca(2+), respectively, cultured INS-1 cells were loaded with fura-2/AM or furaptra/AM. [Ca(2+)](i) was repetitively increased by caffeine stimulation in normal Ca(2+) buffer. However, peak [Ca(2+)](i) was only observed after the first caffeine stimulation in Ca(2+) free buffer and this increase was markedly blocked by ruthenium red, a RyR blocker. KCl-induced elevations in [Ca(2+)](i) were reduced by pretreatment with ruthenium red, as well as by depletion of internal Ca(2+) stores using cyclopiazonic acid (CPA) or caffeine. Caffeine-induced Ca(2+) mobilization ceased after the internal stores were depleted by carbamylcholine (CCh) or CPA. In permeabilized INS-1 cells, Ca(2+) release from internal stores was activated by caffeine, Ca(2+), or ryanodine. Furthermore, ruthenium red completely blocked the CICR response in permeabilized cells. RyRs were widely distributed throughout the intracellular compartment of INS-1 cells. These results suggest that caffeine-sensitive RyRs exist and modulate the CICR response from internal stores in INS-1 pancreatic β-cells.

The Gq/G11-mediated signaling pathway is critical for autocrine potentiation of insulin secretion in mice

A variety of neurotransmitters, gastrointestinal hormones, and metabolic signals are known to potentiate insulin secretion through GPCRs. We show here that beta cell-specific inactivation of the genes encoding the G protein alpha-subunits Galphaq and Galpha11 resulted in impaired glucose tolerance and insulin secretion in mice. Interestingly, the defects observed in Galphaq/Galpha11-deficient beta cells were not restricted to loss of muscarinic or metabolic potentiation of insulin release; the response to glucose per se was also diminished. Electrophysiological recordings revealed that glucose-induced depolarization of isolated beta cells was impaired in the absence of Galphaq/Galpha11, and closure of KATP channels was inhibited. We provide evidence that this reduced excitability was due to a loss of beta cell-autonomous potentiation of insulin secretion through factors cosecreted with insulin. We identified as autocrine mediators involved in this process extracellular nucleotides such as uridine diphosphate acting through the Gq/G11-coupled P2Y6 receptor and extracellular calcium acting through the calcium-sensing receptor. Thus, the Gq/G11-mediated signaling pathway potentiates insulin secretion in response to glucose by integrating systemic as well as autocrine/paracrine mediators.

Modulation of insulin release by adenosine A1 receptor agonists and antagonists in INS-1 cells: the possible contribution of 86Rb+ efflux and 45Ca2+ uptake

Due to the lack of specific agonists and antagonists the role of adenosine receptor subtypes with respect to their effect on the insulin secretory system is not well investigated. The A1 receptor may be linked to different 2nd messenger systems, i.e. cAMP, K+- and 45Ca2+ channel activity. Partial A1 receptor agonists are going to be developed in order to improve diabetes (increase in insulin sensitivity, lowering of FFA and triglycerides). In this study newly synthesized selective A1 receptor agonists and antagonists were investigated thereby integrating three parameters, insulin release (RIA), 45Ca2+ uptake and 86Rb+ efflux (surrogate for K+ efflux) of INS-1 cells, an insulin secretory cell line. The presence of A1-receptors was demonstrated by Western blotting. The receptor nonselective adenosine analogue NECA (5-N-ethylcarboxyamidoadenosine) at high concentration (10 microM) had no effect on insulin release and 45Ca2+ uptake which could be interpreted as the sum of effects mediated by mutual antagonistic adenosine receptor subtypes. However, an inhibitory effect mediated by A1 receptor agonism was detected at 10 nM NECA and could be confirmed by adding the A1 receptor antagonist PSB-36 (1-butyl-8-(3-noradamantyl)-3-(3-hydroxy-propyl)xanthine). NECA inhibited 86Rb+ efflux which, however, did not fit with the simultaneous inhibition of insulin secretion. The selective A1 receptor agonist CHA (N6-cyclohexyladenosine) inhibited insulin release; the simultaneously increased Ca2+ uptake (nifedipine dependent) and inhibition of 86Rb+ efflux did not fit the insulin release data. The CHA effect (even the maximum effect at 50 microM) can be increased by 10 microM NECA indicating that CHA and NECA have nonspecific and physiologically non-relevant effects on 86Rb+ efflux in addition to their A1-receptor interaction. Since PSB-36 did not influence the NECA-induced inhibition of 86Rb+ efflux, the NECA effect is not mediated by potassium channel-linked A1 receptors. The nonselective adenosine receptor antagonist caffeine increased insulin release which was reversed by CHA as expected when hypothesizing that both act via A1 receptors in this case. In conclusion, stimulation of A1 receptors by receptor selective and nonselective compounds reduced insulin release which is not coupled to opening of potassium channels (86Rb+ efflux experiments) or inhibition of calcium channels (45Ca2+ uptake experiments). It may be expected that of all pleiotropic 2nd messengers, the cAMP system (not tested here) is predominant for A1 receptor effects and the channel systems (K+ and Ca2+) are of minor importance and do not contribute to insulin release though being coupled to the receptor in other tissues.

Dual regulation of the ATP-sensitive potassium channel by caffeine

ATP-sensitive potassium (K(ATP)) channels couple cellular metabolic status to changes in membrane electrical properties. Caffeine (1,2,7-trimethylxanthine) has been shown to inhibit several ion channels; however, how caffeine regulates K(ATP) channels was not well understood. By performing single-channel recordings in the cell-attached configuration, we found that bath application of caffeine significantly enhanced the currents of Kir6.2/SUR1 channels, a neuronal/pancreatic K(ATP) channel isoform, expressed in transfected human embryonic kidney (HEK)293 cells in a concentration-dependent manner. Application of nonselective and selective phosphodiesterase (PDE) inhibitors led to significant enhancement of Kir6.2/SUR1 channel currents. Moreover, the stimulatory action of caffeine was significantly attenuated by KT5823, a specific PKG inhibitor, and, to a weaker extent, by BAPTA/AM, a membrane-permeable Ca(2+) chelator, but not by H-89, a selective PKA inhibitor. Furthermore, the stimulatory effect was completely abrogated when KT5823 and BAPTA/AM were co-applied with caffeine. In contrast, the activity of Kir6.2/SUR1 channels was decreased rather than increased by caffeine in cell-free inside-out patches, while tetrameric Kir6.2LRKR368/369/370/371AAAA channels were suppressed regardless of patch configurations. Caffeine also enhanced the single-channel currents of recombinant Kir6.2/SUR2B channels, a nonvascular smooth muscle K(ATP) channel isoform, although the increase was smaller. Moreover, bidirectional effects of caffeine were reproduced on the K(ATP) channel present in the Cambridge rat insulinoma G1 (CRI-G1) cell line. Taken together, our data suggest that caffeine exerts dual regulation on the function of K(ATP) channels: an inhibitory regulation that acts directly on Kir6.2 or some closely associated regulatory protein(s), and a sulfonylurea receptor (SUR)-dependent stimulatory regulation that requires cGMP-PKG and intracellular Ca(2+)-dependent signaling.

Mammalian G proteins and their cell type specific functions

Heterotrimeric G proteins are key players in transmembrane signaling by coupling a huge variety of receptors to channel proteins, enzymes, and other effector molecules. Multiple subforms of G proteins together with receptors, effectors, and various regulatory proteins represent the components of a highly versatile signal transduction system. G protein-mediated signaling is employed by virtually all cells in the mammalian organism and is centrally involved in diverse physiological functions such as perception of sensory information, modulation of synaptic transmission, hormone release and actions, regulation of cell contraction and migration, or cell growth and differentiation. In this review, some of the functions of heterotrimeric G proteins in defined cells and tissues are described.

Ryanodine receptor‐operated activation of TRP‐like channels can trigger critical Ca2+signaling events in pancreatic β‐cells

There is little information available concerning the link between the ryanodine (RY) receptors and the downstream Ca(2+) signaling events in beta-cells. In fura-2 loaded INS-1E cells, activation of RY receptors by 9-methyl 5,7-dibromoeudistomin D (MBED) caused a rapid rise of [Ca(2+)]i followed by a plateau and repetitive [Ca(2+)]i spikes on the plateau. The [Ca(2+)]i plateau was abolished by omission of extracellular Ca(2+) and by SKF 96365. In the presence of SKF 96365, MBED produced a transient increase of [Ca(2+)]i, which was abolished by thapsigargin. Activation of RY receptors caused Ca(2+) entry even when the ER Ca(2+) pool was depleted by thapsigargin. The [Ca(2+)]i plateau was not inhibited by nimodipine or ruthenium red, but was inhibited by membrane depolarization, La(3+), Gd(3+), niflumic acid, and 2-aminoethoxydiphenyl borate, agents that inhibit the transient receptor potential channels. The [Ca(2+)]i spikes were inhibited by nimodipine and ryanodine, indicating that they were due to Ca(2+) influx through the voltage-gated Ca(2+) channels and Ca(2+)-induced Ca(2+) release (CICR). Activation of RY receptors depolarized membrane potential as measured by patch clamp. Thus, activation of RY receptors leads to coherent changes in Ca(2+) signaling, which includes activation of TRP-like channels, membrane depolarization, activation of the voltage-gated Ca(2+) channels and CICR.

Atypical Ca2+-induced Ca2+ release from a sarco-endoplasmic reticulum Ca2+-ATPase 3-dependent Ca2+ pool in mouse pancreatic beta-cells

The contribution of Ca(2+) release from intracellular stores to the rise in the free cytosolic Ca(2+) concentration ([Ca(2+)](c)) triggered by Ca(2+) influx was investigated in mouse pancreatic beta-cells. Depolarization of beta-cells by 45 mm K(+) (in the presence of 15 mm glucose and 0.1 mm diazoxide) evoked two types of [Ca(2+)](c) responses: a monotonic and sustained elevation; or a sustained elevation superimposed by a transient [Ca(2+)](c) peak (TCP) (40-120 s after the onset of depolarization). Simultaneous measurements of [Ca(2+)](c) and voltage-dependent Ca(2+) current established that the TCP did not result from a larger Ca(2+) current. Abolition of the TCP by thapsigargin and its absence in sarco-endoplasmic reticulum Ca(2+)-ATPase 3 (SERCA3) knockout mice show that it is caused by Ca(2+) mobilization from the endoplasmic reticulum. A TCP could not be evoked by the sole depolarization of beta-cells but required a rise in [Ca(2+)](c) pointing to a Ca(2+)-induced Ca(2+) release (CICR). This CICR did not involve inositol 1,4,5-trisphosphate (IP(3)) receptors (IP(3)Rs) because it was resistant to heparin. Nor did it involve ryanodine receptors (RyRs) because it persisted after blockade of RyRs with ryanodine, and was not mimicked by caffeine, a RyR agonist. Moreover, RyR1 and RyR2 mRNA were not found and RyR3 mRNA was only slightly expressed in purified beta-cells. A CICR could also be detected in a limited number of cells in response to glucose. Our data demonstrate, for the first time in living cells, the existence of an atypical CICR that is independent from the IP(3)R and the RyR. This CICR is prominent in response to a supraphysiological stimulation with high K(+), but plays little role in response to glucose in non-obese mouse pancreatic beta-cells.

Ryanodine receptors in human pancreatic beta cells: localization and effects on insulin secretion

It is clear that pancreatic beta-cell dysfunction, including basal hyperinsulinemia and reduced insulin release in response to glucose, is a key determinant of disease progression in type 2 diabetes, but the underlying molecular defects are not known. In diabetes, the expression and function of ryanodine receptor (RyR) Ca2+ release channels are reduced. The present studies were undertaken to define the subcellular location and role of RyR in the control of stimulated and basal insulin release from human pancreatic beta cells. Using confocal microscopy, we observed RyR immunoreactivity in a vesicular pattern. RyRs did not colocalize with insulin secretory granules but partially colocalized with endosomes. Direct activation with nanomolar concentrations of ryanodine evoked increases in cytosolic Ca2+ that were coupled to transient insulin release. Insulin release stimulated by 1 nM ryanodine was sensitive to BAPTA-AM preincubation but independent of thapsigargin-sensitive endoplasmic reticulum (ER) Ca2+ pools. Blocking RyRs with micromolar concentrations of ryanodine led to BAPTA-resistant insulin release that was not associated with an increase in cytosolic Ca2+, which implicated alterations in luminal Ca2+. However, neither Ca2+ signals nor insulin release stimulated by glucose was blocked by 10-50 microM ryanodine, which suggests that the CD38/cyclic ADP-ribose/RyR pathway is not a primary mechanism of glucose action in nontransformed beta cells. We provide the first evidence that RyRs directly control insulin secretion in primary beta cells. Unexpectedly, stimulation of insulin secretion by ryanodine occurs independently of glucose and by two mechanisms, including a novel cytosolic Ca2+-independent mechanism likely involving changes in Ca2+ within the lumens of non-ER organelles, such as endosomes.

Ryanodine receptors of pancreatic beta-cells mediate a distinct context-dependent signal for insulin secretion

The ryanodine (RY) receptors in beta-cells amplify signals by Ca2+-induced Ca2+ release (CICR). The role of CICR in insulin secretion remains unclear in spite of the fact that caffeine is known to stimulate secretion. This effect of caffeine is attributed solely to the inhibition of cAMP-phosphodiesterases (cAMP-PDEs). We demonstrate that stimulation of insulin secretion by caffeine is due to a sensitization of the RY receptors. The dose-response relationship of caffeine-induced inhibition of cAMP-PDEs was not correlated with the stimulation of insulin secretion. Sensitization of the RY receptors stimulated insulin secretion in a context-dependent manner, that is, only in the presence of a high concentration of glucose. This effect of caffeine depended on an increase in [Ca2+]i. Confocal images of beta-cells demonstrated an increase in [Ca2+]i induced by caffeine but not by forskolin. 9-Methyl-7-bromoeudistomin D (MBED), which sensitizes RY receptors, did not inhibit cAMP-PDEs, but it stimulated secretion in a glucose-dependent manner. The stimulation of secretion by caffeine and MBED involved both the first and the second phases of secretion. We conclude that the RY receptors of beta-cells mediate a distinct glucose-dependent signal for insulin secretion and may be a target for developing drugs that will stimulate insulin secretion only in a glucose-dependent manner.

Amiodarone induces a caffeine-inhibited, MID1-depedent rise in free cytoplasmic calcium in Saccharomyces cerevisiae

Calcium signalling is involved in myriad cellular processes such as mating morphogenesis. Mating in yeast induces changes in cell morphology with a concomitant increase in calcium uptake that is dependent on the MID1 and CCH1 genes. Mid1p and Cch1p are believed to function in a capacitive calcium entry (CCE)-like process. Amiodarone alters mammalian calcium channel activity but, despite its clinical importance, its molecular mechanisms are not clearly defined. We have shown previously that amiodarone has fungicidal activity against a broad array of fungi. We show here that amiodarone causes a dramatic increase in cytoplasmic calcium ([Ca2+]cyt) in Saccharomyces cerevisiae. The majority of this increase is dependent on extracellular Ca2+ nonetheless, a significant increase in [Ca2+]cyt is still induced by amiodarone when no uptake of extracellular Ca2+ can occur. The influx of extracellular Ca2+ may be a direct effect of amiodarone on a membrane transporter or may be by a CCE mechanism. Uptake of the extracellular Ca2+ is inhibited by caffeine and reduced in strains deleted for the mid1 gene, but not in cells deleted for cch1. Our data are the first demonstrating control of yeast calcium channels by amiodarone and caffeine.

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.

Ca2+ handling of rat pancreatic beta-cells exposed to ryanodine, caffeine, and glucagon

Reported species differences in the stimulus-secretion coupling of insulin release made it important to compare the Ca2+ handling of rat beta-cells with that previously observed in mice. Single beta-cells and small aggregates were prepared from pancreatic islets of Wistar rats, attached to cover slips and then used for measuring the cytoplasmic Ca2+ concentration ([Ca2+]i) with the ratiometric fura-2 technique. Glucose (11 mM) induced slow oscillations of [Ca2+]i similar to those seen in other species, including humans. Comparison of the oscillations in rat beta-cells with those previously described in mouse revealed that there was a slightly lower frequency and an increased tendency to transformation into sustained [Ca2+]i in response to glucagon or caffeine. Ryanodine (5-20 microM) did not affect existing oscillations but sometimes restored rhythmic activity in the presence of caffeine. Stimulation with glucose resulted not only in oscillations but also in transients of [Ca2+]i sometimes appearing in synchrony in adjacent beta-cells and disappearing after the addition of 200 nM thapsigargin or 20 mM caffeine. The frequency of transients recorded in a medium containing glucagon and methoxyverapamil was higher than seen under similar conditions in mouse beta-cells. Although exhibiting some differences compared with mouse beta-cells, rat beta-cells also have an intrinsic ability to oscillate and to generate the transients of [Ca2+] that are supposed to synchronize the rhythmicity of the islets in the pancreas.

Dynamic imaging of endoplasmic reticulum Ca2+ concentration in insulin-secreting MIN6 Cells using recombinant targeted cameleons: roles of sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA)-2 and ryanodine receptors

The endoplasmic reticulum (ER) plays a pivotal role in the regulation of cytosolic Ca(2+) concentrations ([Ca(2+)](cyt)) and hence in insulin secretion from pancreatic beta-cells. However, the molecular mechanisms involved in both the uptake and release of Ca(2+) from the ER are only partially defined in these cells, and the presence and regulation of ER ryanodine receptors are a matter of particular controversy. To monitor Ca(2+) fluxes across the ER membrane in single live MIN6 beta-cells, we have imaged changes in the ER intralumenal free Ca(2+) concentration ([Ca(2+)](ER)) using ER-targeted cameleons. Resting [Ca(2+)](ER) (approximately 250 micromol/l) was markedly reduced after suppression (by approximately 40%) of the sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA)-2b but not the SERCA3 isoform by microinjection of antisense oligonucleotides, implicating SERCA2b as the principle ER Ca(2+)-ATPase in this cell type. Nutrient secretagogues that elevated [Ca(2+)](cyt) also increased [Ca(2+)](ER), an effect most marked at the cell periphery, whereas inositol 1,4,5-trisphosphate-generating agents caused a marked and homogenous lowering of [Ca(2+)](ER). Demonstrating the likely presence of ryanodine receptors (RyRs), caffeine and 4-chloro-3-ethylphenol both caused an almost complete emptying of ER Ca(2+) and marked increases in [Ca(2+)](cyt). Furthermore, photolysis of caged cyclic ADP ribose increased [Ca(2+)](cyt), and this effect was largely abolished by emptying ER/Golgi stores with thapsigargin. Expression of RyR protein in living MIN6, INS-1, and primary mouse beta-cells was also confirmed by the specific binding of cell-permeate BODIPY TR-X ryanodine. RyR channels are likely to play an important part in the regulation of intracellular free Ca(2+) changes in the beta-cell and thus in the regulation of insulin secretion.

The effect of calcium pump inhibitors on the response of intracellular calcium to caffeine in snail neurones

We have measured intracellular free calcium ([Ca(2+)]i) using Fura-2 or Ca(2+)-sensitive microelectrodes in voltage-clamped neurones of the snail, Helix aspersa. Caffeine-induced transient increases in [Ca(2+)]i were normally followed by a brief fall of [Ca(2+)]i below its pre-caffeine level. We investigated the cause of this undershoot by raising [Ca(2+)]i; and by inhibiting the plasma membrane or endoplasmic reticulum Ca ATPases (PMCA or SERCA respectively). When the cell membrane potential was decreased from -60 to -25mV, steady-state [Ca(2+)]i increased. The caffeine-induced transients were smaller while the undershoots were larger than in control conditions. When the PMCA was inhibited by high pH the steady-state [Ca(2+)]i increased by 100-400nM. The caffeine-induced [Ca(2+)]i increase and the subsequent undershoot both became larger. Injection of orthovanadate, which inhibits the PMCA and increases [Ca(2+)]i, did not block either effect of caffeine. But when the SERCA was inhibited by cyclopiazonic acid the undershoot disappeared. The phosphodiesterase inhibitor IBMX did not influence the undershoot. These results suggest that the undershoot is generated by the Ca(2+)] ATPase of the stores rather than that of the plasma membrane. Since the undershoot increased as [Ca(2+)]i increased, we conclude that at higher levels of [Ca(2+)]i the stores refill more rapidly.

Pancreatic beta-cells from obese-hyperglycemic mice are characterized by excessive firing of cytoplasmic Ca2+ transients

Pancreatic beta-cells from obese-hyperglycemic (ob/ob) mice are widely used for studying the mechanisms of insulin release, including its regulation by the cytoplasmic Ca2+ concentration ([Ca2+]i). In this study, we compared changes of [Ca2+]i in single beta-cells isolated from ob/ob mice with those from lean mice using dual-wavelength microfluorometry and the indicator fura-2. There were no differences in the frequency, amplitude, and half-width of the slow oscillations induced by glucose. Most beta-cells from the obese mice responded to 10 mM caffeine with transformation of the oscillations into sustained elevation of [Ca2+]i, a process counteracted by ryanodine. The beta-cells from the obese mice were characterized by ample generation of [Ca2+]i transients, which increased in number in the presence of glucagon. The transients became less frequent when leptin was added at a concentration as low as 1 nM. It is suggested that the excessive firing of [Ca2+]i transients in the ob/ob mice is owing to the absence of leptin and is mediated by activation of the phospholipase C signaling pathway.

Calcium homeostasis in a clonal pituitary cell line of mouse corticotropes

Calcium homeostasis was studied following a depolarization-induced transient increase in [Ca2+]i in single cells of the clonal pituitary cell line of corticotropes, AtT-20 cells. The KCl-induced increase in [Ca2+]i was blocked in (i) extracellular calcium-deficient solutions, (ii) external cobalt (2.0 mM), (iii) cadmium (200 µM), and (iv) nifedipine (2.0 µM). The mean increase in [Ca2+]i in single cells in the presence of an uncoupler of mitochondrial function [carbonyl cyanide p-(trifluoromethoxy)phenylhydrazone, FCCP, 1 µM] was 54 ± 13 nM (n = 9). The increase in [Ca2+]i produced by FCCP was greater either during or following a KCl-induced [Ca2+]i load. However, FCCP did not significantly alter the clearance of calcium during a KCl-induced rise in [Ca2+]i. Fifty percent of the cells responded to caffeine (10 mM) with an increase in [Ca2+]i (191 ± 24 nM; n = 21) above resting levels; this effect was blocked by ryanodine (10 µM). Thapsigargin (2 µM) and 2,5 di(-t-butyl)-1,4 hydroquinone (BuBHQ, 10 µM) produced increases in [Ca2+]i (47 ± 11 nM, n = 6 and 22 ± 4 nM, n = 8, respectively) that increased cell excitability. These results support a role for mitochondria and sarco-endoplasmic reticulum calcium stores in cytosolic [Ca2+]i regulation; however, none of these organelles are primarily responsible for the return of [Ca2+]i to resting levels following this KCl-induced [Ca2+]i load.Key words: calcium homeostasis, intracellular calcium stores, anterior pituitary cells, mitochondria.

Ca2+-induced Ca2+ release from the endoplasmic reticulum amplifies the Ca2+ signal mediated by activation of voltage-gated L-type Ca2+ channels in pancreatic beta-cells

Stimulus-secretion coupling in pancreatic beta-cells involves membrane depolarization and Ca(2+) entry through voltage-gated L-type Ca(2+) channels, which is one determinant of increases in the cytoplasmic free Ca(2+) concentration ([Ca(2+)](i)). We investigated how the endoplasmic reticulum (ER)-associated Ca(2+) apparatus further modifies this Ca(2+) signal. When fura-2-loaded mouse beta-cells were depolarized by KCl in the presence of 3 mm glucose, [Ca(2+)](i) increased to a peak in two phases. The second phase of the [Ca(2+)](i) increase was abolished when ER Ca(2+) stores were depleted by thapsigargin. The steady-state [Ca(2+)](i) measured at 300 s of depolarization was higher in control cells compared with cells in which the ER Ca(2+) pools were depleted. The amount of Ca(2+) presented to the cytoplasm during depolarization as estimated from the integral of the increment in [Ca(2+)](i) over time (integralDelta[Ca(2+)](i).dt) was approximately 30% higher compared with that in the Ca(2+) pool-depleted cells. neo-thapsigargin, an inactive analog, did not affect [Ca(2+)](i) response. Using Sr(2+) in the extracellular medium and exploiting the differences in the fluorescence properties of Ca(2+)- and Sr(2+)-bound fluo-3, we found that the incoming Sr(2+) triggered Ca(2+) release from the ER. Depolarization-induced [Ca(2+)](i) response was not altered by, an inhibitor of phosphatidylinositol-specific phospholipase C, suggesting that stimulation of the enzyme by Ca(2+) is not essential for amplification of Ca(2+) signaling. [Ca(2+)](i) response was enhanced when cells were depolarized in the presence of 3 mm glucose, forskolin, and caffeine, suggesting involvement of ryanodine receptors in the amplification process. Pretreatment with ryanodine (100 microm) diminished the second phase of the depolarization-induced increase in [Ca(2+)](i). We conclude that Ca(2+) entry through L-type voltage-gated Ca(2+) channels triggers Ca(2+) release from the ER and that such a process amplifies depolarization-induced Ca(2+) signaling in beta-cells.

Multiple effects of caffeine on Ca2+ release and influx in human B lymphocytes

Caffeine has been used as a pharmacological tool to study the ryanodine receptor (RYR)-mediated Ca2+ release from caffeine-sensitive, inositol 1,4,5,-trisphosphate (IP3)-insensitive pools. In the present study, we demonstrate multiple effects of caffeine on Ca2+ homeostasis in human B lymphocytes. Although B cells express a functional RYR, which can be activated by 4-chloro-m-cresol following depletion of IP(3)-sensitive pools, caffeine does not activate RYR-mediated Ca2+ release. Instead, caffeine dose-dependently inhibited IP3 receptor (IP3R)-mediated Ca2+ release, RYR-mediated Ca2+ release and B cell receptor-initiated Ca2+ influx, while high concentrations of caffeine (> or = 25 mM) induced a Ca2+ influx. In contrast with its ability to suppress receptor-stimulated Ca2+ influx, caffeine had no significant effect on the store-operated Ca2+ (SOC) channel-dependent Ca2+ influx induced by thapsigargin. Thus, caffeine may act as an inhibitor on a single or multiple site(s) responsible for regulating the IP3R channel, RYR channel and presumably the receptor-mediated SOC channel. The present report may be the first demonstration of multiple effects of caffeine on Ca2+ mobilization in single cell type. Our results suggest the need for caution regarding use of caffeine simply as a RYR-activator to study Ca2+ homeostasis in eucaryotic cells.

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.

Activation and desensitization of the caffeine-sensitive cation channels and calcium stores have no persistent effect on the electrophysiological properties of leech P neurones

In leech P neurones caffeine activates unselective ion channels in the plasma membrane and induces intracellular Ca2+ release (Schoppe, J., Hochstrate, P., Schlue, W.-R., 1997. Caffeine mediates cation influx and intracellular Ca2+ release in leech P neurones. Cell Calcium 22, 385-397). These effects are prominent only upon the first caffeine exposure, while subsequent applications are largely ineffective; i.e. both plasma membrane channels and intracellular Ca2+ release mechanism desensitize irreversibly. In order to examine whether this desensitization is paralleled by irreversible changes in the electrophysiological parameters of the cells, we investigated the action of caffeine on changes in membrane potential and the cytosolic free Ca2+ concentration, which were induced by varying the ionic composition of the extracellular fluid or by application of 5-hydroxytryptamine. Neither the resting values nor any of the experimentally induced shifts in membrane potential or cytosolic Ca2+ concentration were affected by caffeine, which suggests strongly that activation and/or desensitization of the caffeine-sensitive ion channels and Ca2+ stores have no long-lasting effect on the relevant electrochemical gradients, membrane conductances, or transport mechanisms.

The effects of caffeine on ATP-sensitive K(+) channels in smooth muscle cells from pig urethra

The effects of caffeine on both levcromakalim-induced macroscopic and unitary currents in pig proximal urethra were investigated by the use of patch-clamp techniques (conventional whole-cell configuration and cell-attached configuration). The effects of caffeine were also examined on currents in inside-out patches of COS7 cells expressing carboxy terminus truncated inwardly rectifying K(+) channel (Kir6.2) subunits (i.e. Kir6.2DeltaC36) which form ATP-sensitive K(+) channels (K(ATP) channels). In conventional whole-cell configuration, the levcromakalim (100 microM)-induced inward current (symmetrical 140 mM K(+) conditions) was inhibited by caffeine (> or =1 mM) at a holding potential of -50 mV. In contrast, ryanodine (10 microM) caused no significant inhibitory effect on the gradual decay of the levcromakalim-induced current at -50 mV. The amplitude of the 30 microM levcromakalim-induced current was enhanced by 3-isobutyl-1-methylxanthine (IBMX, 100 microM). In cell-attached configuration, the levcromakalim-induced K(+) channel openings were inhibited by subsequent application of 10 mM caffeine, decreasing the channel open probability at -50 mV. Reverse transcriptase-polymerase chain reaction (RT - PCR) analysis revealed the presence of Kir6.2 transcript in pig urethra. Caffeine (> or =3 mM) inhibited the channel activity of Kir6.2DeltaC36 expressed in COS7 cells (3 mM caffeine, 65+/-6%, n=4; 10 mM caffeine, 29+/-2%, n=4). These results suggest that caffeine can inhibit the activity of K(ATP) channels through a direct blocking effect on the pore-forming Kir subunit.

Cholera toxin and extracellular Ca2+ induce adherence of non-piliated Neisseria: evidence for an important role of G-proteins and Rho in the bacteria-cell interaction

In this study, we characterize the interaction between non-piliated (P-) Neisseria gonorrhoeae and human epithelial cells. P- mutants lacking the pilus subunit protein PilE attach at low levels to cells. Although the binding may not lead to heavy inflammatory responses, the interaction between P- Neisseria and host cells most probably play a role in colonization and asymptomatic carriage of the pathogen. Here we show that the adherence of P N. gonorrhoeae is blocked by GDP-beta-S [guanosine 5'-O(thio)diphosphate], a non-hydrolyzable GTP analogue, and by C3 exotoxin, an inhibitor of the small G-protein Rho. G-protein activators such as cholera toxin, that activates Gs, and fluoroaluminate, a general G-protein activator, induced bacterial adherence. Furthermore, increase of the extracellular free [Ca2+] dramatically enhanced adherence of non-piliated Neisseria. The pharynx and the urogenital tract are natural entry sites of the pathogenic Neisseria species, and at both sites the epithelial cells can be exposed to wide variations in Ca2+ concentration. Taken together, these data show the importance of extracellular Ca2+ in the pathogenic Neisseria-host interaction, and reveal a novel function of cholera toxin, namely induction of bacterial adherence.

Evidence of functional ryanodine receptor involved in apoptosis of prostate cancer (LNCaP) cells

BACKGROUND

Very little is known about the functional expression and the physiological role of ryanodine receptors in nonexcitable cells, and in prostate cancer cells in particular. Nonetheless, different studies have demonstrated that calcium is a major factor involved in apoptosis. Therefore, the calcium-regulatory mechanisms, such as ryanodine-mediated calcium release, may play a substantial role in the regulation of apoptosis.

METHODS

We assessed the presence of such functional receptors in LNCaP prostate cancer cells, using fluorimetric measurements of intracellular calcium and expression assays of mRNA encoding ryanodine receptors.

RESULTS

We show here that LNCaP cells responded to caffeine, a ryanodine receptor agonist, by mobilizing calcium. Another ryanodine receptor agonist, 4-chloro-m-cresol, had a similar effect and promoted calcium release. These effects were inhibited by pretreatment with ryanodine or thapsigargin. In addition to a calcium release, caffeine was able to produce a calcium entry blocked by nickel. We used a reverse transcription-polymerase chain reaction assay to investigate the expression of ryanodine receptors in LNCaP cells. Two types of ryanodine receptor mRNAs were expressed in LNCaP cells: RyR1 and RyR2 mRNAs. Finally, we show that ryanodine receptor activation by caffeine slightly stimulates apoptosis of prostate cancer cells, and that the inhibition of these receptors by ryanodine protects the cells against apoptosis.

CONCLUSIONS

The combination of results showed that LNCaP cells, derived from a human prostate cancer, express functional RyRs able to mobilize Ca(2+) from intracellular stores and which might control apoptosis.

Caffeine interaction with fluorescent calcium indicator dyes

We report that caffeine, in millimolar concentrations, interacts strongly with four common calcium indicator dyes: mag-fura-2, magnesium green, fura-2, and fluo-3. Fluorescence intensities are either noticeably enhanced (mag-fura-2, fura-2) or diminished (magnesium green, fluo-3). The caffeine-induced changes in the fluorescence spectra are clearly distinct from those of metal ion binding at the indicator chelation sites. Binding affinities for calcium of either mag-fura-2 or magnesium green increased only slightly in the presence of caffeine. Caffeine also alters the fluorescence intensities of two other fluorescent dyes lacking a chelation site, fluorescein and sulforhodamine 101, implicating the fluorophore itself as the interaction site for caffeine. In the absence of caffeine, variation of solution hydrophobicity by means of water/dioxane mixtures yielded results similar to those for caffeine. These observations suggest that hydrophobic substances, in general, can alter dye fluorescence in a dye-specific manner. For the particular case of caffeine, and perhaps other commonly used pharmacological agents, the dye interactions can seriously distort fluorescence measurements of intracellular ion concentrations with metal indicator dyes.

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+.

Caffeine releases a glucose-primed endoplasmic reticulum Ca2+ pool in the insulin secreting cell line INS-1

Caffeine mobilized an intracellular Ca2+ pool in intact fura-2-loaded INS-1 cells in suspension exposed to high (16 mM) [glucose], while a minor effect was observed with low (2 mM) [glucose]. Cells were kept in a medium containing diaxozide or no Ca2+ to prevent the influx of extracellular Ca2+. The caffeine-sensitive intracellular Ca2+ pool was within the endoplasmic reticulum since it was depleted by the inhibitor of the reticular Ca2+ pumps thapsigargin and the InsP3-dependent agonist carbachol. No effect of caffeine was observed in the parent glucose-insensitive RINmF5 cells. In microsomes from INS-1 but not RINmF5 cells, the type 2 ryanodine receptor was present as revealed by Western blotting. It was concluded that the endoplasmic reticulum of INS-1 cells possesses caffeine-sensitive type 2 ryanodine receptors Ca2+ channels.

Stimulatory transducing systems in pancreatic islet cells

We have determined the cellular distribution of different alpha subtypes of G proteins and adenylyl cyclase (AC) isoforms in endocrine, exocrine, and established pancreatic cell lines. VIP, PACAP, and tGLP-1 receptor proteins are expressed to varying extents in A and B cells, whereas the expression of G alpha subunits is cell specific. Thus, G(olf) alpha is detected in normal rodent B cells and immortalized pancreatic B cell lines, whereas Gs alpha is more ubiquitously expressed. The cellular density of AC isoforms labeling (I, II, III, IV, V/VI) is also islet cell-specific and their distribution is age- and species-dependent. The identification of numerous signaling molecule subtypes, together with the discovery of their specific subcellular distribution, will help the functional characterization of their intraregulatory pathways, leading to the extrusion of insulin or glucagon secretory granules, and those leading to differentiation and apoptosis of islet cells.

Cell signaling by the type IV pili of pathogenic Neisseria

Neisseria gonorrhoeae and Neisseria meningitidis are Gram-negative bacterial pathogens that infect human mucosal epithelia. Type IV pilus-mediated adherence of these bacteria is a crucial early event for establishment of infection. In this work, we show that the type IV pili transduce a signal into the eucaryotic host cell. Purified adherent pili, but not pili from a low binding mutant, trigger an increase in the cytosolic free calcium ([Ca2+]i) in target epithelial cells, a signal known to control many cellular responses. The [Ca2+]i increase was blocked by antibodies against CD46, a putative pilus receptor, suggesting a role for this protein in signal transduction. Pilus-mediated attachment was inhibited by depletion of host cell intracellular Ca2+ stores but not by removal of extracellular Ca2+. Further, kinase inhibition studies showed that pilus-mediated adherence is dependent on casein kinase II. In summary, these data reveal a novel function of the type IV pili, namely induction of signal transduction pathways in host cells.

Effects of ryanodine receptor agonist 4-chloro-m-cresol on myoplasmic free Ca2+ concentration and force of contraction in mouse skeletal muscle

In single mouse skeletal muscle fibers injected with fluorescent Ca2+ indicator Indo-1, 4-chloro-m-cresol (chlorocresol, 4-CmC) and its lipophilic analogue 4-chloro-3-ethylphenol (4-CEP) increased resting myoplasmic free [Ca2+] ([Ca2+]i) in a dose-dependent manner. In this regard, 4-CEP was more potent than 4-CmC and both were more potent than caffeine. High concentrations of 4-CmC (1 mM) or 4-CEP (500 microM) caused large and irreversible increase in resting [Ca2+]i leading to contracture. 4-CmC potentiated the [Ca2+]i increase and force of contraction induced by tetanic stimulation. Unlike caffeine, 4-CmC did not affect the activity of sarcoplasmic reticulum Ca2+ pump or the myofibrillar Ca2+ sensitivity. A low concentration of 4-CEP (20 microM) had no effect on resting [Ca2+]i on its own, but it enhanced the resting [Ca2+]i increase induced by caffeine and also potentiated the [Ca2+]i increase and contraction induced by tetanic stimulation. However, a relatively high concentration of 4-CEP (200 microM) inhibited tetanic stimulation-induced [Ca2+]i increase and contraction. Dantrolene, a muscle relaxant, inhibited 4-CmC-induced [Ca2+]i increase under resting conditions. However, when 4-CEP was applied in the presence of dantrolene, there was an exaggerated increase in [Ca2+]i. We conclude that 4-CmC and 4-CEP are potent agonists that can increase [Ca2+]i rapidly and reversibly by activating ryanodine receptors in situ in intact skeletal muscle fibers. These compounds, specially 4-CmC, may be useful for mechanistic and functional studies of ryanodine receptors and excitation-contraction coupling in skeletal muscles.

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.

Blockade of cerebral blood flow response to insulin-induced hypoglycemia by caffeine and glibenclamide in conscious rats

The possibility that adenosine and ATP-sensitive potassium channels (KATP) might be involved in the mechanisms of the increases in cerebral blood flow (CBF) that occur in insulin-induced hypoglycemia was examined. Cerebral blood flow was measured by the [14C]iodoantipyrine method in conscious rats during insulin-induced, moderate hypoglycemia (2 to 3 mmol/L glucose in arterial plasma) after intravenous injections of 10 to 20 mg/kg of caffeine, an adenosine receptor antagonist, or intracisternal infusion of 1 to 2 mumol/L glibenclamide, a KATP channel inhibitor. Cerebral blood flow was also measured in corresponding normoglycemic and drug-free control groups. Cerebral blood flow was 51% higher in untreated hypoglycemic than in untreated normoglycemic rats (P < 0.01). Caffeine had a small, statistically insignificant effect on CBF in normoglycemic rats, but reduced the CBF response to hypoglycemia in a dose-dependent manner, i.e., 27% increase with 10 mg/kg and complete elimination with 20 mg/kg. Chemical determinations by HPLC in extracts of freeze-blown brains showed significant increases in the levels of adenosine and its degradation products, inosine and hypoxanthine, during hypoglycemia (P < 0.05). Intracisternal glibenclamide had little effect on CBF in normoglycemia, but, like caffeine, produced dose-dependent reductions in the magnitude of the increases in CBF during hypoglycemia, i.e., +66% with glibenclamide-free artificial CSF administration, +25% with 1 mumol/L glibenclamide, and almost complete blockade (+5%) with 2 mumol/L glibenclamide. These results suggest that adenosine and KATP channels may play a role in the increases in CBF during hypoglycemia.

Effects of caffeine on the influx of extracellular calcium in GH4C1 pituitary cells

Caffeine increases intracellular Ca2+ concentrations ([Ca2+]i) in a variety of cell types by triggering the mobilization of Ca2+ from intracellular Ca2+ stores. Caffeine also can change [Ca2+]i by affecting Ca2+ influx through voltage-operated Ca2+ channels (VOCCs). In the present study, we investigated the effects of caffeine on Ca2+ entry in GH4C1 pituitary cells. Pretreatment of the cells with caffeine attenuated the high K+-evoked influx of 45Ca2+ in a dose-dependent manner. This inhibition was not secondary to the caffeine-evoked elevation of [Ca2+]i because caffeine was able to inhibit VOCCs also in the presence of the intracellular Ca2+ chelator BAPTA. However, the inhibitory effect of caffeine on 45Ca2+ entry appeared to be dependent on the degree of depolarization of the plasma membrane. Only in cells depolarized with relatively high concentrations of K+ (20, 35, and 50 mM) was the caffeine-induced inhibition observed. A similar inhibitory effect of caffeine on the high K+-evoked calcium and barium entry was observed in experiments using Fura 2. Neither IBMX, forskolin nor dibutyryl cAMP reduced the enhanced [Ca2+]i induced by 50 mM K+, suggesting that the effect of caffeine was not due to increased intracellular cAMP. Furthermore, high doses of caffeine inhibited the plateau level of the TRH-induced increase in [Ca2+]i, which is caused partly by influx of Ca2+ through VOCCs. The inhibitory effect of caffeine was, in part, due to an hyperpolarization of the plasma membrane observed at high doses of caffeine. On the other hand, low doses of caffeine enhanced depolarization-evoked Ba2+ entry as well as the TRH-evoked plateau level of [Ca2+]i. We conclude that caffeine has a dual effect on Ca2+ entry through activated VOCCs in GH4C1 cells: at low concentrations caffeine enhances Ca2+ entry, whereas high concentrations of caffeine block Ca2+ entry.

Thiol oxidation by 2,2'-dithiodipyridine causes a reversible increase in cytoplasmic free Ca2+ concentration in pancreatic beta-cells. Role for inositol 1,4,5-trisphosphate-sensitive Ca2+ stores

2,2'-Dithiodipyridine (2,2'-DTDP), a reactive disulphide that mobilizes Ca2+ from ryanodine-sensitive Ca2+ stores in muscle, induced a biphasic increase in cytoplasmic free Ca2+ concentration ([Ca2+]i) in pancreatic beta-cells loaded with fura 2. This increase consisted of an early transient followed by a second, slower, rise. The [Ca2+]i transient was dependent on extracellular Ca2+ and disappeared on treatment with nimodipine. The reactive disulphide caused plasma membrane depolarization, as studied by the perforated-patch configuration of the patch-clamp technique. Hence membrane depolarization and opening of the L-type voltage-gated Ca2+ channels were responsible for the first transient in [Ca2+]i. The second slower increase in [Ca2+]i was prolonged but readily reversed by the disulphide-reducing agent 1,4-dithiothreitol. This increase in [Ca2+]i was not decreased by nimodipine or by omission of extracellular Ca2+, but was eliminated when the Ins(1,4,5)P3-sensitive Ca2+ pool was first depleted by carbachol. Ryanodine or its beta-alanyl analogue did not release Ca2+ from intracellular stores, and a high concentration of ryanodine did not inhibit Ca2+ release by 2,2'-DTDP. The disulphide compound suppressed glucose metabolism and decreased the mitochondrial inner-membrane potential. We conclude that thiol oxidation by 2,2'-DTDP affects Ca2+ homeostasis in beta-cells by multiple mechanisms. However, unlike the situation in muscle, in beta-cells 2,2'-DTDP releases Ca2+ from intracellular pools by mechanisms that do not involve activation of ryanodine receptors. Instead, in these cells the Ins(1,4,5)P3-sensitive intracellular Ca2+ store comprises an alternative target for the Ca(2+)-mobilizing action of the reactive disulphide compound.

Modulation of membrane currents and mechanical activity by niflumic acid in rat vascular smooth muscle

The effects of niflumic acid on whole-cell membrane currents and mechanical activity were examined in the rat portal vein. In freshly dispersed portal vein cells clamped at -60 mV in caesium (Cs+)-containing solutions, niflumic acid (1-100 microM) inhibited calcium (Ca2+)-activated chloride currents (IC1(Ca)) induced by caffeine (10 mM) and by noradrenaline (10 microM). In a potassium (K+)-containing solution and at a holding potential of - 10 mV, niflumic acid (10-100 microM) induced an outward K+ current (IK(ATP)) which was sensitive to glibenclamide (10-30 microM). At concentrations < 30 microM and at a holding potential of -2 mV, niflumic acid had no effect on the magnitude of the caffeine- or noradrenaline-stimulated current (IBK(Ca)) carried by the large conductance, Ca(2+)-sensitive K+ channel (BKCa). However, at a concentration of 100 microM, niflumic acid significantly inhibited IBK(Ca)) evoked by caffeine (10 mM) but not by NS1619 (1-(2'-hydroxy-5'-trifluoromethylphenyl)-5-trifluoromethyl-2(3 H) benzimidazolone; 20 microM). In Cs(+)-containing solutions, niflumic acid (10-100 microM) did not inhibit voltage-sensitive Ca2+ currents. In intact portal veins, niflumic acid (1-300 microM) inhibited spontaneous mechanical activity, an action which was partially antagonised by glibenclamide (1-10 microM), and contractions produced by noradrenaline (10 microM), an effect which was glibenclamide-insensitive. It is concluded that inhibition of ICl(Ca) and stimulation of IK(ATP) both contribute to the mechano-inhibitory actions of niflumic acid in the rat portal vein.

Oscillations of cytosolic free calcium in bombesin-stimulated HIT-T15 cells

The mechanism underlying the generation of cytosolic free Ca2+ ([Ca2+]i) oscillations by bombesin, a receptor agonist activating phospholipase C, in insulin secreting HIT-T15 cells was investigated. At 25 microM, 61% of cells displayed [Ca2+]i oscillations with variable patterns. The bombesin-induced [Ca2+]i oscillations could last more than 1 h and glucose was required for maintaining these [Ca2+]i fluctuations. Bombesin-evoked [Ca2+]i oscillations were dependent on extracellular Ca2+ entry and were attenuated by membrane hyperpolarization or by L-type Ca2+ channel blockers. These [Ca2+]i oscillations were apparently not associated with fluctuations in plasma membrane Ca2+ permeability as monitored by the Mn2+ quenching technique. 2,5-di-(tert-butyl)-1,4-benzohydroquinone (tBuBHQ) and 4-chloro-m-cresol, which interfere with intracellular Ca2+ stores, respectively, by inhibiting Ca(2+)-ATPase of endoplasmic reticulum and by affecting Ca(2+)-induced Ca2+ release, disrupted bombesin-induced [Ca2+]i oscillations. 4-chloro-m-cresol raised [Ca2+]i by mobilizing an intracellular Ca2+ pool, an effect not altered by ryanodine. Caffeine exerted complex actions on [Ca2+]i. It raised [Ca2+]i by promoting Ca2+ entry while inhibiting bombesin-elicited [Ca2+]i oscillations. Our results suggest that in bombesin-elicited [Ca2+]i oscillations in HIT-T15 cells: (i) the oscillations originate primarily from intracellular Ca2+ stores; and (ii) the Ca2+ influx required for maintaining the oscillations is in part membrane potential-sensitive and not coordinated with [Ca2+]i oscillations. The interplay between intracellular Ca2+ stores and voltage-sensitive and voltage-insensitive extracellular Ca2+ entry determines the [Ca2+]i oscillations evoked by bombesin.

Effects of caffeine on intracellular calcium release and calcium influx in a clonal beta-cell line RINm5F

We studied the effects of caffeine on intracellular calcium in a clonal insulin-secreting cell line RINm5F. Caffeine (1-30 mM) induced a dose-dependent intracellular Ca2+ release and Ca2+ influx. Pretreatment with combination of ryanodine (10 microM) and caffeine (10 mM), but not ryanodine alone, abolished subsequent caffeine-induced release of intracellular Ca2+. Pretreatment with another ryanodine receptor blocker procaine (0.1, 0.3, and 1mM) antagonized the caffeine-induced release of intracellular Ca2+. Pretreatment with thapsigargin (2 microM), the endoplasmic reticular Ca2+-ATPase inhibitor, abolished both the caffeine- and arginine vasopressin (AVP)-induced Ca2+ release. AVP induces Ca2+ release by increasing the formation of IP3. Pretreatment with AVP greatly reduced caffeine-induced Ca2+ release whereas pretreatment with caffeine also reduced AVP-induced Ca2+ release. The L-type Ca2+ channel blocker nimodipine (1 microM) inhibited caffeine-induced Ca2+ influx. In addition, depletion of intracellular Ca2+ stores with thapsigargin did not affect caffeine-induced Ca2+ influx. These results suggested that: 1) the ryanodine- and caffeine-sensitive Ca2+ store exists in RINm5F cells, 2) most of these Ca2+ release channels in RINm5F cells are closed in the resting state, since pretreatment with ryanodine alone failed to block the caffeine-induced Ca2+ release, 3) there is a "cross-talk" between the caffeine- and IP3-sensitive pools in RINm5F cells, 4) caffeine increases Ca2+ entry in RINm5F cells through L-type voltage-dependent Ca2+ channels, and 5) the caffeine-induced Ca2+ influx is independent of the Ca2+ release from the intracellular Ca2+ stores.

Caffeine inhibits Ca2+ uptake by subplasmalemmal calcium stores ('alveolar sacs') isolated from Paramecium cells

Caffeine inhibits 45Ca2+ sequestration by subplasmalemmal calcium stores ('alveolar sacs') of low thapsigargicin sensitivity which we have isolated from the ciliated protozoan, Paramecium tetraurelia. Inhibition depends on caffeine concentration, with an IC50 of 31.8 mM. According to kinetic evaluation this is compatible with non-competitive inhibition of Ca2+ uptake, rather than with superimposed 45Ca2+ release during sequestration. It remains to be analysed whether this mechanism might be of possible relevance also for Ca2+-mediated activation in vivo in this or in any other secretory system. Such an effect could also operate indirectly, e.g., by Ca2+-release induction via sequestration inhibition. This is the first description of caffeine-mediated inhibition of Ca2+ uptake by calcium stores from a secretory system. Our data are compatible with some observations with sarcoplasmic reticulum from striated muscle fibers.

Caffeine-induced oscillations of cytosolic Ca2+ in GH3 pituitary cells are not due to Ca2+ release from intracellular stores but to enhanced Ca2+ influx through voltage-gated Ca2+ channels

Caffeine, a well known facilitator of Ca2+-induced Ca2+ release, induced oscillations of cytosolic free Ca2+ ([Ca2+]i) in GH3 pituitary cells. These oscillations were dependent on the presence of extracellular Ca2+ and blocked by dihydropyridines, suggesting that they are due to Ca2+ entry through L-type Ca2+ channels, rather than to Ca2+ release from the intracellular Ca2+ stores. Emptying the stores by treatment with ionomycin or thapsigargin did not prevent the caffeine-induced [Ca2+]i oscillations. Treatment with caffeine occluded phase 2 ([Ca2+]i oscillations) of the action of thyrotropin-releasing hormone (TRH) without modifying phase 1 (Ca2+ release from the intracellular stores). Caffeine also inhibited the [Ca2+]i increase induced by depolarization with high-K+ solutions (56% at 20 mM), suggesting direct inhibition of the Ca2+ entry through voltage-gated Ca2+ channels. We propose that the [Ca2+]i increase induced by caffeine in GH3 cells takes place by a mechanism similar to that of TRH, i.e. membrane depolarization that increases the firing frequency of action potentials. The increase of the electrical activity overcomes the direct inhibitory effect on voltage-gated Ca2+ channels with the result of increased Ca2+ entry and a rise in [Ca2+]i. Consideration of this action cautions interpretation of previous experiments in which caffeine was assumed to increase [Ca2+]i only by facilitating the release of Ca2+ from intracellular Ca2+ stores.

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

It is controversial whether the Ca2+ mobilizing agent, cADP-ribose (cADPR), is implicated in secretagogue-mediated intracellular Ca2+ responses of pancreatic beta-cells. In this study we utilised a potent antagonist of cADPR, 8-amino-cADPR, to determine whether cADPR is involved in glucose-, acetylcholine-, caffeine- and nitric oxide-induced intracellular Ca2+ responses of isolated rat beta-cells. The antagonist was found to be effective in the complete inhibition of cADPR-induced Ca2+ release from sea urchin egg microsome preparations, when used at equivalent concentrations to cADPR (between 0.1-10 microM) in the assay. Isolated beta-cells were co-loaded with up to 50 microM 8-amino-cADPR, and Fura-2 or Fluo-3, by the whole-cell patch technique. At this concentration, the antagonist failed to affect standard glucose- and acetylcholine-induced increases in the intracellular free Ca2+ ([Ca2+]i) of isolated rat pancreatic beta-cells, as assessed by video ratio imaging and single wavelength microfluorimetry. Applying the same methodology, the antagonist also failed to affect NO- and caffeine-induced intracellular Ca2+ responses of rat beta-cells. These results suggest that cADPR does not appear to play a fundamental role in beta-cell Ca2+ signalling. As a control, patch-loading with heparin (2 mg/ml) however, abolished the acetylcholine response but neither affected the NO- or caffeine-induced mobilization of intracellular Ca2+. These results support the involvement of the IP3-receptor in acetylcholine-induced mobilization of intracellular Ca2+, but not that invoked by caffeine.