Simultaneous measurements of changes in sarcoplasmic reticulum and cytosolic

Serum-withdrawal-dependent apoptosis of hippocampal neuroblasts involves Ca++ release by endoplasmic reticulum and caspase-12 activation

Apoptotic death caused by diseases or toxic insults is preceded and determined by endoplasmic reticulum dysfunction and altered intraluminar calcium homeostasis in many different cell types. With the present study we have explored the possibility that the ER stress could be involved also in apoptotic death induced by serum deprivation in neuronal cells. We have chosen as a model of study the cell line HN9.10e, constituted by immortalized hippocampal neuroblasts. The Ca(++) concentration in the lumen of the ER has been evaluated by using the low affinity Ca(++) probe Mag-fluo-4. We show that serum deprivation lowers the ER Ca(++) concentration with a time course closely related to the increase of apoptosis incidence. Serum deprivation also enhances the expression of a well-known marker of ER stress, the glucose-regulated protein-78 (GRP-78), a member of the heat shock/stress response protein family. Moreover, in serum-deprived neuroblasts, following GRP-78 up-regulation, the ER-associated procaspase-12 is cleaved with a time course which parallels the ER calcium loss while activation of caspase-3 is a later event. Depletion of ER Ca(++) by thapsigargin, a specific inhibitor of the ER-associated Ca(++) ATPase, also produces caspase-12 processing and apoptotic cell death, whereas agents capable of reducing the ER calcium loss protect the cells from serum-deprivation-induced apoptosis. These findings indicate that, in hippocampal neuroblasts, Ca(++) mobilization from ER and caspase-12 activation are components of the molecular pathway that leads to apoptosis triggered by serum deprivation and may constitute an amplifying loop of the mitochondrial pathway.

In Situ Calcium Signaling: No Calcium Sparks Detected in Rat Myometrium

Controlled uterine smooth muscle activity is essential for our reproductive health. While we understand reasonably well the steps that produce contraction following a rise in intracellular [Ca], the mechanism controlling excitability and thus the rise of Ca, is less well understood. Here we examine the role of the internal Ca sore, the sarcoplasmic reticulum (SR), and its relation to surface membrane ion channels. We show that despite having a well-developed SR, the rat uterus does not produce the elemental and local Ca signals, known as Ca sparks. This in turn has consequences for excitability, as the negative feedback loop between these Ca signals and Ca-activated K (BK) channels on the surface membrane is lost. This may be important for producing the powerful long-lasting contractions of the uterus required during labor.

Biphasic modulation of ryanodine receptors by sulfhydryl oxidation in rat ventricular myocytes

To understand better the modulation of ryanodine receptors (RyRs) during oxidative stress, the effect of 4,4'-dithiodipyridine (DTDP), a cell-permeant and thiol-reactive oxidant, on global Ca(2+) signal and spontaneous Ca(2+) sparks of rat ventricular myocytes was investigated. It was shown that a brief Ca(2+) transient was elicited by DTDP, when its concentration was raised to 100 microM DTDP. In addition a dose-dependent increase of cytoplasmic free Zn(2+) concentration was induced by DTDP. An increase of the frequency of spontaneous Ca(2+) sparks appeared at 3 microM DTDP, whereas higher concentration of DTDP caused a biphasic change of the frequency in both intact and permeabilized myocytes. Consistent with the biphasic effect, caffeine-induced Ca(2+) transients were similarly affected. Because DTDP did not reduce the free Ca(2+) concentration in the sarcoplasmic reticulum lumen, it is likely that the effects of DTDP on the frequency and caffeine-induced Ca(2+) transients are due mainly to sulfhydryl oxidation-induced activation and subsequent inactivation of RyRs. Unlike the frequency, the spatio-temporal properties of Ca(2+) sparks were not influenced by DTDP. The finding that DTDP does not affect the duration of Ca(2+) sparks is inconsistent with that the DTDP-induced increase of the open time of reconstituted RyR channels. The mechanism underlying this discrepancy, especially the possible role of the interaction between arrayed RyRs in myocytes, is discussed. This study suggests that, even if oxidative stress is mild enough not to cause intracellular Ca(2+) accumulation, it may affect signaling pathways through directly modulating the RyR or its complex and in turn changing the frequency of spontaneous Ca(2+) sparks. Thus, the functional importance of moderate oxidative stress should not be overlooked.

Multiple mechanisms involved in oxytocin-induced modulation of myometrial contractility

Oxytocin is a small peptide hormone with multiple sites of action in human body. It regulates a large number of reproduction-related processes in all species. Particularly important is its ability to stimulate uterine contractility. This is achieved by multiple mechanisms involving sarcoplasmic reticulum Ca2+ release and sensitization of the contractile apparatus to Ca2+. In this paper, we review the data published by us and other groups on oxytocin-induced modulation of uterine contractility. We conclude that sensitization of contractile apparatus to Ca2+ is the most relevant physiological effect of oxytocin on human myometrium.

Depletion of intracellular Ca2+ stores enhances flow-induced vascular dilatation in rat small mesenteric artery

The effect of depleting intracellular Ca2+ stores on flow-induced vascular dilatation and the mechanism responsible for the vasodilatation were examined in rat isolated small mesenteric arteries. The arteries were pressurized to 50 mmHg and preconstricted with phenylephrine. Intraluminal flow reversed the effect of phenylephrine, resulting in vasodilatation. Flow dilatation consisted of an initial transient peak followed by a sustained plateau phase. The magnitude of dilatation was markedly reduced by removing Ca2+ from the intraluminal flow medium. Depletion of intracellular Ca2+ stores with either cyclopiazonic acid (CPA, 2 microM) or 1,4-dihydroxy-2,5-di-tert-butylbenzene (BHQ, 10 microM) significantly augmented the magnitude of flow dilatation. Flow-induced endothelial cell Ca2+ influx was also markedly enhanced in arteries pretreated with CPA or BHQ.Flow-induced dilatation was insensitive to Nw-nitro-L-arginine methyl ester (100 microM) plus indomethacin (3 microM) or to oxyhemoglobin (3 microM), but was markedly reduced by 30 mM extracellular K+ or 2 mM tetrabutylammonium (TBA), suggesting an involvement of EDHF. Catalase at 1200 U ml-1 abolished the flow-induced dilatation, while the application of exogenous H2O2 (90-220 microM) induced relaxation in phenylephrine-preconstricted arteries. Relaxation to exogenous H2O2 was blocked in the presence of 30 mM extracellular K+, and H2O2 (90 microM) hyperpolarized the smooth muscle cells, indicating that H2O2 can act as an EDHF. In conclusion, flow-induced dilatation in rat mesenteric arteries can be markedly enhanced by prior depletion of intracellular Ca2+ stores. Furthermore, these data are consistent with a role for H2O2 as the vasodilator involved.

Complex effects of ryanodine on the sarcoplasmic reticulum Ca2+ levels in smooth muscle cells

We have studied the effects of ryanodine and inhibition of the sarco/endoplasmic reticulum Ca(2+) ATPase (SERCA) with thapsigargin, on both [Ca(2+)](i) and the sarcoplasmic reticulum (SR) Ca(2+) level during caffeine-induced Ca(2+) release in single smooth muscle cells. Incubation with 10 microM ryanodine did not inhibit the first caffeine-induced [Ca(2+)](i) response, although it abolished the [Ca(2+)](i) response to a second application of caffeine. To assess whether ryanodine was inducing a permanent depletion of the internal Ca(2+) stores, we measured the SR Ca(2+) level with Mag-Fura-2. The magnitude of the caffeine-induced reduction in the SR Ca(2+) level was not augmented by incubating cells with 1 microM ryanodine. Moreover, on removal of caffeine, the SR Ca(2+) levels partially recovered in 61% of the cells due to the activity of thapsigargin-sensitive SERCA pumps. Unexpectedly, 10 microM ryanodine instead of inducing complete depletion of SR Ca(2+) stores markedly reduced the caffeine-induced SR Ca(2+) response. It was necessary to previously inhibit SERCA pumps with thapsigargin for ryanodine to be able to induce caffeine-triggered permanent depletion of SR Ca(2+) stores. These data suggest that the effect of ryanodine on smooth muscle SR Ca(2+) stores was markedly affected by the activity of SERCA pumps. Our data highlight the importance of directly measuring SR Ca(2+) levels to determine the effect of ryanodine on the internal Ca(2+) stores.

Modulation of agonist-induced Ca2+ release by SR Ca2+ load: direct SR and cytosolic Ca2+ measurements in rat uterine myocytes

Release of Ca2+ from sarcoplasmic reticulum (SR) is one of the most important mechanisms of smooth muscle stimulation by a variety of physiologically active substances. Agonist-induced Ca2+ release is considered to be dependent on the Ca2+ content of the SR, although the mechanism underlying this dependence is unclear. In the present study, the effect of SR Ca2+ load on the amplitude of [Ca2+]i transients elicited by application of the purinergic agonist ATP was examined in uterine smooth muscle cells isolated from pregnant rats. Measurement of intraluminal Ca2+ level ([Ca2+]L) using a low affinity Ca indicator, mag-fluo-4, revealed that incubation of cells in a high-Ca2+ (10 mM) extracellular solution leads to a substantial increase in [Ca2+]L (SR overload). However, despite increased SR Ca2+ content this did not potentiate ATP-induced [Ca2+]i transients. Repetitive applications of ATP in the absence of extracellular Ca2+, as well as prolonged incubation in Ca2+-free solution without agonist, depleted the [Ca2+]L (SR overload). In contrast to overload, partial depletion of the SR substantially reduced the amplitude of Ca2+ release. ATP-induced [Ca2+]i transients were completely abolished when SR Ca2+ content was decreased below 80% of its normal value indicating a steep dependence of the IP3-mediated Ca2+ release on the Ca2+ load of the store. Our results suggest that in uterine smooth muscle cells decrease in the SR Ca2+ load below its normal resting level substantially reduces the IP3-mediated Ca2+ release, while Ca2+ overload of the SR has no impact on such release.

Confocal imaging of [Ca2+] in cellular organelles by SEER, shifted excitation and emission ratioing of fluorescence

Intracellular calcium signals regulate multiple cellular functions. They depend on release of Ca2+ from cellular stores into the cytosol, a process that appears to be tightly controlled by changes in [Ca2+] within the store. A method to image free [Ca2+] within cellular organelles was devised, which provided the first quantitative confocal images of [Ca2+] inside the sarcoplasmic reticulum (SR) of skeletal muscle. The method exploits, for greater sensitivity, the dual spectral shifts that some fluorescent dyes undergo upon binding Ca2+. It was implemented with mag-indo-1 trapped in the intracellular organelles of frog skeletal muscle and validated showing that it largely monitors [Ca2+] in a caffeine-sensitive compartment with the structure of the SR cisternae. A tentative calibration in situ demonstrated an increase in the dye's dissociation constant, not unlike that observed for other dyes in cellular environments. This increase, together with other characteristics of the ratioing method, placed the half-signal [Ca2+] near 1 mM, a value suitable for cellular stores. Demonstrated advantages of the technique include accuracy (that of a calibrated ratiometric method), dynamic range and sensitivity (from the combination of two spectral shifts), spatial and temporal resolution, and compatibility with a vast array of visible dyes to monitor diverse aspects of cellular function. SEER (shifted excitation and emission ratioing) also provides a [Ca2+]-independent measure of dye concentration in the cell. Store and mitochondrial [Ca2+] ([Ca2+]SR and [Ca2+]mito could be measured separately using the high spatial resolution of SEER. Evolution of [Ca2+]SR was followed upon changes in cytosolic [Ca2+] ([Ca2+]cyto). At [Ca2+]cyto = 100 nM, [Ca2+]mito remained near the lower limit of detection and [Ca2+]SR stabilized at values that were submillimolar according to our tentative calibration. Steady [Ca2+]SR was only slightly higher in 800 nM [Ca2+]cyto, and essentially did not decrease unless [Ca2+]cyto was reduced below 10 nM. While the increase of [Ca2+]SR was limited by loss through Ca2+ release channels, its decrease in low [Ca2+]cyto was largely dependent on leaks through the SR Ca2+ pump.

Pharmacological modulation of sarcoplasmic reticulum function in smooth muscle

The sarco/endoplasmic reticulum (SR/ER) is the primary storage and release site of intracellular calcium (Ca2+) in many excitable cells. The SR is a tubular network, which in smooth muscle (SM) cells distributes close to cellular periphery (superficial SR) and in deeper aspects of the cell (deep SR). Recent attention has focused on the regulation of cell function by the superficial SR, which can act as a buffer and also as a regulator of membrane channels and transporters. Ca2+ is released from the SR via two types of ionic channels [ryanodine- and inositol 1,4,5-trisphosphate-gated], whereas accumulation from thecytoplasm occurs exclusively by an energy-dependent sarco-endoplasmic reticulum Ca2+-ATPase pump (SERCA). Within the SR, Ca2+ is bound to various storage proteins. Emerging evidence also suggests that the perinuclear portion of the SR may play an important role in nuclear transcription. In this review, we detail the pharmacology of agents that alter the functions of Ca2+ release channels and of SERCA. We describe their use and selectivity and indicate the concentrations used in investigating various SM preparations. Important aspects of cell regulation and excitation-contractile activity coupling in SM have been uncovered through the use of such activators and inhibitors of processes that determine SR function. Likewise, they were instrumental in the recent finding of an interaction of the SR with other cellular organelles such as mitochondria. Thus, an appreciation of the pharmacology and selectivity of agents that interfere with SR function in SM has greatly assisted in unveiling the multifaceted nature of the SR.

Functional architecture of the SR calcium store in uterine smooth muscle

Sarcoplasmic reticulum (SR) is abundant in uterine smooth muscle cells. The functional role of this organelle in the regulation of uterine myocytes is not fully understood. The data available in the literature suggest that SR plays a dual role: as a source of calcium and as a calcium sink shaping calcium transients produced by membrane depolarisation and uterotonic agonists. Advances in digital imaging techniques including confocal microscopy of isolated living cells, and the development of methods for direct measurement of intraluminal calcium, has triggered a substantial increase in the number of publications elucidating the role of intracellular stores in calcium signalling. In this paper we review the literature and our own work on the SR calcium store in uterine smooth muscle cells.

Alpha1-adrenergic signaling mechanisms in contraction of resistance arteries

Our goal in this review is to provide a comprehensive, integrated view of the numerous signaling pathways that are activated by alpha(1)-adrenoceptors and control actin-myosin interactions (i.e., crossbridge cycling and force generation) in mammalian arterial smooth muscle. These signaling pathways may be categorized broadly as leading either to thick (myosin) filament regulation or to thin (actin) filament regulation. Thick filament regulation encompasses both "Ca(2+) activation" and "Ca(2+)-sensitization" as it involves both activation of myosin light chain kinase (MLCK) by Ca(2+)-calmodulin and regulation of myosin light chain phosphatase (MLCP) activity. With respect to Ca(2+) activation, adrenergically induced Ca(2+) transients in individual smooth muscle cells of intact arteries are now being shown by high resolution imaging to be sarcoplasmic reticulum-dependent asynchronous propagating Ca(2+) waves. These waves differ from the spatially uniform increases in [Ca(2+)] previously assumed. Similarly, imaging during adrenergic activation has revealed the dynamic translocation, to membranes and other subcellular sites, of protein kinases (e.g., Ca(2+)-activated protein kinases, PKCs) that are involved in regulation of MLCP and thus in "Ca(2+) sensitization" of contraction. Thin filament regulation includes the possible disinhibition of actin-myosin interactions by phosphorylation of CaD, possibly by mitogen-activated protein (MAP) kinases that are also translocated during adrenergic activation. An hypothesis for the mechanisms of adrenergic activation of small arteries is advanced. This involves asynchronous Ca(2+) waves in individual SMC, synchronous Ca(2+) oscillations (at high levels of adrenergic activation), Ca(2+) sparks, "Ca(2+)-sensitization" by PKC and Rho-associated kinase (ROK), and thin filament mechanisms.

Cellular Distribution and Functions of P2 Receptor Subtypes in Different Systems

This review is aimed at providing readers with a comprehensive reference article about the distribution and function of P2 receptors in all the organs, tissues, and cells in the body. Each section provides an account of the early history of purinergic signaling in the organ?cell up to 1994, then summarizes subsequent evidence for the presence of P2X and P2Y receptor subtype mRNA and proteins as well as functional data, all fully referenced. A section is included describing the plasticity of expression of P2 receptors during development and aging as well as in various pathophysiological conditions. Finally, there is some discussion of possible future developments in the purinergic signaling field.

Organellar junctions promote targeted Ca2+ signaling in smooth muscle: why two membranes are better than one

Numerous cellular processes are regulated by fluctuations in the concentration of a single cation, Ca(2+). To accomplish this feat, cells have developed mechanisms that target Ca(2+) signals to specific effectors in both space, by strategically localizing effectors and ion-transporting molecules, and time, by encoding the regulation of the frequency of Ca(2+) oscillations. With an emphasis on smooth muscle, we have analyzed how the interaction of Ca(2+) transporters located on closely apposing membranes of the plasma membrane, sarcoplasmic reticulum and mitochondria provides the structural foundation for site-specific and time-specific Ca(2+) signaling. These junctional membrane complexes can either control the concentration of Ca(2+) in the microdomain that surrounds an effector molecule or deliver Ca(2+) from the translocator on one membrane to a second translocator on the opposing membrane without significant diffusion into the bulk cytosol, an event we term 'linked Ca(2+) transport'.

A calcium-induced calcium influx factor, nitric oxide, modulates the refilling of calcium stores in astrocytes

The roles of nitric oxide are primarily undefined in astrocytes, cells that are active partners in synaptic transmission. Because nitric oxide synthases are present in astrocytes, we imaged the formation of nitric oxide in cultured murine cortical astrocytes using DAF-FM (4-amino-5-methylamino-2',7'-difluorofluorescein diacetate). We demonstrated that physiological concentrations of ATP induced a Ca2+-dependent production of nitric oxide. We then investigated the roles of nitric oxide in astrocytic Ca2+ signaling by exogenous application of a nitric oxide donor and found that nitric oxide induced an influx of external Ca2+. Because these observations raise the possibility that nitric oxide-dependent Ca2+ influx could lead to the refilling of internal stores with Ca2+, we directly monitored the Ca2+ levels of the cytosol and of internal stores while manipulating nitric oxide. Cultures were coloaded with mag-fluo-4 and X-rhod-1 to differentially load the internal stores and cytosol, respectively. ATP induced a cytosolic increase in Ca2+ that results from the IP3-dependent release of Ca2+ from internal stores, detected as a simultaneous reduction in mag-fluo-4 and an increase in X-rhod-1 fluorescence. To monitor store refilling, we measured the recovery of mag-fluo-4 fluorescence after removal of ATP. When nitric oxide signaling was blocked by the nitric oxide scavenger 2-phenyl-4,4,5,5-ketramethyl-imidazoline-1-oxyl-3-oxide or by the nitric oxide synthase inhibitor NG-monomethyl-l-arginine, fluorescence recovery was significantly reduced. These data suggest that transmitters that induce Ca2+ signaling in astrocytes lead to the Ca2+-dependent synthesis of nitric oxide. This in turn stimulates a Ca2+ influx pathway that is, in part, responsible for the refilling of internal Ca2+ stores.

Adult rat cardiomyocytes exhibit capacitative calcium entry

Capacitative Ca(2+) entry (CCE) refers to the influx of Ca(2+) through plasma membrane channels activated on depletion of endoplasmic-sarcoplasmic reticulum Ca(2+) stores. We utilized two Ca(2+)-sensitive dyes (one monitoring cytoplasmic free Ca(2+) and the other free Ca(2+) within the sarcoplasmic reticulum) to determine whether adult rat ventricular myocytes exhibit CCE. Treatments with inhibitors of the sarcoplasmic endoplasmic reticulum Ca(2+)-ATPases were not efficient in releasing Ca(2+) from stores. However, when these inhibitors were coupled with either Ca(2+) ionophores or angiotensin II (an agonist generating inositol 1,4,5 trisphosphate), depletion of stores was observed. This depletion was accompanied by a significant influx of extracellular Ca(2+) characteristic of CCE. CCE was also observed when stores were depleted with caffeine. This influx of Ca(2+) was sensitive to four inhibitors of CCE (glucosamine, lanthanum, gadolinium, and SKF-96365) but not to inhibitors of L-type channels or the Na(+)/Ca(2+) exchanger. In the whole cell configuration, an inward current of approximately 0.7 pA/pF at -90 mV was activated when a Ca(2+) chelator or inositol (1,4,5)-trisphosphate was included in the pipette or when Ca(2+) stores were depleted with a Ca(2+)-ATPase inhibitor and ionophore. The current was maximal at hyperpolarizing voltages and inwardly rectified. The channel was relatively permeant to Ca(2+) and Ba(2+) but only poorly to Mg(2+) or Mn(2+). Taken together, these data support the existence of CCE in adult cardiomyocytes, a finding with likely implications to physiological responses to phospholipase C-generating agonists.

A calcium-induced calcium influx factor, nitric oxide, modulates the refilling of calcium stores in astrocytes

The roles of nitric oxide are primarily undefined in astrocytes, cells that are active partners in synaptic transmission. Because nitric oxide synthases are present in astrocytes, we imaged the formation of nitric oxide in cultured murine cortical astrocytes using DAF-FM (4-amino-5-methylamino-2',7'-difluorofluorescein diacetate). We demonstrated that physiological concentrations of ATP induced a Ca2+-dependent production of nitric oxide. We then investigated the roles of nitric oxide in astrocytic Ca2+ signaling by exogenous application of a nitric oxide donor and found that nitric oxide induced an influx of external Ca2+. Because these observations raise the possibility that nitric oxide-dependent Ca2+ influx could lead to the refilling of internal stores with Ca2+, we directly monitored the Ca2+ levels of the cytosol and of internal stores while manipulating nitric oxide. Cultures were coloaded with mag-fluo-4 and X-rhod-1 to differentially load the internal stores and cytosol, respectively. ATP induced a cytosolic increase in Ca2+ that results from the IP3-dependent release of Ca2+ from internal stores, detected as a simultaneous reduction in mag-fluo-4 and an increase in X-rhod-1 fluorescence. To monitor store refilling, we measured the recovery of mag-fluo-4 fluorescence after removal of ATP. When nitric oxide signaling was blocked by the nitric oxide scavenger 2-phenyl-4,4,5,5-ketramethyl-imidazoline-1-oxyl-3-oxide or by the nitric oxide synthase inhibitor NG-monomethyl-l-arginine, fluorescence recovery was significantly reduced. These data suggest that transmitters that induce Ca2+ signaling in astrocytes lead to the Ca2+-dependent synthesis of nitric oxide. This in turn stimulates a Ca2+ influx pathway that is, in part, responsible for the refilling of internal Ca2+ stores.

Ca2+ scraps: local depletions of free [Ca2+] in cardiac sarcoplasmic reticulum during contractions leave substantial Ca2+ reserve

Free [Ca2+] inside the sarcoplasmic reticulum ([Ca2+]SR) is difficult to measure yet critically important in controlling many cellular systems. In cardiac myocytes, [Ca2+]SR regulates cardiac contractility. We directly measure [Ca2+]SR in intact cardiac myocytes dynamically and quantitatively during beats, with high spatial resolution. Diastolic [Ca2+]SR (1 to 1.5 mmol/L) is only partially depleted (24% to 63%) during contraction. There is little temporal delay in the decline in [Ca2+]SR at release junctions and between junctions, indicating rapid internal diffusion. The incomplete local Ca2+ release shows that the inherently positive feedback of Ca2+-induced Ca2+ release terminates, despite a large residual driving force. These findings place stringent novel constraints on how excitation-contraction coupling works in heart and also reveal a Ca2+ store reserve that could in principle be a therapeutic target to enhance cardiac function in heart failure.

SERCA pump optimizes Ca2+ release by a mechanism independent of store filling in smooth muscle cells

Thapsigargin-sensitive sarco/endoplasmic reticulum Ca(2+) pumps (SERCAs) are involved in maintaining and replenishing agonist-sensitive internal stores. Although it has been assumed that release channels act independently of SERCA pumps, there are data suggesting the opposite. Our aim was to study the relationship between SERCA pumps and the release channels in smooth muscle cells. To this end, we have rapidly blocked SERCA pumps with thapsigargin, to avoid depletion of the internal Ca(2+) stores, and induced Ca(2+) release with either caffeine, to open ryanodine receptors, or acetylcholine, to open inositol 1,4,5-trisphosphate receptors. Blocking SERCA pumps produced smaller and slower agonist-induced [Ca(2+)](i) responses. We determined the Ca(2+) level of the internal stores both indirectly, measuring the frequency of spontaneous transient outward currents, and directly, using Mag-Fura-2, and demonstrated that the inhibition of SERCA pumps did not produce a reduction of the sarco/endoplasmic reticulum Ca(2+) levels to explain the decrease in the agonist-induced Ca(2+) responses. It appears that SERCA pumps are involved in sustaining agonist-induced Ca(2+) release by a mechanism that involves the modulation of Ca(2+) availability in the lumen of the internal stores.

Monitoring of free calcium in the neuronal endoplasmic reticulum: an overview of modern approaches

The concentration of free calcium within the lumen of the endoplasmic reticulum ([Ca2+]L) fluctuates between 100 and 1000 microM. High [Ca2+]L provides an electro-driving force for Ca2+ release and supports high Ca2+ diffusion rate within the endoplasmic reticulum lumen. Fluctuations in [Ca2+]L also regulate numerous chaperones, responsible for postranslational protein processing. Thus, [Ca2+]L integrates various signalling events and establishes a link between fast signalling, associated with the endoplasmic reticulum Ca2+release/uptake, and long-lasting adaptive responses relying primarily on the regulation of protein synthesis. This paper overviews modern approaches for the direct monitoring of [Ca2+]L which rely on three classes of low-affinity Ca2+ probes: ER-targeted aequorin, synthetic fluorescent Ca2+ dyes and GFP-based ER-targeted Ca2+ probes. These techniques, especially as applied to neurones, may substantially widen our appreciation of the endoplasmic reticulum as a universal signalling organelle.

Luminal Ca(2+) and the activity of sarcoplasmic reticulum Ca(2+) pumps modulate histamine-induced all-or-none Ca(2+) release in smooth muscle cells

We have studied histamine (HA)-evoked intracellular Ca(2+) release in single, freshly isolated myocytes from the guinea pig urinary bladder. Short applications of histamine (5 s) produced a thapsigargin (TG)-sensitive transient increase in intracellular calcium concentration ([Ca(2+)](i)). It was established that histamine and caffeine (Caff) released Ca(2+) from the same intracellular stores in these cells. Reducing the Ca(2+) content of internal stores by incubating cells with U-73343 or cyclopiazonic acid (CPA) inhibited the histamine-evoked Ca(2+) release in 69% and 60% of cells, respectively. Under these conditions, all cells released Ca(2+) in response to either caffeine or acetylcholine (ACh). However, decreasing internal Ca(2+) stores by removing external Ca(2+) inhibited histamine-induced Ca(2+) mobilization in only 22% of cells. A similar small fraction of cells was inhibited when sarcoplasmic reticulum (SR) Ca(2+) pumps were quickly blocked to avoid a significant reduction of luminal Ca(2+). In conclusion, lowering the luminal Ca(2+) content in combination with an impairment of the SR Ca(2+) pump activity significantly diminishes the ability of histamine to evoke an all-or-none intracellular Ca(2+) release.

Sequential opening of IP(3)-sensitive Ca(2+) channels and SOC during alpha-adrenergic activation of rabbit vena cava

Alpha(1)-aderenoceptor-mediated constriction of rabbit inferior vena cava (IVC) is signaled by asynchronous wavelike Ca(2+) oscillations in the in situ smooth muscle. We have shown previously that a putative nonselective cationic channel (NSCC) is required for these oscillations. In this report, we show that the application of 2-aminoethoxyphenyl borate (2-APB) to antagonize inositol 1,4,5-trisphosphate (InsP(3))-sensitive Ca(2+) release channels (IP(3)R channels) can prevent the initiation and abolish ongoing alpha(1)-aderenoceptor-mediated tonic constriction of the venous smooth muscle by inhibiting the generation of these intracellular Ca(2+) concentration ([Ca(2+)](i)) oscillations. The observed effects of 2-APB can only be attributed to its selective inhibition on the IP(3)R channels, not to its slight inhibition of the L-type voltage-gated Ca(2+) channel and the sarco(endo)plasmic reticulum Ca(2+) ATPase. Furthermore, 2-APB had no effect on the ryanodine-sensitive Ca(2+) release channel and the store-operated channel (SOC) in the IVC. These results indicate that the putative NSCC involved in refilling the sarcoplasmic reticulum (SR) and maintaining the tonic contraction is most likely an SOC-type channel because it appears to be activated by IP(3)R-channel-mediated SR Ca(2+) release or store depletion. This is in accordance with its sensitivity to Ni(2+) and La(3+) (SOC blockers). More interestingly, RT-PCR analysis indicates that transient receptor potential (Trp1) mRNA is strongly expressed in the rabbit IVC. The Trp1 gene is known to encode a component of the store-operated NSCC. These new data suggest that the activation of both the IP(3)R channels and the SOC are required for PE-mediated [Ca(2+)](i) oscillations and constriction of the rabbit IVC.

Imaging of changes in sarcoplasmic reticulum [Ca(2+)] using Oregon Green BAPTA 5N and confocal laser scanning microscopy

We describe experiments in which the low affinity indicator Oregon Green BAPTA 5N was used to record the spatially resolved changes in [Ca(2+)] from intracellular stores in rat gastric myocytes. Cells were loaded with the membrane permeant form of the indicator and imaged using a confocal scanning laser microscope. In doubly stained cells the Oregon Green signal colocalized with BIODIPY 558/568 Brefeldin A, a label for the endo/sarcoplasmic reticulum (SR) and Golgi apparatus. Oregon Green BAPTA 5N was calibrated in gastric myocytes, giving an in situ K(d) of 90 microM. The resting free [Ca(2+)] within the SR averaged 65 microM. A reversible decrease in Oregon Green fluorescence was observed on bath application of Inositol triphosphate (IP(3)) (10 microM) to permeabilized cells. Similar changes were also observed when cyclopiazonic acid (5 microM) was applied to intact myocytes, again with recovery of store [Ca(2+)] following drug washout. Identical patterns of Ca(2+) depletion were seen when caffeine (1 microM) and carbachol (10 microM) were applied sequentially to the same cells, suggesting that activation of ryanodine and IP(3)-sensitive channels can result in the release of Ca(2+) from the same regions of the SR.

Effect of inhibiting the sarcoplasmic reticulum on spontaneous and oxytocin-induced contractions of human myometrium

OBJECTIVE

1. To assess the contribution of the sarcoplasmic reticulum calcium store in the generation of uterine smooth muscle contractions; 2. to evaluate the contribution of calcium induced calcium release or ryanodine gated calcium channels to myometrial force production.

DESIGN

Laboratory scientific study.

METHODS

Myometrial strips were obtained from women undergoing elective prelabour caesarean section at term. These were loaded with the calcium sensitive indicator Indo-1 allowing simultaneous assessment of intracellular calcium concentrations and force production. The effect of exposing the strips to ryanodine (which abolishes calcium induced calcium release), caffeine (which activates calcium induced calcium release) and cyclopiazonic acid (which abolishes the sarcoplasmic reticulum calcium store) was examined.

RESULTS

Exposure to ryanodine had no appreciable effect on either the amplitude or the duration of the myometrial calcium and force transients but did increase the frequency of contractions (139+/-5%). Caffeine did not potentiate force. Cyclopiazonic acid increased frequency, duration and amplitude of both calcium and force transients. The ability of oxytocin to provoke calcium and force transients in the absence of extracellular calcium was abolished by cyclopiazonic acid but not by ryanodine.

CONCLUSIONS

These results demonstrate that calcium induced calcium release does not play a significant role in human myometrium and that no functioning role for the ryanodine receptors in human myometrial tissue could be shown. These data suggest that the sarcoplasmic reticulum may act to limit contractions and act as a calcium sink, rather than to amplify contractions.

Effect of nitric oxide donors and noradrenaline on Ca2+ release sites and global intracellular Ca2+ in myocytes from guinea-pig small mesenteric arteries

In smooth muscle the spontaneous Ca2+ release from the sarcoplasmic reticulum (SR) occurs at preferred locations called frequent discharge sites (FDSs) giving rise to localized intracellular Ca2+ transients (Ca2+ sparks). Laser scanning confocal microscopy of fluo-3-loaded single myocytes freshly isolated from small mesenteric arteries of guinea-pig was used to investigate the action of nitric oxide (NO) donors and noradrenaline on the position and activity of FDSs and on global intracellular Ca2+ concentration ([Ca2+]i). In 8 % of cells 'microsparks', Ca2+ release events smaller in duration, spread and amplitude than Ca2+ sparks were observed. The location of the initiation point of Ca2+ sparks observed during line-scan imaging was found to 'jitter' by +/- 0.41 microm. However, the general position of an FDS within the cell did not change; most FDSs were close (within 1.2 +/- 0.1 microm) to the cell membrane and often multiple FDSs occurred in one confocal plane of the cell. In the resting state, NO donors S-nitroso-N-acetylpenicillamine (SNAP; 50 microM) and sodium nitroprusside (SNP; 100 microM) did not change the general position of FDSs and slightly depressed their activity, but did not affect the global [Ca2+]i significantly. Application of noradrenaline (1-10 microM) increased Ca2+ spark frequency at existing FDS(s) leading to a Ca2+ wave. The increase in FDS activity and in global [Ca2+]i produced by noradrenaline were inhibited by the presence of SNAP or SNP but not by 8-bromoguanosine cyclic 3',5'-monophosphate (8-Br-cGMP; 100 microM). In the presence of 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), inhibitor of soluble guanylate cyclase, SNAP and SNP still exerted their effects on the noradrenaline response. These results suggest that SNAP and SNP inhibit the noradrenaline-evoked rise in global [Ca2+]i by a cGMP-independent mechanism and that part of this effect is due to inhibition of the activity of FDSs; moreover, only the activity, but not the position, of FDSs is changed by either stimulant or inhibitory substances.

Ca(2+) regulation in guinea-pig colonic smooth muscle: the role of the Na(+)-Ca(2+) exchanger and the sarcoplasmic reticulum

To study the contribution of the Na(+)-Ca(2+) exchanger to Ca(2+) regulation and its interaction with the sarcoplasmic reticulum (SR), changes in cytoplasmic Ca(2+) concentration ([Ca(2+)](c)) were measured in single, voltage clamped, smooth muscle cells. Increases in [Ca(2+)](c) were evoked by either depolarisation (-70 mV to 0 mV) or by release from the SR by caffeine (10 mM) or flash photolysis of caged InsP(3) (InsP(3)). Depletion of the SR of Ca(2+) (verified by the absence of a response to caffeine and InsP(3)) by either ryanodine (50 microM), to open the ryanodine receptors (RyRs), or thapsigargin (500 nM) or cyclopiazonic acid (CPA, 10 microM), to inhibit the SR Ca(2+) pumps, reduced neither the magnitude of the Ca(2+) transient nor the relationship between the influx of and the rise in [Ca(2+)](c) evoked by depolarisation. This suggested that Ca(2+)-induced Ca(2+) release (CICR) from the SR did not contribute significantly to the depolarisation-evoked rise in [Ca(2+)](c). However, although Ca(2+) was not released from it, the SR accumulated the ion following depolarisation since ryanodine and thapsigargin each slowed the rate of decline of the depolarisation-evoked Ca(2+) transient. Indeed, the SR Ca(2+) content increased following depolarisation as assessed by the increased magnitude of the [Ca(2+)](c) levels evoked each by InsP(3) and caffeine, relative to controls. The increased SR Ca(2+) content following depolarisation returned to control values in approximately 12 min via Na(+)-Ca(2+) exchanger activity. Thus inhibition of the Na(+)-Ca(2+) exchanger by removal of external Na(+) (by either lithium or choline substitution) prevented the increased SR Ca(2+) content from returning to control levels. On the other hand, the Na(+)-Ca(2+) exchanger did not appear to regulate bulk average Ca(2+) directly since the rates of decline in [Ca(2+)](c), following either depolarisation or the release of Ca(2+) from the SR (by either InsP(3) or caffeine), were neither voltage nor Na(+) dependent. Thus, no evidence for short term (seconds) control of [Ca(2+)](c) by the Na(+)-Ca(2+) exchanger was found. Together, the results suggest that despite the lack of CICR, the SR removes Ca(2+) from the cytosol after its elevation by depolarisation. This Ca(2+) is then removed from the SR to outside the cell by the Na(+)-Ca(2+) exchanger. However, the exchanger does not contribute significantly to the decline in bulk average [Ca(2+)](c) following transient elevations in the ion produced either by depolarisation or by release from the store.

Role of phosphate and calcium stores in muscle fatigue

Intensive activity of muscles causes a decline in performance, known as fatigue, that is thought to be caused by the effects of metabolic changes on either the contractile machinery or the activation processes. The concentration of inorganic phosphate (P(i)) in the myoplasm ([P(i)](myo)) increases substantially during fatigue and affects both the myofibrillar proteins and the activation processes. It is known that a failure of sarcoplasmic reticulum (SR) Ca(2+) release contributes to fatigue and in this review we consider how raised [P(i)](myo) contributes to this process. Initial evidence came from the observation that increasing [P(i)](myo) causes reduced SR Ca(2+) release in both skinned and intact fibres. In fatigued muscles the store of releasable Ca(2+) in the SR declines mirroring the decline in SR Ca(2+) release. In muscle fibres with inoperative creatine kinase the rise of [P(i)](myo) is absent during fatigue and the failure of SR Ca(2+) release is delayed. These results can all be explained if inorganic phosphate can move from the myoplasm into the SR during fatigue and cause precipitation of CaP(i) within the SR. The relevance of this mechanism in different types of fatigue in humans is considered.