Receptor-regulated calcium entry

Selective modulation of gene expression in activated normal human peripheral blood mononuclear cells by store-operated calcium entry blocker BTP2

Calcium is a critical signaling molecule in many cell types including immune cells. The calcium-release activated calcium channels (CRAC) responsible for store-operated calcium entry (SOCE) in immune cells are gated by STIM family members functioning as sensors of Ca2+ store content in the endoplasmic reticulum. We investigated the effect of SOCE blocker BTP2 on human peripheral blood mononuclear cells (PBMC) stimulated with the mitogen phytohemagglutinin (PHA). We performed RNA sequencing (RNA-seq) to query gene expression at the whole transcriptome level and identified genes differentially expressed between PBMC activated with PHA and PBMC activated with PHA in the presence of BTP2. Among the differentially expressed genes, we prioritized genes encoding immunoregulatory proteins for validation using preamplification enhanced real time quantitative PCR assays. We performed multiparameter flow cytometry and validated by single cell analysis that BTP2 inhibits cell surface expression CD25 at the protein level. BTP2 reduced significantly PHA-induced increase in the abundance of mRNAs encoding proinflammatory proteins. Surprisingly, BTP2 did not reduce significantly PHA-induced increase in the abundance of mRNAs encoding anti-inflammatory proteins. Collectively, the molecular signature elicited by BTP2 in activated normal human PBMC appears to be tipped towards tolerance and away from inflammation.

Small molecules under development for psoriasis: on the road to the individualized therapies

Psoriasis is an incurable cutaneous illness characterized by the presence of well-delimited reddish plaques and silvery-white dry scales. So far, there is a limited understanding of its pathogenesis, though recent discoveries on the immunological, genetic and molecular aspects of this disease have significantly contributed to the identification of new targets and the development of novel drugs. Despite these advances, many patients are still dissatisfied, so to improve patient satisfaction, reliability, and clinical outcomes, the individualization of the treatments for this disease becomes a necessity. This review summarizes recent findings related to psoriasis pathogenesis and describes new small molecules and targets recently identified as promising for treatments. Additionally, the current status, challenges and the future directions for achieving individualized therapy for this disease and the need for more collaborative studies are discussed. The individualization of treatments for psoriasis, rather than a goal, is analyzed as a process where a dynamic integration between the needs and characteristics of the patients, the pharmacological progress, and the clinical decisions takes place.

Long-term dexamethasone treatment diminishes store-operated Ca2+ entry in salivary acinar cells

Corticosteroids are used in the treatment of many diseases; however, they also induce various side effects. Dexamethasone is one of the most potent corticosteroids, and it has been reported to induce the side effect of impaired salivary gland function. This study aimed to evaluate the effects of dexamethasone on mouse submandibular gland function to gain insight into the mechanism of dexamethasone-induced salivary hypofunction. The muscarinic agonist carbachol (CCh) induced salivary secretion and was not affected by short-term dexamethasone treatment but was decreased following long-term dexamethasone administration. The expression levels of the membrane proteins Na⁺-K⁺-2Cl^- cotransporter, transmembrane member 16A, and aquaporin 5 were comparable between the control and long-term dexamethasone treatment groups. The CCh-induced increase in calcium concentration was significantly lower in the presence of extracellular Ca²⁺ in the long-term dexamethasone treatment group compared to that in the control group. Furthermore, CCh-induced salivation in the absence of extracellular Ca²⁺ and Ca²⁺ ionophore A23187-induced salivation was comparable between the control and long-term dexamethasone treatment groups. Moreover, salivation induced by the Ca²⁺-ATPase inhibitor thapsigargin was diminished in the long-term dexamethasone treatment group. In summary, these results demonstrate that short-term dexamethasone treatment did not impair salivary gland function, whereas long-term dexamethasone treatment diminished store-operated Ca²⁺ entry, resulting in hyposalivation in mouse submandibular glands.

Recent Advances in the Development of Anticancer Drugs that Act against Signalling Pathways

Cancer can be considered a disease of deranged intracellular signalling. The intracellular signalling pathways that mediate the effects of oncogenes on cell growth and transformation present attractive targets for the development of new classes of drugs for the prevention and treatment of cancer. This is a new approach to developing anticancer drugs and the potential, as well as some of the problems, inherent in the approach are discussed. Anticancer drugs that produce their effects by disrupting signalling pathways are already in clinical trial. Some properties of these drugs, as well as other inhibitors of signalling pathways under development as potential anticancer drugs, are reviewed.

Physiology of blood pressure relevant to managing hypertension in pregnancy

PURPOSE

Pregnancy causes physiological changes in maternal organ systems, and blood pressure (BP) is one of the variables affected. This review is focusing on the physiology of BP relevant to the management of hypertension in pregnancy.

MATERIALS AND METHODS

A detailed literature search was performed using electronic databases (including WorldCat, PubMed, MEDLINE, Google Scholar) to retrieve and review reports related to physiology of BP in pregnancy.

RESULTS

During pregnancy, there is vasodilation caused by mediators such as increased levels of progesterone and nitric oxide. The vasodilation leads to a reduction in vascular resistance, BP, and renal blood flow. In compensation, the following postulated events occur: activation of renin-angiotensin-aldosterone axis, resetting of osmotic threshold for thirst, and an increase in the production of vasopressin. Sodium and water conservation ensue to increase the total body water, end-diastolic volume, cardiac output, and BP. The increase in cardiac output incompletely compensates for the decreased vascular resistance, and BP therefore decreases in midpregnancy and returns to prepregnancy level toward term.

CONCLUSIONS

An understanding of the physiological changes in BP is essential for appropriate management of pregnancy-related hypertension.

Pore opening mechanism of CRAC channels

Three decades ago, James W. Putney Jr. conceptualized the idea of store-operated calcium entry (SOCE) to explain how depletion of endoplasmic reticulum (ER) Ca2+ stores evokes Ca2+ influx across the plasma membrane. Since the publication of this highly influential idea, it is now established that SOCE is universal among non-excitable and probably even many types of excitable cells, and contributes to numerous effector functions impacting immunity, muscle contraction, and brain function. The molecules encoding SOCE, the STIM and Orai proteins, are now known and our understanding of how this pathway is activated in response to ER Ca2+ store depletion has advanced significantly. In this review, we summarize the current knowledge of how Orai1 channels are activated by STIM1, focusing on recent work supporting a hydrophobic gating mechanism for the opening of the Orai1 channel pore.

Effect of yessotoxin on cytosolic calcium levels in human hepatocellular carcinoma cells in vitro

Yessotoxin (YTX) and its analogs are a type of marine toxins found in marine environments in numerous coastal countries. These toxins tend to accumulate in filter-feeding molluscs and may threaten the shellfish industry and public health. Several previous studies indicated that YTX may induce apoptosis in different types of cell lines, although the exact underlying mechanisms have not yet been elucidated. The aim of this study was to mainly focus on the effect of YTX on cytosolic Ca²⁺ levels in human hepatocellular carcinoma cells. In order to investigate the exact mechanism of YTX-evoked Ca²⁺ increase, laser scanning confocal microscopy was used, with the addition of the chelator ethylene glycol tetraacetic acid (EGTA) and nifedipine, an L-type Ca²⁺ channel blocker, to the reaction system. The results demonstrated that YTX caused cytosolic Ca²⁺ level increase in Bel7402 cells and the YTX-evoked Ca²⁺ increase was successfully blocked by EGTA and nifedipine. Therefore, our results indicated that YTX may cause apoptosis via inducing Ca²⁺ entry in Bel7402 cells.

Vascular Smooth Muscle Cell Signaling Mechanisms for Contraction to Angiotensin II and Endothelin-1

Vasoactive peptides, such as endothelin-1 and angiotensin II are recognized by specific receptor proteins located in the cell membrane of target cells. Following receptor recognition, the specificity of the cellular response is achieved by G-protein coupling of ligand binding to the regulation of intracellular effectors. These intracellular effectors will be the subject of this brief review on contractile activity initiated by endothelin-1 and angiotensin II.Activation of receptors by endothelin-1 and angiotensin II in smooth muscle cells results in phopholipase C (PLC) activation leading to the generation of the second messengers insitol trisphosphate (IP(3)) and diacylglycerol (DAG). IP(3) stimulates intracellular Ca(2+) release from the sarcoplasmic reticulum and DAG causes protein kinase C (PKC) activation. Additionally, different Ca(2+) entry channels, such as voltage-operated (VOC), receptor-operated (ROC), and store-operated (SOC) Ca(2+) channels, as well as Ca(2+)-permeable nonselective cation channels (NSCC), are involved in the elevation of intracellular Ca(2+) concentration. The elevation in intracellular Ca(2+) is transient and initiates contractile activity by a Ca(2+)-calmodulin interaction, stimulating myosin light chain (MLC) phosphorylation. When the Ca(2+) concentration begins to decline, Ca(2+)-sensitization of the contractile proteins is signaled by the RhoA/Rho-kinase pathway to inhibit the dephosphorylation of MLC phosphatase (MLCP) thereby maintaining force generation. Removal of Ca(2+) from the cytosol and stimulation of MLCP initiates the process of smooth muscle relaxation. In pathological conditions such as hypertension, alterations in these cellular signaling components can lead to an over stimulated state causing maintained vasoconstriction and blood pressure elevation.

Treatment of mast cells with carbon dioxide suppresses degranulation via a novel mechanism involving repression of increased intracellular calcium levels

BACKGROUND

Intranasal noninhaled delivery of carbon dioxide (CO₂) is efficacious in the symptomatic treatment of seasonal allergic rhinitis. The goal of this study was to determine whether and how 100% CO₂ inhibits mast cell degranulation, thereby possibly contributing to the reduction of symptoms in seasonal allergic rhinitis.

METHODS

  Peritoneal mast cells isolated from rats and labelled with sulforhodamine-B (SFRM-B) were used to determine whether CO₂ treatment could block mast cell degranulation and histamine release in response to 48/80. In addition, the effect of CO₂ on intracellular calcium levels in unstimulated and stimulated mast cells was determined by fluorescent microscopy.

RESULTS

Treatment with 48/80 caused >90% of mast cells containing SFRM-B to degranulate, resulting in a marked decrease in the fluorescent intensity within the mast cells, and simultaneously causing a significant increase in histamine release. Significantly, the stimulatory effect of 48/80 on fluorescent intensity and histamine levels was greatly inhibited (>95%) to near control levels by pretreatment with 100% CO₂. Treatment with 48/80 also caused a robust transient increase in intracellular calcium, whereas pretreatment with CO₂ repressed the increase in calcium (>70%) in response to 48/80.

CONCLUSIONS

Results from this study provide the first evidence of a unique regulatory mechanism by which CO₂ inhibits mast cell degranulation and histamine release by repressing stimulated increases in intracellular calcium. Thus, our data provide a plausible explanation for the reported therapeutic benefit of noninhaled intranasal delivery of 100% CO₂ to treat allergic rhinitis.

Regulatory role of neuron-restrictive silencing factor in expression of TRPC1

Neuron-restrictive silencer factor (NRSF) binds its consensus element to repress the transcription of various genes. The dominant-negative form (dnNRSF) has a hypertrophic effect on cardiogenesis through an unidentified mechanism. We examined the involvement of transient receptor potential (TRP) channel proteins, using transgenic mice overexpressing dnNRSF (dnNRSF mice). Electrophoretic mobility-shift assays revealed an interaction between NRSF and a neuron-restrictive silencer element-like sequence in intron 4 of TRPC1 genomic DNA. According to RT-PCR and Western analyses, TRPC1 was up-regulated in dnNRSF mouse heart. Transient overexpression of TRPC1 in HEK 293T cells increased the activity of the nuclear factor in activated T cells (NFAT) promoter and stimulated store-operated Ca(2+) channel (SOCC)-mediated Ca(2+) entry. Transfection of TRPC1 into primary cardiomyocytes increased NFAT activity, indicating a major role for TRPC1 in NFAT activation. Our findings strongly suggest that NRSF regulates TRP1 gene expression and causes changes in the levels of calcium entry through SOCCs.

Calcium signalling in human spermatozoa: a specialized 'toolkit' of channels, transporters and stores

Ca(2+) is a ubiquitous intracellular messenger which encodes information by temporal and spatial patterns of concentration. In spermatozoa, several key functions, including acrosome reaction and motility, are regulated by cytoplasmic Ca(2+) concentration. Despite the very small size and apparent structural simplicity of spermatozoa, evidence is accumulating that they possess sophisticated mechanisms for regulation of cytoplasmic Ca(2+) concentration and generation of complex Ca(2+) signals. In this review, we consider the various components of the Ca(2+)-signalling 'toolkit' that have been characterized in somatic cells and summarize the evidence for their presence and activity in spermatozoa. In particular, data accumulated over the last few years show that spermatozoa possess one (and probably two) Ca(2+) stores as well as a range of plasma membrane pumps and channels. Selective regulation of the various components of the 'toolkit' by agonists probably allows spermatozoa to generate localized Ca(2+) signals despite their very small cytoplasmic volume, permitting the discrete and selective activation of cell functions.

Calcium channels, transporters and exchangers in placenta: a review

Calcium (Ca2+) entry in cells is crucial for development and physiology of virtually all cell types. It acts as an intracellular (second) messenger to regulate a diverse array of cellular functions, from cell division and differentiation to cell death. Among candidates for Ca2+ entry in cells are-voltage-dependant Ca2+ channels (VDCCs), transient receptor potential (TRP)-related Ca2+ channels and store-operated Ca2+ (SOC) channels. Plasma membrane Ca2+-ATPases (PMCA) and Na+/Ca2+ exchanger (NCX) are mainly responsible for Ca2+ extrusion. These different Ca2+channels/transporters and exchangers exhibit specific distribution and physiological properties. During pregnancy, the syncytiotrophoblast layer of the human placenta transfers as much as 30 g of Ca2+ from the mother to the fetus, especially in late gestation where Ca2+ transport through different channels must increase in response to the demands of accelerating bone mineralization of the fetus. The identification and characterization of the different Ca2+ channels/transporters and exchangers on the brush-border membrane (BBM) facing the maternal circulation, and the basal plasma membrane (BPM) facing the fetal circulation; placental membrane of the syncytiotrophoblasts have been the focus of numerous studies. This review discusses current views in this field regarding localization and functions during transcellular Ca2+ entry and extrusion from cells particularly in the placenta.

Angiotensin II causes calcium entry into bovine adrenal chromaffin cells via pathway(s) activated by depletion of intracellular calcium stores

The characteristics and properties of the increase in cytosolic [Ca2+] that occurs in bovine adrenal medullary chromaffin cells on exposure to angiotensin 11 have been investigated. In fura-2 loaded cells exposure to a maximally effective concentration of angiotensin II (100 nM) caused a rapid, but transient increase in cytosolic [Ca2+] followed by a lower plateau that was sustained as long as external Ca2+ was present. In the absence of external Ca2+ only the initial brief transient was observed. In cells previously treated with thapsigargin in Ca2+-free medium to deplete the internal Ca2+ stores, angiotensin II caused no increase in cytosolic [Ca2+] when external Ca2+ was absent. Reintroduction of external Ca2+ to thapsigargin-treated, store-depleted cells caused a sustained increase in cytosolic [Ca2+] that was not further increased upon exposure to angiotensin II. Analysis of the data suggests that in bovine chromaffin cells angiotensin II causes Ca2+ entry via a pathway(s) activated as a consequence of internal store mobilization, and entry through this pathway(s) forms the majority of the sustained Ca2+ influx evoked by angiotensin II.

TRPC1 is required for functional store-operated Ca2+ channels. Role of acidic amino acid residues in the S5-S6 region

The exact role of TRPC1 in store-operated calcium influx channel (SOCC) function is not known. We have examined the effect of overexpression of full-length TRPC1, depletion of endogenous TRPC1, and expression of TRPC1 in which the proposed pore region (S5-S6, amino acids (aa) 557-620) was deleted or modified by site-directed mutagenesis on thapsigargin- and carbachol-stimulated SOCC activity in HSG cells. TRPC1 overexpression induced channel activity that was indistinguishable from the endogenous SOCC activity. Transfection with antisense hTRPC1 decreased SOCC activity although characteristics of SOCC-mediated current, I(SOC), were not altered. Expression of TRPC1 Delta 567-793, but not TRPC1 Delta 664-793, induced a similar decrease in SOCC activity. Furthermore, TRPC1 Delta 567-793 was co-immunoprecipitated with endogenous TRPC1. Simultaneous substitutions of seven acidic aa in the S5-S6 region (Asp --> Asn and Glu --> Gln) decreased SOCC-mediated Ca(2+), but not Na(+), current and induced a left shift in E(rev). Similar effects were induced by E576K or D581K, but not D581N or E615K, substitution. Furthermore, expressed TRPC1 proteins interacted with each other. Together, these data demonstrate that TRPC1 is required for generation of functional SOCC in HSG cells. We suggest that TRPC1 monomers co-assemble to form SOCC and that specific acidic aa residues in the proposed pore region of TRPC1 contribute to Ca(2+) influx.

Cell side-specific sensitivities of intracellular Ca2+ stores for inositol 1,4,5-trisphosphate, cyclic ADP-ribose, and nicotinic acid adenine dinucleotide phosphate in permeabilized pancreatic acinar cells from mouse

In pancreatic acinar cells hormonal stimulation leads to a cytosolic Ca(2+) wave that starts in the apical cell pole and subsequently propagates toward the basal cell side. We used permeabilized pancreatic acinar cells from mouse and the mag-fura-2 technique, which allows direct monitoring of changes in [Ca(2+)] of intracellular stores. We show here that Ca(2+) can be released from stores in all cellular regions by inositol 1,4,5-trisphosphate. Stores at the apical cell pole showed a higher affinity to inositol 1,4,5-trisphosphate (EC(50) = 89 nm) than those at the basolateral side (EC(50) = 256 nm). In contrast, cADP-ribose, a modifier of Ca(2+)-induced Ca(2+) release, and nicotinic acid adenine dinucleotide phosphate (NAADP) were able to release Ca(2+) exclusively from intracellular stores located at the basolateral cell side. Our data agree with observations that upon stimulation Ca(2+) is released initially at the apical cell side and that this is caused by high affinity inositol 1,4,5-trisphosphate receptors. Moreover, our findings allow the conclusion that in Ca(2+) wave propagation from the apical to the basolateral cell side observed in pancreatic acinar cells Ca(2+)-induced Ca(2+) release, modulated by cADP-ribose and/or NAADP, might be involved.

Effect of low temperatures on compound 48/80-induced intracellular Ca2+ changes and exocytosis of rat peritoneal mast cells

It has been well documented that compound 48/80-induced exocytosis of mast cells is accompanied by changes in intracellular Ca2+ concentration ([Ca2+]i) showing a biphasic pattern: an initial phase which constitutes an abrupt increase, followed by a plateau phase. The former is caused by Ca2+ release from intracellular Ca2+ stores, and the latter is the result of secondary Ca2+ influx. Low temperatures lead to the inhibition of exocytosis, but the precise mechanism remains unclear. The present study aims to reveal whether [Ca2+]i changes are affected by the environmental temperature. To this end, we developed a novel imaging method to record [Ca2+]i changes and exocytotic processes simultaneously. Rat peritoneal mast cells were loaded by Indo-1/AM or Fluo-3/AM for measuring [Ca2+]i, and the exocytosed granule matrices were stained by sulforhodamine-B. Cells were stimulated by compound 48/80, and [Ca2+]i changes and exocytosis were recorded by means of a real-time confocal microscope. At 37 degrees C, [Ca2+]i changes in stimulated mast cells showed a sustained plateau phase. Granule discharge was observed at the cell surface, and, in addition, most of the intracellular granule matrices were involved in compound exocytosis. The granule discharge and compound exocytosis proceeded over a period of a few minutes. At 4 degrees C, the plateau phase of [Ca2+]i changes declined rapidly, although the initial phase was not suppressed. Granule discharge occurred at the cell surface, but compound exocytosis ceased within a few minutes. These findings indicate that a low temperature inhibits compound exocytosis which can be caused by Ca2+ influx. The present imaging method represents a powerful tool for investigating the stimulus-secretion coupling of mast cells.

SERCA pump isoform expression in endothelium of veins and arteries: every endothelium is not the same

Endothelium from rat aorta expresses sarco/endoplasmic reticulum Ca2+(SERCA) pump gene SERCA3 where as the smooth muscle expresses SERCA2. This has led to the postulate that vascular endothelium expresses SERCA3. To test this postulate, we examined the SERCA2 and SERCA3 mRNA expression in endothelium and smooth muscle dissected from coronary artery, coronary vein, aorta and vena cava of pig. Smooth muscle from all arteries and veins expressed only the SERCA2 mRNA. Endothelium from coronary artery, coronary vein and aorta expressed both SERCA2 and SERCA3 mRNA but the endothelium from vena cava did not express SERCA3 mRNA although it expressed SERCA2. These observations support the postulate that vascular endothelium expresses SERCA3 but the affirmation is equivocal because vena cava endothelium does not express SERCA3.

Oxytocin-stimulated capacitative calcium entry in human myometrial cells

OBJECTIVE

Our purpose was to investigate the relative contribution of extracellular calcium recruitment and release of calcium from intracellular stores in an immortalized myometrial cell line derived from a pregnant woman (PHM1-41) and to determine the importance of capacitative calcium entry in the oxytocin-stimulated rise in intracellular free calcium.

STUDY DESIGN

The PHM1-41 immortalized myometrial cell line, which retains smooth muscle phenotype, estrogen, and oxytocin receptors and responds to oxytocin with an increase in intracellular free calcium, was used for this study. Intracellular free calcium was measured directly in cells loaded with Fura 2-AM.

RESULTS

The oxytocin-stimulated rise in intracellular free calcium decreased in the absence of extracellular calcium or in the presence of phospholipase C inhibitors, suggesting mobilization of calcium from both extracellular and intracellular sources to increase intracellular free calcium. Phospholipase C inhibitors resulted in greater inhibition of the oxytocin-stimulated rise in intracellular free calcium than expected on the basis of experiments performed in the absence of extracellular calcium. This implies interdependence of the intracellular and extracellular pathways for elevation of intracellular free calcium and suggests some capacitative entry of calcium as a consequence of depletion of intracellular stores. The oxytocin-stimulated intracellular free calcium increase resulting from calcium entry was blocked by store depletion by thapsigargin or cyclopiazonic acid, consistent with a capacitative calcium entry mechanism.

CONCLUSION

Oxytocin stimulates both capacitative and noncapacitative calcium entry in a pregnant human myometrium cell line.

Effects of peroxide on contractility of coronary artery rings of different sizes

Reactive oxygen species (ROS, free radicals) produced during cardiac ischemia and reperfusion can damage the contractile functions of arteries. The sarcoplasmic reticulum (SR) Ca2+ pump in coronary artery smooth muscle is very sensitive to ROS. Here we show that contractions of de-endothelialized rings from porcine left coronary artery produced by the hormone Angiotensin II and by the SR Ca2+ pump inhibitors cyclopiazonic acid and thapsigargin correlate negatively with the tissue weight. In contrast, the contractions due to membrane depolarization by high KCl correlate positively. Peroxide also produces a small contraction which correlates negatively with the tissue weight. When artery rings are treated with peroxide and washed, their ability to contract with Angiotensin II, cyclopiazonic acid and thapsigargin decreases. Thus, the SR Ca2+ pump may play a more important role in the contractility of the smaller segments of the coronary artery than in the larger segments. These results are consistent with the hypothesis that ROS which damage the SR Ca2+ pump affect the contractile function of the distal segments more adversely than of the proximal segments.

Peroxide resistance of ER Ca2+ pump in endothelium: implications to coronary artery function

We examined the effects of peroxide on the sarco(endo)plasmic reticulum Ca2+ (SERCA) pump in pig coronary artery endothelium and smooth muscle at three organizational levels: Ca2+ transport in permeabilized cells, cytosolic Ca2+ concentration in intact cells, and contractile function of artery rings. We monitored the ATP-dependent, azide-insensitive, oxalate-stimulated 45Ca2+ uptake by saponin-permeabilized cultured cells. Low concentrations of peroxide inhibited the uptake less effectively in endothelium than in smooth muscle whether we added the peroxide directly to the Ca2+ uptake solution or treated intact cells with peroxide and washed them before the permeabilization. An acylphosphate formation assay confirmed the greater resistance of the SERCA pump in endothelial cells than in smooth muscle cells. Pretreating smooth muscle cells with 300 microM peroxide inhibited (by 77 +/- 2%) the cyclopiazonic acid (CPA)-induced increase in cytosolic Ca2+ concentration in a Ca2+-free solution, but it did not affect the endothelial cells. Peroxide pretreatment inhibited the CPA-induced contraction in deendothelialized arteries with a 50% inhibitory concentration of 97 +/- 13 microM, but up to 500 microM peroxide did not affect the endothelium-dependent, CPA-induced relaxation. Similarly, 500 microM peroxide inhibited the angiotensin-induced contractions in deendothelialized arteries by 93 +/- 2%, but it inhibited the bradykinin-induced, endothelium-dependent relaxation by only 40 +/- 13%. The greater resistance of the endothelium to reactive oxygen may be important during ischemia-reperfusion or in the postinfection immune response.

Sarco(endo)plasmic reticulum Ca2+ pump isoform SERCA3 is more resistant than SERCA2b to peroxide

Sarco(endo)plasmic reticulum Ca2+ pumps are encoded by genes SERCA1, SERCA2, and SERCA3. Most tissues express SERCA2 Ca2+ pumps (splice SERCA2b) which are inactivated by reactive oxygen. In contrast, SERCA3 is expressed in tissues such as tracheal epithelium, mast cells, lymphoid cells, and aortic endothelium, which are frequently exposed to oxidative stress. Therefore, we compared SERCA3 and SERCA2b proteins for their sensitivity to oxidation. We isolated microsomes from HEK-293 cells overexpressing SERCA3 or SERCA2b. We incubated the microsomes with different concentrations of hydrogen peroxide and then determined Ca2+ pump activities in them in the following three assay systems: ATP-dependent oxalate-stimulated azide-insensitive 45Ca2+ uptake by the microsomal vesicles, Ca(2+)-Mg(2+)-ATPase, and Ca(2+)-dependent acylphosphate formation. Peroxide inhibited the pump activities in microsomes with half-maximal inhibitory concentration (IC50) values of 69 +/- 14, 66 +/- 13, and 84 +/- 15 microM for the 45Ca2+ uptake, Ca(2+)-Mg(2+)-ATPase, and the acylphosphate formation reactions, respectively. However, for microsomes from SERCA3-expressing cells, the corresponding values of IC50 for peroxide were 274 +/- 47, 857 +/- 110, and 746 +/- 40 microM. Thus, in each assay system, the resistance to inactivation by peroxide was significantly (P < 0.05) higher for the SERCA3 protein than for SERCA2b. The SERCA3 resistance to oxidants may aid the cells expressing it to function during exposure to oxidative stress.

Barium influx mediated by the cardiac sodium-calcium exchanger in transfected Chinese hamster ovary cells

We examined Ba2+ influx using isotopic and fura-2 techniques in transfected Chinese hamster ovary cells expressing the bovine cardiac Na+/Ca2+ exchanger (CK1.4 cells). Ba2+ competitively inhibited exchange-mediated 45Ca2+ uptake with a Ki approximately 3 mM. Ba2+ uptake was stimulated by pretreating the cells with ouabain and by removing extracellular Na+, as expected for Na+/Ba2+ exchange activity. The maximal velocity of Ba2+ accumulation was estimated to be 50% of that for Ca2+. When the monovalent cation ionophore gramicidin was used to equilibrate internal and external concentrations of Na+, Ba2+ influx was negligible in the absence of Na+ and increased to a maximum at 20-40 mM Na+. At higher Na+ concentrations, Ba2+ influx declined, presumably due to the competition between Na+ and Ba2+ for transport sites on the exchanger. Unlike Ca2+, Ba2+ did not appear to be taken up by intracellular organelles. Thus, 133Ba2+ uptake in ouabain-treated cells was not reduced by mitochondrial inhibitors such as-Cl-CCP or oligomycin-rotenone. Moreover, intracellular Ca2+ stores that had been depleted of Ca2+ by pretreatment of the cells with ionomycin (a Ca2+ ionophore) remained empty during a subsequent period of Ba2+ influx. Ca2+ uptake or release by intracellular organelles secondarily regulated exchange activity through alterations in [Ca2+]i. Exchange-mediated Ba2+ influx was inhibited when cytosolic [Ca2+] was reduced to 20 nM or less and was accelerated at cytosolic Ca2+ concentrations of 25-50 nM We conclude that (a) Ba2+ substitutes for Ca2+ as a transport substrate for the exchanger, (b) cytosolic Ba2+ does not appear to be sequestered by intracellular organelles, and (c) exchange-mediated Ba2+ influx is accelerated by low concentrations of cytosolic Ca2+.

Acceleration of sodium-calcium exchange activity during ATP-induced calcium release in transfected Chinese hamster ovary cells

The P2U purinergic agonist ATP (0.3 mM) elicited an increase in [Ca2+]i due to Ca2+ release from intracellular stores in transfected Chinese hamster ovary cells that express the bovine cardiac Na+/Ca2+ exchanger (CK1.4 cells). The following observations indicate that ATP-evoked Ca2+ release was accompanied by a Ca(2+)-dependent regulatory activation of Na+/Ca2+ exchange activity: Addition of extracellular Ca2+ (0.7 mM) 0-1 min after ATP evoked a dramatic rise in [Ca2+]i in Na(+)-free media (Li+ substitution) compared to Na(+)-containing media; no differences between Na(+)- and Li(+)-based media were observed with vector-transfected cells. In the presence of physiological concentrations of extracellular Na+ and Ca2+, the ATP-evoked rise in [Ca2+]i declined more rapidly in CK1.4 cells compared to control cells, but then attained a long-lived plateau of elevated [Ca2+]i which eventually came to exceed the declining [Ca2+]i values in control cells. ATP elicited a transient acceleration of exchange-mediated Ba2+ influx, consistent with regulatory activation of the Na+/Ca2+ exchanger. The acceleration of Ba2+ influx was not observed in vector-transfected control cells, or in CK1.4 cells in the absence of intracellular Na+ or when the Ca2+ content of the intracellular stores had been reduced by prior treatment with ionomycin. The protein kinase C activator phorbol 12-myristate 13-acetate attenuated the exchange-mediated rise in [Ca2+]i under Na(+)-free conditions, but did not inhibit the ATP-evoked stimulation of Ba2+ influx. The effects of PMA are therefore not due to inhibition of exchange activity, but probably reflect the influence of protein kinase C on other Ca2+ homeostatic mechanisms. We conclude that exchange activity is accelerated during ATP-evoked Ca2+ release from intracellular stores through regulatory activation by increased [Ca2+]i. In the absence of extracellular Ca2+, the stimulation of exchange activity is short-lived and follows the time course of the [Ca2+]i transient; in the presence of extracellular Ca2+, we suggest that the exchanger remains activated for a longer period of time, thereby stabilizing and prolonging the plateau phase of store-dependent Ca2+ entry.

Modulation of the platelet-derived-growth-factor-induced calcium signal by extracellular/intracellular pH in Syrian hamster embryo cells. Implications for the role of calcium in mitogenic signalling

Studies have been performed to understand the interactions and the role which intracellular calcium and intracellular pH have in mediating mitogen-stimulated cellular proliferation. Stimulation of Syrian hamster embryo (SHE) cells with the mitogen platelet-derived growth factor A/B (PDGF) results in intracellular acidification and capacitative calcium entry involving the intracellular release of calcium via the inositol trisphosphate gamma receptor calcium channel, followed by an extracellular influx of calcium through a dihydropyridine-sensitive plasma membrane calcium channel. Chronic extracellular/intracellular acidification results in the inactivation of both these calcium channels due to slowly reversible protein alterations. Paradoxically, transient intracellular acidification, like that following PDGF stimulation, could not stimulate the activation of either calcium channel. In addition, even though intracellular calcium fluxes by themselves could intiate intracellular acidification, loss of the PDGF-induced calcium signal did not result in the loss of the PDGF-induced transient intracellular acidification. Importantly with regard to the role intracellular calcium and pH have in mediating the mitogenic signal leading to cellular proliferation, chronic extracellular/intracellular acidification, which leads to a complete loss of the PDGF-induced calcium signal, did not result in the loss of PDGF-induced mitogenesis. These results indicate that the PDGF-induced calcium signal is not essential for PDGF-stimulated mitogenesis in Syrian hamster embryo cells. In contrast, blocking the PDGF-induced transient intracellular acidification completely blocks PDGF-induced mitogenesis, indicating that the mitogen-induced transient intracellular acidification, unlike the intracellular calcium ion signal, is indispensible for cellular proliferation in Syrian hamster embryo cells.

Pharmacologic characterization of refilling inositol 1,4,5-trisphosphate-sensitive Ca2+ stores in NG108-15 cells

Following mobilization with the inositol 1,4,5-trisphosphate (IP3)-generating agonist bradykinin, Ca2+ stores in neuroblastoma x glioma hybrid, NG108-15 cells require extracellular Ca2+ to refill. The process by which this store refills with Ca2+ was characterized by recording bradykinin-induced intracellular free Ca2+ concentration transients as an index of the degree of refilling of the store. Cyclopiazonic acid, a microsomal Ca2+ ATPase inhibitor, reversibly depleted intracellular Ca2+ stores in these cells, but did not recruit detectable Ca2+ influx, suggesting that these cells lack substantial capacitative Ca2+ entry. The paucity of voltage-sensitive Ca2+ channels in undifferentiated NG108-15 cells, suggested that a channel analogous to that proposed to mediate capacitative Ca2+ entry in nonexcitable cells might assist refilling IP3-sensitive Ca2+ stores in these cells. The possibility that compounds shown previously to inhibit capacitative Ca2+ entry in nonexcitable cells might inhibit the refilling of the IP3-sensitive store in NG108-15 cells was explored. The IP3-sensitive store was depleted by exposure to bradykinin, allowed to refill briefly in the presence of the test compound and then challenged again with bradykinin to evaluate the degree of refilling of the store. The imidazole derivatives, econazole (10 microM), L-651582 (10 microM) and SKF 96365 (20 microM), all completely blocked the bradykinin-induced Ca2+ response. Calmodulin antagonists, W-7 (100 microM) and trifluoperazine (10 microM), were also effective, although at concentrations well above those required to inhibit calmodulin. Because of the high concentrations required to inhibit bradykinin responses, the possibility that these agents might have additional effects was explored. Compounds were tested in a paradigm in which the store was preloaded with Ca2+ before treatment. All of these agents depleted, at least partially, the preloaded store. Econazole was the least effective of the compounds tested for releasing stores, although it was comparable to the other compounds for inhibition of refilling. Although NG108-15 cells refill intracellular Ca2+ stores by a plasmalemmal Ca2+ leak, this leak shares a pharmacology similar to the capacitative Ca2+ entry pathway described for nonexcitable cells.

Clotrimazole inhibits cell proliferation in vitro and in vivo

Cell proliferation is critically dependent on the regulated movement of ions across various cellular compartments. The antimycotic drug clotrimazole (CLT) has been shown to inhibit movement of Ca2+ and K+ across the plasma membrane. Our results show that CLT inhibits the rate of cell proliferation of normal and cancer cell lines in a reversible and dose-dependent manner in vitro. Moreover, CLT depletes the intracellular Ca2+ stores and prevents the rise in cytosolic Ca2+ that normally follows mitogenic stimulation. In mice with severe combined immunodeficiency disease (SCID) and inoculated intravenously with MM-RU human melanoma cells, daily subcutaneous injections of CLT induced a significant reduction in the number of lung metastases. Modulation of early ionic mitogenic signals and potent inhibition of cell proliferation both in vitro and in vivo are new and potentially useful clinical effects of CLT.

Influence of BAY k-8644, a calcium channel agonist, on the anticonvulsant activity of conventional anti-epileptics against electroconvulsions in mice

BAY k-8644, an agonist at the dihydropyridine binding site of the L-type voltage dependent calcium channel, at the dose of 5 mg/kg (s.c.) did not significantly affect the threshold for electroconvulsions, but impaired the protective efficacy of flunarizine (15 and 20 mg/kg, i.p.) in the electroconvulsive test. Interestingly, the calcium channel agonist (at 1 and 5 mg/kg) distinctly diminished the protection offered by conventional anti-epileptic drugs (carbamazepine, diphenylhydantoin and phenobarbital) against maximal electroshock-induced seizures in mice. A pharmacokinetic interaction does not seem to be involved in the effect of BAY k-8644, since total plasma levels of these anti-epileptics (measured by immunofluorescence) were not affected by the calcium channel agonist. The only anti-epileptic drug resistant to BAY k-8644 (up to 5 mg/kg) was valproate, whose ED50 (in mg/kg) was not changed in the presence of the calcium channel agonist. Further, BAY k-8644 (5 mg/kg) did not influence the flunarizine (a calcium channel blocker)-induced potentiation of the protective action of valproate against maximal electroshock-induced convulsions. The calcium channel agonist (5 mg/kg) reversed the flunarizine-induced augmentation of the anticonvulsive activity of carbamazepine. It may be concluded that carbamazepine, diphenylhydantoin and phenobarbital partially exert their anticonvulsive effects via blockade of calcium influx whilst valproate does not seem to. In this context, the flunarizine-induced potentiation of the anticonvulsive activity of valproate is probably independent of calcium channel blockade.

The pharmacological profile of cloned and stably expressed alpha 1b-adrenoceptor in CHO cells

Using Chinese hamster ovary (CHO) cells stably expressing the alpha 1B-adrenoceptor (CHO alpha 1B cells) as a model, we investigated whether the transfected cells that express alpha 1B subtype of adrenoceptor can show the pharmacologic characteristics as previously defined in native tissues. Radioligand binding studies with 2-[beta-(4-hydroxy-3-[125I]iodophenyl)ethylamino-methyl]tetralone ([125I]HEAT) in CHO alpha 1B cells showed the similar Ki values of the alpha 1-adrenoceptor selective drugs as previously observed in rat liver and spleen, and that pretreatment with chlorethylclonidine markedly inactivated the binding sites (94.7-98.6%). In CHO alpha 1B cells alpha 1-adrenoceptor agonists caused a dose-dependent increase in transients of cytosolic Ca2+ concentrations ([Ca2+]i), and the potency order of antagonists in inhibiting norepinephrine-induced [Ca2+]i response was similar to that observed in radioligand binding assays. In summary, the present study shows that the ligand binding property, the pharmacological characteristics and the intracellular transduction mechanisms of alpha 1B-adrenoceptors stably expressed in CHO cells appear to be the same as those defined in native tissues. Thus they can be a useful model system for further characterization of the receptor as well as for the development of specific ligands.

Chronic hypocalcemia of vitamin D deficiency leads to lower intracellular calcium concentrations in rat hepatocytes

Several lines of evidence indicate that calcium deficiency is associated with cellular defects in many tissues and organs. Owing to the large in vivo gradient between ionized extra- and intracellular Ca2+ concentrations ([Ca2+]i), it is generally recognized that the prevailing circulating Ca2+ does not significantly affect resting cytosolic Ca2+. To probe the consequences of hypocalcemia on [Ca2+]i, a model of chronic hypocalcemia secondary to vitamin D (D) deficiency was used. Hepatocytes were isolated from livers of hypocalcemic D-deficient, of normocalcemic D3-repleted, or of normal control rats presenting serum Ca2+ of 0.78 +/- 0.02, 1.24 +/- 0.03, or 1.25 +/- 0.01 mM, respectively (P < 0.0001). [Ca2+]i was measured in cell couplets using the fluorescent probe Fura-2. Hepatocytes of normocalcemic D3-repleted and of normal controls exhibited similar [Ca2+]i of 227 +/- 10 and 242 +/- 9 nM, respectively (NS), whereas those of hypocalcemic rats had significantly lower resting [Ca2+]i (172 +/- 10 nM; P < 0.0003). Stimulation of hepatocytes with the alpha 1-adrenoreceptor agonist phenylephrine illicited increases in cytosolic Ca2+ leading to similar [Ca2+]i and phosphorylase a (a Ca(2+)-dependent enzyme) activity in all groups but in contrast to normocalcemia, low extracellular Ca2+ was often accompanied by a rapid decay in the sustained phase of the [Ca2+]i response. When stimulated with the powerful hepatic mitogen epidermal growth factor (EGF), hepatocytes isolated from hypocalcemic rat livers responded with a blunted maximal [Ca2+]i of 237.6 +/- 18.7 compared with 605.2 +/- 89.9 nM (P < 0.0001) for their normal counterparts, while the EGF-mediated DNA synthesis response was reduced by 50% by the hypocalcemic condition (P < 0.03). Further studies on the possible mechanisms involved in the perturbed [Ca2+]i homeostasis associated with chronic hypocalcemia revealed the presence of an unchanged plasma membrane Ca2+ ATPase but of a significant decrease in agonist-stimulated Ca2+ entry as indicated using Mn2+ as surrogate ion (P < 0.03). Our data, thus indicate that, in rat hepatocytes, the in vivo calcium status significantly affects resting [Ca2+]i, and from this we raise the hypothesis that this lower than normal [Ca2+]i may be linked, in calcium disorders, to inappropriate cell responses mediated through the calcium signaling pathway as illustrated by the response to phenylephrine and EGF.

Signalling pathways as targets for anticancer drug development

Intracellular signalling pathways mediating the effects of oncogenes on cell growth and transformation offer novel targets for the development of anticancer drugs. With this approach, it may be sufficient to target a component of the signalling pathway activated by the oncogene rather than the oncogene product itself. In this review, the abilities of some antiproliferative drugs to inhibit signalling targets are considered. There are some anticancer drugs already in clinical trial that may act by inhibiting signalling targets, as well as drugs in preclinical development. Some problems that may be encountered in developing this new class of anticancer drugs are discussed.

Bradykinin B2- and 5-hydroxytryptamine (5-HT2)-receptor stimulated increases in intracellular calcium in cultured guinea-pig aortic smooth muscle cells

The effect of 5-hydroxytryptamine (5-HT) and bradykinin on intracellular free calcium concentration ([Ca2+]i) has been studied in cultured guinea-pig aortic smooth muscle cells loaded with the Ca(2+)-sensitive fluorescent dye fura-2. Bradykinin and 5-HT elicited rapid increases in [Ca2+]i with log EC50 values of -7.9 +/- 0.05 M (N = 3) and -6.1 +/- 0.1 M (N = 4), respectively. Both agonists stimulated the release of intracellular calcium (Ca2+ responses still present in the absence of extracellular calcium) and to a lesser extent Ca2+ influx (observed when extracellular calcium was re-applied to cells initially stimulated in nominally Ca(2+)-free buffer containing 0.1 mM EGTA). The B1-receptor antagonist des-Arg9,[Leu8]-bradykinin had no effect on calcium responses elicited 100 nM bradykinin, whereas the B2-receptor antagonist D-Arg[Hyp3,Thi5.8,D-Phe7]-bradykinin (10 microM) inhibited the response to 100 nM bradykinin by 43%. The bradykinin B2-receptor antagonist D-Arg[Hyp3,Thi5.8,D-Phe7]-bradykinin virtually abolished the response to 10 nM bradykinin (IC50 4 microM). No increase in [Ca2+]i was observed with the B1-receptor agonist des-Arg9-bradykinin (300 nM). The response to 5-HT (100 microM) was abolished by the 5-HT2 receptor antagonist ketanserin (IC50 = 2 +/- 0.4 nM; N = 3). These data suggest that in cultured guinea-pig aortic smooth muscle cells bradykinin B2- and 5-HT2-receptor activation stimulates the release of Ca2+ from intracellular stores and Ca2+ influx through Ca2+ entry pathways in the plasma membrane.

Inhibiting intracellular signalling as a strategy for cancer chemoprevention

The intracellular signalling pathways that mediate the effects of growth factors and oncogenes on cell growth and transformation offer potential targets for the development of chemopreventive agents that prevent the progression of premalignant cells to invasive cancer. Agents acting on signalling targets would be expected to be cytostatic rather than cytotoxic agents. A number of existing chemopreventive agents exhibit, among their properties, inhibition of intracellular signalling enzymes. It is possible that this activity accounts, at least in part, for their chemopreventive properties.

Increased intracellular Ca2+ signaling caused by the antitumor agent helenalin and its analogues

The antitumor sesquiterpene lactone helenalin, which is found in species of the plant genus Helenium, caused a marked potentiation of the increases in intracellular free Ca2+ concentration ([Ca2+]i) produced by mitogens such as vasopressin, bradykinin, and platelet-derived growth factor in Swiss mouse 3T3 fibroblasts. Removing external Ca2+ partly attenuated the increased [Ca2+]i responses caused by helenalin. The increased [Ca2+]i responses occurred at concentrations of helenalin that inhibited cell proliferation. At higher concentrations, helenalin inhibited the [Ca2+]i responses. No change in resting [Ca2+]i was caused by helenalin even at high concentrations. Other helenalin analogues also increased the [Ca2+]i response. Helenalin did not inhibit protein kinase C (PKC) and PKC appeared to play a minor role in the effects of helenalin on [Ca2+]i responses in intact cells. Studies with saponin-permeabilized HT-29 human colon carcinosarcoma cells indicated that helenalin caused an increased accumulation of Ca2+ into nonmitochondrial stores and that the potentiating effect of helenalin on mitogen-stimulated [Ca2+]i responses was due in part to an increase in the inositol-(1,4,5)-trisphosphate-mediated release of Ca2+ from these stores.

Intracellular calcium waves generated by Ins(1,4,5)P3-dependent mechanisms

Cellular oscillations of cytosolic free Ca2+ ([Ca2+]i) have been observed in many cell types in response to cell surface receptor agonists acting through inositol 1,4,5-trisphosphate (InsP3). In a number of cases where appropriate spatial and temporal resolution have been used to examine these [Ca2+]i oscillations, they have been found to be organized as repetitive waves of Ca2+ increase that propagate through the cytosol of individual cells. In some cases Ca2+ waves also occur as a single pass through stimulated cells. This review discusses the factors underlying the spatial organization of [Ca2+]i signals in the form of Ca2+ waves. In addition, potential mechanisms for the initiation and subsequent propagation of these Ca2+ waves are described.

Econazole inhibits thapsigargin-induced platelet calcium influx by mechanisms other than cytochrome P-450 inhibition

Cytochrome P-450 has been suggested as a mediator of the signal between depleted platelet calcium stores and an increase in plasma membrane permeability to calcium which follows depletion of the stores. This hypothesis is based on the observations that inhibitors of cytochrome P-450, such as the imidazole antifungal agents, also inhibit influx of a calcium surrogate (manganese) into calcium-depleted platelets. We tested the effects of econazole and of a cytochrome P-450 inhibitor, carbon monoxide (CO), on thapsigargin (TG)-induced platelet 45Ca2+ influx. TG specifically depletes internal calcium stores and activates store-regulated calcium influx. Econazole blocked 45Ca2+ influx when it was added before TG (IC50 11 microM). Econazole at a concentration (20 microM) that inhibited 83% of TG-induced calcium influx was not inhibitory to TG-induced calcium efflux from 45Ca(2+)-loaded platelets, and did not affect calcium fluxes in resting platelets. This econazole concentration was also inhibitory to calcium influx even when it was added after the stores had been calcium-depleted by EGTA and TG for 15 min and the signal to increase calcium influx had already been generated. Inhibition of cytochrome P-450 with CO bubbled through platelet suspensions did not change calcium influx in resting cells and potentiated TG-induced calcium influx (160% of control calcium accumulation at 20 min). This effect appeared to be concentration-dependent, such that a 5 min exposure to CO produced a greater influx potentiation than a 3 min exposure. These observations indicate that (1) cytochrome P-450 does not mediate store-regulated calcium influx, and (2) econazole probably inhibits store-regulated calcium influx by an alternative mechanism, such as interaction with plasma membrane calcium channels.

Vasodilator action of cromakalim on human mesenteric artery

Human mesenteric artery rings (1-3 mm diameter obtained from bowel resections), precontracted with KCl 80 mM, were relaxed by cromakalim (IC50 = 0.39 +/- 0.04 microM, maximum inhibition 69 +/- 2%). Cromakalim was more effective at inhibiting KCl 40 mM than KCl 80 mM. Cromakalim also inhibited contraction produced by noradrenaline, and was a more potent inhibitor of the second phase of the noradrenaline contraction than of the first phase.

Inhibition of human brain tumor cell growth by a receptor-operated Ca2+ channel blocker

SK&F 96365, a reported receptor-operated Ca2+ channel blocker, inhibited the growth of U-373 MG human astrocytoma and SK-N-MC human neuroblastoma cell lines in a dose-dependent manner. Carbachol and serum which act as growth factors for these cells induced a rapid, transient increase of intracellular Ca2+ concentration without a sustained increase. SK&F 96365 also exerted a significant inhibition of carbachol or serum-induced intracellular Ca2+ mobilization. These results suggest that SK&F 96365 is a potent inhibitor of brain tumor cell growth and that its effect may be mediated by the inhibition of agonist-induced intracellular Ca2+ mobilization.

Nonselective cation channels: physiological and pharmacological modulations of channel activity

Cation channels play a major role in fast and sustained cellular responses to hormones and neurotransmitters. They contribute to depolarization of the membrane and--in most cases--to an increase in the intracellular Ca2+ concentration. Nonselective cation channels presumably form a large family of diverse channels which are modulated by various extracellular and intracellular signals. Structure and regulation of ligand-operated and cyclic nucleotide-activated nonselective cation channels found in synapses and sensory receptor cells, respectively, are well documented; none of the structures of other cation channels are known. Except for ligand-operated and stretch-activated channels, G-proteins form the link between the involved receptors and signalling cascades stimulating nonselective cation channels. Observed in numerous cellular systems is hormonal activation of cation channels by hormones or neurotransmitters interacting with heptahelical receptors inducing a phosphoinositide breakdown (PI response); several pathways stimulated within the PI response may generate signals involved in cation channel activation. Pharmacological modifications of nonselective cation channels by inorganic and organic blockers are so far extremely limited; various blockers have been described but unfortunately lack high specificity for these channels.

Caffeine-induced release and reuptake of Ca2+ by Ca2+ stores in myocytes from guinea-pig urinary bladder

1. Voltage-clamped isolated smooth muscle cells from guinea-pig urinary bladder were studied with 3.6 mM extracellular Ca2+ at 36 degrees C. The fluorescence of the Ca(2+)-sensitive dye Indo-1 was used to monitor the cytosolic calcium concentration ([Ca2+]i) and its changes ([Ca2+]i transient). Fast application of caffeine (10 mM) to the cell was used to release the intracellular Ca2+ from a 'caffeine-sensitive Ca2+ store'. 2. At the holding potential -60 mV, a short (1 s) caffeine application increased [Ca2+]i within less than 1 s from the resting 118 +/- 22 nM to 1490 +/- 332 nM. Following the caffeine wash-out, [Ca2+]i fell from this peak to a subresting level of 47 +/- 12 nM, i.e. an 'undershoot' of [Ca2+]i occurred. Subsequent caffeine-induced [Ca2+]i transients had attenuated peaks suggesting that the caffeine-sensitive Ca2+ store had lost a part of the releasable Ca2+. 3. In the continuous presence of caffeine, [Ca2+]i decayed from its peak to control resting [Ca2+]i values. The wash-out of caffeine following prolonged (10-30 s) treatment also resulted in [Ca2+]i undershoot. Subsequent caffeine-induced [Ca2+]i transients were largely abolished as if the caffeine-sensitive Ca2+ store had lost a large part of releasable Ca2+. During the undershoot, hyperpolarization to -100 mV did not affect [Ca2+]i. In most cells studied, recovery of [Ca2+]i from the undershoot to the resting level required depolarizations inducing Ca2+ influx through L-type Ca2+ channels. 4. Block of plasmalemmal Ca(2+)-ATPase (PMCa) with extracellular La3+ (3 mM) did not modify the decay of the [Ca2+]i transients induced by depolarization or by a 1 s caffeine application suggesting that decay rate of both is not limited by PMCa rate. La3+ abolished the undershoot of [Ca2+]i. In the continuous presence of caffeine, La3+ largely prevented the decay of [Ca2+]i. 5. When the depolarizing steps from -60 to 0 mV (160 ms duration) were applied during the period of [Ca2+]i undershoot, the half-time of decay of the corresponding [Ca2+]i transients was up to three times faster than in control. Repetitive depolarizations restored the rate of decay and [Ca2+]i recovered to the resting value. Both processes recovered along a similar time course. 6. Application of the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX; 0.1 mM) or of 8-Br-cAMP (0.1 mM) did not mimic the above caffeine effects suggesting that stimulation of sarcoplasmic reticulum (SR) Ca(2+)-ATPase (SERCa) by cAMP-dependent phosphorylation is not the underlying mechanism.(ABSTRACT TRUNCATED AT 400 WORDS)

Control of plasma-membrane Ca2+ entry by the intracellular Ca2+ stores. Kinetic evidence for a short-lived mediator

We have studied the correlation between the degree of filling of the intracellular Ca2+ stores and the plasma-membrane permeability to Mn2+, a Ca2+ surrogate for plasma-membrane Ca2+ channels, in human neutrophils loaded with fura-2. Refilling of the stores of cells previously depleted of Ca2+ decreased the entry of Mn2+, but the magnitude of this effect depended on the refilling protocol. When refilling was allowed to proceed to steady-state levels by a 3 min incubation with different external Ca2+ concentrations (0.05-1 mM), almost complete inhibition of Mn2+ entry was observed at 40% of maximum refilling. In contrast, when different degrees of store refilling were attained by incubation with 1 mM-Ca2+ for short periods (10-40 s), inhibition of Mn2+ entry was smaller at comparable degrees of refilling. When quick refilling was allowed to proceed up to 40% (about 20 s at 37 degrees C) and then stopped at this level by removal of external Ca2+, the rate of Mn2+ uptake was high just after refilling and then decreased with time within the next few seconds (half-times approximately 7 s at 37 degrees C and approximately 20 s at 25 degrees C). We have proposed previously that the Ca2+ stores, when emptied of Ca2+, may generate a second messenger able to open the plasma-membrane Ca2+ channels by a mechanism involving cytochrome P-450. The results here are consistent with the existence of such a messenger and suggest that it is cleared from the cytoplasm with a half-time of about 7 s at 37 degrees C. In addition, inhibition of Mn2+ entry in cells with empty Ca2+ stores by cytochrome P-450 inhibitors showed a time lag consistent with the clearance kinetics proposed above.

Subtypes of α1-adrenoceptors in DDT1 MF-2 and BC3H-1 clonal cell lines

We examined which subtype(s) of alpha 1-adrenoceptors are expressed in the widely used DDT1 MF-2 and BC3H-1 cell lines. Pretreatment with chloroethylclonidine (CEC) inactivated 76-85% of the specific [125I]BE 2254 binding sites in membrane preparations from both cell lines. Competition with subtype-selective competitive antagonists showed primarily the alpha 1B subtype in both cell lines. However, in BC3H-1 cells 5-methyl-urapidil showed complex behavior suggesting that about half of the binding sites had a lower affinity. Chloroethylclonidine pretreatment eliminated [3H]inositol phosphate responses to norepinephrine in both cell lines. Measurement of intracellular Ca2+ with fura-2 in DDT1 MF-2 cells showed that norepinephrine induced a complex response involving both transient and sustained components. Chloroethylclonidine pretreatment blocked both responses, while chelation of extracellular Ca2+ left the transient response intact but eliminated the sustained component. These results support previous work that these cell lines contain alpha 1B-adrenoceptors linked to inositol phosphate formation and mobilization of intracellular Ca2+. However, these results show that alpha 1B-adrenoceptors can be linked to Ca2+ influx as well as intracellular mobilization, and support the existence of pharmacologically distinct alpha 1B variants.