Mechanical strain-induced Ca(2+) waves are propagated via ATP release and purinergic receptor activation

Purinergic regulation of neutrophil chemotaxis

Chemotaxis allows polymorphonuclear neutrophils (PMN) to rapidly reach infected and inflamed sites. However, excessive influx of PMN damages host tissues. Better knowledge of the mechanisms that control PMN chemotaxis may lead to improved treatments of inflammatory diseases. Recent findings suggest that ATP and adenosine are involved in PMN chemotaxis. Therefore, these purinergic signaling processes may be suitable targets for novel therapeutic approaches to ameliorate host tissue damage.

Mechanical loading stimulates ecto-ATPase activity in human tendon cells

Response to external stimuli such as mechanical signals is critical for normal function of cells, especially when subjected to repetitive motion. Tenocytes receive mechanical stimuli from the load-bearing matrix as tension, compression, and shear stress during tendon gliding. Overloading a tendon by high strain, shear, or repetitive motion can cause matrix damage. Injury may induce cytokine expression, matrix metalloproteinase (MMP) expression and activation resulting in loss of biomechanical properties. These changes may result in tendinosis or tendinopathy. Alternatively, an immediate effector molecule may exist that acts in a signal-dampening pathway. Adenosine 5'-triphosphate (ATP) is a candidate signal blocker of mechanical stimuli. ATP suppresses load-inducible inflammatory genes in human tendon cells in vitro. ATP and other extracellular nucleotide signaling are regulated efficiently by two distinct mechanisms: purinoceptors via specific receptor-ligand binding and ecto-nucleotidases via the hydrolysis of specific nucleotide substrates. ATP is released from tendon cells by mechanical loading or by uridine 5'-triphosphate (UTP) stimulation. We hypothesized that mechanical loading might stimulate ecto-ATPase activity. Human tendon cells of surface epitenon (TSC) and internal compartment (TIF) were cyclically stretched (1 Hz, 0.035 strain, 2 h) with or without ATP. Aliquots of the supernatant fluids were collected at various time points, and ATP concentration (ATP) was determined by a luciferin-luciferase bioluminescence assay. Total RNA was isolated from TSC and TIF (three patients) and mRNA expression for ecto-nucleotidase was analyzed by RT-PCR. Human tendon cells secreted ATP in vitro (0.5-1 nM). Exogenous ATP was hydrolyzed within minutes. Mechanical load stimulated ATPase activity. ATP was hydrolyzed in mechanically loaded cultures at a significantly greater rate compared to no load controls. Tenocytes (TSC and TIF) expressed ecto-nucleotidase mRNA (ENTPD3 and ENPP1, ENPP2). These data suggest that motion may release ATP from tendon cells in vivo, where ecto-ATPase may also be activated to hydrolyze ATP quickly. Ecto-ATPase may act as a co-modulator in ATP load-signal modulation by regulating the half-life of extracellular purine nucleotides. The extracellular ATP/ATPase system may be important for tendon homeostasis by protecting tendon cells from responding to excessive load signals and activating injurious pathways.

Effect of P2 receptor on the intracellular calcium increase by cancer cells in human umbilical vein endothelial cells

One of the important functions of vascular endothelial cells is as a barrier between blood and vascular tissue. This led us to speculate that cancer cells affect endothelial cells during metastasis. In the present study, we investigated the influence of human fibrosarcoma cells (HT-1080) on human umbilical vein endothelial cells (HUVEC), particularly intracellular calcium ion levels ([Ca2+]i), which are known to be an important intracellular signal transduction factor. HUVEC were treated with a fluorescent marker, and the fluorescence intensity of [Ca2+]i was then measured by phase contrast microscopic imaging. Extracellular adenosine triphosphate (ATP) release was measured using the chemiluminescence of luciferin-luciferase and a photon counting imaging system. HT-1080 (5x10(4) cells per dish) was found to increase [Ca2+]i in HUVEC. This [Ca2+]i rise was significantly reduced by U-73122 (phospholipase C inhibitor, 1 microM) and thapsigargin (calcium pump inhibitor, 1 microM). Interestingly, the [Ca2+]i rise in HUVEC was also significantly reduced by pyridoxalphosphare-6-azophenyl-2', 4'-disulfonic acid, a P2Y receptor antagonist (100 microM) and apyrase, a nucleotidase inhibitor (2 U/ml). In addition, we observed ATP release from HT-1080. These results suggest that [Ca2+]i in HUVEC was increased through the phospholipase C-IP3 pathway via ATP release from cancer cells. We previously reported that extracellular ATP increased [Ca2+]i and enhanced macromolecular permeability via the P2Y receptor. In tumor metastasis, cancer cells may exploit these regulatory mechanisms in the endothelial cell layer.

Loss of apical monocilia on collecting duct principal cells impairs ATP secretion across the apical cell surface and ATP-dependent and flow-induced calcium signals

Renal epithelial cells release ATP constitutively under basal conditions and release higher quantities of purine nucleotide in response to stimuli. ATP filtered at the glomerulus, secreted by epithelial cells along the nephron, and released serosally by macula densa cells for feedback signaling to afferent arterioles within the glomerulus has important physiological signaling roles within kidneys. In autosomal recessive polycystic kidney disease (ARPKD) mice and humans, collecting duct epithelial cells lack an apical central cilium or express dysfunctional proteins within that monocilium. Collecting duct principal cells derived from an Oak Ridge polycystic kidney (orpk ( Tg737 ) ) mouse model of ARPKD lack a well-formed apical central cilium, thought to be a sensory organelle. We compared these cells grown as polarized cell monolayers on permeable supports to the same cells where the apical monocilium was genetically rescued with the wild-type Tg737 gene that encodes Polaris, a protein essential to cilia formation. Constitutive ATP release under basal conditions was low and not different in mutant versus rescued monolayers. However, genetically rescued principal cell monolayers released ATP three- to fivefold more robustly in response to ionomycin. Principal cell monolayers with fully formed apical monocilia responded three- to fivefold greater to hypotonicity than mutant monolayers lacking monocilia. In support of the idea that monocilia are sensory organelles, intentionally harsh pipetting of medium directly onto the center of the monolayer induced ATP release in genetically rescued monolayers that possessed apical monocilia. Mechanical stimulation was much less effective, however, on mutant orpk collecting duct principal cell monolayers that lacked apical central monocilia. Our data also show that an increase in cytosolic free Ca(2+) primes the ATP pool that is released in response to mechanical stimuli. It also appears that hypotonic cell swelling and mechanical pipetting stimuli trigger release of a common ATP pool. Cilium-competent monolayers responded to flow with an increase in cell Ca(2+) derived from both extracellular and intracellular stores. This flow-induced Ca(2+) signal was less robust in cilium-deficient monolayers. Flow-induced Ca(2+) signals in both preparations were attenuated by extracellular gadolinium and by extracellular apyrase, an ATPase/ADPase. Taken together, these data suggest that apical monocilia are sensory organelles and that their presence in the apical membrane facilitates the formation of a mature ATP secretion apparatus responsive to chemical, osmotic, and mechanical stimuli. The cilium and autocrine ATP signaling appear to work in concert to control cell Ca(2+). Loss of a cilium-dedicated autocrine purinergic signaling system may be a critical underlying etiology for ARPKD and may lead to disinhibition and/or upregulation of multiple sodium (Na(+)) absorptive mechanisms and a resultant severe hypertensive phenotype in ARPKD and, possibly, other diseases.

Detection of immune danger signals by NALP3

The innate immune system in animals has been forged to detect microbes, coordinate symbiotic responses, and mount immune defenses against pathogens. Recently, innate immunity was shown to detect signals released by damaged cells or tissues such as uric acid or ATP. These danger signals were proposed to be important in promoting and regulating inflammation upon trauma or pathogen insults. The physiological relevance of these signals in the immune response and their mechanisms of action are still unclear. Recent findings suggest that some danger signals activate the NALP3 inflammasome, an innate immune complex that controls inflammatory caspases and IL-1 activation.

Extracellular ATP: an unexpected role as a signaler in plants

ATP and other nucleoside triphosphates not only drive energy-dependent reactions inside cells, but can also function outside the plasma membrane in the extracellular matrix, where they function as agonists that can induce diverse physiological responses without being hydrolyzed. This external role of ATP is well established in animal cells but only recently has it become apparent that extracellular ATP (eATP) can also function as a signaling agent in plants. Recent data have shown that eATP and other nucleotides can induce an increase in the cytosolic Ca(2+) concentration and diverse downstream changes that influence plant growth and defense responses. Ectoapyrase enzymes that regulate the eATP concentration also have an impact on plant growth. These results beg the question of whether there is a receptor that can bind to eATP and transduce this into signaling changes. Answering this will be key to understanding how eATP and ectoapyrases influence plant growth and development.

Cell mechanics: integrating cell responses to mechanical stimuli

Forces are increasingly recognized as major regulators of cell structure and function, and the mechanical properties of cells are essential to the mechanisms by which cells sense forces, transmit them to the cell interior or to other cells, and transduce them into chemical signals that impact a spectrum of cellular responses. Comparison of the mechanical properties of intact cells with those of the purified cytoskeletal biopolymers that are thought to dominate their elasticity reveal the extent to which the studies of purified systems can account for the mechanical properties of the much more heterogeneous and complex cell. This review summarizes selected aspects of current work on cell mechanics with an emphasis on the structures that are activated in cell-cell contacts, that regulate ion flow across the plasma membrane, and that may sense fluid flow that produces low levels of shear stress.

Purinergic control of apical plasma membrane PI(4,5)P2 levels sets ENaC activity in principal cells

Activity of the epithelial sodium channel (ENaC) is limiting for Na(+) reabsorption at the distal nephron. Phosphoinositides, such as phosphatidylinositol 4,5-biphosphate [PI(4,5)P(2)] modulate the activity of this channel. Activation of purinergic receptors triggers multiple events, including activation of PKC and PLC, with the latter depleting plasma membrane PI(4,5)P(2). Here, we investigate regulation of ENaC in renal principal cells by purinergic receptors via PLC and PI(4,5)P(2). Purinergic signaling rapidly decreases ENaC open probability and apical membrane PI(4,5)P(2) levels with similar time courses. Moreover, inhibiting purinergic signaling with suramin rescues ENaC activity. The PLC inhibitor U73122, but not U73343, its inactive analog, recapitulates the action of suramin. In contrast, modulating PKC signaling failed to affect purinergic regulation of ENaC. Unexpectedly, inhibiting either purinergic receptors or PLC in resting cells dramatically increased ENaC activity above basal levels, indicating tonic activation of purinergic signaling in these polarized renal epithelial cells. Increased ENaC activity was associated with elevation of apical membrane PI(4,5)P(2) levels. Subsequent treatment with ATP in the presence of inhibited purinergic signaling failed to decrease ENaC activity and apical membrane PI(4,5)P(2) levels. Dwell-time analysis reveals that depletion of PI(4,5)P(2) forces ENaC toward a closed state. In contrast, increasing PI(4,5)P(2) levels above basal values locks the channel in an open state interrupted by brief closings. Thus our results suggest that purinergic control of apical membrane PI(4,5)P(2) levels is a major regulator of ENaC activity in renal epithelial cells.

Cyclic compression of chondrocytes modulates a purinergic calcium signalling pathway in a strain rate- and frequency-dependent manner

Mechanical loading modulates cartilage homeostasis through the control of matrix synthesis and catabolism. However, the mechanotransduction pathways through which chondrocytes detect different loading conditions remain unclear. The present study investigated the influence of cyclic compression on intracellular Ca2+ signalling using the well-characterised chondrocyte-agarose model. Cells labelled with Fluo4 were visualised using confocal microscopy following a period of 10 cycles of compression between 0% and 10% strain. In unstrained agarose constructs, not subjected to cyclic compression, a subpopulation of approximately 45% of chondrocytes exhibited spontaneous global Ca2+ transients with mean transient rise and fall times of 19.4 and 29.4 sec, respectively. Cyclic compression modulated global Ca2+ signalling by increasing the percentage of cells exhibiting Ca2+ transients (population modulation) and/or reducing the rise and fall times of these transients (transient shape modulation). The frequency and strain rate of compression differentially modulated these Ca2+ signalling characteristics providing a potential mechanism through which chondrocytes may distinguish between different loading conditions. Treatment with apyrase, gadolinium and the P2 receptor blockers, suramin and basilen blue, significantly reduced the percentage of cells exhibiting Ca2+ transients following cyclic compression, such that the mechanically induced upregulation of Ca2+ signalling was completely abolished. Thus cyclic compression appears to activate a purinergic pathway involving the release of ATP followed by the activation of P2 receptors causing a combination of extracellular Ca2+ influx and intracellular Ca2+ release. Knowledge of this fundamental cartilage mechanotransduction pathway may lead to improved therapeutic strategies for the treatment of cartilage damage and disease.

Plant responses to extracellular nucleotides: Cellular processes and biological effects

Higher plants exhibit cellular responsiveness to the exogenous application of purine nucleotides in a manner consistent with a cell–cell signaling function for these molecules. Like animals, plants respond to extracellular ATP, ADP, and stable analogues (e.g., ATPγS and ADPβS) by increasing the cytoplasmic concentration of calcium. Agonist substrate specificity and concentration dependency suggest receptor mediation of these events, and, although the identity of the plant receptor is currently unknown, pharmacological analysis points to the involvement of a plasma membrane-localized calcium channel. Extracellular ATP can also induce the production of reactive oxygen species and stimulate an increase in the mRNA levels of a number of stress- and calcium-regulated genes, suggesting a role for nucleotide-based signaling in plant wound and defense responses. Furthermore, the growth and development of plants can also be altered by the application of external ATP. Recent studies are only beginning to uncover the complexities of plant signaling networks activated in response to extracellular ATP and how these might interact to affect plant physiological processes.

Glutamate acts at NMDA receptors on fresh bovine and on cultured human retinal pigment epithelial cells to trigger release of ATP

The photoreceptors lie between the inner retina and the retinal pigment epithelium (RPE). The release of glutamate by the phototoreceptors can signal changes in light levels to inner retinal neurons, but the role of glutamate in communicating with the RPE is unknown. Since RPE cells are known to release ATP, we asked whether glutamate could trigger ATP release from RPE cells and whether this altered cell signalling. Stimulation of the apical face of fresh bovine RPE eyecups with 100 mum NMDA increased ATP levels more than threefold, indicating that both receptors for NMDA and release of ATP occurred across the apical membrane of fresh RPE cells. NMDA increased ATP levels bathing cultured human ARPE-19 cells more than twofold, with NMDA receptor inhibitors MK-801 and d-AP5 preventing this release. Blocking the glycine site of the NMDA receptor with 5,7-dichlorokynurenic acid prevented ATP release from ARPE-19 cells. Release was also blocked by channel blocker NPPB and Ca(2+) chelator BAPTA, but not by cystic fibrosis transmembrane conductance regulator (CFTR) blocker glibenclamide or vesicular release inhibitor brefeldin A. Glutamate produced a dose-dependent release of ATP from ARPE-19 cells that was substantially inhibited by MK-801. NMDA triggered a rise in cell Ca(2+) that was blocked by MK-801, by the ATPase apyrase, by the P2Y(1) receptor antagonist MRS2179 and by depletion of intracellular Ca(2+) stores with thapsigargin. These results suggest that glutamate stimulates NMDA receptors on the apical membrane of RPE cells to release ATP. This secondary release can amplify the glutaminergic signal by increasing Ca(2+) inside RPE cells, and might activate Ca(2+)-dependent conductances. The interplay between glutaminergic and purinergic systems may thus be important for light-dependent interactions between photoreceptors and the RPE.

Spontaneous oscillation and mechanically induced calcium waves in chondrocytes

The characteristics of spontaneous calcium (Ca(2+)) oscillation and mechanically induced Ca(2+) waves in articular chondrocytes were studied. In some, but not all, chondrocytes in sliced cartilage and primary cultures, we observed spontaneous oscillation of intracellular Ca(2+) that never spread to adjacent cells. In contrast, a mechanical stimulus to a single cell by touching with a glass rod induced an increase of intracellular Ca(2+) that spread to neighboring cells in a wave-like manner, even though there was no physical contact between the cells. This indicated the release of some paracrine factor from the mechanically stimulated cells. Application of ultrasonic vibration also induced an oscillation of intracellular Ca(2+). The application of a uridine 5'-triphosphate (UTP), UTP, induced a transient increase in intracellular Ca(2+) and the release of adenosine 5'-triphosphate (ATP) in cultured chondrocytes. A P2 receptor antagonist (suramin) and blockers of Cl(-) channels, niflumic acid and 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS), reduced the UTP-induced ATP release. The results indicated that Cl(-) channels were involved in the extracellular release of ATP following mechanical or P2Y receptor stimulation. Thus, ATP stimulation of P2Y receptors elicits an increase in intracellular Ca(2+), triggering further release of ATP from adjacent cells, thereby expanding the Ca(2+) wave in chondrocytes.

Transduction mechanisms in airway sensory nerves

The induction of action potentials in airway sensory nerves relies on events leading to the opening of cation channels in the nerve terminal membrane and subsequent membrane depolarization. If the membrane depolarization is of sufficient rate and amplitude, action potential initiation will occur. The action potentials are then conducted to the central nervous system, leading to the initiation of various sensations and cardiorespiratory reflexes. Triggering events in airway sensory nerves include mechanical perturbation, inflammatory mediators, pH, temperature, and osmolarity acting through a variety of ionotropic and metabotropic receptors. Action potential initiation can be modulated (positively or negatively) through independent mechanisms caused mainly by autacoids and other metabotropic receptor ligands. Finally, gene expression of sensory nerves can be altered in adult mammals. This neuroplasticity can change the function of sensory nerves and likely involve both neurotrophin and use-dependent mechanisms. Here we provide a brief overview of some of the transduction mechanisms underlying these events.

Chondrocyte response to cyclic hydrostatic pressure in alginate versus pellet culture

Cells are often cultured at high density (e.g., confluent monolayer and as pellets) to promote chondrogenic differentiation and to maintain the chondrocyte phenotype. They are also frequently suspended in hydrogels such as agarose or alginate for the same purposes. These culture techniques differ markedly with respect to frequency of direct contact between cells and overall intercellular spacing. Because these factors may significantly affect mechanotransduction, the purpose of this study was to determine if the response of articular chondrocytes to cyclic hydrostatic pressure would depend on the culture condition. Primary articular chondrocytes from young and mature pigs were cultured either as pellets or suspended in alginate beads. Both groups were exposed to dynamic hydrostatic pressure (4 MPa, 1 Hz, 5400 cycles per day) for 7 days. Cell proliferation was unaffected by pressure, but pressurized chondrocytes in pellet culture had significantly greater sGAG content and incorporated [3H]proline at a higher rate than nonpressurized controls. Electron microscopy revealed a fibrous extracellular matrix (ECM) surrounding pellets, but not cells in alginate. In addition, expression of Connexin 43 (Cx43) mRNA was slightly lower in alginate than in pellet cultures and was not significantly altered by loading. Thus, metabolic response of chondrocytes to dynamic hydrostatic pressure was affected by culture technique; chondrocytes cultured as pellets exhibited the classical anabolic response to dynamic hydrostatic pressure, but those in alginate did not. Although cell-ECM interaction could be important, the differential response is not likely attributable to differential expression of Cx43 mRNA.

Changes in galanin immunoreactivity in rat micturition reflex pathways after cyclophosphamide-induced cystitis

Alterations in the expression of the neuropeptide, galanin, were examined in micturition reflex pathways of rat after cyclophosphamide (CYP)-induced cystitis of variable duration: acute (4 h), intermediate (48 h), or chronic (10 days). In control animals, galanin expression was present in specific regions of the gray matter in the rostral lumbar and caudal lumbosacral spinal cord, including: (1) the dorsal commissure (DCM); (2) superficial dorsal horn; (3) the regions of the intermediolateral cell column (L1-L2) and the sacral parasympathetic nucleus (SPN, L6-S1); and (4) the lateral collateral pathway (LCP) in lumbosacral spinal segments. Densitometry analysis demonstrated significant decreases (P< or =0.01) in galanin immunoreactivity (IR) in these regions of the L1-S1 spinal cord after acute or intermediate CYP-induced cystitis. In contrast, increases (P< or =0.01) in galanin-IR were observed in the DCM, SPN, or LCP regions in the L6-S1 spinal segments in rats with chronic cystitis. No changes in the number of galanin-immunoreactive cells were observed in the L1-S1 dorsal root ganglia (DRG) after CYP-induced cystitis of any duration. A small percentage of bladder afferent cells (Fast-blue-labeled) in the DRG expressed galanin-IR in control rats; this was not altered with cystitis. Galanin-IR was observed encircling DRG cells after chronic cystitis. These changes may contribute to urinary bladder dysfunction, altered sensation, and referred somatic hyperalgesia after cystitis.

Feed forward cycle of hypotonic stress-induced ATP release, purinergic receptor activation, and growth stimulation of prostate cancer cells

ATP is released in many cell types upon mechanical strain, the physiological function of extracellular ATP is largely unknown, however. Here we report that ATP released upon hypotonic stress stimulated prostate cancer cell proliferation, activated purinergic receptors, increased intracellular [Ca(2+)](i), and initiated downstream signaling cascades that involved MAPKs ERK1/2 and p38 as well as phosphatidylinositol 3-kinase (PI3K). MAPK activation, the calcium response as well as induction of cell proliferation upon hypotonic stress were inhibited by preincubation with the ATP scavenger apyrase, indicating that hypotonic stress-induced signaling pathways are elicited by released ATP. Hypotonic stress increased prostaglandin E(2) (PGE(2)) synthesis. Consequently, ATP release was inhibited by antagonists of PI3K (LY294002 and wortmannin), phospholipase A(2) (methyl arachidonyl fluorophosphonate (MAFP)), cyclooxygenase-2 (COX-2) (indomethacin, etodolac, NS398) and 5,8,11,14-eicosatetraynoic acid (ETYA), which are involved in arachidonic acid metabolism. Furthermore, ATP release was abolished in the presence of the adenylate cyclase (AC) inhibitor MDL-12,330A, indicating regulation of ATP-release by cAMP. The hypotonic stress-induced ATP release was significantly blunted when the ATP-mediated signal transduction cascade was inhibited on different levels, i.e. purinergic receptors were blocked by suramin and pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS), the Ca(2+) response was inhibited upon chelation of intracellular Ca(2+) by 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA), and ERK1,2 as well as p38 were inhibited by UO126 and SB203580, respectively. In summary our data demonstrate that hypotonic stress initiates a feed forward cycle of ATP release and purinergic receptor signaling resulting in proliferation of prostate cancer cells.

Activation of chondrocytes calcium signalling by dynamic compression is independent of number of cycles

Mechanical loading is necessary for the development and maintenance of healthy articular cartilage through the control of extracellular matrix synthesis and catabolism. However, the underlying process of chondrocyte mechanotransduction remains unclear. This study examined the influence of cyclic compression on intracellular calcium (Ca(2+)) signalling within isolated articular chondrocytes cultured in agarose constructs. A validated experimental system was developed for applying controlled cyclic cell deformation. Cell-agarose constructs were subjected to 1Hz cyclic compression between 0 and 10% gross strain for 1, 10, 100 or 300 cycles. The cells were subsequently visualised for 300s in the unstrained state using confocal microscopy and the Ca(2+) indicator, Fluo-4 AM. Within unloaded control constructs, a sub-population of approximately 50% of chondrocytes exhibited characteristic spontaneous Ca(2+) transients each lasting approximately 40-60s. Cyclic compression, for only 1 cycle, significantly up-regulated the percentage of cells exhibiting Ca(2+) transients in the subsequent 5min period (p<0.05). Increasing the number of cycles to 10 or 100 had no additional effect. The up-regulated Ca(2+) signalling was maintained for up to 5min before returning to basal levels. By contrast, 300 cycles were followed by Ca(2+) signalling that was not significantly different from that in unloaded controls. However, this response was shown to be due to the increased time following the start of compression. In conclusion, this study indicates that chondrocyte Ca(2+) signalling is stimulated by dynamic compression, probably mediated by cyclic cell deformation. The overall response appears to be independent of the number of cycles or duration of cyclic compression. The sustained up-regulation of Ca(2+) signalling after 1, 10 or 100 cycles suggests the involvement of an autocrine-paracrine signalling mechanism. Furthermore, the reduced response following 300 cycles indicates a possible receptor desensitisation mechanism. Therefore, Ca(2+) signalling may be part of a mechanotransduction pathway through which chondrocyte populations can modulate their metabolic activity in response to changing mechanical stimuli.

Distribution and fate of cocaine- and amphetamine-regulated transcript peptide (CARTp)-expressing cells in rat urinary bladder: a developmental study

We examined the distribution and fate of cocaine- and amphetamine-regulated transcript peptide (CARTp)(55-102)-immunoreactive (IR) structures in the neonatal and adult rat urinary bladder. Double-labeling studies examining CARTp with tyrosine hydroxylase (TH), neuronal nitric oxide synthase (nNOS), or choline acetyltransferase (ChAT) were performed in wholemounts of urothelium or detrusor or cryostat sections of the bladder. In younger animals (postnatal day [P]1, P3), CARTp-IR cell bodies in detrusor smooth muscle were observed in large clusters ( approximately 100 cells/cluster) at the ureteral insertion and along thick bundles of nerve fibers at the bladder base. The total number of CARTp-IR cells was significantly reduced (by five-fold) at P14, and this reduced number persisted into adulthood. The decrease in the number of CARTp-expressing cells was complemented with positive staining for cleaved caspase-3, suggesting that apoptosis contributed to this decrease. At birth (P1), all CARTp-IR cells expressed the neuronal marker Hu. After birth, CARTp was expressed by some neurons (CARTp-IR, Hu-IR) that represent intramural ganglion cells and by cells that lacked a neuronal phenotype (CARTp-IR, Hu-) but did express TH. Neither of these cell populations expressed ChAT immunoreactivity in adult bladder. These cells (CARTp-IR, Hu-, TH-IR) may represent paraganglion or small intensely fluorescent (SIF) cells. The percentage of colocalization of CARTp-IR and nNOS or TH was dependent on postnatal age and showed an inverse relationship. At P1, 67.1 % of CARTp-IR cells expressed nNOS immunoreactivity. Decreased colocalization was observed with increasing postnatal age. In contrast, 19.5% of CARTp-IR cells expressed TH at P1, but colocalization increased with postnatal age. The suburothelial plexus lacked CARTp-IR nerve fibers until P14, when nerve fibers with varicosities were observed in the urethra and bladder neck region. In summary, we demonstrate 1) a decrease in the number of CARTp-IR cells in rat detrusor in early postnatal development; 2) apoptotic events in the bladder during early postnatal development; 3) rostral migration of CARTp-IR cells from the ureteral insertion toward the bladder body during postnatal development; 4) the presence of different populations of CARTp-IR cells, some with and others without a neuronal phenotype; and (5) age-dependent changes in chemical coding of CARTp-IR cells with postnatal development. This study demonstrates that CARTp-IR intramural ganglia and CARTp-IR paraganglion or SIF cells exist in the postnatal and adult rat bladder, although the role of these cell types remains to be determined.

Ligament cells stretch-adapted on a microgrooved substrate increase intercellular communication in response to a mechanical stimulus

An in vitro model was used to investigate the effect of mechanical stimuli on adaptation to load and calcium signaling in aligned medial collateral ligament cells (MCL). This model used a patterned silicone membrane to align the cells parallel with the direction of the microgrooves. Alignment created an architecture that simulated a degree of cell orientation in native ligament tissue. It was hypothesized that aligned ligament cells would be more efficient at calcium wave propagation than cells that were randomly oriented. It was further hypothesized that calcium wave propagation would be greater among cells that were both aligned and subjected to mechanical stretch compared to cells that were aligned but not stretched. Rat MCL cells were loaded with Fura-2AM, a calcium-binding dye, and mechanically indented using a micropipette tip. A ratio-imaging fluorescence technique was used to quantitate the calcium (Ca2+) response. It was concluded that stretching ligament cells prior to stimulation increased their sensitivity to load and their ability to propagate a calcium wave. However, the ability of aligned cells to propagate this wave was not significantly different when compared to nonaligned cells. Treatment of cultures with inhibitors such as apyrase and suramin significantly reduced the number of cells recruited in the calcium response. Hence, it was concluded that ATP released from mechanically stimulated cells was a principal mediator responsible for the rise in intracellular calcium in ligament cells. Further, purinoceptor activation may amplify the signal to alert and recruit more cells in a response to mechanical stimulation.

Mechanisms of mechanotransduction by specialized low-threshold mechanoreceptors in the guinea pig rectum

The guinea pig rectum, but not the colon, is innervated by a specialized class of distension-sensitive mechanoreceptors that have transduction sites corresponding to rectal intraganglionic laminar endings (rIGLEs). Rectal mechanoreceptors recorded in vitro had low threshold to circumferential stretch, adapted slowly, and could respond within 2 ms to mechanical stimulation by a piezo-electric probe. Antagonists to ionotropic N-methyl-D-aspartate (NMDA; CGS 19755, memantine) and non-NMDA (6,7-dinitroquinoxaline-2,3-dione) glutamate receptors did not affect mechanotransduction. In normal Krebs solution, the P2X purinoreceptor agonist alpha,beta-methylene ATP reduced mechanoreceptor firing evoked by distension but simultaneously relaxed circular smooth muscle and inhibited stretch-induced contractions. Neither ATP nor alpha,beta-methylene ATP affected mechanotransduction when transduction sites were directly compressed with von Frey hairs. The P2 purinoreceptor antagonist pyridoxal phosphate-6-azophenyl-2',4'-disulfonic acid did not affect stretch-induced firing but reduced the inhibitory effect of alpha,beta-methylene ATP on stretch-induced firing. Under isometric conditions, blocking synaptic transmission in Ca2+-free solution reduced stretch-evoked firing but not when basal tension was restored to control levels. Under isotonic condition, Ca2+-free solution did not significantly affect load-evoked firing. The blockers of mechanogated and/or transient receptor potential channels, benzamil, Gd3+, SKF 96365, and ruthenium red inhibited stretch-induced firing but, in parallel, significantly reduced stretch-induced contractions. Benzamil and SKF 96365 were able to inhibit mechanotransduction when transduction sites were compressed with von Frey hairs. The results show that mechanotransduction is rapid but does not depend on fast exocytotic release of mediators. It is likely that stretch-activated ion channels on rIGLEs are involved in direct, physical mechanotransduction by rectal low-threshold mechanoreceptors.

Leukotriene and purinergic receptors are involved in the hyperpolarizing effect of glucagon in liver cells

The pancreatic hormone glucagon hyperpolarizes the liver cell membrane. In the present study, we investigated the cellular signalling pathway of glucagon-induced hyperpolarization of liver cells by using the conventional microelectrode method. The membrane potential was recorded in superficial liver cells of superfused mouse liver slices. In the presence of the K+ channel blockers tetraethylammonium (TEA, 1 mmol/l) and Ba2+ (BaCl2, 5 mmol/l) and the blocker of the Na+/K+ ATPase, ouabain (1 mmol/l), no glucagon-induced hyperpolarization was observed confirming previous findings. The hyperpolarizing effect of glucagon was abolished by the leukotriene B4 receptor antagonist CP 195543 (0.1 mmol/l) and the purinergic receptor antagonist PPADS (5 micromol/l). ATPgammaS (10 micromol/l), a non-hydrolyzable ATP analogue, induced a hyperpolarization of the liver cell membrane similar to glucagon. U 73122 (1 micromol/l), a blocker of phospholipase C, prevented both the glucagon- and ATPgammaS-induced hyperpolarization. These findings suggest that glucagon affects the hepatic membrane potential partly by inducing the formation and release of leukotrienes and release of ATP acting on purinergic receptors of the liver cell membrane.

Do rat cardiac myocytes release ATP on contraction?

ATP is released by numerous cell types in response to mechanical strain. It then acts as a paracrine or autocrine signaling molecule, inducing a variety of biological responses. In this work, we addressed the question whether mechanical force acting on the membranes of contracting cardiomyocytes during periodic longitudinal shortening can stimulate the release of ATP. Electrically stimulated isolated adult rat cardiomyocytes as well as spontaneously contracting mouse cardiomyocytes derived from embryonic stem (ES) cells were assayed for ATP release with the use of luciferase and a sensitive charge-coupled device camera. Sensitivity of soluble luciferase in the supernatant of cardiomyocytes was 100 nM ATP, which is approximately 10-fold below the EC(50) values for most purinergic receptors expressed in the heart (1.5-20 microM). Light intensities were not different between resting or contracting adult rat cardiomyocytes. Similar results were obtained with ES-cell-derived contracting mouse cardiomyocytes. ATP release was measurable only from obviously damaged or permeabilized cells. To increase selectivity and sensitivity of ATP detection we have targeted a recombinant luciferase to the sarcolemmal membrane using a wheat germ agglutinin-IgG linker. Contraction of labeled adult rat cardiomyocytes was not associated with measurable bioluminescence. However, when human umbilical vein endothelial cells were targeted with membrane-bound luciferase, shear stress-induced ATP release could be clearly detected, demonstrating the sensitivity of the detection method. In the present study, we did not detect ATP release from contracting cardiomyocytes on the single cell level, despite adequate sensitivity of the detection system. Thus deformation of the contracting cardiomyocyte is not a key stimulus for the release of cellular ATP.

ATP-conducting maxi-anion channel: a new player in stress-sensory transduction

The regulated release of ATP is a fundamental process in cell-to-cell signaling. The electrogenic translocation of ATP via an anion channel has been suggested as one possible mechanism of the release. In this review, we survey possible candidate channels for this pathway. The maxi-anion channel characterized by an exceedingly large unitary conductance has been a stray channel with regard to its function. A newly discovered property, its ATP conductivity and its activation in response to stress signals, indicates that this channel has a central role in stress-sensory transduction for cell volume regulation and tubuloglomerular feedback.

Cell swelling-induced ATP release is tightly dependent on intracellular calcium elevations

Mechanical stresses release ATP from a variety of cells by a poorly defined mechanism(s). Using custom-designed flow-through chambers, we investigated the kinetics of cell swelling-induced ATP secretion, cell volume and intracellular calcium changes in epithelial A549 and 16HBE14o- cells, and NIH/3T3 fibroblasts. Fifty per cent hypotonic shock triggered transient ATP release from cell confluent monolayers, which consistently peaked at around 1 min 45 s for A549 and NIH/3T3, and at 3 min for 16HBE14o- cells, then declined to baseline within the next 15 min. Whereas the release time course had a similar pattern for the three cell types, the peak rates differed significantly (294 +/- 67, 70 +/- 22 and 17 +/- 2.8 pmol min(-1) (10(6) cells)(-1), for A549, 16HBE14o- and NIH/3T3, respectively). The concomitant volume changes of substrate-attached cells were analysed by a 3-dimensional cell shape reconstruction method based on images acquired from two perpendicular directions. The three cell types swelled at a similar rate, reaching maximal expansion in 1 min 45 s, but differed in the duration of the volume plateau and regulatory volume decrease (RVD). These experiments revealed that ATP release does not correlate with either cell volume expansion and the expected activation of stretch-sensitive channels, or with the activation of volume-sensitive, 5-nitro-2-(3-phenylpropylamino) benzoic acid-inhibitable anion channels during RVD. By contrast, ATP release was tightly synchronized, in all three cell types, with cytosolic calcium elevations. Furthermore, loading A549 cells with the calcium chelator BAPTA significantly diminished ATP release (71% inhibition of the peak rate), while the calcium ionophore ionomycin triggered ATP release in the absence of cell swelling. Lowering the temperature to 10 degrees C almost completely abolished A549 cell swelling-induced ATP release (95% inhibition of the peak rate). These results strongly suggest that calcium-dependent exocytosis plays a major role in mechanosensitive ATP release.

Nucleotide-mediated mucin secretion from differentiated human bronchial epithelial cells

Most current cell-based models for examining the regulation of mucin secretion demonstrate low signal-to-noise ratios, making experimental manipulation and data interpretation difficult. Using adenosine triphosphate (ATP) as a mucin secretagogue, we have developed a model of agonist-induced mucin secretion in differentiated human bronchial epithelial cells. Mucin secretory signals were estimated using enzyme-linked lectin assay, and typical signals of 300-400% of baseline were observed in response to a 30-min exposure to ATP (100 microM). ATP and uridine triphosphate equipotently stimulated mucin secretion consistent with mediation via P2Y2 receptor activation. Suramin and AR-C118925XX, a competitive P2Y2 receptor antagonist, inhibited adenosine 5'-o-(3-thiotriphosphate) (ATP-gammaS)-induced mucin secretion. A selective Gq G-protein antagonist (GP-ANT)-2A completely abrogated ATP-gammaS-induced mucin secretion. Pertussis toxin and the G(i/o)-specific, GP-ANT-2, had no effect. The phospholipase C inhibitor, D609, and the protein kinase C inhibitor, calphostin C, substantially inhibited ATP-gammaS-induced mucin secretion. Phorbol myristate acetate also stimulated mucin secretion in a calphostin C-sensitive manner. ATP-gammaS-induced mucin secretion was inhibited by the Ca2+ chelator, 1,2-bis(o-aminophenoxy) ethane-N,N,N',N'-tetra-acetic acid tetra (acetoxymethyl) ester. Ionomycin and thapsigargin both stimulated mucin secretion. Our data are broadly consistent with known G-protein-coupling and downstream signaling events associated with the P2Y2 receptor. The exceptional signal-to-noise ratios obtained using this model have permitted clear evaluation of the involvement of these mechanisms in agonist-induced mucin secretion from differentiated human bronchial epithelial cells.

Localized transient increases in endothelial cell Ca2+ in arterioles in situ: implications for coordination of vascular function

Intracellular Ca2+ transients were identified in endothelial cells (ECs) in intact blood-perfused arterioles. ECs in cremaster muscle arterioles (diameter approximately 45 microm) in anesthetized mice were loaded with the Ca2+ indicator fluo 4-AM by intraluminal perfusion, after which blood flow was reestablished. Confocal microscopy was used to visualize Ca2+ as a function of fluo-4 intensity in real time. Separate sets of experiments were performed under the following conditions: control, ischemia, during inhibition of P(2x) or P(1) purinoreceptors, and with the application of exogenous adenosine. In controls, spontaneous EC Ca2+ transients displayed a wide range of activity frequency (1-32 events/min) and about one-third of these transient events were synchronized between adjacent ECs. The increase in Ca2+ remained localized and did not spread to encompass the entire cell body. Ca2+ transient activity decreased significantly with ischemia (from 9.9 +/- 0.6 to 3.1 +/- 0.3 events/min, n = 135) but was unaffected by P(2x) or P(1) receptor inhibition. Exogenous adenosine significantly increased the frequency of Ca2+ transients (to 12.8 +/- 0.9 events/min) and increased synchronization so that 50% of all Ca2+ events were synchronized between ECs. This response to adenosine was not due to an increase in shear stress. These data indicate that localized Ca2+ transients are sensitive to flow conditions and, separately, to metabolically active pathways (exogenous adenosine), although the basal activity occurs independently of P(2x) or P(1) receptors. These transients may represent a mechanism by which individual EC responses are integrated to result in coordinated arteriolar responses in situ.

Changes in galanin immunoreactivity in rat lumbosacral spinal cord and dorsal root ganglia after spinal cord injury

Alterations in the expression of the neuropeptide galanin were examined in micturition reflex pathways 6 weeks after complete spinal cord transection (T8). In control animals, galanin expression was present in specific regions of the gray matter in the rostral lumbar and caudal lumbosacral spinal cord, including: (1) the dorsal commissure; (2) the superficial dorsal horn; (3) the regions of the intermediolateral cell column (L1-L2) and the sacral parasympathetic nucleus (L6-S1); and (4) the lateral collateral pathway in lumbosacral spinal segments. Densitometry analysis demonstrated significant increases (P < or = 0.001) in galanin immunoreactivity (IR) in these regions of the S1 spinal cord after spinal cord injury (SCI). Changes in galanin-IR were not observed at the L4-L6 segments except for an increase in galanin-IR in the dorsal commissure in the L4 segment. In contrast, decreases in galanin-IR were observed in the L1 segment. The number of galanin-IR cells increased (P < or = 0.001) in the L1 and S1 dorsal root ganglia (DRG) after SCI. In all DRG examined (L1, L2, L6, and S1), the percentage of bladder afferent cells expressing galanin-IR significantly increased (4-19-fold) after chronic SCI. In contrast, galanin expression in nerve fibers in the urinary bladder detrusor and urothelium was decreased or eliminated after SCI. Expression of the neurotrophic factors nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) was altered in the spinal cord after SCI. A significant increase in BDNF expression was present in spinal cord segments after SCI. In contrast, NGF expression was only increased in the spinal segments adjacent and rostral to the transection site (T7-T8), whereas spinal segments (T13-L1; L6-S1), distal to the transection site exhibited decreased NGF expression. Changes in galanin expression in micturition pathways after SCI may be mediated by changing neurotrophic factor expression, particularly BDNF. These changes may contribute to urinary bladder dysfunction after SCI.

A novel form of cellular communication among thymic epithelial cells: intercellular calcium wave propagation

We here describe intercellular calcium waves as a novel form of cellular communication among thymic epithelial cells. We first characterized the mechanical induction of intercellular calcium waves in different thymic epithelial cell preparations: cortical 1-4C18 and medullary 3-10 thymic epithelial cell lines and primary cultures of thymic "nurse" cells. All thymic epithelial preparations responded with intercellular calcium wave propagation after mechanical stimulation. In general, the propagation efficacy of intercellular calcium waves in these cells was high, reaching 80-100% of the cells within a given confocal microscopic field, with a mean velocity of 6-10 microm/s and mean amplitude of 1.4- to 1.7-fold the basal calcium level. As evaluated by heptanol and suramin treatment, our results suggest the participation of both gap junctions and P2 receptors in the propagation of intercellular calcium waves in thymic nurse cells and the more prominent participation of gap junctions in thymic epithelial cell lines. Finally, in cocultures, the transmission of intercellular calcium wave was not observed between the mechanically stimulated thymic epithelial cell and adherent thymocytes, suggesting that intercellular calcium wave propagation is limited to thymic epithelial cells and does not affect the neighboring thymocytes. In conclusion, these data describe for the first time intercellular calcium waves in thymic epithelial cells and the participation of both gap junctions and P2 receptors in their propagation.

ATP-dependent mechanism for coordination of intercellular Ca2+ signaling and renin secretion in rat juxtaglomerular cells

A change in intracellular Ca2+ is considered to be the common final signaling pathway through which renin secretion is governed. Therefore, information relating to the generation, control, and processing of Ca2+ signaling in juxtaglomerular cells (JG) will be critical for understanding JG cell behavior. In this study, we investigated the means by which JG cells harmonize their intracellular Ca2+ signals and explored the potential role of these mechanisms in renin secretion. Mechanical stimulation of a single JG cell initiated propagation of an intercellular Ca2+ wave to up to 11.9+/-4.1 surrounding cells, and this was prevented in the presence of the ATP-degrading enzyme, apyrase (1.7+/-0.7 cells), or by desensitization of purinergic receptors via pretreatment of cells with ATP (1.8+/-0.9 cells), thus implicating ATP as a mediator responsible for the propagation of intercellular Ca2+ signaling. Consistent with this, JG cells were demonstrated not to express the gap junction protein connexin43, and neither did they possess functional gap junction communication. Furthermore, massive mechanical stretching of JG cells elicited a 3-fold increase in ATP release. Administration of ATP into isolated perfused rat kidneys induced a rapid, potent, and persistent inhibition of renin secretion, together with a transient elevation of renal vascular resistance. ATP (1 mmol/L) caused up to 79% reduction of the renin secretion activated by lowering the renal perfusion flow (P<0.01). Taken together, our results indicate that under mechanical stimulation, ATP functions as a paracellular mediator to regulate renin secretion, possibly through modulating intra- and intercellular Ca2+ signals.

Casein binds to the cell membrane and induces intracellular calcium signals in the enteroendocrine cell: a brief communication

Dietary protein but not amino acids stimulates cholecystokinin (CCK) secretion in rat mucosal cells. However, the dietary protein sensory mechanisms and the intracellular signal pathway in the enteroendocrine cells have not yet been clarified. The relationship between dietary protein binding to cell membrane and intracellular calcium responses were examined in the CCK-producing enteroendocrine cell line STC-1. The binding of solubilized STC-1 cell membrane to proteins was analyzed using a surface plasmon resonance sensor. Intracellular calcium concentrations of STC-1 cell suspensions loaded with Fura-2 AM were measured using a spectrafluorophotometer system with continuous stirring. Intracellular calcium concentrations in STC-1 cells were increased by exposure to alpha-casein or casein sodium, but not to bovine serum albumin. Solubilized STC-1 membranes bound to alpha-casein and casein sodium but did not bind to bovine serum albumin. alpha-Casein demonstrated higher membrane binding and intracellular calcium stimulating activities than casein sodium. Thus, protein binding to the STC-1 cell membrane and intracellular calcium responses were correlated. Intracellular calcium responses to alpha-casein were suppressed by an L-type calcium channel blocker. These results suggest that casein, a dietary protein, binds to a putative receptor on the CCK-producing enteroendocrine cell membrane and elicits the subsequent intracellular calcium response via an L-type calcium channel.

Hypertonic stress increases T cell interleukin-2 expression through a mechanism that involves ATP release, P2 receptor, and p38 MAPK activation

Hypertonic stress (HS) can alter the function of mammalian cells. We have reported that HS enhances differentiated responses of T cells by increasing their ability to produce interleukin (IL)-2, a finding of clinical interest because hypertonic infusions may modulate immune function in patients. HS shrinks cells and mechanically deforms membranes, which results in ATP release from many cell types. Here we investigate if ATP release is an underlying mechanism through which HS augments T cell function. We found that mechanical stress and HS induced rapid ATP release from Jurkat T cells. HS and exogenous ATP mobilized intracellular Ca(2+), activated p38 MAPK, and increased IL-2 expression. Ca(2+) mobilization was attenuated in the presence of EGTA or by removal of extracellular ATP with apyrase. Adenosine did not increase IL-2 expression, as did ATP. Apyrase, inhibition of P2 receptors, or inhibition of p38 MAPK with SB203580 reduced the stimulatory effects of HS, indicating that HS enhances IL-2 expression through a mechanism that involves ATP release, P2 (perhaps P2X7) receptors, and p38 MAPK activation. We conclude that release of and response to ATP plays a key role in the mechanism through which hypertonic stress regulates the function of T cells.

A model for mechanotransduction in bone cells: the load-bearing mechanosomes

The skeleton's response to mechanical force, or load, has significance to space travel, the treatment of osteoporosis, and orthodontic appliances. How bone senses and processes load remains largely unknown. The cellular basis of mechanotransduction, however, likely involves the integration of diffusion-controlled signaling pathways with a solid-state scaffold linking the cell membrane to the genes. Here, we integrate various concepts from models of connective membrane skeleton proteins, cellular tensegrity, and nuclear matrix architectural transcription factors, to describe how a load-induced deformation of bone activates a change in the skeletal genetic program. We propose that mechanical information is relayed from the bone to the gene in part by a succession of deformations, changes in conformations, and translocations. The load-induced deformation of bone is converted into the deformation of the sensor cell membrane. This, in turn, drives conformational changes in membrane proteins of which some are linked to a solid-state signaling scaffold that releases protein complexes capable of carrying mechanical information, "mechanosomes", into the nucleus. These mechanosomes translate this information into changes in the geometry of the 5' regulatory region of target gene DNA altering gene activity; bending bone ultimately bends genes. We identify specific candidate proteins fitting the profile of load-signaling mechanosomes.

Stretch activation of Ca2+ transients in pancreatic beta cells by mobilization of intracellular stores

INTRODUCTION

Nonadrenergic, noncholinergic neurons have been proposed to synchronize pulsatile insulin release from the islets in the pancreas by triggering transient increases of the cytoplasmic Ca2+ concentration ([Ca2+]i) in beta-cells via an inositol trisphoshate-dependent mechanism.

AIMS

To test whether pancreatic beta-cells respond to stretch activation with similar types of transients and whether these Ca signals propagate to other beta-cells in the presence and absence of cell contacts.

METHODOLOGY

Single cells and small aggregates were prepared from beta-cell-rich islets from mice. After 2-5 days of culture, [Ca2+]i was measured with digital imaging and the indicator fura-2 during superfusion with a medium containing 20 mmol/L glucose and 50 micromol/L methoxyverapamil. Membrane stretch was induced by osmotic swelling or focal touch stimulation.

RESULTS

Lowering the medium osmolarity with 100-102 mOSM/L by removal of sucrose or by dilution resulted in a 2-3-fold increase in the number of transients during an initial 5-minute period. Sucrose omission was stimulatory also after isosmolar replacement with readily penetrating urea. The intracellular Ca2+-ATPase inhibitor thapsigargin suppressed both the spontaneously occurring transients and those initiated by volume expansion. Touch stimuli induced [Ca2+]i transients, which rapidly propagated to cells within the same aggregate or lacking contact.

CONCLUSION

The observations support the idea that beta-cells both receive and regenerate extracellular signals triggering [Ca2+]i transients. Touch stimulation is a useful tool for investigating the propagation of [Ca2+]i signals between pancreatic beta-cells lacking physical contact.

The role of P2Y1 purinergic receptors and cytosolic Ca2+ in hypotonically activated osmolyte efflux from a rat hepatoma cell line

Exposure of HTC rat hepatoma cells to a 33% decrease in extracellular osmolality caused the cytosolic Ca(2+) concentration ([Ca(2+)](i)) to increase transiently by approximately 90 nm. This rise in [Ca(2+)](i) was inhibited strongly by apyrase, grade VII (which has a low ATP/ADPase ratio) but not by apyrase grade VI (which has a high ATP/ADPase ratio) or hexokinase, indicating that extracellular ADP and/or ATP play a role in the [Ca(2+)](i) increase. The hypotonically induced rise in [Ca(2+)](i) was prevented by the prior discharge of the intracellular Ca(2+) store of the cells by thapsigargin. Removal of extracellular Ca(2+) or inhibition of Ca(2+) influx by 1-10 microm Gd(3+) depleted the thapsigargin-sensitive Ca(2+) stores and thereby diminished the rise in [Ca(2+)](i). The hypotonically induced rise in [Ca(2+)](i) was prevented by adenosine 2'-phosphate-5'-phosphate (A2P5P) and pyridoxyl-5'-phosphate-6-azophenyl-2',4'-disulfonate, inhibitors of purinergic P2Y(1) receptors for which ADP is a major agonist. Both inhibitors also blocked the rise in [Ca(2+)](i) elicited by addition of ADP to cells in isotonic medium, whereas A2P5P had no effect on the rise in [Ca(2+)](i) elicited by the addition of the P2Y(2) and P2Y(4) receptor agonist, UTP. HTC cells were shown to express mRNA encoding for rat P2Y(1), P2Y(2), and P2Y(6) receptors. Inhibition of the hypotonically induced rise in [Ca(2+)](i) blocked hypotonically induced K(+) ((86)Rb(+)) efflux, modulated the hypotonically induced efflux of taurine, but had no significant effect on Cl(-) ((125)I-) efflux. The interaction of extracellular ATP and/or ADP with P2Y(1) purinergic receptors therefore plays a role in the response of HTC cells to osmotic swelling but does not account for activation of all the efflux pathways involved in the volume-regulatory response.

Behavior of junction channels between rat glomus cells during normoxia and hypoxia

The activity of gap junction channels between cultured and clustered carotid body glomus cells of the rat was studied with dual voltage clamping during normoxia (PO(2) 300 Torr) and hypoxia induced by sodium dithionite (Na(2)S(2)O(4)) or 100% N(2). Na(2)S(2)O(4) reduced the saline PO(2) to approximately 10 Torr, whereas 100% N(2) reduced ambient O(2) to approximately 60 Torr. The following observations were made. 1) In normoxia, the intercellular macroconductance (G(j) = 3.0 +/- 1.01 ns, mean +/- SE) was changed unevenly (increased and decreased) under hypoxic conditions by either agent, although N(2) produced the largest changes. 2) The intercellular microconductances of the channels (g(j) = 104.44 +/- 10.16 pS under normoxic conditions) significantly decreased in 100% N(2) but showed depressions and enhancements in Na(2)S(2)O(4). 3) The conductance of single-junction channels (SChs), calculated as g(j) variance/mean g(j), yielded a mean of approximately 17.6 pS. Larger values were obtained with manual measurements of the data (approximately 34 pS). Hypoxic hypoxia (induced by 100% N(2)) significantly depressed the conductance of SChs when calculated from digitized records or from manual measurements. Hypoxia induced by Na(2)S(2)O(4) did not significantly change junctional conductance. 4) The number of intercellular channels, calculated as g(j)/SCh g(j), had a mean of approximately 452 (range 1 to 2,471). During N(2)-induced hypoxia, this number significantly decreased to approximately 84 but remained unchanged during Na(2)S(2)O(4) hypoxia. 5) The mean open time of junction channels varied from 4 to 30 ms in different experiments, having an overall mean of mu = 11.33 +/- 0.33 ms. This value was significantly reduced by 100% N(2) but was not changed by Na(2)S(2)O(4). 6) Intracellular calcium ([Ca(2+)](i)), 46.2 +/- 4.84 nM under normoxia, significantly increased to 77.32 +/- 11.27 nM with Na(2)S(2)O(4) and to 66.39 +/- 11.64 nM with 100% N(2). It is concluded that 100% N(2) uncouples glomus cells by significantly reducing intercellular macro- and microconductances. Hypoxia induced by Na(2)S(2)O(4) had variable effects. The coupling effects of hypoxia may depend on, or be aided by, increases in [Ca(2+)](i) and/or intracellular pH changes. However, secreted transmitters and ATP plus the effects of hypoxia on second messengers and other cytoplasmic components may also play an important role in this phenomenon.

Role of luminal ATP in regulating electrogenic Na(+) absorption in guinea pig distal colon

Extracellular ATP regulates a variety of functions in epithelial tissues by activating the membrane P2-receptor. The purpose of this study was to investigate the autocrine/paracrine regulation by luminal ATP of electrogenic amiloride-sensitive Na(+) absorption in the distal colon from guinea pigs treated with aldosterone by measuring the amiloride-sensitive short-circuit current (I(sc)) and (22)Na(+) flux in vitro with the Ussing chamber technique. ATP added to the luminal side inhibited the amiloride-sensitive I(sc) and (22)Na(+) absorption to a similar degree. The concentration dependence of the inhibitory effect of ATP on amiloride-sensitive I(sc) had an IC(50) value of 20-30 microM, with the maximum inhibition being approximately 50%. The effects of different nucleotides and of a nucleoside were also studied, the order of potency being ATP = UTP > ADP > adenosine. The effects of ATP were slightly, but significantly, reduced in the presence of suramin in the luminal solution. The inhibitory effect of luminal ATP was more potent in the absence of both Mg2+ and Ca2+ from the luminal solution. Pretreatment of the tissue with ionomycin or thapsigargin in the absence of serosal Ca2+ did not affect the percent inhibition of amiloride-sensitive I(sc) induced by ATP. Mechanical perturbation with a hypotonic luminal solution caused a reduction in amiloride-sensitive I(sc), this effect being prevented by the presence of hexokinase, an ATP-scavenging enzyme. These results suggest that ATP released into the luminal side by hypotonic stimulation could exert an inhibitory effect on the electrogenic Na(+) absorption. This effect was probably mediated by a P2Y(2) receptor on the apical membrane of colonic epithelial cells, and a change in the intracellular Ca2+ concentration may not be necessary for this process.

Coordination of mesangial cell contraction by gap junction--mediated intercellular Ca(2+) wave

Gap junction intercellular communication (GJIC) plays a fundamental role in mediating intercellular signals and coordinating multicellular behavior in various tissues and organs. Glomerular mesangial cells (MC) are rich in GJ, but the functional associations of these intercellular channels are still unclear. This study examines the potential role of GJ in the transmission of intercellular Ca(2+) signals and in the coordination of MC contraction. First, the presence of GJ protein Cx43 and functional GJIC was confirmed in MC by using immunochemical staining or transfer of Lucifer yellow (LY) after a single cell injection, respectively. Second, mechanical stimulation of a single MC initiated propagation of an intercellular Ca(2+) wave, which was preventable by the GJ inhibitor heptanol but was not altered by pretreatment of MC with ATP or addition of apyrase into the assay system. Third, the phospholipase C (PLC) inhibitor U73122 could largely eliminate the mechanically elicited propagation of intercellular Ca(2+) waves, suggesting a possibly mediating role of inositol trisphosphate (IP(3)) in the initiation and transmission of intercellular Ca(2+) signaling. Fourth, injection of IP(3) into a single cell caused contraction, not only in the targeted cell, but also in the adjacent cells, as indicated by the reduction of cellular planar area. Fifth, addition of two structurally unrelated GJ inhibitors, heptanol and alpha-glycyrrhetinic acid (GA), into MC embedded in collagen gels significantly attenuated the reduction of gel areas after exposure to serum. This study provides the first functional evidence supporting the critical role of GJIC in the synchronization of MC behaviors.

Up-regulation of tyrosine kinase (Trka, Trkb) receptor expression and phosphorylation in lumbosacral dorsal root ganglia after chronic spinal cord (T8-T10) injury

Previous studies have demonstrated changes in urinary bladder neurotrophic factors after bladder dysfunction. We have hypothesized that retrograde transport of neurotrophin(s) from the bladder to lumbosacral dorsal root ganglia (DRG) may play a role in bladder reflex reorganization after spinal cord injury (SCI). In this study, we determined whether the expression of tyrosine kinase receptors (TrkA, TrkB) is altered in lumbosacral DRG after SCI through immunofluorescence techniques. Complete transection of the spinal cord (T8-T10) was performed in female Wistar rats (120-150 g), and animals were studied 5-6 weeks after SCI. One week before killing, Fast Blue (FB) was injected into the bladder to label bladder afferent cells in the L1, L2, L6, and S1 DRG. After SCI, a significant increase in the number of TrkA-immunoreactive (IR) positive cells was detected in the L6-S1 DRG (L6: 1.9-fold, P < or = 0.01; S1: 1.7-fold, P < or = 0.05) and in the L1 DRG (3.0-fold; P < or = 0.01) but not in the L4-L5 DRG compared with spinal-intact (control) rats. After SCI, a significant increase in the number of TrkB-IR cells was also detected in the L6-S1 DRG (L6: 2.2-fold, P < or = 0.01; S1: 1.5-fold, P < or = 0.05) and in the L1-L2 DRG (L1: 1.5-fold, P < or = 0.01; L2: 1.3-fold, P < or = 0.05) but not in the L4-L5 DRG compared with control rats. After SCI, the percentage of FB-labeled cells expressing TrkA immunoreactivity (approximately 68%) or TrkB immunoreactivity (approximately 65%) in L1 and L6 DRG significantly (P < or = 0.01) increased compared with control (20-30%) DRG. After SCI, the percentage of TrkA-IR cells expressing phosphorylated (p)-Trk immunoreactivity significantly increased (1.5- to 2.3-fold increase) in the L1, L6, and S1 DRG. The percentage of TrkB-IR cells expressing p-Trk immunoreactivity after SCI also increased (1.3-fold increase) in the L1 and L6 DRG. These results demonstrate that (1) TrkA and TrkB immunoreactivity is increased in bladder afferent cells after SCI and (2) TrkA and TrkB receptors are phosphorylated in DRG after SCI. Neuroplasticity of lower urinary tract reflexes after SCI may be mediated by both nerve growth factor and brain-derived neurotrophic factor.

Volume-regulated anion channels serve as an auto/paracrine nucleotide release pathway in aortic endothelial cells

Mechanical stress induces auto/paracrine ATP release from various cell types, but the mechanisms underlying this release are not well understood. Here we show that the release of ATP induced by hypotonic stress (HTS) in bovine aortic endothelial cells (BAECs) occurs through volume-regulated anion channels (VRAC). Various VRAC inhibitors, such as glibenclamide, verapamil, tamoxifen, and fluoxetine, suppressed the HTS-induced release of ATP, as well as the concomitant Ca(2+) oscillations and NO production. They did not, however, affect Ca(2+) oscillations and NO production induced by exogenously applied ATP. Extracellular ATP inhibited VRAC currents in a voltage-dependent manner: block was absent at negative potentials and was manifest at positive potentials, but decreased at highly depolarized potentials. This phenomenon could be described with a "permeating blocker model," in which ATP binds with an affinity of 1.0 +/- 0.5 mM at 0 mV to a site at an electrical distance of 0.41 inside the channel. Bound ATP occludes the channel at moderate positive potentials, but permeates into the cytosol at more depolarized potentials. The triphosphate nucleotides UTP, GTP, and CTP, and the adenine nucleotide ADP, exerted a similar voltage-dependent inhibition of VRAC currents at submillimolar concentrations, which could also be described with this model. However, inhibition by ADP was less voltage sensitive, whereas adenosine did not affect VRAC currents, suggesting that the negative charges of the nucleotides are essential for their inhibitory action. The observation that high concentrations of extracellular ADP enhanced the outward component of the VRAC current in low Cl(-) hypotonic solution and shifted its reversal potential to negative potentials provides more direct evidence for the nucleotide permeability of VRAC. We conclude from these observations that VRAC is a nucleotide-permeable channel, which may serve as a pathway for HTS-induced ATP release in BAEC.

Altered calcium-mediated cell signaling in keratinocytes cultured from patients with neurofibromatosis type 1

Capacitative calcium entry and calcium wave propagation were studied in keratinocytes cultured from control persons and patients with type 1 neurofibromatosis. The cells were stimulated mechanically in the presence of inhibitors of gap-junctional or ATP-mediated communication to determine which pathways are operative in Ca(2+) signaling between these cells. Keratinocytes cultured from patients with type 1 neurofibromatosis (NF1) had a tendency to form cultures with markedly altered calcium-related signaling characteristics. Specifically, the resting Ca(2+) levels, intracellular Ca(2+) stores, capacitative calcium influx, and gap-junctional signal transduction were defective in NF1 keratinocytes. Western transfer analysis revealed apparently equal connexin 43 protein levels in normal control and in NF1 keratinocytes. Indirect immunofluorescence, however, demonstrated that connexin 43 was relatively evenly distributed in NF1 cells and did not form typical gap-junctional plaques between keratinocytes. Furthermore, the speed of the calcium wave was reduced in NF1 cells compared to normal keratinocytes. The results demonstrate that keratinocytes cultured from patients with NF1 display altered calcium-mediated signaling between cells.

K+ channel currents in rat ventral prostate epithelial cells

BACKGROUND

Electrophysiological function of the normal prostate has not been extensively studied. In particular, ion channel currents and their regulation have not been studied in freshly-isolated prostate cells.

METHODS

Rat prostate secretory epithelial (RPSE) cells were isolated by collagenase treatment. Columnar epithelial cells were used for nystatin-perforated, whole-cell voltage clamp, and the intracellular Ca(2+) concentration ([Ca(2+)](i)) was measured using fura-2.

RESULTS

Step-like depolarizing pulses (900 msec) starting from - 90 mV induced outwardly rectifying K(+) currents without inactivation. ACh (10 microM) or ATP (100 microM) increased the outward current and hyperpolarized the cell membrane potential. Ionomycin (0.1 microM), a Ca(2+) ionophore, induced a similar increase in the outward current. TEA (5 mM), charybdotoxin (50 nM), and iberiotoxin (30 nM) inhibited the effect of ACh (or ATP) on the outward current, whereas apamin (100 nM) had no effect. The [Ca(2+)](i) of RPSE cells was increased by ACh, ATP, and UTP.

CONCLUSIONS

RPSE cells have iberiotoxin-sensitive Ca(2+)-activated K(+) channels that may play an important role in the exocrine secretions of the prostate.

ATP masks stretch activation of epithelial sodium channels in A6 distal nephron cells

The mechanosensitivity of the epithelial sodium channel (ENaC) is controversial. Using cell-attached patch-clamp techniques, we found that mechanical stretch stimulated ENaC in A6 distal nephron cells in only three of nine cell-attached patches. However, stretch consistently activated ENaC after apical ATP was scavenged with apical hexokinase plus glucose or after P(2) receptors in the patch were blocked. The mean open probability (P(o)) of ENaC was increased from 0.31 +/- 0.04 to 0.61 +/- 0.06 (P < 0.001; n = 9) when patch pipettes contained hexokinase and glucose, or from 0.24 +/- 0.05 to 0.55 +/- 0.11 (P < 0.01; n = 7) when patch pipettes contained suramin, respectively. A poorly hydrolyzable ATP analog, ATPgammaS, in the patch pipettes inhibited ENaC, reducing the P(o) from 0.41 +/- 0.06 to 0.19 +/- 0.05 (P < 0.01; n = 8). Pretreatment of A6 cells with the phospholipase C (PLC) inhibitor U-73122 abolished the effect of ATP on ENaC activity. These data together suggest that ATP, acting through a PLC-dependent purinergic pathway, masks stretch-induced ENaC activation.

Muscle contraction under capillaries in hamster muscle induces arteriolar dilatation via K(ATP) channels and nitric oxide

We tested the hypothesis that adenosine and nitric oxide can be sensed by capillaries and are implicated in the remote arteriolar dilatation initiated by muscle contraction. We also explored a role for K(ATP) channel activity in this response. Small bundles of muscle fibres underlying a group of capillaries in cremaster muscles of anaesthetized hamsters were electrically stimulated to contract for 2 min at each of 2, 4 and 8 Hz. Diameter changes were measured in the inflow arteriole to the group of capillaries after muscle contraction in the presence or absence of 10(-6) M xanthine amine congener (XAC) to block A(1) and A(2) adenosine receptors, 10(-4) or 10(-3) M N(omega)-nitro-L-arginine (LNNA) to block nitric oxide production, or 10(-5) M glibenclamide to block K(ATP) channel activity. Dilatations were unchanged with XAC (3.0 +/- 0.5, 3.9 +/- 0.7 and 6.1 +/- 1.0 microm), but were significantly reduced with LNNA (to 1.8 +/- 0.6, 3.5 +/- 0.7 and 4.9 +/- 0.7 microm) or glibenclamide (to 0.4 +/- 0.3, 0.8 +/- 0.7 and 1.9 +/- 0.6 microm). Neither K(ATP) channel activity nor nitric oxide was required for transmission or manifestation of the dilator response. Thus, muscle contraction can be sensed by capillaries and the signalling mechanism for the ensuing remote dilatation depends on K(ATP) channel activity and on NO, but not adenosine. Local application of 10(-4) M adenosine, 10(-4) M sodium nitroprusside or 10(-5) M pinacidil directly to capillaries initiated remote arteriolar dilatations. Thus, capillaries can respond directly to known mediators of metabolic vasodilatation, but these signalling pathways are not invariably implicated in the response to muscle contraction.

ATP is released from guinea pig ureter epithelium on distension

Distension of the perfused guinea pig ureter at pressures from 20 to 700 cmH(2)O increased the amount of ATP released from the epithelium in a pressure-dependent manner. During basal perfusion (40 microl/min), the perfusate contained 10 pmol/ml ATP; this increased 10- to 50-fold at various distending pressures. ATP was released from epithelial cells during distension as mechanical removal of the urothelium blocked release. No lactate dehydrogenase was detected in the perfusate, and scanning electron microscopy confirmed an intact urothelium after distension. ATP was not released due to the activation of stretch-activated channels, as gadolinium (10 microM) failed to affect ATP release. Glibenclamide (10 microM), known to inhibit two members of the ATP-binding cassette (ABC) protein family, did not affect ATP release after distension; nor did verapamil (10 microM). In contrast, both monensin (100 microM) and brefeldin A (10 microM), which interfere with vesicular formation or trafficking, inhibited distension-evoked ATP release, which was Ca(2+)-dependent. This suggests that ATP release from the ureter epithelium might be mediated by vesicular exocytosis. The role of ATP released by distension of hollow visceral organs is discussed in relation to the concept of purinergic mechanosensory transductions, with special reference to nociception and the activation of P2X(3) receptors on the subepithelial sensory nerves.

Cell swelling-induced ATP release and gadolinium-sensitive channels

ATP release induced by hypotonic swelling is an ubiquitous phenomenon in eukaryotic cells, but its underlying mechanisms are poorly defined. A mechanosensitive (MS) ATP channel has been implicated because gadolinium (Gd(3+)), an inhibitor of stretch-activated channels, suppressed ATP efflux monitored by luciferase bioluminescence. We examined the effect of Gd(3+) on luciferase bioluminescence and on ATP efflux from hypotonically swollen cells. We found that luciferase was inhibited by < or =10 microM Gd(3+), and this may have contributed to the previously reported inhibition of ATP release. In ATP efflux experiments, luciferase inhibition could be prevented by chelating Gd(3+) with EGTA before luminometric ATP determinations. Using this approach, we found that 10-100 microM Gd(3+), i.e., concentrations typically used to block MS channels, actually stimulated hypotonically induced ATP release from fibroblasts. Inhibition of ATP release required at least 500, 200, or 100 microM Gd(3+) for fibroblasts, A549 cells, and 16HBE14o(-) cells, respectively. Such biphasic and cell-specific effects of Gd(3+) are most consistent with its action on membrane lipids and membrane-dependent processes such as exocytosis.

Effects of noise on the off rate of Ca(2+) binding proteins in a coupled biochemical cell system

We investigated numerically the kinetic properties of calcium binding proteins by using a three-calcium store model and discussed the response of a two-way coupled biochemical cell system, whose subsystems were coupled via diffusion-like cytosolic calcium transfer through gap junctions, to the external stimulation. When we used noise to modulate the off rate of the Ca(2+) binding proteins in simulation, an SR-like phenomenon of synchronous oscillations is observed. In addition, the interaction involving noise, coupling and the dynamics of Ca(2+) binding proteins is discussed.

Activation of p90RSK and growth stimulation of multicellular tumor spheroids are dependent on reactive oxygen species generated after purinergic receptor stimulation by ATP

Mitogenic stimulation by growth factors may be mediated through intracellular reactive oxygen species (ROS) acting as signaling molecules. Incubation of multicellular prostate tumor spheroids with adenosine 5' triphosphate (ATP) dose-dependently stimulated tumor growth. ATP, uridine 5'-triphosphate (UTP), adenosine 5'-diphosphate (ADP), and 2-methylthio-ATP (2-MeS-ATP) increased intracellular ROS levels significantly. ROS generation by ATP was inhibited by the P2 receptor antagonist suramin, by the reduced nicotinamide adenine dinucleotide phosphate (NADPH) oxidase inhibitors diphenylene iodonium chloride (DPI) and 4-(2-aminoethyl) benzenesulfonylfluoride (AEBSF), as well as by the Ca2+-dependent phospholipase A2 (PLA2) inhibitors indomethacin and methyl arachidonyl fluorophosphonate (MAFP). The generation of ROS was dependent on the intracellular Ca2+ response evoked by ATP. Exogenous ATP activated the extracellular signal-regulated kinase 1/2 (ERK1/2) mitogen-activated protein kinase (MAPK) pathway, which was blunted by the MAPK/ERK kinase 1/2 (MEK1/2) antagonist PD98059. The radical scavengers vitamin E, dimethyl thiourea (DMTU), and N-acetyl cysteine (NAC) failed to inhibit ERK1/2 activation but abolished p90 ribosomal S6 kinase (p90RSK) activation downstream of ERK1/2, as well as the growth stimulation of tumor spheroids. Our data indicate that p90RSK downstream of ERK1/2 is the molecular target for ROS generated through stimulation of purinergic receptors by ATP.