Cell swelling-induced ATP release and gadolinium-sensitive channels

Amplified P2X3 pathway activity in muscle afferent dorsal root ganglion neurons and exercise pressor reflex regulation in hindlimb ischaemia-reperfusion

Hindlimb ischaemia-reperfusion (IR) is among the most prominent pathophysiological conditions observed in peripheral artery disease (PAD). An exaggerated arterial blood pressure (BP) response during exercise is associated with an elevated risk of cardiovascular events in individuals with PAD. However, the precise mechanisms leading to this exaggerated BP response are poorly elucidated. The P2X3 signalling pathway, which plays a key role in modifying the exercise pressor reflex (EPR), is the focus of the present study. We determined the regulatory role of P2X3 on the EPR in a rat model of hindlimb IR. In vivo and in vitro approaches were used to determine the expression and functions of P2X3 in muscle afferent nerves and EPR in IR rats. We found that in IR rats there was (1) upregulation of P2X3 protein expression in the L4-6 dorsal root ganglia (DRG); (2) amplified P2X currents in isolated isolectin B4 (IB4)-positive muscle DRG neurons; and (3) amplification of the P2X-mediated BP response. We further verified that both A-317491 and siRNA knockdown of P2X3 significantly decreased the activity of P2X currents in isolated muscle DRG neurons. Moreover, inhibition of muscle afferents' P2X3 receptor using A-317491 was observed to alleviate the exaggerated BP response induced by static muscle contraction and P2X-induced BP response by α,β-methylene ATP injection. P2X3 signalling pathway activity is amplified in muscle afferent DRG neurons in regulating the EPR following hindlimb IR.

Propidium uptake and ATP release in A549 cells share similar transport mechanisms

The lipid bilayer of eukaryotic cells' plasma membrane is almost impermeable to small ions and large polar molecules, but its miniscule basal permeability in intact cells is poorly characterized. This report describes the intrinsic membrane permeability of A549 cells toward the charged molecules propidium (Pr²⁺) and ATP^4-. Under isotonic conditions, we detected with quantitative fluorescence microscopy, a continuous low-rate uptake of Pr (∼150 × 10^-21 moles (zmol)/h/cell, [Pr]_o = 150 μM, 32°C). It was stimulated transiently but strongly by 66% hypotonic cell swelling reaching an influx amplitude of ∼1500 (zmol/h)/cell. The progressive Pr uptake with increasing [Pr]_o (30, 150, and 750 μM) suggested a permeation mechanism by simple diffusion. We quantified separately ATP release with custom wide-field-of-view chemiluminescence imaging. The strong proportionality between ATP efflux and Pr²⁺ influx during hypotonic challenge, and the absence of stimulation of transmembrane transport following 300% hypertonic shock, indicated that ATP and Pr travel the same conductive pathway. The fluorescence images revealed a homogeneously distributed intracellular uptake of Pr not consistent with high-conductance channels expressed at low density on the plasma membrane. We hypothesized that the pathway consists of transiently formed water pores evenly spread across the plasma membrane. The abolition of cell swelling-induced Pr uptake with 500 μM gadolinium, a known modulator of membrane fluidity, supported the involvement of water pores whose formation depends on the membrane fluidity. Our study suggests an alternative model of a direct permeation of ATP (and other molecules) through the phospholipid bilayer, which may have important physiological implications.

ASIC3 knockout alters expression and activity of P2X3 in muscle afferent nerves of rat model of peripheral artery disease

In peripheral artery disease (PAD), the metaboreceptor and mechanoreceptor in muscle afferent nerves contribute to accentuated sympathetic outflow via a neural reflex termed exercise pressor reflex (EPR). Particularly, lactic acid and adenosine triphosphate (ATP) produced in exercising muscles respectively stimulate acid sensing ion channel subtype 3 (ASIC3) and P2X3 receptors (P2X3) in muscle afferent nerves, inducing the reflex sympathetic and BP responses. Previous studies indicated that those two receptors are spatially close to each other and AISC3 may have a regulatory effect on the function of P2X3. This inspired our investigation on the P2X3‐mediated EPR response following AISC₃ abolished, which was anticipated to shed light on the future pharmacological and genetic treatment strategy for PAD. Thus, we tested the experimental hypothesis that the pressor response to P2X3 stimulation is greater in PAD rats with 3 days of femoral artery occlusion and the sensitizing effects of P2X3 are attenuated following ASIC₃ knockout (KO) in PAD. Our data demonstrated that in wild type (WT) rats femoral occlusion exaggerated BP response to activation of P2X3 using α,β‐methylene ATP injected into the arterial blood supply of the hindlimb, meanwhile the western blot analysis suggested upregulation of P2X3 expression in dorsal root ganglion supplying the afferent nerves. Using the whole cell patch‐clamp method, we also showed that P2X3 stimulation enhanced the amplitude of induced currents in muscle afferent neurons of PAD rats. Of note, amplification of the P2X3 evoked‐pressor response and expression and current response of P2X3 was attenuated in ASIC3 KO rats. We concluded that the exaggerated P2X3‐mediated pressor response in PAD rats is blunted by ASIC₃ KO due to the decreased expression and activities of P2X3 in muscle afferent neurons.

Lytic Release of Cellular ATP: Physiological Relevance and Therapeutic Applications

The lytic release of ATP due to cell and tissue injury constitutes an important source of extracellular nucleotides and may have physiological and pathophysiological roles by triggering purinergic signalling pathways. In the lungs, extracellular ATP can have protective effects by stimulating surfactant and mucus secretion. However, excessive extracellular ATP levels, such as observed in ventilator-induced lung injury, act as a danger-associated signal that activates NLRP3 inflammasome contributing to lung damage. Here, we discuss examples of lytic release that we have identified in our studies using real-time luciferin-luciferase luminescence imaging of extracellular ATP. In alveolar A549 cells, hypotonic shock-induced ATP release shows rapid lytic and slow-rising non-lytic components. Lytic release originates from the lysis of single fragile cells that could be seen as distinct spikes of ATP-dependent luminescence, but under physiological conditions, its contribution is minimal <1% of total release. By contrast, ATP release from red blood cells results primarily from hemolysis, a physiological mechanism contributing to the regulation of local blood flow in response to tissue hypoxia, mechanical stimulation and temperature changes. Lytic release of cellular ATP may have therapeutic applications, as exemplified by the use of ultrasound and microbubble-stimulated release for enhancing cancer immunotherapy in vivo.

One-Time Acute Heat Treatment Is Effective for Attenuation of the Exaggerated Exercise Pressor Reflex in Rats With Femoral Artery Occlusion

The purpose of this study was to determine the effects of one-time acute heat treatment (HT) on the exaggerated exercise pressor reflex in a model of peripheral arterial insufficiency induced by ligation of the femoral artery and was to further examine the underlying mechanism of ATP-P2X₃ signal activity during this process. The blood pressure (BP) response to static muscle contraction and muscle tendon stretch was recorded to determine the exercise pressor reflex. Also, αβ-methylene ATP (αβ-me ATP) was injected into the arterial blood supply of the hindlimb muscles to stimulate P2X₃ receptors in the muscle afferent nerves. To process one-time acute HT, a heating pad was placed locally on the hindlimb and the muscle temperature (Tm) was increased by ~1.5°C and maintained for 5 min. Compared with control rats, a greater mean arterial pressure (MAP) response to muscle contraction was observed in rats with femoral occlusion in a pre-heat control session (28 ± 2 mmHg in occluded rats/n = 12 vs. 18 ± 2 mmHg in control rats/n = 9; p < 0.05). The one-time acute HT attenuated the amplification of the BP response in rats with femoral artery occlusion (MAP response: 19 ± 8 mmHg in occluded rats + HT/n = 11; p < 0.05 vs. occluded rats). In contrast, HT did not significantly attenuate amplification of MAP response to muscle stretch and αβ-me ATP injection in rats with femoral artery occlusion and controls (all p > 0.05). Our data suggest that one-time acute HT selectively attenuates the amplified pressor response induced by activation of the metabolic and mechanical components of the reflex in rats after femoral artery occlusion. The suppressing effects of acute HT on the exaggerated exercise pressor reflex are likely mediated through a reduction in metabolites (e.g., ATP) stimulating the muscle afferent nerves in contracting muscle, but unlikely through direct alteration of P2X receptors per se.

Heat treatment improves the exaggerated exercise pressor reflex in rats with femoral artery occlusion via a reduction in the activity of the P2X receptor pathway

KEY POINTS

During exercise, the blood pressure (BP) response is exaggerated in peripheral artery disease (PAD). We examined whether heat treatment (HT) has beneficial effects on the exaggerated exercise pressor reflex in PAD rats. With HT (increase in basal muscle temperature of ∼1.5°C for 30 min, twice daily for three continuous days), the amplified BP response to muscle contraction is alleviated in PAD. We demonstrated that HT attenuates the enhancement of the BP response induced by stimulation of P2X in muscle afferent nerves of PAD rats. HT also attenuates the upregulation of the P2X₃ and the increase in P2X currents in the muscle afferent neurons of PAD rats. Previous heat exposure plays a beneficial role in modifying the exaggeration of the exercise pressor reflex in PAD and a reduction in the activity of the P2X receptor pathway is probably a part of the mechanism mediating this improvement.

ABSTRACT

The current study was performed to examine if heat treatment (HT) has beneficial effects on the exaggerated exercise pressor reflex in rats with peripheral artery disease (PAD). We further determined if the temperature-sensitive P2X receptor is involved in the effects of HT. The pressor response to static muscle contraction and α,β-methylene ATP (αβ-me ATP, a P2X agonist) was examined. Western blot analysis was used to determine the protein levels of P2X₃ in the dorsal root ganglion (DRG), and the whole cell patch clamp was used to examine the amplitude of P2X currents in the DRG neurons. The basal muscle temperature (T_m ) was lower in PAD rats than in control rats. T_m was increased by ∼1.5°C and this increase was maintained for 30 min. This HT protocol was performed tweice daily for three continuous days. A greater blood pressure (BP) response to contraction was observed in PAD rats. HT attenuated the amplification of the BP response in PAD rats. HT also attenuated the enhancement of the BP response induced by the arterial injection of αβ-me ATP in PAD rats. In addition, HT attenuated the upregulation of the P2X₃ and increased P2X currents in the DRG neurons of PAD rats. In conclusion, previous heat exposure plays an inhibitory role in modifying the exaggeration of the exercise pressor reflex in PAD and a reduction of the activity of the P2X receptor pathway is probably a part of mechanisms leading to the beneficial effects of HT.

Wide field of view quantitative imaging of cellular ATP release

Although several mechanical stressors promote ATP secretion from eukaryotic cells, few mechanosensitive pathways for ATP release have been precisely characterized and none have been clearly identified. To facilitate progress, we report here a wide field of view (∼20 × 20 mm sample area) imaging technique paired with a quantitative image analysis to accurately map the dynamics of ATP release from a cell population. The approach has been tested on A549 cells stretched at high initial strain rate (2-5 s^-1) or swelled by hypotonic shock. The amount of ATP secreted in response to a series of five graded stretch pulses (5-37% linear deformation, 1-s duration at 25°C) changed nonmonotonically with respect to strain amplitude and was inhomogeneous across the cell monolayer. In a typical experiment, extracellular ATP density averaged 250 fmol/mm², but the area of detectable signal covered only ∼40% of the cells. In some areas, ATP accumulation peaked around 900 fmol/mm², which corresponded to an estimated concentration of 4.5 µM. The total amount of ATP released from the combined stretch pulses reached 384 ± 224 pmol/million cells (n = 4). Compared with stretch, hypotonic shock (50%, 30°C) elicited a more homogeneous ATP secretion from the entire cell population but at a lower yield totaling 28 ± 12 pmol/million cells (n = 4). The quantitative extracellular ATP mapping of several thousand cells at once, with this wide field of view imaging system, will help identify ATP release pathways by providing unique insights on the dynamics and inhomogeneities of the cellular ATP secretion that are otherwise difficult to assess within the smaller field of view of a microscope.

Elevated hydrostatic pressure stimulates ATP release which mediates activation of the NLRP3 inflammasome via P2X4 in rat urothelial cells

Partial bladder outlet obstruction (pBOO) is a prevalent urological condition commonly accompanied by increased intravesical pressure, inflammation, and fibrosis. Studies have demonstrated that pBOO results in increased NLRP3 inflammasome and caspase-1 activation and that ATP is released from urothelial cells in response to elevated pressure. In the present study, we investigated the role of elevated pressure in triggering caspase-1 activation via purinergic receptors activation in urothelial cells. Rat urothelial cell line, MYP3 cells, was subjected to hydrostatic pressures of 15 cmH₂O for 60 min, or 40 cmH₂O for 1 min to simulate elevated storage and voiding pressure conditions, respectively. ATP concentration in the supernatant media and intracellular caspase-1 activity in cell lysates were measured. Pressure experiments were repeated in the presence of antagonists for purinergic receptors to determine the mechanism for pressure-induced caspase-1 activation. Exposure of MYP3 cells to both pressure conditions resulted in an increase in extracellular ATP levels and intracellular caspase-1 activity. Treatment with P2X₇ antagonist led to a decrease in pressure-induced ATP release by MYP3 cells, while P2X₄ antagonist had no effect but both antagonists inhibited pressure-induced caspase-1 activation. Moreover, when MYP3 cells were treated with extracellular ATP (500 µM), P2X₄ antagonist inhibited ATP-induced caspase-1 activation, but not P2X₇ antagonist. We concluded that pressure-induced extracellular ATP in urothelial cells is amplified by P2X₇ receptor activation and ATP-induced-ATP release. The amplified ATP signal then activates P2X₄ receptors, which mediate activation of the caspase-1 inflammatory response.

Cationic control of Panx1 channel function

The sequence and predicted membrane topology of pannexin1 (Panx1) places it in the family of gap junction proteins. However, rather than forming gap junction channels, Panx1 forms channels in the nonjunctional membrane. Panx1 operates in two distinct open states, depending on the mode of stimulation. The exclusively voltage-gated channel has a small conductance (<100 pS) and is highly selective for the flux of chloride ions. The Panx1 channel activated by various physiological stimuli or by increased concentrations of extracellular potassium ions has a large conductance (~500 pS, however, with multiple, long-lasting subconductance states) and is nonselectively permeable to small molecules, including ATP. To test whether the two open conformations also differ pharmacologically, the effects of di-and trivalent cations on the two Panx1 channel conformations were investigated. The rationale for this venture was that, under certain experimental conditions, ATP release from cells can be inhibited by multivalent cations, yet the literature indicates that the ATP release channel Panx1 is not affected by these ions. Consistent with previous reports, the Panx1 channel was not activated by removal of extracellular Ca²⁺ and the currents through the voltage-activated channel were not altered by Ca²⁺, Zn²⁺, Ba²⁺, or Gd³⁺. In contrast, the Panx1 channel activated to the large channel conformation by extracellular K⁺, osmotic stress, or low oxygen was inhibited by the multivalent cations in a dose-dependent way. Thus, monovalent cations activated the Panx1 channel from the closed state to the "large" conformation, while di- and trivalent cations exclusively inhibited this large channel conformation.

Fate of tenogenic differentiation potential of human bone marrow stromal cells by uniaxial stretching affected by stretch-activated calcium channel agonist gadolinium

The role for mechanical stimulation in the control of cell fate has been previously proposed, suggesting that there may be a role of mechanical conditioning in directing mesenchymal stromal cells (MSCs) towards specific lineage for tissue engineering applications. Although previous studies have reported that calcium signalling is involved in regulating many cellular processes in many cell types, its role in managing cellular responses to tensile loading (mechanotransduction) of MSCs has not been fully elucidated. In order to establish this, we disrupted calcium signalling by blocking stretch-activated calcium channel (SACC) in human MSCs (hMSCs) in vitro. Passaged-2 hMSCs were exposed to cyclic tensile loading (1 Hz + 8% for 6, 24, 48, and 72 hours) in the presence of the SACC blocker, gadolinium. Analyses include image observations of immunochemistry and immunofluorescence staining from extracellular matrix (ECM) production, and measuring related tenogenic and apoptosis gene marker expression. Uniaxial tensile loading increased the expression of tenogenic markers and ECM production. However, exposure to strain in the presence of 20 μM gadolinium reduced the induction of almost all tenogenic markers and ECM staining, suggesting that SACC acts as a mechanosensor in strain-induced hMSC tenogenic differentiation process. Although cell death was observed in prolonged stretching, it did not appear to be apoptosis mediated. In conclusion, the knowledge gained in this study by elucidating the role of calcium in MSC mechanotransduction processes, and that in prolonged stretching results in non-apoptosis mediated cell death may be potential useful for regenerative medicine applications.

Cell culture: complications due to mechanical release of ATP and activation of purinoceptors

There is abundant evidence that ATP (adenosine 5′-triphosphate) is released from a variety of cultured cells in response to mechanical stimulation. The release mechanism involved appears to be a combination of vesicular exocytosis and connexin and pannexin hemichannels. Purinergic receptors on cultured cells mediate both short-term purinergic signalling of secretion and long-term (trophic) signalling such as proliferation, migration, differentiation and apoptosis. We aim in this review to bring to the attention of non-purinergic researchers using tissue culture that the release of ATP in response to mechanical stress evoked by the unavoidable movement of the cells acting on functional purinergic receptors on the culture cells is likely to complicate the interpretation of their data.

Suppression of cell membrane permeability by suramin: involvement of its inhibitory actions on connexin 43 hemichannels

BACKGROUND AND PURPOSE

Suramin is a clinically prescribed drug for treatment of human African trypanosomiasis, cancer and infection. It is also a well-known pharmacological antagonist of P2 purinoceptors. Despite its clinical use and use in research, the biological actions of this molecule are still incompletely understood. Here, we investigated the effects of suramin on membrane channels, as exemplified by its actions on non-junctional connexin43 (Cx43) hemichannels, pore-forming α-haemolysin and channels involved in ATP release under hypotonic conditions.

EXPERIMENTAL APPROACH

Hemichannels were activated by removing extracellular Ca(2+) . The influences of suramin on hemichannel activities were evaluated by its effects on influx of fluorescent dyes and efflux of ATP. The membrane permeability and integrity were assessed through cellular retention of preloaded calcein and LDH release.

KEY RESULTS

Suramin blocked Cx43 hemichannel permeability induced by removal of extracellular Ca(2+) without much effect on Cx43 expression and gap junctional intercellular communication. This action of suramin was mimicked by its analogue NF023 and NF449 but not by another P2 purinoceptor antagonist PPADS. Besides hemichannels, suramin also significantly blocked intracellular and extracellular exchanges of small molecules caused by α-haemolysin from Staphylococcus aureus and by exposure of cells to hypotonic solution. Furthermore, it prevented α-haemolysin- and hypotonic stress-elicited cell injury.

CONCLUSION AND IMPLICATIONS

Suramin blocked membrane channels and protected cells against toxin- and hypotonic stress-elicited injury. Our finding provides novel mechanistic insights into the pharmacological actions of suramin. Suramin might be therapeutically exploited to protect membrane integrity under certain pathological situations.

Hemolysis is a primary ATP-release mechanism in human erythrocytes

The hypothesis that regulated ATP release from red blood cells (RBCs) contributes to nitric oxide-dependent control of local blood flow has sparked much interest in underlying release mechanisms. Several stimuli, including shear stress and hypoxia, have been found to induce significant RBC ATP release attributed to activation of ATP-conducting channels. In the present study, we first evaluated different experimental approaches investigating stimulated RBC ATP release and quantifying hemolysis. We then measured ATP and free hemoglobin in each and every RBC supernatant sample to directly assess the contribution of hemolysis to ATP release. Hypotonic shock, shear stress, and hypoxia, but not cyclic adenosine monophosphate agonists, significantly enhanced ATP release. It tightly correlated, however, with free hemoglobin in RBC supernatants, indicating that lysis was responsible for most, if not all, ATP release. Luminescence ATP imaging combined with simultaneous infrared cell imaging showed that ATP was released exclusively from lysing cells with no contribution from intact cells. In summary, with all stimuli tested, we found no evidence of regulated ATP release from intact RBCs other than by cell lysis. Such a release mechanism might be physiologically relevant in vivo, eg, during exercise and hypoxia where intravascular hemolysis, predominantly of senescent cells, is augmented.

Hypotonic shock modulates Na(+) current via a Cl(-) and Ca(2+)/calmodulin dependent mechanism in alveolar epithelial cells

Alveolar epithelial cells are involved in Na⁺ absorption via the epithelial Na⁺ channel (ENaC), an important process for maintaining an appropriate volume of liquid lining the respiratory epithelium and for lung oedema clearance. Here, we investigated how a 20% hypotonic shock modulates the ionic current in these cells. Polarized alveolar epithelial cells isolated from rat lungs were cultured on permeant filters and their electrophysiological properties recorded. A 20% bilateral hypotonic shock induced an immediate, but transient 52% rise in total transepithelial current and a 67% increase in the amiloride-sensitive current mediated by ENaC. Amiloride pre-treatment decreased the current rise after hypotonic shock, showing that ENaC current is involved in this response. Since Cl^- transport is modulated by hypotonic shock, its contribution to the basal and hypotonic-induced transepithelial current was also assessed. Apical NPPB, a broad Cl^- channel inhibitor and basolateral DIOA a potassium chloride co-transporter (KCC) inhibitor reduced the total and ENaC currents, showing that transcellular Cl^- transport plays a major role in that process. During hypotonic shock, a basolateral Cl^- influx, partly inhibited by NPPB is essential for the hypotonic-induced current rise. Hypotonic shock promoted apical ATP secretion and increased intracellular Ca²⁺. While apyrase, an ATP scavenger, did not inhibit the hypotonic shock current response, W7 a calmodulin antagonist completely prevented the hypotonic current rise. These results indicate that a basolateral Cl^- influx as well as Ca²⁺/calmodulin, but not ATP, are involved in the acute transepithelial current rise elicited by hypotonic shock.

Mechanical stimulation-induced calcium wave propagation in cell monolayers: the example of bovine corneal endothelial cells

Intercellular communication is essential for the coordination of physiological processes between cells in a variety of organs and tissues, including the brain, liver, retina, cochlea and vasculature. In experimental settings, intercellular Ca(2+)-waves can be elicited by applying a mechanical stimulus to a single cell. This leads to the release of the intracellular signaling molecules IP3 and Ca(2+) that initiate the propagation of the Ca(2+)-wave concentrically from the mechanically stimulated cell to the neighboring cells. The main molecular pathways that control intercellular Ca(2+)-wave propagation are provided by gap junction channels through the direct transfer of IP3 and by hemichannels through the release of ATP. Identification and characterization of the properties and regulation of different connexin and pannexin isoforms as gap junction channels and hemichannels are allowed by the quantification of the spread of the intercellular Ca(2+)-wave, siRNA, and the use of inhibitors of gap junction channels and hemichannels. Here, we describe a method to measure intercellular Ca(2+)-wave in monolayers of primary corneal endothelial cells loaded with Fluo4-AM in response to a controlled and localized mechanical stimulus provoked by an acute, short-lasting deformation of the cell as a result of touching the cell membrane with a micromanipulator-controlled glass micropipette with a tip diameter of less than 1 μm. We also describe the isolation of primary bovine corneal endothelial cells and its use as model system to assess Cx43-hemichannel activity as the driven force for intercellular Ca(2+)-waves through the release of ATP. Finally, we discuss the use, advantages, limitations and alternatives of this method in the context of gap junction channel and hemichannel research.

Maxi-anion channel and pannexin 1 hemichannel constitute separate pathways for swelling-induced ATP release in murine L929 fibrosarcoma cells

The maxi-anion channel plays a classically recognized role in controlling the membrane potential through the chloride conductance. It also has novel functions as a regulated pathway for the release of the anionic signaling molecules ATP and excitatory amino acids from cells subjected to osmotic perturbation, ischemia, or hypoxia. Because hemichannels formed by pannexins and connexins have been reported to mediate ATP release from a number of cell types, these hemichannels may represent the molecular correlate of the maxi-anion channel. Here, we found that L929 fibrosarcoma cells express functional maxi-anion channels which mediate a major portion of swelling-induced ATP release, and that ATP released via maxi-anion channels facilitates the regulatory volume decrease after osmotic swelling. Also, it was found that the cells express the mRNA for pannexin 1, pannexin 2, and connexin 43. Hypotonicity-induced ATP release was partially suppressed not only by known blockers of the maxi-anion channel but also by several blockers of pannexins including the pannexin 1-specific blocking peptide (10)Panx1 and small interfering (si)RNA against pannexin 1 but not pannexin 2. The inhibitory effects of maxi-anion channel blockers and pannexin 1 antagonists were additive. In contrast, maxi-anion channel activity was not affected by pannexin 1 antagonists and siRNAs against pannexins 1 and 2. Although a connexin 43-specific blocking peptide, Gap27, slightly suppressed hypotonicity-induced ATP release, maxi-anion channel activity was not affected by Gap27 or connexin 43-specific siRNA. Thus, it is concluded that the maxi-anion channel is a molecular entity distinct from pannexin 1, pannexin 2, and connexin 43, and that the maxi-anion channel and the hemichannels constitute separate pathways for swelling-induced ATP release in L929 cells.

Neuroepithelial bodies as mechanotransducers in the intrapulmonary airway epithelium: involvement of TRPC5

In rodent lungs, a major part of the myelinated vagal airway afferents selectively contacts pulmonary neuroepithelial bodies (NEBs). Because most myelinated vagal airway afferents concern physiologically characterized mechanoreceptors, the present study aimed at unraveling the potential involvement of NEB cells in transducing mechanosensory information from the airways to the central nervous system. Physiological studies were performed using confocal Ca(2+) imaging of airway epithelium in murine lung slices. Mechanical stimulation by short-term application of a mild hypoosmotic solution (230 mosmol) resulted in a selective, fast, reversible, and reproducible Ca(2+) rise in NEB cells. Other airway epithelial cells could only be activated using more severe hypoosmotic stimuli (< 200 mosmol). NEB cells selectively expressed the Ca(2+)-permeable osmo- and mechanosensitive transient receptor potential canonical channel 5 (TRPC5) in their apical membranes, whereas immunoreactivity for TRP vanilloid-4 and TRP melastatin-3 was abundant in virtually all other airway epithelial cells. Hypoosmotic activation of NEB cells was prevented by GsMTx-4, an inhibitor of mechanosensitive ion channels, and by SKF96365, an inhibitor of TRPC channels. Short application of gadolinium, reported to activate TRPC5 channels, evoked a transient Ca(2+) rise in NEB cells. Osmomechanical activation of NEB cells gave rise to a typical delayed activation of Clara-like cells due to the release of ATP from NEB cells. Because ATP may activate the NEB-associated P2X(2/3) ATP receptor expressing myelinated vagal afferents, the current observations strongly suggest that pulmonary NEB cells are fully equipped to initiate mechanosensory signal transduction to the central nervous system via a purinergic signaling pathway.

Imaging exocytosis of ATP-containing vesicles with TIRF microscopy in lung epithelial A549 cells

Nucleotide release constitutes the first step of the purinergic signaling cascade, but its underlying mechanisms remain incompletely understood. In alveolar A549 cells much of the experimental data is consistent with Ca(2+)-regulated vesicular exocytosis, but definitive evidence for such a release mechanism is missing, and alternative pathways have been proposed. In this study, we examined ATP secretion from A549 cells by total internal reflection fluorescence microscopy to directly visualize ATP-loaded vesicles and their fusion with the plasma membrane. A549 cells were labeled with quinacrine or Bodipy-ATP, fluorescent markers of intracellular ATP storage sites, and time-lapse imaging of vesicles present in the evanescent field was undertaken. Under basal conditions, individual vesicles showed occasional quasi-instantaneous loss of fluorescence, as expected from spontaneous vesicle fusion with the plasma membrane and dispersal of its fluorescent cargo. Hypo-osmotic stress stimulation (osmolality reduction from 316 to 160 mOsm) resulted in a transient, several-fold increment of exocytotic event frequency. Lowering the temperature from 37°C to 20°C dramatically diminished the fraction of vesicles that underwent exocytosis during the 2-min stimulation, from ~40% to ≤1%, respectively. Parallel ATP efflux experiments with luciferase bioluminescence assay revealed that pharmacological interference with vesicular transport (brefeldin, monensin), or disruption of the cytoskeleton (nocodazole, cytochalasin), significantly suppressed ATP release (by up to ~80%), whereas it was completely blocked by N-ethylmaleimide. Collectively, our data demonstrate that regulated exocytosis of ATP-loaded vesicles likely constitutes a major pathway of hypotonic stress-induced ATP secretion from A549 cells.

Thrombin-induced ATP release from human umbilical vein endothelial cells

ATP and its degradation products play an important role as signaling molecules in the vascular system, and endothelial cells are considered to be an important source of nucleotide release. To investigate the mechanism and physiological significance of endothelial ATP release, we compared different pharmacological stimuli for their ability to evoke ATP release from first passage cultivated human umbilical vein endothelial cells (HUVECs). Agonists known to increase intracellular Ca(2+) levels (A23187, histamine, thrombin) induced a stable, non-lytic ATP release. Since thrombin proved to be the most robust and reproducible stimulus, the molecular mechanism of thrombin-mediated ATP release from HUVECs was further investigated. ATP rapidly increased with thrombin (1 U/ml) and reached a steady-state level after 4 min. Loading the cells with BAPTA-AM to capture intracellular calcium suppressed ATP release. The thrombin-specific, protease-activated receptor 1 (PAR-1)-specific agonist peptide TFLLRN (10 μM) fully mimicked thrombin action on ATP release. To identify the nature of the ATP-permeable pathway, we tested various inhibitors of potential ATP channels for their ability to inhibit the thrombin response. Carbenoxolone, an inhibitor of connexin hemichannels and pannexin channels, as well as Gd(3+) were highly effective in blocking the thrombin-mediated ATP release. Specifically targeting connexin43 (Cx43) and pannexin1 (Panx1) revealed that reducing Panx1 expression significantly reduced ATP release, while downregulating Cx43 was ineffective. Our study demonstrates that thrombin at physiological concentrations is a potent stimulus of endothelial ATP release involving PAR-1 receptor activation and intracellular calcium mobilization. ATP is released by a carbenoxolone- and Gd(3+)- sensitive pathway, most likely involving Panx1 channels.

Negative-feedback regulation of ATP release: ATP release from cardiomyocytes is strictly regulated during ischemia

Extracellular ATP acts as a potent agonist on cardiomyocytes, inducing a broad range of physiological responses via P2 purinoceptors. Its concentration in the interstitial space within the heart is elevated during ischemia or hypoxia due to its release from a number of cell types, including cardiomyocytes. However, the exact mechanism responsible for the release of ATP from cardiomyocytes during ischemia is not known. In this study, we investigated whether and how the release of ATP was strictly regulated during ischemia in cultured neonatal rat cardiomyocytes. Ischemia was mimicked by oxygen-glucose deprivation (OGD). Exposure of cardiomyocytes to OGD resulted in an increase in the concentration of extracellular ATP shortly after the onset of OGD (15 min), and the increase was reversed by treatment with blockers of maxi-anion channels. Unexpectedly, at 1 and 2h after the onset of OGD, the blocking of maxi-anion channels increased the concentration of extracellular ATP, and the increase was significantly suppressed by co-treatment with blockers of hemichannels, suggesting that ATP release via maxi-anion channels was involved in the suppression of ATP release via hemichannels during persistent OGD. Here we show the possibility that the release of ATP from cardiomyocytes was strictly regulated during ischemia by negative-feedback mechanisms; that is, maxi-anion channel-derived ATP-induced suppression of ATP release via hemichannels in cardiomyocytes.

Molecular mechanisms of purine and pyrimidine nucleotide release

Given the widespread importance of purinergic receptor-evoked signaling, understanding how ATP and other nucleotides are released from cells in a regulated manner is an essential physiological question. Nonlytic release of ATP, UTP, UDP-glucose, and other nucleotides occurs in all cell types and tissues via both constitutive mechanisms, that is, in the absence of external stimuli, and to a greater extent in response to biochemical or mechanical/physical stimuli. However, a molecular understanding of the processes regulating nucleotide release has only recently begun to emerge. It is generally accepted that nucleotide release occurs in two different scenarios, exocytotic release from the secretory pathway or via conductive/transport mechanisms, and a critical review of our current understanding of these mechanisms is presented in this chapter.

Potentiation of regulatory volume decrease by a p2-like receptor and arachidonic acid in american alligator erythrocytes

This study examined the role of a P2 receptor and arachidonic acid (AA) in regulatory volume decrease (RVD) by American alligator red blood cells (RBCs). Osmotic fragility was determined optically, mean cell volume was measured by electronic sizing, and changes in intracellular Ca(2+) concentration were visualized using fluorescence microscopy. Gadolinium (50 μM), hexokinase (2.5 U/ml), and suramin (100 μM) increased osmotic fragility, blocked volume recovery after hypotonic shock, and prevented a rise in intracellular Ca(2+) that normally occurs during cell swelling. The P2X antagonists PPADS (50 μM) and TNP-ATP (10 μM) also increased fragility and inhibited volume recovery. In contrast, ATPγS (10 μM), α,β-methylene-ATP (50 μM) and Bz-ATP (50 μM) had the opposite effect, whereas 2-methylthio-ATP (50 μM) and UTP (10 μM) had no effect. In addition, the phospholipase A(2) (PLA(2)) inhibitors ONO-RS-082 (10 μM), chlorpromazine (10 μM), and isotetrandrine (10 μM) increased osmotic fragility and blocked volume recovery, whereas AA (10 μM) and its nonhydrolyzable analog eicosatetraynoic acid (ETYA, 10 μM) had the reverse effect. Further, AA (10 μM), but not ATPγS (10 μM), prevented the inhibitory effect of a low Ca(2+)-EGTA Ringer on RVD, whereas both AA (10 μM) and ATPγS (10 μM) caused cell shrinkage under isosmotic conditions. In conclusion, our results are consistent with the presence of a P2-like receptor whose activation stimulated RVD. In addition, AA also was important for volume recovery.

Evidence for sustained ATP release from liver cells that is not mediated by vesicular exocytosis

Extracellular ATP regulates many important cellular functions in the liver by stimulating purinergic receptors. Recent studies have shown that rapid exocytosis of ATP-enriched vesicles contributes to ATP release from liver cells. However, this rapid ATP release is transient, and ceases in ~30 s after the exposure to hypotonic solution. The purpose of these studies was to assess the role of vesicular exocytosis in sustained ATP release. An exposure to hypotonic solution evoked sustained ATP release that persisted for more than 15 min after the exposure. Using FM1-43 (N-(3-triethylammoniumpropyl)-4-(4-(dibutylamino)styryl)pyridinium dibromide) fluorescence to measure exocytosis, we found that hypotonic solution stimulated a transient increase in FM1-43 fluorescence that lasted ~2 min. Notably, the rate of FM1-43 fluorescence and the magnitude of ATP release were not correlated, indicating that vesicular exocytosis may not mediate sustained ATP release from liver cells. Interestingly, mefloquine potently inhibited sustained ATP release, but did not inhibit an increase in FM1-43 fluorescence evoked by hypotonic solution. Consistent with these findings, when exocytosis of ATP-enriched vesicles was specifically stimulated by 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB), mefloquine failed to inhibit ATP release evoked by NPPB. Thus, mefloquine can pharmacologically dissociate sustained ATP release and vesicular exocytosis. These results suggest that a distinct mefloquine-sensitive membrane ATP transport may contribute to sustained ATP release from liver cells. This novel mechanism of membrane ATP transport may play an important role in the regulation of purinergic signaling in liver cells.

Cyclic loading opens hemichannels to release ATP as part of a chondrocyte mechanotransduction pathway

The process of chondrocyte mechanotransduction is poorly understood. However, recent studies suggest the involvement of a purinergic calcium signaling pathway although the mechanism of ATP release has not been identified. The present study tests the hypothesis that cyclic compression opens hemichannels thereby triggering the release of ATP into the extracellular milieu activating P2 receptors. The well-established chondrocyte-agarose model was utilized enabling chondrocytes to be subjected to a 40-min period of cyclic compression at 0-15% strain and 1 Hz. The opening of hemichannels was determined using Lucifer yellow (LY) incorporation and fluorescence microscopy, whereas the release of ATP into the surrounding media was quantified using the luciferin-luciferase assay. Results indicated that cyclic compression activated hemichannels such that the percentage of cells showing LY incorporation increased from 50 to 70%. This was associated with a sevenfold increase in the release of ATP. Both LY incorporation and ATP release in response to mechanical loading were blocked by the hemichannel inhibitor, flufenamic acid. Treatment with apyrase or P2 receptor antagonists, suramin or oxidated-ATP, did not prevent the mechanically induced response. In conclusion, mechanical loading triggers release of ATP via hemichannels. Hence, this study provides the first evidence of hemichannel involvement in chondrocyte mechanobiology.

Extracellular ATP and P2Y2 receptors mediate intercellular Ca(2+) waves induced by mechanical stimulation in submandibular gland cells: Role of mitochondrial regulation of store operated Ca(2+) entry

Coordination of Ca(2+) signaling among cells contributes to synchronization of salivary gland cell function. However, mechanisms that underlie this signaling remain elusive. Here, intercellular Ca(2+) waves (ICW) in submandibular gland cells were investigated using Fura-2 fluorescence imaging. Mechanical stimulation of single cells induced ICW propagation from the stimulated cells through approximately 7 layers of cells or approximately 120microm. Our findings indicate that an extracellular ATP-dependent pathway is involved because the purinergic receptor antagonist suramin and the ATP hydrolyzing enzyme apyrase blocked ICW propagation. However, the gap junction uncoupler oleamide had no effect. ATP is released from mechanically stimulated cells possibly through opening of mechanosensitive maxi-anion channels, and does not appear to be directly linked to cytosolic Ca(2+). The ICW is propagated by diffusing ATP, which activates purinergic receptors in neighboring cells. This purinergic signaling induces a Ca(2+) transient that is dependent on Ca(2+) release via IP(3) receptors in the ER and store operated Ca(2+) entry (SOCE). Finally, inhibition of mitochondrial Ca(2+) uptake modified ICW indicating an important role of these organelles in this phenomenon. These studies increase our understanding of purinergic receptor signaling in salivary gland cells, and its role as a coordination mechanism of Ca(2+) signals induced by mechanical stimulation.

Pannexins, distant relatives of the connexin family with specific cellular functions?

Intercellular communication (IC) is mediated by gap junctions (GJs) and hemichannels, which consist of proteins. This has been particularly well documented for the connexin (Cx) family. Initially, Cxs were thought to be the only proteins capable of GJ formation in vertebrates. About 10 years ago, however, a new GJ-forming protein family related to invertebrate innexins (Inxs) was discovered in vertebrates, and named the pannexin (Panx) family. Panxs, which are structurally similar to Cxs, but evolutionarily distinct, have been shown to be co-expressed with Cxs in vertebrates. Both protein families show distinct properties and have their own particular function. Identification of the mechanisms that control Panx channel gating is a major challenge for future work. In this review, we focus on the specific properties and role of Panxs in normal and pathological conditions.

Regulation of cellular contractile force in response to mechanical stretch by diphosphorylation of myosin regulatory light chain via RhoA signaling cascade

Fibroblasts regulate their contractile force in response to external stretch; however, the detailed mechanism by which the force is regulated is unclear. Here, we show that diphosphorylation and dephosphorylation of myosin regulatory light chain (MRLC) are involved in the stretch-induced force response. Cellular stiffness, which reflects the cellular contractile force, under external stretch was measured by mechanical-scanning probe microscopy. Fibroblasts (NIH-3T3) expressing green fluorescent protein (GFP)-tagged mutant-type MRLC (MRLC(T18A)-GFP), which cannot be diphosphorylated, did not show any stretch-induced stiffness response, whereas the stiffness in cells expressing GFP-tagged wild-type MRLC (MRLC(WT)-GFP) increased immediately after the stretch and subsequently decreased after 1-2 h. Urea-PAGE western blot analysis showed that the proportion of diphosphorylated MRLC (PP-MRLC) transiently increased after the stretch and decreased after 1-2 h. Dominant-negative RhoA (RhoA(N19))-expressing cells did not show the stiffness response to the stretch, whereas wild-type RhoA-expressing cells did. It was concluded that the cellular force response originates in the stretch-induced diphosphorylation and dephosphorylation of MRLC and is regulated via the RhoA signaling cascade.

Involvement of respiratory chain in the regulatory volume decrease process in turbot hepatocytes

Regulatory volume decrease (RVD) constitutes a fundamental process that turbot (Scophthalmus maximus) hepatocytes are able to perform when exposed to hypo-osmotic stress. RVD is an integrative mechanism that involves various elements of the cellular machinery. Among others, ATP is an essential protagonist: released following hypo-osmotic shock, it acts as an auto/paracrine factor to trigger other signalling events. The origin of this ATP remains unclear and, to the best of our knowledge, no information exists about the role of mitochondrial respiration in RVD. Therefore, we propose to analyse the potential link between RVD and the respiratory chain, with a focus on ATP release and exocytosis. Using inhibitors of mitochondrial respiration, RVD was shown to be dependent on respiratory chain activity. However, we demonstrated an indirect role of mitochondrial respiration: ATP could be synthesized and then stored in intracellular vesicles until the moment cells release it to face hypo-osmotic swelling. However, the involvement of exocytosis in this process needs to be further investigated.

ATP release from non-excitable cells

All cells release nucleotides and are in one way or another involved in local autocrine and paracrine regulation of organ function via stimulation of purinergic receptors. Significant technical advances have been made in recent years to quantify more precisely resting and stimulated adenosine triphosphate (ATP) concentrations in close proximity to the plasma membrane. These technical advances are reviewed here. However, the mechanisms by which cells release ATP continue to be enigmatic. The current state of knowledge on different suggested mechanisms is also reviewed. Current evidence suggests that two separate regulated modes of ATP release co-exist in non-excitable cells: (1) a conductive pore which in several systems has been found to be the channel pannexin 1 and (2) vesicular release. Modes of stimulation of ATP release are reviewed and indicate that both subtle mechanical stimulation and agonist-triggered release play pivotal roles. The mechano-sensor for ATP release is not yet defined.

Autocrine regulation of T-cell activation by ATP release and P2X7 receptors

T-cell activation requires the influx of extracellular calcium, although mechanistic details regarding such activation are not fully defined. Here, we show that P2X(7) receptors play a key role in calcium influx and downstream signaling events associated with the activation of T cells. By real-time PCR and immunohistochemistry, we find that Jurkat T cells and human CD4(+) T cells express abundant P2X(7) receptors. We show, using a novel fluorescent microscopy technique, that T-cell receptor (TCR) stimulation triggers the rapid release of ATP (<100 microM). This release of ATP is required for TCR-mediated calcium influx, NFAT activation, and interleukin-2 (IL-2) production. TCR activation up-regulates P2X(7) receptor gene expression. Removal of extracellular ATP by apyrase or alkaline phosphatase treatment, inhibition of ATP release with the maxi-anion channel blocker gadolinium chloride, or siRNA silencing of P2X(7) receptors blocks calcium entry and inhibits T-cell activation. Moreover, lymphocyte activation is impaired in C57BL/6 mice that express poorly functional P2X(7) receptors, compared to control BALB/c mice, which express fully functional P2X(7) receptors. We conclude that ATP release and autocrine, positive feedback through P2X(7) receptors is required for the effective activation of T cells.

The maxi-anion channel: a classical channel playing novel roles through an unidentified molecular entity

The maxi-anion channel is widely expressed and found in almost every part of the body. The channel is activated in response to osmotic cell swelling, to excision of the membrane patch, and also to some other physiologically and pathophysiologically relevant stimuli, such as salt stress in kidney macula densa as well as ischemia/hypoxia in heart and brain. Biophysically, the maxi-anion channel is characterized by a large single-channel conductance of 300-400 pS, which saturates at 580-640 pS with increasing the Cl(-) concentration. The channel discriminates well between Na(+) and Cl(-), but is poorly selective to other halides exhibiting weak electric-field selectivity with an Eisenman's selectivity sequence I. The maxi-anion channel has a wide pore with an effective radius of approximately 1.3 nm and permits passage not only of Cl(-) but also of some intracellular large organic anions, thereby releasing major extracellular signals and gliotransmitters such as glutamate(-) and ATP(4-). The channel-mediated efflux of these signaling molecules is associated with kidney tubuloglomerular feedback, cardiac ischemia/hypoxia, as well as brain ischemia/hypoxia and excitotoxic neurodegeneration. Despite the ubiquitous expression, well-defined properties and physiological/pathophysiological significance of this classical channel, the molecular entity has not been identified. Molecular identification of the maxi-anion channel is an urgent task that would greatly promote investigation in the fields not only of anion channel but also of physiological/pathophysiological signaling in the brain, heart and kidney.

Purinergic activation of anion conductance and osmolyte efflux in cultured rat hippocampal neurons

The majority of mammalian cells demonstrate regulatory volume decrease (RVD) following swelling caused by hyposmotic exposure. A critical signal initiating RVD is activation of nucleotide receptors by ATP. Elevated extracellular ATP in response to cytotoxic cell swelling during pathological conditions also may initiate loss of taurine and other intracellular osmolytes via anion channels. This study characterizes neuronal ATP-activated anion current and explores its role in net loss of amino acid osmolytes. To isolate anion currents, we used CsCl as the major electrolyte in patch electrode and bath solutions and blocked residual cation currents with NiCl(2) and tetraethylammonium. Anion currents were activated by extracellular ATP with a K(m) of 70 microM and increased over fourfold during several minutes of ATP exposure, reaching a maximum after 9.0 min (SD 4.2). The currents were blocked by inhibitors of nucleotide receptors and volume-regulated anion channels (VRAC). Currents showed outward rectification and inactivation at highly depolarizing membrane potentials, characteristics of swelling-activated anion currents. P2X agonists failed to activate the anion current, and an inhibitor of P2X receptors did not block the effect of ATP. Furthermore, current activation was observed with extracellular ADP and 2-(methylthio)adenosine 5'-diphosphate, a P2Y(1) receptor-specific agonist. Much less current activation was observed with extracellular UTP, suggesting the response is mediated predominantly by P2Y(1) receptors. ATP caused a dose-dependent loss of taurine and alanine that could be blocked by inhibitors of VRAC. ATP did not inhibit the taurine uptake transporter. Thus extracellular ATP triggers a loss of intracellular organic osmolytes via activation of anion channels. This mechanism may facilitate neuronal volume homeostasis during cytotoxic edema.

Interstitial adenosine triphosphate modulates muscle afferent nerve-mediated pressor reflex

Previous work has shown that muscle contraction elevates interstitial adenosine triphosphate concentration ([ATP]i), which is likely due to the release of ATP from active skeletal muscle. ATP activation of purinergic receptors P2X on thin muscle afferent fibers further enhances cardiovascular responses to contraction. Thus, the purposes of this study were: (1) to examine the mechanisms by which ATP is released from muscle in response to mechanical stimulation; and (2) to study the effects of interstitial ATP concentrations on modulating pressor response to muscle contraction. Static contraction of the triceps surae muscle was evoked by electrical stimulation (at 5 HZ and 2.5 times motor threshold) of the tibial nerve in 9 anesthetized cats. Muscle interstitial ATP samples were collected from microdialysis probes inserted into the muscles. Dialysate ATP concentrations were determined using the luciferin-luciferase assay. In a control experiment, contraction was induced after 0.5 ml of saline was injected into the arterial blood supply of the hindlimb muscles. This increased [ATP]i by 220% (P < 0.05 vs. baseline). After gadolinium (1 mM), a blocker of mechanically sensitive channels, was injected into the muscles, contraction increased [ATP]i by 112% (P < 0.05 vs. control). In contrast, glibenclamide (an inhibitor of the ATP-binding cassette protein), monensin, and brefeldin A, which interfere with vesicular formation (or trafficking) and inhibit exocytosis, did not significantly affect ATP release by muscle contraction. In addition, a regression analysis showed that [ATP]i was linearly related to the pressor response to muscle contraction. The data suggest that ATP release from skeletal muscle is mediated via involvement of mechanosensitive channels. These findings further support a physiological role for release of ATP in modulating cardiovascular responses during static muscle contraction.

Autocrine signaling involved in cell volume regulation: the role of released transmitters and plasma membrane receptors

Cell volume regulation is a basic homeostatic mechanism transcendental for the normal physiology and function of cells. It is mediated principally by the activation of osmolyte transport pathways that result in net changes in solute concentration that counteract cell volume challenges in its constancy. This process has been described to be regulated by a complex assortment of intracellular signal transduction cascades. Recently, several studies have demonstrated that alterations in cell volume induce the release of a wide variety of transmitters including hormones, ATP and neurotransmitters, which have been proposed to act as extracellular signals that regulate the activation of cell volume regulatory mechanisms. In addition, changes in cell volume have also been reported to activate plasma membrane receptors (including tyrosine kinase receptors, G-protein coupled receptors and integrins) that have been demonstrated to participate in the regulatory process of cell volume. In this review, we summarize recent studies about the role of changes in cell volume in the regulation of transmitter release as well as in the activation of plasma membrane receptors and their further implications in the regulation of the signaling machinery that regulates the activation of osmolyte flux pathways. We propose that the autocrine regulation of Ca2+-dependent and tyrosine phosphorylation-dependent signaling pathways by the activation of plasma membrane receptors and swelling-induced transmitter release is necessary for the activation/regulation of osmolyte efflux pathways and cell volume recovery. Furthermore, we emphasize the importance of studying these extrinsic signals because of their significance in the understanding of the physiology of cell volume regulation and its role in cell biology in vivo, where the constraint of the extracellular space might enhance the autocrine or even paracrine signaling induced by these released transmitters.

Acute inhibition of the betaine transporter by ATP and adenosine in renal MDCK cells

Extracellular ATP interacts with purinergic P2 receptors to regulate a range of physiological responses, including downregulation of transport activity in the nephron. ATP is released from cells by mechanical stimuli such as cell volume changes, and autocrine signaling by extracellular ATP could occur in renal medullary cells during diuresis. This was tested in Madin-Darby canine kidney (MDCK) cells, a model used frequently to study P1 and P2 receptor activity. ATP was released within 1 min after transfer from 500 to 300 mosmol/kgH2O medium. A 30-min incubation with ATP produced dose-dependent inhibition (0.01-0.10 mM) of the renal betaine/GABA transporter (BGT1) with little effect on other osmolyte transporters. Inhibition was reproduced by specific agonists for P2X (alpha,beta-methylene-ATP) and P2Y (UTP) receptors. Adenosine, the final product of ATP hydrolysis, also inhibited BGT1 but not taurine transport. Inhibition by ATP and adenosine was blocked by pertussis toxin and A73122, suggesting involvement of inhibitory G protein and PLC in postreceptor signaling. Both ATP and adenosine (0.1 mM) produced rapid increases in intracellular Ca2+, due to the mobilization of intracellular Ca2+ stores and Ca2+ influx. Blocking these Ca2+ increases with BAPTA-AM also blocked the action of ATP and adenosine on BGT1 transport. Finally, immunohistochemical studies indicated that inhibition of BGT1 transport may be due to endocytic accumulation of BGT1 proteins from the plasma membrane. We conclude that ATP and adenosine, through stimulation of PLC and intracellular Ca2+, may be rapidly acting regulators of BGT1 transport especially in response to a fall in extracellular osmolarity.

Oxygen-glucose deprivation induces ATP release via maxi-anion channels in astrocytes

ATP represents a major gliotransmitter that serves as a signaling molecule for the cross talk between glial and neuronal cells. ATP has been shown to be released by astrocytes in response to a number of stimuli under nonischemic conditions. In this study, using a luciferin-luciferase assay, we found that mouse astrocytes in primary culture also exhibit massive release of ATP in response to ischemic stress mimicked by oxygen-glucose deprivation (OGD). Using a biosensor technique, the local ATP concentration at the surface of single astrocytes was found to increase to around 4 μM. The OGD-induced ATP release was inhibited by Gd³⁺ and arachidonic acid but not by blockers of volume-sensitive outwardly rectifying Cl⁻ channels, cystic fibrosis transmembrane conductance regulator (CFTR), multidrug resistance-related protein (MRP), connexin or pannexin hemichannels, P2X₇ receptors, and exocytotic vesicular transport. In cell-attached patches on single astrocytes, OGD caused activation of maxi-anion channels that were sensitive to Gd³⁺ and arachidonic acid. The channel was found to be permeable to ATP⁴⁻ with a permeability ratio of P_ATP/P_Cl = 0.11. Thus, it is concluded that ischemic stress induces ATP release from astrocytes and that the maxi-anion channel may serve as a major ATP-releasing pathway under ischemic conditions.

Mechanosensor Channels in Mammalian Somatosensory Neurons

Mechanoreceptive sensory neurons innervating the skin, skeletal muscles and viscera signal both innocuous and noxious information necessary for proprioception, touch and pain. These neurons are responsible for the transduction of mechanical stimuli into action potentials that propagate to the central nervous system. The ability of these cells to detect mechanical stimuli impinging on them relies on the presence of mechanosensitive channels that transduce the external mechanical forces into electrical and chemical signals. Although a great deal of information regarding the molecular and biophysical properties of mechanosensitive channels in prokaryotes has been accumulated over the past two decades, less is known about the mechanosensitive channels necessary for proprioception and the senses of touch and pain. This review summarizes the most pertinent data on mechanosensitive channels of mammalian somatosensory neurons, focusing on their properties, pharmacology and putative identity.

Exercise training normalizes beta-adrenoceptor expression in dogs susceptible to ventricular fibrillation

Previous studies demonstrated an enhanced beta(2)-adrenoceptor (AR) responsiveness in animals susceptible to ventricular fibrillation (VF) that was eliminated by exercise training. The present study investigated the effects of endurance exercise training on beta(1)-AR and beta(2)-AR expression in dogs susceptible to VF. Myocardial ischemia was induced by a 2-min occlusion of the left circumflex artery during the last minute of exercise in dogs with healed infarctions: 20 had VF [susceptible (S)] and 13 did not [resistant (R)]. These dogs were randomly assigned to either 10-wk exercise training [treadmill running; n = 9 (S) or 8 (R)] or an equivalent sedentary period [n = 11 (S) or 5 (R)]. Left ventricular tissue beta-AR protein and mRNA were quantified by Western blot analysis and RT-PCR, respectively. Because beta(2)-ARs are located in caveolae, caveolin-3 was also quantified. beta(1)-AR gene expression decreased ( approximately 5-fold), beta(2)-AR gene expression was not changed, and the ratio of beta(2)-AR to beta(1)-AR gene expression was significantly increased in susceptible compared with resistant dogs. beta(1)-AR protein decreased ( approximately 50%) and beta(2)-AR protein increased (400%) in noncaveolar fractions of the cell membrane in susceptible dogs. Exercise training returned beta(1)-AR gene expression to levels seen in resistant animals but did not alter beta(2)-AR protein levels in susceptible dogs. These data suggest that beta(1)-AR gene expression was decreased in susceptible dogs compared with resistant dogs and, further, that exercise training improves beta(1)-AR gene expression, thereby restoring a more normal beta-AR balance.

Salt-sensing mechanisms in blood pressure regulation and hypertension

High salt consumption contributes to the development of hypertension and is considered an independent risk factor for vascular remodeling, cardiac hypertrophy, and stroke incidence. In this review, we discuss the molecular origins of primary sensors involved in the phenomenon of salt sensitivity. Based on the analysis of literature data, we conclude that the kidneys and central nervous system (CNS) are two major sites for salt sensing via several distinct mechanisms: 1) [Cl(-)] sensing in renal tubular fluids, primarily by Na(+)-K(+)-Cl(-) cotransporter (NKCC) isoforms NKCC2B and NKCC2A, whose expression is mainly limited to macula densa cells; 2) [Na(+)] sensing in cerebrospinal fluid (CSF) by a novel isoform of Na(+) channels, Na(x), expressed in subfornical organs; 3) sensing of CSF osmolality by mechanosensitive, nonselective cation channels (transient receptor potential vanilloid type 1 channels), expressed in neuronal cells of supraoptic and paraventricular nuclei; and 4) osmolarity sensing by volume-regulated anion channels in glial cells of supraoptic and paraventricular nuclei. Such multiplicity of salt-sensing mechanisms likely explains the differential effects of Na(+) and Cl(-) loading on the long-term maintenance of elevated blood pressure that is documented in experimental models of salt-sensitive hypertension.

Calcium-dependent release of adenosine and uridine nucleotides from A549 cells

Extracellular nucleotides play an important role in lung defense, but the release mechanism and relative abundance of different nucleotide species secreted by lung epithelia are not well defined. In this study, to minimize cell surface hydrolysis, we used a low-volume, flow-through chamber and examined adenosine and uridine nucleotide concentrations in perfusate aliquots of human lung A549 cells challenged by 50% hypotonic shock. Adenosine triphosphate (ATP), adenosine diphosphate (ADP), adenosine monophosphate (AMP), and adenosine (Ado) were quantified in high-performance liquid chromatography (HPLC) analysis of fluorescent etheno derivatives, and uridine triphosphate (UTP) and uridine diphosphate (UDP) were measured using HPLC-coupled radioenzymatic assays. After the onset of hypotonic shock, ATP, ADP, UTP, and UDP in the perfusates increased markedly and peaked at approximately 2.5 min, followed by a gradual decay in the next 15-20 min; peak changes in Ado and AMP were relatively minor. The peak concentrations and fold increment (in parentheses) were: 34 +/- 13 nM ATP (5.6), 11 +/- 5 nM ADP (3.7), 3.3 +/- 1.2 nM AMP (1.4), 23 +/- 7 nM Ado (2.1), 21 nM UTP (>7), and 11 nM UDP (27). Nucleotide release was almost completely abolished from cells loaded with the calcium chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA). Under isotonic conditions, elevation of intracellular calcium with the calcium ionophore ionomycin (5 muM, 3 min) also released nucleotides with kinetics and relative abundance as above, albeit less robust. ADP:ATP (1:3) and UDP:UTP (1:2) ratios in perfusates from stimulated cells were markedly higher than the cytosolic ratios of these species, suggesting that a nucleotide diphosphate (NDP)-rich compartment, e.g., the secretory pathway, contributed to nucleotide release. Laser confocal microscopy experiments illustrated increased FM1-43 uptake into the plasma membrane upon hypotonic shock or ionomycin treatment, consistent with enhanced vesicular exocytosis under these conditions. In summary, our results strongly suggest that calcium-dependent exocytosis is responsible, at least in most part, for adenosine and uridine nucleotide release from A549 cells.

Purinergic inhibition of Na⁺,K⁺,Cl⁻ cotransport in C11-MDCK cells: Role of stress-activated protein kinases

Previously, we observed that sustained activation of P2Y₁ leads to inhibition of Na⁺,K⁺,Cl⁻ cotransport (NKCC) in C11 cells resembling intercalated cells from collecting ducts of the Madin-Darby canine kidney. This study examined the role of stress-activated protein kinases (SAPK) in NKCC inhibition triggered by purinergic receptors. Treatment of C11 cells with ATP led to sustained phosphorylation of SAPK such as JNK and p38. Activation of these kinases also occurred in anisomycin-treated cells. Surprisingly, we observed that compounds SP600125 and SB202190, known as potent inhibitors of JNK and p38 in cell-free systems, activated rather than inhibited phosphorylation of the kinases in C11 cells. Importantly, similarly to ATP, all the above-listed activators of JNK and p38 phosphorylation inhibited NKCC. Thus, our results suggest that activation of JNK and/or p38 contributes to NKCC suppression detected in intercalated-like cells from distal tubules after their exposure to P2Y₁ agonists.

Regulation of the epithelial sodium channel by phosphatidylinositides: experiments, implications, and speculations

Recent studies suggest that the activity of epithelial sodium channels (ENaC) is increased by phosphatidylinositides, especially phosphatidylinositol 4,5-bisphosphate (PI(4,5)P(2)) and phosphatidylinositol 3,4,5-trisphosphate (PI(3,4,5)P(3)). Stimulation of phospholipase C by either adenosine triphosphate (ATP)-activation of purinergic P2Y receptors or epidermal growth factor (EGF)-activation of EGF receptors reduces membrane PI(4,5)P(2), and consequently decreases ENaC activity. Since ATP and EGF may be trapped in cysts formed by the distal tubule, it is possible that ENaC inhibition induced by ATP and EGF facilitates cyst formation in polycystic kidney diseases (PKD). However, some results suggest that ENaC activity is increased in PKD. In contrast to P2Y and EGF receptors, stimulation of insulin-like growth factor-1 (IGF-1) receptor by aldosterone or insulin produces PI(3,4,5)P(3), and consequently increases ENaC activity. The acute effect of aldosterone on ENaC activity through PI(3,4,5)P(3) possibly accounts for the initial feedback for blood volume recovery after hypovolemic hypotension. PI(4,5)P(2) and PI(3,4,5)P(3), respectively, interacts with the N terminus of beta-ENaC and the C terminus of gamma-ENaC. However, whether ENaC selectively binds to PI(4,5)P(2) and PI(3,4,5)P(3) over other anionic phospholipids remains unclear.

Swelling-Activated K+ Efflux and Regulatory Volume Decrease Efficiency in Human Bronchial Epithelial Cells

This study describes the correlation between cell swelling-induced K+ efflux and volume regulation efficiency evaluated with agents known to modulate ion channel activity and/or intracellular signaling processes in a human bronchial epithelial cell line, 16HBE14o(-1). Cells on permeable filter supports, differentiated into polarized monolayers, were monitored continuously at room temperature for changes in cell height (T(c)), as an index of cell volume, whereas (86)Rb efflux was assessed for K+ channel activity. The sudden reduction in osmolality of both the apical and basolateral perfusates (from 290 to 170 mosmol/kg H(2)O) evoked a rapid increase in cell volume by 35%. Subsequently, the regulatory volume decrease (RVD) restored cell volume almost completely (to 94% of the isosmotic value). The basolateral (86)Rb efflux markedly increased during the hyposmotic shock, from 0.50 +/- 0.03 min(-1) to a peak value of 6.32 +/- 0.07 min(-1), while apical (86)Rb efflux was negligible. Channel blockers, such as GdCl(3) (0.5 mM), quinine (0.5 mM) and 5-nitro-2-(3-phenyl-propylamino) benzoic acid (NPPB, 100 microM), abolished the RVD. The protein tyrosine kinase inhibitors tyrphostin 23 (100 microM) and genistein (150 microM) attenuated the RVD. All agents decreased variably the hyposmosis-induced elevation in (86)Rb efflux, whereas NPPB induced a complete block, suggesting a link between basolateral K(+) and Cl(-1) efflux. Forskolin-mediated activation of adenylyl cyclase stimulated the RVD with a concomitant increase in basolateral (86)Rb efflux. These data suggest that the basolateral extrusion of K+ and Cl(-1) from 16HBE14o(-1) cells in response to cell swelling determines RVD efficiency.

Purinergic pathway suppresses the release of .NO and stimulates proteoglycan synthesis in chondrocyte/agarose constructs subjected to dynamic compression

Mechanical loading plays a fundamental role in the physiological and pathological processes of articular cartilage. The application of dynamic compression to chondrocytes cultured in agarose, downregulates the release of nitric oxide (NO) and enhances cell proliferation and proteoglycan synthesis. We hypothesize that the observed metabolic changes in response to dynamic compression involve a purinergic signaling pathway. Chondrocyte/agarose constructs were subjected to dynamic compression (15%, 1 Hz, 48 h) in the presence of antagonists for the purinergic pathway. Gadolinium was used as a putative inhibitor of stretch-activated calcium ion channels including adenosine 5'-triphosphate (ATP) release channels; suramin was employed as a P2 receptor antagonist and apyrase was used to catalyze the hydrolysis of extracellular ATP. The data presented demonstrate that in the absence of the inhibitor, dynamic compression suppressed .NO release. Treatment with gadolinium and suramin caused a compression-induced upregulation of .NO release, a response abolished with apyrase. Compression-induced stimulation of cell proliferation was reversed with gadolinium, suramin, or apyrase. By contrast, compression-induced stimulation of proteoglycan synthesis was abolished under all treatment conditions. Thus, the purinergic pathway is important in suppressing the release of .NO and stimulation of proteoglycan synthesis. Indeed, high levels of .NO could trigger a downstream catabolic response and mediate the compression-induced inhibition of cell proliferation. The current study demonstrates for the first time the importance of a purinergic pathway in mediating the metabolic response to dynamic compression and suppressing an inflammatory effect.

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.

Mechanosensitivity of mouse tracheal ciliary beat frequency: roles for Ca2+, purinergic signaling, tonicity, and viscosity

Mechanosensitivity is hypothesized to participate in the regulation of ciliary beat frequency (CBF) in airway epithelia. To investigate this hypothesis, CBF in excised mouse trachea was monitored (microscopy image analysis) while varying mucosal shear (perfusate velocity and/or viscosity; planar flow). CBF increased within minutes of step increase to steady shear stress as small as 10(-3) Pa and decreased within minutes of shear reduction (<or=10(-4) Pa). CBF response was directional, being less with cephalad vs. caudal flow, and was reduced in trachea from mutant mice lacking P2Y2 receptors, as well as by administration of the Ca2+ chelator EGTA, the Ca2+ channel inhibitor La3+, the nucleotide phosphohydrolase apyrase, the metabolically stabilized adenosine receptor agonist 5'-(N-ethylcarboxamido)adenosine, the osmotic agent mannitol, and the viscosity modifier dextran. Brief exposure to exogenous ATP, a candidate mediator, augmented CBF response, although augmentation declined with higher ATP concentration (5.0 vs. 0.1 mM) or longer ATP exposure before shear (55 vs. 20 min). Prolonged extended exposure (45 min) to the metabolically stabilized ATP analog ATPgammaS [adenosine 5'-(3-thiotriphosphate), 0.1 mM] inhibited CBF response to shear. Furthermore, neither ATP nor ATPgammaS substantially increased CBF in the relative absence of shear. With viscosity increase or shear withdrawal apyrase evoked CBF stimulation, inhibitable by the adenosine receptor antagonist 8-(p-sulfophenyl)theophylline. Thus CBF response to shear is finely tuned, directional, La3+ sensitive, likely dependent on extracellular Ca2+ and ATP, involving P2Y2 and adenosine receptor activations, influenced by shear history, tonicity, viscosity, and metabolism/exposure of ATP, and thus reflective of a complex interplay of physical and biochemical actions.

International Union of Pharmacology LVIII: update on the P2Y G protein-coupled nucleotide receptors: from molecular mechanisms and pathophysiology to therapy

There have been many advances in our knowledge about different aspects of P2Y receptor signaling since the last review published by our International Union of Pharmacology subcommittee. More receptor subtypes have been cloned and characterized and most orphan receptors de-orphanized, so that it is now possible to provide a basis for a future subdivision of P2Y receptor subtypes. More is known about the functional elements of the P2Y receptor molecules and the signaling pathways involved, including interactions with ion channels. There have been substantial developments in the design of selective agonists and antagonists to some of the P2Y receptor subtypes. There are new findings about the mechanisms underlying nucleotide release and ectoenzymatic nucleotide breakdown. Interactions between P2Y receptors and receptors to other signaling molecules have been explored as well as P2Y-mediated control of gene transcription. The distribution and roles of P2Y receptor subtypes in many different cell types are better understood and P2Y receptor-related compounds are being explored for therapeutic purposes. These and other advances are discussed in the present review.