cAMP activates an ATP-permeable pathway in neonatal rat cardiac myocytes

Fetal supraventricular tachycardia and cystic fibrosis: Coincidence or association? Two case reports

Prenatal diagnosis of cystic fibrosis (CF) is difficult and is mainly considered upon identification of digestive sonographic signs. Although such an association has never been described until now to our knowledge, we report two cases of fetal arrhythmia associated with cystic fibrosis. This association may be explained by the physiopathology of heart in the context of CF, but nevertheless needs to be confirmed by other reports. The prenatal diagnosis of CF is important in order to implement early appropriate care, with better prognosis. The finding of possibly new associated prenatal signs may then improve the global management of the disease.

cAMP/protein kinase A activates cystic fibrosis transmembrane conductance regulator for ATP release from rat skeletal muscle during low pH or contractions

We have shown that cystic fibrosis transmembrane conductance regulator (CFTR) is involved in ATP release from skeletal muscle at low pH. These experiments investigate the signal transduction mechanism linking pH depression to CFTR activation and ATP release, and evaluate whether CFTR is involved in ATP release from contracting muscle. Lactic acid treatment elevated interstitial ATP of buffer-perfused muscle and extracellular ATP of L6 myocytes: this ATP release was abolished by the non-specific CFTR inhibitor, glibenclamide, or the specific CFTR inhibitor, CFTR_inh-172, suggesting that CFTR was involved, and by inhibition of lactic acid entry to cells, indicating that intracellular pH depression was required. Muscle contractions significantly elevated interstitial ATP, but CFTR_inh-172 abolished the increase. The cAMP/PKA pathway was involved in the signal transduction pathway for CFTR-regulated ATP release from muscle: forskolin increased CFTR phosphorylation and stimulated ATP release from muscle or myocytes; lactic acid increased intracellular cAMP, pCREB and PKA activity, whereas IBMX enhanced ATP release from myocytes. Inhibition of PKA with KT5720 abolished lactic-acid- or contraction-induced ATP release from muscle. Inhibition of either the Na⁺/H⁺-exchanger (NHE) with amiloride or the Na⁺/Ca²⁺-exchanger (NCX) with SN6 or KB-R7943 abolished lactic-acid- or contraction-induced release of ATP from muscle, suggesting that these exchange proteins may be involved in the activation of CFTR. Our data suggest that CFTR-regulated release contributes to ATP release from contracting muscle in vivo, and that cAMP and PKA are involved in the activation of CFTR during muscle contractions or acidosis; NHE and NCX may be involved in the signal transduction pathway.

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.

Multidrug resistance protein transporter and Ins(1,4,5)P₃-sensitive Ca²+-signaling involved in adenosine triphosphate export via Gq protein-coupled NK₂-receptor stimulation with neurokinin A

The purpose of this study is to identify the membrane transport machinery and cell signaling involved in the neurokinin A-inducible release of adenosine triphosphate (ATP) as an autocrine/paracrine signal from cultured guinea-pig taenia coli (T. coli) smooth muscle cells (SMCs). ATP release evoked by neurokinin A was inhibited by L-659877, a NK(2)-receptor antagonist; by modulators for Ins(1,4,5)P(3)-sensitive Ca(2+)-signaling, U-73122, thapsigargin, and 2-APB; and by W-7, a calmodulin inhibitor, and staurosporine, a protein kinase C (PKC) inhibitor, but not by wortmannin, a phosphoinositide 3-kinase inhibitor. The evoked release was suppressed by a multidrug resistance protein (MRP)-transporter inhibitors, MK-571, indomethacin, and benzbromarone, but not by CFTR-inh 172, a CFTR-Cl(-) channel blocker, and α-glycyrrhetinic acid, a gap junction hemichannel blocker. Neurokinin A caused a marked accumulation of Ins(1,4,5)P(3) and an increase in [Ca(2+)](i) in the cultured cells. These findings suggest that stimulation of Gq/(11) protein-coupled NK(2) receptor with neurokinin A caused a substantial release of ATP from cultured T. coli SMCs and that the evoked release may be mediated by Ins(1,4,5)P(3)-sensitive Ca(2+)-signaling, further by PKC and Ca(2+)/calmodulin signals, and finally by an activation of MRP transporters as the membrane device.

Cystic fibrosis transmembrane conductance regulator in human muscle: Dysfunction causes abnormal metabolic recovery in exercise

OBJECTIVE

Individuals with cystic fibrosis (CF) have exercise intolerance and skeletal muscle weakness not solely attributable to physical inactivity or pulmonary function abnormalities. CF transmembrane conductance regulator (CFTR) has been demonstrated in human bronchial smooth and cardiac muscle. Using (31)P-magnetic resonance spectroscopy of skeletal muscle, we showed CF patients to have lower resting muscle adenosine triphosphate and delayed phosphocreatine recovery times after high-intensity exercise, suggesting abnormal muscle aerobic metabolism; and higher end-exercise pH values, suggesting altered bicarbonate transport. Our objective was to study CFTR expression in human skeletal muscle.

METHODS AND RESULTS

We studied CFTR expression in human skeletal muscle by Western blot with anti-CFTR antibody (Ab) L12B4 and demonstrated a single band with expected molecular weight of 168kDa. We isolated the cDNA by reverse transcription polymerase chain reaction and directly sequenced a 975bp segment (c. 3,600-4,575) that was identical to the human CFTR sequence. We showed punctate staining of CFTR in sarcoplasm and sarcolemma by immunofluorescence microscopy with L12B4 Ab and secondary Alexa 488-labeled Ab. We confirmed CFTR expression in the sarcotubular network and sarcolemma by electron microscopy, using immunogold-labeled anti-CFTR Ab. We observed activation of CFTR Cl(-) channels with iodide efflux, on addition of forskolin, 3-isobutyl-1-methyl-xanthine, and 8-chlorphenylthio-cyclic adenosine monophosphate, in wild-type C57BL/6J isolated muscle fibers in contrast to no efflux from mutant F508del-CFTR muscle.

INTERPRETATION

We speculate that a defect in sarcoplasmic reticulum CFTR Cl(-) channels could alter the electrochemical gradient, causing dysregulation of Ca(2+) homeostasis, for example, ryanodine receptor or sarco(endo)plasmic reticulum Ca(2+) adenosine triphosphatases essential to excitation-contraction coupling leading to exercise intolerance and muscle weakness in CF.

MRP transporters as membrane machinery in the bradykinin-inducible export of ATP

Adenosine triphosphate (ATP) plays the role of an autocrine/paracrine signal molecule in a variety of cells. So far, however, the membrane machinery in the export of intracellular ATP remains poorly understood. Activation of B2-receptor with bradykinin-induced massive release of ATP from cultured taenia coli smooth muscle cells. The evoked release of ATP was unaffected by gap junction hemichannel blockers, such as 18alpha-glycyrrhetinic acid and Gap 26. Furthermore, the cystic fibrosis transmembrane regulator (CFTR) coupled Cl(-) channel blockers, CFTR(inh)172, 5-nitro-2-(3-phenylpropylamino)-benzoic acid, Gd3(+) and glibenclamide, failed to suppress the export of ATP by bradykinin. On the other, the evoked release of ATP was greatly reduced by multidrug resistance protein (MRP) transporter inhibitors, MK-571, indomethacin, and benzbromarone. From western blotting analysis, blots of MRP 1 protein only, but not MRP 2 and MRP 3 protein, appeared at 190 kD. However, the MRP 1 protein expression was not enhanced after loading with 1 muM bradykinin for 5 min. Likewise, niflumic acid and fulfenamic acid, Ca2(+)-activated Cl(-) channel blockers, largely abated the evoked release of ATP. The possibility that the MRP transporter system couples with Ca2(+)-activated Cl(-) channel activities is discussed here. These findings suggest that MRP transporters, probably MRP 1, unlike CFTR-Cl(-) channels and gap junction hemichannels, may contribute as membrane machinery to the export of ATP induced by G-protein-coupled receptor stimulation.

Effect of acute hypoxia on the concentrations of potassium and phosphorus in the cardiomyocyte of a pregnant animal during early organogenesis

The concentrations of potassium and phosphorus in the cardiomyocyte cytoplasm of the Wistar rat were measured by means of electron probe microanalysis. Pregnancy was accompanied by a decrease in cytoplasmic phosphorus concentration and an increase in cytoplasmic potassium concentration. Acute hypoxia modified the concentrations of these elements in the cytoplasm.

Caffeine-inducible ATP release is mediated by Ca2+-signal transducing system from the endoplasmic reticulum to mitochondria

Adenosine triphosphate (ATP) is released as an autocrine/paracrine signal from a variety of cells. The present study was undertaken to clarify the Ca2+-signal pathway involved in the caffeine-inducible release of ATP from cultured smooth muscle cells (SMC). The release of ATP induced by caffeine (3 mM) was almost completely inhibited by ryanodine and tetracaine, but not by 2-APB, thus being mediated by ryanodine receptors (RyR). The expression of messenger RNA from only RyR-2 was detected in the cells. Furthermore, the induced release was attenuated by mitochondrial inhibitors, rotenone and oligomycin and by Cl- channel blockers, niflumic acid, and 5-nitro-2-(3-phenylpropylamino)-benzoic acid. Increase in Ca2+-signals with fluo 4 and rhod-2 caused by caffeine were reduced by tetracaine and oligomycin plus carbonyl cyanide m-chlorophenylhydrazone, respectively. A close spatial relation between the endoplasmic reticulum (ER) and mitochondria was electromicroscopically observed in the SMC, supporting the existence of a Ca2+-signaling bridge on both the organelli. These results suggest that caffeine stimulates ryanodine receptor (RyR-2) and facilitates a Ca2+-signal transducing system from ER to mitochondria, and then, the signal appears to accelerate the ATP synthesis in mitochondria. In addition, the mitochondrial event may lead further cell signaling to the cell membrane and activates Cl- channels, resulting in the extracellular release of cytosolic ATP.

Endoplasmic reticulum is a key organella in bradykinin-triggered ATP release from cultured smooth muscle cells

ATP has broad functions as an autocrine/paracrine molecule. The mode of ATP release and its intracellular source, however, are little understood. Here we show that bradykinin via B(2)-receptor stimulation induces the extracellular release of ATP via the inositol 1,4,5-trisphosphate [Ins(1,4,5)P(3)]-signaling pathway in cultured taenia coli smooth muscle cells. It was found that bradykinin also increased the production of Ins(1,4,5)P(3) and 2-APB-inhibitable [Ca(2+)](i). The evoked release of ATP was suppressed by the Ca(2+)-channel blockers, nifedipine, and verapamil. Moreover, the extracellular release of ATP was elicited by photoliberation of Ins(1,4,5)P(3). Bradykinin caused a quick and transient accumulation of intracellular ATP from cells treated with 1% perchloric acid solution (PCA), but not with the cell lysis buffer. Peak accumulation was prevented by 2-APB and thapsigargin, but not by nifedipine or verapamil, inhibitors of extracellular release of ATP. These findings suggest that bradykinin elicits the extracellular release of ATP that is mediated by the Ins(1,4,5)P(3)-induced Ca(2+) signaling and, finally, leads to a Ca(2+)-dependent export of ATP from the cells. Furthermore, the bradykinin-induced transient accumulation of ATP in the cells treated with PCA may imply a possible release of ATP from the endoplasmic reticulum.

Effects of hypo-osmotic stress on ATP release in isolated turbot (Scophthalmus maximus) hepatocytes

BACKGROUND INFORMATION

ATP is released from many cell types exposed to hypo-osmotic shock and is involved in RVD (regulatory volume decrease). Purinergic signalling events have been extensively investigated in mammals, but not in marine teleosteans.

RESULTS

The effect of hypo-osmotic shock on ATP release was examined in isolated hepatocytes from turbot (Scophthalmus maximus), a marine flatfish. Hypo-osmotic stress (240 mOsm x kg(-1)) induced a significant increase in ATP efflux, and was inhibited by a potential CFTR (cystic fibrosis transmembrane conductance regulator) inhibitor, glibenclamide, but not by the MDR1 (multidrug resistance 1) P-glycoprotein inhibitor, verapamil. ATP efflux could be a cAMP-dependent process, as IBMX (isobutylmethylxanthine) and forskolin triggered the process under iso-osmotic conditions. Protein kinases, including protein kinase C, could also be involved, as staurosporine and chelerythrine inhibited the mechanism. Calcium could contribute to ATP efflux as ionomycin, a calcium ionophore, elicited a rapid release under iso-osmotic conditions, and chelation using EGTA abolished ATP release under hypo-osmotic conditions. RVD was partially abolished by apyrase, an ATP scavenger, and suramin, a purinoceptor antagonist. Moreover, hypo-osmotic shock induced a rise in intracellular calcium which could be involved in RVD. Since extracellular ATP triggered an increase in cellular free-calcium content under iso-osmotic conditions, our results could indicate that hypo-osmotic-induced ATP efflux contributes to RVD in turbot hepatocytes by stimulating purinergic receptors, which may lead to activation of a calcium signalling pathway.

CONCLUSIONS

These data provide the first evidence of volume-sensitive ATP signalling for volume maintenance in a marine teleost fish cell type.

Rab27a negatively regulates CFTR chloride channel function in colonic epithelia: Involvement of the effector proteins in the regulatory mechanism

Cystic fibrosis, an autosomal recessive disorder, is caused by the disruption of biosynthesis or function of CFTR. CFTR regulatory mechanisms include channel transport to plasma membrane and protein-protein interactions. Rab proteins are small GTPases involved in vesicle transport, docking, and fusion. The colorectal epithelial HT-29 cells natively express CFTR and respond to cAMP with an increase in CFTR-mediated currents. DPC-inhibited currents could be completely eliminated with CFTR-specific SiRNA. Over-expression of Rab27a inhibited, while isoform specific SiRNA and Rab27a antibody stimulated CFTR-mediated currents in HT-29 cells. CFTR activity is inhibited both by Rab27a (Q78L) (constitutive active GTP-bound form of Rab27a) and Rab27a (T23N) (constitutive negative form that mimics the GDP-bound form). Rab27a mediated effects could be reversed by Rab27a-binding proteins, the synaptotagmin-like protein (SLP-5) and Munc13-4 accessory protein (a putative priming factor for exocytosis). The SLP reversal of Rab27a effect was restricted to C2A/C2B domains while the SHD motif imparted little more inhibition. The CFTR-mediated currents remain unaffected by Rab3 though SLP-5 appears to weakly bind it. The immunoprecipitation experiments suggest protein-protein interactions between Rab27a and CFTR. Rab27a appears to impair CFTR appearance at the cell surface by trapping CFTR in the intracellular compartments. Munc13-4 and SLP-5, on the other hand, limit Rab27a availability to CFTR, thus minimizing its effect on channel function. These observations decisively prove that Rab27a is involved in CFTR channel regulation through protein-protein interactions involving Munc13-4 and SLP-5 effector proteins, and thus could be a potential target for cystic fibrosis therapy.

Rab4GTPase modulates CFTR function by impairing channel expression at plasma membrane

Cystic fibrosis (CF), an autosomal recessive disorder, is caused by the disruption of biosynthesis or the function of a membrane cAMP-activated chloride channel, CFTR. CFTR regulatory mechanisms include recruitment of channel proteins to the cell surface from intracellular pools and by protein-protein interactions. Rab proteins are small GTPases involved in regulated trafficking controlling vesicle docking and fusion. Rab4 controls recycling events from endosome to the plasma membrane, fusion, and degradation. The colorectal cell line HT-29 natively expresses CFTR and responds to cAMP stimulation with an increase in CFTR-mediated currents. Rab4 over-expression in HT-29 cells inhibits both basal and cAMP-stimulated CFTR-mediated currents. GTPase-deficient Rab4Q67L and GDP locked Rab4S22N both inhibit channel activity, which appears characteristically different. Active status of Rab4 was confirmed by GTP overlay assay, while its expression was verified by Western blotting. The pull-down and immunoprecipitation experiments suggest that Rab4 physically interacts with CFTR through protein-protein interaction. Biotinylation with cell impermeant NHS-Sulfo-SS-Biotin implies that Rab4 impairs CFTR expression at cell surface. The enhanced cytosolic CFTR indicates that Rab4 expression restrains CFTR appearance at the cell membrane. The study suggests that Rab4 regulates the channel through multiple mechanisms that include protein-protein interaction, GTP/GDP exchange, and channel protein trafficking. We propose that Rab4 is a dynamic molecule with a significant role in CFTR function.

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.

ATP release via anion channels

ATP serves not only as an energy source for all cell types but as an 'extracellular messenger' for autocrine and paracrine signalling. It is released from the cell via several different purinergic signal efflux pathways. ATP and its Mg(2+) and/or H(+) salts exist in anionic forms at physiological pH and may exit cells via some anion channel if the pore physically permits this. In this review we survey experimental data providing evidence for and against the release of ATP through anion channels. CFTR has long been considered a probable pathway for ATP release in airway epithelium and other types of cells expressing this protein, although non-CFTR ATP currents have also been observed. Volume-sensitive outwardly rectifying (VSOR) chloride channels are found in virtually all cell types and can physically accommodate or even permeate ATP(4-) in certain experimental conditions. However, pharmacological studies are controversial and argue against the actual involvement of the VSOR channel in significant release of ATP. A large-conductance anion channel whose open probability exhibits a bell-shaped voltage dependence is also ubiquitously expressed and represents a putative pathway for ATP release. This channel, called a maxi-anion channel, has a wide nanoscopic pore suitable for nucleotide transport and possesses an ATP-binding site in the middle of the pore lumen to facilitate the passage of the nucleotide. The maxi-anion channel conducts ATP and displays a pharmacological profile similar to that of ATP release in response to osmotic, ischemic, hypoxic and salt stresses. The relation of some other channels and transporters to the regulated release of ATP is also discussed.

Ischemia-induced enhancement of CFTR expression on the plasma membrane in neonatal rat ventricular myocytes

Pathophysiological functions of cardiac cystic fibrosis transmembrane conductance regulator (cCFTR) in ischemia are not well known. Using neonatal rat ventricular cardiomyocytes in primary culture in this study, we thus examined whether the CFTR protein is expressed and is functioning as a cAMP-activated anion channel on the plasma membrane under ischemic conditions. After the cells were subjected to simulated ischemia (O(2) and glucose deprivation), an up-regulation of the CFTR expression was transiently observed in the membrane fraction by Western blot. A peak expression of mature CFTR protein was found at 3 h of ischemia, and thereafter the signal diminished gradually. In contrast, the results of Northern blot indicated that the expression level of CFTR mRNA changed little until 3 h of ischemia, whereas the level slightly decreased after 8 h of ischemia. An immunohistochemical examination showed, in agreement with the results of Western blot analysis, that the expression of CFTR protein on the plasma membrane became most prominent at 3 h of ischemia, whereas the plasmalemmal CFTR signal was markedly reduced after 8 h of ischemia. Whole-cell recordings showed that the cardiomyocytes responded to cAMP with an activation of time- and voltage-independent currents that contained an anion-selective component sensitive to CFTR Cl(-) channel blockers (NPPB and glibenclamide) but not to a stilbene-derivative conventional Cl(-) channel blocker (SITS). This cAMP-activated Cl(-) channel current was found to be enhanced after an application of ischemic stress for 3 to 4 h. These findings indicate that a plasmalemmal expression of CFTR is transiently enhanced under glucose-free hypoxic conditions presumably because of a posttranslational control.

Hypotonicity-induced ATP release is potentiated by intracellular Ca2+ and cyclic AMP in cultured human bronchial cells

We have examined the cultured human bronchial epithelial cells (16HBE) to learn if changes in Cl(-) concentration or osmolality stimulate the cells to release ATP and to determine whether its release is cyclic AMP (cAMP)- and/or Ca(2+)-dependent by using the luciferin-luciferase luminometric assay. In a control solution (290 mosmol kg H(2)O(-1)), the external ATP concentration and the rate of ATP release were 0.52 +/- 0.20 nM and 0.036 +/- 0.034 pmol min(-1), respectively. Upon hypotonicity (205 mosmol kg H(2)O(-1)), they increased to 7.0 +/- 1.3 nM and 3.1 +/- 0.6 pmol min(-1), respectively, at 6 min, then decreased. At the peak, the rate of ATP release is estimated to be 6.2x10(4) ATP molecules s(-1) per cell. An accumulation of the released ATP for the initial 10 min increased significantly (p < 0.005) by 71.5% in the presence of forskolin (10 microM), adenylyl cyclase activator, however, it was abolished (p < 0.001) by pretreatment with BAPTA-AM (25 microM), a membrane permeable Ca(2+) chelator. On the other hand, neither low Cl(2-) (75 mM, isotonic) nor hypertonicity (+NaCl or +mannitol, 500 mosmol kg H(2)O(-1)) could significantly increase the ATP release. Further, forskolin or ionomycin (a Ca(2+) ionophore) or, both, failed to stimulate ATP release under the isotonic condition. In conclusion, first, hypertonicity and changes in Cl(-) concentrations are not effective signals for the ATP release; second, hypotonicity-induced ATP release is potentiated by the level of intracellular Ca(2+) and cAMP; and third, a biphasic increase in ATP release and its low rate at the peak support the hypothesis that ATP is released through a non-conducting pathway model, such as exocytosis, or through a volume-dependent, ATP-conductive anion channel.

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.

Coupled oscillator systems of cultured cardiac myocytes: fluctuation and scaling properties

Isolated and cultured neonatal cardiac myocytes exhibit autonomous rhythmic contraction, and their dynamics vary dramatically depending on the extent of mutual coupling among individual myocytes. We study the temporal changes of interbeat interval series in aggregated systems of spontaneously beating cultured neonatal rat cardiac myocytes and observe a rich variety of complex, nonlinear features such as frequent alternations, bistability, and periodic spikes. Fluctuation analysis of the interval series reveals that there occurs a transition in scaling behavior from persistent correlations to antipersistent correlations as the coupling develops with incubation time. Additionally, we perform computer simulations using interacting Bonhoeffer-van der Pol oscillators to understand the effects of coupling on the fluctuation dynamics of each constituent oscillator. We find that the formation of strong and heterogeneous coupling among the oscillators is a key factor to yield the complexity in the interval series as well as in the scaling behavior.

Role of ATP-conductive anion channel in ATP release from neonatal rat cardiomyocytes in ischaemic or hypoxic conditions

It is known that the level of ATP in the interstitial spaces within the heart during ischaemia or hypoxia is elevated due to its release from a number of cell types, including cardiomyocytes. However, the mechanism by which ATP is released from these myocytes is not known. In this study, we examined a possible involvement of the ATP-conductive maxi-anion channel in ATP release from neonatal rat cardiomyocytes in primary culture upon ischaemic, hypoxic or hypotonic stimulation. Using a luciferin-luciferase assay, it was found that ATP was released into the bulk solution when the cells were subjected to chemical ischaemia, hypoxia or hypotonic stress. The swelling-induced ATP release was inhibited by the carboxylate- and stilbene-derivative anion channel blockers, arachidonic acid and Gd3+, but not by glibenclamide. The local concentration of ATP released near the cell surface of a single cardiomyocyte, measured by a biosensor technique, was found to exceed the micromolar level. Patch-clamp studies showed that ischaemia, hypoxia or hypotonic stimulation induced the activation of single-channel events with a large unitary conductance (approximately 390 pS). The channel was selective to anions and showed significant permeability to ATP4- (PATP/PCl approximately 0.1) and MgATP2- (PATP/PCl approximately 0.16). The channel activity exhibited pharmacological properties essentially identical to those of ATP release. These results indicate that neonatal rat cardiomyocytes respond to ischaemia, hypoxia or hypotonic stimulation with ATP release via maxi-anion channels.

CFTR in K562 human leukemic cells

In this study, the expression and functional characterization of CFTR (cystic fibrosis transmembrane regulator) was determined in K562 chronic human leukemia cells. Expression of the CFTR gene product was determined by RT-PCR and confirmed by immunohistochemistry and Western blot analysis. Functional characterization of CFTR Cl- channel activity was conducted with patch-clamp techniques. Forskolin, an adenylyl cyclase activator, induced an anion-selective channel with a linear current-voltage relationship and a single-channel conductance of 11 pS. This cAMP-activated channel had a Pgluconate/PCl or PF/PCl perm-selectivity ratio of 0.35 and 0.30, respectively, and was inhibited by the CFTR blocker glibenclamide and the anti-CFTR antibody MAb 13-1, when added to the cytoplasmatic side of the patch. Glibenclamide decreased the open probability increasing the frequency of open-to-closed transitions. Addition of 200 microM DIDS caused an irreversible block of the channels when added to the cytosolic side of inside-out patches. These and other observations indicate a widespread distribution of CFTR gene expression and suggest that this channel protein may function in most human cells to help maintain cellular homeostasis.

Paracrine factors in tubuloglomerular feedback: adenosine, ATP, and nitric oxide

The tubuloglomerular feedback response, the change in afferent arteriolar tone caused by a change in NaCl concentration at the macula densa, is likely initiated by the generation of a vasoactive mediator within the confines of the juxtaglomerular apparatus. Substantial progress has been made in identifying the nature of this mediator and the factors that modulate its effect on vascular tone. In support of earlier studies using P1 purinergic antagonists, the application of the knockout technique has shown that adenosine 1 receptors are absolutely required for eliciting TGF responses. The background level of angiotensin II appears to be an important cofactor determining the efficiency of A1AR-induced vasoconstriction, probably through a synergistic interaction at the level of the G protein-dependent transduction mechanism. The source of the adenosine is still unclear, but it is conceivable that adenosine is generated extracellularly from released ATP through a cascade of ecto-nucleotidases. There is also evidence that ATP may activate P2 receptors in preglomerular vessels, which may contribute to autoregulation of renal vascular resistance. Nitric oxide (NO), generated by the neuronal isoform of nitric oxide synthase in macula densa cells, reduces the constrictor effect of adenosine, but the regulation of NO release and its exact role in states of TGF-induced hyperfiltration are still unclear.

Regulation of an ATP-conductive large-conductance anion channel and swelling-induced ATP release by arachidonic acid

Mouse mammary C127 cells responded to hypotonic stimulation with activation of the volume-dependent ATP-conductive large conductance (VDACL) anion channel and massive release of ATP. Arachidonic acid downregulated both VDACL currents and swelling-induced ATP release in the physiological concentration range with K(d) of 4- 6 microM. The former effect observed in the whole-cell or excised patch mode was more prominent than the latter effect observed in intact cells. The arachidonate effects were direct and not mediated by downstream metabolic products, as evidenced by their insensitivity to inhibitors of arachidonate-metabolizing oxygenases, and by the observation that they were mimicked by cis-unsaturated fatty acids, which are not substrates for oxygenases. A membrane-impermeable analogue, arachidonyl coenzyme A was effective only from the cytosolic side of membrane patches suggesting that the binding site is localized intracellularly. Non-charged arachidonate analogues as well as trans-unsaturated and saturated fatty acids had no effect on VDACL currents and ATP release, indicating the importance of arachidonate's negative charge and specific hydrocarbon chain conformation in the inhibitory effect. VDACL anion channels were inhibited by arachidonic acid in two different ways: channel shutdown (K(d) of 4- 5 microM) and reduced unitary conductance (K(d) of 13-14 microM) without affecting voltage dependence of open probability. ATP(4-)-conducting inward currents measured in the presence of 100 mM ATP in the bath were reversibly inhibited by arachidonic acid. Thus, we conclude that swelling-induced ATP release and its putative pathway, the VDACL anion channel, are under a negative control by intracellular arachidonic acid signalling in mammary C127 cells.

Inositol(1,4,5)trisphosphate signal triggers a receptor-mediated ATP release

Intracellular signal transduction pathways involved in ATP release evoked by angiotensin II (Ang II) were investigated in cultured guinea pig Taenia coli smooth muscle cells. Ang II (0.3-1 microM) elicited substantial release of ATP from the cells, but not from a human fibroblast cell line. However, Ang II even at 10 microM failed to cause a leakage of lactate dehydrogenase (LDH) from the smooth muscle cells. The release of ATP by Ang II was suppressed by 10 microM SC52458, an AT1 receptor antagonist, not by 10 microM PD123319, an AT2 receptor antagonist. The evoked release of ATP was almost completely inhibited in the presence of 10 microM U73122, a phospholipase C inhibitor, and 0.5 microM thapsigargin, a Ca2+-ATPase inhibitor. Furthermore, the release was hampered by 50 microM BAPTA/AM, an intracellular Ca2+ chelator, but not by 0.1 microM nifedipine, a voltage gated Ca2+ channel inhibitor. The basal release of ATP was increased by BAPTA/AM, but was reduced by U-73122. Ang II enhanced instantaneously inositol(1,4,5)trisphosphate (Ins(1,4,5)P3) accumulation in the cells. The enhancing effect was perfectly antagonized by SC52458. These findings suggest that intracellular Ca2+ signals activated via stimulation of Ins(1,4,5)P3 receptor are involved in the release of ATP evoked by Ang II.

A pulsed DC electric field affects P2-purinergic receptor functions by altering the ATP levels in in vitro and in vivo systems

Recently it was shown that extracellular ATP, acting through purinergic receptors, has many physiological functions, including opening of Ca(2+)-ion channels, activation and mediation of signal transduction mechanisms as well as activation of the pain sensation. Since electrical stimulation is also known to affect many signal transduction processes as well as the alleviation of pain, we hypothesized that electric stimulation may affect the extracellular release of ATP. We investigated the effects of a small DC electric field (10(1)--10(2) V m(-1) range and with frequencies below 150 Hz) on the release of ATP in vitro (HeLa cells), and on the levels of ATP in vivo (the plasma of healthy volunteers). In HeLa cells ATP release was increased 50 fold, while the total amount of ATP in the cells was increased by 163%. In the plasma a significant decrease (P<0.05) in ATP concentration was seen after electrical stimulation, in all the volunteers. The small DC electric field also affected the cAMP signal transduction system in vitro (HeLa cells and human lymphocytes) and in vivo (human plasma). Decreased levels of cAMP (P<0.05) were seen in HeLa cells and increased levels of cAMP (P<0.05) in isolated human lymphocytes. The cAMP levels in the plasma of the electrically treated volunteers were lower than control values. These results show that the frequency, waveform and signal strength of the applied electric field are suitable for effecting measurable changes on signal transduction in vitro and in vivo.

Erythrocyte membrane ATP binding cassette (ABC) proteins: MRP1 and CFTR as well as CD39 (ecto-apyrase) involved in RBC ATP transport and elevated blood plasma ATP of cystic fibrosis

In addition to the better-known roles of the erythrocyte in the transport of oxygen and carbon dioxide, the concept that the red blood cell is involved in the transport and release of ATP has been evolving (J. Luthje, Blut 59, 367, 1989; G. R. Bergfeld and T. Forrester, Cardiovasc. Res. 26, 40, 1992; M. L. Ellsworth et al., Am. J. Physiol. 269, H2155, 1995; R. S. Sprague et al., Am. J. Physiol. 275, H1726, 1998). Membrane proteins involved in the release of ATP from erythrocytes now appear to include members of the ATP binding cassette (ABC) family (C. F. Higgins, Annu. Rev. Cell Biol. 8, 67, 1992; C. F. Higgins, Cell 82, 693, 1995). In addition to defining physiologically the presence of ABC proteins in RBCs, accumulating gel electrophoretic evidence suggests that the cystic fibrosis transmembrane conductance regulator (CFTR) and the multidrug resistance-associated protein (MRP1), respectively, constitute significant proteins in the red blood cell membrane. As such, this finding makes the mature erythrocyte compartment a major mammalian repository of these important ABC proteins. Because of its relative structural simplicity and ready accessibility, the erythrocyte offers an ideal system to explore details of the physiological functions of ABC proteins. Moreover, the presence of different ABC proteins in a single membrane implies that interaction among these proteins and with other membrane proteins may be the norm and not the exception in terms of modulation of their functions.

Volume-dependent ATP-conductive large-conductance anion channel as a pathway for swelling-induced ATP release

In mouse mammary C127i cells, during whole-cell clamp, osmotic cell swelling activated an anion channel current, when the phloretin-sensitive, volume-activated outwardly rectifying Cl(-) channel was eliminated. This current exhibited time-dependent inactivation at positive and negative voltages greater than around +/-25 mV. The whole-cell current was selective for anions and sensitive to Gd(3)+. In on-cell patches, single-channel events appeared with a lag period of approximately 15 min after a hypotonic challenge. Under isotonic conditions, cell-attached patches were silent, but patch excision led to activation of currents that consisted of multiple large-conductance unitary steps. The current displayed voltage- and time-dependent inactivation similar to that of whole-cell current. Voltage-dependent activation profile was bell-shaped with the maximum open probability at -20 to 0 mV. The channel in inside-out patches had the unitary conductance of approximately 400 pS, a linear current-voltage relationship, and anion selectivity. The outward (but not inward) single-channel conductance was suppressed by extracellular ATP with an IC(50) of 12.3 mM and an electric distance (delta) of 0.47, whereas the inward (but not outward) conductance was inhibited by intracellular ATP with an IC(50) of 12.9 mM and delta of 0.40. Despite the open channel block by ATP, the channel was ATP-conductive with P(ATP)/P(Cl) of 0.09. The single-channel activity was sensitive to Gd(3)+, SITS, and NPPB, but insensitive to phloretin, niflumic acid, and glibenclamide. The same pharmacological pattern was found in swelling-induced ATP release. Thus, it is concluded that the volume- and voltage-dependent ATP-conductive large-conductance anion channel serves as a conductive pathway for the swelling-induced ATP release in C127i cells.

Release of ATP by a human retinal pigment epithelial cell line: potential for autocrine stimulation through subretinal space

1. Stimulation of purinergic receptors on retinal pigment epithelial (RPE) cells can increase the rate of fluid transport or decrease phagocytosis. This study aims to: determine whether the purine ATP can be released from RPE cells, begin probing the mechanism of any release and test whether cells degrade ATP extracellularly. 2. ATP release was monitored from cultured human ARPE-19 cells with the luciferin-luciferase assay. Biphasic release of ATP was triggered by basic fibroblast growth factor (bFGF), by the pyrimidine uridine triphosphate (UTP) and by hypotonicity. 3. The Cl(-) channel blocker 5-nitro-2-(3-phenylpropylamino)-benzoate (NPPB) inhibited release of ATP, suggesting that release was associated with Cl(-) channels. 4. Elevating intracellular Ca(2+) directly with ionomycin was insufficient to trigger ATP release. 5. UTP induced a biphasic elevation in intracellular Ca(2+). NPPB inhibited the second phase, suggesting autostimulation by released ATP. 6. Cells grown on permeable supports showed apical release of ATP, analogous to release into subretinal space in vivo. 7. The presence of ecto-ATPases on ARPE-19 cell membranes was suggested by the degradation of ATP added to intact cells. 8. Phagocytosis of fluorescent beads was inhibited by ATP, but the ecto-5'-nucleotidase inhibitor alpha, beta-methylene ADP prevented this, suggesting that inhibition was mediated by extracellular conversion of ATP to adenosine. 9. These results suggest that growth factors, pyrimidines and changes in tonicity could trigger ATP release into subretinal space. The levels of ATP released may be capable of autocrine stimulation of ATP receptors, while conversion to adenosine by ecto-enzymes could alter phagocytosis.

Studies on the expression of mRNA for anion transport related proteins in corneal endothelial cells

PURPOSE

Chloride and bicarbonate are necessary for maintenance of fluid transport by the corneal endothelium, however there is little information on the identity of anion transport proteins that could serve as anion efflux mechanisms in endothelial cells. Therefore, we ask whether mRNA for the anion transport related proteins, CFTR, CLC-2, ClC-3, ClC-5 and AE2, are expressed in human, bovine or rabbit corneal endothelium.

METHODS

RT-PCR was performed for CFTR, CLC-2, ClC-3, ClC-5 and AE2 using total RNA from fresh human, bovine and rabbit corneal endothelium as well as cultured bovine corneal endothelial cells (CBCEC). Specificity of PCR products was confirmed by sequencing.

RESULTS

RT-PCR analysis gave positive bands at the predicted size for CLC-3 and CLC-5 from fresh human, rabbit and bovine as well as CBCEC. However, for CLC-2, no band was apparent around the predicted size from fresh and cultured corneal endothelium. A band at the predicted size was obtained for CFTR from fresh human, rabbit and bovine endothelium, as well as from CBCEC. RT-PCR analysis for AE2 produced specific bands from fresh human, rabbit and bovine corneal endothelium, but no positive band was obtained from CBCEC. Sequencing analysis further confirmed the identities of CLC-3, CLC-5, CFTR and AE2 in corneal endothelium.

CONCLUSIONS

CFTR, CLC-3 and ClC-5 are expressed in fresh and cultured corneal endothelial cells. However, consistent with previous immunoblots studies, AE2 is only expressed in fresh corneal endothelium. These results have implications for modeling possible apical anion efflux mechanisms in corneal endothelium.