Electrodiffusional ATP movement through the cystic fibrosis transmembrane conductance regulator

On P2X receptors in the brain: microvessels. Dedicated to the memory of the late Professor Geoffrey Burnstock (1929-2020)

This tribute article presents selected immunocytochemical and transmission electron microscope data on the location of ATP-gated P2X receptor in the rat brain, as studied in the 1990s in Prof G. Burnstock's laboratory at University College London. There are examples of immuno-ultrastructural findings and introductory information about pre- and post-synaptic location of P2X receptors in the rat cerebellum and endocrine hypothalamus to support the concept of purinergic transmission in the central nervous system. Then findings of diverse immunoreactivity for P2X1, P2X2, P2X4, and P2X6 receptors associated with brain microvessels are shown, including vascular endothelium and pericytes as well as perivascular astrocytes and neuronal components. These findings imply the involvement of P2X receptors and hence purinergic signalling in the neurovascular unit, at least in microvessels in the rat cerebellum and hypothalamic paraventricular and supraoptic nuclei examined here. Various aspects of P2X receptors in brain microvessels are discussed.

Mitochondrial dysfunction in diabetic kidney disease

Globally, diabetes is the leading cause of chronic kidney disease and end-stage renal disease, which are major risk factors for cardiovascular disease and death. Despite this burden, the factors that precipitate the development and progression of diabetic kidney disease (DKD) remain to be fully elucidated. Mitochondrial dysfunction is associated with kidney disease in nondiabetic contexts, and increasing evidence suggests that dysfunctional renal mitochondria are pathological mediators of DKD. These complex organelles have a broad range of functions, including the generation of ATP. The kidneys are mitochondrially rich, highly metabolic organs that require vast amounts of ATP for their normal function. The delivery of metabolic substrates for ATP production, such as fatty acids and oxygen, is altered by diabetes. Changes in metabolic fuel sources in diabetes to meet ATP demands result in increased oxygen consumption, which contributes to renal hypoxia. Inherited factors including mutations in genes that impact mitochondrial function and/or substrate delivery may also be important risk factors for DKD. Hence, we postulate that the diabetic milieu and inherited factors that underlie abnormalities in mitochondrial function synergistically drive the development and progression of DKD.

ATP-induced Ca2+-signalling mechanisms in the regulation of mesenchymal stem cell migration

The ability of cells to migrate to the destined tissues or lesions is crucial for physiological processes from tissue morphogenesis, homeostasis and immune responses, and also for stem cell-based regenerative medicines. Cytosolic Ca²⁺ is a primary second messenger in the control and regulation of a wide range of cell functions including cell migration. Extracellular ATP, together with the cognate receptors on the cell surface, ligand-gated ion channel P2X receptors and a subset of G-protein-coupled P2Y receptors, represents common autocrine and/or paracrine Ca²⁺ signalling mechanisms. The P2X receptor ion channels mediate extracellular Ca²⁺ influx, whereas stimulation of the P2Y receptors triggers intracellular Ca²⁺ release from the endoplasmic reticulum (ER), and activation of both type of receptors thus can elevate the cytosolic Ca²⁺ concentration ([Ca²⁺]_c), albeit with different kinetics and capacity. Reduction in the ER Ca²⁺ level following the P2Y receptor activation can further induce store-operated Ca²⁺ entry as a distinct Ca²⁺ influx pathway that contributes in ATP-induced increase in the [Ca²⁺]_c. Mesenchymal stem cells (MSC) are a group of multipotent stem cells that grow from adult tissues and hold promising applications in tissue engineering and cell-based therapies treating a great and diverse number of diseases. There is increasing evidence to show constitutive or evoked ATP release from stem cells themselves or mature cells in the close vicinity. In this review, we discuss the mechanisms for ATP release and clearance, the receptors and ion channels participating in ATP-induced Ca²⁺ signalling and the roles of such signalling mechanisms in mediating ATP-induced regulation of MSC migration.

Purinergic signalling in the kidney in health and disease

The involvement of purinergic signalling in kidney physiology and pathophysiology is rapidly gaining recognition and this is a comprehensive review of early and recent publications in the field. Purinergic signalling involvement is described in several important intrarenal regulatory mechanisms, including tuboglomerular feedback, the autoregulatory response of the glomerular and extraglomerular microcirculation and the control of renin release. Furthermore, purinergic signalling influences water and electrolyte transport in all segments of the renal tubule. Reports about purine- and pyrimidine-mediated actions in diseases of the kidney, including polycystic kidney disease, nephritis, diabetes, hypertension and nephrotoxicant injury are covered and possible purinergic therapeutic strategies discussed.

CFTR activity and mitochondrial function

Cystic Fibrosis (CF) is a frequent and lethal autosomal recessive disease, caused by mutations in the gene encoding the Cystic Fibrosis Transmembrane Conductance Regulator (CFTR). Before the discovery of the CFTR gene, several hypotheses attempted to explain the etiology of this disease, including the possible role of a chloride channel, diverse alterations in mitochondrial functions, the overexpression of the lysosomal enzyme α-glucosidase and a deficiency in the cytosolic enzyme glucose 6-phosphate dehydrogenase. Because of the diverse mitochondrial changes found, some authors proposed that the affected gene should codify for a mitochondrial protein. Later, the CFTR cloning and the demonstration of its chloride channel activity turned the mitochondrial, lysosomal and cytosolic hypotheses obsolete. However, in recent years, using new approaches, several investigators reported similar or new alterations of mitochondrial functions in Cystic Fibrosis, thus rediscovering a possible role of mitochondria in this disease. Here, we review these CFTR-driven mitochondrial defects, including differential gene expression, alterations in oxidative phosphorylation, calcium homeostasis, oxidative stress, apoptosis and innate immune response, which might explain some characteristics of the complex CF phenotype and reveals potential new targets for therapy.

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.

Involvement of the cystic fibrosis transmembrane conductance regulator in the acidosis-induced efflux of ATP from rat skeletal muscle

The present study was performed to investigate the effect of acidosis on the efflux of ATP from skeletal muscle. Infusion of lactic acid to the perfused hindlimb muscles of anaesthetised rats produced dose-dependent decreases in pH and increases in the interstitial ATP of extensor digitorum longus (EDL) muscle: 10 mM lactic acid reduced the venous pH from 7.22 ± 0.04 to 6.97 ± 0.02 and increased interstitial ATP from 38 ± 8 to 67 ± 11 nM. The increase in interstitial ATP was well-correlated with the decrease in pH (r(2) = 0.93; P < 0.05). Blockade of cellular uptake of lactic acid using α-cyano-hydroxycinnamic acid abolished the lactic acid-induced ATP release, whilst infusion of sodium lactate failed to depress pH or increase interstitial ATP, suggesting that intracellular pH depression, rather than lactate, stimulated the ATP efflux. Incubation of cultured skeletal myoblasts with 10 mM lactic acid significantly increased the accumulation of ATP in the bathing medium from 0.46 ± 0.06 to 0.76 ± 0.08 μM, confirming the skeletal muscle cells as the source of the released ATP. Acidosis-induced ATP efflux from the perfused muscle was abolished by CFTR(inh)-172, a specific inhibitor of the cystic fibrosis transmembrane conductance regulator (CFTR), or glibenclamide, an inhibitor of both K(ATP) channels and CFTR, but it was not affected by atractyloside, an inhibitor of the mitochondrial ATP transporter. Silencing of the CFTR gene using an siRNA abolished the acidosis-induced increase in ATP release from cultured myoblasts. CFTR expression on skeletal muscle cells was confirmed using immunostaining in the intact muscle and Western blotting in the cultured cells. These data suggest that depression of the intracellular pH of skeletal muscle cells stimulates ATP efflux, and that CFTR plays an important role in the release mechanism.

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.

Adenosine 5'-triphosphate stimulates the increase of TGF-beta1 in rat mesangial cells under high-glucose conditions via reactive oxygen species and ERK1/2

AIM

To investigate the role of adenosine 5'-triphosphate (ATP)-induced generation of reactive oxygen species (ROS) and phosphorylation of extracellular signal-regulated kinase 1/2 (ERK1/2) in the production of transforming growth factor-beta1 (TGF-beta1) in cultured rat glomerular mesangial cells under high-glucose conditions.

METHODS

Subconfluent glomerular mesangial cells were serum-starved for 24 h and pretreated with suramin, diphenylenechloride iodonium (DPI) or PD98059 followed by stimulation with a high concentration of glucose (30 mmol/L D-glucose) or ATP (300 micromol/L). Extracellular and total ATP and ROS production were detected using commercially available kits. Phosphorylation of ERK1/2 was evaluated by Western blot. TGF-beta1 mRNA expression was examined by real-time PCR.

RESULTS

Suramin had a dose-dependent inhibitory effect on the generation of ROS induced by high glucose. Extracellular ATP production by mesangial cells increased markedly after a 2-h incubation with high glucose. ROS production was upregulated in mesangial cells after 5 min incubation with 300 micromol/L ATP and was sustained for 120 min. ERK1/2 was significantly activated after 5 min incubation of mesangial cells with ATP, this activation was partially inhibited by DPI. The effects of high glucose on TGF-beta1 mRNA were markedly inhibited by suramin, DPI or PD98059.

CONCLUSION

Our results suggest that a high concentration of glucose increases the extracellular levels of ATP in mesangial cells within a short time-frame. ATP, in turn, activates ERK1/2, an effect which is at least partially dependent on ROS, which results in the upregulation of TGF-beta1.

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.

Uridine adenosine tetraphosphate acts as an autocrine hormone affecting glomerular filtration rate

Recently, uridine adenosine tetraphosphate (Up(4)A) was described as a strong vasoconstrictor released from endothelial cells after stimulation with mechanical stress. In this study, we isolated and identified Up(4)A from kidney tissue, and we characterized the essential varying effects of Up(4)A on the afferent and efferent arterioles. Porcine and human kidney tissue was fractionated by size exclusion chromatography, affinity chromatography, anion exchange chromatography and reverse phase chromatography. In fractions purified to homogeneity, Up(4)A was identified by matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry (MALDI-TOF MS), MALDI-LIFT fragment mass spectrometry (MALDI-TOF/TOF MS), retention-time comparison and enzymatic cleavage analysis. We analysed the release of Up(4)A from cultivated renal proximal tubule cells after stimulation of protein kinase C with oleoyl-2-acetyl-sn-glycerol (OAG). Up(4)A was identified in renal tissue, and the effect of Up(4)A on the vascular tone of isolated perfused afferent and efferent arterioles was tested. Stimulation of tubule cells with OAG increased the release rate of Up(4)A from tubule cells about tenfold. Up(4)A acts as a strong vasoconstrictive mediator on afferent arterioles, but has no significant effect on the tone of efferent arterioles, suggesting a functional role of Up(4)A as an autocrine hormone for glomerular perfusion. Because of the predominant effect of the Up(4)A on afferent arterioles, we assume that Up(4)A may decrease glomerular perfusion, intra-glomerular pressure and, hence, glomerular filtration rate. The release of Up(4)A from renal tubular cells may be an additional mechanism whereby tubular cells could affect renal perfusion. Up(4)A release may further contribute to renal vascular autoregulation mechanisms. In conclusion, as Up(4)A occurs in renal tissue and has marked effects on afferent but not efferent arterioles, Up(4)A may play a role in renal hemodynamics and possibly blood pressure regulation.

P2 receptors in cardiovascular regulation and disease

The role of ATP as an extracellular signalling molecule is now well established and evidence is accumulating that ATP and other nucleotides (ADP, UTP and UDP) play important roles in cardiovascular physiology and pathophysiology, acting via P2X (ion channel) and P2Y (G protein-coupled) receptors. In this article we consider the dual role of ATP in regulation of vascular tone, released as a cotransmitter from sympathetic nerves or released in the vascular lumen in response to changes in blood flow and hypoxia. Further, purinergic long-term trophic and inflammatory signalling is described in cell proliferation, differentiation, migration and death in angiogenesis, vascular remodelling, restenosis and atherosclerosis. The effects on haemostasis and cardiac regulation is reviewed. The involvement of ATP in vascular diseases such as thrombosis, hypertension and diabetes will also be discussed, as well as various heart conditions. The purinergic system may be of similar importance as the sympathetic and renin-angiotensin-aldosterone systems in cardiovascular regulation and pathophysiology. The extracellular nucleotides and their cardiovascular P2 receptors are now entering the phase of clinical development.

Relationships between cystic fibrosis transmembrane conductance regulator, extracellular nucleotides and cystic fibrosis

Cystic fibrosis (CF) is one of the most common lethal autosomal recessive genetic diseases in the Caucasian population, with a frequency of about 1 in 3000 livebirths. CF is due to a mutation in the cystic fibrosis transmembrane conductance regulator (CFTR) gene encoding the CFTR protein, a cyclic adenosine 5'-monophosphate (cAMP)-regulated chloride channel localized in the apical membrane of epithelial cells. CFTR is a multifunctional protein which, in addition to be a Cl-channel, is also a regulator of multiple ion channels and other proteins. In particular CFTR has been reported to play a role in the outflow of adenosine 5'-triphosphate (ATP) from cells, but this remains controversial. Extracellular nucleotides are signaling molecules that regulate ion transport and mucociliary clearance by acting on P2 nucleotide receptors, in particular the P2Y(2) receptor. Nucleotides activating the P2Y(2) receptor represent thus one pharmacotherapeutic strategy to treat CF disease, via improvement of mucus hydration and mucociliary clearance in airways. Phase II clinical trials have recently shown that aerosolized denufosol (INS37217, Inspire(R)) improves pulmonary function in CF patients: denufosol was granted orphan drug status and phase III trials are planned. Here, we review what is known about the relationship between extracellular nucleotides and CFTR, the role of extracellular nucleotides in epithelial pathophysiology and their putative role as therapeutic agents.

Anion channels transport ATP into the Golgi lumen

Anion channels provide a pathway for Cl(-) influx into the lumen of the Golgi cisternae. This influx permits luminal acidification by the organelle's H(+)-ATPase. Three different experimental approaches, electrophysiological, biochemical, and proteomic, demonstrated that two Golgi anion channels, GOLAC-1 and GOLAC-2, also mediate ATP anion transport into the Golgi lumen. First, GOLAC-1 and -2 were incorporated into planar lipid bilayers, and single-channel recordings were obtained. Low ionic activities of K(2)ATP added to the cis-chamber directly inhibited the Cl(-) subconductance levels of both channels, with K(m) values ranging from 16 to 115 microM. Substitution of either K(2)ATP or MgATP for Cl(-) on the cis, trans, or both sides indicated that ATP is conducted by the channels with a relative permeability sequence of Cl(-) > ATP(4-) > MgATP(2-). Single-channel currents were observed at physiological concentrations of Cl(-) and ATP, providing evidence for their importance in vivo. Second, transport of [alpha-(32)P]ATP into sealed Golgi vesicles that maintain in situ orientation was consistent with movement through the GOLACs because it exhibited little temperature dependence and was saturated with an apparent K(m) = 25 microM. Finally, after transport of [gamma-(32)P]ATP, a protease-protection assay demonstrated that proteins are phosphorylated within the Golgi lumen, and after SDS-PAGE, the proteins in the phosphorylated bands were identified by mass spectrometry. GOLAC conductances, [alpha-(32)P]ATP transport, and protein phosphorylation have identical pharmacological profiles. We conclude that the GOLACs play dual roles in the Golgi complex, providing pathways for Cl(-) and ATP influx into the Golgi lumen.

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.

Wide nanoscopic pore of maxi-anion channel suits its function as an ATP-conductive pathway

The newly proposed function of the maxi-anion channel as a conductive pathway for ATP release requires that its pore is sufficiently large to permit passage of a bulky ATP(4-) anion. We found a linear relationship between relative permeability of organic anions of different size and their relative ionic mobility (measured as the ratio of ionic conductance) with a slope close to 1, suggesting that organic anions tested with radii up to 0.49 nm (lactobionate) move inside the channel by free diffusion. In the second approach, we, for the first time, succeeded in pore sizing by the nonelectrolyte exclusion method in single-channel patch-clamp experiments. The cutoff radii of PEG molecules that could access the channel from intracellular (1.16 nm) and extracellular (1.42 nm) sides indicated an asymmetry of the two entrances to the channel pore. Measurements by symmetrical two-sided application of PEG molecules yielded an average functional pore radius of approximately 1.3 nm. These three estimates are considerably larger than the radius of ATP(4-) (0.57-0.65 nm) and MgATP(2-) (approximately 0.60 nm). We therefore conclude that the nanoscopic maxi-anion channel pore provides sufficient room to accommodate ATP and is well suited to its function as a conductive pathway for ATP release in cell-to-cell communication.

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.

The cystic fibrosis transmembrane conductance regulator: an intriguing protein with pleiotropic functions

Cystic fibrosis is a frequent autosomal recessive disorder that is caused by the malfunctioning of a small chloride channel, the cystic fibrosis transmembrane conductance regulator. The protein is found in the apical membrane of epithelial cells lining exocrine glands. Absence of this channel results in imbalance of ion concentrations across the cell membrane. As a result, fluids secreted through these glands become more viscous and, in the end, ducts become plugged and atrophic. Little is known about the pathways that link the malfunctioning of the CFTR protein with the observed clinical phenotype. Moreover, there is no strict correlation between specific CFTR mutations and the CF phenotype. This might be explained by the fact that environmental and additional genetic factors may influence the phenotype. The CFTR protein itself is regulated at the maturational level by chaperones and SNARE proteins and at the functional level by several protein kinases. Moreover, CFTR functions also as a regulator of other ion channels and of intracellular membrane transport processes. In order to be able to function as a protein with pleiotropic actions, CFTR seems to be linked with other proteins and with the cytoskeleton through interaction with PDZ-domain-containing proteins at the apical pole of the cell. Progress in cystic fibrosis research is substantial, but still leaves many questions unanswered.

Release of ATP from retinal pigment epithelial cells involves both CFTR and vesicular transport

The retinal pigment epithelium (RPE) faces the photoreceptor outer segments and regulates the composition of the interstitial subretinal space. ATP enhances fluid movement from the subretinal space across the RPE. RPE cells can themselves release ATP, but the mechanisms and polarity of this release are unknown. The RPE expresses the cystic fibrosis transmembrane conductance regulator (CFTR), and CFTR is associated with ATP release in other epithelial cells. However, an increasing number of reports have suggested that the exocytotic pathway contributes to release. In the present study, we examined the involvement of CFTR and the vesicular pathway in ATP release from RPE cells. Release from cultured human ARPE-19 cells and across the apical membrane of fresh bovine RPE cells in an eyecup was studied. A cAMP cocktail to activate CFTR triggered ATP release from fresh and cultured RPE cells. Release from both RPE preparations was largely prevented by the broad-acting blocker glibenclamide and the specific thiazolidinone CFTR inhibitor CFTR-172. The block by CFTR-172 was enhanced by preincubation and prevented ATP release with 3.5 microM IC(50). The rise in intracellular Ca(2+) accompanying hypotonic challenge was prevented by CFTR-172. The vesicular transport inhibitor brefeldin A prevented ATP release after stimulation with both hypotonic and cAMP conditions, suggesting vesicular insertion was also involved. These results show an intimate involvement of CFTR in ATP release from RPE cells which can autostimulate receptors on the apical membrane to modify Ca(2+) signaling. The requirement for both CFTR and vesicular transport pathways suggests vesicular insertion of CFTR may underlie the release of ATP.

P2X receptor-stimulated calcium responses in preglomerular vascular smooth muscle cells involves 20-hydroxyeicosatetraenoic acid

The current study tested the hypothesis that endogenous 20-hydroxyeicosatetraenoic acid (20-HETE) contributes to the increase in intracellular calcium ([Ca2+]i) elicited by P2X receptor activation in renal microvascular smooth muscle cells. Vascular smooth muscle cells obtained from rats were loaded with fura-2 and studied using standard single cell fluorescence microscopy. Basal renal myocyte [Ca2+]i averaged 96 +/- 5 nM. ATP (10 and 100 microM) increased vascular smooth muscle cell [Ca2+]i by 340 +/- 88 and 555 +/- 80 nM, respectively. The cytochrome P450 hydroxylase inhibitor, N-methylsulfonyl-12,12-dibromododec-11-enamide (DDMS), or the 20-HETE antagonist, 20-hydroxyeicosa-6(Z),15(Z)-dienoic acid (20-HEDE), significantly attenuated the peak myocyte [Ca2+]i responses to 10 and 100 microM ATP. ATP (100 microM) increased vascular smooth muscle cell [Ca2+]i by 372 +/- 93 and 163 +/- 55 nM in the presence of DDMS or 20-HEDE, respectively. The P2X receptor agonist, alpha,beta-methylene-ATP (10 microM), increased myocyte [Ca2+]i by 78 +/- 12 nM, and this response was significantly attenuated by DDMS (40 +/- 15 nM). In contrast, the vascular smooth muscle cell [Ca2+]i evoked by the P2Y agonist, UTP (100 microM), was not altered by DDMS or 20-HEDE. The effect of 20-HETE on [Ca2+]i was also assessed, and the peak increases in [Ca2+]i averaged 62 +/- 12 and 146 +/- 70 nM at 20-HETE concentrations of 1 and 10 microM, respectively. These results demonstrate that 20-HETE plays a significant role in the renal microvascular smooth muscle cell [Ca2+]i response to P2X receptor activation.

Ion transport across Xenopus alveolar epithelium is regulated by extracellular ATP, UTP and adenosine

Native alveolar epithelium from Xenopus lung was used for electrophysiological Ussing chamber experiments to investigate ion transport regulation. The tissue exhibits a considerable absorption of Na(+) ions and this transepithelial transport is largely up-regulated after treatment of donor animals with ACTH. Extracellular ATP, UTP and adenosine were tested for their regulating effects and all three increased I(sc), which was mainly due to a stimulation of amiloride sensitive Na(+) transport (increase of I(ami) 32% for ATP, 21% for UTP, 25% for adenosine). Solely the effect of UTP was completely abolished in the presence of amiloride. In contrast, the effects of ATP or adenosine disappeared under Cl(-)-free conditions. ATP and UTP proved to have additive effects and pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS), an antagonist of purinergic receptors, inhibited selectively the effect of UTP on I(sc). Further, I(sc) was increased by the P2X selective agonist beta,gamma-meATP. We were able to demonstrate, that extracellular purines and pyrimidines play a possible role as auto/paracrine messengers for alveolar ion transport regulation in Xenopus lung.

Mechanisms of secretion of ATP from cortical astrocytes triggered by uridine triphosphate

The mechanisms involved in autocrine ATP release from cultured astrocytes isolated from the rat cortex were investigated using an online bioluminescence technique. Astrocytes released ATP in response to application of 10 microM uridine triphosphate, which was blocked by the non-specific purinergic receptor antagonist suramin. Intracellular pathways of the uridine triphosphate-stimulated ATP release were seen to involve inositol triphosphate and calcium with the assistance of the Golgi-complex and cytoskeleton as the release was inhibited by phospholipase C antagonist lithium, endoplasmic reticulum calcium-dependent ATPase inhibitor thapsigargin, F-actin interruptor cytochalasin D and Golgi-complex interruptor brefeldin A. The uridine triphosphate-stimulated ATP release was also potently blocked by exocytosis inhibitor botulinum toxin A and anion transporter blockers furosemide and glibenclamide. These results suggest that calcium-dependent exocytosis and transportation via anion transporters are the predominant secretion mechanisms for uridine triphosphate-stimulated ATP release from cortical astrocytes.

Pharmacotherapy of the ion transport defect in cystic fibrosis: role of purinergic receptor agonists and other potential therapeutics

Cystic fibrosis (CF), is an autosomal recessive disease frequently seen in the Caucasian population. It is caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene. CF is characterized by enhanced airway Na(+) absorption, mediated by epithelial Na(+) channels (ENaC), and deficient Cl(-) transport. In addition, other mechanisms may contribute to the pathophysiological changes in the CF lung, such as defective regulation of HCO(3)(-) secretion. In other epithelial tissues, epithelial Na(+) conductance is either increased (intestine) or decreased (sweat duct) in CF. CFTR is a cyclic AMP-regulated epithelial Cl(-) channel, and appears to control the activity of several other transport proteins. Accordingly, defective epithelial ion transport in CF is likely to be a combination of defective Cl(-) channel function and impaired regulator function of CFTR, which in turn is linked to impaired mucociliary clearance and development of chronic lung disease. As the clinical course of CF is determined primarily by progressive lung disease, novel pharmacological strategies for the treatment of CF focus on correction of the ion transport defect in the airways. In recent years, it has been demonstrated that activation of purinergic receptors in airway epithelia by extracellular nucleotides (adenosine triphosphate/uridine triphosphate) has beneficial effects on mucus clearance in CF. Activation of the dominant class of metabotropic purinergic receptors, P2Y(2) receptors, appears to have a 2-fold benefit on ion transport in CF airways; excessive Na(+) absorption is attenuated, most likely by inhibition of the ENaC and, simultaneously, an alternative Ca(2+)-dependent Cl(-) channel is activated that may compensate for the CFTR Cl(-) channel defect. Thus activation of P2Y(2) receptors is expected to lead to improved hydration of the airway surface liquid in CF. Furthermore, purinergic activation has been shown to promote other components of mucociliary clearance such as ciliary beat frequency and mucus secretion. Clinical trials are under way to test the effect of synthetic purinergic compounds, such as the P2Y(2) receptor agonist INS37217, on the progression of lung disease in patients with CF. Administration of these compounds alone, or in combination with other drugs that inhibit accelerated Na(+) transport and help recover or increase residual activity of mutant CFTR, is most promising as successful therapy to counteract the ion transport defect in the airways of CF patients.

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.

CFTR-dependent and -independent swelling-activated K+ currents in primary cultures of mouse nephron

The role of CFTR in the control of K(+) currents was studied in mouse kidney. Whole cell clamp was used to identify K(+) currents on the basis of pharmacological sensitivities in primary cultures of proximal (PCT) and distal convoluted tubule (DCT) and cortical collecting tubule (CCT) from wild-type (WT) and CFTR knockout (KO) mice. In DCT and CCT cells, forskolin activated a 293B-sensitive K(+) current in WT, but not in KO, mice. In these cells, a hypotonic shock induced K(+) currents blocked by charybdotoxin in WT, but not in KO, mice. In PCT cells from WT and KO mice, the hypotonicity-induced K(+) currents were insensitive to these toxins and were activated at extracellular pH 8.0 and inhibited at pH 6.0, suggesting that the corresponding channel was TASK2. In conclusion, CFTR is implicated in the control of KCNQ1 and Ca(2+)-sensitive swelling-activated K(+) conductances in DCT and CCT, but not in proximal convoluted tubule, cells. In KO mice, impairment of the regulatory volume decrease process in DCT and CCT could be due to the loss of an autocrine mechanism, implicating ATP and adenosine, which controls swelling-activated Cl(-) and K(+) channels.

Macula densa cell signaling involves ATP release through a maxi anion channel

Macula densa cells are unique renal biosensor cells that detect changes in luminal NaCl concentration ([NaCl](L)) and transmit signals to the mesangial cellafferent arteriolar complex. They are the critical link between renal salt and water excretion and glomerular hemodynamics, thus playing a key role in regulation of body fluid volume. Since identification of these cells in the early 1900s, the nature of the signaling process from macula densa cells to the glomerular contractile elements has remained unknown. In patch-clamp studies of macula densa cells, we identified an [NaCl](L)-sensitive ATP-permeable large-conductance (380 pS) anion channel. Also, we directly demonstrated the release of ATP (up to 10 microM) at the basolateral membrane of macula densa cells, in a manner dependent on [NaCl](L), by using an ATP bioassay technique. Furthermore, we found that glomerular mesangial cells respond with elevations in cytosolic Ca(2+) concentration to extracellular application of ATP (EC(50) 0.8 microM). Importantly, we also found increases in cytosolic Ca(2+) concentration with elevations in [NaCl](L), when fura-2-loaded mesangial cells were placed close to the basolateral membrane of macula densa cells. Thus, cell-to-cell communication between macula densa cells and mesangial cells, which express P2Y(2) receptors, involves the release of ATP from macula densa cells via maxi anion channels at the basolateral membrane. This mechanism may represent a new paradigm in cell-to-cell signal transduction mediated by ATP.

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.

Intercellular calcium signaling mediated by point-source burst release of ATP

Calcium signaling, manifested as intercellular waves of rising cytosolic calcium, is, in many cell types, the result of calcium-induced secretion of ATP and activation of purinergic receptors. The mechanism by which ATP is released has hitherto not been established. Here, we show by real-time bioluminescence imaging that ATP efflux is not uniform across a field of cells but is restricted to brief, abrupt point-source bursts. The ATP bursts emanate from single cells and manifest the transient opening of nonselective membrane channels, which admits fluorescent indicators of < or = 1.5 kDa. These observations challenge the existence of regenerative ATP release, because ATP efflux is finite and restricted to a point source. Transient efflux of cytosolic nucleotides from a subset of cells may represent a conserved pathway for coordinating local activity of electrically nonexcitable cells, because identical patterns of ATP release were identified in human astrocytes, endothelial cells, and bronchial epithelial cells.

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

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

Cellular and biophysical evidence for interactions between adenosine triphosphate and P-glycoprotein substrates: functional implications for adenosine triphosphate/drug cotransport in P-glycoprotein overexpressing tumor cells and in P-glycoprotein low-level expressing erythrocytes

P-glycoprotein is involved with the removal of drugs, most of them cations, from the plasma membrane and cytoplasm. Pgp is also associated with movement of ATP, an anion, from the cytoplasm to the extracellular space. The central question of this study is whether drug and ATP transport associated with the expression of Pgp are in any way coupled. We have measured the stoichiometry of transport coupling between drug and ATP release. The drug and ATP transport that is inhibitable by the sulfonylurea compound, glyburide (P. E. Golstein, A. Boom, J. van Geffel, P. Jacobs, B. Masereel, and R. Beauwens, Pfluger's Arch. 437, 652, 1999), permits determination of the transport coupling ratio, which is close to 1:1. In view of this result, we asked whether ATP interacts directly with Pgp substrates. We show by measuring the movement of Pgp substrates in electric fields that ATP and drug movement are coupled. The results are compatible with the view that substrates for Pgp efflux are driven by the movement of ATP through electrostatic interaction and effective ATP-drug complex formation with net anionic character. This mechanism not only pertains to drug efflux from tumor cells overexpressing Pgp, but also provides a framework for understanding the role of erythrocytes in drug resistance. The erythrocyte consists of a membrane surrounding a millimolar pool of ATP. Mammalian RBCs have no nucleus or DNA drug/toxin targets. From the perspective of drug/ATP complex formation, the RBC serves as an important electrochemical sink for toxins. The presence in the erythrocyte membrane of approximately 100 Pgp copies per RBC provides a mechanism for eventual toxin clearance. The RBC transport of toxins permits their removal from sensitive structures and ultimate clearance from the organism via the liver and/or kidneys.

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.

Brefeldin A block of integrin-dependent mechanosensitive ATP release from Xenopus oocytes reveals a novel mechanism of mechanotransduction

Many animal cells release ATP into the extracellular medium, and often this release is mechanosensitive. However, the mechanisms underlying this release are not well understood. Using the luciferin-luciferase bioluminescent assay we demonstrate that a Xenopus oocyte releases ATP at a basal rate approximately 0.01 fmol/s, and gentle mechanical stimulation can increase this to 50 fmol/s. Brefeldin A, nocodazole, and progesterone-induced- maturation block basal and mechanosensitive ATP release. These treatments share the common feature of disrupting the Golgi complex and vesicle trafficking to the cell surface and thereby block protein secretion and membrane protein insertion. We propose that ATP release occurs when protein transport vesicles enriched in ATP fuse with the plasma membrane. Collagenase, integrin-binding peptides, and cytochalasin D also block ATP release, indicating that extracellular, membrane and cytoskeletal elements are involved in the release process. Elevation of intracellular Ca(2+) does not evoke ATP release but potentiates mechanosensitive ATP release. Our study indicates a novel mechanism of mechanotransduction that would allow cells to regulate membrane trafficking and protein transport/secretion in response to mechanical loading.

Renal microvascular effects of P2 receptor stimulation

1. The field of extracellular nucleotides and purinoceptors has undergone a resurgence of interest and enthusiasm in the past decade. More and more investigators are probing the physiological and pathophysiological roles of P2 receptors in virtually every organ system, including the kidney. 2. With this renewed interest has come a new appreciation for the roles extracellular adenine nucleotides can play in regulating or modulating renal function. In the past 5 years, investigators have provided compelling evidence that extracellular nucleotides, working through activation of P2 purinoceptors, have a significant impact on renal microvascular function, mesangial cell function and on renal epithelial transport. 3. Evidence has been uncovered that implicates P2 receptor activation in mediating renal microvascular autoregulatory behaviour. Locally released ATP has a direct paracrine and/or autocrine effect modulating renal epithelial transporters and tubular epithelial channels to influence tubular fluid composition. 4. While the specific roles of extracellular nucleotides and their receptors in the kidney have not been absolutely identified, it now appears clear that endogenously released ATP may play a significant role in regulating kidney function. 5. The purpose of the present review is to update our current understanding of the effect of P2 receptor activation on renal microvascular function and to detail the signal transduction mechanisms known to be involved.

Molecular basis of mechanotransduction in living cells

The simplest cell-like structure, the lipid bilayer vesicle, can respond to mechanical deformation by elastic membrane dilation/thinning and curvature changes. When a protein is inserted in the lipid bilayer, an energetic cost may arise because of hydrophobic mismatch between the protein and bilayer. Localized changes in bilayer thickness and curvature may compensate for this mismatch. The peptides alamethicin and gramicidin and the bacterial membrane protein MscL form mechanically gated (MG) channels when inserted in lipid bilayers. Their mechanosensitivity may arise because channel opening is associated with a change in the protein's membrane-occupied area, its hydrophobic mismatch with the bilayer, excluded water volume, or a combination of these effects. As a consequence, bilayer dilation/thinning or changes in local membrane curvature may shift the equilibrium between channel conformations. Recent evidence indicates that MG channels in specific animal cell types (e.g., Xenopus oocytes) are also gated directly by bilayer tension. However, animal cells lack the rigid cell wall that protects bacteria and plants cells from excessive expansion of their bilayer. Instead, a cortical cytoskeleton (CSK) provides a structural framework that allows the animal cell to maintain a stable excess membrane area (i.e., for its volume occupied by a sphere) in the form of membrane folds, ruffles, and microvilli. This excess membrane provides an immediate membrane reserve that may protect the bilayer from sudden changes in bilayer tension. Contractile elements within the CSK may locally slacken or tighten bilayer tension to regulate mechanosensitivity, whereas membrane blebbing and tight seal patch formation, by using up membrane reserves, may increase membrane mechanosensitivity. In specific cases, extracellular and/or CSK proteins (i.e., tethers) may transmit mechanical forces to the process (e.g., hair cell MG channels, MS intracellular Ca(2+) release, and transmitter release) without increasing tension in the lipid bilayer.

Membrane transport in the malaria-infected erythrocyte

The malaria parasite is a unicellular eukaryotic organism which, during the course of its complex life cycle, invades the red blood cells of its vertebrate host. As it grows and multiplies within its host blood cell, the parasite modifies the membrane permeability and cytosolic composition of the host cell. The intracellular parasite is enclosed within a so-called parasitophorous vacuolar membrane, tubular extensions of which radiate out into the host cell compartment. Like all eukaryote cells, the parasite has at its surface a plasma membrane, as well as having a variety of internal membrane-bound organelles that perform a range of functions. This review focuses on the transport properties of the different membranes of the malaria-infected erythrocyte, as well as on the role played by the various membrane transport systems in the uptake of solutes from the extracellular medium, the disposal of metabolic wastes, and the origin and maintenance of electrochemical ion gradients. Such systems are of considerable interest from the point of view of antimalarial chemotherapy, both as drug targets in their own right and as routes for targeting cytotoxic agents into the intracellular parasite.

Renal interstitial atp responses to changes in arterial pressure during alterations in tubuloglomerular feedback activity

We recently demonstrated a direct relationship between autoregulation-related changes in renal vascular resistance (RVR) and renal interstitial ATP concentrations. To assess the possible role for extracellular ATP in the regulation of tubuloglomerular feedback (TGF)-mediated autoregulatory adjustments in RVR, renal interstitial ATP concentrations were measured with microdialysis probes in anesthetized dogs at different renal arterial pressures (RAPs) within the autoregulatory range during augmented and diminished activity of the TGF mechanism. Stepwise reductions in RAP from ambient pressure (129+/-3 mm Hg) to 102+/-2 mm Hg (step 1) and 75+/-1 mm Hg (step 2) resulted in significant decreases in ATP concentrations from 9.0+/-0.8 to 6.3+/-0.6 nmol/L in step 1 and to 4.2+/-0.5 nmol/L in step 2. Changes in RVR were highly correlated with changes in ATP concentrations (r=0.86, P<0.001, n=12). Acetazolamide (100 microgram. kg(-1). min(-1), n=6), which increases solute delivery to the macula densa, thus augmenting TGF activity, significantly decreased renal blood flow (RBF) by -16+/-2% and glomerular filtration rate (GFR) by -22+/-4% and increased ATP concentrations from 8.4+/-0.7 to 15.5+/-1.4 nmol/L. Although basal RBF and GFR levels were reduced by the acetazolamide infusion, autoregulation efficiency was maintained, and interstitial ATP concentrations were significantly decreased in response to reductions in RAP by -36+/-4% in step 1 and by -54+/-2% in step 2. The relationship between changes in RVR and interstitial ATP concentrations was preserved during acetazolamide treatment (r=0.80, P<0.01). Inhibition of the TGF mechanism by furosemide significantly increased RBF by 33+/-6% and GFR by 13+/-2% and decreased ATP concentrations from 8.9+/-1.4 to 5.0+/-0.8 nmol/L (n=6). Furosemide caused marked impairment of RBF and GFR autoregulatory efficiency (by -14+/-3% and -11+/-3% in step 1 and by -26+/-2% and -18+/-4% in step 2, respectively). In the furosemide-treated kidneys, interstitial ATP levels remained low and were not altered during reductions in RAP (4.7+/-0.7 nmol/L in step 1 and 4.7+/-0.8 nmol/L in step 2), and changes in RVR did not exhibit a correlation with changes in ATP concentrations (r=0.22, P=0.30). These data support the hypothesis that extracellular ATP contributes to autoregulatory adjustments in RVR that are mediated by changes in activity of the TGF mechanism.

Ultrastructural localisation of ATP-gated P2X2 receptor immunoreactivity in vascular endothelial cells in rat brain

In this report, we show for the first time that P2X2 receptors--ATP-gated cation channels--can be demonstrated in endothelial cells of small cerebral vessels of rat. Immunoreactivity to P2X2 receptors was visualised at the ultrastructural level with electron-immunocytochemistry (ExtrAvidin-horseradish peroxidase technique) using a polyclonal antibody against the fragment of an intracellular domain of the receptor. The possibilities that these receptors may regulate the formation of gap and/or tight junctions between adjacent endothelial cells influencing the blood-brain barrier, or modulate the contractility of capillary endothelial cells are discussed.

Regulation of cell volume via microvillar ion channels

A novel mechanism of cellular volume regulation is presented, which ensues from the recently introduced concept of transport and ion channel regulation via microvillar structures (Lange K, 1999, J Cell Physiol 180:19-35). According to this notion, the activity of ion channels and transporter proteins located on microvilli of differentiated cells is regulated by changes in the structural organization of the bundle of actin filaments in the microvillar shaft region. Cells with microvillar surfaces represent two-compartment systems consisting of the cytoplasm on the one side and the sum of the microvillar tip (or, entrance) compartments on the other side. The two compartments are separated by the microvillar actin filament bundle acting as diffusion barrier ions and other solutes. The specific organization of ion and water channels on the surface of microvillar cell types enables this two-compartment system to respond to hypo- and hyperosmotic conditions by activation of ionic fluxes along electrochemical gradients. Hypotonic exposure results in swelling of the cytoplasmic compartment accompanied by a corresponding reduction in the length of the microvillar diffusion barrier, allowing osmolyte efflux and regulatory volume decrease (RVD). Hypertonic conditions, which cause shortening of the diffusion barrier via swelling of the entrance compartment, allow osmolyte influx for regulatory volume increase (RVI). Swelling of either the cytoplasmic or the entrance compartment, by using membrane portions of the microvillar shafts for surface enlargement, activates ion fluxes between the cytoplasm and the entrance compartment by shortening of microvilli. The pool of available membrane lipids used for cell swelling, which is proportional to length and number of microvilli per cell, represents the sensor system that directly translates surface enlargements into activation of ion channels. Thus, the use of additional membrane components for osmotic swelling or other types of surface-expanding shape changes (such as the volume-invariant cell spreading or stretching) directly regulates influx and efflux activities of microvillar ion channels. The proposed mechanism of ion flux regulation also applies to the physiological main functions of epithelial cells and the auxiliary action of swelling-induced ATP release. Furthermore, the microvillar entrance compartment, as a finely dispersed ion-accessible peripheral space, represents a cellular sensor for environmental ionic/osmotic conditions able to detect concentration gradients with high lateral resolution. Volume regulation via microvillar surfaces is only one special aspect of the general property of mechanosensitivity of microvillar ionic pathways.

Increased circulating levels of plasma ATP in cystic fibrosis patients

Recent studies have shown that the cystic fibrosis transmembrane conductance regulator (CFTR), an ATP-binding cassette (ABC) transporter whose mutations are responsible for cystic fibrosis (CF), permeates ATP. However, little information is available concerning extracellular ATP concentrations in CF patients. Thus, the goal of this preliminary study was to determine the circulating levels of plasma ATP in CF patients. Circulating levels of plasma ATP were determined by the luciferin-luciferase assay in both CF patients and healthy volunteer control subjects. The two groups were compared using an analysis of variance. CF genotype and age, which ranged from 7 to 56 years, were also used to compare data by single-blind analysis. With comparable sample numbers, CF patients had statistically higher levels of circulating ATP (34%, P<0.01) when compared by analysis of covariance with the age of the subjects as the cofactor. The CF patients bearing the DeltaF508 genotype had a 54% (n=33, P<0.01) higher plasma ATP concentration compared to controls, while patients bearing other CF genotypes were similar to controls (n=10, P<0.4). We conclude that CF patients have higher circulating levels of ATP when compared to controls. Increased levels of plasma ATP, which is an important autocrine/paracrine hormone in many cell types, may be associated with chronic manifestations of the disease.

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

Mechanical strain applied to prostate cancer cells induced an intracellular Ca(2+) (Ca(i)(2+)) wave spreading with a velocity of 15 microm/s. Ca(i)(2+) waves were not dependent on extracellular Ca(2+) and membrane potential because propagation was unaffected in high-K(+) and Ca(2+)-free solution. Waves did not depend on the cytoskeleton or gap junctions because cytochalasin B and nocodazole, which disrupt microfilaments and microtubules, respectively, and 1-heptanol, which uncouples gap junctions, were without effects. Fluorescence recovery after photobleaching experiments revealed an absence of gap junctional coupling. Ca(i)(2+) waves were inhibited by the purinergic receptor antagonists basilen blue and suramin; by pretreatment with ATP, UTP, ADP, UDP, 2-methylthio-ATP, and benzoylbenzoyl-ATP; after depletion of ATP by 2-deoxyglucose; and after ATP scavenging by apyrase. Waves were abolished by the anion channel inhibitors 5-nitro-2-(3-phenylpropylamino)benzoic acid, tamoxifen, 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid, niflumic acid, and gadolinium. ATP release following strain was significantly inhibited by anion channel blockers. Hence, ATP is secreted via mechanosensitive anion channels and activates purinergic receptors on the same cell or neighboring cells in an autocrine and paracrine manner, thus leading to Ca(i)(2+) wave propagation.

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

The molecular mechanisms associated with intracellular ATP release by the heart are largely unknown. In this study the luciferin-luciferase assay and patch-clamp techniques were used to characterize the pathways responsible for ATP release in neonatal rat cardiac myocytes (NRCM). Spontaneous ATP release by NRCM was significantly increased after cAMP stimulation under physiological conditions. cAMP stimulation also induced an anion-selective electrodiffusional pathway that elicited linear, diphenylamine-2-carboxylate (DPC)-inhibitable Cl(-) currents in either symmetrical MgCl(2) or NaCl. ATP, adenosine 5'-O-(3-thiotriphosphate), and the ATP derivatives ADP and AMP, permeated this pathway; however, GTP did not. The cAMP-induced ATP currents were inhibited by DPC and glibenclamide and by a monoclonal antibody raised against the R domain of the cystic fibrosis transmembrane conductance regulator (CFTR). The channel-like nature of the cAMP-induced ATP-permeable pathway was also determined by assessing protein kinase A-activated single channel Cl(-) and ATP currents in excised inside-out patches of NRCM. Single channel currents were inhibited by DPC and the anti-CFTR R domain antibody. Thus the data in this report demonstrate the presence of a cAMP-inducible electrodiffusional ATP transport mechanism in NRCM. Based on the pharmacology, patch-clamping data, and luminometry studies, the data are most consistent with the role of a functional CFTR as the anion channel implicated in cAMP-activated ATP transport in NRCM.

Relation between renal interstitial ATP concentrations and autoregulation-mediated changes in renal vascular resistance

The present study was performed to examine the hypothesis that autoregulation-related changes in renal vascular resistance (RVR) are mediated by extracellular ATP. By use of a microdialysis method, renal interstitial concentrations of ATP and adenosine were measured at different renal arterial pressures (RAPs) within the autoregulatory range in anesthetized dogs (n=12). RAP was reduced in steps from the ambient pressure (131+/-4 mm Hg) to 105+/-3 mm Hg (step 1) and 80+/-2 mm Hg (step 2). Renal blood flow and glomerular filtration rate exhibited efficient autoregulation in response to these changes in RAP. RVR decreased by 22+/-2% in step 1 (P<0.01) and 38+/-3% in step 2 (P<0.01). The control renal interstitial concentration of ATP was 6.51+/-0.71 nmol/L and decreased to 4. 51+/-0.55 nmol/L in step 1 (P<0.01) and 2.77+/-0.47 nmol/L in step 2 (P<0.01). In contrast, the adenosine concentrations (117+/-6 nmol/L) were not altered significantly. Changes in ATP levels were highly correlated with changes in RVR (r=0.88, P<0.0001). Further studies demonstrated that stimulation of the tubuloglomerular feedback (TGF) mechanism by increasing distal volume delivery elicited with acetazolamide also led to increases in renal interstitial ATP concentrations, whereas furosemide, which is known to block TGF responses, reduced renal interstitial fluid ATP concentrations. The data demonstrate a positive relation between renal interstitial fluid ATP concentrations and both autoregulation- and TGF-dependent changes in RVR and thus support the hypothesis that changes in extracellular ATP contribute to the RVR adjustments responsible for the mechanism of renal autoregulation.

Swelling-activated, cystic fibrosis transmembrane conductance regulator-augmented ATP release and Cl- conductances in murine C127 cells

1. A hypotonic challenge, but not cAMP stimulation, was found to induce release of ATP measured by the luciferin-luciferase assay from both the murine mammary carcinoma cell line C127i and C127 cells stably transfected with the cDNA for human cystic fibrosis transmembrane conductance regulator (CFTR) protein (C127/CFTR). CFTR expression augmented swelling-induced ATP release by 10-20 times under hypotonic conditions (< or = 80 % osmolality). 2. Glibenclamide failed to suppress swelling-induced ATP release from C127/CFTR cells. In contrast, whole-cell patch-clamp recordings showed that both the cAMP-activated ohmic Cl- currents and volume-sensitive outwardly rectifying (VSOR) Cl- currents were prominently suppressed by glibenclamide. 3. Gd3+ markedly blocked swelling-induced ATP release but failed to suppress both cAMP- and swelling-activated Cl- currents in the CFTR-expressing cells. Even after pretreatment and during treatment with Gd3+, VSOR Cl- currents were activated normally. 4. The continuous presence of an ATP-hydrolysing enzyme, apyrase, in the bathing solution did not prevent activation of VSOR Cl- currents in C127/CFTR cells. 5. The rate of regulatory volume decrease (RVD) in C127/CFTR cells was much faster than that in C127i cells. When apyrase was added to the bathing solution, the RVD rate was retarded in C127/CFTR cells. 6. On balance, the following conclusions can be deduced. First, swelling-induced ATP release is augmented by expression of CFTR but is not mediated by the CFTR Cl- channel. Second, swelling-induced ATP release is not mediated by the VSOR Cl- channel. Third, the released ATP facilitated the RVD process but is not involved in the activation of VSOR Cl- channels in C127/CFTR cells.

cAMP-activated anion conductance is associated with expression of CFTR in neonatal mouse cardiac myocytes

In this study, patch-clamp techniques were applied to cultured neonatal mouse cardiac myocytes (NMCM) to assess the contribution of cAMP stimulation to the anion permeability in this cell model. Addition of either isoproterenol or a cocktail to raise intracellular cAMP increased the whole cell currents of NMCM. The cAMP-dependent conductance was largely anionic, as determined under asymmetrical (low intracellular) Cl(-) conditions and symmetrical Cl(-) in the presence of various counterions, including Na(+), Mg(2+), Cs(+), and N-methyl-D-glucamine. Furthermore, the cAMP-stimulated conductance was also permeable to ATP. The cAMP-activated currents were inhibited by diphenylamine-2-carboxylate, glibenclamide, and an anti-cystic fibrosis transmembrane conductance regulator (CFTR) monoclonal antibody. The anti-CFTR monoclonal antibody failed, however, to inhibit an osmotically activated anion conductance, indicating that CFTR is not linked to osmotically stimulated currents in this cell model. Immunodetection studies of both neonatal mouse heart tissue and cultured NMCM revealed that CFTR is expressed in these preparations. The implication of CFTR in the cAMP-stimulated Cl(-)- and ATP-permeable conductance was further verified with NMCM of CFTR knockout mice [cftr(-/-)] in which cAMP stimulation was without effect on the whole cell currents. In addition, stimulation with protein kinase A and ATP induced Cl(-)-permeable single-channel activity in excised, inside-out patches from control, but not cftr(-/-) NMCM. The data in this report indicate that cAMP stimulation of NMCM activates an anion-permeable conductance with functional properties similar to those expected for CFTR, thus suggesting that CFTR may be responsible for the cAMP-activated conductance. CFTR may thus contribute to the permeation and/or regulation of Cl(-)- and ATP-permeable pathways in the developing heart.