Extracellular 2-chloroadenosine and ATP stimulate volume-sensitive Cl- current and calcium mobilization in human tracheal 9HTEo- cells

The regulation of ATP release from the urothelium by adenosine and transepithelial potential

WHAT'S KNOWN ON THE SUBJECT? AND WHAT DOES THE STUDY ADD?: Stretch of the urothelium, as occurs during bladder filling, is associated with a release of ATP that is postulated to act as a sensory neurotransmitter. The regulation of ATP release is poorly understood and in particular if there is a feedback mechanism provided by ATP itself. Adenosine, a breakdown product of ATP, is a potent inhibitor of stretch-induced ATP release, acting through and A1 receptor; endogenous levels are about 0.6μM. Data are consistent with ATP release relying on the rise of intracellular Ca2+. Transepithelial potential also controls ATP release, also acting via an A1 receptor-dependent pathway.

OBJECTIVES

To test the hypothesis that distension-induced ATP release from the bladder urothelium is regulated by adenosine as well as changes to transurothelial potential (TEP). To examine the role of changes to intracellular [Ca(2+) ] in ATP release.

MATERIALS AND METHODS

Rabbit urothelium/suburothelium membranes were used in an Ussing chamber system. Distension was induced by fluid removal from the chamber bathing the serosal (basolateral) membrane face. The TEP and short-circuit current were measured. ATP was measured in samples aspirated from the serosal chamber by a luciferin-luciferase assay. Intracellular [Ca(2+) ] was measured in isolated urothelial cells using the fluorochrome Fura-2. All experiments were performed at 37°C.

RESULTS

Distension-induced ATP release was decreased by adenosine (1-10 μm) and enhanced by adenosine deaminase and A1- (but not A2-) receptor antagonists. Distension-induced ATP release was reduced by 2-APB, nifedipine and capsazepine; capsaicin induced ATP release in the absence of distension. ATP and capsaicin, but not adenosine, generated intracellular Ca(2+) transients; adenosine did not affect the ATP-generated Ca(2+) transient. ATP release was dependent on a finite transepithelial potential. Changes to TEP, in the absence of distension, generated ATP release that was in turn reduced by adenosine.

CONCLUSION

Adenosine exerts a powerful negative feedback control of ATP release from the urothelium via A1 receptor activation. Distension-induced ATP release may be mediated by a rise of the intracellular [Ca(2+) ]. Modulation of distension-induced ATP release by adenosine and TEP may have a common pathway.

TMEM16A, a membrane protein associated with calcium-dependent chloride channel activity

Calcium-dependent chloride channels are required for normal electrolyte and fluid secretion, olfactory perception, and neuronal and smooth muscle excitability. The molecular identity of these membrane proteins is still unclear. Treatment of bronchial epithelial cells with interleukin-4 (IL-4) causes increased calcium-dependent chloride channel activity, presumably by regulating expression of the corresponding genes. We performed a global gene expression analysis to identify membrane proteins that are regulated by IL-4. Transfection of epithelial cells with specific small interfering RNA against each of these proteins shows that TMEM16A, a member of a family of putative plasma membrane proteins with unknown function, is associated with calcium-dependent chloride current, as measured with halide-sensitive fluorescent proteins, short-circuit current, and patch-clamp techniques. Our results indicate that TMEM16A is an intrinsic constituent of the calcium-dependent chloride channel. Identification of a previously unknown family of membrane proteins associated with chloride channel function will improve our understanding of chloride transport physiopathology and allow for the development of pharmacological tools useful for basic research and drug development.

Purinergic regulation of epithelial transport

Purinergic receptors are a family of ubiquitous transmembrane receptors comprising two classes, P1 and P2 receptors, which are activated by adenosine and extracellular nucleotides (i.e. ATP, ADP, UTP and UDP), respectively. These receptors play a significant role in regulating ion transport in epithelial tissues through a variety of intracellular signalling pathways. Activation of these receptors is partially dependent on ATP (or UTP) release from cells and its subsequent metabolism, and this release can be triggered by a number of stimuli, often in the setting of cellular damage. The function of P2Y receptor stimulation is primarily via signalling through the G(q)/PLC-beta pathway and subsequent activation of Ca(2+)-dependent ion channels. P1 signalling is complex, with each of the four P1 receptors A(1), A(2A), A(2B), and A(3) having a unique role in different epithelial tissue types. In colonic epithelium the A(2B) receptor plays a prominent role in regulating Cl(-) and water secretion. In airway epithelium, A(2B) and A(1) receptors are implicated in the control of Cl(-) and other currents. In the renal tubular epithelium, A(1), A(2A), and A(3) receptors have all been identified as playing a role in controlling the ionic composition of the lumenal fluid. Here we discuss the intracellular signalling pathways for each of these receptors in various epithelial tissues and their roles in pathophysiological conditions such as cystic fibrosis.

Ecto 5'-nucleotidase and nonspecific alkaline phosphatase. Two AMP-hydrolyzing ectoenzymes with distinct roles in human airways

In human airways, extracellular adenosine regulates epithelial functions supporting mucociliary clearance, an important airway defense mechanism against bacterial infection. Thus, defining the mechanisms of adenosine generation is critical for elucidating the role of this nucleoside in airway homeostasis. In this study, we identified the source of adenosine on the mucosal surface of human airway epithelia. Polarized primary cultures of human nasal or bronchial epithelial cells were assayed for transepithelial transport, cytosolic and cell surface adenosine production. Ussing chamber experiments indicated that serosal 1 microM [(3)H]adenosine was not transported to the mucosal compartment. Messenger RNA for the cytosolic AMP-specific 5'-nucleotidase (CN-I) was not detected in human bronchial epithelial cells, suggesting that mucosal adenosine did not originate from intracellular pools. In contrast, extracellular 0.1 mm ATP was rapidly dephosphorylated into adenosine on the mucosal epithelial surface. We identified two ectonucleotidases that mediated the conversion of AMP to adenosine: ecto 5'-nucleotidase (ecto 5'-NT, CD73) and alkaline phosphatase (AP). Both mucosal and serosal epithelial surfaces displayed ecto 5'-NT activity (K(m) = 14 microM, V(max) = 0.5 nmol x min(-1) x cm(-2)), whereas AP activity was restricted to the mucosal surface (K(m,)(high) = 36 microM, V(max) = 1.2 nmol x min(-1) x cm(-2); K(m,)(low) = 717 microM, V(max) = 2.8 nmol x min(-1) x cm(-2)). In bronchial cultures and tissues, ecto 5'-NT accounted for >80% of total activity toward 0.01 mm AMP, compared with <15% for 5 mm AMP. The proximal airway AP isoform was identified as nonspecific AP (NS AP) by levamisole sensitivity and mRNA expression. The two ectoenzymes presented opposite airway distributions, ecto 5'-NT and NS AP mRNA dominating in higher and lower airways, respectively. Collectively, these experiments support a major role for extracellular nucleotide catalysis and for ecto 5'-NT and NS AP in the regulation of adenosine concentrations on airway surfaces.

Complex host cell responses to antisense suppression of ACHE gene expression

3'-End-capped, 20-mer antisense oligodeoxynucleotides (AS-ODN) protected with 2'-O-methyl (Me) or phosphorothioate (PS) substitutions were targeted to acetylcholinesterase (AChE) mRNA and studied in PC12 cells. Me-modified AS-ODN suppressed AChE activity up to 50% at concentrations of 0.02-100 nM. PS-ODN was effective at 1-100 nM. Both AS-ODN displayed progressively decreased efficacy above 10 nM. In situ hybridization and confocal microscopy demonstrated dose-dependent decreases, then increases, in AChE mRNA. Moreover, labeling at nuclear foci suggested facilitated transcription or stabilization of AChE mRNA or both under AS-ODN. Intracellular concentrations of biotinylated oligonucleotide equaled those of target mRNA at extracellular concentrations of 0.02 nM yet increased only 6-fold at 1 microM ODN. Above 50 nM, sequence-independent swelling of cellular, but not nuclear, volume was observed. Our findings demonstrate suppressed AChE expression using extremely low concentrations of AS-ODN and attribute reduced efficacy at higher concentrations to complex host cell feedback responses.

Influence of calcium on regulatory volume decrease: role of potassium channels

In most cell types, hyposmotic swelling consistently elicits an increase in the concentration of cytosolic Ca2+ - [Ca2+]i - with contributions of extracellular and intracellular sources. The mechanisms of Ca2+ entry and release from endogenous sources are not fully clarified and may be cell specific. The ubiquity of the swelling-evoked [Ca2+]i rise makes Ca2+ a likely candidate for a role as osmotransducing signal. However, the regulatory volume decrease (RVD) which follows swelling and the osmolyte fluxes involved in this process are not always Ca2+ dependent. It was found that, with a few exceptions, in most cell types the osmosensitive Cl- efflux pathway and the swelling-activated organic osmolyte fluxes are Ca2+ independent. In contrast, Ca2+-dependent or Ca2+-independent K+ fluxes activated by swelling are detected, depending on the cell type. The close correlation found in this review between the Ca2+ dependence of RVD and that of the K+ channels activated by swelling led to the conclusion that it is the type of osmosensitive K+ pathway which largely confers the Ca2+ dependence to RVD. Interestingly, this coincidence of Ca2+-dependent K+ efflux and RVD is found predominantly in epithelial cells, whereas in nonepithelial cells both processes are largely Ca2+ independent. In these cells, the [Ca2+]i rise elicited by swelling may be an epiphenomenon.

Calcium response to adenosine and ATP in rabbit afferent arterioles

The effects of purine compounds in the renal vasculature are almost exclusively restricted to pre-glomerular vessels. Although their physiological role as extracellular messengers is not clear, there are extensive data indicating the importance of adenosine and ATP in the regulation of renal haemodynamics. This study was undertaken to characterize the calcium response of rabbit afferent arteriole to adenosine, ATP and other nucleotides. Experiments were performed in isolated afferent arterioles, microdissected from rabbit kidneys and loaded with fura-2. Intracellular calcium concentration ([Ca2+]i) was measured by video in proximal and distal parts of the afferent arteriole. Application of 100 microM adenosine or ATP increased [Ca2+]i in both arteriolar regions. In all cases the response had two well distinguishable phases: a quick peak increase and a plateau phase that equilibrated at a [Ca2+]i significantly higher than the basal level. UTP (100 microM) had no effect on the arteriole. Removal of extracellular calcium (2.5 mM EGTA) abolished only the plateau phase in response to adenosine, without significantly changing the peak increase. In contrast, the response to ATP was completely abolished in both arteriolar regions, where [Ca2+]i decreased upon application of the agonist and rapidly increased after restoration of calcium concentration to plasma level. We conclude that P1 and P2X receptors are present along the rabbit afferent arteriole and mediate calcium mobilization, with the same distribution in the proximal and distal segments.

Autocrine regulation of volume-sensitive anion channels in airway epithelial cells by adenosine

The activity of volume-sensitive Cl- channels was studied in human tracheal epithelial cells (9HTEo-) by taurine efflux experiments. The efflux elicited by a hypotonic shock was partially inhibited by adenosine receptor antagonists, by alpha,beta-methyleneadenosine 5'-diphosphate (alphabetaMeADP), an inhibitor of the 5'-ectonucleotidase, and by adenosine deaminase. On the other hand, dipyridamole, a nucleoside transporter inhibitor, increased the swelling-induced taurine efflux. Extracellular ATP and adenosine increased taurine efflux by potentiating the effect of hypotonic shock. alphabetaMeADP strongly inhibited the effect of extracellular ATP but not that of adenosine. These results suggest that anion channel activation involves the release of intracellular ATP, which is then degraded to adenosine by specific ectoenzymes. Adenosine then binds to purinergic receptors, causing the activation of the channels. To directly demonstrate ATP efflux, cells were loaded with [3H]AMP, and the release of radiolabeled molecules was analyzed by high performance liquid chromatography. During hypotonic shock, cell supernatants showed the presence of ATP, ADP, and adenosine. alphabetaMeADP inhibited adenosine formation and caused the appearance of AMP. Under hypotonic conditions, elevation of intracellular Ca2+ by ionomycin caused an increase of ATP and adenosine in the extracellular solution. Our results demonstrate that volume-sensitive anion channels are regulated with an autocrine mechanism involving swelling-induced ATP release and then hydrolysis to adenosine.

CFTR is a conductance regulator as well as a chloride channel

CFTR Is a Conductance Regulator as well as a Chloride Channel. Physiol. Rev. 79, Suppl.: S145-S166, 1999. - Cystic fibrosis transmembrane conductance regulator (CFTR) is a member of the ATP-binding cassette (ABC) transporter gene family. Although CFTR has the structure of a transporter that transports substrates across the membrane in a nonconductive manner, CFTR also has the intrinsic ability to conduct Cl- at much higher rates, a function unique to CFTR among this family of ABC transporters. Because Cl- transport was shown to be lost in cystic fibrosis (CF) epithelia long before the cloning of the CF gene and CFTR, CFTR Cl- channel function was considered to be paramount. Another equally valid perspective of CFTR, however, derives from its membership in a family of transporters that transports a multitude of different substances from chemotherapeutic drugs, to amino acids, to glutathione conjugates, to small peptides in a nonconductive manner. Moreover, at least two members of this ABC transporter family (mdr-1, SUR) can regulate other ion channels in the membrane. More simply, ABC transporters can regulate somehow the function of other cellular proteins or cellular functions. This review focuses on a plethora of studies showing that CFTR also regulates other ion channel proteins. It is the hope of the authors that the reader will take with him or her the message that CFTR is a conductance regulator as well as a Cl- channel.

Bioluminescence detection of ATP release mechanisms in epithelia

Autocrine and paracrine release of and extracellular signaling by ATP is a ubiquitous cell biological and physiological process. Despite this knowledge, the mechanisms and physiological roles of cellular ATP release are unknown. We tested the hypothesis that epithelia release ATP under basal and stimulated conditions by using a newly designed and highly sensitive assay for bioluminescence detection of ATP released from polarized epithelial monolayers. This bioluminescence assay measures ATP released from cystic fibrosis (CF) and non-CF human epithelial monolayers in a reduced serum medium through catalysis of the luciferase-luciferin reaction, yielding a photon of light collected by a luminometer. This novel assay measures ATP released into the apical or basolateral medium surrounding epithelia. Of relevance to CF, CF epithelia fail to release ATP across the apical membrane under basal conditions. Moreover, hypotonicity is an extracellular signal that stimulates ATP release into both compartments of non-CF epithelia in a reversible manner; the response to hypotonicity is also lost in CF epithelia. The bioluminescence detection assay for ATP released from epithelia and other cells will be useful in the study of extracellular nucleotide signaling in physiological and pathophysiological paradigms. Taken together, these results suggest that extracellular ATP may be a constant regulator of epithelial cell function under basal conditions and an autocrine regulator of cell volume under hypotonic conditions, two functions that may be lost in CF and contribute to CF pathophysiology.

Functional significance of cell volume regulatory mechanisms

To survive, cells have to avoid excessive alterations of cell volume that jeopardize structural integrity and constancy of intracellular milieu. The function of cellular proteins seems specifically sensitive to dilution and concentration, determining the extent of macromolecular crowding. Even at constant extracellular osmolarity, volume constancy of any mammalian cell is permanently challenged by transport of osmotically active substances across the cell membrane and formation or disappearance of cellular osmolarity by metabolism. Thus cell volume constancy requires the continued operation of cell volume regulatory mechanisms, including ion transport across the cell membrane as well as accumulation or disposal of organic osmolytes and metabolites. The various cell volume regulatory mechanisms are triggered by a multitude of intracellular signaling events including alterations of cell membrane potential and of intracellular ion composition, various second messenger cascades, phosphorylation of diverse target proteins, and altered gene expression. Hormones and mediators have been shown to exploit the volume regulatory machinery to exert their effects. Thus cell volume may be considered a second message in the transmission of hormonal signals. Accordingly, alterations of cell volume and volume regulatory mechanisms participate in a wide variety of cellular functions including epithelial transport, metabolism, excitation, hormone release, migration, cell proliferation, and cell death.

Volume expansion-sensing outward-rectifier Cl- channel: fresh start to the molecular identity and volume sensor

The maintenance of a constant volume in the face of extracellular and intracellular osmotic perturbation is essential for the normal function and survival of animal cells. Osmotically swollen cells restore their volume, exhibiting a regulatory volume decrease by releasing intracellular K+, Cl-, organic solutes, and obligated water. In many cell types, the volume regulatory effluxes of Cl- and some organic osmolytes are known to be induced by swelling-induced activation of anion channels that are characterized by their moderate outward rectification, cytosolic ATP dependency, and intermediate unitary conductance (10-100 pS). Recently, simultaneous measurements of cell size by light microscopy and whole cell Cl- current have shown that the Cl- current density is proportionally increased with an increase in the outer surface area, which is mainly achieved through unfolding of membrane invaginations by volume expansion. Thus this anion channel can somehow sense volume expansion and can be called the volume expansion-sensing outwardly rectifying (VSOR) anion channel. Its molecular identity and activation mechanism are yet to be elucidated. Three cloned proteins, ClC-2, P-glycoprotein, and pIcln, have been proposed as candidates for the VSOR anion channel. The unitary conductance, voltage dependency, anion selectivity, pH dependency, and pharmacology of the VSOR anion channel are distinct from the ClC-2 Cl- channel, which is also known to be sensitive to volume changes. Recent patch-clamp studies in combination with molecular biological techniques have shown that P-glycoprotein is not itself the channel protein but is a regulator of its volume sensitivity. Although there is still debate about another candidate protein, pIcln, the most recent study has suggested that this is likely to be a regulator of some other distinct Cl- channel. Identification of the VSOR anion channel protein per se, its volume-sensing mechanism, and its accessory/regulatory proteins at the molecular level is currently a subject of utmost physiological importance.

Characterization of volume-sensitive taurine- and Cl(-)-permeable channels

Volume-sensitive Cl- channels [ICl(vol)] were studied using taurine efflux and patch-clamp experiments in 9HTEo- human tracheal cells. Cells were stimulated with the Ca(2+)- elevating agents ATP and ionomycin in isotonic medium or in hypotonic solutions. ATP (100 microM) or ionomycin (1 microM) and hypotonic shock produced a synergic effect. Indeed, the resulting taurine efflux was much higher than the sum of the single effects elicited by ATP, ionomycin, or hypotonic medium. The taurine release elicited by hypotonic shock and the potentiation by ATP and ionomycin were markedly inhibited by using a Ca(2+)-free extracellular medium and by incubating the cells with the membrane-permeable 1,2-bis(2-amino- phenoxy)ethane-N,N,N',N'-tetraacetic acid acetoxymethyl ester chelating agent. Patch-clamp experiments confirmed the role of Ca2+ on ICl(vol) channels. Swelling-induced taurine efflux was inhibited by reactive blue 2, suramin, and pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid. Patch-clamp experiments demonstrated that these compounds shift the voltage-dependent inactivation of ICl(vol) channels toward more negative values. This study indicates that the sensitivity of ICl(vol) to cell volume changes is modulated by intracellular Ca2+ and that purinergic receptor antagonists represent a new class of CI- channel blockers.

Volume regulatory taurine release in human tracheal 9HTEo- and multidrug resistant 9HTEo-/Dx cells

The intracellular taurine release evoked by hypotonic shock is accomplished by volume-activated Cl- channels whose activity has been related to the expression of the multidrug resistance protein (MDR-1). We studied taurine transport in 9HTEo- cells and in the derived cell line 9HTEo-/Dx expressing MDR-1. [3H]taurine release from preloaded cells increased upon reduction of extracellular osmolality. This process was not inhibited by preincubation with phorbol 12-myristate 13-acetate but was reduced by inhibitors of volume-sensitive Cl- channels such as 1,9-dideoxiforskolin, La3+, and arachidonate. Verapamil, a substrate of MDR-1, increased the osmotically evoked taurine efflux. Replacement of extracellular Cl- with I- or gluconate or of extracellular Na+ with Li+ significantly reduced the taurine efflux, whereas substitution of N-methyl-D-glucamine for Na+ increased it. Application of ATP and 2-chloroadenosine stimulated the efflux in isotonic medium. No differences were seen between 9HTEo- and 9HTEo-/Dx cells with respect to hypotonically induced taurine efflux and the response to phorbol ester, channel blockers, ion replacement, and purinergic agents. Our results reveal novel properties of the osmotically induced taurine release and demonstrate its independence from MDR-1 gene expression.

Apical and basolateral ATP stimulates tracheal epithelial chloride secretion via multiple purinergic receptors

Stimulation of Cl- secretion across the airway epithelium by ATP or UTP as agonists has therapeutic implications for cystic fibrosis. Our results demonstrate that ATP stimulates Cl- secretion in rat tracheal epithelial cell monolayers in primary culture from the apical or basolateral side of the monolayer. Multiple types of ATP-sensitive Cl- conductances in intact monolayers were elucidated through inhibition by Cl- channel-blocking drugs. Multiple Cl- conductances stimulated by ATP and adenosine 3',5'-cyclic monophosphate (cAMP) (tested for comparison) were also deciphered more specifically by nystatin permeabilization of the basolateral membrane, subsequent imposition of symmetrical Cl-, I-, or Br- solutions to test halide permselectivity, inhibition by Cl- channel-blocking drugs, and construction of current-voltage plots to study time and voltage dependence of the currents. Apical ATP stimulates Cl- secretion through P2U (or P2Y2) purinergic receptors via both intracellular Ca2+ (Ca(2+)i)-dependent and Cai(2+)-independent signaling pathways by opening outwardly rectifying Cl- channels (ORCCs), cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channels, and Cai(2+)-dependent Cl- channels. Basolateral ATP stimulates Cl- secretion via a combination of receptor subtypes (P2T and P2U) or a novel type of receptor (P2Y3), independent of Cai2+ or cAMP signaling by opening only CFTR channels. cAMP also stimulated multiple types of Cl- conductances, consistent with simultaneous activation of CFTR and ORCCs. Together, these results suggest that ATP as an agonist stimulates Cl- secretion via multiple purinergic receptors and multiple signal transduction pathways activated in different membrane domains of tracheal epithelia.

Adenosine A1 receptor-mediated changes in basal and histamine-stimulated levels of intracellular calcium in primary rat astrocytes

1. The effects of adenosine A1 receptor stimulation on basal and histamine-stimulated levels of intracellular free calcium ion concentration ([Ca2+]i) have been investigated in primary astrocyte cultures derived from neonatal rat forebrains. 2. Histamine (0.1 microM-1 mM) caused rapid, concentration-dependent increases in [Ca2+]i over basal levels in single type-2 astrocytes in the presence of extracellular calcium. A maximum mean increase of 1,468 +/- 94 nM over basal levels was recorded in 90% of type-2 cells treated with 1 mM histamine (n = 49). The percentage of type-2 cells exhibiting calcium increases in response to histamine appeared to vary in a concentration-dependent manner. However, the application of 1 mM histamine to type-1 astrocytes had less effect, eliciting a mean increase in [Ca2+]i of 805 +/- 197 nM over basal levels in only 30% of the cells observed (n = 24). 3. In the presence of extracellular calcium, the A1 receptor-selective agonist, N6-cyclopentyladenosine (CPA, 10 microM), caused a maximum mean increase in [Ca2+]i of 1,110 +/- 181 nM over basal levels in 30% of type-2 astrocytes observed (n = 53). The size of this response was concentration-dependent; however, the percentage of type-2 cells exhibiting calcium increases in response to CPA did not appear to vary in a concentration-dependent manner. A mean calcium increase of 605 +/- 89 nM over basal levels was also recorded in 23% of type-1 astrocytes treated with 10 microM CPA (n = 30). 4. In the absence of extracellular calcium, in medium containing 0.1 mM EGTA, a mean increase in [Ca2+]i of 504 +/- 67 nM over basal levels was recorded in 41% of type-2 astrocytes observed (n = 41) after stimulation with 1 microM CPA. However, in the presence of extracellular calcium, pretreatment with the A1 receptor-selective antagonist, 8-cyclopentyl-1,3-dipropylxanthine, for 5-10 min before stimulation with 1 microM CPA, completely antagonized the response in 100% of the cells observed. 5. In type-2 astrocytes, prestimulation with 10 nM CPA significantly increased the size of the calcium response produced by 0.1 microM histamine and the percentage of responding cells. Treatment with 0.1 microM histamine alone caused a mean calcium increase of 268 +/- 34 nM in 41% of the cells observed (n = 34). After treatment with 10 nM CPA, mean calcium increase of 543 +/- 97 nM was recorded in 100% of the cells observed (n = 33). 6. These data indicate that adenosine Al receptors couple to intracellular calcium mobilization and extracellular calcium influx in type-1 and type-2 astrocytes in primary culture. In addition, the simultaneous activation of adenosine Al receptors on type-2 astrocytes results in an augmentation of the calcium response to histamine H1 receptor stimulation.

Interactions between adenosine A1- and histamine H1-receptors

The interactions or "cross-talk" between adenosine A1-receptors and receptors coupled to phospholipase C (leading to the hydrolysis of inositol phospholipids) have been well documented in the literature. For example, activating the A1-receptor selectively potentiates the histamine H1-receptor stimulated hydrolysis of inositol phospholipids in guinea-pig cerebral slices. In contrast, when the adenosine receptor is activated in the cerebral cortex of mouse or man the histamine response is selectively inhibited. Our studies have focused on the smooth muscle cell line, DDT1 MF-2, derived from hamster vas deferens. These cells express A1-receptors which, in addition to the expected negative coupling to adenylate cyclase, also stimulate inositol phospholipid hydrolysis and Ca2+ mobilization. These A1-receptors also potentiate histamine H1-receptor responses, i.e. inositol phospholipid hydrolysis and Ca2+ mobilization. The mechanism(s) underlying the potentiation or inhibition of histamine H1-receptor responses by the adenosine A1-receptor remain to be unravelled. One mechanism may involve intracellular "cross-talk" at the G-protein level. This review will discuss how beta gamma subunits from G(i) proteins could be involved in augmenting responses to calcium mobilizing receptors.

Activation of Ca(2+)-dependent K+ and Cl- currents by UTP and ATP in CFPAC-1 cells

Activation of Cl- and K+ conductances by nucleotide receptor-operated mobilization of intracellular Ca2+ was investigated in CFPAC-1 cells with the perforated-patch technique. Adenosine 5'-triphosphate (ATP) and uridine 5'-triphosphate (UTP) caused a dose-dependent fast and transient membrane hyperpolarization. UTP was more effective than ATP. In voltage-clamped cells, two currents with different ionic permeability and kinetics were activated by the nucleotides. The first one was carried by Cl- ions, peaked in the first few seconds after addition of nucleotides, and lasted for 1 +/- 0.3 min. Its amplitude was about 2.7 nA at -100 mV with 100 mumol/l of either ATP or UTP. The second current was carried by K+ ions and was blocked by Cs+. This current peaked more slowly and had a mean duration of 4.6 +/- 0.7 min. Its amplitude was 0.9 nA and 0.5 nA at -20 mV with 100 mumol/l UTP and ATP, respectively. Activation of the nucleotide receptor caused a transient increase in intracellular Ca2+ concentration ([Ca2+]i) that was similar in the presence or absence of extracellular Ca2+. The ED50 for UTP was 24 mumol/l and that for ATP was 94 mumol/l. Depletion of the inositol 1,4,5-trisphosphate-sensitive Ca2+ store by thapsigargin prevented both the nucleotide-induced [Ca2+]i increase and the activation of membrane currents. Addition of 2 mmol/l Ca2+ to thapsigargin-treated cells produced a sustained increase of Cl- and K+ currents, which was reversed by Ca2+ removal. The present study demonstrates that CFPAC-1 cells respond to nucleotide receptor activation with a transient increase in [Ca2+]i that stimulates Ca(2+)-dependent Cl- and K+ currents.(ABSTRACT TRUNCATED AT 250 WORDS)

Intercellular calcium signaling induced by extracellular adenosine 5′-triphosphate and mechanical stimulation in airway epithelial cells

Airway epithelial cells in culture respond to extracellular adenosine 5′-triphosphate (ATP) by increasing their intracellular Ca2+ concentration ([Ca2+]i). The effective concentration of ATP that elicited a Ca2+ response equal to 50% of the maximal response (EC50) was 0.5 microM. Release of ATP from a pipette to form a local gradient of ATP increased [Ca2+]i of individual cells in a sequential manner. Cells closest to the pipette showed an immediate increase in [Ca2+]i while more distal cells displayed a delayed increase in [Ca2+]i. This response to the local release of ATP appeared as a wave of increasing [Ca2+]i that spread to several cells and, in this respect, was similar to the intercellularly communicated Ca2+ waves initiated by mechanical stimulation in airway epithelial cells (Sanderson et al., Cell Regul. 1, 585–596, 1990). In the presence of a unidirectional fluid flow, the Ca2+ response to a local release of ATP was biased such that virtually all the cells responding with an increase in [Ca2+]i were downstream of the release site. By contrast, an identical fluid flow did not bias the radial propagation of intercellular Ca2+ waves induced by mechanical stimulation. Suramin, a P2-purinergic receptor antagonist, did attenuate the Ca2+ response induced by ATP but did not block the propagation of mechanically induced Ca2+ waves. Cells from young cultures (3-5 days) or those at the leading edge of an outgrowth elevated their [Ca2+]i in response to ATP. However, these cells do not respond to mechanical stimulation by the propagation of a Ca2+ wave. From these results we conclude that the intercellular Ca2+ waves elicited by mechanical stimulation are not the result of ATP or another compound released from the stimulated cell, diffusing through the extracellular fluid. This conclusion is consistent with previous experimental evidence suggesting that intercellular Ca2+ signaling in epithelial cells is mediated by the movement of inositol trisphosphate through gap junctions.

Adenosine A1-receptor stimulated increases in intracellular calcium in the smooth muscle cell line, DDT1MF-2

1. The effect of a range of adenosine receptor agonists on intracellular free calcium concentration ([Ca2+]i) has been studied in the hamster vas deferens smooth muscle cell line DDT1MF-2. 2. Adenosine receptor agonists elicited a rapid and maintained increase in [Ca2+]i in fura-2 loaded DDT1MF-2 cells. The initial rise could be maintained in the absence of extracellular calcium, whereas the maintained or plateau phase was dependent upon the presence of extracellular calcium and appeared to be associated with calcium influx. The rank order of agonist potencies was N6-cyclopentyladenosine > 5'-N-ethylcarboxamidoadenosine > 2-chloroadenosine > adenosine. 3. The response to 2-chloroadenosine was antagonized by the antagonists 8-cyclopentyl-1,3-dipropylxanthine (DPCPX, KD 0.14 nM) and 8-phenyltheophylline (KD 112 nM). 4. Pretreatment with the 5-lipoxygenase inhibitor AA861 (20 microM) produced only a small (14 +/- 2%) inhibition of the [Ca2+]i response elicted by N6-cyclopentyladenosine (300 nM), in nominally Ca(2+)-free buffer containing 0.1 mM EGTA. The cyclo-oxygenase inhibitor, indomethacin (2 microM) was without effect. 5. The Ca(2+)-influx associated with the plateau phase required the continued presence of agonist on the receptor. The antagonist DPCPX (100 nM) attenuated the rise in [Ca2+]i observed when extracellular Ca2+ was re-applied after the cells had been stimulated with N6-cyclopentyladenosine (CPA; 300 nM) in experiments initiated in nominally Ca(2+)-free buffer. 6. Pretreatment with pertussis toxin (200 ng ml-1 for 4 h) inhibited the CPA (100 nM) stimulated intracellular Ca2+ release and Ca2+ influx but was without effect on the response to histamine (100 microM). 7.These data suggest that adenosine A(1)-receptor activation in DDT(1)MF-2 cells stimulates release of Ca(2+) from intracellular stores and influx of extracellular Ca(2+) through Ca(2+) entry pathways in the plasma membrane which required the continued presence of agonist on the receptor.