A DNA-transfection protocol has been developed that makes use of a synthetic cationic lipid, N-[1-(2,3-dioleyloxy)propylJ-N,N,N-trimethylammonium chloride (DOTMA). Small unilamellar liposomes containing DOTMA interact spontaneously with DNA to form lipid-DNA complexes with 100% entrapment of the DNA. DOTMA facilitates fusion of the complex with the plasma membrane of tissue culture cells, resulting in both uptake and expression of the DNA. The technique is simple, highly reproducible, and effective for both transient and stable expression of transfected DNA. Depending upon the cell line, lipofection is from 5-to >100-fold more effective than either the calcium phosphate or the DEAEdextran transfection technique.

Recovery of cell volume in response to osmotic stress is mediated in part by increases in the Cl1 permeability of the plasma membrane. These studies evaluate the hypothesis that ATP release and autocrine stimulation of purinergic (P2) receptors couple increases in cell volume to opening of Cl-channels. In HTC rat hepatoma cells, swelling Epithelial cells express a variety of plasma membrane proteins that bind solutes in a selective manner and transport them to the cell interior. Although the action of such transport proteins is essential for normal cell function, the resulting increase in intracellular solute concentration leads to an influx of water and cell swelling. However, cell volume is maintained within a relatively narrow physiologic range by adaptive responses that generally involve solute efflux through opening of K+ and Clchannels and restoration of cell volume toward its resting state (1-3). Despite extensive study, the mechanisms that couple increases in cell volume to opening of ion channels remain incompletely understood.In a number of cell types, swelling-induced opening of Clchannels is dependent on the presence of intracellular ATP (4, 5). This ATP dependence of volume-stimulated channel opening is attributable, in part, to the action of intracellular protein kinases on channels or channel regulatory proteins (6, 7). However, other mechanisms are possible as well. Recent experimental evidence suggests that ATP can also function outside of the cell as an autocrine factor to increase membrane Cl-permeability (8). In secretory epithelia, increases in cAMP stimulate cellular release of ATP in concentrations sufficient to activate purinergic (P2) receptors in the plasma membrane, which leads to Cl-channel opening (9). The mechanisms involved in ATP trafficking in secretory cells are not fully defined, with evidence for (9, 10) and against (11) ATP release through the cystic fibrosis transmembrane conductance regulator (CFTR), a member of the ATP-binding cassette (ABC) family of proteins. Other members of the ABC family, including the multidrug resistance P-glycoprotein, are thought to regulate volume-activated channels through cellular release of ATP or other pathways (6, 12).Like secretory epithelia, activation of P2 receptors in liver cells increases membrane Cl-permeability through opening of Cl-channels (13). However, unlike secretory cells, liver cells are not thought to exhibit Cl--dependent secretion (14), and the physiologic importance of purinergic signaling in liver has not been defined. Based on the prominent role of Cl-ions in liver cell volume regulation (15), we have proposed that volume-dependent activation of Cl-channels is mediated by an autocrine mechanism involving ATP release and activation of a P2 receptor. In a model liver cell line that expresses P2 receptors (13, 16), we show that (i) cell swelling increases membrane ATP permeability, and (ii) Cl-channel opening and cell volume recovery from swelling each require stimulation of P2 receptors by extracellular ATP. These...

Extracellular ATP is a potent signaling factor that modulates a variety of cellular functions through the activation of P 2 purinergic receptors in the plasma membrane. These receptors are widely distributed among different liver cell types, including hepatocytes, cholangiocytes, macrophages, and endothelial cells, but the physiologic roles have not been fully defined. Cells release ATP in response to both osmotic and mechanical stimuli, and one mechanism may involve opening of a channel-like pathway (1, 2). In respiratory epithelia, ATP release stimulated by cytosolic cAMP activates outwardly rectified Cl Ϫ channels coupled to P 2U receptors and enhances Cl Ϫ secretion (3).Recent studies in a model liver cell line support an alternative pathway where increases in cell volume induce conductive ATP efflux. In these cells, removal of extracellular ATP or P 2 receptor blockade prevents both Cl Ϫ channel activation and volume recovery (1). These findings suggest functional interactions between ATP release, P 2 receptor stimulation, and Cl Ϫ channel opening in epithelial secretion and volume regulation.Members of the ATP-binding cassette (ABC) 1 protein family are likely to be relevant to this volume regulatory pathway for two reasons. First, while the molecular basis for the transmembrane ATP conductance has not been established, heterologous expression or up-regulation of ABC family members in some cell models is associated with enhanced electrodiffusional ATP release. In cystic fibrosis respiratory epithelia, cAMP fails to stimulate channel-mediated ATP efflux, a response that is present in native epithelia; CFTR gene transfer restores the ATP conductance (3, 4). In other cell lines, ATP release is proportional to the expression of mammalian and Drosophila Mdr1 P-glycoproteins (5, 6). Second, in some but not all cell types, Mdr1 P-glycoproteins regulate swelling-activated Cl Ϫ currents (I Cl-swell ). Effects include enhancement of I Cl-swell and endowment of Cl Ϫ channel sensitivity to protein kinase C (mdr1 gene transfer) and increase in I Cl-swell for a given hypotonic stress (P-glycoprotein overexpression) (7,8). The cellular mechanisms involved in these responses and the implications for other cell types have yet to be clarified.In hepatocytes, P-glycoproteins transport both amphipathic compounds and phospholipids across canalicular membranes into bile (9, 10). However, the functions of multiple other ABC members present in liver cells are unknown. In light of the putative association of certain ABC proteins with channelmediated ATP and Cl Ϫ transport, we sought to investigate the role of hepatocellular ABC proteins in these processes. Findings in rat HTC hepatoma cells were compared with those in a selected population of HTC cells (HTC-R) that overexpress both endogenous and novel Mdr proteins (11). These studies demonstrate that inhibition of P-glycoprotein transport prevents recovery from swelling and that overexpression of Mdr proteins is associated with enhanced ATP release, volume recovery, and cell surv...

These studies provide evidence that cystic fibrosis transmembrane conductance regulator (CFTR) potentiates and accelerates regulatory volume decrease (RVD) following hypotonic challenge by an autocrine mechanism involving ATP release and signaling. In wild-type CFTR-expressing cells, CFTR augments constitutive ATP release and enhances ATP release stimulated by hypotonic challenge. CFTR itself does not appear to conduct ATP. Instead, ATP is released by a separate channel, whose activity is potentiated by CFTR. Blockade of ATP release by ion channel blocking drugs, gadolinium chloride (Gd 3؉ ) and 4,4-diisothiocyanatostilbene-2,2di-sulfonic acid (DIDS), attenuated the effects of CFTR on acceleration and potentiation of RVD. These results support a key role for extracellular ATP and autocrine and paracrine purinergic signaling in the regulation of membrane ion permeability and suggest that CFTR potentiates ATP release by stimulating a separate ATP channel to strengthen autocrine control of cell volume regulation.ATP and its metabolites function as potent autocrine and paracrine agonists that act within tissues to control cell function through activation of P2 purinergic receptors (1-3) expressed by all cells and tissues. Purinergic agonists are essential for many specialized physiological functions (1-10). In cystic fibrosis (CF), 1 ATP and a related triphosphate nucleotide, UTP, stimulate epithelial chloride (Cl Ϫ ) channels alternative to CFTR via purinergic receptors (11-16). Supraphysiological concentrations of ATP also stimulate CFTR (17). Metabolites of ATP can also act as Cl Ϫ secretagogues (15,16,18). Despite the diverse roles of purinergic signaling, the cellular mechanisms that govern ATP release are not fully defined. CFTR and related ATP-binding cassette (ABC) transporters such as mdr-1 or P-glycoprotein have been implicated as facilitators of ATP release in some cell models (14, 19 -24), while other laboratories have failed to show evidence of CFTRfacilitated ATP conduction or release (25-30).Release of ATP via a conductive pathway has been implicated as an essential autocrine regulator of cell volume in rat hepatoma cells (5). Moreover, ABC transporters have been shown to modulate volume-sensitive Cl Ϫ channels and cell volume (31-34). As such, we tested the hypotheses that CFTR facilitates ATP release under constitutive and hypotonic conditions for autocrine control of cell volume regulation. These hypotheses were also based on the fact that airway surface liquid on CF epithelia is hypertonic with respect to NaCl (35) and/or reduced in volume (36) or both (37, 38) when compared with non-CF epithelia. These airway surface liquid composition abnormalities may reflect an inability of CF epithelial cells to sense changes in external mucosal environment and/or an inability of CF cells to regulate their own cell volume.To this end, complimentary observations using a variety of techniques suggest that expression of CFTR enhances ATP release and modulates the dynamic relationship between cell volume, puriner...

P2Y receptor stimulation increases membrane Cl− permeability in biliary epithelial cells, but the source of extracellular nucleotides and physiological relevance of purinergic signaling to biliary secretion are unknown. Our objectives were to determine whether biliary cells release ATP under physiological conditions and whether extracellular ATP contributes to cell volume regulation and transepithelial secretion. With the use of a sensitive bioluminescence assay, constitutive ATP release was detected from human Mz-ChA-1 cholangiocarcinoma cells and polarized normal rat cholangiocyte monolayers. ATP release increased rapidly during cell swelling induced by hypotonic exposure. In Mz-ChA-1 cells, removal of extracellular ATP (apyrase) and P2 receptor blockade (suramin) reversibly inhibited whole cell Cl− current activation and prevented cell volume recovery during hypotonic stress. Moreover, exposure to apyrase induced cell swelling under isotonic conditions. In intact normal rat cholangiocyte monolayers, hypotonic perfusion activated apical Cl−currents, which were inhibited by addition of apyrase and suramin to bathing media. These findings indicate that modulation of ATP release by the cellular hydration state represents a potential signal coordinating cell volume with membrane Cl− permeability and transepithelial Cl−secretion.

tion of biliary secretion through apical purinergic receptors in cultured rat cholangiocytes. Am. J. Physiol. 273 (Gastrointest. Liver Physiol. 36): G1108-G1117, 1997.-To evaluate whether ATP in bile serves as a signaling factor regulating ductular secretion, voltage-clamp studies were performed using a novel normal rat cholangiocyte (NRC) model. In the presence of amiloride (100 µM) to block Na ϩ channels, exposure of the apical membrane to ATP significantly increased the shortcircuit current (I sc ) from 18.2 Ϯ 5.9 to 52.8 Ϯ 12.7 µA (n ϭ 18). The response to ATP is mediated by basolateral-to-apical Cl Ϫ transport because it is inhibited by 1) the Cl Ϫ channel blockers 4,4Ј-diisothiocyanostilbene-2,2Ј-disulfonic acid (1 mM), diphenylanthranilic acid (1.5 mM), or 5-nitro-2-(3-phenylpropylamino)benzoic acid (50 or 100 µM) in the apical chamber, 2) the K ϩ channel blocker Ba 2ϩ (5 mM), or 3) the Na ϩ -K ϩ -2Cl Ϫ cotransport inhibitor bumetanide (200 µM) in the basolateral chamber. Other nucleotides stimulated an increase in I sc with a rank order potency of UTP ϭ ATP ϭ adenosine 5Ј-O-(3)-thiotriphosphate, consistent with P 2u purinergic receptors. ADP, AMP, 2-methylthioadenosine 5Ј-triphosphate, and adenosine had no effect. A cDNA encoding a rat P 2u receptor (rP 2u R) was isolated from a liver cDNA library, and functional expression of the corresponding mRNA in Xenopus laevis oocytes resulted in the appearance of ATP-stimulated currents with a similar pharmacological profile. Northern analysis identified hybridizing mRNA transcripts in NRC as well as other cell types in rat liver. These findings indicate that exposure of polarized cholangiocytes to ATP results in luminal Cl Ϫ secretion through activation of P 2u receptors in the apical membrane. Release of ATP into bile may serve as an autocrine or paracrine signal regulating cholangiocyte secretory function.

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