Two distinct receptors for ATP can be distinguished in Swiss 3T6 mouse fibroblasts by their desensitization

Origin, duplication and reshuffling of plasmid genes: Insights from Burkholderia vietnamiensis G4 genome

Using a computational pipeline based on similarity networks reconstruction we analysed the 1133 genes of the Burkholderia vietnamiensis (Bv) G4 five plasmids, showing that gene and operon duplication played an important role in shaping the plasmid architecture. Several single/multiple duplications occurring at intra- and/or interplasmids level involving 253 paralogous genes (stand-alone, clustered or operons) were detected. An extensive gene/operon exchange between plasmids and chromosomes was also disclosed. The larger the plasmid, the higher the number and size of paralogous fragments. Many paralogs encoded mobile genetic elements and duplicated very recently, suggesting that the rearrangement of the Bv plastic genome is ongoing. Concerning the "molecular habitat" and the "taxonomical status" (the Preferential Organismal Sharing) of Bv plasmid genes, most of them have been exchanged with other plasmids of bacteria belonging (or phylogenetically very close) to Burkholderia, suggesting that taxonomical proximity of bacterial strains is a crucial issue in plasmid-mediated gene exchange.

Exploring the evolutionary dynamics of plasmids: the Acinetobacter pan-plasmidome

BACKGROUND

Prokaryotic plasmids have a dual importance in the microbial world: first they have a great impact on the metabolic functions of the host cell, providing additional traits that can be accumulated in the cell without altering the gene content of the bacterial chromosome. Additionally and/or alternatively, from a genome perspective, plasmids can provide a basis for genomic rearrangements via homologous recombination and so they can facilitate the loss or acquisition of genes during these events, which eventually may lead to horizontal gene transfer (HGT). Given their importance for conferring adaptive traits to the host organisms, the interest in plasmid sequencing is growing and now many complete plasmid sequences are available online.

RESULTS

By using the newly developed Blast2Network bioinformatic tool, a comparative analysis was performed on the plasmid and chromosome sequence data available for bacteria belonging to the genus Acinetobacter, an ubiquitous and clinically important group of gamma-proteobacteria. Data obtained showed that, although most of the plasmids lack mobilization and transfer functions, they have probably a long history of rearrangements with other plasmids and with chromosomes. Indeed, traces of transfers between different species can be disclosed.

CONCLUSIONS

We show that, by combining plasmid and chromosome similarity, identity based, network analysis, an evolutionary scenario can be described even for highly mobile genetic elements that lack extensively shared genes. In particular we found that transposases and selective pressure for mercury resistance seem to have played a pivotal role in plasmid evolution in Acinetobacter genomes sequenced so far.

Mechanisms for inhibition of P2 receptors signaling in neural cells

Trophic factors are required to ensure neuronal viability and regeneration after neural injury. Although abundant information is available on the factors that cause the activation of astrocytes, little is known about the molecular mechanisms underlying the regulation of this process. Nucleotides released into the extracellular space from injured or dying neural cells can activate astrocytes via P2 nucleotide receptors. After a brief historical review and update of novel P2 receptor antagonists, this article focuses on recent advancements toward understanding molecular mechanisms that regulate G protein-coupled P2Y receptor signaling. Among P2Y receptor subtypes, the heptahelical P2Y2 nucleotide receptor interacts with vitronectin receptors via an RGD sequence in the first extracellular loop, and this interaction is required for effective signal transduction to activate mitogen-activated protein kinases ERK1/2, to mobilize intracellular calcium stores via activation of phospholipase C, protein kinase C isoforms, and to activate focal adhesion kinase and other signaling events. Ligation of vitronectin receptors with specific antibodies caused an inhibition of P2Y2 receptor-induced ERK1/2 and p38 phosphorylation and P2Y2 receptor-induced cytoskeleton rearrangement and DNA synthesis. Structure-function studies have identified agonist-induced phosphorylation of the C-terminus of the P2Y2 receptor, an important mechanism for receptor desensitization. Understanding selective mechanisms for regulating P2Y2 receptor signaling could provide novel targets for therapeutic strategies in the management of brain injury, synaptogenesis, and neurological disorders.

Cellular Distribution and Functions of P2 Receptor Subtypes in Different Systems

This review is aimed at providing readers with a comprehensive reference article about the distribution and function of P2 receptors in all the organs, tissues, and cells in the body. Each section provides an account of the early history of purinergic signaling in the organ?cell up to 1994, then summarizes subsequent evidence for the presence of P2X and P2Y receptor subtype mRNA and proteins as well as functional data, all fully referenced. A section is included describing the plasticity of expression of P2 receptors during development and aging as well as in various pathophysiological conditions. Finally, there is some discussion of possible future developments in the purinergic signaling field.

Nucleotide-regulated calcium signaling in lung fibroblasts and epithelial cells from normal and P2Y(2) receptor (-/-) mice

To test for the role of the P2Y(2) receptor (P2Y(2)-R) in the regulation of nucleotide-promoted Ca(2+) signaling in the lung, we generated P2Y(2)-R-deficient (P2Y(2)-R(-/-)) mice and measured intracellular Ca(2+)(i) responses (DeltaCa(2+)(i)) to nucleotides in cultured lung fibroblasts and nasal and tracheal epithelial cells from wild type and P2Y(2)-R(-/-) mice. In the wild type fibroblasts, the rank order of potencies for nucleotide-induced DeltaCa(2+)(i) was as follows: UTP >/= ATP >> ADP > UDP. The responses induced by these agonists were completely absent in the P2Y(2)-R(-/-) fibroblasts. Inositol phosphate responses paralleled those of DeltaCa(2+)(i) in both groups. ATP and UTP also induced Ca(2+)(i) responses in wild type airway epithelial cells. In the P2Y(2)-R(-/-) airway epithelial cells, UTP was ineffective. A small fraction (25%) of the ATP response persisted. Adenosine and alpha,beta-methylene ATP were ineffective, and ATP responses were not affected by adenosine deaminase or by removal of extracellular Ca(2+), indicating that neither P1 nor P2X receptors mediated this residual ATP response. In contrast, 2-methylthio-ADP promoted a substantial Ca(2+)(i) response in P2Y(2)-R(-/-) cells, which was inhibited by the P2Y(1) receptor antagonist adenosine 3'-5'-diphosphate. These studies demonstrate that P2Y(2)-R is the dominant purinoceptor in airway epithelial cells, which also express a P2Y(1) receptor, and that the P2Y(2)-R is the sole purinergic receptor subtype mediating nucleotide-induced inositol lipid hydrolysis and Ca(2+) mobilization in mouse lung fibroblasts.

Purinergic signalling: pathophysiological roles

In this review, after a summary of the history and current status of the receptors involved in purinergic signalling, we focus on the distribution and physiological roles of purines and pyrimidines in both short-term events such as neurotransmission, exocrine and endocrine secretion and regulation of immune cell function, and long-term events such as cell growth, differentiation and proliferation in development and regeneration. Finally, the protective roles of nucleosides and nucleotides in events such as cancer, ischemia, wound healing, drug toxicity, inflammation and pain are explored and some suggestions made for future developments in this rapidly expanding field, with particular emphasis on the involvement of selective agonists and antagonists for purinergic receptor subtypes in therapeutic strategies.

Extracellular ATP increases intracellular calcium in cultured adult Schwann cells

We have previously reported that extracellular ATP causes a transient rise in intracellular calcium concentration ([Ca2+]i) in cultured Schwann cells derived from adult animals [Ansselin A. D. et al. (1994) Int. J. Neurosci. 74, 148]. In this study, the receptor mediating this response has been characterized. Established adult rat and rabbit Schwann cell cultures were loaded with fura-2 (acetoxymethyl ester, 10 micromol/l, 40 min, 37 degrees C). which indicated, by fluorescence imaging, a resting [Ca2+]i of 34.7 +/- 1.4 nmol/l (mean S.E., n=591). The cells were exposed to 100 micromol/l ATP, ADP, AMP, UTP and adenosine in defined medium for 1-2 min, and the change in [Ca2+]i was observed as a change in the Fura-2 ratio. Seventy-seven percent of adult rat Schwann cells (n=235) and 88% adult rabbit Schwann cells (n=356) responded to the presence of extracellular ATP (100 mmol/l) with a transient increase in [Ca2+]i (41 and 90 nmol/l from resting value, respectively), independent of the presence of [Ca2+]o. Calcium waves were observed in one experiment. The following order of agonist potency was observed: UTP= ATP>>ADP>AMP=adenosine. The agonists alpha,beta-methylene-ATP and 2-methylthio-ATP had a small effect on the cells, similar to AMP, and were mutually desensitizing. The ATP antagonist suramin blocked the response. We conclude that adult Schwann cells express a purinergic ATP receptor belonging to the G-protein-coupled P2u alpha subtype [O'Connor S. et al. (1991) Trends pharmac. Sci. 12, 137-141].

Extracellular nucleotides stimulate proliferation in MCF-7 breast cancer cells via P2-purinoceptors

Nucleotides such as ATP can act as extracellular effector molecules by interaction with specific cellular receptors known as P2-purinoceptors. Recently, we cloned the human P2U purinoceptor from osteoclastoma and demonstrated its expression in skeletal tissues. In the current study we have investigated the expression of P2U purinoceptors in human breast tumour cell lines and examined functional effects of extracellular nucleotides on these cells. By reverse transcription-linked polymerase chain reaction (RT-PCR) the expression of mRNA for P2U purinoceptors was demonstrated in four human breast cancer cell lines, Hs578T, MCF-7, SK-Br3 and T47-D. In MCF-7 cells, extracellular ATP (1-100 microM) elevated intracellular free calcium concentration [Ca2+]i, indicating that these cells express functional P2-purinoceptors. UTP elevated [Ca2+]i in an identical manner to ATP, whereas 2-methylthioATP was completely ineffective, and ADP only partially effective. This pharmacological profile suggests that the P2U subtype may be the only P2-purinoceptor expressed by these cells. The functional significance of P2U purinoceptor expression by MCF-7 cells was investigated by analysing the effects of extracellular ATP on cell proliferation. The slowly hydrolysed analogue of ATP, ATPgammaS (which was also shown to elevate [Ca2+]i), induced proliferation of MCF-7 cells when added daily to serum-free cultures over a period of 3 days. ATPgammaS-induced proliferation was demonstrated by three separate methods, detection by scintillation counting of [3H]thymidine incorporation, immunocytochemical detection of 5-bromo-2-deoxyuridine incorporation and direct counting of cell numbers. These data suggest that ATP, possibly released at sites of tissue injury or inflammation, may be capable of growth factor action in promotion of tumour proliferation or progression.

Homologous desensitization of ATP-stimulated mitogenesis: mechanism involves desensitization of arachidonic acid release and cAMP elevation but not the activation of protein kinase A

Prolonged incubation of quiescent 3T3, 3T6, and A431 cells with the P2Y purinoceptor agonists ATP, ADP, or AMPPNP reduced the mitogenic responses of target cells to a further challenge by these agonists, as measured by [3H]thymidine incorporation. The mitogenic desensitization was agonist-specific, for no effect was seen on DNA synthesis stimulated by epidermal growth factor, insulin, bombesin, 12-O-tetradecanoyl-phorbol-12 acetate (TPA), or adenosine. The desensitization was completely reversible, since after a 24 hr incubation in the absence of ATP, the cells responded fully to the mitogenic action of ATP. The presence of a low level of cycloheximide blocked recovery, suggesting that down-regulation of the P2Y receptor may have occurred during desensitization. In Swiss 3T3 cells, stimulation of DNA synthesis occurs predominantly by activation of arachidonic acid release, followed by its oxidation to prostaglandin E2 and stimulation of adenylyl cyclase. Interestingly, prolonged preincubation with ATP produced a similar degree of desensitization of DNA synthesis and of ATP-dependent arachidonic acid release and cAMP accumulation. Furthermore, this was true for both wild type cells and mutants with a defective cAMP-dependent protein kinase (PKA). We conclude that homologous desensitization is likely due to uncoupling of the P2Y purinoceptor from phospholipase A2, and this process does not require activation of protein kinase A.

Activation of cell growth inhibitor by ectoprotein kinase-mediated phosphorylation in transformed mouse fibroblasts

Our previous studies have shown that exogenous ATP induces cell growth inhibition in transformed mouse fibroblasts, 3T6 cells, whereas the growth of their nontransformed counterparts, Swiss 3T3 cells, is only slightly affected. In this study a similar selective, ATP-induced growth inhibition was found in Balb/c SV40-3T3 cells and in primary cultures of adenovirus-transformed murine fibroblasts. The inhibitory activity was found in the conditioned media of ATP-treated cultures. Several lines of evidence have shown that ectoprotein kinase (ecto-PK) plays a major role in the ATP-induced growth inhibition. (a) There is a good correlation between the activity of ecto-PK and the ability of ATP to induce cell growth inhibition. (b) The removal of the ecto-PK from the cell surface prevents the ATP-induced growth inhibition. (c) Addition of the removed enzyme to the cell culture reconstitutes the ability of ATP to induced growth inhibition. (d) Serum-containing, or serum-free, conditioned media from untreated cultures gain an inhibitory activity after their phosphorylation, and dephosphorylation of conditioned media from ATP-treated cultures results in the loss of the inhibitory activity. (e) Growth medium by itself does not inhibit cell proliferation after its phosphorylation. The findings described in d and e indicate, as well, that the ATP-induced growth inhibitor is produced by the cells. The putative inhibitor was found to be a protein, with an apparent molecular mass of 13 kDa. The selectivity of the inhibition for transformed cells is due to the higher level of ecto-PK in these cells, as well as to their higher susceptibility to the inhibitor, as compared with their non-transformed counterparts.

Biphasic regulation of paracellular permeability in human cervical cells by two distinct nucleotide receptors

The effects of extracellular ATP (ATPe) were characterized in human cervical cells grown on filters. ATPe changed the transepithelial conductance (GT) in a biphasic manner with an initial acute increase of 13 +/- 3% (phase I), lasting approximately 2 min, followed by a sustained decrease of 49 +/- 17% (phase II). ATPe also effected decreases of permeabilities to pyranine and to sucrose, which correspond to the phase II decrease in GT. Both phase I and II effects appear to be mediated by apical nucleotide receptors. However, the phase I effect differed from the phase II effect as follows: 1) a higher 50% effective concentration for ATPe, 22 vs. 3 microM; 2) different nucleotide specificity; 3) lack of influence of pretreatment with pertussis toxin; 4) independence from time in culture after seeding; and 5) involvement of cytosolic calcium, rather than diacylglycerol, as intracellular messenger. These differences suggest the presence of two different types of nucleotide receptors that use different intracellular signaling systems and have opposite effects on the paracellular permeability and electrical conductance. These results suggest that ATPe could regulate cervical mucus production in vivo by regulating the paracellular permeability. Depending on the specific receptors present, ATPe could either increase or decrease mucus production.

Growth inhibition of breast cancer cells induced by exogenous ATP

Addition of ATP (> 0.1 mM) to cultures of human breast cancer T47D cells resulted in an inhibition of cell proliferation. The inhibition was found to be specific for ATP, and dependent on its concentration. Growth inhibition continued for at least three days, although ATP and its hydrolysis products were metabolized within one day. Conditioned medium from ATP-treated cultures (CM+) was found to inhibit the growth of cells that were not exposed to ATP. This is an indication that extracellular factors, besides ATP, are involved in the inhibition process. The inhibition was maintained after dialysis of the CM+, using an 8 kDa cut-off membrane. Conditioned medium from untreated cultures (CM-), however, only slightly affected cell growth. The data suggest that the CM(+)-induced cell growth inhibition is mediated by an ATP-activated growth inhibiting factor. Flow microfluorometry and thymidine incorporation experiments have shown that the growth arrest is mainly due to the elongation of the S-phase of the cell cycle.

Signal transduction via P2-purinergic receptors for extracellular ATP and other nucleotides

Extracellular ATP, at micromolar concentrations, induces significant functional changes in a wide variety of cells and tissues. ATP can be released from the cytosol of damaged cells or from exocytotic vesicles and/or granules contained in many types of secretory cells. There are also efficient extracellular mechanisms for the rapid metabolism of released nucleotides by ecto-ATPases and 5'-nucleotidases. The diverse biological responses to ATP are mediated by a variety of cell surface receptors that are activated when ATP or other nucleotides are bound. The functionally identified nucleotide or P2-purinergic receptors include 1) ATP receptors that stimulate G protein-coupled effector enzymes and signaling cascades, including inositol phospholipid hydrolysis and the mobilization of intracellular Ca2+ stores; 2) ATP receptors that directly activate ligand-gated cation channels in the plasma membranes of many excitable cell types; 3) ATP receptors that, via the rapid induction of surface membrane channels and/or pores permeable to ions and endogenous metabolites, produce cytotoxic or activation responses in macrophages and other immune effector cells; and 4) ADP receptors that trigger rapid ion fluxes and aggregation responses in platelets. Current research in this area is directed toward the identification and structural characterization of these receptors by biochemical and molecular biological approaches.

Extracellular ATP stimulates inositol phospholipid turnover and calcium influx in C6 glioma cells

Extracellular ATP caused a dose-dependent accumulation of inositol phosphates and a rise in cytosolic free Ca2+ ([Ca2+]i) in C6 glioma cells with an EC50 of 60 +/- 4 and 10 +/- 5 microM, respectively. The threshold concentration of ATP (3 microM) for increasing [Ca2+]i was approximately 10-fold less than that for stimulating phosphoinositide (PI) turnover. The PI response showed a preference for ATP; ADP was about 3-fold less potent than ATP but had a comparable maximal stimulation (11-fold of the control). AMP and adenosine were without effect at concentrations up to 1 mM. ATP-stimulated PI metabolism was found to be partially dependent on extracellular Ca2+ and Na+ but was resistant to tetrodotoxin, saxitoxin, amiloride, ouabain, and inorganic blockers of Ca2+ channels (Co2+, Mn2+, La3+, or Cd2+). In Ca(2+)-free medium, ATP caused only a transient increase in [Ca2+]i as opposed to a sustained [Ca2+]i increase in normal medium. The ATP-induced elevation of [Ca2+]i was resistant to Na+ depletion and treatment with saxitoxin, verapamil and nisoldipine, but was attenuated by La3+. The differences in the characteristics of ATP-caused P1 hydrolysis and [Ca2+]i rise suggest that ATP receptors are independently coupled to phospholipase C and receptor-gated Ca2+ channels. Because of the robust effect of ATP in stimulating PI turnover and the apparent absence of P1-purinergic receptors, the C6 glioma cell line provides a useful model for investigating the transmembrane signalling pathway induced by extracellular ATP. The mechanisms underlying the unexpected finding of [Na+]o dependency for ATP-induced PI turnover require further investigation.

Effect of ionophore A23187 on the membrane permeability in mouse fibroblasts

Addition of the divalent cation ionophore A23187 to transformed mouse fibroblasts (3T6) resulted in an increase in the cell membrane permeability to normally impermeant solutes (e.g., nucleotides). The membrane permeability was assessed by following the efflux of prelabeled adenine nucleotides, the influx of p-nitrophenyl phosphate in cells attached to plastic dishes and reconstitution of intracellular protein synthesis in the presence of exogenously added normally impermeant factors required for macromolecular synthesis. The permeability change of 3T6 cells was found to be dependent on the specific presence of external calcium ion. The permeabilization was found to occur preferably in alkaline pH and specific to certain transformed cells. It is preceded by rapid efflux of K+, influx of Na+ and partial hydrolysis of cellular nucleotides in 3T6 cells. Similar ion fluxes were previously found to precede cell permeabilization by electrogenic ionophores for monovalent ions and by exogenous ATP. Our data suggest that a calcium dependent process caused the K+ release and excess Na+ entry, causing dissipation of the membrane potential and subsequent formation of aqueous channels.

Extracellular ATP and some of its analogs induce transient rises in cytosolic free calcium in individual canine keratinocytes

Changes in intracellular free calcium ([Ca++]i) play an important role in a variety of biochemical reactions that lead to cellular responses such as proliferation and differentiation. The response of [Ca++]i to extracellular nucleotides (ATP, UTP, ITP, and AMP-PNP) was determined in individual canine keratinocytes using the fluorescent probe fura-2 and digital video fluorescence imaging microscopy. In the presence of 1.8 mM extracellular Ca++, 100 and 500 microM ATP caused a rapid (less than 9 sec) three- to twelvefold rise in [Ca++]i above resting levels of 50-150 nM followed by occasional fluctuations. Small responses were elicited with doses as low as 0.1 microM ATP. The response of cells stimulated with 500 microM ATP in Ca(++)-free medium was characterized by 1.5 to 3 times rapid initial peak followed by a decrease of [Ca++]i below resting levels. Loss of response occurred in the majority of keratinocytes preincubated for 30 min in Ca(++)-free medium. UTP was as effective as ATP in stimulating rises in [Ca++]i in keratinocytes. Smaller elevations in [Ca++]i up to four- to fivefold resting levels were noted with 100 microM AMP-PNP or 500 microM ITP. Desensitization of cells was demonstrated when a second stimulation followed the primary ATP or UTP treatment. These results are suggestive of the presence of purinergic receptors in the cytoplasmic membrane of canine keratinocytes. Experiments using the calcium channel blocker lanthanum suggest that ATP-induced initial rises and sustained levels of [Ca++]i are dependent on the release of Ca++ from intracellular stores. These intracellular Ca++ stores appear to be rapidly depleted after removal of extracellular calcium ([Ca++]e), thereby abolishing ATP-induced [Ca++]i increases.

Dual effects of ATP on phosphatidylinositol breakdown in rat hepatocyte membranes

The mechanisms whereby adenosine-5'-triphosphate (ATP) regulates the inositol phospholipid-signalling system were studied in rat hepatocytes. Intact hepatocytes respond to extracellular ATP, adenosine-5'-O-(3-thiotriphosphate) (ATP gamma S), ADP and weakly to guanosine-5'-triphosphate (GTP), but not to other purine nucleotides (GDP or AMP). This is consistent with the idea that a P2 purinergic receptor is coupled to the phosphatidylinositol metabolism in these cells. Partially purified plasma membranes prepared from myo-[3H]inositol prelabelled hepatocytes exhibit a phosphatidylinositol-4,5-bisphosphate phospholipase C activity sensitive to ATP, ATP gamma S and guanosine-5'-O-(3-thiotriphosphate) (GTP gamma S). Moreover the GTP gamma S effect is greatly enhanced by ATP and ATP gamma S. These potentiating effects differ according to the adenylnucleotide considered. ATP produces (1) an increase in the GTP gamma S-PLC sensitivity, (2) a potentiation of the phospholipase C (PLC) response induced by maximal dose of GTP gamma S, and (3) an increase in the inositol lipids pools. At variance, ATP gamma S, a nonhydrolysable analogue of ATP, only increases the PLC-sensitivity towards GTP gamma S. These results may signify that ATP stimulates inositol phosphate accumulation via at least two distinct mechanisms (i) a direct activation of a P2 purinergic receptor coupled to a PLC via a GTP binding protein and (ii) a stimulation of the phosphatidylinositol (PI) and phosphatidylinositol-4-phosphate (PIP) kinases which increased the pool of phospholipase C substrates.

Activation of early events of the mitogenic response by a P2Y purinoceptor with covalently bound 3'-O-(4-benzoyl)-benzoyladenosine 5'-triphosphate

3'-O-(4-Benzoyl)benzoyl-ATP (BzATP), a photoaffinity analog of ATP, was used as a ligand for a P2Y purinoceptor (adenine nucleotide receptor) present in intact Swiss 3T3 and 3T6 cells and A-431 epidermoid carcinoma cells. Photolysis of serum-starved cells in the presence of 10-50 microM BzATP, followed by extensive washing to remove unincorporated BzATP, induced the release of arachidonic acid. A trace (less than 0.01%) of photoincorporated BzATP was as effective as when 50 microM BzATP or ATP was contained in the incubation medium during the assay. Photoincorporated BzATP also stimulated the production of prostaglandin E2 and the accumulation of cyclic AMP. In previous studies, we demonstrated that these three events are obligatory early steps in a pathway leading to DNA synthesis in the above cell lines. The evidence indicated that the purinoceptor activated by extracellular ATP or BzATP was linked to a pertussis toxin-sensitive GTP-binding protein. Consistent with these observations, we now find that pertussis toxin inhibits the effect of photoincorporated BzATP on arachidonic acid release. These results indicate that BzATP is an effective agonist for the P2Y purinoceptor concerned with stimulation of DNA synthesis in 3T3, 3T6, and A-431 cells. Furthermore, after photolysis it becomes irreversibly associated with intact cells and promotes the activation of early events required for DNA synthesis.

Evidence for two Ca2(+)-mobilizing purinoceptors on rat hepatocytes

Aequorin measurements of cytosolic free Ca2+ in single rat hepatocytes show that ADP and ATP, thought to act through the same P2Y purinoceptor, elicited very different responses in the majority of cells tested. ADP invariably induced transients of short duration (approx. 9 s), whereas ATP induced either similar transients or transients with a much longer duration (approx. 49 s). We explain this variability in terms of two separate purinoceptors on rat hepatocytes, one of which responds to either ATP or ADP to generate free-Ca2+ transients of short duration, and the other responds to ATP only, with transients of longer duration.