ATP activates a Ca(2+)-dependent Cl- current in the rat thyroid cell line, FRTL-5

Canonical transient receptor potential channel 2 (TRPC2): old name-new games. Importance in regulating of rat thyroid cell physiology

In addition to the TSH-cyclic AMP signalling pathway, calcium signalling is of crucial importance in thyroid cells. Although the importance of calcium signalling has been thoroughly investigated for several decades, the nature of the calcium channels involved in signalling is unknown. In a recent series of investigations using the well-studied rat thyroid FRTL-5 cell line, we showed that these cells exclusively express the transient receptor potential canonical 2 (TRPC2) channel. Our results suggested that the TRPC2 channel is of significant importance in regulating thyroid cell function. These investigations were the first to show that thyroid cells express a member of the TRPC family of ion channels. In this review, we will describe the importance of the TRPC2 channel in regulating TSH receptor expression, thyroglobulin maturation, intracellular calcium and iodide homeostasis and that the channel also regulates thyroid cell proliferation.

Purinergic signalling in endocrine organs

There is widespread involvement of purinergic signalling in endocrine biology. Pituitary cells express P1, P2X and P2Y receptor subtypes to mediate hormone release. Adenosine 5'-triphosphate (ATP) regulates insulin release in the pancreas and is involved in the secretion of thyroid hormones. ATP plays a major role in the synthesis, storage and release of catecholamines from the adrenal gland. In the ovary purinoceptors mediate gonadotrophin-induced progesterone secretion, while in the testes, both Sertoli and Leydig cells express purinoceptors that mediate secretion of oestradiol and testosterone, respectively. ATP released as a cotransmitter with noradrenaline is involved in activities of the pineal gland and in the neuroendocrine control of the thymus. In the hypothalamus, ATP and adenosine stimulate or modulate the release of luteinising hormone-releasing hormone, as well as arginine-vasopressin and oxytocin. Functionally active P2X and P2Y receptors have been identified on human placental syncytiotrophoblast cells and on neuroendocrine cells in the lung, skin, prostate and intestine. Adipocytes have been recognised recently to have endocrine function involving purinoceptors.

Functional coupling of TRPC2 cation channels and the calcium-activated anion channels in rat thyroid cells: implications for iodide homeostasis

The initial step in a synthesis of thyroid hormones is the uptake of iodide from the circulation. Iodide (I(-)) is transported into thyroid cells via a Na(+)/I(-) symporter (NIS), which is electrogenic and thus sensitive to alterations in membrane potential (V(m)). I(-) is then released to the lumen of thyroid follicles where the hormones are synthesised and stored. The mechanisms of I(-) release to follicle lumen are poorly characterised. Our whole-cell voltage clamp recordings revealed the presence of a Ca(2+) activated Cl(-) current (CaCC) in Fisher rat thyroid cell line 5 (FRTL-5). Transcripts of anoctamin 1 (ANO1) and anoctamin 10 (ANO10), putative molecular constituents of CaCC, were detected. The anion channels underlying CaCC are highly permeable to I(-). Both niflumic acid (NFA) and 2-aminoethyl diphenylborinate (2-APB), antagonists of CaCC and transient receptor potential channels, respectively, inhibited CaCC. Canonical transient receptor potential channel 2 (TRPC2) is the only TRPC member present in FRTL-5 cells. The activation rate of CaCC was markedly slower in shTRPC2 knock-down cells, indicating that Ca(2+) entry via TRPC2 contributes to CaCC activation. The uptake of iodide was enhanced and the resting V(m) was more depolarised in TRPC2 knock-down cells. We suggest that the interplay between TRPC2 and ANO1 may have dual effects on iodide transport, modulating I(-) release via ANO channels and I(-) uptake via the V(m) sensitive NIS.

Fluorescence quantitation of thyrocyte iodide accumulation with the yellow fluorescent protein variant YFP-H148Q/I152L

The thyroid gland accumulates iodide for the synthesis of thyroid hormones. The aim of the current study was to quantify iodide accumulation in cultured thyroid cells by live cell imaging using the halide-sensitive yellow fluorescent protein (YFP) variant YFP-H148Q/I152L. In vivo calibrations were performed in FRTL-5 thyrocytes to determine the sensitivity of YFP-H148Q/I152L to iodide. In the presence of ion-selective ionophores, YFP-H148Q/I152L fluorescence was suppressed by halides in a pH-dependent manner with 20-fold selectivity for iodide versus chloride and competition between the two halides. At a physiological pH of 7 and a chloride concentration of 15mM, the affinity constant of YFP-H148Q/I152L for iodide was 3.5mM. In intact FRTL-5 cells, iodide induced a reversible decrease in YFP-H148Q/I152L fluorescence. FRTL-5 cells concentrated iodide to 60 times the extracellular concentration. Iodide influx exhibited saturation kinetics with respect to extracellular iodide with a K(m) of 35 microM and a V(max) of 55 microM/s. Iodide efflux exhibited saturation kinetics with respect to intracellular iodide concentration with a K(m) of 2.2mM and a V(max) of 43 microM/s. The results of this study demonstrate the utility of YFP-H148Q/I152L as a sensitive and selective biosensor for the quantification of iodide accumulation in thyroid cells.

Physiology and pathophysiology of purinergic neurotransmission

This review is focused on purinergic neurotransmission, i.e., ATP released from nerves as a transmitter or cotransmitter to act as an extracellular signaling molecule on both pre- and postjunctional membranes at neuroeffector junctions and synapses, as well as acting as a trophic factor during development and regeneration. Emphasis is placed on the physiology and pathophysiology of ATP, but extracellular roles of its breakdown product, adenosine, are also considered because of their intimate interactions. The early history of the involvement of ATP in autonomic and skeletal neuromuscular transmission and in activities in the central nervous system and ganglia is reviewed. Brief background information is given about the identification of receptor subtypes for purines and pyrimidines and about ATP storage, release, and ectoenzymatic breakdown. Evidence that ATP is a cotransmitter in most, if not all, peripheral and central neurons is presented, as well as full accounts of neurotransmission and neuromodulation in autonomic and sensory ganglia and in the brain and spinal cord. There is coverage of neuron-glia interactions and of purinergic neuroeffector transmission to nonmuscular cells. To establish the primitive and widespread nature of purinergic neurotransmission, both the ontogeny and phylogeny of purinergic signaling are considered. Finally, the pathophysiology of purinergic neurotransmission in both peripheral and central nervous systems is reviewed, and speculations are made about future developments.

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.

Ceramide 1-phosphate increases intracellular free calcium concentrations in thyroid FRTL-5 cells: evidence for an effect mediated by inositol 1,4,5-trisphosphate and intracellular sphingosine 1-phosphate

Sphingolipid (SP) derivatives have diverse effects on the regulation of intracellular free calcium concentrations ([Ca2+]i) in a multitude of non-excitable cells. In the present investigation, the effect of C2-ceramide 1-phosphate (C1P) on [Ca2+]i was investigated in thyroid FRTL-5 cells. C1P evoked a concentration-dependent increase in [Ca2+]i, both in a calcium-containing and a calcium-free buffer. A substantial part of the C1P-evoked increase in [Ca2+]i was due to calcium entry. The effect of C1P was attenuated by overnight pretreatment of the cells with pertussis toxin. Similar results were obtained with C8-ceramide 1-phosphate, although the magnitude of the responses was smaller than with C1P. The phospholipase C inhibitor U73122 attenuated the effect of C1P. C1P invoked a small, but significant, increase in inositol 1,4,5-trisphosphate (IP3). However, the effect of C1P on [Ca2+]i was inhibited by neither Xestospongin C, 2-aminoethoxydiphenylborate nor neomycin. C1P mobilized calcium from an IP3-sensitive calcium store, as C1P did not increase [Ca2+]i in cells pretreated with thapsigargin. The effect of C1P on [Ca2+]i was potently attenuated by dihydrosphingosine and dimethylsphingosine, two inhibitors of sphingosine kinase, but not by the inactive SP-derivative N -acetyl sphingosine. Stimulating the cells with C1P evoked an increase in the production of intracellular sphingosine 1-phosphate. C1P did not modulate DNA synthesis or the forskolin-evoked production of cAMP. The results indicate that C1P may be an important SP participating in cellular signalling.

Ceramide 1-(2-cyanoethyl) phosphate enhances store-operated Ca2+ entry in thyroid FRTL-5 cells

Sphingolipid derivatives cause diverse effects towards the regulation of intracellular free Ca(2+) concentrations ([Ca(2+)](i)) in a multitude of nonexcitable cells. In the present investigation, the effect of C-8 ceramide-1-(2-cyanoethyl) phosphate (C1CP) on store-operated Ca(2+) (SOC) entry was investigated. C1CP evoked a modest increase in [Ca(2+)](i). The increase was inhibited by the SOC channel antagonist 1-(beta-[3-(4methoxyphenyl)propoxy]-4-methoxyphenethyl)-1H-imidazole (SKF96365) but not by overnight pretreatment of the cells with pertussis toxin. C1CP did not invoke the production of inositol phosphates. When cells were stimulated with both C1CP and thapsigargin, the thapsigargin-invoked increase in [Ca(2+)](i) was enhanced in comparison to control cells. When Ca(2+) was added to cells treated with both C1CP and thapsigargin in a Ca(2+)-free buffer, the increase in [Ca(2+)](i) was enhanced in comparison to control cells. In patch-clamp experiments, C1CP hyperpolarized the membrane potential (E(m)) of the cells and attenuated the thapsigargin-invoked depolarization of the E(m). The effects of C1CP came, in part, as a result of a decreased conductance of the cell membrane towards Cl(-) ions, as C1CP in a Cl(-)-free solution also enhanced Ca(2+) entry. Barium 2-cyanoethylphosphate (Ba2Cy), which also contains the 2-cyanoethyl group, did not modulate thapsigargin-invoked changes in [Ca(2+)](i) nor did it modulate the E(m). In conclusion, C1CP enhances SOC entry, in part, via hyperpolarization of the E(m) and attenuation of the thapsigargin-invoked membrane depolarization, thus increasing the electrochemical gradient for Ca(2+) ions. Hence, C1CP may be a useful reagent for investigating the cellular effects of ceramide derivatives.

Immunohistochemical identification of cells expressing ATP-gated cation channels (P2X receptors) in the adult rat thyroid

We carried out immunohistochemistry and western blotting of fresh frozen sections and crude extracts from adult rat thyroids. The histochemical and immunoblotting studies were performed with P2X receptor antibodies from 2 different sources. P2X-immunopositive cells were identified by fluorescence double labelling and confocal microscopy. Results of the western blotting experiments showed double bands of approximately 70 kDa and 140 kDa for all 7 P2X receptor subtypes with both sets of antibodies. Histochemical stains with antibodies from both sources also gave essentially identical results. P2X1, P2X, and P2X6 receptors were detected exclusively in vascular smooth muscle; P2X5 and P2X7 receptors were also present on vascular smooth muscle. Endothelial cells stained for P2X3, P2X4 and P2X7 receptors. Thyroid follicular cells displayed immunoreactivity for P2X3, P2X4 and P2X5 receptors. No immunostaining for P2X receptors was observed on C-cells. Possible roles for the broad expression of P2X receptor subtypes in the rat thyroid are discussed.

UTP-preferring P2 receptor mediates inhibition of sodium transport in porcine thyroid epithelial cells

1. The effects of adenosine 5'-triphosphate (ATP), uridine 5'-triphosphate (UTP) and analogues on forskolin-stimulated absorption of Na+ by porcine thyroid epithelial cells were analysed in cultures grown as confluent monolayers on permeable supports in Transwell Ussing chambers. 2. 85% of the forskolin (10 microM)-stimulated short-circuit current was inhibited by phenamil (1 microM), which is a selective antagonist for epithelial type Na+ channels. 3. Phenamil-sensitive current was inhibited in a dose dependent manner by nucleotides added to the apical compartment of Ussing chambers. In contrast, the phenamil-resistant current, previously shown to represent anion secretion, was unaffected by nucleotides. 4. The order of potency (with EC50 values given in microM) was UTP (0.08)>>ATP (6.3)=uridine 5'-diphosphate (UDP) (6. 6)>2methyl-thio-adenosine-5'-triphosphate (2MeSATP) (84.5)>adenosine 5'-diphosphate (ADP) (147.8)>alpha,beta-methylene ATP (>150)>>adenosine (>1000). 5. P2 receptors mediating inhibition of sodium absorption were present on the apical membrane of the cells since addition of UTP (1-1000 microM) to the basal compartment of the Ussing chambers had little effect while subsequent addition to the apical compartment produced a normal response. 6. Cibachron blue (Reactive blue 2) (1-100 microM), an antagonist at some P2 receptor subtypes, inhibited phenamil sensitive current in a dose dependent manner with half maximal inhibition occurring at 14.25 microM. 7. Suramin (100 microM), pyridoxalphosphate-6-azophenyl-2', 4'-disulphonic acid (PPADS) (100 microM) and pyridoxal 5'-phosphate (P5P) (100 microM) showed only slight competitive antagonism against the response to UTP. 8 These results indicate that a UTP-preferring P2 receptor located on the apical membrane of thyroid epithelial cells mediates inhibition of Na+ absorption.

Extracellular ATP-induced inward current in isolated epithelial cells of the endolymphatic sac

Using whole-cell patch-clamp technique and Fura-2 fluorescence measurement, the presence of ATP-activated ion channels and its dependence on intracellular Ca2+ concentration ([Ca2+]i) in the epithelial cells of the endolymphatic sac were investigated. In zero current-clamp configuration, the average resting membrane potential was -66.8+/-1.3 mV (n=18). Application of 30 microM ATP to the bath induced a rapid membrane depolarization by 43.1+/-2.4 mV (n=18). In voltage-clamp configuration, ATP-induced inward current at holding potential (VH) of -60 mV was 169.7+/-6.3 pA (n=18). The amplitude of ATP-induced currents increased in sigmoidal fashion over the concentration range between 0.3 and 300 microM with a Hill coefficient (n) of 1.2 and a dissociation constant (Kd) of 11.7 microM. The potency order of purinergic analogues in ATP-induced current, which was 2MeSATP>ATPgammas>/=ATP>alpha, beta-ATP>ADP=AMP>/=adenosine=UTP, was consistent with the properties of the P2Y receptor. The independence of the reversal potential of the ATP-induced current from Cl- concentration suggests that the current is carried by a cation channel. The relative ionic permeability ratio of the channel modulated by ATP for cations was Ca2+>Na+>Li+>Ba2+>Cs+=K+. ATP (10 microM) increased [Ca2+]i in an external Ca2+-free solution to a lesser degree than that in the external solution containing 1.13 mM CaCl2. ATP-induced increase in [Ca2+]i can be mimicked by application of ionomycin in a Ca2+-free solution. These results indicate that ATP increases [Ca2+]i through the P2Y receptor with a subsequent activation of the non-selective cation channel, and that these effects of ATP are dependent on [Ca2+]i and extracellular Ca2+.

DIDS (4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid) increases iodide trapping, inhibits thyroperoxidase and antagonizes the TSH-induced apical iodide efflux in porcine thyroid cells

4,4'-Di-isothiocyanatostilbene-2,2'-disulfonic acid (DIDS), an inhibitor of several anionic channels and transporters including the band 3 protein of the red blood cell membrane was tested on iodide metabolism in cultured porcine thyroid cells. We used three experimental cell culture models: (i) forskolin-stimulated correctly inside-in polarized follicle-associated thyroid cells cultured onto plastic support (ii) suspensions of isolated cells derived from such cultures (iii) polarized monolayers in bicameral chambers. DIDS was observed to increase free-iodide trapping in all conditions. Organification of iodide by follicle-associated cell cultures incubated for 6 h decreased as a function of DIDS concentration with an IC50 of 5 x 10(-5) M. This block in organification is accounted for a block in thyroperoxidase activity as in vitro both purified lactoperoxidase and purified porcine thyroperoxidase were inhibited by DIDS with a similar dose-dependency the IC50 being also of 5 x 10(-5) M. Both control and DIDS-treated cells in suspension, actively trapped iodide and reached a steady concentration in about 50 min; however the plateau was 4.4-fold higher in (10(-3) M) DIDS-treated cells. Acute TSH-stimulation at this plateau of 125I-preloaded cells in suspension in the presence of 2 mM methimazole (MMI) induced a fast release of iodide from these cells as expected (first step of the TSH-biphasic effect). This TSH-induced iodide efflux was however completely inhibited by DIDS (10(-3) M). Furthermore, addition of DIDS to the apical compartment of TSH-prestimulated cell monolayers in bicameral chambers resulted in an increase in intracellular-iodide concentration and in an inhibition of iodide efflux into the apical medium. Taken together, the present results demonstrate that DIDS mainly interacts with two main components of the thyroid apical cell membrane: thyroperoxidase and a cAMP-sensitive iodide channel.

Thyrotropin regulation of basolateral Cl- and I- effluxes in thyroid follicles in culture

This report describes chloride and iodide effluxes across the basolateral membrane of porcine thyroid follicles reconstituted in culture. Basolateral chloride efflux is activated by thyrotropin (TSH). TSH (10 mU/ml) induces a twofold increase in the initial rate of chloride efflux. Forskolin (FSK, 5 microM) which increases intracellular cAMP also stimulates the initial rate of chloride efflux 3.5-fold, whereas an increase in the free cytosolic Ca2+ with the ionophore A23187 or thapsigargin, fails to mimic the TSH effect. The chloride channel blocker 5-nitro-2(3-phenylpropylamino)benzoic acid (NPPB) dose dependently inhibits chloride efflux rates with the maximal and half maximal effects observed for 100 microM and 30 microM, respectively. Basolateral chloride efflux rates are also inhibited in the presence of the organic anion transporter blocker probenecid (5 mM) or the Cl-/HCO3- exchanger blocker 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid (SITS, 250 microM), respectively, by 60% and 40%, whereas it is not affected by ClO4 (100 microM). The initial rate of iodide efflux is weakly activated (1.4-fold) by TSH (10 mU/ml). TSH effect could be reproduced by agents known to activate Ca(2+)-dependent processes as A23187, ionomycin (1 microM), phorbol 12-myristate 13-acetate (TPA, 0.1 microM) and epidermal growth factor (EGF, 0.1 microM) which increase the initial rate of iodide efflux from 1.2- to 1.8-fold, whereas FSK is without effect. The chloride channel blocker NPPB (500 microM) is required to significantly inhibit the initial rate of iodide efflux by 30%. The initial rate of iodide efflux is also reduced by 30% in the presence of SITS (250 microM) or probenecid (5 mM) whereas it is activated by ClO4 (100 microM). We conclude that basolateral chloride and iodide effluxes are both regulated by TSH, using two different transduction pathways. Chloride efflux regulation may involve a cAMP transduction signal, whereas the regulation of iodide efflux may involve a Ca2+ signal. Furthermore, as the sensitivities of chloride and iodide effluxes for the anion transporter blockers (especially NPPB) are different, it seems likely that chloride and iodide use two different transport pathways.

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)

Norepinephrine, unlike ATP, induces all-or-none increase in cytosolic calcium in thyroid cells. The role of inositol-trisphosphate-sensitive stores and calcium channels

The mechanism of action of norepinephrine and ATP has been analyzed in single FRTL5 cells (a normal thyroid cell line), loaded with the fluorescent Ca2+ probe Fura2. ATP increased the cytosolic Ca2+ in an apparently concentration-dependent manner with a maximal effect at 10 microM (413 +/- 26% over basal levels of 135 +/- 7 nM). In contrast, the norepinephrine-induced increase (198 +/- 5% over basal) was concentration independent in individual cells, the minimal effective concentration being 1 nM. However, the number of cells responding to norepinephrine was concentration dependent. The ATP-induced Ca2+ rise was biphasic, consisting of a rapid rise (2-4 s, 252 +/- 15%), resembling the effect of norepinephrine, followed by a slower and longer component, which reached a plateau in 0.5-2 min. The second component appeared to be related to the opening of a channel, since it required extracellular Ca2+ and was abolished by SC38249, an inhibitor of the second-messenger-operated and voltage-operated channels. Moreover, it was inhibited by 4 beta-phorbol 12-myristate 13-acetate, suggesting that protein kinase C might be involved in the modulation of this Ca2+ channel.

Mobilization of intracellular calcium in cultured vascular smooth muscle cells by uridine triphosphate and the calcium ionophore A23187

The known action of uridine triphosphate (UTP) to contract some types of vascular smooth muscle, and the present finding that it is more potent than adenosine triphosphate in eliciting an increase in cytosolic Ca2+ concentration in aortic smooth muscle, led us to investigate the mode of action of this nucleotide. With this aim, cultured bovine aorta cells were subjected to patch-clamp methodologies under various conditions. Nucleotide-induced variations in cytosolic Ca2+ were monitored by using single channel recordings of the high conductance Ca(2+)-activated K+ (Maxi-K) channel within on-cell patches as a reporter, and whole-cell currents were measured following perforation of the patch. In cells bathed in Na(+)-saline, UTP (> 30 nM) induced an inward current, and both Maxi-K channel activity and unitary current amplitude of the Maxi-K channel transiently increased. Repetitive exposures elicited similar responses when 5 to 10 min wash intervals were allowed between challenges of nucleotide. Oscillations in channel activity, but not oscillation in current amplitude were frequently observed with UTP levels > 0.1 microM. Cells bathed in K+ saline (150 mM) were less sensitive to UTP (approximately 5-fold), and did not show an increase in unitary Maxi-K current amplitude. Since the increase in amplitude occurs due to depolarization of the cell membrane, a change in amplitude was not observed in cells previously depolarized with K+ saline. The enhancement of Maxi-K channel activity in the presence of UTP was not diminished by Ca2+ entry blockers or by removal of extracellular Ca2+. However, in the latter case, repetitive responses progressively declined. These observations, as well as data comparing the action of low concentrations of Ca2+ ionophores (< 5 microM) to that of UTP indicate that both agents elevate cytosolic Ca2+ by mobilization of this ion from intracellular pools. However, the Ca2+ ionophore did not cause membrane depolarization, and thus did not change unitary current amplitude. The effect of UTP on Maxi-K channel activity and current amplitude was blocked by pertussis toxin and by phorbol 12-myristate 13-acetate (PMA), but was not modified by okadaic acid, or by inhibitors of protein kinase C (PKC). Our data support a model in which a pyrimidinergic receptor is coupled to a G protein, and this interaction mediates release of Ca2+ from intracellular pools, presumably via the phosphatidyl inositol pathway. This also results in activation of membrane channels that give rise to an inward current and depolarization. Ultimately, smooth muscle contraction ensues.(ABSTRACT TRUNCATED AT 400 WORDS)

Adenosine 5'-[alpha beta-methylene]triphosphate potentiates the oscillatory cytosolic Ca2+ responses of hepatocytes to ATP, but not to ADP

Single rat hepatocytes microinjected with aequorin generate oscillations in cytosolic free Ca2+ concentration ([Ca2+]i) when stimulated with agonists acting through the phosphoinositide signalling pathway. The duration of these transients has been shown to be characteristic of the stimulating agonist, so that transients of very different duration can be induced in the same individual hepatocyte by different agonists. In a previous study we have shown that ADP and ATP, which are believed to act through a single P2y-purinoceptor species, elicit very different [Ca2+]i responses in most of the hepatocytes. We have interpreted this as evidence for two Ca(2+)-mobilizing purinoceptors. The methylated derivative of ATP, adenosine 5'-[alpha beta-methylene]-triphosphate (pp[CH2]pA), is only a weak P2y-purinoceptor agonist. When 100 microM pp[CH2]pA was supplied to aequorin-injected hepatocytes, there was no effect on [Ca2+]i. However, 25 microM pp[CH2]pA co-supplied with ATP causes a potentiation of the [Ca2+]i response in most of the hepatocytes. The effect was specific for ATP-induced transients; [Ca2+]i transients induced by other agonists, and importantly by ADP, were not affected by addition of pp[CH2]pA. This further illustrates differences in the actions of ADP and ATP, strengthening the argument for separate receptors for these nucleotides.

Effects of ATP on cultured smooth muscle cells from rat aorta

1. Membrane ionic currents provoked by externally applied ATP were studied by patch-clamp techniques in cultured aortic smooth muscle cells of the rat. 2. Using standard bath and pipette solutions and whole-cell voltage-clamp, ATP evoked an inward current when the cell membrane potential was held at -50 mV and an outward current when the potential was held at 30 mV, with a reversal potential near -10 mV. 3. Application of ATP gamma S gave results similar to those obtained with ATP, while adenosine, AMP and alpha,beta-methylene ATP were ineffective. The ATP-activated current was inhibited by suramin, 100 microM. 4. ATP also induced a biphasic rise in internal free Ca levels as shown directly by Fura-2 measurements and by the increase in Ca-dependent K single-channel activity in cell-attached patches. 5. With outward current through K channels blocked by internal Cs and TEA, modification of the ionic composition of bath and pipette solutions revealed that the reversal potential for the ATP-induced whole-cell current closely followed ECl, the chloride equilibrium potential, and was insensitive to manipulations of the monovalent cation gradient. 6. These results indicate that in rat cultured aortic smooth muscle cells, ATP binding to P2-purinoceptors produces increases of internal free Ca levels and subsequent activation of both Ca-dependent K and Cl currents.