Evidence for two adenine derivative receptors in rat ileum which are not involved in the nonadrenergic, noncholinergic response.

The receptors mediating inhibition of the rat ileum by adenosine and adenine nucleotides were studied. ATP and ADP were more potent than AMP or adeonsine. Theophylline antagonized the effects of adenosine and AMP but not those of ATP or ADP. Preparations desensitized to ATP or ADP were still inhibited by adenosine and vice versa. The nonadrenergic, noncholinergic inhibition produced by field stimulation or nicotine was not attenuated by the presence of theophylline or desensitization to ATP. These data indicate that more than one adenine derivative receptor is present in rat ileum and that ATP and adenosine are unlikely candidates for the unknown transmitter.     

Stimulation of maximal intracellular insulin action on glycogen synthase by preincubation of adipocytes with adenosine 5'-triphosphate

Preincubation of rat adipocytes with ATP further stimulated maximal insulin action on glycogen synthase. Half-maximum concentration of ATP was 5 X 10(-5) M. ATP, ADP, adenosine, inosine, and GTP were effective, while beta-gamma-methylene ATP was without effect. ADP and GTP were less potent than ATP, adenosine, or inosine. Inosine was active without insulin but was without effect in the presence of insulin. The mechanism of action of adenosine was clearly different from ATP. While ATP required both Mg2+ and Ca2+ for effectiveness, adenosine required only Ca2+. The effect of ATP, but not of adenosine, was preserved after cells were washed. The adenosine effect was completely blocked by theophylline, but the ATP effect was inhibited only 40%. The ATP effect was thus not due to adenosine generated by ATP breakdown.     

Mouse neuroblastoma adenylate cyclase. Adenosine and adenosine analogues as potent effectors of adenylate cyclase activity.

1. Intact mouse neuroblastoma NS20 cells, in the presence of cyclic adenosine 3':5'-monophosphate (cAMP) phosphodiesterase inhibitor, responded to adenosine (200 muM) and 2-chloroadenosine (200 muM) with a 20-fold increase in intracellular cAMP levels. AMP (200 muM) additions caused only a 3.5-fold cAMP level elevation. ATP, ADP, guanosine, cytidine, uridine, and guanine, all at 200 muM, had no effect on the cAMP level of these cells. 2. Homogenate NS20 adenylate cyclase activity was increased 2.5- to 4-fold by addition of 200 muM adenosine, 2-chloroadenosine, 2-hydroxyadenosine, or 8-methylaminoadenosine. Prostaglandin E1 additions (1.4 muM) produced about an 8-fold stimulation of homogenate cyclase activity. The Km of homogenate cyclase activation by adenosine and 2-chloroadenosine was 67.6 and 6.7 muM, respectively. Addition of 7-deazaadenosine, tolazoline, yohimbine, guanosine, cytosine, guanine, 2-deoxy-AMP, and adenine 9-beta-D-xylopyranoside, all at 200 muM were found to be without effect on homogenate NS20 adenylate cyclase. Two classes of inhibitors of homogenate NS20 adenylate cyclase activity were observed. One class, which included AMP, adenine, and theophylline, blocked 2-chloroadenosine but not prostaglandin E1 stimulation of cyclase. Theophylline was shown to be a competitive inhibitor of 2-chloroadenosine, with a Ki of 35 muM. The second class of inhibitors, which included 2'- and 5'-deoxyadenosine, inhibited unstimulated, 2-chloroadenosine and prostaglandin E1-stimulated homogenate cyclase activity to about the same degree. 3. Activation of NS20 homogenate adenylate cyclase by adenosine appears to be noncooperative. 4. The inhibitory action of putative "purinergic" neurotransmitters is postulated to be due to their effects on adenylate cyclase activity.     

Light-induced exchange of nucleotides into coupling factor 1 in spinach chloroplast thylakoids.

The method of centrifugation of chloroplast thylakoids through silicone fluid, previously used to estimate the uptake of solutes by thylakoids, is shown to be an excellent method for measuring binding of nucleotides to thylakoids. This binding, which is probably an exchange (Harris, D. A. and Slater, E. C. (1975) Biochim. Biophys. Acta 387, 335-348), is enhanced by light and is sensitive to uncoupling. Half-maximal binding of adenosine 5'-triphosphate (ATP) or adenosine 5'-diphosphate (ADP) at 10 mjM was reached within less than 0.1 s. With illumination times sufficient to elicit maximal binding, saturation of the site(s) is approached at 20 muM nucleotide and dissociation constants of 5 muM and 7 muM were calculated for ADP and ATP, respectively. At saturation, the binding corresponds to 1 mol/mol of coupling factor 1 or less. Although the light-dependent binding of ADP does not require Mg2+, that of ATP is markedly enhanced by Mg2+. A 10-fold molar excess of guanosine di- or triphosphate or adenyl-5'-yl imidodiphosphate had little effect on the binding. Adenosine 5'-phosphosulfate, a competitive inhibitor of phosphorylation with respect to ADP, decreases the binding. Thylakoids, previously illuminated in the absence of added nucleotides, retain the capacity to bind ADP or ATP in the dark long after the H+ electrochemical gradient has decayed. The conformation of coupling factor 1 in darkened thylakoids following illumination in the absence of added nucleotides may thus differ from that in thylakoids either illuminated in the presence of nucleotides or kept in the dark. Approximately 20% of the ADP bound to coupling factor 1 in thylakoids is converted to ATP by a 2-s illumination. Bound inorganic phosphate, derived either from ATP or from inorganic phosphate itself, serves as the phosphoryl donor. Bound ADP may, therefore, be of catalytic significance in the mechanism of phosphorylation.     

Effects of adenosine and its derivatives on protein kinase activity of beef thyroid.

Effects of adenosine and some of its derivatives on beef protein kinase activity were investigated in vitro. Adenosine rapidly inhibited protein kinase activity in a dose-dependent manner. Significant inhibition occurred with 10 muM and half-maximal inhibition at 100 muM adenosine. Inhibition was almost complete with 5 mM adenosine. Inhibition was similar whether protein kinase activity was assayed with or without cyclic AMP. The inhibition by adenosine was reversed by increasing the concentration of ATP and Lineweaver-Burk analysis indicated that adenosine inhibition was competitive with ATP. Addition of adenosine deaminase to the incubation medium prevented the inhibition induced by adenosine. Intact 1 and N6 positions of adenosine were important for the inhibition since their modification was associated with loss of inhibition. Modification of the 8 position of adenosine decreased, but did not abolish, the inhibition. The 2 and 3 position of ribose did not seem to be critical since 2- and 3-deoxyadenosine produced inhibition similar to that of adenosine.     

The effects of adenosine and adenine nucleotides on the guinea-pig vas deferens: evidence for existence of purinergic receptors.

The effect of adenosine 5′-triphosphate (ATP) and its congeners on the alpha-adrenergic neuroeffector transmission in the isolated vas deferens of the guinea pig was evaluated. Both intracellular activity and contractile response of the smooth muscle of the vas deferens were recorded by using the sucrose-gap method. Adenosine, adenosine diphosphate (ADP) and adenosine monophosphate (AMP) influenced alpha-adrenergic receptor-mediated excitatory responses by depolarizing the cell membrane potential. ATP, on the other hand, produced action potentials rather than sustained depolarization, and its activity was blocked by theophylline and 2, 2′-pyridylisatogen, an ATP antagonist, but not blocked by either phentolamine or phenoxybenzamine, which inhibit alpha-adrenoreceptor responsiveness caused by norepinephrine or phenylephrine. Furthermore, dipyridamole, an adenosine uptake blocker, potentiated both ATP and adenosine activities. These findings indicate that adenosine and adenine nucleotides may exert their action at an extracellular site. From these results, it may be speculated that alpha adrenoreceptors and purinergic receptors do indeed exist on the smooth muscle of the vas deferens.     

Dual effect of adenosine triphosphate on the apical small conductance K+ channel of the rat cortical collecting duct

We used the patch-clamp technique to study the effects of ATP on the small-conductance potassium channel in the apical membrane of rat cortical collecting duct (CCD). This channel has a high open probability (0.96) in the cell-attached mode but activity frequently disappeared progressively within 1-10 min after channel excision (channel "run-down"). Two effects of ATP were observed. Using inside-out patches, low concentrations of ATP (0.05-0.1 mM) restored channel activity in the presence of cAMP-dependent protein kinase A (PKA). In contrast, high concentrations (1 mM) of adenosine triphosphate (ATP) reduced the open probability (Po) of the channel in inside-out patches from 0.96 to 0. 1.2 mM adenosine diphosphate (ADP) also blocked channel activity completely, but 2 mM adenosine 5'-[beta,gamma-imido]triphosphate (AMP-PNP), a nonhydrolyzable ATP analogue, reduced Po only from 0.96 to 0.87. The half-maximal inhibition (Ki) of ATP and ADP was 0.5 and 0.6 mM, respectively, and the Hill coefficient of both ATP and ADP was close to 3. Addition of 0.2 or 0.4 mM ADP shifted the Ki of ATP to 1.0 and 2.0 mM, respectively. ADP did not alter the Hill coefficient. Reduction of the bath pH from 7.4 to 7.2 reduced the Ki of ATP to 0.3 mM. In contrast, a decrease of the free Mg2+ concentration from 1.6 mM to 20 microM increased the Ki of ATP to 1.6 mM without changing the Hill coefficient; ADP was still able to relieve the ATP-induced inhibition of channel activity over this low range of free Mg2+ concentrations. The blocking effect of ATP on channel activity in inside-out patches could be attenuated by adding exogenous PKA catalytic subunit to the bath. The dual effects of ATP on the potassium channel can be explained by assuming that (a) ATP is a substrate for PKA that phosphorylates the potassium channel to maintain normal function. (b) High concentrations of ATP inhibit the channel activity; we propose that the ATP-induced blockade results from inhibition of PKA-induced channel phosphorylation.     

Differential effects of UTP, ATP, and adenosine on ciliary activity of human nasal epithelial cells

The purinergic regulation of ciliary activity was studied using small, continuously superfused explants of human nasal epithelium. The P2Y(2) purinoceptor (P2Y(2)-R) was identified as the major purinoceptor regulating ciliary beat frequency (CBF); UTP (EC(50) = 4.7 microM), ATP, and adenosine-5'-O-(3-thiotriphosphate) elicited similar maximal responses, approximately twofold over baseline. ATP, however, elicited a post-peak sustained plateau in CBF (1.83 +/- 0.1-fold), whereas the post-peak CBF response to UTP declined over 15 min to a low-level plateau (1.36 +/- 0.16-fold). UDP also stimulated ciliary beating, probably via P2Y(6)-R, with a maximal effect approximately one-half that elicited by P2Y(2)-R stimulation. Not indicated were P2Y(1)-R-, P2Y(4)-R-, or P2Y(11)-R-mediated effects. A(2B)-receptor agonists elicited sustained responses in CBF approximately equal to those from UTP/ATP [5'-(N-ethylcarboxamido)adenosine, EC(50) = 0.09 microM; adenosine, EC(50) = 0.7 microM]. Surprisingly, ADP elicited a sustained stimulation in CBF. The ADP effect and the post-peak sustained portion of the ATP response in CBF were inhibited by the A(2)-R antagonist 8-(p-sulfophenyl)theophylline. Hence, ATP affects ciliary activity through P2Y(2)-R and, after an apparent ectohydrolysis to adenosine, through A(2B)AR.     

Interrelationships between the membrane-bound ATP-dependent energy systems of the gastric mucosa and the gastric cytoprotective effect of beta-carotene on the development of gastric mucosal damage produced by 0.6 M HCl in rats

The effects of different doses (0.01-0.1-1.0-10.0/mg/kg-1) of beta-carotene were studied on gastric secretory responses of 4 hr pylorus-ligated rats: development of gastric mucosal damage (as assessed by number and severity of lesions) produced by intragastric administration of 0.6 M HCl; tissue level of adenosine triphosphate (ATP), adenosine diphosphate (ADP), adenosine monophosphate (AMP), adenylate pool (ATP + ADP + AMP), ratio of ATP X ADP-1, "energy charge" (ATP + 0.5 ADP X X (ATP + ADP + AMP)-1) (during the development of gastric mucosal damage by 0.6 M HCl and of gastric cytoprotection by beta-carotene. It was found that beta-carotene did not decrease the gastric secretory responses of 4 hr pylorus-ligated rats; The development of gastric mucosal damage could be decreased dose-dependently by the administration of beta-carotene; the ATP transformation could be decreased by beta-carotene; the tissue levels of cAMP and AMP could be increased significantly and dose-dependently by beta-carotene; the ratio of ATP X ADP-1 could be increased significantly and dose-dependently by beta-carotene; the values of adenylate pool and "energy charge" remained unchanged.(ABSTRACT TRUNCATED AT 250 WORDS)     

Inhibition by adenine compounds of the induced formation of photosynthetic pigments in Rhodopseudomonas spheroides cells under dark-semiaerobic conditions

Effects of adenine, adenosine, AMP, ADP and ATP on the inducedformation of bacteriochlorophyll and carotenoids in cell suspensionsof dark-aerobically grown Rhodopseudomonas spheroides were examinedunder dark-semiaerobic conditions where no significant cellgrowth occurred. Pigment formation was strongly inhibited by3 mM adenine, adenosine, AMP or ATP, but less strongly by ADP.Inhibition by either adenosine or ATP was completely reversible.Addition of 3 mM adenosine resulted in complete inhibition ofpigment formation, while inhibition by more than 10 min ATPdid not exceed 80%. No accumulation of any precursor-like pigmentsof either bacteriochlorophyll or carotenoids was observed incells incubated in the presence of adenine compounds. Amountsof exogenously-added adenine, adenosine, or AMP decreased significantlyduring incubation, whereas the amount of exogeneously-addedATP or ADP did not appreciably decrease. Addition of 3 mM ATPor adenosine also significantly suppressed ³H-leucine incorporationinto bacterial proteins. Nucleosides other than adenosine wereineffective in inhibiting the induced formation of photosyntheticpigments, indicating that the inhibitory action is specificto adenine compounds. It was assumed that both adenosine andATP inhibit chromatophore formation rather than a particularstep(s) in the biosynthetic pathways of the photosynthetic pigment,and that ATP exerts its effect from outside the cells, whereasadenosine does so after being taken up by the cells. (Received July 24, 1972; )     

Effects of metronidazole and tinidazole on NTPDase1 and ecto-5'-nucleotidase from intact cells of Trichomonas vaginalis

Here we report the effects of metronidazole and tinidazole on NTPDase1 and ecto-5'-nucleotidase from intact cells of Trichomonas vaginalis. Adenosine triphosphate (ATP) and adenosine diphosphate (ADP) hydrolysis was 5- to 7-fold higher for the fresh clinical strain, when compared with the ATCC (American Type Culture Collection) strain. ATP hydrolysis was activated in the presence of metronidazole in the ATCC strain, whilst it was inhibited 33% by 50 microM tinidazole in a fresh clinical isolate. The treatment of cells in the presence of metronidazole for 2 h inhibited ATP and ADP hydrolysis, whilst treatment with tinidazole inhibited ATP and ADP hydrolysis only in the fresh clinical isolate. The drugs did not change the ecto-5'-nucleotidase activity for both strains. Our results suggest that the modulation of extracellular ATP and ADP levels during treatment with these drugs could be a parasitic defence strategy as a survival mechanism in an adverse environment.     

Involvement of inositol 1,4,5-trisphosphate and calcium in the action of adenine nucleotides on aortic endothelial cells

ADP and ATP, in the 1-100 microM range of concentrations, increased the formation of inositol phosphates in bovine aortic endothelial cells. The accumulation of inositol trisphosphate in response to adenine nucleotides was rapid (maximum at 15 s) and transient. This material was identified as the biologically active isomer inositol 1,4,5-trisphosphate on the basis of its retention time by high-performance liquid chromatography on an anion-exchange resin. AMP and adenosine have no effect on inositol phosphates. The action of ATP and ADP was mimicked with an equal potency and activity by their phosphorothioate analogs, ATP gamma S and ADP beta S, and with a lower potency by adenosine 5'-(beta,gamma-imido)triphosphate, whereas adenosine 5'-(alpha,beta-methylene)triphosphate, was inactive. In the same range of concentrations, ADP and ATP induced an efflux of 45Ca2+ from prelabeled bovine aortic endothelial cells and increased the fluorescence emission by cells loaded with quin-2. Here, too, AMP and adenosine were completely inactive. The outflow of 45Ca2+ induced by ADP was partially maintained in a calcium-free medium. These data suggest that in aortic endothelial cells, P2-purinergic receptors, of the P2Y subtype, are coupled to the hydrolysis of phosphatidylinositol bisphosphate by a phospholipase C. It is likely that the release of prostacyclin and endothelium-derived relaxing factor in response to ADP and ATP is a consequence of this initial event.     

Acute Caffeine Treatment Increases Extracellular Nucleotide Hydrolysis from Rat Striatal and Hippocampal Synaptosomes

The psychostimulant caffeine promotes behavioral effects such as hyperlocomotion, anxiety, and disruption of sleep by blockade of adenosine receptors. The availability of extracellular adenosine depends on its release by transporters or by the extracellular ATP catabolism performed by the ecto-nucleotidase pathway. This study verified the effect of caffeine on NTP-Dase 1 (ATP diphosphohydrolase) and 5-nucleotidase of synaptosomes from hippocampus and striatum of rats. Caffeine and theophylline tested in vitro were unable to modify nucleotide hydrolysis. Caffeine chronically administered in the drinking water at 0.3 g/L or 1 g/L for 14 days failed to affect nucleotide hydrolysis. However, acute administration of caffeine (30 mg/kg, ip) produced an enhancement of ATP (50%) and ADP (32%) hydrolysis in synaptosomes of hippocampus and striatum, respectively. This activation of ATP and ADP hydrolysis after acute treatment suggests a compensatory effect to increase adenosine levels and counteract the antagonist action of caffeine.     

Human airway ecto-adenylate kinase. A mechanism to propagate ATP signaling on airway surfaces

Mechanically induced ATP release from human airway epithelial cells regulates mucociliary clearance through cell surface nucleotide receptors. Ectoenzymes detected on these cells were recently shown to terminate ATP-mediated responses by sequential dephosphorylation of extracellular ATP into ADP, AMP, and adenosine. We now demonstrate that an ecto-adenylate kinase (ecto-AK) contributes to the metabolism of adenine nucleotides on human airway epithelial surfaces by the reversible reaction: ATP + AMP 2ADP. This phosphotransferase exhibited a bilateral distribution on polarized primary cultures of human bronchial epithelial cells with a 4-fold higher activity on the mucosal surface. Ecto-AK presented an absolute requirement for magnesium and adenine-based nucleotides. UMP, GMP, and CMP could not substitute for AMP as gamma-phosphate acceptor, and UDP could not replace ADP. Apparent K(m) and V(max) values were 23 +/- 5 microM and 1.1 +/- 0.1 nmol x min(-1) x cm(-2) for ATP and 43 +/- 6 microM and 0.5 +/- 0.1 nmol x min(-1) x cm(-2) for ADP. Ecto-AK accounted for 20% of [gamma-(32)P]ATP dephosphorylation, and the impermeant AK inhibitor, diadenosine pentaphosphate, reduced ADPase activity by more than 70% on both epithelial surfaces. Time course experiments on ATP metabolism demonstrated that ecto-AK significantly prolongs effective ATP and ADP concentrations on airway epithelial surfaces for P2 receptor signaling and reduces by 6-fold adenosine production. Our data suggest a role for this nucleotide entrapment cycle in the propagation of purine-mediated mucociliary clearance on human airway epithelial surfaces.     

ATP hydrolysis pathways and their contributions to pial arteriolar dilation in rats

ATP is thought to be released to the extracellular compartment by neurons and astrocytes during neural activation. We examined whether ATP exerts its effect of promoting pial arteriolar dilation (PAD) directly or upon conversion (via ecto-nucleotidase action) to AMP and adenosine. Blockade of extracellular direct ATP to AMP conversion, with ARL-67156, significantly reduced sciatic nerve stimulation-evoked PADs by 68%. We then monitored PADs during suffusions of ATP, ADP, AMP, and adenosine in the presence and absence of the following: 1) the ecto-5'-nucleotidase inhibitor α,β-methylene adenosine 5'-diphosphate (AOPCP), 2) the A(2) receptor blocker ZM 241385, 3) the ADP P2Y(1) receptor antagonist MRS 2179, and 4) ARL-67156. Vasodilations induced by 1 and 10 μM, but not 100 μM, ATP were markedly attenuated by ZM 241385, AOPCP, and ARL-67156. Substantial loss of reactivity to 100 μM ATP required coapplications of ZM 241385 and MRS 2179. Dilations induced by ADP were blocked by MRS 2179 but were not affected by either ZM 241385 or AOPCP. AMP-elicited dilation was partially inhibited by AOPCP and completely abolished by ZM 241385. Collectively, these and previous results indicate that extracellular ATP-derived adenosine and AMP, via A(2) receptors, play key roles in neural activation-evoked PAD. However, at high extracellular ATP levels, some conversion to ADP may occur and contribute to PAD through P2Y(1) activation.     

Modulation of cyclic AMP in purified rat mast cells. I. Responses to pharmacologic, metabolic, and physical stimuli.

The cyclic adenosine 3', 5'-monophosphate (cAMP) content of isolated unstimulated mast cells and the changes induced by a variety of pharmacologic, metabolic, and physical stimuli were studied. A modified bovine serum albumin density gradient purification method consistently provided mast cell preparations which were 95% or more pure, without apparent damage, and a 73% recovery of the mast cells applied to the gradients. The measured cAMP in unstimulated mast cells was high, a mean of 16 picomoles per million cells. Moderate agitation or contact with glass increased cAMP content about 2-fold. When calcium was omitted from the medium mast cell cAMP was markedly elevated; incremental increases in added calcium ion concentration from 1 muM to 1 mM caused a linear decrease in cAMP content. Theophylline (3 to 20 mM) caused a dose-related increase in mast cell cAMP content, approximately 2-fold at 20 mM theophylline. Epinephrine (0.01 to 1 mM) caused a modest, dose-related increase in cAMP. In the presence of theophylline, epinephrine increased cAMP levels equal to or greater than the sum of the effects of the agents used individually. The increase in cAMP induced by epinephrine was completely inhibited by 100 muM propranolol and partially inhibited by 10 muM propranolol, thus suggesting that a beta adrenergic receptor is involved. Prostaglandin E1 (PGE1) and histamine (in the presence of theophylline) also raised cAMP. Carbamylcholine (1 nM) lowered cAMP 38%; Atropine (0.1 mM) inhibited the decrease in cAMP induced by 1 nM carbamylcholine by 83% indicating that a muscarinic receptor is involved. In these homogeneous single cell suspensions, therefore, cholinergic and beta adrenergic agents have opposing effects on cAMP levels. Diazoxide (10 muM) and adenine (1 muM) caused 37 and 32% decreases in cAMP, respectively. The availability of highly purified mast cells and the identification of agents which either decrease or increase cAMP content allows a direct examination of the role of cAMP in histamine release.     

P2Y-purinoceptor mediated inhibition of L-type Ca2+ channels in rat pancreatic beta-cells

We used the patch-clamp technique to study the effects of extracellular ATP on the activity of ion channels recorded in rat pancreatic beta-cells. In cell-attached membrane patches, action currents induced by 8.3 mM glucose were inhibited by 0.1 mM ATP, 0.1 mM ADP or 15 microM ADPbetaS but not by 0.1 mM AMP or 0.1 mM adenosine. In perforated membrane patches, action potentials were measured in current clamp, induced by 8.3 mM glucose, and were also inhibited by 0.1 mM ATP with a modest hyperpolarization to -43 mV. In whole-cell clamp experiments, ATP dose-dependently decreased the amplitudes of L-type Ca2+ channel currents (ICa) to 56.7+/-4.0% (p<0.001) of the control, but did not influence ATP-sensitive K+ channel currents observed in the presence of 0.1 mM ATP and 0.1 mM ADP in the pipette. Agonists of P2Y purinoceptors, 2-methylthio ATP (0.1 mM) or ADPbetaS (15 microM) mimicked the inhibitory effect of ATP on ICa, but PPADS (0.1 mM) and suramin (0.2 mM), antagonists of P2 purinoceptors, counteracted this effect. When we used 0.1 mM GTPgammaS in the pipette solution, ATP irreversibly reduced ICa to 58.4+/-6.6% of the control (p<0.001). In contrast, no inhibitory effect of ATP was observed when 0.2 mM GDPbetaS was used in the pipette solution. The use of either 20 mM BAPTA instead of 10 mM EGTA, or 0.1 mM compound 48/80, a blocker of phospholipase C (PLC), in the pipette solution abolished the inhibitory effect of ATP on ICa, but 1 microM staurosporine, a blocker of protein kinase C (PKC), did not. When the beta-cells were pretreated with 0.4 microM thapsigargin, an inhibitor of the endoplasmic reticulum (ER) Ca2+ pump, ATP lost the inhibitory effect on ICa. These results suggest that extracellular ATP inhibits action potentials by Ca2+-induced ICa inhibition in which an increase in cytosolic Ca2+ released from thapsigargin-sensitive store sites was brought about by a P2Y purinoceptor-coupled G-protein, PI-PLC and IP3 pathway.     

Effects of various physiologic adenine derivatives on the secretion of acid in isolated gastric glands in rabbits

The physiological adenine derivatives, adenosine (ADO), adenosine 5'-monophosphate (AMP), adenosine 5'-diphosphate (ADP) and adenosine 5'-triphosphate (ATP) at concentrations ranging from 10 microM to 1 mM caused concentration-related modifications on gastric H+ secretion, as measured by the aminopyrine accumulation method, in resting and histamine-stimulated rabbit gastric glands. In resting glands, ADO caused significant concentration-related increases of the basal H+ secretion, whereas no changes were obtained in response to the other purines tested. In histamine-stimulated glands, ADO and AMP caused concentration-related potentiation of the histamine-raised H+ secretory rate, while ATP and ADP induced graded inhibition. The results suggest the involvement of purinergic mechanisms in the physiological regulation of the gastric acid secretory process.