Inhibitory effects of some purinergic agents on ecto-ATPase activity and pattern of stepwise ATP hydrolysis in rat liver plasma membranes

Calcium Oscillatory Behavior and Its Possible Role during Wound Healing in Bovine Corneal Endothelial Cells in Culture

In epithelial layers in culture, immediately after an injury a fast calcium wave (FCW) propagates from the wound borders toward the rest of the monolayer. We show here that similarly to other tissues, during the FCW in bovine corneal endothelial (BCE) cells in culture many cells exhibit calcium oscillations mediated by IP3 signaling. In this study we perform a detailed characterization of this oscillatory behavior and explore its possible role in the process of wound healing. In previous work we showed that, in BCE cells in culture, the healing cells undergo two stages of caspase-dependent apoptosis, at approximately two and eight hours after wounding. We determined that inhibition of the FCW greatly increases the apoptotic rate of the two stages, suggesting that the wave prevents excessive apoptosis of the healing cells. Taking this into account, we investigated the possible participation of the calcium oscillations during the FCW in apoptosis of the healing cells. For this, we employed ARL-67156 (ARL), a weak competitive inhibitor of ecto-ATPases, and the calcium chelator EGTA. We show here that, in healing BCE cells, ARL enhances cellular calcium oscillations during the FCW, while EGTA decreases oscillations. We found that ARL produces a significant decrease (to about half the control value) in the apoptotic index of the first stage of apoptosis, while EGTA increases it. Neither drug noticeably affects the second stage. We have interpreted the effect of ARL on apoptosis as due to the maintenance of moderately risen ATP levels during the FCW, which is in turn the cause for the enhancement of ATP-dependent calcium oscillations. Correspondingly, EGTA would increase the apoptotic index of the first stage by promoting a decrease in the calcium oscillatory rate. The fact that the second stage of apoptosis is not affected by the drugs suggests that the two stages are at least partially subject to different signaling pathways.

P2X receptors up-regulate the cell-surface expression of the neuronal glycine transporter GlyT2

Glycinergic inhibitory neurons of the spinal dorsal horn exert critical control over the conduction of nociceptive signals to higher brain areas. The neuronal glycine transporter 2 (GlyT2) is involved in the recycling of synaptic glycine from the inhibitory synaptic cleft and its activity modulates intra and extracellular glycine concentrations. In this report we show that the stimulation of P2X purinergic receptors with βγ-methylene adenosine 5'-triphosphate induces the up-regulation of GlyT2 transport activity by increasing total and plasma membrane expression and reducing transporter ubiquitination. We identified the receptor subtypes involved by combining pharmacological approaches, siRNA-mediated protein knockdown, and dorsal root ganglion cell enrichment in brainstem and spinal cord primary cultures. Up-regulation of GlyT2 required the combined stimulation of homomeric P2X₃ and P2X₂ receptors or heteromeric P2X_2/3 receptors. We measured the spontaneous glycinergic currents, glycine release and GlyT2 uptake concurrently in response to P2X receptor agonists, and showed that the impact of P2X3 receptor activation on glycinergic neurotransmission involves the modulation of GlyT2 expression or activity. The recognized pro-nociceptive action of P2X₃ receptors suggests that the fine-tuning of GlyT2 activity may have consequences in nociceptive signal conduction.

ATP metabolism in skeletal muscle arterioles

The purpose of this study was to investigate the metabolism of Adenosine triphosphate (ATP) in skeletal muscle resistance arterioles and to determine whether this metabolism is altered during the rapid growth phase of the rat. We attempted to quantify ATP metabolism in gastrocnemius first‐order arterioles from 8‐, 10‐, and 12‐week‐old rats. We measured ATP metabolism using an ATPase/GTPase assay with whole vessel segments as well as using a real‐time adenosine biosensor following electric field stimulation. Our first method of measuring ATP metabolism allowed us to measure the amount of free phosphate produced with ATP as a substrate. When ecto‐nucleotidase activity was inhibited by ARL67156, pyridoxal phosphate‐6‐azophenly‐2′, 4′‐disulfonic acid (PPADS), or suramin prior to adding ATP, we found that the rate of phosphate production was significantly reduced by 27%, 21%, and 22%, respectively (P < 0.05). Our second method of measuring ATP metabolism allowed us to measure the amount of adenosine produced following electric field stimulation of the arteriole with and without nucleotidase inhibitors. Surprisingly, we found that adenosine overflow was not attenuated by nucleotidase inhibitors. We concluded that ecto‐phosphodieterase/phyrophophatase (E‐NPP), ecto‐diadenosine polyphosphatase (ApnA), NTPDase1 and 2, and E5NT may be present on the gastrocnemius 1A arteriole and do play a role in ATP metabolism. Between the ages of 8 weeks and 12 weeks, however, overall ATP metabolism may not change.

The purpose of this study was to investigate the metabolism of Adenosine triphosphate (ATP) in skeletal muscle resistance arterioles and whether it is altered during the rapid growth phase of the rat. We concluded that ecto‐phosphodieterase/phyrophophatase (E‐NPP), ecto‐diadenosine polyphosphatase (ApnA), NTPDase1 and 2, and E5NT may be present on the gastrocnemius 1A arteriole and do play a role in ATP metabolism. Between the ages of 8 weeks and 12 weeks, however, overall ATP metabolism may not change.

Purinergic receptors and regulatory volume decrease in seabream (Sparus aurata) hepatocytes: a videometric study

The response of isolated hepatocytes of Sparus aurata to hypotonic stress was studied by the aid of videometric methods with the aim to investigate the possible involvement of ATP in the regulatory volume decrease (RVD). This study confirms our previous observations showing the ability of these cells to undergo RVD. In addition, it shows that the homeostatic response was inhibited by apyrase, an ATP scavenger, thus suggesting the involvement of extracellular ATP in the RVD response. Experiments performed in the presence of ATPγS or adenosine, agonists of P(2) and P(1) receptors respectively, and in the presence of suramin or 8-PT, antagonists of P(2) and P(1) receptors respectively, suggest that ATP exerts its stimulatory effect on the homeostatic response by interacting with P(2) receptors. On the other hand, the activation of P(1) receptors by ATP metabolites produces opposite effects. In an attempt to clarify the mechanisms involved in ATP release from the cell, we performed some experiments with known inhibitors of the possible mechanisms of regulated ATP release. The results we obtained let us to suppose that the mechanism allowing the exit of ATP from the cell is verapamil sensitive suggesting the involvement of the P-glycoprotein.

Uridine adenosine tetraphosphate is a novel vasodilator in the coronary microcirculation which acts through purinergic P1 but not P2 receptors

Uridine adenosine tetraphosphate (Up4A) has been identified as an endothelium-derived contracting factor, which acts through purinergic P2X and P2Y receptors. Since the coronary vascular actions of Up4A are unknown, we investigated the vasoactive profile of Up4A in coronary microvessels, and studied the involvement of purinergic receptor subtypes. Studies were performed in isolated porcine coronary small arteries (diameter∼250 μm), with and without endothelial denudation, mounted on a Mulvany wire myograph. Purinergic receptor expression was assessed by real-time PCR. Up4A (10(-9)-10(-5) M) failed to induce contraction at basal tone, but produced concentration-dependent vasorelaxation in precontracted microvessels. Up4A was slightly less potent than adenosine, ATP, and ADP in producing vasorelaxation, but significantly more potent than UTP and UDP. mRNA expression of P2X(4), P2Y(1), P2Y(2), P2Y(4), P2Y(6) and A(2A), but not P2X(1), receptors was observed. Up4A-induced vasodilation was unaffected by non-selective P2 receptor antagonist PPADS, P2X(1) antagonist MRS2159, P2Y(1) antagonist MRS2179 and P2Y(6) antagonist MRS2578, but was markedly attenuated by non-selective P1 receptor antagonist 8PT and A(2A) antagonist SCH58261. Up4A-induced vasodilation was not affected by ectonucleotidase inhibitor ARL67156, suggesting that A(2A) stimulation was not the result of Up4A breakdown to adenosine. Up4A-induced vasodilation was blunted in denuded vessels; additional A(2A) receptor blockade further attenuated Up4A-induced vasodilation, suggesting that A(2A) receptor-mediated vasodilation is only partly endothelium-dependent. In conclusion, Up4A exerts a vasodilator rather than a vasoconstrictor influence in coronary microvessels, which is mediated via A(2A) receptors and is partly endothelium-dependent.

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

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

OBJECTIVES

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

MATERIALS AND METHODS

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

RESULTS

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

CONCLUSION

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

Metabolism of circulating ADP in the bloodstream is mediated via integrated actions of soluble adenylate kinase-1 and NTPDase1/CD39 activities

Extracellular ATP and ADP trigger inflammatory, vasodilatatory, and prothrombotic signaling events in the vasculature, and their turnover is governed by networks of membrane-associated enzymes. The contribution of soluble activities to intravascular nucleotide homeostasis remains controversial. By using thin-layer chromatographic assays, we revealed transphosphorylation of [γ-(32)P]ATP and AMP by human and murine sera, which was progressively inhibited by specific adenylate kinase (AK) inhibitor Ap(5)A. This phosphotransfer reaction was diminished markedly in serum from knockout mice lacking the major AK isoform, AK1, and in human serum immunodepleted of AK1. We also showed that ∼75% ADP in cell-free serum is metabolized via reversible AK1 reaction 2ADP ↔ ATP + AMP. The generated ATP and AMP are then metabolized through the coupled nucleotide pyrophosphatase/phosphodiesterase and 5'-nucleotidase/CD73 reactions, respectively. Constitutive presence of another nucleotide-converting enzyme, nucleoside triphosphate diphosphohydrolase-1 (NTPDase1, known as CD39), was ascertained by the relative deficiency of serum from CD39-null mice to dephosphorylate [(3)H]ADP and [γ-(32)P]ATP, and also by diminished [(3)H]ADP hydrolysis by human serum pretreated with NTPDase1 inhibitors, POM-1 and ARL-67156. In summary, we have identified hitherto unrecognized soluble forms of AK1 and NTPDase1/CD39 that contribute in the active cycling between the principal platelet-recruiting agent ADP and other circulating nucleotides.

cAMP production mediated through P2Y(11)-like receptors in rat striatum due to severe, but not moderate, carbon monoxide poisoning

We examined the effect of carbon monoxide (CO) poisoning on the production of cAMP, an intracellular second messenger, in rat striatum in terms of extracellular cAMP, which is highly correlated with intracellular cAMP, by using microdialysis. Severe poisoning due to 3000ppm CO, but not moderate poisoning due to 1000ppm CO, caused an increase in cAMP, which was susceptible to a voltage-dependent Na(+) channel blocker, tetrodotoxin, and more profound than that under comparable hypoxia caused by 5% O(2). These results were similar to our previous findings on the production of hydroxyl radical ((•)OH), suggesting a close relationship between cAMP and (•)OH production. The increase in cAMP was suppressed by a non-selective purine P2 receptor antagonist, suramin. However, other non-selective P2 receptor antagonists, pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid and reactive blue 2, exhibited no effect and weak non-significant suppression, respectively. A P2Y(11) receptor antagonist, NF157, dose-dependently suppressed the increase in cAMP, although rats lack the P2Y(11) receptor. These results suggest that a threshold for cAMP production mediated through P2Y(11)-like receptors following depolarization triggered by Na(+) influx exists in rat striatum during CO poisoning, and that the threshold is reached only in cases of severe CO poisoning. It is also likely that the threshold is related to the generation of (•)OH, contributing to the toxicity of CO in the brain.

Apyrase (nucleoside triphosphate-diphosphohydrolase) and extracellular nucleotides regulate cotton fiber elongation in cultured ovules

Ectoapyrase enzymes remove the terminal phosphate from extracellular nucleoside tri- and diphosphates. In Arabidopsis (Arabidopsis thaliana), two ectoapyrases, AtAPY1 and AtAPY2, have been implicated as key modulators of growth. In fibers of cotton (Gossypium hirsutum), transcript levels for GhAPY1 and GhAPY2, two closely related ectoapyrases that have high sequence similarity to AtAPY1 and AtAPY2, are up-regulated when fibers enter their rapid growth phase. In an ovule culture system, fibers release ATP as they grow, and when their ectoapyrase activity is blocked by the addition of polyclonal anti-apyrase antibodies or by two different small molecule inhibitors, the medium ATP level rises and fiber growth is suppressed. High concentrations of the poorly hydrolyzable nucleotides ATPgammaS and ADPbetaS applied to the medium inhibit fiber growth, and low concentrations of them stimulate growth, but treatment with adenosine 5'-O-thiomonophosphate causes no change in the growth rate. Both the inhibition and stimulation of growth by applied nucleotides can be blocked by an antagonist that blocks purinoceptors in animal cells, and by adenosine. Treatment of cotton ovule cultures with ATPgammaS induces increased levels of ethylene, and two ethylene antagonists, aminovinylglycine and silver nitrate, block both the growth stimulatory and growth inhibitory effects of applied nucleotides. In addition, the ethylene precursor, 1-aminocyclopropane-1-carboxylic acid, lowers the concentration of nucleotide needed to promote fiber growth. These data indicate that ectoapyrases and extracellular nucleotides play a significant role in regulating cotton fiber growth and that ethylene is a likely downstream component of the signaling pathway.

Dinucleoside polyphosphates: strong endogenous agonists of the purinergic system

The purinergic system is composed of mononucleosides, mononucleoside polyphosphates and dinucleoside polyphosphates as agonists, as well as the respective purinergic receptors. Interest in the role of the purinergic system in cardiovascular physiology and pathophysiology is on the rise. This review focuses on the overall impact of dinucleoside polyphosphates in the purinergic system. Platelets, adrenal glands, endothelial cells, cardiomyocytes and tubular cells release dinucleoside polyphosphates. Plasma concentrations of dinucleoside polyphosphates are sufficient to cause direct vasoregulatory effects and to induce proliferative effects on vascular smooth muscle cells and mesangial cells. In addition, increased plasma concentrations of a dinucleoside polyphosphate were recently demonstrated in juvenile hypertensive patients. In conclusion, the current literature accentuates the strong physiological and pathophysiological impact of dinucleoside polyphosphates on the cardiovascular system.

Involvement of P2Y receptors in pyridoxal-5'-phosphate-induced cardiac preconditioning

Using an isolated non-working rat heart model, this study investigated the mechanisms of pharmacological pre-conditioning (PC) induced by P2Y receptor stimulation with pyridoxal-5'-phosphate (PLP). After 6-hydroxydopamine pretreatment and a 15-min stabilization period, isolated rat hearts were perfused for 25 min then subjected to 40 min of global ischemia and 30 min of reperfusion (I/R); exposed for 15 min to 0.05 microM PLP bracketed for 25 min with broad-spectrum P2 antagonists (suramin or PPADS) or with more specific P2Y antagonists (AMPalphaS or MRS2578), 1 microM each, followed by a 5-min PLP-free perfusion before I/R; treated during 25 min with either glybenclamide (GLY, 1 microM), 5-hydroxydecanoic acid (5-HD, 100 microM), U73122 (0.5 microM), H89 (1 microM), or KN93 (1 microM), with an infusion starting 5 min before PLP. The main endpoints were the rate-pressure product (RPP), creatine kinase (CK) release and area necrosis. Recovery of RPP, measured 5 min after reperfusion, was rapidly improved by PLP, blocked by the P2 antagonists, and decreased with the different inhibitors. Fifteen minutes after the end of ischemia, CK release reached maximal values in all groups. PLP provided significant protection, whereas the P2 antagonists, 5-HD, a mitochondrial selective K(ATP) antagonist and GLY a non-selective K(ATP) channel blocker, suppressed the protective effect on myocardial injury. The suppression of the cardioprotective effects of PLP by AMPalphaS, the PKA inhibitor (H89), and phospholipase C blocker (U73122) is in agreement with the P2Y11 receptor as a receptor for PLP-induced PC. The suppression of the cardioprotective effects of PLP by MRS2578 and U73122 is in agreement with the P2Y6 receptor as a receptor for PLP-induced PC. Pre-ischemic exposure to nanomolar concentrations of PLP is protective against I/R. P2Y11 and P2Y6 represents the most likely candidate receptors for PLP-induced cardiac PC.

Identification of hydrolytically stable and selective P2Y(1) receptor agonists

P2Y nucleotide receptors (P2YRs) are attractive pharmaceutical targets. Most P2YR agonists proposed as drugs consist of a nucleotide scaffold, but their use is limited due to their chemical and enzymatic instabilities. To identify drug candidates, we developed non-hydrolyzable P2YR agonists. We synthesized ATP-beta,gamma-CH(2) analogues 2-4, and evaluated their chemical and metabolic stabilities and activities at P2Y(1,2,4,6) receptors. Analogues 2-4 exhibited t(1/2) values of 14.5-65 h in gastric juice pH. They were completely resistant to alkaline phosphatase for 30 min at 37 degrees C and slowly hydrolyzed in human blood serum (t(1/2) 12.7-71.9 h). In comparison to ATP, analogues 2-4 were barely hydrolyzed by nucleoside triphosphate diphosphohydrolases, NTPDase1,2,3,8 (< 8% hydrolysis), and nucleotide pyrophosphatases, NPP1,3 (< or = 10% hydrolysis). Analogues 2 and 4B were selective agonists of the P2Y(1)R with EC(50)s of 0.08 and 17.2 microM, respectively. These features make analogues 2 and 4B potential therapeutic agents for health disorders involving the P2Y(1)R.

Relative contribution of ecto-ATPase and ecto-ATPDase pathways to the biphasic effect of ATP on acetylcholine release from myenteric motoneurons

BACKGROUND AND PURPOSE

The relative contribution of distinct ecto-nucleotidases to the modulation of purinergic signalling may depend on differential tissue distribution and substrate preference.

EXPERIMENTAL APPROACH

Extracellular ATP catabolism (assessed by high-performance liquid chromatography) and its influence on [(3)H]acetylcholine ([(3)H]ACh) release were investigated in the myenteric plexus of rat ileum in vitro.

KEY RESULTS

ATP was primarily metabolized via ecto-ATPDase (adenosine 5'-triphosphate diphosphohydrolase) into AMP, which was then dephosphorylated into adenosine by ecto-5'-nucleotidase. Alternative conversion of ATP into ADP by ecto-ATPase (adenosine 5'-triphosphatase) was more relevant at high ATP concentrations. ATP transiently increased basal [(3)H]ACh outflow in a 2',3'-O-(2,4,6-trinitrophenyl)adenosine-5'-triphosphate (TNP-ATP)-dependent, tetrodotoxin-independent manner. ATP and ATPgammaS (adenosine 5'-[gamma-thio]triphosphate), but not alpha,beta-methyleneATP, decreased [(3)H]ACh release induced by electrical stimulation. ADP and ADPbetaS (adenosine 5'[beta-thio]diphosphate) only decreased evoked [(3)H]ACh release. Inhibition by ADPbetaS was prevented by MRS 2179 (2'-deoxy-N(6)-methyl adenosine 3',5'-diphosphate diammonium salt, a selective P2Y(1) antagonist); blockade of ADP inhibition required co-application of MRS 2179 plus adenosine deaminase (which inactivates endogenous adenosine). Blockade of adenosine A(1) receptors with 1,3-dipropyl-8-cyclopentyl xanthine enhanced ADPbetaS inhibition, indicating that P2Y(1) stimulation is cut short by tonic adenosine A(1) receptor activation. MRS 2179 facilitated evoked [(3)H]ACh release, an effect reversed by the ecto-ATPase inhibitor, ARL67156, which delayed ATP conversion into ADP without affecting adenosine levels.

CONCLUSIONS AND IMPLICATIONS

ATP transiently facilitated [(3)H]ACh release from non-stimulated nerve terminals via prejunctional P2X (probably P2X(2)) receptors. Hydrolysis of ATP directly into AMP by ecto-ATPDase and subsequent formation of adenosine by ecto-5'-nucleotidase reduced [(3)H]ACh release via inhibitory adenosine A(1) receptors. Stimulation of inhibitory P2Y(1) receptors by ADP generated alternatively via ecto-ATPase might be relevant in restraining ACh exocytosis when ATP saturates ecto-ATPDase activity.

Dinucleotide polyphosphates contribute to purinergic signalling via inhibition of adenylate kinase activity

Dinucleoside polyphosphates are well described as direct vasoconstrictors and as mediators with strong proliferative properties, however, less is known about their effects on nucleotide-converting pathways. Therefore, the present study investigates the effects of Ap(4)A (diadenosine tetraphosphate), Up(4)A (uridine adenosine tetraphosphate) and Ap(5)A (diadenosine pentaphosphate) and the non-selective P2 antagonist suramin on human serum and endothelial nucleotide-converting enzymes. Human serum and HUVECs (human umbilical vein endothelial cells) were pretreated with various concentrations of dinucleotide polyphosphates and suramin. Adenylate kinase and NDP kinase activities were then quantified radiochemically by TLC analysis of the ATP-induced conversion of [(3)H]AMP and [(3)H]ADP into [(3)H]ADP/ATP and [(3)H]ATP respectively. Endothelial NTPDase (nucleoside triphosphate diphosphohydrolase) activity was additionally determined using [(3)H]ADP and [(3)H]ATP as preferred substrates. Dinucleoside polyphosphates and suramin have an inhibitory effect on the serum adenylate kinase [pIC(50) values (-log IC(50)): Ap(4)A, 4.67+/-0.03; Up(4)A, 3.70+/-0.10; Ap(5)A, 6.31+/-0.03; suramin, 3.74+/-0.07], as well as on endothelial adenylate kinase (pIC(50) values: Ap(4)A, 4.17+/-0.07; Up(4)A, 2.94+/-0.02; Ap(5)A, 5.97+/-0.04; suramin, 4.23+/-0.07), but no significant effects on serum NDP kinase, emphasizing the selectivity of these inhibitors. Furthermore, Ap(4)A, Up(4)A, Ap(5)A and suramin progressively inhibited the rates of [(3)H]ADP (pIC(50) values: Ap(4)A, 3.38+/-0.09; Up(4)A, 2.78+/-0.06; Ap(5)A, 4.42+/-0.11; suramin, 4.10+/-0.07) and [(3)H]ATP (pIC(50) values: Ap(4)A, 3.06+/-0.06; Ap(5)A, 3.05+/-0.12; suramin, 4.14+/-0.05) hydrolyses by cultured HUVECs. Up(4)A has no significant effect on the endothelial NTPDase activity. Although the half-lives for Ap(4)A, Up(4)A and Ap(5)A in serum are comparable with the incubation times of the assays used in the present study, secondary effects of the dinucleotide metabolites are not prominent for these inhibitory effects, since the concentration of metabolites formed are relatively insignificant compared with the 800 mumol/l ATP added as a phosphate donor in the adenylate kinase and NDP kinase assays. This comparative competitive study suggests that Ap(4)A and Ap(5)A contribute to the purinergic responses via inhibition of adenylate-kinase-mediated conversion of endogenous ADP, whereas Up(4)A most likely mediates its vasoregulatory effects via direct binding-mediated mechanisms.

Nucleotide- and nucleoside-converting ectoenzymes: Important modulators of purinergic signalling cascade

The involvement of extracellular nucleotides and adenosine in an array of cell-specific responses has long been known and appreciated, but the integrative view of purinergic signalling as a multistep coordinated cascade has emerged recently. Current models of nucleotide turnover include: (i) transient release of nanomolar concentrations of ATP and ADP; (ii) triggering of signalling events via a series of ligand-gated (P2X) and metabotropic (P2Y) receptors; (iii) nucleotide breakdown by membrane-bound and soluble nucleotidases, including the enzymes of ecto-nucleoside triphosphate diphosphohydrolase (E-NTPDase) family, ecto-nucleotide pyrophosphatase/phosphodiesterase (E-NPP) family, ecto-5'-nucleotidase/CD73, and alkaline phosphatases; (iv) interaction of the resulting adenosine with own nucleoside-selective receptors; and finally, (v) extracellular adenosine inactivation via adenosine deaminase and purine nucleoside phosphorylase reactions and/or nucleoside uptake by the cells. In contrast to traditional paradigms that focus on purine-inactivating mechanisms, it has now become clear that "classical" intracellular ATP-regenerating enzymes, adenylate kinase, nucleoside diphosphate (NDP) kinase and ATP synthase can also be co-expressed on the cell surface. Furthermore, data on the ability of various cells to retain micromolar ATP levels in their pericellular space, as well as to release other related compounds (adenosine, UTP, dinucleotide polyphosphates and nucleotide sugars) gain another important insight into our understanding of mechanisms regulating a signalling cascade. This review summarizes recent advances in this rapidly evolving field, with particular emphasis on the nucleotide-releasing and purine-converting pathways in the vasculature.

Mechanical loading stimulates ecto-ATPase activity in human tendon cells

Response to external stimuli such as mechanical signals is critical for normal function of cells, especially when subjected to repetitive motion. Tenocytes receive mechanical stimuli from the load-bearing matrix as tension, compression, and shear stress during tendon gliding. Overloading a tendon by high strain, shear, or repetitive motion can cause matrix damage. Injury may induce cytokine expression, matrix metalloproteinase (MMP) expression and activation resulting in loss of biomechanical properties. These changes may result in tendinosis or tendinopathy. Alternatively, an immediate effector molecule may exist that acts in a signal-dampening pathway. Adenosine 5'-triphosphate (ATP) is a candidate signal blocker of mechanical stimuli. ATP suppresses load-inducible inflammatory genes in human tendon cells in vitro. ATP and other extracellular nucleotide signaling are regulated efficiently by two distinct mechanisms: purinoceptors via specific receptor-ligand binding and ecto-nucleotidases via the hydrolysis of specific nucleotide substrates. ATP is released from tendon cells by mechanical loading or by uridine 5'-triphosphate (UTP) stimulation. We hypothesized that mechanical loading might stimulate ecto-ATPase activity. Human tendon cells of surface epitenon (TSC) and internal compartment (TIF) were cyclically stretched (1 Hz, 0.035 strain, 2 h) with or without ATP. Aliquots of the supernatant fluids were collected at various time points, and ATP concentration (ATP) was determined by a luciferin-luciferase bioluminescence assay. Total RNA was isolated from TSC and TIF (three patients) and mRNA expression for ecto-nucleotidase was analyzed by RT-PCR. Human tendon cells secreted ATP in vitro (0.5-1 nM). Exogenous ATP was hydrolyzed within minutes. Mechanical load stimulated ATPase activity. ATP was hydrolyzed in mechanically loaded cultures at a significantly greater rate compared to no load controls. Tenocytes (TSC and TIF) expressed ecto-nucleotidase mRNA (ENTPD3 and ENPP1, ENPP2). These data suggest that motion may release ATP from tendon cells in vivo, where ecto-ATPase may also be activated to hydrolyze ATP quickly. Ecto-ATPase may act as a co-modulator in ATP load-signal modulation by regulating the half-life of extracellular purine nucleotides. The extracellular ATP/ATPase system may be important for tendon homeostasis by protecting tendon cells from responding to excessive load signals and activating injurious pathways.

P2 purinoceptor-mediated cardioprotection in ischemic-reperfused mouse heart

P2 purinoceptor modulation of injury during ischemia-reperfusion was studied in murine hearts. Effects of P2 agonism or antagonism, and interstitial accumulation of P2 agonists (UTP, ATP, and ADP), were assessed in Langendorff perfused hearts during 20 min of ischemia and 45 min of reperfusion. In control hearts, ventricular pressure development recovered to 68 +/- 4 mm Hg (63 +/- 3% baseline), diastolic pressure remained elevated (23 +/- 2 mm Hg), and 26 +/- 4 U/g lactate dehydrogenase (LDH) was released during reperfusion, evidencing necrosis. Treatment with 250 nM UTP improved pressure development (85 +/- 5 mm Hg, or 77 +/- 2%) and reduced diastolic contracture (by approximately 70%, to 7 +/- 1 mm Hg) and LDH loss (by approximately 60%, to 11 +/- 2 U/g). In contrast, P2Y1 agonism with 50 nM 2-methyl-thio-ATP (2-MeSATP) was ineffective. In the presence of the P2Y antagonist suramin (10 or 200 microM), UTP no longer improved postischemic outcomes. Ischemia also substantially elevated interstitial [UTP], [ATP], and [ADP], potentially activating P2 receptors. This was supported in part by effects of antagonists: 200 microM suramin worsened LDH efflux (53 +/- 9 IU/g) and contractile dysfunction (41 +/- 2 mm Hg diastolic pressure; 28 +/- 3 mm Hg developed pressure), as did P2Y antagonism with either 10 or 100 microM reactive blue 2. However, a 10 microM concentration of suramin failed to alter outcome. P2X antagonism with 10 microM pyridoxal phosphate-6-azo-(benzene-2,4-disulfonic acid and P2X1-selective pyridoxal-alpha5-phosphate-6-phenylazo-4'-carboxylic acid (MRS2159) (30 microM) was ineffective. Data collectively support cardioprotection with low concentrations of UTP, and they are consistent with P2Y2 involvement. Endogenous nucleotides may also play a protective role, as evidenced by effects of P2 antagonists, although this warrants further investigation.

A capillary electrophoresis method for the characterization of ecto-nucleoside triphosphate diphosphohydrolases (NTPDases) and the analysis of inhibitors by in-capillary enzymatic microreaction

A capillary electrophoresis (CE) method for the characterization of recombinant NTPDases 1, 2, and 3, and for assaying NTPDase inhibitors has been developed performing the enzymatic reaction within the capillary. After hydrodynamic injection of plugs of substrate solution with or without inhibitor in reaction buffer, followed by a suspension of an enzyme-containing membrane preparation, and subsequent injection of another plug of substrate solution with or without inhibitor, the reaction took place close to the capillary inlet. After 5 min, the electrophoretic separation of the reaction products was initiated by applying a constant current of -60 muA. The method employing a polyacrylamide-coated capillary and reverse polarity mode provided baseline resolution of substrates and products within a short separation time of less than 7 min. A 50 mM phosphate buffer (pH 6.5) was used for the separations and the products were detected by their UV absorbance at 210 nm. The Michaelis-Menten constants (K (m)) for the recombinant rat NTPDases 1, 2, and 3 obtained with this method were consistent with previously reported data. The inhibition studies revealed pronounced differences in the potency of reactive blue 2, pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS), suramin, and N (6)-diethyl-beta,gamma-dibromomethylene-ATP (ARL67156) towards the NTPDase isoforms. Notably, ARL67156 does not inhibit all NTPDases, having only a minor inhibitory effect on NTPDase2. Dipyridamole is not an inhibitor of the NTPDase isoforms investigated. The new method is fast and accurate, it requires only tiny amounts of material (nanoliter scale), no sample pretreatment and can be fully automated; thus it is clearly superior to the current standard methods.

Purinergic P2Y6 receptors induce Ca2+ and CFTR dependent Cl- secretion in mouse trachea

In airways Cl- secretion is activated and Na+ absorption is inhibited when P2Y2 receptors are stimulated by ATP or UTP. Both nucleotides are subject to degradation to ADP and UDP by ecto-nucleotidases. Here we show that these metabolites change electrolyte transport by stimulation of P2Y6 receptors in mouse trachea. Immunohistochemistry confirmed luminal and basolateral expression of P2Y6 receptors. In Ussing chamber experiments luminal ADP, UDP or the P2Y6 receptor agonist INS48823 induced both transient and persistent increase in short circuit currents (ISC). Activation of ISC was inhibited by the P2Y6 receptor blocker PPADS. The transient response was inhibited by DIDS, whereas the persistent ISC was inhibited by glibenclamide and by the protein kinase A (PKA) blocker H-89. Moreover, sustained activation of ISC by luminal UDP was inhibited by blocking basolateral K+ channels with 293B. Possible effects of diphosphates on P2Y1 or adenosine receptors were excluded by the inhibitors MRS2179 and 8-SPT, respectively. Inhibition of amiloride sensitive Na+ absorption was only seen after blocking basolateral K+ channels with 293B. In contrast, Cl- secretion activated by basolateral ADP or UDP was only transient and was blocked by the sk4 K+ channel blocker clotrimazole. In summary, activation of luminal P2Y6 receptors in the airways shifts electrolyte transport towards secretion by increasing intracellular Ca+ and activation of PKA.

Characterization of the ATP-hydrolysing activity of alpha-sarcoglycan

Alpha-Sarcoglycan is a glycoprotein associated with the dystrophin complex at sarcolemma of skeletal and cardiac muscles. Gene defects in alpha-sarcoglycan lead to a severe muscular dystrophy whose molecular mechanisms are not yet clear. A first insight into the function of alpha-sarcoglycan was obtained by finding that it is an ATP-binding protein and that it probably confers ability to hydrolyse ATP to the purified dystrophin complex [Betto, Senter, Ceoldo, Tarricone, Biral and Salviati (1999) J. Biol. Chem. 274, 7907-7912]. In the present study, we present definitive evidence showing that alpha-sarcoglycan is an ATP-hydrolysing enzyme. The appearance of alpha-sarcoglycan protein expression was correlated with the increase in ecto-nucleotidase activity during differentiation of C2C12 cells. Approx. 25% of ecto-nucleotidase activity displayed by the C2C12 myotubes was inhibited by preincubating cells with an antibody specific for the ATP-binding motif of alpha-sarcoglycan. This demonstrates that alpha-sarcoglycan substantially contributes to total ecto-nucleotidase activity of C2C12 myotubes. To characterize further this activity, human embryonic kidney 293 cells were transfected with expression plasmids containing alpha-sarcoglycan cDNA. Transfected cells exhibited a significant increase in the ATP-hydrolysing activity that was abolished by the anti-alpha-sarcoglycan antibody. The enzyme had a substrate specificity for ATP and ADP, did not hydrolyse other triphosphonucleosides, and the affinity for ATP was in the low mM range. The ATPase activity strictly required the presence of both Mg2+ and Ca2+ and was completely inhibited by suramin and reactive blue-2. These results show that alpha-sarcoglycan is a Ca2+, Mg2+-ecto-ATPDase. The possible consequences of the absence of alpha-sarcoglycan activity in the pathogenesis of muscular dystrophy are discussed.

Methylene ATP analogs as modulators of extracellular ATP metabolism and accumulation

1 Transient accumulation of extracellular ATP reflects both release of ATP from intracellular stores and altered rates of ATP metabolism by ecto-enzymes. Ecto-nucleoside triphosphate diphosphohydrolases (eNTPDases) and ecto-nucleotide pyrophosphatases (eNPPs) degrade ATP, while ecto-nucleotide diphosphokinases (eNDPKs) synthesize ATP from ambient ADP. 2 Although the methylene ATP analogs betagamma-meATP and alphabeta-meATP are widely used as metabolically stable tools for the analysis of purinergic signaling, their specific effects on eNTPDase, eNPP, and eNDPK activities have not been defined. This study compared the actions of these analogs on extracellular ATP metabolism by human 1321N1 astrocytes, rat PC12 pheochomocytoma cells, and rat C6 glioma cells. 3 Both analogs significantly reduced clearance of extracellular ATP by 1321N1 cells that express both eNTPDases and eNPPs, as well as by C6 cells that exclusively express eNPPs. In contrast, both analogs were much less efficacious in inhibiting ATP clearance by PC12 cells that predominantly express eNTPDases. Betagamma-meATP, but not alphabeta-meATP, was effectively hydrolyzed by the 1321N1 and C6 cells; PC12 cells did not significantly degrade this analog. 4 Alphabeta-meATP, but not betagamma-meATP, acted as a substrate for purified yeast NDPK to generate ATP via trans-phosphorylation of ADP. alphabeta-meATP also acted as substrate for the eNDPK activities expressed by 1321N1, PC12, and C6 cells and thereby induced extracellular ATP accumulation in the presence of ambient or exogenously added ADP. 5 These results indicate that methylene ATP analogs exert complex and cell-specific effects on extracellular ATP metabolism that can significantly modify interpretation of studies that use these reagents as probes of purinergic signal transduction in intact tissues.

Identification and characterization of a cell-surface receptor, P2Y15, for AMP and adenosine

AMP and adenosine are found in all cell types and can be released by cells or created extracellularly from the breakdown of ATP and ADP. We have identified an orphan G protein-coupled receptor with homology to the P2Y family of nucleotide receptors that can respond to both AMP and adenosine. Based on its ability to functionally bind the nucleotide AMP, we have named it P2Y15. Upon stimulation, P2Y15 induces both Ca2+ mobilization and cyclic AMP generation, suggesting coupling to at least two different G proteins. It is highly expressed in mast cells and is found predominantly in the tissues of the respiratory tract and kidneys, which are known to be affected by AMP, adenosine, and adenosine antagonists. Until now, the effects of AMP have been thought to depend on its dephosphorylation to adenosine but we demonstrate here that P2Y15 is a bona fide AMP receptor by showing that it binds [(32)P]AMP. Because AMP and adenosine have bronchoconstrictive effects that can be inhibited by theophylline, we tested whether theophylline and other adenosine receptor antagonists can block P2Y15. We found inhibition at a theophylline concentration well within the therapeutic dose range, indicating that P2Y15 may be a clinically important target of this drug.

Effect of subchronic treatment with mercury chloride on NTPDase, 5'-nucleotidase and acetylcholinesterase from cerebral cortex of rats

The aim of the present investigation was to evaluate the effect of a subchronic treatment (30 days/30 doses) with subcutaneous injections (0.1 mg/kg) of HgCl2 on NTPDase (E.C. 3.6.1.5), 5'-nucleotidase (E.C 3.1.3.5) and acetylcholinesterase (AChE, E.C. 3.1.1.7) activities in brain from adult rats. NTPDase and 5'-nucleotidase were measured in cortical synaptosomal fraction and AChE was measured in the homogenate of cerebral cortex and hippocampus. After the subchronic treatment (30 days), NTPDase activity was enhanced approximately 35% (p < 0.05) with ATP and ADP as substrates and no difference was observed in 5'-nucleotidase activity (AMP hydrolysis). In addition, AChE activity was enhanced in the cerebral cortex (22%, p < 0.05) and hippocampus (26%, p < 0.05) after the subchronic treatment. Mercury deposited in brain was measured by cold vapor (atomic absorption spectrometry) and no difference between the control and the subchronically treated group was observed. Here we showed for the first time that exposure to low levels of Hg2+, which resembles occupational exposure to low levels of mercury, caused a marked increase in NTPDase and AChE activities. The relationship of these alterations with the neurotoxicity of inorganic mercury deserves further studies.

Extracellular ATP as a signaling molecule for epithelial cells

The charge of this invited review is to present a convincing case for the fact that cells release their ATP for physiological reasons. Many of our "purinergic" colleagues as well as ourselves have experienced resistance to this concept, because it is teleologically counter-intuitive. This review serves to integrate the three main tenets of extracellular ATP signaling: ATP release from cells, ATP receptors on cells, and ATP receptor-driven signaling within cells to affect cell or tissue physiology. First principles will be discussed in the Introduction concerning extracellular ATP signaling. All possible cellular mechanisms of ATP release will then be presented. Use of nucleotide and nucleoside scavengers as well as broad-specificity purinergic receptor antagonists will be presented as a method of detecting endogenous ATP release affecting a biological endpoint. Innovative methods of detecting released ATP by adapting luciferase detection reagents or by using "biosensors" will be presented. Because our laboratory has been primarily interested in epithelial cell physiology and pathophysiology for several years, the role of extracellular ATP in regulation of epithelial cell function will be the focus of this review. For ATP release to be physiologically relevant, receptors for ATP are required at the cell surface. The families of P2Y G protein-coupled receptors and ATP-gated P2X receptor channels will be introduced. Particular attention will be paid to P2X receptor channels that mediate the fast actions of extracellular ATP signaling, much like neurotransmitter-gated channels versus metabotropic heptahelical neurotransmitter receptors that couple to G proteins. Finally, fascinating biological paradigms in which extracellular ATP signaling has been implicated will be highlighted. It is the goal of this review to convert and attract new scientists into the exploding field of extracellular nucleotide signaling and to convince the reader that extracellular ATP is indeed a signaling molecule.

Soluble purine-converting enzymes circulate in human blood and regulate extracellular ATP level via counteracting pyrophosphatase and phosphotransfer reactions

Extracellular ATP and other purines play a crucial role in the vasculature, and their turnover is selectively governed by a network of ectoenzymes expressed both on endothelial and hematopoietic cells. By studying the whole pattern of purine metabolism in human serum, we revealed the existence of soluble enzymes capable of both inactivating and transphosphorylating circulating purines. Evidence for this was obtained by using independent assays, including chromatographic analyses with 3H-labeled and unlabeled nucleotides and adenosine, direct transfer of gamma-terminal phosphate from [gamma-32P]ATP to NDP/AMP, and bioluminescent measurement of ATP metabolism. Based on substrate-specificity and competitive studies, we identified three purine-inactivating enzymes in human serum, nucleotide pyrophosphatase (EC 3.6.1.9), 5'-nucleotidase (EC 3.1.3.5), and adenosine deaminase (EC 3.5.4.4), whereas an opposite ATP-generating pathway is represented by adenylate kinase (EC 2.7.4.3) and NDP kinase (EC 2.7.4.6). Comparative kinetic analysis revealed that the Vmax values for soluble nucleotide kinases significantly exceed those of counteracting nucleotidases, whereas the apparent Km values for serum enzymes were fairly comparable and varied within a range of 40-70 micro mol/l. Identification of soluble enzymes contributing, along with membrane-bound ectoenzymes, to the active cycling between circulating ATP and other purines provides a novel insight into the regulatory mechanisms of purine homeostasis in the blood.

In vitro effects of L-arginine and guanidino compounds on NTPDase1 and 5'-nucleotidase activities from rat brain synaptosomes

Tissue accumulation of arginine (Arg), N-acetylarginine (NA), argininic acid (AA) and homoarginine (HA) occurs in hyperargininemia, an inborn error of the urea cycle. In the present study, we investigated the in vitro effects of Arg, NA, AA and HA on NTPDase1 and 5'-nucleotidase activities from synaptosomal cerebral cortex of rats. The results showed that Arg enhances NTPDase1 activity at the high concentrations tested (1.5 and 3.0mM) for both the ATP and ADP nucleotides. Activation was also observed with other guanidino compounds tested: NA, AA and HA activated ATP and ADP hydrolysis in all experiments at the concentration of 25 microM. Besides this, NA and AA activated ATP hydrolysis at a lower concentration (1 microM). In another set of experiments, we verified the effect of Arg on purified apyrase at pH 8.0 and 6.5 and observed an increase in the enzyme activity at all Arg concentrations tested (0.01-3.0mM). In contrast, Arg and the other guanidino compounds tested did not alter 5'-nucleotidase activity. These results suggest that changes in nucleotide hydrolysis may be involved in the brain dysfunction caused by hyperargininemia amongst other potential pathophysiological mechanisms involved in this condition.

Involvement of P2Y receptors in the differentiation of haematopoietic cells

The effects of extracellular nucleotides are mediated by multiple P2X ionotropic receptors and G protein-coupled P2Y receptors. These receptors are ubiquitous, but few physiological roles have been firmly identified. In this review article, we present a survey of the functional expression of P2Y receptors in the different haematopoietic lineages by analyzing the selectivity of these cells for the various adenine and uracil nucleotides as well as the second messenger signaling pathways involved. The pharmacological profiles of metabotropic nucleotide receptors are different among myeloid, megakaryoid, erythroid, and lymphoid cells and change during differentiation. A role of P2Y receptors in the differentiation and maturation of blood cells has been proposed: In particular the P2Y(11)receptor seems to be involved in the granulocytic differentiation of promyelocytes and in the maturation of monocyte-derived dendritic cells. It is suggested that the role of P2Y receptors in the maturation of blood cells may be more important than believed so far.

The role of P2Y1 purinergic receptors and cytosolic Ca2+ in hypotonically activated osmolyte efflux from a rat hepatoma cell line

Exposure of HTC rat hepatoma cells to a 33% decrease in extracellular osmolality caused the cytosolic Ca(2+) concentration ([Ca(2+)](i)) to increase transiently by approximately 90 nm. This rise in [Ca(2+)](i) was inhibited strongly by apyrase, grade VII (which has a low ATP/ADPase ratio) but not by apyrase grade VI (which has a high ATP/ADPase ratio) or hexokinase, indicating that extracellular ADP and/or ATP play a role in the [Ca(2+)](i) increase. The hypotonically induced rise in [Ca(2+)](i) was prevented by the prior discharge of the intracellular Ca(2+) store of the cells by thapsigargin. Removal of extracellular Ca(2+) or inhibition of Ca(2+) influx by 1-10 microm Gd(3+) depleted the thapsigargin-sensitive Ca(2+) stores and thereby diminished the rise in [Ca(2+)](i). The hypotonically induced rise in [Ca(2+)](i) was prevented by adenosine 2'-phosphate-5'-phosphate (A2P5P) and pyridoxyl-5'-phosphate-6-azophenyl-2',4'-disulfonate, inhibitors of purinergic P2Y(1) receptors for which ADP is a major agonist. Both inhibitors also blocked the rise in [Ca(2+)](i) elicited by addition of ADP to cells in isotonic medium, whereas A2P5P had no effect on the rise in [Ca(2+)](i) elicited by the addition of the P2Y(2) and P2Y(4) receptor agonist, UTP. HTC cells were shown to express mRNA encoding for rat P2Y(1), P2Y(2), and P2Y(6) receptors. Inhibition of the hypotonically induced rise in [Ca(2+)](i) blocked hypotonically induced K(+) ((86)Rb(+)) efflux, modulated the hypotonically induced efflux of taurine, but had no significant effect on Cl(-) ((125)I-) efflux. The interaction of extracellular ATP and/or ADP with P2Y(1) purinergic receptors therefore plays a role in the response of HTC cells to osmotic swelling but does not account for activation of all the efflux pathways involved in the volume-regulatory response.

Purines and pyrimidines are not involved in NANC relaxant responses in the rabbit vaginal wall

1. Non-adrenergic non-cholinergic (NANC) relaxant responses were elicited by electrical field stimulation (EFS) in rabbit vaginal wall strips after treatment with guanethidine and scopolamine and raising smooth muscle tone with phenylephrine. Under these conditions treatment with NOS inhibitors revealed a non-nitrergic NANC relaxant response. The possible role of purines and pyrimidines in these non-nitrergic NANC responses was investigated. 2. Exogenous application of ATP, ADP, adenosine, UTP, or UDP (all at 0.03-10 mM) induced concentration-dependent relaxant responses. 3. Responses to exogenous application of ATP were reduced by the general P2 antagonist cibacron blue (500 micro M), but not by suramin (100 micro M) and were unaffected by L-NAME (500 micro M), omega-conotoxin GVIA (omega-CTX, 500 nM) or tetrodotoxin (TTX, 1 micro M). 4. Responses to exogenous application of adenosine were reduced by the A(2A) antagonist ZM-241385 (30 micro M). 5. ATP- and ADP-induced responses were unaffected by the G-protein inhibitor pertussis toxin (100 ng ml(-1)), whilst ADP- but not ATP-induced responses were reduced by GDPbetaS (100 micro M), which stabilizes G-proteins in their inactive state. 6. EFS-induced non-nitrergic NANC relaxant responses were unaffected by suramin, cibacron blue, ZM-241385, pertussis toxin or GDPbetaS, but were completely inhibited by TTX. 7. Exogenous application of ATP (10 mM) and adenosine (10 mM) increased intracellular cyclic adenosine-3', 5'-monophosphate (cAMP). However, non-nitrergic NANC responses were not associated with increased cAMP. Neither non-nitrergic NANC responses nor responses to ATP or adenosine were associated with increased intracellular cyclic guanosine-3', 5'-monophosphate (cGMP) concentrations. 8. These results suggest that adenosine A(2A) receptors and P2 receptors are present in the rabbit vaginal wall, but that they are not involved in non-nitrergic NANC relaxant responses.

Evidence that release of adenosine triphosphate from endothelial cells during increased shear stress is vesicular

In response to increased shear stress, vascular endothelial cells release adenosine triphosphate (ATP) by an unknown mechanism. We have investigated this mechanism using different approaches. First, we discovered that quinacrine, used to locate intracellular stores of ATP bound to peptides, displayed a granular fluorescence, typical of vesicular storage. Second, we found that two inhibitors of vesicular transport (monensin and N-ethylmaleimide) produced a highly significant reduction in the release of ATP from vascular endothelial cells in response to increased shear stress. Preliminary experiments using inhibitors of the cystic fibrosis transmembrane regulator, the sulfonylurea receptor, and the multidrug resistance protein showed no involvement of these ATP-binding cassette transporter proteins (previously characterized in endothelial cells) in the mechanism of release of ATP. We suggest, therefore, that the release of ATP from vascular endothelial cells, like that of nerve cells, is probably by vesicular exocytosis.