Presynaptic A1-purinoceptor-mediated inhibitory effects of adenosine and its stable analogues on the mouse hemidiaphragm preparation

A1 Adenosine Receptor-Mediated Inhibition of Parasympathetic Neuromuscular Transmission in Human and Murine Urinary Bladder

The potential role of A1 adenosine receptors in modulating neuromuscular transmission in the detrusor muscle of the urinary bladder has been tested in human and murine preparations with the intent to determine the viability of using adenosine receptor agonists as adjuncts to treat overactive bladder. In human detrusor muscle preparations, contractile responses to electrical field stimulation were inhibited by the selective A1 adenosine receptor agonists 2-chloro-N(6)-cyclopentyladenosine, N(6)-cyclopentyladenosine (CPA), and adenosine (rank order of potency: 2-chloro-N(6)-cyclopentyladenosine > CPA > adenosine). Pretreatment with 8-cyclopentyl-3-[3-[[4(fluorosulphonyl)benzoyl]oxy]propyl]-1-propylxanthine, an irreversible A1 antagonist, blocked the effects of CPA, thus confirming the role of A1 receptors in human detrusor preparations. In murine detrusor muscle preparations, contractions evoked by electrical field stimulation were reduced by CPA or adenosine. Amplitudes of the P2X purinoceptor-mediated excitatory junctional potentials (EJPs) recorded with intracellular microelectrodes were reduced in amplitude by CPA and adenosine with no effect on the spontaneous EJP amplitudes, confirming the prejunctional action of these agents. 8-Cyclopentyltheophylline, a selective A1 receptor antagonist, reversed the effects of CPA on EJP amplitudes with no effect of spontaneous EJPs, confirming the role of A1 receptors in mediating these effects.

Evidence for constitutively-active adenosine receptors at mammalian motor nerve endings

A study was made to determine if constitutively active adenosine receptors are present at mouse motor nerve endings. In preparations blocked by low Ca(2+)/high Mg(2+) solution, 8-cyclopentyl-1,3,dipropylxanthine (CPX, 10-100 nM), which has been reported to be both an A(1) adenosine receptor antagonist and inverse agonist, produced a dose-dependent increase in the number of acetylcholine quanta released by a nerve impulse. Adenosine deaminase, which degrades ambient adenosine into its inactive congener, inosine, failed to alter the response to 100 nM CPX. 8-Cyclopentyltheophylline (CPT, 3 μM), a competitive inhibitor at A(1) adenosine receptors, prevented the increase in acetylcholine release produced by CPX. At normal levels of acetylcholine release, neither adenosine deaminase nor CPX affected acetylcholine release at low frequencies of nerve stimulation in (+)-tubocurarine blocked preparations. The results suggest that a proportion of the acetylcholine release process is controlled by constitutively active adenosine receptors at murine motor nerve endings, providing the first evidence for constitutive activity of G-protein-coupled receptors that modulate the function of mammalian nerve endings.

Selective disruption of the mammalian secretory apparatus enhances or eliminates calcium current modulation in nerve endings

Modulation of secretion via G protein-coupled receptors (GPCRs) serves an important regulatory function in neuronal and nonneuronal secretory cells. Most secretory cells possess voltage-gated calcium channels, share homologues of the core complex of three proteins (the SNAREs) that constitute the secretory apparatus, and are modulated by GPCR activation. Activators of GPCRs generally inhibit the release of neurotransmitter substances to a maximum of only 50-60% of the control level, suggesting that complex protein-protein interactions may govern the efficacy of this form of modulation. In this article, molecular genetic approaches are used in combination with botulinum toxins (selective molecular scalpels that cleave the SNAREs at highly restricted loci) to address this issue. The results suggest that the cleavage of either of the plasma membrane SNAREs (syntaxin or SNAP-25) prevents modulation of calcium currents by A(1) adenosine receptors at mammalian motor nerve endings. In contrast, cleavage of the synaptic vesicle SNARE (synaptobrevin) in conjunction with deletion of the vesicle-docking protein Rab3A greatly enhances the efficacy of calcium current modulation.

P2 purinoceptors: historical perspective and classification

This article presents an overview that gives some historical perspective to the detailed papers at the cutting edge of P2 purinoceptor research that follow. I consider the proposal, first put forward by Abbracchio & Burnstock (Pharmacol Ther 64:445-475, 1994), that P2 purinoceptors should be regarded as members of two main families: a P2X purinoceptor family consisting of ligand-gated ion channels, and a P2Y purinoceptor family consisting of G protein-coupled receptors. The latest subclasses of these two families (P2X1-4 and P2Y1-5), identified largely on the basis of molecular cloning and expression, are tabled. Finally, I suggest some future directions for P2 purinoceptor research, including studies of the long-term (trophic) actions of purines, the evolution and development of purinoceptors and therapeutic applications.

Drugs to facilitate recovery of neuromuscular blockade and muscle strength

Several drugs that quicken recovery from neuromuscular blockade caused by vecuronium in anesthetized patients are reviewed. Ulinastatin, a protease inhibitor, is thought to promote the release of acetylcholine at the neuromuscular junction and increases hepatic blood flow and urine volume. For this reason, ulinastatin quickens recovery from neuromuscular blockade in anesthetized patients receiving vecuronium. Additionally, pretreatment with ulinastatin avoids prolongation of vecuronium-induced neuromuscular blockade in patients with hepatic cirrhosis. Gabexate mesilate is also a protease inhibitor. During a continuous infusion of gabexate mesilate, recovery from neuromuscular blockade was quickened. Amino acid-enriched solution supplies energy to the skeletal muscles and causes an increase in muscle strength. An infusion of amino acid-enriched solution hastens recovery from neuromuscular blockade in anesthetized patients. When amino acids supply energy to the skeletal muscles, they simultaneously produce heat in the skeletal muscles. This thermal generation may be closely related to fast recovery from neuromuscular blockade. Amino acid-enriched solution makes recovery from neuromuscular blockade quick and avoids hypothermia during general anesthesia. Milrinone, a phosphodiesterase III inhibitor, is supposed to increase the release of acetylcholine at the neuromuscular junction and make the neuromuscular junction sensitive to acetylcholine. Therefore, recovery from neuromuscular blockade is hastened. Nicorandil enhances membrane K+ conductance in skeletal muscle and increases contraction of the skeletal muscle. Thus, nicorandil quickens recovery from neuromuscular blockade.

Modulation of calcium currents is eliminated after cleavage of a strategic component of the mammalian secretory apparatus

Adenosine inhibits neurotransmitter secretion from motor nerves by an effect on the secretory apparatus in amphibia. In contrast, the inhibitory effect of adenosine is associated with decreases in calcium currents at mouse motor nerve endings. To determine if the action of adenosine in the mouse is mediated thorough a direct effect on calcium channels or through the secretory machinery, the effects of cleavage of the SNARE proteins on the action of adenosine were examined. Cleavage of the SNARE syntaxin with botulinum toxin type C (Botx/C) prevented the inhibitory effect of adenosine on nerve terminal calcium currents. Cleavage of the other SNAREs (synaptobrevin with Botx/D or SNAP-25 with Botx/A) failed to affect the inhibitory action of adenosine. The results provide evidence for an intimate coupling of nerve terminal calcium channels with a plasma membrane component of the SNARE complex, such that modulation of calcium currents by a G-protein coupled receptor cannot occur when syntaxin is cleaved.

Properties of presynaptic P2X7-like receptors at the neuromuscular junction

Adenosine triphosphate is released into the synaptic cleft of the neuromuscular junction during normal synaptic transmission, and in much greater quantities following injury and ischaemia. There is much data to suggest roles for presynaptic P2 receptors but little to demonstrate which specific receptor subunits are present. Here we show P2X7 receptor subunits on presynaptic motor nerve terminals from birth, but no evidence for P2X1, P2X2, P2X3, P2X4, P2X5 or P2X6 receptor subunits. Further, P2X receptor subunits are present as multimeric, membrane-inserted receptors. A selective agonist, 2'-3'-O-(4-benzoylbenzoyl)-adenosine 5'-triphosphate (BzATP: 100 microM), triggers vesicle release from motor nerve terminals, which is blocked by P2X7RS-specific concentrations of periodate oxidised ATP (OxATP: 100 microM) and brilliant blue G (BBG: 1 microM), but not by suramin (100 microM). Vesicle release is enhanced in the absence of extracellular divalent cations and occurs through activation of the ion channel and not any associated large pore, as we failed to label nerve terminals with large membrane-impermeant molecules after addition of BzATP. We conclude that a P2X7-like receptor is present at mouse motor nerve terminals, and that their activation promotes vesicle release.

Reversal of neuromuscular effects of adenosine by specific adenosine A1-receptor antagonist in live rats

Intravenous adenosine in-vivo was shown to potentiate the effects of non-depolarizing neuromuscular blocking agents. This study aimed to determine whether adenosine A1-receptors mediated this potentiation. The authors investigated the effects of intravenous adenosine, N6-cyclopentyladenosine, specific A1-receptor agonist, and 8-cyclopentyl-1,3-dipropylxanthine, specific A1-receptor antagonist, on neuromuscular block by vecuronium, in in-vivo rat sciatic nerve-tibialis anterior preparations. In the presence of 50% steady state block by vecuronium, adenosine, and N6-cyclopentyladenosine caused similar degree of depressions of twitch tension. Twitch tension returned to its pre-injection value more rapidly when 8-cyclopentyl-1,3-dipropylxanthine was given at the maximal block than when it was allowed to recover spontaneously. It was concluded that in in-vivo adenosine potentiated the neuromuscular effects of vecuronium through adenosine A1-receptors in rats.

The effect of adenosine triphosphate on vecuronium-induced neuromuscular block

Continuous IV adenosine triphosphate administration has been used during surgery in the expectation of analgesic and vasodilative effects. Because adenosine triphosphate inhibits neuromuscular transmission, we investigated whether the neuromuscular effect of vecuronium was enhanced by IV adenosine triphosphate in 29 patients randomly given either continuous IV adenosine triphosphate 0.1 mg.kg(-1).min(-1) or 0.9% NaCl when undergoing elective minor surgery. Anesthesia was induced and maintained with propofol. Neuromuscular monitoring was recorded from the adductor pollicis muscle using electromyography with train-of-four stimulation of the ulnar nerve. Vecuronium 25, 30, or 40 microg/kg was given and lag time, onset time, and maximum block were recorded. ED50 and ED95 values for each group were derived from least squares linear regression analysis. ED50 and ED95 values were 29 microg/kg and 44 microg/kg, respectively, for the adenosine triphosphate group and 26 microg/kg and 46 microg/kg, respectively, for the controls. Differences in lag time, onset time, and neuromuscular responses between the two groups were not statistically significant. A significantly larger number of patients in the adenosine triphosphate group showed hypotension (systolic blood pressure <80 mm Hg). Our results demonstrated that adenosine triphosphate 0.1 mg.kg(-1).min(-1) did not enhance the neuromuscular block induced by vecuronium.

Adenosine decreases both presynaptic calcium currents and neurotransmitter release at the mouse neuromuscular junction

A controversy currently exists as to the mechanism of action by which adenosine, an endogenous mediator of neurotransmitter depression, reduces the evoked release of the neurotransmitter acetylcholine (ACh) at the skeletal neuromuscular junction. Specifically, it is uncertain whether adenosine inhibits ACh release from mammalian motor nerve endings by reducing Ca(2+) calcium entry through voltage-gated calcium channels or, as is the case at amphibian motor nerve endings, by an effect downstream of Ca(2+) entry. In an attempt to address this controversy, the effects of adenosine on membrane ionic currents and neurotransmitter release were studied at neuromuscular junctions in adult mouse phrenic nerve hemidiaphragm preparations. In wild-type mice, adenosine (500 microm-1 mm) reduced prejunctional Ca(2+) currents simultaneously with a reduction in evoked ACh release. In Rab3A knockout mice, which have been shown to have an increased sensitivity to adenosine, the simultaneous reduction in Ca(2+) currents and ACh secretion occurred at significantly lower adenosine concentrations (< or = 50 microM). Measurements of nerve terminal Na(+) and K(+) currents made simultaneously with evoked ACh release demonstrated that the decreases in Ca(2+) currents were not attributable to changes in cation entry through voltage-gated Na(+) or K(+) channels. Furthermore, no effects of adenosine on residual ionic currents were observed when P/Q-type calcium channels were blocked by Cd(2+) or omega-agatoxin-IVA. The results demonstrate that inhibition of evoked neurotransmitter secretion by adenosine is associated with a reduction in Ca(2+) calcium entry through voltage-gated P/Q Ca(2+) channels at the mouse neuromuscular junction. Whilst it may be that adenosine inhibits ACh release by different mechanisms at amphibia and mammalian neuromuscular junctions, it is also possible that the secretory apparatus is more intimately coupled to the Ca(2+) channels in the mouse such that an effect on the secretory machinery is reflected as changes in Ca(2+) currents.

Effect of milrinone on vecuronium-induced neuromuscular block

We examined the effect of milrinone, a phosphodiesterase III inhibitor, on neuromuscular block induced by vecuronium. Thirty adult patients were randomly assigned to one of two equal groups: the milrinone group and the control group. Subjects in the milrinone group received an intravenous loading dose of milrinone 5 microg x kg-1x min-1 for 10 min, followed by an infusion at a rate of 0.5 microg x kg-1x min-1. Subjects in the control group received normal saline at a rate of 0.1 ml x kg-1 x h-1. Thirty minutes after the beginning of the infusion of milrinone, anaesthesia was induced with intravenous thiopental 4 mg x kg-1 and fentanyl 2 microg x kg-1, and was maintained with isoflurane in oxygen and nitrous oxide. Neuromuscular blockade was monitored electromyographically at the adductor pollicis muscle. The times from the administration of vecuronium 0.1 mg.kg-1 to the onset of neuromuscular block and the return of the first, second, third, and fourth response of the train-of-four were compared between the two groups. Times to the recovery of the ratio of the first twitch to the control twitch to 25%, 50% and 75%, and times to the recovery of train-of-four ratio to 25%, 50% and 75% were also compared between the two groups. The onset of neuromuscular block in the milrinone group was significantly slower than in the control group. The times to the returns of the four twitches of the train-of-four, times to recovery of the ratio of the first twitch to the control twitch to 25% and 50%, and the times to the recovery of the train-of-four ratio to 25% and 50% were significantly shorter in the milrinone group than in the control group. We conclude that milrinone delays the onset of neuromuscular blockade but hastens its recovery in anaesthetised patients receiving vecuronium.

Modulatory role of adenosine receptors in insect motor nerve terminals

The effects of adenosine and ATP were studied on blowfly larvae Calliphora vicina neuromuscular preparation. Adenosine diminished (IC50 = 40 +/- 3 microM) the amplitude of nerve-evoked postsynaptic currents (EPSCs) and slightly decreased the frequency of spontaneous currents without affecting their amplitude. EPSCs were slightly reduced by ATP, and this effect was prevented by concanavalin A. Presynaptic inhibition by adenosine was temperature-dependent and insensitive to pertussis toxin. A1 agonists of vertebrate adenosine receptor CPA and NECA failed to reproduce the effect of adenosine, and 2-CADO enhanced the EPSCs. A1 antagonist DPCPX competitively inhibited adenosine action. A2 agonist DPMA potentiated EPSCs, and its effect was abolished by A2 antagonist DMPX. Adenosine and ATP failed to affect the nonquantal release of glutamate. The results show for the first time the presence of presynaptic adenosine receptors regulating transmitter release at insect motor nerve terminals and point to differences in pharmacological properties of adenosine receptor subtypes in insects and vertebrates.

Bisindole alkaloids from Strychnos guianensis are effective antagonists of nicotinic acetylcholine receptors in cultured human TE671 cells

Several mono- and bisindole quaternary alkaloids isolated from the stem bark of Strychnos guianensis have recently been shown to be effective blockers of neuromuscular transmission in mice. In this study, we used a human clonal cell line (TE671) expressing muscle-type nicotinic acetylcholine receptors. The agonist carbamylcholine activated a receptor-mediated (86)Rb(+) efflux and this activation was antagonized by the indole alkaloids, the most active being bisindole bisquaternary compounds. The most effective antagonist, guiachrysine, had an IC(50) around 0.43 microM in the presence of 0.5 mM carbamylcholine, compared to 0.16 microM for d-tubocurarine, the most potent curarizing alkaloid. Guiaflavine and 5',6'-dehydroguiaflavine were slightly less effective. Monoindole compounds were 10 to 100 times less potent than bisindole alkaloids. Kinetic analysis showed that the inhibition of the carbamylcholine-dependent (86)Rb(+) efflux by guiaflavine was of mixed competitive and uncompetitive type. The competitive component (K(I)=0.21 microM) is presumably due to binding at the acetylcholine site, while the uncompetitive component (K'(I)=0.92 microM) may be due to open channel block.

Inhibition of spontaneous acetylcholine secretion by 2-chloroadenosine as revealed by a protein kinase inhibitor at the mouse neuromuscular junction

1. Previous studies have reported discrepancies in the potencies of A(1) adenosine receptor agonists at mouse motor nerve terminals. In addition, conflicting results on the role of protein kinase A (PKA) in mediating the inhibitory effects of A(1) receptor agonists have been published. We thus decided to investigate the possibility of endogenous control of adenosine receptor sensitivity by protein kinases, using a variety of protein kinase inhibitors in conjunction with the adenosine receptor agonist 2-chloroadenosine (CADO). 2. CADO, at the concentration employed previously to study spontaneous ACh release in the mouse (1 microM), did not inhibit spontaneous ACh release in our experiments. However, a higher concentration of CADO (10 microM) produced highly statistically-significant reductions in spontaneous ACh release. 3. In the presence of the non-selective protein kinase inhibitor, H7 (50 microM), the potency of CADO was increased such that 1 microM CADO now reduced spontaneous quantal ACh release to approximately 63% of control. 4. Both H7, and the selective PKA inhibitor, KT5720 (500 nM) prevented increases in ACh release produced by CPT cyclic AMP (250 microM), suggesting these kinase inhibitors were blocking PKA. In contrast to H7, however, KT5720, did not reveal an inhibitory effect of 1 microM CADO. A number of other non-selective PKA inhibitors also failed to increase the potency of CADO. 5. The results suggest that an endogenous H7-sensitive process modulates the sensitivity of the mouse A(1) adenosine receptor and that the inhibitory effects of CADO are independent of cyclic AMP accumulation or PKA inhibition.

Activation of presynaptic A1-receptors by endogenous adenosine inhibits acetylcholine release in the guinea-pig ileum

1. It is well established that presynaptic adenosine A1-receptor activation inhibits acetylcholine (ACh) release in the guinea-pig ileum. The present study extends this observation and examines a possible role for endogenous adenosine in modulating cholinergic nerve function. 2. The actions of the adenosine uptake blocker, dipyridamole, the adenosine deaminase inhibitor, erythro-9(2-hydroxy-3-nonyl)adenine (EHNA) and the A1-receptor antagonist, 1,3-dipropyl-8-cyclopentylxanthine (DPCPX) were examined on electrically evoked neurogenic, cholinergic twitch contractions of the guinea-pig ileum. Some additional studies measuring [3H]-ACh release were also performed. 3. Adenosine and the selective A1-receptor agonist, 2-chloroadenosine (2-CA), inhibited electrically evoked contractions and, in the case of 2-CA, [3H]-ACh release. The actions were antagonized by DPCPX. At low concentrations, dipyridamole and EHNA enhanced the effect of adenosine causing a leftward shift of the concentration-response curve. In contrast, inhibition induced by 2-CA was unaffected by either dipyridamole or EHNA. 4. When applied alone at higher concentrations, EHNA and dipyridamole produced a concentration-dependent suppression of cholinergic neurotransmission. In both cases, the effect could be reversed by DPCPX. At the same concentration, DPCPX alone produced a small but consistent increase in twitch height and [3H]-ACh release. 5. The data confirm the existence of inhibitory presynaptic adenosine A1-receptors modulating cholinergic nerve function in the guinea-pig ileum and suggests that these receptors can be activated by endogenous adenosine released either as adenosine itself or as an ATP metabolite.

Stimulation-dependent release, breakdown, and action of endogenous ATP in mouse hemidiaphragm preparation: the possible role of ATP in neuromuscular transmission

In this study the in vitro mouse phrenic nerve- hemidiaphragm preparation was utilized to study the release and extracellular catabolism of endogenous ATP and its action on the postsynaptic site, i.e. on the contraction force evoked by nerve stimulation. ATP, measured by the luciferin-luciferase assay, was released stimulation-dependently from the mouse hemidiaphragm in response to electrical field stimulation at 10 Hz. Blockade of the Na(+) channel activity by tetrodotoxin inhibited the majority of the release of ATP in response to stimulation, showing that it is related to neuronal activity. The nicotinic receptor antagonists d-tubocurarine, and alpha-bungarotoxin and cooling the bath temperature to 7 degrees C also reduced stimulation-induced ATP outflow, suggesting that nicotinic receptors are responsible for the part of the release of ATP that is released from postsynaptic sites in a carrier-mediated manner. Exogenous ATP (20-500 microM) added to the bath was degraded to ADP and AMP by the action of ectoATPase and ectoATPdiphosphohydrolase; the K(m) and v(max) values of these enzymes were 185.8 microM and 55.16 nmol/min.g respectively. However, the total amount of nucleotides ([ATP+ADP+AMP]) was increased after the addition of ATP, indicating that ATP itself promoted further adenine nucleotide release. Twitch contractions of the rat hemidiaphragm preparation evoked by low frequency electrical stimulation was blocked concentration-dependently by the non-depolarizing muscle relaxants d-tubocurarine and pancuronium. Suramin (100 microM-1 mM) reversed neuromuscular blockade by d-tubocurarine and pancuronium; i.e., it shifted their concentration-response curves to the right Taken together our data, that endogenous ATP is released by stimulation and subsequently catabolized in the hemidiaphragm preparation and that suramin inhibits ecto-ATPase activity could be interpreted as meaning that suramin prolongs the action of endogenous ATP to elicit twitch contraction, which points to a new, undefined role of ATP in neuromuscular transmission. The source of ATP is partly postsynaptic, released from the muscle in response to activation of nicotinic ACh receptors expressed on the muscle.

Evaluation of intrinsic modulation of synaptic transmission by ATP in mouse fast twitch muscle

This study aims to evaluate whether endogenous ATP or adenosine modulates the neurotransmission and contractile function of mouse phrenic nerve-diaphragm. Bath application of ATP (1 mM) and alpha, beta-methylene ATP (m-ATP, 0.1 mM) elevated muscle tones, depressed contractions (approximately 12%), and depolarized muscle membranes (approximately 20 mV). Adenosine (1 mM) or low concentrations of ATP (0.1 mM) had no effect. In a low Ca2+ media, ATP caused prolonged inhibitions of endplate potentials (EPPs), whereas m-ATP augmented EPPs while both agents produced slight effects in normal Tyrode solution. When applied by puff ejection, ATP and m-ATP additionally elicited fast transient suppressions of EPPs in association with inhibitions of high K+-evoked releases of miniature EPPs. Blockades of P2 purinoceptors with suramin antagonized all the effects of ATP and m-ATP except the prolonged inhibitions of EPPs induced by ATP, which were antagonized instead by 8-cyclopentyl-1,3-dipropylxanthine (CPDPX), an A1 adenosine receptor antagonist. Suramin and CPDPX did not change contractions nor alter EPPs evoked by a low- or high-frequency nerve stimulation. The results indicate that exogenously applied ATP and m-ATP, via activations of distinct pre- and postsynaptic purinoceptors, exert inhibitory and facilitatory pharmacological modulations on the mature neuromuscular junction. However, because of intrinsic high efficiency of the synaptic transmission under physiological conditions, endogenously released ATP and its degradation product-adenosine-do not build up to concentrations high enough to alter motor functions.

Age-dependent changes of presynaptic neuromodulation via A1-adenosine receptors in rat hippocampal slices

The presynaptic neuromodulation of stimulation-evoked release of [3H]-acetylcholine by endogenous adenosine, via A1-adenosine receptors, was studied in superfused hippocampal slices taken from 4-, 12- and 24-month-old rats. 8-Cyclopentyl-1,3-dimethylxanthine (0.25 microM), a selective A1-receptor antagonist, increased significantly the electrical field stimulation-induced release of [3H]-acetylcholine in slices prepared from 4- and 12-month-old rats, showing a tonic inhibitory action of endogenous adenosine via stimulation of presynaptic A1-adenosine receptors. In contrast, 8-cyclopentyl-1,3-dimethylxanthine had no effect in 24-month-old rats. 2-Chloroadenosine (10 microM), an adenosine receptor agonist decreased the release of [3H]-acetylcholine in slices taken from 4- and 12-month-old rats, and no significant change was observed in slices taken from 24-month-old rats. In order to show whether the number/or affinity of the A1-receptors was affected in aged rats, [3H]-8-cyclopentyl-1,3-dimethylxanthine binding was studied in hippocampal membranes prepared from rats of different ages. Whereas the Bmax value was significantly lower in 2-year-old rats than in younger counterparts, the dissociation constant (Kd) was not affected by aging, indicating that the density rather than the affinity of adenosine receptors was altered. Endogenous adenosine levels present in the extracellular space were also measured in the superfusate by high performance liquid chromatography (HPLC) coupled with ultraviolet detection, and an age-related increase in the adenosine level was found. In summary, our results indicate that during aging the level of adenosine in the extracellular fluid is increased in the hippocampus. There is a downregulation and reduced responsiveness of presynaptic adenosine A1-receptors, and it seems likely that these changes are due to the enhanced adenosine level in the extracellular space.

Modulation by presynaptic adenosine A1 receptors of nicotinic receptor antagonist-induced neuromuscular block in the mouse

We have investigated how altering the activation of adenosine A1 receptors modifies nicotinic receptor antagonist-induced fade of tetanic contractions in the mouse isolated hemi-diaphragm. Vecuronium-induced tetanic fade was attenuated by an adenosine A1 receptor antagonist (8-cyclopentyl-1,3-dipropylxanthine, DPCPX, 10(-7) M) and by an inhibitor of the synthesis of extracellular adenosine from ATP (alpha,beta-methylene ADP, MeADP, 5 x 10(-5) M). Conversely, vecuronium-induced tetanic fade was potentiated by an adenosine A1 receptor agonist (N6-cyclohexyladenosine, CHA, 10(-7) M) and an inhibitor of the extracellular destruction of adenosine (erythro-9-[2-hydroxy-3-nonyl]adenine, EHNA, 10(-4) M). The ability of an adenosine A1 receptor antagonist to attenuate vecuronium-induced tetanic fade indicates that a component of this fade is due to endogenous adenosine. Further, the ability of the inhibitor of adenosine synthesis to attenuate vecuronium-induced tetanic fade indicates that this endogenous adenosine is derived from ATP. Hexamethonium-induced tetanic fade was also potentiated by increasing adenosine A1 receptor activation, albeit with a higher concentration of CHA (10(-4) M). However, unlike for vecuronium, hexamethonium-induced tetanic fade was not attenuated by reducing adenosine A receptor activation. This latter observation suggests that the tetanic fade produced by hexamethonium and vecuronium does not share a common mechanism of action.

The effects of purine compounds on the isolated aorta of the frog Rana temporaria

1. In the isolated aorta of the frog, Rana temporaria, adenosine concentration-dependently, endothelium-independently relaxed adrenaline pre-constricted vessels. None of the adenosine analogues including D-5'-(N-ethylcarboxamide) adenosine (NECA), R- and S-N6-(2-phenylisopropyl) adenosine (R-and S-PIA) and 2-chloroadenosine (2-CA), or the more selective A1, A2 and A3 agonists cyclopentyladenosine (CPA), CGS 21680 and N6-(3-iodobenzyl) adenosine-5'-N-methylcarboxamide (IB-MECA) respectively, had any effect. 2. The non-selective adenosine antagonist, 8-p-sulphophenyl-theophylline (8-pSPT; 30 microM) failed to inhibit adenosine relaxations, as did NG-nitro-L-arginine methyl ester (L-NAME; 0.1 mM) and indomethacin (30 microM). 3. Adenosine 5'-triphosphate (ATP), alpha, beta-methylene ATP (alpha, beta-MeATP), beta, gamma-methylene ATP (beta, gamma-MeATP), 2-methylthio ATP (2-MeSATP) and uridine 5'-triphosphate (UTP) all concentration-dependently contracted the frog aorta. ATP and alpha, beta-MeATP were equipotent and more potent than UTP and beta, gamma-MeATP; 2-MeSATP had little activity. 4. The P2-purinoceptor antagonist, suramin (0.1 mM) inhibited contractions to alpha, beta-MeATP but not to ATP. Pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS; 30 microM) also inhibited contractions to alpha, beta-MeATP but not to ATP. Contractions to ATP were, however, inhibited by indomethacin (30 microM). 5. In conclusion, in the frog aorta there appears to be a novel subclass of P1-purinoceptor mediating vasodilatation, although like the A3 subclass it is not blocked by methylxanthines; a P2-purinoceptor mediates vasconstriction which resembles a P2x subtype, based on the agonist potency of alpha, beta-MeATP being more potent than 2-MeSATP (UTP has moderate activity) and PPADS is an effective antagonist. There is no evidence for the presence of a P2y-purinoceptor, mediating vasodilatation, in this preparation.

A1-receptor-mediated effect of adenosine on the release of acetylcholine from the myenteric plexus: role and localization of ecto-ATPase and 5'-nucleotidase

No attempt has been made so far to classify the subtypes of presynaptic inhibitory adenosine receptors located in the myenteric plexus and to localize ecto-ATPase and 5'-nucleotidase in the intestine. The release of [3H]acetylcholine and smooth muscle responses to acetylcholine were measured and the effect of selective adenosine receptor ligands was studied using field-stimulated isolated longitudinal muscle strips of guinea-pig ileum. Release of ATP and its hydrolysis rate were also measured using the luciferin-luciferase technique. A histochemical method combined with electron microscopy was used for localization of ecto-ATPase and 5'-nucleotidase, enzymes responsible for destruction of extracellular ATP, ADP and AMP. Subtype-selective A1-receptor agonists and antagonists inhibited and enhanced, respectively, the release of acetylcholine associated with neuronal activity. A significant amount of ATP was released in response to electrical stimulation and administration of carbamylcholine. The release of ATP was inhibited by atropine and 4-diphenylacetoxy-N-methylpiperidine methiodide, an M3-receptor antagonist. Hydrolysis of ATP was rapid and resulted in an accumulation of extracellular adenosine involved in presynaptic A1-receptor-mediated inhibition of acetylcholine release. While the inhibitory effect of adenosine and ATP was significantly potentiated by dipyridamol, an adenosine uptake blocker, that of 2-ms ATP was not. The effect of ATP was not competitively antagonized by 8-cyclopentyl-1,3-dipropylxanthine, a selective A1-receptor antagonist. In conclusion, axon terminals of cholinergic interneurons are equipped with inhibitory A1- and P2 gamma-receptors. Therefore, both adenosine and ATP control the release of acetylcholine through these receptors. ATP is mainly released from the smooth muscle in response to stimulation of M3-muscarinic receptors by endogenous acetylcholine (cascade transmission [Vizi E. S. et al. (1992) Neuroscience 50, 455-465]) and is rapidly hydrolysed by ecto-ATPase localized on the surface of the smooth muscle and axon terminals producing ADP and AMP, and by 5'-nucleotidase present only on the surface of smooth muscle cells producing adenosine.

Ability of some K+ channel blockers to reverse inhibition of electrically evoked contractions of longitudinal muscle-myenteric plexus

Blockers selective for different potassium (K+) channels were examined for their ability to reverse inhibition of electrically evoked contractions of longitudinal muscle-myenteric plexus (lm-mp) by adenosine analogs. Cyclohexyl adenosine (CHA) was selected for these studies, since it effectively inhibited contraction (EC50 33 nM). 4-aminopyridine (4-AP) antagonized the inhibition by the adenosine analog, but also stimulated contraction by itself. alpha- and gamma-dendrotoxin produced the most profound reversal of CHA-induced inhibition, while producing a minimal contraction alone. Other blockers produced only nominal reversal of the CHA-induced inhibition. These results suggest that inhibition by CHA is mediated via activation of an alpha- and gamma-dendrotoxin-sensitive K+ channel.

1,3-Dipropyl-8-cyclopentyl xanthine (DPCPX): a useful tool for pharmacologists and physiologists?

There is now ample evidence in the literature to demonstrate the selectivity of action of DPCPX for adenosine A1 vs other adenosine receptor types in tissues derived from a wide range of species. However, care has to be exercised to ensure that its physiochemical properties do not result in the production of quantitatively misleading data. In experiments using canine tissues the still limited data available in the literature clearly and consistently demonstrate that DPCPX has a lower affinity than expected in preparations which would be anticipated to contain A1 receptors. A range of in vitro experiments also demonstrate that DPCPX is not always a "neutral" or "silent" antagonist. The mechanism underlying these additional effects is unclear, but may result from an ability of the compound to disrupt the normal interaction of the A1 receptor with Gi, or may be indicative of a lack of specificity of action. The limited evidence available suggests that the compound retains its selectivity and specificity of action in vivo, and early work indicates that the compound is proving to be a useful tool with which to explore the potential of activation of adenosine A1 receptors as an important mechanism in physiological and pathophysiological processes.