Corelease of noradrenaline and ATP by brief pulse trains in guinea-pig vas deferens

ATP as a cotransmitter in sympathetic and parasympathetic nerves - another Burnstock legacy

Geoff Burnstock created an outstanding scientific legacy that includes identification of adenosine 5'-triphosphate (ATP) as an inhibitory neurotransmitter in the gut, the discovery and characterisation of a large family of purine and uridine nucleotide-sensitive ionotropic P2X and metabotropic P2Y receptors and the demonstration that ATP is as an excitatory cotransmitter in autonomic nerves. The evidence for cotransmission includes that: 1) ATP is costored with noradrenaline in synaptic vesicles in postganglionic sympathetic nerves innervating smooth muscle tissues, including the vas deferens and most arteries. 2) When coreleased with noradrenaline, ATP acts at postjunctional P2X1 receptors to elicit depolarisation, Ca²⁺ influx, Ca²⁺ sensitisation and contraction. 3) ATP is also coreleased with acetylcholine from postganglionic parasympathetic nerves innervating the urinary bladder, where it stimulates postjunctional P2X1 receptors, and a second, as yet unidentified site to evoke contraction of detrusor smooth muscle. In both systems membrane-bound ecto-enzymes and soluble nucleotidases released from postganglionic nerves dephosphorylate ATP and so terminate its neurotransmitter actions. Currently, the most promising potential area of therapeutic application relating to cotransmission is treatment of dysfunctional urinary bladder. This family of disorders is associated with the appearance of a purinergic component of neurogenic contractions. This component is an attractive target for drug development and targeting it may be a rewarding area of research.

ATP as a cotransmitter in the autonomic nervous system

The role of adenosine 5'-triphosphate (ATP) as a major intracellular energy source is well-established. In addition, ATP and related nucleotides have widespread extracellular actions via the ionotropic P2X (ligand-gated cation channels) and metabotropic P2Y (G protein-coupled) receptors. Numerous experimental techniques, including myography, electrophysiology and biochemical measurement of neurotransmitter release, have been used to show that ATP has several major roles as a neurotransmitter in peripheral nerves. When released from enteric nerves of the gastrointestinal tract it acts as an inhibitory neurotransmitter, mediating descending muscle relaxation during peristalsis. ATP is also an excitatory cotransmitter in autonomic nerves; 1) It is costored with noradrenaline in synaptic vesicles in postganglionic sympathetic nerves innervating smooth muscle preparations, such as the vas deferens and most arteries. When coreleased with noradrenaline, ATP acts at postjunctional P2X1 receptors to evoke depolarisation, Ca(2+) influx, Ca(2+) sensitisation and contraction. 2) ATP is also coreleased with acetylcholine from postganglionic parasympathetic nerves innervating the urinary bladder and again acts at postjunctional P2X1 receptors, and possibly also a P2X1+4 heteromer, to elicit smooth muscle contraction. In both cases the neurotransmitter actions of ATP are terminated by dephosphorylation by extracellular, membrane-bound enzymes and soluble nucleotidases released from postganglionic nerves. There are indications of an increased contribution of ATP to control of blood pressure in hypertension, but further research is needed to clarify this possibility. More promising is the upregulation of P2X receptors in dysfunctional bladder, including interstitial cystitis, idiopathic detrusor instability and overactive bladder syndrome. Consequently, these roles of ATP are of great therapeutic interest and are increasingly being targeted by pharmaceutical companies.

ATP release and its prejunctional modulation

We studied some properties of the release of noradrenaline and ATP in isolated sympathetically innervated tissues. Release was elicited by electric stimulation and assessed as overflow of tritiated compounds (after labelling with [3H]noradrenaline) and enzymically measured ATP, respectively. Evans blue, which inhibits ectonucleotidases, greatly increased the evoked overflow of ATP, indicating that a major part of the ATP was metabolized after release. Much of the ATP was postjunctional in origin. The neural fraction was isolated when postjunctional release was suppressed by prazosin (alpha 1-adrenoceptor antagonist) and suramin (P2 purinoceptor antagonist). Comparison of neural ATP and [3H]-noradrenaline release showed that prostaglandin E2 reduced the release of both co-transmitters to a similar extent. Activation of prejunctional alpha 2-adrenoceptors, however, preferentially reduced the release of [3H]noradrenaline, and activation of prejunctional A1 purinoceptors reduced preferentially the release of ATP. Nucleotides such as ATP depressed the release of [3H]noradrenaline through two receptors: the well-known prejunctional A1 receptors and a separate group of prejunctional P2 purinoceptors. P2 antagonists increased the release of [3H]-noradrenaline. Overall, the results indicate differential storage, release and modulation of release of the two sympathetic co-transmitters. They also indicate that postganglionic sympathetic axons possess receptors for both co-transmitters: alpha 2 and P2 autoreceptors.

Possible role of sildenafil in inhibiting rat vas deferens contractions by influencing the purinergic system

AIM

To evaluate the effect of sildenafil, a selective inhibitor of cyclic guanosine monophosphate (cGMP)-selective type 5 phosphodiesterase, on isolated rat vas deferens and its connections with the purinergic system.

METHODS

Epididymal and prostatic portions of isolated vas deferens were placed in organ baths containing Krebs' solution. Contractions were induced by noradrenaline (NA), adenosine triphosphate (ATP), alpha,beta-methylene ATP and electrical field stimulation (EFS). The effect of sildenafil on the contractions was compared with suramin and Evans blue (EB).

RESULTS

NA, ATP, alpha,beta-methylene ATP and EFS caused contractions in both portions of vas deferens. NA-induced contractions were unaffected by sildenafil and suramin but potentiated by EB. ATP-induced contractions were non-competitively inhibited in both portions by sildenafil and suramin but potentiated by EB. alpha,beta-methylene ATP-induced contractions were unaffected by sildenafil but were inhibited in both portions by suramin and EB. EFS-induced contractions were inhibited by sildenafil and suramin while potentiated by EB.

CONCLUSION

Sildenafil inhibited the contractions in both portions of vas deferens, as did suramin. We have suggested that purinergic system has a role in this antagonism and it seems to be mediated by an ATP-dependent mechanism instead of a receptor interaction.

Novel dual 'small' vesicle model of ATP- and noradrenaline-mediated sympathetic neuromuscular transmission

The main question asked was if sympathetic nerves in guinea-pig vas deferens release the co-transmitters ATP and noradrenaline from the same, or different vesicles, i.e. in fixed combinations or independently. The extracellularly recorded excitatory junction current (EJC) and the fractional increase in overflow of tritium (delta T) were used to monitor the per pulse secretion of ATP and [3H]NA, respectively, during electrical stimulation with 1-3000 pulses at 0.1-40 Hz. The frequency- and train length-dependence and alpha 2-adrenoceptor-mediated autoinhibition of these parameters, and of the ATP-mediated twitch contraction, were compared first in the presence of cocaine (to block noradrenaline reuptake), then after brief exposure to phenoxybenzamine (PBA, to irreversibly 'destroy' alpha 2-autoreceptors). Parallel variations of EJC/p(ulse) and delta T/p(ulse) under all conditions would support, non-parallel variations argue against exocytosis of ATP and noradrenaline from the same vesicles. The main findings were that facilitation and alpha 2-autoinhibition of EJC/p and delta T/p were remarkably similar during stimulation at 2 Hz but increasingly dissimilar at higher frequencies. delta T/p remained strongly facilitated and tightly controlled by activation of alpha 2-autoreceptors at 10-40 Hz, but both the facilitation and the sensitivity to alpha 2-autoinhibition of EJC/p were inversely related to frequency. At 40 Hz EJCs were 'small', minimally facilitated and totally unaffected by cocaine or PBA, i.e. insensitive to alpha 2-autoinhibition. Nevertheless, activation of alpha 2-receptors during the 40 Hz train strongly restricted the 'post-tetanic augmentation' (PTA) of the first EJC 10 s after the tetanus. Comparison between the frequency dependence of EJCs and the twitch contraction in the presence of cocaine or after PBA treatment indicates that it is the 'summed EJC per second', i.e. the ATP-driven current injection per unit time into smooth muscle, which triggers the twitch. The working hypothesis is proposed that these nerves use two classes of 'small vesicles' (SVs) to store and release either 'big' or 'small' ATP and noradrenaline 'quanta', and that differences in properties (Ca2+ affinity, capacity) of Ca2+ receptors in the SV membranes enable the nerves to selectively secrete 'big quanta' at low frequency and 'small quanta' during trains at high frequency.

Extracellular ATP-induced apoptosis in PC12 cells

Studies in our laboratory indicate that extracellular ATP (ATP)o may induce cell death by reactive oxygen insults. We have also shown that the Ca(2+)-induced oxidative stress as elicited by ATP may lead to an activation of a specific AP-1 activity. Since early impairment of mitochondria constitutes a critical event of the apoptotic cell death, we have examined whether (ATP)o will affect mitochondrial damage and cell injury by using mitochondrial specific probes, dihydrorhodamine and 3-(4,5-dimethylthiazo-2-yl)-2,5-diphenyl tetrazolium bromide (MTT). We have found that (ATP)o induced cell death in a concentration dependent manner by MTT assay. The (ATP)o induced cell death correlated well with the reactive oxygen species (ROS) generation in mitochondria, since (ATP)o enhanced both cell death and ROS production and antioxidant blocked both of these processes. We found (ATP)o treatment led to apoptotic cell death by examining DNA laddering and the TUNEL assay. Interestingly, vitamin C and vitamin E combined treatment appeared to attenuate the (ATP)o-induced apoptosis. Results indicated that (ATP)o may cause oxidative damage of mitochondria leading to apoptotic cell death. Antioxidants may be useful in preventing apoptosis by preventing ROS formation in mitochondria.

Effects of heptanol on the neurogenic and myogenic contractions of the guinea-pig vas deferens

1. The effects of the putative gap junction uncoupler, 1-heptanol, on the neurogenic and myogenic contractile responses of guinea-pig vas deferens were studied in vitro. 2. Superfusion of 2.0 mM heptanol for 20-30 min produced the following reversible changes in the biphasic neurogenic contractile response (8 trials): (i) suppression of both phases; (ii) delayed development of both the first as well as the second phase, accompanied by complete temporal separation of the two phases; (iii) prominent oscillations of force during the second (noradrenergic) phase only. 3. To eliminate prejunctional effects of heptanol, myogenic contractions were evoked by field stimulation of the vas in the presence of suramin (200 microM) and prazosin (1 microM). Heptanol (2.0 mM) abolished these contractions reversibly. 4. These results show that (i) heptanol inhibits both excitatory junction potential (EJP)-dependent and non EJP-dependent contractions of the vas; (ii) a postjunctional site of action of heptanol, probably intercellular uncoupling of smooth muscle cells, contributes to the inhibition of contraction.

Activation of transcription factor AP-1 by extracellular ATP in PC12 cells

We have previously shown that extracellular ATP caused cell death in PC12 cells through activation of its receptors. Oxidative stress has been implicated as a mechanism of cell death caused by extracellular ATP. In the present study we examined the possible signal transduction cascades leading to cell death by extracellular ATP. We found, using the electrophoretic mobility shift assay, that transcription factor AP-1 DNA binding activity was stimulated by extracellular ATP. Northern blot analysis showed that mRNA levels of c-fos, c-jun were elevated after treatment with ATP. The stimulation was receptor mediated, since it was blocked by the ATP receptor antagonist, suramin. The stimulated AP-1 binding was also blocked by the antioxidant N-acetyl-L-cysteine, indicating that reactive oxygen species generated following ATP stimulation were involved in the induction of AP-1 activity. It appears that both translational and posttranslational events contributed to the increased AP-1 DNA binding since cyclohexamide (a protein synthesis inhibitor), genistein (tyrosine kinase inhibitor) and staurosporine (PKC inhibitor) each partially blocked the AP-1 activation. Changes in AP-1 DNA binding activity may modulate expression of target genes involved in cell death pathways.

Beta-adrenoceptor-mediated inhibition of alpha 1-adrenoceptor-mediated and field stimulation-induced contractile responses in the prostate of the guinea pig

1. The prostate of the guinea pig responds to electrical field-stimulation (2 s trains, 0.1 ms pulses at 3-60 Hz, supramaximal voltage) with contractile responses. At 18 Hz these responses were inhibited (82 +/- 2%) by the L-type Ca2+ channel blocker, nifedipine (10 microM) and (by 100%) by the neurotoxin, tetrodotoxin (500 nM). The alpha 1A-selective adrenoceptor antagonist, 5-methylurapidil, inhibited responses to field stimulation in the absence and presence of nifedipine (10 microM) with -log molar (p) IC50 (+/- s.e. mean) values of 7.95 +/- 0.14 and 7.01 +/- 0.07, respectively. 2. The non-selective beta-adrenoceptor agonist, isoprenaline, reduced (56 +/- 8%) field stimulation induced contractile responses (pEC50 6.91 +/- 0.11). The non-selective beta-adrenoceptor antagonist propranolol (50 nM) and the beta 1-adrenoceptor selective antagonist, atenolol (3 microM), but not the beta 2-adrenoceptor antagonist ICI 118,551 ((+/-)-1 -[2,3-(dihydro-7-methyl-1H-inden-4-yl)oxyl]-3-[1-methylethyl)amino ]-2-butanol HCl; 100 nM) antagonized this effect (apparent pKB values 8.44 +/- 0.22 and 6.92 +/- 0.21, respectively) indicating an effect mediated through beta 1-like adrenoceptors. In the presence of nifedipine (10 microM) isoprenaline (up to 10 microM) did not inhibit the remaining response to field-stimulation. 3. Phenylephrine elicited contractile responses (pEC50 4.47 +/- 0.30) from preparations of guinea pig prostate which were reduced (63 +/- 25%) by nifedipine (10 microM). This response was antagonized by 5-methylurapidil (100 nM, apparent pKB 8.24 +/- 0.33), but was not affected by preincubation chloroethylclonidine (50 microM, 30 min). Responses to phenylephrine (30 microM) were inhibited (by up to 52 +/- 5%) by isoprenaline (pIC50 6.40 +/- 0.35, the beta 2-adrenoceptor selective agonist, salbutamol was weakly effective). Propranolol (300 nM), ICI 118,551 (100 nM) and atenolol (3 microM) shifted isoprenaline concentration-response curves to the right (apparent pKB +/- s.e. values 7.68 +/- 1.10; 8.00 +/- 0.72 and 6.62 +/- 0.95, respectively). In the presence of nifedipine (10 microM) responses to phenylephrine (30 microM,) were inhibited (by up to 51 +/- 4%) by isoprenaline (pIC50 6.88 +/- 0.17): propranolol (300 nM) and ICI 118,551 (100 nM), but not atenolol (3 microM) antagonized this effect (apparent pKB values 8.85 +/- 1.53 and 8.35 +/- 1.18, respectively). Thus beta 1-like and beta 2-like adrenoceptors may be involved in the isoprenaline-stimulated inhibition of phenylephrine concentration-response curves. 4. Phenylephrine stimulated [3H]-inositol phosphate accumulation (pEC50 4.47 +/- 0.83), an effect insensitive to chloroethylclonidine pre-treatment (50 microM, 30 min) and to nifedipine (10 microM), but inhibited by 5-methylurapidil (apparent pKD 7.90 +/- 0.22). Isoprenaline (up to 1 microM) did not affect the phenylephrine-stimulated maximal increase in [3H]-inositol phosphates but did increase [3H]-cyclic adenosine monophosphate ([3H]-cAMP) accumulation (pEC50 6.77 +/- 0.66); propranolol (30 nM) and ICI 118,551 (110 nM), but not atenolol (up to 3 microM), antagonized this effect. These responses may therefore be mediated through beta 2-like adrenoceptors. 5. These results show that the alpha 1-adrenoceptor mediated and field stimulation-induced contractions of the guinea pig prostate are partly dependent upon intracellular and extracellular sources of Ca2+. We conclude that both beta 1- and beta 2-like adrenoceptors inhibit responses to phenylephrine in the prostate of the guinea pig. The beta 1-like adrenoceptor-mediated inhibition of these responses is evident upon the field stimulation-induced and nifedipine-sensitive component of the response to phenylephrine and may not involve the activation of adenylyl cyclase. The beta 2-like adrenoceptor may inhibit both nifedipine sensitive and insensitive components of the response to phenylephrine, possibly through the activation of adenylyl cyclase, but not through the i

Effects of heptanol on electrical activity in the guinea-pig vas deferens

1. The effects of the putative intercellular uncoupling agent I-heptanol on electrical activity in the guinea-pig vas deferens were studied by use of intracellular and extracellular recording techniques. 2. At concentrations of 0.5, 1 and 2 mM, heptanol rapidly, monotonically and reversibly attenuated intracellularly recorded excitatory junction potential (e.j.p.) amplitude without affecting its time course, while spontaneous excitatory junction potentials (s.e.j.ps) were left unaffected. 3. Heptanol did not affect either the extracellularly recorded evoked excitatory junction current (e.j.c.), or the nerve terminal impulse that preceded it. These observations indicate that heptanol does not affect nerve impulse conduction, neurotransmitter release, or the postjunctional receptors involved in the production of the e.j.p. 4. E.j.ps appear to be suppressed by heptanol due to its intercellular uncoupling effects. Therefore, functional intercellular coupling may be necessary for the generation of the e.j.p. in smooth muscle.

Evidence for the differential release of the cotransmitters ATP and noradrenaline from sympathetic nerves of the guinea-pig vas deferens

1. Experiments were carried out to quantify the stimulation-evoked overflow of catecholamines and purines (ATP, ADP, AMP and adenosine) from an in vitro sympathetic nerve-smooth muscle preparation of the guinea-pig vas deferens and from isolated bovine adrenal chromaffin cells. The superfused preparations were stimulated for 60 s with electrical field stimulation (EFS; vas deferens), dimethylphenylpiperazinium (chromaffin cells) or KCl (both preparations). 2. Samples of superfusate were taken at 10 s intervals during the 60 s stimulation period for analysis of purines by HPLC-fluorescence detection and catecholamines by HPLC-electrochemical detection. 3. The evoked overflow of catecholamines and purines from chromaffin cells occurred with the same time course and in a constant ratio of approximately 4:1 (catecholamine to purine). These findings are compatible with the release of catecholamines and purines from a homogeneous population of exocytotic vesicles in the chromaffin cells. 4. The evoked overflow of purines and noradrenaline (NA) from the vas deferens preparation differed from the pattern of overflow from chromaffin cells and there was also some temporal disparity in the overflow of the two cotransmitters. The evoked overflow of ATP exceeded that of NA. In addition, the overflow of NA was tonic while the overflow of ATP and the other purines was phasic. 5. The EFS-evoked overflow of NA and the purines from the guniea-pig vas deferens preparation was examined after treatment with the neuronal amine-uptake inhibitors desipramine and cocaine, the alpha 1-adrenoceptor agonist methoxamine, the alpha 1-adrenoceptor antagonist prazosin, the alpha 2-adrenoceptor antagonists idazoxan and yohimbine, the noradrenaline-depleting drug reserpine and the adrenergic neuron-blocking agent guanethidine. The results of these studies, together with an analysis of the metabolic degradation of extracellular ATP, indicated that the temporal disparity in the overflow of NA and ATP is unlikely to be due to differences in the clearance of the cotransmitters or to the release of purines from non-neuronal sites. These results indicate that evoked overflow of the cotransmitters accurately reflects release from nerves. This pattern of release from nerves suggests that the two cotransmitters are released from two separate populations of exocytotic vesicles. 6. Superfusion of the vas deferens with exogenous epsilon-ATP, a fluorescent derivative of ATP, revealed that there was essentially no metabolism of the nucleotide over 60 s unless the tissue was subjected to EFS. Upon EFS, there was a rapid and nearly complete degradation of ATP with a corresponding increase in ADP, AMP and adenosine. This indicates the presence of a nerve stimulation-dependent metabolism of ATP.

[Cys(O2NH2)2]enkephalin analogues and dalargin: selectivity for delta-opioid receptors

To investigate the structure-activity relationships for potent and selective action of enkephalins at the delta-opioid receptors, two newly synthesized analogues, [Cys(O2NH2)2,Leu5]enkephalin and [Cys(O2NH2)2, Met5] enkephalin and the hexapeptide [D-Ala2,Leu5]enkephalyl-Arg (dalargin) were tested and compared with [Leu5]enkephalin and [Met5]enkephalin, for their effectiveness to inhibit electrically evoked contractions of the mouse vas deferens (predominantly enkephalin-selective delta-opioid receptors) and the guinea pig ileum (mu- and kappa-opioid receptors). The mouse vas deferens assays included evaluation of the effects of opioid agonists on the first, purinergic, and the second, adrenergic, components of electrically evoked biphasic responses (10 Hz and 20 Hz) and on ATP- or noradrenaline-evoked, tetrodotoxin-resistant responses. The opioids tested inhibited in a similar manner: (i) the purinergic and the adrenergic components of the electrically evoked contractions; and (ii) the ATP- and noradrenaline-induced postjunctional responses of the mouse vas deferens. Extremely low IC50 values (of 2-5 orders) were found for [Cys(O2NH2)2,Leu5] enkephalin, whose relative potency was between 239 and 1316 times higher than that of [Leu5]enkephalin. The order of potency for the other peptides in this tissue was: [Cys(O2NH2)2,Met5]enkephalin > [Leu5]enkephalin > dalargin > [Met5]enkephalin. The highest IC50 values in the guinea pig ileum assays, indicating the lowest affinity for mu-/kappa-opioid receptors, were obtained for the cysteine sulfonamide analogues, while dalargin showed a potency four times higher than that of [Met5]enkephalin. The order of potency in this tissue was: dalargin > [Met5]enkephalin > [Leu5]enkephalin > [Cys(O2NH2)2,Met5]enkephalin > [Cys(O2NH2)2,Leu5]enkephalin. The ratio, IC50 in guinea pig ileum: IC50 in mouse vas deferens, indicating selectivity of the respective peptide for delta-opioid receptors, was extremely high for [Cys(O2NH2)2,Leu5]enkephalin and especially for the adrenergic component of the responses. This ratio for [Cys(O2NH2)2,Met5]enkephalin was higher than the ratios for dalargin, [Leu5]enkephalin and [Met5]enkephalin, which were about 3 orders of magnitude lower. The results suggest that incorporation of hydrophilic Cys(O2NH2) in the enkephalin molecule greatly increases the potency and selectivity of the analogues at delta-opioid receptors, while both D-Ala2 substitution and lengthening of the peptide chain by Arg6 in the molecule of [Leu5]enkephalin decrease them.

Failure of tyramine to release neuronal ATP as a cotransmitter of noradrenaline in the guinea-pig vas deferens

Contractions, release of noradrenaline and release of ATP elicited by the indirectly acting sympathomimetic amine tyramine and responses elicited by exogenous noradrenaline were studied in the isolated vas deferens of the guinea pig. Release of noradrenaline was assessed as overflow of tritium after preincubation with [3H]-noradrenaline. ATP was measured by means of the luciferin-luciferase technique. In tissues pretreated with pargyline 1 mM, tyramine 300 microM, when added to the superfusion medium for 2 min, elicited contraction and an overflow of tritium (mainly [3H]-noradrenaline) and ATP. Contraction and ATP overflow responses were prevented and tritium overflow was greatly reduced by desipramine 10 microM. Prazosin 0.3 microM abolished contractions and evoked ATP overflow without changing tritium overflow. Blockade of postjunctional P2-purinoceptors by suramin 300 microM caused a marked decrease of tyramine-evoked contractions and a slight reduction of tritium overflow whereas evoked ATP overflow was markedly increased. The effect on contraction was not shared by two other P2-purinoceptor antagonists, namely pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS) 32 microM and diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS) 32 microM: PPADS increased contractions about fourfold, whilst DIDS had no effect at all. When the vas deferens was superfused for 24 min with medium containing tyramine 300 microM, evoked contractions and tritium overflow continued throughout whereas ATP overflow faded rapidly to basal values. In the presence of prazosin 0.3 microM, tyramine 300 microM again failed to elicit contractions as well as an overflow of ATP. Application of noradrenaline 10 microM instead of tyramine also resulted in prolonged contraction and an overflow of ATP that declined rapidly. It is concluded that all ATP released by tyramine is non-neuronal in origin, secondary to the activation of postjunctional alpha 1-adrenoceptors by released noradrenaline. The non-neural ATP does not seem to play a functional role in smooth muscle contraction and derives from a postjunctional source which is subject to a rapid depletion upon sustained alpha 1-adrenoceptor activation.

Enhanced lipid peroxidation by extracellular ATP in PC12 cells

Recently we have demonstrated that extracellular ATP acts as an excitatory neurotransmitter and enhances cell death in the presence of ferrous ions. By using a newly developed cis-parinaric acid fluorescence technique, we demonstrated that ATP, in a dose dependent manner, enhanced the increased membrane lipid peroxidation in PC12 cells when cells were incubated with micromolar FeCl2/DTP. P2 purinoceptor agonists, alpha,beta-methylene ATP and 2-methylthio-ATP, induced PC12 cell lipid peroxidation, but to a lesser extent than ATP. ATP-induced Ca(2+) influx via P2 purinoceptor activation significantly increased the intracellular Ca(2+)concentration, which may have triggered a free radical generating cascade(s), and led to membrane lipid peroxidation and cell death. Since oxidative stress has been implicated in certain neurodegenerative diseases such as aging, extracellular ATP may contribute to neuronal cell death by an oxidative mechanism involving lipid peroxidation.

Alpha-adrenoceptor modulation of norepinephrine and ATP release in isolated kidneys of spontaneously hypertensive rats

The present study investigates sympathetic cotransmission and its alpha-adrenoceptor-mediated modulation in kidneys of spontaneously hypertensive rats (SHR, 12 to 14 weeks) and age-matched normotensive Wistar-Kyoto rats (WKY). In the presence of cocaine and corticosterone, renal nerve stimulation at 1 Hz (30 seconds) induced a greater outflow of norepinephrine in SHR (4.2 +/- 0.2 pmol/g kidney) than in WKY (3.0 +/- 0.2 pmol/g kidney). The alpha 2-adrenoceptor antagonist rauwolscine (0.01 to 1 mumol/L) increased the stimulation-induced norepinephrine outflow to a greater extent in SHR than in WKY. In contrast, the alpha 1-adrenoceptor antagonist prazosin (0.03 to 3 mumol/L) increased the stimulation-induced norepinephrine outflow to a greater extent in WKY than in SHR. This difference was not observed in the presence of the P1-purinoceptor antagonist 8-(p-sulfophenyl)theophylline (100 mumol/L). Stimulation at 4 Hz (30 seconds) induced an outflow of ATP (SHR, 12.7 +/- 3.3 pmol/g kidney; WKY, 16.7 +/- 2.1 pmol/g kidney; perfusion solution without cocaine and corticosterone). Prazosin (0.03 mumol/L) markedly reduced pressor responses to stimulation and inhibited the induced ATP outflow by 60% to 70%. When prazosin (0.03 mumol/L) was present, rauwolscine (0.1 mumol/L) increased the induced outflow of norepinephrine and ATP and markedly enhanced prazosin-resistant pressor responses. These pressor responses were abolished by the P2-purinoceptor antagonist suramin (300 mumol/L). The results demonstrate an increased alpha 2-adrenoceptor-mediated automodulation of norepinephrine release in SHR kidneys caused by increased intrasynaptic norepinephrine levels.(ABSTRACT TRUNCATED AT 250 WORDS)

Modulation of neural noradrenaline and ATP release by angiotensin II and prostaglandin E2 in guinea-pig vas deferens

Effects of angiotensin II and prostaglandin E2 on contractions, release of noradrenaline and release of ATP elicited by electrical stimulation (210 pulses, 7 Hz) were studied in the isolated vas deferens of the guinea pig. Release of noradrenaline was assessed as overflow of tritium after preincubation with [3H]-noradrenaline. ATP was measured by means of the luciferin-luciferase technique. In some experiments postsynaptic alpha 1-adrenoceptors and P2X-purinoceptors were blocked by prazosin and suramin, respectively, to isolate the neural fraction of the overflow of ATP. Electrical stimulation elicited an overflow of tritium and ATP and, in the absence of prazosin and suramin, contraction. In the absence of prazosin and suramin, angiotensin II (1-100 nM) enhanced contractions as well as the evoked overflow of tritium and ATP. All parameters were increased by about the same percentage for a given concentration of angiotensin II. The effect of prostaglandin E2 (1-100 nM) was complex. Contractions were mainly enhanced, the evoked overflow of tritium was reduced, whereas the evoked overflow of ATP was predominantly increased. No or almost no contraction remained in the presence of prazosin and suramin, and the evoked overflow of ATP was decreased to about 16%. Angiotensin II (1-100 nM) again enhanced the evoked overflow of tritium and ATP. Both were increased by about the same percentage for a given concentration of angiotensin II and also were increased by about the same percentage as obtained in the absence of prazosin and suramin.(ABSTRACT TRUNCATED AT 250 WORDS)