Extracellular ATP in the human kidney: mode of release and vascular effects

1. We have previously shown that ATP is a co-transmitter of noradrenaline in the rat kidney. In the present study the release of ATP and noradrenaline from human kidney cortex was investigated. Vascular effects of ATP and stable analogues were tested in human and rabbit isolated renal blood vessels. 2. Sympathetic nerve stimulation (20 Hz for 1 min) in human kidney slices released 89 +/- 16 fmol noradrenaline per mg wet weight and 99 +/- 20 fmol ATP per mg wet weight in controls (n = 12). The Na+ channel blocker tetrodotoxin (1 microM) abolished ATP and noradrenaline release. 3. In human isolated extrarenal arteries the P2X-purinoceptor agonist beta, gamma-methylene-L-ATP caused almost no constrictor responses, beta, gamma-methylene-L-ATP induced moderate constrictor responses in intrarenal arteries. In preconstricted human intrarenal arteries ATP induced vasodilation. 4. ATP and the P2Y-receptor agonist 2-methyl-thio-ATP (2-MeSATP) dilated preconstricted rabbit renal arteries. The P2Y-receptor antagonist Reactive Blue 2 (3 microM) shifted the concentration response curves of ATP and 2-MeSATP to the right. 5. In conclusion, sympathetic nerve stimulation induces the release of ATP and noradrenaline in human renal cortex. ATP activates vasoconstrictory P2X- and vasodilatory P2Y-receptors in human renal blood vessels. The net vascular response to ATP in vivo will depend on the tissue distribution of these purinoceptors.

Vasoconstrictor responses to the P2x-purinoceptor agonist beta, gamma-methylene-L-ATP in human cutaneous and renal blood vessels

1. Strips of human saphenous veins and of human renal arteries and veins were superfused with Krebs-Henseleit solution at 37 degrees C. Constrictor responses were elicited by exogenous noradrenaline and the P2x-purinoceptor-selective agonist, beta, gamma-methylene-L-ATP. 2. In human saphenous veins, beta, gamma-methylene-L-ATP (0.3-30 microM; EC50 2.2 microM) induced marked constrictor responses. The maximal response to beta, gamma-methylene-L-ATP was similar to the maximal response to noradrenaline. The P2-purinoceptor antagonist suramin (30 microM) shifted the concentration-response curve of beta, gamma-methylene-L-ATP to the right (apparent pKB value 4.8); suramin (100 microM) markedly inhibited the responses to beta, gamma-methylene-L-ATP. The preferential P2x-purinoceptor antagonist, pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS; 3 microM) slightly reduced the response to beta, gamma-methylene-L-ATP. At a ten times higher concentration (30 microM), PPADS almost abolished the responses to beta, gamma-methylene-L-ATP. PPADS (30 microM), in contrast, caused no significant change in the concentration-response curve of noradrenaline. 3. In extrarenal and intrarenal arteries, EC50 values and maximal responses to noradrenaline were similar when compared with responses to noradrenaline in saphenous veins. Noradrenaline also constricted extrarenal veins. However, in contrast to the results obtained on saphenous veins, beta, gamma-methylene-L-ATP caused almost no constrictor responses in extrarenal veins and arteries and only moderate responses in intrarenal arteries. 4. The results demonstrate marked differences in responsiveness of human blood vessels to the selective P2x-purinoceptor agonist, beta, gamma-methylene-L-ATP, suggesting tissue differences in the occurrence or operation of P2x-purinoceptors in human vascular tissues. Moreover, the results indicate that PPADS blocks P2x-purinoceptors in human isolated blood vessels as previously demonstrated in animal blood vessels.

Comparison of corelease of noradrenaline and ATP evoked by hypogastric nerve stimulation and field stimulation in guinea-pig vas deferens

Contractions and overflow of tritium and ATP elicited by hypogastric nerve stimulation (HNS) and field stimulation (FS) were studied in the guinea-pig isolated vas deferens preincubated with [3H]-noradrenaline. ATP was measured by means of the luciferin-luciferase technique. HNS and FS elicited contraction, tritium overflow and ATP overflow. HNS at supramaximal current strength produced smaller responses than did FS at supramaximal current strength (210 pulses/7 Hz). Supramaximal HNS and submaximal FS were used in the remainder of the study. Prazosin (0.3 mumol/l) reduced contractions and the overflow of ATP elicited by both HNS and FS; the evoked overflow of tritium was not changed (210 pulses/7 Hz). Combined administration of prazosin (0.3 mumol/l) and suramin (300 mumol/l) abolished contractions and reduced the overflow of ATP elicited by both HNS and FS slightly more than did prazosin alone; tritium overflow again was not changed (210 pulses/7 Hz). Contractions, tritium overflow and ATP overflow increased with the frequency of both HNS and FS (from 7 to 25 Hz; 210 pulses); the increase in ATP overflow with frequency was more marked than the increase in tritium overflow. The preferential increase of ATP overflow with the frequency of HNS and FS persisted in the combined presence of prazosin (0.3 mumol/l) and suramin (300 mumol/l). The study confirms for HNS, a more physiologic way of sympathetic nerve stimulation, several observations previously obtained with FS. First, HNS-evoked ATP release is detectable as an overflow of ATP into the superfusion fluid.(ABSTRACT TRUNCATED AT 250 WORDS)

Cultured chick sympathetic neurons: modulation of electrically evoked noradrenaline release by P2-purinoceptors

The present study investigates the pharmacological profile of P2-purinoceptors modulating noradrenaline release from cultured chick sympathetic neurons. ATP (30 microM-3 mM) and 2-methylthio-ATP (3-100 microM), but not alpha, beta-methylene-ATP (up to 100 microM), caused a significant facilitation of electrically evoked [3H]-noradrenaline release when added 2 min before depolarization. The facilitation declined with time of exposure suggesting receptor desensitization. The facilitatory effect was markedly diminished by the P2-purinoceptor antagonists reactive blue 2 (3 microM) and suramin (300 microM), but not changed by mecamylamine (10 microM), a nicotinic receptor antagonist. At 1 mM and higher concentrations, ATP added for 12 min, inhibited noradrenaline release; release was virtually abolished by 6 mM ATP. The inhibitory effect of ATP was slightly diminished by suramin but not affected by reactive blue 2. Electrically evoked [3H]-noradrenaline release remained unaffected in the presence of the adenosine (P1)-receptor agonists R(-)N6-(2-phenylisopropyl)adenosine (R-PIA), 2-[p-(2-carboxyethyl) phenylethylamino]-5'-N-ethylcarboxamidoadenosine (NECA), and N6-2-(4-aminophenyl)ethyladenosine (APNEA), used up to 1 microM. The present results confirm the existence of two P2-purinoceptors affecting noradrenaline release: 1) a facilitatory receptor which is activated by 2-methylthio-ATP as well as ATP, and blocked by suramin as well as reactive blue 2, and 2) an inhibitory receptor which is activated by ATP, only slightly affected by suramin but not at all by reactive blue 2 and does not belong to the established P2-purinoceptor subtypes.

Cultured chick sympathetic neurons: ATP-induced noradrenaline release and its blockade by nicotinic receptor antagonists

The ATP-induced increase in tritium outflow from cultured chick sympathetic neurons prelabelled with [3H]-noradrenaline was investigated. Seven days-old dissociated cell cultures of embryonic paravertebral ganglia, loaded with [3H]-noradrenaline (0.05 microM), were superfused in the presence of (+)-oxaprotiline and exposed to ATP, ATP-analogues, or 1,1-dimethyl-4-piperazinium (DMPP) for 2 min. ATP (3 microM-3 mM), 2-methylthio-ATP (3-100 microM), as well as DMPP (10 and 100 microM) induced a significant overflow of tritium. The EC50-value of ATP was 20 microM. Both the ATP-induced and the DMPP-induced tritium overflow was Ca(2+)-dependent and sensitive to tetrodotoxin (0.3 microM) and omega-conotoxin (0.1 microM); in addition, it was inhibited by the alpha 2-adrenoceptor agonist 5-bromo-6-(2-imidazoline-2-ylamino)-quinoxaline (UK-14,304; 1 microM). The effects of ATP and DMPP were not additive. The ATP-induced as well as the DMPP-induced overflow of tritium was diminished by the P2-purinoceptor antagonists suramin (300 microM) and reactive blue 2 (3 microM); in all 4 cases, the inhibition amouted to approximately 40%. The tritium overflow induced by ATP or DMPP was almost abolished by the nicotinic receptor antagonist mecamylamine (10 microM) and markedly inhibited by hexamethonium (100 microM). Neither ATP nor electrical stimulation caused an overflow of tritium from cultures loaded with [3H]-choline. The results suggest that ATP at mumolar concentrations induces noradrenaline release from cultured chick sympathetic neurons via an action on a subclass of the nicotinic cholinoceptor.

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)

P2-purinoceptor-mediated inhibition of noradrenaline release in rat atria

1. We looked for P2-purinoceptors modulating noradrenaline release in rat heart atria. Segments of the atria were preincubated with [3H]-noradrenaline and then superfused with medium containing desipramine (1 microM) and yohimbine (1 microM) and stimulated electrically, by 30 pulses/1 Hz unless stated otherwise. 2. The adenosine A1-receptor agonist, N6-cyclopentyl-adenosine (CPA; EC50 9.7 nM) and the nucleotides, ATP (EC50 6.6 microM) and adenosine-5'-O-(3-thiotriphosphate) (ATP gamma S; EC50 4.8 microM), decreased the evoked overflow of tritium. The adenosine A2a-agonist, 2-p-(2-carbonylethyl)-phenethylamino-5'-N-ethylcarboxamido-a denosine (CGS-21680; 0.03-0.3 microM) and the P2x-purinoceptor agonist beta, gamma-methylene-L-ATP (30 microM) caused no change. 3. The concentration-response curve of CPA was shifted to the right by the adenosine A1-receptor antagonist, 8-cyclopentyl-1,3-dipropyl-xanthine (DPCPX; 3 nM; apparent pKB value 9.7) but hardly affected by the P2-purinoceptor antagonist, cibacron blue 3GA (30 microM). In contrast, the concentration-response curves of ATP and ATP gamma S were shifted to the right by DPCPX (3 nM; apparent pKB values 9.3 and 9.4, respectively) as well as by cibacron blue 3GA (30 microM; apparent pKB values 5.0 and 5.1, respectively). Combined administration of DPCPX and cibacron blue 3GA caused a much greater shift of the concentration-response curve of ATP than either antagonist alone. The concentration-response curve of ATP was not changed by indomethacin, atropine or the 5'-nucleotidase blocker alpha, beta-methylene-ADP. 4. Cibacron blue 3GA (30 microM) increased the evoked overflow of tritium by about 70%. The increase was smaller when the slices were stimulated by 9 pulses/O00 Hz instead of 30 pulses/I Hz.5. The results indicate that the postganglionic sympathetic axons in rat atria possess P2-purinoceptors in addition to the known adenosine Al-receptor. Both mediate inhibition of noradrenaline release. Some adenine nucleotides such as ATP and ATP gamma S act at both receptors. The presynaptic P2-purinoceptor seems to be activated by an endogenous ligand, presumably ATP, under the condition of these experiments. This is the first evidence for presynaptic P2-purinoceptors at cardiac postganglionic sympathetic axons.

Release of ATP in rat vas deferens: origin and role of calcium

Release of endogenous ATP elicited by electrical (neural) stimulation and exogenous agonists was studied in the rat isolated vas deferens. The aims were to dissect neural and postjunctional contributions to the nerve activity-evoked overflow of ATP and to clarify the role of transmitter receptors and calcium in postjunctional ATP release. In tissues preincubated with [3H]-noradrenaline, electrical stimulation (100 pulses/10 Hz) elicited contraction and an overflow of tritium and ATP. Contractions as well as ATP overflow were reduced by prazosin 0.3 microM and even more so by prazosin 0.3 microM combined with suramin 300 microM. They were also reduced by nifedipine 10 microM and even more so by nifedipine 10 microM combined with ryanodine 20 microM (the additional effect of ryanodine on ATP overflow was not significant). In tissues not pretreated with [3H]-noradrenaline, exogenous noradrenaline 10 microM and alpha,beta-methylene ATP 10 microM elicited contraction and an overflow of ATP. Responses to noradrenaline were blocked by prazosin 0.3 microM but not suramin 300 microM and were greatly reduced by nifedipine 10 microM and in Ca(2+)-free medium. Responses to alpha,beta-methylene ATP were blocked by suramin 300 microM but not prazosin 0.3 microM, were reduced by nifedipine 10 microM (effect on ATP overflow not significant) and were reduced even more in Ca(2+)-free medium. Neuropeptide Y 0.3 microM caused only very small contraction and ATP overflow. The electrically as well as the agonist-evoked ATP overflow correlated well with the contraction responses except in experiments with suramin which retarded the removal, by vas deferens tissue, of ATP from the medium.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence for P2-purinoceptor-mediated inhibition of noradrenaline release in rat brain cortex

1. Some postganglionic sympathetic axons possess P2Y-like P2-purinoceptors which, when activated, decrease the release of noradrenaline. We examined the question of whether such receptors also occur at the noradrenergic axons in the rat brain cortex. Slices of the brain cortex were preincubated with [3H]-noradrenaline, then superfused with medium containing desipramine (1 microM) and stimulated electrically, in most experiments by trains of 4 pulses/100 Hz. 2. The selective adenosine A1-receptor agonist, N6-cyclopentyl-adenosine (CPA; 0.03-3 microM) as well as the non-subtype-selective agonist 5'-N-ethylcarboxamido-adenosine (NECA; 0.3-3 microM) reduced the evoked overflow of tritium, whereas the adenosine A2a-receptor agonist, 2-p-(2-carbonylethyl)-phenethylamino-5'-N-ethylcarboxamido-a denosine (CGS-21680; 0.003-30 microM) and the adenosine A3-receptor agonist N6-2-(4-aminophenyl)ethyl-adenosine (APNEA; 0.03-3 microM) caused no change. Of the nucleotides tested, ATP (30-300 microM), adenosine-5'-O-(3-thiotriphosphate) (ATP gamma S; 30-300 microM), adenosine-5'-O-(2-thiodiphosphate) (ADP beta S; 30-300 microM), P1,P4-di(adenosine-5')-tetraphosphate (Ap4A; 30-300 microM) and the preferential P2Y-purinoceptor agonist, 2-methylthio-ATP (300 microM) decreased the evoked overflow of tritium. The P2X-purinoceptor agonist, alpha,beta-methylene-ATP (3-300 microM) caused no change. 3. The A1-selective antagonist, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX; 10 nM) attenuated the effects of the nucleosides CPA (apparent pKB value 9.8) and NECA as well as of the nucleotides ATP (apparent pKB 9.3), ATP gamma S (apparent pKB 9.2) and ADP beta S (apparent pKB 8.7). CGS-21680 and APNEA were ineffective also in the presence of DPCPX. The A2-selective antagonist 1,3-dipropyl-8-(3,4-dimethoxystyryl)-7-methylxanthine (KF-17837) reduced the effects of CPA, NECA and ATP gamma S only when given at a concentration of 300 nM but not at 1O nM.4. The P2-purinoceptor antagonists, suramin (300 micro M), reactive blue 2 (30 micro M) and cibacron blue 3GA(30 micro M) did not change the effect of CPA. Suramin and cibacron blue 3GA shifted the concentration response curve of ATP gamma S to the right (apparent pKB values 3.7 and 5.0, respectively). Reactive blue 2 also attenuated the effect of ATPyS, and cibacron blue 3GA attenuated the effect of ATP, but in these cases the agonist concentration-response curves were not shifted to the right. There was no antagonistic effect of suramin against ATP and ADP beta S.5. The results indicate that rat cerebrocortical noradrenergic axons possess, in addition to the knownadenosine Al-receptor, a separate purinoceptor for nucleotides (P2) which, in contrast to the Al-receptor,is blocked by suramin, reactive blue 2 and cibacron blue 3GA. Nucleotides such as ATP and ATP gamma S activate both receptors. Inconsistencies in antagonist effects against nucleotides are probably due to this activation of two receptors. The presynaptic P2-purinoceptor is P2Y-like, as it is in the peripheral sympathetic nervous system.

The fade of the purinergic neurogenic contraction of the guinea-pig vas deferens: analysis of possible mechanisms

The purinergic response of the guinea-pig vas deferens to long trains of pulses at high frequency consists of an initial twitch followed by a much lower plateau. Mechanical, neurochemical and electrophysiological techniques were used to examine the reason for the fade. Mechanical measurements. In tissues stimulated by trains of 180 pulses/10 Hz and treated with prazosin to suppress the noradrenergic contraction component, the response to alpha, beta-methylene ATP and to exogenous ATP was as high during the secondary plateau of the purinergic neurogenic contraction as it was outside electrical stimulation periods; the response to 50 pulses/100 Hz was also unchanged during the low plateau. The plateau was not increased by reactive blue 2,8-(p-sulphophenyl)theophylline, propranolol or capsaicin. Neurochemical measurements. In tissues preincubated with [3H]-noradrenaline, electrical stimulation elicited an overflow of tritium and of ATP. In the absence of drugs as well as in the presence of prazosin and suramin to suppress contractions, the overflow of tritium per pulse decreased slightly in the course of trains of 90 pulses/10 Hz; the overflow of ATP per pulse decreased to a greater extent on average, but the decrease was not statistically significant. In the presence of prazosin and nifedipine, also to suppress contractions, the overflow of tritium per pulse again decreased slightly in the course of trains of 105 pulses/10 Hz, but the overflow of ATP per pulse if anything tended to increase. Electrophysiological measurements.(ABSTRACT TRUNCATED AT 250 WORDS)

A new class of compounds, peptide derivatives of adenosine 5'-carboxylic acid, includes inhibitors of ATP receptor-mediated responses

A new type of ligand for the study of P2-purinergic receptor subtypes was synthesized by combining and modifying conventional nucleoside chemistry with Fmoc solid phase peptide synthesis techniques. The tri- and tetra-aspartic acid derivatives of adenosine-5'-carboxylic acid (AdoCAsp3 and AdoCAsp4) were found to act as weak agonists at P2-purinergic receptors, (activated by ATP and UTP respectively) present on C6 glioma cells. AdoCAsp4 induced inositol 1,4,5-trisphosphate formation in the C6 cells with an EC50 of 73 microM. In addition, AdoCAsp4 was found to inhibit (IC50 approximately 80 microM) ATP-induced cytosolic [Ca2+] transients in these glioma cells. The glycine derivative, AdoCGly, increased evoked release of noradrenaline from mouse vas deferens slices, probably due to the blockade of presynaptic P2-autoreceptors. The possibility that aspartic, glutamic or gamma-carboxyglutamic residues may be used to replace phosphate groups on an ATP receptor ligand, opens up new ways in ligand design.

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

Contractions and overflow of tritium and ATP elicited by single electrical pulses or short pulse trains were studied in the guinea-pig isolated vas deferens preincubated with [3H]-noradrenaline. ATP was measured using the luciferase technique. A single pulse caused only a small contraction and minimal tritium and ATP overflow. In contrast, trains of 6 pulses elicited marked contractions as well as tritium and ATP overflow. In experiments with 6 pulses/100 Hz, prazosin 0.3 microM reduced the contraction by 73%, did not change the evoked overflow of tritium, and reduced the evoked overflow of ATP by 85%. Suramin 300 microM reduced the contraction by 69% but changed neither the evoked overflow of tritium nor that of ATP. The combination of prazosin 0.3 microM and suramin 300 microM abolished the contraction, did not change the evoked overflow of tritium, and reduced the evoked overflow of ATP by 70%. When 6 pulses were applied at frequencies of 1, 2, 10 or 100 Hz, all responses increased with frequency up to a maximum at 10 Hz, but contractions and the evoked overflow of ATP increased with frequency to a greater extent than the evoked overflow of tritium. A similar frequency overflow relationship was observed when the medium contained prazosin 0.3 microM and suramin 300 microM (and evoked ATP overflow was greatly reduced). Yohimbine 1 microM did not affect the overflow of tritium evoked by 6 pulses/100 Hz but increased that evoked by 6 pulses/10 Hz. The results demonstrate an overflow of both noradrenaline and ATP in response to short pulse trains.(ABSTRACT TRUNCATED AT 250 WORDS)

P1-purinoceptor-mediated modulation of neural noradrenaline and ATP release in guinea-pig vas deferens

The effect of P1-purinoceptor activation on contractions, release of noradrenaline and release of ATP elicited by electrical field stimulation (210 pulses, 7 Hz) was studied in the superfused vas deferens of the guinea pig. Release of noradrenaline was assessed as overflow of total tritium after preincubation with [3H]-noradrenaline. ATP was measured by means of the luciferin-luciferase technique. Electrical stimulation elicited reproducible contraction, tritium overflow and ATP overflow. In the absence of other drugs, adenosine (10-100 microM) did not change evoked contractions but reduced the evoked overflow of tritium and ATP. In subsequent experiments alpha 1-adrenoceptors were blocked by prazosin, P2-purinoceptors by suramin and alpha 2-adrenoceptors by rauwolscine. No or almost no contraction remained under these conditions. The evoked overflow of tritium was 505% and the evoked overflow of ATP 34% of that observed in the absence of prazosin, suramin and rauwolscine. Adenosine (1-100 microM) again reduced the evoked overflow of tritium and ATP, and so did the A1-selective agonist 2-chloro-N6-cyclopentyladenosine (CCPA; 0.032-0.32 microM). Adenosine and CCPA decreased the evoked overflow of ATP to a greater extent than the evoked overflow of tritium. It is concluded that neural release of both postganglionic sympathetic cotransmitters, noradrenaline and ATP, is decreased upon activation of prejunctional P1- (A1-) purinoceptors in guinea-pig vas deferens. The A1-receptor-mediated inhibition of the release of ATP is more marked than the inhibition of the release of noradrenaline, a pattern opposite to the inhibition produced by activation of prejunctional alpha 2-autoreceptors.

Co-release of noradrenaline and ATP from cultured sympathetic neurons

The vesicles of postganglionic sympathetic axons store both noradrenaline and ATP. The theory of noradrenaline-ATP co-transmission implies that both compounds are released by nerve action potentials and elicit postjunctional effects. Many properties of postjunctional responses support the theory. However, neural release of ATP has been difficult to detect biochemically: by far the major part of the overflow of ATP from intact tissues upon sympathetic nerve stimulation comes from non-neural elements, especially smooth muscle and endothelial cells. Here we describe a parallel electrically evoked overflow of [3H]noradrenaline and endogenous ATP from cultured chick sympathetic neurons. The overflow was abolished by tetrodotoxin, omega-conotoxin and withdrawal of Ca2+, was increased by tetraethylammonium and 4-aminopyridine, and was not changed by prazosin or suramin. The results demonstrate directly the action potential-evoked, Ca(2+)-dependent and presumably vesicular and exocytotic release of ATP from postganglionic sympathetic neurons. They support the co-transmitter theory and suggest that cultured sympathetic neurons are a preparation in which noradrenaline-ATP co-release can be examined free from postjunctional components.

Presynaptic modulation of the release of the co-transmitters noradrenaline and ATP

The release of both sympathetic co-transmitters, noradrenaline and ATP, is modulated via presynaptic receptors. However, the degree of the modulation may differ indicating that the ratio of the released co-transmitters changes upon presynaptic receptor activation. For example, alpha 2-autoinhibition affects the release of noradrenaline more markedly than the release of ATP. Some sympathetic axon terminals possess presynaptic P2-purinoceptors which are activated by endogenous ATP. These receptors are a novel kind of auto-receptor: they mediate a presynaptic negative feedback mechanism in which released ATP inhibits subsequent co-transmitter release.

P2-purinoceptor-mediated autoinhibition of sympathetic transmitter release in mouse and rat vas deferens

Effects of drugs acting at P2-purinoceptors on the release of newly taken up [3H]-noradrenaline were studied in slices of mouse and rat vas deferens. The slices were superfused and stimulated electrically, in most experiments by trains of 60 pulses/8 Hz. In mouse vas deferens, the P2-purinoceptor antagonists reactive blue 2 (1.8-100 microM) and brilliant blue G (10-300 microM) increased the stimulation-evoked overflow of tritium in a concentration-dependent manner as shown previously for suramin. Reactive blue 2, which preferentially blocks the P2Y-subtype, was the most potent compound and the compound with highest maximal effect, an increase by 104%. Pyridoxal phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS), in contrast, caused a small increase only at a single concentration (30 microM). The effects of reactive blue 2, brilliant blue G and suramin were not additive. The P2 agonist adenosine 5'-O-(3-thio)-triphosphate (ATP gamma S) reduced the evoked overflow of tritium. As shown previously for suramin, reactive blue 2 30 microM and brilliant blue G 100 microM antagonized the effect of ATP gamma S. From the shift of the ATP gamma S concentration-response curve to the right, an apparent pKB value of 5.3 was estimated for reactive blue 2 and an apparent pKB of 4.5 for brilliant blue G. In rat vas deferens, reactive blue 2 (3-30 microM), brilliant blue G (10 microM) and suramin (30-300 microM) also increased the evoked overflow of tritium. As in the mouse, reactive blue 2 was the most potent compound and the compound with highest maximal effect, an increase by 90%.(ABSTRACT TRUNCATED AT 250 WORDS)

A study of ATP as a sympathetic cotransmitter in human saphenous vein

1. Strips of human saphenous veins were superfused with Krebs-Henseleit solution at either 25 degrees C or 37 degrees C. Constrictor responses to electrical stimulation (10 Hz, 40 s) but not to exogenous noradrenaline (0.1, 1 microM) were abolished by guanethidine (10 microM) and tetrodotoxin (1 microM). Hence, responses to electrical stimulation are due to action potential-induced release of sympathetic neurotransmitters. 2. Constrictor responses to electrical stimulation and noradrenaline were reduced by the alpha 1-adrenoceptor antagonist, prazosin (0.3 microM) as well as by the alpha 2-adrenoceptor antagonist, rauwolscine (1 microM). The combination of prazosin and rauwolscine abolished constrictor responses to noradrenaline at 25 degrees C and 37 degrees C. However, constrictor responses to electrical stimulation were partly resistant to alpha-adrenoceptor blockade by prazosin and rauwolscine (at 25 degrees C about 30%). Residual constrictor responses to electrical stimulation were also observed in the presence of the combination of prazosin (3 microM) and rauwolscine (10 microM) as well as in the presence of phenoxybenzamine (10 microM). 3. Veins, incubated with [3H]-noradrenaline, released tritium upon electrical stimulation (10 Hz, 40 s). Moreover, electrical stimulation also induced an overflow of ATP amounting to 4.8 +/- 1.5 pmol g-1 at 25 degrees C and 2.0 +/- 0.5 pmol g-1 at 37 degrees C. Both tritium and ATP overflow were abolished by tetrodotoxin (0.5 microM). The combination of prazosin (0.3 microM) and rauwolscine (1 microM) increased tritium overflow at either 25 degrees C or 37 degrees C by about 120%, but reduced ATP overflow by about 70%. Hence, a significant percentage of the electrically evoked ATP overflow seems to be released from non-neuronal cells upon activation of alpha-adrenoceptors by endogenous noradrenaline. The remaining ATP overflow, which was resistant to alpha-adrenoceptor blockade, may reflect neuronally released ATP.4. ATP (300 MicroM) and alpha,Beta-methylene-ATP (1, 10 MicroM), both induced constrictor responses. The P2-purinoceptor antagonist, suramin (300 MicroM) markedly inhibited constrictor responses to ATP and alpha, beta-methylene-ATP, but not those to electrical stimulation and to noradrenaline. Moreover, suramin(300 MicroM) failed to diminish the alpha-adrenoceptor blockade-resistant constrictor response to 10 Hz.5. In conclusion, constrictor responses to sympathetic nerve stimulation in human saphenous veins are mainly but not exclusively mediated by neuronally released noradrenaline. There is a concomitant release of ATP and noradrenaline. P2-purinoceptors which mediate vasoconstriction are present; however,a role of neuronally released ATP in constrictor responses to electrical stimulation could not be established. Therefore, the nature of the sympathetic transmitter responsible for alpha-adrenoceptor blockade-resistant constrictor responses remains unknown.

Ionotropic glutamate receptor types leading to adenosine-mediated inhibition of electrically evoked [3H]-noradrenaline release in rabbit brain cortex slices

1. Glutamate inhibits the electrically evoked release of noradrenaline in rabbit brain cortex slices; the inhibition is mediated by adenyl compounds, presumably adenosine. The aim of the present study was to identify the receptors involved in this indirect inhibitory effect of glutamate. Slices of the occipitoparietal cortex were preincubated with [3H]-noradrenaline and then superfused and stimulated by trains of 6 pulses, 100 Hz. 2. The ionotropic glutamate receptor agonists alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AM-PA; 10-100 microM), kainate (10-100 microM) and N-methyl-D-aspartate (NMDA; 30-300 microM) but not the metabotropic glutamate receptor agonist, 1-amino-1,3-cyclopentanedicarboxylate (ACPD; 10-100 microM) reduced the electrically evoked overflow of tritium. 3. The effects of AMPA, kainate and NMDA were attenuated or abolished by the adenosine A1-receptor antagonist, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) as well as by adenosine A1-receptor antagonist, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) as well as by adenosine deaminase but not by the alpha 2-adrenoceptor antagonist yohimbine, the gamma-aminobutyric acid (GABA) receptor antagonists, bicuculline and 2-hydroxysaclofen and the NO synthase inhibitor NG-nitro-L-arginine methyl ester (L-NAME). 4. The NMDA receptor antagonist, 2-amino-5-phosphonopentanoate (AP5) blocked the inhibitory effect of NMDA but not that of AMPA and kainate. The non-NMDA-receptor antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) blocked the effect of AMPA but not of kainate and NMDA. 5. In addition to decreasing the electrically evoked overflow of tritium, AMPA, kainate and NMDA but not ACPD caused a steep but transient rise of basal tritium efflux. This immediate releasing effect was not significantly changed by DPCPX, adenosine deaminase, yohimbine, bicuculline, 2-hydroxysaclofen and L-NAME (except that L-NAME enhanced the effect of kainate). AP5 and CNQX antagonized the immediate releasing effects in the same way that they antagonized the inhibition by AMPA, kainate and NMDA of the electrically evoked overflow of tritium.6. It is concluded that AMPA, kainate and NMDA, like glutamate, reduce the electrically evoked release of noradrenaline by releasing adenosine or an adenine nucleotide which is then degraded to adenosine. Activation of each of the three ionotropic glutamate receptors, AMPA, kainate and NMDA receptors, but not activation of metabotropic glutamate receptors can initiate this indirect inhibitory effect on the release of noradrenaline (as well as the known noradrenaline releasing effect).

Prejunctional modulation of noradrenaline release in mouse and rat vas deferens: contribution of P1- and P2-purinoceptors

1. Prejunctional purinoceptors modulating the release of noradrenaline were compared in mouse and rat vas deferens. Tissue slices were preincubated with [3H]-noradrenaline and then superfused and stimulated electrically, in most experiments by trains of 60 pulses, 1 Hz. 2. In mouse vas deferens, 2-chloroadenosine (IC50 0.24 microM), beta,gamma-methylene-ATP (IC50 3.8 microM), alpha,beta-methylene-ATP (IC50 2.9 microM) and 2-methylthio-ATP (only 30 microM tested) reduced the evoked overflow of tritium. 8-Cyclopentyl-1,3-dipropylxanthine (DPCPX), 10 nM, antagonized the effect of 2-chloro-adenosine (apparent pKB 10.2) as well as of beta,gamma-methylene-ATP (apparent pKB 9.6) and alpha,beta-methylene-ATP. Suramin, 300 microM, attenuated the effect of 2-chloroadenosine at best very slightly, antagonized the effect of beta,gamma-methylene-ATP (apparent pKB 4.5) and, when combined with DPCPX 10 nM, caused a further marked shift to the right of the concentration-response curve of beta,gamma-methylene-ATP beyond the shift produced by DPCPX alone. 3. In rat vas deferens, 2-chloroadenosine (IC50 0.20 microM), beta,gamma-methylene-ATP (IC50 4.8 microM), alpha,beta-methylene-ATP (IC50 3.0 microM) and 2-methylthio-ATP (only 30 microM tested) also reduced the evoked overflow of tritium. DPCPX, 10 nM, antagonized the effect of 2-chloroadenosine (apparent pKB 9.7) as well as of beta,gamma-methylene-ATP (apparent pKB 9.6) and alpha,beta-methylene-ATP. Suramin, 300 microM, did not change the effect of 2-chloroadenosine, attenuated the effect of beta,gamma-methylene-ATP at best very slightly and, when combined with DPCPX, caused at best a very small shift to the right of the concentration-response curve of beta,gamma-methylene-ATP beyond the shift produced by DPCPX alone.4. It is concluded that prejunctional purinoceptor mechanisms in mouse and rat vas deferens are similar. In either species, both nucleosides such as adenosine and nucleotides such as beta,gamma-methylene-ATP activate a common release-inhibiting receptor which is a Pl- or, more specifically, A1-purinoceptor.There seems to be no need to postulate the existence of a novel prejunctional P3-purinoceptor.Moreover, the sympathetic terminal axons possess an additional P2-purinoceptor in both species which is activated by some nucleotides such as beta,gamma-methylene-ATP and 2-methylthio-ATP, although the activation of the P2-purinoceptor by beta,gamma-methylene-ATP is difficult to demonstrate in the rat.

Neural ATP release and its alpha 2-adrenoceptor-mediated modulation in guinea-pig vas deferens

Contractions, release of previously stored [3H]-noradrenaline (measured as overflow of total tritiated compounds) and release of ATP elicited by electrical field stimulation (210 pulses, 7 Hz) were studied in the superfused vas deferens of the guinea pig. Prazosin and suramin were used to suppress non-neural ATP release, and effects of bromoxidine and rauwolscine on the neural release thus isolated were examined. Electrical stimulation elicited reproducible contraction, tritium overflow and ATP overflow. Both prazosin (0.03-3 microM) and suramin (30-300 microM) reduced contractions as well as the evoked overflow of ATP. No visible contraction remained in 21 of 28 tissues exposed to prazosin 0.3 microM combined with suramin 300 microM. The evoked overflow of ATP under these conditions was about 17% of that observed in the absence of drugs. In the presence of prazosin 0.3 microM and suramin 300 microM, bromoxidine (0.01-1 microM) decreased and rauwolscine (0.1-10 microM) increased the evoked overflow of both tritium and ATP. Rauwolscine increased the evoked overflow of tritium to a significantly greater extent than the overflow of ATP. It is concluded that the overflow of ATP elicited by electrical (neural) stimulation in the presence of prazosin 0.3 microM and suramin 300 microM reflects purely neural release of ATP. This release of ATP, like the release of noradrenaline, is modulated through prejunctional alpha 2-adrenoceptors. The alpha 2-adrenoceptor modulation of the release of noradrenaline seems to be more marked than the modulation of the release of ATP.

Axon terminal P2-purinoceptors in feedback control of sympathetic transmitter release

Extracellular ATP acts on P2-purinoceptors of peripheral effector cells, and this is the basis for its function as a (co-)transmitter in peripheral efferent neurons. ATP also acts on P2-receptors of neuronal cell bodies or dendrites, and this is the basis for its function as a fast excitatory transmitter at neuroneural synapses. A third site of action is axon terminals. In the vas deferens of the mouse, noradrenaline and ATP are postganglionic sympathetic co-transmitters, and exogenous ATP acts on P2-purinoceptors of the sympathetic terminals to inhibit release of noradrenaline. Here we show that two P2 antagonists, suramin and Reactive Blue 2, increase the release of noradrenaline in mouse vas deferens. The increase is only obtained when there has been preceding nerve activity and is largely independent of the postjunctional response. These findings indicate a physiological function for axon terminal P2-purinoceptors: they mediate a novel prejunctional negative feedback in which released ATP inhibits subsequent transmitter release.

Effects of nifedipine and ryanodine on adrenergic neurogenic contractions of rat vas deferens: evidence for a pulse-to-pulse change in Ca2+ sources

1. The effects of nifedipine and ryanodine on the adrenergic component of neurogenic contractions of the rat isolated vas deferens were studied in an attempt to identify the sources of Ca2+ mediating the contraction. The tissue was electrically stimulated by single pulses or pairs of widely spaced pulses. The purinergic component of contraction was suppressed by the presence of 300 microM suramin. 2. In Mg(2+)-free medium, nifedipine (0.01-10 microM) reduced the first and, to a greater extent, the second twitch elicited by two pulses 3 s apart. This pattern of inhibition was observed both in the absence of rauwolscine (when twitch 2 was smaller than twitch 1) and in the presence of 0.1 microM rauwolscine (when, due to interruption of prejunctional alpha 2-adrenoceptor-mediated autoinhibition, twitch 2 was of similar height to twitch 1). Nifedipine reduced only twitch 2 but not twitch 1 in medium containing 1.2 mM Mg2+. 3. Single pulses of increasing current strength elicited increasing contraction. Nifedipine reduced contractions by about the same absolute extent at all current strengths, so that the relative contribution of the nifedipine-sensitive component decreased with increasing current strength. 4. When the pulse interval in a pair was increased from 5 to 60 s, the inhibition by nifedipine of the second twitch was most marked at an interval of 5 s and declined as the interval increased. 5. In contrast to nifedipine, 20 microM ryanodine reduced the first twitch of a pair to a greater extent than a second twitch 5 s later, so that twitch 2 became greater than twitch 1. The inhibition by ryanodine of twitch 2 increased with increasing pulse interval.6. In vasa deferentia preincubated with [3H]-noradrenaline, I microM nifedipine and 20 microM ryanodine did not change the electrically evoked overflow of tritium, whereas 10 microM nifedipine increased it.7. It is concluded that, when the sympathetic axons of the vas deferens are stimulated by a single pulse(or the first pulse of a pair) in Mg2+-free medium, both Ca2+ mobilization inside the smooth muscle cells and Ca2+ entry contribute to the ensuing adrenergic contraction. The relative contribution of Ca2+ entry is small at maximal stimulus strength but increases with decreasing stimulus strength. When a second pulse follows the first after an appropriate interval, a switch of Ca2+ sources occurs: intracellular Ca2+mobilization is decreased during twitch 2, whereas Ca2+ entry is increased.

Effect of opioid receptor subtype-selective agonists on purinergic and adrenergic components of neurogenic contractions of mouse vas deferens

1. Effects of opioid agonists on the purinergic and adrenergic components of neurogenic contractions and in some experiments on transmitter overflow were studied in the mouse isolated vas deferens. 2. When the vas deferens was stimulated every 2 min by pairs of pulses 2 s apart in the presence of prazosin 0.3 microM (to isolate the purinergic component) or alpha,beta-methylene-ATP 3 microM (to isolate the adrenergic component), each pulse elicited a separate twitch. The opioid agonists [D-Ala2,N-Me-Phe4,Gly5-ol]enkephalin (DAMGO, mu-receptor-selective), [D-Pen2,D-Pen5]enkephalin (DPDPE, delta-selective) and trans-(+/-)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl]- benzeneacetamide (U-50488, kappa-selective) concentration-dependently reduced both purinergic and adrenergic contractions. For each agonist, maximal effects and concentrations causing half-maximal effects were very similar for inhibition of the purinergic component on the one hand and for inhibition of the adrenergic component on the other hand, although the adrenergic component was inhibited with a slight preference. Moreover, effects on contractions elicited by the first and the second pulse of the pairs were very similar. 3. When vasa deferentia preincubated with [3H]-noradrenaline were stimulated with trains of 100 pulses delivered at 20 Hz, morphine 10 microM reduced significantly both evoked tritium overflow and evoked contractions. Its effect was antagonized by naloxone. 4. It is concluded that, in contrast to drugs acting at some other presynaptic receptors, opioid mu-, delta- and kappa-agonists inhibit purinergic and adrenergic neurogenic contractions of the mouse vas deferens in a similar manner. In contrast to a previous report, no enhancement by morphine of the release of noradrenaline elicited by high frequency pulse trains was observed.

Adenosine but not an adenine nucleotide mediates tonic purinergic inhibition, as well as inhibition by glutamate, of noradrenaline release in rabbit brain cortex slices

A possible contribution of adenine nucleotides to the endogenous purinergic, A1-receptor-mediated inhibition of noradrenaline release was studied in rabbit occipito-parietal cortex slices. The slices were preincubated with [3H]-noradrenaline and then superfused and stimulated electrically, in most experiments by trains of 6 pulses/100 Hz. A few experiments were carried out in rat occipito-parietal cortex slices. The A1-purinoceptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX; 1-100 nmol/l) as well as the enzyme adenosine deaminase (0.1-10 U/ml) increased the electrically evoked overflow of tritiated compounds. The maximal increase was by about 85% for both DPCPX and adenosine deaminase. The increases obtained with maximally effective concentrations of DPCPX and adenosine deaminase were not additive. The alpha 1-adrenoceptor-selective agonist methoxamine (10 but not 1 mumol/l) reduced the evoked overflow. Its effect was antagonized by yohimbine 1 mumol/l but then not attenuated further by DPCPX 100 nmol/l. L-Glutamate (300 mumol/l-2.3 mmol/l) also reduced the evoked overflow of tritium. Its effect was not changed by yohimbine 1 mumol/l but greatly, and to the same extent, attenuated by DPCPX 100 nmol/l and adenosine deaminase 3 U/ml. Neither the N-methyl-D-aspartate (NMDA) receptor antagonist dizocilpine nor omission of Mg++ changed the inhibition by glutamate. Glutamate did not alter the basal efflux of tritium from rabbit cortex slices under any experimental condition. In contrast, glutamate (100 mumol/l and 1 mmol/l) caused an immediate, marked and transient acceleration of tritium outflow from rat occipitoparietal cortex slices (medium without Mg++).(ABSTRACT TRUNCATED AT 250 WORDS)