Effect of lithium chloride on the neurotransmitter release from adrenergic nerve terminals of guinea-pig atria

Lithium chloride at 1 and 10 mM produced a dose-dependent inhibition of the cardiac response to field stimulation of the adrenergic nerve terminals, without affecting myocardial contractility in electrically stimulated guinea-pig atria. This effect was calcium-independent and was also present in preparations superfused with 10 mM myoinositol. Moreover, 10 mM lithium chloride reduced the positive inotropic effect of tyramine. These results indicate that lithium reduces the evoked release of noradrenaline from the adrenergic nerve endings, probably lowering the content of releasable neurotransmitter.     

Presynaptic modulation by endogenous ouabain-like substances of noradrenaline release from blood vessels

The effect of an endogenous ouabain-like compound (OCL) and of ouabain was studied on [3H]noradrenaline release and on the tension of rabbit pulmonary arterial strip. Similarly to ouabain, the OLC enhanced release of [3H] NA in resting and in stimulated condition. Moreover, in the presence of OLC and ouabain, the tension of the rabbit artery increased gradually and the contraction evoked by electrical stimulation was potentiated. It is suggested that this mechanism might be involved in the physiological regulation of blood pressure or in the genesis of hypertension.     

Muscarinic modulation of acetylcholine release evoked by dimethylphenylpiperazinium and high potassium from guinea-pig myenteric plexus

The effects of oxotremorine and atropine on the ACh release evoked from the guinea-pig myenteric plexus by dimethylphenylpiperazinium (DMPP) or by high potassium were investigated. DMPP caused an output of ACh by stimulating nicotine receptors that are probably situated on the soma-dendritic part of the cholinergic neuron. The DMPP-induced release of ACh was dose-dependently inhibited by oxotremorine and increased by atropine. The ACh output evoked by either 45 or 108 mM potassium was enhanced by atropine. Oxotremorine did not affect the ACh release by high potassium but prevented the facilitatory effect of atropine. It is concluded that the inhibitory muscarinic mechanism modulates similarly the ACh release evoked by DMPP or high potassium and the release caused by electrical stimulation. From the experiments with high potassium it is concluded that the inhibitory muscarine receptors are localized at the site of ACh release.     

Muscarinic inhibition of endogenous noradrenaline release from rabbit isolated trachea: receptor subtype and receptor reserve

The aim of the present study was to characterize putative muscarine receptors on sympathetic nerve terminals in the rabbit trachea. Release of endogenous noradrenaline from in vitro incubated rabbit tracheae was evoked by electrical field stimulation (3 Hz, 540 pulses) and quantified by high performance liquid chromatography with electrochemical detection.The muscarine receptor agonist oxotremorine inhibited the evoked release of noradrenaline completely at 1 ol/l (EC₅₀: 64 nmol/l). The concentration response curve was very steep (Hill coefficient of 2.3). Scopolamine shifted the concentration response curve of oxotremorine to the right (–log K_B 8.48) demonstrating specific, inhibitory muscarine receptors. Several subtype-preferring muscarine receptor antagonists also shifted the concentration response curve of oxotremorine to the right. The rank order of potency was (–log K_B or pA^* ₂): scopolamine (8.48) > AF DX 384 (7.88^*; slope of Schild plot 1.1) > (R)-trihexyphenidyl (7.87) > 4-DAMP (7.85) > AQ-RA 741 (7.77) methoctramine 6.18 > pirenzepine (6.0) >p-fluoro-hexahydrosiladifenidol (p-FHHSiD, 5.68). When these affinity constants were plotted against reported –log K_i values determined in binding studies on human cloned muscarine receptor subtypes (m₁-m₅), the best correlation was obtained for m₂. Indomethacin (3 mol/l), which on its own increased the evoked noradrenaline release by about 45%, affected neither the inhibitory effect of oxotremorine nor the antagonistic potency of methoctramine or p-FHHSiD. After preincubation for 48 min with 300 mol/l phenoxybenzamine, which has been shown to inactivate muscarine receptors irreversibly, the concentration response curve of oxotremorine was shifted 5.2 fold to the right and the maximal inhibition was reduced by 50%, whereas the slope remained steep (Hill coefficient 2.6). These experiments indicated that a fraction of about 22% of the muscarine receptors has to be occupied by oxotremorine to produce half-maximum inhibition of noradrenaline release; the dissociation constant of oxotremorine at the prejunctional muscarine receptors was 0.33 mol/l.In conclusion, the sympathetic nerve terminals in the rabbit trachea are endowed with inhibitory M₂-like muscarine receptors for which methoctramine displayed a low affinity. Since a large receptor reserve could be excluded, the steep concentration response curve of oxotremorine suggests that activation of muscarine receptors has to reach a threshold level before the onset of an inhibitory effect. Correspondence to: K. Racké at the above address     

Presynaptic alpha 2-adrenoceptor modulation of 5-hydroxytryptamine and noradrenaline release from vascular adrenergic nerves

Modulation of the stimulation-evoked release of 5-hydroxytryptamine (5-HT) and noradrenaline (NA) by presynaptic alpha 2-adrenoceptors was characterized in the perfused mesenteric vascular bed of the rat. The vasoconstrictor response to periarterial nerve stimulation (PNS; 8 Hz), previously abolished in the presence of 30 nM prazosin, was restored after 15 min treatment with 10 microM 5-HT, without a significant effect on the pressor response to 1 nmol of infused NA, which was previously abolished with prazosin. The restored pressor response to PNS was abolished by 100 nM tetrodotoxin and 100 nM ketanserin. Clonidine (1-10 microM) in the presence of prazosin induced a dose-dependent potentiation of the restored pressor response to PNS after 5-HT treatment while BHT 920 (10 nM-1 microM) and 100 nM clonidine inhibited the restored response. In the presence of 100 nM phentolamine, the restored pressor response to PNS was not altered by clonidine, but was inhibited by BHT 920. The PNS (8 Hz)-evoked tritium release in a preparation labeled with [3H]5-HT was facilitated by clonidine (100 nM-10 microM) while BHT 920 (10 nM-1 microM) and cocaine (1-10 microM) reduced the release. Yohimbine (1 microM) antagonized the effects of clonidine and cocaine but not of BHT 920 on the PNS-evoked tritium release. In the preparation labeled with [3H]NA, clonidine did not alter the PNS-evoked tritium release while BHT 920 inhibited it and cocaine facilitated it. Yohimbine did not antagonize the effect of BHT 920.(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 luciferinluciferase technique.Electrical stimulation elicited reproducible contraction, tritium overflow and ATP overflow. In the absence of other drugs, adenosine (10–100 M) did not change evoked contractions but reduced the evoked overflow of tritium and ATP. In subsequent experiments ₁-adrenoceptors were blocked by prazosin, P₂-purinoceptors by suramin and 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 M) 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 M). Adenosine and CCPA decreased the evoked overflow of ATP to a greater extent than the evoked overflow of tritrium.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 ₂-autoreceptors. Correspondence to: B. Driessen at the above address     

Further evidence that prostaglandins inhibit the release of noradrenaline from adrenergic nerve terminals by restriction of availability of calcium.

1 Guinea-pig vasa deferentia were continuously superfused after labelling the transmitter stores with [3H](-)-noradrenaline. Release of [3H]-(-)-noradrenaline was induced by transmural nerve stimulation. 2 Prostglandin E2 (14 nM) drastically reduced the release of [3H]-(-)-noradrenaline, while tetraethylammonium (2 mM), rubidium (6 mM), phenoxybenzamine (3 muM) each in the presence or absence of Uptake 1 or 2 blockade, and prolonged pulse duration (from 0.5 to 2.0 ms) all significantly increased the release of [3H]-(-)-noradrenaline per nerve impulse. 3 The inhibitory effect of prostaglandin E2 on evoked release of [3H]-(-)-noradrenaline was significantly reduced by tetraethylammonium, rubidium and prolonged pulse duration, whilst it was actually enhanced by phenoxybenzamine. This indicates that increased release of noradrenaline per nerve impulse does not per se counteract the inhibitory effect of prostaglandin E2. 4 It is concluded that tetraethylammonium, rubidium and prolonged pulse duration counteracted the inhibitory effect of prostaglandin E2 on T3H]-(-)-noradrenaline release by promoting calcium influx during the nerve action potential. The results are consistent with, and add more weight to the view that prostaglandins inhibit the release of noradrenaline by restriction of calcium availability.     

Mechanism of nicotine-induced release of noradrenaline from adrenergic nerve endings

1 A study of the mechanism of release of [³H]-noradrenaline ([³H]-NA) by nicotine from isolated vas deferens of the rat was made using incubation media of different ionic composition.

2 Nicotine (20 μg/ml)-induced release of [³H]-NA was significantly potentiated in K⁺-free Krebs solution as compared to that in normal Krebs-Ringer solution.

3 Nicotine-induced release of [³H]-NA was significantly reduced in Na⁺-deficient Krebs solution (containing only 11 mM Na⁺) and was abolished in Na⁺-free Krebs solution.

4 In totally depolarized tissues, nicotine failed to cause an outflow of [³H]-NA but Ca²⁺ (5 mM) did so.

5 Nicotine required the presence of Ca²⁺ in the incubation medium to cause release of [³H]-NA from adrenergic nerve terminals, the magnitude of release being dependent upon the concentration of Ca²⁺.

6 Nicotine-induced release of [³H]-NA was demonstrated in high Ca²⁺, Na⁺-free Krebs solution in which all Na⁺ had been replaced with Ca²⁺.

7 It is concluded that nicotine increases the membrane permeability to both Na⁺ and Ca²⁺. It is also suggested that the increase in permeability to Ca²⁺ alone is not sufficient but a local depolarizing action of nicotine is necessary to cause release of noradrenaline from adrenergic nerve endings.

     

Metabolism of endogenous and exogenous noradrenaline in guinea-pig atria

Summary The outflow of noradrenaline, 3,4-dihydroxyphenylglycol (DOPEG) and 3,4-dihydroxymandelic acid (DOMA) from guinea-pig isolated atria was studied by chromatography on alumina followed by high pressure liquid chromatography with electrochemical detection. In the absence of drugs, the outflow of endogenous noradrenaline over a period of 3 h averaged 1.6 pmol×g^–1×min^–1 and the outflow of DOPEG 17 pmol×g^–1×min^–1. The outflow of DOMA was below the detection limit (<0.31 pmol×g^–1×min^–1). Tyramine greatly increased the outflow of noradrenaline and DOPEG, and the reserpine-like compound Ro 4-1284 selectively increased the outflow of DOPEG; DOMA remained below the detection limit. When atria were exposed to (–)-noradrenaline 1.7 or 17 M, the subsequent outflow of noradrenaline and DOPEG was enhanced. Moreover, substantial amounts of DOMA were now found. This outflow of DOMA was prevented when atria were exposed to (–)-noradrenaline in the presence of cocaine or after an initial incubation with amezinium. Exposure to (–)-noradrenaline 1.7 M mainly enhanced the formation of DOPGE, while exposure to (+)-noradrenaline 1.7 M mainly enhanced the formation of DOMA.Our experiments confirm some and qualify other conclusions drawn from studies in which exogenous ³H-noradrenaline had been used to examine the metabolism of noradrenaline in guinea-pig atria. In agreement with the isotope studies, DOPEG is a major metabolite of endogenous noradrenaline. In contrast to what the isotope studies had suggested, however, endogenous DOMA is a very minor product, at least as long as the neurones are at rest. DOMA is only formed when the tissue is exposed to high concentrations of exogenous noradrenaline. In further contrast to previous conclusions, DOMA is then formed intra- and not extraneuronally.     

Effect of progesterone on norepinephrine release from mouse adrenergic terminals in vitro

1. The in vitro effect of progesterone on norepinephrine (NE) release and contractile activity was analyzed in uterine horns from estrogen-primed and progesterone-primed mice. 2. Progesterone (6-10 nmol/ml) evoked the release of [3H]NE above basal levels from uterine horns in both experimental conditions, the effect of progesterone on estrogen-primed being more important than on progesterone-primed mice uterus. 3. Progesterone also increased electrically evoked [3H]NE release in estrogen-primed uterine tissue, nevertheless no effect was observed in progesterone-primed ones. 4. Progesterone (0.6-10 nmol/ml) inhibited uterine horn isometric contractions only in estradiol-primed mice. This effect was partially blocked in uterine horns from reserpine-treated mice and when propanolol (1 microM) was added to the preparation of estradiol-primed mice uterus.     

Local modulation of noradrenaline release in vivo: presynaptic beta 2-adrenoceptors and endogenous adrenaline

Isoprenaline bitartrate (0.5 microgram/kg/min i.v.) increased the rate of noradrenaline release into the circulation of pentobarbitone-anesthetized rabbits. This increase was much greater than that produced by an equi-hypotensive dose of the vasodilator hydralazine (0.2 mg/kg i.v.), suggesting that it was only partly due to baro-reflex activation of sympathetic nerves. This facilitatory effect of isoprenaline was also observed in the nephrectomized, pithed rabbit, with electrically stimulated sympathetic outflow, ruling out central nervous system and renin-angiotensin effects. ICI 118,551 HCl (0.3 mg/kg + 0.1 mg/kg/h i.v.) blocked the isoprenaline-induced hypotension, but did not affect the isoprenaline-induced tachycardia, suggesting that it selectively blocked beta 2-adrenoceptors. ICI 118,551 totally abolished the isoprenaline-induced increase in noradrenaline release, suggesting a beta 2-effect. Atenolol (0.3 mg/kg + 0.1 mg/kg/h) blocked the isoprenaline-induced tachycardia, a beta 1-effect, but only slightly attenuated the isoprenaline-induced increase in noradrenaline release. Atenolol by itself decreased heart rate and arterial pressure, but there was no reflex rise in the noradrenaline release rate, which suggests that atenolol impairs baroreceptor activation of sympathetic nerves. In another series of experiments, also in the pentobarbitone-anesthetized rabbit, adrenaline was released into the circulation by splanchnic nerve stimulation. This resulted in prolonged increases of adrenaline levels in heart tissue. After the plasma adrenaline levels had returned to prestimulation values, the rate of noradrenaline release into the plasma was enhanced. This increase was not observed in rabbits treated with either desipramine HCl (1 mg/kg i.v.) or propranolol HCl (2 mg/kg i.p.).(ABSTRACT TRUNCATED AT 250 WORDS)     

Dopaminergic modulation of hippocampal noradrenaline release

Summary ³H-Noradrenaline release in the rabbit hippocampus and its possible modulation via presynaptic dopamine receptors was studied. Hippocampal slices were preincubated with ³H-noradrenaline, continuously superfused in the presence of cocaine (30 mol/l) and subjected to electrical field stimulation. The electrically evoked tritium over-flow from the slices was reduced by 0.1 and 1 mol/l dopamine and apomorphine, but significantly enhanced by 10 mol/l apomorphine or by 0.1 and 1 mol/l bromocriptine. If the ₂-adrenoceptor antagonist yohimbine (0.1 mol/l) was present throughout superfusion, the inhibitory effects of dopamine and apomorphine were more pronounced and even 10 mol/l apomorphine and 1 mol/l bromocriptine inhibited noradrenaline release. Qualitatively similar observations were made in the presence of another ₂-antagonist, idazoxane (0.1 mol/l). In the presence of the D2-receptor antagonist domperidone (0.1 mol/l) the inhibitory effects of dopamine were almost abolished, whereas both apomorphine (>1 mol/l) and bromocriptine (>0.01 mol/l) greatly facilitated noradrenaline release. The D2-receptor agonist LY 171555 (0.1 and 1 mol/l) significantly reduced the evoked noradrenaline release whereas the D1-selective agonist SK & F 38393 was ineffective at similar concentrations. The effects of LY 171555 were abolished in the presence of domperidone (0.1 mol/l) but remained unchanged in the presence of yohimbine or idazoxane (0.1 mol/l, each).At 1 mol/l the D2-receptor antagonists domperidone and (-)sulpiride significantly increased the evoked noradrenaline release by about 10%. However, at this concentration, domperidone (but not (-)sulpiride) affected also basal tritium outflow. Bulbocapnine and the preferential D1-receptor antagonists SCH 23390 enhanced the evoked noradrenaline release already at 0.1 mol/l. Their marked facilitatory effects (50 to 60% increase at 1 mol/l) were reduced in the presence of idazoxane (0.1 mol/l) and almost abolished in the presence of 0.1 mol/l yohimbine, whereas the increase due to 1 mol/l (-)sulpiride persisted under these conditions.The evoked tritium efflux from rabbit hippocampal slices preincubated with ³H-serotonin was not affected by dopamine receptor agonists.From our results we conclude that hippocampal noradrenaline, but not serotonin release, is modulated via D2-dopamine receptors. In addition, our results provide evidence for more or less pronounced ₂-adrenoceptor agonistic properties of dopamine and ₂-adrenoceptor antagonistic properties of apomorphine, bromocriptine, SCH 23390 and bulbocapnine in this noradrenaline release model from CNS tissue.     

Muscarinic inhibition of acetylcholine release from a novel in vitro preparation of the guinea-pig trachea

Summary An isolated preparation of the guinea-pig trachea is described which allows the simultaneous measurement of acetylcholine release and smooth muscle contraction. Incubation of the epithelium-free preparation with [³H]choline resulted in the formation of [³H]acetylcholine. Electrical stimulation caused the release of [³H]acetylcholine and a contractile response. Tetrodotoxin and omission of calcium from the medium abolished both the evoked release and contractions.The muscarinic agonists oxotremorine, carbachol and pilocarpine concentration-dependently inhibited the electrically evoked acetylcholine release and contracted the tracheal smooth muscle. Pre- and postsynaptic EC₅₀ values for a given agonist were not different. Atropine (100 nmol/l) significantly faciliated the evoked acetylcholine release. A concentration of 10 nmol/l atropine did not change the evoked release but antagonized the inhibitory effect of oxotremorine. It is concluded that presynaptic muscarine autoreceptors inhibit the release of acetylcholine from parasympathetic nerves of the guinea-pig trachea.Send offprint requests to G. D'Agostino at the above address     

The recovery of noradrenaline in adrenergic nerve terminals of the rat after reserpine treatment

Tissue concentrations of endogenous noradrenaline in heart, submaxillary gland, and gastrocnemic muscle have been examined after one large dose of reserpine (10 mg/kg) to rats. After the initial depletion of the amine, the concentration started to rise between 24 and 36 h. For about one week thereafter the amine recovery proceeded comparatively fast, then the rate of the recovery slowed. Between the 4th and the 6th weeks there was a pronounced drop in the noradrenaline concentration in all three tissues, apparently beginning in the 4th week with a maximal decrease of about 20% in the 5th week after reserpine. Thereafter the concentrations increased to approach normal about 6 weeks after reserpine. These results are discussed in relation to the axonal down-transport of newly formed amine storage granules and to the life-span of these granules in the nerve terminals. The different parts of the noradrenaline recovery curve appeared to reflect the axonal down-flow of granules. A theoretical recovery curve was calculated, based on granular transport. This curve was similar to the observed recovery curve. The claim is made that the recovery of adrenergic function and noradrenaline levels after reserpine is due to a down-transport of newly formed, amine storage granules to the nerve terminals. There seems little need for the theory that the storage function reappears in old, reserpine-blocked granules, as a mechanism for noradrenaline recovery after a large dose of reserpine.     

Calcium channel blockers apparently decrease noradrenaline release from nerve-skin terminals in Caudiverbera caudiverbera

• 1.1. The effects of three calcium channel blockers, verapamil, diltiazem and nifedipine were examined on the response of the skin neuroepithelial synapse of the toad Caudiverbera caudiverbera to electrical stimulation.
• 2.2. The stimulus induced a significant rise in potential difference (PD) and short-circuit current (SCC). The three calcium antagonists reduced the responses in a dose-dependent manner. The greatest reduction was induced by verapamil followed by diltiazem and nifedipine.
• 3.3. Amiloride treatment did not affect the responses to electrical stimulation, indicating that the response to nerve stimulation is not due to current flowing through sodium channels.
• 4.4. When the preparation was blocked by either of the three antagonists, the skin response to noradrenaline was not affected.
• 5.5. It may be concluded that verapamil, diltiazem or nifedipine reduce the release of noradrenaline at the neuroepithelial synapse of C. caudiverbera.

     

Acetylcholine release from the nerve terminals of the guinea-pig ileum by Naja venoms.

N. nigricollis, N. naja, N. nivea and N. melanoleuca produced contraction of the guinea-pig ileum which was not blocked by hexamethonium tetrodotoxin, or mepyramine but markedly reduced by atropine. In small doses the venoms increased the tone and twitch height of the electrically stimulated guinea-pig ileum. However, in large doses they produced a blockade of the twitches. All Naja venoms studied significantly increased the resting output of acetylcholine from the longitudinal muscle strip. This increase in acetylcholine was not prevented by hexamethonium, tetrodotoxin, mepyramine or by desensitization of 5-HT receptors, but was blocked by adrenaline, calcium lack and magnesium excess. It is concluded that Naja venoms increase the spontaneous release of acetylcholine from the longitudinal muscle strip of the guinea-pig ileum through inhibition of (Na⁺K⁺Mg²⁺) activated ATP-ase in the nerve terminal membrane.     

Assessment of acetylcholine release from myenteric plexus of guinea-pig colon

Due to a complex mechanical tissue response to electrical stimulation, a colonic smooth muscle-myenteric plexus (SMMP) preparation, combined with radiolabelling techniques for assessing the acetylcholine release, has been developed to investigate intrinsic cholinergic nerve activity in the guinea-pig colon. Electrical field stimulation of the preparation gave reproducible release of label which was inhibited by tetrodotoxin. Release increased proportionally with the strength of stimulus (130, 150, 195 and 250 mA), but was inversely proportional to frequency of stimulation for a fixed number of pulses, 1 Hz releasing more than 10 Hz. Electrical stimulation of the tissue during incubation with [3H]choline enhanced the release by subsequent field stimulation. Release of label increased progressively towards the distal end of the colon.     

Facilitatory and inhibitory modulation by endogenous adenosine of noradrenaline release in the epididymal portion of rat vas deferens

Summary The present study aimed at determining the modulation by adenosine of the release of noradrenaline in the epididymal portion of the rat vas deferens. The tissues were treated with pargyline and perifused in the presence of desipramine and yohimbine. Up to four periods of electrical stimulation were applied (5 Hz, 9 min).The A₁-adenosine receptor selective agonist R-N⁶-phenylisopropyladenosine (R-PIA; 100–900 nmol·l^–1) reduced, whereas the A_2A-receptor selective agonist 2-p-(2-carboxyethyl)phenethylamino-5-N-ethylcarboxamidoadenosine (CGS21680; 3–30nmol·l^–1) increased the electrically-evoked noradrenaline overflow in a concentration-dependent manner. The nonselective agonist 5-N-ethy1carboxamidoadenosine (NECA; 30–300 nmol·l^–1) reduced noradrenaline overflow, but the effect did not depend on the concentration. Adenosine deaminase at the concentration of 0.5 ·ml^–1 decreased but at that of 2.0 ·ml^–1 increased noradrenaline overflow. The inhibitors of adenosine uptake, S-(4-nitrobenzyl)-6-thioinosine (NBTI; 50 nmol·l^–1) and dipyridamole (3 mol·l^–1), increased the electrically-evoked noradrenaline overflow. The A₁-adenosine receptor antagonist 1,3-dipropyl-8-cyclopentylxanthine (DPCPX; 20 nmol·l^–1) caused an increase whereas the A₂-adenosine receptor antagonist 3,7-dimethyl-1-(2-propynyl)xanthine (DMPX; 0.1 mol·l^–1) caused a decrease. NBTI (50 nmol·l^–1), partially antagonized the effect of both DPCPX (20 nmol·l^–1) and DMPX (0.1 mol·l^–1).It is concluded that, in the epididymal portion of the rat vas deferens, endogenous adenosine tonically modulates the release of noradrenaline evoked by electrical stimulation, through activation of both inhibitory (A₁) and facilitatory (A_2A) adenosine receptors.Abbreviations CGS 21680 2-p-(2-carboxyethyl)phenethylamino-5-N-ethylcarboxamidoadenosine - DMPX 3,7-dimethyl-l-(2-propynyl)xanthine - DPCPX 1,3-dipropyl-8-cyclopentylxanthine - NBTI S-(4-nitrobenzyl)-6-thioinosine - NECA 5-N-ethylcarboxamidoadenosine - R-PIA R-N⁶-phenylisopropyladenosine Correspondence to J. Gongalves at the above address     

Co-release of endogenous ATP and noradrenaline from guinea-pig mesenteric veins exceeds co-release from mesenteric arteries

1. The present study was designed to compare the overflow of sympathetic neurotransmitters of guinea-pig inferior mesenteric artery and mesenteric vein evoked by electrical field stimulation (EFS) with special emphasis on the simultaneous release of ATP and noradrenaline (NA). The stimulation-evoked overflow of ADP, AMP and adenosine was also evaluated. 2. Endothelium-denuded segments of inferior mesenteric arteries or veins were superfused in a small volume (200 microL)-chamber for EFS and subsequent detection of NA (HPLC- electrochemical detection) and adenine nucleotides and adenosine (HPLC-fluorescence detection) in samples of the superfusate. 3. Both arteries and veins responded to EFS (15 V, 4-16 Hz, 0.3 msec for 60 s) with overflow of ATP and NA in a tetrodotoxin (1 micromol/L)- and guanethidine (10 micromol/L)-sensitive manner. The EFS-evoked overflow of NA in veins exceeded the overflow of NA in arteries at all frequencies of stimulation, whereas the EFS-evoked overflow of ATP, ADP and AMP in veins exceeded the overflow of adenine nucleotides in arteries at 4 and 8 Hz but not at 16 Hz stimulation. The EFS-evoked overflow of adenosine was similar in arteries and veins. 4. Activation of alpha1-adrenoceptors with methoxamine (10 micromol/L) did not produce overflow of ATP. 5. Blockade of alpha1/alpha2-adrenoceptors with phentolamine (1 micromol/L) did not affect EFS-evoked overflow of ATP, ADP, AMP and adenosine. 6. It is concluded that overflow of ATP and NA from sympathetic nerves may constitute an effective mechanism in the complex balance between capacitance and resistance in splanchnic circulation.