Adenosine modulation of non-adrenergic non-cholinergic neurotransmission in isolated guinea-pig atria

Summary Transmural stimulation of non-adrenergic, non-cholinergic sensory nerves in guinea-pig atria, isolated from reserpine-pretreated animals, in the presence of atropine and the beta-adrenoceptor-blocking drug CGP 20712A, induced a positive inotropic effect. Adenosine (0.1–10 M) concentration-dependently reduced the cardic response to transmural nerve stimulation, without modifying the response to exogenous calcitonin gene-related peptide; the inhibitory effect of adenosine was antagonized by 1 M 8-phenyltheophylline. Moreover, the cardiac response to field stimulation was enhanced by 8-phenyltheophylline (0.1, 1 M) and by adenosine deaminase (1 g/ml), but was reduced by dipyridamole (1 M). These findings indicate the presence of inhibitory adenosine receptors on cardiac sensory nerves and suggest a modulatory effect of endogenous adenosine on cardiac non-adrenergic, non-cholinergic neurotransmission.Send offprint requests to A. Rubino at the above address     

Multiple inhibitory mechanisms mediate non-adrenergic non-cholinergic relaxation in the circular muscle of the guinea-pig colon

Summary The mechanisms responsible for nerve-mediated, non-adrenergic, non-cholinergic (NANC) relaxation in mucosa-free circular muscle strips from the proximal colon of the guinea-pig were investigated. Electrical field stimulation (EFS, 1–20 Hz, trains of 5 s duration, 100 V, 0.25 ms pulse width) in the presence of atropine (1 mol/l) and guanethidine (3 mol/l) evoked a triphasic motor response consisting of. (a) a primary relaxation, (b) a rebound contraction and (c) a secondary relaxation. These three responses were abolished by tetrodotoxin (1 mol/l). Both apamin (0.01–0.3 mol/l), a known blocker of low conductance, calcium-activated potassium channels in smooth muscles, and L-nitroarginine (L-NOARG) (1–100 mol/l), a known blocker of nitric oxide (NO) synthase, increased the tone of the strips. Maximum effects on tone were observed with 0.1 mol/l apamin (21 ± 3% of KCl-induced contraction) and 30 mol/l L-NOARG (26 ± 4% of KCl response). The combined administration of 0.1 mol/l apamin and 30 mol/l L-NOARG produced an increase in tone (47 ± 5% of KCl response) that was larger than that produced by either compound alone. Neither apamin (0.1 mol/l) nor L-NOARG (30 mol/l) affected the isoprenaline-induced relaxation.Apamin (0.1 mol/l) depressed, but did not abolish, the primary relaxation to EFS at all frequencies without affecting the secondary relaxation. Apamin also enhanced the rebound contraction at a frequency of 1 Hz. L-NOARG (30 mol/l) depressed, but did not abolish, the primary relaxation to EFS at all frequencies, had no effect on the rebound contraction and inhibited the secondary relaxation evoked at frequencies of 1–5 Hz, but not 10–20 Hz. L-arginine (300 mol/l) reversed the effect of L-NOARG on tone and the inhibitory effect on the EFS-evoked relaxation. In the presence of apamin and L-NOARG, the primary relaxation was suppressed at all frequencies; the secondary relaxation was inhibited at 1–5 Hz and unchanged at 10–20 Hz, as observed with L-NOARG alone. We conclude that three distinct mechanisms mediate the NANC relaxation of the circular muscle of the proximal colon of the guinea-pig in response to EFS. One mechanism can be operationally defined as apamin-sensitive and a second as L-NOARG-sensitive, the latter implying a possible role of NO as an inhibitory transmitter. A third NANC inhibitory mechanism, which is apamin- and L-NOARG-resistant, is also suggested.Correspondence to: C. A. Maggi at the above address     

Adenosine deaminase antagonizes inhibitory responses to adenosine and non-adrenergic, non-cholinergic inhibitory nerve stimulation in isolated preparations of guinea-pig trachea.

In guinea-pig isolated tracheal tube preparations treated with dipyridamole, adenosine deaminase antagonized inhibitory responses to adenosine and non-adrenergic, non-cholinergic nerve stimulation. Inhibitory responses to vasoactive intestinal peptide (VIP) were unaffected and hyoscine-sensitive excitatory responses to field stimulation were not reduced. The evidence supports a role of adenosine as an inhibitory neurotransmitter in non-adrenergic, non-cholinergic nerves in trachea.     

The coexistence of adenosine A1 and A2 receptors in guinea-pig aorta

The effects of adenosine, 5'-(N-ethyl)carboxamidoadenosine (NECA), 2-chloroadenosine (2-CA), No-cyclohexyladenosine (CHA) and N6(R-2-phenylisopropyl)-adenosine (R-PIA) on the tone of phenylephrine-constricted guinea-pig isolated aorta have been examined. For aortic relaxation the analogues exhibited the following rank order of potency: NECA > adenosine > 2-CA > R-PIA > CHA. This is consistent with previous reports that relaxation of this tissue is mediated by the adenosine A₂ receptor. An unexpected finding was that R-PIA, 2-CA and CHA all induced contractions at concentrations lower than were required for relaxation, giving a biphasic dose-response curve. Neither NECA nor adenosine contracted the aorta. This is consistent with activation of vascular A₁ receptors. An A₁-selective concentration of the antagonist l,3-dipropyl-8-cyclopentyl xanthine abolished the contraction elicited by R-PIA in the guinea-pig aorta. This further suggests that the contraction is mediated by a₁ receptors.     

Characterization of opioid receptors modulating the function of capsaicin-sensitive neurons in guinea-pig atria

Transmural nerve stimulation of isolated atria, obtained from reserpine-pretreated guinea-pigs, in the presence of atropine and the β₁-adrenoceptor-blocking drug CGP 20712A, induced a positive inotropic effect. [D-Ala², N-Me-Phe⁴, Gly⁵-ol] enkephalin (DAGO), [D-Ala², D-Leu⁵]enkephalin (DADLE), morphine and dynorphin dose dependently reduced the cardiac response to transmural nerve stimulation. The δ receptor selective agonist [D-Pen², D-Pen⁵]enkephalin (DPDPE), and the κ receptor agonist, U50488, were unable to modify the response. The inhibitory effect of all the active opioid agonists was antagonized by naloxone but not by the selective δ and κ opioid receptor antagonists, ICI 174.864 and MR 2266. These results suggest the presence on sensory nerve terminals of inhibitory opioid receptors belonging to the μ, but not to the δ and κ subtypes.     

Neurotensin receptors on the ileum of the guinea-pig: evidence for the coexistence of inhibitory and excitatory receptors

Neurotensin caused a complex muscular response of the longitudinal muscle-myenteric plexus preparation of guinea-pig ileum: picomoles of neurotensin produced inhibition while larger concentrations caused an inhibitory effect followed by a delayed dose-dependent contraction. The inhibitory phase of the neurotensin-induced muscular activity was not modified by tetrodotoxin but was potently antagonized in a non-competitive manner by apamin, a bee venom toxin. The contractile component was blocked by tetrodotoxin but not by apamin. These toxins were used to dissect the neurotensin muscular response into an inhibitory phase and an excitatory component. It was possible to further characterize the two neurotensin muscular components by their kinetics of desensitization. The inhibitory neurotensin response showed a fast rate of desensitization and presented a relatively low spare receptor capacity. In contrast, desensitization to the excitatory action of neurotensin was much slower, the excitatory receptors apparently having a larger spare receptor capacity. Desensitization to the action of neurotensin was selective for the neuropeptide not altering the contractile activity of substance P, angiotensin II, bradykinin, histamine or acetylcholine. These results strongly suggest the presence of two subsets of neurotensin receptors in the ileum: the inhibitory set probably localized at the postsynaptic effector level and excitatory neurotensin receptors probably of neuronal origin whose function is probably to modulate the release of neurotransmitters. The physiological implications of these two subtypes of neurotensin receptors in the control of gastrointestinal motility are discussed.     

Role of Ca2+ in non-cholinergic,non-adrenergic responses to nerve stimulation of the guinea-pig small intestine

Summary The effects of calcium channel blockers (D-600, verapamil), sodium nitroprusside, papaverine, indomethacin, local anaesthetics and blockade of sodium pump activity on the non-cholinergic, non-adrenergic transmission in the guinea-pig duodenum, jejunum, proximal and terminal ileum were analysed in the presence of atropine and guanethidine.A decrease of the extracellular Ca²⁺ concentration inhibited the primary and rebound contractions but only in Ca²⁺-free solution was the primary relaxation diminished. D-600, verapamil, sodium nitroprusside and papaverine inhibited both the primary and rebound contractions to the same degree and their effects on the primary relaxation were less pronounced than on the contractions.Indomethacin dissolved in alkaline solution did not depress the non-cholinergic, non-adrenergic responses in any region of the small intestine, whereas indomethacin dissolved in ethanol antagonized both the primary and rebound contractions in the muscles.Local anaesthetics (procaine, trimecaine) in low concentrations inhibited only the primary contraction. Higher concentrations also inhibited both the rebound contraction and primary relaxation. Procaine in low concentrations did not markedly affect the non-cholinergic, non-adrenergic i.j.p.s and e.j.p.s., but did block the action potentials induced by e.j.p.s.Our findings indicate that the primary relaxation, and primary and rebound contractions are probably induced by different mechanisms and are not mediated by ATP. We confirmed that prostaglandins did not participate in the generation of the rebound contraction.This study was supported in part by JSPS Japan     

Different effector mechanisms for ATP and adenosine hyperpolarization in the guinea-pig taenia coli

The hyperpolarizations in response to ATP and adenosine in the guinea-pig taenia coli, measured by the sucrose-gap technique at room temperature, were compared in solutions of modified ionic composition. ATP hyperpolarization was increased in low chloride and in low potassium, but was reduced to 12% of control in calcium-free solution on second application of the agonist. The response to adenosine, however, was decreased in low chloride, unchanged in low potassium and was 45% of control in calcium-free solution. The different mechanisms for ATP and adenosine hyperpolarization provide evidence for the presence of separate receptors.     

The role of nitric oxide in inhibitory non-adrenergic non-cholinergic neurotransmission in the canine lower oesophageal sphincter.

1. The role of nitric oxide (NO) in non-adrenergic non-cholinergic (NANC) neurotransmission was studied on circular muscle strips of the canine lower oesophageal sphincter (LOS). Electrical field stimulation evoked frequency-dependent relaxations, which were resistant to adrenergic and cholinergic blockade and abolished by tetrodotoxin. 2. Exogenous administration of NO induced concentration-dependent and tetrodotoxin-resistant relaxations which mimicked those in response to electrical stimulation. 3. NG-nitro-L-arginine (L-NNA), a stereospecific inhibitor of NO-biosynthesis, inhibited the relaxations induced by electrical stimulation but not those by exogenous NO or vasoactive intestinal polypeptide (VIP). 4. The effect of L-NNA was prevented by L-arginine, the precursor of the NO biosynthesis but not by its enantiomer D-arginine. 5. Haemoglobin abolished the NO-induced responses and reduced those evoked by electrical stimulation. 6. Cumulative administration of VIP induced concentration-dependent relaxations, which were slow in onset and sustained. A complete relaxation to VIP was not achieved and the relaxations were not affected by L-NNA. 7. In conclusion, our results provide evidence that NANC relaxations are mediated by NO, suggesting NO or a NO releasing substance as the final inhibitory NANC neurotransmitter in the canine LOS.     

Separate receptors mediating the positive inotropic and chronotropic effect of histamine in guinea-pig atria

The direct positive inotropic effect of histamine was studied on paced left atrial preparations from guinea pigs. Histamine (10^?8 to 10^?4 M) increased the maximum tension developed in left atria incubated at 35°C and driven at 2 Hz. The maximum increase in tension was 60% of that observed with norepinephrine. Metiamide (3 × 10^?5 M; a specific H₂-receptor antagonist) did not alter the inotropic response of left atria to histamine. However, tripelennamine (a typical H₁-receptor antagonist) competitively shifted the histamine inotropic dose—response curve to the right at concentrations from 10^?8 to 10^?7 M. Higher concentrations (3 × 10^?7 and 10^?6 M) caused little further additional shift to the right. The positive chronotropic effect of histamine on spontaneously beating atria was competitively antagonized by metiamide (10^?6 and 3 × 10^?6 M). These results demonstrate that in guinea-pig atria histamine increases myocardial contractility by an interaction with receptors closely related to classical H₁-receptors while its chronotropic effects is mediated by interaction with H₂-receptors.     

The coexistence of adenosine A1 and A2 receptors in guinea-pig aorta

The effects of adenosine, 5'-(N-ethyl)carboxamidoadenosine (NECA), 2-chloroadenosine (2-CA), No-cyclohexyladenosine (CHA) and N6(R-2-phenylisopropyl)-adenosine (R-PIA) on the tone of phenylephrine-constricted guinea-pig isolated aorta have been examined. For aortic relaxation the analogues exhibited the following rank order of potency: NECA > adenosine > 2-CA > R-PIA > CHA. This is consistent with previous reports that relaxation of this tissue is mediated by the adenosine A₂ receptor. An unexpected finding was that R-PIA, 2-CA and CHA all induced contractions at concentrations lower than were required for relaxation, giving a biphasic dose-response curve. Neither NECA nor adenosine contracted the aorta. This is consistent with activation of vascular A₁ receptors. An A₁-selective concentration of the antagonist l,3-dipropyl-8-cyclopentyl xanthine abolished the contraction elicited by R-PIA in the guinea-pig aorta. This further suggests that the contraction is mediated by a₁ receptors.     

Pharmacological characterization of adenosine A1 and A2 receptors in the bladder: evidence for a modulatory adenosine tone regulating non-adrenergic non-cholinergic neurotransmission.

1. The nerve-evoked contractions elicited by transmural electrical stimulation of mouse urinary bladders superfused in modified Krebs Ringer buffer containing 1 microM atropine plus 3.4 microM guanethidine were inhibited by adenosine (ADO) and related nucleoside analogues with the following rank order of potency: R-phenylisopropyladenosine (R-PIA) greater than cyclohexyladenosine (CHA) greater than 5'N-ethylcarboxamido adenosine (NECA) greater than ADO greater than S-phenylisopropyladenosine (S-PIA). Tissue preincubation with 8-phenyltheophylline (8-PT) displaced to the right, in a parallel fashion, the NECA concentration-response curve. 2. The contractions elicited by application of exogenous adenosine 5'-triphosphate (ATP) were also inhibited by ADO and related structural analogues. The rank order of potency to reduce the motor response to ATP was: NECA greater than 2-chloroadenosine (CADO) greater than R-PIA greater than ADO greater than CHA greater than S-PIA. 3. The ADO-induced ATP antagonism was of a non-competitive nature and was not specific. Tissue incubation with 10 microM NECA not only reduced the motor responses elicited by ATP, but also 5-hydroxytryptamine, acetylcholine and prostaglandin F2 alpha. The action of NECA was antagonized following tissue preincubation with 8-PT. The inhibitory action of NECA was not mimicked by 10 microM CHA. 4. The maximal bladder ATP contractile response was significantly increased by tissue preincubation with 5-30 microM 8-PT. 5. The 0.15 Hz evoked muscular twitch was significantly increased by 8-PT while dipyridamole consistently reduced the magnitude of the twitch response. These results are consonant with the hypothesis that an endogenous ADO tone modulates the bladder neurotransmission.(ABSTRACT TRUNCATED AT 250 WORDS)     

Evidence that prostaglandin is responsible for the 'rebound contraction' following stimulation of non-adrenergic, non-cholinergic ('purinergic') inhibitory nerves.

The prostaglandin synthesis inhibitor, indomethacin, blocks the "rebound contractions" which characteristically follow the inhibitory responses of the guinea-pig taenia coli to non-adrenergic, non-cholinergic (purinergic) nerves and to exogenously applied ATP, without affecting the responses to periaterial adrenergic nerves. Since adenine nucleotides are known to induce prostaglandin synthesis, this result is consistent with the purinergic hypothesis and suggests that purinergic nerves may form a link with prostaglandin in the physiological regulation of a variety of organs.     

Modulation of non-adrenergic non-cholinergic inhibitory neurotransmission in rat gastric fundus by the alpha 2-adrenoceptor agonist, UK-14,304.

1. The influence of the alpha 2-adrenoceptor agonist, UK-14,304, on non-adrenergic non-cholinergic (NANC) relaxation induced by electrical field stimulation was investigated in longitudinal muscle strips of the gastric fundus of reserpinized rats. 2. In tissues where tone was raised by 3 x 10(-7) M prostaglandin F2 alpha (PGF2 alpha), the inhibitory effect of 10(-6) M UK-14,304, on the NANC relaxations induced by short train stimulation (40 V, 1 ms, 20 s) was inversely related to the stimulus frequency (1-4-16 Hz). UK-14,304 (10(-6) M) did not influence relaxations induced by administration of exogenous nitric oxide (NO, 2 x 10(-6) M-10(-4) M). The inhibitory effect of UK-14,304 on the electrically induced relaxations was antagonized by 10(-6) M rauwolscine but not by 10(-6) M prazosin. 3. UK-14,304 (10(-6) M) also reduced the amplitude of the sustained NANC relaxation, induced by electrical field stimulation (40 V, 1 ms, 4 Hz) for 5 min. The effect of UK-14,304 was also antagonized by 10(-6) M rauwolscine but not by 10(-6) M prazosin. UK-14,304 (10(-6) M) did not reduce the relaxation induced by 3 x 10(-9) M vasoactive intestinal polypeptide (VIP). 4. These results suggest that the release of the inhibitory NANC neurotransmitter during short train stimulation, thought to be NO, and during sustained stimulation, thought to be VIP, is inhibited by stimulation of presynaptic alpha 2-adrenoceptors in the rat gastric fundus.     

Adenosine receptor modulation of sympathetic neurotransmission in rat isolated kidney

In the present study we set out to define, using discriminatory agonists and antagonists, the adenosine receptors modulating sympathetic neurotransmission in the rat kidney. Isolated kidneys from male Wistar rats were perfused with modified Krebs-Henseleit buffer solution at constant flow. The neuronal noradrenaline stores were labeled with ³H-noradrenaline and the renal nerves stimulated electrically (2 Hz, 3 msec, 9 mA, during 20 sec at intervals of 6 min). ³H overflow was taken as an index of ³H-noradrenaline release. The A₁ receptor selective agonists N⁶-cyclopentyladenosine (CPA), N⁶-cyclohexyladenosine (CHA), and N⁶-[R(−)-1-phenyl-2-propyl]adenosine (R-PIA), and the mixed A₁/A_2A receptor agonists 5′-N-ethylcarboxamidoadenosine (NECA) and 2-chloroadenosine (CADO) inhibited evoked ³H outflow concentration-dependently. The selective A_2A receptor agonist 2-[p-(2-carboxyethyl)phenylethylamino]-5′-N-ethylcarboxamidoadenosine (CGS 21680), at concentrations selective for A_2A receptors, failed to modify ³H outflow, whereas at higher concentrations it induced inhibition. The rank order of potency of agonists, CPA > CHA = R-PIA > NECA > CADO >> CGS 21680, is typical for an interaction with the A₁ receptor. 1,3-Dipropyl-8-cyclopentylxanthine (DPCPX), at concentrations selective for blockade of A₁ receptors, blocked concentration-dependently the inhibitory effects of CPA and NECA; no evidence of an increase in outflow was seen with NECA in the presence of DPCPX. The selective A_2A receptor antagonist 9-chloro-2-(2-furanyl)[1,2,4]triazol[1,5-c] quinazoline-5-amine (CGS 15943) did not influence the agonist effects at concentrations interacting selectively with A_2A receptors but antagonized them concentration-dependently at higher, non-selective concentrations. Taken together, our data establish the presence of inhibitory adenosine A₁ receptors on the terminal sympathetic neurons of rat kidney. No evidence was obtained for the presence of functional A_2A receptors in this preparation. © 1996 Wiley-Liss, Inc.     

Purine antagonists in the identification of adenosine-receptors in guinea-pig trachea and the role of purines in non-adrenergic inhibitory neurotransmission

1 To test the possibility that adenosine receptors exist within the trachea of the guinea-pig, an attempt has been made to identify a compound with adenosine antagonist activity in this tissue.

2 Quinidine, phentolamine, phenoxybenzamine, 2-2′-pyridylisatogen tosylate (PIT) and caffeine were tested for antagonism of spasmolytic responses to adenosine, adenosine 5′-triphosphate (ATP) and adenine on the guinea-pig isolated trachea.

3 Quinidine (10 and 25 μg/ml), phentolamine (10 and 30 μg/ml) and phenoxybenzamine (10 μg/ml) had little or no effect on response to adenosine, ATP and adenine. PIT (21 μg/ml) potentiated responses to adenosine, ATP and adenine by an unexplained mechanism.

4 Caffeine (25 μg/ml) partially relaxed the trachea and inhibited spasmolytic responses to both adenosine and ATP, but not to adenine, isoprenaline, aminophylline or prostaglandin E₂ (PGE₂).

5 A number of compounds related to caffeine (xanthine, hypoxanthine, theophylline and theobromine) were tested for adenosine antagonist activity. Xanthine (300 μg/ml) and hypoxanthine (300 μg/ml) did not relax the trachea or antagonize spasmolytic responses to adenosine. Both theophylline (10 μg/ml) and theobromine (30 μg/ml) partially relaxed the trachea; theophylline, but not theobromine, antagonized spasmolytic responses to adenosine.

6 pA₂ values for caffeine and theophylline as antagonists of adenosine were 4.3 and 4.7 respectively. However, the slopes of the Schild plot regressions were significantly less than 1.0 for both compounds.

7 Four compounds, adenine, AH 8883, M30966 and ICI 63197, which like caffeine and theophylline, have phosphodiesterase inhibitory activity were tested for adenosine antagonist activity in the trachea. Adenine and AH 8883 had no effect and M30966 and ICI 63197 caused significant potentiation.

8 The effects of caffeine and theophylline were also investigated on the non-adrenergic inhibitory response to nerve stimulation (NAIR). Both caffeine (100 μg/ml, n = 4) and theophylline (30 μg/ml, n = 4) enhanced the NAIR (20 Hz) while virtually abolishing matched responses to exogenous adenosine.

9 The results support the existence of adenosine receptors in the guinea-pig trachea.

     

Non-cholinergic,non-adrenergic responses to nerve stimulation of different regions of the guinea-pig small intestine

Summary Non-cholinergic, non-adrenergic responses to nerve stimulation recorded from smooth muscles of the guinea-pig duodenum, jejunum, proximal and terminal ileum were investigated in an attempt to characterize these responses.In the presence of atropine (1–2 mol · l^–1) and guanethidine (10 mol · l^–1) coaxial stimulation induced in all regions of the guinea-pig small intestine an initial relaxation (primary relaxation) upon which contraction (primary contraction) appeared, followed by rebound contraction.Noradrenaline decreased the cholinergic smooth muscle twitches, predominantly at low stimulation frequencies, and had a similar effect on the non-cholinergic, non-adrenergic primary relaxation, primary and rebound contractions.ATP decreased the smooth muscle twitches; however, this agent had only a transient influence on the non-cholinergic, non-adrenergic responses of muscle (tension and membrane potential) to single stimuli. With higher stimulus frequencies ATP increased the primary relaxation and decreased the contraction phases. ATP also inhibited the post-tetanic inhibition induced by non-cholinergic, non-adrenergic nerve stimulation.In most of the muscle cells of the guinea-pig proximal and terminal ileum the non-cholinergic, non-adrenergic nerve stimulation generated i.j.p.s., while about 15–20% of the cells responded with e.j.p.s. During long-lasting stimulation (10s) the i.j.p.s. were sometimes interrupted by action potentials or by a gradual depolarization of the membrane. The i.j.p.s. were followed by a marked rebound depolarization accompanying the action potentials. Those cells which generated i.j.p.s. in response to field stimulation, were depolarized by ATP, while those cells, which generated e.j.p.s, were hyperpolarized by ATP.A reduction in the concentration of extracellular sodium chloride decreased both the primary and rebound contractions; the primary contraction was, however, more sensitive than was the rebound contraction.Theophylline increased the primary and rebound contractions with no marked influence on the primary relaxation, lowered the action potential threshold, increased the rebound depolarization and did not markedly influence the i.j.p.s.Quinidine enhanced the primary relaxation and inhibited the primary contraction in a concentration-dependent manner. Inhibition of the rebound contraction by quinidine was slight (less than 50%).The present results demonstrate that primary relaxation, primary and rebound contractions are associated with i.j.p.s and e.j.p.s., and rebound depolarization with action potentials, respectively; they are typical responses of various regions of the guinea-pig small intestine to activation of inhibitory and excitatory non-cholinergic, non-adrenergic nerves. The P₁ and P₂ receptors, proposed by Burnstock (1975), probably do not mediate the non-cholinergic, non-adrenergic postsynaptic responses of the guinea-pig small intestine. A possible physiological function of ATP as a mediator of non-cholinergic, non-adrenergic nerves of the guinea-pig small intestine is discussed.This study was supported in part by JSPS Japan     

Nucleotide pyrophosphatase antagonizes responses to adenosine 5''-triphosphate and non-adrenergic, non-cholinergic inhibitory nerve stimulation in the guinea-pig isolated taenia coli

The enzyme nucleotide pyrophosphatase converted adenosine 5'-triphosphate (ATP) to adenosine 5'-monophosphate (AMP). In the isolated taenia coli of the guinea-pig it reduced the inhibitory responses to exogenously applied ATP. This could be explained on the basis that the ATP was rapidly converted to AMP which is less potent. The enzyme also reduced inhibitory responses to stimulation of non-adrenergic, non-cholinergic nerves but failed to reduce inhibitory responses to either perivascular sympathetic nerve stimulation or to noradrenaline. The results support the hypothesis that ATP is the transmitter released by non-adrenergic, non-cholinergic ("purinergic') inhibitory nerves.     

Nitrergic neurotransmission mediates the non-adrenergic non-cholinergic responses in the clitoral corpus cavernosum of the rabbit

The corpus cavernosum is the erectile tissue in the penis and clitoris. Although nitrergic neurotransmission has been characterized in detail in the penile corpus cavernosum, functional studies on the inhibitory non-adrenergic non-cholinergic (NANC) transmission in the clitoral corpus cavernosum have been lacking. Here we demonstrate that electrical field stimulation (EFS) induces NANC relaxation responses in the clitoral corpus cavernosum of the rabbit. These responses were inhibited by N^G-nitro-L-arginine methylester (L-NAME), 1H-[1,2,4]oxadiazolo[4,3,-a]quinoxalin-1-one (ODQ) or tetrodotoxin. The inhibitory effect of L-NAME was partially reversed by L-arginine but not by D-arginine. EFS-induced relaxations were enhanced by an inhibitor of type V cyclic GMP phosphodiesterase, zaprinast. These results suggest that nitrergic neurotransmission is responsible for the NANC relaxation responses in the clitoral corpus cavernosum of the rabbit.