Investigation of relaxations of the rabbit anococcygeus muscle by nerve stimulation and ATP using the ATP antagonist ANAPP3

Neurotransmitters responsible for purinergic motor neurotransmission and regulation of GI motility

Classical concepts of peripheral neurotransmission were insufficient to explain enteric inhibitory neurotransmission. Geoffrey Burnstock and colleagues developed the idea that ATP or a related purine satisfies the criteria for a neurotransmitter and serves as an enteric inhibitory neurotransmitter in GI muscles. Cloning of purinergic receptors and development of specific drugs and transgenic mice have shown that enteric inhibitory responses depend upon P2Y₁ receptors in post-junctional cells. The post-junctional cells that transduce purinergic neurotransmitters in the GI tract are PDGFRα⁺ cells and not smooth muscle cells (SMCs). PDGFRα⁺ cells express P2Y₁ receptors, are activated by enteric inhibitory nerve stimulation and generate Ca²⁺ oscillations, express small-conductance Ca²⁺-activated K⁺ channels (SK3), and generate outward currents when exposed to P2Y₁ agonists. These properties are consistent with post-junctional purinergic responses, and similar responses and effectors are not functional in SMCs. Refinements in methodologies to measure purines in tissue superfusates, such as high-performance liquid chromatography (HPLC) coupled with etheno-derivatization of purines and fluorescence detection, revealed that multiple purines are released during stimulation of intrinsic nerves. β-NAD⁺ and other purines, better satisfy criteria for the purinergic neurotransmitter than ATP. HPLC has also allowed better detection of purine metabolites, and coupled with isolation of specific types of post-junctional cells, has provided new concepts about deactivation of purine neurotransmitters. In spite of steady progress, many unknowns about purinergic neurotransmission remain and require additional investigation to understand this important regulatory mechanism in GI motility.

Biology of the anococcygeus muscle

The anococcygeus is a smooth muscle tissue of the urogenital tract which, in the male, runs on to form the retractor penis. The motor innervation is classically sympathetic with noradrenaline as transmitter, but the relaxant parasympathetic transmitter has only recently been identified as nitric oxide. Indeed, the anococcygeus has provided an extremely useful model with which to probe the mechanisms underlying this novel nitrergic system, including the importance of physiological antioxidants in maintaining the potency of nitric oxide as a neurotransmitter. The cellular mechanisms of contraction and relaxation are slowly being clarified, with particular interest in the contribution of capacitative calcium entry and the guanylyl cyclase/cyclic GMP system. Many questions remain unanswered, however, including the precise physiological role of the muscle, the identity of substances released from subcellular vesicles of nitrergic nerves, the unusual sensitivity of the tissue to certain peptides (oxytocin and urotensin II), and the nature of store-operated channels through which calcium enters the cell to maintain contraction.

Evidence for nitric oxide as an inhibitory neurotransmitter in rabbit isolated anococcygeus

The role of nitric oxide (NO) and ATP as putative inhibitory non-adrenergic, non-cholinergic (NANC) neurotransmitters was investigated in rabbit isolated anococcygeus after block of adrenergic and cholinergic responses, and raising tone with histamine. NANC nerve stimulation produced rapid relaxations which were completely abolished by tetrodotoxin. The magnitude of the NANC inhibitory responses was significantly reduced by the nitric oxide synthase inhibitors NG-nitro-L-arginine (NO-Arg) and NG-nitro-L-arginine methyl ester (L-NAME). This effect could be partially reversed by L-arginine but not by D-arginine. Oxyhaemoglobin inhibited NANC nerve responses and sodium nitroprusside mimicked the effects of NANC nerve stimulation. NO-Arg also reduced the magnitude of the inhibitory junction potentials recorded from the smooth muscle cells during NANC nerve stimulation. Exogenously applied ATP and adenosine each produced concentration dependent relaxations which were unaffected by the NO-synthase inhibitor NO-Arg. Relaxations to adenosine were virtually abolished by the P1 purinoceptor antagonist 8-(p-sulphophenyl)theophylline. Relaxations to ATP were also significantly reduced, indicating that part of the response to exogenous ATP is due to its breakdown to adenosine and subsequent action on P1 purinoceptors. Relaxations of the tissue to ATP and adenosine were unaffected by the P2 purinoceptor antagonist suramin. NANC nerve mediated responses were not significantly changed by either 8-(p-sulphophenyl)theophylline or suramin. These results suggest that NO is involved in inhibitory NANC neurotransmission in the rabbit isolated anococcygeus, but do not support a role for ATP as a NANC neurotransmitter in this tissue.

N-methylhydroxylamine inhibits and M&B 22948 potentiates relaxations of the mouse anococcygeus to non-adrenergic, non-cholinergic field stimulation and to nitrovasodilator drugs

1. The effects of N-methylhydroxylamine (NMH) and of M&B 22948 on relaxations of the mouse anococcygeus to non-adrenergic, non-cholinergic (NANC) field stimulation and to a number of smooth muscle relaxant drugs were investigated. 2. Relaxations to NANC field stimulation (10 Hz; 60 s train) were reversibly blocked by NMH (1-5 mM), which also caused weak, transient reductions of carbachol (50 microM)-induced tone. N,N-dimethylhydroxylamine (2 mM) and hydroxylamine (5 microM) reduced tone to the same extent as NMH, but neither produced any inhibition of NANC relaxations. 3. M&B 22948 10 microM, which by itself reduced tone by 12%, potentiated submaximal but not maximal relaxations to NANC field stimulation; overall the log frequency-response curve was displaced to the left by a factor of 2. 4. Sodium nitroprusside (0.01-1 microM), hydroxylamine (0.5-100 microM), and nitric oxide (2-200 microM) all relaxed carbachol-induced tone; relaxations to submaximal concentrations of these nitrovasodilators were reduced in the presence of 2 mM NMH, and potentiated in the presence of 10 microM M&B 22948. 5. Neither NMH (2 mM) nor M&B 22948 (10 microM) affected relaxations induced by submaximal concentrations of vasoactive intestinal peptide (VIP; 1 microM), papaverine (10 microM), 3-isobutyl-1-methyl-xanthine (10 microM), or 8-bromo-cyclic guanosine monophosphate (100 microM); relaxations to adenosine 5'-triphosphate (ATP, 2 mM) were unaffected by M&B 22948, but were potentiated by NMH. 6. The selective inhibition by NMH, and potentiation by M&B 22948, of NANC and nitrovasodilator-induced relaxations of the mouse anococcygeus suggests that the NANC transmitter is neither VIP nor ATP, but resembles the nitrovasodilator drugs in its mode of action. The NANC transmission system is therefore similar to that recently described in the bovine retractor penis.

Arylazido aminopropionyl ATP (ANAPP3): interaction with adenosine receptors in longitudinal smooth muscle of the guinea-pig ileum

Arylazido aminopropionyl ATP (ANAPP3), an ATP-receptor antagonist containing a photosensitive arylazido moiety coupled to the 3' hydroxyl of ATP, inhibited the twitch response of electrically stimulated ileal longitudinal muscle strips in a dose-dependent manner. These agonist responses to ANAPP3 were attenuated by the enzyme adenosine deaminase and antagonized by the adenosine receptor antagonist 8-phenyltheophylline. Schild analysis yielded similar pA2 values for ANAPP3 and adenosine suggesting a common receptor site. Several 3'-ribose-modified adenosine analogs were tested for agonist activity and found to be inactive. Results suggest that ANAPP3 interacts at the presynaptic adenosine receptor of the ileum following its metabolism to adenosine, which explains the lack of antagonism at adenosine receptors of ileal smooth muscle following photolysis of ANAPP3.

Characteristics of P2 (nucleotide) receptors mediating contraction and relaxation of rat aortic strips: possible physiological relevance

ATP and ADP relaxed rat aortic strips precontracted with noradrenaline by an endothelium-dependent mechanism. 5'-AMP was much less potent and adenosine was essentially without effect. The metabolically stable analogues alpha,beta-methylene ATP and beta,gamma-methylene ATP further contracted precontracted aorta. Aortic strips, which had not been precontracted with noradrenaline, contracted when exposed to either ATP or alpha,beta-methylene ATP, the latter nucleotide being much more potent than the former. Removal of the endothelium increased the contractions to ATP. ANAPP3 had no effect on the endothelium-dependent relaxations produced by ATP but it antagonized contractions produced by alpha,beta-methylene ATP. These results provide evidence for the possible existence of two subtypes of P2 receptors in rat aorta; a P2 receptor mediating contraction residing on smooth muscle which can be antagonized by ANAPP3 and where alpha,beta-methylene ATP is more potent than ATP, and a P2 receptor mediating relaxation located on the endothelium which cannot be antagonized by ANAPP3 and where ATP is much more potent than alpha,beta-methylene ATP.

Is there a basis for distinguishing two types of P2-purinoceptor?

It is suggested that the P2-purinoceptor may be separated into two subtypes largely on the basis of the rank order of agonist potency of structural analogues of ATP and also on the activity of antagonists at the P2-purinoceptor: Subtype 1 (designated P2X), potency order: alpha, beta-methyleneATP, beta, gamma-methyleneATP greater than ATP = 2 methylthioATP; antagonism by ANAPP3 and selectively desensitisation following administration of alpha, beta-methyleneATP; present in the vas deferens and urinary bladder of guinea-pig and rat, frog and rat ventricle, and also in the smooth muscle of the rat femoral artery and rabbit central ear artery, where they mediate excitation. Subtype 2 (designated P2Y), potency order: 2-methylthioATP much greater than ATP greater than alpha, beta-methyleneATP, beta, gamma-methyleneATP; weak antagonism by ANAPP3 and desensitisation following administration of alpha, beta-methyleneATP; present in the guinea-pig taenia coli and the longitudinal muscle layer of the rabbit portal vein, where they mediate relaxation and also on the vascular endothelial cells of the rat femoral artery and pig aorta (where occupation leads to the production of endothelium-derived relaxing factor). Differences in the structure of the P2-purinoceptor in various tissues may be useful in the development of drugs for the treatment of vascular, gastrointestinal and urinoglenital disorders.

Pharmacological evidence that adenosine triphosphate and noradrenaline are co-transmitters in the guinea-pig vas deferens

The contractile response of the guinea-pig vas deferens to tetanic nerve stimulation was biphasic. The first phase was mimicked by exogenously applied ATP. The second more tonic phase was mimicked by exogenously applied noradrenaline (NA). Intracellular micro-electrodes were used to record the electrical response of the vas deferens to nerve stimulation and to exogenously applied ATP and NA. Local application of ATP (10(-5) to 10(-3)M), by pressure ejection from a micropipette, produced a depolarization similar in magnitude and time course to the excitatory junction potential (e.j.p.). NA produced no such response. Superfusion of the vas deferens with ATP and NA (10(-6) to 10(-4)M) produced a depolarization. The depolarization produced by NA was more gradual than that produced by the same concentration of ATP. The ATP-receptor antagonist ANAPP3 (arylazido aminopropionyl-ATP) preferentially antagonized the first component of the neurogenic contractile response and also antagonized the e.j.p. The alpha-receptor antagonist prazosin preferentially antagonized the second phase of the neurogenic contractile response and enhanced the e.j.p. Similar results were obtained using the irreversible alpha-receptor antagonists phenoxybenzamine and dibenamine. Cocaine (10(-6) and 10(-5)M) enhanced the second phase of the contractile response to nerve stimulation, but reduced the first phase. Lidocaine (10(-5) and 10(-4)M) had no such effect. Cocaine (10(-6) and 10(-5)M) reduced the magnitude of e.j.p.s. at all stimulation frequencies from 1 to 8 Hz. In the presence of the selective alpha 2-receptor antagonist yohimbine (10(-7)M), both phases of the contractile response to nerve stimulation were enhanced to the same degree. This concentration of yohimbine also increased the magnitude of e.j.p.s. In the presence of 10(-7) M-yohimbine, cocaine (10(-6) and 10(-5)M) still enhanced the second phase of the contractile response, but no longer reduced the initial phase of the contraction or e.j.p.s to the same degree. In vas deferens from animals pre-treated with reserpine (2 mg/kg.day), the second phase of the contractile response to nerve stimulation was reduced but neither the first phase of the contraction nor the e.j.p.s was blocked. These results suggest that the first phase of the neurogenic contractile response of the vas deferens and the e.j.p. are mediated by ATP acting on P2-purinoreceptors, whereas NA mediates phase two, via alpha 1-adrenoceptors. The results also suggest that release of ATP and NA is influenced by a negative feed-back mechanism involving presynaptic alpha 2-adrenoceptors.

Direct evidence against a role of ATP as the nonadrenergic, noncholinergic inhibitory neurotransmitter in guinea pig tenia coli

Electrical field stimulation of the isolated guinea pig tenia coli in the presence of a muscarinic receptor antagonist (atropine) and an adrenergic neuron blocker (guanethidine) produces relaxation. A large amount of indirect evidence has suggested that the neurotransmitter that is released from these nonadrenergic, noncholinergic inhibitory neurons is ATP or a related nucleotide, and the nerves have been termed "purinergic." A photoaffinity analog of ATP, arylazido aminopropionyl ATP, which produces a specific pharmacological antagonism of P2 purinergic receptors in isolated guinea pig vas deferens and urinary bladder, was utilized in the present study to evaluate directly whether ATP is the nonadrenergic, noncholinergic inhibitory neurotransmitter in tenia coli. By blocking postjunctional P2 receptors, arylazido aminopropionyl ATP produced a pronounced antagonism of relaxations induced by exogenously added ATP. Responses produced by ADP, AMP, and adenosine also were antagonized by arylazido aminopropionyl ATP, but to a lesser extent. Inhibitory responses to isoproterenol were not antagonized. Under these conditions of established, specific P2-receptor blockade of responses to exogenously added ATP, relaxations induced by field stimulation of intrinsic inhibitory nerves in the presence of atropine (1 microM) and guanethidine (1 microM) were not antagonized. Though these results provide no indication of the actual substance involved, they suggest strongly that the nonadrenergic, noncholinergic inhibitory neurotransmitter in the guinea pig tenia coli is not ATP.

Comparison of contractions of the smooth muscle of the guinea-pig vas deferens induced by ATP and related nucleotides

The shape of contractile responses of the isolated guinea-pig vas deferens changes as ATP concentration is increased from 10(-7) to 10(-2) M. The ATP concentration-response curve is bimodal and reflects the change in response profile. Initially spike-like (10(-7) -3 x 10(-5) M) in nature, contractions acquire a secondary, slower tonic phase in transitional ATP concentrations (greater than or equal to 3 x 10(-5) M). At high ATP concentrations (10(-2) M) the secondary phase predominates. To determine if there are structural requirements for these complex effects, responses to ATP were compared to those elicited with analogs containing phosphate-chain, ribose and adenine modifications. In general, substitution of 5'-anhydride linkages with methylene or imido bridges prolonged responses to low concentrations but at high concentrations both potentiated and abbreviated the responses. ATP gamma S, a substrate for phosphohydrolases which incorporate phosphate but which are less able to remove thiophosphate, produced responses with a greatly prolonged tonic phase. Removal of the 2'-hydroxyl of ATP resulted in reduced potency at low concentrations while removal of the 3'-hydroxyl was without effect. Modification of both the 1 and N6 positions of adenine substantially reduced agonist activity. Responses to ATP and the beta, gamma-methylene congener were unaffected by treatment with 10(-5) M indomethacin. The results indicate that more than one simple interaction of ATP with a receptor is involved in the production of responses to ATP. Of several hypotheses discussed, we favor one which suggests that the phasic responses to low concentrations of ATP are receptor-mediated and modified by tissue enzymes, while those to high concentrations are mediated, in part, by hydrolysis per se. The hypothesis, which realizes that conformational preferences may exist as well, is proposed for the vas deferens only.