1. In the presence of atropine (1 μM), guanethidine (3 μM), indomethacin (3 μM), nifedipine (1 μM), L-nitroarginine (L-NOARG, 100 μM), and the selective tachykinin NK1 and NK2 receptor antagonists, SR 140,333 and GR 94,800, respectively (0.1 μM each), a single pulse of electrical field stimulation (EFS) produced a monophasic non-adrenergic non-cholinergic (NANC) inhibitory junction potential (i.j.p., about 10 mV in amplitude) in the circular muscle of guinea-pig proximal colon, recorded by the modified single sucrose gap technique. 2. The P2 purinoceptor agonist, α,β methylene ATP (α,β mATP, 100 μM) and the pituitary adenylyl cyclase activating peptide (PACAP, 1 μM) both produced hyperpolarization (11 ± 0.8 mV, n = 14 and 10.2 ± 0.8 mV, n = 19, respectively) and relaxation (1.1 ± 0.2 mV, n = 14 and 1.5 ± 0.2 mN, n = 19, respectively) of the circular muscle. 3. Apamin (0.1 μM) nearly abolished (about 90% inhibition) the NANC i.j.p. and the α,β mATP-induced hyperpolarization, markedly reduced the α,β mATP-induced relaxation (73% inhibition) and the PACAP-induced hyperpolarization (65% inhibition), while the PACAP-induced relaxation was unaffected. 4. Tetraethylammonium (TEA, 10 mM) increased the EFS-evoked i.j.p. and revealed an excitatory junction potential (e.j.p.). In the presence of TEA, α,β mATP induced a biphasic response:transient depolarization and contraction followed by hyperpolarization and relaxation. The hyperpolarization to PACAP was reduced by TEA (45% inhibition) but the relaxation was unaffected. 5. The combined application of apamin (0.1 μM) and TEA (10 mM) abolished the i.j.p. and single pulse EFS evoked a pure e.j.p. with latency three times longer than that of the i.j.p. In the majority of strips tested, α,β mATP and PACAP elicited a biphasic response:depolarization and small contraction followed by hyperpolarization and relaxation. 6. The P2 purinoceptor antagonist, pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS) inhibited the NANC i.j.p. in concentration-dependent manner and inhibited the α,β mATP-induced hyperpolarization and relaxation, without affecting the hyperpolarization and relaxation induced by PACAP. On the other hand, the P2 purinoceptor antagonist, suramin (100 μM) inhibited to a similar extent (60-80%) the NANC i.j.p, and the hyperpolarization and relaxation induced by α,β mATP or PACAP. 7. PPADS and suramin reduced the NANC e.j.p. evoked by a single pulse EFS in the presence of apamin and TEA (100 μM of PPADS and 300 μM of suramin inhibited the e.j.p. by about 40%). 8. We conclude that ATP but not PACAP, mediates the apamin-sensitive NANC i.j.p. in the circular muscle of the guinea-pig colon. After blockade of the NANC i.j.p., ATP may act as an excitatory transmitter by activating excitatory P2 purinoceptors. The subtypes of P2 purinoceptor involved in the inhibitory and excitatory responses remain to be established. The data suggest that excitatory P2 purinoceptors may be located extrajunctionally.
|Number of pages||8|
|Journal||British Journal of Pharmacology|
|Publication status||Published - Nov 1996|
- Guinea-pig colon
- Non-adrenergic non-cholinergic (NANC)
- Pituitary adenylyl cyclase activating peptide (PACAP)