Possible transsynaptic cholinergic neuromodulation by ATP released from ileal longitudinal muscles of guinea pigs

The effects of α, β-methylene ATP (α, β-mATP) and β, γ-methylene ATP (β, γ-mATP) on endogenous acetylcholine (ACh) release evoked by electrical nerve stimulation were evaluated in guinea-pig ileal longitudinal muscles. Release of ACh was measured with an HPLC-electrochemical detector system and release of ATP by luciferinluciferase essay. Electrically evoked endogenous ACh release was reduced by both α, β-mATP and β, γ-mATP at concentrations of 3 and 30 μM. The inhibitory effect of α, β-mATP (30 μM) on ACh release was not detectable in the presence of theophylline (100 μM), a P 1-purinoceptor antagonist, that itself enhanced ATP release. When exogenuos ATP (0.1 μM) was added to the bath in which the ileal segment was suspended, it was rapidly metabolized, presumably by ecto-ATPase, and disappeared from the medium within 15 min. At 30 μM, α, β-mATP induced ATP release in a suramin-sensitive but Ca 2+-and atrophine-insensitive manner, suggesting P 2-receptor-mediated release of ATP from the smooth muscle. We conclude from these findings that α, β-mATP and, probably, also β, γ-mATP, do not reduce ACh release by direct stimulation of presynaptic P 1-purinoceptors, and that endogenous ATP released postjunctionally by these ATP analogs is decomposed metabolically to adenosine in the synapse and this adenosine triggers P 1-purinoceptor sensitive neuromodulation of cholinergic transmission.