Effects of purine compounds on cholinergic nerves. Specificity of adenosine and related compounds on acetylcholine release in electircally stimulated guinea pig ileum.

The action of 21 purine compounds on the twitch response of the electrically stimulated guinea pig isolated ileum has been investigated. Adenosine and related compounds produced a dose-dependent depression of the response. Adenosine was the most potent and 2'-deoxyadenosine had one hundredth the potency of adenosine. Adenine, hypoxanthine, inosine, IMP, ITP, xanthine, xanthosine, XMP, XTP, guanine, GMP and GTP were ineffective at concentrations less than 1 mM. Adenosine (30 microgram) reduced the electrically induced ACh output from the ileal strips. The dose--depression curve for adenosine (0.1--30 microgram) was shifted to the right in the presence of xanthine derivatives and of these, theophylline was the most potent inhibitor of adenosine. On the other hand, dipyridamole (0.1--1 microgram) and hexobendine (0.1--1 microgram) shifted the curve to the left. They markedly inhibited 3H-adenosine uptake into the ileum. Theophylline (0.1 mM), dipyridamole (0.3 microgram) and hexobendine (0.3 microgram) did not affect tetrodotoxin-, adrenaline-, strychnine- and morphine-induced inhibition of the twitch response. The present investigations have revealed that adenosine and related compounds reduce ACh release from the intramural cholinergic nerves in the guinea pig ileum possibly in a specific manner (or through a specific receptor site) different from that of other inhibitors such as morphine.