Effects of adenosine on 45Ca uptake and [3H]acetylcholine release in synaptosomal preparation from guinea-pig ileum myenteric plexus.

The effects of adenosine on acetylcholine (ACh) release and calcium uptake were examined in a synaptosomal fraction prepared from guinea-pig ileum myenteric plexus-longitudinal muscle. A high concentration of potassium (40 mM) and electrical pulses (ES:10Hz) caused a marked increase in the output of [3H]ACh from [3H]choline-preloaded crude synaptosomes. This [3H]ACh output was calcium- and temperature-dependent. Adenosine reduced the high potassium-induced release significantly, and the electrically stimulated release completely. When the preparation was depolarized by high potassium or electrical pulses, the 45Ca uptake by synaptosomes was significantly enhanced. The uptake of 45Ca induced by high potassium was significantly reduced and that induced by electrical stimulation was completely abolished by adenosine. From these results, it may be suggested that adenosine inhibits neurotransmitter release by suppressing the presynaptic influx of calcium ion during depolarization of the cholinergic nerve terminals in guinea-pig ileum.