Toshio Maeda

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.

Sites of actions of adenosine in intrinsic cholinergic nerves of ileal longitudinal muscle from guinea pig

The sites of presynaptic actions of adenosine on electrically stimulated longitudinal muscle strip of guinea pig ileum were studied with a special dual organ bath. Local stimulation (0.1 Hz) of the anal part of the strip induced a twitch response in the stimulated part and in the non-stimulated oral part; both of these twitch responses were depressed by an application of adenosine to the anal, stimulated part of the strip. The present results suggested that electrical stimulation applied to the anal part of the strip travelled along the network of Auerbachs plexus in the oral direction, and adenosine might depress such conduction in cholinergic nerve fibers in addition to nerve terminals.

Effect of ascorbate on the contractile response induced by DMPP in guinea-pig ileal longitudinal muscle strip

The study concerned the effect of ascorbate on the contractile response induced by DMPP in the guinea-pig ileal longitudinal muscle. At 0.5-10 mM, sodium ascorbate shifted the dose-response curve for DMPP to the left and enhanced the maximum contraction. On the other hand, in the presence of ascorbate, the high potassium (40 mM)-induced contraction was not altered, and the contraction by acetylcholine and histamine in concentrations higher than 1 microM and 0.3 microM, respectively, was slightly reduced. Thus, ascorbate enhanced the contractile response caused by DMPP which induced acetylcholine release from nerve terminals indirectly by stimulating the ganglia of the Auerbachs plexus elements. This potentiating effect of ascorbate was studied with a dual organ bath which was partitioned into two compartments. When the oral half of the muscle strip was directly stimulated by DMPP applied to the oral compartment, a contraction of the unstimulated anal part was observed. This contraction was blocked by atropine or adenosine applied to the unstimulated part. Tetrodotoxin, applied to the stimulated part, abolished the contraction of the unstimulated part. The contraction of the unstimulated part was enhanced by ascorbate applied to the stimulated part but not to the unstimulated part. These results indicate that the contraction observed in the unstimulated part may have been caused by acetylcholine release from cholinergic nerves as a result of conduction of the oral part excitation by DMPP along cholinergic nerve fibers; ascorbate may have affected the ganglion cells in Auerbachs plexus to potentiate the action of DMPP.