CA2-DEPENDENT CHANGES OF ACETYLCHOLINE-RELEASE AND IP3 MASS IN TORPEDO CHOLINERGIC SYNAPTOSOMES()

The aim of the present study was to investigate possible changes of in ositol 1,4,5-trisphosphate (IP3) mass in Torpedo cholinergic synaptoso mes in conditions promoting stimulated acetylcholine (ACh) release. Fo r this purpose, we used a radioreceptor IP3 mass assay and a chemilumi nescent method for ACh detection. Torpedo cholinergic synaptosomes hav e consistent IP3 mass levels under resting conditions. The IP3 mass wa s neither modified by changes in external Ca2+ nor by a Ca2+-free medi um containing EGTA. IP3 mass and ACh release, measured in the same con ditions and in parallel, were increased by depolarization with high K and by the ionophores A-23187 and gramicidin-D in a manner dependent on external Ca2+ emphasizing that Ca2+ entry, independently of the inf lux mechanism involved, leads to an IP3 increase. The phospholipase C- beta inhibitors U-73122 and U-73343 reduced K+ stimulated IP3 levels w hile K+-evoked ACh release was almost completely blocked suggesting an additional effect of these drugs on depolarization-neurotransmitter s ecretion coupling. The effect reported showing an increase of IP3, by agents that stimulate ACh release may suggest a possible link between IP3 metabolism and the neurotransmitter release mechanism. However, su ch a link is probably not a direct one as implied by the results obtai ned with the inhibitors of phospholipase C. (C) 1997 Elsevier Science Ltd. All rights reserved.