Inhibition of acetyl- and butyrylcholinesterase and amylase release from canine pancreas.

There are two tissue-fixed cholinesterases in dog pancreas: acetylcholinesterase and butyrylcholinesterase. In the present experiments, an organophosphate that only inhibits butyrylcholinesterase (isopropylpyrophosphoramide, or iso-OMPA) was compared with echothiophate and a nonorganophosphate compound, physostigmine. The latter two agents inhibit both cholinesterases. Fresh canine pancreas from anesthetized dogs was minced into fragments and suspended in Eagle's solution gassed with 100% O2. Amylase release was measured by the Phadebas method. In 2-h dose-response studies, there was a fivefold increase in sensitivity to acetylcholine when fragments were preincubated 1 h with iso-OMPA. There was a 1,000-fold increase in sensitivity when fragments were preincubated for 1 h in echothiophate. Basal amylase release in incubates with echothiophate were also increased. In dose-response studies with CCK-8, iso-OMPA was without effect, but echothiophate treatment resulted in a greater total response to CCK-8. There was a corresponding increase in basal output with echothiophate alone. Physostigmine also potentiates the response to CCK-8. Cumulative responses up to 3 h with half-maximal acetylcholine or half-maximal CCK-8 doses showed enhanced total output in fragments preincubated with echothiophate (p less than 0.05). The enhancement effect was atropine-sensitive to hexamethonium ganglionic blockade. In calcium-free medium, the enhancement with echothiophate was greatly reduced but still present. Inhibitors of both cholinesterases in the pancreas cause a greater total amylase release to sub-maximal doses of acetylcholine and CCK-8 than agents that only inhibit butyrylcholinesterase. Though our data do not provide direct proof, the results could be explained by a greater accumulation of endogenous acetylcholine when both cholinesterases are inhibited.