Action of atropine on rabbit blood cholinesterase.

Abstract

There have been some investigations as to the influence of atropine on the activity of cholinesterase (ChE). Roepke (1) demonstrated that atropine inhibited horse serum ChE competitively with an affinity for the enzyme much higher than that of acetylcholine, and Schaller (2) also pointed out that atropine inhibited human blood ChE. On the other hand, Scoz and Michole (3) found that very small quantity of atropine increased blood ChE in vitro, and Yoshida et al. (4) reported that atropine intensified the destruction of acetylcholine by rabbit serum in vivo and in vitro. I have also undertaken this work using rabbit blood to investigate this problem.The activity of abbit blood ChE was determined by the Warburg's manometer. Into the main compartment of each flask were placed definite quantities of enzyme material diluted twenty times with bicarbonate-Ringer solution saturated with C02. One cc of substrate (acetylcholine, to make a 0.0025 molar solution) saturated with CO2 was added to a side arm of each series of flasks. Atropine sulfate was added in varying concentrations to the enzyme solution. The total fluid content of each flask was 5.0 cc. After equilibrating at 37.5°C, the content of the side arm was tipped into the reaction chamber. Controls and blanks were run simultaneously. Gas evolution resulting from the liberation of acetic acid and its action on bicarbonate was measured in the conventional manner. The results of the experiment are presented in the accompanying tables. Activity of enzyme was indicated in the vo'ume (cmm) of CO2 gas liberated by 0.1 cc blood or serum in one hour.In vivo: The activity of rabbit serum ChE after intravenous injection of atropine was compared with that before injection. There was no change between them (Table 1).In vitro: Firstly, when atropine was added to rabbit serum directly, the activity of its ChE was inhibited (Table 2). Secondly, after defibrinated rabbit blood was incubated with atropine at 37.5°C for ten minutes, the activity of ChE in serum from the blood was determined. In that case the activity was higher than control in high concentration of atropine. But when whole blood under the same treatment was used, the activity of its ChE was inhibited in high concentration of atropine (Table 3). It was probably because atropine liberated the ChE contained in blood cell into serum that the activity of ChE in serum from the blood with atropine was higher than that without atropine.There is an interpretation that the influence of atropine on the parasympathomimetic effect or acetylcholine action can be attributed to the activating action.of atropine on the ChE (4). However, this interpretation has been found inappropriate from my observation, because atropine did not activate rabbit blood ChE both in vivo and in vitro.Acknowledgement: The author wishes to express his gratitude to Professor Z. Kanda for suggesting this investigation and for his continued. guidance and help throughout the course of this work.