CONTRIBUTION TO THE STUDY OF THE MECHANISM OF ACTION OF CHOLINESTERASE INHIBITORS

¹ Department of Pharmacology and Materia Medica, Georgetown University School of Medicine, Washington, D. C.

DFP, HETP, TEPP, Parathion and dimethylcarbamate of (2-hydroxy-5-phenyl-benzyl) triinethylammonium bromide (Nu 683) potentiate the pressor effect of ACh in atropinized dogs as shown previously for physostigmine and neostigmine. Anticholinesterases generally potentiate also the pressor effects of nicotine and of the stimulation of the cephalic end of the vagosympathetic trunk. Atropinization serves to prevent the interference of the muscarinic effects; control experiments were carried out to show that it does not interfere with the nicotinic effects of ACh.

The potentiation of the pressor ganglionic effects of ACh was used as a quantitative assay of anticholinesterases. The measurements of the duration of this potentiation yielded data generally comparable with those on duration of inhibition of ChE activity by anticholinesterases in vivo.

The relationship between the pressor response and the dose of the anticholinesterases studied (dose-effect curves) yielded the following reference points: minimum effective, optimum effective, maximum and paralytic doses. The optimum and maximum effective doses correspond to two peaks on the plot of the pressor responses. These points characterize physostigmine, neostigmine and TEPP. The dose-effect curves of other anticholinesterases have only one peak. The anticholinesterases differ also as to the other reference points.

Manometric determination of ChE activity in sympathetic ganglia revealed that the 100 per cent inactivation occurs with doses of DFP which are about one-fifth of the dose causing the maximum potentiation of the ACh pressor effect. Also, for potent anticholinesterases with a low minimum effective dose, optimum and/or maximum effective doses were far beyond the concentration of the anticholinesterases in the body necessary for complete inactivation of specific, ganglionic esterase. Finally, the maximum pressor effect obtained with ACh alone does not equal the pressor effect of even small doses of ACh injected following the administration of moderate doses of TEPP.

The paralytic dose is many times larger than the optimum or the maximum dose. It is, however, relatively low for Nu 683, intermediate for DFP and high for TEPP (10, 65 and 150 mgm./kgm., respectively). Independent experiments indicated that the paralysis is not caused by ACh accumulation. Since there is little relationship between minimum effective, optimum and paralytic dose, it is unlikely that ganglionic paralysis depends on the enzyme inhibitory potency of the anticholinesterases.

The ganglionic paralysis by anticholinesterases can be simulated by agents causing calcium withdrawal, such as citrates and phosphates. This paralysis is relieved by calcium administration. Since TEPP relieves it also while calcium does not reverse the ganglionic paralytic effects even of anticholinesterases which are phosphate esters, these effects can not be due to calcium withdrawal.

Small doses of physostigmine and neostigmine prevent the potentiation of ACh pressor effects by TEPP, HETP and DFP. On the other hand, there is a mutual additive synergism or potentiation when the latter are administered prior to the injection of physostigmine and neostigmine.

Whenever the effects described seemed to be due to the anticholinesterase effect of the compound studied, there was also an indication that true rather than pseudo ChE was involved. However, it is suggested that many of their effects described at present are dependent on other factors than ChE inhibition. These extra ChE factors are discussed.

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