INDIRECT PARASYMPATHOMIMETIC ACTIVITY OF METOCLOPRAMIDE - REVERSIBLE INHIBITION OF CHOLINESTERASES FROM HUMAN CENTRAL-NERVOUS-SYSTEM AND BLOOD

Inhibitory effects of the dopamine D-2-receptor antagonistic benzamide compound metoclopramide (MCP) on acetylcholinesterase (AChE; EC 3.1.1 .7) isoenzymes of both erythrocytes and human caudate nucleus and on h uman serum cholinesterase (ChE; EC 3.1.1.8) were studied in vitro usin g a spectrophotometric assay with acetylthiocholine (ASCh) as substrat e. MCP concentrations in the assays varied from 0.30 mu m to 0.15 mM, All isoenzymes studied were inhibited by metoclopramide in a concentra tion-dependent manner. MCP inhibition of AChE and ChE isoenzymes was n ot time-dependent and of the reversible type. Double reciprocal plots of the reaction velocity against varying ASCh concentrations revealed that, for AChE isoenzymes of erythrocytes and of the caudate nucleus, MCP reduced both maximal reaction velocity (V-max) and substrate affin ity (apparent Michaelis constant, K-M, increased). Thus, MCP inhibitio n of both AChE isoenzymes was of mixed competitive/non-competitive typ e. MCP constants for reversible competitive (K-i) and non-competitive (K-i') inhibition could be determined for erythrocyte AChE (K-i=10 mu M; K-i'=70 mu M) and caudate nucleus AChE (K-i=9.3 mu m; K-i'=82 mu m) . In contrast to MCP inhibition of AChE isoenzymes, the type of revers ible MCP inhibition of human serum ChE depended on substrate concentra tion, If substrate concentration exceeded 0.2 mM, MCP inhibition was o f mixed competitive/non-competitive type (K-i=0.19 mu m; K-i'=1.4 mu M ). MCP inhibition was of uncompetitive type, if substrate concentratio n was below 0.2 mM (K-i(u)=1.0 mu M). The mixed-type MCP inhibition of cholinesterase isoenzymes, because of its non-competitive component, can only partially be overcome by increased concentrations of the chol inergic transmitter acetylcholine (ACh), Since, with intravenous infus ions, peak MCP plasma concentrations in humans reach 4 mu M, MCP inhib ition of ACh hydrolysis in vivo may contribute both to prokinetic and anti-emetic actions of the substance and to its extrapyramidal side ef fects, (C) 1996 The Italian Pharmacological Society