Cardiotonic drugs inhibit purified mammalian acetylcholinesterase

Oxime‐ and non‐oxime‐related drugs, as well as cardiotonic drugs (CDs), have been used to treat the effects of organophosphorus (OP) poisoning. We conducted our experiments to determine what effects CDs may have on acetylcholinesterase (AChE), and how CDs interact with other treatment drugs as well as with OP‐inhibited AChE. True AChE (EC 3.1.1.7) was purified from fetal bovine serum, and enzyme activity was measured according to Ellman et al. The CDs coumingine, cassaine, proscillaridin and convallatoxin were incubated with AChE at 550 μM at pH 7.6 and 25°C. The CD ouabain was incubated with AChE at 500 μM. The CDs inhibited AChE by 97%, 89%, 10%, 7% and 6%, respectively. The mean AChE activities for these experiments, except for ouabain, were significantly different ( P = 0.05) from their controls, as determined by the two‐tailed Student's t ‐test. In a separate experiment, the oxime TMB‐4·2Br (100 μM), which did not inhibit AChE, increased the inhibitory effect of proscillaridin from 4% to 11% (a 3.7‐fold increase). When AChE was inhibited 39% with 37 nM VX, the addition of proscillaridin increased the inhibition to 51% (a 1.3‐fold increase). When TMB‐4 was added to the proscillaridin‐ and VX‐inhibited AChE mixture, the inhibition decreased from 50% to 32% (a 0.37‐fold decrease), whereas TMB‐4 alone added to VX‐inhibited AChE decreased the inhibition from 39% to 24% (a 0.38‐fold decrease). The results show that TMB‐4 increases the inhibition of AChE by proscillaridin. However, TMB‐4 decreases the inhibition of AChE by VX and proscillaridin combined. The inhibition of AChE by the CDs alone may help in part to account for the cardiac parasympathetic response of CDs.