Characterization of plasma cholinesterase in rabbit and evaluation of the inhibitory potential of diazinon

Several studies indicate that more than one cholinesterase form may be present in the blood of mammals. In this study the predominant plasma cholinesterase activity, the physiological cholinesterase activity as well as cholinesterase sex-dependent changes in non-exposed individuals of rabbit have been established. Plasma cholinesterase was characterized using three substrates (acetylthiocholine iodide, propionylthiocholine iodide, and S-butyrylthiocholine iodide) and three cholinesterase inhibitors (eserine sulfate, BW284C51 and iso-OMPA). The results indicated that propionylthiocholine was the preferred substrate by plasma cholinesterase followed by acetylthiocholine and butyrylthiocholine, and the predominant enzymatic activity was acetylcholinesterase. Physiological plasma cholinesterase activity was 198.9±5.8 nmol/min/ml for male and 205.2±5.0 nmol/min/ml for female using acetylthiocholine as substrate. Thus, sex had no significant effect on the physiological cholinesterase activity (p>0.05). In addition, the in vivo and in vitro sensitivity of plasma cholinesterase to diazinon was also investigated. In rabbits exposed to single doses of diazinon (25 or 125 mg/kg) the higher inhibitions of plasma cholinesterase were reached 9 h after oral administration (53% and 87% inhibition, respectively). Cholinesterase activity significantly recovered up to values similar to pre-administration between 3 and 7 d depending on the administered dose and sex of the animals. Plasma cholinesterase activity decreased to 24%, 53% and 74% of the initial activity at 9 h of in vitro exposure to 1.25, 3.13 and 6.25 mg/l of diazinon, respectively, and it remained steadily depressed throughout the experimental period (10 d). This study has demonstrated the sensitivity of cholinesterase activity in plasma of rabbits following both in vivo and in vitro exposure to sub-lethal concentrations of diazinon.