Influence of storage conditions on the stability of cholinesterase activity in plasma and brain tissue taken from carbamate or organophosphorus pesticide-treated rats

Cholinesterase (ChE) activity in tissues front carbamate-treated animals is especially difficult to analyze because the inhibited ChE tends to decarba mylate, leading to an underestimation of ChE inhibition. Given this instabi lity during analysis, reactivation of the carbamylated ChE may occur during storage. The present study was designed to investigate the degree of react ivation of ChE in plasma and brain tissue taken from carbamate-treated anim als. Plasma and brain were taken from carbaryl-treated (50 mg/kg, po) made, Long-Evans rats and stored at either -20 or -80 degrees C and analyzed for ChE activity after 31, 46, and 138 days of storage. A "control" group of t issues was taken from rats treated (20 mg/kg, po) with chlorpyrifos (consid ered a stable inhibitor of ChE; not prone to reactivation) and handled and analyzed in exactly the same manner. Plasma ChE activity from carbaryl- or chlorpyrifos-treated animals remained stable when stored at -80 degrees C; in the -20 degrees C storage condition, the chlorpyrifos-inhibited plasma C hE levels also remained stable, but the carbaryl-inhibited plasma ChE level s increased sharply. Brain tissue was stored as either a homogenate (one ha lf of brain) or as intact tissue (other half of brain). The stability of br ain homogenate ChE from carbaryl-treated animals was similar to that of car baryl-inhibited plasma ChE; it was constant at -80 degrees C, but reactivat ion occurred when the brain was stored at -20 degrees C (though the degree of reactivation of ChE was less in brain). Interestingly chlorpyrifos-inhib ited brain homogenate ChE reactivated whether stored at -80 or -20 degrees C. Intact brain tissue from animals treated with either carbaryl ol chlorpy rifos showed no reactivation even if kept for up to 207 days at -80 degrees C. In conclusion, tissues taken from antiChE-treated animals should be sto red at -80 degrees C (-20 degrees C is inappropriate) and brain samples sho uld be stored as intact tissue.