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 decarbamylate, leading to an underestimation of ChE inhibition. Given this instability during analysis, reactivation of the carbamylated ChE may occur during storage. The present study was designed to investigate the degree of reactivation of ChE in plasma and brain tissue taken from carbamate-treated animals. 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 tissues was taken from rats treated (20 mg/kg, po) with chlorpyrifos (considered 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 ChE levels also remained stable, but the carbaryl-inhibited plasma ChE levels increased sharply. Brain tissue was stored as either a homogenate (one half of brain) or as intact tissue (other half of brain). The stability of brain homogenate ChE from carbaryl-treated animals was similar to that of carbaryl-inhibited plasma ChE; it was constant at -80 degrees C, but reactivation occurred when the brain was stored at -20 degrees C (though the degree of reactivation of ChE was less in brain). Interestingly chlorpyrifos-inhibited brain homogenate ChE reactivated whether stored at -80 or -20 degrees C. Intact brain tissue from animals treated with either carbaryl ol chlorpyrifos 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 stored at -80 degrees C (-20 degrees C is inappropriate) and brain samples should be stored as intact tissue.