In Vitro Inhibition of Blood Cholinesterase Activities From Horse, Cow, and Rat by Tetrachlorvinphos

Abstract

The organophosphorus insecticide tetrachlorvinphos (TCVP) is commonly used as a feed-through larvicide in many livestock species, including cattle and horses. Cholinesterase (Ch E) activity in blood (generally plasma or whole blood) is often employed to assess organophosphorus insecticide intoxication in animals as well as humans. In many species, including horse and man, plasma contains predominantly butyrylcholinesterase whereas red blood cells in all species express exclusively acetylcholinesterase. To evalulate the comparative interaction of TCVP with blood Ch Es in different species, we compared the in vitro sensitivity of Ch E activity in plasma and erythrocytes from horse, cow, and rat. Horse plasma Ch E was most sensitive (IC50, 30 minutes, 30°C = 97 n M), whereas horse erythrocyte Ch E activity was least sensitive (IC50 > 1 m M). In contrast, cow plasma Ch E showed lower sensitivity (IC50 = 784 μM) to inhibition by TCVP than erythrocyte Ch E (IC50 = 216 μM). Rat plasma and erythrocyte Ch E activities had relatively similar sensitivity to TCVP (IC50 = 54 μM and 78 μM, respectively). The results suggest that plasma and erythrocyte Ch E from horse, cow, and rat show marked species- and blood fraction-dependent differences in sensitivity to TCVP. Knowledge of such differences in sensitivity of blood Ch E activities to TCVP may be important in the clinical interpretation of intoxication with this pesticide across species.