Effect of phenobarbitone treatment against signal grass (Brachiaria decumbens) toxicity in sheep

The effect of phenobarbitone against signal grass (Brachiaria decumbens) to xicity was studied in 26 male crossbred sheep. Grazing on signal grass sign ificantly decreased the concentration of cytochrome P-450 and the activity of drug metabolizing enzymes, viz. aminopyrine-N-demethylase, aniline-4-hyd roxylase, UDP-glucuronyltransferase and glutathione-S-transferase in liver and kidneys of affected sheep. Oral administration of phenobarbitone (30 mg/kg body weight) for five conse cutive days before grazing on B. decumbens pasture, and thereafter, for thr ee consecutive days every two weeks, resulted in significant increases in h epatic and renal activities of drug-metabolizing enzymes. The induction of drug metabolizing activity in sheep grazing on signal grass group was found to be lower than in animals given phenobarbitone alone. Induction by pheno barbitone provided a degree of protection against the toxic effects of B. d ecumbens as indicated by the delay in the appearance of signs of toxicity. Furthermore, these were much milder compared to those in the sheep not trea ted with phenobarbitone. The present study suggests that phenobarbitone-typ e cytochrome P-450 isoenzyme-induction may increase resistance against sign al grass (B. decumbens) toxicity in sheep. (C) 2000 Harcourt Publishers Ltd .