Rumen involvement in sheep tannic acid metabolism

Tannic acid (TA) metabolism was studied in sheep after p.o., intra-abomasal and i.p. administration. TA and its phenolic metabolites in ruminal fluid, abomasal fluid, plasma and urine were determined using high-performance liquid chromatography at various intervals post-dosing. After po administration, gallic acid (GA) and pyrogallol (PYR) were present in ruminal fluid and plasma, and GA, PYR and 4-O-methyl gallic acid (40MGA) were found in urine. The concentration of GA in ruminal fluid and urine gradually decreased after dosing, while PYR concentration continued to rise. By contrast, after intra-abomasal dosing, GA and ellagic acid (EA) as well as TA were found in abomasal fluid, GA, 40MGA, EA and TA were present in plasma, and GA, 40MGA and PYR were in urine. The latter 3 metabolites were also detected in the urine after ip administration of TA. The plasma concentration of TA after intra-abomasal administration was significantly correlated with liver necrosis, and PYR concentration in ruminal fluid following po administration was significantly correlated to blood methemoglobin level. It was concluded that the methemoglobinemia seen in sheep given TA po is caused by high levels of the phenolic metabolite PYR, which is produced in the rumen. Liver and kidney necrosis, seen in sheep given TA directly into the abomasum, appears caused by unmetabolized TA rather than by metabolites.