Residues of Ractopamine and Identification of its Glucuronide Metabolites in Plasma, Urine, and Tissues of Cattle.

Ractopamine was approved by the US Food and Drug Administration for use as a growth promoter on cattle a decade ago is still banned by the European Union and most Asian countries. While, ractopamine residues in living samples and some organs (besides liver, muscle and kidney) of cattle under the recommended feed condition (10-30 mg/kg in dry matter, 28-42 days) remains unclear. In this study, nine cattle (246.22 ± 18.17 kg) were fed 0.67 mg/kg body weight (equivalent to 30 mg/kg in dry matter) of ractopamine for 28 days to investigate the residues of ractopamine in plasma, urine and organs. Plasma and urine were sampled during treatment and withdrawal period. Three cattle were slaughtered on withdrawal days 0 and 3 for organs collection. Ractopamine was determined by LC-MS/MS and its glucuronide metabolites were identified by Q-TOF/MS. Ractopamine concentrations in plasma and urine reached highest on treatment day 14 (2.88 ng/mL) and day 7 (4713.25 ng/mL), respectively. On withdrawal day 28, ractopamine concentrations in plasma and urine and were undetectable (limit of quantitation, 0.2 ng/mL) and 4.21 ng/mL, respectively. On withdrawal day 0, ractopamine residue in tissues were as follows: liver > eye > lung > spleen > aqueous fluid > heart > bile > kidney > gluteus > rib eye muscle. Compared with those on withdrawal day 0, ractopamine contents in most tissues that sampled on withdrawal day 3 were lower (P < 0.05), while that in the eye tissues, aqueous fluid, and kidney were stable or higher. These results provide extensive data for risk management in ractopamine approved countries and monitoring of the illegal usage in countries that ban ractopamine.