Determination of residual ractopamine concentrations by enzyme immunoassay in treated pig's tissues on days after withdrawal

The objective of this study was to measure residual ractopamine concentrations in tissues of pigs as experimental animals after treatment with dietary ractopamine for 28 consecutive days. Ractopamine was administered orally to the experimental group (n = 9) in a dose of 0.1 mg/kg body mass per day, whereas control animals (n = 3) were left untreated. Treated pigs (60 kg) were sacrificed on days 1, 3 and 8 of treatment discontinuation and residues were determined in kidney, liver, muscle, brain and heart tissues using previously validated enzyme-linked immunosorbent assay (ELISA) as a quantitative screening method. Validation showed good mean recoveries (approx. 70-90%) with acceptable inter- and intra-day relative standard deviations (RSD < 13%), demonstrating the method efficiency in determination of ractopamine tissue concentrations. The highest ractopamine concentration on day 1 (24 h) after the last exposure was recorded in the kidney (12.49 ± 7.96 ng/g), followed by the liver (7.21 ± 2.73 ng/g), heart (1.26 ± 0.12 ng/g) and brain (0.63 ± 0.05 ng/g); at this time of withdrawal, residues were not detected in the muscle. Ractopamine was depleted rapidly from all study tissues, with mostly no detectable residues on day 8 of withdrawal.