Studies on elimination regularity of doxycycline residues in Ictalurus punctatus at different temperatures.

Doxycycline(DOTC) is a tetracycline broad-spectrum antibiotic widely used for the treatment of systemic bacterial infections.The broad-spectrum antibiotic doxycycline(DOTC) has commonly been used in channel catfish aquaculture to prevent and treat bacterial diseases.In order to study the tissue distribution and elimination of doxycycline(DOTC) in channel catfish(Ictalurus punctatus),continuous(for 5 days) oral administration(20 mg/kg body weight) of DOTC has been investigated in healthy channel catfish at(18±1) ℃ and(28±1) ℃water temperature.This is first report on tissue distribution and residues of DOTC in channel catfish.After repeated oral administration for 5 consecutive days,channel catfish(Ictalurus punctatus) were killed and the muscle and skin,liver,kidney tissues were collected after 1,3,5,7,9,12,15,24,30,40 d of oral administration.Doxycycline(DOTC) in tissues was determined by using high performance liquid chromatography(HPLC) with UV detection.The mobile phase was 0.01 mol/L oxalic acid∶acetonitrile∶methanol(70∶20∶10,V/V/V) and UV detection set at 350 nm.Analyte was extracted with Mcllvaine buffer-EDTA extraction,whose pH was 4.0.DOTC was eluted from Oasis HLB column with 2 mL methanol.The standard curves were linear in the investigated ranges,0.05-10.0 μg/g of DOTC in tissues.The correlation coefficient for DOTC was both r≥0.9964.The recovery of DOTC was from 75.13%to 92.05%.The within-day and day-to-day precision expressed by CV was less than 10% at two drug levels(0.1 and 1.0 μg/g).The detection limit for DOTC in muscle and skin,liver,kidney was 10,20 and 20 μg/kg respectively.The results show that the depletion speed of DOTC in channel catfish closely relates with the water temperature.The elimination rate was markedly different at different water temperature in the same tissues.The depletion of DOTC residue in channel catfish was faster at high water temperature.To compare with other tissues,the elimination of DOTC residue in muscle and skin was other slowest,liver was the main reservoir of DOTC residue in channel catfish.The speed from fast to slow is muscle and skin,kidney,liver.It took liver as target tissue in this experiment.According to the maximum residue limit(MRL) of 300 μg/kg in liver,the withdrawl period should not be less than 30 d and 55 d respectively at(18±1) ℃ and(28 ± 1) ℃water temperature.This study aims to provide a theoretical basis to make dosage regimen and withdrawl time at different water temperatures.