Comparative pharmacokinetics of chlortetracycline in milk-fed versus conventionally fed calves.

Plasma and tissue concentration and pharmacokinetics of chlortetracycline (CTC) was determined in milk-fed and conventionally fed Holstein calves. A two-compartment open model was used after a single intravenous dose (11 mg CTC/kg body weight). There were no significant differences between dietary treatments. The drug was rapidly distributed from plasma into the peripheral compartment but was slowly eliminated, with detectable concentration of CTC continuing for 72 h after dosing. A single-compartment model was used after a single oral dose (22 mg CTC/kg body weight). All but four of the kinetic parameters were significantly different for the two dietary treatments. Milk-fed calves had a larger area under the plasma level curve, a larger fraction of the dose absorbed, a smaller volume of distribution and a smaller overall body clearance rate. Estimated recovery of CTC in the urine of the milk-fed calves was greater, regardless of route of administration. The concentration of CTC in tissues following an oral dose was greatest in kidney, followed by liver, heart, skeletal muscle, spleen and brain. Tissue depletion of CTC closely paralleled the decline in plasma concentration.