Pharmacokinetics and tissue residues in channel catfish, Ictalurus punctatus, given intracardiac and intramuscular injections of gentamicin sulfate.

We studied the rate of absorption, distribution and elimination of gentamicin sulfate in the channel catfish, Ictalurus punctatus. It was determined that the plasma half-life of immunoreactive gentamicin was 12 to 15 hours in the catfish and the rate of elimination from fish after intracardiac and intramuscular (im) routes are not significantly different. Compared to mammals, the very extended half-life is probably the result of a greater unionized fraction of drug in the central compartment and increased reabsorption of drug from the alkaline urine of the fish. Tissue levels at the end of one dose experiments approximate what was predicted based on the intracardiac microconstants. A dosage regimen was established on a 24 hr interval that would keep peak plasma concentration between 10-13 micrograms/ml and allow trough concentrations of 3-4 micrograms/ml. Intramuscular gentamicin appears to have a bioavailability of approximately 37%. Finally, a renal lesion was produced by giving gentamicin im at 10 mg/kg every 33 hr for 6 treatments. The lesion was identical to that described for other mammals and was reversible even while medication was continued.