Pharmacokinetics of Intravascularly Administered 65Zinc in Channel Catfish (Ictalurus punctatus)

Comparison was made of the pharmacokinetics of the radioisotope 65Zinc (65Zn) in blood, plasma, and whole body of adult channel catfish (Ictalurus punctatus) following intravascular (iv) administration. A two-compartment model described the pharmacokinetics of 65Zn in plasma and blood during the first 40 days following iv administration, but was unable to describe the long-term disposition of 65Zn. Whole-body counting revealed that approximately half of the 65Zn dose was sequestered in a slowly exchangeable pool with a half-life of 1.5 years. Greater than 99% of the circulating 65Zn was bound to plasma proteins, whereas there was less than 1% binding to red blood cells. Synthesis of the results for channel catfish and existing data in other species indicates three phases in the pharmacokinetics of zinc. The first phase consists of initial distribution outside the vascular system to kidney, liver, and other organs (alpha phase in blood and plasma; t1/2 of 4 to 5 h). The second phase involves distribution from organs to a slowly exchangeable zinc pool, likely consisting of bone (beta phase in blood and plasma; alpha phase in whole body; t1/2 of 4 to 20 days). The third phase appears to involve a slow turnover of sequestered zinc (t1/2 greater than 1 year). Blood sampling or short-term whole-body measurements will underestimate the persistence of zinc in fish, thus prolonged sampling and measurement of whole-body concentrations are necessary to characterize the pharmacokinetics of zinc.