A cellular mechanism for imidocarb retention in edible bovine tissues.

Imidocarb dipropionate, formulated as Imizol, is used for the treatment and prophylaxis of bovine babesiosis. Several studies have shown that imidocarb remains detectable in edible ovine and bovine tissues for several months after dosing but the mechanism of retention remains unknown. In this study, the mechanism of imidocarb retention was investigated by measuring the binding of [14C]imidocarb to bovine hepatocytes, erythrocytes, sub-cellular fractions and isolated bovine macromolecules. The proportion of [14C]imidocarb (10 microM) bound to cells in suspension culture (1 x 10(7) cells.ml-1) was found to be substantially greater to hepatocytes (56.5%) than to erythrocytes (4.6%). Studies with washed erythrocytes reconstituted in plasma indicated that approximately 70% of the [14C]imidocarb was bound to plasma proteins, 10% to erythrocytes, and 20% remained free. Measurement of [14C]imidocarb binding to sub-cellular fractions prepared from bovine liver revealed preferential accumulation in the nuclear, rather than in the mitochondrial, microsomal or cytosolic fractions. Binding capacities of selected bovine macromolecules for [14C]imidocarb were in the order deoxy-ribonucleic acid (DNA) = ribonucleic acid (RNA) > > alpha 1-acid glycoprotein (AGP) > serum albumin (BSA) > haemoglobin (Hb). DNA binding sites for imidocarb remained unsaturated over the concentration range 0-100 microM [14C]imidocarb. Competitive binding studies between imidocarb and pentamidine or spermidine provided evidence for common DNA binding sites. These studies indicated that preferential binding of [14C]imidocarb to hepatocytes compared with erythrocytes observed in vitro was a result of substantial reversible binding to nucleic acids and that the same cellular mechanism may be implicated in the slow elimination of imidocarb from edible tissues in vivo.