A CELLULAR MECHANISM FOR IMIDOCARB RETENTION IN EDIBLE BOVINE-TISSUES

Imidocarb dipropionate, formulated as Imizol(TM), is used for the trea tment 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 inve stigated by measuring the binding of [C-14]imidocarb to bovine hepatoc ytes, erythrocytes, sub-cellular fractions and isolated bovine macromo lecules. The proportion of [C-14]imidocarb (10 mu M) bound to cells in suspension culture (1 x 10(7) cells . ml(-1)) was found to be substan tially greater to hepatocytes (56.5%) than to erythrocytes (4.6%). Stu dies with washed erythrocytes reconstituted in plasma indicated that a pproximately 70% of the [C-14]imidocarb was bound to plasma proteins, 10% to erythrocytes, and 20% remained free. Measurement of [C-14]imido carb binding to sub-cellular fractions prepared from bovine liver reve aled preferential accumulation in the nuclear, rather than in the mito chondrial, microsomal or cytosolic fractions. Binding capacities of se lected bovine macromolecules for [C-14]imidocarb were in the order deo xy-ribonucleic acid (DNA) = ribonucleic acid (RNA) much greater than a lpha(1)-acid glycoprotein (AGP) > serum albumin (BSA) > haemoglobin (H b). DNA binding sites for imidocarb remained unsaturated over the conc entration range 0-100 mu M [C-14]imidocarb. Competitive binding studie s between imidocarb and pentamidine or spermidine provided evidence fo r common DNA binding sites. These studies indicated that preferential binding of [C-14]imidocarb to hepatocytes compared with erythrocytes o bserved in vitro was a result of substantial reversible binding to nuc leic acids and that the same cellular mechanism may be implicated in t he slow elimination of imidocarb from edible tissues in vivo.