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[¹⁴C] Monensin Balance in Bile-Fistulated Ponies¹

U.S. Department of Agriculture^{,3, 4,}, Fargo ND 58105

Abstract

To measure absorption of monensin or its metabolites and its elimination from the body, [¹⁴C] monensin sodium was given orally (1 mg/kg body wt) to two bile-fistulated ponies and iv (8.7 mg) to one bile-fistulated pony. For one orally-dosed pony, 4.7% of the ¹⁴C was eliminated in bile, 52% in feces, .7% in urine and 33% remained in the gastrointestinal (GI) tract after 3 d. Total ¹⁴C recovery was 90%. For the other orally-dosed pony, 18.3% of the ¹⁴C was eliminated in bile, 69% in feces, 1.7% in urine and 7% remained in the GI tract after 4 d. Total ¹⁴C recovery was 98%. For the iv-dosed pony, 72% of the ¹⁴C was eliminated in bile, .5% in feces, 4.9% in urine and 15% remained in the GI tract after 23 h. Total ¹⁴C recovery was 96%. Maximum rate of ¹⁴C elimination in feces from the orally-dosed ponies occurred on the third and fourth days, and the maximum rate of biliary elimination occurred between 16 and 24 h for one pony and 48 and 56 h for the other. For the iv-dosed pony, ¹⁴C became immeasurable in plasma within 12 h after dosing and appeared in bile within 30 min, with maximum biliary concentration occurring at about 1 h. Monensin accounted for a minimum of 19% of the ¹⁴C in a 56- to 72-h sample of feces for one orally-dosed pony and a minimum of 25% of the ¹⁴C in feces from the same time period for the other. Monensin could not be isolated from bile from the orally-dosed ponies, but accounted for 13% of the ¹⁴C in the 0 to 8 h bile from the iv-dosed pony. Metabolites that could not be isolated were present in all cases. These data show that monensin (or its metabolites) is absorbed to at least 18% of the dose by ponies, and that bile is the primary pathway for eliminating monensin (or its metabolites) from the body.

¹ No warranties are herein implied by the USDA.

² USDA, ARS, Vet. Toxicol, and Entomol. Res. Lab., College Station, TX 77841.

³ We thank Kristin McDonald, Jean Picard, Gene Davee, and Clayton Meyers for their assistance, and A. L. Donoho, Lilly Res. Lab., Greenfield, IN, for the [¹⁴C] monensin.

⁴ Agr. Res. Serv., Metabolism and Radiation Res. Lab.