13C Phenylalanine for Assessment of Hepatic Function in Horses
ACVIM 2008
N. Sasaki; N. Tsuzuki; H. Yamada
Department of Veterinary Clinical Science, Obihiro University of Agriculture and Veterinary Medicine
Japan

It is difficult to establish a definitive diagnostic method for hepatitis in horses as this condition is similar to other colicky diseases. Based on its metabolic pathway, 13C Phenylalanine (L-[1-13C] Phenylalanine) is expected to be a substratum for diagnose of liver diseases. However, basic study on 13C Phenylalanine breath test in horses has not been undertaken. In this study, we examined the validity and experimental conditions for use of 13C Phenylalanine breath test in horses.

Six healthy thoroughbreds (1 stallion, 3 mares and 2 geldings) were dosed with 1 ml/kg of distilled water and 2.5 mg/kg, 5 mg/kg, or 10 mg/kg 13C Phenylalanine (Tokyo Gas Chemical Corporation, Japan) through a nasogastric tube. Breath samples were collected at 5 minute intervals for 20 minutes, at 10 minute intervals for 60 minutes, at 15 minute intervals for 180 minutes, and at 30 minutes intervals for 240 minutes after dosing. Analysis of breath samples was performed using a spectrophotometer (POCone, Otsuka Electronics Corporation, Japan). The time corresponding to maximum Δ13CO2 in Δ13CO2 graph was marked as tmax and maximum Δ13CO2 was marked as Cmax.

Two horses, suspected to have hepatic disease by serum biochemical analysis, were considered as cases (case 1 and case 2) and were dosed with 13C Phenylalanine at 10 mg/kg. Breath test in these two cases was carried out under the same conditions as for healthy horses, except for the dosage.

In both the 5 mg/kg and 10 mg/kg groups, clear maximum Δ13CO2 was observed earlier than the respective healthy horses under the test. However, in the 2.5 mg/kg group tmax points were scattered over test time horizon, and no distinctive maximum Δ13CO2 was observed. Standard deviation of Cmax in the 5 mg/kg group was lower than that in the 10 mg/kg group.

In case 1, tmax was not distinct from that obtained for healthy horses. . However, tmax in case 2 appeared little lower than that observed for healthy horses. Cmax in case 1 was remarkably lower than that of healthy horses. In addition, Δ13CO2 values and cumulative 13CO2 output for both case 1 and case 2 were lower than those of healthy horses over the test time.

In the 5 mg/kg and 10 mg/kg groups, clear maximum Δ13CO2 was observed, and Cmax in the 5 mg/kg group had a standard deviation lower than that in the 10 mg/kg group. These findings indicate that 5 mg/kg is the appropriate dose of 13C Phenylalanine for breath test in horses. During the test, distinctive tmax and decreased Δ13CO2 (after a peak) were observed. Thus, the time framework set for the test was judged to be sufficient. Reference values of tmax and Cmax for healthy horses under this test were 32.5±14.6 min and 32.1±11.0‰, respectively.

Case 1 had lower Cmax than those of the healthy horses, and both case 1 and case 2 showed cumulative 13CO2 output lower than those of the healthy horses. This implies that Cmax and cumulative 13CO2 output could be considered as symptomatic indicators for liver disease.

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Naoki Sasaki


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