Oral Administration of L-Citrulline Increases Plasma Concentrations of L-Citrulline and Arginine in Horses

L-citrulline can influence circulating concentrations of arginine and thus enhance nitric oxide (NO) generation to increase blood flow to tissue. This experiment was conducted to determine the dose response of horses to oral L-citrulline administration. The day before blood collection, mature gelding American Quarter Horses (n = 5; bw = 521 ± 27 kg) were sedated with xylazine (0.5 mg/kg IV) and fitted with an IV catheter in the jugular vein. The horses were then placed in individual stalls and allowed water and bermudagrass hay ad libitum. The following day, one h prior to treatment administration, hay was removed. Blood samples were collected via jugular catheter at -60,-30, 0, 30, 60, 90, 120, 240, 360, 720, and 1440 min relative to treatment administration. At time zero, horses received treatments of 0 g/kg (water), 0.018 g/kg (low), 0.09 g/kg (medium), or 0.18 g/kg (high) of L-citrulline dissolved in 200 ml of distilled water orally via nasal gastric tube, and hay was returned. Plasma was harvested and frozen at -20^oC for analysis of L-citruline and arginine concentrations in circulation by HPLC, as well as nitrate and nitrite levels by colorimetric assay (Cayman Chemical, Ann Arbor, MI). Horses were administered a 7 d wash out period, and the study was repeated until all horses received all treatments. Data were tested for effects of replication, dose, time and dose × time interaction using procedures for repeated measures with JMP software (version 10, SAS Inst. Inc., Cary, NC). Circulating concentrations of L-citruline were greater with the high dose compared to those of the water or low dose (P < 0.04). Circulating concentrations of arginine were greater with the high and medium dose than water or low dose (P < 0.01). There was also a dose × time effect on circulating concentrations of arginine such that the medium dose had greater arginine concentrations than water at 120 min as well as the water and low dose at 240, 360 and 720 m. The high dose had greater arginine concentrations than the water and low dose at 120, 360, and 720 min. There was no effect of dose or dose × time interaction on plasma concentration of nitrate and nitrites (P > 0.56). These results indicate oral L-citruline administration can alter circulating concentrations of both L-citruline and arginine in horses.