The effect of reflex closure of the esophageal groove on bioavailability of oral sulfamethoxazole-trimethoprim in ruminating calves.

The effect of reflex closure of the esophageal groove induced by the accustomed milking procedure on absorption of sulfamethoxazole (SMX)-trimethoprim (TMP) combination was examined, using calves trained to suck through a nipple-bucket. The experiment was carried out with a two-way cross-over design in which 2 groups of 5 calves of 6 weeks old were used. SMX-TMP combination was administered to the calves in each group through a nipple-bucket or an esophageal catheter. Tmax of SMX after nipple-bucket dosing was about 40 min shorter than that after catheter dosing. Cmax and AUC of SMX after the nipple-bucket dosing were 7.5 times greater and 6.9 times larger than those after the catheter dosing, respectively. Plasma TMP concentration was measurable only after the nipple-bucket dosing. These results suggest that a large portion of the drugs administered by the nipple-bucket dosing, which induces closure of the esophageal groove, were transferred directly into the abomasum resulting in higher bioavailabilities of the drugs.