Abstract

Two experiments were conducted to determine the effect of protein supplementation strategies on the expression and distribution of urea transporter-B (UT-B) in lambs fed low-quality forage. In Experiment 1, fifteen Dorset wether lambs were fed increasing levels of dietary crude protein to determine its effect on expression and distribution of UT-B. In Experiment 2, the effects of supplementation frequency and ruminal protein degradability on expression and distribution of UT-B in thirteen Dorset wether lambs were studied. Nine tissues of importance to the N recycling process were sampled for Western blot analyses for UT-B using a polyclonal antibody created against the carboxy-terminal 19 amino acids known for human erythrocyte UT-B1. Western blots confirmed the presence of distinct 32 kDa (consistent with a non-glycosylated UT-B protein) and 47 kDa (consistent with an N-glycosylated form of UT-B) protein bands in all tissues analyzed. In both experiments, the liver, dorsal rumen, reticulum, and ventral rumen displayed strong bands at 32 kDa and lighter bands at 47 kDa, while the cecum, large colon, spiral colon, and parotid salivary gland displayed slight 32 kDa bands and stronger, more visible bands at 47 kDa. The abundance of the 47 kDa UT-B band in the ventral rumen was greater (P = 0.03) in lambs fed RDP daily in Experiment 2, but no other treatment differences (P = 0.15 to 0.99) in the abundance of the 32 or 47 kDa UT-B proteins within tissues were observed in either experiment. Although protein supplementation strategies may have little effect on UT-B expression and distribution, greater expression of the N-glycosylated form of UT-B in the ventral rumen in response to daily supplementation with ruminally degradable protein suggests that UT-B may serve an excretory function for urea, rather than recycling of urea into the gastrointestinal tract. Nonetheless, further research into the physiological significance of N-glycosylation in the regulation of UT-B function is needed.