Influence of abomasal carbohydrates on small intestinal sodium-dependent glucose cotransporter activity and abundance in steers

Most animals adapt readily to increased supplies of carbohydrate in the intestinal lumen by increasing enzymes for degradation and increasing glucose transporter activity. However, the extent of upregulation of Na ⁺-dependent glucose cotransporter 1 (SGLT1) activity and content in response to increased delivery of carbohydrate to the small intestinal lumen of ruminants is unclear. Therefore, an experiment was conducted to determine the effect of glucose and starch hydrolysate on the activity and abundance of SGLT1 in the small intestine of steers. In a randomized complete block design, 40 cross-bred beef steers (243 ± 2 kg BW) were fed 0.163 Mcal of ME/(kg BW^0.75(d; W), 0.215 Mcal of ME/(kg BW^0.75·d; 2M), or 0.163 Mcal ME/(kg BW^0.75·d) and infused for 35 d into the rumen (R) or abomasum (A) with 12.6 g/(kg BW^0.75·d) of starch hydrolysate (S) or into the abomasum with 14.4 g/(kg BW^0.75d) of glucose (G). Steers were slaughtered, and brush-border membrane vesicles were prepared from the small intestinal samples obtained from five equidistant sites along the intestine. Maltase activity in vesicles and homogenates differed with intestinal sampling site (quadratic, P < 0.001). Steers on the AG treatment yielded a greater intestinal maltase activity (38 nmol glucose·mg protein^-1·min^-1) compared with the AS, RS, W, or 2M treatments (34, 26, 23, and 23 nmol glucose·mg protein^{- 1}·min^-1, respectively [SEM = 3; P = 0.02]). Sodium-dependent glucose uptake averaged 18.4 ± 3.94 pmol glucose/(mg protein·s) and was not affected by treatment, but uptake decreased distally along the intestine (P < 0.001). There was no effect of treatment on SGLT1 protein abundance, but SGLT1 protein abundance increased linearly from the duodenum to the ileum (P = 0.05). The inverse relationship between glucose uptake and SGLT1 abundance suggests that the regulation of brush border Na ⁺-dependent glucose transport capacity is complex, involving factors other than the presence of luminal carbohydrate.