Functional and molecular biological evidence of SGLT-1 in the ruminal epithelium of sheep

Because of the effective catabolism of D-glucose to short-chain fatty acids by intraruminal microorganisms, the absorption of D-glucose from the rumen was thought to be of minor importance. However, clinical studies suggested that significant quantities of D-glucose are transported from the ruminal contents to the blood. We therefore tested the ruminal epithelium of sheep for the presence of Na+-glucose cotransporter 1 (SGLT-1) on both the functi onal and mRNA levels. In the absence of an electrochemical gradient, 3-O-me thylglucose (3-OMG) was net absorbed across isolated ruminal epithelia moun ted in Ussing chambers. The net transport of 3-OMG followed Michaelis-Mente n kinetics and was sensitive to phlorizin or decreasing Na 1 concentrations . The mucosal addition of 10 mM D-glucose induced an immediate, phlorizin-s ensitive increase in short-circuit current (I-sc). I-sc could also be incre ased by serosal addition of D-glucose or D-mannose, but electrogenic uptake of D-glucose or 3-OMG added on the mucosal side was still detectable after serosal stimulation of I-sc. RT-PCR using primers specific for the ovine i ntestinal SGLT-1 with subsequent TA cloning and sequencing revealed 100% id entity between the cloned cDNA and mRNA fragment 187-621 of ovine intestina l SGLT-1. In conclusion, the ruminal epithelium has a high-affinity SGLT-1, which indicates that it maintains the capacity for D-glucose absorption.