In vitro studies on intestinal peptide transport in horses

Published data on the physiology of nutrient transport across the equine intestine are limited, and the existence and relevance of peptide transporters are still unknown in the horse. In the present study, the equine intestinal peptide transport was investigated by Ussing chamber experiments using the radioisotope tracer technique and by uptake studies into brush border membrane vesicles (BBMV). Jejunal mucosae of 16 healthy adult horses were used. Tissue samples were mounted in Ussing chambers, and electrophysiological parameters as well as unidirectional flux rates of the radiolabelled dipeptide glycylglutamine (Gly-Gln) were determined. The short-circuit current (Isc) response to the luminal addition of Gly-Gln was significantly greater compared to the Isc response to glycylsarcosine (Gly-Sar) addition (P<0.01). Positive net flux rates were determined indicating absorption of the dipeptide. The addition of Gly-Sar reduced the flux rates significantly (P<0.01), suggesting that both peptides compete for the same transport system. The flux rates were not affected by changes in luminal pH value. Uptake studies into BBMV demonstrated an uphill transport in both the absence and the presence of an inwardly directed H+-gradient with the H+-mediated uphill transport being significantly greater than the transport under equilibrium conditions (P<0.001). A Na+-gradient did not cause an uphill transport. The Gly-Gln uptakes displayed Michaelis-Menten kinetics with the Km value for the H+-dependent Gly-Gln uptake being significantly different from the Km value for the Gly-Gln uptake under equilibrium conditions (P<0.05). In conclusion, the study demonstrated for the first time that dipeptides are transcellularly transported across the equine small intestine. The results indicate the presence of at least 2 transport systems for peptide absorption in the horse: 1 secondary active H+-mediated cotransport and 1 that is capable of an uphill transport energized by a mechanism other than a H+ or a Na+-gradient.