In vitro studies on intestinal peptide transport in horses.

This study investigates the nature and importance of peptide transporters in horses. It provides the first evidence that dipeptides are physically transported across the small intestine of horses through two distinct mechanisms: a hydrogen ion-dependent co-transport system and a separate mechanism that powers uphill transport.

Research method and result

• The research used samples from the jejunum, a part of the small intestine, taken from 16 healthy adult horses. These were studied in vitro using Ussing chamber experiments and brush border membrane vesicles (BBMV) techniques.
• Ussing chambers were used to determine the electrophysiological parameters and the unidirectional flux rates of the radiolabelled dipeptide glycylglutamine (Gly-Gln).
• BBMV techniques involved studies on the uptake of dipeptides into brush border membrane vesicles. The aim was to see how absorption of the dipeptides changed in the absence and presence of a hydrogen ion gradient, and whether a sodium ion gradient had any effect.
• The researchers found that a Gly-Gln isotopic tracer had a significantly greater short-circuit current response compared to glycylsarcosine (Gly-Sar), a dipeptide used as a transporter substrate.
• This research determined positive net flux rates, which suggested that dipeptides were absorbed, and supplemented this observation with the finding that adding Gly-Sar significantly reduced these flux rates. This implies that the two peptides competed for the same transport mechanism.
• The study could not identify any influences of luminal pH value changes on the flux rates, proving the efficiency of these transport systems under varying conditions.

Conclusion

• The data obtained during this study suggests that there are at least two absorption transport systems for peptides within horses.
• The first is a secondary active hydrogen ion-mediated co-transport system, as supported by the greater influx rates seen in the presence of an inwardly directed hydrogen ion gradient and unaffected by a sodium ion gradient.
• The second is capable of uphill transport powered by a mechanism other than a hydrogen or sodium ion gradient, which elucidates the transport of dipeptides even under equilibrium conditions where these ion gradients are not present.

In conclusion, this is the first piece of research to demonstrate that dipeptides, pairs of amino acids, are transported transcellularly across the equine small intestine. It gives a greater understanding of the physiology of nutrient transport in horses, which had been lacking in published data prior to this study.