QUANTIFICATION OF CIRCULATING PEPTIDES AND ASSESSMENT OF PEPTIDE UPTAKE ACROSS THE GASTROINTESTINAL-TRACT OF SHEEP

Gastrointestinal absorption of peptides was examined in sheep fed a fo rage-based diet. Peptide concentrations were determined in arterial, p ortal, and mesenteric blood and plasma by quantification of amino acid concentrations before and after acid hydrolysis of samples that had b een first deproteinized then subjected to Sephadex G-15 gelfiltration to remove residual protein. In contrast to other studies of ruminants, peptide concentrations for individual amino acids were lower than for the corresponding free amino acids with peptide (expressed as a propo rtion of total nonprotein amino acid) representing not more than .25 t o .3 of total amino acid. Peptide concentrations in arterial, mesenter ic, and portal blood and plasma were similar, indicating that on this diet there was no net uptake of peptides from the small intestine (mes enteric-drained viscera, MDV) or the whole tract (portal-drained visce ra, PDV). Increasing the intake of alfalfa pellets from 800 to 1,200 g /d, while increasing the absorption and net flux across the MDV and PD V of free amino acids, had no effect on peptide absorption. Preparatio n of blood and plasma samples for peptide analysis with methods used i n studies in which substantial peptide absorption has been reported in dicated no net MDV or PDV flux of peptide. Such conflicting data on th e extent of gastrointestinal peptide flux are discussed in the context of methodological differences and the importance of diet and physiolo gical state of the animal.