AMINO-ACID USE BY THE GASTROINTESTINAL-TRACT OF SHEEP GIVEN LUCERNE FORAGE

Essential amino acid (EAA) utilization by gastrointestinal tract (GIT) tissues has been investigated in sheep given 800 and 1,200 g/day luce rne pellets. Animals prepared with indwelling catheters into the aorta and the portal drained viscera plus cannulas into the small intestine were infused with mixed U-C-13-labeled amino acid or [1-C-13]leucine tracers into the jugular vein or directly into the small intestine. GI T sequestration of EAA from arterial and luminal AA pools was determin ed from tracer and tracee arterioportal concentration differences at b oth levels of intake. Proportional tracer C-13-labeled EAA extraction of the arterial supply, on first pass across the GIT during jugular in fusion, ranged from 0.063 for histidine to 0.126 for leucine. Recovery of intestinally infused tracer C-13-EAA at the portal vein ranged fro m 0.61 for histidine to 0.83 for valine. These data were independent o f intake. Calculated rates of tracee sequestration by GIT tissues repr esented 0.45-0.65 of whole body EAA flux, except for histidine, for wh ich the values were much lower (0.25-0.32). With the exception of phen ylalanine, more than 0.8 of the EAA used by the GIT was extracted from circulating blood, thus calling into question the theory that GIT tis sues make preferential use of EAA during absorptive metabolism, restri cting supply to peripheral tissues such as skeletal muscle (growth) or mammary glands (lactation). Instead the GIT seems to compete very suc cessfully with these tissues for circulating blood EAA.