THE POTENCY OF DIETARY AMINO-ACIDS IN ELEVATING PLASMA CHOLECYSTOKININ IMMUNOREACTIVITY IN CATS IS RELATED TO AMINO-ACID HYDROPHOBICITY

Incomplete agreement exists on the relative potency of amino acids in stimulating endocrine secretion of cholecystokinin (CCK). Species and methodological variations have been suggested to account for the appar ent inconsistencies. In the present research, the CCK-releasing potenc y of dietary amino acids was evaluated in cats using plasma CCK-like i mmunoreactivity (CCK-LI) as an indicator of CCK secretion rather than pancreatic protein and enzyme secretion, as has been used in past rese arch. Oral-gastric administrations of a casein-simulating amino acid m ixture increased (P < 0.05) plasma CCK-LI but not to the extent of tha t observed for casein or sodium oleate. The response in plasma CCK-LI to administrations of 50 mM solutions of amino acids was significant ( P < 0.05) for tryptophan, phenylalanine, leucine, and isoleucine and t he response increased linearly (P < 0.01) with increasing amino side-c hain hydrophobicity. Control administrations of water and saline also evoked elevation in plasma CCK-LI, but the responses were so transient that amino acid effects were not obscured. This was substantiated by the finding of a significant linea; (P < 0.001) dose response to pheny lalanine administration. Cholecystokinin-8, 33 and 58 were among the C CK molecular forms identified by HPLC in plasma after administrations of phenylalanine and water. The present findings indicate that lipophi lic amino acids released during digestion account for at least part of the endocrine CCK response in cats to ingested protein. The greater C CK-releasing potency observed for intact protein relative to free amin o acids may have been the result of a slow digestive release of amino acids, elaboration of peptide secretogogues or protection of protease- sensitive releasing factors. (C) 1997 Elsevier Science B.V.