The potency of dietary amino acids in elevating plasma cholecystokinin immunoreactivity in cats is related to amino acid hydrophobicity

Abstract

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 apparent inconsistencies. In the present research, the CCK-releasing potency of dietary amino acids was evaluated in cats using plasma CCK-like immunoreactivity (CCK-LI) as an indicator of CCK secretion rather than pancreatic protein and enzyme secretion, as has been used in past research. Oral–gastric administrations of a casein-simulating amino acid mixture increased ( P<0.05) plasma CCK-LI but not to the extent of that 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 the response increased linearly ( P<0.01) with increasing amino side-chain 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 linear ( P<0.001) dose response to phenylalanine administration. Cholecystokinin-8, 33 and 58 were among the CCK molecular forms identified by HPLC in plasma after administrations of phenylalanine and water. The present findings indicate that lipophilic amino acids released during digestion account for at least part of the endocrine CCK response in cats to ingested protein. The greater CCK-releasing potency observed for intact protein relative to free amino 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.