Dietary regulation of amino acid transport activity in the exocrine pancreatic epithelium.

Dietary-induced alterations in exocrine pancreatic amino acid transport were investigated in rats adapted for 14 days to isocaloric diets of varying casein and carbohydrate content. The kinetics of unidirectional (15 s) L-phenylalanine and L-lysine transport were measured relative to D-mannitol (extracellular tracer) in the perfused pancreas isolated from dietary adapted animals. In rats adapted to a 20% casein diet a weighted non-linear regression analysis of phenylalanine transport (1-24 mM) indicated an apparent Kt = 9.4 +/- 1.1 mM and Vmax = 14.8 +/- 0.9 mumol/min per g pancreas (n = 6). Saturation of lysine transport occurred at lower concentrations (0.05-10 mM) with an apparent Kt = 2.40 +/- 0.09 mM and Vmax = 2.44 +/- 0.18 mumol/min per g (n = 6). The characteristics of phenylalanine transport were modified after adaptation to either high (Kt = 3.6 +/- 1 mM, Vmax = 8.2 +/- 0.9 mumol/min per g, n = 3) or low (Kt = 4.2 +/- 0.9 mM, Vmax = 6.8 +/- 0.5 mumol/min per g, n = 3) carbohydrate diets. Increasing the dietary protein content (0-45% casein) led to a linear increase in the Kt for phenylalanine transport whereas Vmax values remained unchanged. Unlike phenylalanine, adaptation to a 0% casein diet significantly elevated the Vmax for lysine transport (4.82 +/- 0.21 mumol/min per g, n = 4) without altering the Kt (2.54 +/- 0.23 mM). The present findings suggest that changes in dietary composition induce select adaptive responses in the transport activities of System L (phenylalanine) and System y+ (lysine) in the basolateral membrane of the exocrine pancreatic epithelium.