Uptake of L-leucine and L-phenylalanine across the basolateral cell surface in isolated oxyntic glands.
The time course, kinetic, specificity and sodium-dependence of L-leucine and L-phenylalanine uptake by rabbit isolated oxyntic glands were studied in order to identify the systems involved in the transport of branched-chain and aromatic neutral amino acids through the basolateral cell membrane. The uptake was measured directly in the disrupted cells after incubation of the glands with the 3H-labelled amino acid both in a sodium-containing and a sodium-free medium. The uptake of L-leucine was largely carrier-mediated whilst L-phenylalanine was taken up by either carrier-mediated and nonsaturable processes. Both amino acids were taken up by a Na(+)-independent process. The kinetic parameters of L-leucine and L-phenylalanine carrier-mediated influx were, respectively: Kt = 2.71 mM and Jmax = 1390 nmol mg-1 s-1, Kt = 1.03 mM and Jmax = 176 nmol mg-1 s-1. From cross-inhibition studies it can be inferred that L-leucine is primarily transported by a Na(+)-independent system which shows specificity for bulky side chains dipolar amino acids. The system displays similar affinities for L-phenylalanine (Ki = 2.81 mM) and L-isoleucine (Ki = 2.62 mM). Similar results were obtained from self-inhibition experiments: the Ki of the carrier-mediated uptake of L-leucine and L-phenylalanine were 2.12 and 2.40 mM (from a Hanes plot) or 3.2 and 0.8 mM (from a Dixon plot), respectively. It is concluded that a sodium-independent transport system, like Christensen's 'L' type, is shared by branched-chain and aromatic dipolar amino acids, which only shows slight differences in their affinities for the carrier.