Mechanisms of transport of L-histidine and beta-alanine in hamster small intestine.

Whole rings of hamster jejunum and ileum were used to study the uptake of L-histidine (L-His) and beta-alanine (beta-Ala), the constituents of the dipeptide carnosine. The rate of uptake of L-His and beta-Ala (1 mM) was not significantly different in the jejunum compared with the ileum. Results of total influx (2 min) of 0.5-100 mM L-His suggested that transport was by more than one pathway, and the contribution of nonmediated component was calculated to be 0.24 mumol X g-1 X 2 min-1 X mM-1 for both jejunum and ileum. The apparent affinity of L-His for a transporter was higher in the ileum (K iota, 8.0 mM) than the jejunum (Kt, 11.7 mM). Influx (2 min) of beta-Ala was found to be linearly related to substrate concentration over the range 0.5-100 mM. The Kd (rate constant for nonmediated uptake of beta-Ala) was 0.23 and 0.14 mumol X g-1 X 2 min-1 X mM-1 for jejunum and ileum, respectively. Steady-state (20-min) uptake of L-His was significantly higher in the ileum than jejunum at substrate concentrations of 75 mM. L-His accumulated in the tissue up to a medium concentration of 50 mM in the jejunum and 75 mM in the ileum. In contrast, no evidence of tissue accumulation of beta-Ala was found in 20-min incubations. beta-Ala steady-state uptake in the ileum was significantly higher than in the jejunum at substrate concentrations of 30 and 75 mM.